Laser Rod Assembly for Side Pumped Lasers

1. Field of the Invention

The present invention is related generally to lasers and more specifically to side-pumped composite laser rods and a mounting structure for increased efficiency of such lasers.

2. State of the Prior Art

Side-pumped solid-state laser rods have an advantage over longitudinal pumped rods in that they are more readily scaled to higher output powers, because they are more conducive to pumping with a plurality of light sources. However, conventional side-pumped rods are often not very efficient absorbers of the pumping radiation, primarily because the rods are fairly transparent and not very thick, so a substantial portion of the radiation from the pump source can pass all the way through the rod without being absorbed. Some pump source radiation can also be lost by reflection from curved lateral surfaces of cylindrical-shaped laser rods, and there can be problems in maintaining TEM.sub.00 mode operation when too much of the pump light that is absorbed by the rods gets absorbed close to the incident surface of the rod. Limited heat dissipation at higher average power operation can also result in serious thermo-optic distortions in the rod due to excessive heat build-up. Physical damage due to stress-fracture may also occur with high heat loads.

There have been several developments to alleviate the problems with side-pumped lasers. For example, G. E. Devlin et al., "Composite Rod Optical Masers," Applied Optics, Vol. 1, No. 1, Jan. 1962, reported the development and testing of a composite rod in which a core of doped, light absorbant gain medium is surrounded by a transparent cladding. This composite structure enhanced radiation absorption in the doped core, and the larger diameter cladding provided increased heat exchange surface area for more efficient heat dissipation to the surrounding environment. R. Byer et al., in their U.S. Pat. No. 4,860,295, issued on Aug. 22, 1989, disclosed improvements in such composite rods, comprising a neodymium doped yttrium aluminum garnet (Nd: YAG) core surrounded by a cladding made of the undoped base material. The undoped base material of the cladding is not absorptive, while the doped core does absorb some of the transversely directed side-pump light and functions as the gain medium. However, the Byer et al. core and cladding materials have the same atomic structure, except for the doping ions in the core, to minimize index of refraction differences between the core and the cladding so that the TEM.sub.00 mode diameter can be greater than the core portion diameter without extensive index of refraction discontinuity, which enhances utilization of absorbed energy. The Byer et al. patent also discloses a reflecting element positioned diametrically opposite the pump light source to reflect the pump light that passes unabsorbed through the rod back into the rod for another opportunity to be absorbed, thereby effectively lowering the threshold energy necessary for laser action.

However, further improvements in pumping efficiency, heat dissipation, and mounting structures are still needed to improve the performance of side-pumped lasers for a multitude of potential commercial applications, such as medical and industrial lasers and transmitter sources for remote sensing.

Accordingly, it is a general object of the present invention to improve the efficiency and gain as well as to lower the energy threshold necessary for amplification of transverse, or sidepumped, solid-state laser rods.

A more specific object of the present invention is to increase absorption of transversely directed pump light in composite laser rods.

A mother specific object of the present invention is to enhance heat dissipation efficiency in side-pumped laser rod crystals.

A still further specific object of the present invention is to provide a convenient and efficient mounting for elongated cylindrical laser rods that accommodates multiple side-pump light sources and effective heat transfer from the laser rod.

Additional objects, advantages, and novel features of the invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by the practice of the invention. The objects and the advantages may be realized and attained by means of the instrumentalities and in combinations particularly pointed out in the appended claims.

To achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described therein, the laser red assembly according to this invention comprises an elongated, crystalline or glass rod, preferably having a doped, pump light-absorbing core portion surrounded by a non-doped, transparent cladding portion, with a reflective surface that substantially surrounds the peripheral surface of the cladding portion to confine unabsorbed pump light within the crystalline rod and to reflect and re-reflect the unabsorbed pump light through the core portion. One or more entrance slits or apertures is provided in the reflective surface to admit pump light projected from the side transversely into the crystalline rod. A clamp assembly comprising a massive, heat conducting body has an axial bore for receiving and holding the crystalline rod, and the massive, heat conducting material is for conducting heat away from the crystalline rod. Compression slots in the clamp body extending radially outward from the bore serve the dual functions of providing sufficient flexibility in the clamp body to allow the clamp body to be drawn snugly around the crystalline rod and to admit pump light into the lateral or peripheral side of the crystalline rod. The slot can be wide enough to not affect the pump light, or it can be narrower with either parallel, converging, or diverging sides to channel the pump light and direct it in a desired manner through the core portion. The reflective surface can be smooth for spectral reflection, textured for diffuse reflection, or grooved for dispersed reflection in substantially one plane. The cladding, or the interface between the cladding and the clamp body, may also incorporate materials absorptive at the laser wavelength in order to prevent parasitic laser oscillation in the plane transverse to the laser rod axis.

The accompanying drawings, which are incorporated in and form a part of the specifications, illustrate the preferred embodiments of the present invention, and together with the descriptions serve to explain the principles of the invention.

In The Drawing

FIG. 1 is a perspective conceptual view of a composite, solid-state laser rod with a reflective peripheral coating according to this invention;

FIG. 2 is an elevation view of the composite, solid-state laser rod of this invention;

FIG. 3 is a top plan view of the composite, solid-state laser rod of this invention;

FIG. 4 is a cross-sectional view of the composite, solid-state laser rod of this invention taken along lines 4--4 of FIG. 3;

FIG. 5 is a cross-sectional view of the composite, solid-state laser rod of this invention taken along lines 5--5 of FIG. 2;

FIG. 6 is a perspective view of a mounting structure for the laser rod according to this invention;

FIG. 7 is a plan view of the clamp assembly for mounting the laser rod according to this invention;

FIG. 8 is an elevation view of the clamp assembly;

FIG. 9 is an enlarged cross-sectional view of the clamp assembly securing a laser rod that has a reflective coating according to this invention;

FIG. 10 is a cross-sectional view of the clamp assembly similar to FIG. 9, but with the reflective coating on the inside surface of the clamp assembly rather than on the peripheral surface of the laser rod;

FIG. 11 is a diagrammatic cross-sectional view of the laser rod illustrated with a direct coupling with the laser diode pump source;

FIG. 12 is a diagrammatic view similar to FIG. 11, but illustrated with an intermediate lens coupling of the laser rod to the laser diode pump;

FIG. 13 is a diagrammatic view similar to FIG. 11, but illustrating a light guide coupling of the laser rod to the laser diode pump;

FIG. 14 is a diagrammatic view similar to FIG. 11, but illustrating a tapered light guide coupling of the laser rod to the laser diode pump;

FIG. 15 is a diagrammatic view similar to FIG. 11, but illustrating a tapered, converging light guide coupling of the laser rod to the laser diode pump;

FIG. 16 is a diagrammatic cross-sectional view similar to FIG. 5 of the laser rod assembly of this invention, but illustrating a grooved or textured reflective surface for dispensing or diffusing pump light in the laser rod; and

FIG. 17 is a diagrammatic cross-sectional view similar to FIG. 4 of the laser rod assembly of this invention, but illustrating a textured reflective surface for diffusing pump light in the laser rod.

FIG. 18 is a diagrammatic cross-sectional view similar to FIG. 5, but illustrating an outer coating of a light absorptive material surrounding the reflective coating to absorb the lateral laser emissions from the laser rod.

FIG. 19 is a perspective conceptual view of a composite, solid-state laser rod with a reflective peripheral coating according to invention, but illustrated with only a single-diode system exhibiting multiple emitting junctions.

FIG. 20 is a diagrammatic cross-sectional view of the laser rod illustrated with multiple light guide channels in the clamp body corresponding to the emitting junctions of the diode and oriented parallel to one another.

FIG. 21 is a diagrammatic view similar to FIG. 20, but illustrating converging channel walls in the clamp body.

