Abstract:
An optical gain device preferably uses stimulated Raman scattering and is in a ring configuration. Fused fiber couplers are used in one embodiment in combination with wavelength selective reflectors to input pump energy to the ring, and to output the desired output wavelengths. The couplers and reflectors are arranged so that the relative phase shifting induced by the couplers allows the optical energy to be directed as desired. The system may also make use of a redirecting reflector at the output wavelength that keeps output light in the ring propagating in the proper direction. A plurality of reflectors may also be used in the ring to select the desired wavelengths at the different Stokes orders being used. In an alternative embodiment, thin film type couplers are used rather than fused fiber couplers. The thin film type couplers may include separate input and output devices, each with its own cutoff wavelength, or a single coupler may include two wavelength selective materials, and accomplish both the input and the output coupling functions. A single wavelength selective coupler is also provided that accomplishes both the input and the output coupling functions.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to optical gain devices and, more specifically, to optical gain devices using stimulated Raman scattering. 
     BACKGROUND OF THE INVENTION 
     An optical gain medium is a device that, when provided with pumping energy, increases the amplitude of a desired optical signal. Optical gain media may be constructed using optical fiber, and used as fiber lasers or fiber-based optical amplifiers. One form of optical gain mechanism known in the art is based on stimulated Raman scattering (SRS). In such a device, optical pumping energy is injected into an optical fiber medium. The optical pumping energy, via SRS, allows for a transfer of optical power to a wavelength longer than the pumping wavelength, due to the excitation of a vibrational mode in the medium that provides gain at the longer wavelength. 
     The longer wavelengths to which optical power is transferred may be predetermined relative to the wavelength of the pumping energy. Each wavelength shift is referred to as a “Stokes” shift and, since it is a known amount, the resulting wavelength may be selected by proper selection of the pumping wavelength. Some Raman gain media use only a single wavelength shift to produce optical energy at a desired wavelength. In another type of Raman device, overlapping resonant cavities are constructed for a number of shifted wavelengths all based on the same initial pumping wavelength. That is, the shifted wavelength resulting from the pumping wavelength, referred to as the “first Stokes order” is resonated within the gain medium, generating its own shifted wavelength that is a predetermined amount longer than the first order wavelength. This “second order” wavelength is, in turn, resonated within the cavity to generate a “third Stokes order” wavelength. By accurate selection of the different resonant reflectors and the initial pumping wavelength, a number of Stokes shifts may be used to generate an optical signal at a desired wavelength significantly longer than the pumping wavelength. This type of optical device is generally referred to as a “cascaded Raman resonator” (CRR), and an example of such a device is described in U.S. Pat. No. 5,323,404. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an optical gain apparatus is provided that includes a gain medium that produces optical gain in response to optical energy at a pump wavelength. In a preferred embodiment, the gain medium provides gain via SRS, and the gain is therefore at a wavelength longer than the pump wavelength. A plurality of fused fiber couplers are used, including one coupled to an input port of the system and one coupled to an output port. The fused fiber couplers each have two signal paths on each of a first side and a second side. Each coupler is arranged such that signals directed to the coupler along the first and second signal paths exit the coupler along the third and fourth signal paths, and vice versa. The first coupler has its first path coupled to an input port and its second path coupled to a first optical side of the gain medium. The second coupler has its fourth path coupled to an output port and its third path coupled to a second optical side of the gain medium. 
     In the third and fourth signal paths of the first coupler are located input reflectors, each of which is reflective at an input wavelength. Thus, optical energy at the input wavelength that is directed into the first coupler along its first signal path is directed to both input reflectors and, as such, is directed back into the first coupler. These two signal portions are in phase only along the second signal path of the first coupler, and all of the reflected energy is therefore coupled along the second signal path toward the gain medium. Similarly, in the first and second signal paths of the second coupler are located output reflectors, each of which is reflective at an output wavelength. Thus, optical energy at the output wavelength that is directed from the gain medium into the third signal path of the second coupler is directed to both output reflectors and, as such, is directed back into the second coupler. The two signal portions reflected back to the second coupler are in phase only along the fourth signal path, and all of the reflected energy is therefore coupled toward the output port of the system. The optical energy not reflected by the output reflectors is coupled back toward the gain medium. 
