ASML 1127
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`1
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`ASML 1127
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`Library of Congress Cotaloglngdn-Publication Data
`Kuhn. Klelin J.
`_
`_
`Laser engineering I Keltn .l. Kuhn
`p.
`cm.
`Includes index.
`ISBN 0-U2-36692!-7 (hardcover)
`I _§_;uLasers—Desrgn and construction. 2. Nonlinear optics.
`I e.
`.
`TAl675.l{B4
`I993
`97-532 I lCIP
`
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`
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`2
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`

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`
`.l___'
`
` [Convedfiiiiliiiii
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`
` 1.-. '. 'U.'.—u.ua a-.--4 n.u- I-—.
`
`he
`
`Lasers
`
`Objectives
`
`To summarize the sequence of historical events leading to the development of the
`modern commerciai conventional solid-state laser.
`
`To summarize the commercial applications of conventional solid-state lasers.
`
`To describe the major laser host materials for conventional solid-state lasers.
`
`To describe the various energy states of the Nd:YAG laser and to summarize how
`these states interact with each other.
`
`To compare and contrast noble gas discharge lamp pumping versus semiconductor
`diode laser pumping for conventional solid-state lasers.
`
`To describe the design and construction of noble gas discharge lamp pumped conven-
`tional solid-state lasers. This includes the details of the noble gas discharge lamps.
`the power supplies, and the pump cavities.
`
`To describe the construction of a modern noble gas discharge lamp pumped Nd:YAG
`laser.
`
`To describe the design and COI‘|Sll1.lCliOn of semiconductor diode laser pumped conven-
`lional solid-state lasers. This includes the details of the semiconductor pump lasers.
`the power supplies. and the pump cavities.
`
`a To describe the construction of a modem semiconductor laser-pumped Nd:YAG laser.
`
`302
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`3
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`Sac. 10.1
`
`Hlstory
`
`1 0.1 HISTORY
`
`303
`
`One application that has driven a great deal of laser development is the idea of a directed
`beam weapon. From Buck Rogers to Captain Janeway, the concept of aiming a beam of
`energy at a target and vaporizing it has been very compelling to the military. In 1960. soon
`after the invention of the ruby laser, the Department of Defense (DOD) spent approximately
`$1.5 million on laser development.
`In 1961 the DOD spent $4 million on laser-related
`research, increasing to $12 million in 1962 and $19 to 24 million in 1963.‘ From 1960
`to roughly 1980. the majority of academic research and much of industrial research was
`funded (directly or indirectly) by the military community.’
`Very early in the development of lasers, it was recognized that noble gas lasers were
`far too inefficient for any possibility of a directed energy weapon. Three-state ruby lasers
`were not much better. However,
`in 1961 Johnson and Nassau demonstrated laser action
`using trivalent neodymium in calcium tungstate.3 This was a significant development. as it
`opened the class of efficient four-state trivalent rare earth laser dopants. Later in 1961. Elias
`Snitzer succeeded in obtaining laser action from trivalent neodymium (Nd) doped in barium
`crown glass.‘ This development showed that laser action could be obtained in a material
`that could be fabricated in large sizes (see Figure 10.1 for a photograph of the very large
`NOVA glass laser).
`In the period following the experiments of Johnson, Nassau. and Snitzer, solid-state
`laser research exploded into a flurry of experiments demonstrating laser action from a wide
`variety of di- and trivalent rare earth dopants in an incredible variety of laser hosts. More
`details on this history can be found in the review papers of the era by Kiss and Pressley5
`and by Young.‘ For a modern list of di- and trivalent rare earth dopants (and their hosts).
`see Weber.’ (Laser dopants and their hosts are discussed in Section 10.3.)
`Hundreds of important research results were obtained during this prolific period. Of
`special interest is the demonstration of laser action in Nd-doped yttrium aluminum gar-
`net (YAG) by Joseph Geusic et al. at Bell Laboratories“ and the demonstration of laser
`action from the trivalent dopants thulium. holmium. ytterbiurn. and erbium in YAG by
`Johnson et al.. also at Bell? Other Nd:YAG milestones are the first demonstration of
`
`‘Joan L. Bromberg. The Laser in America. 1950-1970 (Cambridge. MA: The MIT Press. 1991). p. 102.
