33
CHAPTER
Lasers
THE WORD LASER IS AN ACRONYM THAT STANDS FOR THE TECHNICAL TERM LIGHT AMPLIFICATION by stimulated emission of radiation. Lasers have diverse applications in electronic systems.
How a Laser Works
In theory, laser radiation differs from ordinary EM radiation in two ways. First, all of the energy exists at a single wavelength, and therefore, at a single frequency. At visible-light wavelengths, this property gives the light a vivid hue (“color”). Second, the “wave packets” (known as photons) in a laser beam all propagate in phase coincidence, so all the wave peaks and troughs “line up” with one another. When we talk about lasers, we usually refer to devices that produce coherent rays in the ultraviolet (UV), visible-light, or infrared (IR) portions of the spectrum, expressing wavelengths in nanometers (nm), where 1 nm = 10−9 m.
Spectral Distribution and Coherence
The visible light from a conventional source, such as the sun or a house lamp, is not a clean, sine-wave EM disturbance. Components exist in the red, or longest, visible wavelength (upwards from about 770 nm) through the violet, or shortest, visible wavelength (up to about 390 nm). Energy components usually exist outside this range as well. If we look at a graph of light intensity as a function of wavelength for a particular light source, we get a spectral distribution for that source.
The spectral distribution for an incandescent lamp skews toward the red end of the visible spectrum, as shown in Fig. 33-1A. A common fluorescent tube produces energy oriented toward the middle visible wavelengths (Fig. 33-1B). Some lamps emit light at several discrete wavelengths, producing a distribution that shows multiple well-defined spectral lines, as shown in Fig. 33-1C.
33-1 At A, the spectral distribution of light from an incandescent lamp. At B, the spectral distribution of light from a fluorescent lamp. At C, the spectral distribution of light from a hypothetical lamp that emits energy at multiple discrete wavelengths.
Even a lamp that emits light at discrete wavelengths, such as a mercury-vapor lamp, sodium-vapor lamp, or neon-gas lamp, produces myriad waves in random phase for each specific wavelength (Fig. 33-2A). Because of the random phase, we can call such light incoherent, whether it’s monochromatic (single-colored, existing at a single wavelength) or not. We use the term coherent to describe the visible light or other EM energy from a source that radiates at a single wavelength (Fig. 33-2B) and for which all of the wavefronts line up with each other. No ordinary lamp produces coherent light, but lasers do.
33-2 At A, waves of monochromatic light with waveforms aligned at random. At B, waves of coherent monochromatic light.
The Bohr Atom
When an electron is bound by an atomic nucleus, that electron can normally exist only at certain discrete energy levels. All the way back in Chap. 1 (Fig. 1-2), we saw a simplified model of an atom, known as the Bohr atom after its inventor, physicist Niels Bohr, who hypothesized it in the early 1900s. Look at that figure again now. The light dashed circles represent cross sections of spheres called electron shells. The electrons orbit around the nucleus in the shells, each of which has a fixed radius. The amount of energy in an electron increases as its shell radius increases. An electron can “jump” to a higher level or “fall” to a lower level. The heavy dashed path shows a hypothetical example of such a transition.
If a photon having precisely the right amount of energy (and therefore, exactly the right wavelength) strikes an electron, the electron “jumps” from a given energy level to a higher one. Conversely, if an electron “falls” from a higher-energy level to a lower-energy level, the atom emits a photon of a wavelength corresponding to the amount of energy that the electron loses.
The Bohr model oversimplifies the actual behavior of electrons. In the modern view, electrons “swarm” around atomic nuclei so fast, and in such a complicated way, that we can never pinpoint the location of any single electron. Instead, we must define electron movement in terms of probabilities. When we say that an electron resides in a certain shell, we mean that at any point in time, the electron is just as likely to exist inside that shell as outside it.
Physicists assign electron shells principal quantum numbers within atoms, using the lowercase, italic letter n to generalize. The smallest principal quantum number is n = 1, which represents the smallest possible electron shell and, therefore, the lowest possible electron energy level. Progressively larger principal quantum numbers are n = 2, n = 3, n = 4, and so on, representing shells of increasing radius. As the principal quantum number of an electron increases, so does the amount of energy that it has, assuming all other factors remain constant.
