This version reflects the comments of the core participants as reviewed and incorporated in accordance with CORD's FIPSE-supported Curriculum Morphing Project.

MODULE 1-11
LASER CLASSIFICATIONS AND CHARACTERISTICS

(1) The use of lasers in practical applications for industry and re-search is increasing at a rapid rate. Lasers are being used in such diverse applications as the cutting of cloth in the garment industry, the scribing of silicon wafers and micromachining processes in semiconductor manufacturing, eye surgery, pipeline alignment, nondestructive testing of automobile tires, and communications systems of many types. Additional uses include optical data storage and readout, production and inspection of small parts, and the interferometric testing of turbine blades. A system that is suitable for a given application can be chosen on the basis of the characteristics of that laser system relative to the specific requirements of the job at hand.

(2) The intent of this module is to examine the output parameters and characteristics of typical commercially available laser systems.

Upon completion of this module, the student should be able to:

1. With the aid of a wavelength scale, label the principal lasing wavelength(s) for the following types of lasers:

   a. Helium­neon.
   b. Argon.
   c. Helium­cadmium.
   d. Carbon monoxide.
   e. Carbon dioxide.
   f. Ruby.
   g. Nd:YAG.
   h. Gallium arsenide.
   i. Rhodamine 6G dye.

2. Draw and label a diagram that illustrates the general time duration of laser output for the following modes of operation:

   a. Mode-locked.
   b. Q-switched.
   c. Repetitively Q-switched.
   d. Normal pulsed.
   e. Continuous-wave (CW) mode.

3. Draw and label a diagram that demonstrates the range of laser output power for the modes of operation in the above objective.
4. Define the term "operating efficiency" as it applies to the laser. On the efficiency diagram provided, label the approximate operating efficiency of the following lasers:

   a. Helium-neon.
   b. Argon.
   c. Carbon dioxide.
   d. Ruby.
   e. Nd:YAG.
   f. Nd:glass.
   g. Gallium arsenide.
   

5. List four types of gases that can be used as the active media of lasers.
6. State the type of lamp used for optical pumping in the following types of lasers:

   a. Ruby.
   b. High-power CW Nd:YAG.
   c. Low-power CW Nd:YAG.

7. State three applications of semiconductor lasers.
8. List the pumping sources that can be utilized for CW and pulsed dye lasers.
9. State the wavelength range spanned by chemical lasers.
10. 10. Draw and label diagrams of the following lasers:

    a. Argon ion.
    b. CW CO₂.
    c. TEA CO₂.
    d. Ruby.
    e. CW pumped AO Q-switched Nd:YAG.
    f. Nd:glass oscillator/amplifier system.
    g. Single-diode GaAs laser.
    h. CW dye laser.
    i. Nitrogen laser-pumped dye laser.
    j. Electrically pulsed chemical laser.
    k. Gas dynamic laser.

11. Take a field trip to a laser manufacturing facility or to an industry or institution at which lasers are operated. Take notes on the types of lasers fabricated and/or operated, on details of their construction, on their output parameters, and on intended applications. Make drawings of the lasers and laser systems, and obtain manufacturer's literature and data sheets on their physical and optical characteristics, as well as any applications notes available. Write a report on the types of laser observed on your field trip, including a description of laser safety procedures and equipment used. Specifications of the report shall be left to the instructor.

CLASSIFICATION OF LASERS BY OUTPUT PARAMETERS

(3) A few of the many different kinds of lasers that have been developed were introduced briefly in Module 1-1; however, only one specific example, the HeNe gas laser, has beer studied in detail. This section will use the parameters discussed in Modules I-9 and 1-10 to describe the output of some commercially-available lasers. The choice of a laser system for a particular application is strongly dependent upon such parameters as wavelength pulse width, energy per pulse, pulse repetition frequency, and beam divergence.

LASER WAVELENGTH

(4) The output wavelength of a laser is determined by the properties of its active medium and, to a certain extent, by how that active medium is excited. The output wavelengths of some prominent types of lasers are listed in Figure 1. Altogether, several thousand lasing lines have been reported thus far; new lines appear frequently in optics and laser journals. The range of losing wavelengths is bracketed between the ultraviolet region at the shorter end and the microwave region at the longer end of the E­M spectrum. A great deal of current research in new lasing wavelength regions is directed toward extension into the "extreme" regions, the far ultraviolet (UV), and the far infrared (IR). Significant interest was generated when lasing was observed in ArC1 in the far UV at 175 nm. At the far infrared end of the spectrum, losing has been reported in a number of molecular gases such as water vapor (N₂O) and hydrogen cyanide (HCN). The wavelengths radiated by far infrared lasers are typically on the order of a few tenths of a millimeter. This submillimeter wavelength region is of great interest to environmentalists for use in the monitoring of air pollution, and to spectroscopists for basic chemical reaction studies.

