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    Introduction to Amateur Laser Construction

    So You Think You Really Want to Build a Laser

    In this chapter and the one that follows, we provide basic information on the construction of various types of lasers from scratch.

    Extensive basic information is provided on home-built laser safety, setting up a home laser lab, sources of supplies and chemicals, vacuum systems, glass working, structural materials, power supplies, and more.

    Then, a variety of specific types of home-built lasers are described in more detail. Much of this material is derived from the Scientific American collection "Light and its Uses" [5] and from the email, Web sites, articles, and experiences of those who have been successful in building their own lasers from basic components and getting them to work (not taking the easy way out and using commercial tubes or laser diodes!) - or have given it their best shot trying!

    While this will not substitute the hands-on of actually having built one of these lasers or detailed construction plans, it may provide the spark to get you started!

    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: 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 :-(.

    Reasons TO Build a Laser from Scratch

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

    Some Photos of Home-Built Lasers

    Note: There may be some reduncdancy with some of these photos as they may be present on multiple locations including the Amateur Laser Constructors Web site.

    (From: Chris Chagaris (pyro@grolen.com)).

    (From: Thieu Asselbergs (asselber@fys.ruu.nl)). (From: Laserist (laserist@geocities.com)).

    Diagrams Showing Major Components of Typical Home-Built Lasers

    These drawings show the structure and power supplies for some of the lasers built by amateurs. These diagrams are based on the laser articles from Scientific American (including the book: "Light and its Uses" - see the section: Light and its Uses - Table of Contents for an explanation as to why copies of or links to the original artwork could not be provided). Their purpose is to give you a flavor of what this type of laser construction entails - but are NOT intended as dimensioned plans and are NOT drawn to scale. Refer to the more detailed description in the chapter: Home-Built Laser Types, Information, and Links and the relevant Scientific American articles.

    General Resources for Amateur Laser Construction

    Start by locating back issues of Scientific American and/or their reprint collections such as "Light and Its Uses" [5]. There have been many practical articles and Amateur Scientist columns on lasers, laser construction, and other laser related subjects, particularly during the initial laser craze of the 1960s and 1970s but extending to the present particularly for more exotic types of lasers and laser applications. A large public or university library will likely have all of these somewhere though you may have to request them from their storage vaults and/or they may be on microfilm or microfiche.

    See the section: Light and its Uses - Table of Contents for a list of all the articles that constitute this valuable collection and an explanation of why I cannot provide on-line access to it.

    Here is a list of some of the laser articles that have been published in the Amateur Scientist columns of Scientific American. The first 7 of these constitute the chapters on laser construction found in "Light and its Uses":

    1. Helium-Neon Laser, September, 1964, pg. 227.
    2. More on the Helium-Neon Laser, December, 1965, pg. 106.
    3. Argon Ion Laser, February, 1969, pg. 118.
    4. Tunable Dye Laser, February, 1970, pg. 116.
    5. Carbon Dioxide Laser, September, 1971, pg. 218.
    6. Infrared Diode Laser, March, 1973, pg. 114.
    7. Nitrogen Laser, June, 1974, pg. 122.
    8. Mercury-Vapor Laser, October, 1980, pg. 204.
    9. Copper Chloride Laser, April, 1990, pg. 114.
    Except for the one in (6) which for all practical purposes you can ignore (it tells you how to hook up a long obsolete type of laser diode), all the others are built from the ground up using basic materials (i.e., glass tubing, pieces of plastic and metal, mirrors and other optics, glue, duct tape, various bottled gasses and other chemical supplies, high voltage transformers, resistors, capacitors, diodes, wire, etc.).

    As an aside, I lament the fact that few of the more recent Amateur Scientist columns have nearly as much sophistication and depth as those from that era. On the other hand, experiments that are presented may be performed by nearly anyone who is reasonably handy using parts from the local home center and Radio Shack and yet this is definitely real science. There is no need for high vacuum systems, glass working skills, strange gas mixtures and other chemicals, or fancy test equipment!

    While the Scientific American Web site has many interesting articles, they do not go far enough back to be of much use for laser construction. There is an Index to the Amateur Scientist articles maintained by the Society of Amateur Scientists (SAS) or see the section: Light and its Uses - Complete Table of Contents. However, the articles are not on-line (see the explanation there of why this was not possible) so you still have to do the leg-work!

    The Society for Amateur Scientist (SAS) Web site includes among other things, a technical forum devoted to the interests of, you guessed it, amateur scientists like you:

    Laser specific traffic on this forum is quite small but the high chance of finding someone with similar interests balances this out to some extent!

    Check out the Amateur Laser Constructors Web site and the other links to home-built lasers and related projects in the section: Amateur Laser Construction/Laser Communications Sites and Links to see what others have done. Contact them via email. One thing is certain: since there are so few hobbyist types interested in this sort of thing anymore, these people should jump at any opportunity to discuss their passion to build lasers with you. The site also has a variety of useful links and pointers to Scientific American and other articles on lasers and related topics.

    Information Unlimited has what are supposedly complete plans argon ion, carbon dioxide, nitrogen, copper vapor, and tunable dye lasers. (In the case of the CO2 laser, they have parts, kits, and completely assembled versions as well - supposedly.) I doubt that it is coincidental that these are also most of the types of lasers covered in the Amateur Scientist columns of Scientific American! I do not know whether if the plans are of any value beyond those or whether they are indeed just poor reproductions or transcriptions. I also have no idea of whether what they provide is credible for the price or whether it is likely to result in a successfully completed project. Also see the section: Electronic and Laser Project Parts, Plans, Specialized Components for additional comments about Information Unlimited.

