WO2001065646A2

WO2001065646A2 - Multiple stage optical fiber amplifier

Info

Publication number: WO2001065646A2
Application number: PCT/US2001/006406
Authority: WO
Inventors: Bernard G. Fidric; Stephen G. Grubb; David F. Welch
Original assignee: Jds Uniphase Corporation
Priority date: 2000-02-29
Filing date: 2001-02-27
Publication date: 2001-09-07
Also published as: WO2001065646A3

Abstract

A multiple stage optical amplifier includes a Raman fiber amplifier first stage coupled to a rare earth doped fiber amplifier which provides a second amplifier stage. Pump signals are injected into the Raman amplifier and propagate in a direction opposite to input signals. Since the Raman amplifier does not completely absorb the pump signals, a separate optical path is provided to apply the excess pump energy to the second stage fiber amplifier to pump it. The gain and bandwidth characteristics of the Raman amplifier are determined by the spectral properties of the pump signal(s) and gain bandwidth characteristic of the Raman amplifier can be selected by appropriately selecting the wavelengths of the pump signals. In one embodiment, the pump signals are selected so that the first stage Raman amplifier has a relatively low noise characteristic. The second stage fiber amplifier, on the other hand, is designed to have a relatively high power characteristic. Furthermore, to increase the gain bandwidth of the entire optical amplifier, the gain bandwidth characteristics of the first and second amplifier stages are selected to overlap rather than coincide. Pump energy may be used to counter-propagate through a Raman first amplifier stage. Resonant pumping may also be used, including a cascaded Raman resonator type arrangement that encompasses a rare-earth doped gain medium.

Description

MULTIPLE STAGE OPTICAL FIBER AMPLIFIER

FIELD OF THE INVENTION This invention relates generally to optical fiber amplifiers and, more particularly to, low noise, high power optical fiber amplifier circuits.

BACKGROUND OF THE INVENTION

As is known in the art, an optical amplifier is a device that increases the amplitude of an input optical signal fed thereto. If the optical signal at the input to such an amplifier is monochromatic, the output will also be monochromatic, with the same frequency. A conventional fiber amplifier comprises a gain medium, such as a single mode glass fiber doped with a rare earth material, connected to a WDM coupler which provides low insertion loss at both the input signal and pump wavelengths. The input signal is provided, via the coupler, to the medium. Excitation occurs through optical pumping from the pumping source, which is combined with the optical input signal within the coupler, which is within the absorption band of the rare earth dopant, and an amplified output signal is emitted from the other end of the fiber.

Such amplifiers are typically used in a variety of applications including but not limited to amplification of weak optical pulses such as those that have traveled through a long length of optical fiber in communication systems. Optical amplification can take place in a variety of materials including those materials, such as silica, from which optical fibers are formed.

One type of fiber amplifier referred to as an erbium (Er) amplifier typically includes a silica fiber having a single-mode core doped with erbium (specifically doped with erbium ions conventionally denoted as Er^34"). It is well known that an erbium optical fiber amplifier operating in its standard so-called three level mode is capable, when pumped at a wavelength of 980 nanometers (nm), of amplifying optical signals having a wavelength of approximately 1.5 micrometers (μm). Since 1.5 μm is the lowest loss wavelength of conventional single-mode glass fibers, erbium amplifiers are well suited for inclusion in fiber systems that propagate optical signals having wavelengths around 1.5 μm.

However, there are several limitations with erbium fiber amplifiers used in practical communication systems. One-problem is that the gain characteristic is relatively uniform only within a relatively narrow gain bandwidth. In addition, the output power characteristic developed by erbium amplifiers is limited by the range of power available from pump sources used to pump the fiber. Also, with the advent of dense wavelength division multiplexed (WDM) systems, the available gain bandwidth and uniformity of gain flatness has become a critical issue in optical amplifier design. Another type of well-known optical fiber amplifier is a so-called Raman amplifier. A Raman amplifier provides amplification of signals, via stimulated Raman scattering in an optical fiber, and does not have the gain bandwidth problems associated with erbium amplifiers. One problem with Raman amplifiers, however, is that they are relatively inefficient and thus must be pumped with pump signals having relatively high power levels in order to obtain suitable output power. Much of the energy from the high power pump signals is not utilized and becomes wasted and, even. when pumped with high power pump signals, the Raman amplifier provides output signals having relatively low power levels due to the inefficient use of the energy provided by the pump signal. This is especially true when the Raman amplifier is not operated in its "saturation mode".

Another problem with Raman amplifiers is that the upper atomic energy level of the amplifier has essentially a zero lifetime. In a wavelength division multiplexed system, the rapid depletion of the upper amplifier energy level can result in crosstalk between the required high power pump signal and an input signal to be amplified. It would therefore be desirable to provide an optical fiber amplifier having a relatively wide bandwidth, a relatively low noise characteristic and a relatively high output power characteristic.

SUMMARY OF THE INVENTION In accordance with the present invention, a multiple stage optical amplifier includes a first fiber amplifier stage optically coupled to a second fiber amplifier stage. In accordance with one embodiment, a pump signal is injected into the first amplifier stage at the connection between the first and second stages and propagates in a direction opposite to the input signals. The first amplifier stage does not completely absorb the pump signal so that it does not saturate and an optical path may also be provided to apply the excess pump signal to the second amplifier stage to pump it. The optical path is arranged so that the pump signal also propagates in a direction opposite to input signals in the second amplifier stage. In accordance with another embodiment, a pump signal is injected into the second amplifier stage and propagates in a direction opposite to input signals. The second amplifier stage does not completely absorb the pump signal and a separate optical path is provided to apply the excess pump signal to the first amplifier stage to pump it. The pump signal also propagates in a direction opposite to input signals in the first amplifier stage. The pump signal may also be coupled into the second stage so that it propagates in the same direction as the amplified input signal from the first stage, and the excess pump energy from the second stage coupled into the first stage so that it propagates in an opposite direction as the input signal. In accordance with still another embodiment, the first fiber amplifier stage is a

Raman amplifier and the second amplifier stage is a rare earth doped fiber amplifier. The gain and bandwidth characteristics of the Raman amplifier stage are selected by choosing the wavelengths of the pump signals so that the Raman amplifier stage has a relatively low noise characteristic. The rare earth doped amplifier stage, on the other hand, is designed to have a relatively high power characteristic. Furthermore, to increase the gain bandwidth of the entire optical amplifier, the gain bandwidth characteristics of the first and second amplifier stages are selected to overlap and/or to cover separate wavelength ranges, rather than to simply coincide.