Fig. 1 Operating wavelengths for typical lasers

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OUTPUT DURATION

(5) Time duration of the various laser outputs is controlled by the mode of operation, as shown in Fiqure 2. Mode-locked lasers emit a series of ultrashort pulses, each of which usually is a few to several hundred picoseconds in length. A typical Q-switched pulse is about a few to 50 nsec in duration, and repetitively Q-switched lasers have individual pulses from tens of uses to a few microseconds in length. Lasers that operate in the normal (long) pulse mode usually deliver a train of output pulses from a few hundred microseconds to a few milliseconds in duration. Finally, CW lasers emit a continuous beam in time, although there may be small fluctuations in the output (approximately 1-2% of the output power or less).

Fig. 2 Time duration of laser outputs

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OUTPUT POWER

(6) Different types of lasers will produce different power output levels. The output power range available for various modes of laser operation is displayed in Figure 3. Power levels shown for pulsed modes refer to the peak power output per pulse.

Fig. 3 Output power for various
modes oflaser operation

(7) The output power of a laser operating in the continuous-wave or CW mode varies from a fraction of a milliwatt (10­3 W) to several hundreds of kilowatts (lOs W). Pulsed normal mode laser outputs are generally a series of pulses (or spikes) with peak outputs in the range of 10³ to 10⁵ watts. Lasers that are repetitively Q-switched produce output pulses of about 1-5O kilowatts (kN). The peak output power of a single Q-switched pulse (typica1ly 10⁶-10⁹ watts) may be increased to typically 10⁹-10¹² watts with one or more laser amplifier stages, as discussed in a later section of this module. Mode-locked lasers have a wide range of peak outputs, from about 10⁴ to 10¹⁴ watts; the upper limit, again, is obtained by the use of laser amplifiers.

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OPERATING EFFICIENCY

(8) An additional comparison of lasers can be made in terms of how efficiently typical lasers operate. The operating efficiency of a laser may be defined as its "optical output power, P_laser, divided by its electrical input power, P_in," as in Equations 1 and 2.

Equation 1

Operating efficiency = P_laser/P_in

or

Equation 2

% Efficiency = P_laser/P_in(100%)

(9) Example A demonstrates the use of these two equations.

EXAMPLE A: CALCULATION OF OPERATING EFFICIENCY
Given:	The operating plasma tube voltage of a 1-mW HeNe laser is 1500 V at 5 mA.
Find:	The operating efficiency and % efficiency of the HeNe laser.
Solution:

(10) Figure 4 demonstrates that, in general, lasers are fairly inefficient power converters. Usually, only a small portion of the electrical power delivered to lasers is converted into coherent light output. Carbon dioxide and semiconductor lasers are among the most efficient laser systems, with the HeNe being among the least efficient type of laser in common use today.

Fig. 4 Efficiency of typical lasers

(11) Great effort is being expended to achieve a better coupling of energy from the excitation efficiencies. Much research has been performed on both flashlamp and optical-cavity design of optically pumped solid lasers to deliver as much useful energy as possible to the active laser crystal.

(12) Cooling systems significantly affect the overall performance of lasers, as well; for example, in a CW-pumped Nd:YAG system, a change in the water flow rate around the outside of the cavity not only can cause a change in power output, but also can produce a shift in the transverse mode structure observed (from multimode to TEM₀₀ mode).

(13) Table 1 lists the output characteristics of several laser systems commercially available. The remainder of the discussion describes the operation of these lasers, as well as other systems currently used in scientific research.

GAS LASERS

(14) Gas lasers may be grouped into four categories according to the type of gas used as an active medium:

• Neutral-atom gas lasers employ electrically-neutral gas atoms as the active medium. The HeNe laser is the most important neutral-atom gas laser.
• Ion lasers contain ionized gas molecules as their active medium. The most important of this group are the argon and krypton gas lasers. Some lasers, such as helium-cadmium (HeCd), contain metal ions in a gas.
• The active media of molecular lasers consist of gas molecules. CO₂ is, by far, the most common molecular laser, but several other molecular gases are employed as well, such as CO, HE, OF, etc.
• Each molecule of the active medium of an exciter laser is composed of an inert gas atom and a halogen gas atom. Among others, these include krypton fluoride (KrF) and xenon fluoride (XeF).

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HELIUM-NEON LASERS

(15) Commercially available HeNe lasers operating at the 632.8-nm wavelength can be obtained with CW outputs ranging from 0.5 mW to 50 mW, with higherpower systems often being used in holographic work. Some HeNe lasers have interchangeable sets of mirrors for operation at 1.15 mm and 3.39 mm. HeNe laser systems have the following characteristics:

• HeNe beams have a low divergence.
• HeNe beams have a high temporal/spatial coherence.
• HeNe lasers can utilize a built-in modulator.
• The HeNe plasma tube has a long lifetime.
• Construction of a HeNe laser is rugged and can withstand hostile environmental conditions (temperature, humidity, mechanical shock).
• HeNe lasers are relatively inexpensive.

