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

[Apprenticez]

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!

[swansont]

If the PZT only moves 375 nm you should only see one fringe, but if you are scanning the PZT the number of fringes on the scope might depend on the time setting, i.e. you could be seeing the same fringe repeatedly. Is the scope triggering off of the PZT source?

 

How are you getting the reference mirror to move in 8.8 mm increments?

[Apprenticez]

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).

Edited October 29, 2013 by Apprenticez

[Enthalpy]

I can't imagine the light as emitted by the diode regaining coherence every 8.8mm.

But if you have a resonator on the path - with many echoes in the resonator, since you see several coherences - it could be an explanation.

Something like a mirror that is metallized at its rear face, or a dichroic mirror reflecting from the second face, or maybe a filter, a beamsplitter...

Look for something as thick as 4.4mm air, for instance 3mm glass.

[Apprenticez]

Thanks!

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

[Enthalpy]

Semiconductors have indices like 12, so phase coherence might - could have - recover within the diode if it's some 0.4mm long. Though, this would hint at a bizarre lasing mode, something like superradiance, where coherence lasts for 150µm but is recovered cleanly after a full ping-pong path in the lasing medium.

 

I'd search at other parts of the experiment, like the splitter, the mirrors...

 

You can decide between the diode and the rest by building a simpler experiment: a resonating cavity without the interferometer. Take semireflective mirrors with antireflection coating, check if you obtain resonances when the mirrors are 4.4mm apart. Or more convincing: make two-slit interferences, offset the diode, observe if you get fringes again when the paths differ by 8.8mm.

[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.

[Enthalpy]

"one TEM00 mode but multi longitudinal modes" and

"substructure with periodically spaced longitudinal modes"

are other words for

"superradiance modes".

 

I suppose electrons and holes in a laser diode have energies too varied, so they don't engage all on a common transition energy. Instead, light is created at (around) one energy, this depletes within 150µm/c the corresponding available stocks of carriers, then lasing goes on at a different energy, and so on until the echo of a pulse around one energy comes back and finds again enough carriers to lase.

 

This is compatible with a constant emission power. Only the emitted wavelength hops every 150µm/c. And if carriers are replenished or thermalized quickly enough, more pulses can happen around one energy within a ping-pong delay in the material.

 

As opposed, He-Ne offer a more uniform transition energy, enabling a quiet operation with long coherence.