29 March 2000 Wavelength modulation spectroscopy of water vapor and line center stabilization at 1.462 μm for lidar applications
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Wavelength modulation spectroscopy was employed to investigate water vapor absorption lines in the 1.462 micrometer wavelength region using an external-cavity diode laser. These measurements were necessary in the development of a lidar (light detection and ranging) instrument for differential absorption measurement of the concentration and movement of water vapor in the Earth's atmosphere. Differential absorption measurements require that the laser frequency remain stable throughout the duration of the measurement. To ensure this stability, the laser output wavelength is monitored and a feedback control loop set up to minimize laser line drift. Three lines were investigated in the 1.462 micrometer region. The first-harmonic spectroscopic signal of the strongest of these lines was used as an error signal for the stabilization feedback loop. The derivative-like nature of harmonic signals provides a zero crossing for odd harmonics which can be used to determine the polarity of the requisite feedback voltage and compensate the laser wavelength accordingly. The feedback control loop utilized the virtual instrument capabilities of Labview and locking to within plus or minus 5.2 MHz was achieved using this method.
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Colleen M. Fitzgerald, Grady J. Koch, Audra Michelle Bullock, Amin N. Dharamsi, "Wavelength modulation spectroscopy of water vapor and line center stabilization at 1.462 μm for lidar applications", Proc. SPIE 3945, Laser Diodes and LEDs in Industrial, Measurement, Imaging, and Sensors Applications II; Testing, Packaging, and Reliability of Semiconductor Lasers V, (29 March 2000); doi: 10.1117/12.380525; https://doi.org/10.1117/12.380525

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