# Measuring components of circularly polarised light.

1. Nov 18, 2009

### quantumlolz

Hi there,

Having a bit of trouble working something out for a lab experiment at uni - not sure if this really counts as a 'homework' question - but if it does, moderators, feel free to move it there!

So we've got linearly polarised laser light hitting a quarter waveplate to create circularly polarised light, where the incident laser light is incident at 45 degrees to one of the fast/slow axes on the waveplate. Then this circular light is going through a linear polariser before hitting a photodiode, connected to what is basically a voltmeter.

A quick note about the linear polariser - we have that there to ensure the quarter waveplate is set up correctly - the linear polariser is crossed with the laser so that, on their own, nothing gets through to the photodiode.

To ensure the quarter waveplate was set up correctly to create circular light, we rotated the quarter waveplate until no light was being picked up by the diode, noting that angle, and then rotating it again through 90 degrees so that nothing was being picked up by the photodiode again (the scale on the waveplate is arbitrary as it'd been cleaned and the dial had just been put back in some random position), so that halfway between the two angles (on the scale) gave the 45 degrees needed to create circularly polarised light. (is this right??)

So, here's the problem: how do we use this set up to measure the voltages for the left handed and right handed circular components of the circular polarised light. The main problem here really is that we're not sure what's going on in between the linear polariser and the photodiode.

Sorry if this all sounds rreally confused - I definitely am!

2. Nov 19, 2009

### Redbelly98

Staff Emeritus
Welcome to Physics Forums.

Your setup sounds correct for getting circularly polarized light, by rotating the waveplate to be 45 degrees from the two null-signal positions.

I'm not sure what your are asking here. The beam is circularly polarized after the waveplate -- it's either left or right handed circular, one or the other, not some combination of both. Are you asking whether it's left or right handed? To find that you would have to know the direction of the fast axis of the waveplate, as well as the direction of linear polarization before the waveplate.

At any rate, it sounds like you need to remove the linear polarizer in order to have circular polarization incident on the photodiode, if the goal is to measure what signal you get for the circularly polarized beam.