FIBER STUB END-PUMPED LASER

Abstract: A laser diode package (1) couples laser diode outputs into a plurality of fibers (21), and these are bundled and brought to an output face (2') that produces a divergent composite beam from the fiber ends (21'). The beam end pumps a solid state laser (100) across a gap (6), and the divergence allows a wide tolerance in alignment of the pump and crystal (3). Preferably, one cavity mirror (7) is a focusing mirror that reconcentrates residual pump light in the desired mode. In a preferred embodiment, the output face of the package is at a short stub (2) or ferrule that provides a simple and effective pump beam centering alignment. The solid state laser (100) may be a rod or crystal (3) and is preferably sufficiently short, in relation to pump beam diameter and divergence in the rod, that the pump beam within the rod lies in the TEMoo mode volume of the laser cavity. A concave mirror then refocuses residual pump light back into that mode. The rod preferably has a high index at the pump wavelength, but need not have high absorption. A common mounting plate (10) and heat sinking arrangement are provided for the package (1) and crystal (3).

Description: FIBER STUB END-PUMPED LASER Background of the Invention The present invention relates to end-pumped solid-state lasers, and more particularly, to lasers which are pumped by one or more semiconductor laser diodes. In general, such systems have required a fair degree of ingenuity or purposeful design to inject the laser diode output light into the solid-state rod or crystal in a manner to obtain a useful output. Generally, for end-pumped lasers, this has meant selecting optics that match the pump light to the TEM 00 crystal mode, or entailed using crystals of very high absorbance to limit the region in which pump light is concentrated; it has further required a number of elements to be precisely adjusted and aligned to achieve effective operation. Descriptions in patents tend to be written at an early stage ofthe development of a technology, and may not accurately reflect either the technology, or the true scope of results achieved by the techniques sought to be patented. However, a brief review of patents in this area offers some insight as to constructions which the patentees believe to be promising or useful for effecting laser pumping systems of this type. U.S. Patent No. 3,982,201 of Rosenkrantz claims an end-pumped solid-state laser using an array of pump light emitting semiconductor diode lasers arranged to direct their light onto an end ofthe solid-state laser. It suggests that the laser diode array could be pulsed at such a rate as to induce continuous wave (CW) operation of the solid-state laser. The array is a physical configuration such that diode light enters the Nd:YAG laser crystal through an end face. U.S. Patent No. 4,847,851 of Dixon claims end pumping a solid-state laser by "butt-coupling" a laser diode to one end ofthe rod and using a "very short absoφtion length... less than 500 micrometers" so as to absorb all ofthe diode pump light within the TEM 00 mode volume. It further has a rear laser mirror coated directly onto a face of the diode. U.S. Patent No. 4,710,940 of Sipes claims a configuration in which an array of laser diodes are positioned to pump the laser medium in the direction ofthe axis, and requires a means for concentrating diode light inside the lasing volume. This is -2- illustrated as a series of lenses which tailor the light from diode array to provide a focused spot inside the laser rod that overlaps the TEM 00 mode volume of the laser. U.S. Patent No. 4,653,056 of Baer, et al., claims a laser diode array secured in a housing behind and in optical alignment with a laser rod. The preferred construction of this device also uses a series of lenses to provide focused spots inside the laser rod in the mode volume. In summary, these prior art and pumping technologies have been restricted to laser diode arrays mounted in optical alignment to the laser rod, within a laser head. Generally, the diode array is a linear array with astigmatic properties and may require fairly sophisticated optical arrangements to bring its output into a symmetrical, semi¬ circular output suitable for end pumping (see for example, the papers of Zhou, et al. (1985), Berger, et al, (1988), and Oka and Kubota (1988)). In addition to these constructions, various researchers have suggested using one or more optical fibers to couple pump light to a laser. Recently, a new type of laser diode source has emerged which produces a large output at a single aperture by coupling many single diodes through separate optical fibers which are then bundled together to form a single, circular emission aperture. The entire assembly is a self-contained package with a short metal ferrule protruding from one side; the ferrule collects the individual fibers into a bundle of circular cross-section. This bundle is coupled via a permanently bonded interface into an S A connector which couples the output light into a single-core fiber. This homogenizes the diode outputs, and produces an output of Gaussian circular intensity profile at the fiber