# US 8,787,411 B2 · Mode-locked fiber laser based on narrowband optical spectral filtering and amplifier similaritons

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- Number: US 8,787,411 B2
- Document ID: US8787411B2
- Status: Granted
- Kind: B2
- Application number: 13/530,044
- Filed: 2012-06-21
- Published: 2012-12-27
- Granted: 2014-07-22
- Earliest claimed: 2011-06-21
- Assignees: Cornell University
- Applicants: Frank Wise; William Renninger; Andy Chong
- Inventors: Frank Wise; William Renninger; Andy Chong
- Web page: https://patentgrep.com/patent/US8787411B2

## CPC

- H01S 3/06791: {Fibre ring lasers (fibre laser gyrometers G01C19/72)}
- H01S 3/06725: {Fibre characterized by a specific dispersion, e.g. for pulse shaping in soliton lasers or for dispersion compensating [DCF]}
- H01S 3/0675: {Resonators including a grating structure, e.g. distributed Bragg reflectors [DBR] or distributed feedback [DFB] fibre lasers}
- H01S 3/08027: {by a filter, e.g. a Fabry-Perot filter is used for wavelength setting}
- H01S 3/1118: Semiconductor saturable absorbers, e.g. semiconductor saturable absorber mirrors [SESAMs]; Solid-state saturable absorbers, e.g. carbon nanotube [CNT] based
- H01S 3/1618: {ytterbium}
- H01S 3/0014: {Monitoring arrangements not otherwise provided for (photometry G01J1/00, e.g. G01J1/4257; radiation pyrometry G01J5/00; measuring coherence of light G01J9/00; measuring wavelength of light G01J9/00, e.g. G01J9/0246; measuring optical pulses G01J11/00; calorimetrically measuring power of laser beams G01K17/003)}
- H01S 3/06712: {Polarising fibre; Polariser}
- H01S 2301/08: Generation of pulses with special temporal shape or frequency spectrum
- H01S: DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL

## Abstract

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.

## Claims

### Claim 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.

### Claim 2 (depends on 1)

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.

### Claim 3 (depends on 1)

The device as in claim 1, wherein:
the gain fiber section of the fiber amplifier has a length of meters.

### Claim 4 (depends on 1)

The device as in claim 1, wherein:
the gain fiber section of the fiber amplifier has a length of tens of meters.

### Claim 5 (depends on 1)

The device as in claim 1, wherein:
the optical spectral filter has a bandwidth of nanometers.

### Claim 6 (depends on 5)

The device as in claim 5, wherein:
the optical spectral filter has a bandwidth of 2 nanometers.

### Claim 7 (depends on 5)

The device as in claim 5, wherein:
the optical spectral filter has a bandwidth of 4 nanometers.

### Claim 8 (depends on 1)

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.

### Claim 9 (depends on 1)

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.

### Claim 10 (depends on 9)

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.

### Claim 11 (depends on 9)

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.

### Claim 12 (depends on 1)

The device as in claim 1, wherein:
the saturable absorber includes an artificial saturable absorber.

### Claim 13 (depends on 12)

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.

### Claim 14 (depends on 1)

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.

### Claim 15 (depends on 1)

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.

### Claim 16 (depends on 1)

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.

### Claim 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.

### Claim 18 (depends on 17)

The method as in claim 17, wherein the optical spectral filter has a bandwidth of nanometers.

### Claim 19 (depends on 17)

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.

### Claim 20 (depends on 17)

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.

### Claim 21 (depends on 17)

The method as in claim 17, comprising:
reducing a frequency chirp in laser pulses output by the laser cavity to reduce a pulse duration.

### Claim 22 (depends on 17)

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.

### Claim 23 (depends on 17)

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.

### Claim 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.

### Claim 25 (depends on 24)

The device as in claim 24, wherein:
the gain fiber section of the fiber amplifier has a length of meters.

### Claim 26 (depends on 24)

The device as in claim 24, wherein:
the gain fiber section of the fiber amplifier has a length of tens of meters.

### Claim 27 (depends on 24)

The device as in claim 24, wherein:
the optical spectral filter has a bandwidth of nanometers.

### Claim 28 (depends on 24)

The device as in claim 24, wherein:
the optical spectral filter has a bandwidth of 2 nanometers.

### Claim 29 (depends on 24)

The device as in claim 24, wherein:
the optical spectral filter has a bandwidth of 4 nanometers.

### Claim 30 (depends on 24)

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.

### Claim 31 (depends on 24)

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.

### Claim 32 (depends on 31)

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.

### Claim 33 (depends on 31)

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.

### Claim 34 (depends on 24)

The device as in claim 24, wherein:
the saturable absorber includes an artificial saturable absorber.

### Claim 35 (depends on 34)

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.

### Claim 36 (depends on 24)

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.

### Claim 37 (depends on 24)

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.

### Claim 38 (depends on 24)

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.