Slanted FBG for SRS suppression

Abstract

An example apparatus includes an optical fiber including a core and cladding, the core being situated to propagate an optical beam along a propagation axis associated with the core, and at least one fiber Bragg grating (FBG) situated in the core of the optical fiber, the fiber Bragg grating including a plurality of periodically spaced grating portions situated with respect to the propagation axis so that light associated with Raman scattering is directed out of the core so as to reduce the generation of optical gain associated with stimulated Raman scattering (SRS).

We claim: 
     
       1. An apparatus, comprising:
 an optical fiber including a core and cladding, the core being situated to propagate an optical beam along a propagation axis associated with the core; and 
 at least one fiber Bragg grating (FBG) situated in the core of the optical fiber, the fiber Bragg grating including a plurality of periodically spaced grating portions situated with respect to the propagation axis so that light associated with Raman scattering is directed out of the core and cladding so as to reduce the generation of optical gain associated with stimulated Raman scattering (SRS); 
 wherein the FBG defines a reflection bandwidth that is offset from a center wavelength of a first Raman Stokes wavelength range of the optical beamso as to compensate for a shift of the reflection bandwidth associated with localized heating of the FBG from the propagation of the optical beam. 
 
     
     
       2. The apparatus of  claim 1 , wherein the plurality of periodically spaced grating portions are situated at a non-perpendicular angle with respect to the propagation axis. 
     
     
       3. The apparatus of  claim 1 , wherein the at least one FBG is situated within a fiber oscillator defined between a high-reflecting FBG and a partially-reflecting FBG. 
     
     
       4. The apparatus of  claim 3 , wherein the at least one FBG is situated adjacent to the partially-reflecting FBG. 
     
     
       5. The apparatus of  claim 4 , wherein the at least one FBG and the partially-reflecting FBG are situated in a passive section of optical fiber that is spliced to an active fiber of the fiber oscillator. 
     
     
       6. The apparatus of  claim 1 , wherein the at least one FBG comprises a plurality of FBGs spaced apart from each other and distributed along the length of the optical fiber. 
     
     
       7. The apparatus of  claim 6 , wherein the optical fiber includes an active core and the plurality of FBGs are spaced apart along the length to produce a distributed SRS loss in the optical fiber. 
     
     
       8. The apparatus of  claim 6 , wherein each FBG of the plurality of FBGs includes a single refractive index varied portion. 
     
     
       9. The apparatus of  claim 1 , wherein the at least one FBG is chirped. 
     
     
       10. The apparatus of  claim 1 , wherein the offset is between about 0.01 nm and 10 nm. 
     
     
       11. An apparatus, comprising:
 a gain fiber including an actively doped core defining a propagation axis and a pump cladding surrounding the actively doped core, the gain fiber being situated to generate a signal beam; 
 one or more pump sources optically coupled to the gain fiber to provide pump light for generation of the signal beam; 
 a high reflector fiber Bragg grating (FBG) optically coupled to an end of the gain fiber active core and situated to reflect the signal beam propagating in the active core of the gain fiber; 
 a partial reflector FBG optically coupled to an opposite end of the gain fiber active core and situated to partially reflect the signal beam and to transmit an output beam; and 
 at least one slanted stimulated Raman scattering (SRS) FBG situated in the gain fiber so as to direct light associated with Raman scattering out of the gain fiber so as to reduce the generation of optical gain associated with SRS in the gain fiber; 
 wherein the at least one slanted SRS FBG defines a reflection bandwidth that is offset from a center wavelength of a first Raman Stokes wavelength range of the signal beamso as to compensate for a shift of the reflection bandwidth associated with localized heating of the at least one slanted SRS FBG from the propagation of the signal beam. 
 
     
     
       12. The apparatus of  claim 11 , wherein the at least one slanted SRS FBG includes a first SRS FBG situated in a passive core of a passive fiber section of the gain fiber. 
     
     
       13. The apparatus of  claim 12 , wherein the first SRS FBG is situated between the actively doped core of the gain fiber and the partial reflector FBG. 
     
     
       14. The apparatus of  claim 11 , wherein the output beam is single-mode or multi-mode. 
     
     
       15. The apparatus of  claim 11 , wherein the gain fiber is an oscillator fiber and the apparatus further comprises:
 an amplifier fiber situated to receive the output beam from the oscillator fiber and to amplify the output beam; and 
 at least another slanted FBG situated in the amplifier fiber so as to direct light associated with Raman scattering out of the amplifier fiber so as to reduce the generation of optical gain associated with SRS in the amplifier fiber. 
 