     In the preferred embodiment, the optical gain apparatus is configured as an optical ring, such that the optical energy travels continuously through the ring in one direction until the output wavelength is reached, and the light is reflected by the output reflectors toward the system output port. Light not at the output wavelength bypasses the output reflectors and remains in the ring. In a preferred embodiment, the light that bypasses the output reflectors is coupled through two more fused fiber couplers. This light is then directed toward the input reflectors adjacent to the first coupler, but approaches from the side of the input reflectors opposite the first coupler. This light bypasses the input reflectors and, being at the correct relative phase, is all coupled through the first fused coupler, and out its second signal path toward the first optical side of the gain medium. 
     In one variation of the invention, a reflector is used in the ring structure that is reflective at the output wavelength. This reflector is positioned so that any light at the output wavelength that is traveling opposite to the desired ring direction is reflected back to the desired direction. In another variation, a plurality of reflectors are used in the ring, each of which has a narrow band reflectivity at a different Stokes order relative to the input wavelength. Thus, when the apparatus is functioning as a CRR, the different reflectors allow the wavelength at each different Stokes order to be selected with precision. 
     In an alternative embodiment of the invention, a ring configuration is used, but without the fused fiber couplers. The ring includes a Raman gain medium, and operates as a CRR. Wavelength selective couplers, preferably thin film type couplers, are used to couple input optical energy into the ring and to couple output optical energy out of the ring. In the preferred version of this embodiment, the input coupler has a cutoff wavelength, such that wavelengths significantly longer than that of the input optical energy are prevented from passing through the coupler, while the input wavelength passes through unimpeded. Similarly, the output coupler has a cutoff wavelength such that wavelengths significantly shorter than a desired output wavelength are prevented from passing through the coupler, while optical energy at the output wavelength passes out through the coupler to an output port. Shorter wavelengths are maintained in the ring, and continue propagating through the Raman gain medium. The couplers may be arranged such that input and output light is transmitted through the coupler filter elements into and out of the ring, respectively, or they may be arranged so that the input and output light is reflected by the coupler filter elements into and out of the ring. As in aforementioned embodiments, a reflector may be used at the wavelength of the output light to maintain a single direction of propagation through the ring. Likewise, one or more reflectors in the ring may be used to select intermediate Raman wavelengths at the various Stokes orders being used with the system, if the device is being used as a CRR. 
     In accordance with the invention, a single coupler is provided that accomplishes both the input and the output functions of a thin film type coupler embodiment. This coupler has a pair of graded index (GRIN) lenses separated by a filter element. The filter element includes two separate materials, each of which has a different wavelength filter function. In one embodiment of the invention, the materials are coatings on opposite surfaces of the filter element. A first one of the materials may have a “high-pass” type wavelength characteristic, such that wavelengths at or below a first cutoff wavelength coupled into a first optical path of the coupler are reflected into a second optical path by the first material, while longer wavelengths are transmitted through it. Similarly, the other material may have a “low-pass” type wavelength characteristic, such that wavelengths at or above a second cutoff wavelength coupled into a third optical path of the coupler are reflected into a fourth optical path by the second material. Wavelengths below the second cutoff wavelength that enter along the third optical path are transmitted through the filter element to a desired optical path, such as the second optical path of the coupler. With this arrangement, the filter element has an overall reflectivity characteristic that allows optical energy between the first and second cutoff wavelengths to be trapped within an optical ring, continuously coupled between the third optical path and the second optical path. Meanwhile, optical energy with wavelengths at or below the first cutoff wavelength may be coupled into the ring using the reflectivity of the first material, and optical energy with wavelengths at or above the second cutoff wavelength may be coupled out of the ring using the reflectivity of the second material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which: 