For example, the wavelengths of the pump signals may be selected such that the gain characteristic provided by the Raman amplifier stage complements the gain characteristic of the rare earth doped amplifier stage. This provides an amplifier having a high gain characteristic and a low noise characteristic over a relatively wide bandwidth. In addition, the power levels of the pump signals may be selected such that the Raman amplifier operates in an unsaturated amplifier mode. Since the Raman amplifier does not efficiently use all of the power of the pump signals, the excess pump signal power from the Raman amplifier stage pumps the rare earth doped amplifier. Thus the consequences of the inefficiency of the Raman amplifier are minimized since the pump signals are recycled to pump the rare earth amplifier. Moreover, the pump signals may be arranged to propagate through the Raman amplifier stage and the rare earth doped amplifier stage in a direction which is opposite to the direction in which the input signal propagates through the amplifier stages. This minimizes crosstalk between the input signal and the pump signals. Such minimization of crosstalk results because the walk-off characteristic between the input signal and pump signal is large and, accordingly, the pump-mediated signal crosstalk mechanism is thus defeated.

In one embodiment, the rare earth doped amplifier stage may use an erbium- doped silica fiber or, in an alternate embodiment, an erbium-doped fluoride fiber. Since a fluoride fiber provides amplification to signals having relatively long wavelengths, use of a fluoride fiber may increase the overall amplifier gain bandwidth. In another embodiment, two pump signals having wavelengths of 1465 nm and 1485 nm, respectively, are used to pump the Raman amplifier stage. This approach results in the optical amplifier having a relatively wide and flat gain characteristic with a gain bandwidth of 50 nm or greater.

In yet another embodiment, two pump wavelengths are used to pump the multiple stage amplifier so as to provide a desired pumping of both stages.

In still another embodiment, a single pump source is used to pump the Raman amplifier stage. The pump source may be a Raman fiber laser which provides an output signal having a wavelength typically of about 1470 nm. Alternatively, the single pump source may be provided as two high-power diodes which provide an output signal having a wavelength typically of about 1470 nm, or by using another broadband type source with broad or multiple wavelength peaks.

In another embodiment, the first amplifier stage is an erbium doped fiber amplifier and the second stage is a Raman amplifier.

In another embodiment, the first amplifier stage is a fiber amplifier with a thulium doped ZBLAN core and the second stage is an erbium/ytterbium doped fiber amplifier. The first amplifier stage is cladding pumped from a 1.06 μm pump source. In still another embodiment, the first amplifier stage is a fiber amplifier with a praseodymium doped ZBLAN core and the second amplifier stage is an erbium/ytterbium doped fiber amplifier. The first amplifier stage is cladding pumped from a 1.04 μm pump source.

In a different embodiment of the invention, a first stage is a Raman amplifier stage that provides gain in a band centered about a first wavelength. A second amplifier stage is a rare earth doped stage, and provides gain to the signal passed from the first stage in a band centered about a second wavelength. A gain adjusting filter may be used to filter the amplified signal so that the effective gain of the signal is approximately equal across the two wavelength bands. The filter may be located between the second stage and an optional third amplifier stage that may also be a rare-earth doped fiber amplifier.

Another embodiment has a Raman amplifier stage and a rare-earth doped amplifier stage, and uses a reflector pair that develops a resonant condition through both the Raman gain medium and the rare-earth doped gain medium. In a preferred version of this embodiment, a set of reflectors is used in the form of a cascaded Raman resonator, such that a shorter wavelength initial pump signal is converted to progressively longer wavelengths. The longest of the resonated wavelengths is preferably used for pumping the rare-earth gain medium. A portion of that longest wavelength may also be coupled out and returned to pump a preamplifier stage that precedes the Raman amplifier stage. The preamplifier may also be based on stimulated Raman scattering, and another optional rare-earth doped stage may also be used as well. The preamplifier may also take the form of a transmission fiber that is capable of providing Raman gain when pumped, and which caries a transmitted optical signal to be amplified. In a variation of this embodiment, a resonant condition may be developed through a rare-earth doped stage alone at a desired pumping wavelength. A portion of that pumping energy may be coupled out and used to counterpump another amplifier stage, such as a Raman preamplifier stage. A second rare earth doped stage may also be used in combination with the other stages.

BRIEF DESCRIPTION OF THE DRAWING

The foregoing features of the invention, as well as the invention itself may be more fully understood from the following detailed description of the drawing, in which:

FIG. 1 is a schematic view of an optical amplifier constructed in accordance with the principles of the present invention with a Raman amplifier first stage and a rare earth doped second stage;

FIG. 2 is a schematic view of an optical amplifier constructed in accordance with the principles of the present invention with a rare earth doped first stage and a Raman amplifier second stage; FIG. 3 is a plot of the gain spectrum of the multiple stage amplifier illustrated in

FIGS. 1 and 2;

FIG. 4 is a schematic view of an optical amplifier constructed in accordance with the principles of the present invention in which the first amplifier stage is a fiber amplifier with a thulium doped ZBLAN core and the second stage is an erbium/ytterbium doped fiber amplifier;

FIG. 5 is a plot of the gain spectrum of the multiple stage amplifier illustrated in FIG. 4; FIG. 6 is a schematic view of an optical amplifier constructed in accordance with the principles of the present invention in which the first amplifier stage is a fiber amplifier with a praseodymium doped ZBLAN core and the second stage is an erbium/ytterbium doped fiber amplifier;

FIG. 7 is a plot of the gain spectrum of the multiple stage amplifier illustrated in FIG. 6;

FIG. 8 is a schematic view of an optical amplifier constructed in accordance with the principles of the present invention in which the pumping signals are applied to the second amplifier stage;

