WO2004061486A2

WO2004061486A2 - Optical amplifier

Info

Publication number: WO2004061486A2
Application number: PCT/US2003/040477
Authority: WO
Inventors: David Harris; Stephen C. Guy; John D. Minelly
Original assignee: Avanex Corporation
Priority date: 2002-12-31
Filing date: 2003-12-17
Publication date: 2004-07-22
Also published as: AU2003299720A8; WO2004061486A3; AU2003299720A1

Abstract

An optical amplifier with first and second serially connected optical amplifier stages, each of which includes an input and an output, the second of which includes a length of amplification fiber connected to a source of pump light, a signal input connected to the input of the first amplifier stage that receives an optical signal, a reflector connected to the output of the second amplifier stage that reflects an amplified signal back through the second amplifier stage, an isolator device connected between the output of the first amplifier stage and the input of the second amplifier stage that separates incoming and outgoing amplified signals to and from said second amplifier stage, and a signal output optically connected to said isolator device. In one embodiment, the isolator device is a three-way isolator such as a circulator.

Description

OPTICAL AMPLIFIER

FIELD OF THE INVENTION [0001] The present invention relates to erbium doped fiber amplifiers (EDFAs) that utilize reflected pump light to reduce the amount of gain fiber needed to achieve a desired gain level. In particular, the present invention relates to such amplifiers having an optical circulator disposed between the stages of an EDFA for improved utilization of pump energy and increased gain coefficient without the increases in amplifier noise associated with the prior art.

BACKGROUND OF THE INVENTION

[0002] In conventional erbium-doped fiber amplifiers (EDFAs), the lengths of erbium-doped fiber (EDF) used in C-band and L-band amplifiers are between 10-20 meters and 60-100 meters, respectively. The long length of erbium-doped fiber (EDF) used in optical amplifiers is an area of concern and attention in the fiber optics industry.

[0003] Use of shorter EDFs is especially well suited for L-band amplifiers where the increase in passive loss and four-wave mixing (FWM) effects caused by fiber non-linearity significantly limits the performance of the amplifier. Such nonlinear impairments have been found to increase with fiber length. Consequently, various techniques have been investigated to allow use of shorter fiber lengths in optical amplifiers while maintaining a desired gain level. For instance, increases in erbium concentration and/or reconfiguration of conventional amplifier design have been investigated which have resulted in some improvement in the performance of erbium- doped fiber amplifiers.

[0004] Unfortunately, performance increases attained by increasing the erbium concentration are limited by a phenomenon known as concentration quenching. Concentration quenching can arise from upconversion effects that result from the cross relaxation between two adjacent ions. This results in only one of two adjacent ions to be promoted to the highest energy level for amplification purposes, while the second ion is promoted to only an intermediate energy level before nonradiatively decaying to the ground level. In this regard, Figure 1 illustrates the upconversion process for a pair of erbium ions 2 and 4. As can be seen, upconversion effects causes ion 2 to be promoted to a relatively high level such as the ⁴I_9/2 level before eventually cascading down to the ⁴Io/₂ metastable level, then to the ⁴Iι_5/2 ground level, while ion 4 is promoted only to the Ij_3/ metastable level before nonradiatively decaying to the ⁴Iι_5/2 ground level. Accordingly, once the concentration of erbium atoms in the glass becomes high enough for such upconversion effects to occur, further increases in concentration do not result in any increases in gain.

[0005] Reconfiguration of optical amplifier design using various pump schemes including co-propagating, counter propagating, and bi-directional pumping have made improvements to gain and noise characteristics in conventional optical amplifiers utilizing long EDFs. For instance, Figure 2 shows a schematic of a conventional optical amplifier 10 that utilizes a counter-propagating pumping scheme. The optical amplifier 10 is provided with an input 12 provided by source 14 into which an input signal enters the optical amplifier 10. An optical isolator 16 and a coil of erbium doped fiber 18 is provided downstream of the input 12. In the illustrated example, the length of the coil of erbium doped fiber 18 may be between 4-20 meters. In addition, a wavelength division multiplexer (WDM) 20 is provided that allows entry of pump power provided by the optical pump 22, the pump power being passed through an optical isolator 24. The output from the WDM 20 is then passed through the optical isolator 26 and out of the optical amplifier 10 through the output 28 so as to provide an output signal which has been amplified.

