Lasers and Fiber optics

Most everyone has encountered fiberoptics at one time or another. Many telephone calls—both local and long distance— are now carried at least partway by light shuttling through a strand of plastic or glass. Fiberoptics are now used on some of the higher end audio systems as a means to prevent digital signals from interfering with analog signals. And fiberoptic sculptures, in vogue in the late 1960s but coming back in style today, look like high-tech flowers that seem to burst out in brightly colored lights.

An optical fiber is to light what PVC pipe is to water. Though the fiber is a solid, it channels light from one end to the other. Even if the fiber is bent, the light will follow the path in whatever course it takes. Because light acts as the information carrier, a strand of optical fiber no bigger than a human hair can carry the same information as about 900 copper wires. This is one reason why fiberoptics is used increasingly in telephone communications.

Laser light exhibits unique behavior when transmitted through optical fiber. This section discusses how to work with optical fibers, ways to interface fibers to a laser, and many interesting applications of laser/fiberoptic links.

HOW FIBEROPTICS WORK

The idea of optical fibers is over 100 years old. British physicist John Tyndall once demonstrated how a bright beam of light was internally reflected through a stream of water flowing out of a tank. Serious research into light transmission through solid material started in 1934 when Bell Labs was issued a patent for the light pipe.

In the 1950’s, the American Optical Corporation developed glass fibers that transmitted light over short distances (a few yards). The technology of fiberoptics really took off in about 1970 when scientists at Corning Class Works developed long-distance optical fibers.

All optical fibers are composed of two basic materials, as illustrated in ill. 15-1: the core and the cladding. The core is a dense glass or plastic material where the light actually passes through as it travels the length of the fiber. The cladding is a less dense sheath, also of plastic or glass, that serves as a refracting medium. An optical fiber may or may not have an outer jacket, a plastic or rubber insulation for protection.

Optical fibers transmit light by total internal reflection (TIR). Imagine a ray of light entering the end of an optical fiber strand. If the fiber is perfectly straight, the light will pass through the medium just as it passes through a plate of glass. But if the fiber is bent slightly, the light will eventually strike the outside edge of the fiber.

Tithe angle of incidence is great (greater than the critical angle), the light will be reflected internally and will continue its path through the fiber. But ii the bend is large and the angle of incidence is small (less than the critical angle), the light will pass through the fiber and be lost. The basic operation of fiberoptics is shown in ill. 15-2.

Note the cone of acceptance; the cone represents the degree to which the incoming light can be off-axis and still make it through the fiber. The angle of acceptance (usually 30 degrees) of an optical fiber determines how far the light source can be from the optical axis and still manage to make it into the fiber. Though the angle of acceptance might seem generous, fiberoptics perform best when the light source (and detector) are aligned to the optical axis.

Optical fibers are made by pulling a strand of glass or plastic through a small orifice. The process is repeated until the strand is just a few hundred (or less) micrometers in diameter. Although single strands are sometimes used in special applications, most optical fibers consist of many strands bundled and fused together. There might be hundreds or even thousands of strands in one fused optical fiber bundle. Separate fused bundles can also be clustered to produce fibers that measure /I6 of an inch or more in diameter.

ill. 15-1. Design of the typical optical fiber.

TYPES OF OPTICAL FIBERS

The classic optical fiber is made of glass, also called silica. Glass fibers tend to be expensive and are more brittle than stranded copper wire. But they are excellent conductors of light, especially light in the infrared region between 850 and 1300 nm. Less expensive optical fibers are made of plastic. Though light loss through plastic fibers is greater than with glass fibers, they are more durable. Plastic fibers are best used in communications experiments with near-infrared light sources—in the 780 to 950 rim range. This nicely corresponds to the output wavelength and sensitivity of commonplace infrared emitters and detectors.

Optical fiber bundles can be coherent or incoherent. The terms don’t directly relate to laser light or its properties but to the arrangement of the individual strands in the bundle. If the strands are arranged so that the fibers can transmit a pictorial image from one end to the other, it's said to be coherent. The vast majority of optical fibers are incoherent, where an image or special pattern of light is lost when it reaches the other end of the fiber.

