GB2322228A

GB2322228A - Optical amplifiers and WDM transmission systems

Info

Publication number: GB2322228A
Application number: GB9703233A
Authority: GB
Inventors: Kevin Wallace Bennett; Fiona Davis; Richard Achile Habel; Paul Anthony Jakobson; Nigel Edward Jolley; Robert William Keys; Kim Byron Roberts
Original assignee: Northern Telecom Ltd; Corning Inc
Current assignee: Nortel Networks Ltd; Corning Inc
Priority date: 1997-02-17
Filing date: 1997-02-17
Publication date: 1998-08-19
Also published as: TW369613B; GB9703233D0

Abstract

In a WDM transmission system having a transmission path including a concatenation of laser diode pumped optical amplifiers 20, the gain spectrum of an amplifier is controlled at least in part by a feedback loop regulating the temperature, and thus the emission wavelength, of its laser diode pump 21. The feedback loop may for instance derive its control signal from a measure of the drive current applied to the pump (as shown), of the emission wavelength of the pump, or of the disparity between the power output from the amplifier in one of the multiplexed signal channels and that from at least one other of the channels.

Description

Optical Amplifiers and WDM Transmission Systems Background to the Invention In wavelength division multiplexed (WDM) transmission systems that empioy known optically pumped optical amplifiers in their transmission paths the phenomenon of gain tilt presents problems. Under different operating conditions the amplifier amplifies the different channels to different relative extents such that any passive system that is designed to equalise the power output of the channels for one specific set of operating conditions is liable to fail to provide equalisation when those conditions are changed.

It has been previously reported that in the case of optically pumped optical waveguide amplifiers that exhibit homogenous broadening, if the gain at any one wavelength is by some means stabilised to a particular value, then the gain at the other wavelengths is similarly stabilised (the stabilised value of the gain being different at different wavelengths).

It is also known, for instance from the paper by A K Srivastava et al, entitled 'Room temperature spectral hole-burning in erbium-doped fiber amplifiers' OFC '96 Tu G7, that spectral hole burning effects can occur as the result of a degree of inhomogenous broadening in the amplifier, this effect having a line width (an affected portion of the spectrum) of about lOnm. This effect is dependent upon the degree of saturation of the amplifier, but is always centred at the saturating signal wavelength.

Summary of the Invention A new and hitherto unreported effect has been observed by us, namely the fact that in respect of an optically pumped optical amplifier saturated at a particular wavelength within its gain spectrum, for instance an erbium doped fibre amplifier pumped in the 980nm band and amplifying in the 1525 to 1570nm band, the shape of the gain spectrum varies significantly with pump wavelength within a spectral range over which there is no significant change in absorption of the pump power by the amplifier. This effect has been observed by setting the power delivered to the amplifier by the pump to a fixed power level, and then making a change of about 5nm to the pump wavelength. Over this range of wavelength there is about an 8% change in absorption of the pump power by the fibre amplifier and, as expected, the shape of the gain spectrum in the wavelength range from 1540nm to 1570nm remains substantially unchanged. In contrast, the shape of the gain spectrum in the wavelength range from 1525nm to 1540nm is unexpectedly found to alter significantly. The effect is therefore much more sensitive to pump wavelength than previously observed effects. The shape of the gain spectrum within the 1525nm to 1540nm wavelength range has been seen to change by 0.5dB for a 0.8nm change in pump wavelength. Lack of adequate control of pump wavelength can therefore seriously impair the operation of optical amplifiers of an optically amplified transmission system amplifying WDM channels with this band.

According to a first aspect of the present invention there is provided a method of operating an optical amplifier having an optically amplifying medium optically pumped with a semi-conductor laser, in which method the shape of the gain spectrum is regulated by dynamic control of the emission wavelength of the laser.

According to a second aspect of the present invention there is provided an optical amplifier having an optically amplifying medium optically pumped with a semiconductor laser, the amplifier including control means that regulates the shape of the gain spectrum by dynamic control of the emission wavelength of the laser.