[0006] It has been demonstrated that a doubling of the gain coefficient may be attained using a gain fiber length only half to one-third as long as in conventional optical amplifiers by using a reflector to propagate the pump light twice through the gain fiber. Figure 3 is a schematic illustration of such an optical amplifier 30. Such an optical amplifier 30 is prpvided with an input 32 into which an input signal enters the optical amplifier 30, the input signal being provided by a signal input source 34. The input signal is passed through an optical isolator 36, a circulator 38, and a WDM 40. The WDM 40 allows entry of pump power provided by the optical pump 42, the pump power being passed through an optical isolator 44. The input signal is then co-propagated along with the pump light through the erbium doped fiber 46 where it is reflected off a reflector 48 so that it becomes counter propagating output signal. In the illustrated example, the reflector 48 is a mirror and the length of the erbium doped fiber 46 may be between 2-8 meters which is significantly less than that of the optical amplifier 10 shown in Figure 2. The counter propagating output signal passes through the WDM 40 and is separated from the input signal by the optical circulator 38. The counter propagating output signal exits the optical amplifier 30 through the output 50 so as to provide an output signal which has been amplified. However, the reflector 48 reflects not only the signal and pump light, but the amplified spontaneous emission (ASE) which accompanies the EDF amplification process. This has the effect of undesirably increasing the noise figure for such amplifiers at a particular gain level over more conventional EDFAs as shown in Figure 2.

[0007] The relatively higher noise figure for the amplifier illustrated in Figure 3 is shown in Figures 4a, 4b and 4c. These graphs show the signal output power P_out (dBm), Gain (dB), and Noise Figure (dB) for the optical amplifiers 10 and 30 shown in Figure 2 and Figure 3, respectively. The clear circles and clear diamonds indicate characteristics of the optical amplifier 10 of Figure 2 with the signal wavelength parameters at 1530 nm and 1545 nm, respectively as shown in the legend with a saturating input power of -15 dBm. Likewise, the darkened squares and triangles indicate characteristics of the optical amplifier 30 of Figure 3, with the signal wavelength parameters at 1530 nm and 1545 nm, respectively as shown in the legend with a saturating input power of -15 dBm. All measurements were made at 120 mW with 977 nm pump wavelength. As can be seen from the data presented in these figures, amplifier power output P_out and gain is substantially higher for the Figure 3 amplifier 30 over the Figure 2 amplifier 10 for a given gain fiber length. But the noise figure is also much higher for the Figure 3 amplifier 30 relative to the Figure 2 amplifier 10 for a given gain level. Compare, for example, the noise figures for a 25 dB gain wherein the Figure 3 and Figure 2 amplifiers use 4 and 9 meters of gain fiber, respectively. The relative noise figures are 6-8 dBs vs. 3-4 dBs, respectively.

[0008] Therefore, despite the higher gain per length of gain fiber realized in the Figure 3 amplifier 30, there still exits a need for an optical amplifier that provides both high gain and low noise figure characteristics with a relatively shorter length of gain fiber. In this regard, the inventors have observed that the relatively higher noise figure in prior art Figure 3 type amplifiers probably comes about from a combination of the insertion losses caused by the optical isolator that is present at the amplifier input, and the reduced inversion caused by the saturation of the EDF coil at the coil input. However, up to now, there has been no recognition of these sources of the problem nor any known techniques for overcoming the increased noise.

