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

TL;DR

This paper explores the use of frequency modulation and optical feedback locking in cavity-enhanced absorption spectroscopy, demonstrating a sensitivity improvement for detecting low concentrations of gases like C2H2.

Contribution

It introduces a novel combination of optical feedback self-locking and phase modulation at the cavity's free spectral range for enhanced absorption spectroscopy.

Findings

Achieved a noise-equivalent absorption sensitivity of 2.1×10^-11 cm^-1 Hz^-1/2.

Sensitivity is 11.7 times above the shot-noise limit.

Demonstrated effective detection of C2H2 absorption in a 1.5-cm cell.

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 C2H2 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 C2H2 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 C2H2 absorption spectra extracted from the heterodyne beat signals recorded at the transmission maxima intensity peaks of the successive TEM00 resonances.