FIG. 9 is a schematic view of an amplifier apparatus that includes a Raman stage and a rare-earth doped stage, and in which excess pumping energy from the rare earth doped stage is used to counterpump the Raman stage;

FIG. 10 is a schematic view of a multi-stage amplifier apparatus that provides gain in different gain bandwidths for different stages;

FIG. 11 is a schematic view of an amplifier apparatus that uses a cascaded Raman resonator type arrangement that includes a rare-earth doped gain medium; and

FIG. 12 is a schematic view of an amplifier apparatus that uses resonant pumping of a rare-earth doped stage, with excess pump energy being used to counterpump a Raman preamplifier stage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 , an optical amplifier 10 having an input port 10a and an output port 10b includes a first wavelength division multiplexer (WDM) 12 having an input port 12a coupled to amplifier input port 10a. An output port 12b of WDM 12 is coupled to an input port 14a of a Raman fiber amplifier 14 which serves as a first amplifier stage of amplifier 10. Raman amplifier 14 has a quantum-limited noise figure which causes optical amplifier 10 to have a relatively low noise figure. Amplifier 14 is comprised of a cladded optical fiber which may be, for example, a germano-silica fiber having a core diameter typically in the range of about 2 μm - 5 μm and having an insertion loss characteristic not greater than 0.6 decibels per kilometer (db/km) at a wavelength of about 1470 nm. Also, when the fiber is to amplify radiation of wavelength λ,, where λ, = λ_M, + Δλ; in which Δλ, is a length within the appropriate Stokes band associated with the fiber, 1=1 , ... , n (n is an integer equal to or greater than 2) and λ_M is defined to be the wavelength λ_p of the pump radiation when 1=1 , a fiber having a relatively high value of delta is preferred. As is known in the art, a conventional single mode transmission fiber has an effective core diameter of at least 8 μm, thereby minimizing scattering and connection losses. However, the discrete amplifier 14 of FIG. 1 preferably uses a fiber with a smaller effective core diameter, thereby taking advantage of the increased Raman gain resulting from the correspondingly higher intensity. For the short length of the fiber (typically less than 1 km), the losses do not present a significant problem. As shown in the figure, the fiber is typically coiled so that its input and output ports are in close proximity to each other relative to the overall length of the fiber. Amplifier 14 is coupled to an input port of an erbium-doped fiber amplifier 22 through a second WDM 16 and a first isolator 20. Specifically, an output port 14b of amplifier 14 is coupled to an input port 16a of second WDM 16 and an output port 16b of WDM 16 is coupled to an input port 20a of isolator 20. Isolator 20 is provided having an isolation characteristic typically of about 40 dB. An output port 20b of isolator 20 is coupled to a first input port 22a of erbium-doped (Er) amplifier 22 which provides a second amplifier stage of amplifier 10.

Er amplifier 22 is provided from a single mode fiber section having a core doped with Er^3'' ions using conventional doping techniques. Although, in a preferred embodiment, second stage amplifier 22 is an Er amplifier, it should be appreciated that amplifier stage 22 may alternatively be fabricated from a single mode fiber having a core doped with a rare earth material other than erbium. In addition, the fiber may be a silica glass fiber or, alternatively, fluoride glass compositions based on ZrF₄, for example ZBLA and ZBLAN, may also be used.

Amplifier stage 22 is pumped with a signal having a predetermined wavelength. The output of amplifier 22 is coupled to an input port 24a of a third WDM 24 and a first output port 24b of the WDM 24 is coupled to a second isolator 26 at an input port 26a. Isolator 26 may be provided, for example, as a multistage isolator having a relatively low insertion loss in a pass band which is at least as wide as the gain bandwidth provided by amplifier stages 14, 22. An output port 26b of isolator 26 is coupled to amplifier output port 10b.

A third WDM 18 has input ports 18a, 18b through which are fed respective ones of pump signals P.,, P₂ having respective wavelengths λ_{1 ;} λ₂. An output port 18c of WDM 18 is coupled to a second input port 16c of WDM 16. Thus pump signals P.,, P₂ are fed through WDM 18 to input port 16c of WDM 16.

The pump signals P.,, P₂ propagate through WDM 16 and pump Raman amplifier 14. An optical path 28 has a first end 28a coupled to an output port 12c of WDM 12 and a second end 28b coupled to a third port 24c of WDM 24. In operation, pump signals P., and P₂ having respective wavelengths λ₁ and λ₂ and pump powers pump the Raman amplifier 14 at a level which is below the saturation level of the amplifier 14. Thus, the excess pump power from the first unsaturated Raman amplifier stage 14 is coupled via optical path 28 through WDM 24 and fed to the second amplifier stage 22 to thus pump the Er amplifier 22. Consequently, the pump signal energy not used in the first amplifier stage 14 is used in the second amplifier stage 22. Thus, the Raman amplifier 14 is intentionally not operated in saturation mode because it is desirable to make the pump power provided by pump signals, P., and P₂, available to the second amplifier stage 22 where the pump power is used in a manner which is relatively efficient when compared with the efficiency with which the Raman amplifier 14 uses the pump signal. In typical applications, the percentage of the pump power fed to Raman amplifier 14 which is coupled through signal path 28 to the second stage amplifier 22 is in the range of about 40% to 90% with typically about 75% of the pump power being preferred. As mentioned above, Raman amplifiers are relatively inefficient and thus require pump signals having a relatively high power level. This is especially true when the Raman amplifier is not pumped at a level which drives the amplifier into its saturation region. Thus, by utilizing optical path 28 to couple the pump power from the first amplifier stage to the second amplifier stage, the inefficiency and high power pump signal requirement of the Raman amplifier is mitigated. Moreover, since the pump signal propagates through the Raman amplifier in a direction which is opposite that of the input signal, crosstalk between the pump signal and the input signal is minimized. This is due to the fact that the walk off between the input signal and pump signal is relatively large and thus pump-mediated signal crosstalk is reduced. The gain and bandwidth characteristics of the Raman amplifier are determined by the spectral properties of the pump signal. Thus, the Raman amplifier can be designed to have preferred gain and bandwidth characteristics by appropriate selection of the wavelength and power characteristics of the pump signals, P., and P₂. It is relatively easy to modify the gain and bandwidth characteristics of the Raman amplifier 14 in this manner.

