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Apparatus and method for amplifying laser signals using segments of fibers of differing core diameters and/or differing cladding diameters to suppress amplified spontaneous emission and non-linear effects such as four-wave mixing (FWM), self-phase modulation, and stimulated Brillouin and/or Raman sc

Apparatus and method for amplifying laser signals using segments of fibers of differing core diameters and/or differing cladding diameters to suppress amplified spontaneous emission and non-linear effects such as four-wave mixing (FWM), self-phase modulation, and stimulated Brillouin and/or Raman scattering (SBS/SRS). In some embodiments, different core sizes have different sideband spacings (spacing between the desired signal and wavelength-shifted lobes). Changing core sizes and providing phase mismatches prevent buildup of non-linear effects. Some embodiments further include a bandpass filter to remove signal other than the desired signal wavelength and/or a time gate to remove signal at times other than during the desired signal pulse. Some embodiments include photonic-crystal structures to define the core for the signal and/or the inner cladding for the pump. Some embodiments include an inner glass cladding to confine the signal in the core and an outer glass cladding to confine pump light in the inner cladding.

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1. An apparatus comprising: an optical-fiber amplifier having a plurality of optically coupled gain-fiber segments, including: a first gain-fiber segment, wherein the first segment has a first core that is surrounded by a first inner cladding, wherein the first inner cladding is surrounded by a firs

