OPTICAL AMPLIFIER METHOD AND APPARATUS

Abstract: An L-band Er-doped optical amplifier has a 60m to 110m length of doped optical fiber. Pump light in the wavelength range from 1435nm to 1460nm is coupled into the fiber. Substantially no light in the range 1465nm to 1505 nm is coupled into the fiber. The gain of the amplifier is greater than when using pump light of 1480nm.

Description: OPTICAL AMPLIFIER METHOD AND APPARATUS Field of the Invention The invention relates to an optical amplifier, and more particularly to a doped-fiber optical amplifier using a pump. Background of the Invention Optical communications systems in which optical signals are transmitted using optical fibers have become widespread. When optical signals are transmitted over long distances, an optical amplifier is required at intervals, for example every 80km or thereabouts. Optical amplifiers may also be required in other applications, for example to compensate for losses in optical modulators or other equipment. Figure 1 schematically illustrates one type of optical amplifier, a doped fiber optical amplifier. The central component is a length of optical fibre 10 doped in this example with Erbium (Er). As well as Er doping, it is known to use other dopant atoms, such as Neodymium or Thulium. The fiber 10 may be around 10m long. A pump 20 transmits pump light into the Er-doped optical fiber through an input optical coupler 12 to excite Er atoms into an inverted state. Signal light is passed into the Er-doped optical fiber 10 from an input optical fiber 16 via the input optical coupler 12 and output light is output from the fiber through an isolator 14 to an output optical fiber 18. The isolator 14 prevents light travelling in the reverse direction. The pump light is absorbed by excitation of doping states in the Er-doped optical fiber 10 and causes an inversion. This causes any signal light entering the Er-doped optical fiber 10 to be amplified by stimulated emission in the Er-doped optical fiber. Optical communications systems have traditionally used signals in the C band, a wavelength band between 1530 and 1560 nm. Typically, optical amplifiers for signals in the C band use a pump wavelength of 980nm, i.e. the wavelength of the light emitted by the pump 20 and input into the fiber is at 980nm. 980nm is at one absorption peak of the Er-doped fiber, and pumping at this frequency works well for low noise. Another absorption peak of Er-doped fiber is at 1480nm, and pump light in the wavelength range 1474nm to 1483nm is also used, especially for high power at low cost. It is possible also to combine these two pump wavelengths in the arrangement illustrated in Fig. 2. In this arrangement a first length of Er-doped fiber 10 is provided, pumped by a 980nm pump light source 20, and a second length of Er-doped fiber 22 is provided, pumped by a 1480nm pump light source 24. Since low noise is most important in the first amplification stage, this arrangement combines to some extent the low noise benefits of a 980nm pump with the low-cost power requirements of a 1480nm pump. Known pump light sources include an LED and/or a laser emitting light at a suitable wavelength. Increasing use is now being made of the L band, using signal light having a wavelength higher than that in the C band, between 1570nm and 1620nm. Optical amplification in the L band is carried out in generally the same manner as in the C band. However, the length of Er-doped optical fiber needed to obtain equivalent amplification to a C band amplifier is much longer in the L band so long lengths of Er-doped optical fiber are needed, typically 60m to 110m, in order to obtain sufficient amplification. In order to pump these lengths of optical fibers, high pump powers are required; the amplification efficiency is low. Accordingly, there is a desire to increase the efficiency and reduce the required pump power of optical fiber type optical amplifiers, and especially those using long lengths of doped optical fiber such as typically used for the L band. An increase in efficiency may reduce the cost of the optical pump within the optical amplifier. Summary of Invention According to a first aspect of the invention there is provided an optical amplifier for amplifying L-band optical signals, having an Er-doped optical fiber and an optical pump coupling light having a pump wavelength from 1435nm to 1460nm into the Er-doped optical fiber. Er-doped optical fiber amplifiers have previously used a pump light source with a wavelength that is very close to the absorption of the Er dopant atoms at around 1480nm. Typical pump wavelengths might be 1479nm or 1483nm. When such an Er-doped optical fiber is used to amplify a C-band optical signal a length of Er-doped optical fiber of typically 10m to 20m may be used. A pump wavelength of 1480nm gives good results in this band. An optical fiber amplifier for the L-band may have an Er-doped optical fiber more than 60m long, typically in the range 60m to 110m. Such greater lengths of optical fiber are needed to get sufficient amplification of L-band signals in view of the lower amplification gain achieved with such L-band signals as compared with C-band signals. As mentioned above, high pump powers are required to pump such long lengths of Er-doped optical fibers. • The inventors have investigated the transmission of 1480nm pump light in a long optical fiber amplifier, and have deteimined that the light is absorbed over a short length of fiber, leaving a long length of fiber that does not have significant inversion. Since the non-inverted fiber is lossy at the signal wavelength, loss