Build a He-Ne Laser Experimenter’s System

Bare laser tubes have a “shocking” personality—you’ll want to enclose yours in a protective housing to make using it easier. You can place just the tube in the enclosure or make an all-in-one lab laser that incorporates both laser and -high voltage power sup ply in one compact package. You might also want to house a cylindrical laser head in a box to keep it from rolling around the table.

This section provides plans on how to bundle a bare or cylindrical laser head in an easy-to-manage plastic enclosure. Several versions of laser housings are provided, including hand-held cylindrical pointers, box-type lab lasers, and mini optical breadboards.

BUILD AN ALL-IN-ONE LAB LASER

Small, 1- to 2-mW bare He-Ne’s—along with their high-voltage power supplies— can be neatly packaged within a small plastic enclosure. Depending on the size of the enclosure, you should be able to use any commercially made power supply, as well as the home-brew power supply presented in Section 11.

Although plastic is a poor conductor of heat, neither power supply nor tube generate enough heat to do damage. However, you should avoid placing the tube or power supply components directly against the plastic of the enclosure. Allow some breathing room and provide ventilation slots or holes.

Basic Assembly

An assembly diagram for the all-in-one lab laser, using a 1. 12-by-.9-inch, 1 milli-watt He-Ne tube and compact, commercially made 12-volt dc power supply, is shown in ill. 6-i (the power supply is available new and surplus). A parts list is provided in TABLE 6-1. The dimensions provided in the plans are for the power supply and He-Ne tube specified and fit well in a 7¾- by-4¾- by-2 plastic project box (available at Radio Shack).

ill. 6-1. Drilling guide for the lab laser enclosure, using the components detailed in the text. (A) Drilling guide for the inside bottom of the enclosure; (B) Drilling guide for the top cover; (C) Mounting detail for the laser tube, pipe hanger, and grommet.

Note that the actual size of your enclosure depends on the dimensions of your components. Measure the power supply and tube and buy parts to fit. Don’t be afraid of making the enclosure on the large side. Unless you are specifically after miniaturization, the larger the enclosure, the better the cooling of the components within.

Place stand-offs under the power supply and large grommets to support the tube. The grommets provide ventilation space between the laser and enclosure as well as shock absorption for the tube. You can use rubber weather stripping, O-rings, or high-voltage dielectric tape for the grommets.

Electrical Connection

You must connect the high voltage anode and cathode leads to the terminals on the tube. With most tubes, the terminals are the metal mirror mounts on each end of the laser. It’s decidedly a bad idea to solder the leads directly to the terminals, yet the electrical connection must be solid and stable.

One method for attaching the high-voltage leads to the tube is to use a ¾-inch compression spring (such as Century Spring S-676), as illustrated in ill. 6-2A. These are readily available at most hardware stores. If the spring is too long, cut it with a pair of heavy clippers. Use a file to remove the outer coating of the spring (many are plastic coated), and then solder the power lead to the spring. Slip the spring over the terminal and inch it into place. If the spring won’t fit over the terminals on your laser, use a larger or smaller type. Don’t use a spring that’s too heavy. A lightweight compression spring maintains electrical contact without stressing the mirror mount.

Another method is to use ¼-inch fuse clips (see ill. 6-2B). You can buy these at most any electronics store, including Radio Shack. The clip is just about the right size for most laser tube terminals and can be easily soldered to the high- voltage leads. If the clip has small indentations for holding the fuses, bend these out with a pair of needle nose pliers. You might need to tweak the clip a bit to get it to fit around the ends of the mirror mounts.

ill. 6-2. Two ways to attach the high-voltage leads to the laser tube terminals: by compression spring, and fuse clip. Be sure to file away any coating that might be on the spring, or electrical contact will be impaired.

Final Assembly

Before permanently mounting the laser and power supply, note the position of the output mirror and drill a hole for the beam to escape. A 5 hole provides plenty of room for mounting error and is about the size of the output mirror. You can drill a smaller hole, but you must be sure to precisely align the tube in the enclosure.

Next, drill holes in the top cover for the 12 Vdc power socket, on off switch, and LED indicator. Use a 1/4- or 9 bit for the ¼-inch phone jack; measure the diameter of the switch and LED and use the proper size drill bits for each. When drilling plastic, start with a small pilot hole, then enlarge it with bigger drill bits until the hole is the proper size. Place a block of wood behind the plastic to prevent chipping.

You can use a metal enclosure only if the power supply is completely sealed and all interconnections are insulated (this includes the leads that connect to the terminals on the tube). Use heat-shrinkable tubing rated for at least 20 kV, high-voltage heat- shrinkable dielectric tape, or high-voltage putty. You might also want to coat the inside of the enclosure with a non-conductive paint. I’ve had great success using brush-on plastic coating, the stuff designed as a covering for tool handles. Many plastics outlets sell this coating, which is available in brush- and dip-on forms.

