HK1017785B - Method and apparatus for transmitting signals in an optical fibre - Google Patents

Description

Method and apparatus for transmitting signals in optical fiber

Technical Field

The invention relates on the one hand to a method for transmitting signals in an optical fiber, comprising amplitude-modulating a high-frequency optical carrier with the transmitted signal and transmitting the thus modulated optical carrier in the optical fiber, and on the other hand to a device for transmitting signals in an optical fiber, comprising an amplitude modulator for amplitude-modulating the high-frequency optical carrier with the transmitted signal.

Background

In today's development, one of the main problems of high frequency or high speed fiber transmission systems, 10Gbit/s or higher, is group velocity dispersion in the fiber causing distortion of the transmitted signal. Most of the installed fibers had zero dispersion at 1.3 μm, but the loss was minimal at 1.55 μm, and the group velocity dispersion was about-17 ps/(nm km). This distortion has been a problem for long distance transmissions of more than 100km at 2.5Gbit/s, for which purpose an external modulator has to be used instead of a directly modulated semiconductor laser. At 10Gbit/s, dispersion is the main limiting factor for transmission distance, and the higher the bit rate, the more the limit, since the spectral broadening of the signal is proportional to the bit rate.

The reason why dispersion causes signal distortion is that it causes the phase to vary with the square of the modulation frequency. Both sidebands acquire this phase with the same sign. After detection with a square-law detector, such as a photodiode, this results in a small signal response of the form:

H(v)＝cos(F Lν²) Where v is the modulation frequency and L is the transmission distance, and if L becomes large enough, the response causes transmission to be zero in the desired frequency band. These nulls are responsible for signal distortion.

Since the problem increases with the spectral width of the transmitted signal, one way to improve this situation is to limit this spectral width. In the radio and television fields, spectral reduction is used to block more channels in the available frequency band and is commonly achieved with Single Sideband (SSB) modulation or Vestigial Sideband (VSB) modulation with or without carrier suppression. The problem with suppressing the carrier is that a very stable narrow-band local oscillator is required to recover the signal. This makes it similar to coherent modulation systems (FM, PM), but commercial optical transmission systems today use only Amplitude Modulation (AM), since a simple square-law detector, such as a photodiode, can then be used in the receiver.

Other methods have previously been proposed to overcome the problems caused by chromatic dispersion in optical fibres. The two most promising approaches appear to be spectral inversion with four-wave mixing and the use of a length of fiber with dispersion of opposite sign. The problem with spectral inversion is that it is complex, has low efficiency, must be implemented in the middle of a fiber link and is difficult to use in wavelength multiplexed systems. The main problem with having a compensating fiber is the additional loss, which must be compensated with an optical amplifier, which causes a reduction in the signal-to-noise ratio.

Brief introduction to the invention

The object of the invention is to overcome the limitation of transmission distance due to group velocity dispersion in optical fibers in a simple manner and at low cost in the optical field.

This is achieved with the method according to the invention by suppressing at least a part of one sideband of the modulated optical carrier to reduce the effect of group velocity dispersion in the optical fibre.

This object is also achieved with the device according to the invention in that it comprises a suppression means for suppressing at least a part of one sideband of the modulated optical carrier to reduce the effect of group velocity dispersion in the optical fiber.

This sideband suppression can be achieved before transmission in the optical fiber or vice versa after transmission in the optical fiber but before detection of the optical signal.

An electronic circuit for phase correction (and final equalization) may also be added to the receiver.

The method and the apparatus according to the invention are simple and can be implemented at low cost, so that they can be at least as low as the methods and apparatuses that have been proposed hitherto.

Brief description of the drawings

The present invention will be described in further detail with reference to the accompanying drawings. The drawings are

Figure 1 shows a first embodiment of the device according to the invention,

figures 2a and 2b show diagrams illustrating the function of the embodiment according to figure 1,

figure 3 shows a second embodiment of the device according to the invention,

figures 4a, 4b and 4c show diagrams illustrating the function of the embodiment according to figure 3,

figure 5 shows a third embodiment of the device according to the invention,

fig. 6a, 6b and 6c show diagrams illustrating the functionality of the embodiment according to fig. 5.

Detailed description of the invention

To achieve the goal of the present invention, i.e., to preserve the low cost and high reliability advantages of a simple receiver, a simple SSB or VSB is employed in accordance with the present invention.