According to a third aspect of the present invention there is provided a method of operating an optical amplifier having an optically amplifying medium optically pumped with a semiconductor laser, in which method a feedback control loop is employed to control the magnitude of pump power delivered to the amplifying medium by the laser, and adjustment means is employed to modify the change in emission wavelength that results directly from changes in said delivered pump power.

According to a fourth aspect of the present invention there is provided an optical amplifier having an optically amplifying medium optically pumped with a semiconductor laser, the amplifier including feedback means controlling the magnitude of pump power delivered to the amplifying medium by the laser, and including adjustment means that modifies the change in emission wavelength that results directly from changes in said delivered pump power.

One way of exercising dynamic control over the emission wavelength of the semiconductor laser involves the use of a dynamically wavelength adjustable narrow-band laser optical cavity defining reflector as an external reflector of an external cavity semiconductor laser. Such a reflector may be constituted by a Bragg reflection grating in a length of single mode optical fibre that is dynamically stretched, for instance piezo electrically. An alternative way involves the use of a three-terminal semiconductor laser such as described by Y. Yoshikuni and G Motosugi in the paper entitled 'Multielectrode Distributed Feedback Laser for Pure Frequency Modulation and Chirping Suppressed Amplitude Modulation', Journal of Lightwave Technology Vol. LT5, No. 4, April 1987, pp 516522. A preferred way involves exercising dynamic control over the temperature of the semiconductor laser.

According to a fifth aspect of the present invention there is provided an optical amplifier having an optically amplifying medium optically pumped with a laser diode whose temperature is dynamically regulated at least in part by a signal derived from a measured operating parameter of the amplifier.

According to sixth aspect of the invention, in a wavelength division multiplexed transmission system having a receiver optically coupled with a transmitter via a wavelength division multiplexed transmission path that includes a concatenation of laser diode pumped optical amplifiers, each of which exhibits a gain spectrum, the invention also provides a method of adjusting the gain spectrum of at least one of said amplifiers by dynamically regulating the temperature of its laser diode pump in response to a measured operating parameter of said amplifier, or of said system.

The emission wavelength of a typical diode laser emitting at 980nm can be shifted by 0.8nm by a 2.5"C temperature change, or a 40mA change in drive current. This corresponds to wavelength change coefficients of 0.32 nm/ C and 0.02 nm/mA respectively, and represents intrinsic properties of the diode laser. Accordingly, if an amplifier specification calls for the pump drive current to be adjustable over a range of not more than 200 mA, keeping the diode at a constant temperature will ensure that the emission wavelength is kept with a 5nm range, thereby ensuring that absorption of the pump power by the amplifier remains substantially constant. On the other hand it will not ensure that the pump wavelength mediated differential gain effects discussed above are kept to an insignificant level. In other words, whereas stabilising the temperature of a diode pump laser is sufficient to eliminate amplifier operational problems attributable to pump power absorption efficiency, it is by no means sufficient to eliminate those attributable to the newly discovered pump wavelength mediated differential gain effects.

According to one preferred feature of the present invention, the pump's temperature regulating signal is derived from a measure of the pump's drive current.

According to an alternative preferred feature, the signal is derived from a measure of the pump's emission wavelength.

According to a further alternative preferred feature, the signal is derived from a measure of differential gain afforded by the amplifier to traffic in different wavelengths.

In a transmission system having a concatenation of optical amplifiers in its transmission path, the signal derived from a measure of the differential gain afforded by an amplifier to traffic in different wavelengths can be employed not just simply for regulation of the temperature of the diode laser pump of that particular amplifier, but also that of diode laser pumps of some or all of the other optical amplifiers in the transmission path, particularly of those amplifiers upstream in the transmission path.