output. This output is well matched for end-pumping the TEM 00 mode of a crystal, since that mode also has a Gaussian intensity profile. At present, this type of multi-fiber-coupled and single-fiber-output high power CW diode array is available from two commercial suppliers, SDL, Inc., and Opto Power Corp. Typical output powers of these devices approach or exceed six watts. Because of this high output power, one would expect these diode packages to be useful as pump lasers for various laser media. Indeed, Baer, et al., in U.S. Patent No. 4,723,257 have previously suggested using a fiber-coupled, laser-diode source to power a remote laser head, since this would allow the head to be entirely free of electronics and, thus, to have a relatively simple and robust architecture. The fiber output of these current laser diode arrays would appear to be amenable to positioning and focusing so as to implement at least some ofthe particular pumping configurations described in one or more ofthe above patents. However, as a trade-off for the ease of handling obtained by having a diode source with fibers already attached and bundled, one must settle for a permanently attached and aligned high-power single fiber jumper. One might expect that highly specialized optics, for example, relatively large aperture long focal length optics, would be required to match the output to an end- pumped crystal. Moreover, the pumping qualities of such a strong source with its beam derived of multiple small fiber output beams are not fully explored. Summary of the Invention In accordance with the present invention, a laser diode source is fabricated to terminate in a short stub bundle, and the output of this stub is butt-coupled directly to a crystal to pump the solid-state laser. Such a stub-output device may be achieved by cutting away the sealant/interface from the junction ofthe stub and the single-core fiber coupler of a commercial diode package device, and removing the single-core fiber. The remaining stub has a simple and uniform packing of fibers that individually produce small, divergent outputs, and no separate optics provided to shape the pump output beam. Applicant has found quite unexpectedly, that by pumping a laser medium with this dispersed source having neither a fiber jumper nor a coupling lens, the solid-state laser is more efficiently excited: both the output power and the beam quality are increased. In a preferred embodiment, the ferrule defines an axially-directed bundle of fibers each having an end. The ends are positioned spaced across an air gap from the solid-state crystal, and they emit a beam which diverges in the air gap and, after entering the crystal, still diverges, although generally at a lesser rate in the crystal. Thus, the beam intensity decreases as it propagates through the crystal, in addition to the attenuation which occurs due to absoφtion. When used for generating TEM 00 mode laser radiation, this pump beam distribution renders the system relatively immune to tilt and misalignment ofthe pump laser so that a relatively coarse alignment suffices to achieve effective operation. Concentricity is easily obtained due to the cylindrical stub mounting. Furthermore, the solid-state crystal may be so short in relation to pump divergence and the crystal attenuation depth that the pump beam passes out ofthe distal face ofthe crystal before it has diverged outside the TEM 00 mode volume. Preferably, the laser diode, the crystal, and any intracavity elements such as doublers, modulators or switches are all mounted on a common sink or temperature -4- control device. A first mirror is coated on one end ofthe laser crystal through which the pump beam enters. A distal mirror is spaced from the other end and acts to define the lasing cavity. The distal mirror is preferably concave, to refocus the diverged beam back within a small mode volume ofthe crystal. This architecture is also amenable to a passive stabilization construction as described in applicant's co-pending U.S. Patent Application Serial No. 08/300,137. Advantageously, applicant has found that by end-pumping a crystal with a distributed array of divergent beamlets one avoids the high degree of stress, optical and physical distortion, for example, thermal lensing, that occur when focusing a single-fiber beam of comparable power, and which has heretofore frustrated attempts to maintain TEM 00 operation with a laser diode pump at such powers. Brief Description of the Drawings These and other features ofthe invention will be understood from the description below taken in conjunction with illustrative drawings, wherein: FIGURE 1 A shows a prior art packaged laser diode array with fiber bundle output; FIGURE 1 shows a side view of one embodiment ofthe present invention; FIGURE 2 shows a typical end-pumping configuration ofthe prior art; FIGURE 3 shows another prior art configuration for end pumping a Nd:YAG laser rod; FIGURE 4 shows a packaged laser diode array with fiber bundle stub output; FIGURE 4A illustrates beam shape and intensity ofthe array of FIGURE 4; FIGURE 5 shows a prior art end-pumped laser using a fiber-coupled diode source; FIGURE 6 shows the diode