We describe the development and characterization of a near-infrared diode-laser-based sensor to measure the vapor from trace gases having unstructured absorption spectra. The technique uses two equal amplitude-modulated laser beams, with the modulation of the two lasers differing in phase by 180 deg. One of the laser beams is at a wavelength absorbed by the gas [for these experiments, vapor is from pyridine (C<sub>5</sub>H<sub>5</sub>N)], and the second laser beam is at a wavelength at which no absorption occurs. The two laser beams are launched onto near-coincident paths by graded-index lens-tipped optical fibers. The mixed laser beam signal is detected by use of a single photodiode and is demodulated with standard phase-sensitive detection. Data are presented for the detection and measurement of vapor from pyridine (C<sub>5</sub>H<sub>5</sub>N) by use of the mixed laser technique. The discussion focuses on experimental determination of whether a compound exhibits unstructured absorption spectra (referred to here as a broadband absorber) and methods used to maximize sensitivity.
© 2000 Optical Society of America
(120.0120) Instrumentation, measurement, and metrology : Instrumentation, measurement, and metrology
(120.1880) Instrumentation, measurement, and metrology : Detection
(140.2020) Lasers and laser optics : Diode lasers
(280.0280) Remote sensing and sensors : Remote sensing and sensors
Kevin L. McNesby, Richard T. Wainner, Robert G. Daniel, Andrzej W. Miziolek, William M. Jackson, and Ian A. McLaren, "High-Sensitivity Laser Absorption Measurements of Broadband Absorbers in the Near-Infrared Spectral Region," Appl. Opt. 39, 5006-5011 (2000)