Brief Description of the Drawings There follows a description of WDM transmission systems and their optical amplifiers embodying the invention in preferred forms. The description refers to the accompanying drawings in which: Figure 1 schematically depicts a WDM transmission system having a receiver connected with a transmitter via a transmission path including a concatenation of optical amplifiers, Figures 2, 3 and 4 schematically depict alternative forms of optical amplifier for use in the system of Figure 1, and Figure 5 schematically depicts a modified form of the WDM transmission system of Figure 1.

Detailed Description of Preferred Embodiments Referring to Figure 1, a WDM receiver indicated generally at 10 is optically coupled with a WDM transmitter indicated generally at 11 by means of a transmission path 12 that includes a concatenation of optical amplifiers 13. The transmitter 11 has a plurality of data-modulated sources 14 (for convenience of illustration only four such sources are indicated in the figure) operating at different wavelengths, typically wavelengths in the waveband extending from about 1525nm to 1570nm.

These wavelengths are multiplexed on to the common transmission path 12 by means of a wavelength multiplexer 15. Optionally, the transmitter may include one of the concatenation of amplifiers 13. The receiver has a wavelength demultiplexer 16, the counterpart to the multiplexer 15 of the transmitter. The outputs of the demultiplexer 16 feed the individual demultiplexed signal channels to associated detectors 17. The receiver may similarly include one of the concatenation of amplifiers 13 as a preamplifier located upstream of the demultiplexer.

Each of the amplifiers 13 includes a laser diode pumped amplifying medium whose laser diode has its temperature dynamically regulated at least in part by a signal derived from a measured operating parameter of the amplifier or of the system.

Figure 2 depicts an amplifier 13 in which this measured operating parameter is the pump laser diode drive current. The amplifier has an optically waveguiding amplifying medium 20. Typically this is constituted by a length of erbium doped optical fibre. This amplifying medium is optically pumped by means of a diode laser pump source 21, typically a source emitting at a wavelength of approximately 980nm. The output of the source 21 is coupled into the common transmission path by means of a wavelength multiplexing coupler 22. The overall gain of the amplifying medium 20 is stabilised by controlling the drive current applied to the pump laser diode.

The drive current for the laser diode of the pump 21 is supplied by a current controller 24 regulated by a control signal from a feedback loop 25 that derives its error signal from an optical coupler 26 that taps a small proportion of optical power from the output of the amplifier. The laser diode of pump 21 is temperature regulated, for instance by means of a Peltier cooler. The regulation is provided by a temperature controller 27 driven by a wavelength control unit 28. The wavelength control unit 28 receives an input from the current controller 24 that provides the pump laser diode drive current, and employs stored data to determine the value of laser diode temperature that is required for this value of drive current in order for the laser diode to emit at a specific wavelength. The data may be stored in the form of a look-up table whose values are obtained prior to installation by plotting the emission wavelength of the laser diode as a function of drive current for different values of laser diode temperature. An alternative way of obtaining these values is to use a reduced number of measurement points and interpolate for the required operating point.

The amplifier of Figure 2 uses the pump laser diode drive current as the regulating parameter for control of the laser temperature. A drawback of this approach is that it inherently assumes that the relationship between emission wavelength, drive current and temperature does not change as the laser ages. This problem is better addressed by the amplifier of Figure 3.

The amplifier of Figure 3 has many components in common with the amplifier of Figure 2, and these components are identified in Figure 3 with the same index numerals as those employed in respect'of their counterparts in Figure 2. The difference between the two amplifiers comprises the replacement of the wavelength control unit 28 of Figure 2 by a second feedback loop 30 which controls the operation of the temperature controller 27 and derives its error signal from the output of an optical wavelength discriminator 31 applied with an input from an optical coupler 32 that taps a small proportion of optical power supplied to the wavelength multiplexing coupler 22 from the pump 21. Instead of using an optical coupler 32 to tap power emitted from the front facet of the laser diode, the input to the discriminator 31 can be taken from power emitted from the rear facet. The optical wavelength discriminator 31 may, for instance, be constructed in known manner from a parallel arrangement of two Mach Zehnders. One of these is constructed to have a falling edge of its characteristic centred at the desired emission wavelength, while the other Mach Zehnder is constructed to have a rising edge centred at that wavelength.

