Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy

[citation needed] First, due to the presence of high intensity counter-propagating beams in the cavity, both Doppler-broadened and Doppler-free signals can be obtained.

[citation needed] The mode of operation to be preferred depends on the particular application of the technique and on the prevailing experimental conditions, mainly the type of noise or background signal that limits the detectability.

Frequency modulated Doppler-broadened signals can be modeled basically as ordinary fm-signals, although an extended description has to be used if the transition is optically saturated.

Each of these signals can, in turn, originate from interactions between several groups of molecules with various pairs of modes (e.g. carrier-carrier, sideband-carrier, sideband-sideband in various combinations).

[citation needed] Some typical Doppler-broadened NICE-OHMS signals, from 13 ppb (10 μTorr, 13•10−9 atm) of C2H2 detected in a cavity with a finesse of 4800, are shown in the figure.

[3][4][5][6][7][8][9][10][11] However, although the NICE-OHMS technique has shown to possess an extremely high detectability, it has so far only sparsely been developed towards trace gas analysis.

Fiber lasers with free-running linewidths as narrow as 1 kHz (measured over a fraction of a second), thus two to three orders of magnitude below those of ECDLs, are available today.

Evidently, this feature simplifies the feedback electronics (bandwidths as low as 10 kHz are sufficient) and the locking procedure considerably.