Re: Problem with Laser-fiber coupler

On Fri, 28 Nov 2008 12:28:40 -0800 (PST), Jepessen
<jepessen@xxxxxxxxx> wrote:

Thanks for your answers

@Mr. jk: I've seen this Application note:
http://www.ozoptics.com/ALLNEW_PDF/DTS0071.pdf
This is the company from which the local reseller has taken fiber and
coupler. In the last two pages you can see the note about alignment.
The connector key is aligned with the slow axis, so, to mantain
polarization with my He-Ne, laser, I must to have this key connector
at the top, considering that the laser is vertically-polarized, right?
Do you think that changing the angle can reduce power oscillations?
And, considering that I actually don't know who have built the fiber,
the slow axis of the fiber core aligned with the key connector, it's a
standard or I can have some other combination? Because for me it's
difficult to rotate the fiber, considering that we must to create a
mechanical adapter between the coupler and the laser head.


Nice drawing they have on the last page !

Could you rotate the laser ? Some small HeNe lasers are tubes.

I know that adjustments for optimum throughput through such a single
mode fiber are extremely critical.

It could be a try and error to find the best alignement of input
polarization. It does not matter if you use the slow or fast axis.

I would make a test setup doing so:

1) get a cheap pol. filter of plastic or use a pol. sun glass or pol.
filter for photo. Use this as an analyzer at the output end. Look at
the thoughput of the beam (at a wall) and turn the analyzer to get
minimum through. Then swing or move the fiber cable and se what
happens. See how much the minimum changes during this, - how sensitive
the setup is to movements.

2) Rotate the laser 10 degrees, readjust foucusing. Now touch the
fiber again and see if it has improved, - less sensitivity or worse

3) repeat

Worst case of misalignment of polarization is 45 degrees. So you
should come to an end.

As I mentioned: a half-wave retarder plate yields an easy adjustment,
since it will swing a linear polarization around. But this component
is quite expensive, say 200$.
They are sold as zero order plates with antiflections coatings. They
are not achromatic, but should be for HeNe.

I developed fiber links for HeNe and Argon-ion laser in the early 1980
...

With High Power Argon laser it was really some kind of a stunt.
Misfocusing could lead to thermal burn away. Further we had thermal
degradation of the fiber core over time (laser induced damage), when
going over 10 watt. The intensity on the area is enormous.
Laser induced damage was not very well understood. But it had
something to do with the creation of longitudinal index variations, so
that the fiber core become an interference filter sending the light
back into the laser. Very funny show !

best regards

John
.

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