OPTICAL FIBRE AMPLIFIER HAVING A CONTROLLED GAIN

Abstract: An optical fiber amplifier for WDM-signals comprises an active optical fiber (1, 3) pumped with pumping light from a pump source (17). In order to give the amplifier a constant gain for the WDM-signals irrespectively of the number of active WDM-signals an additional light source (29) is arranged to inject, when required, extra light into the active optical fiber, at least in a portion (3) thereof, through a power combiner (27). The injected light has a wavelength different from that of the WDM-channels and preferably longer so that it will not cause an amplifying or pumping effect in the active optical fiber but so that it still can be amplified in the active fiber. The light source (29) is controlled by a signal derived from an output power measurement device (23) so that it will inject extra light when a WDM-channel becomes inactive. The power of the extra light is thus selected to saturate the optical fiber amplifier at a constant gain irrespectively of the number of signal channels, i.e. of the total power of the input light signals. A light filter (18) can be connected to the output end of the active optical fiber (1, 3) to block light having a wavelength corresponding to the wavelength of the light issued from the light source (29). Controlling the gain by this injection of "extra signal light" is much quicker than controlling the power of the pumping light.

Description: OPTICAL FIBER AMPLIFIER HAVING A CONTROLLED GAIN TECHNICAL FIELD The present invention relates to an optical amplifier based on erbium-doped fibers and also to a fiber optical network including such an optical amplifier. BACKGROUND Optical fibers are presently widely used for communicating information such as in large telecommunication systems, primarily owing to their large rehability, their insensitivity to electrical interference and their high capacity. Of course, there is a desire in the existing telecommunication networks to use the available optical fibers in their networks as efficiently as possible, in particular for communication over long distances, since such fibers obviously have high installation costs. By introducing wavelength division multiplexing WDM in existing communication systems using optical fibers and in new communication systems to be built a plurality of individual wavelength channels can be transmitted on the same optical fiber and thus the information transmitted over the fiber can be multiplied. In optical fiber networks for long distance communication there may be a need for amplifying the optical signals. Such amplification can of course be achieved by a repeater built in a straight-forward way, including components converting the optical signals to electrical signals, amplifying the electrical signals and converting the electrical signals to optical signals. For WDM signals this will require one optoelectrical and one electrooptical converter per wavelength channel used in the WDM transmission and also one filter or demultiplexer for filtering out the different wavelengths in the incoming signal. This will obviously be very costly and also results in rehability problems owing the large number of components, both electronic and optical, which are required. Another type of amplifier comprises optical fiber amplifiers based on optical fibers doped with rare-earth metals, primarily erbium-doped fiber amplifiers. Such amplifiers have great advantages when used in optical fiber systems owing to e.g. their compatibility with the optical fibers and their high gain, and they are in particular advantageous when used in wavelength multiplexed transmission systems, since they are capable of simultaneously amplifying a number of WDM channels and only require a limited amount of electronic components. The basic design of an erbium-doped fiber amplifier includes one length of an active, erbium-doped optical fiber, connected at its input end to the output of a 2-to-l optical coupler, the coupler receiving on one of its inputs the signal to be amplified and on the other input more energetic hght providing the power for amplifying the signal. This more energetic input light is called the pump light and is obtained from an optical power source, called the optical pump. The pump light has a shorter wavelength than that of the signal and is generally more energetic and is capable of lifting erbium ions from lower energy states to higher energy states in the erbium- doped fiber. Light is then generated in the fiber when the ions return to lower energy levels. In a typical WDM system the number of used wavelength channels normally varies at random. The gain of an erbium-doped fiber amphfier operated in the conventional saturated way is dependent on the number of wavelength channels, see the diagram shown in Fig. 1 , owing to the total constant output power of the amphfier. In Fig. 1 the gain for a typical erbium-doped fiber amphfier is plotted as a function of wavelength for the case of only using one WDM wavelength channel, see the upper curve, and for the case of all channels being active or used, see the lower curve. The gain has a difference of at least about 10 dB for the important wavelength band of 1540 - 1560 nm. This means that also the gain will vary at random when the erbium-doped fiber amphfier is used in a typical WDM system used for