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Noise Immune Cavity Enhanced - Optical Heterodyne Molecular Spectroscopy (NICE-OHMS)



•Resonant cavity
•Increased interaction length
•Enhanced power

Optical Heterodyne

Frequency modulation for noise reduction

Molecular Spectroscopy

•High sensitivity
•Detectability 10-10 - 10-13 cm-1
•Close to shot-noise-limited performance
•Trace gas detection

Noise-immune cavity-enhanced optical-heterodyne molecular spectroscopy (NICE-OHMS) combines cavity enhanced absorption spectroscopy with laser frequency stabilization by means of frequency modulation. The laser is locked to an optical cavity by the Pound-Drever-Hall method [1], which matches the laser frequency to the center of a cavity resonance. It is this ability of the PDH locking technique to provide corrections for frequency fluctuations over a broad bandwidth that makes it powerful for laser frequency stabilization.

We require a high modulation frequency to decrease 1/f noise and a high cavity finesse to increase the signal.  By choosing the modulation frequency equal to the free spectral range of the cavity, all components of the spectral triplet are ideally transmitted through the cavity. Therefore the cavity does not compromise the balance of the triplet and does not convert frequency noise into amplitude noise, which is why this technique is called noise-immune. All this implies that frequency modulation spectroscopy can be performed as if the cavity were not present, yet fully benefiting from the prolonged interaction length.

NICE-OHMS_feedback loop

Schematic of a feedback loop.

NICE-OHMS has been used for spectroscopic investigations as well as chemical sensing and trace species detection. Solid-state lasers have been used for frequency standard applications, in which the absolute frequency stability of the locked laser is of more importance than its tunability. First demonstration of NICE-OHMS involved Nd:YAG laser mainly due to its low intrinsic noise. External cavity diode lasers have been used for spectroscopic investigations of weak molecular transitions. The main advantage of this type of laser is its wide mode-hop-free tuning range. As described in other sections, are quantum cascade lasers a reasonable choice for infrared trace gas detection due to their small size, high power, substantial tuning range and great robustness. The latest implementation of NICE-OHMS was based on an erbium-doped fiber laser with an exceptionally narrow free-running linewidth of 1 kHz over 120 μs, which is below that of a typical cavity mode[2].

1. Drever, R. W. P. (1983). "Laser phase and frequency stabilization using an optical resonator". Appl Phys B 31 (2): 97.

2. F. M. Schmidt, A. Foltynowicz, W. Ma, and O. Axner, "Fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry for Doppler-broadened detection of C2H2 in the parts per trillion range," JOSA B 24 (6), 1392-1405 (2007)