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ARGON AND KRYPTON LASERS

(16) Ion-gas lasers are high-current (20-30 A), low-voltage (200-300 V) devices that employ an ionized gas (arc plasma) as the active medium. Argon, with a number of 1asing lines in the blue-green portion of the EM spectrum, and krypton, with lines spanning virtually the entire visible spectrum, are the two most important ion-gas lasers. Only a few of the many losing lines in singly-ionized argon and krypton are shown in Figure 1. The two strong lines in Ar⁺, at 488.0 nm and 514.5 nm, should be noted. Laser systems with active media composed of argon, krypton, or a mixture of these two gases are obtainable with a CW output power of several watts, low beam divergence, and good coherence properties. Pulsed ion lasers also are available, with pulse energies in the range of 100 to 200 mJ at repetition rates of 1 to 120 pulses/second (pps).

(17) Ion lasers employ expensive plasma tubes constructed of graphite or beryllium oxide (BeO). Normally, a solenoid is placed around the tube (Figure 5). The magnetic field generated by the solenoid "squeezes" the plasma in order to increase the current density (current per unit area) in the active medium, providing for more efficient excitation. The large amount of current passing through the tube necessitates some type of cooling system. Normally, either water or forced airflow provides maintenance of stable operation temperatures.

Fig. 5 Typical ion gas laser

(18) The ballast tank provides a source of gas (connected to the plasma tube by a valve) to maintain the tube at optimum operating pressure. Argon/krypton systems usually are equipped with a device known as a "prism wavelength selector," which is placed between the high-reflectance mirror and the plasma tube. The wavelength selector makes cavity losses high, except at one wavelength, by the tilting of a prism at an angle with respect to the axis of the plasma tube. The laser, therefore, can be "tuned" to oscillate on only one of the losing lines available with the gas or gas mixture used. Argon and krypton ion lasers are valuable in many applications, such as holography and spectroscopy, in which a tunable source of coherent light is required.

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HELIUM-CADMIUM LASERS

(19) The helium-cadmium (HeCd) laser is one of a number of ion-gas lasers that utilize an ionized metal vapor (cadmium in this case) as the active medium. HeCd laser systems have a typical CW output of 10-20 mW at 441.6 nm (in the blue) and 2-3 mW at 325 nm (in the near ultraviolet). Again, output wavelengths may be selected by changing dielectric-coated mirrors. The UV HeCd line is useful in the "erasing" function of experimental optical devices utilized for data storage and manipulation.

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CARBON DIOXIDE LASERS

(20) Although the number of molecular gas lasers being marketed is increasing, only CO₂ laser systems are finding fairly widespread application in industry and research. These systems are most useful in certain materials processing applications such as hole drilling, seam welding, paper perforation, and cloth cutting. CO₂ lasers are used in air pollution monitoring and spectroscopy, as well.

(21) Commercial CO₂ systems are available in CW output of 5-10,000 watts, and repetitively pulsed units may deliver up to several kilowatts peak power at high pulse-repetition frequencies (-10³ Hz). Q-switched operation by use of a rotating prism can yield peak power of about 1-100 kW.

(22) CW CO₂ lasers are excited by a high-voltage (5-10 kV), low-current (5-30 mA) electrical discharge. Coherent light output is available at a large number of wavelengths in the infrared, centered around 9.6 mm and 10.6 mm. Carbon dioxide lasers normally contain a mixture of CO₂, N₂, and He gases. The output power obtained is proportional to the volume of gas used (approximately 70 watts/liter). Various ratios of the three constituents of the gas mixture have been employed. A fairly common partial pressure ratio is as follows:

PHE : PN₂ : P_CO2 = 8 : 3 : 1

(23) Nitrogen serves much the same purpose in a CO₂ laser as helium in a neon system. Nitrogen molecules are pumped to a certain excited state (the upper losing level for CO₂) by the electrical discharge. CO₂ molecules then are pumped from the ground state to the upper lasing level by resonant transfer collisions with N₂ molecules. (Refer to Module 1-10). After losing, collisions between He atoms and CO₂ molecules result in the return of CO₂ molecules from the lower losing level to the ground state, where the pumping can be repeated.

(24) Most CO₂ systems are excited by an electrical discharge along the axis of the plasma tube or optical axis; that is, the direction of current flow through the plasma tube is the same as the direction of coherent light output, as in a HeNe or argon laser tube. CO₂ systems are available with either sealed-off tubes or a flowing-gas configuration. With the flowing-gas unit, the higher the rate of gas flow through the tube, the greater the laser output power will be. Figure 6 is a schematic representation of a 250-W CW CO₂ laser system. The plasma tubes in the losing system are double walled and water cooled in order that waste heat will be removed from the system.

Fig. 6 Optical and electrical system of 250-W CW CO₂
laser (overall length of laser head—3 m).

(25) Another type of CO² laser of increasing importance is the so-called "TEA" laser; the letters denote a system "Transversely Excited at Atmospheric pressure." In a TEA CO₂ laser, the electrical discharge direction is perpendicular to the optical axis, as illustrated in Figure 7.

Fig. 7 TEA CO₂ laser

(26) A high-voltage (-50 kV) power supply is used to charge a capacitor. When the switch is closed, the gas inside the plasma tube is excited by a series of short, transverse discharges. Since the power output per unit volume is directly proportional to the gas pressure, a TEA CO₂ laser can deliver pulses whose peak power is much greater than axially excited systems (which operate at lower pressures). TEA lasers are available with a peak output of several megawatts, pulse widths of several hundred nanoseconds, and pulse repetition rates that can be varied from a fraction of a pulse/second (pps) up to several hundreds of pps.

