METHOD AND APPARATUS FOR LOCKING A LASER WITH A RESONANT CAVITY

• US 20110216311A1
• Filed: 05/11/2011
• Published: 09/08/2011
• Est. Priority Date: 03/02/2010
• Status: Active Grant

• Current Assignee: Li & S Company Limited (Yesco Holdings Co., Ltd.)
• Original Assignee: Li & S Company Limited (Yesco Holdings Co., Ltd.)
• Inventors: Kachanov, Alexander, Koulikov, Serguei
• Granted Patent: US 8,539,816 B2
• Time in Patent Office: Days
• US Class Current: 356/213
• CPC Class Codes: G01N 2021/1704 in gases
• CPC Class Codes: G01N 2021/1708 with piezotransducers
• CPC Class Codes: G01N 21/1702 with opto-acoustic detection, e.g. for gases or analysing solids
• CPC Class Codes: G01N 21/39 using tunable lasers
• CPC Class Codes: 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
• CPC Class Codes: H01S 5/0064 Anti-reflection components, e.g. optical isolators
• CPC Class Codes: 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

Abstract

A method and apparatus for the photo-acoustic identification and quantification of one or more analyte species present in a gaseous or liquid medium in low concentration utilizing a laser and a resonant optical cavity containing the medium and having within the cavity at least two partially transparent mirrors, one of which is a cavity coupling mirror and one of which is moveably mounted on an assembly responsive to an input signal.

Claims

1. A method of locking a laser with a resonant optical cavity having at least two cavity mirrors, one of which is a cavity coupling mirror, the method comprising:

coupling the laser beam to the cavity via the cavity coupling mirror using mode matching optics;

applying a periodic dither or modulation waveform signal to the optical frequency of the laser beam or to the laser itself to thereby induce modulation of the intracavity optical power; and

enabling a portion of the light emerging from the cavity coupling mirror to reenter 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.

2. The method of claim 1, wherein enabling includes:

i) obtaining a time varying photo-detector output signal proportional to the intracavity optical power using a photo-detector located external to the cavity;

ii) obtaining a magnitude of an in-phase component of the time-varying photo-detector output signal at a dither frequency of the dither waveform;

iii) obtaining a magnitude of a quadrature component of the time-varying photo-detector output signal at the dither frequency;

iv) using the in-phase component to maintain a lock between the mean optical frequency of the laser and the resonance peak of the cavity; and

v) increasing or decreasing an optical path difference between the laser and the cavity coupling mirror by inputting to an optical element adapted to vary the optical path difference between the laser and the cavity coupling mirror 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.

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

4. The method of claim 1, further including optimizing the amplitude and shape of the periodic dither or modulation waveform signal.

5. The method of claim 4, wherein optimizing includes:

monitoring the intracavity optical power using a photo-detector located external to the cavity; and

repeatedly;

obtaining 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 signal 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.

6. The method of claim 5, wherein obtaining the magnitude of the in-phase and quadrature components of the photo-detector output signal includes multiplying the 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.

7. The method of claim 1, wherein applying the periodic dither or modulation waveform causes the laser to periodically move into and out of resonance with the cavity one or more times per dither period, thereby resulting in modulation of the intracavity optical power.

8. The method of claim 1, further including scanning an optical frequency of the cavity resonance peak while maintaining a lock between the mean optical frequency of the laser and the cavity resonance peak.

9. The method of claim 8, wherein scanning the optical frequency of the cavity includes changing an optical length of the cavity.

10. The method of claim 9, wherein at least one of the cavity mirrors is moveable, and wherein changing the optical length of the cavity includes changing a physical length of the cavity by moving a movable cavity mirror.

11. The method of claim 10, further including maintaining a position of the movable cavity mirror within a displacement range, and if the mirror'"'"'s position approaches a displacement range limit, resetting the mirror'"'"'s position to a new position within the displacement range such that a new cavity resonance mode matches a mean optical frequency of the laser.

12. The method of claim 8, wherein maintaining the laser locked to the cavity includes obtaining an error signal and applying the error signal to control the laser mean optical frequency to thereby lock the mean optical frequency of the laser to the cavity resonance peak.

13. The method of claim 8, wherein maintaining the laser locked to the cavity includes obtaining an error signal and applying the error signal to control an optical length of the cavity to thereby lock the cavity resonance peak to the mean optical frequency of the laser.

14. 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.

15. The system of claim 14, 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.

16. The system of claim 14, 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.

17. The system of claim 16, 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.

18. The system of claim 16, 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.

19. The system of claim 18, 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.

20. The system of claim 18, 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.

21. The system of claim 14, 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.

22. The system of claim 14, further including means for optimizing the amplitude and shape of the applied periodic dither or modulation waveform signal.

23. The system of claim 22, 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.

24. The system of claim 14, 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.

25. The system of claim 14, 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.