The Rydberg-Ritz Formula
Within an atom, changes in an electron’s energy can occur in conjunction with absorption or emission of EM waves. Let’s consider the special case of hydrogen gas. Hydrogen atoms are the simplest in the universe, normally containing a single proton in the nucleus and a single electron in “orbit” around the nucleus.
Imagine a glass cylinder from which all the air has been pumped out. Suppose that we introduce a small amount of hydrogen gas into the cylinder. Then we connect a DC source to electrodes at either end of the cylinder, obtaining a device called a gas-discharge tube. If we apply a high enough DC source voltage V, the gas inside the tube ionizes, and an electrical current I flows. As a result, the gas dissipates a certain amount of power P. The power, voltage, and current relate according to the formula
P = VI
where we express P in watts, V in volts, and I in amperes. We use V, rather than E, to represent voltage here because later in this chapter we’ll use E to represent energy. The standard unit of energy is the joule (abbreviated J). One watt (1 W) of power represents energy dissipated at the rate of one joule per second (1 J/s).
If the DC voltage source remains active and stays connected to the electrodes, the gas in the tube absorbs energy as time goes by, causing the electrons in the atoms to attain higher energy levels, and therefore, to reside in larger shells, than they would do if no voltage source were connected to the electrodes. As a result of the overall energy gain, the gas gets hot. This process cannot continue indefinitely. Therefore, as current continuously flows through the ionized hydrogen gas, electrons not only “rise” to higher energy levels (larger shells), but they also constantly “fall” back to lower energy levels (smaller shells), emitting photons at various EM wavelengths.
Once an electron has “fallen” from a certain shell to another shell having a lower energy level, that electron is ready to absorb some energy from the DC source again, “rise” again, and then eventually “fall” again. After a short initial heating-up time, the amount of energy absorbed by the gas from the DC source, which causes electrons to “rise” from smaller shells to larger ones, balances the energy radiated by the gas as electrons “fall” from larger shells back to smaller ones.
A hydrogen atom has several electron shells, so an electron can gain or lose any one of numerous specific energy quantities as it moves from one shell to another. Energy, therefore, radiates from ionized hydrogen gas in the form of photons having multiple discrete wavelengths λ as follows:
λ = 1/[RH (n1−2 − n2−2)]
where we express λ in meters, we let RH represent a constant called the Rydberg constant, n1 represents the principal quantum number of the smaller shell to which a particular electron “falls,” and n2 represents the principal quantum number of the larger shell from which that same electron has just “fallen.” Physicists call the above equation the Rydberg-Ritz formula, named after Johannes Rydberg and Walter Ritz who first published an academic paper about it in 1888. The value of the Rydberg constant, accurate to six significant figures, is
RH = 1.09737 × 107
This figure is sometimes rounded off to three significant figures as 1.10 × 107. We define it in units of per meter (/m or m−1). Some texts denote the constant as R with an infinity-symbol subscript (R∞). Once in a while you’ll see it symbolized simply as R, but in that case you must not confuse it with the universal gas constant.
We’ve examined how electrons relate to EM energy for hydrogen gas. Other gases, as well as liquids and solids, can behave in a similar way, but their Rydberg constants all differ. Every element, mixture, and compound has a unique Rydberg constant, and a unique set of wavelengths at which it can absorb or emit energy under certain conditions. With some materials, we can exploit this phenomenon to generate coherent light.
Wave Amplification
In a gas laser, the atoms behave somewhat differently than they do in a simple gas-discharge tube because we keep the gas in a super-energized condition in which an abnormally large number of electrons exist in high-energy shells. This state of affairs, called a population inversion, constitutes the critical factor responsible for the stimulated emission of photons that makes a laser work.
When a photon strikes an electron in a gas laser tube, that photon is not absorbed. Instead, it continues to travel in the same direction, and with the same energy, as it did before the collision. The electron, instead of “rising” to a higher shell, actually “falls” to a lower shell, as shown in Fig. 33-3. When this transition occurs, the electron loses energy instead of gaining energy. Therefore, a new photon P2 emanates from the electron, leaving the atom along with the original photon P1.
33-3 Amplification of EM energy by means of a photon interacting with an atom.