telecommunication. In WDM systems it is obviously important to have a constant and flat output gain in the used wavelength band, since this will allow e.g. an optimization of other components. Also, it is inefficient to use gain flattening filters, see e.g. the article by Paul F. Wysocki et al., "Broad-band Erbium-Doped Fiber Amphfier Flattened Beyond 40 nm Using Long-Period Grating Filter" , IEEE Photonics Techn. Lett., Vol. 9, 10, Oct. 1997, since such filters must be designed according to the actual gain curve of the amphfier. A possible solution would be to control the pump power provided to the active fiber, but this includes disadvantages resulting from the long lifetime of excited erbium ions in the active fiber. In the published European patent application 0 777 346 an optical amphfier used in a communication system is disclosed comprising an optical amplifying medium, i.e. a length of an erbium doped optical fiber, a pump hght source and a probe hght source, which sources both inject hght into the amplifying medium. The probe hght has a wavelength included in the amplification band of the optical amplifying medium and is preferably different from the wavelength of the signal hght. Control means maintain the gain for the input signal constant. In the control the power of the probe hght and the power of the input signal hght are added to each other, the powers being given suitable weights in the adding operation. The result of the weighted addition is controlled to be constant by adjusting the power of the probe hght. SUMMARY It is an object of the invention to provide an optical fiber amphfier having a gain curve which is independent of the number of active wavelength channels. It is another object of the invention to provide an optical fiber amplifier having a gain for different wavelength channels, which for each considered channel is approximately constant in time being affected as little as possible and/or during as short time as possible by another channel becoming inactive or active. It is another object of the invention to provide an optical fiber amplifier having a gain for different wavelength channels, which for each considered channel is approximately constant, the optical fiber amphfier requiring only a simple control unit. The problem solved by the invention is thus how to provide an optical fiber amphfier used for amplifying hght signals of different wavelengths and having substantially the same gain in time for all mcoming hght signals independently of the number of input signals present and how to achieve that the gain for wavelength channel is as little as possible affected by other channels. When designing an erbium-doped fiber amphfier the gain of the amphfier is dimensioned for the case of all WDM-channels being active. The gain of the amphfier depends on the number of excited erbium ions and this number is approximately constant for a sufficient input pump hght power, the amphfier then being operated in a saturated state. This results in an approximately constant output power of the amphfier irrespective of the number of input signals to be amplified. This in turn results in that the gain will increase when one or more WDM-channels become/becomes inactive. The number of excited erbium ions can be changed by altering the pump hght power provided to the amphfier which results in a change of the gain characteristic. For a suitable control the amphfier can be made to have a substantially constant gain independent of the number of active WDM-channels, this being a commonly used method of controlling optical fiber amplifiers. However, the response time of such a control can be too long, as indicated above, since excited erbium ions in an active fiber have too long lifetimes, of about 10 ms. Instead, in the optical fiber amphfier the switched-off channel or channels can be replaced with a dummy or idling channel having a wavelength outside the wavelength band used by the WDM-channels or generally having a wavelength different from those used by the WDM-channels such as at wavelength between two wavelength bands. In practical cases the wavelength used for the idling channel can be a wavelength larger than the wavelength band used for signal transmission, such as larger than the commonly used wavelength band around 1530 - 1550 nm. A laser diode supplying the idling wavelength channel can be rapidly controlled to modify its output power so that the output of the total amphfier is kept substantially constant what results in better response times and more constant gain than what is possible to achieve by controlling the input pump power. Thus generally, an optical fiber amphfier for WDM-signals, i.e. hght signals of distinct wavelength channels usually included in a wavelength band or wavelength region, comprises an active optical fiber of the type which is doped with a rare-earth metal. The active fiber length is pumped with pumping hght from a pump source such as a laser diode. In order to give the amphfier a constant gain for the WDM-signals irrespectively of the number of active WDM-signals an additional hght source is arranged to inject, when required, i.e. when one or more WDM-signals become inactive, extra hght into the active optical fiber to replace the failing input hght power. The extra injection can be made in only a portion of the active fiber length and then in a