Laser vibrometer based on optical-feedback-induced frequency modulation of a single-mode laser diode

We describe a sensitive and inexpensive vibrometer based on optical feedback by diffuse scattering to a single-mode diode laser. Fluctuations in the diode laser's operating frequency that are due to scattered light from a vibrating surface are used to detect the amplitude and frequency of surface vibrations. An additional physical vibration of the laser provides an absolute amplitude calibration. The fundamental bandwidth is determined by the laser response time of roughly [10.sup.-9] s. A noise floor of 0.23 nm/[Hz.sup.1/2] at 30 kHz with 5 x [10.sup.-5] of the incident light returning is demonstrated. This instrument provides an inexpensive and sensitive method of noncontact measurement in solid materials with low or uneven reflectivity. It can be used as a vibration or velocity sensor. Key words: Vibrometer, optical sensor, noncontact measurement, surface displacement.