Optical gain fiber having fiber segments with different-sized cores and associated method
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 cores for the signal and/or photonic-crystal inner cladding for the pump and/or use reflector connector segments. 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.
1. An apparatus comprising:
an optical-fiber laser 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, wherein a first end of the first gain-fiber segment includes a first reflector;
a connecting segment; and
a second gain-fiber segment that is joined to the first segment via the connecting segment, 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 a second distal end of the second gain-fiber segment includes a second partially-transmissive reflector, wherein an outer diameter of the first inner cladding of the first gain-fiber segment at its connection to the connecting segment is smaller than an outer diameter of the second inner cladding of the second gain-fiber segment at its connection to the connecting segment, 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.
2. The apparatus of claim 1 , wherein the first reflector includes a fiber-Bragg grating.
3. The apparatus of claim 1 , wherein the first reflector includes a high-reflectivity fiber-Bragg grating.
4. The apparatus of claim 1 , wherein the second partially-transmissive reflector includes a fiber-Bragg grating.
5. The apparatus of claim 1 , wherein the second partially-transmissive reflector includes a low-reflectivity fiber-Bragg grating.
6. The apparatus of claim 1 , wherein the first reflector includes a high-reflectivity fiber-Bragg grating, and wherein the second partially-transmissive reflector includes a low-reflectivity fiber-Bragg grating.
7. The apparatus of claim 1 , further comprising a plurality of pump-light-input ports joined to the optical-fiber amplifier, wherein the plurality of pump-light-input ports includes a fiber pigtail-fused to an end of the first gain-fiber segment.
8. The apparatus of claim 1 , further comprising a plurality of pump-light-input ports joined to the optical-fiber amplifier, wherein the plurality of pump-light-input ports includes a free-space optical-coupling system.
9. The apparatus of claim 1 , further comprising a plurality of pump-light-input ports joined to the optical-fiber amplifier, 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 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.
10. The apparatus of claim 1 , further comprising a plurality of pump-light-input ports joined to the optical-fiber amplifier, 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 plurality of pump-light-input ports and co-propagates in the first direction.
11. The apparatus of claim 1 , wherein the connecting 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, wherein the outer diameter of the second core of the second gain-fiber segment at its connection to the connecting segment is larger than an outer diameter of the third core of the connecting segment, wherein the signal light is coupled from the first core to the third core and from the third core to the second core.
12. A method comprising:
providing an optical-fiber laser that includes a plurality of amplifying fiber segments including a first amplifying segment and a second amplifying segment that are joined to one another at a connecting segment, 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 its connection to the connecting segment is smaller than an outer diameter of the second inner cladding of the second amplifying segment at its connection to the connecting segment;
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;
further amplifying the first-amplified signal light in the second segment to generate second-amplified signal light;
reflecting at least a portion of the signal light at a first end of the first segment; and
reflecting at least a portion of the signal light and transmitting at least a portion of the signal light at a second distal end of the second segment in order to provide lasing of the signal light.
13. The method of claim 12 , further comprising:
providing a plurality of pump-light-input ports joined to the optical-fiber amplifier, wherein the providing includes pigtail fusing a fiber to the first end of the first amplifying segment; and
injecting pump light through the plurality of pump-light-input ports.
14. The method of claim 12 , further comprising:
injecting pump light into the optical-fiber amplifier through a free-space optical-coupling system.
15. The method of claim 12 , wherein the signal light propagates through the plurality of amplifying fiber segments in a first direction, the method further comprising:
providing a plurality of pump-light-input ports joined to the optical-fiber amplifier; and
injecting pump light at or near an end of the second inner cladding from the plurality of pump-light-input ports such that the pump light counter-propagates in a second direction, opposite the first direction.
16. The method of claim 12 , wherein the signal light propagates through the plurality of amplifying fiber segments in a first direction, the method further comprising:
providing a plurality of pump-light-input ports joined to the optical-fiber amplifier; and
injecting pump light into an end of the second inner cladding from the plurality of pump-light-input ports such that the pump light co-propagates in the first direction.
17. The method of claim 12 , wherein the reflecting of at least a portion of the signal light at the first end of the first segment includes fiber-Bragg reflecting at least a portion of the signal light at the first end of the first segment.
18. The method of claim 12 , wherein the reflecting of at least a portion of the signal light and transmitting of at least a portion of the signal light at the second distal end of the second segment includes fiber-Bragg reflecting at least a portion of the signal light and transmitting at least a portion of the signal light at the second distal end of the second segment.
19. The method of claim 12 , wherein the reflecting of at least a portion of the signal light at the first end of the first segment includes fiber-Bragg reflecting at least a portion of the signal light at the first end of the first segment, and wherein the reflecting of at least a portion of the signal light and transmitting of at least a portion of the signal light at the second distal end of the second segment includes fiber-Bragg reflecting at least a portion of the signal light and transmitting at least a portion of the signal light at the second distal end of the second segment.
20. 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 via a connecting segment, 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 its connection to the connecting segment is smaller than an outer diameter of the second inner cladding of the second amplifying segment at its connection to the connecting segment;
means for amplifying 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;
means for further amplifying the first-amplified signal light in the second amplifying segment to generate second-amplified signal light;
means for reflecting at least a portion of the signal light at a first end of the first segment; and
means for reflecting at least a portion of the signal light and transmitting at least a portion of the signal light at a second distal end of the second segment in order to provide lasing of the signal light.
21. The apparatus of claim 20 , further comprising:
means for selectively absorbing light of the signal wavelength in the first inner cladding of the first amplifying segment.