Laser Doppler Displacement Measurement

Existing precision displacement-measuring devices are primarily based on linear encoders (optical or magnetic) or laser interferometers. Most encoders use grating scales and frequency counters, while most interferometers use stabilized lasers and fringe counters. In general, interferometers are more expensive and more accurate than linear encoders. However, there are large gaps in price and performance between these two technologies. Based on current developments in laser radar technology, a new device, a Laser Doppler Displacement Meter (LDDMTM) has been developed. This device is an electro-optical assembly, which uses the Doppler shift of a laser frequency caused by the movement of a target to measure displacement accurately over a range of a few meters. As shown in Figure 1, the LDDM consists of a laser head, a retro-reflector, a processor (not shown) and a notebook. Using the latest in microelectronic, electro-optic and optical heterodyning technologies, an LDDM provides a high precision and low-cost alternative to submicrometer positioning. The key feature of the LDDM is that its performance compares with that of a laser interferometer, while its price compares with that of a linear encoder.