Digital control of actively mode-locked lasers

• US 20020176452A1
• Filed: 03/18/2002
• Published: 11/28/2002
• Est. Priority Date: 03/16/2001
• Status: Active Grant

• Current Assignee: Calmar Optcom, Inc.
• Original Assignee: Calmar Optcom, Inc.
• Inventors: Neos, Perry, Lee, Chunglin, Lin, Hong Tony
• Granted Patent: US 6,839,363 B2
• Time in Patent Office: Days
• US Class Current: 372/18
• CPC Class Codes: H01S 3/067 Fibre lasers
• CPC Class Codes: H01S 3/06712 Polarising fibre; Polariser
• CPC Class Codes: H01S 3/06791 Fibre ring lasers fibre laser gyrometers G01C19/72
• CPC Class Codes: H01S 3/105 by controlling the mutual position or the reflecting properties of the reflectors of the cavity , e.g. by controlling the cavity lengthH01S3/10076, H01S3/13 take precedence
• CPC Class Codes: H01S 3/1109 Active mode locking
• CPC Class Codes: H01S 3/1317 by controlling the temperature
• CPC Class Codes: H01S 3/136 by controlling devices placed within the cavity
• CPC Class Codes: H01S 3/139 by controlling the mutual position or the reflecting properties of the reflectors of the cavity , e.g. by controlling the cavity length

Abstract

Digital control of actively mode-locked lasers where an active feedback control system is implemented to include a digital processor such as a microprocessor. The digital processor digitally extracts noise information from multiple monitor signals generated from the laser output and digitally diagnoses the operating condition of the laser based on the noise information. Based on the operating condition of the laser, the digital processor generates control signals to adjust the laser cavity to establish or regain the mode locking condition and to maintain the mode locking by reducing the output noise.

Claims

1. A laser system, comprising:

an actively mode-locked laser including a laser cavity, and an optical modulator in said laser cavity responsive to a modulation control signal to modulate at least one of an amplitude and a phase of light in said laser cavity at a modulation frequency to lock optical modes of said laser cavity,

a cavity length control element engaged to said laser cavity to control and adjust a cavity length of said laser cavity in response to a cavity length control signal;

a thermal control unit to control at least a temperature of a portion of said laser cavity in response to a temperature control signal;

a thermal sensor coupled to measure a temperature of said portion of said laser cavity under control of said thermal control unit to produce a temperature signal to said thermal control unit;

an optical detector receiving a portion of a laser output from said laser cavity to produce a detector output;

an electrical signal mixer to mix a reference signal split from said modulation control signal with said detector output to produce a mixer output that includes an error signal indicating a frequency difference between said modulation frequency and a multiplicity of a longitudinal mode spacing of said laser cavity;

a phase delay unit coupled to a signal path of said reference signal to cause a phase delay in said reference signal in response to a phase delay control signal;

at least one bandpass filter, having a spectral bandwidth covering at least frequencies associated with energy relaxation processes in said laser cavity, and coupled to respectively receive and filter another portion of said detector output to produce a filter output signal indicating noise in said laser output;

a digital control module to digitally process said filter output signal to extract noise information of said laser output, to digitally process said error signal to extract said frequency difference caused by said laser cavity length, to digitally process said temperature signal to determine a measured temperature of said portion of said laser cavity, wherein said digital processor is operable to produce said cavity length control signal in response to said frequency difference, said temperature control signal, and said phase delay control signal in response to said noise information of said laser output.

2. The laser system as in claim 1, wherein said optical modulator includes a bias input port to receive a bias control signal to bias said optical modulator, and wherein said digital control module is operable to generate said bias control signal to reduce noise energy in said filter output signal.

3. The laser system as in claim 2, wherein said digital control module is operable to periodically adjust said bias control signal to reduce said noise energy.

4. The laser system as in claim 1, wherein said digital control module is operable to set said phase delay of said phase delay element to minimize energy of said error signal.

5. The laser system as in claim 1, wherein said digital control module is operable to control said cavity length control element to scan said cavity length to determine a selected cavity length from a noise energy pattern as a function of said cavity length to establish mode locking for a first time.

