Introduction to Semiconductor Lasers

The typical 5 mW helium-neon gas laser measures almost two inches in diameter by 10 to 15 inches in length. Imagine stuffing it all into a size no larger than the dot in the letter i! Such is the semiconductor laser. A close relative to the ordinary light-emitting diode, the semiconductor laser is made in mass quantities from wafers of gallium arsenide or similar crystals.

In quantity, low- to medium-power semiconductor lasers cost from $10 to $65. Such lasers are used in consumer products such as DVD/compact audio disc players and laser disc players, as well as bar-code readers and fiberoptics data links. With the proliferation of these and other devices, the cost of laser semiconductors (or laser diodes) is expected to drop even more.

This section presents an overview of the diode laser: how it’s made, the various types that are available, and how to use them in your experiments. The low cost of semiconductor lasers— typically $20 on the surplus market—make them ideal for school or hobbyist projects where a tight budget doesn’t allow for more expensive gas lasers.

THE INSIDES OF A SEMICONDUCTOR LASER

The basic configuration of the diode laser (sometimes called an injection laser) is shown in ill. 10-1. The laser is composed of a pn junction, similar to that found in transistors and LEDs. A chunk of this material is cut from a larger silicon wafer, and the ends are cleaved precisely to make the diode chip. Wires are bonded to the top and bottom. When current is applied, light is produced inside the junction. As it stands, the device is an LED—the light isn't coherent.

ill. 10-1. Design of a semiconductor laser chip showing cleaved face and pn junction.

An increase in current causes an increase in light output. The cleaved faces act as partially reflective mirrors that bounce the emitted light back and forth within the junction. Once amplified, the light exits the chip. This light is temporally and spatially coherent, but because of the design of the diode chip, isn't very directional. The beam of most laser diodes is elliptical, with a spread of about 10 to 35 degrees.

The first laser diodes, created in 1962 shortly after the introduction of the ruby and helium-neon lasers, were composed of a single material forming one junction—a homo-junction. These could be powered only in short pulses because the heat produced within the junction would literally cause the diode to explode. Continuous output could only be achieved by dipping the diode in a cryogenic fluid such as liquid nitrogen (with a temperature of —196 degrees C, or —320 degrees F).

As manufacturing techniques improved, additional layers were added in varying thicknesses to produce a hetero-junction diode. The simplest heterojunction semiconductor lasers have a gallium arsenide (GaAs) junction topped off by layers of aluminum gallium arsenide (A1GaAs). These can produce from 3 to 10 watts of optical output when driven by a current of approximately 10 amps. At such high outputs, the diode must be operated in pulsed mode.

Typical specifications for single heterostructure (sh) lasers call for a pulse duration of less than 200 nanoseconds. Most drive circuits operate the diode laser conservatively with pulse durations under 75 or 100 nanoseconds. Output wavelength is generally between 780 nm and 904 nm.

A double heterostructure (dh) laser diode is usually made by sandwiching a GaAs junction between two AIGaAs layers. This helps confine the light generated within the chip and allows the diode to operate continuously (called continuous wave, or cw) at room temperature. The wavelength can be altered by varying the amount of aluminum in the AIGaAs material. The output wavelength can be between 680 nm and 900 nm, with 780 nm being most common.

Power output of a double heterostructure laser is considerably less than with a single heterostructure diode. Most dh lasers produce 3 to 5 mW of light, although some high- output varieties can generate up to 500 mW yet can still be operated at room temperature (indeed, some high-cost cw lasers can produce up to 2.6 watts of optical power, but these are rare and very expensive). High-output laser diodes come in T0-3 transistor- type cases and are mounted on suitable heat sinks. A typical application for high-output lasers is long-haul (long distance) fiberoptic data links.

POWERING A DIODE LASER

Drive circuits for both sh and dh lasers are presented in Section 11, “Laser Power Supplies.” But it’s worthwhile here to discuss the drive requirements necessary for operating diode lasers.