The amplifier of Figure 3 operates to stabilise the wavelength emission of its pump laser diode. In this way changes in gain spectrum attributable to changes in pump wavelength are suppressed. A primary reason for desiring stability in the gain spectrum is to enable measures to be taken to avoid signal to noise ratio problems that arise if the power levels of signals launched into different channels of a WDM transmission system are allowed to become increasingly disparate as those signals are amplified by successive amplifiers in the transmission path.

The amplifier of Figure 4 regulates the temperature of the laser diode of its pump 21, not so as to stabilise the emission wavelength, but so as to minimise the disparity in power at the output of the amplifier appearing in the spectral bands of two or more of the multiplexed signal channels that the amplifier amplifies. The amplifier of Figure 4 has the same components as that of Figure 2 except for the replacement of the feedback loop 25 and wavelength control unit 28 of the amplifier of Figure 2 with a detector unit 40 and microprocessor 41. The signal tapped off the output of the amplifier by the optical coupler 26 and fed to the detector unit 40 is demultiplexed in that unit and detected to provide separate outputs P(x1), P(X2) .. P(kn) to the microprocessor 41 representative, typically in digital form, of the relative powers detected in each of the WDM channel bands X ...... xn- For this purpose the detector unit 40 may be implemented by a WDM demultiplexing filter that separates the WDM channels into separate fibres, each fibre terminating in a PIN diode followed by a transimpedance amplifier. The voltage output of each transimpedance amplifier is converted to digital form by an A to D converter operating for instance at 1kHz. An alternative implementation of the function of the detector unit 40 is as disclosed in US Patent No 5 513 029 and employs orthogonal dithers modulated on each of the WDM channels to enable the determination of the power present in each channel without requiring the use of wavelength selective filters or multiple PIN diodes.

The microprocessor 41 provides a first output, fed to the current controller 24, to regulate the power output of the pump 21, and provides a second output fed to the temperature controller 27 to regulate the temperature of the laser diode of the pump 21 in such a way as to minimise the power disparity between some or all of the power levels P(Bl), to P(kn). Typically this will be arranged to involve only those power levels P(Al), ... P(kn) that exceed a certain threshold value. This is so that no account is taken of channels that are inoperative because of a fault condition, or because they have been deliberately shut down.

In the transmission system of Figure 5 the final optical amplifier of the transmission path 12 is included as part of the receiver 10, and employs essentially the same sort of temperature regulation as employed in the amplifier of Figure 4. In this instance the output of the microprocessor 41 is not used to control solely to the temperature controller of the laser diode of the pump 21 of that amplifier alone, but instead is used to control the temperatures of the pumps 21 of all the amplifiers 21 in the transmission path 12 via a system management unit 50.

The version of the detector unit 40 of the amplifier unit of Figure 4 described above that employs multiple PIN diodes performs three functions. First it demultiplexes the channels, then it separately detects the demultiplexed channels, and finally it converts those detected signal into power measurement signals for input into the microprocessor 41. In the case of the transmission system of Figure 5, the first two of these functions are already performed respectively by the demultiplexer 16 and detectors 17 of the receiver, while the third function is incorporated into the microprocessor 41 so as to be able to dispense with the need for the separate detector unit 40.

The transmission system of Figure 5 will function in essentially the same way even though the final amplifier of the concatenation be located not in the receiver but at a point in the transmission path physically remote from the receiver. Under these circumstances, rather than saying that it is the disparity in power in different spectral bands at the output of the final amplifier that is being used for regulating the temperature of the pump laser diodes, it is more correct to say that it is the disparity at the receiver which is being so employed.

