# US 8,539,816 B2 · Method and apparatus for locking a laser with a resonant cavity

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- Number: US 8,539,816 B2
- Document ID: US8539816B2
- Status: Granted
- Kind: B2
- Application number: 13/105,795
- Filed: 2011-05-11
- Published: 2011-09-08
- Granted: 2013-09-24
- Earliest claimed: 2010-03-02
- Assignees: LI-COR, Inc.
- Applicants: Alexander Kachanov; Serguei Koulikov
- Inventors: Alexander Kachanov; Serguei Koulikov
- Web page: https://patentgrep.com/patent/US8539816B2

## CPC

- G01N 21/1702: {with opto-acoustic detection, e.g. for gases or analysing solids}
- G01N 21/39: using tunable lasers
- H01S 5/0064: {Anti-reflection components, e.g. optical isolators}
- H01S 5/0656: {Seeding, i.e. an additional light input is provided for controlling the laser modes, for example by back-reflecting light from an external optical component (H01S5/14, H01S5/4062 and H01S5/4006 take precedence)}
- G01N 2021/1704: {in gases}
- G01N 2021/1708: {with piezotransducers}
- H01S 5/005: {Optical components external to the laser cavity, specially adapted therefor, e.g. for homogenisation or merging of the beams or for manipulating laser pulses, e.g. pulse shaping (H01S5/026, H01S5/18388 take precedence)}
- G01N: INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES (measuring or testing apparatus or processes other than immunoassay, involving enzymes or microorganisms C12M, C12Q)
- 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

A system for locking a laser with a resonant optical cavity includes a laser that emits a laser beam, a resonant optical cavity having at least two cavity mirrors, one of which is a cavity coupling mirror, mode matching optics configured to couple the laser beam to the cavity via the cavity coupling mirror, means for applying a periodic dither or modulation waveform signal to the optical frequency of the incident laser beam or to the laser itself to thereby induce modulation of the intracavity optical power, and means for enabling a portion of the light emerging from the cavity coupling mirror to enter a cavity of the laser while maintaining an optical phase that results in periodic optical feedback locking whereby a mean optical frequency of the laser matches a resonance peak of the cavity.

## Claims

### Claim 1

A system for locking a laser with a resonant optical cavity, the system comprising:
a laser that emits a laser beam;
a resonant optical cavity having at least two cavity mirrors, one of which is a cavity coupling mirror;
mode matching optics configured to couple the laser beam to the cavity via the cavity coupling mirror;
means for applying a periodic dither or modulation waveform signal to the optical frequency of the incident laser beam or to the laser itself to thereby induce modulation of the intracavity optical power; and
means for enabling a portion of the light emerging from the cavity coupling mirror to enter a cavity of the laser while maintaining an optical phase that results in periodic optical feedback locking whereby a mean optical frequency of the laser matches a resonance peak of the cavity.

### Claim 2 (depends on 1)

The system of claim 1, further including a photodetector located external to the cavity for use in locking a mean optical frequency of the laser to the frequency of the cavity transmission peak using first or other odd harmonics of the modulation waveform signal.

### Claim 3 (depends on 1)

The system of claim 1, further including means for controlling a position of one of the cavity mirrors so as to scan an optical frequency of the cavity resonance peak while maintaining a lock between a mean optical frequency of the laser and the cavity resonance peak.

### Claim 4 (depends on 3)

The system of claim 3, wherein the means for controlling the position of one of the cavity coupling mirrors includes a means for resetting the mirror's position to a new position within the displacement range such that a new cavity resonance mode matches a laser mean optical frequency of the laser when the mirror's position approaches a displacement range limit.

### Claim 5 (depends on 3)

The system of claim 3, further including:
a photodetector located external to the cavity for use in obtaining an electronic waveform proportional to the intracavity optical power emerging from a cavity mirror;
a feedback element coupled with the photodetector, with the means for controlling the position of one of the cavity mirrors, and with the laser, the feedback element being configured to:
determine an error signal based at least in part on signals received from the photodetector; and
provide control signals to one of the laser or the means for controlling the position of one of the cavity mirrors so as to maintain a lock between the cavity resonance peak and the mean optical frequency of the laser.

### Claim 6 (depends on 5)

The system of claim 5, wherein the feedback element provides control signals to the means for controlling the position of one of the cavity mirrors such that said means for controlling controls an optical length of the cavity to thereby lock the cavity resonance peak to the mean optical frequency of the laser.

### Claim 7 (depends on 5)

The system of claim 5, wherein the feedback element provides control signals to the laser such as to control the laser mean frequency to thereby lock the mean optical frequency of the laser to the cavity resonance peak.

### Claim 8 (depends on 1)

The system of claim 1, wherein the means for enabling includes an optical isolator positioned between the laser and the cavity coupling mirror, wherein the optical isolator is partially transmitting such that a portion of the light emerging from the cavity coupling mirror enters a cavity of the laser, the system further including:
a photo-detector located external to the cavity for detecting the intracavity optical power emerging from a cavity mirror;
an optical element adapted to vary the optical path difference between the laser and the cavity coupling mirror; and
an intelligence module adapted to:
i) receive a time varying photo-detector output signal;
ii) determine a magnitude of an in-phase component of the time-varying photo-detector output signal at a dither frequency of the dither waveform signal;
iii) determine a magnitude of a quadrature component of the time-varying photo-detector output signal at the dither frequency;
iv) use the in-phase component to maintain a lock between a mean optical frequency of the laser and a resonance peak of the cavity; and
v) increase or decrease an optical path difference between the laser and the cavity coupling mirror by inputting to the optical element a value proportional to the magnitude of the quadrature component of the time varying photo-detector output signal at the dither frequency until the magnitude of the quadrature component zeroes down.

### Claim 9 (depends on 1)

The system of claim 1, further including means for optimizing the amplitude and shape of the applied periodic dither or modulation waveform signal.

### Claim 10 (depends on 9)

The system of claim 9, further including an intelligence module and a photo-detector located external to the cavity for detecting the intracavity optical power emerging from a cavity mirror, wherein the intelligence module is adapted to optimize the amplitude and shape of the applied periodic dither or modulation waveform by repeatedly:
determining a magnitude of in-phase and quadrature components of an output signal of the photo-detector at the second harmonic of a dither frequency of the dither waveform signal;
determining a phase of second harmonic sine and cosine reference functions that zeroes out the quadrature component, and setting a constant phase relative to the phase of the dither waveform to this determined phase; and
determining an optimum laser wavelength dither amplitude that results in a maximum value of the in-phase component of the photo-detector signal at the second harmonic of the dither frequency.

### Claim 11 (depends on 1)

The system of claim 1, wherein the intelligence module determines the magnitude of the in-phase and quadrature components of the photo-detector output signal by multiplying said photo-detector output signal by a sine and a cosine reference function having the same frequency as the dither frequency and said constant phase relative to the phase of the dither waveform signal.

### Claim 12 (depends on 1)

The system of claim 1, wherein the means for enabling includes an optical isolator positioned between the laser and the cavity coupling mirror, wherein the optical isolator is partially transmitting such that a portion of the light emerging from the cavity coupling mirror enters the cavity of the laser.