`‘Arthur Schalow has said. "I often joked that no matter what you told the press about lasers. it always came
`out as a ‘death ray‘ or a cure for eancer—-or both!" Quoted in Jefi Hecht. Laser Pioneers. revised ed.
`(Boston.
`MA: Academic Press. I992). p. 92.
`3L. F. Johnson and K. Nassau. Prac. IRE (Correspondence) 49:l7D4 (I961); L. F. Johnson. G. D. Boyd.
`K. Nassau. and R. R. Soden. Phys. Rev.
`l26:l4D6 (I962); and L. F. Johnson. G. D. Boyd. K. Nassau. and R. R.
`Soden. Proc. IRE (Correspondence) 50:2l3 (1962).
`‘E. Snitzcr. Phys. Rev. Len‘. 7:444 (1961).
`52. J. Kiss and R. J. Pressley. Proc. JEEE 54:l236 (1966).
`‘C. Gilbert Young. Proc. IEEE 57:l267 (I969).
`7Marvin J. Weber. ed. Handbook of hirer Science and Technology. Vol. 1. Laser: and Maser: (Boca Raton.
`FL: CRC Press. Inc.. 1982): and the more recent supplement: Marvin J. Weber. ed. Handbook of Laser Science
`and Technology. Sup. 1. laser: (Boea Raton. FL: CRC Press. Inc.. I991).
`‘J. E. Geusic. H. M. Marcus. and L. G. Van Uitert. Appl. Phys. Len. 4:182 ( I964).
`9L. F. Johnson. J. E. Geusic. and L. G. Van Uitert. Appl. Phys. Left. 7:127 (1965).
`
`4
`
`

`
`304
`
`Conventional Solid-State Lasers
`
`Chap. 10
`
`Q-switched Nd:YAG operation by Geusic et al.,”’ and the first mode-locking of Nd:YAG
`by DiDomenico et al."
`(Q-switching is discussed in Section 6.2 and mode-locking in
`Section 6.4.)
`Another important pattern began to emerge during the solid-state laser development
`period of the 19605. Unlike gas laser development, solid-state laser development was driven
`by the available materials technology. This pattern was certainly apparent during the early
`days of Nd:YAG laser development. When Geusic had originally identified YAG as a
`possible laser host, there were no high optical quality YAG crystals available. Geusic worked
`with LeGrand G. Van Uitert (of the Chemistry Research Department at Bell Laboratories)
`to develop a Czochralski crystal-growth process specifically for making high optical quality
`Nd:YAG crystals. This growth process was then handed off to Union Carbide. The laser
`developments in Nd:YAG precisely paralleled the continuing improvements in the crystal-
`growth technology."
`During this developmental period. Nd:YAG began to emerge as the leading solid-
`state material for commercial solid-state laser development. YAG is a mechanically robust
`and high optical quality material with good then-rial properties. Neodymium is a four-state
`laser ion and (when doped into YAG) yields a laser system whose pump bands match
`well with standard commercial light sources. The combination of the two led to the in-
`troduction of the lamp-pumped Nd:YAG laser as a standard commercial laser source (see
`Section 10.4).
`Nd:glass was another laser material to achieve prominence during this period. Glass is
`a less robust material than ‘(AG and has significantly poorer thermal conductivity. However,
`the isotropic nature of glass permits higher doping concentrations for the laser ion and glass
`can be manufactured in very large sizes. Thus, Nd:glass became a favorite laser material
`for large laser systems. The crown jewels of these efforts are the huge glass lasers built by
`Lawrence Livermore National Laboratories for inertial confinement fusion (see Figure 10.1).
`These lasers began with Janus (1974; with an output energy on the order of 80 joules), and
`proceeded through Cyclops (I975), Argus (I976). Shiva (1977). and Nova (1984); gaining
`roughly an order of magnitude energy during each development cycle. An overview of
`the LLNL laser program is given by Emmett et al.” and Lawrence Livertnore National
`Laboratory publishes a detailed annual report.
