Laser Experimenter’s System: Constructing an Optical Bench

An optical bench is a device that allows you to experiment with lasers and optics by mounting them securely on a rigid base. Components can be easily added or deleted, using any of a number of fastening systems, including nuts and bolts, magnets, sand, Styrofoam, or even clay. The optical bench is the equivalent of the solderless breadboard used in electronics. Once you are satisfied that your project works, you can disassemble it and start on another project or re-assemble the components in a permanent housing.

There are a number of useful design approaches to optical benches in this section. You can make an optical bench of just about any size, up to a practical limit of 4 by 8 feet. Projects that follow show how to build a 2-by-4 foot bench as well as a 4-by-4 foot model. Construction materials are cheap and easy to get, and you’re free to use more exotic materials if you desire.

You’ll also learn how to construct components for an optical breadboard system. Small, Lego-like blocks can combine to facilitate just about any design. The mounting of lenses, mirrors, and other optical parts is tricky business; this section includes several affordable approaches that make the job easier.

BASIC 2-BY-4 OPTICAL BENCH

The basic 2-by-4-foot optical bench is made completely of wood. Using the parts indicated in TABLE 1, start with a 2-by-4-foot chunk of ¼-inch pegboard.

After sizing the pegboard, sand the edges as well as the front and back (the back will have a coarse finish and the front will be smooth). Seal the wood by spraying or brushing on one or more coats of clear lacquer or enamel. Let dry completely, then proceed to the next step.

Now for the frame of the bench. Cut two 4-foot lengths of 2-by-2 lumber. Use nails or screws and attach the lumber pieces to the long sides of the pegboard. Next, cut two 18-inch lengths of 2-by-2 lumber and secure them to the ends. Center the 18-inch lengths so that there is an even gap on either side. You can use just about any size lumber for the frame, but you should be consistent in case you build more optical benches. By using the same framing lumber, you maintain a constant height for each bench. Several benches can be used together on a large table.

While you can use the bench as is, it’s a good idea to paint it with flat black paint. The black paint helps cut down light scatter and also imparts a more professional look. Rubber feet (for electronics projects) or rubber or foam weather stripping make for good cushions and shock absorbers, and they prevent marring desks and tabletops. Apply the rubber to the underside of the frame.

ENHANCED 4 x 4 OPTICAL BENCH

The enhanced 4-by-4-foot optical bench is also made with ¼-inch pegboard, but the frame is of all-metal construction. You may use aluminum or steel shelving standards or 4 extruded aluminum channel for the frame.

The parts list for the 4-by-4-foot bench is included in TABLE 2. Use a hacksaw (with a fine-tooth blade) to cut the metal to size as shown in ill. 1. Be sure to cut each piece with the proper 45-degree miter. Use a miter box and C-clamps for best results.

After cutting, remove the flash and rough edges from the ends of the metal with a file.

Secure the framing pieces to the underside of the pegboard using 8/32-by- ½-inch hardware.

A diagram of the completed bench is shown in ill. 2.

Because of the size of the pegboard sheet, it might buckle in the middle under the weight of heavy object. If this is a problem, add one or more reinforcing struts to the underside of the bench. An extra piece of shelving (standard or aluminum channel) can be mounted down the center of the bench. Secure the extra piece to the frame using ½-inch angle-iron brackets (available at the hardware store).

ill. 1. (A) Cutting guide for the aluminum channel used as a frame for the 4-by4-foot optical bench. Be sure to miter the ends as shown. (B) Use 1½-by 3/8-inch flat corner irons to secure the ends. (C) Assembly detail for the framing pieces and corner irons showing 8/32 hardware.

Note: when drilling through metal, use only a new or sharpened bit. The drill motor should turn slowly—less than 1,000 rpm. Exert only enough pressure to bite into the metal, not enough to bend the metal or bit while drifting. Remove the flash around the drilled hole with a file.

As with the smaller version of the optical bench, paint this one a flat black to reduce light reflections. Also be sure to paint over the shiny heads of the machine screws.

ill. 2. The underside of the optical bench, showing aluminum channel framing pieces, corner irons, and mounting hardware.