SUMMARY OF THE INVENTION

[0009] In view of the above, an advantage of the preferred embodiment of the present invention is in providing a novel optical amplifier that reduces the length of optical fiber used in optical amplifiers, such as C-band and L-band amplifiers, without significantly increasing the noise figure for a given gain level.

[0010] Another advantage of the preferred embodiment of the present invention is in providing an optical amplifier that minimizes filter depth of a filtration device used in the optical amplifier.

[0011] The above noted advantages and others are attained by an optical amplifier including first and second serially connected optical amplifier stages, each of which includes an input and an output, the second of which includes a length of amplification fiber connected to a source of pump light, a signal input connected to the input of the first amplifier stage that receives an optical signal, a reflector connected to the output of the second amplifier stage that reflects an amplified signal back through said second amplifier stage, an isolator device connected between the output of the first amplifier stage and the input of the second amplifier stage that separates incoming and outgoing amplified signals to and from said second amplifier stage, and a signal output optically connected to said isolator device. In one embodiment, the isolator device is a three- way isolator such as a circulator.

[0012] In another embodiment, the first amplifier stage also includes a length of amplification fiber and a source of pump light. The amplification fiber may include an erbium dopant and is less than about 6 meters, preferably between 3 and 5 meters. In various embodiments, the reflector of the optical amplifier is a mirror or a cleaved fiber end having a coating of a reflective material. Optional gain flattening filter and/or a dispersion compensation filter which may be coupled between the output of the second amplifier stage and the reflector may also be provided.

[0013] In accordance with still another embodiment, the source of pump light for the first amplifier stage fully inverts an energy state of dopant atoms in the amplification fiber. In yet another embodiment, the source of pump light may be the same for both of the amplifier stages so that a single source of pump light is used for both of the amplifier stages. Optical isolators may also be coupled between the second amplifier stage and its source of pump light as well as between the input of the first amplifier stage and its length of amplification fiber.

[0014] These and other features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments of the invention when viewed in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

[0015] Figure 1 is an energy level diagram showing a pair of erbium ions undergoing an upconversion process indicated by arrows.

[0016] Figure 2 shows a schematic of a conventional optical amplifier.

[0017] Figure 3 shows a schematic of an optical amplifier having a circulator and a reflector.

[0018] Figure 4a shows empirical data of Signal Output Power as a function of fiber length of the optical amplifiers shown in Figure 2 and Figure 3.

[0019] Figure 4b shows empirical data of Gain as a function of fiber length of the optical amplifiers shown in Figure 2 and Figure 3.

[0020] Figure 4c shows empirical data of Noise Figure as a function of fiber length of the optical amplifiers shown in Figure 2 and Figure 3.

[0021] Figure 5 shows a schematic of an optical amplifier in accordance with one embodiment of the present invention. [0022] Figure 6a shows empirical data of Signal Output Power as a function of signal input powers for the embodiment of the optical amplifier in accordance with the present invention shown in Figure 5.

[0023] Figure 6b shows empirical data of Gain as a function of signal input powers for the embodiment of the optical amplifier shown in Figure 5.

[0024] Figure 6c shows empirical data of Noise Figure as a function of signal input powers for the embodiment of the optical amplifier shown in Figure 5.

[0025] Figure 7 shows a schematic of an optical amplifier in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0026] As will be evident from the discussion herein below, the present invention provides an improved optical amplifier that reduces the length of optical fiber used in optical amplifiers such as C-band and L-band amplifiers without significantly increasing the noise figure for a given gain level. In addition, it will also be evident that present invention also provides such an optical amplifier that also minimizes filter depth of a filtration device used. ^'

[0027] In the above regard, Figure 5 shows a schematic of an optical amplifier 50 in accordance with one embodiment of the present invention. It should initially be noted that whereas various embodiments of the present invention are described in detail herein below, theses specific embodiments should be understood to be examples only and the present invention is not limited thereto.

