# Question for the use of Woolaston Prism in OPTICAL TWEEZER

## Main Question or Discussion Point

I studied optical tweezer,
So I saw the optical path in the diagram.

Then I can't understand why they use the Wollaston Prism?

After launch the laser, the beam through the half-wave plate
So the beam polarized, I know this.

and after through the half-wave plate, the beam through the Wollaston Prism.

But this time, I can't understand why they use it.

Does they have a special purpose for using the Wollaston Prism?

^^

Related Other Physics Topics News on Phys.org
The half wave plate does not polarize the light into a single direction. This is why the prisim is needed, in order to polarize the light before it enters a spacial light modulator. It can also be used to reduce the power of the laser so you don't burn out anything in the path. But from my understanding it's mainly used to polarize the light either horizontally or vertically.

In many optics book, they explain a half wave plate to the things that make laser beam
linear polarization.
is it different your explain and book?

http://en.wikipedia.org/wiki/Wollaston_prism
I read the webpage that explain the Wollaston Prism.

but, I can't understand this sentence..
"The half wave plate does not polarize the light into a single direction."

I'm use the Nd:Yag laser, So light already linearly polarized.
So I think that sentence doesn't make sense for me anymore, right?

Cthugha
In many optics book, they explain a half wave plate to the things that make laser beam
linear polarization.
is it different your explain and book?
If you already have a linearly polarized laser beam, you can rotate the linear polarization using a half wave plate, but you cannot create linearly polarized light from arbitrarily polarized light using a half wave plate.

I'm use the Nd:Yag laser, So light already linearly polarized.
So I think that sentence doesn't make sense for me anymore, right?
That depends. For most real setups you will have some mirrors between your laser and your experiment. These can spoil your linear polarization pretty bad. When using my Ti:Sa laser (well, the one of my department), I always use a Glan-Thompson or Glan-Taylor prism to ensure I have a good degree of linear polarization before using half wave plates to rotate the polarization around.

Andy Resnick
I studied optical tweezer,
So I saw the optical path in the diagram.
There are as many different designs for tweezers as there are constructed tweezers- can you provide a link to the diagram you saw?

http://www.stanford.edu/group/blocklab/Optical%20Tweezers%20Introduction.htm [Broken]
that page's Figure 3.
I just setup for seeing the trapped particle.
So now I need more optical elements for my experiment.

I just saw that diagram, and just setup, just one part of it.

Now I studied how can I research the result,
so I concerned and study other optics....

Above answer, and my other people who answer my question help me to know and
understand the optics that align in Figure..

Last edited by a moderator:
Andy Resnick
That setup looks like a nightmare....

As best I can tell, the prisms are there for DIC imaging while trapping, and don't pertain to the trapping/monitoring beams- but it's difficult to tell becasue of the 1/2-wave plate and dichroic mirrors (which have a polarization-dependent performance).

Personally, I keep the prisms well away from the trapping beam to prevent damage.

The prisms belong to the DIC (differential interference contrast) microscope, as Andy REsnick has mentioned. I worked on a similar setup. You don't need them for the tweezers effect and I would remove them most of the time, at least during the alignment of the trapping laser.

Ok, so I've been working with a set up similar to this one this summer and I can kind of tell you how we used each thing. First off why do you need the Wollaston between the lamp and the specimen? We just used the half wave plate to control the amount of power that was entering the objective (an easy way to control it rather than using the laser itself). the prisim was used to polarize the light and to help further reduce the power.

Split Ratio..Re: Question for the use of Woolaston Prism in OPTICAL TWEEZER

If you already have a linearly polarized laser beam, you can rotate the linear polarization using a half wave plate, but you cannot create linearly polarized light from arbitrarily polarized light using a half wave plate.

That depends. For most real setups you will have some mirrors between your laser and your experiment. These can spoil your linear polarization pretty bad. When using my Ti:Sa laser (well, the one of my department), I always use a Glan-Thompson or Glan-Taylor prism to ensure I have a good degree of linear polarization before using half wave plates to rotate the polarization around.
Yes, it is correct.
Half wave plate is used to change the linear polarization direction, by rotating.
Wollaston prism is to get 2 polarizations devided with big angle.