last portion or end portion thereof, which is located close to the output end of the active fiber length. The injection is made through some kind of power combiner connected at the intended position in the active fiber length. The injected hght is selected not to interfere with the propagation of signal hght in the active fiber. It thus has a wavelength different from that of each of the WDM-channels. Furthermore the wavelength is preferably longer than the wavelengths of all of the signal hght, so that it will not cause an amplifying or pumping effect in the active optical fiber. However, the wavelength is selected so that it still can be amplified in the active fiber. The hght source is controlled by a signal derived from an output power measurement device so that it will inject extra hght when a WDM-channel becomes inactive. The power of the extra hght is thus selected to saturate the optical fiber amphfier which will give it a constant gain irrespectively of the number of signal channels, i.e. of the power of the input hght signals. A hght filter is preferably connected to the active optical fiber to block the extra injected hght from propagating through more than the intended portion of the active fiber. Thus, an optical fiber amphfier intended for amplifying WDM-signals is provided which has a gain for the different channels, which for each considered channel has been made constant in time. The gain for a considered channel is little affected, during a very short time, by another channel becoming inactive or active. When another wavelength channel becomes inactive or active, the constant gain for the considered channel is maintained with a short response or regulating time, so that there will no appreciable gain increase or gain decrease or a gain increase or gain decrease for a very short time at the instant when the other channel becomes active or inactive. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in detail by way of non-limiting embodiments with reference to the accompanying drawings, in which: Fig. 1 is a diagram of the gain as a function of the wavelength for an optical fiber amphfier according to prior art, Fig. 2 is a schematic picture showing the general configuration of an optical fiber amphfier having improved gain stability, Fig. 3 is a schematic picture showing the general configuration of an alternative design of an optical fiber amphfier having improved gain stability, and Fig. 4 is a schematic picture showing a simple fiber optical network using optical fiber amplifiers. DESCRIPTION OF PREFERRED EMBODIMENTS Optical fiber amplifiers are presently used at various positions inside fiber optical networks as illustrated in Fig. 4. Here a transmitting node 101 receives an electrical signal which is converted to a hght signal by the laser 103. The hght signal is amplified by a booster amphfier 105 and therefrom transmitted on a long optical fiber 107. The signal can be attenuated when propagating along the optical fiber 107 and then it has to amplified by a line amphfier 109. The hght signal on the optical fiber 107 is received by a receiving node 111, in which it is first amplified by a preamplifier 113. The amplified signal is detected by a hght detector 115 and therein converted to an electric signal, which is provided from the node. In such a network, the booster 105, the line amphfier 109 and the preamplifier 113 usually all comprise optical fiber amplifiers using a length of an active optical fiber as the amplifying medium or element. In Fig. 2 the general configuration of an optical fiber amphfier is illustrated. The active parts thereof are two lengths 1 , 3 of erbium-doped fiber, which are connected in series with each other. The first length 1 receives an optical signal from some source, not shown, arriving to a connector 7, passing on an optical fiber from the connector 7 to a power splitting device or coupler 9, tapping off a small part, e.g. 1 % , of the hght power on the fiber to a power measurement device such as a PIN-diode 11. The substantial part of the hght power continues to an optical isolator 13 and from the optical isolator to an input of a power combiner or coupler 15, the power combiner 15 receiving on its other input pumping hght power from a pump hght source or first hght source 17, e.g. generating hght of the wavelength 980 nm or 1480 nm. The output of the power combiner 15 is connected to the input end of the first fiber length 1. The output end of the second fiber length 3 is through an optional filter 18 connected to the input of an optical isolator 19, the output of which is connected to a power splitter coupler 21 , like the power splitter 9 at the input side tapping off a small part, e.g. 1 % , of the power of hght on the fiber to an output power measurement device 23. The substantial part of the output power is forwarded to some destination, not shown, from the power splitter 21 through a connector 25. Between the first and second fiber lengths 1 , 3 is another power combiner or coupler 27 connected. It receives saturating hght from a second hght source 29 issuing hght of a wavelength of e.g. the range of 1550 - 1570 nm and in any case of a wavelength different from and preferably larger than those of the wavelength band used for wavelength channels in WDM systems. The hght issued by the second hght source 29 can thus not be used for "pumping" the amphfier but will instead be amplified in the second length 3 of active fiber and then filtered away by the optional filter 18 which for instance can be a long-period grating filter, see e.g. A. Vengsarkar, "Long-period fiber gratings", in Conf. Optical Fiber Communications, 1996 Tech. Dig. Ser. Washington DC: Opt. Soc. Amer. 1996, Vol. 2, pp. 269 - 270, paper ThP4. An erbium-doped fiber amphfier is usually operated in a saturated state, in which it has an approximately constant output power independent of the input power, provided that the input pump power is sufficient. When designing an erbium-doped fiber amphfier the gain of the amphfier has to be dimensioned to have a desired magnitude or equivalently for an amphfier operated in a saturated state so that the output power of the amphfier is the desired one. Then the case must be considered, in which all WDM- channels are active, i.e. that hght of the wavelengths of all WDM-channels is forwarded to the input end of the amphfier. The gain of a typical optical fiber amphfier for that 5 case is illustrated by the lower curve in the diagram of Fig. 1. If one or more channels are not used, the input power will decrease, the output power is about constant resulting in that the gain will increase. The case of having all channels active or used thus results in the lowest gain curve. The effect resulting from channels becoming inactive is illustrated by the upper curve of Fig. 1 showing the gain for the case where only one ιo WDM-channel is used or is active. In the case in which there is at least one inactive channel the hght generated by the laser 29 replaces the input hght corresponding to the not used channels in order to keep the gain constant, not allowing it to increase in the way illustrated by the curves of Fig. 1. The laser diode 29 is thus controUed accordingly, this being possible by the fact that i5 the saturating signal power is correlated to the optical output power of the amphfier. The output power can be determined by an electrical circuit, such as the control unit 31 shown in Fig. 2, from the signals received from the output measurement 23 and indicating the optical output power. Hence, in the case of all channels being active the optical output power of the 20 saturating laser 29 is controUed to be equal to zero. In the case where a WDM-channel is not active, the saturating laser 29 is controUed to replace the hght power of this wavelength channel and to generate a sirmlar population inversion in the active fiber, see D.M. Baney and J. Stimple, "WDM EDFA Gain Characterization with a Reduced Set of Saturating Channels", IEEE Photonics Technology Letters, Vol. 8, No. 12, Dec. 1996, 25 pp. 1615 - 1617. The second hght source 29 is thus aU the time controUed to vary the intensity of its issued hght depending on a control signal derived from the output signal of the power measurement device 23 at the output side. This power measurement device is designed to in some way measure the output power and in particular it can be designed to measure 30 the presence and absence of aU the WDM-channels which the amphfier is intended to amplify. This can be done for instance by using pUot tones, see the pubhshed European patent application No. 0 637 148 Al. Then there is an electronic control unit 31 connected to receive measurement signals from the output power detector 23 and to send a control signal to the second hght source 29 for controlling the output power thereof to 35 provide a saturating hght power. The first fiber length 1 can possibly have zero length, so that it is completely omitted. Then the hght issued by second hght source wiU travel through aU of the active fiber to be amplified therein. It is also possible to connect the power combiner 27 used for injecting hght issued by the second hght source 29 before the combiner 15 which injects hght from the pump source 17. For these alternative embodiments there wiU be an added insertion loss of the wavelength corresponding to that issued by the second hght source 29 resulting in a higher total noise figure. Another possible embodiment, see Fig. 3, includes that the extra hght from the second hght source 29 is injected into the total active fiber length to propagate in a backward direction. Then the power combiner/splitter 27 is connected to the output end of the second active fiber length 3. The optional filter 18 is connected between the two filter lengths 1 , 3 in order to remove hght having a wavelength corresponding to that of the hght produced by the second hght source 29. The amplifiers described above are primarily intended for amplifying hght signals of the electromagnetic single mode type. This implies that ah components used should not affect the electromagnetic modes of the hght, both of the signal hght and of the pump hght. The components, in particular the optical power combiner 15 and the optional filter 18, should then also be designed for transmitting hght of the respective wavelengths of signal hght and of pump light in single mode. Thus, an optical fiber amphfier to be used for amplifying WDM-signals such as in WDM-networks has been described herein providing a substantiaUy constant gain for the WDM-channels used independently of the actual number of channels used and having a short control or regulating time when a WDM-channel becomes inactive or active.