[0028] As can be seen in Figure 5, the optical amplifier 50 includes a first amplifier stage 60 and a second amplifier stage 70 which are serially connected to each other, each amplifier stage being described in detail below. The first amplifier stage 60 has an input 62 and output 68. A signal input 61 is connected to the input 62 of the first amplifier stage 60 to provide an input signal which is amplified by the optical amplifier 50 in accordance with the present invention. The optical signal provided to the input 62 by the signal input 61 is passed through an optical isolator 63. In the present illustrated embodiment, the first amplifier stage 60 also includes a length of amplification fiber 64. The amplification fiber 64 in the embodiment shown is a coil of fiber that includes a dopant such as erbium or other dopant. The amplification fiber 64 is less than about 6 meters in length and preferably, is between 3 and 5 meters in length, and is connected to a source of pump light 65 via a wavelength division multiplexer (WDM) 66 in a conventional manner. As can be appreciated by the directional arrow provided, the source of pump light 65 in the illustrated embodiment provides a reverse pumping of the input signal. Of course, in other embodiments, the source of pump light may configured to provide forward pumping of the input signal instead.

[0029] The output of the first amplifier stage 60 is connected to an isolator device which in the present embodiment, is a three-way isolator such as the circulator 69 shown. The circulator 69 is provided mid-stage between the first amplifier stage 60 and the second amplifier stage 70. The circulator 69 functions to double pass and separate the optical signal amplified by the second amplifier stage 70 from the output of the first amplifier stage 60 in the manner further described below.

[0030] The circulator 69 is optically connected to the input 71 of the second amplifier stage 70 that receives the output signal from the circulator 69. The circulator 69 also has an output 79 to allow an output signal amplified by the second amplifier stage 70 to exit the optical amplifier 50. The second amplifier stage 70 also includes a source of pump light 72 which is connected to a length of amplification fiber 75 via a wavelength division multiplexer (WDM) 74 in a conventional manner. Thus, the circulator 69 of the present illustrated embodiment is placed in the mid-stage between two amplifier fibers 64 and 75 of the first and second amplifier stages 60 and 70, respectively. The amplification fiber 75 in the illustrated embodiment is a coil of fiber that includes a dopant such as erbium, and is less than about 6 meters in length and preferably, is between 3 and 5 meters in length. The second amplifier stage 70 shown also includes an optical isolator 73 provided between the source of pump light 72 and the WDM 74.

[0031] Furthermore, the optical amplifier 50 in accordance with the present invention includes a reflector 77 connected to the output 76 of the second amplifier stage 70, the reflector 77 being adapted to reflect the amplified signal back through the second amplifier stage 70 and the isolator device such as the circulator 69 shown. In one embodiment, the reflector 77 of the optical amplifier 50 may be a mirror. For practicality, the reflector 77 may be provided directly at the fiber end of the output 76 by cleaving the fiber end and coating the fiber end with an appropriate reflective material such as gold, silver, aluminum, or other material. In addition, as evident in Figure 5, the illustrated embodiment of the source of pump light 72 of the second amplifier stage 70 provides forward pumping of the output signal from the circulator 69 in one direction, and also provides reverse pumping of the output signal which has been reflected off the reflector 77. In the above regard, the reflector 77 provides a two way amplification and pumping by reflecting the pump and signal beam.

[0032] As previously noted, the circulator 69 separates incoming input signal and outgoing amplified output signals to and from the second amplifier stage 70. In this regard, the optical amplifier 50 includes a signal output 79 which is optically connected to the circulator 69 in the present embodiment to allow the input signal which has been amplified by the optical amplifier 50 to leave the optical amplifier 50 as an output signal.

