MIT Demonstrations on Lasers and Optics. College level videos.

[Alaskan]

List of free laser video courses with links: Demonstrations in laser fundamentals | Video Demonstrations in Lasers and Optics | MIT OpenCourseWare

The above shows an unpolarized beam of light split to linearly polarized by a calcite polarizer which is made circularly polarized by a quarter waveplate and split into transmitted -p and reflected -s linear polarizations by a thin film polarizing cube.

Course Description

This resource contains demonstrations used to illustrate the theory and applications of lasers and optics. A detailed listing of the topics can be found below.

Lasers today are being used in an ever-increasing number of applications. In fact, there is hardly a field that has not been touched by the laser. Lasers are playing key roles in the home, office, hospital, factory, outdoors, and theater, as well as in the laboratory.

To learn about lasers and related optics, one usually takes a course or two, or acquires the necessary information from books and journal articles. To make this learning more vivid and more exciting, and, one hopes, more understandable, one needs to see some of the basic phenomena involved. To fill this need, Professor Ezekiel has videotaped 48 demonstrations that illustrate most of the fundamental phenomena relating to lasers and physical optics.

By using split-screen inserts and a wide range of video-recording capabilities, it is possible to show real-time effects in lasers and optics with the simultaneous manipulation of the components that cause these effects. In this way, one can see effects in close up that would be difficult, if not impossible, to display in front of an audience or in the classroom.

These video demonstrations are designed for:
•The individual student of lasers and optics who wants to observe the various phenomena covered in theoretical treatments in courses, books, and technical papers.
•The Instructor in lasers and optics in a company, university, college, or high school who wants to illustrate, in class, many of the fundamental phenomena in optics and lasers.

These videos were produced by the MIT Center for Advanced Engineering Study.

 

[paul1598419]

Re: MIT Video Demonstrations in Lasers and Optics

Nice find, Alaskan. This seems to be from the 2008 spring semester and is absolutely free. I plan on using it as there is always something more to be learned. Thanks for the link.

Edit: I got a chance to watch this for awhile and it is really for the uninitiated, with no math or anything more than very basic information. I prefer my physics to be in the language of physics; math. This would be great for someone who is very new to lasers and needs a good basic understanding.

 

[Alaskan]

I was pleased to find the video courses, they should help to answer many questions for me.

 

[paul1598419]

I bought a piece of calcite a few months ago for this very reason. I got a nice piece of optical grade Icelandic calcite about 4 inches by 3 inches by 2 inches for under $15.00. It is interesting to see how light can be polarized by different methods and applied to everyday needs. MIT does a great job of using lasers and video to give a clearer understanding to the student. I also purchased some mica quarter wave plates, but for use in holography. I have some polarized beam splitters too. Also for holography.

 

[RedCowboy]

Questions

Aren't our 445nm laser already basically polarized?

If I used a non polarizing beam splitter cube and two 445nm lasers, one run through a 1/2 wave plate and combined the beams with the non polarizing cube then used a 2nd non polarizing beam splitter cube and pass the combined beam through and fire a 3rd 445nm laser beam with a circular polarization into the 2nd cube could I effectively combine the 3 beams that way?

This clip is from something else but offers a visualization.

 

[Alaskan]

I don't know but I'd like to know if something can be done to increase the combined power by manipulating polarities like that. I've researching the difference between non-polarized and polarized and think I've found the problem for us using non-pol, they have a high amount of loss, I found two web pages which indicate this:

5mm VIS, Non-Polarizing Cube Beamsplitter

Beam Splitter | Non polarizing Beam Splitter | Broad band beam splitter

From what I can make of these pages, a non-polarized beamsplitter looses half of the power due to transmission losses. Is that what you see too?

 

[paul1598419]

If you are talking about polarized lasers, you need to rotate the beam 90 degrees, otherwise you will end up with destructive interference. That was the reason for the quarter wave plate that Alaskan was talking about. A half wave plate will rotate the beam 180 degrees and it will give you the same destructive interference as two polarized beams that are out of phase. The reason for this is the longitudinal, electrical and magnetic wave fronts are all at 90 degrees with respect to each other. Since the longitudinal wave is, lets say along the Z axis, the M and E wave fronts will be along the X and Y axes and be 90 degrees with respect to each other. To put them where they won't destructively interfere with each other one needs to have the E wave front of one laser matching the M wave front of the other laser. It is not quite analogous to compare this with alternating current as in this case, if the phase relationship is not very close you set up ostensibly a short circuit in your power grid.

The video reminded me of my first statics course in EE school where the TA who taught the course was from the Middle East and I had a very difficult time understanding what he was saying because his English was not very good. For that reason, I dropped that course and took it later from someone else. I watched about a minute of the video.

 

[RedCowboy]

When linearly polarized light is passed through a quarter wave plate the electric and magnetic waves are slowed by a quarter wavelength adding as vectors and is converted into circularly polarized light.

Our 445 diodes produce polarized light.

So there is no easy way to combine 3 beams of the same wavelength, correct?

With a quarter wave plate you get circularly polarized light to cube combine with linearly polarized light using 2 of our 445nm diodes.

But there is no reason to use a polarizing cube.