The laser rod assembly 10 according to the present invention is shown conceptually in FIGS. 1-5, including an elongated composite crystalline light amplifying composite laser rod 11, preferably, but not necessarily, in the shape of a cylinder, with a doped, light-absorbing core portion 12 surrounded by a non-doped, non-absorbing cladding portion 14. Except for the doping, the absorbing core portion 12 is preferably the same base crystalline material as the cladding portion 14, so they have substantially the same thermal and mechanical properties. Such crystalline material can be, for example yttrium aluminum garnet (YAG), and the core portion 12 can be doped with thulium (Tm: YAG), although this invention is applicable to any laser material, regardless of whether the core and cladding portions are comprised of the same base crystalline materials and regardless of whether all or part of the composite laser rod 11 is doped. While the doped core 12 is illustrated as a substantially circular cylinder, it does not have to be confined to that shape. In fact, with current technology, it might be easier to make a core 12 that does not have a circular cross-section, such as by slicing or otherwise producing and assembling multiple pieces of core material in other shapes that have planar surfaces, such as triangular or hexagonal prisms, and then assembling the pieces together in a composite "nearly" cylindrical core configuration. A reflective coating 16 is provided around substantially the entire peripheral surface of the cladding portion 14 of the composite laser rod 11, except for one or more narrow, side light entrance slits 18. As best seen in FIG. 4, the ends of the laser rod assembly 10 may have anti-reflection (AR) coatings 30, 32 at the laser wavelength so that laser emission can be transmitted clear through the entire laser rod. Although not shown in these drawings, it is well-known that most lasers, including the laser rod assembly 10 of this invention, have reflectors positioned in axial relation to the composite laser rod 11 to reflect laser emission and pump radiation back into the composite laser rod 11 to complete the laser resonator. One of such axially positioned reflectors is not totally reflective so that it emits the amplified laser beam 42 produced by the laser rod assembly 10 while the other reflector is totally reflective of the laser emission, as is also well-known to persons skilled in this art and which is not part of this invention. However, as is also well known in this art, laser rods can be used simply to amplify, rather than oscillate, laser light. In such amplifier configurations, the external reflectors described above may not be used. Instead, a light beam may be passed one or more times axially through the rod to amplify its power. The peripheral reflector and other features of this invention can be used to enhance either the oscillation or the amplifier configurations.

In this embodiment of this invention, as illustrated in FIGS. 1-5, side-pump light beams 34 are directed transversely through the entrance slits 18 into the composite laser rod 11. Such side-pump light can be provided by one or more laser diodes 24, for example an infrared (IR) laser diode, which are also well-known in the art and readily available from a number of manufacturers. For example, but not for the purposes of limitation, the laser diode can be a Model 3245-C1 manufactured by SDL, Inc. of San Jose, Calif., which emits an elongated, narrow beam 34 of laser radiation from a semiconductor junction. The length of the composite laser rod 11 is preferably similar in length to the elongated pump light beam 34 to avoid long, unpumped regions in the composite laser rod 11 and the potentially high losses and lower performance that would result from such long, unpumped regions.

As best seen in FIG. 5, the side-pump light beams 34 are preferably directed into the composite laser rod 11 with the axis 50 substantially perpendicular to the longitudinal axis 40 of the composite laser rod 11 and in a pattern that is substantially confined to passing into the doped, light-absorbing core portion 12, where at least some of the pump light beam 34 is absorbed by the core 12 to excite ions in the crystal lattice therein to higher energy states and to produce laser light. The undoped cladding portion 14 is substantially transparent to the pump light beam 34, but light in beam 34 that is not absorbed initially in the doped core portion 12 gets reflected and re-reflected by the reflective coating 16, which substantially surrounds the composite laser rod 11, back and forth through the composite laser rod 11 many times, thereby increasing significantly the chances that a much larger proportion of the photons in pump light beam 34 will be absorbed in the core 12. Consequently, the pump light absorption efficiency is improved substantially, and the power input necessary for threshold laser oscillation is reduced. The entrance slits 18 should be as small as possible, essentially just large enough to admit the pump light beam 34, to avoid loss of power by loss of reflected radiation through them. Also, if more than one entrance slit 18 and pump light source 24 are used, it is preferred, but not essential, to not locate them diametrically opposite each other in relation to the composite laser rod 11 to ensure that substantially all of the pump light beam 34 that is not absorbed by the core 12 on the first pass therethrough gets reflected by coating 16 at least once and preferably many more times. For example, the three entrance slits 18 shown in FIGS. 1-5 are disposed radially at about 120 degree angles in relation to each other with respect to the longitudinal axis 40 of the composite laser rod 11.

