Lab 18 Prep / Materials

Preparedness will be demonstrated by sharing presentation status - you should have slide titles and content identified. Final versions are due Wednesday.

Today is lab notebook review for those who don't have a "bye".

The table will be open until 7:50.

Some possibilities for work on the table during class time include: a Sagnac (ring laser) interferometer; a Mach-Zehnder interferometer with a microscope slide in one arm to generate path differences and fringe modulation, solutions to various made-up alignment puzzles, michelson fringe imaging w/ the camera to assess wavefront error.

After 7:50 there is a holography experiment scheduled on the table that needs the room dark.

Lab 17 Wrap Up

Tonight Team "Wednesday" was tasked with creating a Mach-Zehnder interferometer (MZI) from scratch on the table. While some online notes suggest that an MZI is easier to align than a Michelson, I think everyone in the class would disagree. The MZI has an extra degree of freedom at each of the two 45-degree angle mirrors that is coupled to its rotation around the vertical axis. Of these two degrees of freedom, the Michelson only has the rotational degree of freedom - the return mirrors can be placed anywhere along the direction of the arms and still obtain laser fringes. Here's a diagram showing the MZI coupled degrees of freedom:

Aligning optics on a table is a little like riding a bicycle. After a while your body just "gets it" as you learn the effect of exercising each of the many degrees of freedom in an optic's mount. Of course you need to have an overall alignment strategy as well, such as working optic by optic down-beam. In this alignment, the "aha" moment is when you translate and rotate the mirror at the same time, keeping the location "A" constant. Having a second location "B" helps a lot.

We learned several other alignment tricks. "Chopping" a card in and out of beam rapdily by hand as the card is moved downstream gives a strong and fairly persistent visual impression of the beam's location in 3D akin to as if there were smoke or mist to mark the beam's location. This trick is especially helpful if there are features (bolt-hole row, tape w/ line down middle) on the table showing the direction the beam should go. I'll post up the other trick later.

Here are some in-progress alignment photos:

This one showing mirror translation along a bolt-hole row, and below, showing by-eye alignment looking down a bolt-hole row from one end of the table to make sure everything is in the right place. It is amazing how important it is to keep moving around the table and eye-balling optic locations! It is just too darned easy to plop things down off-by-one because of the repetitive pattern on the table top.

Oh, by the way, the fringes were good:

I'll hold open lab Friday 1PM on.
Good Thanksgiving everyone!

Lab 17

... is a repeat of 16 with team assignments swapped.


Above: Team "Monday MZ" fringes from their Mach-Zehnder interferometer.

Look ahead:
Your FINAL presentation in digital or (scannable) paper form is due next Wednesday, December 2.
Your DRAFT presentation (slide titles and content identified) is due next Monday, November 30.
Your lab notebooks will be reviewed Monday (Lab 18). Some of you have byes on this for having stupendous notebooks last time and already have full credit on this score.

Prep today will be a status on your presentations:
Lab work begun
Lab work finished
Lab work written up
Presentation slide titles complete
Presentation draft content complete
Presentation final content complete

Remember that I will be reviewing your lab notebook at the time of the presentations.

Lab 16 Materials

Lab 16 Plan of the Day
Lab 16: Mach-Zehnder Interferometer
Self-Assessment (note addition of more of/less of items)

Today and Wednesday we'll break into 2 groups. One group will build a Mach-Zehnder interferometer, and the other will learn optics cleaning techniques. Then on Wednesday we'll switch. The interferometer build will take 1-2 hours.

Here's a pdf with optics cleaning instructions:
Meilse Griot cleaning instructions.
Because of the importance of minimizing damage to an optic's surface, cleaning is done very carefully. Many years of experience has developed a practical and safe approach. The key ingredient is patience and losing all desire to rush. A little practice helps too.

Lab 16 Prep

There is no separate prep package. Work on your projects. Be prepared to give status of work accomplished (not time spent, or work planned).
I have a pile of photos from Friday's open lab. Too many to email or upload. Instead I'll burn them to CDs and bring them to lab. I note, however, that with 5 dozen photos it is hard to keep track. My advice for future photo taking is that you write a little placard to appear in the frame when the photo is taken to identify it better.
I'll post up this one photo though. It was the highlight of the day when Sy's Mach-Zehnder interferometer gave fringes.

Lab 15 Materials

This post is the plan of the day.
1) Demonstrate preparedness
2) Quantum optics snippit - polarization tagging
3) One-on-one presentation topic & plan review
4) Open Lab w/ Michelson layout & polarizers

Lab 15 Prep

There is no separate prep package. Your preparation has 2 parts:
1) Write a paragraph (or more) describing your plan for the final presentation. Your plan should include a short technical description, an estimate of the amount of table time you will need, and a timeline for accomplishing the project. Refer to the presentation rules given during lab 12. Key dates are: presentation is evening of 12/7 and a hardcopy/digital version is due on the last day of class. I will spend about 10 minutes with each of you discussing your project during lab 15.
2) Review your lab notes for the last experiment, where we polarization tagged the arms of a Michelson interferometer. Generate a 1-page description of the experiment and the results. Make a note of something you would like to try differently or in addition. Write a simple procedure for doing that and incorporate it into your table work during the open lab.

