Breaking the coherence of an optical signal

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Discussion Overview

The discussion revolves around methods to convert a coherent optical signal into an incoherent one, exploring both passive and active techniques. Participants consider various approaches, including the use of nonlinear media, scattering, and optical diffusers, while addressing the complexities of coherence in optical signals.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that a nonlinear medium could dephase parts of a coherent signal, but notes the complexity of coherence length in optical signals, particularly with lasers.
  • Another proposes using a scattering medium or low-pressure gas to achieve incoherence, mentioning that the random positions of scatterers can lead to incoherent light.
  • A different participant mentions the use of counter-rotating ground glass sheets for producing spatially incoherent light, while expressing uncertainty about reducing temporal coherence without complex nonlinear processes.
  • Another suggestion includes using photonic-crystal fiber to potentially decrease temporal coherence.
  • One participant recommends optical diffusers as a method, and also mentions the possibility of using fluorescent centers or non-phase matched second harmonic generation (SHG) signals, albeit with a change in wavelengths.

Areas of Agreement / Disagreement

Participants express various methods and ideas, but there is no consensus on a single effective approach. Multiple competing views and techniques are presented, indicating that the discussion remains unresolved.

Contextual Notes

Participants highlight the complexity of coherence length and the challenges in manipulating temporal coherence, suggesting that some methods may require specific conditions or may not be practical.

narra
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Hi,

I am curious to know if there any passive (or simple active) methods to convert a very coherent signal into an incoherent one? I suppose I would need to find a way to chirp the signal and a non-linear medium might be one method. Any ideas?

Thanks

narra
 
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This is a pretty vague question... certainly a nonlinear medium could be used to dephase parts of a signal... but it is true that any coherent optical signal has a finite "coherence length", which basically is the length over which the light travels while maintaining a certain degree of coherence. So you could just increase length of the path over which your signal travels (maybe if you have a long fiber optic cable?). But coherence of an optical signal can be a tricky thing, especially for lasers, where coherence can come in and out as you increase path lengths.

Can you be any more specific with your question?
 
In theory, you can lead the light into scattering medium, low pressure gas or other rarified substance and collect the radiation at right angle. If the distances of the scatterers are larger than the wavelength, they will give you incoherent light of the same frequency, as they have random positions. I do not know whether it is practicable though.
 
narra said:
I am curious to know if there any passive (or simple active) methods to convert a very coherent signal into an incoherent one?<snip>

The standard way to produce (spatially) incoherent light is to pass it through counter-rotating ground glass sheets. If, on the other hand, you want to decrease the *temporal* coherence, I'm not sure you can do it without complicated nonlinear processes, although it may be possible to simply use a length of photonic-crystal ("holey") fiber.
 

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