It is also preferred that the reflective surface of the coating 16 be smooth to provide specular reflectance to avoid loss of pump light energy through the end 30. However, a textured reflective surface, as illustrated in FIGS. 15 and 16, could be used to increase chances of reflections or re-reflections of the pump light beam 34 being propagated into the core 12 at some point. Such a textured reflective surface would be particularly advantageous where the angular divergence of the pump light beam 34 in the plane transverse to laser rod axis 40 is high enough that a substantial portion of the pump light beam 34 does not pass through the doped core 12.

The pump light, as is well-known in the art, excites atoms or ions in the crystal lattice of the core 12, where it is absorbed, causing electrons to move to higher energy levels. When they relax to lower energy levels, the atoms produce light or other radiation at a characteristic wavelength for that atomic species, which is confined in the composite laser rod 11 by the axially positioned reflectors (not shown), as is also well-known in the art as explained above. When a sufficient number of ions in the doped core 12 are promoted to the higher energy state, the radiation confined by the axially positioned reflectors experiences gain. It also experiences loss due to scattering, transmission through the axially positioned partial reflector (not shown), and other effects. When the gain is high enough to offset the losses, the laser oscillation threshold is reached. Further increases in the radiation of pump beam 34 increases the mount of radiation in the amplified laser beam 42 extracted through the partial reflector. The confinement and re-reflection of the pump light beam 34 by the surrounding reflective coating 16 facilitates reaching the laser threshold with less pump light input, which increases the efficiency of the laser rod assembly 10.

Because of the effective confinement of the pump light in the composite laser rod 11 and consequent increased absorption of the light by the core 12, the power production and heat dissipation in the composite laser rod 11 is very high. The crystalline structure of the cladding portion 14 is an effective heat transfer medium to conduct heat and transmit infrared radiation away from the core 12, where most of the heat is produced. The peripheral surface of the cladding portion 14, from where heat is dissipated to the surrounding environment, is substantially larger than the core portion 12, so this structure does accommodate greater heat transfer than non-composite laser rods. Also, the reflective coating 16 which preferably comprises a metal, such as indium, gold, aluminum, silver, or another highly reflective material, including dielectrics, also enhances heat transfer away from the composite laser rod 11. Assistance in heat dissipation is desirable to avoid damage and thermo-optic distortions in the composite laser rod 11 while maximizing power input and output.

The clamp assembly 60 illustrated in FIGS. 6-10 provides an effective mount for the laser rod assembly 10 as well as providing an effective additional heat dissipation medium. The laser rod assembly 10 is positioned and retained within an axial bore 62 in the clamp body 61. The clamp body 61 is preferably a metal or some other excellent heat conducting material which is clamped snugly in contact with the reflective coating 16, not only to hold the laser rod assembly 10 in position, but also to establish a good, large surface area heat conductive path directly from the laser rod assembly 10 to the clamp body 61, which acts as a heat sink as well as a rigid mounting structure for the laser rod assembly 10.