Further analysis of the SSB and VSB modulation schemes has shown that their benefits are not merely to reduce the spectral spread of the signal.

However, in a pure SSB system, there is only one sideband, and thus the response becomes:

H_SSB(ν)＝(1/2)exp(jFLν²) (2) where j is the square root of-1, which is a pure frequency dependent phase. This can be compensated for by an electronic circuit that performs phase correction.

True SSB is difficult to implement in the optical domain. Furthermore, as a way of commonly implementing AM modulation in optical systems, i.e. simulation of the transmission of a pseudorandom bit pattern with a large extinction ratio for this modulation, it is shown that pure SSB modulation may not be the best solution.

However, the three VSB modulated embodiments of the apparatus according to the present invention described below give a significant improvement in the achievable transmission distance.

The three embodiments of VSB optical modulators described are based on the use of asymmetric Mach-Zehnder (Mach-Zehnder) interference filters (fig. 1), Bragg grating filters (fig. 3), and Fabry-Perot filters (fig. 5), respectively.

Other embodiments of VSB modulators can be readily implemented by using a different type of optical filter in place of one of the three filters listed above. Examples include multilayer dielectric filters, interference filters, dual fabry-perot filters, and the like. The only requirement is that the filter suppresses a large portion of one sideband while transmitting a large portion of the carrier and the other sideband.

As an alternative to the VSB modulator, it is also possible to use instead a conventional amplitude modulator of one of the types described below and to perform the filtering of the optical signal in the receiver after transmission in the optical fibre. The problems and advantages of this solution are as follows.

In all cases, an electronic circuit for phase correction of the received signal may be required, as shown in equation (2). However, some optical filters, such as fabry-perot filters, also add a phase to the optical signal, which may make electronic phase correction unnecessary. Furthermore, equalization of electronic signals may be advantageous in some instances. This is because most filter filters not only add a phase as in the SSB example described in equation (2), but also change amplitude (see, for example, equation (8) below for asymmetric mach-zehnder filters). VSB modulator

The three embodiments of VSB modulators described herein are all based on the use of an amplitude modulator and an optical filter. Other methods of implementing VSB (or SSB) modulators are well known, but are either quite difficult or even impossible to implement at very high bit rates, or are quite complex and expensive.

It is assumed below that the modulator itself is a pure AM modulation (without chirp). This is in the case of semiconductor electro-absorption modulators or in symmetric Mach-Zehnder modulators (in LinbO)₃Or implemented in a semiconductor) are possible.

The apparatus shown IN fig. 1 comprises an amplitude modulator 1 for amplitude modulating an optical carrier IN with a signal directed to a receiving end at high speed. According to a first embodiment of the apparatus according to the invention, the modulator 1 is followed by an asymmetric mach-zehnder interferometer, generally designated 2, having two arms, a longer arm 3 and a shorter arm 4, between the splitter 5 and the combiner 6.

Three parameters described below are important for the proper operation of this device:

1. the two arms 3 and 4 of the interferometer 2 must have almost equal losses, so that the recombination of the different phases at the combiner 6 will result in a large extinction ratio. Longer arms 3 will generally have higher losses due to absorption and scattering. Several compensation methods are possible depending on the materials and fabrication methods used to fabricate the device. One method that is commonly applied is to use an asymmetric splitting ratio on the splitter 5 in the interferometer 2, so that the power is equal on the combiner 6. Another approach is to have additional loss or gain on one of the arms 3 and 4 to balance the total loss.

2. The optical path difference between the two arms 3 and 4 will determine the filter characteristics, since its transfer function can be written as:neglecting losses in the formula, ω is the angular frequency of the light, c is the speed of light, P_iIs the optical path length of one arm of the interferometer 2, defined as:where s is the distance along the arm, n_effIs the effective propagation index of light in that arm. If n is_effThe same everywhere and constant, equation (3) is simplified as:where ad is the length difference between the arms and phi is taken into account as any additional phase difference, for example due to the phase control means 7 in one arm.

As can be seen from equation (3) or (5), the transfer function of the filter is a periodic function of ω, whose period depends on the optical path difference (or Δ d).

