Measuring components of circularly polarised light.

[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!

 

[Redbelly98]

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.

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.

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.

 

[astrokreng]

Hi there,

Thank you for your question. It sounds like you are conducting a very interesting experiment on measuring components of circularly polarised light. From what you have described, it seems like you have set up a good method for creating circularly polarised light using a quarter waveplate and linear polariser. Your idea to rotate the quarter waveplate and note the angles where no light is picked up by the photodiode is a good way to determine the correct angle for circular polarisation.

To measure the voltages for the left and right handed circular components of the circularly polarised light, you will need to use a technique called "polarimetry". This involves using a polarimeter, which is a device that measures the state of polarisation of light. In your set up, the polarimeter would be placed between the linear polariser and the photodiode. It will measure the intensity of the light passing through and allow you to calculate the voltages for each component.

You can also use a technique called "rotating analyser method" to measure the components of circularly polarised light. This involves rotating the linear polariser while keeping the quarter waveplate fixed at the angle that produces circular polarisation. As you rotate the linear polariser, the intensity of the light passing through will change and you can use this to calculate the voltages for each component.

I hope this helps and good luck with your experiment! If you have any further questions, please feel free to ask.