    MWK Industries also has plans for most of the types of lasers included in "Light and its Uses". As with the plans, above, I do not know if these are derived from there or elsewhere or whether there is any benefit to buying these as opposed to digging up the info in "Light and its Uses" in conjunction with this document!

    Acknowledgements

    Thanks to Chris Chagaris (Email: pyro@grolen.com) for his comments and additions to this document. His first-hand experience in constructing several lasers from scratch has been extremely valuable in polishing and enhancing this and the chapter on Home-Built Laser Types, Information, and Links that follows.


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    Setting up a Home Laser Lab

    Safety Issues in a Lab for Home-Built Lasers

    There are a variety of issues that are important for any sort of home lab or workshop but the following, in particular, apply directly to lasers and laser construction: There didn't appear to be a critical mass of lawyers present at the time most of the articles in "Light and its Uses" were written. Therefore, they tend not to deal with the safety issues as emphatically as might be desired. Most of these projects have aspects (most often the high voltage power supplies) that are potentially dangerous or lethal. Safety must be at the top of your list of priorities when undertaking such an endeavor!

    Work Area - Setting up a Laser Lab

    Since any of these lasers represents a long term comittment, it is essential that an area be set aside for your laser lab. Therefore, the kitchen or dining room table is NOT an appropriate place to be constructing a laser. It is possible to do without the sort of setup depicted in the section: Possible Laser Lab Layout but there are some basic requirements for a safe, functional, and convenient space:

    Possible Laser Lab Layout

    I wish I had this! Note: Two means of exit and two fire extinguishers!

    Also note the chair - most important - and the bench for your guest (though probably should be s eleep-sofa so they can snooze while you spend the afternoon adjusting your gas mixture or performing mirror alignment. :-) 

    
    |<------------------------------- 12' ------------------------------>|
 ___|____________________________________________________________________|
  ^ |    |                                                         |     |
  | |    |        Storage Cabinets/Shelves (above work area)       |     |
  | |    '---------------------------------------------------------'     |
  | |          Electrical Outlets (two circuits) all along wall          |
  | |                                                                    |
  | |            Work Surface - thick hard-plywood (3' x 12')            |
  | |____________________________________________________________________|
  | |           |                                                        |
  | |           |  Vacuum System on floor (beneath work area)     Gas    | 
  | |           |                                              Cylinders |
  | | Test      |                                              __________|
  | | Equip.,   |                    ________                 |          |
  | | Power     |                   |        |                | Wet area |
    | Supplies, |                   | Office |                | Glass-   |
 10'| Misc.     |                  (| Chair  |)               |  working |
    |           |                   |________|                | Ventila- |
  | |           |                   '--------'                |  tion    |
  | |___________|                                             |__________|
  | |                                                                    |
  | |S Power Switch            _ _        __________                     |
  | |(on Wall)          .-======'======-.|          |                    |
  | |        /          |               ||  Bench   |           \        |
  | |      /       Fire |    Storage    ||          | Fire        \      |
  | |    /         Ext. |               ||==========| Ext.          \    |
 _v_|__/         _______|_______________|____________________         \__|

    Sources of Supplies

    Develop a relationship with a teacher/instructor/professor/researcher at a high school/technical school/college/university/industrial lab. Some people will be more than eager to help and mentor you - even to the extent of loaning equipment or donating small quantities of chemicals, electronic components, hard to find optics, etc. to your cause. Use of their lab may even be possible. There are various programs as well to encourage students to go into science and technology fields. Who knows, they may even pay you to do this!

    Call up laser and optics manufacturers. Sure, many won't give you the time of day unless they think you will be ordering $1,000,000 worth of equipment. But, all you need is one to say yes! There are always such things as cosmetic rejects or seconds - that are useless to them because they cannot sell the parts - but fine for your needs. The trick is to hold their attention long enough - or be such a (polite) pain in the neck that the easy way out is for the company to provide what you want! I have heard of people obtaining all sorts of material, parts, equipment - some of it quite expensive - in this manner.

    In summary - possible places to find useful stuff:

    (From: Chris Chagaris (pyro@grolen.com)).

    Here are a couple of resources that I have not seen mentioned anywhere on the Net:

    For chemicals used in various aspects of laser construction and laboratory glassware at unbeatable prices, a fine source is:

    For quartz tubing and quartz windows of all sizes, at very good prices, may I suggest: I would be glad in assisting other individuals in locating some of the more difficult to procure items needed in some aspects of constructing these various lasers.

    (From: Steve Roberts (osteven@akrobiz.com)).

    If you are into building your own HeNe (or other) laser from the ground up, these suppliers may come in handy.

    (From: Joe or JoEllen (joenjo@pacbell.net)).

    A good resource for components found in "Light and it's Uses" is:


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    Introduction to Vacuum Systems and Technology

    Vacuum Systems

    This and the sections that follow represent the barest introduction to vacuum technology and systems. See The Electronic Bell Jar for additional articles, links, and references on vacuum technology of relevance to the hobbyist and experimenter. In particular, Vacuum Basics provides a nice introduction including vacuum terminology and applications.

    Its parent site, the Bell Jar has an index to many additional articles available only in hard copy and/or by subscription.

    There have also been a number of articles on vacuum systems and technology in the Amateur Scientist column of Scientific American. See the Index to the Amateur Scientist articles maintained by the Society of Amateur Scientists.

