Diode for laser doppler detection
In this wiki page, it says that "The initial frequency of the laser is very high (> 1014 Hz), which is higher than the response of the detector. The detector does respond, however, to the beat frequency between the two beams."
This confused me that photodiode response to light intensity, not light's wavelength. Why diode has no response to the reference beam itself?
Thanks,
Neo
1014 Hz means 10^14 Hz, of course. Maximum frequency to be detected by a photodiode is in several 10^9, maximal 10^10 Hz range.
The phototiode will give you a response to the reference beam and it would be a DC signal proportional to the beam intensity. But what you want to measure here is a relative (to the reference beam) frequency shift of back-reflected light beam from your vibrating object. For today we do not have such a instruments to directly measure frequency of the light. Although one can easily measure frequency difference between two beams by doing a spatial overlap of those beams at the active area of the photodetector. See https://www.rp-photonics.com/beat_note.html
I believe the detection frequency is the ON/OFF frequency of the light intensity, not light's wavelength. Right?
"ON/OFF", or more generally intensity modulation frequency.
Some time ago I read about laser sensor which used ICs that designed for handheld radios. And those radio usually below 1GHz frequency, something around 400MHz. I think diode must handle frequency a little higher than that. You can google laser sensor teardown pics. Not sure if it related to your question, but maybe it will help. What would be Doppler shift for 3km/h? I think the same formula may be used as for radars. Also from my understanding some laser sensors only measure distance, and then estimate speed based on continuous distance readings. Because long time ago I have seen PCB of laser sensor which contained pretty old FPGA connected to diode through some simple ICs, I think just amplification and some conditioning. I doubt there may be anything higher than 100MHz, pretty old design. But I may be wrong. Did not researched this topic for a long time. Main problem with lasers is good optics, lenses. At last time I've checked this laser related topic
I see. This makes sense. It is a type of light interference to generate the beat. Thanks.