The cladding used in optical fibers can be one of two types—step-index and graded- index. Step-index fibers provide a discrete boundary between more dense and less dense regions between core and cladding. They are the easiest to manufacture, but their design causes a loss of coherency when laser light passes through the fiber. That means coherent light in, largely incoherent light out. The loss of coherency, which is due to light rays traveling on slightly different paths through the fiber, reduces the efficiency of the laser beam. Still, it offers some practical benefits, as you’ll see later in this section.

ill. 15-2. Operation of an optical fiber, with cone of acceptance.

There is no discrete refractive boundary in graded-index fibers. The core and cladding media slowly blend, like an exotic tropical drink. The grading acts to refract light evenly, at any angle of incidence. This preserves coherency and improves the efficiency of the fiber. As you might have guessed, graded-index optical fibers are the most expensive of the bunch. Unless you have a specific project in mind (like a 10-mile fiber link), graded- index fibers are not needed, even when experimenting with lasers.

WORKING WITH FIBEROPTICS

Where do you buy optical fibers? While you could go directly to the source, such as Dow Coming, American Optical, or Dolan-Jenner, a cheaper and easier way is through electronic and surplus mail order. Radio Shack sells a 5-meter length of jacketed plastic fiber; Edmund Scientific offers a number of different types and diameters of optical fibers. You can buy most any length you need, from a sampler containing a few feet of several types to a spool containing thousands of feet of one continuous fiber.

A number of surplus outfits offer optical fibers from time to time (stocks change, so write for the latest catalog before ordering). You might not have much choice over the type of fiber you buy, but the cost will be more than reasonable.

Although it’s possible to use an optical fiber by itself, serious experimentation requires the use of fiber couplings and connectors. These are mechanical splices used to connect two fibers together or to link a fiber to a light emitter or detector. Be aware that good fiberoptic connectors are expensive. Look for the inexpensive plastic types meant for non-military applications. You can also make your own home-built couplings. Details follow later in this section.

Optical fibers can be cut with wire cutters, nippers, or even a knife. But care must be exercised to avoid injury from shards of glass that can fly out when the fiber is cut (plastic fibers don’t shatter when cut). Wear heavy cotton gloves and eye protection when working with optical fibers. Avoid working with fibers around any kind of food serving or preparation areas, because tiny bits of glass can inadvertently and invisibly settle on food, plates, etc.

One good way to cut glass fiber is to gently nick it with a sharp knife or razor, then snap it in two. Position your thumb and index finger of both hands as close to the nick as possible, then break the fiber with a swift downward motion (snapping upwards increases the chance of glass shards flying off toward you).

Whether snapped apart or cut, the end of the fiber should be prepared before splicing it to another fiber or connecting it to a light emitter or detector. The ends of the cut fiber can be polished using extra fine grit aluminum oxide wet/dry sandpaper (330 grit or higher). Wet the sandpaper and gently grind the end of the fiber on it. You can obtain good results by laying the sandpaper flat on a table and holding the fiber in your hands. Rub in a circular motion and take care to keep the fiber perpendicular to the surface of the sandpaper. If the fiber is small, mount it in a pin vise.

Inspect the end of the fiber with a high-powered magnifying glass (a record player stylus magnifier works well). Shine a light through the opposite end of the fiber. The magnified end of the fiber should be bright and round. Re-cut the fiber if the ends look crescent shaped or have nicks in them.

FIBEROPTIC CONNECTORS

Commercially made fiberoptic connectors are pricy, even the plastic AMP Optimate Dry Non-Polish (DNP) variety. A number of mail-order firms such as DigiKey and Jameco offer splices, for joining two fibers, and connectors, for attaching the fiber to emitters and detectors. Depending on the manufacturer and model, the connectors are made to work with either the round- or flat-style phototransistors and emitters.

ill. 15-3. Ways to connect fiberoptics to phototransistors, LEDs, and laser diodes.