1. An apparatus comprising: an optical-fiber amplifier having a plurality of optically coupled gain-fiber segments, including: a first gain-fiber segment, wherein the first segment has a first core that is surrounded by a first inner cladding, wherein the first inner cladding is surrounded by a first outer cladding;a second gain-fiber segment that is joined to the first segment at a first junction, wherein the second segment includes a second core that is surrounded by a second inner cladding, wherein the second inner cladding is surrounded by a second outer cladding, wherein an outer diameter of the first inner cladding of the first gain-fiber segment at the first junction is smaller than an outer diameter of the second inner cladding of the second gain-fiber segment at the first junction, wherein signal light of a signal-light wavelength is coupled into the first core, and wherein the signal light is coupled from the first core into the second core; anda first plurality of pump-light-input ports joined to the optical-fiber amplifier at the first junction. 2. The apparatus of claim 1, wherein the signal light propagates through the optical-fiber amplifier in a first direction, and wherein pump light is injected into an end of the second inner cladding from the first plurality of pump-light-input ports and co-propagates in the first direction. 3. The apparatus of claim 1, wherein the signal light propagates through the optical-fiber amplifier in a first direction, and wherein pump light is injected into the second inner cladding from the first plurality of pump-light-input ports at or near an end of the second inner cladding and counter-propagates in a second direction, opposite the first direction. 4. The apparatus of claim 1, further comprising a light gate that increases transmission of light of the signal-light wavelength between the first segment and second segment during a signal-pulse time as compared to other times. 5. The apparatus of claim 1, further comprising: a third gain-fiber segment that is joined to the second segment at a second junction, wherein the third segment has a third core that is surrounded by a third inner cladding, wherein the third inner cladding is surrounded by a third outer cladding, wherein the outer diameter of the second inner cladding of the second gain-fiber segment at the second junction is smaller than an outer diameter of the third inner cladding of the third gain-fiber segment at the second junction, wherein the signal light is coupled from the second core to the third core; anda second plurality of pump-light-input ports joined to the optical-fiber amplifier at the second junction. 6. The apparatus of claim 1, wherein a first end of the first segment includes a high-reflectivity surface and a second distal end of the second segment includes a low-reflectivity surface, and wherein the optical-fiber amplifier is configured to reflect the signal light between the high-reflectivity surface at the first end of the first segment and the low-reflectivity surface at the second distal end of the second segment in order to provide lasing. 7. The apparatus of claim 1, wherein a first end of the first segment includes a high-reflectivity fiber Bragg grating and a second distal end of the second segment includes a low-reflectivity fiber Bragg grating, and wherein the optical-fiber amplifier is configured to reflect the signal light between the high-reflectivity Bragg grating at the first end of the first segment and the low-reflectivity fiber Bragg grating at the second distal end the second segment in order to provide lasing. 8. A method comprising: providing a plurality of amplifying fiber segments includes a first amplifying segment and a second amplifying segment that are joined to one another at a first junction, and a first plurality of pump-light-input ports connected at the first junction, wherein the first amplifying segment includes a first core that is surrounded by a first inner cladding, wherein the first inner cladding is surrounded by a first outer cladding, wherein the second amplifying segment includes a second core that is surrounded by a second inner cladding, wherein the second inner cladding is surrounded by a second outer cladding, wherein an outer diameter of the first inner cladding of the first amplifying segment at the first junction is smaller than an outer diameter of the second inner cladding of the second amplifying segment at the first junction;injecting pump light through the first plurality of pump-light-input ports at the first junction;coupling signal light of a signal-light wavelength into the first core;amplifying the signal light in the first amplifying segment to generate first-amplified signal light;coupling the first-amplified signal light from the first core into the second core; andfurther amplifying the first-amplified signal light in the second segment to generate second-amplified signal light. 9. The method of claim 8, wherein the signal light propagates through the plurality of amplifying fiber segments in a first direction, and wherein the injecting of the pump light is done into an end of the second inner cladding from the first plurality of pump-light-input ports such that the pump light co-propagates in the first direction. 10. The method of claim 8, wherein the signal light propagates through the plurality of amplifying fiber segments in a first direction, and wherein the injecting of the pump light is done into the second inner cladding from the first plurality of pump-light-input ports at or near an end of the second inner cladding such that the pump light counter-propagates in a second direction, opposite the first direction. 11. The method of claim 8, further comprising gating light transmission between the first segment and second segment such that more light of the signal-light wavelength is transmitted during a signal-pulse time as compared to other times. 12. The method of claim 8, further comprising: providing a third amplifying segment that is joined to the second amplifying segment at a second junction, and a second plurality of pump-light-input ports connected at the second junction wherein the third amplifying segment includes a third core that is surrounded by a third inner cladding, wherein the third inner cladding is surrounded by a third outer cladding, and wherein the outer diameter of the second inner cladding of the second amplifying segment at the second junction is smaller than an outer diameter of the third inner cladding of the third amplifying segment at the second junction;injecting pump light through the second plurality of pump-light-input ports at the second junction;coupling the second-amplified signal light from the second core to the third core; andfurther amplifying the second-amplified signal light in the third segment to generate third-amplified signal light. 13. The method of claim 8, further comprising: reflecting substantially all of the signal light at a first end of the first segment, andreflecting some of the signal light and transmitting some of the signal light at a second distal end of the second segment in order to provide lasing of the signal light. 14. The method of claim 8, further comprising: fiber-Bragg reflecting substantially all of the signal light at a first end of the first segment; andfiber-Bragg reflecting some of the signal light and transmitting some of the signal light at a second distal end of the second segment in order to provide lasing of the signal light. 15. An apparatus comprising: a plurality of amplifying fiber segments that includes a first amplifying segment and a second amplifying segment that are joined to one another at a first junction, wherein the first amplifying segment includes a first core that is surrounded by a first inner cladding, wherein the first inner cladding is surrounded by a first outer cladding, wherein the second segment includes a second core that is surrounded by a second inner cladding, wherein the second inner cladding is surrounded by a second outer cladding, wherein an outer diameter of the first inner cladding of the first amplifying segment at the first junction is smaller than an outer diameter of the second inner cladding of the second amplifying segment at the first junction;means for injecting pump light at the first junction;means for coupling signal light of a signal-light wavelength into the first core;means for amplifying the signal light in the first amplifying segment to generate first-amplified signal light;means for coupling the first-amplified signal light from the first core into the second core; andmeans for further amplifying the first-amplified signal light in the second amplifying segment to generate second-amplified signal light. 16. The apparatus of claim 15, wherein the signal light propagates through the plurality of amplifying fiber segments in a first direction, wherein the means for injecting pump light includes means for injecting pump light into an end of the second inner cladding such that the pump light co-propagates in the first direction. 17. The apparatus of claim 15, wherein the signal light propagates through the plurality of amplifying fiber segments in a first direction, wherein the means for injecting pump light includes means for injecting pump light into the second inner cladding at or near an end of the second inner cladding such that the pump light counter-propagates in a second direction, opposite the first direction. 18. The apparatus of claim 15, further comprising; a third amplifying segment that is joined to the second amplifying segment at a second junction, wherein the third amplifying segment includes a third core that is surrounded by a third inner cladding, wherein the third inner cladding is surrounded by a third outer cladding, and wherein the outer diameter of the second inner cladding of the second amplifying segment at the second junction is smaller than an outer diameter of the third inner cladding of the third amplifying segment at the second junction;means for injecting pump light at the second junction;means for coupling the second-amplified signal light from the second core to the third core; andmeans for further amplifying the second-amplified signal light in the third amplifying segment to generate third-amplified signal light. 19. The apparatus of claim 15, further comprising means for reflecting the signal light between a first end of the first segment and a second distal end of the second segment in order to provide lasing. 20. The apparatus of claim 15, wherein the first-amplified signal light has a desired first signal wavelength, the apparatus further comprising means for wavelength filtering the first-amplified signal light in order to remove light other than the desired first signal wavelength.