Self-mixing diode laser interferometry for velocity measurements of different targets

Supersonic gas jets can be used as a profile monitor for charged particle beams, as well as a cold target for collision experiments. For the optimisation of these experiments, it is important to know the velocity and density distribution of the jet. In these applications, gas jet velocities can be up to 2000 m/s. A diode laser velocimeter based on laser self-mixing method is currently being developed as an easy to build and compact alternative measurement technique. The technique seems a promising way for a complete characterisation of the gas jet parameters. It should be pointed out, however, that laser self-mixing is usually used for measurement of low velocities and vibrations. In this contribution, the heterodyne principle and design of the laser diode velocimeter are first discussed. The laser velocimeter is a self-aligning device, based on the self-mixing method where the laser is both, transmitter and receiver of the signal. The here presented theoretical analysis shows the possibility to extend measurement capabilities also to high velocities by altering the design. Experimental results from measurements with different targets are presented. The set-up for testing the sensor allows investigations into the limitation of the method to be made as well as the amount of feedback which is required for a detailed study of a gas jet.