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NITROGEN LASERS

(27) Another molecular gas laser that should be mentioned is the nitrogen (N₂) superradiant system. Operating in the near UV at l = 337.1 nm in a repetitively pulsed mode, nitrogen gas lasers can yield fairly large peak power (< 100 kW) at high pulse-repetition rates (< 100 Hz). The N₂ system is used primarily as an efficient means of optically pumping tunable dye lasers.

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EXCIMER LASERS

(28) Excimer lasers that utilize seven different molecules as active media have been constructed. The molecules consist of an atom of an inert gas and atom of a highly reactive halogen gas. Table 2 lists some exciter lasers and their output wavelengths in nanometers. These lasers produce pulses of from 10 to 40 ns with pulse energies ranging up to 100 J. Commercially available models typically emit a few tens of millijoules per pulse with repetition rates of a few hertz.

TABLE 2. EXCIMER LASERS
(Wavelength in nanometers)

(29) Excimer lasers are excited by a current flow through a gas mixture that contains both the inert gas atoms and the halogen gas atoms. High current densities are achieved by the use of short-duration discharges or electron beams. The excimer molecules are created in an excited state during the discharge. Excimer lasers are of great interest in many areas of research because of their high peak powers (10⁹ W) at ultraviolet wavelengths.

OPTICALLY PUMPED SOLID LASERS

(30) A large number of different kinds of optically pumped solid lasers have been developed, being distinguished from one another by the host material, by the active lasing ions with which the host is doped, and by the output characteristic of the beam. Of these, only ruby, Nd:YAG, and Nd-doped glass-laser systems are of major importance in industrial and laboratory environments. Like CO₂ systems, these crystalline systems are particularly useful for the group of applications known as materials processing, which includes drilling, welding, cutting, and scribing.

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RUBY LASERS

(31) Practical ruby systems are normally operated in pulsed modes, although CW operation has been achieved. The energy per pulse varies from a few millijoules (10^-3 joule) to several tens of joules; and in exceptional cases, several hundred joules per pulse are available. Operation normally is restricted to relatively low pulse repetition rates (a maximum of approximately 3 pulses/second).

(32) As an example, ruby lasers are used to drill holes in diamond dies through which copper is drawn to standard wire sizes. Such systems exhibit output energies of 2-4 joules, pulse widths of about 0.5 msec, and pulse repetition rates of 1 pulse per second (pps). In another application, thin copper wires can be welded with ruby systems having a 100-kW peak output at repetition rates of <1 pps and pulse widths of 0.2-5 msec. Other uses of ruby laser systems include (1) holographic applications with ruby systems having rather long coherence lengths (1-2 meters) for recording holograms for large scenes, and (2) rangefinding operations.

(33) Ruby lasers can be Q-switched with a rotating prism or with various other electro-optic devices. Typical outputs are 100-200 mW peak, pulse widths of 3-30 nsec, and repetition rates of 1-6 ppm. Mode-locked operation is capable of routinely yielding outputs in the gigawatt (10⁹ watts) peak power range with pulse durations of a few picoseconds.

(34) Xenon-filled flashlamps of either linear or helical design are employed in pumping ruby lasers. The great amount of heat generated in these systems requires water and/or forced-air cooling to prevent damage to the laser rod and to other system components by overheating.

(35) In Figure 8, both the laser crystal and the flashlamp are water cooled. The cavity often is silver plated because silver has a high reflectance in the pumping bands of ruby.

Fig. 8 Ruby laser head

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Nd:YAG LASERS

(36) A wide variety of industrial Nd:YAG lasers are manufactured at present. The usefulness and versatility of this type of system is attributable, in part, to the fact that such lasers may be operated in a variety of modes, including the following:

• Continuous wave (CW).
• CW pumped, repetitively Q-switched.
• Pulse pumped, repetitively Q-switched.
• Mode-locked.

(37) Acousto-optically (A-O) Q-switched,* CW-pumped Nd:YAG systems are available with high pulse repetition rates (< 100 kHz) and good control of pulse width (50-700 nsec)--two important factors in metalworking, resistor trimming, and laser scribing. A typical peak power range of 5 kW to 50 kW may be obtained, and an average power for CW YAGs varies from 1 to 200 watts. A typical layout for an A-O Q-switched system is illustrated in Figure 9. The transverse mode selector indicated in the illustration usually is an iris diaphragm or pinhole aperture placed between the laser rod and the output mirror. As mentioned previously (Module 1-9), the presence of such an aperture can force the cavity to oscillate in various transverse modes, in addition to TEM₀₀. This feature is useful in certain materials processing applications.

Fig. 9 CW pumped A-O Q-switched Nd:YAG system

*In an acousto-optical Q-switch, sonic waves are generated in a quartz block attached to a transducer driven by a radio frequency (rf) oscillator. The waves produce changes in the refractive index of the block in such a manner that it behaves like a diffraction grating, deviating the beam out of the cavity.

