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

Seven orders of magnitude shortening of photon lifetime to τ p =90ps as compared with the fluorescence lifetime of τ =3.4ms was established in a laser-diode pumped 5 mm thick ruby (Cr:Al 2 O 3 ) laser with coated end mirrors. The resulting huge lifetime ratio of K = τ / τ p =3.78×10 7 enabled a wide-band self-mixing modulation at Doppler-shift frequency, f D , up to 36 times the relaxation oscillation frequency, f RO . The drastic enhancement of self-mixing interference took place as f D was decreased toward f RO , and the ultimate sensitivity at f D =2MHz was measured to be 0.38 feedback photon on average per the single observation time of 160 µs for data acquisition.