Moreover, the wavelength and the respective power of the pump signals may be selected such that the gain of the Raman amplifier complements the gain of the Er amplifier at longer wavelengths, yet the pump signals can still be recycled through optical path 28 to pump the Er fiber in the second amplifier stage 22. For example, if the amplifier 22 had a gain characteristic which linearly decreased for signals having wavelengths between 1550 nm and 1570 nm, then the wavelengths of the pump signals P.,, P₂ could be selected to provide the Raman amplifier 14 having a gain characteristic which increased linearly between wavelengths of 1550 nm and 1570 nm. Thus, the hybrid Raman amplifier/Er power amplifier configuration of optical amplifier 10 utilizes the advantages of each amplifier type to provide a relatively wide band, high gain, high power amplifier 10.

A still further improvement can be obtained by using a dispersion compensating fiber for the Raman gain fiber. Dispersion compensating fibers are well-known and can be used to reverse the dispersion of pulses caused by passage through conventional optical fibers.

If the second stage Er fiber is a fluoride fiber, it will provide gain at relatively long wavelengths and cause the amplifier 10 to have a wider bandwidth than it would have if a silica fiber were used in the second stage amplifier 22. In accordance with another embodiment, pump signals P.,, P₂ are designed to have wavelengths of 1465 nm and 1485 nm, respectively. In this case, the composite gain spectrum of amplifier 10 may be wider and flatter than gain spectrums of conventional amplifiers. Thus, using the techniques of the present invention, an optical amplifier having a gain bandwidth in excess of 40 nm may be provided. A pump source (not shown) which provides pump signals P., P₂ could be a

Raman fiber laser which generates an output signal having a wavelength typically of about 1470 nm. Alternatively, the pump source may be provided from two high-power laser diodes, each of which provides a signal having different output wavelengths in the band of 1420 nm to 1520 nm. The selection of a particular type of pump source depends upon a variety of factors including, but not limited to, the total output power desired from the optical amplifier 10.

FIG. 2 illustrates another embodiment in which the first amplifier stage 22 is a rare earth doped fiber amplifier and the second amplifier stage 14 is a Raman amplifier. The pumping arrangements are the same as shown in FIG. 1. This arrangement has similar properties to the amplifier shown in FIG. 1 but may have a better noise figure. The gain spectra of the amplifiers shown in FIGs. 1 and 2 are illustrated in FIG. 3 which shows the overlap in the spectra which can be obtained in accordance with one embodiment of the invention. In particular, the gain spectrum 300 of the erbium amplifier extends from about 1530 nm to 1560 nm whereas the gain spectrum 302 of the Raman amplifier can be adjusted to span from 1560 nm to 1580 nm. The result is a wider overall gain spectrum.

FIG. 4 is a block diagram of an optical amplifier constructed in accordance with the principles of the present invention in which both the first amplifier stage and the second amplifier stage are rare earth doped fiber amplifiers. Specifically, the first amplifier stage 30 is a fiber amplifier with a thulium doped ZBLAN core and the second stage 22 is an erbium/ytterbium doped fiber amplifier. The amplifier is pumped with pump signals 32 having a single frequency of 1.06 μm. The gain spectrum of such an amplifier is illustrated in FIG. 5. The thulium doped amplifier gain spectrum 500 is centered at 1.48 μm whereas the erbium/ytterbium spectrum 502 is centered at 1.55 μm. There is no overlap between the two spectra.

FIG. 6 is a block diagram of another optical amplifier constructed in accordance with the principles of the present invention in which the first amplifier stage 34 is a fiber amplifier with a praseodymium doped ZBLAN core and the second stage 22 is an erbium/ytterbium doped fiber amplifier. The amplifier is pumped with pump signal 36 having a single frequency of 1.04 μm. The gain spectrum of such an amplifier is illustrated in FIG. 7. The praseodymium doped amplifier gain spectrum 700 is centered at 1.3 μm whereas the erbium/ytterbium spectrum 702 is centered at 1.55 μm. There is no overlap between the two spectra. FIG. 8 illustrates still another embodiment of the invention in which an erbium amplifier 804 is coupled to a Raman amplifier 814. Specifically, the amplifier has an input port 800 which is connected, via an isolator 802, to the erbium amplifier 804. The signal output of the erbium amplifier 806 is connected by means of a first wavelength division multiplexer (WDM) 804, a second isolator 808 and a second WDM 810 to the signal input of Raman amplifier 814. The signal output of amplifier 814 is connected to output port 818 via WDM 816. Pump signals 820, for example, at 1470 nm, are applied via WDM 816 to the system and propagate in a direction opposite to the input signals in Raman amplifier 814. The excess pump signal which remains after pumping Raman amplifier 814, is bypassed around isolator 808 by means of WDMS 810 and 804 and pumps erbium amplifier 804. The pump signal also propagates in a direction opposite to the input signals in amplifier 804. Although a single pumping frequency is shown in FIG. 8, two different frequencies can be used as described above. In another embodiment of the invention as shown in FIG. 9, a Raman amplifier

914 is again used in combination with an erbium doped fiber amplifier 922. In this embodiment, however, pump energy source 902 couples pump energy, via WDM 916, into the erbium amplifier 922 such that it co-propagates with the signal being amplified. The pump source may be centered about a single wavelength or, as in previous embodiments, may be a multiple wavelength source. Unabsorbed pump energy exiting the erbium amplifier 922 is directed, using WDM 924, to WDM 912. WDM 912 couples the recirculated pump energy into Raman amplifier 914 such that it counter-propagates with the signal being amplified. Both WDM 924 and WDM 912 are wavelength selective such that optical energy at the wavelength of the output signal is not diverted by the WDMs, and passes straight from Raman amplifier 914, through isolator 920 to erbium amplifier 922, through isolator 926 and on to output port 930. For example, the pump energy may be at a wavelength of 1470 nm, which is sufficient to pump erbium amplifier to produce gain at a signal wavelength of 1550 nm. The 1470 nm excess pump energy that is returned to the Raman amplifier 914 is also sufficient to pump that amplifier, which provides Raman-shifted gain at the 1550 nm signal wavelength. Such a configuration is particularly useful if the two amplifiers are not physically close to each other or if the Raman amplifier is in the form of a lengthy transmission line along which the Raman gain is distributed. That is, the amplifier 914 may be a portion of a transmission fiber through which the signal being amplified travels. In such a case, the Raman gain is distributed over the portion of the transmission fiber that is adjacent to the coupler 912 and that receives some of the recycled pumping energy before it is fully consumed.