(38) For the efficient coupling of pump energy into the laser rod, optical cavities of elliptical cross section often are frequently used with the rod and flashlamp (or CW pumping lamp) placed at the foci of the ellipse. A double-elliptical cavity (Figure 10) frequently is utilized in CW pumped, Nd:YAG systems, in which case, pumping is accomplished with two high-power (1-5 kW) krypton arclamps. A major portion of the intense spectral output of the lamp is conveniently in the pumping bands of triplyionized neodymium (Nd3+). CW YAG lasers of lower output power (0.25-5 W) often are pumped by tungsten-filament lamps (discussed in Modules 1-1 and 1-6) filled with a gas such as iodine to retard filament oxidation. Such lamps exhibit typical outputs of 800-1500 watts and are in common use as high-intensity light sources in many areas, for example, in TV studios. Most laser cavity pump reflectors are gold plated to provide a high-reflectance surface that reflects pump light from the cavity walls onto the laser crystal. CW-pumped Nd:YAG laser heads often utilize water cooling to both the rod and the pumping lamp similar to that of the ruby system of Figure 8.

Fig. 10 Double-elliptical cavity

(39) Flashlamp-pumped, repetitively Q-switched Nd:YAG lasers have peak outputs of from 5 MW to 200 MW, pulse durations of 10-300 uses, and repetition rates up to 60 pps. The pulse widths of normal, or free-running, pulsed YAG systems are about 200-800 sec with pulse energies in the range of 0.1-10 joules and pulse repetition rates up to 30 ppm. As in the case of ruby units, mode-locked Nd:YAG lasers may have peak outputs of several gigawatts and a time history that consists of a series of ultrashort pulses, each of several tens of picoseconds duration.

(40) The growing of large Nd3+:doped YAG crystals of sufficient optical quality is a difficult process. Cut and polished rods are available in a number of sizes from approximately 3-mm diam x 25 mm in length to 6-mm diam x 100 mm in length. Since the amount of energy that can be stored in a laser rod is proportional to its size, a restriction is placed on the amount of energy obtainable in a single pulse from a Nd:YAG laser. On the other hand, for high-energy applications, Nd3+:doped glass rods of excellent optical properties can be manufactured in rather large sizes (7.5-cm diam x 100 cm in length).

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Nd: GLASS LASERS

(41) Nd:glass systems capable of high-pulse energy output (> 10³ joules) have been constructed. With a combination of Q-switching and mode-locking, several gigawatts of peak output power is achievable in ultrashort pulses.

(42) Even greater peak powers (> 1 terawatt = 10¹² watts) may be obtained by use of a laser oscillator and several stages of amplification, as illustrated in Figure 11. Considerable effort is expended in obtaining high beam quality in the oscillator and, subsequently, in amplifying the oscillator output in later stages. If the amplifier elements are of high optical homogeneity and pumping uniformity, they can increase the high-quality beam output of the oscillator without additional beam distortion since, unlike the oscillator, amplifiers are single-pass elements.

(43) In Figure 11, the oscillator is a flashlamp-pumped Nd:glass laser. The amplifier stages are lasers less the mirrors. The rod in such an amplifier is pumped to a state of population inversion by the xenon flashlamp. When the beam from the oscillator is incident upon the Nd:glass rod in the first amplifier stage, it stimulates emission of light at 1.06 mm. The incident beam thus increases in intensity upon passing through each amplifier stage.

Fig. 11 Laser oscillator/amplifier

The purpose of the optical isolators is to prevent feedback between the oscillator and amplifier or between the two amplifiers by allowing passage of light in one direction only (from left to right in Figure 11).

(44) Another type of laser oscillator/amplifier system is illustrated in Figure 12. A series of elliptically shaped Nd:glass discs is used for the amplifier stages instead of a cylindrical Nd:glass rod. Each disc usually is set at Brewster's angle for maximum transmission of light polarized in a plane parallel to the plane of incidence.

Fig. 12 Disc amplifier

Disc amplifier such as these are being employed in experiments on controlled fusion designed to demonstrate the feasibility of the production of fusion reactions by focusing an extremely energetic laser pulse onto a nuclear fuel pellet. This disc design also is beneficial in later stages of high-power oscillator/amplifier systems, as the discs are pumped more evenly and easily than solid rods. They can be fabricated in larger diameters, and are less susceptible to certain high-intensity, laser-induced damage processes.

SEMICONDUCTOR DIODE LASERS

(45) Laser diodes, described briefly in Module 1-1, are being used in voice communications, intrusion alarm systems, optical data transmission, and other applications. In ranging, terrain-following laser radar units have been developed; other semiconductor laser systems (celiometers) have been employed in the measurement of cloud altitude and in the detection of fog banks at airports. Injection lasers, whose outputs normally are in the near IR (from about 0.8 m to 0.9 m) are operated in a repetitively-pulsed mode, although in certain instances CW operation is possible when the devices are cooled to cryogenic temperatures. They are small, rugged, portable (i.e., battery operation is possible), inexpensive, can be modulated easily at high pulse repetition rates (1-6 kHz), and can be arranged in diode arrays to provide peak output of several hundred watts or more.