     FIG. 1 is a schematic view of an optical gain apparatus using fused fiber couplers to introduce an input signal and collect an output signal; 
     FIG. 1A is a schematic view of a portion of a fused fiber coupler as used with the embodiment of FIG. 1, showing different coupling paths and signal phases; 
     FIG. 2 is a schematic view of an embodiment similar to that of FIG. 2 in which one or more reflectors are used in the system to redirect light and/or select desired wavelengths; 
     FIG. 3A is a schematic view of an alternative embodiment of the system that uses thin film type input and output couplers via which input and output wavelengths are transmitted through the couplers into and out of an optical ring, respectively; 
     FIG. 3B is a schematic view of an embodiment similar to that of FIG. 3A in which one or more reflectors are used in the system to redirect light and/or select desired wavelengths; 
     FIG. 4A is a schematic view of another embodiment of the invention in which thin film type input and output couplers are again used, but in which the couplers are such that the input and output wavelengths are reflected into and out of an optical ring, respectively; 
     FIG. 4B is a schematic view of an embodiment similar to that of FIG. 4A in which one or more reflectors are used in the system to redirect light and/or select desired wavelengths; 
     FIG. 5 is a schematic view of an embodiment similar to that of FIG. 4B, but in which a single coupler accomplishes both the input and the output functions of the system; and 
     FIG. 5A is a schematic view of an optical coupler like that used in the embodiment of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Shown in FIG. 1 is a schematic view of an embodiment of the invention in which a Raman amplifier makes use of a ring configuration. An input signal at a wavelength of λ 0  is injected into a fiber ring via input port  12 . The light at the input port  12  is coupled into the system by a fused fiber coupler  14  that, in the present embodiment, is a 3 dB or “50/50” coupler. That is, the λ 0  light is split roughly equally between the two fibers of the coupler, with each portion being directed toward one of fiber Bragg gratings  16 ,  18 . However, as described below, the coupler separates the light by relative phase shift, allowing appropriate use of the couplers to direct the light as desired. 
     FIG. 1A shows an isolated view of the coupler  14  of FIG. 1, with arrows indicating the relative portions of light entering and exiting the coupler. The light is initially coupled into the input port  12 , and that input branch is therefore labeled in the figure as “100%-0 deg.” This label indicates that 100% of the light is input to the coupler along this branch and that it has a relative phase shift of zero. Those skilled in the art will recognize that the absolute phase shift of the light in this context is irrelevant, provided that the relative phase shift is known. This initial light is therefore arbitrarily identified as having a phase shift of zero degrees. As the light passes through the coupler  14 , it is divided into two portions roughly equal in power. As shown in FIG. 1A, one of these portions is directed into the opposite fiber than that containing port  12 , and maintains the same relative phase. This is therefore identified in the figure by the label “50%-0 deg.” However, as is known in the art of such fiber couplers, the portion of the light that passes through the coupler, but stays in the same fiber as contains port  12 , undergoes a 90° phase shift, and therefore exits the coupler with a relative phase (relative to the input signal) of 90°. This is identified in the figure by the label “50%-90 deg.” 
     Each of gratings  16 ,  18  is highly reflective at the input wavelength λ 0 . Thus, each portion of light exiting the coupler  14  toward the gratings  16 ,  18  is reflected back toward the coupler  14  by its respective grating. The relative phase shift of these two portions is maintained, such that the two light portions reenter the coupler at a 90° phase shift relative to each other. The light portion reflected from grating  16  reenters the coupler, and the portion of it that is coupled into the same fiber as contains grating  16  enters the ring, and is subject to a 90° phase shift. If any portion of the light were to be coupled into the other fiber (i.e., toward input port  12 ), no additional phase shift would be introduced. As for the light reflected from grating  18 , it reenters the coupler, and that portion of the light that crosses over to the other fiber is not subject to any additional phase shift. As such, this light is in phase with the light reflected from grating  16 , and enters the ring. Any portion of the light reflected from grating  18  that would stay within the same fiber upon reentering the coupler  14  (i.e., and be directed back toward input port  12 ), would be subjected to an additional phase shift of 90°. Thus, the overall phase shift of any such light relative to the original input signal would be 180°. 
