Abstract

We demonstrated a new type of cw non-coherent laser Doppler velocimeter based on a variant of a Faraday magneto-optic atomic line filter with a time dependent transmission spectrum. From a detected optical signal of only 10 nW, we measured the velocity of a spinning disk with a resolution of 4 cm/sec, corresponding to a frequency resolution of about 100 kHz. Like harmonic detection, the sensitivity of our technique benefits from ac signal detection at intermediate frequencies, as opposed to the direct detection schemes used in most non-coherent laser Doppler velocimeters. The ac detection is implemented through the use of a dynamic atomic line filter as a Doppler analyzer. Features in the transmission spectrum of the Faraday filter are dithered in frequency by modulating the magnetic field of the filter. To measure the Doppler shift, signal light is passed through the filter, modulating the beam, which is then synchronously detected at the first and second harmonic of the dither frequency using lock-in amplifiers. In the detected waveform, the ratio of the first to the second harmonic of dither frequency are proportional to the Doppler shift. Compared to dc non-coherent Doppler velocimetry techniques, the harmonic detection technique offers significant advantages in noise rejection. At modulation frequencies of a few tens of kHz, the spectral amplitudes are substantially out the low frequency regime where 1/f noise dominates. In addition, lock-in amplifiers are ideally suited to implement harmonic detection, providing very narrow electronic bandpasses and high out-of-band rejection. The utilization of Faraday atomic line filters provide a practical approach to obtaining precise, absolute frequency registration between a diode laser probe and the Doppler analyzer, as well as a fortuitous means to obtain a frequency dithered filter. In this paper, we describe this technique and a proof of principle experiment using a cw diode laser tuned to the stop band of a cesium vapor based Faraday filter near 852 nm.

© 1995 Optical Society of America