Re: Scattered light from AOTF

Posted by Mark Cannell on
URL: http://confocal-microscopy-list.275.s1.nabble.com/Scattered-light-from-AOTF-tp5352866p5352994.html

Hi Matt

Its hard to diagnose remotely what you can't see, but is it related to
laser speckle from your sources? Can you take the lid off the AOTF and
clean the faces to reduce/elimiminate scatter there?

Hope this helps

Mark


Matthew Nicholas wrote:

> Hi All,
> I have what seems like a trivial question, but I cannot seem to sort
> it out, and wondered if others have experience/solutions to offer. I
> have two excitation lasers combined (i.e. overlapped) via a dichroic
> mirror and directed into an AA AOTF (opto.braggcell.com). The AOTF is
> used to shutter the beams and control their intensities. Overall, the
> system works well. Back reflections from the front face of the AOTF
> are very nearly colinear with the incoming beams (purposely slightly
> mis-aligned to prevent back-reflection into the lasers), as are
> first-order diffracted beams. The zero-order beams are reflected by a
> pick-off mirror into a light trap. Following the AOTF, the first-order
> beams are expanded by a 10X beam expander (i.e. telescope), after
> which they enter our microscope. The faces of the crytsal in the AOTF
> and dichroic mirror are free of dust, fingerprints, etc. The input
> beam diameter is approximately 1-2 mm, and strikes the AOTF input
> aperture very nearly at the center.
>
> The problem is, even when all laser lines are off (or when the AOTF
> itself is powered-down), there is minimal, but noticable scattered
> light exiting the AOTF. Some of this light is collected by the beam
> expander and enters the microscope (this is what I want to eliminate).
> I've more or less ruled-out that the scattering occurs after the AOTF,
> and believe it may be due to reflections at the front surface
> (entrance) of the AOTF or within it. Since it occurs even when the
> AOTF is off, it does not seem to be an acousto-optic effect. My
> current thought is to move the AOTF farther from the telescope, and to
> use a smaller aperture lens at the telescope input, but what I'd
> really like to do is eliminate the scattered light. I've also thought
> of using a pinhole as a spatial filter at the telescope focus, but
> since the scattered light is focused at more or less the same spot,
> the hole would have to be fairly small, and I'd like to avoid adding
> another element to the system that is time-consuming to align.
>
> Any advice or ideas would be much appreciated!
>
> Thanks in advance,
> Matt
>
>