If the original photon P1 has exactly the right amount of energy, the new photon P2 emerges at the same wavelength, and going in the same direction, as the original photon P1. Photon P1 then travels along with P2 in such a way that the two wave disturbances exist in phase coincidence. When transitions of this sort occur in large numbers, the super-energized gas, in effect, boosts the amplitude of the incoming EM energy, a phenomenon called wave amplification. That’s how the gas laser generates its intense energy beam. A similar phenomenon can also occur in certain liquids and solids.
The Cavity Laser
One of the most common types of lasers, the cavity laser, has a resonant cavity, usually in the shape of a cylinder or prism, and an external source of energy. The cavity is designed so that EM waves bounce back and forth inside it, reinforcing each other in phase coincidence.
Basic Configuration
The cavity contains (or consists of) a gas, liquid, or solid substance called the lasing medium. We place mirrors at each end so the EM waves reflect back and forth many times between them. One mirror reflects all the energy that strikes it, and the other mirror reflects approximately 95 percent of the incident energy. The mirrors can be flat or concave. The ends of the cavity can be perpendicular to the lengthwise cavity axis or oriented at a slant.
Figure 33-4 illustrates two common cavity laser designs. These drawings are “squashed horizontally” for clarity. The waves represent the EM energy, usually IR or visible light, resonating between the reflectors. Only a few wave cycles appear in this simplified functional drawing. In a practical cavity laser, thousands or millions of wave cycles occur between each reflection.
33-4 Two simple cavity lasers. At A, a flat-ended cavity and flat reflectors. At B, an angle-ended cavity and concave reflectors.
Pumping
Energy is supplied to the lasing medium by means of a process called pumping, so that the intensity of the beam builds up as the internal beam repeatedly reflects from the mirrors. The waves emerge from the partially silvered (95-percent reflective) mirror in the form of coherent radiation, which concentrates the wave energy into the most intense possible beam.
Beam Radius
The beam radius depends on the physical radius of the cavity and also on its length. Generally, small-radius cavities produce narrower beams at close range than large-radius cavities do. However, large-diameter cavities usually have less beam divergence, which quantifies the extent to which the beam radius increases with increasing distance from the source, once the beam gets several cavity lengths away from the partially silvered mirror. At a great distance in free space (a vacuum), we can expect a large-radius laser to produce a narrower beam than a small-radius laser of the same type. Beam divergence occurs as a result of imperfections in the hardware, and also because of scattering and diffusion in the air or other medium through which the laser beam travels.
Output Wavelength
The output wavelength from a cavity laser depends on the length of the cavity and on the atomic resonant wavelengths of the lasing medium. Many different sizes of cavity lasers can produce an output of a given wavelength. Some cavity lasers are continuously tunable (adjustable) over a range of wavelengths. Most operate in the IR or visible part of the EM spectrum.
Beam Energy Distribution
We can portray the beam energy distribution at a given distance from a laser device by generating a cross-sectional graph of beam intensity versus the radius from the beam center in a plane perpendicular to the beam axis. Figures 33-5 and 33-6 show hypothetical examples for so-called “spot” and “ring” distributions, respectively. In all four of these graphs, the relative intensity appears on the horizontal axis, and the relative distance from the beam center is shown on the vertical axis, with the intersection of the two axes representing the beam center. Figure 33-7 illustrates typical examples of “spot” and “ring” beams as they would look if we projected them onto a white screen in a dark room.
33-5 Hypothetical intensity-versus-radius graphs of “spot” laser-beam distributions. At A, sharp peak and gradual dropoff. At B, flat peak and abrupt dropoff.
33-6 Hypothetical intensity-versus-radius graphs of “ring” laser-beam distributions. At A, sharp peaks and gradual dropoffs. At B, flat peaks and abrupt dropoffs.
33-7 At A, a typical “spot” laser beam as it would look if projected onto a white screen. At B, the projection of a “ring” beam on the same screen.
Focusing and Collimating the Beam
Because the rays from a distant light source follow essentially parallel paths within any small zone in space, we can bring the rays to a sharp focus so that they cover a minuscule area. The radius of the focused beam image depends on the distance to the source, the size of the source, and the focal length of the lens or mirror. Have you ever used a convex lens to focus sunlight and cause something to catch on fire? As the diameter of the lens or mirror increases, it gathers more light and the focused beam becomes more intense, assuming that the focal length remains constant.
