| 摘要 |
A velocimeter comprises a laser diode (10) driven to provide an output beam of sinusoidally varying frequency. The output beam is directed into one port (21) of a four port single mode optical fibre coupler (20). The output beam forms a probe beam (22) which is directed onto a moving surface (5) and a reference beam (23). The reference beam is formed by a portion of the beam which is reflected at the interface (26) with a graded index lens (25) which terminates an output port (22) of the coupler (20). Light backscattered from the moving surface (5) along the direction of the probe beam is recombined with the reference beam (23) at a photodetector (30). The output from the photodetector (30) is demodulated using a sinusoidally-varying signal to give a signal indicative of the speed of movement of the moving surface (5). Feedback control of the laser diode drive is also provided. The invention may be applied to measuring vibrations. <IMAGE> |