Single heterostructure lasers are typically driven by applying a high-voltage, short- duration pulse. The duration of the pulse is controlled by an RC network, as shown in the basic schematic in ill. 10-2, and the pulse is delivered by a power transistor. Care must be exercised to ensure that the pulse duration does not exceed the maximum specified by the manufacturer. Longer pulses cause the laser to overheat, annihilating itself in a violent puff of smoke.

Double heterostructure semiconductor lasers can be operated either in pulsed or cw mode. In pulsed mode, the diode is driven by short, high-energy spikes, as with an sh laser. Power output may be on the order of several watts, but because the pulses are short in duration, the average power is considerably less. In cw mode, a low-voltage constant current is applied to the laser outputs in a steady stream of light. Cw lasers and drive circuits are used in compact disc players where the light emitted by the laser is even more coherent than the beam from the revered He-Ne tube.

Forward-drive current for most cw lasers is in the neighborhood of 60 to 80 mA. That’s 50 to 200 percent higher than the forward current used to power light-emitting diodes. If a cw laser is provided less current, it can still emit light, but it won’t be laser light. The device lases only when the threshold current is exceeded—typically a minimum of 50 to 60 mA. Conversely, if the laser is provided too much current, it generates excessive heat and is soon destroyed.

ill. 10-2. One way to drive a single heterostructure laser diode. The transistor is driven in avalanche mode, producing short-duration pulses of current.

Monitoring Power Output

All laser diodes are susceptible to changes in temperature. As the temperature of a semiconductor laser increases, the device becomes less efficient and its light output falls. If the temperature decreases, the laser becomes far more efficient. With the increase in output power, there is a risk of damaging the laser, so most cw drive circuits incorporate a feedback loop to monitor the temperature or output power of the device and adjust its operating current accordingly.

Sensing temperature change requires an elaborate thermal sensing device and complicated constant-current reference source. An easier approach is to monitor the light output of the laser. When the output increases, current is decreased. Conversely, when the output decreases, current is increased.

To facilitate the feedback system, the majority of cw laser diodes now incorporate a built-in photodiode monitor. This photodiode is positioned at the opposite end of the diode chip, as shown in ill. 10-3, and samples a small portion of the output power. The photodiode is connected to a relatively simple comparator or op amp circuit. As the power output of the laser varies, the current (and voltage) of the photodiode monitor changes. The feedback circuit tracks these changes and adjusts the voltage (or current) supplied to the laser. The feedback circuit can be designed around discrete parts or a custom- made IC. Actual driving circuits using both designs are presented in the following section. There is also a schematic for driving a cw laser in pulsed mode.

ill. 10-3. The orientation of the laser and monitor photodiode chips in a typical double-hetero-structure semiconductor laser.

Connecting the Laser to the Drive Circuit

The laser and photodiode are almost always ganged together, using one of two approaches. Either the anode of the laser is connected to the cathode of the photodiode, or the cathodes are grouped together. That leaves three terminals for connecting the diode to the control circuits. Schematic diagrams for the two approaches are illustrated in ill. 10-4. A sample terminal layout for the popular Sharp laser diodes (as used in bar code readers and compact disc players) is shown in ill. 10-5.

ill. 10-4. Two ways of internally connecting laser and monitor photodiode.

ill. 10-5. Package outline and terminal configuration for the Sharp LT020 laser diode.

There is a danger of damaging a laser diode by improperly connecting it to the drive circuit. Connecting a 60 to 80 mA current source to the photodiode will probably burn it out and can destroy the entire laser. Moral: follow the hook-up diagram carefully. If no diagram came with the laser diode you received, write to the seller or manufacturer and ask for a copy of the specifications sheet or application note.

HANDLING AND SAFETY PRECAUTIONS

While the latest semiconductor lasers are hearty, well-made beasts, they do require certain handling precautions. And, even though they are small, they still emit laser light that can be potentially dangerous to your eyes. Keep these points in mind:

* Always make sure the terminals of a laser diode are connected properly to the drive circuit (I’ve covered this already but it’s most crucial).

* Never apply more than the maximum forward current (as specified by the manufacturer), or the laser will burn up. Use the pulser drive (see Section 11) if you are not using the laser with a monitor photodiode feedback circuit.

