Fiber-based Near-Infrared Heterodyne Interferometer

This is the setup for the initial static ground tests, and quasi-static sky tests (north-south direction at highest elevation). For the east-west sky experiment the fringing has to be stopped. For this, two fiber-frequency shifters are inserted before the laser signal enters the 90/10 splitters at the telescopes. The one is tuned for 50MHz, the other for 50 MHz+b*47Hz/m (b is the projected baseline length).  The beat on the photodiode would then occur at a fixed frequency of 2*b*47Hz/m instead of at DC. In front of the PID will then be added a frequency-to-voltage converter.