A beam position sensor for low power IR laser diodes

This article presents the design criteria, the prototype development, and application results of a low-cost four quadrants infrared ~IR! detector. The detector was specifically developed as a position sensor for the alignment of low power IR laser diode beams, especially for spectroscopic applications. The detector consists of a polyvinilidene difluoride pyroelectric radiation sensor of 434 mm2 total surface area and of a four-channel high gain transimpedance amplifier, followed by a narrow bandwidth resonant filter for noise rejection. The overall single pixel responsivity resulted 3590Vroot mean square /W¯ at 5.8 mm, and modulation frequency of 85 Hz. The beam spot centroid of a square-wave modulated laser diode emitting 85mW¯ was determined using an algorithm evaluating the difference signals between diagonal elements along vertical and horizontal scanning directions. An uncertainty in the centroid position of 6 mm, in both directions, was obtained, mainly limited by the digitizing resolution of the 12-bit acquisition system.