`Another important thread in solid-state laser systems is development of semiconductor
`diode pumping of solid-state lasers (see Section 10.5.5). It was recognized extremely early
`that the light from semiconductor junctions could efficiently pump solid-state lasers. The
`first proposal of this concept was by Newman in 1963, and the experimental demonstra-
`tion consisted of using 880 nm radiation from an LED-like source to excite fluorescence in
`Nd:CaWO4."‘ Ochs and Pankove in 1964 used an array of LEDs in a transverse geometry
`to pump a Dy2+:CaFg laser.” The first semiconductor diode laser pumped solid-state laser
`
`
`‘"1. E. Geusic. M. L. Hansel. and R. G. Smith. Appl. Phys. Lett. 6:175 (1965).
`"M. DiDomenico. I. E. Geusic, H. M. Marcos and R. G. Smith, Appl. Phys. Letr. 8:180 (I966).
`“Juan 1.. Bromberg. The Laser in America. 1950-1970 (Cambridge. MA: The MIT Press. I991). pp. 176-7.
`"J. L. Emmett. William F. Kruplte. and J. 1. Davis. IEEE J. of Quantum Electron on-20.-591 (I984).
`"IL Newman. J’. Appl. Phys. 34:43? (1963).
`'53. A. Ochs and J. t. Pankove. Proc. IEEE 52:71: (I964).
`
`
`
`5
`
`

`
`
`
`305
`
`The NOVA laser hey at
`Figure 10.1
`Lawrence Livermore National Laboratory.
`Note the technician in the center right for
`scale. (Courtesy of LLNL. Livennore. CA)
`
`(GaAs laser diodes transversely pumping a U3‘*':CaF2 laser) was demonstrated in 1964 by
`Keyes and Quist.'°
`However, during the early 1960s. semiconductor diode pumping of lasers was unim-
`pressive compared to lamp pumping. Lamp technology was very well developed (primarily
`driven by photographic applications) and semiconductor laser sources were still in their in-
`fancy. Thus, research into semiconductor diode pumping of solid-state lasers took a distinct
`second place to development of high-power lamp-pumped solid-state lasers.
`This situation began to change in the mid-1960s and early 19705. With higher-power
`laser diodes available commercially, interest was rekindled in using semiconductor diode
`lasers to pump solid-state laser hosts. In 1968. Ross demonstrated the first semiconductor
`diode laser pumped Nd:YAG laser.” In Ross's initial experiment. a single Ga.As laser diode
`was used in a transverse geometry. The Nd:YAG rod was operated at room temperature,
`but the laser diode was cooled to 170K so that the output spectrum of the laser diode would
`better match the Nd:YAG pump bands.
`
`"R. J. Keyes and T. M. Quist. Appl. Phys. ban. 4:50 (1964).
`“M. Ross. Prat‘. IEEE 56:l96 ([968).
`
`6
`
`

`
`306
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`Conventional Solid-State Lasers
`
`Chap. 10
`
`ficient. since much of the pump light passes through the rod and does not contribute to
`laser action.
`In I973, end~pumping was demonstrated by Rosenkrantz” and modeled by
`Chesler and Singh.'9 In end-pumping the diode laser source is colinear with the laser beam.
`End-pumping has the advantage of efficiency. since the majority of pump light is absorbed
`by the laser rod. However,
`it has a disadvantage in that only a limited number of laser
`diodes can be packed around the end of a rod. Ross's demonstration of semiconductor
`
`Significant research in semiconductor diode pumping of solid-state lasers did not ap-
`pear until the mid-l9'i0s. This was quite late in the development cycle for laser technology.
`As a consequence. the maturation of semiconductor diode laser pumping leveraged off many
`
`a Q-switching. mode-locking. and injection seeding are all possible in diode laserpumped
`solid-state lasers. Representative papers include Owyoung et al.,'*'3 Schmitt et al.,”
`and Alcock et al.“
`
`0 Doubling and frequency up-conversion are widely used and representative papers in-
`clude Pollack et al.,25 Fan et al..2° Risk et al..27 and Dixon et al.”
`0 More complex diode pumped geometries can be used in order to meet specific needs.
`Representative papers include Trutna et al..'-""Kane et al..3° and Reed et al.3'
`_.._j——_:_j_j
`
`“See for example. R. L. Byer. Science 239:742 (19885; and T. Y. Fan and R. L. Byer. IEEEJ. ofQuantum
`Eiectron. 24:B95 ( I988}.