to the signal can occur in this non-inverted region. Thus, the high pump power used in conventional approaches is used to compensate for this loss. Instead of this approach, the inventors propose using pump light at a wavelength that is not at the usual wavelength. Rather, the wavelength proposed is a little away from this wavelength which reduces the absorption in the fiber per unit length so that the light is absorbed over a much longer length of fiber. This increases the length of optical fiber that has significant inversion. Contrary to what might be supposed, the reduction in the efficiency with which the pump light is absorbed increases the overall efficiency by reducing rather than increasing the total pump power requirement. The following considerations are relevant in determining the pump wavelength. Firstly, the undoped optical fiber has an absorption peak due to water at 1380nm, so that the chosen pump wavelength has to be away from this peak. Secondly, the pump wavelength must remain able to excite the doped atoms, and so although it should not be at the absorption peak some absorption must still be obtained. Thirdly, optical fibers are likely to carry both C-band and L- band signals, possibly also S-band (lower wavelength) signals, and the pump wavelength should be kept away from the signal wavelengths. These considerations suggest that the pump wavelength for an Er-doped fiber is to be between 1435nm and 1460nm, preferably between 1445nm and 1455nm, further preferably at substantially 1450nm. Preferably, substantially no pump light above this frequency range, i.e. from 1465nm to 1510nm, is coupled into the optical fiber. Notwithstanding the foregoing arguments, in a further preferred embodiment a pump wavelength in the range 1490nm to 1510 nm, rather than 1435nm, is employed. The input light signal may be brought in on a signal input optical fiber. An optical coupler may be provided to couple the light from the signal input optical fiber and the pump into the Er- doped optical fiber. An isolator may be provided on the Er-doped optical fiber to prevent back reflections. A second stage of amplification may be provided, having a second-stage Er-doped optical fiber and a second-stage pump source. The second-stage pump source may emit at a wavelength of 1480nm. A second-stage optical coupler may couple the outputs from the first Er-doped optical fiber and the second pump source into the second Er-doped optical fiber. An isolator may be provided between the first Er-doped optical fiber and the second-stage optical coupler to prevent light in the second Er-doped optical fiber having an effect on the first optical fiber. A variable optical attenuator (NOA) may be provided in front of the second-stage optical coupler. A C-band optical amplifier may be provided in parallel to the L-band optical amplifier described above. The invention is also directed to a method by which the described apparatus operates and including method steps for carrying out every function of the apparatus. A preferred embodiment provides a method of amplifying an L-band optical signal including pumping an Er-doped optical fiber with a pump signal at 1435nm to 1455 nm and feeding an L- band optical signal through the Er-doped optical fiber to amplify the L-band optical signal. The invention has wider application than simply to an Er-doped optical fiber in the L-band. Accordingly, in further aspect, there is provided an optical amplifier having a doped length of optical fiber having dopant atoms which absorb light in an absorption wavelength band having a maximum absorption at a maximum absorption wavelength, a pump hght source for providing pump light at a pump wavelength, an optical coupler, having a signal input, for coupling the pump light source and the signal input to the doped length of optical fiber, wherein the doped length of optical fiber is sufficiently long that pump hght at the said maximum absorption wavelength is substantially absorbed and inversion occurs along only part of the length of optical fiber, and the pump wavelength is at the edge of the absorption wavelength band away from the maximum absorption wavelength so that inversion occurs along a greater length of the optical fiber than for a pump light at the said maximum absorption wavelength. The wavelength may differ by at least 1% from the maximum absorption wavelength. The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the invention. Brief Description of the drawings For a better understanding of the invention specific embodiments will now be described, purely by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic of a conventional optical amplifier; Figure 2 is a schematic of a conventional 2-stage optical amplifier; Figure 3 is a schematic of a first embodiment of an optical amplifier according to the invention; Figure 4 is a graph of the inversion in an Er-doped optical fiber as a function of distance down the fiber; Figure 5 is an expanded view of a section of the graph of Figure 4; Figure 6 is a graph of the total inversion and the amplifier gain of an Er-doped fiber amplifier as a function of pump wavelength; Figure 7 is a graph of amplifier gain of an Er-doped fiber amplifier as a function of signal wavelength for a number of different pump wavelengths; Figure 8 is a schematic of a second embodiment of the invention; and Figure 9 is a schematic of an embodiment of a combined C- and L- band amplifier. Detailed Description Referring to Fig. 3, an input optical coupler 12 has a signal input 36 to which a signal optical fiber 16 is connected, and a pump input 34 to which a