[0033] In operation, input signal is provided by the signal input 61 to input 62 of the first amplifier stage 60. The input signal is passed through an optical isolator 63 and the amplification fiber 64, and is further provided with pump light via the source of pump light 65 as the input signal passes through the WDM 66. The input signal, which has now been amplified by the first amplifier stage 60, is passed through the isolator device such as the circulator 69, and is provided to the input 71 of the second amplifier stage 70. The amplified input signal is then, again, provided with forward pumping' light from the source of pump light 72 as the signal passes through the WDM 74, the pump light itself being passed through an optical isolator 73. The signal is then passed through the amplification fiber 75 and allowed to exit out of the second amplifier stage 70 through the output 76. The signal is then reflected off the reflector 77 back into the second amplifier 75 so that the amplified signal is a reverse propagating signal that propagates backward through the second amplifier stage 70. The reverse propagating sigla is again amplified through the amplification fiber 75 and is reverse pumped by the source of pump light 72. At the circulator 69, the amplified signal propagating backward through the second amplifier stage 70 is separated from the amplified input signal from the output 68 of the first amplifier stage 60. Thus, the input signal is passed to the second amplifier stage 70 while the amplified output signal exits out of the optical amplifier 50 through the signal output 79 optically connected to the circulator 69.

[0034] In the conventional double-pass type amplifiers such as that shown in Figure 3, the inclusion of an optical circulator and reflector allowed reduction in the fiber length but it also resulted in corresponding increases in amplifier noise as shown in Figures 4a-4c and discussed above. In contrast, the embodiment in accordance with the present invention as shown in Figure 5 has been found to offer many of the benefits of conventional double-pass type amplifiers while also minimizing noise so that measured noise is comparable to conventional amplifiers such as that shown in Figure 2 discussed previously.

[0035] In the above regard, utilization of an optical amplifier 50 having a first amplifier stage 60 and second amplifier stage 60 where an isolator device such as the circulator 69 is placed at the mid-stage of the optical amplifier 50 between the first and second amplifier stages 60 and 70 respectively, has been found by the inventors of the present invention to minimize the propagation of amplifier spontaneous emission back to the first amplifier stage 60. In particular, the reduction in amplifier noise was found to be mainly attributable to minimizing insertion loss at the front end of the amplifier. This minimization of amplifier noise was attained by initially amplifying the input signal via the first amplifier stage 60 prior to further amplification by the second amplifier stage 70 which utilizes a reflector 77 to reverse propagate the signal. Moreover, another major advantage of using an isolator device such as the circulator 69 described above is that it provides a separated and double-passed signal at the signal output 79.

[0036] Figures 6a to 6c show empirical data obtained utilizing the embodiment of the optical amplifier 50 in accordance with the present invention shown in Figure 5 at 1530 nm and 1545 nm. The results shown and discussed above were attained by providing the first amplifier stage 60 with backward pump light from the source of pump light 65 at 977 nm pump wavelength to achieve full inversion of the energy state of dopant atoms in the amplification fiber 64 and hence, a better noise performance. In this regard, the amplification fiber 64 was doped with erbium and fuller inversion of the erbium ions were attained. In addition, the second amplifier stage 70 was provided with pump light from the source of pump light 72 at 975 nm pump wavelength, the amplification fiber 75 also being doped with erbium.

[0037] As shown by Figure 6a, the power output P_out of optical amplifier 50 was comparable to the power output of the optical amplifier 30 shown in Figure 3 that also used a reflector and a circulator, the maximum P_out for the optical amplifier 50 being approximately 15 dBm while the maximum P_out for the optical amplifier 30 being approximately 16 dBm as shown in Figure 4a. In addition, the total gain was increased using the optical amplifier 50 in accordance with the embodiment of the present invention, the maximum gain ranging between 31 to 48 dB as compared to the optical amplifier 30 which had a maximum gain ranging between 28 to 32 dB. What is most significant is that as most clearly seen in Figure 6c when compared to Figure 4c, large reduction in the noise figure was attained utilizing the optical amplifier 50 in accordance with the embodiment of the invention as shown in Figure 5. In particular, the noise figure ranged between 3.8 to 6.2 dBm for the optical amplifier 50 while the noise figure ranged between 5.8 to 10.6 dBm for the optical amplifier 30 of the prior art. Thus, the present invention provided an improved optical amplifier that reduces the length of optical fiber used without significantly increasing the noise figure for a given gain level.

