Design of Closed Loop Mach-Zehnder Interferometer for Path Length Stability

Optical coherent detection techniques are important techniques for accurate detection of optical signals. Optical coherent detection consists of Mach-Zehnder interferometer which provides beating between two optical beams. Inside an interferometer an optical signal is divided into two parts with a beam splitter. These two beams travel in test and reference arms of optical interferometer and gets combined at another beam splitter. If the path lengths traveled by test beam and reference beam is equal inside an interferometer, then test beam and reference beam shows constant phase relationship with each other. The constant phase relationship between two beams is the most important characteristics of an interferometer as it enables extraction of modulated frequency components in the test arm. If one of the mirrors in Mach-Zehnder interferometer is mounted on piezo actuator, then it is possible to precisely control the path difference between two beams. In this research paper, a PID controlled closed loop operation of Mach-Zehnder interferometer is demonstrated.