Optical heterodyne detection for cavity ring-down spectroscopy

• US 6,094,267 A
• Filed: 04/21/1999
• Issued: 07/25/2000
• Est. Priority Date: 04/21/1999
• Status: Expired due to Term

• Current Assignee: US Department of Energy (Government of the United States of America)
• Original Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford University)
• Inventors: Levenson, Marc D., Zare, Richard N., Paldus, Barbara A.
• Primary Examiner(s): Turner, Samuel A.
• Application Number:
  US09/295,213
• Time in Patent Office: 461 Days
• Field of Search: 356/346, 356/349, 250/343
• US Class Current: 356/484
• CPC Class Codes: G01J 3/30 Measuring the intensity of spectral lines directly on the spectrum itself G01J3/42, G01J3/44 take precedence
• CPC Class Codes: G01J 3/42 Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry beam switching arrangements G01J3/08
• CPC Class Codes: G01N 2021/391 Intracavity sample
• CPC Class Codes: G01N 21/39 using tunable lasers

Abstract

A cavity ring-down system for performing cavity ring-down spectroscopy (CRDS) using optical heterodyne detection of a ring-down wave E_RD during a ring-down phase or a ring-up wave E_RU during a ring up phase. The system sends a local oscillator wave E_LO and a signal wave E_SIGNAL to the cavity, preferably a ring resonator, and derives an interference signal from the combined local oscillator wave E_LO and the ring-down wave E_RD (or ring-up wave E_RU). The local oscillator wave E_LO has a first polarization and the ring-down wave E_RD has a second polarization different from the first polarization. The system has a combining arrangement for combining or overlapping local oscillator wave E_LO and the ring-down wave E_RD at a photodetector, which receives the interference signal and generates a heterodyne current I_H therefrom. Frequency and phase differences between the waves are adjustable.

Claims

1. A cavity ring-down system for optical heterodyne detection of a ring-down wave E_RD decaying at a first decay rate 1/τ

, said system comprising;

a) a light source for providing a light beam;

b) a set of optics for generating from said light beam a local oscillator wave E_LO at a local oscillator frequency ν

_LO and a signal wave E_SIGNAL at a signal frequency ν

_SIGNAL ;

c) a ring-down cavity for receiving said local oscillator wave E_LO and said signal wave E_SIGNAL ;

d) a switching means for interrupting said signal wave E_SIGNAL to initiate a ring-down phase during which said ring-down cavity issues said local oscillator wave E_LO and said ring-down wave E_RD at said signal frequency ν

_SIGNAL ;

e) an optical combining means for overlapping said local oscillator wave E_LO with said ring-down wave E_RD to produce an interference signal having a heterodyne frequency related to a frequency difference δ

ν

between said local oscillator frequency ν

_LO and said signal frequency ν

_SIGNAL ; and

f) a photodetector for receiving said interference signal and generating a heterodyne current I_H having a second decay rate 1/2τ

.

2. The cavity ring-down system of claim 1, wherein said set of optics comprises a polarizing means for imparting said local oscillator wave E_LO with a first polarization and imparting said ring-down wave E_RD with a second polarization different from said first polarization.

3. The cavity ring-down system of claim 2, wherein said optical combining means comprises one or more elements selected from the group consisting of polarization rotators, linear birefringent elements, circular birefringent elements, beam splitters and waveplates.

4. The cavity ring-down system of claim 2, wherein said light source is a continuous wave laser.

5. The cavity ring-down system of claim 4 further comprising a locking means for locking said ring-down cavity to said continuous wave laser using said local oscillator wave E_LO.

6. The cavity ring-down system of claim 1 wherein said photodetector is selected from the group consisting of photodiodes and phototransistors.

7. The cavity ring-down system of claim 1, wherein the intensity of said local oscillator wave E_LO is larger than an initial intensity of said ring-down wave E_RD.

8. The cavity ring-down system of claim 1, wherein said frequency difference δ

ν

is equal to the frequency difference between different modes of said ring-down cavity.

9. The cavity ring-down system of claim 1, wherein said frequency difference δ

ν

is equal to the frequency difference between two polarizations of the same mode of said ring-down cavity.

10. The cavity ring-down system of claim 1, wherein said frequency difference δ

ν

is equal to an integral multiple of the free spectral range of said ring-down cavity.

11. The cavity ring-down system of claim 1, further comprising a phase adjustment means for adjusting the phase between said local oscillator wave E_LO and said ring-down wave E_RD.

12. The cavity ring-down system of claim 11, wherein said frequency difference δ

ν

is approximately zero.

13. The cavity ring-down system of claim 1, further comprising a frequency adjustment means for adjusting said frequency difference δ

ν

.

14. The cavity ring-down system of claim 13, wherein said frequency adjustment means is an acousto-optic modulator.

15. The cavity ring-down system of claim 1, wherein said ring-down cavity is a ring resonator.

16. The cavity ring-down system of claim 1, wherein said switching means is an acousto-optic modulator.

17. A cavity ring-down system for optical heterodyne detection of a ring-down wave E_RD decaying at a first decay rate 1/τ

, said system comprising;

a) a light source for providing a light beam;

b) a set of optics for generating from said light beam a local oscillator wave E_LO at a local oscillator frequency ν

_LO and a signal wave E_SIGNAL at a signal frequency ν

_SIGNAL ;

c) a ring-down cavity for receiving said local oscillator wave E_LO and said signal wave E_SIGNAL ;

d) a switching means for altering said signal frequency ν

_SIGNAL of said signal wave E_SIGNAL to initiate a ring-down phase during which said ring-down cavity issues said local oscillator wave E_LO and said ring-down wave E_RD at said signal frequency ν

_SIGNAL ;

e) an optical combining means for overlapping said local oscillator wave E_LO with said ring-down wave E_RD to produce an interference signal having a heterodyne frequency related to a frequency difference δ

ν

between said local oscillator frequency ν

_LO and said signal frequency ν

_SIGNAL ; and

f) a photodetector for receiving said interference signal and generating a heterodyne current I_H having a second decay rate 1/2τ

.