You can make your own connectors and splices for home-brew laser experiments. Ill. 15-3 shows several approaches. An easy way to splice fibers is to use small heat- shrink tubing. Cut a piece of the tubing to about ½ inch. After properly cutting (and polishing) the ends of the fiber, insert them into the tubing and heat lightly to shrink. Best results are obtained when the tubing is thick-walled.

Optical fibers can be directly connected to photodiodes by drilling a hole in the casing, inserting the fiber, and bonding the assembly with epoxy. Be sure that you don’t drill into the semiconductor chip itself. Keep the drill motor at a fairly slow speed to avoid melting the plastic casing. Work slowly.

A strand of optical fiber can be held in place using a pin vise (remove the outer jacket, if any, and tighten the chuck around the fiber) or by using solderless insulated spade tongues. These tongues are designed for terminating copper wire but can be successfully used to anchor almost any size of optical fiber to a bulkhead. The laser, be it He-Ne or semiconductor, can then be aimed directly into the cone of acceptance of the fiber.

ill. 15-4. A solderless crimp lug can be used to secure the end of an optical-fiber to a circuit board or bulkhead.

Spade tongues, as shown in ill. 15-4, are available in a variety of sizes to accommodate different wire gauges. Use #6 (22 to 18 gauge) for small optical fibers and #8 (16 to 14 gauge) for larger fibers. Secure the fiber in the spade tongue by crimping with a crimp tool. Don't exert too much pressure or you will deform the fiber. If the fiber is loose after crimping, dab on a little epoxy to keep everything in place.

ill. 15-5. The popular plastic “FLCS” optical fiber connector.

The “FLCS” package, shown in ill. 15-5, is a low-cost fiberoptic connector available from a variety of sources including Radio Shack, Circuit Specialists, and many Motorola semiconductor representatives. It can be easily adapted for use with laser diodes by cutting off the back portion. This exposes the optical fiber.

After removing the emitter diode, file or grind off the back end of the connector, as shown in ill. 15-6. You can also drill out the back of the connector with a 5 bit. Mount the connector and laser on a circuit board or pen board, and be careful to align the laser so that its beam directly enters the end of the fiber.

ill. 15-6. How to modify an “FLCS” connector to open the back portion. With the back open, you can shine a laser beam inside the connector and into the optical fiber.

BUILD A LASER DATA LINK

A number of educational fiberoptic kits are available (see Section A for sources) and at reasonable cost. E.g., the Edu-Link (available through Advanced Fiberoptics Corp., Circuit Specialists, Edmund, and others) contains pre-etched and drilled PCBs for a small transmitter and receiver along with a short length of jacketed plastic or glass fiber. The emitter LED and photodetector are housed in plastic connectors, and the circuits provide input and output pins for sending and receiving digital data.

You can use the Edu-Link or the circuits shown in FIGS. 15-7 and 15-8 to build your own fiberoptic transmitter and receiver. Parts lists for the two circuits are provided in TABLES 15-1 and 15-2. Note that the Edu-Link, as well as other fiberoptic data communications kits, use a resistor to limit current to the emitter LED. The value of the resistor is typically 100 to 220 ohms. At 100 ohms and a 5 Vdc supply, current to the emitter LED is 35 milliamps (assuming a 1.5-volt drop through the LED). That isn’t enough to operate a dual-heterostructure laser diode, so the resistor must be exchanged for a lower value. A 56 ohm resistor will provide about 62 mA current to the laser diode; a 47-ohm resistor will deliver about 74 mA current. A feedback mechanism for control ling the output of the laser diode isn't strictly required because the device is used in pulsed mode.

If you are using the Edu-Link system and have not yet assembled the boards, construct the transmitter and receiver circuits, but don’t mount the emitter LED. Connect a Sharp LTO2O, LT022, or equivalent low-power dh laser diode to the transmitter circuit. Dismantle the emitter and modify the connector for use with the laser diode as detailed in the previous section.

ill. 15-7. Schematic for experimenter’s data transmitter using optical fibers and a laser diode. Transmission frequency of the free-running oscillator is approximately 3 kHz.