Solder the connections between all components, as shown in the schematic in ill. 6-3, and mount them in the enclosure. Avoid long wire lengths; keep all lead lengths as short as possible. Place the ballast resistor close to the anode terminal of the tube. Be sure that no leads interfere with the output beam of the laser. You might want to tie down the leads to keep them from wandering about in the enclosure. Use plastic tie wraps and secure the leads to the inside of the enclosure with a small piece of double sided foam tape. Keep low-voltage and high-voltage leads separate. Finish the lab laser by adding rubber feet to the bottom of the enclosure. The finished laser is shown in ill. 6.4A and B.

ill. 6-3. Wiring diagram for the lab laser. Be careful of the very high voltages present at the terminals of the laser and the output of the power supply.

ill. 6-4. Tb completed lab laser: (A) With the cover removed, showing the mounting of the components, and (B) with cover in place.

Power Supply

Power for the lab laser comes from a 12 Vdc battery pack or a 12 Vac adapter/battery eliminator. The battery or ac adapter must provide at least 350 mA current, or more, depending on the tube and high-voltage power supply used. Construction details for a battery pack and charger/adapter, suitable for use with the all-in-one lab laser, appear in Section 21.

BUILD AN ENCLOSED LASER HEAD

The all-in-one lab laser doesn’t lend itself to hand-held portability. A bare laser tube can be easily shielded in a pipe that's suitable for hand-held use, or you can even mount one on an optical bench. You can use metal or plastic for the laser enclosure. Metal conducts heat more readily and is recommended if you plan on keeping the laser turned on for long periods of time. However, a plastic enclosure provides insulation against the high-voltage potentials present at the terminals of the laser.

The laser head used in this project is designed around a 2-mW Melles Griot plasma tube. The tube measures 1.45 inches in diameter by 10.6 inches in length. A piece of 1½-inch PVC pipe, cut to a length of 12 inches, serves as the enclosure. The pipe is capped off on both ends and a removable plug is attached to the enclosure to provide a shutter—a guard against accidental exposure to the beam.

The power supply pack, a separate component to the laser head, is designed to run off 117 Vac and includes a fuse, key switch, and pilot lamp. The design of this laser incorporates many of the safeguards required by the CDRH for a commercially sold Class II or Class IIIa device.

Building the Laser Head

Refer to TABLE 6-2 for a parts list. Ill. 6-5 shows the construction details of the laser head. Cut a piece of 1½-inch schedule 40 PVC to 12 inches. Make sure the cuts on both ends are square. Remove the rough edges with a file or fine-grit sand paper (300-grit wet/dry paper, used dry, is a good choice). Drill a hole, using a number 48 bit, a distance of 5 from the back end of the tube. This hole is for attaching the eyelet used to secure the protective cap chain (see below).

Solder the high-voltage leads to pair of springs or fuse clips, as detailed above. Insert a ballast resistor (80 k-ohm], or more) between the anode terminal and anode lead, as shown in ill. 6-6, and wrap the connections in heat-shrinkable tubing. Waft a lighter or match under the tubing or use a heat gun to shrink it around the resistor and other connections.

ill. 6-5. Construction details for the cylindrical laser head. Adjust pipe diameter and length to accommodate the exact dimensions of the tube you are using.

ill. 6-6. After soldering the leads to the ballast resistor, secure the resistor to the test plug with silicone rubber sealant. For a professional look, feed the high-voltage leads through a grommet mounted in the center of the plug. Be sure to use high-voltage dielectric wire (rated 3 kV or more) or you can receive a bad shock. ill. 6-7. Recommended mounting areas on a bare gloss He-Ne tube. You can use large O-rings or grommets as shock absorbers (shock absorption reduces “microphonic noise” that can impair the coherent operation of the laser.).

Thread the wires through a grommet and poke the grommet inside a ¼-inch hole drilled in the center of a 1½-inch test plug. Secure the grommet and resistor by applying a layer of silicone rubber over the inside surface of the test plug and let dry.

Once the sealant has set, attach the springs or fuse clips to the terminals on the laser tube. Wrap 0-rings, electrical tape, or rubber bands around the tube, as shown in ill. 6-7, and insert the tube in the PVC pipe. The output mirror should be approximately ½-inch from the end of the tube. Secure the test plug to the other end of the tube with a dab of all-purpose adhesive.

Next, drill a ¼-inch hole in the center of another test plug and mount it on the output end of the tube. Secure the plug with glue (not PVC solvent cement).