Claims: CLAIMS 1. An optical fiber amphfier for amplifying hght signals within a wavelength band, the amphfier comprising an active optical fiber having an input end adapted to receive hght signals to be amplified having wavelengths in a wavelength region and an output end adapted to forward the hght signals amplified during the propagation thereof in the active optical fiber by gain factors specific to the wavelengths of the hght signals, a pump source connected to inject pumping hght into the active optical fiber, an output power measurement device connected to the output end of the active optical fiber for measuring the power of hght having wavelengths within the wavelength region at the output end, and a hght source for injecting hght into the active optical fiber, the injected hght having a wavelength outside the wavelength region and selected not to be capable of causing an amplifying or pumping effect in the active optical fiber and selected to be capable of being amplified by the optical fiber amphfier, characterized in that the hght source is connected to the output power measurement device in order to control the intensity of the injected hght injected by the hght source to maintain the gain factors constant. 2. An optical fiber amphfier according to claim 1, arranged to amplify hght signals of signal channels having distinct wavelengths in the wavelength region, characterized in that the hght source is so connected that the power of the hght injected by the hght source saturates the optical fiber amphfier at a constant gain irrespectively of the number of signal channels, i.e. of the total power of the input hght signals. 3. An optical fiber amphfier according to any of claims 1 - 2, characterized in that the active optical fiber is divided into two portions, a first portion having an input end receiving the hght signals to be amplified and a second portion having an output end from which the amplified hght signals are dehvered, the hght source being arranged to inject hght into only the second portion. 4. An optical fiber amphfier according to claim 3, characterized by a hght combining device connected between the first and second portions and to receive hght from the hght source to be injected into the second portion. 5. An optical fiber amphfier according to claim 4, characterized by a hght filter connected to the output end of the second portion to let the hght signals from the second portion through but to block hght having a wavelength corresponding to the wavelength of the hght issued from the hght source. 6. An optical fiber amphfier according to claim 3, characterized by a hght sphttmg/combining device connected to the output end of the second portion to let hght from the second portion through and to receive hght from the hght source to be injected into the second portion in a direction opposite the direction of the hght signals. 7. An optical fiber amphfier according to claim 4, characterized by a hght filter connected between the first and second portion to let the hght signals from the first portion through but to block hght having a wavelength corresponding to the wavelength of the hght issued from the hght source. 8. A fiber optical network comprising at least a transmitting node and a receiving node and at least one optical fiber amphfier for amphfying hght signals within a wavelength band, the amphfier comprising an active optical fiber having an input end adapted to receive hght signals having wavelengths in a wavelength region to be amphfied and an output end adapted to forward the hght signals amphfied during the propagation thereof in the active optical fiber by gain factors specific to the wavelengths of the hght signals, a pump source connected to inject pumping hght into the active optical fiber, an output power measurement device connected to the output end of the active optical fiber for measuring the power of hght having wavelengths within the wavelength region at the output end, and a hght source for injecting hght into the active optical fiber, the injected hght having a wavelength outside the wavelength region and selected not to be capable of causing an amphfying or pumping effect in the active optical fiber and selected to be capable of being amphfied by the optical fiber amphfier characterized in that the hght source is connected to the output power measurement device in order to control the intensity of the injected hght to maintain the gain factors constant.

Inventors: BLIXT PETER, LUTZ DIRK, BLIXT, PETER, LUTZ, DIRK
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 1999
Main classification: H04B10
IPC codes: H01S3, H04B10, H04J14