     From the description above, it can be seen that light that might be reflected from grating  16  and directed back toward the input port  12  would be 180° out of phase with the light that might be reflected from grating  18  and directed toward input port  12 . Thus, the two portions have a canceling effect on each other, the result of which is that no light reflected from either of the gratings  16 ,  18  is redirected through the coupler along the path containing input port  12 . As such, substantially all of the input light is reflected by gratings  16 ,  18  into the ring, albeit at a phase shift of 90° relative to the light input at port  12 . This is indicated in FIG. 1A by the label “100%-90 deg.” located adjacent to an arrow indicating the coupling of light into the ring. 
     The light coupled into the ring from coupler  14  enters gain medium  20 , as shown in FIG.  1 . Although gain medium  20  is shown as a separate coil of fiber in the figure, those skilled in the art will recognize that the entire ring may have the capacity to provide gain to the input light. In the preferred embodiment, the gain is provided via SRS, as the input light is shifted to higher wavelengths. An example of an appropriate gain medium  20  is a single mode fiber that is highly doped with germanium (Ge 3+ ), and is of relatively long length (e.g., 500-1000 meters long). The light passing through the ring reaches coupler  22 , which is of essentially the same construction as coupler  14 . 
     Here the light passes through the coupler, and is divided between the two output fibers at the opposite side of the fiber, and subjected to the resultant phase shifts. 
     Diffraction gratings  24 ,  26  are located in the signal paths to one side of the coupler  22 . In the preferred embodiment, each of these gratings is the same, and has a reflectivity centered about a desired output wavelength of the device. Other wavelengths pass through the gratings unreflected. For the light entering the coupler  22 , the portion exiting through the same fiber as that from which it entered the ring (i.e., the fiber containing grating  24 ), undergoes a 90° phase shift. The light that crosses over to the other fiber (i.e., the fiber containing grating  26 ) maintains the original phase. Any light that is not reflected by gratings  24 ,  26  passes through to fused fiber coupler  28 , to which is connected fiber connection  30 . 
     The portion of light passing from grating  24  to fiber  30  does not undergo an additional phase shift, but maintains the 90° phase shift relative to the light entering coupler  22 . The portion of light passing from grating  26  to fiber  30  was not shifted in phase by coupler  22 , but undergoes a 90° phase shift when it passes through coupler  28 . Thus, the light from grating  26  is in phase with the light from grating  26  as it enters fiber  30 , and is constructively added. However, light from grating  24  that might pass through to the other signal path of the coupler  28  would be 180° out of phase with light from grating  26  that was to pass over to that signal path. That is, light from grating  24  would be subjected to an additional 90° phase shift, giving it a total phase of 180° relative to the light entering coupler  22 . Meanwhile, light from grating  26  would have no additional phase shift, and would therefore remain unshifted in phase relative to the light entering coupler  22 . As such, all of the light entering coupler  28  exits into fiber  30 , since the opposite port suffers from an opposite phase condition that results in a totally destructive interference effect. 
     From the above description, it will be apparent to those skilled in the art that, when two fused couplers of this type are connected one to the other, and light is coupled into one port of one of the couplers, it will all exit from a single port of the other coupler. More particularly, if the two 3 dB couplers are viewed as two fibers fused at two points, the phase shifting provided by the couplers results in light coupled into a first coupler via a first fiber to exit a second coupler into the opposite fiber. Thus, light entering coupler  22  via the port connected to the gain medium  20 , exits into the fiber  30 , albeit with a phase shift of 90°. Moreover, light in the fiber  30 , all having the same relative phase, passes through fused fiber coupler  32 , is separated between the paths passing through gratings  16 ,  18 , and is recombined at the port of coupler  14  that connects to gain medium  20 . Specifically, the light passing from fiber  30  through grating  18  undergoes a 90° phase shift in coupler  32 , but no phase shift as it passes through coupler  14  back into the ring. The light passing from fiber  30  through grating  16 , however, undergoes no phase shift in coupler  32 , but undergoes a 90° phase shift as it passes through the coupler.  14 . The opposite phase condition at the port of coupler  14  leading to input port  12  prevents any of this light from being coupled out in that direction. 
