Characterisation of the potential of frequency modulation and optical feedback locking for cavity-enhanced absorption spectroscopy

Abstract

A combination of optical feedback self-locking of a continuous-wave distributed feedback diode laser to a V-shaped high finesse cavity, laser phase modulation at a frequency equal to the free spectral range of the V-cavity and detection of the transmitted laser beam at this high modulation frequency is described for possible application in cavity-enhanced absorption spectroscopy. In order to estimate an absorbance baseline noise of laser intensity and frequency modulated light triplet passed through the V-cavity in open air, a 1.5-cm long optical cell filled by C 2 H 2 at low pressure was placed behind the cavity output mirror. The performance of the setup was evaluated from the experimental bandwidth normalised relative intensity noise on the cavity output and the frequency modulation absorption signals induced by C 2 H 2 absorption in the 1.5-cm cell.From these data we estimate that the noise-equivalent absorption sensitivity of 2.1×10 -11 cm -1 Hz -1/2 by a factor of 11.7 above a shot-noise limit can be achieved for C 2 H 2 absorption spectra extracted from the heterodyne beat signals recorded at the transmission maxima intensity peaks of the successive TEM 00 resonances.

Cavity-based absorption spectroscopy techniques

“…However, awkward 'spectral ripple' effects are observed in many OF-CRDS measurements, owing to standing waves associated with odd-and even-numbered cavity modes which arise from the dependence of mirror refl ectivity on the phase of the intracavity fi eld; such effects can be minimised in various ways, including use of an equilateral three-mirror ring cavity (Hamilton et al , 2010) or a more elaborate linear cavity design with an intracavity beam splitter (Motto-Ros et al , 2007. Very recently, Kasyutich and Sigrist (2013) have reported a novel frequency-modulated OF-CEAS/ICOS approach which yields an impressive NEA sensitivity of 2.1 × 10 − 11 cm − 1 Hz − 1/2 , ~12 times above the shot-noise limit.…”