* Handle laser diodes with the same care you extend to CMOS devices. Wear an anti-static wrist strap while handling the laser, and keep the device in a protective, anti-static bag until ready for use.

* Never connect the probes of a volt-ohmmeter across the terminals of a laser diode (the current from the internal battery of the meter can damage the laser).

* Use only batteries or well-filtered ac power supplies. Laser diodes are susceptible to voltage transients and can be ruined when powered by poorly filtered line-operated supplies.

* Take care not to short the terminals of the laser during operation.

* Avoid looking into the window of the laser while it's operating, even if you can’t see any light coming out. This is especially important if you have added focusing or collimating optics.

* Mount the laser diode on a suitable heatsink, preferably larger than 1 inch square. Use silicone heat transfer paste to assure a good thermal contact between the laser and the heatsink. You can buy heatsinks ready made or construct your own. Some ideas for heatsinks appear in the next section.

* Insulate the connections between the laser diode and the drive to minimize the chance of short circuits. Use shielded three-conductor wire to reduce induction from nearby high-frequency sources.

* Laser diodes are subject to the same CDRH regulations as any other laser in its power class. Apply the proper warning stickers and advise others not to stare directly into the laser when it's on.

* Use only a grounded soldering pencil when attaching wires to the laser diode terminals. Limit soldering duration to less than 5 seconds per terminal.

* Unless otherwise specified by the manufacturer, clean the output window of the laser diode with a cotton swab dipped in ethanol. Alternatively, you can use optics-grade lens cleaning fluid.

MOUNTING AND HEAT SINKS

Most laser diodes lack any means by which to mount them in a suitable enclosure. Their compact size does not allow for mounting holes. However, with a bit of ingenuity, you can construct mounts that secure the laser in place as well as provide the recommended heatsinking. One approach is to clip the laser in place using a fuse holder, as shown in ill. 10-6. You might have to bend the holder out a bit to accommodate the laser. Mount the clip on a small piece of aluminum or a TO-220 heatsink. Use silicone paste at the junction of all-metal pieces; this assists in proper heat transfer.

Another method, detailed in ill. 10-7, is to drill a hole the same diameter as the laser in an aluminum heatsink. Use copper retaining clips (available at the hobby store) to secure the laser in place. Once again, apply silicone paste to aid in heat transfer.

Some lasers are available on the surplus market, like that shown in ill. 10-8, are already attached to a heatsink and mount. The mount doubles as a rail for collimating and beam-shaping optics. You can use the laser with or without these optics, of course, or substitute with your own.

ill. 10-6. A fuse clip can be used as a simple heatsink for a semiconductor laser.

ill. 10-7. Use a flexible copper retaining ring to hold a diode to the heat-sink. Use silicone heatsink paste to aid in proper heat transfer.

ill. 10-8. .A commercially made with “sled” laser diode (left side) and beam-shaping optics installed.

SOURCES FOR LASER DIODES

Laser diodes are seldom sold at the neighborhood electronics store, and as of this writing, Radio Shack does not carry the device as a replacement or experimenter’s item. That leaves buying your laser diodes directly from the manufacturer, through an authorized manufacturer’s representative, or through surplus. Buying direct from the manufacturer or rep assures you of receiving prime, new goods, but the cost can be high. Average cost for a new 3 to 5 mW laser cw diode is about $30. Names and addresses of manufacturers are in Section A. You can locate the manufacturer online, or look in the phone book under “Electronics—Wholesale and Retail.”

The same or similar device on the surplus market is about $19 to $25, depending on the power output. Several of the surplus mail-order dealers listed in Section A offer sh and dh laser diodes; write them for a current catalog. Many also provide kits and ready-made drive/power supply circuits. Be aware that, at this time, most surplus laser diodes are take-outs, meaning that they were used in some product that was later retired and scrapped. While buying used He-Ne tubes can be a chancy affair, the risk of buying pre-owned laser diodes is minimal. Like all solid-state electronics, the life span of a laser diode is extremely long—in excess of 5,000 to 10,000 hours of continuous use.