`RA. Owyoung. G. R. Hadley. P. Esherick. R. L. Schmitl. and L. A. Rnhn. Opt. Len.
`l0:484 (1935).
`"R. L. Schmitt and L. A. Rahn. Appi. Opt. 21629 (1986).
`3‘t. P. Alcock and A. l. Ferguson. Opt. Comm. 53.417 0986).
`75$. A. Pollack. D. B. Chang. and N. L Moise. J. Appl. Phys.
`
`l60:407? (I986).
`
`
`
`7
`
`

`
`
`
`Sec. 10.2
`
`Applications
`
`307
`
`the
`- Guided wave structures. such as fiber lasers. offer many advantages over
`diode pumping of conventional geometry lasers.
`Fiber lasers are reviewed by
`Digonnet.”
`a A class of materials called sroichiomerrics exist where neodymium is a component of
`the crystal rather than a dopant. These materials can sustain a higher ion concentration
`than Nd:YAG and thus offer more efficient absorption of pump light. Stoichiometric
`materials are reviewed by Danielmeyer et al.” and Huber.“
`
`10.2 APPLICATIONS
`
`The primary commercial example of a conventional solid-state laser is Nd:YAG. Nd:YAG
`lases at 1.064 um and can be frequency doubled (see Chapter 7) to 532 nm. tripled to 355 nm
`and quadrupled to 266 nm. Continuous wave Nd:YAG lasers are available in power levels
`up to several hundred watts and pulsed Nd:YAG lasers are available with pulse energies up
`to a few joules per pulse.
`Nd:YAG lasers can also be operated Q-switched (which produces pulses in the tens
`of nanoseconds) or mode-locked (which produces pulses in the tens to hundreds of picosec-
`onds).
`(See Section 6.2 for more information on Q~switching and Section 6.4 for more
`infonnation on mode-locking.)
`Q-switched Nd:YAG lasers are frequently used for laser machining processes that
`require rapid removal of relatively small quantities of material. Examples include trace
`and link blowing in the electronics industry, laser marking, and laser hole drilling. An
`important market niche is the use of Q-switched and frequency-doubled Nd:YAG lasers
`for laser marking of silicon wafers. A developing market niche is the use of Q-switched
`Nd:YAG lasers for removal of unwanted body hair.”
`Frequency-doubled cw Nd:YAG lasers compete with argon-ion lasers for the mod-
`erate power green laser market. Thus. Nd:YAG lasers compete with argon ion lasers in
`such applications as printing, display technology. stereolithography. and retinal photocoag-
`ulation.
`A developing application for Nd:YAG lasers is laser-enhanced bonding.“ The pro-
`cess uses a laser to drive a polymer adhesive into the material being joined. The process
`provides a replacement for more traditional sewing, taping. gluing. or ultrasonic welding.
`Specific applications for the technology include bookbinding, laminating. textiles. injection
`molding. and carpeting.
`
`
`“Michael J. F. Digonnet. Rare-Earth Doped Fiber Laser: andAmplifier: (New York: Marcel Dekkcr. I993};
`and Selected Papers on Rare-Ermlr Doped FiberLaserSources andAmplifiers, ed M. J. F. Digonnet, SPIE Milestone
`Series Vol. M537, 1992.
`33H. G. Danielmeyer and F. W. Ostermayer. Jr., J. Appl. Phys. 43:29ll (1972).
`"G. Huber. "Miniature Neodymium Lasers." in Current Topics‘ in Materials Science. Vol. 4. ed E. Kaldis
`(Amsterdam: North-Holland. I980). pp. I-40.
`“Laser Focus World January: 62-3 (I996).
`"Laser Focus World August: 32 (I995)-
`
`8
`
`

`
`'
`
`i"
`-
`
`3oa
`
`Conventional Solld-State Lasers
`
`Chap. 10
`
`Additional medical applications for Nd:YAG lasers include bleaching birthmarks,”
`removing tattoos,“ and photothermolysis to remove large-diameter leg veins.”
`Nd:YAG lasers also have a large scientific market. They are commonly used in laser
`radar applications (Lidar). laser spectroscopy, laser spectrophotometry, and laser metrology
`applications (such as seafloor mapping“) that require moderate power in the green and
`rugged design.
