NO341810B1 - Optical wavelength grid interrogation system and wavelength grid sensor system - Google Patents

Abstract

An optical interrogation system 10 comprises an optical amplifying and signal selecting means, in the form of a semiconductor optical amplifier (SOA) 14 and an optical source 12, 14. Operating means 22 (an electric pulse generator driven by a variable frequency oscillator) are arranged to generate electric operating pulses (see insert (a)) applied to the SOA 14 to turn the SOA 14 on and off. The optical source comprises a super-light emitting diode (SLD) 12, the continuous wave output from which is gated into optical pulses of the SOA 14. The SOA 14 is optically connected to the waveguide 16 which contains an array of reflecting optical elements (gratings G) to be interrogated. The interrogation system further comprises optical detection means 18, optically coupled to the SOA 14, which can evaluate the wavelength of a returned optical pulse transmitted by the SOA 14.

Description

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The invention relates to an optical waveguide grating interrogator system and an optical waveguide grating sensor system, as appears from the introductory part of patent claims 1 and 12 respectively.

Background

The two most used methods for interrogation gratings in a multi-grating matrix are wavelength multiplexing 5 ("WDM (Wavelength Division Multiplexing)") and time multiplexing ("TDM (Time Division Multiplexing)"). In the case of WDM-based systems, each grating within the array is fabricated to have a different resonant wavelength, placed in a different wavelength window. A continuous wave broad bandwidth optical source is used to illuminate all the gratings simultaneously, and the resulting reflected signal from each grating is simultaneously present in the output spectrum produced for a wavelength measuring system. The wavelength measuring system must therefore be able to measure several wavelengths simultaneously. This is a distinct feature for a scanning instrument, such as an optical spectrum analyzer, but creates a problem for instruments such as wave meters. Another disadvantage of WDM-based systems is that as the number of gratings in the array increases, so does the required operating bandwidth of the optical source and wavelength measurement system.

15 In contrast, TDM-based systems usually allow all gratings in the array to be placed in the same wavelength window, with the gratings illuminated by a pulsed optical source. In the simplest TDM-based system, a separate short broad bandwidth pulse ("single short broad bandwidth pulse"), sent into one end of a fiber, will reach a particular grating in the array at a particular time. The grating will reflect part of the optical pulse and the reflected signal will propagate back in the fiber towards the optical source and a wavelength measuring system.

Pulses reflected from other gratings inside the array will arrive at the measurement system at different times, since they will have traveled different distances. Differentiating the absolute wavelength for each grating makes use of the fact that light travels in a fiber at a calculated speed of about 200000000 meters per second. The difficulty in operating this type of TDM-based system is that it requires a wavelength measurement system that can operate at high speed. That's because the wavelength of each reflected pulse must be determined before the next pulse arrives. Due to the speed of light and regular grating spacing, the reflected pulses are typically only nanoseconds apart.

An alternative TDM technique, which does not depend on high-speed wavelength determination, utilizes short optical pulses of a separately known wavelength. Only gratings in the array whose resonance wavelength corresponds to that pulse will reflect the pulse. By changing the wavelength of the optical signal between pulses, the entire spectrum of the grating array can be scanned and the wavelength of 341810

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each lattice is determined. The difficulty with this TDM technique is that it requires high-speed photodiodes to detect the reflected pulses and high-speed signal processing electronics. The need to have an optical scanning source, such as a tunable laser diode or a bandwidth light source or a scanning filter, is also a problem, as these devices are often large and expensive.

5 A problem that is faced with all these grating interrogator systems is that it is difficult to obtain optical sources with highly efficient wide bandwidth. Since a wide bandwidth source by its nature covers a large range of wavelengths, the power for each wavelength is usually very low (typically -30db/nm). In a TDM-based system, this problem is compounded by the fact that only short pulses with long pulse interval delays are received by the measurement system, which further results (often substantially) in reduction in the average optical power received by the measurement system.

In order to maximize the available effect, it is therefore necessary to ensure that the reflectivity of the gratings in the matrix is relatively large. Increasing grating reflection increases the available signal at the measurement system, but has the disadvantage that it limits the number of gratings that can be in the array.

15 This is because for each grating in the matrix that incident pulse must pass through, it loses some of its strength as it is reflected back from the grating. The more gratings the pulse passes through, the more effect it loses. In addition, the reflected light from a grating that propagates back towards the measuring system will also experience degradation in effect due to further reflections from other gratings between the grating and the measuring system.

20 For gratings with 10% or higher reflectivity, the rate of reduction in power can be dramatic.

This can limit the total number of gratings in an array to around five to ten, and can mean that the measurement system must be able to cope with a wide range of optical input power levels. A further disadvantage of using highly reflective gratings in a TDM-based sensor system is that significant interference can occur due to the multiple reflections of the illuminated optical pulses.

Another type of optical interrogation system is an optical time domain reflectometer ("OTDR (optical time domain reflectometer)"). In an ODTR, a light source (light emitting diode or laser diode) is turned on for a short period of time to generate an output optical pulse. The light travels along, for example, a section of an optical fiber under test and is reflected from any reflection point inside the fiber.

30 The reflection points are common breaks in the fiber or optical components, such as patch cord connectors. Reflections can also occur due to general fiber bireflexes. The reflected signals are registered as a series of pulses through a detector. Management of the 341810

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the reflected pulses relative to the outgoing optical pulse determine the position of the breaks or components along the fiber. The magnitude of the reflected optical signal depends on the type of reflection point. For example, a reflection from a patch cord connector will be around 4%, while the magnitude of a reflection from a broken fiber will be less than this.

5 The main problem with existing OTDR technique is that it requires very fast signal processing electronics to separate the arrival times of different reflected pulses. The use of fast electronics means that the measurement accuracy of the size of the reflected pulses is small. In addition, the reflected optical signals usually have low power, resulting in poor signal-to-noise ratio.

10 US 2002/025097 describes a method and devices for time domain demultiplexing of serial fiber Braggitter sensor groups where the optical signals reflected from the gratings are synchronized by an electronically regulated optical modulator before any wavelength measurement is performed. The device also includes an optical amplifier for amplifying the reflected optical signals before they are synchronized by the optical modulator.

15 Stubkjer, K.E. et al "Recent advances in semiconductor optical amplifiers and their applications", Proceeding of the International Conference on Indium Phosphide and related materials, 1992, IEEE, vol. Conf.4, April 21, 1992, pp. 242-245 reports that semiconductor optical amplifiers (SOAs) can be used as gates in high-speed switching circuits, where the light is regulated by modulating the injection current supplied to the SOA.

20 The invention

An optical wavelength grating interrogator system and an optical wavelength grating sensor system according to the invention appear from the characterizing part of patent claims 1 and 12, respectively. Further advantageous features appear from the associated independent claims.

In accordance with a first feature of the invention, an optical waveguide grating interrogator system 25 has been created which comprises:

an optical source capable of generating optical pulses to be sent into an optical waveguide comprising one or more gratings to be interrogated,

wavelength evaluation apparatus;

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and two-way optical amplifying and signal selecting means optically coupled to the waveguide, wherein the wavelength evaluation apparatus is optically coupled to the optical amplifying and signal exchanging means, and which can evaluate the wavelength of a returned optical pulse transmitted by the optical amplifying and signal exchanging means, characterized in that the two-way 5 optical amplifying and signal selecting means is a single optical device and operable for selective transmission of an optical pulse returned from a grating under interrogation and for simultaneous optical amplification of an optical signal transmitted therethrough.

The two-way optical amplifying and signal selecting means is preferably an optical amplifier device, such as an optical semiconductor amplifier, with a switching function; wherein when the optical amplifying and signal selecting means is turned on, transmits and amplifies an optical signal, and when it is turned off, the transmission and amplification of optical signals is prevented.

The optical waveguide grating interrogator system further preferably comprises an operating means for the bidirectional optical amplifying and signal selecting means, which operating means is useful for generating electrical operating pulses of variable frequency and for switching the optical amplifying and signal selecting means on and off.

The operating means are preferably usable for generating electrical operating pulses with variable frequency and duration. The operating means can alternatively generate pairs of electrical operating pulses. The operating means can further alternatively generate a continuous stream of electrical operating pulses, at a selected frequency and each having the same duration. The duration of an electrical operating pulse is preferably shorter than twice the time required for an optical signal to travel between the nearest adjacent reflective optical elements.

The operating means may comprise an electric pulse generator which is triggered by a frequency oscillator, or alternatively comprise a digital programmable pulse generating device.

A plurality of optical amplifying and signal selecting means can be arranged, each optically coupled to a respective optical waveguide comprising one or more reflective optical elements to be interrogated. Preferably, each of the optical amplifying and signal selecting means is controlled by a separate operating means, which operating means can be selectively connected to the respective optical amplifying and signal selecting means of the optical waveguide comprising a selected reflective optical element to be interrogated.

30 The optical source preferably comprises the two-way optical amplifying and signal selecting means, wherein when the optical amplifying and signal selecting means are switched on simultaneously generate 341810

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an optical signal, in the form of amplified spontaneous emission, and ports the optical signal into an optical pulse.

The optical source can alternatively comprise a continuous optical wave source ("continuous wave optical source") which can generate an optical wave signal ("continuous wave optical signal"), such as a super-luminescent optical diode, connected to the two-way optical amplifying and the signal selecting means, wherein when the optical amplifying and signal selecting means is turned on and off, it gates the continuous optical wave signal into optical pulses.

The optical source may additionally comprise an optical filter arranged between the continuous optical wave source and the optical amplifying and signal selecting means, or between the optical amplifying and signal selecting means and the optical waveguide.

The optical source can further alternatively comprise a pulsating optical source which can generate optical pulses. The optical source may additionally comprise an optical filter arranged after the pulsating optical source. The interrogation system further preferably comprises optical coupling means for coupling the optical pulses into the optical waveguide. The optical coupling means may comprise an optical waveguide routing device ("optical waveguide routing device"), such as an optical waveguide coupling, or alternatively comprise an optical coupling device.

The optical filter has a transmission profile which mainly corresponds to the spectral range occupied by the one or more reflective optical elements to be interrogated, where the optical filter thus limits the optical bandwidth of the pulses with regard to wavelength.