    These include:

    1. Vacuum pump, December, 1958, pg. 134.
    2. Vacuum pumps, March, 1960, pg. 187.
    3. Vacuum leak detection, February, 1961 Feb, pg. 159.
    4. How to make a Valveless pump, January, 1965, pg. 118.
    5. How to make a McLeod gauge, December, 1965, pg. 106.
    All but one of the gas lasers described in chapter: Home-Built Laser Types and Information require a decent vacuum system to remove air from the laser tube so that it can be back-filled with the required lasing gasses at a low pressure - a vacuum. These include the HeNe, Ar/Kr, CO2, HeHg, and CuCl/CuBr lasers. The N2 laser requires only a 'low' vacuum since it runs at a substantial fraction (e.g., perhaps 20%) of atmospheric pressure and some versions can run ambient pressure (1 atm).

    The vacuum system serves three functions:

    By the standards of the vacuum industry, our requirements are modest and are not really termed a 'high' vacuum but they are still not the sort of thing you come across in daily life.

    But first, how about all this talk of pressure?

    What does a Pressure of Such-and-Such Really Mean?

    We always hear about the barometric pressure - or the level of a vacuum - in terms of 'mm or mercury' or 'inches'. 1 atmosphere (at sea level under some unidentified ideal conditions) is also said to be 14.7 pounds per square inch. Why?

    The earth is covered with a vast ocean of air. Despite common experiences, even air has mass and mass implies weight. We know it has volume or else your automobile would have a real problem with flat tires. Most of the volume (the contribution from the volume of the the protons, neutrons, and electrons in the atoms are negligible but not precisely zero) results from the constant motion of the molecules (in air or other gas) bouncing against each-another due to their thermal motion. This also keeps the air in a gaseous state. At really low temperatures, the motion is reduced resulting in liquid and solid phases of even air. At exactly absolute zero (-459 degree F or -273 degrees C) all motion ceases. However, even then most of the volume of the frozen air is still empty space - but that is another story.

    At sea level under average conditions, the column (actually an inverted truncated pyramid if you want to be strictly correct) of air above 1 square inch of area would weigh 14.7 pounds if you could capture, compress, and package them and plop them down on a delicatessen scale! As you move away from the earth, this 'column' of air becomes increasingly rarified approaching a prefect vacuum at 50 miles or so - else low earth orbit satellites would not stay up very long due to air friction.

    It turns out that a column of mercury with an area of 1 square inch and 29.92 inches (760 mm) high weighs exactly 14.7 pounds as well (what a coincidence, huh?). So, if you take a closed-end tube a little more than 30 inches long, fill it with mercury, and invert it in a pool of mercury, the pressure of the surrounding air will be able to support a column of mercury 30 inches high. The space above the mercury will be a decent vacuum. You have made a mercury barometer.

    If you were to take this barometer and place it inside a vacuum vessel and start up the pump, the column would go down until at the point of a perfect vacuum (not achievable but close), it would be precisely level with the surrounding pool of mercury.

    Note that the diameter of the tube doesn't matter - wider implies a heavier column of mercury but the area of the air acting on the column changes by the same factor. In fact, it can have pretty much any convoluted shape you want (except that if portions are too thin, surface tension becomes a factor) as long as it is sealed and totally filled with mercury. Why this is so is left as an exercise for the student!

    The corresponding height of 1 atmosphere for water is about 34 feet - a column of water with a cross sectional area of 1 inch and height of 34 feet weighs 14.7 pounds. This also means that for a diver, the water pressure increases by 1 atm for each 34 feet of depth. Thus it is not surprising that there are significant problems in deep sea diving! You have to go up by MILES in air for the pressure to decrease by a substantial fraction of 1 atm but need only go down 34 feet in water to increase pressure by 1 atm!

    Note that the most likely form of a pressure you are familiar with is the reading on the gauge you use when checking or filling your automobile or bicycle tires. However, this is calibrated relative to the surrounding pressure of around 1 atm. Thus, the actual pressure inside a tire will actually be 1 atm + the reading on the gauge. And you thought you had a perfect vacuum inside that flat tire when the reading was 00.0! :-)

    What is a Low, Medium, or High Vacuum?

    Vacuums come in all shapes and sizes - and I am not referring to vacuum cleaners! Any local reduction in air pressure significantly below standard atmospheric pressure (760 mm of mercury, 14.7 pounds per square inch) is termed a vacuum (except by your local weather person who talks about 'low pressure areas'). For convenience (and because there must have been a meeting of elder statesman with nothing better to do), the Torr in honor of some Italian named Torrecelli is used to designate a pressure of 1 mm of mercury I guess referring to 'Torrecellis' all the time would be too confusing. :-)

    The Vacuum Chart provides a nice instant summary of pump types, gauges, and applications, as a function of the level of vacuum.

    The following dividing lines between low, medium, high, and ultra-high vacuums are somewhat arbitrary but will be convenient for discussion:

    You may also hear the term 'hard vacuum'. I don't know if there is a precise definition for this either but I would assume that anything with a low enough pressure to behave similarly to a perfect vacuum from the normal experiences point of view would qualify.

    How Good a Vacuum System is Really Needed?

    None of the gas lasers we will be discussing requires a vacuum better than about .5 Torr when operating. However, in order to clear them of contaminants in a timely and economical manner (without a semi-inifinite number of purge and back-fill cycles), it is desirable to be able to pump down to a much lower pressure than this. The better your vacuum capability - to a point - the easier it will be to obtain a pure gas fill. Less gas will be needed (due to fewer pump-down and back-fill cycles) and time will be saved. However, there is no need to go overboard. My rule-of-thumb (read: wild guess) is that a vacuum system capable of reliably pumping down to 1/100th of the lowest operating pressure is adequate for dealing with a laser tube that has a single vacuum/gas fill port. Pumping to 1/10th the desired final pressure may even be good enough if the laser tube is fabricated to have a gas-fill port at one end and a vacuum port at the other. For a flowing gas design (e.g., CO2 laser), the requirements are even less stringent and just being able to maintain the desired operating pressure may be good enough. If you think you will be building more than one type of gas laser, make sure this applies to the one with the lowest operating pressure. Also keep in mind that some types of lasers (like the HeNe) are particularly sensitive to the slightest traces of unwanted gasses and a better vacuum system would be advantageous for these.