Another embodiment of the invention is shown in FIG. 10. In this embodiment, Raman amplification is combined with erbium amplification to provide a particularly broad gain profile, such as that shown in FIG. 3. In this particular embodiment, two erbium amplifiers 1022, 1023 are used, and one Raman amplifier 1014 is used. The Raman amplifier is pumped by pump energy generated by pump source 1006, which may be, e.g., a laser diode. That pump energy is coupled into the Raman amplifier in a counter-propagating direction by WDM 1012. The Raman amplifier 1014 may therefore be a section of transmission fiber that is capable of producing Raman gain, such that the gain is distributed over a length of the transmission fiber adjacent to the coupler 1012. By proper selection of the wavelength output from pump 1006, a desired gain bandwidth may also be obtained. For example, to have a gain band like that of band 302 of FIG. 3, a Raman pump wavelength of 1470 nm may be used.

After being initially amplified by the Raman amplifier, the optical signal passes through isolator 1020 to erbium-doped amplifier 1022. Amplifier 1022 is pumped by optical energy from pump source 1004, which may have a center wavelength of, e.g., 980 nm. The pump energy from pump source 1004 is coupled into the amplifier gain medium 1022 via WDM 1018. This gives the first erbium amplifier a gain profile that is similar to profile 300 shown in FIG. 3. Thus, the gain profiles of the Raman amplifier 1014 and the erbium amplifier 1022 combine to provide an overall gain profile that is relatively wide. The signal output from gain medium 1022 is directed through a gain flattening filter (GFF) 1008. Such GFFs are known in the art, and provide gain limit across the entire gain spectrum to allow a flatter overall gain profile. In a preferred embodiment, the GFF operates by wavelength selective attenuation of the signal being amplified so that the effective gain in different wavelength bands is approximately equal.

The amplified signal output from the GFF 1008 is passed through isolator 1028, and input to a second erbium amplifier 1023. This amplifier provides additional gain to the signal, and may be pumped at a different wavelength than erbium amplifier 1022. For example, pump source 1002 may provide a pump signal at a wavelength of 1470 nm, coupled into the gain medium 1023 via WDM 1016. Typically, the gain provided by this additional stage would be in the same band as the gain of amplifier 1022. Finally, the output from the gain medium 1023 passes through isolator 1026 to output port 1030.

FIG. 11 shows an amplifier embodiment for which an input signal is first amplified by a Raman preamplifier 1114 that is pumped by pump energy that is recirculated from the output side of the amplifier and coupled into the preamplifier 1114 in a counter-propagating direction via WDM 1112. In the preferred embodiment, the input signal is at a wavelength of 1550 nm, and the pump signal for the Raman preamplifier is at a wavelength of 1470 nm. As in the embodiment of FIG. 9, the preamplifier 1114 may be a distributed amplifier in that it consists of a portion of transmission fiber that is adjacent to the coupler 1112 and that is capable of producing Raman gain. The output of preamplifier 1114 passes through isolator 1120, and into erbium-doped fiber amplifier 1122. Pump energy at a wavelength of 980 nm is generated by pump source 1104, which may be a laser diode, and coupled into the gain medium 1122 via WDM 1118. The signal undergoes additional amplification in the amplifier 1122, and is output through isolator 1128.

After exiting the erbium-doped amplifier stage 1128, the optical signal enters an amplifier stage consisting of an erbium-doped gain medium 1123 and a Raman- shifting gain medium 1132. The Raman gain medium 1132 is pumped with optical pump energy at a wavelength of 1117 nm that is coupled into the gain medium 1132 from pump source 1134 via WDM 1136. Fiber diffraction gratings are located to either side of the Raman gain medium, and create a resonant cavity through the gain medium 1132 at a selected set of Stokes order wavelengths. That is, this portion of the system functions as a "cascaded Raman resonator" (CRR), progressively shifting the initial pump wavelength to longer wavelengths until a desired pumping wavelength of 1470 nm is reached. The general functionality of a CRR is known in the art, and will not be discussed in extensive detail herein.

The diffraction gratings are used to control where various wavelengths resonate. The set of gratings 1138 to the input side of the Raman gain medium 1132 have resonance peaks, respectively, at 1175 nm, 1240 nm, 1311 nm, 1375 nm and 1470 nm. These wavelengths correspond to the first five Stokes orders relative to the pump wavelength 1117 nm. The set of gratings 1140 to the output side of the Raman gain medium 1132 have resonance peaks, respectively, at 1117 nm, 1175 nm, 1240 nm, 1311 nm, and 1375 nm. These wavelengths correspond to the pump wavelength and the first four Stokes orders. Thus, the first four Stokes orders (relative to the pump wavelength) are resonated only through the Raman gain medium 1132. Optical energy at the fifth Stokes order, however, passes the grating set 1140 and is coupled into erbium-doped fiber amplifier 1123. On the output side of amplifier 1123 is diffraction grating 1142, which has a reflection peak at 1470 nm. Thus, the fifth Stokes order resonates through both the Raman gain medium 1132 and the erbium- doped gain medium 1123.

The resonance of the 1470 nm wavelength energy through gain media 1132 and 1123 provides the optical signal (at 1550 nm) to be amplified, both by Raman gain in the gain medium 1132 and by gain produced by pumping of the erbium-doped fiber. Moreover, in the preferred embodiment, the grating 1142 is made only partially (e.g., 80%) reflective at the 1470 nm wavelength, such that some of the 1470 nm optical energy reaches WDM 1144. The WDM 1144 is wavelength selective such that the 1550 nm signal energy is passed to the output port of the system though isolator 1146, while the 1470 nm energy is coupled back to the Raman preamplifier 1114 via WDM 1112, where it is used for pumping purposes, as discussed above.

