An adaptive robust computational method for removing unwanted etalon fringes from diode laser absorption spectra

Abstract

Optical interference fringes due to unwanted etalons are often the limiting uncertainty in diode laser spectroscopic trace gas measurements. Temporal variations in the fringe spacings, phases, and amplitudes introduce systematic baseline changes that limit useful signal averaging times to ~1000 seconds, and constrain minimum detectable absorbances to between one and three orders of magnitude worse than the fundamental limiting noise sources (shot noise and/or detector thermal noise). We describe an adaptive numerical filtering method based on singular value decomposition (SVD) that shows, for one system studied, a five-fold reduction in baseline drift due to unwanted etalons over a one week measurement period. The adaptive algorithm is fast (< 1 msec per computation), robust, and uses linear methods. It is computationally equivalent to principal component analysis (PCA). The test systems were acetylene detected using a near-infrared telecommunications laser operating near 6542 cm^-1 and methane detected using a vertical cavity surface emitting laser (VCSEL) operating at 6057cm^-1. The acetylene detection limit was 20 ppb (1 &#963;) over a 1 week measurement.