`
`10.3 LASER MATERIALS
`
`Two major classes of solid-state laser host materials exist: crystalline solid-state hosts (such
`as Nd:YAG) and isotropic solid-state hosts (principally glass).
`In these solid-state lasers.
`a host material with desirable mechanical and thermal properties (such as YAG) is doped
`with an impurity with desirable laser properties (such as neodymium).
`The structural and laser properties of the material are inter-related since the atomic
`environment around the dopant atom determines the exact nature of the laser transition. As
`an example, neodymium is used as the dopant atom in both Nd:glass and Nd:YAG lasers. In
`Nd:YAG lasers, the atom is caged in a crystalline lattice where the immediate environment
`around each atom is well-ordered and symmetric. The resulting laser transition is relatively
`narrow (approximately 5 angstroms at 300K) with a wavelength of [.064 pm. In Ndzglass
`lasers.
`the atom is in an amorphous structure where the immediate environment around
`each atom is poorly ordered and different for each atom. The resulting laser transition is
`broader than Nd:YAG (approximately 300 angstroms at 300K) with a wavelength ranging
`from [.062 um (silicate glass) to 1.054 um (phosphate glass).
`Many solid-state lasers use trivalent rare earths as the active ions. The rare earths have
`a partially filled 4f shell, and the various observed transitions occur near this shell. All the
`trivalent laser transitions are four-state, and the general transitions are given in Figure 10.2.
`Nd:YAG is a good example of a trivalent rare-earth doped laser material.
`An important aspect of the efficient operation of solid-state lasers is effective transfer
`of the pump energy to the upper laser state. It is possible to co-dope many of the rare earth
`lasers with transition metal ions. This pennits the wide absorption band of the transition
`metal ion to pump the narrower rare earth laser transition. Chromium-doped Nd:GSGG is
`a good example of a co-doped laser material.
`There are a number of good references on laser host materials. Perhaps the most
`comprehensive are the detailed summaries given by Weber." Industry and manufacturer
`
`
`"tam Focus World May: 57 (1995).
`“Laser Focus World August: 62 (1995).
`"’£a:er Focus World August: 6 ([995).
`'“’Larer Focus World November: 53 (1995).
`"Marvin J. Weber. ed. "Solid State Lasers." Handbook ofuuer Science and Technology. Vol. 1. Laser: and
`Marerr. (Boca Raton. FL: CRC Press. lnc.. I982) Marvin J. Weber. ed. pp. 21-295; and the updated version.
`"Solid State Lasers." Handbook ofLarer Science and Technology. Supp.
`l'. Lasers (Boea Raton. FL: CRC Press.
`Int:.. 1991). Pp. 3-216.
`
`
`
`9
`
`

`
`
`
`Sec. 10.3
`30.000
`
`Laser Materials
`
`309
`
`Pr-3+
`
`Nd3+
`
`Eu3+
`
`Ha3+
`
`Er“
`
`1'm3+
`
`vb3+
`
`30’
`_j 3D‘
`3oo
`
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`
`Fa”
`
`4|
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`4|m2
`
`TF2
`
`532
`
`5'6
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`
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`
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`
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`
`3
`
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`
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`5
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`
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`
`10 000
`'
`
`0
`
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`0
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`
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`
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`Figure 10.2 Many solid-state lasers use trivalent rare earths as the active ions. The
`rare earths have 0 partially filled 41' shell. and the various observed transitions occur near
`this shell. All the the trivalent laser transitions are four-level.
`(From Z. J. Kiss and
`R. J. Pressley. "Crystalline Solid Lasers.“ Prat-. IEEE 54:12:36 (I966). figure 2. ©1966
`IEEE.)
`
`references are also extremely valuable.” In addition. some of the more classic references
`such as Kiss and Pressley.“ Nassau.“ Kaminskii.“ and Danielmeyer“ remain very relevant.
`
`10.3.1 Crystalline Laser Hosts
`
`A very large number of crystalline laser hosts have been explored over the years. However.
`as the laser industry has matured. only a few hosts have survived the transition into the
`commercial marketplace. These “workhorse” hosts are the garnets. glasses. sapphire, and
`the lithium fluorides (such as LiSAF).
