SiliziumQPDs

SiliziumQPDs are silicon quadrant photodiodes designed for fast two-dimensional beam position sensing. A typical device contains four identical photodiode elements arranged in a square with a common back contact. When light falls on the surface, each quadrant produces a photocurrent proportional to the local intensity. The four currents are read out separately, and the beam position is inferred from their relative values by forming sums and differences. A common approach defines I_right = I1 + I4, I_left = I2 + I3, I_top = I1 + I2, and I_bottom = I3 + I4; the normalized x position is (I_right − I_left)/Itot and the normalized y position is (I_top − I_bottom)/Itot, where Itot is the total photocurrent.

SiliziumQPDs have fast response times, enabling closed-loop beam steering and vibration measurements. They are most sensitive in the visible range and up to roughly 1100 nm with decreasing sensitivity; typical responsivity depends on wavelength. They’re used in laser beam alignment and pointing, optical trapping, autofocus in imaging systems, scanning and microscopy, and as centroid detectors in optical metrology. Variants include monolithic QPDs in a single silicon die, and infrared QPDs using InGaAs for longer wavelengths. Some devices include built-in transimpedance amplifiers to improve signal-to-noise and enable single-ended or differential readout. In practice SiliziumQPDs are valued for fast, low-noise position sensing in compact form factors, though they require careful calibration and linearization to maximize accuracy.