Anti-reflection coatings and optical adhesive

[Frank McDermott]

Hey all,

Looking for some input on this project:

I'm looking to make a polarization sensitive detector - split s and p into their own photodiode. Unfortunately I've got make do with the parts I have. I've got a polarization beam splitter cube, some thin film polarizers and the photodiodes.

The PBS cube isn't ideal since it there is poor extinction in the reflected wave , so I thought I'd use the thin film polarizers to increase the extinction.

The PBS cube and the polarizers are anti-reflection coated for my wavelength of interest (1550 nm). I was initially going to cement everything together with an optical adhesive (Norland 61).

But now I'm wondering if it would be better to leave out the adhesive since the AR coatings are designed to operate that way.

I'm just looking to limit etalon effects.

Any suggestions?

Thanks,
Frank

[anorton]

"Frank McDermott" <frankjosep...@gmail.com> wrote in message
news:8ed48e5e-ae21-4539...@googlegroups.com...

First of all, I am a little confused how you would bond a thin film
polarizer to a beamsplitter cube since a thin film polarizer is meant to be
used at a steep angle. Do you mean a dichroic polarizer which is usually a
sheet of plastic?

Assuming the later, certainly reflections from the AR coating itself will be
worse with cement. The real question is whether they will be worse than the
extra uncoated interface of the polarizer if it is not cemented. You could
try measuring this using temporary coupling gel or mineral oil in place of
the cement at first.

Make sure your polarizer is designed for 1550nm, most common dichroic
polarizers will not work there. You can also have your polarizer sandwiched
between glass plates that are AR coated on the outside.
--
Adam Norton
Norton Engineered Optics
www.nortonoptics.com

(Remove antispam feature before replying)

[Phil Hobbs]

Adam's right, Polaroid sheet doesn't work in the IR. Depending on the
cube, you may be able to improve its polarization purity a lot by
tilting it just right. If the beam diameter is small enough, the tilt
will also make the multiple reflections miss each other, which will get
rid of the etalon effects pretty well. Putting dots of black paint
where the first reflections hit will help some more, and if the beam
profile is reasonably smooth (no diffraction artifacts) the two together
will pretty much solve the problem.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

[anorton]

"Phil Hobbs" <pcdhSpamM...@electrooptical.net> wrote in message
news:4FE480BC...@electrooptical.net...

There are some dichroic polarizers that work inthe NIR. See
http://www.codixx.de/cms/polarizers/polarizer/polarizing-filters/ht-polarizing-filter.html
http://www.codixx.de/cms/polarizers/polarizer/polarizing-filters/ir-infrared-vis-visible.html
But if you buy these through the usual catalog houses they are not cheap.

I like the idea of tilting the cube and blocking the second reflection with
a spot of black paint.

[Frank McDermott]

Thanks for the reply, I misspoke - the polarizers are supposedly from Thorlabs and based on the photos on the site I would guess that they are in fact this exact model : http://www.thorlabs.com/thorProduct.cfm?partNumber=LPNIR050

They were in a drawer full of unlabeled optics in the lab - someone has a vague memory of buying them so I'm pretty confident that they are the item above. I've tested them at 1550nm and they do work.

Now that I've seen the price on them though I'm think I might try to split one in half instead of using two since the beam is 2 mm at most (it is coming from a GRIN lens collimator).

Tilting the cube is an interesting idea, the only problem is the cube is very small - 4 mm on a side.

[anorton]

"Frank McDermott" <frankjosep...@gmail.com> wrote in message

news:7dd7d94f-4158-4591...@googlegroups.com...

>
> Thanks for the reply, I misspoke - the polarizers are supposedly from
> Thorlabs and based on the photos on the site I would guess that they are
> in fact this exact model :
> http://www.thorlabs.com/thorProduct.cfm?partNumber=LPNIR050
>
> They were in a drawer full of unlabeled optics in the lab - someone has a
> vague memory of buying them so I'm pretty confident that they are the item
> above. I've tested them at 1550nm and they do work.
>
> Now that I've seen the price on them though I'm think I might try to split
> one in half instead of using two since the beam is 2 mm at most (it is
> coming from a GRIN lens collimator).
>
> Tilting the cube is an interesting idea, the only problem is the cube is
> very small - 4 mm on a side.
>
>

Given that these are laminates of three layers,you will not have much luck
cutting them by the usual scribing and breaking method. You will need a
glass saw.

[Phil Hobbs]

> There are some dichroic polarizers that work inthe NIR. See
> http://www.codixx.de/cms/polarizers/polarizer/polarizing-filters/ht-polarizing-filter.html
> http://www.codixx.de/cms/polarizers/polarizer/polarizing-filters/ir-infrared-vis-visible.html
> But if you buy these through the usual catalog houses they are not cheap.
>
> I like the idea of tilting the cube and blocking the second reflection with
> a spot of black paint.

Thanks. Polarcor and wire grids work fine in the IR--it's just the
iodine-doped PVA Polaroid film that doesn't.

A 4 mm prism doesn't leave much space for adjustment. Maybe the OP
could shop around on eBay for a 25 mm cube.

[Phil Hobbs]

The other thing one can do is to polish the coating off one face of the
cube and attach a wedge prism using index oil or well-matched UV epoxy.
The polishing job will leave a bit of orange peel due to the coating
coming off irregularly, but that doesn't matter much in a glued joint.

[Samuel M. Goldwasser]

Another thought is that depending on how you're using the output of the
PBS, you may be able to electronically subtract out the leak-through (T or R).

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