Adding a Sheet-Metal Top

The pegboard optical bench is suitable for most hobbyist applications. The wood is sturdy, absorbs many vibrations, and is easy to drill. However, experiments that require greater precision need an all-metal optical bench. One can be constructed using medium- to thin-gauge aluminum, steel, or regular sheet metal. Large sheets or plates of metal are not routinely available at a hardware store, so look in the Yellow Pages under headings such as aluminum, metal specialties, sheet metal work, and steel.

Many sheet-metal shops work with thin-gauge metal—20- to 24-gauge or thinner. This is too thin to be used alone, but you can laminate it over the pegboard. The thickness of plate aluminum and steel is most commonly listed in inches, not gauge. A 1/8- or 3/16- inch thick piece of aluminum or steel does nicely as the top of the optical bench. Steel is cheaper, but aluminum is easier to work with. The average price for a 2-by-4-foot piece of 3/16 hot roll steel is about $25 to $30. The same size plate in un-anodized aluminum is approximately three times as much.

You might be lucky and find an outlet that sells pre-drilled stock. More than likely, however, you have to do the work yourself if you want the convenience of pre-drilled holes in your optical bench. Use a large carpenter’s square to lay out a matrix of lines spaced either ½- or 1-inch apart (I prefer holes at ½-inch centers, but obviously this requires you to drill four times as many holes).

You might find it easier to place masking tape on the metal and mark the lines with a pencil. Use a carbide-tipped bit (#19 bit for 8/32 hardware) and heavy-duty drill motor.

A drilling alignment tool, available as an option for many brands of drill motors, helps to make perpendicular holes.

Using Plastic Instead of Metal

Another approach to the benchtop is acrylic plastic (Plexiglas, Lexan, and so forth). The advantages of plastic are that it's extremely strong for its weight and is easier to work with than metal. A steel top adds considerable weight to the bench (which in some cases is desirable), while a plastic top adds little weight.

A 2-by-4-foot piece of 3/16- or ¼-inch acrylic plastic costs $12 to $16; you can buy sheets of plastic at most plastic fabricator outlets. Have them cut the plastic to size and finish the edges. You can drill through the plastic for the mounting holes using a regular bit, but better results are obtained when using a special plastic/glass drill bit.

USING OPTICAL BENCHES

Because pegboard stock already has holes, you probably won’t need to drill new ones to mount the laser and other optical components. The holes are pre-drilled with better-than-average accuracy so you can use them for alignment. You can place a series of mirrors and lenses along one row of holes, for example, and they will all line up to one another.

There might be occasion, however, to drill new holes in the pegboard. Extra holes (5/32-inch or #19 bit for 8/32 hardware) should be ¼ inch or more away from existing holes, or the bit might slip while you are drilling. Schemes for mounting the laser and optical components appear later in this section.

As you complete a project or make changes to an existing experiment, keep notes and record the placement of all components. You might want to mark the rows and columns of holes as a reference for indicating the location of components. E.g., you can mark all the rows with a letter and all columns with a number. A mirror placed at the intersection of C6 could be marked on a piece of graph paper. You can develop your own shorthand for identifying various optical components, but here are some suggestions:

* S-FSM—small front-surface mirror

* L-FSM—large front-surface mirror

* P-BS—plate beam splitter

* C-BS—Cube beam splitter

* RAP—right-angle prism

* EQP—equilateral prism

* SPP—special prism

* POL—polarizer

* EXP—beam expander

* SPA—spatial filter

* L(XXX)—Lens (with type, size, and focal length)

You might also want to provide shorthand notation for any special components you use, such as single and double slits, diffraction gratings, optical fibers and fiber couplers, diode lasers, LEDs, sensors, and more.

OPTICAL BREADBOARD COMPONENTS

Small versions of the optical benches described above make perfect optical breadboards. Each breadboard, measuring perhaps 8- by 12-inches, can contain a complete optical sub-assembly, such as a laser and power supply (as described in the previous section), beam expander, beam director or splitter, sensing element, or lens array. You can construct a breadboard for each sub-assembly you commonly use. E.g., if you often experiment with lasers and fiberoptics, one breadboard would consist of a la ser/power supply and the other a fiberoptic coupling, cable, and sensor.

ill. 3. Suggestions for possible sizes for optical breadboards, but you can make your breadboards any size you wish.

Construct the small breadboards in the same fashion as the larger optical benches detailed earlier in this section. Use the same materials for each breadboard to maintain consistency. Several sizes of breadboards are shown in ill. 3. Use the scrap from the construction of the optical bench for the short lengths of framing pieces.