When we want to focus a laser beam, the area of the lens or mirror need only be large enough so that it captures the whole beam. In theory, this minimum capture area never changes, no matter what the distance of the laser from the lens. In the “real world,” the laser beam spreads a little, but the beam-divergence effect is not significant unless the distance from the laser is great.
With a coherent light beam, we can obtain an extremely small region of light at the focus of a convex lens or a concave mirror. Theoretically, this region constitutes a geometric point regardless of the focal length of the lens or mirror (Fig. 33-8). In practice, of course, we never actually get a perfect point. If we could, it would contain infinite energy density, a physical impossibility. Instead, we obtain an intense spot or ring having a tiny radius. Some laser beams can be focused to microscopic precision, forming energy spots or rings so small that they can alter the genes in living biological cells.
33-8 In theory, a coherent light beam can be focused to a geometric point by a convex lens (A) or by a concave mirror (B).
Continuous versus Pulsed
Some lasers deliver output power at a constant rate as time passes, and therefore, have equal peak and average power output. We call such devices continuous-wave (CW) lasers. Other lasers produce brief bursts having extreme peak power output but much lower average power output. We call them pulsed lasers. Pulsed-laser output can be achieved in a variety of ways.
A pulsed-laser technology known as Q-switching involves allowing the intensity of the light in the cavity to build up, while the cavity remains in a state unfavorable for lasing. When the intensity reaches a certain level, the cavity conditions are adjusted, producing resonance so that we get an intense, brief burst of coherent radiation. The process is repeated indefinitely, producing energy bursts at regular intervals.
Pulsed lasing can also be obtained by supplying the cavity with bursts of energy, rather than with a continuous supply. This technique is useful with lasing media that cannot maintain CW operation. A brilliant flash lamp can serve as the pumping source in this mode.
Energy, Efficiency, and Power
The output energy and average output power obtainable from a cavity laser depend on the physical size of the cavity, and on the amount of energy or power supplied by the pumping source.
We can determine the output energy by measuring the total radiant energy Eout of the beam, in joules, over a period of time. We might carry out the measurement by allowing the laser to strike a perfectly absorbing target (a so-called black body) that converts all the incident radiant energy to heat, and then measuring the amount of heat produced with a calorimeter or joule meter.
The efficiency eff of the laser can be determined by measuring the total pumping energy Ein over a certain period, measuring Eout for the same period, and then finding the ratio
eff = Eout/Ein
The average output power Pavg-out in watts is determined by measuring the output energy Eout in joules produced over a certain period of time t in seconds, and then dividing the energy in joules by the number of seconds over which the energy measurement has been made, giving us
Pavg-out = Eout/t
In terms of power, we can calculate the efficiency eff by dividing the average output power Pavg-out by the average input power Pavg-in to obtain
eff = Pavg-out/Pavg-in
The peak power output Ppk-out of a pulsed laser can prove difficult to determine unless the pulses have a rectangular shape, meaning a near-instantaneous rise, a near-instantaneous decay, and a constant amplitude for the duration of the pulse. In this situation (the theoretical ideal), the peak power output equals the average power output divided by the duty cycle D, which tells us the proportion of the time the laser is active. Mathematically, we have
Ppk-out = Pavg-out/D
In a pulsed laser, D is always smaller than 1 (usually much smaller), so Ppk always exceeds Pavg (usually by a large factor). If we know the peak output power and the duty cycle, then we can determine the average power using the formula
Pavg-out = Ppk-out D
If we know the peak power and the average power, then we can calculate the duty cycle using the formula
D = Pavg-out/Ppk-out
Semiconductor Lasers
As we’ve learned, some semiconductor diodes emit radiant energy when forward-biased so that current flows. This phenomenon, called photoemission, occurs as electrons in the atoms, driven to higher shells from the energy provided by a DC source, “fall” back down to lower energy states. The wavelength depends on the mixture of semiconductors. Most diodes emit energy in the visible or IR range.
