A DIODE LASER SOURCE FOR DIAL METHANE MEASUREMENTS IN COAL MINES

The research reported in this dissertation was directed toward the development of a coal mine methanometer source. The project encompassed two tasks. The first was a study conducted to determine if room-temperature injection lasers could be used as the source for a DIAL methanometer for coal mone applications. As part of that study, prior DIAL measurements of methane were reviewed and the measurement technique was explored in a general way. The absorption spectroscopy of methane was then examined in detail and found to contain an appropriate absorption feature near a wavelength of 1.65 (mu)m provided the spectral width of the light source was approximately 0.2 cm('-1). As an additional part of the study the operating characteristics of broad area high optical output InGaAs/InP injection lasers were examined. A detailed calculation of the requirements for a diode laser DIAL measurement of methane in a coal mine was then performed. By comparing the observed diode laser properties with the DIAL requirements, it was found that broad area InGaAs/InP injection lasers were suitable in most respects for coal methane measurements. The exception was the diode laser spectral bandwidth. It was found to be some 400 times too large.;The second task was the design and fabrication of an external grating cavity to reduce the bandwidth of the injection lasers. The optical design of an off-axis parabolic mirror grating cavity was developed. Geometric optics was used to determine the physical parameters for an external cavity which would produce the desired spectral bandwidth of 0.2 cm('-1). The fabrication and performance of this external resonator source is described. Measurement of the source linewidth showed it to be two to three times larger than the design value. An explanation of the resultant bandwidth based on coupled mode theory is presented. In summary, an external grating cavity for a diode laser was designed, assembled, and tested. It was shown to approximate the optical properties required for a DIAL coal mine methane measurement.