pump light source 32 is connected. The input optical coupler 12 couples both inputs 34, 36 to an output 38 coupled to an 80m coiled length of Er-doped optical fiber 10. The pump light source 32 is a semiconductor laser emitting at 1450nm. Any other suitable light source may also be used, such as a semiconductor LED. The pump light emitted by the pump light source 32 is absorbed by dopant atom energy states in the Er-doped optical fiber 10 which causes an inversion there. The fiber amplifies signal light transmitted on the input optical fiber 16 by stimulated emission. After passing through the fiber and being amplified, light then passes through an isolator 14 which passes light in only one direction, indicated in the drawings by an arrow. After passing through the isolator 14, the light passes through a filter 28 for filtering out the pump light and a variable optical attenuator (NO A) 30. The light is then output into output optical fiber 18. The inventors have investigated inversion in an Er-doped optical fiber. A 200mW pump is injected into a 60m length of Er-doped fiber at 0m. The amount of inversion as a function of -1 distance along the fiber is shown in Figure 4 for a number of different pump wavelengths. Figure 5 shows in greater detail the portion of this graph between 30 and 50m from the pump injection point. Figure 6 shows the total inversion and the total amplifier gain in an 80m length of Er-doped optical fiber as a function of pump wavelength. Unexpectedly, the best gain is obtained with a pump wavelength of 1450nm, which is at the edge of the Er absorption line. A simple explanation of increased gain using pump light at 1450nm is as follows. The pump light of wavelength 1450nm is at the edge of the 1480nm absorption band and so absorbs much less efficiently in a short length of fiber than light at 1480nm. Pump light at 1480nm accordingly only efficiently inverts a short length of fiber, of order 10m, before being substantially absorbed, whereas pump light at 1450nm is capable of inverting a much longer length of fiber. The results bear this out: at distances of 30m to 50m from the injection point the inversion created by a 1450nm pump is significantly greater than for a 1480 nm pump, as can be seen by inspection of Figure 5. Since optical fiber is lossy, long lengths of non-inverted fiber will produce no amplification of the signal but instead simply result in losses. Thus, by using a pump wavelength of 1450nm the loss to the signal in long lengths of non-inverted fiber may be minimised. It turns out that this effect improves the overall gain. Figure 7 shows the total gain of an 80m Er-doped fiber optical amplifier as a function of the input wavelength for different pump wavelengths. An approximately ldB increase in gain using a pump at 1450nm as compared to 1480nm is achieved over almost all of the L-band. To test the advantage of this effect, the amplifier with the 1450nm pump was tested again, reducing the optical pump power until the same gain as was achieved with a 200mW 1470nm pump. A pump power of 155mW was required which is a saving of 45mW, i.e. 23%. The invention is not restricted to the single length of fiber shown above. Figure 8 illustrates a two-stage optical amplifier using the invention. The first stage corresponds to the optical fiber amplifier as described above with reference to Figure 3. The output of the optical fiber 10 is input through an optical isolator 14 to a second-stage optical coupler 26 driven by a second- -stage optical pump 24. The second-stage optical amplifier includes a second-stage optical fiber 22 that is also doped with Er. The output of the second-stage Er-doped optical fiber passes through an output isolator 14 as in the embodiment described with reference to Figure 3. In this arrangement, the first optical pump 32 has a wavelength of 1450nm, whereas the second optical pump is less critical and has a wavelength of 1480nm. Of course, as an alternative 1450nm pump light could be used in both stages. Where it is required to amplify both L- and C-band optical signals, the arrangement as described with reference to Figure 9 may be used. In this arrangement, an optical signal is amplified by C- and L-band optical amplifiers in parallel. The input signal of an input optical fiber 16 is passed to a splitter 50 which splits the signal to pass through an L-band optical amplifier 52 and a C-band optical amplifier 54. The amplified signals are then combined in an optical combiner 56 to produce the final signal. The L-band optical amplifier 52 may be an optical amplifier as described above. This approach makes use of the ready availability of C band optical amplifiers. The invention is not limited to the above embodiments. The invention may also find application to optical amplifiers doped by different materials such as neodymium or thulium. The technique of pumping optical fibers with a wavelength that is different to the wavelength of the maximum absorption in the optical fiber may have application in any doped-fiber optical amplifier, especially to those of lower gain where long lengths of doped optical fiber are required. The pump hght source 32 may be any suitable light source that emits light at the required wavelength. In particular, the pump light source 32 may be an LED, a laser, for example a semiconductor laser, or a number of LEDs or lasers. Other pump arrangements may be used, such as counter pumping, i.e. pumping the Er- doped optical fiber 10 in the reverse direction to the signal direction. The input optical coupler may be of any suitable type, such as a fused fiber coupler.