Laser Diode
A laser diode, also called an injection laser, is a special LED or IRED with a relatively large and flat P-N junction. If the forward current through the junction remains below a certain threshold level, the device behaves like an ordinary LED or IRED, producing incoherent light or IR radiation. When the threshold current is reached, the emissions become coherent. Laser diodes, as well as conventional LEDs and IREDs, are designed to work properly only when forward-biased. They operate like ordinary semiconductor diodes when reverse-biased; they fail to radiate whether or not the reverse bias exceeds the avalanche voltage.
Gallium arsenide (GaAs) is a compound used in the manufacture of various semiconductor devices, including diodes, LEDs, IREDs, FETs, and ICs. Pure GaAs constitutes an N type semiconductor material. The charge carriers move fast and easily, so engineers say that GaAs has excellent, or high, carrier mobility.
GaAs LEDs are used in numeric displays for consumer items, such as electronic clocks. We’ll also find them in sophisticated systems, such as radio transceivers, test instruments, calculators, and computers. GaAs IREDs, which have a primary emission wavelength of approximately 900 nm, find applications in fiber-optic systems, robotic proximity sensors, handheld remote-control boxes for home appliances, and motion detectors for security systems.
All GaAs LEDs and IREDs can be rapidly switched or modulated because of the high carrier mobility. We can modulate some GaAs laser diodes at frequencies in excess of 1 GHz (109 Hz), making them ideal for use in broadband or high-speed optical communications systems. Figure 33-9 illustrates the construction of a typical GaAs laser diode. The P and N type semiconductor materials are built up on top of a base called the substrate, which also serves to carry away excess heat by means of thermal conduction.
33-9 Simplified functional diagram of a laser diode. Coherent radiation is emitted in the plane of the P-N junction.
Hybrid Silicon Laser
In order to meet the demand for inexpensive, mass-producible silicon-based LEDs and IREDs, engineers developed the hybrid silicon laser (HSL) by combining silicon in layers with certain other semiconductor compounds called III-V materials. These compounds, which include indium phosphide (InP) and GaAs, allow for energy emission when excited by either an electrical current or an external source of coherent light. The lasing medium is surrounded by mirrors to form an enclosed medium called a waveguide, which produces coherent light by creating a resonant state in which the waves reinforce each other.
Quantum Cascade Laser
In some semiconductor lasers, electrons make “cascading” jumps among multiple atoms. Figure 33-10 shows, in simplified form, how this process occurs. This type of device, called a quantum cascade laser (QCL), produces more output than a conventional semiconductor laser because of a “chain reaction” of energy transitions. Figure 33-10 shows only two semiconductor layers, but a typical QCL has many layers, producing a massive cascade effect. The emission wavelength, which depends on the thickness of the semiconductor layers, lies in the middle-IR to far-IR range, considerably longer than the waves of visible red light, but shorter than radio microwaves.
33-10 In a QCL, multiple energy transitions produce greater output than can be obtained with a conventional semiconductor laser.
Vertical-Cavity Surface-Emitting Laser
In a vertical-cavity surface-emitting laser (VCSEL), a coherent visible or IR beam emerges perpendicular to the plane of the P-N junction, in contrast to the usual laser diode in which the beam emerges in the plane of the junction. Lasing takes place inside a region called the quantum well, surrounded on either side by semiconductor layers that also serve as end reflectors (Fig. 33-11).
33-11 Simplified functional diagram of a VCSEL. Coherent radiation is emitted at right angles to the plane of the P-N junction.
In practice, the VCSEL offers a couple of advantages over a conventional laser diode. First, technicians can test a VCSEL at multiple stages prior to production, a procedure not always practical with other types of semiconductor devices. Second, a VCSEL generally emits a narrower beam than a conventional laser diode does.
Vertical-cavity devices based on GaAs operate at wavelengths from the visible red (about 650 nm) to the near-IR (about 1300 nm). Devices based on InP can function at wavelengths up to approximately 2000 nm. The wavelength can be adjusted by varying the thicknesses of the reflecting layers.
Solid-State Lasers
Solid-state lasers include the ruby laser, the crystal laser, and various lasers using a solid combination of yttrium, aluminum, and garnet (called YAG). A solid-state laser is optically pumped by a flash tube that surrounds the lasing medium.
