Novel optical vibration sensor using external cavity feedback

A novel low cost, non-contact optical vibration sensor requiring only a single optoelectronic component has been developed. It consists of a CW semiconductor laser operating with external optical feedback. The laser beam reflected from a target generates a series of lasing modes set by the external cavity length. Beating of the modes produces an RF signal and this signal is detected as a variation in the junction voltage. Any change in the external cavity length induces corresponding beat frequency variations in the RF signal, which are transformed into amplitude variations using a simple edge-detection filter system. Using this sensor, low amplitude vibrations have been measured at frequencies of up to 600 Hz. Successful results have been achieved with target reflectivities lower than 5%. When calibrated, the sensor demonstrates satisfactory output for submicron vibration amplitudes. Maximum amplitudes of 1 mm have been measured with an accuracy of 0.2%.