Most of my development tasks are usually in the digital world, so this detector was a welcome new task.
The Metrology lasers are picked up by commercial laser detector diodes. The signal from the detector is noisy, suffers interference and has an amplitude of a view micro volts only. A high-speed JFET opamp stage amplifies the signal first before it is fed into a Sallen-Key bandpass filter to remove erroneous signals. An additional amplification is applied before the conditioned signal is fed into a precise zero-crossing detector. The resulting digital signal is transported to the remote Metrology system using LVDS. Because the phase shift between the detected signals is essential for the Metrology, the Sallen-Key filter is optimized to meet a minimum phase- shift difference response over the signal spectrum.
The system also measures the signal amplitude which can be read remotely for future automatic alignment systems.