To use the breadboard pieces, lay them out like building blocks on a large, flat surface. The dining room table is a good choice, but be sure to place a drop cloth or large piece of paper on the table surface to prevent marring and scratching. You can also add rubber feet, rubber weather stripping, or foam weather stripping to the bottom of the breadboard pieces.

Lay out the pieces in the orientation you require for the project. If you’ve used pa per as a drop cloth, you can draw on it to mark the edges of the breadboards. This is helpful in case you want to repeat an experiment and need to replace the breadboard pieces in the same spot.

LASER AND OPTICS MOUNTS

The optical bench or breadboard serves as a universal surface for mounting lasers and various optical components. The type of mounting you use for your laser and optics depends on their individual design, and most parts can be successfully secured to a bench or breadboard using one or more of the following techniques. Keep in mind that you might need to adjust the dimensions and design for each mounting configuration to conform to your particular components.

Laser Mounts

One of the most versatile yet easiest to construct laser mounts uses a piece of 2-inch (inside diameter) PVC cut in half lengthwise. Building plans are provided in Section 6. The mount is designed for use with cylindrical laser heads, either commercially-made versions or ones you build yourself using bare laser tubes.

ill. 4. Cylindrical laser heads can be secured to optical benches and breadboards using pipe hangers. Choose a hanger large enough to accommodate the diameter of the laser, and avoid overtightening.

Another approach to laser mounting is shown in ill. 4. A pipe hanger, designed for electrical and plumbing pipe, secures a bare laser tube to the optical bench. This approach isn't recommended unless you shield the tube with an insulating box or cover the anode and cathode terminals on the laser with high-voltage putty or high-voltage heat-shrinkable tubing.

Pipe hangers come in various sizes to accommodate pipe from ½-inch to 2½-inches (outside dimension is approximately ¾ inch to 3 inches). Get a size large enough to easily fit the tube. Add a few layers of plastic tape around the tube where the hanger touches. Don't overtighten the hanger, or you run the risk of cracking or breaking the tube! You may use bigger hangers to mount cylindrical laser heads. Most commercially manufactured cylindrical heads measure 1.74 inches in diameter and can be successfully mounted using a 1½- or 2-inch hanger. All-in-one lab lasers (commercial or home-built) don’t usually need mounting, because their housing lets you place them just about anywhere, yet some projects require you to secure the laser to the optical bench so that it doesn’t move. Use straps or wood blocks to hold the laser in place, or drill mounting holes in the housing and attach the laser to the bench using nuts and bolts. The latter technique isn't recommended if you are using a commercially made lab laser, because any modification voids its CDRH certification. Your own home-built lab laser (as detailed in Section 6, can be modified by locating a spot inside where the hardware won't interfere with the tube or power supply.

Table 3. Tube Hanger Laser MountSystem Parts List

Lens Mounts

Lenses present a problem to the laser experimenter because they come in all sizes and shapes. Unless you are very careful about which lenses you buy (and purchase them new from prime sources), you will have little choice over the exact diameter of lenses you use. That makes it hard to build lens mounts using tubing and retaining rings, which come in only a few standard sizes. Another problem is that many lenses suitable for la ser experiments are small, which makes them difficult to handle.

With a bit of ingenuity, however, you can make simple lens holders for most any size lens you encounter. The basic ingredient is patience and a clean working environment. As much as possible, handle lenses by the edges only and work in a well-lit, clean area— free from dust, cigarette smoke, and other contaminates.

A basic lens mount is shown in ill. 5, and is useful for lenses as small as 4 mm to as large as 30 mm. Start by cutting a ledge in one end of a piece of ¾-inch PVC pipe coupling. Use a fine-toothed hacksaw to make two right-angle cuts. Smooth the rough edges with a file or wet/dry sandpaper (used dry). Next, use a caliper or accurate steel rule to measure the diameter of the lens. Choose a drill bit just slightly smaller than the diameter of the lens, and drill a hole in the remaining stub on the pipe coupler. Remember: you can always enlarge a hole to accommodate the lens, but you can’t make it smaller.

ill. 5. One of the best (and easiest) ways to mount a lens is inside one wall of a piece of PVC pipe or coupler. You can make the hole the same size as the lens (for a friction fit) or slightly smaller (glue the lens on the outside wall of the pipe using all-purpose adhesive).