     Given the construction shown in FIG. 1, input light at λ 0  is coupled into the ring as described above, and is progressively shifted upward in wavelength through various Stokes levels until a desired output wavelength λ 0  is reached. This is the wavelength at which the reflectivity of gratings  24 ,  26  is centered. Assuming the output wavelength is more than one Stokes shift longer than the input wavelength, the system will operate as a cascaded Raman resonator. Thus, by the time the Raman-shifted wavelength λ n  is reached by the light circulating in the ring, it enters the coupler  22 , and is divided into two portions, one directed toward grating  24  and one directed toward grating  26 . Due to the nature of the coupler, the portion of light directed toward grating  26  is not shifted in phase. However, the light directed toward coupler  24  is shifted 90°. The λ n  light is reflected by both grating  24  and grating  26 , and reenters coupler  22 . Light directed toward output port from grating  24  retains its 90° phase shift, but is not further shifted in phase as it passes through the coupler. The light directed to port  34  from grating  26  undergoes a 90° phase shift, such that it is in phase with the light from grating  24 . However, the light from grating  24  that would be redirected toward the gain medium  20  from coupler  22  would undergo a second phase shift, and would therefore be 180° out of phase relative to the light that would return to that same port from grating  26 . As such, a cancellation condition exists in that direction, and all of the light reentering the coupler is output via port  34 . 
     The arrangement of FIG. 1 allows for Raman-shifted gain over a predetermined number of Stokes levels. For example, with an input wavelength λ 0 =1117 nm (at which the gratings  16 ,  18  would have their peak reflectivity), and gratings  24 ,  26  having a peak reflectivity at λ n =1480 nm, the system would function as a cascaded Raman resonator, and would shift from the 1117 nm input wavelength through intermediate Stokes levels at 1175 nm, 1240 nm, 1315 nm and 1395 nm until ultimately reaching the desired output wavelength of 1480 nm. It will be understood that it is not necessary that λ n  be at any particular Stokes order. The selectivity of gratings  24  and  26  may be used to select the desired output wavelength. For example, given an input wavelength λ 0  of 1117 nm, an output wavelength λ n  may be selected at the third order of 1315 nm, or even the first order of 1175 nm, by choosing the wavelength selectivity of the gratings  24 ,  26 . Indeed, any desired wavelength within the range of the gain medium  18  may be output by proper selection of the input wavelength λ 0  and the wavelength selectivity of gratings  16 ,  18 ,  24 ,  26 . 
     Shown in FIG. 2 is an alternative embodiment of the invention having essentially the same structure as that shown in FIG. 1, except that an additional grating set is provided. Grating set  36  is located in fiber  30  and, in the preferred embodiment, includes highly reflective diffraction gratings at each of the wavelengths of the intermediate Stokes orders. For example, using wavelengths from the example described above, the grating set would include wavelengths at each of 1175 nm, 1240 nm, 1315 nm and 1395 nm. Use of this grating set would ensure that the tuning of the system for each of the Stokes orders would be at the desired wavelengths, since the grating set would act to select each of those wavelengths for resonance through the system. Although, in the system of FIG. 1, the input wavelength would step progressively through Stokes wavelengths, the use of the gratings  36  in FIG. 2 makes the selection of those intermediate wavelengths more precise, and helps to minimize wavelength drift. In the preferred embodiment, the gratings are combined together in a single grating structure having the desired selectivity peaks. However, those skilled in the art will recognize that multiple discrete gratings may also be used. 
     In another embodiment associated with FIG. 2, the grating set  36  could contain a single grating having a peak reflectivity at λ n  that is located so as to reflect counter-clockwise (relative to the orientation of the figure) optical energy at λ n . With a grating located in this fashion, any optical energy that was generated in the ring at λ n  in the counter-clockwise direction would be reflected back in the clockwise direction, and would therefore follow the appropriate route to output port  34 , as described above in conjunction with FIG.  1 . 