`The most useful laser materials have traditionally been the garnets. The general for-
`mula for the garnets is (C3A2D30;-1). The C ion is in a relatively large dodecahedrally
`
`“The Photonicr Design ondAppIt‘carion.r Handbook. 1996 Book 3. 42d ed. (Pittsfield. MA: Laurin Publishing
`C0.. 1996).
`"Z. 1. Kiss and R. J. Pressley. Proc. IEEE 54:l23I5 (1966).
`“K. Nassau. "The Chemistry of user Crystals." in Applied Solid State Science. Vol. 2. ed R. Wolfe (New
`York: Academic Press. 1914).
`“A. A. Kaminskii. Laser Crystals (New York: Springer-Verlag. 1980).
`“H. G. Danielrneyer. "Progress in Nd:‘t'AG Lasers." in Lasers. Vol. 4. ed A. K. Levine and A. J. DeMaria.
`(New York: Marcel Dekker. 1976).
`
`10
`
`10
`
`

`
`310
`
`Conventional Solid-State Lasers
`
`Chap. 10
`
`Important laser materials of the garnet structure include
`tetrahedrally coordinated site.
`yttrium aluminum garnet (YAG. Y3Al5O.2). yttrium gallium garnet (YGG. Y3Ga501g).
`gadolinium gallium garnet
`(GGG. Gd3Ga5O.;).
`lanthanum lutetium garnet
`(LLG,
`La3Lu5Ou), yttrium gadolinium garnet (Y3Gd50.2). yttrium scandium gallium garnet
`(YSGG. Y;SczGa30.;). yttrium scandium aluminum garnet (YSAG. Y3SC1Al30]2). and
`gadolinium scandium gallium garnet (GSGG. Gd3Sc;Ga301g).
`To dope garnets. rare earths can be substituted in the C site and metals in the A
`site. Garnets have been doped with the trivalent rare earths Pr”. Nd”. Eu3"'.
`l-lo“.
`Er”, Tm”. and Yb“. Garnets are also co-doped with chromium Cr-""‘ and titanium
`113+.
`
`Sapphire (A1203) can be doped by impurities that replace the aluminum atom. How-
`ever, the aluminum site is relatively small. so transition metals instead of rare earths are used
`as dopants. Examples include ruby (three-state, doped with Cr“) and titanium-sapphire
`(four-state. doped with Ti3"'). Related to sapphire are the beryl crystals such as alexan-
`drite (four-state. BeAl204. doped with Cr3"') and emerald (four-state. BB3A|2Si50|g, doped
`with Cr3*'). Ti:sapphire. alexandrite, and emerald lasers are of special interest because the
`large linewidth permits tunable laser operation. Tunable solid-state lasers are discussed in
`Chapter 5.
`Other popular laser hosts are the cubic fluoride crystals such as CaF2. SrF;. BaF;.
`There is a charge compensation problem in these materials if they are doped with rare
`earths such as Nd“. Thus. the divalent (Nd2"') rare earths are more commonly used as
`dopants for the cubic fluoride crystals.
`‘This charge compensation problem can also be
`addressed by fluoride crystals such as lanthanum fluoride LaF3 and LiYF4 (YLF) or with
`oxide hOSl5 such as Y203, Gd;O3, and EI203.
`
`10.3.2 Glass Laser Hosts
`
`Crystalline solid-state lasers are hard. thermally conductive. and stable at high tempera-
`tures. However. they are also anisotropic. difficult to fabricate in large sizes (the crystals
`must be grown). and possess a maximum limit on the quantity of dopant atoms.
`(In the
`majority of the crystalline solid-state laser materials. the dopant site is restricted in size.
`Thus.
`there is typically a limit to the quantity of doping permitted before the material
`becomes so strained that
`laser operation is impaired.
`This limit is typically 1
`to 3
`atomic 96.)
`
`An alternative approach is to use glass as a host for the dopant atoms. Glass lasers
`are easy (and economical) to fabricate in any size, uniform in material composition. and
`possess high gains due to high limits on doping concentration. However. glass lasers
`have some disadvantages,
`including poor thermal conductivity and greater fragility than
`crystals.
`In addition. glass hosts and crystalline hosts differ significantly in linewidth. Crys-
`talline hosts have linewidths on the order of a few angstroms while glass hosts have
`linewidths on the order of many hundreds of angstroms. Thus. glass lasers are often used as
`tunable lasers or for short pulse generation. while crystalline lasers are used where narrow
`linewidths are required (such as spectroscopy).
