Monday 16 July 2007

A lot has happened since my last journal entry. Most of the work has been done with the open-cavity HeNe laser, after I asked Dr. Noe if we could see if we could get it working. We weren't sure if it would lase because the mirror has been sitting out in the open getting dusty for months, but when we plugged it in it started right up without even having to adjust the mirror. I was hoping to play around with it to see all the different laser modes that I had just been reading about. I was originally going to take the mirror out of the frame and screw it down onto the table to make a longer cavity, but after I moved it a bit, I discovered how hard it really is to align. I had enough trouble putting the mirror back to where it originally was to produce a beam, so I decided to just leave it there when I finally got the beam back again. It would have been much harder to align with a longer cavity. Also, the frame itself is not secured to the table, so if it had been slightly nudged it would have stopped lasing. When the mirror is attached to the cavity instead that would not happen, although a nudge would still probably mess up whatever else it is I'm using it for, so I just have to be careful.

The first thing I did was to simply try to observe the different transverse modes. I sent the beam through a lens and observed the pattern on a screen at the end of the table. Without even touching anything, the beam was already in an HG(0,1) (or 1,0) mode. As I adjusted the mirror in the cavity, I observed other modes as well, though I do not think they can be called pure HG modes. As I did this, I noticed that the beam took an almost perfect "doughnut" shape. This occurred while the beam went between a (0,1) mode and a (1,0) mode, with the doughnut beam appearing when these two modes seemed to overlap, as shown here:

The question at that point was whether what we were seeing was an optical vortex. We knew that vortices have a phase singularity, but does having a phase singularity necessarily imply a vortex? To test this, we decided to build a Mach-Zender interferometer. The reason for this was that Dr. Noe knew that the interference pattern of two vortices gives fringes with a "fork" in the middle, meaning that one fringe turns into two fringes at the center of the pattern (it "bifurcates").

So Mallory and I started to build the interferometer, which consists of the open-cavity laser, two beamsplitters and two mirrors, a lens, and a screen. The beam first hits a beamsplitter, and each of those beams then reflects off of a mirror and hits another beamsplitter, and then goes through a lens and is projected onto the screen. We spent a good amount of time aligning this system. First we leveled the beam by propping up the laser frame, then we tried to make the beams collinear. As we were doing this, the patterns that we saw were not what we expected. The pattern showed straight fringes, as shown in the following photo, just as any normal beam would.

At first we thought that this meant that what we were seeing was in fact not a vortex beam. This seemed true at first because the only methods of making votices that I had heard of were using cracked plastic, spiral plates, or cylindrical lenses. Dr. Noe then suggested that we incorporate a Dove prism into the setup. When looking at an object through one of these prisms, you will see what looks like a mirror image of the object. It reverses the "handedness" of things. So, if this doughnut beam actually was a vortex, this prism would reverse the direction of rotation of the propogating beam. This is like turning a "righty-tighty" screw into a "lefty-tighty" screw. As soon as we inserted the Dove prism into the beam path of one of the arms of the interferometer and realigned it, we saw the characteristic "forked" interference pattern, as shown here:

To be continued tomorrow...

Friday 6 July 2007

This morning I gave my HeNe laser talk, and now I'm looking forward to starting something new to work on. I would like to continue learning about laser modes and gaussian beams, since I still don't really understand them. Every explanation I found just presents the equations describing the electric field of the TEM modes, saying that these are the solutions to the wave equation in various coordinate systems. I think I need to go through the math myself to really understand it all. Otherwise I have a hard time picturing what is happening in the cavity that is giving rise to the TEM modes. I want to start working on a project soon, because we now have less than a month left here. I became interested in possibly continuing an unfinished project of one of last summer's REU students. It would involve learning how to build a feedback circuit to frequency stabilize a two-mode HeNe. It would be a great skill to learn while learning some more optics at the same time. Other possibilities are working with the 80 mW green laser to work on either beam shaping or iodine spectroscopy. I would also enjoy finding a project to do with the open-frame HeNe.

Here are some links to applets I used for my HeNe talk:

Monday 25 June 2007

Today the high school Simons Fellows arrived, and we listened to Dr. Noe give his LTC educational philosophy talk again for them. We've done a lot since my last journal entry. Last Monday we went outside for some more solar optics. We used a photodetector and measured the current induced by the sunlight at various angles and through various filters. The maximum reading occurred when the photoreceptor window was perpendicular to the incoming rays, and this reading diminished when the reading was taken with glass in the beam path, and also when the reading was taken from light reflected off the mirror. When the beam first passed through a magnifying glass, the reading was diminished with the lens touching the photodetector, but increased as the lens moved further away and the image spot size shrank. It reached a maximum when the spot size was slightly bigger than the photoreceptor. This could be due to internal resistance in the battery limiting the current output.

Using our reading of unaltered sunlight (4.4 mA), and the power output of the sun as given by a textbook in the lab (3.85*10^26 W), and the product information of the photodetector as given in the ThorLabs catalog, I calculated the "quantum efficiency" of the device (meaning how many electrons are freed per incoming photon). I got a result of about 0.69.

We also decided that each of the REU students will give a talk to the high-schoolers as they did last year here at the LTC. Dan will start tomorrow talking about geometric optics, Mallory will talk about polarization of light, and I will give a talk about HeNe lasers. I chose this topic because I never got into the finer details of lasers in my optics course, so I should learn a great deal in preparing for this talk.

Friday 15 June 2007

Despite all three of us having had a course in optics, this week served to demonstrate how much we really do not know when it comes to hands on experience and observations. As our first activity of the summer we relived everyone's favorite childhood pastime and headed outside to burn some stuff with magnifying glasses. At first it seemed trivial, but then Dr. Noe began to ask us questions about what we were seeing that none of us really had an adequate explanation for. One that we spent a great deal of time on was why we were not able to focus the spot of sunlight down to a point. Another observation we could not explain at first was why the spot changed shape when we focused the sunlight near the edge of the shadow of the physics building. We even wrote down the thin lens equation and played around inside with the magnifying glass and a lamp to project images onto the wall. But it was not until today when it was finally sunny again outside and we took the magnifying glasses out again that we realized the answers to our questions. When we saw a few clouds floating by on the paper we were trying to burn, we finally realized that we were seeing an inverted image. The reason that we could not focus the spot down to a point is because the sun is not a point! The rays entering the lens are not parallel as we were assuming. Instead, there is a small but very significant angular difference corresponding to rays coming from opposite edges of the sun. This also accounts for the shadow of the building not being a sharp, distinct line, but slightly smeared. Some rays come at a steeper angle than others, and that is enough to explain both the shadow and the spot. This is also why we were unable to burn the paper with the reading glasses: because they have a longer focal length, the image is larger and about the same amount of energy is concentrated in a larger area. Another interesting effect that we observed was dancing colors behind the hole we burned in the paper. This was possibly due to the air being heated up, which causes the index of refraction to change.

Solar optics pictures here

Other things we did:

-learned some basic Linux commands and html coding so we could start our websites.
-listened to Dr. Metcalf talk about negative indices of refraction.
-played with the "pig toy"
-played with the interferometer
-discussed f/#'s, parabolic mirrors, polarization in regions of the sky.

Ian Goyette
June 2007
Laser Teaching Center