array of FIGURE 4 optically coupled to a rod; FIGURE 7 is a schematic top view ofthe embodiment of FIGURE 1 ; FIGURE 8 illustrates beam shape in the laser of FIGURES 1 and 7; FIGURE 9 illustrates mode shape and rod length in a preferred embodiment; FIGURE 10 illustrates another embodiment pumped by butt-coupled fibers; FIGURE 1 1 illustrates a laser cavity for expanding laser mode in a pump input region; FIGURE 12 shows an embodiment with cavity mode expanded at its output coupler end and matched to divergent diode pump light; FIGURE 13 illustrates a pumping scheme with fiber ends arranged for a desired laser mode shape; FIGURE 14 illustrates a pumping arrangement with non-circular fibers to enhance packing and provide a mode matched beam shape; FIGURE 15 illustrates an embodiment with intracavity frequency doubling; FIGURE 16 illustrates an embodiment with a relay lens; FIGURE 17 illustrates an embodiment in which two solid-state rods are pumped by two laser diode packages; FIGURE 18 illustrates the system of FIGURE 17 with a nonlinear crystal positioned to achieve two-pass conversion with output via a folding mirror; FIGURE 18A shows a doubling embodiment with a single pump and crystal; FIGURE 19(a)-(d) show beam intensity and thermal or stress distribution for the prior art and for the present invention; FIGURE 20 shows another embodiment with two rods pumped by two laser diode packages; and FIGURE 21 shows yet another two rod, two laser diode package system embodiment. Detailed Description FIGURE 1 A illustrates a commercially-available laser diode package in which a plurality of laser diodes are mounted with requisite circuitry or controls enclosed within a box to form a standard P5 or P6 assembly in which each laser diode is coupled to an output fiber and the output fibers are bundled together. The output fibers pass through a ferrule as a pigged-tailed fiber bundle that extends outwardly from the box and is coupled to a single large core fiber extension to provide an output face which may be positioned relatively conveniently to suit a desired application. The bundle consists of a great number of fibers, each one of which carries a relatively small amount of power; however, collectively the bundle provides up to about ten watts of output power. The collected bundle has an aperture between approximately one-quarter and three millimeters. The ferrule serves the dual puφoses of gathering the fibers into a sheaf and reinforcing the fibers at the otherwise mechanically vulnerable transition region between the inside ofthe package and the flexible, unsupported length of fiber externally ofthe package. So far as relevant hereto, applicant employs such a laser diode package as a pump source for the systems described further below, and the commercially-available diode package is modified with the external fiber bundle severed or reduced in length, so that the fibers attached to the individual laser diodes extend only to the cut line C at the output end ofthe ferrule rather than coupling into a jumper fiber. Applicant refers to the laser diode package so modified in construction as a stub-output package. In this construction, each fiber has an emission face at the end of the ferrule so that light is emitted directly outward from each fiber at that position and the fiber outputs merge into a beam of large cross-section and relatively uniform intensity distribution. FIGURE 1 shows a diode end-pumped solid-state laser 100 in accordance with the present invention. System 100 includes a laser diode package 1 such as an SDL, Inc. P5 or P6 package, in which the diode outputs are brought to an output aperture through a metal stub 2 fixed to the side ofthe package. The stub 2 is placed in immediate proximity, across a small air gap 6, to a back mirror 5 which is formed on the end of a laser crystal 3. By way of example, stub 2 may have a clear aperture of approximately 500 micrometers, and the gap 6 is typically less than one mm. and, preferably as small as possible without actual contact between the output face of stub 2, and the end face 5 of crystal 3. The gap 6 allows for thermal expansion without damage or misalignment of elements. Spaced across from the other end of the crystal 3 is a front mirror 7 supported in a mounting and alignment mechanism 7', such that the mirror 7 and the face 5 define a lasing cavity about the crystal. The pump light 4 emitted from the stub enters the crystal through the back mirror 5 which transmits at the pump wavelength but is highly reflective at the lasing wavelength ofthe crystal. The front mirror 7 is reflective at the pump wavelength and is transmissive at the crystal laser wavelength such that laser light 8 excited in the crystal by the pump, is coupled therethrough as an output beam 9. The laser crystal 3 is supported in a mount 3* which also provides heat sinking for the crystal. Both the diode array 1 and the crystal 3 are mounted and heat- sunk to a common base plate 10 which in turn is temperature controlled by a thermoelectric or similar cooler assembly 11. A heat sink 12 carries heat from the cooler 11 and may in turn be actively cooled, for example, by a fan or by