The transmitter circuit of ill. 15-7 includes a built-in oscillator. Trigger it by placing the ENABLE and DATA IN lines HIGH. Next, connect a logic probe to the DATA OUT pin of the receiver. Apply power to the two circuits and watch for the pulses at the DATA OUT pin. If pulses are not present, double-check your work and make sure that the fiberoptic connection between the transmitter and receiver is aligned and secure and that the output laser diode is properly aligned to the fiber. Remove power and disconnect the logic probe.

In normal operation, place the ENABLE pin LOW and connect any serial data to the DATA IN pin. Be sure the incoming signal is TTL compatible or does not exceed the supply voltage of the transmitter. You can use the circuit to transmit and receive ASCII computer data, remote-control codes, or other binary information. A number of circuits in Section 14, “Advanced Projects in Laser Communication,” show how to transmit computer data by laser over the air.

ill. 15-8. Schematic for experimenter’s data receiver.

Table 15-1. Optical Fiber Transmitter Parts List

• IC1 4093 CMOS NAND gate IC
• R1 220 kilohm resistor
• R2 33 kilohm resistor
• R3 3.9 kilohm resistor
• R4 33 kilohm resistor
• R5 56 ohm (nominal) resistor
• C1 0.01 F disc capacitor
• Laser diode
• Q1 2N2222 transistor

All resistors are 5 to 10 percent tolerance, ¼ watt. All capacitors are 10 to 20 percent tolerance, rated 35 volts or more.

A number of these projects can easily be adapted for use with the fiberoptic transmitter and receiver. E.g., you can connect the output pin of the Motorola 14557 remote control transmitter chip to the DATA IN pin of the Edu-kit transmitter circuit. The matching Motorola 14558 receiver chip connects to the DATA OUT terminal on the Edu-kit receiver circuit.

The CMOS chips in the transmitter and receiver can be operated with supplies up to 18 volts, but care must be taken to adjust the value of R5, the resistor that limits current to the laser diode. An increase in supply current must be matched with an increase in resistance or the laser diode could be damaged.

Use Ohm’s law to compute the required value (R = E/I). Assume a 1.2- to 1.5-volt drop through the diode.

Example:

Supply voltage—12 volts.

Voltage drop through laser diode—1.5 volts.

Working voltage—10.5 volts (12 — 1.5).

Desired current (60 to 80 mA)—70 mA nominal..

Resistor to use—150 ohms (10.5 / 0.070 = 150).

Table 15-2. Optical Fiber Receiver Parts List

• IC1 4093 CMOS NAND gate IC
• R1 220 kilohm resistor
• R2 1 kilohm resistor
• Q1 Infrared phototransistor
• Q2 2N2222 transistor
• All resistors are 5 to 10 percent tolerance, ¼ watt.

MORE EXPERIMENTS WITH LASERS AND FIBEROPTICS

Besides data transmission, fiberoptics can be used to:

* Transmit analog data (modulate a diode or He-Ne laser and pass it through the fiber).

* Detect vibration and motion.

* Route laser light to remote locations.

* Separate a laser beam into several shafts of light.

This is only a partial listing; there are literally dozens of useful and practical applications for lasers and fiberoptics. Some hands-on projects follow.

Vibration and Movement Detection

A fiberoptic strand doesn’t make the best medium for transmitting laser-light analog data. Why? The fiber itself can contribute to noise. As mentioned earlier, when a beam of coherent laser light is passed through a conventional step-index optical fiber, the rays travel different paths, and the light that exits is largely incoherent. You can see the effects of this interference by shining a helium-neon laser through an optical fiber. Point the exit beam at a white piece of paper and you’ll see a great deal of speckle. The speckle is the constructive and destructive interference, created inside the fiber, as the laser light rays travel from one end to the other.

