physics question #248



Umit K, a 11 year old male from Cambridgeshire asks on November 7, 2001,

Q:

How can I construct a simple laser beam?

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the answer

Samuel Goldwasser answered on November 7, 2001, A:

Lasers are so cheap, it's easier to buy one for $10 or $20. Here's a summary I found on line at Samuel Goldwasser's Laser FAQ. Much more can be found at Sam's FAQ.

Reasons NOT TO Build a Laser from Scratch:

First, let us consider some ill-posed justifications for attempting to build a laser from (almost) raw materials:

"Something can be put together quickly." Forget it. Any of the types of lasers considered in this discussion will require many many hours, more like months, to locate the materials, chemicals, and other supplies and to construct, align, and get working - if you ever succeed at all.

Furthermore, you will be CONSTANTLY fiddling with adjustments like gas fill, mirror alignment, power supply voltage/current. In many cases, total laser lifetime is often short (a few hours) before a total rebuild is needed. These are not generally set-it-and-forget-it type equipment! If you just want a working laser, this is definitely NOT the way to go.

"Lasers are SO expensive." Yes in many cases, but what you get is something that works (relatively) reliably with (relatively) minimal fiddling with adjustments. In the end, you will likely spend more than you might think based on your initial estimates simply because you cannot go and buy just the quantity of materials or chemicals that you will need - 1 screw or 2 inches of pyrex glass 5 mm ID tubing or 6 feet of #24 magnet wire, for example. A surplus $25, 1 mW helium-neon laser head and power supply, or even a $9.95 laser pointer may be more than adequate for your needs.

If these are your only reasons for wanting to do this, you will rapidly tire of the endeavor and the parts will end up in a box alongside that dusty old partially ground telescope mirror you also never completed. :-(

If you want a working laser for a particular application, save your pennies and buy one. The cost of a used laser appropriate for what you have in mind may not be as terrible as you may think. The result of building a laser from scratch isn't likely to be something you can use reliably day in and day out without constant maintenance, repairs, and the occasional disaster. (The one exception to this might be the axial flow CO2 laser which if properly constructed, is less finicky than the other types discussed in the following chapters.) A system that starts life on and under a workbench will also probably never be packaged in a nice self-contained cabinet and may have to coexist with the home washer-drier, family car, or kitchen table. :) Anything home-built is also going to have many potentially serious hazards associated with it unless significant effort has been made to provide the necessary beam blocks, electrical and thermal protection devices, and safety interlocks.

Reasons TO Build a Laser from Scratch

However, there are many justifications for embarking on an adventure of this type:

Educational experience. I guarantee that you will learn a tremendous amount in the process - even if your baby never actually produces a beam. You will need to deal with a variety of disciplines (depending on the type of laser) possibly including: glass working, vacuum and gas supply systems, machining, design of high voltage power supplies, optical alignment, sensors, and more. There is no better way to learn about these disciplines than to have to deal with them first-hand. And, these efforts may stimulate interests in other areas as well.

Challenge. It probably goes without saying that the production of coherent light - even if for a short time - from a totally homemade laser represents a tremendous achievement with enormous satisfaction value. Why do people climb mountains? Building a laser is like climbing from the depths of the Pacific to the top of Everest and beyond. Note: If you don't believe this then you will probably not have enough motivation to complete the project!

Experimentation. There are many things you can try with full access to both the inside and outside of the laser resonator that are impossible with a sealed HeNe or Ar/Kr ion tube. Variations on the lasing medium including (for gas lasers) the types and pressures of the gas fill or (for dye lasers), the types and concentrations of dyes themselves; types of excitation (direct discharge, RF, light); alternative optics like curved and plane mirrors, prisms, lenses, and coatings.

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