polarisation mismatch

[Din]

Not quite sure where this would go (no "laser" topic).

We're setting up a full colour shot with 633, 532 and 476. But, we just noticed that the blue (Lexel 95) and green (351M) have horizontal polarisation and the HeNe (SP 127) has a vertical polarisation for the same geometry. That is, all three beams are co-linear, resulting in a "white" laser beam. But, when we checked polarisation of the white beam, wavelength by wavelength, we found that the HeNe is polarised at right angles to the other two. I find that curious.

[BobH]

The SP-127 is vertically polarized. Horizontal polarization was an option and I know someone who has one of them, but they're ultra-rare. You're not beamsplitting with a PBS cube?!?

[Din]

The SP-127 is vertically polarized. Horizontal polarization was an option and I know someone who has one of them, but they're ultra-rare.

That's what I figured. It's just ---odd.

You're not beamsplitting with a PBS cube?!?

No, We use a circular variable beamsplitter. We used a PBS at Icon and at ABNH, but it was just bulky and took up too much room. For a colour table, we have to cram three sets of optics onto one table, so space is at a premium! Especially, remember, our table has a big hole right on the edge. Besides which, it's probably just tradition. When I first was working with Peter Miller at Richmond, we used a circular beamsplitter, so I guess I just stuck with it over the years. We also used a 910 spatial filter assembly at Richmond and I've always stuck with it. I found the 900 awkward to use. For the longest while, the 910s were unavailable, but I was at a facility recently where they had new 910's. The holographer there told me that they were now back on the catalogue.

[lobaz]

I remember a few years ago there was a thread on the holoforum how to rotate polarization of a laser using mirrors. If I remember correctly, you did not believe it works, Dinesh (but maybe I am wrong and do not remember details). Isn't it the case you accidentally included something that rotated the red polarization?

[jeand]

I remember a few years ago there was a thread on the holoforum how to rotate polarization of a laser using mirrors. <snip>

If my memory is good enough, Kaveh placed mirrors to form a kind of periscope (this is a part of his thesis)
There is a wiki page about this : http://holowiki.nss.rpi.edu/wiki/Hints_and_Kinks

[lobaz]

A bit off-topic, Dinesh: are you sure your wavelengths 633, 532 and 476 are well chosen? I would be scared of blue as all white LEDs I measured have a serious gap around 475 nm, see spectra below.

[Din]

Petr,
Thanks for the curves.

Actually, I have no choice because the material does not record below 476. Normally, for colour work, I use 633, 532 and 456. All I can do is ask the client to carefully choose the leds for reconstruction.

It's often difficult to say to a client that I need exact specs, and that if they want "red" or "green" it's simply not enough information. I need a spec that I can check against a spectrophotometer. I often say that making HOEs is not the same as making display holograms, the criteria for HOEs is a mixture of holographic talents, optical engineering skills and mathematical modelling. You simply cannot just throw photons at a plate. Then a friend of mine showed me a piece from LinkedIn that said exactly the same. That is, the talents required to produce a HOE are a combination of holographer, optical engineer and physicist and a lot of HOEs fail because the engineer/scientist does not understand holography and the holographer does not understand engineering. My friend also sent a note saying, "Looks like you're vindicated!"

I think it's going to be an issue with these so-called "smart glasses". The criteria of interest in these applications is the MTF, not the optical efficiency a la Kogelnik. In this case, the MTF has a non-linear relationship to the Seidel aberrations.

[142laser]

use a 1/2 wave plate on the HeNe to rotate the polarization into alignment. I think most SP 127's are Horizontal and vertical is rare but maybe I remember backlwards? I seem to remmber the VP on the special ones. Maybe 488nm would be better?

[BobH]

use a 1/2 wave plate on the HeNe to rotate the polarization into alignment. I think most SP 127's are Horizontal and vertical is rare but maybe I remember backlwards? I seem to remmber the VP on the special ones. Maybe 488nm would be better?

That's probably right Phil, and I got them mixed up.

[Din]

use a 1/2 wave plate on the HeNe to rotate the polarization into alignment. I think most SP 127's are Horizontal and vertical is rare but maybe I remember backlwards? I seem to remmber the VP on the special ones. Maybe 488nm would be better?

Actually, that's just what Joy did (she does all the setups - there's a reason I'm a theoretical physicist. I can't hammer a nail into a piece of wood without screwing it up! ). We have a 1/2 wave plate "lent" to us on a permanent basis by Bob several years ago.