Where the reflective optical elements to be interrogated are connected to two or more different optical waveguides, an optical source can be arranged for each optical waveguide. Alternatively, a separate optical source can be used, where the optical source is connected to the optical waveguides via an optical routing element ("optical routing element"), such as an optical waveguide coupler or an optical switch.

25 The optical detection means can comprise a photodetector. The optical detection means may alternatively comprise wavelength evaluation apparatus optically coupled to the optical amplifying and signal selecting means, and may evaluate the wavelength of a returned optical pulse, transmitted through the optical amplifying and signal selecting means.

The wavelength evaluation apparatus may comprise an optical filter element having a wavelength dependent filter response followed by optical detection means, such as a 341810

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photodetector, where the electrical output from the optical detection means is processed through signal processing means which can determine the wavelength of a detected optical signal; the signal selectivity of the optical amplifying and signal selecting means identifies which grating it was returned from and the optical strength of the filtered signal identifies its wavelength. The optical filter element may comprise an optical bulk filter ("bulk optic optical filter"). The optical filter may alternatively comprise an optical waveguide grating, such as a high frequency Bragg grating ("chirped Bragg grating") having a reflectivity that varies substantially linearly with wavelength across the spectral profile of the grating, or a tilted optical waveguide grating which has a coupling coefficient that varies with wavelength of 10 across the spectral profile of the grating.

The wavelength evaluation apparatus may alternatively comprise a wavemeter or an optical spectrum analyzer, where the signal selection of the optical amplifying and signal selecting means identifies which reflective optical element from which an optical signal was returned and the wavemeter or optical spectrum analyzer measures the wavelength of the optical signal.

The interrogating system can further comprise a part of an optical waveguide, connected between an optical amplifying and signal selecting means and its respective optical waveguide which contains one or more reflective optical elements to be interrogated.

The interrogating system may alternatively comprise or additionally further comprise optical signal routing means 20 ("optical signal routing means") configured to route an optical pulse returned from a reflective optical element which is interrogated back through the optical amplifying and signal selecting means, in the direction towards the reflective optical the element under query.

The presence of the optical amplifying and signal selecting means creates an optical pulse that has been reflected from a reflective optical element which is asked to reflect back through the optical amplifying and signal selecting means, back into the optical waveguide and back to the reflective optical element which is asked, where it is again reflected, also further. In this way, the optical pulse experiences multiple amplifications as it travels back and forth from the reflective optical element under interrogation, passing through the optical amplifying and signal selecting means each time.

30 The optical singal routing means preferably comprises an optical reflector arranged after the optical amplifying and signal selecting means, where the reflector is placed sufficiently close to the optical amplifying and signal selecting means to ensure that the time it takes an optical signal to be transmitted 341810

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from the optical amplifying and signal selecting means to the reflector and back to the optical amplifying and signal selecting means, is shorter than the duration of the electrical drive pulse that turns on the optical amplifying and signal selecting means.

The spectral profile in the reflection of the optical reflector preferably covers the same spectral range that is occupied by one or more reflective optical elements to be interrogated. The optical reflector can have a reflectivity that is less than 100%, thus a part of the optical signal is transmitted which is then routed to the wavelength evaluation apparatus. The optical reflector can be connected to an optical signal output element, such as an optical waveguide coupler, thus enabling the optical reflector to have a reflectivity of up to 100%, where part of the optical signal reflected from the optical reflector is extracted and routed to the wavelength evaluation apparatus.

The interrogator system can further alternatively comprise a series of optical reflectors arranged after the optical amplifying and signal selecting means, each reflector is placed at different distances from the optical amplifying and signal selecting means, the most distant 15 reflector is placed sufficiently close to the optical amplifying and signal selecting means for to ensure that the time taken for an optical signal to be transmitted, from the optical amplifying and signal selecting means to the most distant reflector and back to the optical amplifying and signal selecting means, is less than the duration of the electrical drive pulse which turns on the optical amplifying and the signal selecting agent. The spectral profile in reflection 20 for each optical reflector preferably covers a different spectral range.

The or each optical reflector is preferably an optical waveguide grating, and is preferably a Bragg grating.

The optical signal routing means may alternatively comprise a second optical waveguide, extending between them, and optically connected to the optical waveguide coupled to the one or more reflective elements and the optical amplifying and signal selecting means, so that optical pulses propagate through the optical amplifying and the signal selecting means in only one direction, which is towards the one or more reflective optical elements.

The interrogating system may further comprise an optical filter arranged in front of the (set of an optical signal) wavelength evaluation apparatus, the optical filter having a transmission profile which mainly corresponds to the spectral range occupied by the one or more reflective elements to be interrogated, where the optical filter thus functions to remove substantially any portion of a returned optical signal at wavelengths outside the spectral range of the one or more reflective optical elements.

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The one waveguide or each optical waveguide can be an optical fiber or a planar optical waveguide.

In accordance with a second feature of the invention, an optical sensor system is arranged which comprises:

5 an optical waveguide connected at one end to one or more reflective elements;

wherein the optical waveguide is connected at its other end to an optical interrogator system in accordance with the first feature of the invention.

The optical sensor system preferably comprises an optical waveguide connected to a matrix of spaced optical waveguide gratings. The resonant wavelength of each grating in the array is preferably within the same wavelength window, and all the gratings thus operate within a separate optical channel.

Alternatively, the gratings in the matrix can be arranged in groups, where each group comprises a substantially identical set of gratings, where the resonance wavelength of each grating in the group lies within a different wavelength window, and thus operates within a different optical channel, so that the time transit time 15 for a returned optical pulse identifies which group a grid being queried belongs to.

The sensor system can further comprise a plurality of optical waveguides comprising a matrix of optical waveguide gratings with spaces, where each waveguide is connected to a respective optical amplifying and signal selecting means.

The one optical waveguide or each optical waveguide may be an optical fiber or a planar optical waveguide. The array of optical waveguide gratings is preferably a Bragg grating.

The one or each reflective optical element can alternatively comprise a Fabry-Perot spectroscope ("Fabry-Perot etalon device") which can be a bulk optical Fabry-Perot spectroscope, an optical fiber Fabry-Perot spectroscope ("optical fiber Fabry-Perot etalon"), or an optical waveguide grating-based Fabry-Perot spectroscope. The one or each reflective optical element 25 can further alternatively comprise an end of an optical fiber, which can be provided with a mirror on the end, the end of an optical fiber patch-cord, a break in part of a optical fiber, a crystal-based reflective optical element, or a mirror element.

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Example

Particular embodiments of the invention will now be described in detail, by way of example only, with reference to the attached drawings, in which:

Fig. 1(a) schematically shows an optical interrogator system in accordance with a first embodiment of the invention and an optical sensor system in accordance with a second embodiment of the invention,

Fig. 1(b) schematically shows a digitally programmable alternative means of operation for use in the query system in Fig. 1(a),

Fig. 2 schematically shows an optical interrogation system in accordance with a third embodiment of the invention and an optical sensor system in accordance with a fourth embodiment of the invention,

10 Fig. 3 shows a schematic illustration of the interference that occurs when several pulses are transmitted in an optical sensor system,

Fig. 4 schematically shows an optical interrogation system in accordance with a fifth embodiment of the invention and an optical sensor system in accordance with a sixth embodiment of the invention,

Fig. 5 schematically shows an optical interrogation system in accordance with a seventh embodiment of the invention and an optical sensor system in accordance with an eighth embodiment of the invention,

Fig. 6 schematically shows an optical interrogation system in accordance with a ninth embodiment of the invention and an optical sensor system in accordance with a tenth embodiment of the invention,

Fig. 7 schematically shows an optical interrogation system in accordance with an eleventh embodiment of the invention and an optical sensor system in accordance with a twelfth embodiment of the invention,

20 Fig.8 schematically shows the wavelength evaluation apparatus in Fig.7,

Fig. 9 schematically shows an optical interrogation system in accordance with a thirteenth embodiment of the invention and an optical sensor system in accordance with a fourteenth embodiment of the invention,

Fig. 10 schematically shows an optical interrogator system in accordance with a fifteenth embodiment of the invention and an optical sensor system in accordance with a sixteenth embodiment of the invention,

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Fig. 11 schematically shows an optical interrogation system in accordance with a seventeenth embodiment of the invention and an optical sensor system in accordance with an eighteenth embodiment of the invention,

Fig. 12 schematically shows an optical interrogation system in accordance with a nineteenth embodiment of the invention and an optical sensor system in accordance with a twentieth embodiment of the invention,

5 Fig.13 schematically shows an electrical operating pulse to be applied to the SOA in Fig.12,

Fig. 14 schematically shows an optical interrogation system in accordance with a twenty-first embodiment of the invention and an optical sensor system in accordance with a twenty-second embodiment of the invention,

Fig. 15 schematically shows an optical interrogator system in accordance with a twenty-third embodiment of the invention and an optical sensor system in accordance with a twenty-fourth embodiment of the invention,

Fig. 16 schematically shows an optical interrogation system in accordance with a twenty-fifth embodiment of the invention and an optical sensor system in accordance with a twenty-sixth embodiment of the invention,

15 Fig. 17 schematically shows an optical interrogation system in accordance with a twenty-seventh embodiment of the invention and an optical sensor system in accordance with a twenty-eighth embodiment of the invention,

Fig. 18 schematically shows an optical interrogator system in accordance with a twenty-ninth embodiment of the invention and an optical sensor system in accordance with a thirtieth embodiment of the invention,

Fig. 19 schematically shows an optical interrogation system in accordance with a thirty-first embodiment of the invention and an optical sensor system in accordance with a thirty-second embodiment of the invention,

Fig. 20 schematically shows an optical sensor system in accordance with the thirty-third embodiment of the invention,

Fig. 21 schematically shows an optical interrogation system in accordance with a thirty-fourth embodiment of the invention,

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Fig. 22 schematically shows an optical interrogation system in accordance with a thirty-fifth embodiment of the invention,

Fig. 23 schematically shows an optical interrogation system in accordance with a thirty-sixth embodiment of the invention,

Fig. 24 schematically shows an optical interrogation system in accordance with a thirty-seventh embodiment of the invention, and

Fig. 25 schematically shows an optical interrogation system in accordance with a thirty-eighth embodiment of the invention,

A first embodiment of the invention creates an optical interrogation system 10, as shown in Fig. 1(a).