Ruby Laser
The lasing medium in a ruby laser comprises aluminum oxide with a trace of chromium added. The ruby crystal is a cylindrical piece of this material, which appears as a reddish solid with reflective surfaces at each end. One reflector is totally silvered to reflect all of the incident light. The other is partially silvered so that it reflects about 95 percent of the incident light. The laser beam emerges from the partially silvered end.
Figure 33-12 is a simplified diagram of a ruby laser. The flash tube emits brilliant, brief pulses of visible light that cause energy transitions in the electrons of the ruby crystal. Coherent emission occurs in pulses at a wavelength of approximately 694 nm, at the extreme red end of the visible spectrum.
33-12 Simplified functional diagram of an optically pumped ruby laser.
With 1 W of power input to the flash tube, a typical ruby laser produces a few milliwatts of coherent light output. The rest of the input power dissipates as heat or radiates as stray light from the flash tube. The output pulses have duration on the order of 0.5 to 1.0 ms. In some arrangements, a small optical ruby laser pumps a large one. The large laser acts as an amplifier to obtain peak power output far greater than a single device can produce.
Crystal Laser
A solid-state laser having multiple compounds in the lasing medium is called a mixed crystal laser. A mixture of gallium arsenide and gallium antimonide, termed gallium-arsenide-antimonide (GaAsSb), constitutes the cavity material in some such devices. Other materials used in mixed crystal lasers include gallium-arsenide-phosphide (GaAsP) and aluminum-gallium-arsenide (AlGaAs).
Engineers “grow” solid-state lasing crystals in a liquid solution, following a process similar to that used to “grow” conventional semiconductor diode and transistor crystals. The semiconductor materials are doped (mixed with trace amounts of impurities) in various concentrations to obtain N and P properties. The emission wavelength can be adjusted by varying the relative concentrations of the substances in the cavity medium. In a GaAsSb laser, the wavelength can be tuned continuously from roughly 900 to 1200 nm in the near-IR range.
An insulating crystal laser consists of a slightly doped sample of a rare-earth element. The resulting laser emission usually occurs at near-IR or visible red wavelengths. The insulating crystal laser is designed to operate at a cool temperature, so the device needs an elaborate heat-dissipation system. Overheating will result in deterioration of the lasing properties of the crystal. Wavelength adjustment is difficult, so the insulating crystal laser is generally used at only a single frequency.
Neodymium Laser
The neodymium-yttrium-aluminum-garnet (or neodymium-YAG) laser is a specialized solid-state laser that combines liquid and solid materials. The elemental neodymium rests in a liquid solution and remains confined within a solid YAG crystal. Energy is delivered into the laser medium by arc lamps or other bright sources of light focused on the crystal.
The wavelength of the neodymium-YAG laser is about 1065 nm in the near-IR portion of the spectrum. The efficiency is on the order of 1 percent. High-power neodymium-YAG lasers, like insulating crystal lasers, require a method of cooling to prevent damage to the crystal. The neodymium-YAG laser can operate in the pulsed mode or in the continuous mode.
Alternatively, a neodymium-based laser can take the form of an insulating crystal device. Glass can be doped with neodymium to obtain laser operation with intense optical pumping. When maintained at 20°C or lower, the efficiency of a neodymium-glass laser is about 3 percent when glass of high optical quality is used. The peak output power can range up to several hundred gigawatts. The device operates at an extremely low duty cycle; the average output power is a minuscule fraction of the peak output power.
Other Noteworthy Lasers
Some of the earliest lasers employed gas and liquid media, and their basic designs have not changed for decades. A few examples follow.
Helium-Neon Laser
Small lasers using gas-filled tubes containing helium and neon are available through scientific hobby companies. The helium-neon (He-Ne) laser emits vivid red light. The gas is excited by an electric current, typically producing emissions at 633 nm. Some designs produce IR output at 1150 nm or 3390 nm. The output power typically ranges from 10 mW to 100 mW, and the efficiency is approximately 5 percent at the visible red wavelength.
