Scattered light from AOTF

[Matthew Nicholas-2]

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


--
Matthew Nicholas
Medical Scientist Training Program Student
Laboratory of Arne Gennerich
Department of Anatomy and Structural Biology
Albert Einstein College of Medicine
Forchheimer Building, Room 628
1300 Morris Park Avenue
Bronx, New York 10461
718.430.3446
[hidden email]

[Craig Brideau]

As a simple solution, can you put a mechanical block directly in front
of each laser?  Some matte black painted metal will do the job.  It
would be a bit of a pain to have to put in and take out the blocks
manually but it would be cheap and get the job done.  This assumes you
have relatively easy access to the front of the lasers to insert the
blocks, of course.

Craig


On Thu, Jul 29, 2010 at 1:59 PM, Matthew Nicholas
<[hidden email]> 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
>
>
> --
> Matthew Nicholas
> Medical Scientist Training Program Student
> Laboratory of Arne Gennerich
> Department of Anatomy and Structural Biology
> Albert Einstein College of Medicine
> Forchheimer Building, Room 628
> 1300 Morris Park Avenue
> Bronx, New York 10461
> 718.430.3446
> [hidden email]

[Mark Cannell]

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
>
>

[Nico Stuurman]

Hi Matthew,

> 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.

How much light is there?  As far as I know, there is always some  
"leak" when using an AOTF, and how much leak there is should be  
specified by the vendor (don't have the specs to ours here, but  
remember it to be somewhere on the order of 10E-6).

Is this really an issue in your system?  In the systems I set up  
(spinning disk, TIRF), this low amount of leak has never been a  
problem, since the emission filter blocks the excitation light  
anyways, and the very low amount of leaking light does not damage the  
sample.

Best,

Nico