Press the lens into the hole. Don’t exert excessive pressure or you may crack the lens. If the hole is too small, use a larger bit or enlarge the hole with a fine rat-tail file (the kind designed for model building works nicely). After careful drilling and filing, you should be able to pop the lens into the hole but still have enough friction-fit to keep it in place. The lens isn't mounted permanently, so it can be removed if you have another use for it. Clean the lens using an approved lens cleaner. Refer to Section 3 for details on lens care and cleaning.

The PVC coupling holder can be mounted on the optical bench in a number of ways. Several methods are shown in ill. 6. The hole for the thumbscrew does not require tapping but tapping makes the job easier. Use a 4/40 or 6/32 thumbscrew or regular pan head machine bolt as the set screw. The hardware used for mounting the holder on the bench is a 10/24- or ¼-inch 20 carriage bolt, secured in place with a threaded rod coupler (cut the coupler if it's too long for the PVC holder). You can adjust the height and angle of the lens by loosening the thumbscrew and adjusting the holder.

ill. 6. Various ways to attach P VC pipe to a pegboard optical bench. If the bench is ferrous metal (steel, not aluminum), you can build a magnetic base by gluing a magnet to the bottom of the pipe.

White PVC plastic can cause light to scatter, so you might want to coat the lens holders with flat black paint. The small spray paints sold by Testor (and available wherever model supplies are sold) adhere well to PVC.

The PVC pipe can be used simply as a holder for mirrors, beam splitters, and other components. A length of 1- or 1 ¼-inch PVC, pressed in a vise, can be used to hold thin Masonite board or acrylic plastic. You can mount mirrors, lenses, plate beam splitters, and other components on the Masonite. Cut the Masonite to a width just slightly larger than the inside diameter of the PVC (1¼-inches for schedule 40 1¼-inch PVC). Clamp the pipe in a vise until it deforms into an ellipse. Stick the Masonite board in the pipe and slowly release the vise. The pipe will spring back into shape, holding the Masonite. You can also directly mount thick plate beam splitters and mirrors into the PVC. Don’t try it with thin optics, or they could shatter as the PVC springs back into shape.

A way to mount lenses of almost any size is to tack them on a piece of ½-inch acrylic plastic. The acrylic pieces needn’t be much larger than the lens itself, but you should allow room for mounting hardware. Black plastic with a matte finish is the all-around best choice, although you can always coat the plastic with black matte paint.

Drill a hole in the plastic just smaller than the lens, leaving enough room for the edges of the lens to make contact with the plastic. After drilling, place the lens directly over the hole. Apply two or three small dabs of all-purpose adhesive (such as Duco cement) to the outside edges of the lens. Use a syringe applicator or toothpick to dab on the adhesive.

Be sure that no “strings” of the cement cover or come in contact with the usable area of the lens. If you make a mistake, immediately remove the lens, wash it in water, and clean the lens with approved cleaner. Although you can remove undried cement with chemicals such as acetone and lacquer thinner, these compounds could have an adverse effect on lens coatings.

Some lenses are square as opposed to round and can be mounted on the top of a short length of PVC pipe or small piece of ¼-inch acrylic plastic. Be sure to mount the lens perpendicular to the plastic, or the laser beam might not follow the path you want.

ill. 7. Two ways to mount lenses to acrylic plastic and how to secure the plastic to the optical bench. You can use angle irons (available at the hardware store) or insert the plastic into a short length of PVC pipe.

ill. 8. A set of nuts and a long bolt allow you to adjust the height of the PVC pipe in relation to the surface of the optical bench.

Let the adhesive dry completely, then mount the lens and holder as shown in ill. 7. The illustration provides a number of mounting techniques. In one, the holder is mounted to a pipe, which is attached to the shaft of a bolt. The length of the bolt depends on the size of the holder and lens but is generally between 1 and 3 inches long. You can alter the height of the bolt by adjusting the mounting nuts, as illustrated in ill. 8. Parts lists for the lens-mounting systems are included in TABLE 4.

