# Can't figure out what polarization the laser has.

1. Aug 2, 2013

### nordmoon

I have a vertically linear polarized laser according to specs. "Polarization ratio 100:1; Vertical linear polarization".

However,

When I place a polarizer in front of the laser its maximum is at 132 deg. and minimum is at 42deg. Thats 90 degrees apart..

Placing two polariazer infront of laser, having both polarizer vertically I have max. transmission. Placing one of them horizontally I have min transmission but....

Placing one at 132 and another at 42 we have min transmission and placing both at 132 we have max. polarization.

I don't understand this.

I compared it to another laser I know is vertically polarized.. and placing just one polarizer infront of the other laser this one has max transmission vertically for one polarizer. If the first laser is also vertically polarized shouldn't it has max. transmission the same, that is at 0 deg?

At zero deg. on my polarizer I have the fast axis set vertically.

What polarization does my laser have?

How can they say its Vertical linear polarization if its not? Or am I do something wrong here?

Last edited: Aug 2, 2013
2. Aug 2, 2013

### johnbbahm

Your methodology sounds good, So I would say the laser is not meeting it's specifications.

3. Aug 2, 2013

### nordmoon

Does this mean that I have an laser that is linearly polarized with 42 degree angle to the fast axes?
I need to obtain vertically or horizontally polarization.

I would guess I need to find max min with an half-wave plate and polarizer. Find the min for vertically or horizontally polarization with rotating half-wave plate. And those angles corresponding to opposite polarization of polarisator? Is this correct?

Is also correct that the polarizators fast axes should be aligned with Zero.. meaning the light passing is vertical polarized?

4. Aug 2, 2013

### johnbbahm

What you have already done with plane polarizer sounds fine.
You said the laser was "Polarization ratio 100:1; Vertical linear polarization"
So lets assume they meant that the light on a polarized line drawn through 0 degrees
and 180 degrees is 100 times greater than the light on the 90 degree axis.
The high energy axis of your laser is on a line across 132 degrees and 312 degrees.
Even if the assumption on which axis is the maximum, is incorrect, the laser
is still not in spec, because nether min or max is on a vertical axis.

5. Aug 2, 2013

### Andy Resnick

Three questions: the spec is 'vertical', but with respect to what orientation? Your polarizer indicates the polarization is linear, oriented at 132 degrees... with respect to what orientation? Finally, what is the extinction ratio- the ratio of intensities at the two polarizer orientations?

A related question- do you need precision control of the polarization? If so, just get a half-wave plate to rotate the polarization as needed and a high-quality calcite polarizer to increase the extinction ratio as needed.

6. Aug 2, 2013

### nordmoon

About orientation: On the polarizer there is two marks opposite each other that is marked with text "fast". I have placed the polarizer so that these marks is going through 0deg and 180deg on the rotation mount. The same thing is done for half wave plate .. although I must admit it has been quite difficult to place it exactly at 0deg with the mount I have. A slight shift will cause the max transmission to end up at 5deg vs min at 95deg instead of 0deg and 90deg respectively. Not sure how to do this more precisely. This is the orientation I have been using on my polarisator and half-wave plate. I am assuming that fast axis is the transmission axis.

My ultimate goal is to find vertical and horizontal polarization of my laser. However since the specs said should be vertically polarized.. I started question myself. It does not say to what orientation on the specs.

With the polarizer I obtain a max. transmission of about 48-50 mW and min of around 2-10 uW.

Since its not according to specs my plan is to use a half wave plate to rotate it to vertical and horizontal polarization.
Is there any chance that these angles for max horizontal and max vertical change? I am assume this could happen if the fast axis marks move from previous calibration?

I noticed that rotating my half-wave plate mount 180 deg around its axis so the other side of the half-wave plate faces the laser.. the rotation of the polarization seem to change. -- It seems where I had a max transmission I get a min transmission at a certain angle of the half wave plate and vice versa.

I don't understand why.

Could this also be a problem for a polarizer or could I use any side?

Last edited: Aug 2, 2013