10 The interrogation system comprises an optical source 12, 14, which can generate optical pulses, which are connected to an optical waveguide 16 containing reflective optical elements, in this example gratings G, which are to be interrogated. The interrogator system 10 further comprises an optical amplifying and signal selecting means, which in this example is formed by a bidirectional semiconductor optical amplifier ("Semiconductor optical amplifier", (SOA)) 14. The SOA 14 is optically connected to the waveguide 16 which 15 contains the gratings G to be asked. The interrogator system 10 also comprises optical detection means, which in the example is formed by a wave meter 18, optically connected to the SOA 14. The wave meter 18 can evaluate the wavelength of the returned optical pulse transmitted through the SOA 14.

In this example, the optical source comprises a continuous wave (CW), in the form of a super-luminescent diode (SLD) 12, which is gated into optical pulses through The SOA 14. The output of the SLD 12 is connected via ports A and B to a signal splitter unit, which in this example is an optical fiber link 20, but could alternatively be a circulator, to the SOA 14.

The interrogator system 10 further comprises operating means 22 which can turn the SOA 14 on and off. SOAs are 25 typically used solely as amplifiers in separate or multi-channel telecommunications networks, where they operate in a "continuously-on" state, and are powered by a DC electrical operating signal. In the invention, the SOA 14 operates in a pulse-driven state, and is required to operate with nanosecond switching times.

The operating means 22 generate electrical operating pulses (see inset (a)) which are applied to the SOA 14. In the 30 example, the operating means comprise an electrical pulse generator which is triggered by a variable 341810

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frequency oscillator (not shown). The variable frequency oscillator can be constructed using a direct digital synthesis (DDS), or frequency synthesizer integrated circuit, with the frequency selection performed digitally under the control of a microprocessor. The pulse generator can be constructed from an analog or digital delay line, a simple RC timing circuit, or logic gate delays.

The operating means 22 can alternatively comprise a separate programmable device which can digitally perform the functions of a variable frequency oscillator, delay generators and pulse generators. A suitable programmable operating means 22 is shown in Fig.1(b). The frequency, duration and delay between pulses can be controlled to quartz crystal precision. With a typical clock frequency of 200Mhz, 10 timing resolutions of at least 5 ns are possible. More advanced designs using multi-phase timing control will further increase the timing resolution.

When an electrical drive pulse is applied to the SOA 14, the SOA 14 is turned on, allowing an optical event signal to be transmitted and amplified through the SOA 14. When no electrical current is applied to the SOA 14, the SOA 14 is turned off. and has a high optical absorption, thus mainly preventing the transmission of an incident optical signal. The SOA 14 can thus selectively transmit an optical signal, in particular an optical pulse returned from a grating G during interrogation.

By applying a series of electrical pulses, in this example a series of pairs of drive pulses to the SOA 14, the optical continuous wave signal from the SLD 12 is thus ported into a series of optical pulses. As the optical signal propagates through to the SOA 14, it undergoes 20 optical amplification. The average spatial length of an optical pulse (for standard optical monomode fiber waveguide) is chosen to be ~0.2 meters per 1 ns.

In order to interrogate one or more reflective optical elements, the interrogating system 10 is connected to an optical waveguide which contains one or more reflective optical elements to be interrogated. In this example, the interrogation system 10 is shown connected to an optical fiber 16 containing a sensor part 24, 25 in which a matrix of fiber Bragg gratings (G1 to Gn) is arranged. The interrogation system 10 and the matrix of grids together form an optical sensor system 26 in accordance with a second embodiment of the invention, where each of the grids G in the matrix forms a sensor element. The resonant wavelength of each of the gratings G is essentially the same in this example, and each grating G therefore operates in the same optical channel. In this example, the gratings G are 2.5 mm in length and have a resonance wavelength of 1550 nm, a spectral bandwidth of 0.2 nm and a transmission loss of 4%.

The four grids shown in Fig. 1(a) are meant to illustrate a matrix of up to n grids, where n can be up to 1000.

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The use of optical waveguide gratings, and in particular optical fiber Bragg gratings, as sensors is well known to those skilled in the art and therefore their operation and use will not be described in detail here.

In use, application of a first electrical drive pulse to the SOA 14 generates a first optical pulse, which is coupled into the fiber 16. As the optical pulse travels along the fiber 16, it will hit each of the 5 gratings G in the array in turn, and part of the pulse will be reflected by each of the gratings. As long as the spatial length of the incident optical pulse is no longer than twice the minimum spatial distance between the gratings in the matrix, the result will be a series of reflected optical pulses, each of a similar length to the original optical pulse, but separated in time and space according to the difference in the total propagation time of each of the 10 reflected optical pulses from their respective gratings G. The reflected optical pulses propagating back towards the SOA 14 will therefore arrive at the SOA 14 at various times. The transit time of each reflected optical pulse therefore identifies the grating from which it was reflected.

It will be appreciated by those skilled in the art that the continuous optical wave source SLD 12 can be replaced by a pulsed optical source. The timing of the electrical pulses of the SOA 14 will then be adjustable to allow selected optical pulses generated by the pulsed optical source to be fed into the optical fiber 16.

The selection of a particular reflective optical element, in this example a grating G, for interrogation is achieved by adjusting the time period T1 between the electrical drive pulses. The frequency of the 20 electrical pulses is set so that the period T1 between the first and second pulses in each pair is equal to the transit time of the optical pulse generated as a result of the first operating pulse from the SOA 14 to a selected grating, G1 in Fig. 1( a), and back, as shown by arrow 28. The SOA 14 therefore receives a second electrical pulse from the operating means 22 while the reflected optical pulse (from grating G1) arrives back at the SOA 14. As a result, the SOA is switched 14 on when the reflected pulse 25 arrives, allowing the reflected pulse to be transmitted through the SOA 14 and the reflected pulse being simultaneously amplified. Any reflected pulse arriving before or after the SOA 14 is turned on, ie from another grating in the array will be absorbed, and thus will not be transmitted. The duration of the optical pulses is chosen to be shorter than twice the transit time of an optical pulse between the nearest adjacent grids in the matrix.

The ported reflected optical pulse is then routed from the SOA 14 to the wavemeter 18, via ports B and C to the coupler 20, where the wavelength of the reflected optical pulse, and thus the wavelength of the interrogated grating G1, is measured and recorded.

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Applying the second electrical drive pulse to the SOA 14 has another effect, in addition to gating the reflected optical pulse: a second optical pulse is generated and coupled into the optical fiber 16.

Where only two electrical drive pulses are used to drive the SOA 14, as described above, only one reflected pulse will arrive back at the wavelength measurement system. This alone can be useful, but will require a fast wave measurement system that can take a measurement from a separate pulse. Alternatively, a series of electrical operating pulses in pairs can be used. Sufficient time must be allowed between the pairs of drive pulses to allow the electrical pulse generated by the second electrical drive pulse of the SOA 14 to be reflected by the gratings and absorbed through the SOA 14, thereby removing it from the system 10 and avoiding interference with the following optical pulses.

An optical interrogation system 30 in accordance with a third embodiment of the invention is shown in Fig. 2. The interrogation system 30 is substantially the same as the interrogation system in accordance with the first embodiment of the invention, with the following modifications. The same reference numbers are used for corresponding features.

15 In this example, the operating means 22 can generate a series of electrical operating pulses, as shown in inset (a). Each electrical operating pulse has the same duration and the period between the pulses is constant. The duration of the electrical pulses is chosen to be no longer than twice the time required for an optical signal to propagate between the nearest adjacent reflective elements, the gratings G, in the matrix. The frequency of the variable frequency oscillator, and thus the frequency of the 20 electrical drive pulses, is changed to select a particular grid in the matrix for querying. The frequency of the variable frequency oscillator is chosen so that the period T1 between electrical drift pulses is equal to the transit time of an optical pulse, from the SOA 14 to the selected grating (G1 in this example) and back to the SOA 14. An approximate number will be 10 ns for each meter of optical fiber between the SOA 14 and the selected grating (assuming standard optical fiber).

During the control of the operating means 22, the SOA 14 is therefore switched on and off at regular intervals. Each time the SOA 14 is turned on, an optical pulse is generated which is fed into the optical fiber 16, simultaneously gating the correct reflected optical pulse, which then propagates to the wavemeter 18, as described above.

Of greater importance is the inclusion of a further portion of an optical waveguide, in this example, a length of optical fiber 32, after the SOA 14, i.e. in use, between the SOA 14 and the optical fiber 16 containing the the reflective elements (the grids G) to be queried. This additional length of optical fiber 32 is included to remove the possibility of interference from 341810

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simultaneous reception of the reflected optical pulses from more than one grating G at the SOA 14. To understand the reason for this, consider the situation in Fig. 3, when the additional fiber 32 is not included and the interrogation system 30 is connected to an optical fiber containing an array of three gratings (G1-3), with 1 meter intervals between those from the SOA 14. If, for example, the interrogation system 30 was set to monitor the wavelength of the first grating G1, located 1 meter from the SOA 14 , the following results would occur.

At time T0, a first electrical pulse is applied to the SOA 14 and an optical pulse A is generated and introduced into the fiber 16, where it propagates towards the gratings G. After a time Ts (where Ts≈ 5 ns, which is the time it takes for an optical signal to propagate 1 meter in a standard optical fiber), part of the optical pulse A will be reflected by the first grating G1 and this first reflected pulse α will propagate back towards the SOA 14. The remainder of the pulse A will continue towards the second and third grids, G2 and G3.

At time 2T2, a second electrical pulse is applied to the SOA 14, which causes a second pulse B to be generated and fed into the fiber 16, where it also propagates towards the gratings G. At the same time 15, the first reflected pulse α arrives back at the SOA' a 14. Since the SOA 14 is switched on (i.e.

port is open and amplifying) it will be transmitted and amplified as described above. The first reflected pulse α will then propagate to the wavemeter, as intended. However, at the same time (2Ts) the remainder of the first optical pulse A will arrive at the second grating G2, where part of it will be reflected, creating a second reflected pulse β.