(46) The fabrication of laser diodes is a complicated manufacturing process. Figure 13 displays three basic types of units. Laser diodes consisting of a p-n junction in a single crystal of gallium arsenide (GaAs) are employed in general applications. Several diodes may be stacked atop one another and electrically connected in series, yielding a unit that has higher peak power than single diodes and a relatively narrow beam. Finally, a number of diodes can be arranged in a side-by-side array, also series connected, providing a device that has (1) a higher peak output than either single or stacked diode lasers and (2) a wider, fanshaped radiation pattern. Typical peak outputs and other parameters are listed in Table 3.

Fig. 13 Single laser diodes, stacked diodes, and
linear arrays (after RCA Tech, Publication OPT-100B)

TABLE 3. PEAK OUTPUT OF LASER DIODES

(47) The output wavelength of a laser diode is dependent upon the material from which it is fabricated and upon the types and amounts of dopants used. More importantly, the diode emission wavelength is a highly sensitive function of operating temperature. Single diodes, stacked diodes, and arrays commonly are operated at room temperature; however, cryogenic diode arrays that contain hundreds of diodes and cooled to liquid nitrogen temperature (77K) also are available. These units have not only high peak output (see Table 2) but are useful in applications that require high average power (20-30 watts). The operating efficiency of laser diodes is dependent upon temperature. A typical efficiency figure for units operating at room temperature is about 4I, although that of cryogenic diode arrays may approach 20-40%.

(48) A relatively new semiconductor diode configuration is the so-called "large-optical-cavity" (LOC), single-diode laser. Its special construction results in a lower operating-current density, higher pulse repetition rates (~I MHz), higher operating temperatures (~100°C), and high duty cycles.

ORGANIC DYE LASERS

(49) Organic dye lasers with outputs in the visible and near IR have been operated in both CW and normal pulsed modes. A fairly large number of dyes is available. The dyes are dissolved in alcohol (methanol or ethanol) or water; and often a small amount of liquid detergent is added to enhance the lasing efficiency of the dye.

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CW DYE LASERS

(50) Most CW dye lasers are optically pumped with an argon ion laser at l = 514.5 nm. They yield a significant fraction of input pump power as output (20-30%); for example, a typical dye laser using rhodamine-6G pumped with 5 watts or argon laser power (@ 514.5 nm) has an output of 1 watt or more at l = 590 nm. Frequency-doubled CW Nd:YAG lasers (l = 533 nm) also are used to pump CW dye lasers.

(51) The active medium in a CW dye laser is a jet stream of dye at Brewster's angle to the optical cavity (Figure 14). The argon pump light is focused into the dye stream by a lens or mirror. The optical cavity of the dye laser has a focal point inside the dye stream at the same point as the focused pump light. Thus, the actual lasing volume of the system is a fraction of a cubic millimeter.

Fig. 14 CW dye laser cavity with Brewster angle jet

(52) The output wavelength of the CW dye laser may be tuned with a bire-fringent filter or with a tuning wedge that acts as a variable thickness etalon. These lasers are tunable throughout the entire visible spectrum. Figure 15 graphically shows the output power of a CW dye laser versus output wavelength for several common dye lasers. Over a hundred laser dyes are commercially available.

Fig. 15 Output versus wavelength for a CW dye laser

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PULSED DYE LASERS

(53) Pulsed dye lasers can be divided conveniently into two classes: those pumped by other lasers and those pumped by flashlamps.

(54) Figure 16 illustrates a dye laser pumped by the beam of a pulsed nitrogen laser. The UV output beam of the N₂ laser is focused into the dye cell by a cylindrical lens to excite the entire length of the cell. Such systems produce hundreds of pulses per second with a few millijoules per pulse. Each pulse has a duration of ~0.4-2.0 msec. Frequency-doubled ruby (l = 394 nm) and Nd:YAG lasers also can be used to pump pulsed-dye lasers.

(55) Flashlamp-pumped dye lasers have a configuration similar to the ruby laser of Figure 8, except that a flowing dye cell replaces the ruby rod. The cell may be water cooled and may have Brewster windows. These lasers can be tuned with diffraction gratings, prisms, birefringent filters, or tuning wedges. Etalons may be installed in the cavity for single-mode operation, and the output may be frequency-doubled. One such commercial system can produce 30 pulses/second with several millijoules per pulse (260 mJ at some wavelengths) at any wavelength between 300 nm and 800 nm. The tunability of dye lasers is useful for spectroscopy, biomedical research, and air-pollution monitoring.

CHEMICAL LASERS

(56) A number of lasers in which population inversion is achieved by means of a chemical reaction have been constructed. Chemical lasers are becoming increasingly important as a research tool and probably will be available on the commercial market in the near future. These devices offer several attractive features, including both CW and pulsed operation; fairly large output powers; shorter IR losing wavelengths (3-4 mm), which allow higher power densities when the beam is focused, compared to that obtainable with CO₂ laser emissions (9.6-10.6 mm); and the promise of high operating efficiency.

