Mode-locked fiber laser based on narrowband optical spectral filtering and amplifier similaritons
Implementations and examples of mode-locked fiber lasers based on fiber laser cavity designs that produce self-similar pulses (“similaritons”) with parabolic pulse profiles with respect to time at the output of the fiber gain media to effectuate the desired mode locking operation. An intra-cavity narrowband optical spectral filter is included in such fiber lasers to ensure the proper similariton conditions.
1. A laser device based on mode locking, comprising a laser cavity that produces laser light in form of laser pulses, wherein the laser cavity includes:
a fiber amplifier having a gain fiber section that exhibits an optical gain with a gain spectral bandwidth and operates to amplify laser pulses within the gain spectral bandwidth and to cause each laser pulse to increase a pulse duration and a pulse spectral width from entry of the gain fiber section to exit of the gain fiber section;
a saturable absorber coupled to receive output light from the fiber amplifier and operable to exhibit saturable absorption of light with the light intensity and to reduce the pulse duration and the pulse spectral width of each laser pulse; and
an optical spectral filter coupled to receive light from the saturable absorber and to further reduce the pulse duration and the pulse spectral width of each laser pulse, the optical spectral filter selectively transmitting light in a single narrowband while rejecting light outside the narrowband and configured to have a bandwidth for the narrowband that is sufficiently narrow to cause each laser pulse to be self similar after propagating through the fiber amplifier and to evolve into a parabolic pulse profile with respect to time at the exit of the fiber amplifier to effectuate mode locking that generates the laser pulses.
2. The device as in claim 1 , wherein:
the laser cavity exhibits normal dispersion at each location along an optical path of the laser pulses within the laser cavity.
3. The device as in claim 1 , wherein:
the gain fiber section of the fiber amplifier has a length of meters.
4. The device as in claim 1 , wherein:
the gain fiber section of the fiber amplifier has a length of tens of meters.
5. The device as in claim 1 , wherein:
the optical spectral filter has a bandwidth of nanometers.
6. The device as in claim 5 , wherein:
the optical spectral filter has a bandwidth of 2 nanometers.
7. The device as in claim 5 , wherein:
the optical spectral filter has a bandwidth of 4 nanometers.
8. The device as in claim 1 , wherein:
the laser cavity is configured to produce a frequency chirp in each pulse at the saturable absorber that is less than a corresponding group velocity dispersion of the laser cavity at the saturable absorber.
9. The device as in claim 1 , wherein:
the laser cavity is configured to produce a varying spectral width in each laser pulse along an optical path within the laser cavity that reaches a maximum spectral width value at the exit of the fiber amplifier and a minimum spectral width value at the output of the optical spectral filter.
10. The device as in claim 9 , wherein:
a spectral breathing ratio between the maximum spectral width value over the minimum spectral width value is greater than 5.
11. The device as in claim 9 , wherein:
a spectral breathing ratio between the maximum spectral width value over the minimum spectral width value is greater than 10.
12. The device as in claim 1 , wherein:
the saturable absorber includes an artificial saturable absorber.
13. The device as in claim 12 , wherein:
the artificial saturable absorber includes a nonlinear polarization evolution device including a half-wave plate, a quarter-wave plate, a polarizer and a second quarter-wave plate.
14. The device as in claim 1 , comprising:
a pulse dechirp device located outside the laser cavity to receive laser pulses output by the laser cavity and operable to reduce a frequency chirp in each laser pulse to reduce a pulse duration.
15. The device as in claim 1 , wherein:
the optical spectral filter is configured to output each laser pulse with a non-parabolic pulse profile with respect to time.
16. The device as in claim 1 , wherein:
the optical spectral filter is configured to output each laser pulse with a Gaussian pulse profile with respect to time.
17. A method for generating laser pulses based on mode locking, comprising:
configuring a laser cavity for generating laser pulses to include a fiber amplifier having a gain fiber section that exhibits an optical gain with a gain spectral bandwidth, a saturable absorber coupled to receive output light from the fiber amplifier and an optical spectral filter coupled to receive light from the saturable absorber and to output filtered light to the fiber amplifier to cause each laser pulse to have a varying spectral width along an optical path within the laser cavity that reaches a maximum spectral width value at the exit of the fiber amplifier and a minimum spectral width value at the output of the optical spectral filter; and
operating the optical spectral filter to selectively transmit light in a single narrowband while rejecting light outside the narrowband and to have a bandwidth for the narrowband that is sufficiently narrow to cause each laser pulse to be self similar after propagating through the fiber amplifier and to evolve into a parabolic pulse profile with respect to time at the exit of the fiber amplifier to effectuate mode locking that generates the laser pulses.
