Re: APC vs. AFC: confused

Thanks to all of you for your kind help. We now made some progress. After stripping down our system to just the laser and the coupler we were able to get good transmission (about 18mW of 30mW)(at least our fiber is in a good shape). After reinserting of the AOTF we were not able to get such a good result. When we tried to reproduce the good coupling we achieved without the AOTF we failed. Since this one glimpse at what is possible we are in the dark again. So I suspect that it is something with the coupler. But we can not think what that might be. We tried different angles of incident onto the coupler (+/- ~2°-3°)or various insertion depth of the fiber into the coupler (searching for the focus of the coupler lens) but we were not able to find the right position. Without AOTF we can see the different lambdas comprised in the beam in separate patterns. We are on the borders of our knowledge, but we have to find a solution. I will tell you if got got it working in the end.
Thanks again.

Reinhard


danek wrote:
You mentioned that the fiber is jacketed. Depending upon the type of jacket you might be able to carefully remove it then you will be able to inspect it. This is done by passing the fiber between your fingertips which are very sensitive. If you feel any bumps look more closely at the location. The fiber should be smooth end to end. I have had problems with the connectors not being installed properly but they were put on by a company other than Fujikura. To check this tug gently on the connector. My bad ones would come right off even with a gentle pull. You could also try wiggling the fiber at the connector to see if it is damaged. If there is a problem you should see the power levels fluctuate drastically while gently moving the fiber.

P. Danek



David M wrote:

JoS wrote:

Thank you for your reply.
Another thing I did not define well in this discussion was that we
observe this pattern even when we used 514nm which is well above the
cutoff of 470nm.
ad 1)As a source we use an Argon multi wavelength Laser. The beam is
modified by an AOTF which selects the wanted lambda and turns the
polarization by 90°. We had an old setup where the laser the AOTF and
the coupler were in line. Now we changed to a setup where we employ two
mirrors for the adjustment of the beam. So our source now is this laser
then two mirrors then the AOTF and finally the coupler. So the beam
should be polarized parallel to the breadboard and parallel to the slow
axes. We changed the setup because the old one was not stable. And now
with the new one we do not get much light through the fiber.
ad 2)Besides the lenses in the coupler (focal length: 3.5mm, achromat
lens) there are no lenses in the optical path prior to the coupler.
ad 3) I have not been able to find out the numerical aperture and the
attenuation coefficient of our fiber. It is a polarization maintaining
fiber (PANDA). The length is 3m.
With the old setup we go much higher output powers after the fiber.

regards
reinhard



The other options

it is possible that you are coupling nearly all you light into the
cladding, and that although the cladding modes are rapidly attenuated,
there is still a residual at the output of your relativeyl short fibre.
The pattern would be more speckle like with this theory, whereas I
suspect that a coupling of different polarisation modes would have
reasonabley defined lobes.

Of course as you first pointed out your fibre could be damaged.

To check this I'd strip the system down to the laser and fibre coupling
arrangement, and test with no AOTF or mirrors so that these can all be
eliminated as potential causses of your problem. As you have achieved
resonable coupling before it is good to that stage to determine wher
thisngs could be going wrong, and then add one component at a time.

cheers

David

.

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