| There is a great deal of interest in monitoring atmospheric greenhouse gases such as methane. Although distributed feedback edge-emitting lasers are often used for atmospheric detection, the development of the vertical cavity surface-emitting laser (VCSEL) has allowed for an alternative source. The VCSEL exhibits several advantages over distributed feedback (DFB), edge-emitting lasers, especially in terms of power requirements and tuning capabilities. A second harmonic spectroscopy system based on a VCSEL laser is presented. Battery operation of the driver, temperature control and receiver is achieved. The system is used to detect methane gas in open path situations, as well as in gas cylinders. Temperature and current scanning are compared as methods for laser wavelength modulation. Mathematical methods for characterizing and filtering absorption signals are investigated. The receiver system is also used with a DFB laser to compare performance with the VCSEL. A software receiver using LabVIEW is implemented, and its performance is compared with the hardware designs. A minimum detectable limit of 1.4 ppm˙m of methane for the hardware receiver is reported. |