This interference—which is most prominent in low-cost plastic optical fibers—is normally an undesirable side effect. However, it can be put to good use as a vibration and motion detection system. Connecting a phototransistor to the exit-end of the fiber lets you monitor the light output. Movement of the fiber causes a change in the way the light is reflected inside, and this changes the coherency (or incoherency, depending on how you look at it) of the beam. A simple audio amplifier connected to the phototransistor allows you to hear the movement.

Ill. 15-9 shows a setup you can use to test the effects of fiberoptic vibration and motion. A parts list for the system is provided in TABLE 15-3. More advanced projects using this technique are in Section 16, “Experiments in Laser Seismology.” The noise is sometimes a hiss and sometimes a “thrum.” Depending on the length of the fiber and type of motion, you might also hear low- or high-pitched squeals. These squeals can change pitch as the fiber or phototransistor is slowly moved.

The squeals are caused by the Doppler effect of optical heterodyning, a process whereby two rays of light at slightly different frequencies meet. The two basic (or fundamental) frequencies mix together, creating two additional frequencies. One is the sum of the two fundamental frequencies and the other is the difference.

How are the different frequencies of light created in the first place? Remember that the speed of light slows down as it passes through a refractive medium. There is a strict relationship between the wavelength, speed, and frequency of light. Because the wave length of light can’t be altered (at least by ordinary refraction), that means the frequency must be shifted in direct proportion to the change in the speed of light.

ill. 15-9. Hookup diagram for connecting a pair of phototransistors to op amp and audio amplifier. (A) Physical connection (showing beam expansion into both fibers) and phototransistor hookup; (B) Op amp and audio amplifier.

The change in frequency caused by refraction is rather small (on the order of a few hundred hertz) and is dependent on the original light frequency and the refractive index of the medium. The sum frequency is extremely high and can’t be heard, but the difference frequency might only be 200 to 500 Hz and can be readily detected with an ordinary phototransistor and audio amplifier.

Optical heterodyning is most conspicuous when only two coherent rays of light meet. In an optical fiber, dozens and even hundreds of rays of internally reflected laser light might meet at the phototransistor, and the result can sound more like cacophonous noise than a distinct tone. A Michelson interferometer (see Section 9) reveals optical heterodyning much more readily.

You might also notice a varying tone when sampling the beam directly from the la ser. Even though lasers are highly monochromatic, they can still emit several frequencies of light, with each frequency spaced only fractions of a nanometer apart. As these frequencies meet on the surface of the photodetector, they cause heterodyning or beat frequencies. When the difference frequency is 20 kHz or less, you can hear them. You can precisely measure the difference frequencies using an oscilloscope. The tones heard when sampling the beam directly from a laser are most prominent with short tubes and when they are first turned on.

Separating Beam with Optical Bundles

By grouping together one end of two or more fused bundles, you can separate the beam of a laser into many individual sub-beams. The laser light enters the common end (where all the bundles are tied together), and exits the opposite end of each individual fiber. Some optical fibers come pre-made with four or more grouped strands (used most often in automotive dashboard application) or you can make your own.

The pencil-thin beam of the typical He-Ne laser is too narrow to enter all the fibers at once, so the beam must be expanded. Place a bi-concave or plano-concave lens in front of the entrance to the bundles. Adjust the distance between the lens to the bundle until the beam is spread enough to enter all the fibers.

Split bundles can be used to experiment with optical heterodyning as well as to split the beam of one laser into several components. Each beam can be used in a separate optical fiber system. E.g., you must use a four-fiber split bundle to provide illumination for four-fiberoptic intrusion detection systems. Each sub-system is placed in a quadrant around the protected area and has its own phototransistor, making it easier to locate the area of disturbance.

Interfacing Fiberoptics to a Computer

Many of the advanced applications of lasers and fiberoptics require interface to a computer. In Section 16, “Experiments in Laser Seismology,” you’ll learn how to connect a phototransistor to a computer via an analog-to-digital converter.