3. Carrier frequency omega_oAnd the relative positions of the maximum and minimum transmissions of equation (3) are important for the device to work well. For example, if ω_oCoinciding with the minimum value, no signal is transmitted, if ω is_oCoinciding with the maximum value, the sidebands are symmetric and no improvement compared to the unfiltered case. The best positions are when:wherein m is an integer. This corresponds to the position indicated by the dashed line in the transmission spectrum in fig. 2 a. There are two ways to adjust this relative position, i.e. to tune the carrier frequency ω_oOr by adjusting the phase change phi in one arm as assumed in fig. 1. Which solution is best dependent on the technology and system requirements used to manufacture the filter.

If the time delay center τ is defined as:and if the bit rate of the modulated signal is B, we can see from fig. 2B and equation (5) how the choice of τ affects the way the signal is filtered. In fig. 2b, the dotted line indicates the typical modulated spectrum and its position relative to the filter transmission spectrum. It can be noted that in an asymmetric mach-zehnder filter tuned as given by equation (6) and shown in fig. 2a, the small-signal modulation response function after detection with a square-law detector becomes:instead of formula (1). From equation (8) we can see that the problem of the occurrence of zeros in equation (1) can be avoided by choosing τ correctly and that the phase distortion can be corrected by appropriate filtering of the electrical signal (as in the case of pure SSB given by equation (2)). Furthermore, it is also possible to achieve amplitude equalization, since there is now no zero point.

The embodiment of the device according to the invention shown in fig. 3 comprises an amplitude modulator, which is denoted 1, since it may be identical to the modulator 1 shown in fig. 1. According to each embodiment, the modulator 1 is followed by a bragg grating filter 8.

The graph shown in fig. 4 illustrates the working principle of the embodiment shown in fig. 3. FIG. 4a shows the optical transfer function, i.e. the transmission spectrum of a typical Bragg grating filter, where ω is_cIs the filter center frequency. FIG. 4B shows a typical modulated spectrum, where B is the bit rate, ω_oIs the optical carrier frequency. Fig. 4c shows the spectrum of the signal after filtering. The bragg grating filter 8 will reflect a certain frequency band and transmit the remaining frequencies, allowing a large portion of one sideband to be suppressed to obtain a VSB optical signal.

Three parameters are also important for the correct operation of this device, namely:

1. the spectral width of the bragg grating reflection band is mainly determined by the grating coupling coefficient k. As a first order approximation, the half maximum (FWHW) of the reflection band is given by (in wavelength):in the formula of_cIs the central wavelength of the grating, L_gIs the length of the grating, n_effIs the effective propagation index (index) of light.

2. Transmission through the grating in the reflection band, which depends on the product kL_g. In a first order approximation, the percentage of power transmitted through the grating (neglecting losses) at the center wavelength is given by:

p_T＝1-tanh²(kL_g) (10)

3. center frequency omega of Bragg grating_c(corresponds to. lambda.)_c) Relative to the central signal frequency omega_oAt the location of (2), here

λ_c＝2n_effAnd Λ (12) where Λ is the physical period of the bragg grating. An example of such a configuration and its effect on the transmitted spectrum is shown in fig. 4. Then, as can be seen from equation (12), if n is_effCenter wavelength, if controllableIt can be adjusted. There are several ways to achieve this (depending on the materials used) as will be seen below. On the other hand, the carrier frequency may be adjusted. Further, which solution is best dependent on the particular technology and system requirements employed.

More accurate calculations of bragg grating properties can be made using the method described in article "New methods for calculating the emission spectra of DFB and DBR lasers" published by j.

The basic structure of the embodiment according to fig. 5 is similar to the first two embodiments and comprises an amplitude modulator 1, which may be identical to the modulator 1 shown in fig. 1 and 3. According to this embodiment of the device according to the invention, the modulator 1 is followed by a filter, generally designated 9, in this example the filter 9 being a fabry-perot filter having two reflecting elements or mirrors 10 and 11. The principle of operation of the embodiment shown in fig. 5 is illustrated by the graph shown in fig. 6. FIG. 6a shows the optical power transfer function, i.e. the transmission spectrum, of a typical Fabry-Perot filter, where ω is_cIs the filter center frequency. FIG. 6B shows a typical modulated spectrum, where B is the bit rate, ω_oIs the optical carrier frequency. Fig. 6c shows the signal spectrum after filtering. The fabry-perot filter 9 is designed and configured to transmit only one sideband and about half the carrier power.