(57) All chemical lasers have the following four elements in common:

• Gas mixing system.
• Some method of initiating the chemical reaction.
• An optical cavity in which 1asing is initiated and sustained.
• Exhaust or venting system for removal of spent reactant gases from the resonator cavity.

(58) Chemical laser reactions can be initiated by flashlamp excitation (UV photolysis), electrical discharges, heating by arc jets or flames, or by direct chemical processes.

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PULSED CHEMICAL LASERS

(59) Among the most important chemical lasers are systems that utilize hydrogen fluoride (HF) and/or deuterium fluoride (OF) as the active laser molecule. Figure 17 depicts a small, pulsed chemical laser developed at the Hughes Research Labs.

Fig. 17 Electrically pulsed chemical laser

(60) Stimulated by an electrical discharge, the laser delivers a peak output of 35 W (single line), with 1-msec pulse duration at a repetition rate of 60 pps, oscillating at both HF and DF wavelengths. (Refer to Table 4.)

(61) The laser illustrated in Figure 17 has a glass tube of 4-mm bore, about 20 cm in length, with Brewster end windows. The Brewster windows are constructed of calcium fluoride (CaF₂), which is transparent to the infrared losing wavelengths of HF and OF. A diffraction grating on a rotatable mount replaces the high reflectance (HR) mirror in the cavity and acts as the tuning element, as in a dye laser. The output coupler is constructed of germanium (Ge) and is similar to that used in CO₂ lasers. A mixture of hydrogen (H₂), sulfur hexafluoride (SF₆), and a diluent¹ gas is employed. Free fluorine atoms are produced by dissociation of SF₆ via the pulsed electrical discharge. The reaction between hydrogen and fluorine that yields hydrogen fluoride is highly energetic. The use of fluorine as a fuel in chemical lasers poses a serious safety problem, however, since it is a most reactive and corrosive substance. Special precautions must be taken for the protection of personnel, and components of such laser systems must be chosen carefully for their resistance to attack by fluorine and hydrogen fluoride.

¹ Diluents are nonreacting gases added for the purpose of controlling the reaction rate in a chemical laser.

TABLE 4. CHEMICAL LASER REACTION SYSTEMS AND THEIR OUTPUT WAVELENGTHS

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CW CHEMICAL LASERS

(62) Figure 18 is a schematic diagram of a gas dynamic CO₂ laser. Cyanogen (C₂N₂) gas fuel burns to produce CO₂ and N₂ molecules. Additional nitrogen is then added to aid in energy transfer. The hot gas is forced through a quick-freeze nozzle that reduces the pressure and temperature of the gas. The gas then flows through the optical cavity with a velocity of several times the speed of sound and a pressure of approximately 50 tore; it then is exhausted through another nozzle at atmospheric pressure and reduced velocity.

Fig. 18 Gas dynamic laser

1. Using a wavelength scale, label the principal losing wavelength(s) for the following lasers:

   a. Helium­neon.
   b. Argon.
   c. Helium­cadmium.
   d. Carbon monoxide.
   e. Carbon dioxide.
   f. Ruby.
   g. Nd:YAG.
   h. Gallium arsenide.
   i. Rhodamine 6G dye.

2. Define the term "operating efficiency" as it applies to the laser. Draw and label a diagram that illustrates the operating efficiency of the following lasers:

   a. Helium­neon.
   b. Argon.
   c. Carbon dioxide.
   d. Ruby.
   e. Nd:YAG.
   f. Nd:glass.
   g. Gallium arsenide.
   h. Rhodamine 6G dye.

3. Draw and label a diagram that depicts the general time duration of laser outputs for the following modes of operation:

   a. Mode­locked.
   b. Q­switched.
   c. Repetitively Q­switched.
   d. Normal pulsed mode.

4. Draw and label a diagram that gives the range of laser output power for the following modes of operation:

   a. CW.
   b. Pulsed normal mode.
   c. Repetitively Q­switched.
   d. Q­switched.
   e. Mode­locked.

5. Draw and label a diagram of an ion­gas laser.
6. State the CW output power(s) and peak outputs for repetitively-pulsed and Q­switched operation of commercial CO2 systems.
7. Draw and label a diagram of a CW CO₂ laser.
8. What is a CO₂ TEA laser? Include in your explanation a drawing of such a system; illustrate plasma tube, electrode, storage capacitor, and ballast resistors.
9. List peak outputs, pulse widths, and repetition rates obtainable with pulse pumped, Q­switched ruby, Nd:YAG, and Nd:glass lasers and with CW-pumped, repetitively Q switched Nd:YAG systems.
10. Draw and label a diagram of a CW­pumped, acousto-optically Q­switched Nd:YAG laser system. State the purpose of the transverse mode selector.
11. Explain why Nd:glass lasers often are found in high-energy applications (in terms of energy storage capabilities). How does a laser oscillator/amplifier operate? Draw and label a diagram of a Nd:glass laser oscillator/amplifier system. State the purpose of an optical isolator.
12. Draw and label a diagram of a CW laser­pumped organic dye laser. State the type of laser pumping source commonly used and its output wavelength. State a typical figure for the conversion efficiency of optical­pump power to dye­laser output power. State how wavelength tuning is accomplished in a CW dye laser.
13. Draw and label a diagram of a nitrogen laser­pumped pulsed­dye laser.
14. State the wavelength range that can be achieved by flashlamp­pumped dye lasers and the methods of tuning these lasers.
15. Draw and label a diagram of a ruby laser.
16. Draw and label a diagram of a gas dynamic laser, and explain its operation.
17. Draw and label a diagram of a single­diode GaAs laser. State three applications of semiconductor lasers.
18. State the approximate range of losing wavelengths for hydrogen fluoride and deuterium fluoride chemical lasers.
19. What is an exciter? What is the wavelength range spanned by exciter lasers?
20. Draw and label a diagram of an electrically­pumped, pulsed­chemical laser.