20 At time 3Ts, the remainder of the pulse A will arrive at the third grating G3, where part of it will be reflected and create a third reflected pulse γ. At the same time, the second optical pulse B will arrive at the first grating G1, where part of it will be reflected, creating a fourth reflected pulse δ. At the same time, the second reflected β will arrive back at the first grating G1. Part of the second reflected pulse β will be reflected at the first grating G1, back towards the grating 25 G2, while the remainder of the second reflected pulse β will be transmitted through the first grating G1. The transmitted part of the second reflected pulse β will therefore propagate along the fourth reflected pulse δ.

At a time 4T, a third electrical drive pulse is applied to the SOA 14 and ensures that a third pulse C is generated and fed into the fiber 16. At this time, the reflected fourth pulse δ (from the 30 selected grating G1) and the second reflected the pulse β (from the second grating G2, i.e. not from the selected grating), both arrive at the SOA 14, where they will be transmitted and amplified. This results in 341810

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that two reflected optical signals will arrive at the wavelength measuring apparatus and make them almost indistinguishable from each other.

If the distance between the SOA 14 and the first grating G1 is not longer than the distance between the first and second gratings in the matrix, this type of interference probably occurs, and the interference is most prominent when the gratings are evenly spaced. Inserting an additional length of fiber 32 between the SOA 14 and the optical fiber 16 removes the most dominant form of this type of interference. The length of the additional fiber 32 must be longer than the distance between the first and second gratings of the matrix to be interrogated. There is still the possibility that third reflection interference can occur, but this can be ignored if the grating reflection values are too low, which is described in more detail below.

As a result of inserting the additional length of optical fiber 32, a series of reflected optical pulses (reflected from a selected grating) are received at the wavemeter, as shown at inset (b) in Fig.2, and a reflected pulse for each optical pulse is generated. and is fed into the fiber 16.

If the additional length of optical fiber 32 is longer than the minimum length required, it may be possible to increase the frequency of the electrical pulses, and thus of the optical pulses, to a majority of base frequencies. In this way, more than one optical pulse will propagate in the optical fiber 16 at any given time. For example, to increase the frequency by a factor of two would require the length of the additional fiber 32 to be at least twice the distance between the first and second gratings in the array.

Arrangement of the additional length of optical fiber 32 therefore ensures that for each optical pulse generated 20 by the SOA 14, and fed into the optical fiber 16, a reflected optical pulse is delivered to the wavemeter.

The skilled person will appreciate that the use of an additional length of optical fiber is described herein, since the optical waveguide containing the array of gratings to be interrogated is an optical fiber. Where the optical waveguide containing the reflective optical elements 25 to be interrogated is instead a planar optical waveguide, an additional portion of a planar optical waveguide may be used instead.

When the interrogator system 30 is connected to the optical fiber 16, which contains an array of gratings G to be interrogated, as described above, they together form an optical fiber grating sensor system 34 in accordance with a fourth embodiment of the invention.

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Figure 4 shows an optical interrogation system 40 in accordance with a fourth embodiment of the invention. The query system 40 in this embodiment is substantially the same as the query system 30 in accordance with the third embodiment, with the following modification. The same reference numbers are used for corresponding features.

In this example, the continuous optical wave source 12 of the third embodiment is replaced with a pulsed optical source 42 which can generate optical pulses. To avoid the need for complicated timing devices to cause the SOA 14 to gate selected optical pulses from a pulsed optical source into the optical fiber 16, the pulsed optical source is coupled to the optical fiber 16 between the SOA 14 and the additional length of of fiber 32.

In use, when coupled to an optical waveguide, such as an optical fiber 16, comprising a sensor portion 24 in which an array of gratings is arranged, optical pulses generated by the pulsed optical source 42 are fed into the additional optical fiber 32 via ports C and B to the optical coupler 20. The optical pulses then propagate along the additional optical fiber 32 and from there along the optical fiber 16, where they strike and are reflected by one or more of the gratings G. The reflected pulses 15 are then propagated back along the fibers 16 and 32 to the SOA 14, via ports B and A of the connector 20. As described above, the frequency of the electrical operating pulses, applied from the operating means 22 to the SOA 14, is set to gate (and amplify) only those pulses that are reflected from a selected lattice of the lattices G in the matrix. The pulses transmitted through the SOA 14 are then routed to the wavelength measuring device 18.

When coupled together, the interrogator 40 and the array of reflective optical elements, in this example gratings in the optical fiber 16, together form an optical sensor system 44 in accordance with a sixth embodiment of the invention.

A seventh embodiment of the invention creates an optical interrogation system 50 as shown in Figure 5. In this embodiment, the interrogation system 50 further comprises a second SOA 52, and a second length 25 of optical fiber 54. The second SOA 52 is connected to the SLD 12 via ports A and D to connector 20.

The second SOA 52 is to be coupled, via the second additional fiber 54 to a second waveguide containing one or more optical elements to be interrogated, in this example a second optical fiber 56 containing an array of fiber Bragg gratings (G12 to G2n ).

The same operating means 22 (an electrical pulse generator triggered by a variable frequency oscillator, 30 as before) is used to control both SOAs 14, 52. An electrical switch 58 is provided to select which SOA 14, 52 is to be operated at a specific time, thus choosing which matrix of 341810

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lattice to be queried. Therefore, only one wavelength measuring device, such as a wavemeter 18, is needed, since only one grating array will be selected at a time.

This setup thus allows the gratings in more than one optical waveguide to be interrogated using a single operating means 22 and a single wavemeter 18. It will be appreciated that the described interrogation system 50 can be further extended to interrogate gratings in more than two optical waveguides by arranging additional SOAs, connected to the SLD 12 and the wavemeter 18, via a more complex optical routing element.

The interrogator system 50 operates in the same manner as previously described when a particular SOA 14, 52 and its respective grid array is selected by actuating the electrical switch 58.

10 A person skilled in the art will appreciate that the SLD 12 and the SOA 14, 52 combination as an optical pulse source can be replaced by a pulsating optical source, as shown in Figure 4. The pulsating optical source will similarly be connected to the optical fibers 16, 56 between the respective SOAs 14, 52, and the additional lengths of fiber 32, 54. A separate pulse source can be connected to the two fibers 16, 56 via the two output ports (B and D) of a separate coupler, or two pulsing sources can are connected to their 15 respective fibers 16, 56 via two separate connectors.

When the interrogation system 50 is connected to the two optical fibers 16, 56, which contain their respective arrays of gratings G11 to G1 and G21 to G2n, the interrogation system 50 and the optical fibers 16, 56 together form an optical sensor system 60 in accordance with an eighth embodiment of the invention.

An optical interrogation system 70 in accordance with a ninth embodiment of the invention is shown in Figure 6.

The query system 70 in this embodiment is essentially the same as the query system 30 as shown in Figure 2, with the following modification. The same reference numbers are used for corresponding features.

In this example, an optical filter 72 is arranged between the SLD 12 and the SOA 14. The filter 72 is inserted because SOAs have a limited maximum total optical output power. As a result, they can be used to amplify an optical signal containing a narrow range of wavelengths to a higher output level than one containing a broad band of wavelengths. The filter 72 has a filter function, shown at inset (a), which has a spectral range corresponding to the operative range of the gratings to be interrogated. As a result, the bandwidth of the continuous optical wave signal reaching the SOA 14 covers the maximum frequency band of the gratings G 30 to be interrogated. All wavelengths in the continuous optical wave signal generated by the SLD 12 that are outside the operative range of the gratings G are blocked. This reduction in 341810

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the bandwidth of the continuous optical wave signal to be pulsed (and amplified) through the SOA 14 means that the SOA 14 can amplify the pulses to a higher average optical power level. The high power level of the pulses improves the performance of the interrogation system 70, particularly the signal-to-noise ratio of the wavelength measuring apparatus, and enables the interrogation system 70 to be used to interrogate low-reflective gratings.

When the interrogator system 70 is connected to the optical fiber 16 comprising the array of gratings G to be interrogated, they together form an optical interrogator system 74 in accordance with a tenth embodiment of the invention.

Figure 7 shows an optical waveguide interrogator system 80 in accordance with an eleventh embodiment of the invention. The interrogation system 80 comprises an optical amplifying and signal selecting means in the form of an SOA 82 which is controlled by operating means 84, which is essentially the same as the operating means 22 in Figures 1(a) and 1(b), and a wavelength evaluation apparatus 86.

In this example, the SOA 82 acts as an optical source in addition to being a bi-directionally usable optical amplifying and signal selecting means. When the SOA 82 receives an electronic operating pulse 15, it simultaneously generates an optical signal, in the form of an amplified spontaneous emission, and converts the optical signal into an optical pulse. As a result, it is possible to completely remove the continuous broad bandwidth optical source from the previous embodiments, since the optical pulse is generated entirely within the SOA 82 and not as a result of that the separate continuous broad bandwidth optical wave signal 20 is carried ("continuous broad bandwidth optical signal"). Consequently, the coupling in the previous embodiments is no longer necessary here. These modifications reduce the cost of the interrogator system 80, compared to the previous embodiments, and also save optical power, since no more power is lost through a link.

When coupled to an optical waveguide containing one or more reflective optical elements 25 to be interrogated, in this example an optical fiber 94 containing an array of gratings G1 to Gn, the SOA operates in the same manner as described above, and the frequency of the the electrical pulses are set to select which grid in the array to interrogate.

The wavelength evaluation apparatus 86, shown in Figure 8 comprises an optical filter element having a wavelength-dependent filter response (see inset (a)), followed by an optical detection means in the form 30 of a photodetector 90. The electrical output of the photodetector 90 is connected to a signal processing means 92 which can determine the wavelength of a detected optical signal. The 341810

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the optical power of the filtered reflected optical pulse identifies the wavelength of the reflected pulse, and thus the wavelength of the interrogated grating.

When the interrogator system 80 is coupled to the optical fiber 94 containing an array of gratings G to be interrogated, they together form an optical sensor system 96 in accordance with a twelfth embodiment of the invention. Like the arrays of gratings arranged in fibers 16 and 56 in previous embodiments, the resonant wavelength of each grating G is substantially the same, and each grating G again operates within the same optical channel.

The query system 100 in the embodiment shown in Figure 9 is substantially the same as the query system 80 in the previous embodiment, with the following modifications. The same 10 reference numbers are used for corresponding moves.

