Laser-diode-pumped thin-slice ruby laser for self-mixing laser Doppler velocimetry with less than one feedback photon per observation period

Seven orders of magnitude shortening of photon lifetime to p = 90 ps as compared with the florescence lifetime of  = 3.4 ms was established in a laser-diode pumped 5-mm-thick ruby (Cr:Al2O3) laser with coated end mirrors. The resulting huge lifetime ratio of K = /p = 3.78x107 enabled a wide-band self-mixing modulation at Doppler-shift frequency, fD, up to 36 times the relaxation oscillation frequency, fRO. The drastic enhancement of self-mixing interference took place as fD was decreased toward fRO and the ultimate sensitivity at fD = 2 MHz was measured to be 0.38 feedback photon on average per observation time.