A laser Doppler velocimeter (LDV) using a semiconductor laser diode (LD) with its self-mixing effect has been developed. A Doppler signal, caused by mixing a returned wave with an originally existing wave inside the LD, is detected with a photodetector in the LD package; it is also picked up from the variation of the LD driving voltage. When the returned light is weak enough, it confirms that there is no change in the single-mode oscillation and its spectral width of the LD. A LDV of this type is compact enough for many applications.
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