    Unless have worked with a decent vacuum system in the past, own a HVAC service business, or just happened to pick up something that looked like a pump of some kind at a garage sale (but you weren't really sure and got lucky), you don't have the needed equipment! However, an adequate 'medium' vacuum system can be put together for less than $400 - possibly a lot less if you are determined and somewhat resourceful.

    Types of Vacuum Pumps

    Various kinds of vacuum pumps are needed to pump down to different levels of vacuum. Generally, mechanical pumps are used for low to medium vacuums and other types are needed to go below this range. However, there are exceptions.

    See Vacuum Pumps Suitable for Various Home-Built Lasers for diagrams of the types of vacuum pumps described below that are relevant for our purposes.

    There are many types of mechanical pumps but they are usually based on one of two basic principles: positive displacement (perhaps these should be called negative displacement in dealing with vacuums!) and turbo-molecular:

    These pumps can be further classified as to the number of stages: While not generally thought of as pumps, the following perform related functions helping to rid the system of moisture and other unwanted volatile materials: In addition to helping to achieve a high vacuum, a dryers and cold traps may also help to prevent contamination to the oil in the vacuum pumps.

    Salvaged Refrigeration Compressors as Vacuum Pumps

    How many refrigerators, window air conditioners, freezers, and dehumidifiers, have you hauled to the dump or passed up on the curb???? The compressor in these systems may be pressed into service as a vacuum pump where a low (and in some cases, medium) vacuum is acceptable. A detailed discussion of this is provided in the hard copy version of the Bell Jar. (The Electronic Bell Jar being the subset of these articles that are on-line. Check that site for contact and subscription info.)

    Salvaged Refrigeration Compressor Wiring

    The following applies to a typical GE refrigerator compressor. YOURS MAY BE DIFFERENT! Don't rip out the compressor without making a wiring diagram and saving all the relevant parts!

    The sealed unit has 3 pins usually marked: S (Start), R or M (Run or Main), and C (Common). The starting relay is usually mounted over these pins in a clip-on box. The original circuit is likely similar to the following: 

                            |<- Starting Relay ->|<---- Compressor Motor ---->|
           
                 ___            L      
       AC H o----o o--------------+--o/   S    S
              "Guardette"         |    o---->>-------------+
               (Thermal           +-+                      |
               Protector)            )||                   +-+
                          Relay Coil )||                      )||
                                     )||                      )|| Start
                                  +-+                         )|| Winding
                                  |                           )||
                                  |      M    R/M          +-+
                                  +-------->>------+       |
                                                    )||    |
                                           Run/Main )||    |
                                            Winding )||    |
                                                    )||    |
                                                 +-+       |
                                              C  |         |
       AC N o------------------------------>>----+---------+
    The Starting Relay engages when power is applied due to the high current through the Run winding (and thus the relay coil) since the compressor rotor is stationary. This applies power to the Start winding. Once the compressor comes up to speed, the current goes down and the Starting Relay drops out.

    Note the Thermal Protector (often called a "Guardette" which I presume is a brand name). Leave this in place - it may save your compressor by shutting it down if the temperature rises too high due to lack of proper cooling or an overload (blocked exhaust port or low line voltage).

    You can use a heavy duty pushbutton switch in its place if you like or if you lost the original starting relay :-(.

    Vacuum Valves

    Two types of valves are required. Fancy expensive types may not be needed so you may find some of this at your local hardware store or home center. However, since common valves are designed to operate in a positive pressure environment, they may not hold up under vacuum conditions - or they may be fine! In addition, the sealing grease used may outgas at low pressure. Some testing will be necessary to be sure.

    Vacuum Gauges

    Some means of determining the precise level of vacuum is perhaps not totally essential but certainly highly desirable. Otherwise, whatever you do is like a shot in the dark. The old 'thumb over the hole' trick really isn't precise enough! There are several types in common use:

    Vacuum Tubing

    The flexible tubing that is used to interconnect various parts of the vacuum system must satisfy several requirements: When in doubt, test a length (e.g., a meter) by comparing the lowest pressure achievable with your pump(s) capped by the vacuum gauge and with the tubing in place. The final pressure should be identical.

    Vacuum Seals and Sealers - Red Glyptal

    Three types of material are used depending on the particular needs: Scientific and vacuum supply companies should carry all of these and other suitable products. Your needs are quite modest compared to say, the CRT industry, so there is no need to go overboard with ultra high vacuum sealers. None of these lasers require anything beyond 10E-3 or 10E-4 Torr anywhere in the vacuum system so the stuff that is guaranteed to 10E-10 Torr is probably a bit of overkill (but won't hurt except in terms of cost).

    Also see: Fabricating Air-Tight Seals for one approach to making inexpensive seals that can be easily opened should a tube need to be regassed.

    Leak Testing

    For our simple vacuum system, leak testing is usually self evident - there are only a few places where leaks can develop. There are a number of approaches:

    A Typical Vacuum System

    For gas laser work, a suitable 'minimalist' vacuum system might consist of: Also see: A Simple Medium Vacuum System for some additional ideas on a low cost approach to a setup that may be adequate for laser construction.