The lasing wavelength depends on the energy transitions shared by the two gases. Helium and neon undergo identical transitions at some wavelengths when their concentration levels have a certain value and occur in the correct proportion with respect to each other. Energy transitions in the electron shells of the gases occur because the applied current ionizes the gases, producing free electrons that frequently collide. If we force high-frequency AC through the ionized gas, the output becomes continuous, offering advantages for optical communications because a continuous laser is easy to modulate with digital or analog data. A continuous-mode He-Ne laser can be modulated by many signals at the same time, each signal having a different frequency.
Figure 33-13 illustrates, in simplified form, a He-Ne laser excited by RF energy. Some He-Ne lasers employ flat mirrors at each end of the resonant tube, and other designs (such as the one shown here) use concave mirrors and a tube with angled ends. The mirrors are of the first-surface type, meaning that the silvered surface is on the inside of the mirror glass, facing the interior of the gas-containing tube.
33-13 Simplified functional diagram of an RF-pumped He-Ne laser.
Nitrogen-Carbon-Dioxide-Helium Laser
A mixture of nitrogen, carbon dioxide, and helium can be used to make a laser that operates at a wavelength of about 1060 nm in the IR spectrum. Energy at this wavelength propagates with exceptionally low loss over long distances through clear air. The nitrogen-carbon-dioxide-helium (N-CO2-He) laser can generate several kilowatts of continuous output power with pulse peaks in the gigawatt range. The gases can be excited by DC, as shown in Fig. 33-14.
33-14 Simplified functional diagram of a gas laser excited by DC from an external voltage source.
Argon-Ion Laser
The argon-ion laser is a gas laser operating at about 480 nm, producing blue visible light. The ionized argon is kept at low pressure. The ionization and energy input are produced by passing an electric current through the gas. The power output is moderate but the efficiency is low, mandating the use of a cooling system. The pictorial representation of the argon-ion laser is essentially the same as that for the N-CO2-He laser (Fig. 33-14).
Miscellaneous Gas Lasers
Many other gases, such as mercury vapor, can generate laser emissions by means of DC electric discharge. Hydrogen and xenon produce UV laser energy. Some gases, such as oxygen and chlorine, emit visible coherent light. Atomic collisions, caused by application of energy in various forms, can result in the emission of laser energy in the IR, visible-light, and UV regions of the spectrum.
Liquid Lasers
A laser cavity can be filled with a liquefied element or compound, forming a colored dye. A typical liquid laser can produce energy in the near-IR, visible light, or UV range. An external source of visible light pumps the device. Some liquid lasers produce continuous output; others produce pulsed output. In the continuous mode, the pumping source is usually a gas laser or solid-state laser. In the pulsed mode, pumping can be done by a source of incoherent light, such as a flash tube.
With certain types of liquid lasers, we can vary the emission wavelength over a continuous range between certain well-defined limits. Such a device, called a tunable laser, operates at a wavelength determined by a chemical dye dissolved in the liquid. Most liquid lasers must have effective cooling systems. Otherwise, overheated liquid can damage or destroy the cavity enclosure.
Quiz
Refer to the text in this chapter if necessary. A good score is 18 correct. Answers are in the back of the book.
1. In a pulsed laser, the duty cycle correlates with the ratio of
(a) average to peak output power.
(b) highest to lowest frequency.
(c) longest to shortest wavelength.
(d) minimum to maximum brilliance.
2. If you shine the beam from an argon-ion laser at a white wall, you’ll see
(a) a blue dot.
(b) a green dot.
(c) a red dot.
(d) nothing because the output is in the UV range.
3. If you reverse-bias a GaAs laser diode beyond the avalanche voltage,
(a) current flows through the P-N junction, and it emits coherent radiation.
(b) current flows through the P-N junction, and it emits incoherent radiation.
(c) current flows through the P-N junction, but it does not radiate anything.
(d) no current flows through the P-N junction, and it does not radiate anything.
4. Assuming all other factors constant, the wavelength of a pulsed laser depends on the
(a) duty cycle.
(b) peak pulse amplitude.
(d) None of the above
5. If you shine the beam from a xenon gas laser at a white wall, you’ll see
(a) a blue dot.
(b) a green dot.
(c) a red dot.
(d) nothing because the output is in the UV range.
6. When an electron moves from a certain shell in an atom to another shell having a larger radius, the electron
(a) gains energy.
(b) loses energy.
(c) emits a photon.
(d) attains a lower frequency.