Claims: We Claim: 1. An optical amplifier comprising a doped length of optical fiber (10) having dopant atoms which absorb light in an absorption wavelength band having a maximum absorption at a maximum absorption wavelength, a pump light source (32) for providing pump Hght at a pump wavelength, wherein the doped length of optical fiber is sufficiently long that pump light at the said maximum absorption wavelength is substantially absorbed and inversion occurs along ι only part of the length of optical fiber, and the pump wavelength is at the edge of the absorption wavelength band away from the maximum absorption wavelength so that inversion occurs along a greater length of the optical fiber than for pump light at the said maximum absorption wavelength. 2. An optical amplifier according to claim 1 wherein the pump wavelength differs by at least 1% from the maximum absorption wavelength. 3. An optical amplifier according to claim according to any one of claims 1 to 2 for amplifying L-band optical signals, wherein the doped optical fiber is doped with Erbium and the pump light source has a pump wavelength between 1435nm and 1460nm. 4. An optical amplifier according to claim 3, wherein the length of the Er-doped optical fiber is 60ffi to 110m. 5. An optical amplifier according to any one of claims 1 to 4 wherein the wavelength of the pump light is between 1445nm and 1455nm. 6. An optical amplifier according to any one of claims 1 to 5 further comprising an optical coupler (12), having a signal input, for coupling the pump light source and the signal input (36) to the doped length of optical fiber, wherein the doped length of optical fiber is sufficiently long that pump light at the said maximum absorption wavelength is substantially absorbed and inversion occurs along only part of the length of optical fiber, and the pump wavelength is at the edge of the absorption wavelength band away from the maximum absorption wavelength so that inversion occurs along a greater length of the optical fiber than for pump light at the said maximum absorption wavelength. 5 7. An optical amplifier according to any one of claims 1 to 6 further comprising a second-stage doped optical fiber (22), a second-stage pump light source (24), and a second-stage optical coupler (26) coupling the outputs of the said doped optical fiber and the second-stage pump light source into the second-stage optical fiber. 10 ' ■■ ■ ' 8. An optical amplifier according to claim 7 wherein the second-stage pump light source emits at a wavelength of between 1470nm and 1490nm. 9. An optical amplifier comprising an L-band optical amplifier (52) according to claim 3 15 and a C-band optical amplifier (54) in parallel to the L-band optical amplifier. 10. An optical amplifier according to claim 3 wherein substantially no pump light having a wavelength from 1465nm to 1515nm is coupled into the fiber. 20 11. An optical amplifier according to any one of claims 1 to 2 wherein the pump light source has a pump wavelength between 1490nm and 1510nm. 12. A method of amplifying an optical signal including: 25 pumping a doped length of optical fiber (10) having dopant atoms which absorb light in an absorption wavelength band having a maximum absorption at a maximum absorption wavelength, feeding an optical signal through the optical fiber to amplify the optical signal, wherein the doped length of optical fiber is sufficiently long that pump light at the said 30 maximum absorption wavelength is substantially absorbed and inversion occurs along only part of the length of optical fiber, and the pump wavelength is at the edge of the absorption wavelength band away from the maximum absorption wavelength so that inversion occurs along a greater length of the optical fiber than for pump light at the said maximum absorption wavelength. 13. A method according to claim 11 in which the optical signal is an L-band signal, the length of optical fiber is doped with Erbium, and the length of optical fiber is pumped with light having a wavelength between 1435nm and 1460nm.

Inventors: DI-MURO RODOLFO, JONES KEVAN, MCCLEAN IAN, DI-MURO, RODOLFO, JONES, KEVAN, MCCLEAN, IAN
Applicants: NORTEL NETWORKS LTD, NORTEL NETWORKS UK LTD, NORTEL NETWORKS LIMITED, NORTEL NETWORKS UK LIMITED
Application year: 2001
Main classification: H01S3
IPC codes: H01S3