    Considerations for Gas Laser Filling/Regassing

    To a large extent, the life expectancy of a HeNe or other low pressure gas laser will be heavily dependent on the cleanliness of the interior of the tube and all its constituent parts and the purity of the final gas fill. Therefore, while it may be possible to use a marginal vacuum system and less than super pure gasses with common basement workshop conditions to get a home-built laser to work for a short time, don't expect optimum output power or stability and a useful lifetime for such a tube if sealed off to be more than a few dozen hours, if that!

    The following comments come from someone who has experience with both HeNe and ion laser refurbishing:

    Letting a HeNe laser tube up to air outside of a inert gas glove box is not a good idea. A class 1000 or better clean room environment with HEPA filtered inert gas is really needed.

    The main things that determine your power are, correct Brewster window angle and material (for external mirror tubes), ultra clean optics, and clean gas fill. We're talking cleaner then cleanroom clean - better than the best surgical suite - semiconductor manufacturing type clean!


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    Introduction to Glass Working

    Laser Tube Fabrication

    The following mainly applies to the traditional gas lasers like the HeNe, Ar/Kr ion, and HeHg where the entire laser discharge tube is generally a single glass structure - it is made in one piece from various individual pieces that are fused together. The N2 and CO2 lasers do not require glass working of this type.

    As laboratory apparatus goes, what you need for any of these lasers is pretty mundane: A few tubes joined together with butt or tee joints, a few dimples or bumps, some angled cuts, and pieces attached with glue.

    Note that at least in principle, it is possible to construct these lasers without actually fusing glass pieces together as Epoxy or other adhesive and/or vacuum rated flexible tubes and clamps can be used. However, such a structure is not nearly as stable and are not recommended. In addition, the added nooks and crannies of clamped pieces and places with glue that can outgas mean that achieving the required level of vacuum and maintaining it is much more difficult.

    There are basically two ways to go about obtaining the needed assembly of tubes, electrodes, Brewster windows, and so forth.

    1. Have someone else do it! Assuming you can find a cooperative individual or pay a neon sign shop or laboratory equipment fabricator, this is by far the easiest especially if you have to start from ground-zero. For someone at all experienced in this sort of stuff, the assembly of the main portion of a typical laser tube (not including the Brewster windows) is a 20 minute job if all the bits and pieces are available. If you can arrange that your design uses the same sizes and types of glass tubing that they commonly deal with, so much the better.

    2. Learn enough of the skill of glass working to do it yourself. This is by far more fun and who knows, maybe you have a talent for the sort of glass art exhibited in museums. This really isn't as difficult as it might seem at first. Glass working is a skill and you will no doubt create some pretty interesting failures at first. But with a little practice (OK, maybe a lot of practice!), butt and tee joints, and dimples and bumps will become second nature. We aren't talking about fancy decorative glass blowing, mostly just basic cutting and joining.
    Glass working as it relates to laboratory apparatus fabrication has been covered in the Amateur Scientist columns as well. See: Glass blowing, technique explained, Scientific American, May, 1964, pg. 129.

    In the following sections, we provide the briefest of introductions to the glass working skills that are needed.

    Types of Glass

    What was call glass is made from silicon dioxide (SiO2) and other additives to produce the wide range of properties of various glasses that we are familiar: from window glass to Pyrex cooking and labware; colored glass bottles and stained glass windows, light bulbs of all shapes and sizes; and optical glass for lenses, mirrors, and prisms. SiO2 is the same stuff that constitutes beach sand and the insulating layers of integrated circuit chips.

    Glass is an amorphous material - it has no crystalline structure and is really a liquid at room temperature. A liquid, you say???? Well, a slow moving liquid at least. As its temperature is increased, glass becomes softer but has no distinct melting point (compared to water, salt, or any other material that forms a crystalline structure where there is a distinct phase transition from a solid to liquid state).

    The two types glass which will be of most interest for laser construction are:

    Fused silica (Vycor) and pure quartz are two highly heat resistant materials that you hopefully won't have to shape since they have even higher softening (or in the case of quartz, a crystal, melting) points as well!

    If you order common laboratory glass tubing, it will likely be made of S-L glass though other types are also available - make sure you specify what you want since for some of the laser parts, heat resistance is an issue. Most beakers, flasks, and anything else that may be heated are made of Pyrex or the equivalent B-S formulation of another manufacturer. However much other labware is of the S-L variety. Since the coefficient of thermal expansion also differs for the two types of glass, there may be problems in attempting to mix them in a given structure.

    Cutting Glass Tubing

    For small tubes - say less tha 1/2" in diameter, cutting is, well, a snap!

    All you need is a small triangular file (new or in excellent condition, not rusty and clogged with something disgusting) and perhaps some spit. :-)

    Sometimes, wetting the filed location with a bit of spit or tap water will aid in the process.

    Practice on some scrap pieces of tubing. In on time you will be turning all the neon tubing in your neighborhood to small bits suitable for making beaded necklaces!

    This also works for larger diameter tubing (like CRT necks) but a longer crevice may be needed - try to keep it straight. In some cases, one pass all the way around will be needed.

    There are also hot wire cutters (the heated wire produces local stress which fractures the glass). For large or irregularly shaped objects, the best tool is a power driven diamond grit glass cutting wet wheel - a water cooled miter saw for glass and ceramics!

    Any sharp edges left by the cutting operation should be smoothed with fine sandpaper or in the flame of your glass working torch.

