Polarization for small lasers September 26 2003 at 5:41 AM
(no login)
I was doing some research yesterday and found a very interesting way to find out the polarization direction of your laser. First set your laser with expanding lens as close to the brewster angle as possible on a nice clean, clear glass plate. If ovehead lighting is used for the brewster angel make sure the side to side is in plane and not on an angle. Then look at the reflection off the glass plate. Start to rotate your laser (small HeNe and diode lasers) and watch the reflection off the glass. When you have minimum reflection off the glass then your laser is polarized in the verticle direction (this is for overhead lighting).
Then as step further, if you can, after finding the correct polarization, vary the angle of the glass plate. Again when the reflection is at its minimum you are closest to the brewster angle.
With perfect brewster angle an perfect polarization you will see no reflection off the glass plate.
If this is incorrect, please feel free to criticize.
this is the right way to find out the polarization. if you make a hologram, use the same method on the reference beam to reduce interferences on the film.
Hold the sunglasses vertical with a clamp and rotate the laser until the beam is totally blocked. The polarization is then vertical - or was it the other way around? I can't count how many times I have had to open Saxby's book practical holography whan making a new setup.
You're right that sunglasses will work as well as the reflection method that John stated, if you know the polarisation of the sunglasses.
I'm not sure what 'vertical' means here, since I think you need to hold it horizontally. As I said this whole p and s business confuses the shell out of me. However, assuming the role of the sunglasses is to block glare from the sun, the plane of incidence of sunlight hitting a surface is vertical. Since the light reflecting off the top of that famous swimming pool that they always use with the girl at the bottom for polaroid commercials is s polarised, I would assume that the sunglasses have to be horizontally polarised.
By the way, either of these methods (glass reflection or sunglasses) is also useful to test that the polarisation of both object and reference is the same just before the plate. Often with multiple reflections at odd angles your beam may stray up or down and the two beam polarisations may not match. If you're using an overhead reference and horizontal object beams then polarisation differences between the beams becomes an issue.
"So if your soul was refused entry to heaven vertically, that would confuse the 'phell' out of you?"
Well firstly, I expect I'd be horizontal before I was vertical. Let's hope heaven can handle birefringence! Secondly, yes it'd confuse the phell out of me, assuming heaven's plane of incidence was vertical and I was linearly polarised. I could be circularly polarised, in which case it'd confuse the chell out of me, or elliptically polarised. Personally, with apologies to the religious amongst us, the hell with heaven! I'm more concerned with the uncertainty in my polarisation at the local pub after a few beers!
By the way, Vidar is right that the glasses are vertically polarised (block horizontal polarisation). Since the reflected light is horizontally polarised and this is what you're trying to block, the glasses allow vertical polarisation and block horizontal polarisation.
I had and lost a polarization axis finder. I bought it from Edmund about 10 years ago. They don't sell it any more but you can get one from Oriel for a little over 100 bucks. It is a really cute item, a glass disk 2 inches in diameter with some polarizing material inside oriented radially. When placed in a laser beam it shows the polarization axis with two dark wedges starting from the center and spreading in opposite directions to the circumferance.
Homebrew optics challenge: describe how to make something that would achieve the same effect, using commonly available materials. (I don't know how, but it would be a neat hack.)
I have the Oriel version. It came in a little wooden box with a selection of linear, 1/4 wave, and 1/2 wave polymer plates (2 ea., 2" square). I love that little toy, its very handy!
Just a thought off the top of my head, untested. Place a light source so that you're looking at it head-on, ie normally. Place a glass bowl between you and the source with the convex side facing you, so that the interior of the bowl is facing the light. Now the description might get tricky. The light source has to be at a point so that it hits the sides of the bowl (the insides, remember) at Brewsters angle - either because the source is sufficently divergent or its cone-shaped. Place a polariser between the light and the bowl. Assume the polariser is passing in the vertical direction. Now, for the light hitting the top and bottom of the bowl, the vertical polarisation is p polarised and so light gets through while for the light hitting the sides of the bowl, the same vertical polarisation is s polarised and so will appear dark. In this "equipment" there will be a "light wedge" along the pass axis of the polariser and a "dark wedge" perpendicular to the pass axis of the polariser.
Hmm.. I like the clever idea of using the Brewster angle phenomenon on a steeply curved convex surface, but I don't think a linear polarizer will help much before or after the surface.
One half-baked idea I had was to cut and bend a sheet of linear polarizing material into a cone to make a radial polarizer, but the material I have on hand is way too stiff to permit this.
"..but I don't think a linear polarizer will help much before or after the surface."
Actually I included a polariser assuming that you had an incoherent source, like the Oriel site. If you're trying to find the polarisation direction of a beam that you know is linearly polarised, but don't know which way, then you don't need the polariser.
"One half-baked idea I had was to cut and bend a sheet of linear polarizing material into a cone to make a radial polarizer"
I took a piece of paper and drew straight parallel lines across it. I then turned it into a cone with the apex both along the lines and across it. In both cases, I didn't get radial lines. I got lines parralel to the sides of the cone in one case (straight lines from apex to base) and across it in the other (circles along the cone from apex to base). I tried cutting a triangle off the bottom of the paper and folding it along the sides of the triangle. got two sets of lines going off at an angle. I'm thinking that if you spiral the paper (or poariser) into a helix and look down the spiral, it may work
try this: find an empty tube, preferably a few inches in diameter and about equal to that in length, with one end cut off at brewster's angle (from the tube long axis). Glue a piece of glass over the angled end.
Place this open end first into your laser beam and look for the reflection on the inside wall of the tube. Now turn the tube until the reflected spot dissappears. You have now identified the direction of p. If you want to get fancy, design it as a sextant (the glass turning angle is a sort of an imaginary plumb-bob), with a stationary markings to measure it against.
The light emiting from an ideal single mode diode laser is linearly polarized parallel to the junction plane, it mean parallel to the minor axis, this is true for Sharp lasers but is true for all?