6. The laser system as in claim 1, wherein said digital control module is operable to retrieve laser parameters including a value for said laser cavity length, a value for said phase delay, a value for said temperature for a previous mode-locked operation of said laser to initialize said laser not in a mode-locking-condition to establish mode locking.

7. The laser system as in claim 6, wherein said digital control module is operable to control said cavity length control element to scan said cavity length to determine a selected cavity length from a noise energy pattern as a function of said cavity length to establish mode locking.

8. The laser system as in claim 1, wherein said thermal control unit includes a thermal control chamber that encloses the entirety of said laser cavity.

9. The laser system as in claim 1, wherein said thermal control unit includes a thermal control chamber that encloses only a portion of said laser cavity.

10. The laser system as in claim 1, wherein said laser cavity is a ring cavity.

11. The laser system as in claim 1, wherein said cavity length control element includes a piezo-electric element.

12. The laser system as in claim 10, wherein said ring cavity comprises a fiber ring.

13. The laser system a sin claim 12, wherein said fiber ring includes a first fiber portion and a second fiber portion that have opposite chromatic dispersions relative to each other.

14. The laser system as in claim 1, wherein said laser cavity is formed of fiber and includes:

a first fiber part formed of a single-mode fiber that maintains a polarization of light;

a polarizing beam splitter having a first optical facet coupled to a first terminal of said first fiber part, a second optical facet coupled to a second terminal of said first fiber part, and third optical facet opposing said first optical facet, said polarizing beam splitter configured to reflect light of a first polarization received from said third optical facet to said first optical facet and into said first terminal and to transmit light of said first polarization received from said second optical facet from said first fiber part to reach said third optical facet;

a second fiber part formed of a single-mode fiber and having a first terminal coupled to said third optical facet to exchange light with said first fiber part and a second terminal; and

a Faraday rotator mirror coupled to said second terminal of said second fiber part to reflect light from said first terminal back with a rotation of 90 degrees in polarization;

15. The laser system as in claim 1 or claim 14, wherein said thermal control unit includes a thermal control chamber that encloses only one portion of said laser cavity, said laser system further comprising a passive thermal control element coupled to a portion of said laser cavity that is not enclosed in said thermal control chamber, wherein said passive thermal control element operable to adjust said cavity length of said laser cavity to negate an effect of thermal expansion without receiving power from a power supply.

16. The laser system as in claim 15, wherein said passive control element includes two different materials with different thermal expansion coefficients.

18. The method as in claim 17, further comprising adjusting a phase delay in a signal path of a reference electrical signal which an electrical signal mixer operates to mix with another portion of the detector output to produce a mixer output that that includes an error signal indicating a frequency difference between the modulation frequency and a multiplicity of a longitudinal mode spacing of the laser cavity,

wherein the reference electrical signal is derived from a fraction of the modulation control signal, and

wherein the adjustment in the phase delay minimizes an amplitude of the error signal.

19. The method as in claim 17, further comprising:

digitally processing the filter output signal to determine a value of the cavity length of the laser cavity at which the energy of noise in the filter output signal is at or near a minimum level; and

controlling the laser cavity length to be at or near the value to establish mode locking.

20. The method as in claim 17, further comprising controlling a temperature of at least a portion of the laser cavity to bias a length of the laser cavity to set a cavity length control element, which operates to adjust a length of the laser cavity, in or near a center of a tuning range of a cavity length control element.

17. A method for controlling an actively mode-locked laser system, comprising:

converting a portion of a laser output of the laser into an electrical monitor signal;

filtering one portion of said electrical monitor signal with at least one bandpass filter which has a spectral bandwidth covering at least frequencies associated with energy relaxation processes in the laser to produce a filter output signal indicating laser noise; and

digitally processing the filter output signal to produce a control signal to adjust at least a DC bias to an optical modulator in a laser cavity of the laser to reduce energy of noise in the filter output signal, the optical modulator responsive to a modulation control signal at a modulation frequency to modulate either an amplitude or a phase of light in the laser cavity at the modulation frequency to lock laser modes.