Apprenticez

Ok. I got the answer. Finally. Asked the manufacturer more info about the laser. The laser has one TEM00 mode but multi longitudinal modes. This causes the coherence function to have substructure with periodically spaced longitudinal modes. The spacing is close to double cavity length. The manufacturer calls coherence length the length of a single substructural peak, not the FWHM of the coherence function. The length of the coherence function is much greater than listed 150 microns.

Thanks! I will try to get more info from the laser manufacturer about the resonator.

I see the same 6-8 fringes repeated on the oscilloscope. I have done rough calculations and looks like 3-4 fringes (oscillations with Doppler frequency) per mirror move is what I should get. Oscilloscope is using internal triggering, but triggering is not the problem here. The problem is I see Doppler modulated coherence function every 8.8 mm when I move the reference mirror with micrometer (towards/away from the beamsplitter).

Hello, I have a problem with OCT setup I am trying to make. The setup is based on Michelson interferometer with mirrors in the sample and the reference arms. Light source is 639 nm high-power (up to 150 mW) diode laser running in CW mode with coherence length ~150 microns. After the laser I have non-polarizing cubic beamsplitter (50%/50%). The mirror in reference arm is modulated by PZT (2.3 kHz, ~375 nm amplitude). After the reference and sample beams combined I have high-speed photodetector connected to the oscilloscope with 50 Ohms termination load. The photodetector is DET36A/M provided by Thorlabs (rise time ~14 ns, 350-1100 nm wavelength range, 13 mm2 active area). I am supposed to get only one reference mirror position when I have constructive interference. In the case of constructive interference on the oscilloscope I see periodic 6-8 Intensity oscillations (mirror goes forward and backward) with Doppler frequency. The problem is in reality I have repetitive constructive interferences ~ every 8.8 mm (reference mirror position is 0 mm, 8.8 mm, 17.6 mm, etc.). And the oscillation amplitude on the oscilloscope is almost the same. Can you please give me any suggestions/comments what can be wrong in the setup? Thanks!