Re: Recommendations for commercial multi-photon system purchase

[Aryeh Weiss]

Thank you for your reply. Many years ago, whenI took a modern optics
lab, we looked at the output of a simple unpolarized HeNe laser with a
scanning Fabry Perot optical spectrum analyzer. I recall that with this
device, we  could see the lasing modes moving through the lasing medium
gain curve (because the medium gain curve is much wider then the
bandwidth of the laser cavity). But now that I think about it, a
linearly polarized laser shoudl also suffer from fluctuation if one mode
is drifting across the gain curve envelope.

I think that I just found the answer in the following (taken from
http://www.repairfaq.org/sam/laserhen.htm#henphlpo)

> Most common HeNe laser tubes are randomly polarized since for many
> applications the polarization of the beam doesn't matter. As noted
> elsewhere, the term "random" here really doesn't mean that the
> polarization is necessarily jumping around to totally arbitrary
> orientations. In fact, such behavior would be rather unusual. It just
> means that nothing special is done to control the polarization. The
> typical HeNe laser will lase on several longitudinal modes (how many
> will depend on tube length of the resonator) with adjacent modes
> having polarizations orthogonal to each-other. Each of the modes will
> change their relative intensities periodically over time or may even
> switch polarizations suddenly as the tube heats and expands. For the
> special case of a short tube where only two modes fit under the gain
> curve (typically 5 or 6 inches in length) at the instants when they
> are equal, the output will appear to be non-polarized (constant
> intensity as an external polarizer is rotated in the beam) but as the
> modes shift under the gain curve, one or the other polarization will
> dominate. For longer tubes, there will be much less of an effect
> because there will be multiple modes with both polarizations at all
> times.

So if the tube is short (like the cheap little HeNe I saw in the modern
optics lab) then there may be only 2 modes lasing at any one time, and
change in the cavity length can make a big difference in output power
and polarization. However, with the longer tubes that are used in 2mW
lasers, there are more modes lasing at once, and the average power may
not change much as a mode slides out of the gain envelope and a new mode
comes in.

--aryeh
--
Aryeh Weiss
School of Engineering
Bar Ilan University
Ramat Gan 52900 Israel

Ph:  972-3-5317638
FAX: [phone]



Sudipta Maiti wrote:

> Ususally the polarization changes pretty fast, certainly it does for
> my unpolarized (randomly polarized) HeNe, so that I do not see any
> problems in the image. I also looked at the output fluorescence
> emission  at a 10 microsecond level through a polarizer, and saw no
> fluctuations. Of course, pixel residence times are somewhat smaller
> than 10 microseconds. Sudipta On Thu, 10 Sep 2009, Aryeh Weiss wrote:
>
>
>> Our Nikon rep has recommended to a C1 user to replace a failing
>> 543nm HeNe laser with an unpolarized 2mW laser. This C1 has no
>> AOTF, and combines three beams into a fiber which feeds the
>> scanhead.
>>
>> I was under the impression that using an unpolarized laser is a bad
>>  idea because I recall that in an unpolarized HeNe, two orthogonal
>>  polarization modes lase together, but they drift across the
>> bandwidth of the cavity as the laser warms up. This results in a
>> slowly varying change in the relative polariation of the beam over
>> time. In a system where reflections from dielectric surfaces can be
>> dependent on polarization, this should be a problem. If the fiber
>> is polarization preserving, it should be a big problem.
>>
>> So -- does anyone out there know if my understanding is correct? Is
>>  anyone using unpolarized HeNe lasers with no problem? I assume
>> that Nikon would not be offering that laser if there was such a
>> problem inherent to it, but still...
>>
>> --aryeh
>>
>