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Thread: What can cause periodic interference (constructive interference occurs every 9 mm when the reference mirror is shifted) in OCT setup?

  1. #1 What can cause periodic interference (constructive interference occurs every 9 mm when the reference mirror is shifted) in OCT setup? 
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    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!


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  3. #2  
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    Quote Originally Posted by Apprenticez View Post

    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!
    There is nothing wrong with your setup, constructive (or destructive) interference is supposed to repeat periodically. See here.


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  4. #3  
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    It should not happen with 150 microns coherence length.
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    Quote Originally Posted by Apprenticez View Post
    It should not happen with 150 microns coherence length.
    The maximum (minimum) interference periodicity is not a function of coherence length, the strength of the maximum (minimum) is.
    (Wave interference is strong when the paths taken by all of the interfering waves differ by less than the coherence length.)
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  6. #5  
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    There might be misunderstanding.
    I see 6-8 periodically repeated Doppler frequency oscillations. And I see them when I shift the reference mirror about 300 microns. Everything is fine here.
    The problem is I see Doppler modulated coherence function every 8.8 mm.
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    Quote Originally Posted by Apprenticez View Post
    There might be misunderstanding.
    I see 6-8 periodically repeated Doppler frequency oscillations. And I see them when I shift the reference mirror about 300 microns. Everything is fine here.
    The problem is I see Doppler modulated coherence function every 8.8 mm.
    There is no Doppler effect in MMX, the source and the receiver are stationary wrt. each other, so you cannot see any Doppler effect.
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  8. #7  
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    The reference mirror is modulated with PZT. Oscillation frequency is 2.3 kHz. Amplitude is ~ 375 nm. There is a Doppler shifted reference beam.
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    Quote Originally Posted by Apprenticez View Post
    The reference mirror is modulated with PZT. Oscillation frequency is 2.3 kHz. Amplitude is ~ 375 nm. There is a Doppler shifted reference beam.
    OK, I see.
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  10. #9  
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    Isn`t it caused by spatial coherence?
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  11. #10  
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    Ok. I got the answer.
    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.
    Last edited by Apprenticez; October 30th, 2013 at 01:06 PM.
    Gere likes this.
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  12. #11  
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    Quote Originally Posted by Apprenticez View Post
    Ok. I got the answer.
    The laser has one TEM00 mode but multi longitudinal modes.
    This causes the coherence function to have substructure with periodicalli 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.
    Aha!
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