Re: Faraday rotators and isolators

Posted by Craig Brideau on
URL: http://confocal-microscopy-list.275.s1.nabble.com/image-databases-tp591976p591982.html

Search the CONFOCAL archive at http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal The Faraday isolator gives the best isolation, but if you use a 1/4 waveplate and a polarizer you can also make a basic isolator.  The drawback is: If your laser is tuneable you will need a BROADBAND 1/4 waveplate, which costs about $800 -starting to approach the cost of the Faraday isolator.  The 1/4 waveplate method also means that your laser (post isolator) will have circular polarization.  If you are doing any non-linear work with your microscope this can reduce the efficiency of some interactions.  The Faraday isolator allows you to retain linear polarization, and tends to have better overall isolation.  Again, if you pick one with a short crystal path you can keep the dispersion down to a minimum.
Also, the dispersion you would expect to see totally depends on the bandwidth of your laser.  If your laser is 130fs or greater, with transform limited bandwidth, your dispersion will be minimal, even through a thick isolator.  If your pulse width is below 130fs, then you will need to find the shortest-path isolator you can.

Craig Brideau

On Feb 15, 2008 10:53 AM, Ann Haberman <[hidden email]> wrote:

Search the CONFOCAL archive at http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal

Dear List,

Thank you all for the many helpful suggestions.

One of the manufacturers of a Faraday isolator feels that a rotator by itself would be insufficient to reduce the reflected laser and indicated that a isolator would be necessary. Given the amount of dispersion I would get with just a rotator, I'm loathe to get an isolator. Their assumption is that the polarization of the reflected laser will be identical to the laser output. Is likely that it would be different ?

The suggestion of using a 1/2 or 1/4 waveplate sounds like a good idea. Would there be less dispersion in this arrangement? What would be disadvantages of this compared to the rotator?

Many thanks again for this helpful advice,

Ann

Search the CONFOCAL archive at http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal A Faraday rotator isolator is a large chunk of crystal.  It will cause a lot of dispersion to your pulses if you are using a femtosecond laser.  Our own laser went from 70fs to 300fs after passing through ours.  This effect can be minimized by getting as short a crystal as possible in your isolator.

EOT http://www.eotech.com/ has decent isolators although it can be hard to tell which one has the shortest crystal.  We originally selected ours based on the assumption that the smallest entrance aperture will have the shortest crystal.  This turned out to not be the case, so check with the company before buying.  Last I heard recently that their 8mm aperture unit:
http://www.eotech.com/store/products.php?categoryParentName=Faraday+Rotators+%26+Isolators&categoryName=Broadband++%28Ti%3ASapphire%29+Rotators+%26+Isolators
actually had the shortest crystal, but make sure to check with the company first.
Regarding the utility of an isolator, if you have any reflections at all having one is critical for the stability of your laser.  In some laser designs the reflected light can actually be physically harmful to the laser as well!

Craig

On Thu, Feb 14, 2008 at 11:51 AM, Ann Haberman <[hidden email]> wrote:

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http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal

Dear List,

I am considering placing a Faraday rotator or isolator in front of a
two-photon laser and am wondering what others have experienced with
such a configuration.

I am currently experiencing a lot of "feedback", ie return reflected
laser, that is wrecking havoc with my laser's stability. When it
senses the reflected light, it can not maintain a mode locked state
and can only lase in CW.

I'm heard that a Faraday rotator should be sufficient, but I am
wondering if an isolator is necessary to stop the reflected light
from entering the cavity of the laser source. Also, I would love to
get a recommendation for a reliable company that makes them.

Many thanks for any advice,
Ann
--

Ann Haberman, PhD
Department of Laboratory Medicine
Yale University School of Medicine
1 Gilbert  St.
TAC S541
New Haven, CT 06510

203-785-7349
203-785-5415 (fax)
[hidden email]

-- 

Ann Haberman, PhD
Department of Laboratory Medicine
Yale University School of Medicine
1 Gilbert  St.
TAC S541
New Haven, CT 06510

203-785-7349
203-785-5415 (fax)
[hidden email]