18. The method as in claim 17 , wherein the optical spectral filter has a bandwidth of nanometers.
19. The method as in claim 17 , comprising:
configuring the laser cavity to produce a frequency chirp in each pulse at the saturable absorber that is less than a corresponding group velocity dispersion of the laser cavity at the saturable absorber.
20. The method as in claim 17 , wherein:
configuring the laser cavity to have a spectral breathing ratio between the maximum spectral width value over the minimum spectral width value to be greater than 10.
21. The method as in claim 17 , comprising:
reducing a frequency chirp in laser pulses output by the laser cavity to reduce a pulse duration.
22. The method as in claim 17 , comprising:
using the optical spectral filter to output each laser pulse with a non-parabolic pulse profile with respect to time.
23. The method as in claim 17 , comprising:
using the optical spectral filter to output each laser pulse with a Gaussian pulse profile with respect to time.
24. A laser device based on mode locking, comprising a laser cavity that produces laser light in form of laser pulses, wherein the laser cavity includes:
a ring optical path to form a closed ring to circulate the produced laser light in form of laser pulses along one circulation direction in the ring optical path;
a fiber amplifier located in the ring optical path and configured to include a gain fiber section that exhibits an optical gain with a gain spectral bandwidth and operates to amplify laser pulses within the gain spectral bandwidth and to cause each laser pulse to increase a pulse duration and a pulse spectral width from entry of the gain fiber section to exit of the gain fiber section;
a saturable absorber located in the ring optical path downstream from the fiber amplifier in the circulation direction and coupled to receive output light from the fiber amplifier and operable to exhibit saturable absorption of light with the light intensity and to reduce the pulse duration and the pulse spectral width of each laser pulse;
an optical spectral filter located in the ring optical path downstream from the saturable absorber in the circulation direction and coupled to receive light from the saturable absorber and to further reduce the pulse duration and the pulse spectral width of each laser pulse, the optical spectral filter structured to selectively transmit light in a single narrowband while rejecting light outside the narrowband and configured to have a bandwidth for the narrowband that is sufficiently narrow to cause each laser pulse to be self similar after propagating through the fiber amplifier and to evolve into a parabolic pulse profile with respect to time at the exit of the fiber amplifier to effectuate mode locking that generates the laser pulses; and
wherein the laser cavity and components inside the laser cavity are structured to exhibit normal dispersion at each location along the ring optical path within the laser cavity.
25. The device as in claim 24 , wherein:
the gain fiber section of the fiber amplifier has a length of meters.
26. The device as in claim 24 , wherein:
the gain fiber section of the fiber amplifier has a length of tens of meters.
27. The device as in claim 24 , wherein:
the optical spectral filter has a bandwidth of nanometers.
28. The device as in claim 24 , wherein:
the optical spectral filter has a bandwidth of 2 nanometers.
29. The device as in claim 24 , wherein:
the optical spectral filter has a bandwidth of 4 nanometers.
30. The device as in claim 24 , wherein:
the laser cavity is configured to produce a frequency chirp in each pulse at the saturable absorber that is less than a corresponding group velocity dispersion of the laser cavity at the saturable absorber.
31. The device as in claim 24 , wherein:
the laser cavity is configured to produce a varying spectral width in each laser pulse along an optical path within the laser cavity that reaches a maximum spectral width value at the exit of the fiber amplifier and a minimum spectral width value at the output of the optical spectral filter.
32. The device as in claim 31 , wherein:
the laser cavity is structured to produce a spectral breathing ratio between the maximum spectral width value over the minimum spectral width value to be greater than 5.
33. The device as in claim 31 , wherein:
the laser cavity is structured to produce a spectral breathing ratio between the maximum spectral width value over the minimum spectral width value to be greater than 10.
34. The device as in claim 24 , wherein:
the saturable absorber includes an artificial saturable absorber.
35. The device as in claim 34 , wherein:
the artificial saturable absorber includes a nonlinear polarization evolution device including a half-wave plate, a quarter-wave plate, a polarizer and a second quarter-wave plate.
36. The device as in claim 24 , comprising:
a pulse dechirp device located outside the laser cavity to receive laser pulses output by the laser cavity and operable to reduce a frequency chirp in each laser pulse to reduce a pulse duration.
37. The device as in claim 24 , wherein:
the optical spectral filter is configured to output each laser pulse with a non-parabolic pulse profile with respect to time.
38. The device as in claim 24 , wherein:
the optical spectral filter is configured to output each laser pulse with a Gaussian pulse profile with respect to time.