RF noise induced laser perturbation for improving the performance of non-resonant cavity enhanced absorption spectroscopy.
Ciaffoni L., Couper J., Hancock G., Peverall R., Robbins PA., Ritchie GA.
We present a novel strategy for suppressing mode structure which often degrades off-axis cavity enhanced absorption spectra. This strategy relies on promoting small, random fluctuations in the optical frequency by perturbing the injection current of the diode laser source with radio frequency (RF) bandwidth-limited white noise. A fast and compact oxygen sensor, constructed from a 764 nm vertical-cavity surface-emitting laser (VCSEL) and an optical cavity with re-entrant configuration, is employed to demonstrate the potential of this scheme for improving the sensitivity and robustness of a field-deployable cavity spectrometer. The RF spectral density of the current noise injected into the VCSEL has been measured, and correlated to the effects on the optical spectral signal-to-noise ratio (SNR) and laser linewidth for a range of re-entrant geometries. A fourfold gain in the SNR has been achieved using the RF noise perturbation for the optimal off-axis alignment, which led to a minimum detectable absorption (MDA) predicted from an Allan variance study as low as 4.3 × 10(-5) at 1 s averaging. For the optically forbidden oxygen transition under investigation, a limit of detection (SNR = 1) of 810 ppm was achieved for a 10 ms acquisition time. This performance level paves the way for a fast, sensitive, in-line oxygen spectrometer that lends itself to a range of applications in respiratory medicine.