A proximity sensor using power variation of a laser diode caused by returned light and its characteristics of stability

The proximity sensor using power variance of a laser diode caused by returned light from the target is examined. The output is taken by a photo-diode included in the laser diode for monitoring the power. The output of the sensor is attenuated exponentially with distance from the target. This attenuation characteristic is assumed in this paper to be caused by attenuation of the light during propagation between the target and the sensor. The details of the influences on the accuracy by drift of the sensor are examined, and then, a control or compensation method is proposed in the paper.

I. Introduction

The power of a laser diode (called LD) is varied by returned light of LD itself caused from reflectance on the target, if LD light beam is illuminated to a target. LD power, therefore, depends on the distance between the LD and the target. The author proposed an optical proximity sensor using the photo-diode (called PD) installed inside of the LD for monitoring the LD power [1]. This sensor is a non-contact and very small sensor, and, moreover, can approach to zero distance from the target. Then, this sensor has possibility of application to control of a robot hand and various automated machines.