    Glass Working 101

    All glass working consists of four steps:
    1. Heating. This is going to be done with a flame of some kind:

      • A common propane torch or natural gas burner using air is just hot enough to soften S-L glass. A bunsen burner works - barely. Other types of lab burners are better.

      • With the use of pure oxygen, the flames from these all run much hotter and that is what you is really needed to be able to do any sort of glass work easily and consistently (or borosillicate glass at all).

      • An oxy-acetylene or oxy-hydrogen torch will be needed to easily deal with some types of heat resistant glass and fused quartz. (CAUTION: Hydrogen flames tend to be invisible!)

      At the proper temperature, glass has the consistency of soft taffy - easy to bend and shape but not so soft that it runs or drips. Part of the skill (and fun) is keeping the glass at just the right temperature as it is worked. As the glass approaches the proper temperature, the flame will take on a yellow tinge from the sodium ions in the glass (the soda part) and the glass itself may appear red or orange-hot itself.

    2. Working: Bending, joining, pulling, dimpling, blowing, etc. is done while the glass is maintained at a relatively constant temperature in the flame. Glass cools quickly so repeated or constant heating is needed. Some positive pressure in the glass parts may be needed to prevent them from collapsing - or to blow bubbles! The surface tension of the soft glass is going to be both our friend (since it will help smooth out much of the damage you will inflict) and foe (since it will tend to want to cause tube ends to collapse or other holes to expand). Usually two hands and a mouth (safely at the end of a length of rubber tubing!) are enough but at times you might wish to be an octopus!

    3. Once the particular joint or whatever is formed to your satisfaction, the piece must be cooled so that it solidifies. However, you cannot just dunk the whole affair in a bucket of water as the sudden temperature change will cause your hard work to shatter into a million pieces (sometimes it will do this even without such help!) It must go through a process of annealing where a lower temperature flame is run back and forth over a large area of the glass - beyond that which was dealt with originally. The cooler flame can be obtained by reducing the air or oxygen supply to the torch. Fortunately, this takes only a couple of minutes for anything we are interested in constructing (unlike the 17 foot diameter Palomar telescope mirror which required over a year of annealing). Note that there is no real way of knowing how much annealing is enough - it is just something that one does based on recommendation or experience.

    4. Cooling. The worked and annealed area will still be very hot. Set it down on a non-flammable material or better yet, in such a way that the hot parts do not touch anything until it is cool enough to touch. This allows it to cool slowly and uniformaly, further minimizing the chances of stress cracks.

    Gas Flames

    A gas flame (natural gas, propane, etc.) adjusted for hottest temperature (optimum fuel:air ratio) is divided into several parts: 
    
         Tip---> /\ (Dark Blue)
                /  \
      Cone --> / /\ \ (Light Blue)
              | |  | |
             _|_|__|_|_
     Burner |          |
    Great diagram, huh?

    Note that it is mostly shades of blue - there should be minimal yellow or orange (indicating that there is adequate air/oxygen) but the flame should not begin to separate from the burner (indicating too much). There should be no smoke or soot from such a flame.

    The hottest location is just above the inner cone.

    With soda-lime glass, once the glass is hot enough to work, the flame will take on a yellow color due to the sodium ions in the glass.

    With the air/oxygen supply cut off, the flame will be long and yellow and may produce black smoke and soot. This will be the proper temperature for the annealing step.

    Note: Where you have control of the air/oxygen supply as with a professional glass working torch (or Oxy-Acetyline welder, for that matter), light it up by first opening just the gas supply a small amount and then adding air/oxygen and adjusting gas flow after the flame is lit. Shut down in the reverse sequence. This avoids unsightly pops, bangs, and other explosive behavior.

    Glass Working Examples

    The only way to really be come proficient at this is to practice. You will create many many interesting disasters at first but glass is cheap. After a while, these sorts of 'simple' procedures will become automatic and second nature. Who knows, even your failures may find a place in the Museum of Modern Art!

  • Back to Amateur Laser Construction Sub-Table of Contents .

    The Laser Assembly and Optics

    Mounting the Laser Components

    The stability and strength of the baseplate are probably the single most important factors in determining how easy it will be to set up and maintain a laser with an external resonator. Multiple optical components have to remain aligned to a small fraction of a degree despite changes in temperature and placement (on another surface like a lumpy tabletop) of the laser assembly.

    Forget about most wood - it is too flexible, absorbs moisture and warps or at least changes size all too readily. It may be possible to totally seal some high quality wood or wood-based composite products but it probably isn't worth the effort.

    Start with a solid metal base. Short of something milled from a big heavy casting or the use of a real optical bench or table or a converted lathe bed, the best is an extruded aluminum box shape since this is very strong for its weight and will resist bending and twisting. A C-channel extrusion will be nearly as good if it is braced at multiple points along its open side - and this is more accessible for attaching screws and whatever from underneath. Or, a thin removable cover plate can be screwed to the open side.

    Buying a big enough piece of this new - say 4" x 2" x 4 feet, more or less depending on the size of your laser - will set you back a few bucks but will save a lot of time in the long run.

    Drill and tap holes for mounting the laser tube, mirror mounts, and whatever else you need. With tapped holes, there is less opportunity to spend your time fishing for lost screws! Add keying holes for assemblies that may need to be removed and replaced without changing their position - like the mirror mounts. Attach some non-slip material on the bottom to force the entire affair to stay put!

    Optical Windows

    There are several considerations when selecting a material to be used for a Brewster or other window through which light must pass undisturbed:
    1. Optical quality. This refers to the surfaces (plane, flat, and polish) and purity of the material.