The clamp body 61 also has a plurality of reference surfaces 74 for mounting the pump light sources or laser diodes 24 in radial relation to the bore 62 and to the laser rod assembly 10. A plurality of slots 64, 66, 68 radiating outwardly from the bore 62 serve the dual function of compression slots for allowing sufficient resilient flexibility or strain in the clamp body 61 to accommodate clamping of the clamp body 61 onto the laser rod assembly 10 and to admit pump light beam 34 from the pump light sources 24 into the bore 62 and laser rod assembly 10. Therefore, these slots 64, 66, 68 are aligned with the entrance slits 18 in the laser rod assembly 10, and they can function as light guides for the pump light beam 34, as will be described in more detail below.

The clamp body 61 also has a plurality of massive radial protrusions or heat sink wings 72, which are part of the clamp body 61 and extend radially outward between the reference surfaces 74, for conducting and dissipating heat away from the bore 62. Solid-state thermal electric cooler (TEC) refrigeration units 76 or other cooling system components can be mounted on the lateral surfaces or ends of the heat sink wings 72. A pair of screws 78 extending between two of the heat sink wings 72 for providing the clamping force to flex or strain the clamp body 61 enough to secure the laser rod assembly 10 in the bore 62. One of the slots 64 extends radially all the way through the clamp block 61 from the bore 62 to the reference surfaces 74 between the two heat sink wings 72 that are pulled toward each other by the screws 78 to allow the clamping flexibility necessary to effectively clamp the laser rod assembly 10 in bore 62. The other slots 66, 68 extend from the bore 62 radially toward the respective reference surfaces 74, but not all the way through the clamp body 61, so that they also allow some clamping flexibility in clamp body 61. The remaining solid portions of the damp body 61 between the terminal ends of slots 66, 68 and the respective adjacent reference surfaces 74 maintain the structural integrity and substantial rigidity of the damp assembly 60 while also acting as strain hinges to accommodate the slight clamping flexibility required to clamp the laser rod assembly 10 in bore 62. However, as illustrated in FIG. 9, the pump light sources 24 do extend into the clamp body 61 a sufficient extent from the reference surfaces 74 to intersect the slots 66, 68, so that the slots 66, 68 as well as slot 64 can act as light guides or otherwise admit pump light beams 34 (FIGS. 1-5) into the laser rod assembly 10. The pump source diodes 24 can be attached to adjustment plates 70 (FIGS. 7 and 8) to facilitate adjustment of the axis 50 of the pump light.

In an alternate embodiment, the reflective surface surrounding the composite laser rod 11, instead of being coated on the peripheral surface of the cladding portion 14, as the reflective coating 16 described above, can be a reflective coating 16' on the surface of the bore 62 in the clamp body 61, as shown in FIG. 10. However, to compensate for the fact that it might be nearly impossible to obtain a perfect fit of the reflective coating surface 16' to the peripheral surface of composite laser rod 11 in this alternate embodiment, a clear, transparent, and heat conducting filler 17, such as Type FGS optical cement, manufactured by Summers Laboratories of Fort Washington, Pa., can be used to preclude air voids that would inhibit heat transfer and scatter light energy. Other than this difference in structure, the composite laser rod 11 and clamp assembly 60 function the same as described above, and the reflective coating 16' confines the pump light to the composite laser rod 11 and results in more passes of the reflected and re-reflected pump light beam 34 through the doped core 12 for enhanced absorption and more gain, as also described above.

The coupling of the pump light beam 34 to the composite laser rod 11 can be accomplished in several different ways, as illustrated in FIGS. 11-15. For example, a direct coupled diode 24, as illustrated in FIG. 11, has the pump light beam 34 emanating from the diode 24 and projecting directly through the entrance slit 18 in the reflective coating 16 into the composite laser rod 11. However, different shaping or accommodation of distance between the diode 24 and the composite laser rod 11 can be accomplished by use of an intermediate lens 25, as shown in FIG. 12.