I have seen builds "triads" I think they were called with 3 diodes, a cube and a bounce mirror, was one diode a different wavelength?

Let me go find it.

Would running one diode at 10% less current cause enough frequency shift to add a 3rd?

I am not finding it but check this out.
http://www.laserfocusworld.com/arti...-laser-beams-combined-with-88-efficiency.html

I think the triple I saw was using a cube to combine 2 and a bounce and dichroic for a 3rd of a different wavelength.
YEP here it is, the 3rd is a mits 638
http://laserpointerforums.com/f48/6w-triple-diode-laser-79196.html

 

[paul1598419]

I think you misunderstood me. The diodes we use for pointers are not what are known as polarized lasers. The lasers used for holography are ones that are examples of polarized lasers. They have a long coherence length, a narrow bandwidth, and a narrow FWHM bell curve. An example would be a 6328 Angstrom He-Ne laser with external mirrors and Brewster's windows. You can actually measure the polarity of this kind of laser. A Michelson interferometer can be set up with any laser and one can see the fringes of a laser with a good coherence length. Try it with a MM diode. You will not be able to see any fringes. Even a diode that can be used to make holograms has generally a 0.2nm bandwidth. That is equivalent to a coherence length of 1.1mm. That is a very flat hologram. So, I guess what I'm trying to say is there are polarized laser and there are nonpolarized lasers. I know this has got nothing to do with what you are trying to do. That was the reason i deleted the post from last night as it wasn't going to help with what Alaskan and you are trying to do.

 

[RedCowboy]

Here he list his WP a 1/2 and this would be a non polarizing...correct or wrong? I'm asking you what I should use.
http://laserpointerforums.com/f48/6w-triple-diode-laser-79196.html#post1138388

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There's a ton of info and it seems that we would need a polarizing cube AR coated for the wavelength your using, I'm not sure about power loss I have never used a cube, maybe cyl correction then knife edging would be better and a large diameter BE after.

 

[paul1598419]

This is a good question that I don't have the answer for off the top of my head. Strictly speaking, the diodes are not polarized. The polarized PBS cubes that were shown in the link Alaskan had yesterday were all for gas lasers. You could tell from the table for the wavelengths the cubes were coated for. Not a single diode wavelength in there. As far as I know, there are companies still working on the best way to do this, but it is not something I haven't spent much time on. I'll look into it, though.

 

[Alaskan]

I referenced the gas laser PBS cubes without realizing those were indeed for gas lasers, I've found other PBS cubes which are advertised to work at 450, 520 and 638nm.

 

[Alaskan]

Here's a really good one page lens tutorial by Thorlabs:

https://www.thorlabs.com/tutorials.cfm?tabID=BA49B425-F85B-4549-8C1A-F111DDBB9099

 

[Rivem]

The video reminded me of my first statics course in EE school where the TA who taught the course was from the Middle East and I had a very difficult time understanding what he was saying because his English was not very good. For that reason, I dropped that course and took it later from someone else. I watched about a minute of the video.

Wow Paul, my EE program must be very different. For the last two years, it's really been nothing but a whole bunch of math and classical physics (sans optics) with a little bit of circuit analysis. I'll be taking some E&M in the fall though, so maybe at least a little taste of optics.

I really want to take an optics class if I get the chance though. There's a laser optical sciences elective that seems very enticing.

 

[paul1598419]

I've been wanting to post to this thread, but my search for a way to combine more than two beams of the same wavelength by PBS cubes has, so far, led nowhere. I'm sure your EE classes are different than mine, Rivem. Mine were back in 1973 to 1978. I took optics my sophomore year and we took Fortran IV as PCs weren't available and we had to keypunch our programs and run them through the huge mainframe computer that took up most of the first floor of the Cherry ( EE ) building. Still took a lot of classical physics and lots of math. There were many circuits classes too, and many hours spent keypunching Bessel functions into a program that would run on our only computer.

 

[Rivem]

I've been wanting to post to this thread, but my search for a way to combine more than two beams of the same wavelength by PBS cubes has, so far, led nowhere. I'm sure your EE classes are different than mine, Rivem. Mine were back in 1973 to 1978. I took optics my sophomore year and we took Fortran IV as PCs weren't available and we had to keypunch our programs and run them through the huge mainframe computer that took up most of the first floor of the Cherry ( EE ) building. Still took a lot of classical physics and lots of math. There were many circuits classes too, and many hours spent keypunching Bessel functions into a program that would run on our only computer.

Yeah. Must have changed a lot. I really was hoping to get into some of the more advanced topics like quantum, optical, and semiconductor physics a lot sooner since I finished my physics before the program, but nothing yet. Might just have to go for a Ph.D. in optics like styro. :crackup:

Definitely haven't even seen Fortran or punch card programming at all, but I will have to learn some assembly language when I finally get to my senior level classes.

Definitely feels like they're pushing more and more theoretical topics in EE and ME though. Every time I talk with an older engineer, it seems like their education pushed a lot more practical experience. In the meantime, I've got classmates that have never soldered once in their life and are afraid to touch their breaker boxes. Might not be necessary for EE majors nowadays, but it just doesn't feel genuine to me if you can't complete your own work if necessary.