Re: Shearing Interferometer with Ti-Sapph

Posted by Craig Brideau on
URL: http://confocal-microscopy-list.275.s1.nabble.com/Shearing-Interferometer-with-Ti-Sapph-tp7585684p7585691.html

***** To join, leave or search the confocal microscopy listserv, go to: http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy Post images on http://www.imgur.com and include the link in your posting. *****

The way I check collimation in a Ti:Saph is using a cheap CCD or CMOS camera placed at a few different points in the beam. As long as the IR filter has been removed, the camera should be able to 'see' the beam projected onto the array. Do note you will want some good reflective ND filters on there to keep from burning the camera! You want to take pictures at several points along the path, or just use mirrors to extend the path while leaving the sensor stationary. You can get a rough measurement of spot size vs. distance which will give you some indication of where the Rayleigh range of the beam is located.

And yes, coherence length of a Ti:Saph is typically ~30um, so it won't work with a shearing interferometer. Also, running the Ti:Saph in CW may produce a slightly different vergence out of the laser. Finally, don't sweat collimation too much, you can always put a pair of good achromats into the beam as a telescope to adjust the beam waist position.

Craig

On Wed, Sep 21, 2016 at 8:50 AM, Michael Giacomelli <[hidden email]> wrote:

*****
To join, leave or search the confocal microscopy listserv, go to:
http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
Post images on http://www.imgur.com and include the link in your posting.
*****

Hi Tim,

A shearing interferometer is not going to work with an ultrafast
laser.  There is a note in the thorlabs catalog that can help you
figure out what it will work with:

https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=2970

The model you have has a 21.16 mm difference in optical pathlength, so
you will need a coherence length at least that long.  Try your laser
in CW mode, although even that may not be quite 2 cm depending on how
stable it is without a mode lock.  Usually though testing for
collimation after a beam expander doesn't require an interferometer
since the beam is large the divergence is low.  Have you tried just
bouncing it across the room and back with a mirror?

Mike

On Wed, Sep 21, 2016 at 12:07 AM, Tim <[hidden email]> wrote:
> *****
> To join, leave or search the confocal microscopy listserv, go to:
> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> Post images on http://www.imgur.com and include the link in your posting.
> *****
>
> Hello, we recently purchased a shearing interferometer from Thorlabs (Part # SI254) to test collimation after beam expansion in our two-photon microscope setup. The strange thing is we cannot seem to get an interference pattern either before or after beam expansion (varying the distance between the lenses in the beam expander). We do see the pattern clearly if we try on a simple laser diode. Does anyone have experience looking at the shearing interferometer pattern with a Ti-Sapphire laser or have any ideas what may be causing our problem?
>
> Thanks for any help,
> Tim