Natural Science Forum / Physics / Optics / December 2008

Thanks for your answer. I also think that the coating or the AFC
connector is the best solution, but I want to be sure that the cause
is back-reflection itself. I'm glad to see that another people thinks
the same thing :-)

I've measured the output power, in an hour, and that's the result.

http://img201.imageshack.us/my.php?image=gruy4.gif

As you see, in this way it's pratically useless...

Do you know any method for decrease backreflection without change the
fiber?

Daniele

IT  is a polarized hene?

Steve Roberts

I'm saying it is one possibility, though the etalon effect of two
parallel surfaces seems more likely in your case but could be a
combination.

Angled optics to divert the backreflection would help.

An isolator would help.

Do the tests we have been suggesting and report back.

--
   sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/ 
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/ 
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm 
       | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line.  Or, you can contact me via the Feedback Form in the FAQs.

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

@Mr. jk: I've tried to align polarization and fiber axis, as you can
see below
@Mr. Goldwasser: As you can see below, it's not easy for me to angle
the optics. I'm trying to put a polarizer in the mechanical adapter to
decrease the laser power, to see if in this case the effect will be
reduced.
@Mr: Abe: I obtain a coupling efficency of about 50%. They want to
mantain the PM fiber because my boss want to have the greater power at
the exit of the fiber. If I use a SM fiber, with the coupling eficency
of 50%, and then I'll cut half power with a polarizer, I'll have the
25% of efficency, and they say that it's not so good.

I've taken also some photos.

Here, you can see the schematic of my laser, that's really basic:

http://img518.imageshack.us/img518/6839/disegnoxg8.png

As you can see, there's only the basic things: laser, fiber, input and
output coupler.

You can see here the input coupler:

http://img389.imageshack.us/img389/263/cimg1478001er8.jpg
http://img370.imageshack.us/img370/2150/cimg1479002ni4.jpg

You can notice the presence of a mechanical adapter, This is necessary
because the coupler must be fixated with three screws at 120°, while
the laser head has four screw holes at 90° (this is always because my
boss has discarder my list and has buy what he wanted :-( ).

In the first image you can see the fiber key, that's on the right. In
this case, according to the datasheet, I must to have the slow axis of
the fiber in vertical position, while in the second photo you can see
the polarization axis of the laser, and them are aligned.

Here you can see the output coupler:

http://img366.imageshack.us/img366/550/cimg1480003kt1.jpg
http://img361.imageshack.us/img361/4746/cimg1481004sq1.jpg

The coupler is mounted with a positioner (because I must to align the
beam, in their experiments). As you can see in the second image, I
have the beam at the output of the laser, and that's the spot with
oscillating power.

I've tried to say to the mechanic that we have, to create in the
mechanical adapter of the input coupler, an hole to place a polarizer
filter, to attenuate the laser beam, but it's not so simple,
considering that I'd like to move it without toggle the coupler (in
this case, I must to toggle the coupler, change the rotation of the
filter, re-attach the coupler and re-align the lens, a long and
tedious work).

Have you any suggestion about hot to insert the filter between the
laser output and the coupler?

Thanks again

Daniele

PS: Sorry for my bad english.