     FIG. 3A shows another embodiment of the invention in which a Raman gain medium  20  is used to amplify an input signal. The signal is input via input port  12 , and is directed to input coupler  40 . The coupler  40  is a wavelength selective input coupler having a “low pass” filter characteristic, in a wavelength sense. That is, it passes shorter wavelengths, while rejecting longer ones. The wavelength cutoff for filter  40  is set to accept wavelengths at the input signal wavelength and shorter, while rejecting the first Stokes order wavelength of the input signal and higher. That is, the cutoff must be at a wavelength long enough to accept the input signal, but short enough to reject the next Raman-shifted wavelength. For example, if the input signal had a wavelength of 1117 nm, the first Raman order would be at a wavelength of 1175 nm. Therefore, a cutoff wavelength of about 1150 nm, for example, would allow the unimpeded input of the signal λ 0 , while preventing the 1175 nm wavelength from exiting toward the ring toward input port  12 . 
     Like input coupler  40 , output coupler  42  is also wavelength selective. However, the output coupler  42  has a “high pass” type filter characteristic, such that it passes longer wavelengths to output port  34 , while maintaining shorter wavelengths in the ring. The cutoff wavelength of the filter  42  is set so as to allow the unimpeded passing of the output signal λ n , while not coupling out any shorter wavelengths. This cutoff wavelength could therefore be, for example, in the vicinity of a midpoint between the output wavelength and the next lowest Raman order. For example, if λ n =1480 nm, than the next lowest Raman order would be 1395 nm. Thus, an appropriate cutoff wavelength for the filter might be at about 1440 nm. In this configuration, the input signal would enter the ring, and propagate through it, undergoing progressive Raman shifts, until it reached the cutoff wavelength of the output coupler. At this wavelength, the optical energy would be transmitted through the filter element of the output coupler  42 , and exit as an output signal directed to output port  34 . 
     In one embodiment of the invention, the wavelength selective couplers of FIG. 3A each make use of a thin film filter type coupler. Such devices are known in the art, and provide coupling of light that is wavelength selective relative to a cutoff wavelength. Typically, these couplers have a filter component that is reflective to wavelengths above or below a particular cutoff wavelength. The filter component has graded index (GRIN) lenses to either side of it to allow coupling into and out of the appropriate optical fibers. Optical energy that enters the coupler through a first fiber, and is reflected by the filter, is directed to a different fiber by a GRIN lens. 
     In the configuration of FIG. 3A, the wavelengths between the cutoff wavelengths of the input coupler  40  and the output coupler  42 , respectively, continue to propagate through the ring. The wavelength selective element of the coupler  40  is reflective to wavelengths above a wavelength in the range of λ 0 , thus allowing the transmission of an input signal from input port  12  into the ring, while providing reflection of higher Stokes orders, so that they are maintained in the ring. This wavelength characteristic is indicated in the figure by the arrow showing λ 0  being coupled into the ring, and the double-headed arrow indicating that wavelengths longer than the input wavelength (i.e., λ&gt;λ 0 ) are reflected by the coupler  40  back into the ring. Similarly, the wavelength selective element in coupler  42  is reflective to wavelengths below a wavelength in the range of λ n , thus allowing the coupling out of the desired output wavelength, while maintaining lower Stokes orders in the ring. This wavelength characteristic is indicated in the figure by the arrow showing λ n  being coupled out of the ring, and the double-headed arrow indicating that wavelengths shorter than the output wavelength (i.e., λ&lt;λ n ) are reflected by coupler  42  back into the ring. 
     Shown in FIG. 3B is an embodiment similar to that of FIG. 3, using similar wavelength selective input and output couplers  40  and  42 . In fact, the configuration of the FIG. 3B embodiment is the same as that of FIG. 3A except for the use of grating set  36 . As in FIG. 2, the grating set  36  may include highly reflective diffraction gratings at each of the wavelengths of the intermediate Stokes orders. Again, use of this grating set ensures that the tuning of the system for each of the Stokes orders would be at the desired wavelengths, since the grating set would act to select each of those wavelengths within the system. For example, gratings of grating set  36  in the FIG. 3B embodiment might include each of 1175 nm, 1240 nm, 1315 nm and 1395 nm, given an input wavelength of 1117 nm. Use of the grating set makes the selection of these intermediate wavelengths more precise, helping to minimize wavelength drift. 