Round lenses measuring between 21 and 26 mm in diameter can be inserted inside ¾-inch PVC coupling. The lenses are held in place with thin pieces of ¾-inch pipe. Follow the instructions shown in ill. 9. Cut a ¾-inch slip coupling in half, discarding the innermost portion to avoid the raised pipe-end stops. Thoroughly smooth the ends with a file, fine-grit sandpaper, or grinding wheel. Next, cut two ¼- to ¾-inch pieces of ¾- inch PVC pipe, sand the edges, and insert one into the coupling (the inside diameter of the coupling is tapered, so the pipe will only fit one way). Drop in the lens, center it, and press in the other small piece of pipe.

ill. 9. Lenses can be mounted inside a ¾-inch PVC clip coupling using the scheme shown in this diagram. The retaining pieces are small slivers of ¾-inch PVC. Paint all parts flat black to prevent light scatter. ill. 10. Two ways to construct advanced lens holders.

You can use the mounted lens in a number of ways, including cementing it to a PVC holder (using PVC solvent cement) or using it in the optical rail system described later in this section. As mentioned before, the white PVC pipe can cause light scattering. Reduce the scatter by painting the coupling and pipe pieces flat black. If you must use smaller lenses, ½-inch PVC pipe couplers accommodate optics between 16 and 21 mm in diameter.

Advanced, adjustable lens holders are shown in ill. 10. These can be made using metal or acrylic plastic and require some precision work on your part. The holders are adjustable and can be used with most any size of circular lenses. On one, change the size by loosening the holding screws and sliding the top along the rails. Clamp the lens in place by turning the top screw. Delete the rounding on the bottom when using cylindrical or square lenses.

ill. 11. Build a simple mirror mount by gluing it to a short length of ¾-inch PVC. Mount the PVC to the optical bench with a nut and bolts, and adjust the height and angle of the mirror by turning the bolts.

Mirror Mounts

ill. 12. A swivel mirror mount (allowing 2 degrees of freedom) can be built using PVC pipe and an angle iron. Choose the right size of angle iron to fit the application.

Front-surface mirrors are often used to redirect the beam or to increase or decrease its height in relation to the surface of the bench. A simple mirror mount, using just about any thickness and size of mirror, is shown in ill. 11. Here, ¾-inch PVC pipe is cut into short ½-inch sections with a mirror glued onto one end and a hole drilled through the top and bottom. Mounting hardware, such as 10/24 or ¼-inch 20 (detailed in TABLE 5), is used to secure the pipe to the optical bench. You can swivel the mirror right and left by loosening the retaining nut. By attaching the mirror/pipe to an angle-iron bracket, as illustrated in ill. 12, you can provide two axes of freedom—right and left, and up and down.

Because you might never know what you’ll need to complete an optical experiment, build several mounted mirrors in the following configurations (note that “small” means approximately ½-inch square and “large” means 1-inch square).

* Small mirror mounted to 10/24-by-1-inch bolt (bench hugger)

* Small mirror mounted to 10/24-by-2½-inch bolt

* Large mirror mounted to ¼-inch 20 by 3-inch bolt

* Small mirror mounted to ½-inch angle-iron bracket and 10/24-by-1-inch bolt

* Small mirror mounted to ½-inch angle-iron bracket and 10/24-by 2-inch bolt

* Large mirror mounted to 1½-inch angle-iron bracket and 1/2-inch 20 by 3-inch bolt

You can make other mounts by using different mirrors, bolt lengths, and angle-iron sizes, as needed. Be sure that the mirror is small enough to clear any hardware. For instance, an overly large mirror might interfere with the angle bracket.

Commercially made mirror mounts are occasionally available from surplus sources. The components are optically flat, dielectric mirrors designed (in this case) for use with helium-neon lasers. The mounts can be secured to the bench in a variety of ways including angle irons, hold-down brackets, and clamps. Many of the mounts come with holes pre drilled and pre-tapped at ½- or 1-inch centers. The holes can be used to facilitate quick and easy mounting to the bench.

The major benefit of commercially made mirror mounts is their precision. Many have fine-threaded screws for adjusting the inclination or angle of the mirror. If you’re handy with tools, you can fashion your own precision mirror mounts from aluminum or plastic. Best results are obtained when you use a precision metal-working lathe or mill.

