B Class Making a T.E.A. Laser-- we have questions...

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1. Jul 24, 2016

whitsona

I run a Maker group for teens in Florida at the Pensacola MESS Hall. We are building a T.E.A. Laser powered by a Wimshurst machine along the lines of a Rimstar project we found on YouTube. But, we have some questions about the laser it will produce:

(1) I read that the TEA Laser has high spatial coherence, but low temporal coherence. But, the pulse is very short. (It's an electric spark discharge in our version. It's very simple.) Since the pulse is so short, does that mean that we actually have pretty coherent light for the entire pulse in both spatial and temporal coherence?

(2) Why is the light coherent? It's not bouncing around inside a medium to get coherent... So, what causes the light to be coherent? How do we know that it is spatially coherent?

(3) Are there laser measurements we can do on a low budget with one of these?

(4) We are doing a quantum eraser again later this year. We usually do it with \$2 cat toy lasers. Any chance we could use our homemade lasers to do it?

(5) (From the kids themselves) What is the most destructive or cool thing we can do with it once it is working?

2. Jul 25, 2016

Andy Resnick

Temporal coherence is set by the frequency spectrum- a broadband pulse has low coherence, while a monochromatic source has high temporal coherence. Because the pulse time Δτ is set by the frequency spread Δƒ (roughly, Δτ Δƒ = 2π), the quantities are all related. Spatial coherence is set by the size of your source (the area of the exit window), so that is somewhat independent of temporal coherence.

Coherence, in this context, essentially means how well you can predict the value of the laser field. The coherence length (temporal coherence) can be measured with a Michaelson-Morley interferometer, by measuring the maximum path length difference that still results in interference fringes- in this sense, coherence is prediction of the laser field at future times. The coherence area (spatial coherence) can be measured using a Young's two-slit interferometer; measuring the maximum separation between slits/pinholes that still results in fringes. In this sense, the coherence is a prediction of the laser field at other locations.

Lots- if you know the average wavelength, you can try measuring film thicknesses, track spacings on a DVD, laser gyroscopes (sagnac interferometer), etc. etc. Since it's pulsed, you should also try and measure the pulse time/pulse shape.

You didn't mention if this is a CO2 laser or some other material- what is the emission wavelength? Safety is essential for IR sources. A fun thing to do is burn paper and pop balloons.

3. Jul 25, 2016

whitsona

Thank you! This is great help.

I think I need to find a textbook and work through some example coherence problems next. This was a great start.

My largest confusion left on this is that when I learned about lasers decades ago in physics class, there was always an amplification medium involved. Somehow solids as an amplification medium make sense to me. But, in this case the gas is the media? Very much lost at this point. Realizing that I may need to step back and think through "what does it mean to amplify light?" Amazing how teaching kids makes you realize that you think you understand things, but when they ask you that one extra question you realize you didn't really.

I found a Michaelson-Morley interferometer diy on instructables! Awesome. Young's 2 slit might be worth investing in some apart us for the science museum, since we do the Quantum eraser project every year. Thanks for the leads on that.

Average wavelength is 337. And laser gyroscope! So cool! And, definitely on pulse time/pulse shape. I'll start looking for instructions on how to do that right away.

It's open to the air in the basic design. So, the gas is Nitrogen, basically. But, your question gave we an idea. Since CO2 displaces air and is heavier, we should be able to build up sides around the lasing channel and fill it with CO2. Then, we can do a bunch of experiments to comoare the two.

We're a very small Maker group at a small museum, so I'm having to be creative about safety. The UV range laser goggles I saw were in the 300 to 900 dollar range. One pair is more than our entire Arduino budget for the year. So, I found some 3M UV film with a high rating and I was planning to make a lid for the whole set up. We are planning to view it using fluorescence in some highlighter ink mixed with water and also chlorophyll in water. (Ideas from Rimstarorg -- his YouTube channel is amazing.) So, I'm hoping the lid idea will keep everyone safe. Any concerns?

Again, than you! I'll send you a thank you pic from the kids when we get it working as well.

4. Jul 25, 2016

Andy Resnick

My favorite introduction for coherence is Wolf's 'introduction to the theory of coherence and polarization of light', I'm sure you can find equivalent information as well.

This makes me nervous. I think an enclosure may be sufficient, if it's rated correctly- you will need to know the pulse energy and repetition rate. 337 nm is within the UV-A band, with an exposure limit at 1 J/cm^2. Read up on ANSI Z-136.1 and ANSI Z-136.5! As a rough calculation, if your pulse duration is 1 us, repetition rate of 10 Hz, and total duration of 10 seconds, each pulse can have a maximum energy of 0.0001 J, so the 3M film needs to have an optical density that attenuates the actual pulse energy down to this level.

Good luck!

5. Jul 25, 2016

whitsona

The repetition rate may be my saving grace. The laser is being powered by a hand-cranked wimshurst machine with a discharge of once every 6 to 12 seconds. Thank you for the safety standards. This is exactly the detailed support we need! I'll try to find the 3M specs and do some calculations. Insurance companies are having a hard enough time trying to figure out how to insure maker spaces at science muse us around the con try right now. In addition to wanting to make sure these kids are safe, this will help us document how we know they are safe for the insurance company.

6. Jul 25, 2016

whitsona

*museums

7. Jul 25, 2016

whitsona

** country

Typing on tablet while teaching a class. :-(

8. Jul 25, 2016

Andy Resnick

You are exactly on the right track- and you see an opportunity to solve a real problem. I'm happy to help and hope your efforts succeed! PM me as needed :)

9. Aug 12, 2016

whitsona

Update:

We are getting sparking at both ends of the lasing channel and some lasing. Heard back from Rimstarorg, he suggested increasing the spark gap before the lasing channel-- as that helped him.

In the meantime, we have enjoyed learning about Rogowski electrodes-- ones that make a uniform electric field, like what the lasing channel apparently uses. There are some beautiful pictures of models of the force emanating from a pair of these kinds of electrodes in different shapes. But, then we hit a bump in our understanding. If the lasing channel is based on having this beautiful symmetry, then how does this configuration (on sparkbangbuzz.com) make sense? See how the pre-ionizing copper wire running down one of the aluminum bars wedges its way in?

http://www.sparkbangbuzz.com/tealaser/tl7664a.jpg

Also-- I get really bogged down with getting from "the Rogowski electrodes are designed to let the charge really, really build up" and "the copper wire pre-ionizes the air, so that when the spark occurs, we will get light amplification" to the big finish where it sparks? but doesn't spark? to initiate the light amplification? And then we get spatially coherent light, but not temporally coherent?

You are so fantastic to help us! I tried all my fellow alum contacts from MIT (I graduated in Building Technology, so not much help from my background)-- including a holographer-- and a two beam physicists. But, you and a friend from Tufts are winning the "actually being helpful" award.