Automatic compensation of thermal drift of laser beam through thermal balancing based on different linear expansions of metals

Abstract

We present a technique for compensating laser beam up-shifting induced by the hot air with temperature-gradient in a 2D-scaning laser radar for monitoring the intruder in the railway system, in which the laser beam is scanning at a height only a few centimeters above the railway tracks so that any object with a size of 5 cm × 5 cm × 5 cm or larger can be detected. It is found that when the laser beam is traveling in the hot air with a temperature gradient, the direction of the laser can be up-shifted. The up-shifted laser beam may miss the obstacle lies on the railway track and fail to detect and report the accidence and it may eventually result in a disaster. The compensation of the upward shifting is necessary to insure the normal operation of the radar system and it can be done by using the difference in linear thermal expansion between metals, which produce an additional elongation when the temperature changes. The temperature dependent elongation is used to drive a rotational stage, which rotates the laser in the opposite direction against the up-shifting. The rotation of the stage compensates the change of angle of the laser beam induced by the temperature gradient of the air. A prototype device is constructed to test the idea. Without thermal compensation, the laser thermal shift can reach up to 15 mm when temperature changes 14 °C from 30 °C to 44 °C above the hot surface after the laser beam travels 30 m and such a shift can be reduced to about 8 mm at the same distance when the compensator system is used. The experimental data agrees very well with the theoretical calculation. Keywords: Laser thermal drift, Temperature gradient, Laser radar