Abstract

We attached a 100-mm core diameter multimode optical fiber along a mechanical arm to sense the position of the arm. The speckle pattern of laser light at the output of the multimode fiber varies as the arm moves. At each of the 128 arm positions we recorded the corresponding speckle pattern as a volume hologram, in the photorefractive crystal Fe:LiNbO₃, using an angle encoded reference beam. The encoded angle is unique to the arm’s position. After the recording sequence is completed, any previously recorded position of the arm reconstructs a particular reference wave, thereby indicating the arm’s position. Interpolation allows encoding a virtually continuous range of arm positions. Our robot arm sensor has encoded a maximum travel of ~45° and has an angular sensitivity of ~2 mrad. The number of allowable positions, the sensitivity of the sensor and its noise, are largely determined by the number of fiber modes, the method of attachment, and the volume of the hologram. Although our mechanical arm has only one degree of freedom, the fiber can be used to record an arbitrarily complex motion of an arm having many degrees of freedom.

© 1988 Optical Society of America