[0038] Similar advantageous results may also be attained by providing the second amplifier stage 70 with pump light from the source of pump light 72 at 1480 nm pump wavelength. In addition, the present invention can also be applied to other embodiments where a single high power source such as a single pump laser diode is used with a pump coupler of appropriate ratios for the first and second amplifier stages 60 and 70 instead of the two separate sources of pump light 65 and 72 for each amplifier stage as shown in Figure 5.

[0039] Figure 7 illustrates an optical amplifier 150 in accordance with another embodiment of the present invention. As can be seen, the optical amplifier 150 is substantially the same as the optical amplifier 50 discussed above relative to Figure 5. As such, the various components of the optical amplifier 150 are indicated using the same numerals except in the one hundreds. Therefore, the optical amplifier 150 includes a first amplifier stage 160 with input 162 and output 168 where a signal input 161 provides an input signal, an optical isolator 163, and an amplification fiber 164 which is connected to a source of pump light 165 via a WDM 166. The output 168 is connected to an isolator device such as a circulator 169 optically connected to the input 171 of the second amplifier stage 170, the circulator 169 having an output 176. The second amplifier stage 170 includes a source of pump light 172 that is connected to an amplification fiber 175 via a WDM 174, and an optical isolator 173. The optical amplifier 150 also includes a reflector 177 and an output 179 connected to the circulator 169. The details and functions of these various components have already been discussed above relative to the optical amplifier 50 and thus, are omitted here to avoid repetition.

[0040] However, in contrast to the previous embodiment, the optical amplifier 150 is also provided with a filter device 178 for filtering the output of the second amplifier stage 170. In this regard, as can be seen, the filter device 178 is provided between the amplification fiber 175 and the reflector 177. The filter device 178 may be a gain flattening filter and/or a dispersion compensation filter known in the art such as chromatic or polarization dispersion gratings. Because a reflector 177 which reflects the output signal of the second amplifier stage 170 to a counter propagating signal is provided in the optical amplifier 150, only approximately half the filter depth is required due to the double passing of the signal. In particular, because the signal is filtered as a forward propagating signal, and also filtered as a reverse propagating signal after it is reflected by the reflector 177, the depth of the filter device 178 can be significantly reduced and even halved. Thus, in the embodiment where the filter device 178 is a gain flattening filter, the gain flatten depth required is only about half that of a conventional optical amplifier. In addition, in the embodiment where the filter device 178 is a dispersion compensation filter, because of the double passing of the signal beam, the dispersion compensation depth required is only about half that of a conventional amplifier.

[0041] In view of the above, it should now be evident how the present invention provides an improved optical amplifier that reduces the length of optical fiber used in optical amplifiers such as C-band and L-band amplifiers without significantly increasing the noise figure for a given gain level. In addition, it should now also be evident how the present invention also provides such an optical amplifier that also minimizes filter depth of a filtration device used.

[0042] While various embodiments in accordance with the present invention have been shown and described, it is understood that the invention is not limited thereto. These embodiments may be changed, modified and further applied by those skilled in the art. Therefore, it should be clear that this invention is not limited to the details shown and described previously but also includes all such changes and modifications.