`
`
`
`11
`
`11
`
`

`
`
`
`sec. 10.3
`
`Laser Materials
`
`311
`
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`
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`
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`
`Disk amplifier
`
`Zig-zag slab
`
`.
`5 Tuxtzi
`YAG Perallelplped
`
`
`
`
`
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`O O N
`
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`
`
`
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`liiivlvrtvmiaafl
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`
`Active mirror
`
`Dlada laser pumped YAG
`
`Figure 10.3 A variety of laser shapes have been developed to meet specific design
`constraints or to solve specific problems.
`
`Crystalline hosts are visualized as having a highly-ordered lattice interspersed with a
`regular array of ions. Glass hosts are more difficult to visualize, because the mechanism
`inhibiting crystallization is poorly understood. The currently accepted model suggests that
`glass is actually composed of a network of energetically favored clusters that form isolated
`domains within the material. Each cluster is strongly coupled within itself. but weakly
`coupled to other clusters. This creates a myriad of different spectroscopic properties for
`the glasses.
`
`10.3.3 The Shape of the Solld-State Laser Material
`
`A rod geometry is the most common shape for a solid-state laser. However, it is not the
`only shape. A variety of other shapes have been used to meet specific design constraints or
`to solve specific problems (see Figure 10.3).
`In very high-energy lasers (such as Shiva and Nova) the diameter of the beam
`is extremely large. The large diameter means that spurious modes (amplified spontaneous
`emission and confined spontaneous emission) are a major problem. In lasers such as these.
`the laser material is fabricated in the form of large disks where the disk may be 2 to 3 feet
`in diameter. but only three to four inches thick.
`‘
`In Nd:YAG and Nd:glass high-average power rod lasers,
`the heat distribution in
`the rod is nonunifonn. This results in thennal-focusing effects that eventually limit the
`maximum average power. Ifthe laser material is fabricated as a zig—zag slab, much of this
`focusing can be eliminated because each bounce of the slab compensates for the focusing
`of the previous bounce.
`In semiconductor diode array pumped lasers. it is often advantageous to fabricate the
`material in the form of a rectangular block. This increases the efficiency of the coupling
`from the semiconductor laser array to the laser material.
`
`12
`
`.......-
`
`12
`
`

`
`312
`
`Conventional Solid-State Lasers
`
`Chap. 10
`
`10.4 THE LASER TRANSITION IN Nd:YAG
`
`Nd:YAG is probably the most widely used solid-state laser material. More details on
`Nd:YAG as used in lasers can be found in Koechner.“ Danielmeyer.“ and from various
`manufacturers.“
`'
`YAG (yttrium aluminum garnet) is a clear. hard 53m materials" whose most familiar
`application is as a diamond replacement in jewelry. YAG is optically isotropic. of good
`optical quality. and quite thermally conductive (0.14 Wlcm-K). YAG is also nonhygroscopic.
`melts at 1970C. and has a Knoop hardness of 1215 (around 8.2 on the Moh‘s hardness scale).
`which makes it one of the most durable of the common laser crystals.
`Doping YAG with neodymium results in a blue to violet crystal. Because the material
`becomes strained at atomic percentages greater than approximately 1.5%. the majority of
`Nd:YAG crystals are doped at approximately 1%.
`It is generally accepted that higher
`doped material (1 to 1.4%) is better for Q-switched laser performance (due to the higher
`energy storage) while lower doped material (0.5 to 0.8%) is better for cw (steady~state) laser
`performance where optical beam quality is important.
`By far the most popular dopant for YAG is neodymium. The neodymium ion in YAG
`forms a characteristic band structure (see Figure 10.4).
`The laser pump bands for cw operation are principally the ‘ Fm. the ‘F‘5,:2. the ‘ Fm.
`the ‘Hm. and the ‘'33,; bands. The ground state is the ‘Igfl band. Titus. the pump bands
`for cw operation fonn a manifold centered around 7500 angstroms and 8100 angstroms
`(see Figure I05). (Notice that for pulsed operation. the higher-energy pump bands become
`important. Therefore.
`the pump bands for pulsed operation also include the manifolds
`centered at 5300 angstroms and 5800 angstroms.)