Another method of mounting mirrors, popular among holographers, is depicted in ill. 13. Here a mirror is cemented inside a nook sawed into a piece of 1¼-inch PVC pipe. The pipe can be mounted on a stem, placed in sand (see below and in Sections 17 and 18), or simply balanced on the bench. You can mount extra large mirrors on the pipe (use bigger pipe) and the pipe pieces can be any length to suit your requirements.

ill. 13. A mounting method popular with holographers is to glue the mirror onto a piece of scooped-out PVC Pipe.

One method of securing the pipe is to use a dowel and short piece of PVC. Secure the dowel inside a short pipe end (¾- to 1-inch long). This pipe holds the mirror. A thumbscrew set in another length of pipe (1 to 4 inches) lets you adjust the height of the mirror. The bottom pipe piece can be secured to the bench using hardware or a magnet. The magnet obviously requires a metal benchtop. You can add metal to a wood benchtop with pieces of galvanized sheet metal. Small squares (up to about 6 by 12 inches) are available at many better hardware stores. The sheet metal comes with holes pre-drilled and can be cut to size with a hacksaw.

Cement the mirror in the pipe using gap-filling glue (Duco works well, and the mirror can be removed later if necessary). On occasion, you may want to make the mirror mount temporary, for example, if you have one extra-nice mirror and don’t want to commit it to any one type of mount. You can temporarily secure the mirror to the pipe using florist or modeling clay. Build up a bead of clay around the pipe and press the mirror into place. You can vary the angle of the mirror by adding extra clay to one edge.

Mounting Prisms, Beam Splitters, and Other Optics

Prisms and beam splitters require unusual mounting techniques. A porro prism, of ten found in binoculars and a common find on the surplus market, can be used as laser retro-reflectors (or corner cubes). The prisms act as mirrors where the axis of the outgoing beam is offset, yet parallel, from the incoming beam. You’ll find plenty of uses for porro prisms, including using them to change the height of the beam over the bench.

The porro prism is easy to mount on a small piece of ½-inch acrylic plastic. Cut the plastic to accommodate the size of prism you have. Secure the plastic holder on a sliding post mount (as described above for mirror mounting) and you have control over the height and angle of the prism.

A prism table is a device that acts as a vise to clamp the component in place. The table can be used with most any type or shape of prism. A basic design is shown in ill. 14; the parts list is provided in TABLE 6. Construct the table using small aluminum, brass, or plastic pieces. The thumbscrew is a 8/32-by-1-inch bolt; the top of the table is threaded to accommodate the bolt.

ill. 15. Plate beam splitters can be attached to an optical bench by inserting them into a length of PVC or by gluing them onto a small angle bracket. Be sure you can adjust the angle of the beam splitter and the transmitted and reflected beams are not obscured.

Beam splitters require an open area for both reflected and refracted beams. Cube beam splitters can often be used in prism tables. Alternatively, you can build a number of different mounts suitable for both cube and plate beam splitters, as shown in ill. 15. The idea is to not block the back or exit surface(s) of the component.

ill. 14. A prism or cube beam splitter table can be constructed using ordinary hardware and acrylic plastic. Secure the component by tightening the thumbscrew.

Filters can be secured using adjustable lens mounts, as shown above. Commercially available filter holders designed for photographic applications (camera and darkroom) can also be used. Filters can be mounted on plastic or, if small enough, held in place inside a PVC pipe.

ADVANCED OPTICAL SYSTEM DESIGN

The projects in this section deal with advanced optical system design. Included are plans on building an optical rack for testing lenses and their effect on the laser beam, as well as motorizing mirrors, beam splitters, and other components for remote control or computerized applications. You can adopt these plans for any number of different optical system requirements such as laser light shows, laboratory experiments, laser rangefinders, and more.

ill. 15. Plate beam splitters can be attached to an optical bench by inserting them into a length of PVC or by gluing them onto a small angle bracket. Be sure you can adjust the angle of the beam splitter and the transmitted and reflected beams are not obscured

Building an Optical Rack

An optical rack provides a simple means of experimenting with optical components such as lenses and filters. A common optical rack used by hobbyists is the meter or yardstick. You place various clamps along the length of the stick to mount optical components. Meter sticks and their clamps, such as the set shown in ill. 16, are available from a number of sources, including Edmund Scientific. The clamps hold only lenses of certain sizes (usually 1 inch or 25 mm), so be sure you get lenses to match.

ill. 16. A yardstick with inexpensive optical components attached. The stick provides an easy way to align optics and measure their distances.

ill. 17. A homemade optical rail can be constructed using an aluminum channel and lenses mounted inside PVC pipe.

ill. 18. A double-concave lens and a double-convex lens make a laser beam expander. Adjust the distance between lenses to match the focal length of the double-convex lens.