Three important parameters for the correct operation of this device are:

1. full Width Half Maximum (FWHM) of the transmission band. It should typically be of the order of the bit rate.

2. Free spectral region, which should be at least several times the bit rate.

3. Carrier frequency omega_oAnd the center wavelength omega of the transmission band_cShould be adjusted so that the carrier frequency is at the half-maximum transmission point of the filter, as shown in fig. 6.

All these parameters can be determined according to well-known fabry-perot filter transmission formulas (see for example m.born and e.wolf, "optical principles", sixth edition, Pergamon Press, Oxford, 1986):where R is the intensity reflection coefficient of the FP filter plate, d is the distance between the two plates, n is the refractive index between the plates, and c is the speed of light in vacuum. The intensity transmission is then:wherein F is given by:clearly, the FWHM is given by (in frequency):and the free spectral region Δ ω is:in practice, the relative position of the filters and the carrier frequency can be adjusted by fine-tuning the distance d.

These VSB modulators may be implemented with different levels of integration using several different techniques. A short summary of some possible implementations will be given for each device. It should be noted that when the devices are integrated, they only work correctly when single mode wavelengths are used. Mach-decoder based modulator

First consider the different technologies available to fabricate the three basic components of a transmitter, namely a laser, a modulator, and an asymmetric mach-zehnder interferometer (hereinafter referred to as MZI).

A laser: semiconductor lasers (usually based on AlGaAs/GaAs or InGaAsP/InP) are typically used, but other lasers, such as diode-pumped YAG lasers, may be used. It needs to operate at a constant output power, stable frequency and narrow line width.

The modulator: only two types of modulators commonly used today have the bandwidth required for high speed transmission systems. The first type is a symmetric mach-zehnder modulator that uses the electro-optic effect (in crystals such as LiNbO)₃Medium) or quantum confined stark line magnetic splitting (in semiconductors) to change the phase in one (or both) arms of the interferometer by changing the refractive index. The second type is an electro-absorption modulator in a semiconductor material with bulk material or quantum wells in the absorption layer. Both types can be made without chirp or with little chirp.

Asymmetric MZI: this is the largest possible number of components: free space (adopt)Mirrors and beam splitters), optical fibres (using fibre splitters), SiO on silicon₂And using LiNbO₃Or an integrated optical dielectric waveguide implemented with a lattice-paired semiconductor such as AlGaAs/GaAs or InGaAsP/InP.

Different solutions are possible for the implementation of the phase control. Some of these solutions are mentioned below:

the piezoelectric element can be used to vary the difference in length of the arms by the total amount required for the free space and fiber conditions

Changing the refractive index of one arm of the MZI by changing the temperature of one arm of the MZI using the thermo-optic effect (e.g. with a resistive heater or a thermo-electric refrigerator). This can be used for optical fibers and all integrated optical waveguides.

For lattice paired semiconductors, we can also use carrier injection or cancellation, BRAQWETS or quantum confined stark line magnetic splitting effects to change the refractive index.

For integrated optical waveguides, the splitter 5 and combiner 6 can be made in several different ways. Among these, we have Y-junctions, coupled waveguides and multimode interference splitters, all of which give any desired splitting ratio.

Now, various integration capabilities can be listed as follows:

1. non-integrated or hybrid integrated: each component may be implemented using different processes and coupled to an optical fiber or free space or waveguide ultimately implemented on a carrier substrate.

2. And (3) complete integration: the laser, modulator and MZI are all fabricated monolithically on the same chip. This is possible with semiconductors such as AlGaAs/GaAs or InGaAsP/InP, with LiNbO₃It is also possible (using erbium doped lasers).

3. Partial integration: there are two possibilities:

laser and modulator integration: by a semiconductor andLiNbO₃is possible (as is the case with full integration)

Modulator and MZI integration: also used are semiconductors and LiNbO₃Is possible.

In some cases, it may be necessary to insert an optical isolator in a position to avoid light reflecting back into the laser, thereby disturbing the stability of the laser. Modulator based on Bragg grating

For lasers and modulators, the possibilities are the same as in the case of the MZI above. The bragg grating filter 8 can be implemented in several ways, including:

UV writing in fiber: the UV interference pattern can be used to form a periodic refractive index variation in the fiber, thereby obtaining a bragg grating.