7. Suppose that the peak output power from a neodymium-glass laser is 500 GW. If the average output power is 50 W and the output occurs as rectangular pulses, what’s the duty cycle?
(a) 10−12
(b) 10−11
(c) 10−10
(d) 10−9
8. If the laser described in Question 7 has an average input power of 1.0 kW, what’s its efficiency?
(a) 1 percent
(b) 5 percent
(c) 10 percent
(d) 20 percent
9. The light from a laser in the visible range has
(a) varying hue with photons in phase coincidence.
(b) varying hue with photons in random phase.
(c) vivid hue with photons in phase coincidence.
(d) vivid hue with photons in random phase.
10. In a VCSEL, the energy beam
(a) comes off at a right angle to the plane of the P-N junction.
(b) is more brilliant than the beam from any other type of laser.
(c) has longer wavelength than the beam from any other type of laser.
(d) All of the above
11. A laser produces rectangular pulses of constant amplitude. The duty cycle is 5.00 percent and the average output power is 25.0 W. What’s the peak output power?
(a) 100 W
(b) 250 W
(c) 500 W
(d) More information is needed to answer this question.
12. What’s the efficiency of the laser described in Question 11 if the average input power is 40.0 W?
(a) 50.0 percent
(b) 62.5 percent
(c) 66.7 percent
(d) 75.0 percent
13. You connect a power source that provides 6.600 kV DC to a gas-discharge tube, ionizing the gas and causing 3.300 mA to flow. How much power does the ionized gas dissipate?
(a) 2.200 W
(b) 21.78 W
(c) 220.0 W
(d) 217.8 W
14. What type of laser produces more output power than a conventional semiconductor laser as a result of a “chain reaction” of energy transitions?
(a) GaAsSb
(b) Neodymium YAG
(c) Ruby
(d) Quantum cascade
15. What type of laser emits pulsed visible light at about 694 nm?
(a) GaAsSb
(b) Neodymium YAG
(c) Ruby
(d) Quantum cascade
16. If you reverse-bias a laser diode below the avalanche voltage,
(a) current flows through the P-N junction, and it emits coherent radiation.
(b) current flows through the P-N junction, and it emits incoherent radiation.
(c) current flows through the P-N junction, but it does not radiate anything.
(d) no current flows through the P-N junction, and it does not radiate anything.
17. A GaAs LED can be modulated or switched rapidly because it has
(a) high carrier mobility.
(b) long-wavelength emissions.
(c) high avalanche thresholds.
(d) high forward-breakover thresholds.
18. You modify the pulsed laser described all the way back in Question 11 so the duty cycle doubles to 10.0 percent, but you take pains to keep the average input and output power the same. What’s the peak output power of the modified laser?
(a) 100 W
(b) 250 W
(c) 500 W
(d) More information is needed to answer this question.
19. What happens to the efficiency of the laser described in Question 11 if you modify it as described in Question 18?
(a) It becomes ¼ as great.
(b) It becomes half as great.
(c) It stays the same.
(d) It doubles.
20. You connect an IR laser diode in series with a current-limiting resistor and a source of variable DC voltage to reverse-bias the P-N junction. Initially, you set the voltage far below the avalanche threshold. Then you gradually increase the reverse voltage until it reaches and passes the avalanche point. How does the P-N junction behave during this process?
(a) At first, the P-N junction does not conduct, and the device radiates nothing. When the avalanche threshold is reached and passed, current flows and the diode emits coherent IR.
(b) At first, the P-N junction does not conduct, and the device radiates nothing. When the avalanche threshold is reached and passed, current flows and the diode emits incoherent IR. However, no laser diode can emit coherent radiation when reverse-biased. If you want lasing to occur, you must forward-bias the junction considerably beyond the forward-breakover threshold.
(c) At first, the P-N junction does not conduct, and the device radiates nothing. When the avalanche threshold is reached and passed, current flows but the diode still radiates nothing. If you want radiation to occur at all, you must forward-bias the junction at or beyond the forward-breakover threshold.
(d) At first, the P-N junction does not conduct, and the device radiates nothing. When the avalanche threshold is reached and passed, current flows and the diode emits incoherent IR. When the voltage rises further and reaches a certain point, the radiation becomes coherent.