In this example, like the interrogator 40 shown in Figure 4, the SOA 82 acts only as an optical amplifying and signal selecting means, the wide bandwidth optical pulses are generated by a separate pulsed optical source 102. The pulsed optical source 102 is coupled in the interrogator system 100 after the SOA 82, via an optical link 104. That is, when the interrogator system 15 100 is connected to an optical fiber 94 containing an array of gratings G to be interrogated, the optical pulses generated by the pulsed optical source are guided 102 into the optical fiber 94 without passing through the SOA 82, as described above in connection with Figure 4.

When the interrogator system 100 is coupled to the optical fiber 94 containing an array of gratings G to be interrogated, they together form an optical sensor system 106 in accordance with a fourteenth embodiment of the invention.

Figure 10 shows a query system 110 in accordance with a fifteenth embodiment of the invention. The query system 110 is substantially the same as the query system 80 of the eleventh embodiment, with the following modification. The same reference numbers are used for corresponding features.

In this embodiment, similar to the interrogator systems 30, 40, 50 and 70 shown in Figures 2, 4, 5 and 6, an additional part of optical waveguide, in this example a length of optical fiber 112, is arranged after the SOA 82, ie .when the interrogator system 110 is connected to the optical fiber 94 containing gratings G to be interrogated, the additional fiber 112 is placed between the SOA 82 and the optical fiber 94. As explained above, the additional length of optical fiber 112 is inserted to remove the possibility of 30 interference from simultaneous reception of reflected optical pulses from more than one grating G at the SOA 82, thus enabling the SOA 82 to be driven by a continuous stream of electrical pulses.

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When the interrogator system 110 is coupled to the optical fiber 94 containing an array of gratings G to be interrogated, an optical sensor system 114 is formed in accordance with a sixteenth embodiment of the invention.

An optical interrogation system 120 in accordance with a seventeenth embodiment of the invention is shown in Figure 11. The interrogation system 120 is substantially the same as the interrogation system 110 shown in Figure 10, with the following modification. The same reference numbers are used for corresponding features.

In this example, an optical filter 122 is arranged between the SOA 82 and the wavelength evaluation apparatus 86. Similar to the filter 72 in the ninth embodiment of the invention, the filter 122 has a top-hat function (see inset (a)) that has a spectral area 10 which corresponds to the operative area of the reflective elements, in this example grid G to be queried. In its application here, the filter 122 acts to remove broadband background noise (ie, wavelengths outside the operative range of the gratings G to be interrogated) present on a reflected optical signal transmitted through the SOA 82. The background noise is generated by the SOA 82 and added a reflected optical signal when transmitted (and amplified) through the SOA 82.

Removal of any such background noise will improve the signal-to-noise ratio of the reflected optical signal reaching the wavelength apparatus 86.

By connecting the interrogator system 120 to an optical fiber 94 containing a matrix of gratings G to be interrogated, an optical sensor system 124 is created in accordance with an eighteenth embodiment of the invention.

Figure 12 shows an optical interrogation system 130 in accordance with a nineteenth embodiment of the invention. The query system 130 in this embodiment is substantially the same as the query system 110 shown in Figure 10, with the following modification. The same reference numbers are used for corresponding features.

The interrogation system 130 comprises an optical reflector, in the form of a high-frequency fiber Bragg grating 25 ("chirped Bragg grating" (CFBG)) 132, in this example, between the SOA 82 and the wavelength evaluation apparatus 86. The CFBG 132 has a reflectivity of less than 100%, ~99% in this example. The function of the CFBG 132 is to reflect a portion (~99%) of a reflected optical pulse back toward the SOA 82, and transmit the remaining (~1%) of the reflected optical pulse to the wavelength evaluator 86.

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The choice of optical reflector depends on the application characteristics required. Some of the design criteria that must be considered when choosing which type of optical reflector to use are as follows:

Using an optical reflector that has a high reflectivity will reduce the number of cycles that a reflected optical pulse must pass through the system before it reaches the maximum optical power. The maximum optical power that a pulse can achieve is determined by the losses in the interrogation system 130 and the optical saturation and amplifier characteristic of the SOA 82. This will be an advantage where the interrogation system 130 must have a fast response. In addition, having a highly reflective optical reflector will allow the interrogation system 130 to withstand a higher level of optical loss in the system. This may mean that an array containing a large number of reflective elements and/or several weakly reflective optical elements may be interrogated.

The use of an optical reflector having a lower reflectivity than that described will result in a higher proportion of the reflected optical signal being delivered to the wavelength evaluation apparatus 86. Having a higher power optical signal will also enable the wavelength evaluation apparatus which requires higher optical signal levels can be used.

If the spectral bandwidth of the optical reflector CFBG 132 only covers the operative range of the reflective optical elements to be interrogated, then only broadband background noise at the wavelength in the spectral range will be reflected back to the SOA 82 for further amplification. This will reduce the output of out-of-band signals from the SOA 82, and concentrate the amplifying effect on wavelengths reflected by the reflective optical elements 20 under interrogation.

In contrast, using an optical reflector having a spectral bandwidth covering the entire spectral range of the SOA 82 will reduce the amount of out-of-band noise transmitted to the wavelength evaluator 86. This can improve the signal-to-noise- ratio at the wavelength evaluation apparatus 86.

25 CFBG 132 is placed sufficiently close (in terms of fiber length) to SOA 182 to ensure that the propagation time TR for a reflected optical pulse to travel from SOA 82 to CFBG 132 and back to SOA 82 (as shown by arrow 134) is shorter than the duration TP of the electrical drive pulse impressed on the SOA 82 (see inset (A)).

In use, when the interrogator system 130 is coupled to an optical fiber 94 containing an array of gratings 30 G to be interrogated, an optically reflected pulse transmitted (and amplified) through the SOA 82 encounters the back-reflected CFBG 132, a portion (~1%) of the reflected optical pulse is transmitted to the 341810

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wavelength evaluator 86 and the remaining (~99%) of the optical pulse is re-reflected back towards the SOA 82. Due to the duration of the electrical drive pulses (TP), and thus also the period that the SOA 82 is switched on (also TP) , is longer than the pulse propagation time TR, the SOA 82 is still on (ie, the gate is still open) when the re-reflected optical pulse returns to the SOA 82. The duration (also TP) of this first re-reflected optical pulse is also longer than a pulse's propagation time TR, and therefore a part of the re-reflected optical pulse, the part of the optical pulse which is longer than a pulse's propagation time, will undergo further amplification when it is again transmitted through the SOA 82. This is illustrated in Figure 13 .

10 The propagation time TR therefore determines the portion of the reflected optical pulse that can undergo multiple amplifications and reflections (cycle) within the interrogator 130. Although the SOA 82 will add some broadband background noise to the reflected optical pulse, the re-reflected optical pulse dominates of the wavelengths reflected by the grating under query. Therefore, although some interference will occur if the electrical drive pulse impressed on the SOA 82 is longer than 15 twice the time it takes an optical pulse to propagate between the nearest adjacent gratings in the array being interrogated, the resulting interference is less significant than that experienced by the previously described interrogation systems where the optical pulses do not undergo cyclic amplification. Adjustment can therefore be made to the distances between the CFBG 132 and the SOA 82 to allow the use of a slower operating SOA 82, or lower speed electronics in the operating means 84.

The re-reflected, re-amplified optical pulse leaving the SOA 82 contains wavelengths originally reflected by the interrogated grating, plus lower level broadband background noise from the SOA 82. The optical pulse is propagated along the optical fiber 94 towards the gratings G, as described previously. Again a portion of the pulse is reflected from each grating in the array and again a series of reflected optical pulses (separated in time and space) arrive back at the SOA 82. As before, only one reflected pulse arrives while the SOA 82 is turned on, since all others will arrive either before or after the electrical drive pulse is applied to the SOA 82. The selected optical pulse is again amplified as it passes through the SOA 82, and propagates to the CFBG 132 where the cycle begins again.

30 The narrow band of wavelengths reflected by the reflective optical element, in this example a fiber grating, which is probed, undergoes multiple amplifications in a cyclic round trip through the SOA 82. A low-power, wide-bandwidth optical start pulse generated by the SOA 'an 82 undergoes several cycles through the interrogation system 130, and as a result of the optical 341810

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the gain it undergoes each time it passes through the SOA 82 is shaped in rapid growth into an optical pulse of strictly narrow bandwidth. The maximum optical power an optical pulse can achieve is dependent on the loss characteristics of the interrogator 130 and the saturation and gain characteristics of the SOA 82 used.

5 As described earlier, the selection of a particular grating in the matrix for interrogation is controlled by adjusting the frequency of the electrical drive pulses applied to the SOA 82. The duration of each pulse is chosen to take into account the effect of the fiber length between the CFBG 132 and the SOA 'an 82.

When the interrogator system 130 is coupled to the fiber 94 containing an array of gratings G to be interrogated, an optical sensor system 136 is formed in accordance with a twentieth embodiment of the invention.

Due to the multiple amplifications undergone by the cycling of the optical pulses in the interrogation system 130, very strong optical pulses can be generated even when the reflective optical elements, in this example fiber gratings, being interrogated have very low reflectivity. The use of low-reflection gratings means that the optical pulses can pass through many gratings without any significant loss of power in each grating. Third reflection interference has also been shown to be negligible for low-reflection gratings. The use of low-reflective gratings in a sensor matrix therefore enables many more gratings to be inserted into the optical sensor system 136, resulting in reduced interference between the gratings.

The cyclic operation of the interrogator system 130 and the grid sensor system 136 creates various advantages. Very strong narrow-bandwidth optical pulses can be rapidly built up around the resonant wavelength of the grating being interrogated. Such a strong optical pulse with a narrow bandwidth is easier to measure for various types of wavelength evaluation apparatus.

The broadband background noise generated through an SOA 82 is reduced when the SOA 82 needs to amplify a strong, narrow-bandwidth optical signal. This transfer, or narrowing of the effect to dominant (highly reflected) wavelengths creates a reduction in the broadband background noise when a reflected optical pulse has traveled a number of cycles in the interrogator 130, and the dominant (desired) optical signal has been built up. This leads to a further increase in the signal-to-noise ratio.

