Re: Shearing Interferometer with Ti-Sapph

Posted by Zdenek Svindrych-2 on
URL: http://confocal-microscopy-list.275.s1.nabble.com/Shearing-Interferometer-with-Ti-Sapph-tp7585684p7585687.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. ***** Hi Tim,

no experience with shearing interferometry, but few thoughts:

As pointed out, 100 fs pulse has 'physical length' (s=c*t) of some 30 um. With the mentioned interferometer (more than 20 mm path difference according to Thorlabs specs!) you end up with two copies of the pulse separated by 70 ps. They will only interfere if you watch them through extremely narrow-band filter (just a theory, probably not practical at all).

Even in CW, as suggested by Mark (not all Ti:saphs can be easily forced to CW) the coherence length may not be enough to see fringes, depending on the laser.

One solution would be zero path difference interferometer, which can be constructed with common path geometry:

http://www.sciencedirect.com/science/article/pii/S0030402605002226

But I'm not aware of it being available commercially.

There are other options to measure collimation, like wavefront sensors (quite costly) or watching the far field of the beam (needs lots of space or some already collimated optics)...

Good luck!

--
Zdenek Svindrych, Ph.D.
W.M. Keck Center for Cellular Imaging (PLSB 003)
University of Virginia, Charlottesville, VA
http://www.kcci.virginia.edu/
tel: [phone]
Annual FRET Workshop: http://kcci.virginia.edu/workshop-2017

---------- Původní zpráva ----------
Od: Joshua Vaughan <[hidden email]>
Komu: [hidden email]
Datum: 21. 9. 2016 0:40:55
Předmět: Re: Shearing Interferometer with Ti-Sapph

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Here's a shot in the dark here for you, since I have never tried doing what you asked about.

Short pulse lasers only interfere over a short distance called their coherence length. It might only be 10-30 microns or so depending on the laser's bandwidth. If the path length difference of the two interfering beams is more than this 10-30 microns, then no fringes.

If the interferometer works by beam 1 reflecting off the front of the glass wedge (going through no glass) and beam 2 reflecting off the back surface of the glass at a little different angle (going through the glass twice), then the optical path length of beam 2 is quite a bit more than that of beam 1. Probably by much more than 10-30 microns since the wedge is probably at least 1 mm thick where you use it. I would guess no fringes.

Basically, I am asking whether your interferometer is designed for short pulse lasers. I bet the Thorlabs folks could weigh in on it if you email them.

Anyway, isn't it easier to check collimation by shooting the beam across the room and checking the diameter is constant?

On Tue, Sep 20, 2016 at 9:07 PM, Tim <[hidden email]> wrote:

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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

--

Joshua C. Vaughan

Assistant Professor

Department of Chemistry

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University of Washington

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