Open path spectroscopy of methane using a battery operated vertical cavity surface-emitting laser system

Abstract

Tunable diode laser spectroscopy (TDLS) is a well-established method for trace gas detection. TDLS systems usually employ edge-emitting diodes with a distributed feedback configuration. Recently long wavelength vertical cavity surface emitting lasers (VCSEL) have emerged as an alternative source for spectroscopic applications. The relatively low cost, low power requirements and large tuning range of VCSELs make them particularly attractive for portable gas detection systems. In this paper we describe a battery-operated VCSEL spectroscopy system operating near 1650 nm for methane detection. Wavelength modulation spectroscopy (WMS) is commonly used in TDLS systems to improve sensitivity. WMS in these systems is usually implemented with a hardware based lock-in amplifier. We report on the construction of a new system with software WMS and compare its operation with a conventional system. The VCSEL TDLS system is used to probe the 2v3 band of methane over an open path. The relative contributions of optical and electrical noise to the system signal to noise ratio and minimum gas detection level is presented. Finally, challenges and future design considerations in VCSEL spectroscopy are discussed.