Construct the protective plug using a 1½-inch PVC end cap. Secure it to the tube using a miniature eyelet and 24-inch length of lightweight metal or plastic chain. Insert another eyelet in the small hole previously drilled in the PVC pipe. Secure the other end of the chain to the eyelet. Be sure that the eyelet doesn’t interfere with the tube, ballast resistor, or high-voltage leads. If the threads of the eyelet contact internal parts, cut the shaft of the eyelet so that it doesn’t protrude inside the pipe. Apply glue to hold the eyelet in place.

Building the Power Pack

Refer to TABLE 6-3 for the parts list for the power pack. The power pack uses a commercially available 117 Vac high-voltage laser power supply. You can use a dc power supply if you need a completely portable and self-contained laser. Many new and surplus ac power supplies are modular, sealed in a box measuring 1¾-by-4¼-by-3 The power supply may be equipped with an Alden high-voltage connector or flying leads (tinned pigtail leads, with no connector). Tithe supply has an Alden connector, cut it off and save it for future use—female Alden connectors are hard to find. Use the two or three holes in the module to mount the supply in a 6½-by-3¾-by-2-inch plastic experimenter’s box (you can use any plastic box that's large enough for the power sup ply, switch, and other components). Don't drill holes in the module—you’ll undoubtedly drill through the components sealed inside.

ill. 6-8. Layout and drilling guide for the laser head power supply. Drill holes large enough to accommodate the fuse holder, key switch, and neon lamp that you are using. The drilling guide assumes you a using a commercially made modular power supply (dimensions indicated in the figure).

ill. 6-9. Wiring diagram for the laser head power supply. Beware of potentially lethal voltages throughout this circuit.

Follow the drilling guide shown in ill. 6-8, and drill holes in the enclosure for the power cord (allowing extra for the grommet), indicator lamp, and key switch. Use a rotary rasp or countersink bit for the larger holes. Drill small pilot holes first to avoid chipping or cracking the plastic with large bits.

Use 16-gauge wire and solder the components as shown in ill. 6-9. Keep wire lengths as short as possible. After soldering is complete, mount the components in the enclosure and route the grommet and power cord in the appropriate hole. Wrap all high- voltage leads in 20 kV heat-shrinkable tubing or tape and apply high-voltage putty to eliminate arcing. You may also insulate the wiring using clear aquarium tubing. Double check your work (use a meter as a safety measure) and insert a 2-amp fast-acting fuse in the fuse holder.

Test the laser by plugging in the power supply and turning the key switch to the ON position. The indicator light should turn on, and within 3 to 5 seconds, the laser should fire. If the indicator lamp and laser don't turn on, it could indicate a wiring problem or blown fuse. Recheck your work to make sure the wiring is correct.

BUILD A COMPACT LASER BREADBOARD

A breadboard allows you to experiment with lasers and other optical components without elaborate mounting and construction. The laser breadboard presented here is a semi-permanent mount for a commercial- or home-made cylindrical laser head and serves as a’ ‘head-end” for the optical breadboards detailed in the next section. The laser and optical breadboards are constructed with the same inexpensive materials, allowing you to mix and match as you desire.

Follow the construction details shown in ill. 6-10 (parts list in TABLE 6-4). Cut a piece of ¼-inch pegboard to 6- by 18-inches. Sand the front, back, and edges and spray on a coat or two of clear lacquer (this seals the wood). Cut lengths of 2-by-2-inch lumber and use nails or screws to attach the wood to the pegboard. You may wish to substitute 4 aluminum channel for the 2-by-2 lumber. Refer to the next section on how to use aluminum channel stock.

ill. 6-10. A simple optical breadboard can be constructed using an 18- by 6-inch piece of ¼-inch Masonite pegboard and lengths of 2-by-2 framing lumber. Cut the lumber as indicated in the parts list in TABLE 64.

Cylindrical laser heads mount easily in a clamp made with PVC pipe. Cut a length of 2-inch PVC to 3 inches. Next, carefully split the pipe in half down the middle using a hacksaw or table saw. Finish all edges with a file. Use the holes already in the pegboard as a template, and drill two holes in each PVC half. Next, mount the pipe halves in the breadboard. Use 8/32 by ¾-inch bolts, 8/32-inch nuts, and washers. Before tightening the hardware, slip the loose end of a 3-inch adjustable hose clamp under each piece of pipe. Tighten the hardware and close the clamp. Mount small rubber feet on top of the four bolt heads, then slide the laser head into the mount, as shown in ill. 6-11. Tighten the clamps.

You can mount the power supply in a number of locations, depending on its size and design. Sealed, modular power supplies can be mounted on top of the breadboard next to the laser. If it’s small enough, you can tuck the power supply underneath the breadboard. Unsealed simply placed along side of it.

ill. 6-11. Secure the laser on the breadboard using lengths of 2-inch PVC cut lengthwise and fastened to the pegboard with 1% hardware. The laser can be lashed in place with an adjustable car radiator hose clamp.