Claims

WE CLAIM:

1. A method of operating an optical amplifier having an optically amplifying medium optically pumped with a semi-conductor laser, in which method the shape of the gain spectrum is regulated by dynamic control of the emission wavelength of the laser.

2. An optical amplifier having an optically amplifying medium optically pumped with a semiconductor laser, the amplifier including control means that regulates the shape of the gain spectrum by dynamic control of the emission wavelength of the laser.

3. A method of operating an optical amplifier having an optically amplifying medium optically pumped with a semiconductor laser, in which method a feedback control loop is employed to control the magnitude of pump power delivered to the amplifying medium by the laser, and adjustment means is employed to modify the change in emission wavelength that results directly from changes in said delivered pump power.

4. An optical amplifier having an optically amplifying medium optically pumped with a semiconductor laser, the amplifier including feedback means controlling the magnitude of pump power delivered to the amplifying medium by the laser, and including adjustment means that modifies the change in emission wavelength that results directly from changes in said delivered pump power.

5. An optical amplifier having an optically amplifying medium optically pumped with a laser diode whose temperature is dynamically regulated at least in part by a signal derived from a measured operating parameter of the amplifier.

6. An optical amplifier as claimed in claim 5, wherein the laser diode emits light in response to a drive current caused to flow through the laser, and said operating parameter is said drive current.

7. An optical amplifier as claimed in claim 5, wherein the laser diode emits light at a wavelength, which wavelength constitutes said operating parameter.

8. In a wavelength division multiplexed transmission system having a receiver optically coupled with a transmitter via a wavelength division multiplexed transmission path that includes a concatenation of laser diode pumped optical amplifiers, each of which exhibits a gain spectrum, a method of adjusting the gain spectrum of at least one of said amplifiers by dynamically regulating the temperature of its laser diode pump in response to a measured operating parameter of said amplifier, or of said system.

9. In a wavelength division multiplexed transmission system having a receiver optically coupled with a transmitter via a wavelength division multiplexed transmission path that includes a concatenation of laser diode pumped optical amplifiers, each of which exhibits a gain spectrum, a method of adjusting, as claimed in claim 11, the gain spectrum of said amplifier in response to variations in magnitude of a drive current applied to the laser diode pump of the amplifier.

10. In a wavelength division multiplexed transmission system having a receiver optically coupled with a transmitter via a wavelength division multiplexed transmission path that includes a concatenation of laser diode pumped optical amplifiers, each of which exhibits a gain spectrum, a method of adjusting, as claimed in claim 8, the gain spectrum of said amplifier in response to variations in wavelength of emission of the laser diode pump of the amplifier.

11. In a wavelength division multiplexed transmission system having a receiver optically coupled with a transmitter via a wavelength division multiplexed transmission path that includes a concatenation of laser diode pumped optical amplifiers, each of which exhibits a gain spectrum, a method of adjusting, as claimed in claim 8, the gain spectrum of said amplifier, which amplifier is amplifying at least two wavelength division multiplexed signals, in response to a difference in power output from the amplifier of said signals.

12. In a wavelength division multiplexed transmission system having a receiver optically coupled with a transmitter via a wavelength division multiplexed transmission path that includes a concatenation of laser diode pumped optical amplifiers, each of which exhibits a gain spectrum, a method of adjusting, as claimed in claim 11, the gain spectrum of said amplifier, and wherein said difference in power output is additionally employed for dynamically regulating the temperature of the laser diode pump of at least one other amplifier of the concatenation.

13. In a wavelength division multiplexed transmission system having a receiver optically coupled with a transmitter via a wavelength division multiplexed transmission path that includes a concatenation of laser diode pumped optical amplifiers, each of which exhibits a gain spectrum, a method of adjusting the gain spectrum of said system by dynamically regulating the temperature of the pump laser diode of at least one of the amplifiers in response to a difference in power output at the receiver between at least two wavelength division multiplexed signals received from the transmitter by the receiver.