Quaddetektor

Quaddetektor, commonly called quad detector or quadrant detector, is a photodetector that uses four sensing regions arranged in a square on a single substrate. It is used to measure the spatial properties of an optical beam, such as its position, centroid, and power distribution, with high speed and simplicity.

Each quadrant outputs a photocurrent proportional to the light it receives. By comparing the four signals, the device derives the beam’s lateral position (x and y) and total optical power. The approach provides fast response and can be built from silicon, germanium, or compound semiconductors to cover visible to infrared wavelengths.

Construction typically involves a four-segment photodiode (quad cell) or a quad arrangement of individual detectors integrated on a single chip. Most devices include a transimpedance amplifier for each quadrant, producing voltage or current outputs suitable for feedback and control systems. Packaging ensures good optical coupling and minimizes crosstalk between regions.

Signal processing uses simple linear combinations of quadrant outputs. For a reference indexing (top-left, top-right, bottom-left, bottom-right) the total current I = I1+I2+I3+I4. The normalized X and Y position can be approximated as X = [(I2+I4) - (I1+I3)] / I, Y = [(I1+I2) - (I3+I4)] / I, with calibration constants to account for geometry and nonuniform response. Some systems apply nonlinear corrections for large displacements or division by the centroid.

Applications include optical beam alignment and stabilization, laser pointing systems, beam profiling, fiber coupling optimization, and optical trapping setups. They are favored for fast response, simple electronics, and compatibility with feedback loops, though they may suffer from nonlinearity near the edges and sensitivity to beam shape variations.

History and variants: Quadrant detectors emerged in the mid-20th century with the development of practical position-sensing photodiodes and were widely adopted in laser instrumentation and metrology. Variants include larger-area quad cells, dark-current-limited devices, and arrays used in imaging.