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How to reduce laser speckle noise for an imaging system that demands integration times of microseconds? I am aware of these two commercial solutions:

  • Optotune laser speckle reducer. It has a frequency of 300 Hz, so it wouldn't work.
  • Nanophoton SK-11 speckle reducer. It is well suited for lasers with coherence length of at most a few meters, mainly pulsed lasers. We are working with a CW solid-state laser.

Up to now the nearest solution I have found is reported in the paper "Low-loss high-speed speckle reduction using a colloidal dispersion".

Does some one now about another solutions? A commercial one or easy and cheap to implement is preferred

Edit:

Our application of interest is Raman imaging, and CW laser is the typical illumination source, so for now I going to stick to it.

Following the suggestion of hsinghal and CuriosOne about modulation and AOM, I looked for references about the use of AOM in speckle reduction and I found this paper "Laser speckle reduction based on angular diversity induced by Piezoelectric Benders" that seems promising. I'll let you know if we decide to give it a try.

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    $\begingroup$ Try modulating your laser (strongly) with a 10-100MHz AC signal. While I am not sure that this will be enough (or even work, at all), it should give some phase variability that might reduce the problem. $\endgroup$
    – CuriousOne
    Commented Jun 29, 2016 at 17:07

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As you know that the speckles in the laser comes from the low bandwidth of the laser. Hence to reduce the speckles one need to increase the bandwidth of the laser or to reduce the coherence properties of the laser.

Similar to the reference given by you same authors also published another findings in nature photonics.

The best solution would be to use a shorter pulsed laser (Q switched Nd:Glass lasers have bandwidth of several nanometers). Pulsed lasers are more powerful than the CW lasers and have large bandwidth. They can also shorten the exposure times but I believe this is out of question.

Next solution is to use different low quality diode lasers. Diodes have larger bandwidth and hence create lesser speckles, this may be a cheaper solution.

A diffuser may certainly help in reducing the speckle pattern but will cost you with added divergence and low illumination.

I think if you can increase your exposure to 100 msec or more Optotune laser speckle reducer may work.

I can suggest one solution but I am not sure it will work. There are several acousto optic modulator (AOM) available in market which can go upto 100 MHz frequency. AOM can modulate the polarization of the laser at MHz frequency and hence you will see reduced speckle in microsecond exposure due to random positioning of speckles.

I hope it will help.

Regards,

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    $\begingroup$ The OP already said that increasing integration time is not an option (and one wouldn't do that by changing the "quality" of the photodiode, to begin with, but by changing the amplifier). The problem here has to be attacked on the ns-scale, i.e. a modulator with >100MHz bandwidth would be necessary. $\endgroup$
    – CuriousOne
    Commented Jun 29, 2016 at 17:00
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    $\begingroup$ AOM of frequencies upto 100 MHz is available. If integration time is of the order of microseconds then it should work. $\endgroup$
    – hsinghal
    Commented Jun 30, 2016 at 0:58
  • $\begingroup$ I know, but is it cheap? $\endgroup$
    – CuriousOne
    Commented Jun 30, 2016 at 1:02
  • $\begingroup$ No may be ~2000 dollar I have one I can tell the exact price tomorrow . $\endgroup$
    – hsinghal
    Commented Jun 30, 2016 at 1:12
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    $\begingroup$ @CarlosMendoza It looks promising, You dont have to fiddle with the laser cavity in this case, hence fairly straight forward. However the bandwidth of the piezo actuators may be limited. Please follow this link, go to specs tab. You will find that Thorlabs piezo actuators (just an indication) supports upto 100-1000 Hz bandwidth. The article you have mentioned have 1/34 sec exposure time and that's why PZT can reduce speckles in that case. SLM are also used but in millisecond exposure times. $\endgroup$
    – hsinghal
    Commented Jun 30, 2016 at 17:38

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