`The primary laser transition in Nd:YAG at room temperature (the transition with the
`lowest threshold energy) is the ‘Fm band to the 4111;; band.
`(In Nd:YAG, crystal-field
`splitting effects divide the principle hands into J + 1/2 doubly degenerate bands. The
`major Nd:YAG 1.064 um laser transition originates at the R; crystal field split compo-
`nent of the ‘Fm band and terminates on the Y3 crystal field split component of the ‘Inn
`band.)
`At room temperature. approximately 40% of the ‘Fm band is in the R2 configuration
`and approximately 60% in R; (from the Boltzrnan's thermal distribution). As the laser lases,
`the R; population is refilled from the R. state.
`
`
`"Walter Koechner. Solid State Laser Engineering. 4th ed. (Berlin: Springer-Verlag. I996).
`"H. G. Danielmeyer. "Progress in Nd:YAG Lasers." in Lasers. Vol. 4. ed A. K. Levine and A. J. DeMaria
`(New York: Marcel Dekker. I976).
`“Litton-Ainron and Union Carbide are two major manufacturers of Nd:YAG laser crystals.
`
`“Crystals are divided into seven major crystal systems defined by their symmetry properties. These classes
`are triclinic. monoclinic. orthorhombic. tetragonal. cubic. trigonal. and hexagonal. Each of these classes is further
`divided into point groups characterized by a number such u 33m or 6. There are two common numbering systems
`for point groups.
`tl1e Hermann-Maugin system (used more commonly by vendors) and the Schoenflies system.
`Boyd. Nonlinear Optics (San Diego: Academic Press. I992) gives a nice overview of point groups in Chapter I
`from a tensor viewpoint (Table l.5.l is especially infonnative). William Leonard and Thomas Martin. in Electronic
`Structure and Transport Properties afMalerr'al.r (Malabar. FL: Robert Krieger. I981). provide a good backround
`in Chapter 5 on crystallographic notation.
`
`
`
`13
`
`13
`
`

`
`
`
`Sec. 10.4
`
`The Laser Transition In Nd:YAG
`
`313
`
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`311
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`132
`0
`
`1111 300K
`
`11:) 77K
`
`Figure NM The energy band diagram for 111: Nd” i
`momcs 21:. by VERDEYEN. 1.1. @1939. Figure
`sion of P1-antic:-Hall. Inc.. Upper Saddle River. NJ)
`
`on in YAG. From LASER ELEC-
`l0.5. p. 317. (Adapted by permis-
`
`14
`
`14
`
`

`
`314
`
`Conventional Solid-State Lasers
`
`Chap. 10
`
`tcm")
`
`Absorptioncoefficient
`
`Wavelength (nm)
`
`Figure 10.5 The pump bands for cw operation form a manifold centered around 7500
`angstroms and 8100 angstroms. The pump bands for pulsed operation also include
`the manifolds centered at 5300 angstroms and 5800 angstroms.
`(From The Pharanics
`Design and Applications Handbook. 1996. Book 3 of the Pltaranics Directory. 42nd
`ed.
`(Pittsfield. MA: Laurin Publishing Co.. Inc.. 1996). p. H-322. Reprinted with the
`permission of Laurin Publishing Co.. Inc.)
`
`The fluorescence efficiency of Nd:YAG is 99.5%. The probability that an ion excited
`to the ‘Fm state will transition to the ‘hm state is 60%. The majority of the remaining
`transitions are E0 the 419/2, 4113/; and 41512. (‘F312 10 ‘[9,; = 25%, 4F;/2 [O ‘In’; = 14%,
`and ‘F3/3 [0 4!”/2 =
`By inserting a prism. grating. or using a selectively coated mirror. it is possible to lase
`on any combination of the crystal field split lines of the ‘F3,-2 band to the 4111,; band. This
`offers a range of transitions from 1.05 pm to l.I2 pm.
`In addition. it is possible to lase
`front the “Fm band R; state to all the field components of the ‘[3,; band. This offers a
`range of transitions from L31 ,u.m to 1.35 um. Finally. if the laser is cooled, it is possible
`to lase in a near three-state configuration from the ‘Fm band R; state to some of the field
`components of the ‘[9,; band. This offers