Subtleties in the use of a quadrant cell photodiode in an optical lever*

Quadrant cell photodiodes are often used in an optical lever to deliver a digital “zero-crossing” signal (either for determining the period or for a static position signal). However, one can also continuously monitor the analog output of a quad-cell photodiode to determine time dependent positioning information (albeit over a narrow range). Unfortunately, the response of a quadrant cell photodiode to the passage of a gaussian laser beam is a non-trivial function of the beam radius. We have created a detailed (python based) computational model of a quadrant cell photodiode, and compared the model to experimental measurements. We remark the accuracy of the model as well as some important frequency spectrum implications of the detector response which are very relevant when analyzing the power spectrum of a quadrant cell signal.