      High quality microscope slides (not the kind that are 100 for $1.00 at your local hobby store) are actually quite good. To check, hold one at arm's length and view a distant scene through it - there should be no detectable distortion or shift of the image as it is inserted/removed from the view. Alternatively, insert/remove the slide from the path of a laser beam projected onto a far wall (reflecting back from a mirror to a nearby screen if you don't have a partner). There should be no noticeable shift in the position of the projected spot with/without the slide in the beam.

    2. Index of refraction (and dispersion if multiple wavelengths are involved). This will mostly affect the Brewster angle, percent reflectance with respect to window angle, and angle of total internal reflection.

    3. Heat absorption/losses. This can be critical where the window is part of a low gain laser resonator as in the case of the Brewster window for the HeNe laser. Quartz should be better than common glass in this regard.

      Keep in mind that the light intensity *inside* the resonator is going to be many many times greater than the actual power in the output beam. This ratio will be approximately 1/(1-Roc) where Roc is the reflectivity of the Output Coupler (mirror reflectivity specified between 0 and 1).

      For example, with a HeNe laser, a typical Roc is .99. So, the power level between the mirrors will be roughly 100 times greater than the actual power in the output beam - or 1 WATT for a 10 mW laser!

      Thus, absorption->heat losses can be significant and need to be minimized. (And no, you cannot stick a mirror in at an angle to extract a high power beam but think about zig-zag paths through laser gain media if you have trouble sleeping some night!)

    See the section: Sources of Supplies for low cost suppliers of high quality optical windows.

    Determining Brewster angle

    The equation for the Brewster angle defined between the window and a plane perpendicular to the direction of the light rays (tube axis) is: 
             Brewster angle = arctan(index of refraction)
    For a quartz window - desirable for an HeNe laser due its lower heat losses at 632.8 nm, the index of refraction is 1.54 resulting in a Brewster angle of 57 degrees.

    So, this is a piece of cake even if you weren't a stellar performer in high school trig. However, suppose you don't know the index of refraction of the material you are using? Ah, no problem if you have a light source (like a laser) of the SAME wavelength since it can be determined experimentally. For the construction of the HeNe laser this should be no problem since you likely already have some sort of HeNe laser! And, we already warned you that you shouldn't be building the HeNe laser if your goal is just to have a working HeNe laser anyhow. :-)

    The light source has to be polarized. This either means a laser outputting a polarized beam (by design or see the section: Unrandomizing the Polarization of a Randomly Polarized HeNe Tube) or the use of a polarizing filter on its output. However, for the latter, common HeNe tubes produce a beam with random polarization - it varies as the tube heats up and just because it feels like it! This means that the intensity will be varying at the output of the polarizer so this will have to be taken into account as you view the reflected beam.

    1. Fasten a sample of your window material to a block of something so it is perfectly vertical and in a way so that it can be adjusted between about 30 and 70 degrees with respect to the direction of the laser beam and that the angle can be accurately measured.

    2. Line everything up with the window at a 45 degree angle and turn on your laser.

    3. Rotate the laser or its polarizer about the axis of the beam until the reflection off the window is minimized (keeping in mind that your actual beam intensity may be varying at the same time if you have a random polarized HeNe tube).

    4. Adjust the angle of the window until the reflection is further minimized.
    Repeat steps 3 and 4 until the reflection is as small as possible. When optimal, the reflection should be extremely faint. Measure the angle. :-)

    Mirrors

    When most people think of mirrors, what they use for shaving or makeup or the rear or side view mirrors of the automobile come to mind. However, none of these would be permitted anywhere near a laser lab. To put it bluntly, their quality and performance stinks!

    The ideal mirror would have a coefficient of reflectivity of 1 (100%) for all wavelengths of interest and produce no distortion.

    Mirrors are used in two sorts of places: as part of the resonator and everywhere else. Needless to say, you aren't going to find resonator-qualified mirrors at the local variety store! Unlike aluminized telescope mirrors which are possible to coat in your basement (at least in principle), this is not an option for dicroic types. They can be obtained from optical supply companies and in the case of the HeNe laser, dead or sacrificial HeNe tubes. (Argon or krypton ion also, but you aren't likely to have any!)

    Adjustable Mirror Mounts

    The HeNe, Ar/Kr ion, HeHg, and other similar gas lasers all require adjustable mirrors outside and usually separate from the tube itself. These should be: You can of course purchase such mounts but one of them will probably set you back more than the entire budget for this laser project! However, there are homemade alternatives.

    The Adjustable Mirror Mount is a simple design that meets all of these requirements. It consists of a right angle aluminum bracket, an aluminum plate to which the mirror is attached with glue (around the edge), screws, or clips, and three spring loaded thumb-screw adjusters. Indexing balls between the base and the mounting surface and an adjustment screw allow it to be removed and replaced with virtually no change in alignment. These can be constructed using common hand tools though a drill-press would be nice and high quality drill bits and taps are a must!

    Parts list (typical):

    A similar design may be found in "Light and its Uses".

    Determining the Radius of an Ion or HeNe Laser Optic (Mirror)

    Basically, the procedure below is a means of using the optic (mirror) to image a source at infinity thus providing the focal length and, from this, the radius since for a mirror, r=2f.

    (From: Steve Roberts (osteven@akrobiz.com)).