BUILD A POCKET LASER DIODE

You can build a complete laser in a box about the size of a pack of cigarettes. Ill. 10-9 shows the basic layout; TABLE 10-1 provides the parts list. You can use just about any of the drive circuits presented in Section 11 to power the laser. In all cases, you can mount the components on a universal solder PCB and fit the whole thing in a 3¼- by-2¼-by-1¼-inch plastic experimenter’s box. Drill holes for the switch, power jack, and lens tube. The lens hole should be ¾-inch in diameter.

Saw off a solderless RG59U coaxial connector and mount the laser inside (check the connector style first to be sure the diode fits snugly). Use all-purpose adhesive to secure a 10-mm double-concave lens (with a focal length of about 15 mm) inside one end of a 1-inch length of a 7 (I.D.) brass tube. The tube is available at most hobby stores. Fit the laser diode in the lens tube and mount the tube in the enclosure. Use all-purpose adhesive to keep it in place. You can adjust the spacing between the laser and lens later.

ill. 10-9. Basic layout for the pocket laser showing ON/OFF switch, power jack, laser assembly, and driver board.

Wire the components as shown in ill. 10-10. Install the switch, power jack, and drive board in the box. Temporarily apply power to the circuit board and dim the lights. Point the lens toward a lightly colored wall at a distance of no more than a few inches. Adjust the distance between laser and lens by sliding the RG59U connector in or out of the brass tube until the spot on the wall is bright and well-defined. You will see rings in the beam; this is normal. A grainy speckle in the spot means that the diode is emitting laser light. If you don’t see the speckle, the laser might not be driven with enough current.

When everything looks ok, dab a small drop of all-purpose adhesive on the RG59U connector and brass tube to keep the laser from coming loose. Don’t apply too much glue, because you might need to readjust the laser later on. Close up the box and fit a set of four “AA” batteries in a battery holder. Place the battery holder in a box measuring at least 2½ by 2½ by 1 inch (see ill. 10-11). Use three-conductor shielded microphone cable as the power cord, as shown, and solder a ½-inch stereo plug on the end. To use the battery pack, simply plug it into the power jack on the laser box.

ill. 10-10. Two ways to wire the pocket laser, Use the dual-ended power supply if the driver board requires it.

Note that the 6-volt battery pack is meant for use with the pulsed drive circuit de scribed in Section 12. Other drive schemes call for a 12-volt supply or for a split ± 5-volt supply. Use the appropriate type of batteries, connected in parallel and /or in series, to provide the required voltage level. You might need to add voltage regulators (small TO-92 case) or zener diodes to maintain or regulate the supply voltages.

You might want to combine the laser/drive components in the same box as the batteries. You can fit everything in a project box measuring 6¼ by 3¾ by 2 inches, or even less if you are careful how you mount the components. Make sure that the batteries are placed in a convenient location so that they can be easily changed when they wear out.

USING THE POCKET LASER

The light from the pocket laser is largely invisible unless you happen to own see-in-the-dark infrared glasses or an JR viewing card (a card coated with a chemical that reacts to infrared radiation). The faint red glow of the laser is discernible only in darkness and when the lens is focused on a nearby wall. That makes applications such as laser pointers out of the question. But the pocket laser is far from useless. As you’ll learn in future sections, you can use this basic configuration to create a collimated free-air laser light communicator.

ill. 10-11. Install the batteries in a project box and terminate the battery holder leads with a ½-inch mini plug (two- or three-conductor as needed).

You can also use the pocket laser as the head-end for a fiberoptic data link or as a means to experiment with interferometry. Although the beam is difficult (if not impossible) to see without some sort of IR viewing device or infrared viewing card, you can detect the interferometric fringes with an ordinary photodetector. Connect the photodetector to an audio amplifier, as shown in Section 9, and you can hear the fringes move. Connected to a counter, you can even count the number of fringes that go by.

An advanced project might be to use the laser to make near-infrared holograms. Although most films are already sensitive to near-infrared radiation, you can obtain better results if you use an emulsion specifically formulated for the 780 to 880 nm range of most laser diodes. Be sure that the film has very high resolution, or the hologram won’t turn out.