Polarization Maintaining Fiber...

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Updated: 2005-03-28 by HoloM (the god)
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Larry

Polarization Maintaining Fiber...

Post by Larry »

Has anyone used PM fiber for a spatial filter, or beam expander or object illuminator? Seem like you can put the light exactly where you need it like this.

Is launching light into the fiber anymore difficult that pinhole alignment?


Colin Kaminski

Polarization Maintaining Fiber...

Post by Colin Kaminski »

I have not done this but it is the same as aligning a pinhole that is the same as the NA of the fiber. This is likely on the order of 4um. Also you have a tilt problem that you don't have in a pinhole.

A easy solution is to use normal fiber. This randomizes the polarization but so does a diffuse object so you can use it for object illumination. A normal fiber has a much larger NA. I have launched into a normal fiber with a ball lens with no problems.

Dinesh, can a regular fiber circularize the polarization?
Dinesh

Polarization Maintaining Fiber...

Post by Dinesh »

"can a regular fiber circularize the polarization?"
Ideally, no. The polarisation axes will rotate, as you said, but a single coherent beam down a straight fiber made of non-birefringent material should not introduce phase shifts causing linear polarisation to become circular polarisation. However, when you bend the fiber, you create birefringence in the fiber and this might cause polarisation to go circular depending on the mode and the phase difference along the fast and slow axes. The fast and slow axes go along the plane and perpendicular to the plane of the bend and this diference, the birefringence, is given by
delta(n_eff) = n_perp - n_para = _C((b/R)^2)
where delta(n_eff) is the birefringence
n_perp is the index perpendicular to the fiber bend plane
n_para is the index parallel to the bend plane
R is the radius of the loop.
The greater the difference between the two indices, the greater the birefringence and, as you can see, this difference is proportional to the inverse square of the radius of the bend. So, the greater the bend, the (much!) greater the birefringence and the more prone to phase differnces that cause circular polarisation. Actually, unless you're bl*o*o*dy lucky, probably elliptical polarisation.
If the material of the fiber is itself birefringent, then phase differences may occur without bend, but as far as I know fibers are not birefringent. There are gadgets on the market called "polarisation controllers" basically introducing particular bend radii that use fibers as quarter and half wave plates.

Greg G

Polarization Maintaining Fiber...

Post by Greg G »

Fiber based holography is a lot of fun. The light sources are all "spatially" filtered by 4-5 uM apertures with no alignment, you're not limited to straight line beam paths, the really isolated part of the bench gets a lot smaller since you aren't using much area for beam splitting,etc..and you can put your laser in the next room if you like.

Unfortunately, visible wavelength single mode fiber and fiber devices aren't very common or cheap compared to near IR telecomm devices. And with a core diameter of 8 to 9 uM, telecomm fiber isn't single mode at visible wavelengths.

I had a nice lab for developing things like interferometric fiber sensors at my previous work life and managed to "justify" playing with fiber holography for a while. I bought 30 meters of SM 633nm fiber from Newport and used our fusion splicer and optical polisher to make my own splitters,etc.. I used connectors in places where I jumped to free space optics and spliced the rest of the "connections". Splicing avoids back reflections from fiber to fiber interfaces ( by removing them ) and also eliminates spending time and lots of money on SM connectors and the mechanicals to link them. The optical polisher was used to expose the fiber cores for making SM splitters in the splicer. You can hand polish connectorized SM fiber ends easily if you have the lapping films and tool for holding the connector. Bare fiber ends should be avoided if possible.( Think of sharp, brittle, Xray invisible 125uM diameter glass needles. In your finger or eye... )

Launching laser light into a fiber is similar to a spatial filter but is a little tricky. The "pinhole" ( core of an SM 633nm fiber ) is 4 uM out of 125uM. The remaining 121 uM is cladding where you don't really want the light to be. ( Fortunately, unlike telecomm, you can look at the output pattern of the fiber. ) I attempted to design/make a decent launcher but found Oz Optics makes a very good, inexpensive one with greater than 70% coupling efficiency for connectorized fiber. Connecting splitters,etc... to the output cable is fairly clean and easy with a fusion splicer. Once you're successfully in fiber, leaving it is pretty easy and the setups are pretty traditional once you're shining on the table.

I've used PM fiber for sensors and other apps in dynamic environments but haven't seen it commercially available for visible wavelengths. If someone thought there was a market for it, I would imagine it's rather expensive. But if you're not moving, heating, vibrating fibers much during exposures, you shouldn't really need it. An optical bench is a pretty benign environment. Regular, non-birefringent SM cable worked fine for me.
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