a channeled flow of coolant. As shown in the FIGURE, the pump illumination 4 forms a diverging beam which increases in diameter from its relatively concentrated point of entry at crystal face 5 to a more diffuse and spreading beam as it exits the crystal at its opposite end. Mirror 7, on the other hand, is a concave mirror which reconcentrates the pump light reflected back toward the crystal so that the returned pump beam attains a relatively high intensity about a focal region through which it had previously passed and which is axially centered in the crystal. Skipping ahead briefly to FIGURE 9, there is shown a detailed view of the pump beam 4 in relation to a preferred geometry of a pumped system with a laser crystal 3. As -7- shown, beam 4 enters crystal 3 at face 5 as a small diameter but diverging beam directed substantially along the axis ofthe crystal. The diameter t ofthe TEM 00 mode ofthe laser cavity within the crystal is indicated at the left side ofthe figure, and the distal end ofthe crystal 5' is shown at the right. In this embodiment, the crystal is selected sufficiently short so that face 5' intersects the pump beam 4 when the beam has not diverged substantially, and is no broader than t. Thus, rather than being absorbed in the crystal, pump energy outside ofthe desired mode passes out ofthe crystal 3 as a residual expanding pump beam 4'. This light, in turn, is reflected back into the crystal by the front mirror 7. Also shown in FIGURE 9 is the laser output 16 of crystal 3. This beam emanating from within the stimulated region ofthe crystal is a narrow beam of high quality. As noted above, the distal front mirror 7 is preferably a focusing mirror which redirects all ofthe residual pump light 4' back into the central mode volume in the crystal 3. By tailoring the crystal length to the divergence ofthe pump beam, in this manner, optimal use is made ofthe available pump energy. Before describing other advantages ofthe invention and additional embodiments thereof, a brief review of typical prior art end-pumped constructions is in order. FIGURE 2 illustrates a typical fiberless construction wherein an array 20 of diodes D have their outputs coupled by a lens L into a laser rod 3. FIGURE 3 shows a variation wherein each diode Dj presents its output via a fiber Fj to a single lens Lj and the fiber/lens arrangements are aligned with precision couplers and aimed into a common focal region f in the laser crystal. FIGURE 5 shows a packaged laser diode assembly of the prior art arranged such that the output from an optical fiber F is coupled via an imaging lens into a solid laser rod. FIGURE 4 shows a packaged laser diode assembly 1 as modified for the present invention, wherein fibers lead the diode outputs to a common output face 40 located in a stub or projecting metal tube 2. This assembly is similar to the commercial device of FIGURE 1 A, but instead of having a permanently attached output fiber bundle, fibers are employed only to reach an output face at the rigid and mechanically-fixed stub 2 projecting from the package casing. This construction has been found by applicant to provide a suφrisingly efficacious solid-state pumping arrangement with enhanced properties as described herein. FIGURE 4A illustrates the output beam 4 formed by the stub end 2 of the laser diode package shown in FIGURE 4. Each fiber produces a beamlet 4/ which is a divergent beam with a half-angle at that depends on fiber size and is typically 5-30°, and a generally Gaussian profile. The beamlets 4/ add together in a central region, and produce an overall beam profile which is generally broad and uniform, with a relatively small amount of energy diverging from the beam sides at edges d in the near field. FIGURE 6 shows a system in accordance with applicant's invention utilizing the package 1 of FIGURE 4. This is a modified laser diode assembly having a stub output, which is relatively directly coupled into a rod. In this case, the laser diode assembly and the laser rod are positioned on a common housing 10 and, preferably also a common heat-sinking arrangement. Furthermore, the cylindrical stub ending is aligned in a straightforward way, within broad mechanical tolerances, to the laser rod itself and a simple relay lens L images the divergent bundle 4 of beamlets from the diode output onto the face ofthe laser rod, and thus allows greater flexibility in placement of mirrors and intracavity optical elements. Returning now to the embodiment of FIGURE 1, FIGURE 7 shows a top view of the preferred embodiment ofthe invention of FIGURE 1, wherein the relay lens L (FIGURE 6) is absent and the diode package 1 is coupled directly through a small fee space or air gap 6 into the rod 3. In this case, the output beam 4 from the diode package diverges in the air gap, and after striking the laser rod 3 refracts to a slightly less divergent beam which, nonetheless, continues diverging within the laser crystal as shown in the detailed drawing, FIGURE 8. The rod and stub may be connected in mutual alignment simply by mechanical alignment of bolt holes in base 10 which match through-holes ofthe laser package, without precision optical