Tunable passively mode-locked lasers with phase-lock feedback for low timing jitters

• US 7,602,825 B1
• Filed: 09/09/2005
• Issued: 10/13/2009
• Est. Priority Date: 10/20/2004
• Status: Active Grant

• Current Assignee: Calmar Optcom, Inc.
• Original Assignee: Calmar Optcom, Inc.
• Inventors: Zhao, Jingjun, Lin, Anthony Hong
• Primary Examiner(s): Fahmy; Wael
• Assistant Examiner(s): Sayadian; Hayr A
• Application Number:
  US11/223,657
• Time in Patent Office: 1,495 Days
• Field of Search: 372/6, 372/9, 372/18, 372/25, 372/29.011, 372/29.02, 372/30, 372/33, 372/38.01
• US Class Current: 372/30
• CPC Class Codes: H01S 3/0064 Anti-reflection devices, e.g. optical isolaters absorbing layers for marking or protecting purposes in laser working B23K26/50; magneto-optical non-reciprocal devices G02F1/093, G02F1/0955
• CPC Class Codes: H01S 3/06712 Polarising fibre; Polariser
• CPC Class Codes: H01S 3/08027 by a filter, e.g. a Fabry-Perot filter is used for wavelength setting
• 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 3/1067 using pressure or deformation
• CPC Class Codes: H01S 3/1118 Semiconductor saturable absorbers, e.g. semiconductor saturable absorber mirrors [SESAMs]; Solid-state saturable absorbers, e.g. carbon nanotube [CNT] based
• CPC Class Codes: H01S 3/1305 Feedback control systems
• CPC Class Codes: H01S 3/1394 by using an active reference, e.g. second laser, klystron or other standard frequency source

Abstract

Techniques and devices to stabilize a passively mode-locked laser against the timing jitter by using a feedback control signal based on a signal component at a high harmonic frequency of the laser pulse repetition frequency in the laser output to control the optical path length of the laser resonator.

Claims

1. A passively mode-locked laser device, comprising:

a tunable passively mode-locked laser comprising a Fabry-Perot laser resonator that produces laser pulses and a laser control mechanism that controls an optical path length of the Fabry-Perot laser resonator to tune a pulse repetition frequency of the laser pulses;

an optical detector to receive at least a portion of the laser pulses from the laser and to produce electronic pulses in response to the received laser pulses; and

a feedback control circuit that extracts a monitor signal whose spectral range centers at a harmonic frequency of the pulse repetition frequency from the electronic pulses and processes the monitor signal to produce an error signal indicating a deviation between the harmonic frequency of the pulse repetition frequency of the laser pulses from either a reference harmonic frequency of a desired pulse repetition frequency in a reference signal, or a reference frequency that is offset from the reference harmonic frequency, wherein the feedback control circuit applies the error signal to the laser control mechanism which, in response, tunes the pulse repetition frequency of the laser pulses to reduce the deviation,

wherein the feedback control circuit comprises;

a signal filter that filters the electronic pulses in frequency and extracts signal spectral components around the harmonic frequency of the pulse repetition frequency within a predetermined bandwidth to produce the monitor signal, wherein the signal filter rejects signals at the pulse repetition frequency;

a signal amplifier that amplifies the monitor signal to output an amplified monitor signal at the harmonic frequency of the pulse repetition frequency;

a signal mixer that is directly connected to the signal amplifier and mixes the amplified monitor signal at the harmonic frequency of the pulse repetition frequency and the reference signal to produce the error signal; and

a signal integrator that integrates an output from the signal mixer and to produce the error signal,

wherein the Fabry-Perot laser resonator comprises;

first and second optical reflectors separated from each other to reflect the laser pulses therebetween;

an optical gain medium located in an optical path between the first and second optical reflectors;

an optical element located in an optical path between the first and second optical reflectors to lock phases of different longitudinal resonator modes to produce the laser pulses; and

optical fiber forming at least part of an optical path between the first and second optical reflectors, and comprising a doped section as the optical gain medium; and

wherein the laser control mechanism comprises;

a variable optical delay element in an optical path of the Fabry-Perot laser resonator to coarsely adjust the optical path length of the Fabry-Perot laser resonator; and

a piezo electrical actuator in an optical path of the Fabry-Perot laser resonator to finely adjust the optical path length of the Fabry-Perot laser resonator.

2. A device as in claim 1, wherein the feedback control circuit further comprises a second signal amplifier that amplifies the error signal from the signal integrator.

3. A device as in claim 1, wherein the optical element is an optical saturable absorber.

4. A device as in claim 1, wherein the optical element is made of a semiconductor material.

5. A device as in claim 1, wherein the variable optical delay element control mechanism comprises:

an optical retroreflector; and

a translation stage engaged to the optical retroreflector and to move a position of the optical retroreflector,

wherein the pieze electrical actuator is also engaged to the retroreflector to change the position of the optical retroreflector.

6. A method, comprising:

converting laser pulses at a pulse repetition frequency generated from a tunable passively mode-locked laser having a Fabry-Perot resonator into electrical pulses, wherein the Fabry-Perot laser resonator comprises (1) first and second optical reflectors separated from each other to reflect the laser pulses therebetween, (2) an optical gain medium located in an optical path between the first and second optical reflectors, (3) an optical element located in an optical path between the first and second optical reflectors to lock phases of different longitudinal resonator modes to produce the laser pulses, and (4) optical fiber forming at least part of an optical path between the first and second optical reflectors and comprising a doped section as the optical gain medium;

filtering the electrical pulses in frequency to obtain a monitor signal at a harmonic frequency of the pulse repetition frequency without signals at the pulse repetition frequency;

mixing the monitor signal at the harmonic frequency of the pulse repetition frequency with a reference signal at a reference frequency related to a harmonic frequency of a desired pulse repetition frequency to produce a beat signal representing a timing jitter in the laser pulses;

using a signal integrator to average the beat signal in time to produce an error signal representing the timing jitter in the beat signal; and

adjusting the laser to change the pulse repetition frequency of the laser pulses in response to the timing jitter in the error signal to reduce the timing jitter, wherein the adjusting includes;

adjusting a variable optical delay element in an optical path of the Fabry-Perot laser resonator to coarsely adjust the optical path length of the Fabry-Perot laser resonator, and

operating a piezo electrical actuator in an optical path of the Fabry-Perot laser resonator to finely adjust the optical path length of the Fabry-Perot laser resonator.

7. A method as in claim 6, further comprising setting the harmonic frequency to amplify a magnitude of the error signal representing the timing jitter to be greater than a noise level in the signal integrator.

8. A method as in claim 6, further comprising amplifying the monitor signal after the filtering in frequency prior to the mixing.

9. A method as in claim 6, further comprising adjusting an optical path length of the laser to change the pulse repetition frequency.

10. The method as in claim 6, comprising:

selecting the reference frequency of the reference signal to be offset from the harmonic frequency.

11. The method as in claim 6, comprising:

selecting the reference frequency of the reference signal to be equal to the harmonic frequency.