PARTS LIST

Optical amplifier

First amplifier stage

Signal input

Input

Optical isolator

Amplification fiber

Pump light

Wavelength division multiplexer (WDM)

Output

Circulator

Second amplifier stage

Input

Pump light

Optical isolator

Wavelength division multiplexer (WDM)

Amplification fiber

Output

Reflector

Signal output

Optical amplifier

First amplifier stage

Signal input

Input

Optical isolator

Amplification fiber

Pump light

Wavelength division multiplexer (WDM)

Output

Circulator

Second amplifier stage

Input 172 Pump light

173 Optical isolator

174 Wavelength division multiplexer (WDM)

175 Amplification fiber

176 Output

177 . Reflector 178 Filter device

179 Output

Claims

WHAT IS CLAIMED IS:

1. An optical amplifier comprising: first and second serially connected optical amplifier stages, each of which includes an input and an output, the second of which includes a length of amplification fiber connected to a source of pump light; a signal input connected to the input of the first amplifier stage that receives an optical signal; a reflector connected to the output of the second amplifier stage that reflects an amplified signal back through said second amplifier stage; an isolator device connected between the output of the first amplifier stage and the input of the second amplifier stage that separates incoming and outgoing amplified signals to and from said second amplifier stage; and a signal output optically connected to said isolator device.

2. The optical amplifier described in claim 1, wherein said amplification fiber includes an erbium dopant.

3. The optical amplifier described in claim 1, wherein said first amplifier stage also includes a length of amplification fiber and a source of pump light.

4. The optical amplifier described in claim 1, wherein said length of amplification fiber is less than about 6 meters.

5. The optical amplifier described in claim 4, wherein said length is between 3 and 5 meters.

6. The optical amplifier described in claim 4, wherein said amplification fiber includes an erbium dopant.

7. The optical amplifier described in claim 1, wherein said reflector is at least one of a mirror and a cleaved fiber end having a coating of a reflective material.

8. The optical amplifier described in claim 3, wherein said source of pump light is the same for both said amplifier stages so that a single source of pump light is used for both of said amplifier stages.

9. The optical amplifier described in claim 1, further comprising a gain flattening filter coupled between the output of said second amplifier stage and said reflector.

10. The optical amplifier described in claim 1, further comprising a dispersion compensation filter coupled between the output of said second amplifier stage and said reflector.

11. The optical amplifier described in claim 1 , wherein said Isolator device is a three-way isolator.

12. The optical amplifier described in claim 1, wherein said isolator device is a circulator.

13. A optical amplifier, comprising: first and second serially connected optical amplifier stages, each of which includes a length of amplification fiber connected to a source of pump light, and an input and an output; a signal input connected to the input of the first amplifier stage that receives an optical signal; a reflector connected to the output of the second amplifier stage that reflects an amplified signal back through said second amplifier stage; an isolator device connected between the output of the first amplifier stage and the input of the second amplifier stage that separates incoming and outgoing amplified signals received by and transmitted by said second amplifier stage; and a signal output.

14. The optical amplifier described in claim 13, wherein said signal output is optically connected to said isolator device.

15. The optical amplifier described in claim 13, wherein said reflector is at least one of a mirror and a cleaved fiber end having a coating of a reflective material.

16. The optical amplifier described in claim 13, wherein said amplification fiber includes an erbium dopant.

17. The optical amplifier described in claim 13, wherein said source of pump light is the same for both said amplifier stages so that a single source of pump light is used for both of said amplifier stages.

18. The optical amplifier described in claim 13, wherein said source of pump light for said first amplifier stage fully inverts an energy state of dopant atoms in said amplification fiber in said first amplifier stage.

19. The optical amplifier described in claim 13, further comprising an optical isolator coupled between said second amplifier stage and its source of pump light.

20. The optical amplifier described in claim 13, further comprising an optical isolator coupled between said input of said first amplifier stage and its length of amplification fiber.

21. The optical amplifier described in claim 13, further comprising a gain flattening filter coupled between the output of said second amplifier stage and said reflector.

22. The optical amplifier described in claim 13, further comprising a dispersion compensation filter coupled between the output of said second amplifier stage and said reflector.

23. The optical amplifier described in claim 13, wherein said isolator device e-way isolator.

24. The optical amplifier described in claim 13, wherein said isolator device ulator.