A low cost position sensor for applications with frequency modulated IR laser emission

The use of IR laser diodes associated to modulation spectroscopy is increasing in importance for many environmental monitoring applications. However, small and cheap sensors are required for the alignment of the involved low power laser beams. Polymer PVDF foils offered the possibility to develop high sensitivity, small dimension, pyroelectric detectors for mid IR laser applications. This paper presents a new position sensitive quadrant detector with a total active area of 4x4 mm, directly connected to a transimpedance amplifier and a band pass filter. The four analog signals from the sensor are sampled, filtered and amplified with a 12 bit data acquisition board, equipped with a DSP that gives the real time rms values and standard deviation of the difference signals between two diagonal elements. The spot centroid of diode laser emitting peak powers of about 20 W is determined with a beam position uncertainty of 20 m, corresponding to 5 % of the beam spot. For this application the overall sensitivity was 3000 V/W and the modulation frequency 85 Hz. The uncertainty is reduced to 0.5 % with 200 W diode power