Periodic fluctuations in the geometry or composition of the dielectric waveguide, which may be made of semiconductor material or SiO₂Si, but also polymers.

Since the center wavelength of the bragg grating is given by equation (12), it can be changed by changing the refractive index in the waveguide. The same approach can be used as in the MZI case above, when the refractive index in one arm of the interferometer is changed.

If a single bragg grating does not cover the desired spectral region, a concatenation of several bragg gratings slightly shifted from the center wavelength or a chirped (chirped) bragg grating, i.e. a bragg grating with a varying period, may be used. It may also be desirable to reduce the side lobes of the bragg grating reflection band. This can be achieved by chirping the grating or by changing the coupling coefficient k along the grating (see J.P.Weber, M.Olofsson, B.Stoltz, "report on Filter optimization", port (Deliverable CT3/D4), RACE 2028 MWTN (Multi-wavelet transform Network) project of the European Commission, 5, Dec.1994).

The integration possibilities of bragg grating based modulators are similar to those of the MZI except that an isolator must be inserted between the grating and the laser to avoid disturbing the stability of the laser in the presence of reflections from the grating. This makes it impossible to fully integrate since the optical isolator cannot be integrated (at least not possible with currently available processes), but other alternatives are possible because the isolator can be added before or after the modulator. Fabry-Perot based modulator

The same possibilities as above are available for the laser and the modulator. The fabry-perot filter 9 can be implemented in several ways (some are commercially available) including:

bulk (Bulk) optics, using parallel-plate mirrors. This device can be tuned mechanically, for example with a piezo actuator.

Fiber fabry-perot: instead of free space, light travels in an optical fiber with high reflection at each end. It can also be tuned with a piezoelectric element.

Integrated waveguide: a section of (single mode) waveguide with large reflections at each end. For example, the reflection may be provided by a bragg grating or a cleaved or etched grid. As in the case of the MZI above, tuning may be achieved by placing a phase control section (not shown) in the waveguide.

Loose optical devices or fiber fabry-perots cannot be integrated, but waveguide devices can be integrated with the modulator (as in the case of bragg gratings). It should be noted that here the same problem exists as in the case of bragg gratings: an isolator is required between the laser and the filter to avoid disturbances caused by reflections. The same limitations apply here, as in the case of bragg gratings.

Examples

An example of implementing each type of device will be outlined below. The first two examples would be devices that integrate the modulator and filter on the same chip as used in InGaAsP/InP. For fabry-perot, one integrated laser/electro-absorption modulator and one fiber fabry-perot filter are assumed. In all examples, the optical filter length is assumed to be around 1.55 μm. These devices do not require any new machining or manufacturing techniques and can be implemented using existing processes. Integrated modulator-MZI

In this example, the structure of fig. 1 can be implemented with a modulator 1 called an electro-absorption modulator, which employs a Y-junction as the splitter 5 and combiner 6, and a forward biased p-i-n impurity structure to inject carriers and control the refractive index in the phase control section 7. Assuming that the waveguide has an InGaAsP core (bandgap wavelength 1.38 μm) that is 0.2 μm thick and 1.3 μm wide, the coating is InP, and the effective propagation index is 3.22 at 1.55 μm wavelength. Carrier injection can reduce the effective refractive index by a maximum of about 0.014 (see "four-channel tunable optical notch filter using InGaAsP/InP reflection grating" by j. -p.weber, b.stoitz, m.dasler and b.koek, IEEE photon.techn.lett., vol.6(1), jan.1994, PP77-79), which means that the phase control section must be greater than about 111 μm long to obtain a 2 pi change. The electro-absorption modulator uses an InGaSaP core with a 1.48 μm bandgap and a reverse biased p-i-n structure. A length between 100 and 200 μm is sufficient to obtain a good extinction ratio. Calculations for some examples show that a good choice of τ is to have (ω o ± π B) correspond to the minimum (maximum) of the filter function (equation 5). Either B is chosen to be 10Gbit/s, which gives τ 1/2B 50ps, or by using equation (7) the arm length difference Δ d is 4.66 mm. (it should be noted that the higher the bit rate, the smaller this.) the splitter 5 can be implemented with a length of the order of 100 μm or less. Thus, the entire device can be fabricated on a single chip with dimensions less than 4mm long by 3mm wide without difficulty. Integrated modulator-bragg grating

