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Diode Laser Velocity Measurements by Modulated Filtered Rayleigh ScatteringThe ability of solid-state lasers to be tuned in operating frequency at MHz rates by input current modulation, while maintaining a relatively narrow line-width, has made them useful for spectroscopic measurements. Their other advantages include low cost, reliability, durability, compact size, and modest power requirements, making them a good choice for a laser source in micro-gravity experiments in drop-towers and in flight. For their size, they are also very bright. In a filtered Rayleigh scattering (FRS) experiment, a diode laser can be used to scan across an atomic or molecular absorption line, generating large changes in transmission at the resonances for very small changes in frequency. The hyperfine structure components of atomic lines of alkali metal vapors are closely spaced and very strong, which makes such atomic filters excellent candidates for sensitive Doppler shift detection and therefore for high-resolution velocimetry. In the work we describe here we use a Rubidium vapor filter, and work with the strong D(sub 2) transitions at 780 nm that are conveniently accessed by near infrared diode lasers. The low power output of infrared laser diodes is their primary drawback relative to other laser systems commonly used for velocimetry. However, the capability to modulate the laser frequency rapidly and continuously helps mitigate this. Using modulation spectroscopy and a heterodyne detection scheme with a lock-in amplifier, one can extract sub-microvolt signals occurring at a specific frequency from a background that is orders of magnitude stronger. The diode laser modulation is simply achieved by adding a small current modulation to the laser bias current. It may also be swept repetitively in wavelength using an additional lower frequency current ramp.
Document ID
Document Type
Conference Paper
Mach, J. J. (Texas Univ. Austin, TX United States)
Varghese, P. L. (Texas Univ. Austin, TX United States)
Jagodzinski, J. J. (Texas Univ. Austin, TX United States)
Date Acquired
August 19, 2013
Publication Date
May 1, 1999
Publication Information
Publication: Fifth International Microgravity Combustion Workshop
Subject Category
Lasers and Masers
Funding Number(s)
Distribution Limits
Work of the US Gov. Public Use Permitted.
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