    Take a working HeNe laser, upcollimate it to at least 10X the size of its normal beam, and make sure it has 1/10 the normal divergence, in other words just expanding it with a lens wont work, it must have all the rays in it parallel. Or, take a large diameter source of projected light focused at infinity, and aim it at a slight angle from the normal to the optic. (Normal means at exactly right angles to the surface.) The optic should be many feet away from the light source. Then you should have the beam coming back toward the source but not hitting it. If it's a flat or convex mirror, the beam will continue to expand. But, if it was figured with a concave radius during polishing, by sweeping a card through the reflected beam you can sometimes find a focal point. Measure the distance from the focal point to the surface of the optic, this is 1/2 of the radius so double the measurement to get the radius. This isn't that accurate, but it will give a measurement within 10 percent. You are probably never going to find a convex ion or HeNe optic, but you might find them in CO2 or YAG lasers.

    For ion laser optics, standard radii are flat, then 60, 100, 200, 300, 400, 800 cm. Generally the optic focal length is at least twice the length of the plasma tube if the rear mirror is flat, for a TEM00 beam.

    Comments on Acquiring Optics for Home-Built Lasers

    The following is from someone who is involved in commercial laser repair and even he has problems finding suitable low cost replacement optics!

    I recently went hunting for laser optics. A pair of standard coated 12.5 mm diameter mirrors for an ion or HeNe laser would set you back $1200-2000 a set, you might get a suitable rear mirror for a Hg, CuBr, or CO2 for much less, but the price of optics for any gas laser will be prohibitive. Large frame argon laser optics, if you could find a used set, are going to be $250 for optics if the coatings are still anywhere near useful, and much more if they are in good shape. Costs seem to stay the same regardless of the substrate material or diameter - buying a smaller optic won't be that much less expensive if at all.

    If you are thinking about going direct to a supplier of laser optics, off the shelf optics similar to what they coat for other laser companies are generally not available as the contract prohibits the optics company from selling them. Thus yours will be a 1 off custom run. The low cost Chinese optics companies do not do ion or HeNe coatings with the needed levels of reflectivity or quality. I tried that too, and I have especially good relations with one of them.

    You have to rip them out of a dead laser of similar size and power, and for HeNe this is a problem as modern sealed HeNe tubes may use at least one mirror that is concave and is only good at the same working distance between the mirrors used in that given tube.

    I know, I just spent two months hunting down an ion set, $750$ a optic new, so $1500 for a full cavity for a 1 meter class laser. That was a relatively inexpensive optics set too. It was for krypton or I could have bought a whole used 1.7 W argon ion laser for not much more.

    Using the short radius semiconfocal cavity optics of an ALC-60X or Omni-532 for the Scientific American tube will not work even though the mirrors are only $300 a set for cheapies. Mirrors are coated to a specific transmission based on tube length, a small air cooled might be .6 percent transmission, where a 2 meter long large frame 25 to 30 watt would be around 8%, but that percentage would be tailered across the range of lasing wavelengths for a specific balance. So if you tried using a 2 watt pair of optics for a 10 mW homemade laser, you would be very sadly disappointed in the output and/or it probably wont lase at all, or if it did you would only see the ultra high gain 488 line lasing. However mirrors for a shorter low power laser might work if you scale up the tube. The problem will be the radius of the mirrors, not the transmission. For example, a 1 meter radius ALC-60X OC might work for Scientific American ion laser, but the usual standard 60 cm radius would not. Plus aligning a non optimized cavity would be a bear, and with a low gain amateur tube, highly unlikely. Funny how the author left the optics specs out entirely!!!!

    For a recent project we put two 45 cm radius optics from a laser with a 1.5 inch longer resonator then a 60X into a 60X, alignment time approached 1 hour instead of the usual 5 minutes, and did not get any quicker. There was exactly one path with respect to the bore that worked, including the offsets in length caused by the X-Y adjustment screws on the end plates, talk about critical!! Only reason we did it was we needed gain on a line not supported by the 60X optics for a experiment.

    So what I'm trying to say, is, unless you have the right optics, you are better off investing in a working laser if you are trying HeNe, or Ar or Kr ion.

    There are only 3 companies in the US who produce hene mirrors, and the one of them that was hobbyist friendly just told me, "no more" as they are tired of coating optics that get returned with the claim "well my tube is good, so it must be your mirrors that aren't working, or for argon, I don't like the green-blue-red balance or transmission of these optics."


  • Back to Amateur Laser Construction Sub-Table of Contents

    Power Supply Considerations for Home-Built Lasers

    Power Supply Components

    Also see the section: Related Power Supply Information for much more on high voltage and other specialized power supply operating principles, design, components, and construction techniques.

    Several types of power supplies are used for these lasers (more than one type may actually be applicable).

    High frequency inverters may also be used as the power source for any of these approaches.

    Related Power Supply Information

    Meters - Voltage and Current

    It is unlikely that you will have or can find exactly the types of meter needed for each of these lasers. However, any sort of mA or uA meter cn be turned into a DC or AC voltage or current meter of almost any full range sensitivity quite easily. This can be a moving coil (D'Arsonval) type or digital panel meter module. For historical reasons, we call these 'movements' whether they have moving parts or not. :-)

    In the discussion below, Im is the full scale sensitivity of the meter movement and Rm is the resistance of the meter movement.

    The DC circuits are discussed first. These are generally simpler than those for reading AC directly and are therefore preferred if a suitable location can be found where the measurement will be just as meaningful and accurate. For AC, there are special AC reading meters but these are much less common than the DC variety. However, where absolute precision and linearity isn't needed, and an average rather than RMS reading is acceptable, it is a simple matter to convert a DC meter to respond to AC.

    The following circuits for AC voltage and current measurements will actually read the average, not RMS if components values are calculated using the same equations as for the DC case. For sinusoids, a simple correction can be made with the calibrate pot. True RMS readings are left as an exercise for the student!