The embodiment of FIG. 11 is useful from the standpoint of providing gain simultaneously with a Raman gain medium and a rare earth doped gain medium, both of which use the same resonating pump energy. However, the configuration shown also provides the opportunity to provide a wider overall gain bandwidth, by selecting the components, materials and signal wavelengths such that the Raman gain medium and the rare earth doped gain medium each provide gain in adjacent wavelength regions. One way to control these adjacent wavelength regions is by selection of the wavelengths of the fifth Stokes order that are resonated between reflector 1142 and its companion reflector in reflector group 1138. When exposed to pumping energy in the range of 1470 nm, the rare erbium-doped gain medium 1123 will produce gain at an output wavelength of approximately 1550 nm. This output wavelength does not change as the wavelength of the pump energy is changed slightly. In contrast, however, the output wavelength range of the Raman gain medium is dependent on the pumping wavelengths at the fifth Stokes order. Therefore, the fifth Stokes order reflector pair may be selected to resonate pump energy at a wavelength that results in a different gain spectrum for each of the amplifier stages 1132, 1123.

In one example of providing a broad overall gain spectrum with the embodiment of FIG. 11 , a gain profile can be provided such as that shown in FIG. 3. The gain profile 300 of this figure, sometimes referred to as the "C-band," could be provided by the erbium-doped stage 1123 of FIG. 11. Meanwhile the gain profile 302 of FIG. 3, sometimes referred to as the "L-band," could be provided by the Raman stage 1132 of FIG. 11. A resonant pumping energy of approximately 1470 nm would allow the erbium-doped amplifier to provide gain in the C-band, while giving the Raman amplifier stage an output wavelength range in the L-band. In this way, the overall gain bandwidth of the system would be relatively wide and, if used for optical telecommunications, would accommodate a relatively large number of transmission channels. In the embodiment of FIG. 12, an input signal is coupled into a Raman preamplifier 1214 that is pumped by recirculated pump energy. This pump energy is coupled into the preamplifier 1214 in a counter-propagating direction via WDM 1212. In the preferred embodiment, the input optical signal is at a wavelength of about 1550 nm, and the pump signal has a wavelength of approximately 1470 nm. As in previously-discussed embodiments, the preamplifier 1214 may again be a distributed amplifier in that it is actually a portion of a transmission fiber such that the Raman gain provided occurs over a relatively long distance. The signal exiting the preamplifier 1214 passes through isolator 1220 and is coupled into erbium-doped gain medium 1222. The gain medium 1222 is pumped by optical energy at a wavelength of 980 nm that is generated by pump source 1204, which may be a laser diode, and coupled into the gain medium 1222 via WDM 1218. The 1550 nm signal undergoes amplification in the erbium amplifier 1222 and is output through isolator 1228.

The next stage through which the optical signal passes includes an erbium/ytterbium (Er/Yb) doped fiber gain medium 1223. A pump source 1234 supplies pump energy to the gain medium 1223 at a wavelength of 920 nm.

Diffraction gratings 1238 and 1240 are located, respectively, to either side of the gain medium 1223, and preferably have a reflectivity peak at 1470 nm. This results in the development of a resonant condition at a wavelength of 1470, which is within the output wavelength range of the gain medium 1223. The gain medium also provides gain to the 1550 nm signal energy that passes through it. Notably, however, the grating 1240 has only a partial reflectivity (e.g., 80%), such that some of the light at 1470 nm is output from the resonant cavity. WDM 1244 is located subsequent to the grating 1240, and has a wavelength selectivity such that the 1470 nm optical energy is coupled back to the preamplifier 1214 via WDM 1212. However, the signal energy at 1550 nm continues past the WDM 1244 to the system output.

Having described preferred embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may be used. For example, it should be noted that while, in a preferred embodiment, amplifier 10 includes isolator 20, in some applications isolator 20 may be omitted from amplifier 10. The removal of isolator 20 allows the excess pump signal counter-propagating through Er amplifier 22 to continue propagating through WDM 16 and into Raman amplifier 14. In general, this excess pump signal will adversely impact the noise performance of Raman amplifier 14, but, in some circumstances this will be acceptable. Similarly, removal of multistage isolator 20 may cause WDM 24 to receive back reflections from components coupled to amplifier output port 10b.

In addition the inventive multistage amplifier can be used in combination with other amplifiers. For example, the disclosed Raman/erbium-doped amplifier can be used as the second and third stage of a three stage amplifier. The erbium first stage can be coupled to the Raman/erbium-doped amplifier by an isolator.

While the invention has been shown and described above with respect to various preferred embodiments, it will be apparent that the foregoing and other changes of the form and detail may be made therein by one skilled in the art without departing from the spirit and scope of the invention. These and other obvious modifications are intended to be covered by the following claims.

What is claimed is:

Claims

1. An optical fiber amplifier apparatus comprising: a first optical fiber amplifier stage having a Raman gain medium and amplifying an optical input signal to produce an intermediate signal with an optical signal power that is higher than that of the input signal; a second optical fiber amplifier stage receiving and amplifying the intermediate signal to produce an output signal with an optical signal power higher than that of the intermediate signal, the second optical fiber amplifier having an active gain medium doped with one or more rare earth materials; and an optical pumping apparatus that couples optical pump energy into the second amplifier stage such that it co-propagates with the intermediate signal and couples a portion of pump energy that passes through the second stage without being absorbed into the first amplifier stage such that it counter- propagates with the optical input signal.

2. An apparatus according to Claim 1 wherein the optical pump energy has a single wavelength peak.

3. An apparatus according to Claim 1 wherein the optical pump energy has multiple wavelength peaks.

4. An optical fiber amplifier apparatus comprising: a first optical fiber amplifier stage having a Raman gain medium and amplifying an optical input signal to produce an intermediate signal with an optical signal power that is higher than that of the input signal, the Raman gain medium providing gain in a first wavelength band centered about a first wavelength; a second optical fiber amplifier stage receiving and amplifying the intermediate signal to produce an output signal with an optical signal power higher than that of the intermediate signal, the second optical fiber amplifier having an active gain medium doped with one or more rare earth materials and providing gain in a second wavelength band centered about a second wavelength significantly different from the first wavelength; and a gain adjusting filter that provides wavelength selective filtering that results in a signal output from the amplifier apparatus having effective gain that is approximately equal in each of the first and second wavelength bands.

