Laser-Doppler velocimetry (LDV) is an established technique for noninvasive monitoring of skin blood flow. The LDV is based on the interference between the photons scattered from the moving red cells and the surrounding stationary tissue. In this work we have developed a new chopper stabilized LDV method. The light bema from the 10 mW HeNe laser was pulsed by a mechanical chopper at a frequency of 130 Hz. The light pulses backscattered from the skin were measured by a dual PIN photodiode electrometer amplifier. In the Fourier spectrum the chopper 130 Hz + harmonics peaks were dominating. The width of the peaks was diminished when measuring necrotic skin areas. The interference signal was demodulated by multiplying the measured pulsed signal by the chopper waveform. The resulting LDV spectra had a negligible amount of electrical distortion. The main advantage of the chopper stabilization is that pulse modulation concentrates the signal power near the chopper frequencies, which are free of electrical 50 Hz + harmonics pick-up.
"Chopper stabilized laser-Doppler skin blood velocimeter", Proc. SPIE 1922, Laser Study of Macroscopic Biosystems, (17 June 1993); doi: 10.1117/12.146203; https://doi.org/10.1117/12.146203