Recently I have read something about the laser surface velocimeter, and after I have to rebuild from scratch (university project), I got a question: In the description they wrote that one of the laser beams is shifted with around 40 MHz compared to the other one. With this shift it is possible to see if the device is working and in which direction the surface is moving.
Second question: How difficult is it to create such an device, and what should I consider?
For explanation as requested:
This project is done for the university racing team, the device should be build into the race car for calibration. Time: Always if I have nothing to do, Equipment: I can use the local university equipment here, and budget: I hope I do not need more than the stuff which is available here (except for finalisation). Is this feasible for a one-man-project (master student, already built some interferometers)?
Ah, and btw., the distance of the moving object is between 5 to 10 cm, but the speed goes up to 30 m/s, so is this a big problem (I thought I just have to use a faster photodetector (~30 MHz clocking rate according to the formulas on wiki))?
Best Answer
There are two typical ways of generating two laser beams with a precise frequency offset from each other.
The first requires you to have a specialized device known an an acousto-optic modulator (AOM). These devices create a standing pressure wave in a specialized crystal. This pressure wave acts as a diffraction grating in which the different diffraction orders are frequency shifted by $\pm nf$ where $n$ is the diffraction order and $f$ is the modulation frequency.
The second way which is easier to work with, and cheaper if you have two lasers but no AOM, is to phase lock the two lasers to each other with an offset. To do so you combine the two lasers on a fast photodetector and tune them until their frequencies are close enough together to see the beatnote. Electronically mixing that beatnote with a reference oscillator provides an error signal for use in a feedback loop to lock one laser to the other. There is a description of the technique here.