Synchronous Detection to Reduce Offsets in Focus Error of an Optical Pickup Unit

The optical pickup unit (OPU) is an optical assembly, which is used for reading and writing digital optical disks. It consists of a laser diode, quadrant photodetector, and other optical components to maintain focus. This arrangement of the laser and photodetector is very similar to those in expensive equipment, such as atomic force microscopes, used for nanoscale metrology. Many researchers have hence investigated the use of an OPU to design low-cost alternatives. However, these nonstandard uses of an OPU are challenging since the OPU has originally been optimized to sense reflections from the highly reflecting surface of an optical disk. In this work, we present a new synchronous detection-based method that improves the performance of the OPU in measuring the reflection from poorly reflecting substrates. This method utilizes the high bandwidth of both the laser and photodetector of the OPU. We excite the laser with a pulsed input and look for a signal at the pulsing frequency in the output from the OPU. This signal is further processed through a phase-sensitive detector. We show that this method reduces the offsets in the measurement and allows us to make effective use of a higher gain to measure the reflection from substrates with poor reflectivity.