18. The cavity ring-down system of claim 17, wherein said set of optics comprises a polarizing means for imparting said local oscillator wave E_LO with a first polarization and imparting said ring-down wave E_RD with a second polarization different from said first polarization.

19. The cavity ring-down system of claim 18, wherein said optical combining means comprises one or more elements selected from the group consisting of polarization rotators, linear birefringent elements, circular birefringent elements, beam splitters and waveplates.

20. The cavity ring-down system of claim 18, wherein said light source is a continuous wave laser.

21. The cavity ring-down system of claim 20 further comprising a locking means for locking said ring-down cavity to said continuous wave laser using said local oscillator wave E_LO.

22. The cavity ring-down system of claim 17, wherein said photodetector is selected from the group consisting of photodiodes and phototransistors.

23. The cavity ring-down system of claim 17, wherein the intensity of said local oscillator wave E_LO is larger than an initial intensity of said ring-down wave E_RD.

24. The cavity ring-down system of claim 17, wherein said frequency difference δ

ν

is equal to the frequency difference between different modes of said ring-down cavity.

25. The cavity ring-down system of claim 17, wherein said frequency difference δ

ν

is equal to the frequency difference between two polarizations of the same mode of said ring-down cavity.

26. The cavity ring-down system of claim 17, wherein said frequency difference δ

ν

is equal to an integral multiple of the free spectral range of said ring-down cavity.

27. The cavity ring-down system of claim 17, further comprising a phase adjustment means for adjusting the phase between said local oscillator wave E_LO and said ring-down wave E_RD.

28. The cavity ring-down system of claim 27, wherein said frequency difference δ

ν

is approximately zero.

29. The cavity ring-down system of claim 17, further comprising a frequency adjustment means for adjusting said frequency difference δ

ν

.

30. The cavity ring-down system of claim 29, wherein said frequency adjustment means is an acousto-optic modulator.

31. The cavity ring-down system of claim 17, wherein said ring-down cavity is a ring resonator.

32. The cavity ring-down system of claim 17, wherein said switching means is an acousto-optic modulator.

33. A cavity ring-down system for optical heterodyne detection of a ring-up wave E_RU growing at a first growth rate 1/τ

, said system comprising;

a) a light source for providing a light beam;

b) a set of optics for generating from said light beam a local oscillator wave E_LO at a local oscillator frequency ν

_LO and a signal wave E_SIGNAL at a signal frequency ν

_SIGNAL ;

c) a ring-down cavity for receiving said local oscillator wave E_LO and said signal wave E_SIGNAL ;

d) a switching means for turning on said signal wave E_SIGNAL to initiate a ring-up phase during which said ring-down cavity issues said local oscillator wave E_LO and said ring-up wave E_RU at said signal frequency ν

_SIGNAL ;

e) an optical combining means for overlapping said local oscillator wave E_LO with said ring-up wave E_RU to produce an interference signal having a heterodyne frequency related to a frequency difference δ

ν

between said local oscillator frequency ν

_LO and said signal frequency ν

_SIGNAL ; and

f) a photodetector for receiving said interference signal and generating a heterodyne current I_H having a second growth rate 1/2τ

.

34. The cavity ring-down system of claim 33, wherein said set of optics comprises a polarizing means for imparting said local oscillator wave E_LO with a first polarization and imparting said ring-up wave E_RU with a second polarization different from said first polarization.

35. A cavity ring-down system for optical heterodyne detection of a ring-up wave E_RU growing at a first growth rate 1/τ

, said system comprising;

a) a light source for providing a light beam;

b) a set of optics for generating from said light beam a local oscillator wave E_LO at a local oscillator frequency ν

_LO and a signal wave E_SIGNAL at a signal frequency ν

_SIGNAL ;

c) a ring-down cavity for receiving said local oscillator wave E_LO and said signal wave E_SIGNAL ;

d) a switching means for altering said signal frequency ν

_SIGNAL of said signal wave E_SIGNAL to initiate a ring-up phase during which said ring-down cavity issues said local oscillator wave E_LO and said ring-up wave E_RU at said signal frequency ν

_SIGNAL ;

e) an optical combining means for overlapping said local oscillator wave E_LO with said ring-up wave E_RU to produce an interference signal having a heterodyne frequency related to a frequency difference δ

ν

between said local oscillator frequency ν

_LO and said signal frequency ν

_SIGNAL ; and

f) a photodetector for receiving said interference signal and generating a heterodyne current I_H having a second growth rate 1/2τ

.

36. The cavity ring-down system of claim 35, wherein said set of optics comprises a polarizing means for imparting said local oscillator wave E_LO with a first polarization and imparting said ring-up wave E_RU with a second polarization different from said first polarization.