Lab 14 Material Available

At the moment it is a draft, I'll do some reorganizing for class but this is the content:
Lab 14 Materials

Lab 14 Prep Package Available

Lab 14 Prep Package
...in which you will further explore the Michelson interferometer, including a quantum mechanical calculation of its fringe pattern.

Lab 13 Photos

Lab 13 was to build a Michelson interferometer and obtain first fringes. Both teams were successful, achieving fringes in an hour or less. Here are some photos.

Team A planning their buildup and alignments. The team came up with a perforated card as an alignment aid that was used to ensure the beam was a constant height above the table.

Team B doing their layout. Fringes obtained at t=1 hour using shimmed and hand-touch alignments. Not bad!

Lab 13 Material Available

Lab 13 Plan of the Day
Lab 13 Michelson Interferometer First Fringes
Self-assessment

Lab 13 Prep Package Available

We are starting the Michelson interferometer sequence. Monday we'll build the first iteration and detect fringes if all goes well. The prep package is to warm you up with a historical perspective and to get you thinking about alignments.
Lab 13 Prep Package

Lab 12 Materials Available

Today's main topics will be lab notebook review and open lab. Be sure to bring your lab notebook!
Lab 12 Plan of the Day
"Final" presentation rules and procedures
Self-Assessment
Please record on your self assessment what you want more of and what you want less of in the lab.

Lab 12 Prep

Lab 12 is Notebook review and open lab. Prep is bringing your lab books up to date and planning your open lab time.

Lab 11 Photos

Last week we brought the old 8-inch 1970-ish telescope up from the store-room where it had been gathering dust. First task: clean it up and prep it for bringing into the lab.

A little elbow grease and damp sponges cleaned off most of the external gunk. Once in the lab, we took out the mirror and found the aluminum coating to be in bad shape. Certainly time for it to be recoated. But first, we'll have fun with it doing a Foucault test to check its figure. Mirrors with pretty bad optical surfaces can still produce images.

A Foucault test is an optical test at center of curvature, often used by amateur telescope makers while "figuring" a mirror to its final shape. The test uses simple equipment to assess the shape of the mirror to within 1/4 wavelength of light or so. It is not as accurate as surface metrology done with an interferometer, but instead of needing $50,000 worth of equipment, you can use stuff that is probably lying around the house. Or lab.

We upgraded the usual Foucault instrument to include a CCD camera to capture the upstream view of the mirror for all to view. In the "classic" Foucault test the procedure is to eyeball it.
Here you see the LED gooseneck lamp that is illuminating a slightly off-axis frosted 1 mm diaphram opening. The frosting is achieved with a piece of Scotch-brand magic tape. This diffuse light source in turn illuminates the mirror, which is located two focal lengths (1 radius of curvature) downstream. Near the focus is a probe on a mounting post. A knife edge is often used. We used a chisel from the toolbox since it comes with a built-in handle for mounting. Later on, Jacob suggested using a pin instead and swapped out the chisel for a mechanical pencil with its 700 micron diameter lead extended, which is what you see here:


Looking upstream from before the pencil, here's what the cell phone camera captured. The bright light on the telescope mirror is the out-of-focus image of the frosted diaphragm, which is located behind and to the left of the camera. To take this picture, I moved the camera until I could see the image of the frosted diaphragm go into the lens of the camera.


Of course, it's not really an experiment without data, so here's the data off the camera when the probe is inserted at a point very near the focus. A perfect mirror would show a uniform disk with varying brightness as the probe was moved to the focal point. But an imperfect mirror doesn't focus all the light at the focal point, some misses. So with the probe at the focal point, the light that misses keeps on going and enters the camera. What you can see here is that the light coming from the edges of the mirror is not blocked, while the light coming off the center is mostly blocked.

The outer edge has a different focal length, by a small amount (several mm) than the middle. By playing with the probe, Paul found that the edge rays came to a focus farther out, so the edge of the mirror has a longer radius of curvature than the middle. This so-called "turned-down-edge" is a common aberration in amateur-class telescopes. The amount present here is pretty small, and probably doesn't noticeably affect image quality for visual observations with this telescope.

Finally, some boardwork on the Zernike functions used to describe aberrations of optical systems. These functions, defined over the unit circle in RxR, form a linear vector space much like sines and cosines do. An arbitrary function on the unit disk can be written as a linear combination of Zernikes. The usefulness comes from the ability to map physical effects onto Zernikes when doing "forensics" in an effort to understand and potentially correct aberrations. Low order terms correspond to mis-pointing, out-of-focus, off-axis, and mount-induced aberrations.

Lab 11 Prep Package Available

Lab 11 Prep Package
Wednesday will be lab notebook review, so be sure to bring yours. We will be making improvement agreements.
There will be open lab this Friday 1PM-5PM.