Velocity measurements by modulated filtered Rayleigh scattering using diode lasers

We describe a novel variation on filtered Rayleigh scattering using solid-s tate near-infrared diode lasers that we call modulated filtered Rayleigh sc attering (MFRS). Solid-state diode lasers are simple, rugged, reliable, and relatively inexpensive. Hence an MFRS instrument would be well suited to f light instrumentation and industrial applications. We exploit the tuning ca pability of the diode laser to set it to the D-2 line of Rb at 780 nm and p erform filtered Rayleigh scattering measurements. The continuous tuning is also exploited to modulate the laser frequency rapidly, which provides the MFRS technique with some unique properties. Because the laser operates with a continuous wave, one can obtain continuous velocity and density measurem ents. In principle, two velocity components can be measured with a single l aser beam, and simultaneous multipoint measurements of velocity and density are possible. We present an early demonstration of the technique by making single point velocity measurements in a supersonic jet of CO2.