Position sensitive device
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A position sensitive device and/or position sensitive detector (PSD) is an optical position sensor (OPS) that can measure a position of a light spot in one or two-dimensions on a sensor surface.
A position sensitive device and/or position sensitive detector (PSD) is an optical position sensor (OPS) that can measure a position of a light spot in one or two-dimensions on a sensor surface. x = k_x \cdot \frac{I_b - I_d}{I_b + I_d} and y = k_y \cdot \frac{I_a - I_c}{I_a + I_c} The k_x and k_y are simple scaling factors, which permit transformation into coordinates. An advantage of this process is the continuous measurement of the light spot position with measuring rates up to over 100 kHz. The dependence of local measurement on form and size of the light spot as well as the nonlinear connection are a disadvantage that can be partly compensated by special electrode shapes. x = k_x \cdot \frac{I_4 - I_3}{I_4+ I_3} and y = k_y \cdot \frac{I_2 - I_1}{I_2 + I_1} The 2-D tetra-lateral PSD has the advantages of fast response, much lower dark current, easy bias application and lower fabrication cost. Its measurement accuracy and resolution is independent of the spot shape and size unlike the quadrant detector which could be easily changed by air turbulence. However, it suffers from the nonlinearity problem. While the position estimate is approximately linear with respect to the real position when the spot is in the center area of the PSD, the relationship becomes nonlinear when the light spot is away from the center. This seriously limits its applications and there are urgent demands for linearity improvement in many applications. To reduce the nonlinearity of 2-D PSD, a new set of formulae have been proposed to estimate the incident light position (Song Cui, Yeng Chai Soh：Linearity indices and linearity improvement of 2-D tetra-lateral position sensitive detector. IEEE Transactions on Electron Devices， Vol. 57, No. 9, pp. 2310-2316, 2010): x = k_{x1} \cdot \frac{I_4 - I_3}{I_0 - 1.02(I_2-I_1)} \cdot \frac{0.7(I_2+I_1) + I_0}{I_0 + 1.02(I_2-I_1)} and y = k_{y1} \cdot \frac{I_2 - I_1}{I_0 - 1.02(I_4-I_3)} \cdot \frac{0.7(I_4+I_3) + I_0}{I_0 + 1.02(I_4-I_3)} where : I_0 = I_1 +I_2 + I_3 + I_4 , and : k_{x1}, k_{y1} are new scale factors. The position estimation results obtained by this set of formulae are simulated below. We assume the light spot is moving in steps in both directions and we plot position estimates on a 2-D plane. Thus a regular grid pattern should be obtained if the estimated position is perfectly linear with the true position. The performance is much better than the previous formulae. Detailed simulations and experiment results can be found in S. Cui's paper. Henrik Andersson, Position Sensitive Detectors : Device Technology and Applications in Spectroscopy. Dissertation, Department of Information Technology and Media, Mid Sweden University, Sundsvall, Sweden, 2008, PDF, Nicola Massari, Lorenzo Gonzo, Massimo Gottardi and Andrea Simoni: High Speed Digital CMOS 2D Optical Position Sensitive Detector. ESSCIRC, Firenze, Italy, September 2002, PDF J. T. Wallmark: A new semiconductor photocell using lateral photoeffect. Proceedings of the IRE, Vol. 45, S. 474-483, 1957
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