alignment. Because the pump output is divergent but centered, any slight angular misalignment has negligible effect on pumping efficiency and the region of highest intensity is automatically centered. FIGURE 10 depicts a related optical pumping scheme wherein the laser diode array 1 consists of a packaged array such as the aforesaid SDL, or the Opto Power CW diode arrays. In this embodiment, a plurality of optical fibers 2 are bundled into a hardy and maneuverable bundle in which the fibers terminate in a common output face 2'. The output face 2' is spaced across a small air gap 6 from the input face 5 ofthe crystal which, as in the first embodiment, is preferably transmissive at the pump wavelength and reflective at the lasing wavelength ofthe crystal. As with the stub embodiment of FIGURE 1 , this injects a divergent pump beam 4 into the crystal. The beam 4 has a minimal diameter approximately equal to that of the bundled output fibers and this diameter increases further away from the input face 5 of a crystal. As in the first embodiment, the laser diode assembly 1 and laser medium 3 are mounted in a common housing 10 which can also include a common heat sink or cooler 11,12 for maintaining these elements at a stable temperature. This construction may be used to form a laser oscillator, or may be used simply to amplify an injected laser beam. As described above, the provision of a composite diode-generated pump beam made up of plural divergent fiber outputs results in the unusual situation of a divergent beam pumping a relatively small crystal, with the result that suitable precision or control of alignment would appear to be unattainable, or achievable only by employing a rather complex arrangement of beam conditioning and alignment elements. However, applicant has realized that by employing a laser medium which is relatively short as depicted, for example, in FIGURE 9, or of relatively low absorbance, the lasing volume can be accurately controlled without requiring a series of precision alignment steps in setting up the laser and its cavity-defining components. Specifically, the crystal 3 can be effectively pumped even when the diode package 1 is held and aimed by hand in a roughly axial direction; only the cavity mirror 7 needs to be precisely positioned. In general, one may expect to gain stability of alignment by forming at least one cavity-defining mirror directly on a face ofthe laser rod. The ideal embodiment of an end-pumped laser uses a back cavity mirror formed by polishing a surface on the pumped end ofthe laser rod, and coating a mirror directly onto this surface (see, for example, US patents numbers 4,710,940 and 4,653,056). The inventor has found that when such a laser is end-pumped by a single-core optical fiber at high power, e.g., above six watts, a strong aberration occurs in the laser medium from thermal lensing together with a distortion ofthe pumped face. Basically, the intense pump radiation at the center ofthe rod causes local heating and uneven distortion ofthe rod end, thereby distorting the back mirror ofthe laser cavity. This distortion is sufficiently non-spherical so as to be effectively un-correctable using standard optics, and it therefore effectively prevents the laser from lasing in a TEM 00 mode. However, applicant has suφrisingly found that when pumping is performed directly with the high power output stub ofa P6 diode package as described above, this problem can be completely overcome. This essentially occurs because ofthe relative uniformity of light distribution; the intensity peaks are small, isolated from each other, and closely spaced where they occur in the near field, so that the induced heating and dimensional changes quickly become highly uniform. This may be understood with reference to FIGURES 4A and 19(a)-(d). FIGURE 19(a) shows the pump light intensity I as a function of radial position in the crystal for a pump beam from a single core fiber. A region of high intensity is established on the axis. FIGURE 19(b) shows the corresponding pattern for illumination by a multi-fiber bundle at a focus, or at region of highest intensity near the output face. The regions of maximum intensity have peaks p which are generally of lower power density than that of a single fiber beam ofthe same total power, and are closely spaced so that any induced heating in the crystal will be smoothed by heat conduction within the crystal, which quickly flattens gradients and equalizes thermal effects. Moreover, away form the input face or focal plane the continuing divergence of the multi-fiber beam allows a somewhat greater level of illumination and heating ofthe outer region of the crystal, eliminating the high levels of strain that occur when the crystal is heated only on the axis. FIGURES 19(c) and 19(d) illustrate the corresponding distributions of heat and consequent distortion or strain. As shown, the high central inhomogeneity of high power single fiber illumination in (c) with its aspheric distortion of the medium, is replaced by the uniform distribution of (d) across the beam width of the pumped area. With such an arrangement, applicant has been able to extract about eighty