Figure 14 shows an optical interrogation system 140 in accordance with a twenty-first embodiment of the invention. The query system 140 is essentially the same as the query system in the previous 341810

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the embodiment, with the following modifications. The same reference numbers are used for the corresponding features.

In this example, the interrogation system additionally comprises an optical routing device ("optical routing device") in the form of an optical fiber link 142. The link 142 is placed between the CFBG 144 and the SOA 82, where the CFBG 144 is connected to the SOA' a 82 via ports C and D to the link 142.

This new optical setup means that it is the optical signal reflected by the CFBG 144 that is transmitted to the wavelength evaluator 86. The CFBG 144 can therefore now be made 100% reflective if required. As described above, the spectral range covered by the CFBG 144 can be selected to cover only the operative range of the gratings to be interrogated.

10 This means that the optical signal routed to the wavelength evaluator 86 will only contain wavelengths located in the spectral range of the CFBG 144. Accordingly, only the SOA-generated noise seen by the wavelength evaluator will be that located in the spectral range of the CFBG 144 (and thus the operative area of the grids G). All of the optical signals at wavelengths that fall outside this spectral range will not be reflected by the CFBG 15 144, and will therefore not reach the wavelength evaluator 86. This will lead to an increase in the signal-to-noise ratio at the wavelength evaluator 86.

In addition, since the spectral profile of the CFBG 144 only covers the operative region of the gratings to be interrogated, the presence of the CFBG 144 will reduce the out-of-band signal amplified through the SOA 82, thus improving its efficiency.

In order to insert the connector 142 into the interrogator system 140, the minimum separation distance between the SOA 82 and the CFBG 144 must be increased. Consequently, the duration of the electrical operating pulses of the SOA 82 must also be increased. The presence of the coupler 142 will cause an increase in the optical loss in the interrogator system 140. The amount of loss (through port A) is determined by the coupling ratio (50:50, 60:40, etc.) of the chosen coupler. Selecting a lower coupling ratio will result in a lower loss, but will create a lower optical signal level to the wavelength evaluator 86. However, the increased gain in the interrogator 140 created by the use of a right-reflecting CFBG 144, and the lower out-of-band- the noise experienced through the SOA 82 can be used to offset some of the loss caused through the link 142.

When the interrogator system 140 is coupled to the fiber 194 comprising an array of gratings G to be interrogated, they together form an optical sensor system 146 in accordance with a twenty-second embodiment of the invention.

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An optical interrogation system in accordance with a twenty-third embodiment of the invention is shown in Figure 15. This embodiment is substantially the same as the previous embodiment with the following modification. The same reference numbers are used for the corresponding features.

In this example, the coupler 142 is located on the other side of the SOA 82, connected via ports 5 A and B, between the SOA 82 and the additional fiber 112. As a result, the wavelength evaluator 86 receives reflected optical pulses directly from the interrogated grating, without that the pulses first pass through the SOA 82. This means that the reflected optical pulses received by the wavelength evaluator 86 do not contain any out-of-band noise from the SOA 82. The signal-to-noise ratio of the optical pulse transmitted to the wavelength evaluator 86 is therefore 10 improved.

When the interrogator system 150 is coupled to the optical fiber 94 comprising an array of gratings to be interrogated, they together form an optical sensor system 152 in accordance with a twenty-fourth embodiment of the invention.

Figure 16 shows an optical waveguide interrogation system 160 in accordance with a twenty-fifth embodiment of the invention, which is substantially the same interrogation system 140 shown in Figure 14, with the following modification. The same reference numbers are used for the corresponding features.

Not all of the increased electrical drive pulse duration is required by the interrogation system 140 (Figure 14), due to the presence of the coupler 142, which helps to amplify the reflected optical signal that cycles. The leading portion of the electrical drive pulses will cause a reflected optical signal arriving from an interrogated grating to be amplified as it passes through the SOA 82. The trailing portion of the electrical drive pulse will cause the re-reflected optical pulse, returned from The CFBG 144 is further amplified as it passes back through the SOA 82 towards the gratings G. The central part of the electrical drive pulses is of no use, because during this part of the electrical drive pulses the reflected optical pulse propagates between the SOA 82 and the CFBG 144. Accordingly, any electrical operating signal applied to the SOA 82 at this time will generate unwanted noise. This causes the extended electrical drive pulses to serve only to reduce the electrical power of the SOA 82, and degrade the signal-to-noise ratio of the interrogator 140.

These disadvantages can be solved by using a stream of electrical operating pulses in pairs, as shown in Figure 16, 30 inset (a). The total length between the SOA 82 and the CFBG 144 via the link thus becomes less critical (in this example it will usually be a couple of meters).

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The duration of each electrical drive pulse is set shorter than twice the time for an optical pulse to propagate between the nearest adjacent gratings G in the array of gratings to be interrogated, as described above. The total repetition time (TT) for a complete cycle of pulse pairs is equal to twice the time required for a reflected optical pulse to propagate from the interrogating grating to the CFBG 144, as described above. The period (TR) between two pulses in each pair is set equal to the time required for a reflected pulse to propagate from the SOA 82 to the CFBG 144, and back to the SOA 82.

Therefore, instead of having a separate broad electrical drive pulse, where the central portion serves no purpose, the central region is effectively removed, leaving two short electrical drive pulses.

The first electrical drive pulse in each pair serves to turn on the SOA 82 when a reflected optical pulse arrives from a grating being interrogated, thereby transmitting and amplifying the reflected optical pulse. The second electrical drive pulse acts to turn the SOA 82 back on when the re-reflected optical pulse arrives back from the CFBG 144, thus transmitting (and amplifying) the optical pulse back to the interrogated grating.

Coupling the interrogator system 160 to the optical fiber 94 containing an array G of gratings G to be interrogated forms an optical sensor system 162 in accordance with a twenty-sixth embodiment of the invention.

An optical interrogation system 170 in accordance with a twenty-seventh embodiment of the invention is shown in Figure 17. This interrogation system 170 is substantially the same as the interrogation system 160 shown in Figure 16, with the following modifications. The same reference numbers are used for the corresponding features.

Where a reflective optical element is shown in previous embodiments at a particular position in an array of reflective optical elements to be interrogated, it is actually possible to place a group of reflective optical elements, in this example gratings 172. Each grating 172 in the group 25 operates in a different wavelength window. The query system 170 in this embodiment is intended for querying such an array of grids. Since the resonant wavelength of each grating in an array 172 lies in different wavelength windows, each grating therefore operates in different optical channels. For example, each array 172 may contain seven FBGs (fiber Bragg gratings), with appropriate C-band resonance wavelengths of 1530 nm, 1535 nm, 1540 nm, 1545 nm, 1550 nm , 30 1555 nm and 1560 nm, which makes each optical channel 5 nm wide. In this example, each grating has a length of 2.5 mm, a bandwidth of 0.2 nm, and a transmission loss of 4%. The grids in each group 341810

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172 are required to be separated by a distance smaller than the propagation distance covered by an optical signal under half of an electrical drive pulse, and in this example separated by 10 cm.

Each group 172 contains the same number of gratings, with the same set of resonant wavelengths. Equal grids in adjacent groups 172 are separated by 1 m; for example, the 1540 nm grating in a 5 group is separated from the 1540 nm grating in the adjacent group or groups by 1 m.

The result of having the gratings to be interrogated arranged in such arrays 172 is that, during operation, for each wide bandwidth optical pulse generated by the SOA 82, multiple reflected optical pulses (the reflections from each grating in array 172) arrive back at The SOA 82 at approximately the same time. Therefore, the first electrical drive pulse in each pair applied to the SOA 82 will cause the SOA 82 to transmit all of the reflected optical pulses corresponding to a particular group of gratings 172.

In order to separate the multiple reflected optical pulses transmitted through the SOA 82, the separate CFBG 144 optical reflector in the previous embodiment is replaced with an array of optical reflectors, in the form of a matrix (only the first two are shown in Figure 17 for clarity) of FBGs 174, 176. Each FBG 174, 176 reflects wavelengths in a different spectral range, each spectral range corresponding to the wavelength window covered by one of the optical channels in which the gratings to be interrogated operate. In this example, the bandwidth of each of the FBG reflectors 174, 176, and thus of the optical channels, is 5 nm.

Each FBG reflector 174, 176 is located at different distances from the SOA 82. In this example, adjacent FBG reflectors 174, 176 are separated by 1 m. As a result, the transit time of a reflected optical pulse from the SOA 82 to its respective FBG reflector 174, 176 and back to the SOA 82, depending on which grating in the array 172 the optical pulse is reflected from.

Therefore, as before, adjusting the period TR between the electrical drive pulses in a pair and the period TT between the pulse pairs will allow a particular grating in a particular group 172 to be selected for interrogation.

By coupling the interrogator system 170 to the fiber 94 containing the groups 172 of gratings to be interrogated, an optical sensor system 178 is formed in accordance with a twenty-eighth embodiment of the invention.

In use, the second electrical drive pulse in the first pair will cause the SOA 82 to generate an optical pulse, which propagates towards the grids to be interrogated. A portion of the optical pulses will 341810

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is reflected by each grating, and a series of reflected optical pulses is produced, each having a wavelength corresponding to its respective grating. The reflected optical signals from the gratings in a particular array 172 will arrive back at the SOA 82 at approximately the same time corresponding to the onset of the first electrical drive pulse in the following electrical drive pulse pair to the SOA 82.

5 This group of reflected signals is therefore transmitted and amplified through the SOA 82, from which they proceed towards the array of FBG reflectors 174, 176.

Since each FBG reflector 174, 176 covers a different optical channel, each of the reflected optical signals will be re-reflected from another FBG reflector 174, 176. The re-reflected optical signals will therefore propagate back towards the SOA 82, separated in time and space. Only one of the re-reflected optical pulses will arrive at the SOA 82 while the SOA 82 is turned on by the second electrical drive pulse of the second pair of electrical drive pulses. All other re-reflected optical signals will arrive at the SOA 82 while it is turned off and will therefore be blocked from further cycling in the sensor system 178.

The selected optical pulse is transmitted and amplified through the SOA 82 and again propagates towards the 15 gratings, where it is again reflected by gratings selected for interrogation and the cycle is repeated.