Another approach to the optical rack is shown in ill. 17. Here, aluminum channel is used as a “trough” for the optical components. The lenses are mounted in tubes such as PVC or brass pipe. Each lens is centered in the tube and the outside dimension of the pipe is the same for all components. That means the light will pass through the optical center of each lens. You may use smaller or larger tubes for some components, but you must adjust the optical axis with shims or blocks.

Cut a piece of 5 aluminum channel stock to whatever length you need for the rack. A 1-to-3-foot length should be sufficient. Mount the lenses (and/or filters) in PVC pipe, as explained in the lens-mounting section previously in this section. With ¾-inch PVC, you are limited to using lenses that measure between 21 mm and 26 mm in diameter. That comprises a large and popular (not to mention inexpensive) group of lenses, so you should have no trouble designing most any optical system of your choice. When using ½-inch PVC, you can use lenses with diameters between 16 mm and 21 mm.

A sample lens layout using the optical rack is shown in ill. 18. A parts list is provided in TABLE 7. The two lenses together comprise a beam expander, which is a common optical system in laser experiments. The laser beam is first diverged using a piano-concave or double-concave lens and is then collimated using a plano-convex or double-convex lens. The focal length of the plano-convex (or double-convex) lens determines whether the beam will be collimated for focusing to a point. You can experiment with different lenses and distances until you get the results you want.

A good way to use the rack is to calculate on paper the effects of the lenses in the system. Then see if your figures hold true. Armed with some basic optical math (see Section B for a list of books that provide formulas), you should be able to compute the result of just about any two- or three-lens combination.

ill. 19. A complete RIG transmitter/receiver/servo setup. This transmitter provides complete digital proportional control of four independent servos (additional channels provide “on/off” functions only).

R/C TRANSMITTER/RECEIVER

Servo motors designed for use in model airplanes and cars can be used as remote- control beam-steering units. Perhaps the easiest way to use the motors is with their intended receiver and transmitter. Unless you need to control many mirrors or other optical components, a two- or three-channel transmitter should do the job adequately. That allows you direct control over two or three servo motors. More sophisticated radio control (RIC) transmitters exist, of course, but they can be expensive. Fortunately, several Korean companies have recently joined the RIC market and offer low-cost alternatives to the more expensive brands. While these might not be as well made as the name brands, they provide similar functionality.

Ill. 19 shows a six-channel RIC transmitter and companion receiver. The servo motors, which contain a small dc motor, feedback potentiometer, and circuit, connect to the receiver. This transmitter is designed for model aircraft use, so some of its channels are dedicated to special purposes, such as raising and lowering the landing gear. These channels are effectively on or off, without intermediate steps, as are the aileron and rudder controls. Even with the extra channels, only three or four can be adequately used for optical bench beam-steering.

The transmitter and receiver operate on battery power. The servos are designed for use in many types of airplane fuselages, so a variety of mounting hardware is included. If you don’t find what you need in the parts included with the R/C transmitter and receiver, you can always take a trip to the hobby store and get more. A well-equipped hobby store, particularly those that specialize in RIC components, will stock everything you need.

Mirrors can be mounted to the servo motors in a number of ways. Gap-filling cyanoacrylate glue can be used to secure the mirror to the various plastic pieces. The hubs on servo motors are interchangeable by removing the set screw. If you can’t find a hub that works for you, you can fashion your own using metal or plastic.

Control the motors by turning on the transmitter and receiver. Rotate the control sticks as necessary to move the motors. If the transmitter is set up correctly, the servo motors should return to their midway position when the control sticks are centered. If this isn't the case, adjust the trimmer pots located on the transmitter. A parts list for a typical servo-controlled system is included in TABLE 8.

ELECTRONIC SERVO CONTROL

You can also control the servos using the circuit and computer program that appears in Section 20, “Advanced Light Shows.” The circuit's designed for interfacing with the Commodore 64 and allows you to control two servo motors. Additional projects in that section and in Section 19 show how to adapt the servos for electronic control using a potentiometer. These approaches can be easily adapted for optical bench beam-steering.