     
     
       16. An apparatus, comprising:
 an optical fiber including a core and cladding, the core being situated to propagate an optical beam along a propagation axis associated with the core; and 
 at least onetwo or more spectrally selective componentcomponents optically coupled to the core of the optical fiber, the spectrally selective componentcomponents including at least onetwo optical redirecting portionportions situated at a non-perpendicular angle angles with respect to the propagation axis and including different respective spectrally selective component pitches wherein the non-perpendicular angles and the pitches are selected so that light associated with Raman scattering is directed out of the core so as to reduce the generation of optical gain associated with stimulated Raman scattering (SRS) and cladding; 
 wherein the at least one of the two or more spectrally selective component components defines a reflection bandwidth that is offset from a center wavelength of a first Raman Stokes wavelength range of the optical beamso as to compensate for a shift of the reflection bandwidth associated with heating proximate the at least one spectrally selective component from the propagation of the optical beam. 
 
     
     
       17. The apparatus of claim 16, wherein the two or more spectrally selective components are FBGs. 
     
     
       18. The apparatus of claim 17, wherein the two optical redirecting portions include slanted elements of the FBG. 
     
     
       19. The apparatus of claim 18, wherein the FBG is situated to reduce the generation of optical gain associated with stimulated Raman scattering (SRS). 
     
     
       20. The apparatus of claim 16, wherein the offset is configured to compensate for a shift of the reflection bandwidth associated with heating proximate the at least one spectrally selective component from the propagation of the optical beam, and wherein the offset is selected based on an operating temperature proximate the at least one spectrally selective component. 
     
     
       21. An apparatus, comprising:
 an optical fiber including a core and cladding, wherein the core is configured to propagate an optical beam; and   a fiber Bragg grating situated in the core, wherein the fiber Bragg grating includes angled reflective elements configured to direct out of the core stimulated Raman scattering (SRS) light associated with the optical beam, wherein the fiber Bragg grating has a first reflectivity spectrum that is offset from and selected to shift to a second reflectivity spectrum coincident with an SRS wavelength responsive to heat loading.   
     
     
       22. The apparatus of claim 21, wherein the offset includes between 0.01 nm and 0.1 nm, between 0.1 nm and 1.0 nm, or between 1 nm and 10 nm. 
     
     
       23. The apparatus of claim 21, wherein the optical fiber is a first optical fiber and the fiber Bragg grating is a first fiber Bragg grating, and further comprising:
 a second fiber Bragg grating situated in the core of the first optical fiber or in a core of a the second optical fiber coupled to the first optical fiber, wherein the second fiber Bragg grating is configured with a reflectivity spectrum offset that is different from the offset of the first fiber Bragg grating.   
     
     
       24. The apparatus of claim 23, wherein the offsets of the first and second fiber Bragg gratings are configured to provide SRS suppression over a temperature range. 
     
     
       25. The apparatus of claim 1, wherein the light directed out of the core and cladding is directed out of the optical fiber. 
     
     
       26. The apparatus of claim 17, wherein at least one of the FBGs is situated in an active fiber portion and at least one of the FBGs is situated in a passive fiber portion. 
     
     
       27. The apparatus of claim 23, wherein the offsets are configured in relation to different localized temperatures associated with the regions in which the respective FBGs are situated. 
     
     
       28. An apparatus, comprising:
 an optical fiber including a core and cladding, the core being situated to propagate an optical beam along a propagation axis associated with the core; and   at least one fiber Bragg grating (FBG) situated in the core of the optical fiber, the fiber Bragg grating including a plurality of periodically spaced grating portions situated with respect to the propagation axis so that light associated with Raman scattering is directed out of the core so as to reduce the generation of optical gain associated with stimulated Raman scattering (SRS);   wherein the at least one FBG comprises a plurality of FBGs spaced apart from each other and distributed along the length of the optical fiber, wherein each of the FBGs of the plurality of FBGs has a single rise and dip or dip and rise of refractive index;   wherein the at least one FBG defines a reflection bandwidth that is offset from a center wavelength of a first Raman Stokes wavelength range of the optical beam.