This example uses the structure of figure 3 and the same InGaSaP/InP waveguide as the previous example. The only difference is that a grating is added on a section of waveguide, and carriers can be injected by a p-i-n structure at the section of waveguide to change the central wavelength. Assuming a FWHM of 2nm is desired, the center wavelength transmission is-10 dB. By using the formulas (9) and (10), the following can be obtainedMust have k equal to 54.3cm^-1And Lg-335 μm. Thus, this device should be implemented on a chip that is less than 600 μm long and less than 100 μm wide. For narrower stop bands, the better the fiber grating may be. However, as mentioned above, an optical isolator must be provided between the laser and the device. Fabry-Perot and integrated laser/modulator

Integrated lasers and electroabsorption modulators will soon become available on the market. For a 10Gbit/s system, a fabry-perot with a FWHM of 10GHz is desired. Assuming that R is 0.9 (which is easy to do), a refractive index of 1.5, a fiber length (between mirrors 10 and 11) of about 2.11mm, and a free spectral region of 298GHz, which is sufficiently large, can be obtained. SiO may also be used₂the/Si waveguide (with temperature adjustment) replaces the optical fiber. As in the case of bragg gratings, an isolator must be placed between the laser and the filter. Filtering at a receiver

As described above, an alternative to a VSB (or SSB) modulator is to use a conventional amplitude modulator and perform sideband suppression by optical filtering after transmission through the fiber. In fig. 1, 3 and 5, this corresponds to inserting optical fibers between the modulator 1 and the filters 2, 8 and 9, respectively. In this case, the same devices as those described above may be used.

If we have a single channel system with sufficiently low optical power (and short distance) in the fiber to ignore the non-linearity, this is equivalent to the previous solution.

The advantage is now that reflection from the filter back to the laser will not be a problem, since there is usually already an optical isolator between the transmitter and the fibre to avoid problems caused by reflection from splices and splices.

However, this solution also has several problems. In the probe case, the total optical power carried in the fiber is higher for the same power than with the VSB modulator. This can be a problem because the non-linear effect increases in proportion to the square of the optical power. In addition, the spectral width of the optical signal in the optical fiber will become large, which means that the channel spacing must be increased in a Wavelength Division Multiplexing (WDM) system.

Because of these problems, it is generally better to use a VSB (or SSB) modulator.

Claims (12)

1. A method of transmitting a signal in an optical fibre comprising amplitude modulating a high frequency optical carrier with the signal to be transmitted and transmitting the so modulated optical carrier in the optical fibre, characterised in that at least a portion of one sideband of the modulated optical carrier is suppressed to reduce the effect of group velocity dispersion in the optical fibre.

2. A method according to claim 1, characterized in that the suppression takes place at the receiving end of the optical fiber.

3. A method according to claim 1, characterized in that the suppression is achieved by means of vestigial sideband modulation.

4. A method according to claim 1, characterized in that the suppression is achieved by means of single sideband modulation.

5. Apparatus for transmitting signals in an optical fibre, comprising an amplitude modulator 1 for amplitude modulating a high frequency optical carrier with the transmitted signal, characterised by suppression means (2, 8, 9) for suppressing at least a part of one sideband of the modulated optical carrier to reduce the effect of group velocity dispersion in the optical fibre.

6. An apparatus according to claim 5, characterized in that said suppressing means (2, 8, 9) is located at the receiving end of the optical fiber.

7. An arrangement according to claim 5, characterized in that said suppressing means (2, 8, 9) comprises a vestigial sideband modulator.

8. Apparatus according to claim 6 or 7, wherein said suppressing means comprises an optical filter.

9. Apparatus according to claim 8, characterized in that said optical filter comprises an asymmetric mach-zehnder interferometric optical filter (2).

10. Apparatus according to claim 8, characterized in that said optical filter comprises a bragg grating optical filter (8).

11. An apparatus according to claim 8, characterized in that said optical filter comprises a fabry-perot optical filter (9).

12. Apparatus according to claim 5, wherein said suppression means comprises a single sideband modulator.