On the other hand, the slot 64, 66, 68 in the damp body, or some similar structure, can provide a light guide for directing the pump light beam 34 into the composite laser rod 11, as shown in FIG. 13. Slanting the sidewalls of the slot 64, 66, 68, to converge or diverge can further shape the pump light beam 34. For example, the diverging slot 66', illustrated in FIG. 14, shows a light guide that results in a narrower projection of pump light beam 34 through the doped portion 12 of the composite laser rod 11, while the converging slot 66" shown in FIG. 15 results in a wider projection. Other angles of divergence or convergence will result in the narrowing or widening of the beam 34 at appropriate places.

The diagrams of FIGS. 11-15 discussed above show only one diode 24 and pump light beam 34, which is for convenience only, and is not intended to limit the number of diodes 24 or pump beams that can be used with these couplings. Also, none of the reflected beams or rays of the beam 34 are shown, but it will be understood by persons skilled in this art that such reflections and re-reflections do occur when light beams or rays are incident on reflected surfaces, such as on the coatings 16 or 16'. The diagram in FIG. 16 shows a roughened reflective surface to enhance diffusion of pump light beam reflections, as discussed above.

As mentioned above, a textured or grooved reflective surface 20, as shown in FIGS. 16 and 17, can be provided to diffuse the pump light beam 34 in the composite laser rod 11 and thereby increase the likelihood of multiple passes of the pump light beam 34 through the core 12. To minimize axial losses of pump light from the composite laser rod 11, it is preferred that such reflective surface 20 be grooved in the direction parallel to the longitudinal axis 40 of the composite laser rod 11, so that reflections will tend to be more transverse than longitudinal. To further minimize such axial end loss of pump light, a totally reflective cap 22 can be provided on the annulus around the end of the core 12, as shown in FIG. 17. The actual size of the axial opening 23 of the end cap 22 can be varied to match the most efficient laser beam 42 projected from the composite laser rod 11. For example, while most of the laser beam 42 is generated in the core 12, a TEM.sub.00 mode might be most efficient with the laser beam 42 extending somewhat into the cladding material 14 adjacent the core 12 (not shown).

The reflective coating 16 can be provided on either a smooth, textured, or grooved peripheral surface of the composite laser rod 11 by any conventional coating process, such as sputtering, chemical vapor deposition, or even just dipping or painting. A coating having a thickness of about one micron is usually sufficient. The reflective coating could also comprise a foil, such as indium or gold pressed onto the surface of the composite laser rod 11. Because of the potential presence of oxygen near the surface of the composite laser rod 11, less oxidizing metal reflectors, such as gold might be more suitable than silver or aluminum. However, this invention is not limited to the use of any particular reflective material. Also, any well-known anti-reflective coating can be provided on the surface of the composite laser rod 11 at the entrance slits 18 to enhance more complete admission of substantially all of the pump light beam 34.

Referring again to FIG. 6, the clamp assembly 60 with the laser rod assembly 10 positioned in the bore 62 is adapted for insertion into a rod cell housing 80. Guide surfaces 82 receive and abut the heat sink wings 72 of the clamp assembly 60, while enlarged cavities 86 receive adjustors 70. The solid-state TEC's 76 align with cooling ports 84 in the housing 80 to permit heat flow and disposal, possibly via a heat exchange fluid medium (not shown) or other suitable structure or material.