     The embodiment of FIG. 4A is similar to that of FIG. 3A, except that thin film type couplers  44  and  46 , used as input and output couplers, respectively, have filter characteristics that allow input and output signals to be reflected by them into the ring, rather than transmitted through them. The couplers  44 ,  46  are similar to couplers  40 ,  42  of the FIG. 3A embodiment, except that different filter elements allow them to be used in a different configuration. In the case of coupler  44 , the wavelength selective element of the coupler reflects wavelengths at and below a wavelength of λ 0 . Thus, an input signal directed from input port  12  is reflected into the ring by the wavelength selective element of the coupler, while higher Stokes orders in the ring pass through the coupler, remaining within the ring. This is indicated in the figure by the arrow showing λ 0  being coupled into the ring, and the double-headed arrow showing wavelengths above the input wavelength (i.e., λ&gt;λ 0 ) passing through the coupler and remaining in the ring. Similarly, the coupler  46  has a wavelength selective element that is reflective for wavelengths at and above a wavelength in the range of output wavelength λ n . This is indicated in the figure by the arrow showing λ n  being coupled out through coupler  46  to output port  34 , and by the double-headed arrow showing wavelengths lower than the output wavelength (i.e., λ&lt;λ n ) passing through the coupler and remaining in the ring. 
     The embodiment of FIG. 4B is similar to the embodiment of FIG. 4A except that grating set  36  is used to select the desired intermediate Stokes wavelengths. Again, gratings of grating set  36  in the FIG. 4B embodiment might include each of 1175 nm, 1240 nm, 1315 nm and 1395 nm, given an input wavelength of 1117 nm. Alternatively, the grating set  36  could contain a single grating having a peak reflectivity at λ n  that is located so as to reflect counter-clockwise (relative to the orientation of the figure) optical energy. With a grating located in this fashion, any optical energy that was generated in the ring at λ n  in the counter-clockwise direction would be reflected back in the clockwise direction, and would therefore be traveling the correct direction to allow it to be coupled out of the ring via coupler  46 . 
     Yet another embodiment of the invention is shown in FIG.  5 . In this embodiment, a thin film type coupler  50  is again used. However, the coupler  50  provides both the input and the output coupling for the ring. The coupler has an input path connected to it in one direction, and has an output path connected to it from an opposite direction. A wavelength selective component in the coupler  50  has two wavelength selective coatings, one on either side of it. On one side is a coating that is reflective at wavelengths at and below a wavelength in the range of λ 0 , such that the input wavelength from input port  12  is reflected by the coupler  50  into the ring. On the other side is a coating that is reflective at wavelengths at and above a wavelength in the range of the output wavelength λ n . This characteristic is indicated by the two arrows showing λ 0  being coupled from input port  12  into the ring and λ n  being coupled from the ring to output port  34 , and by the double-headed arrow showing wavelengths in between (i.e., λ 0 &lt;λ&lt;λ n ) passing through the coupler and remaining in the ring. 
     A coupler structure that may be used for the coupler  50  of the FIG. 5 embodiment is shown in FIG.  5 A. Two GRIN lenses in the coupler are separated by filter element  56 . This filter element has, on a first side, a material (such as a coating on one side of the element  56 ) that is reflective to wavelengths at or below the input wavelength λ 0 . Thus, the wavelength selective material reflects light directed into the coupler along input path  58  into path  60 , but does not interfere with longer wavelength passing through the coupler filter element  56 . The wavelength selective component  56  also includes a material (such as a second coating on a side of the filter element  56  opposite the first coating) that is reflective to wavelengths at or above the output wavelength λ n . Thus, light that reaches the coupler, for example, along path  62  has any components at the output wavelength λ n  reflected to output path  64 . Meanwhile, light at intermediate wavelengths (i.e., λ 1 , λ 2 , . . . λ n-1 ) pass through the wavelength selective component  56  into path  60 . 
     While the structure shown in FIG. 5 is useful for a ring-based Raman amplifier, as discussed above, those skilled in the art will recognize that this configuration may be used for other devices as well. The coupler provides a means to segregate wavelengths above and below a particular wavelength range, and direct them in and/or out along desired paths. It therefore has application beyond the specific Raman amplifier embodiments shown herein. 
     While the invention has been shown and described with reference to a preferred embodiment thereof, it will be apparent to those skilled in the art that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the appended claims.