High-sensitivity laser absorption measurements of broadband absorbers in the near-infrared spectral region

We describe the development and characterization of a near-infrared diode-l aser-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 (C5H5N)], and the second laser beam is at a wavelength at which no absorption occurs. The two laser beams are laun ched 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 (C5H5N) by use of the mixed laser technique. The discussion focuses on experimental determina tion of whether a compound exhibits unstructured absorption spectra (referr ed to here as a broadband absorber) and methods used to maximize sensitivit y. (C) 2000 Optical Society of America OCIS codes: 120.0120, 120.1880, 140. 2020, 280.0280.