5. An apparatus according to Claim 4 further comprising a third amplifier stage having an active gain medium doped with one or more rare earth materials and providing gain in the second wavelength band.

6. An apparatus according to Claim 5 wherein the filter is located between the second amplifier stage and the third amplifier stage.

7. An apparatus according to Claim 5 further comprising a pump apparatus that provides optical pumping energy to the third stage and that couples a portion of pump energy that passes through the third stage without being absorbed into the first amplifier stage such that it counter-propagates with the optical input signal.

8. An apparatus according to Claim 7 further comprising a resonance reflector pair that develops a resonance condition within the third stage for optical energy having a peak wavelength within a pump absorption band of a gain medium of the third stage.

9. An apparatus according to Claim 8 wherein a reflector of the reflector pair is partially reflective and a portion of optical energy within the peak wavelength range exits the third stage and is coupled to the first amplifier stage.

10. An apparatus according to Claim 7 wherein the first amplifier stage comprises an optical transmission link.

11. An apparatus according to Claim 4 further comprising a pump apparatus that provides optical pumping energy to the second stage and that couples a portion of pump energy that passes through the second stage without being absorbed into the first amplifier stage such that it counter-propagates with the optical input signal.

12. An apparatus according to Claim 11 further comprising a resonance reflector pair that develops a resonance condition within the third stage for optical energy having a peak wavelength within a pump absorption band of a gain medium of the third stage.

13. An apparatus according to Claim 12 wherein a reflector of the reflector pair is partially reflective and a portion of optical energy within the peak wavelength range exits the third stage and is coupled to the first amplifier stage.

14. An apparatus according to Claim 4 wherein the gain adjusting filter operates by wavelength selective attenuation.

15. An optical fiber amplifier apparatus comprising: a first optical fiber amplifier stage having a Raman gain medium and amplifying an optical input signal to produce a intermediate signal with an optical signal power that is higher than that of the input signal, the Raman gain medium providing gain in a first wavelength range and a second wavelength range that is a Stokes order relative to the first wavelength range; a second optical fiber amplifier stage receiving and amplifying the intermediate signal to produce an output signal with an optical signal power higher than that of the intermediate signal, the second optical fiber amplifier having an active gain medium doped with one or more rare earth materials and providing gain in a third wavelength range in response to optical energy in the first wavelength range; and a reflector pair that develops a resonant condition through the Raman gain medium and the rare earth doped gain medium for optical energy in the first wavelength range.

16. An apparatus according to Claim 15 further comprising a preamplifier stage through which the optical input signal passes prior to entering the first amplifier stage, the preamplifier stage comprising a Raman gain medium.

17. An apparatus according to Claim 16 wherein a reflector of the reflector pair is partially reflective, and a portion of the resonant optical energy at the first wavelength escapes the resonant cavity, and wherein the apparatus further comprises an optical coupler that couples the escaping portion of the resonant optical energy into the preamplifier stage.

18. An apparatus according to Claim 17 wherein the escaping portion of the resonant optical energy is coupled into the preamplifier stage such that it counter-propagates with the optical input signal.

19. An apparatus according to Claim 16 wherein the preamplifier stage comprises an optical transmission link.

20. An apparatus according to Claim 15 wherein the first amplifier stage is part of a cascaded Raman resonator.

21. An apparatus according to Claim 15 further comprising an additional rare earth doped amplifier stage through which the optical input signal passes prior to being introduced to the first amplifier stage.

22. An apparatus according to Claim 21 further comprising a preamplifier stage through which the optical input signal passes prior to entering the additional rare earth doped amplifier stage, the preamplifier stage comprising a Raman gain medium.

23. An apparatus according to Claim 22 wherein a reflector of the reflector pair is partially reflective, and a portion of the resonant optical energy at the first wavelength escapes the resonant cavity, and wherein the apparatus further comprises an optical coupler that couples the escaping portion of the resonant optical energy into the preamplifier stage.

24. An apparatus according to Claim 23 wherein the first amplifier stage is part of a cascaded Raman resonator.

25. An apparatus according to Claim 15 wherein the third wavelength range overlaps with the second wavelength range.

26. An apparatus according to Claim 15 wherein the third wavelength range is approximately adjacent in wavelength to the second wavelength range such that the second and third wavelength ranges together provide the apparatus with a gain bandwidth that is larger than either of the second and third wavelength ranges taken individually.

27. An optical fiber amplifier apparatus comprising: an optical fiber amplifier having a Raman gain medium and amplifying an optical input signal to produce an output signal; an optical pumping source that provides the Raman gain medium with optical pumping energy having multiple wavelength peaks at each of which the optical pumping energy is at least partially absorbed by the Raman gain medium.

28. An apparatus according to Claim 27 wherein the optical pumping source comprises a plurality of optical sources that each have a different wavelength peak.

29. An apparatus according to Claim 28 further comprising an optical coupling apparatus that couples together the optical pumping energy from the plurality of optical sources.

30. An apparatus according to Claim 27 wherein the optical pumping energy provides the Raman gain medium with a single, relatively wide gain bandwidth.

31. An apparatus according to Claim 27 wherein the optical pumping source couples the optical pumping energy into the Raman gain medium such that it counter-propagates with the optical input signal.

32. A method of amplifying an optical input signal, the method comprising: coupling the optical input signal into a first optical fiber amplifier stage having a Raman gain medium to produce an intermediate signal with an optical signal power that is higher than that of the input signal; coupling the intermediate signal into a second optical fiber amplifier stage to produce an output signal with an optical signal power higher than that of the intermediate signal, the second optical fiber amplifier having an active gain medium doped with one or more rare earth materials; and pumping the first and second amplifier stages with an optical pumping apparatus that couples optical pump energy into the second amplifier stage such that it co-propagates with the intermediate signal and couples a portion of pump energy that passes through the second stage without being absorbed into the first amplifier stage such that it counter-propagates with the optical input signal.

