Broadband short pulse fiber lasers capable of generating output spectra broader than gain bandwidth

1. A method for operating a fiber laser to generate broadband laser pulses with a spectral bandwidth greater than a gain spectral bandwidth of a gain fiber segment in the fiber laser, comprising:

providing a ring laser cavity to include a gain fiber segment having a gain spectral bandwidth, a spectral broadening fiber segment coupled to receive output light from the gain fiber segment, and an optical spectral filter coupled to receive light from the spectral broadening fiber segment to circulate laser pulses in a closed optical loop to, sequentially, pass through the gain fiber segment, the spectral broadening fiber segment and the optical spectral filter;

operating the optical spectral filter to output filtered light to the gain fiber segment downstream in the closed optical loop to selectively transmit light in a narrow spectral band while rejecting light outside the narrow spectral band 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 gain fiber segment; and

directing amplified laser pulses out of the gain fiber segment into the spectral broadening fiber segment to produce chirped self-similar laser pulses circulating in the fiber laser to have a pulse spectral bandwidth in each laser pulse output by the spectral broadening fiber segment to be greater than the gain spectral bandwidth of the gain fiber segment, wherein the spectral broadening fiber segment and the optical spectral filter collected to cause each laser pulse to have a varying spectral width in the closed optical loop that reaches a maximum spectral width value at the exit of the spectral broadening fiber segment and a minimum spectral width value at the output of the optical spectral filter.

2. The method as in claim 1 , comprising:

coupling laser pulses out of the fiber laser at a location between the spectral broadening fiber segment and the optical spectral filter as output laser pulses.

3. The method as in claim 2 , comprising:

de-chirping the output laser pulses to compress a pulse duration of each output laser pulse.

4. The method as in claim 1 , comprising:

configuring the fiber laser to include a saturable absorber coupled to receive output light from the gain fiber segment; and

coupling the spectral broadening fiber segment in the fiber laser between the saturable absorber and the gain fiber segment.

5. The method as in claim 1 , comprising:

using a high nonlinear fiber segment as the spectral broadening fiber segment to have an optical nonlinearity greater than an optical nonlinearity in the gain fiber segment.

6. The method as in claim 5 , comprising:

configuring the nonlinear fiber segment in the fiber laser to have a dispersion lower than a dispersion in the gain fiber segment a fiber.

7. The method as in claim 1 , comprising:

using a high nonlinear fiber segment as the spectral broadening fiber segment to have an optical nonlinearity greater than an optical nonlinearity in the gain fiber segment and a dispersion lower than a dispersion in the gain fiber segment a fiber.

8. The method as in claim 1 , comprising:

using a dispersion decreasing fiber segment which has a dispersion that decreases with position along the dispersion decreasing fiber segment as the spectral broadening fiber segment to produce stable soliton laser pulses circulating in the fiber laser.

9. The method as in claim 8 , comprising:

configuring the fiber laser to include a saturable absorber coupled to receive output light from the gain fiber segment;

coupling dispersion decreasing fiber segment as the spectral broadening fiber segment in the fiber laser between the saturable absorber and the gain fiber segment; and

configuring the dispersion decreasing fiber segment to have, at a beginning segment that connects to the saturable absorber to receive output light from the saturable absorber, a dispersion that matches a dispersion of the gain fiber segment.

10. A fiber laser for generating broadband laser pulses with a spectral bandwidth greater than a gain spectral bandwidth of a gain fiber segment in the fiber laser, comprising:

a gain fiber segment having a gain spectral bandwidth and operable to amplify laser pulses;

a spectral broadening fiber segment coupled to receive output light from the gain fiber segment and operable to cause spectral broadening of laser pulses to have a spectral bandwidth greater than the gain spectral bandwidth of the gain fiber segment; and

an optical spectral filter coupled to receive light from the spectral broadening fiber segment to output filtered light to the gain fiber segment to selectively transmit light in a narrow spectral band while rejecting light outside the narrow spectral band 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 gain fiber segment, wherein the fiber laser is configured to form a unidirectional closed optical loop by the gain fiber segment, the spectral broadening fiber segment and the optical spectral filter to direct each laser pulse, sequentially, through the gain fiber segment, the spectral broadening fiber segment and the optical spectral filter; and

wherein the fiber laser couples light out of the closed optical loop at a location between the spectral broadening fiber segment and the optical spectral filter to produce output laser pulses with a pulse spectral bandwidth greater than the gain spectral bandwidth.

11. The fiber laser as in claim 10 , comprising:

an optical coupler at a location between the spectral broadening fiber segment and the optical spectral filter to couple laser pulses out of the fiber laser as output laser pulses.

12. The fiber laser as in claim 10 , comprising:

an optical element outside the closed optical loop in an optical path of the output laser pulses to de-chirp the output laser pulses to compress a pulse duration of each output laser pulse.

13. The fiber laser as in claim 10 , comprising:

a saturable absorber coupled in the closed optical loop to receive output light from the gain fiber segment at a location so that the spectral broadening fiber segment is between the saturable absorber and the gain fiber segment.

14. The fiber laser as in claim 10 , wherein:

the spectral broadening fiber segment includes a high nonlinear fiber segment which exhibits an optical nonlinearity greater than an optical nonlinearity in the gain fiber segment.

15. The fiber laser as in claim 14 , wherein:

the nonlinear fiber segment in the fiber laser has a dispersion lower than a dispersion in the gain fiber segment a fiber.

16. The fiber laser as in claim 10 , wherein:

the spectral broadening fiber segment includes a high nonlinear fiber segment which exhibits an optical nonlinearity greater than an optical nonlinearity in the gain fiber segment and a dispersion lower than a dispersion in the gain fiber segment a fiber.

17. The fiber laser as in claim 10 , wherein:

the spectral broadening fiber segment includes using a dispersion decreasing fiber segment which has a dispersion that decreases with position along the dispersion decreasing fiber segment to produce stable soliton laser pulses circulating in the closed optical loop.

18. The fiber laser as in claim 17 , comprising:

a saturable absorber coupled in the closed optical loop to receive output light from the gain fiber segment;

wherein the dispersion decreasing fiber segment as the spectral broadening fiber segment in the fiber laser is located between the saturable absorber and the gain fiber segment; and

wherein the dispersion decreasing fiber segment exhibits, at a beginning segment that connects to the saturable absorber to receive output light from the saturable absorber, a dispersion that matches a dispersion of the gain fiber segment.