Although both electrical drive pulses in a pair cause an optical pulse to be generated, due to the cyclic nature of the system, only one group 172 of gratings can undergo cyclic amplification and reach a peak. Therefore, any potential interference from other groups of gratings will be avoided.

The query system 170 and the sensor system 178 create the following advantages. Only a reflected optical signal is passed to the wavelength evaluation apparatus 86. This increases the range of possible wavelength evaluation techniques that can be used. In particular, it allows the use of a colorless wavelength evaluator with a repetitive function that can operate over a range of wavelength windows. For example, the optical filter described above, which has a wavelength-dependent transfer function, can be replaced by a Fabry-Perot filter or a spectroscopic filter ("elaton slope filter"), both of which can be fabricated in an optical fiber to reduce optical loss.

In addition, since only one reflected pulse is supported by the sensor system at any time, the entire optical gain power of the SOA 82 can be concentrated into this one optical pulse. The optical power of the SOA 82 does not need to be shared between a number of reflected optical signals and therefore there is no possibility of instability or power conflict between optical channels.

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Figure 18 shows an optical interrogation system 180 in accordance with a twenty-ninth embodiment of the invention. The query system 180 is essentially the same as the query system 170 in Figure 17, with the following modification. The same reference numbers are used for corresponding features.

In this embodiment, the optical pulses are generated by a pulsed optical source 182, where the SOA 82 is limited to use as an optical amplifying and signal selection means. To avoid the need for complicated timing devices to cause the SOA 82 to selectively port optical pair pulses from the pulsed optical source 182 into the optical fiber 94, the pulsed optical source is coupled via a fiber coupler 184 to the optical fiber 94, between The SOA 82 and the additional length of fiber 112.

By coupling the interrogator system 180 to an optical fiber 94 containing an array of groups 172 of gratings to be interrogated, an optical sensor system 186 is formed in accordance with a thirtieth embodiment of the invention.

An optical waveguide interrogation system 190 in accordance with a thirty-first embodiment of the invention is shown in Figure 19. This interrogation system 190 is substantially the same as the interrogation system 170 shown in Figure 17, with the following modification. The same reference numbers are used for corresponding features.

In this example, the coupler 142 and wavelength evaluator 86 have been moved to the grating array side of the SOA 82. As described above in connection with Figure 17, only one of the optical pulses reflected from the FBG reflectors 174, 176 will arrive back from the SOA 82 while 20 The SOA 82 is switched on. All other re-reflected optical signals will arrive at the SOA 82 while it is turned off and will therefore be blocked from further transmission. Placing the wavelength evaluator 86 on this side of the SOA 82 therefore has the advantage of increasing the selectivity of the desired reflected optical signal and removing other wavelengths from the optical signal fed to the wavelength evaluator 86.

By coupling the interrogator system 190 to the optical fiber 94 containing an array of gratings to be interrogated, an optical sensor system 192 is formed in accordance with a thirty-second embodiment of the invention.

An optical interrogation system 200 in accordance with a thirty-third embodiment of the invention is shown in Figure 20. The sensor system 200 uses an optical interrogation system 170 as shown in Figure 17, although an optical interrogation system 180 as shown in Figure 18 or 190 as shown in Figure 29 alternatively can be used.

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In this example, the sensor system 200 also comprises two fibers 202, 204 containing arrays of sensor gratings 206 and 208, respectively. The fibers 202, 204 are connected to the interrogation system 170 via a WDM multiplexer-demultiplexer ("multiplexer-de-multiplexer", MUX-DEMUX). 210. In use, a MUX-DEMUX 210 demultiplexes a combined optical signal comprising multiple optical channels fed to the separate port 212 on one side into a plurality of optical channels (one for each optical channel). Each of the demultiplexed optical signals is output through different output ports on the opposite side of the MUX-DEMUX. Each output port supports a different optical channel. The MUX-DEMUX 210 also operates in reverse for multiplexing a plurality of optical signals into a single combined optical signal. In this example, only two of the output ports 214, 216 are used, since only two optical channels are used.

Each of the gratings 206, 208 in an array has the same resonant wavelength, ie all the gratings 206, 208 in an array operate in the same optical channel. The optical channel in which the gratings 206, 208 of a particular fiber 202, 204 operate corresponds to the optical channel supported by the output port 214, 216 of the MUX-DEMUX 210 to which the fiber 202, 204 is connected.

15 As described above, the interrogation system 170 can distinguish between the gratings both spatially and by wavelength channel. It is therefore possible to individually query one of the grids 206, 208 inside two fibers 202, 204.

Although only two fibers 202, 204 containing arrays of gratings 206, 208 to be interrogated are shown connected to the output ports of the MUX-DEMUX 210, it will be appreciated that a larger number of fibers, and thus arrays of gratings, may be provided . The number of different grating arrays that can be supported is only limited by the number of wavelength channels that can be interrogated by the interrogator 170 and the configuration of the MUX-DEMUX 210.

The use of WDM MUX-DEMUX 210 to couple multiple arrays of gratings 206, 208 to a single interrogator 170 reduces the optical loss percentage in the sensor system 200, compared to using a coupler (as shown in Figure 5). The ability to have a large number of arrays of grids in a single sensor system 200 is an advantage in sensor applications where measurements must be made at relatively dispersed locations.

An optical interrogation system 220 in accordance with a thirty-fourth embodiment of the invention is shown in Figure 21. The optical interrogation system 220 in this embodiment is substantially the same as the optical interrogation system 80 shown in Figure 7, with the following modification. The same reference numbers are used for corresponding features.

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In this embodiment, the optical detection takes place in the form of a photodetector 222.

The optical interrogation system 220 can operate as an optical reflectometer ("OTDR (optical time domain reflectometer)"). As shown in Figure 21, the OTDR 220 can be used to interrogate, for example, reflective optical elements in the form of breaks 224 or patch cord connections 226 in an optical fiber 228, or the end 230 of the optical the fiber 228.

Figure 22 shows an optical interrogation system 240 in accordance with a thirty-fifth embodiment of the invention. The optical interrogator system 240 is substantially the same as the optical interrogator system 220 of the previous embodiment, with the following modification. The same reference numbers are used for corresponding features.

10 In this embodiment, the optical interrogation system 240 further comprises optical signal routing means ("optical signal routing means") in the form of a fiber Bragg grating ("fibre Bragg grating") (FBG) 242 arranged in the optical path of a returned signal, between the SOA 82 and the photodetector 222. The FBG 242 has a reflectivity of less than 100%, so that most of a returned optical signal is reflected by the FBG 242, while a portion of the returned optical signal is transmitted to the photodetector 222 As described above in connection with Figure 12, the reflected portion of the returned optical signal passes back through the SOA 82 and propagates back toward a reflective optical element under interrogation. The optical interrogator system 240 may operate as an OTDR.

An optical interrogation system 250 in accordance with a thirty-sixth embodiment of the invention is shown in Figure 23. The optical interrogation system 250 is substantially the same as the interrogation system 240 of the previous embodiment, with the following modification. The same reference numbers are used for corresponding features.

In this embodiment, the photodetector 222 is located on the opposite side of the FBG 242, between the FBG 242 and the SOA 82. The photodetector 222 is connected to the optical interrogation system 250 using a low percentage optical coupler 252. This arrangement has the advantage that only the portion of the returned optical signal reflected by the FBG 242 is routed to the photodetector 222. Any amplified spontaneous emission (ASE) noise generated by the SOA 82 outside the reflection bandwidth of the FBG 242 is transferred and thus removed.

An optical interrogation system 260 in accordance with a thirty-seventh embodiment of the invention is shown in Figure 24. The optical interrogation system 260 is substantially the same as the system 250 in accordance with the previous embodiment, with the following modification.

In this embodiment, the photodetector 222 is located on the other side of the SOA 82, connected to the system 260 via the optical coupler 252. This arrangement creates the advantage that the reflected part of the returned signal has been further amplified through the SOA 82, before it is fed to the photodetector 222.

An optical interrogation system 270 in accordance with a thirty-eighth embodiment of the invention is shown in Figure 25. The optical interrogation system 270 in accordance with this embodiment is substantially the same as the system 250 shown in Figure 25, with the following modifications.

In this embodiment, a second optical signal routing means has been created in the form of a second FBG 272, adjacent to the first FBG 242. The two FBGs 242, 272 have different resonance wavelengths.

Similarly to the optical interrogation system 170 shown in Figure 17, the operating means generates 84 pairs of operating pulses. The first drive pulse in each pair enables an optical signal pulse to be generated and loaded into the optical fiber 228 comprising reflective optical elements to be interrogated. The first drive pulse also gates returned optical signals, and selects which signal is transmitted to the FBGs 242, 272. The transmitted returned optical signal is reflected by one of the 15 FBGs 242, 272, depending on its wavelength. The separation between the first operating pulse and the second operating pulse selects which of the FBGs 242, 272 is in use, thereby selecting the operational wavelength bandwidth.

This arrangement is advantageous for an optical system comprising reflective optical elements having wavelength-dependent reflective properties. This enables, for example, that fiber breakage (which will affect all wavelengths) can be distinguished from wavelength drift of specific reflective optical elements, which will not generally affect all wavelengths.

The described embodiments create various advantages, as follows. Using the SOA simultaneously as a broadband source and a combined optical signal selecting and amplifying means allows the interrogation system to be constructed with a separate active optical component, or an active component 25 for each separate array of reflective optical elements to be interrogated. This reduces the overall size and cost of the query systems, and enables improved efficiency and reliability, compared to many known existing systems. In the described optical sensor systems that use a time-division multiplexing based interrogation system, all the gratings can be identical, which simplifies both the manufacturing procedures and the installation procedures. The gratings 30 can alternatively be replaced with another type of reflective optical elements, where each element is again identical.

The provision of an additional length of optical fiber between the SOA and an array of reflective optical elements to be interrogated enables the SOA to be driven by a continuous stream of electrical drive pulses and, when the SOA forms part of the optical pulse source, it enables arrays of reflective optical elements to be interrogated using a continuous stream of 5 optical pulses. This means that the interrogation systems have improved efficiency compared to commonly known TDM-based interrogation systems ("TDM (Time Division Multiplexing)").