33. A method according to Claim 32 wherein the optical pump energy has a single wavelength peak.

34. A method according to Claim 32 wherein the optical pump energy has multiple wavelength peaks.

35. A method of amplifying an optical input signal, the method comprising: coupling the input signal into a first optical fiber amplifier stage having a Raman gain medium to produce an intermediate signal with an optical signal power that is higher than that of the input signal, the Raman gain medium providing gain in a first wavelength band centered about a first wavelength; coupling the intermediate signal into a second optical fiber amplifier stage that receives and amplifies the intermediate signal to produce an output signal with an optical signal power higher than that of the intermediate signal, the second optical fiber amplifier having an active gain medium doped with one or more rare earth materials and providing gain in a second wavelength band centered about a second wavelength significantly different from the first wavelength; and adjusting the gain bandwidth of the apparatus with a gain adjusting filter that provides wavelength selective filtering to provide a signal output from the amplifier apparatus with an overall gain that is approximately equal in each of the first and second wavelength bands.

36. A method according to Claim 35 further comprising coupling the signal output from the second amplifier stage into a third amplifier stage having an active gain medium doped with one or more rare earth materials and providing gain in the second wavelength band.

37. A method according to Claim 36 wherein the filter is located between the second amplifier stage and the third amplifier stage.

38. A method according to Claim 36 further comprising coupling optical pump energy into the third stage and that couples a portion of pump energy that passes through the third stage without being absorbed into the first amplifier stage such that it counter-propagates with the optical input signal.

39. A method according to Claim 38 further comprising developing, with a resonance reflector pair, a resonance condition within the third stage for optical energy having a peak wavelength within a pump absorption band of a gain medium of the third stage.

40. A method according to Claim 39 wherein a reflector of the reflector pair is partially reflective and a portion of optical energy within the peak wavelength range exits the third stage and is coupled to the first amplifier stage.

41. A method according to Claim 35 further comprising coupling optical pump energy into the second stage with a pump apparatus that directs a portion of pump energy that passes through the second stage without being absorbed into the first amplifier stage such that it counter-propagates with the optical input signal.

42. A method according to Claim 41 further comprising developing, with a resonance reflector pair, a resonance condition within the third stage for optical energy having a peak wavelength within a pump absorption band of a gain medium of the third stage.

43. A method according to Claim 42 wherein a reflector of the reflector pair is partially reflective and a portion of optical energy within the peak wavelength range exits the third stage and is coupled to the first amplifier stage.

44. A method according to Claim 41 wherein the first amplifier stage comprises an optical transmission link.

45. A method according to Claim 35 wherein the gain adjusting filter operates by wavelength selective attenuation.

46. A method of amplifying an optical input signal, the method comprising: coupling the input signal into a first optical fiber amplifier stage having a Raman gain medium that amplifies the optical input signal to produce a intermediate signal with an optical signal power that is higher than that of the input signal, the Raman gain medium providing gain in a first wavelength range and a second wavelength range that is a Stokes order relative to the first wavelength range; coupling the intermediate signal into a second optical fiber amplifier stage that receives and amplifies the intermediate signal to produce an output signal with an optical signal power higher than that of the intermediate signal, the second optical fiber amplifier having an active gain medium doped with one or more rare earth materials and providing gain in a third wavelength range in response to optical energy in the first wavelength range; and developing a resonant condition through the Raman gain medium and the rare earth doped gain medium for optical energy in the first wavelength range.

47. A method according to Claim 46 further comprising coupling the input signal through a preamplifier stage prior to its entering the first amplifier stage, the preamplifier stage comprising a Raman gain medium.

48. A method according to Claim 47 wherein a reflector of the reflector pair is partially reflective, and a portion of the resonant optical energy at the first wavelength escapes the resonant cavity, and wherein the apparatus further comprises an optical coupler that couples the escaping portion of the resonant optical energy into the preamplifier stage.

49. A method according to Claim 48 wherein the escaping portion of the resonant optical energy is coupled into the preamplifier stage such that it counter- propagates with the input optical signal.

50. A method according to Claim 47 wherein the preamplifier stage comprises an optical transmission link.

51. A method according to Claim 46 wherein the first amplifier stage comprises a cascaded Raman resonator.

52. A method according to Claim 46 further comprising coupling the optical input signal through an additional rare earth doped amplifier stage prior to its being introduced to the first amplifier stage.

53. A method according to Claim 52 further comprising coupling the input signal through a preamplifier stage prior to its entering the additional rare earth doped amplifier stage, the preamplifier stage comprising a Raman gain medium.

54. A method according to Claim 53 wherein a reflector of the reflector pair is partially reflective, and a portion of the resonant optical energy at the first wavelength escapes the resonant cavity, and wherein the method further comprises coupling the escaping portion of the resonant optical energy into the preamplifier stage.

55. A method according to Claim 54 wherein the first amplifier stage is part of a cascaded Raman resonator.

56. A method according to Claim 46 wherein the third wavelength range overlaps with the second wavelength range.

57. A method according to Claim 46 wherein the third wavelength range is approximately adjacent in wavelength to the second wavelength range such that the second and third wavelength ranges together provide the apparatus with a gain bandwidth that is larger than either of the second and third wavelength ranges taken individually.

58. A method according to Claim 46 wherein said resonant condition is developed using a reflector pair located to either side of the Raman gain medium and the rare earth doped gain medium.

59. A method of amplifying an optical input signal, the method comprising: coupling the input signal into an optical fiber amplifier having a Raman gain medium and amplifying the optical input signal to produce an output signal; pumping the Raman gain medium with optical pumping energy having multiple wavelength peaks each of which is absorbed by the Raman gain medium.

60. A method according to Claim 59 wherein the optical pumping energy is provided by a plurality of optical sources that each has a different wavelength peak.

61. A method according to Claim 60 further comprising coupling together the optical pumping energy from the plurality of optical sources prior to its being coupled into the Raman gain medium.

62. A method according to Claim 59 wherein the multiple wavelength peaks of the pumping energy provide the Raman gain medium with a single, relatively wide gain bandwidth.

63. A method according to Claim 59 wherein the optical pumping source couples the optical pumping energy into the Raman gain medium such that it counter- propagates with the optical input signal.