An enhancement of this invention, as shown in FIG. 18, includes an outer coating 88 of a light absorptive material surrounding the reflective coating 16 for absorbing lateral laser emission from the composite laser rod 11 to reduce inefficiencies that might otherwise result from gain, in such lateral emission. Lateral laser emission is useless, so gain in lateral emission resulting from reflecting lateral emission back through the composite laser rod 11 would be in effect a parasitic and non-productive energy consumption. For best results in this embodiment, the reflective coating 16 should be a material that is highly reflective to light of the wavelength of the pump light beam 34, as described above, but practically non-reflective to light of the wavelength of the laser emission from composite laser rod 11. Therefore, while the pump light beam 34 will still be reflected and re-reflected by the highly reflective coating 16, as described above, any laser emission light produced in the composite laser rod 11 to propagate in a lateral direction in relation to the longitudinal axis 40 of the composite laser rod 11 will pass through the coating 16 to be absorbed by the outer coating 88, instead of being reflected back into the composite laser rod 11 where it could produce more lateral laser emission. The coating 16 could be, for example, a dielectric material custom designed to reflect the pump light wavelength, but to transmit laser emission light. Such custom designing of dielectric materials to selectively reflect certain specified wavelength light while transmitting other wavelengths of light is well-known by persons skilled in that art. For example, depositing multiple layers of materials having different indices of refraction permits the design of a coating that is highly reflective of that incident radiation while remaining transmissive of other wavelength radiation. An absorptive material for the outer coating 88, for purposes of this embodiment, may be for example, Polyimide (trademark), which is obtainable from Goodfellow, Inc., of Malvern, Pa.

While the pump light source 24 has been described above as a laser diode with a single emitting junction that provides a single beam 34 in the shape of an elongated, cone of light, it can also be a laser diode system having multiple emitting junctions, such as the diode 124 shown in FIG. 19. This diode 124 is shown with five light emitting junctions stacked in a common heat sink that emit elongated, thin laser beams 134 through slits 126 (see FIGS. 19-21). Such diodes are readily available with any number of emitting junctions from one to twenty or more, for example, from Laser Diode Array, Inc., of Auburn, N.Y., and as shown generally in U.S. Pat. No. 5,040,187, issued to Karpinski. Of course, multiple pump beams 134 require multiple entrance slits 118 in the reflective coating 16 to allow the pump light to enter the composite laser rod 11. A plurality of light guide channels 166 in the clamp body 61 can be used to channel the pump beams 134 into the composite laser rod 11. The channels 166 can be oriented parallel to each other in alignment with the emission slits 126, as shown in FIG. 20, or they can be oriented to converge toward the longitudinal axis 40 of the composite laser rod 11, as shown in FIG. 21. Of course the walls of each channel 166 or 166' can be parallel, as shown in FIGS. 20 and 21, or on either converging or diverging, which is not shown in FIGS. 20 and 21, but which is described above and illustrated in FIGS. 14 and 15, to optimize passage of light into the doped core portion 12 of composite laser rod 11.

As mentioned above, the composite laser rod 11 can be fabricated of YAG with the core portion doped with Tm. For purposes of example and not for limitation, other crystal hosts may include garnets, fluorides, oxides, LuAG, YLiF.sub.4, LuYAG, YAIO.sub.3, ruby, or sapphire, and other dopant ions can include Nd, Ho, Er, Yb, Dy, or Cr, or combinations of any of these ions or of Tm with any of these ions. Co-doping, as described in the U.S. Pat. No. 4,902,127, issued to R. Byer et al., can be used to provide one dopant that absorbs pump light and another dopant that acts as the laser active ion. Non-crystalline glass can also be used for the composite laser rod 11, as is well-known in the art. The glass can be doped or undoped as described above for crystalline structures. The thermal conductivity of glass is lower than thermal conductivity in crystalline materials, so performance could suffer with glass composite laser rods 11, but it would work for some applications.

When the core portion 12 is doped and the cladding portion 14 is undoped, as described above, the refractive index may change slightly, the extent of such change depending on which host material and which dopant is used. Such a change of refractive index between the cladding 14 and the core portion 12 could adversely affect light absorption. If so, a nonabsorptive dopant that is also not laser active, such as Nd, Ho, Tin, Er, Yb, Dy, or Cr, or combinations of these ions, can be added to the cladding portion 14 to change its index of refraction in a similar manner as the dopant in the core potion 12 to minimize or eliminate such refractive index differentials.

The foregoing description is considered as illustrative only of the principles of the invention. Furthermore, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and process shown as described above. Accordingly, all suitable modifications and equivalents may be resorted to falling within the scope of the invention as defined by the claims which follow.