Insertion of one or more FBG back-reflectors in the interrogation system enables and allows the optical pulses to cycle in the sensor systems, which creates various advantages. Low-reflective optical elements, such as low-reflective FBGs, can be used in the optical sensor systems.

10 Low-reflective FBGs are cheaper to make, suffer less from third-reflection interference and enable a greater number of gratings to be used in an array before the power level drops to unacceptable levels, than high-reflective gratings. That the optical pulses run in cycles also means that high signal-to-noise ratios ("SNR, signa-to-noise ratio) are achieved due to the effect of the SOA being focused on the dominant cycle wavelength, which leads to the broadband spontaneous emission noise ("broad 15 band ASE noise, ASE - amplified spontaneous emission") generated by the SOA is reduced.

Cycle pulses are also more tolerant of slower electrical and optical response times (for a given SNR) than would be expected for non-cycle pulses. This means that the reflective optical elements in an optical interrogation system can have a smaller separation between them than is possible when non-cycle pulses are used.

20 By using pairs of electrical drift pulses, the SNR of an interrogation system and a sensor system can be increased even further, and the electrical power is improved. In addition, adjusting the electrical drive pulse setting enables active selection of which reflected optical signal, and thus which reflective optical element, to interrogate by selecting which FBG back-reflector is the correct distance from the SOA to allow re-reflected signals to again transmitted through the 25 SOA.

The use of multiple reflective rear reflectors enables the interrogation system to use both WDM (Wavelength Division Multiplexing) and TDM. This can further be used to reduce the distance between reflective optical elements to be interrogated, thereby increasing the total number of reflective optical elements in an optical sensor system, or to allow a sensor system to insert multiple separate arrays of reflective optical elements.

Various modifications can be made to the described embodiments without departing from the scope of the invention. In particular, one skilled in the art will appreciate that various types of reflective optical elements can be used in place of the fiber gratings, fiber breaks and patch cord connectors described. A reflective optical element can be one of the following: a Fabry-Perot spectroscope device ("etalon device"), which can be a bulk optical Fabry-Perot spectroscope; an optical waveguide grating-based Fabry-Perot spectroscope, one end of an optical fiber, which may be a mirrored end; the end of an optical fiber patch cord; a break in part of an optical fiber; a crystal-based reflective optical element; or a mirror element.

The three different wide bandwidth optical pulse sources described, i.e. the wide bandwidth continuous optical wave source plus the SOA, the SOA only, and the pulsed optical source, can be used interchangeably. The SLD described can be replaced by any other suitable wide bandwidth continuous optical source. As the skilled person will appreciate, the choice of the optical source will depend at least in part on the wavelength range of the reflective optical elements to be interrogated, since the optical source must be capable of generating an optical signal encompassing those wavelengths.

Where the reflective optical elements to be interrogated are described as gratings, a different number of gratings may be used, and the gratings may have different resonance wavelengths than those described. The grids in a matrix can also be separated by a different distance than that described.

Each Interrogation System can also be used to interrogate more than one array of reflective optical elements, such as the grids shown in Figure 5, and therefore each sensor can comprise more than one array of reflective optical elements.

Where fiber Bragg gratings are described, in-plane Bragg gratings may alternatively be used, and the Bragg gratings may be replaced by other types of optical waveguide gratings. As one skilled in the art will appreciate, if planar optical waveguide gratings are used, the described optical fiber can be replaced with planar optical waveguides. This will enable an entire interrogation system to be fabricated as a hybrid lightwave circuit embedded in the SOA, operating electronics and optical devices such as the optical filters and rear reflectors, and optical connectors or circulators.

The SOA described can be replaced by other optical amplifying and signal-selecting means capable of pulsed operation. In particular, an amplifier-coupled SOA can alternatively be used.

The wavelength evaluation apparatus described, i.e., the wavemeter and the optical filter plus photodetector, can be used interchangeably. The optical filter has a wavelength dependent transfer function may be a bulk optical device, or may be an optical waveguide grating device, such as a tilted Bragg grating (as described in our patent pending UK patent application GB 0013411.4 and PCT application PCT/GB01/02422).

The optical filter makes it possible for it in or accompanying the optical pulse source to have a different spectral transmission profile than described. Similarly, the optical filters arranged in front of the wavelength evaluation apparatus may have a different spectral transmission profile than described. The CFBG back reflectors described may have different spectral profiles than those described, and may be replaced by different optical reflectors, in particular different optical fiber gratings or different optical waveguide gratings in a plane. When more than one rear reflector is used, a different number of rear reflectors can be used than described and there can be a different separation between adjacent rear reflectors.

10

Claims (15)

341810 37 Patent claim 1. An optical waveguide grating tracking system (10, 40, 80, 130, 14, 170), comprising: an optical source capable of generating optical pulses (12 and 14, 42, 82), to be coupled into one end of an optical waveguide (16, 94) comprising one or more gratings (24, G1-n) to be interrogated; wavelength evaluator (18, 82); and on the other hand, optical amplification and signal selection means (14, 82) for optical connection to the waveguide; wherein the wavelength evaluation apparatus is optically coupled to the optical amplifying and signal selecting means, and which can evaluate the wavelength of a returned optical pulse transmitted by the optical amplifying and signal selecting means; characterized in that the two-way optical amplifying and signal selecting means (14, 82) is a single optical device and which can be operated for selective transmission of an optical pulse returned from a grating under interrogation and for simultaneous optical amplification of an optical signal transmitted therethrough. 15 2. An optical waveguide grating interferometer system according to claim 1, characterized in that the two-way optical amplifying and signal selecting means is an optical amplifying device, such as an optical semiconductor amplifier (14, 82) arranged for switching operation; where when the optical amplifying and signal selecting means is turned on, it transmits and amplifies an optical signal, and when it is turned off, the transmission and amplification of optical signals is prevented. 20 3. An optical waveguide grating interrogator system in accordance with claim 1 or 2, characterized in that the interrogator system further comprises operating means (22, 84) for the two-way optical amplifying and signal selecting means, where the operating means can generate electric operating pulses of variable frequency and to obtain the optical amplifying and signal selection means to be turned on and off. 4. An optical waveguide grating interferometer system in accordance with one of the preceding claims, characterized in that the optical source comprises the two-way optical amplifying and signal selecting means (82), wherein when the optical amplifying and signal selecting means is turned on, it simultaneously generates a optical signal in the form of an amplified spontaneous emission and converts the optical signal into an optical pulse. 341810 38 5. An optical waveguide grating detector system according to one of claims 1-3, characterized in that the optical source comprises a continuous optical wave source capable of generating a continuous optical wave signal, such as a super-light-emitting optical diode (12), connected to the two-way optical amplifying and signal selecting means, wherein the optical amplifying and signal selecting means is turned on and off when it reaches the continuous optical wave signal into optical pulses. 6. An optical waveguide grating detection system in accordance with one of claims 1-3, characterized in that the optical source comprises a pulsing optical source (42) which can generate optical pulses. 7. An optical waveguide grating detection system in accordance with one of the preceding claims, characterized in that the wavelength evaluation apparatus comprises a wavelength meter (18), an optical spectrum analyzer or an optical filter element having a wavelength-dependent filter response (88) followed by optical detection means, such as a photodetector (90); where the transit time of the optical signal identifies which grating it was returned from and the wavemeter, optical spectrum analyzer or optical filter and optical detection means measure the wavelength of the optical signal. 8. An optical waveguide grating interrogator system in accordance with one of the preceding patent claims, characterized in that the interrogator system further comprises a part of an optical waveguide (32, 112) coupled between the two-way optical amplifying and signal selection means and its respective 20 optical waveguides containing grid to be queried. 9. An optical waveguide grating interrogator system in accordance with one of the preceding patent claims, characterized in that the interrogator system further comprises an optical reflector (132, 144) arranged after the two-way optical amplification and signal selection means, where the spectral profile in reflection for the optical reflector covers the same spectral region occupied by the one or more reflective optical waveguide gratings to be interrogated, and the reflector is located sufficiently close to the optical amplifying and signal selecting means to ensure that the time it takes an optical signal to propagate from the optical the amplifying and signal selecting means to the reflector and back to the optical amplifying and signal selecting means, is shorter than the duration of the electrical pulses that turn on the optical amplifying and signal selecting means. 10. An optical waveguide grating interrogation system in accordance with patent claim 9, characterized in that a number of optical reflectors (174, 176) are arranged according to the two-way optical amplification and 341810 39 the signal selecting means, where each reflector is placed at a different distance from the optical amplifying and signal selecting means, where the farthest reflector is placed sufficiently close to the optical amplifying and signal selecting means to ensure that the time it takes an optical signal to propagate from it optical amplifying and signal selecting means to the reflector 5 farthest away and back to the optical amplifying and signal selecting means is shorter than the duration of the electrical operating pulse that switches the optical amplifying and signal selecting means on. 11. An optical waveguide grating tracking system in accordance with claim 10, characterized in that the spectral profile in the reflection for each optical reflector covers a different spectral range. 10 12. An optical waveguide grating sensor system (26, 44, 96, 136, 146, 178), which system comprises: an optical waveguide (16, 94) comprising a target system in which a matrix of optical waveguide gratings (24, G1-n) arranged at intervals is provided; wherein the optical waveguide is connected to an optical waveguide grating tracking system (10, 40, 80, 130, 14, 170) as stated in one of the preceding patent claims 1-11. 15 13. An optical waveguide grating sensor system in accordance with patent claim 12, characterized in that the resonant wavelength of each grating in the matrix lies within the same wavelength window, and all the gratings thus operate in a separate optical channel. 14. An optical waveguide grating sensor system in accordance with patent claim 12, characterized in that the gratings in the matrix are arranged in groups, where each group contains a substantially identical set 20 of gratings, where the resonant wavelength of each grating in a group lies in a different wavelength window, and thus operates in a different optical channel, so that the transit time of a returned optical pulse identifies which group a grating that is queried belongs to. 15. An optical waveguide grating sensor system in accordance with one of the patent claims 12-14, characterized in that the sensor system comprises a plurality of optical waveguides (16, 56) each of which comprises a matrix of optical waveguide gratings arranged at intervals, where each waveguide is connected to a respective two-way optical amplification and signal selection means.