Equipment observed during field trip to laser facility
Notebooks and pens or pencils for each student

1. Take a field trip to a laser manufacturing facility or industrial site at which lasers are used.
2. Take notes on the types of lasers fabricated and/or operated, on details of their construction, on their output parameters, on intended applications, and on laboratory safety procedures in use. Make drawings of the lasers and lasers systems, and obtain manufacturers' literature and data sheets on their physical and optical characteristics, as well as any applications notes available.
3. Write a report on the types of lasers observed on your field trip. Specifications shall be left to the instructor.
4. Describe the safety procedures and equipment/materials used in the following areas:

   a. Electrical.
   b. Laser radiation.
   c. Chemical.
   d. General laboratory practice.

Anderson, John D. Gasdynamic Lasers: An Introduction. From the Series Quantum Electronics­­Principles and Applications, edited by Yoh­Han Pao. New York: Academic Press, 1976.

Beesley, M.J. Lasers and Their Applications. New York: Barnes & Noble, Inc., 1971.

Bloom, Arnold. "The Laser Family," in Optical industry and Systems Directory (1972­1973). 19th ed. Pittsfield, MA: The Optical Publishing Co., Inc.

Campbell, Ralph W. and Mims, Forrest M., III. Semiconductor Diode Lasers. Indianapolis, IN Howard W. Sams, Inc., 1972.

Charschan, 5.5., ed. Lasers in Industry. New York: Van Nostrand Reinhold Co., 1972.

Chester, Arthur N. "Chemical Lasers: A Status Report," Laser Focus.November 1971.

Dennis, Jerome E. and Ratoff, Paul. "Applications of Lasers in the Paper industry." Westbury, NY: Hadron.

Duley, W.W. Cob Lasers: Effects and Applications. From the Series Quantum Electronics ­ Principles and Applications, edited by Yoh­Han Pao. New York: Academic Press, 1976.

Eleccion, Marce. "The Family of Lasers: A Survey," IEEE Spectrum, March 1972; "Materials Processing with Lasers," IEEE Spectrum, April 1972.

Goldman, Leon and Rockwell, R. James, Jr. Lasers in Medicine. New York: Gordon and Breach, 1971.

Gross, R.W. and Bott, J.F. Handbook of Chemical Lasers. New York: WileyInterscience, 1976.

Hallmark, Clayton. Lasers, the Light Fantastic. Blue Ridge Summit, PA: TAB Books, 1979.

Kressel, H.; Lockwood, H.F.; and Ettenberg, M. "Progress in Laser Diodes, IEEE Spectrum, May 1973.

Lengyel, Bela A. Lasers, 2nd ed. New York: Wiley Interscience, 1971.

Linford, Gary J. "Zeeman­Tuned Laser Detects Air Pollution," Microwaves, December, 1973.

Lubin, Moshe J. and Fraas, Arthur P. "Fusion by Laser," Scientific American, June, 1971.

The Optical Industry and Systems Directory (1972­73). 19th ed. Pittsfield, MA: The Optical Publishing Co., Inc., 1972­73.

O'Shea, Donald; Callen, W. Russell; and Rhodes, William T. Introduction to Lasers and Their Applications. Reading, MA: Addison­Wesley Publishing Co., 1978.

Pressley, Robert J., ed. CRC Handbook of Lasers, with Selected Data on Optical Technology. Cleveland, OH: The Chemical Rubber Company, 1971.

Ready, John R. Effects of High-Power Laser Radiation. New York Academic Press, 1971.

_______, ed. Lasers in Modern industry. Dearborn, MI: Society of Manufacturing Engineers, 1979.

_______. Laser Systems and Applications. Waco, TX: Engineering Technology, Inc., 1983.

Ross, Monte, ad. Laser Applications, Volume 1. New York: Academic Press, 1971.

"Solid State Infrared Emitting Diodes, Injection Lasers and Silicon Photodetectors," RCA TECH. Publication OPT­1OOB, June 1972.

Verdeyen, Joseph T. Laser Electronics. Englewood Cliffs, NJ: Prentice Hall, Inc., 1981.

Wilson, Leroy E., et al, ads. Electronic Transition Lasers. Cambridge, MA: HIT Press, 1977.

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