percent of the multimode output of the end-pumped laser rod 3 in the TEM 00 mode, even when using thirteen watts of pump power. This contrasts markedly with the heating instability and rapid degeneration that occur when pumping at full power with focused light from a single core fiber. Returning now to a discussion of the pump arrangements with a divergent multi- fiber bundle, within a broad range of alignment possibilities, certain types of beam shaping may be used to efficiently convert the pump beam energy. In the embodiment of FIGURE 11, as in the preceding embodiments, the pump light 4 emitted from the fibers enters the laser rod through a back mirror 5 which is transmissive at the pump wavelength but highly reflective at the lasing wavelength. A front mirror 7 forms a closed optical cavity to allow lasing and output coupling ofthe laser light as an output beam 9. This embodiment differs from the preceding ones, however, in that back mirror 5 is a convex surface polished onto the end ofthe laser crystal, forming a concave mirror surface which focuses return passes of pump light so as to produce a laser mode 8 with a maximum diameter adjacent to the input end 5. In another embodiment, shown in FIGURE 12, a flat input mirror produces a laser mode 8' with a minimum diameter at the input end 5. In this embodiment, the front mirror 7 is curved with a concavity to tailor the expansion of the cavity mode to that ofthe pump light 4 absorbed in the laser crystal 3. Thus, a great degree of control is available over the shape ofthe pump light within the laser crystal despite the nominally divergent nature ofthe end-emitting bundled fiber source. To obtain better control ofthe laser mode, the output ends 2' of the fibers 2 may be arranged in a pattern to match the beam shape with a mode shape ofthe laser cavity, as shown in FIGURE 13. In addition, the ends ofthe fibers can be rounded, or can be doped to provide a graded index, so that they provide some focusing ofthe diode pump light. Alternatively, a microlens array or a hologram equivalent to such an array can be mated to the fiber bundle end to focus or re-direct all outputs, e.g., at a common point. It should be noted that the fibers employed to couple light from the diode facets within package 1 to the output at stub 2 or at the bundle end can be matched better to the diode facets if they are rectangular in aspect. Such a configuration is shown in FIGURE 14, wherein the fibers 21 have rectangular cross-sections. In this case, the fibers may be packed in a shape closely matching the desired mode within the laser cavity as shown by the arrangement of fiber ends 21' in FIGURE 14. As shown, the resultant output face is elliptical or elongated rather than circular. In addition to the basic end-pumped laser constructions described above, in other systems additional elements may be incoφorated in the laser cavity or lasing system to convert the fundamental laser light to another wavelength. FIGURE 15 shows one such system, wherein a nonlinear crystal 18 is provided in the cavity between mirrors 5 and 7 for wavelength conversion ofthe laser light 16 before the output beam 9 is coupled out ofthe cavity. FIGURE 16 shows yet another embodiment, wherein a relay lens 13 collects the multiple source diode output beam from the stub 2 and images the stub output plane onto an end ofthe laser crystal 3 with 1 :1 image magnification. This embodiment allows the pump source to be physically separated from the end ofthe laser crystal so that, for example, a turning mirror 14 may be introduced, as illustrated. This use of a separate mirror, allowing the rod surfaces to be nonreflective, is further advantageous in that it allows other elements, such as Q switches to be placed in appropriate regions ofthe cavity for effecting desired laser control. In addition, in this embodiment, the physical distortion ofthe pump faces due to absorbing the pump light does not directly act on the cavity mirrors, so beam stability is improved with such a construction. FIGURE 17 shows yet another embodiment of the invention, wherein output power of the laser system is increased by employing a second diode pump array la and introducing a second laser crystal 3a within the laser cavity. In this case, the laser cavity is folded by an optical element 14 which is highly reflecting at the lasing crystal wavelength and highly transmissive at the diode wavelength. The second laser crystal 3a has anti -reflection coatings at the laser wavelength on both faces, and the face pumped by diode la also has an anti-reflection coating at the pump wavelength. The diode output of assembly la is collected by a lens 13 and focused into laser crystal 3a through the dichroic mirror 14. This embodiment can be improved by placing an optically-active quartz rotator 15 between the two laser crystals 3 and 3a so as to impart a 90-degree phase shift between the laser beams passing through each rod. This rotator effectively cancels the thermally-induced birefringence in each rod with its mate. The laser output can then be polarized, either by using the polarizing sensitivity ofthe turning mirror 14, or by adding an additional polarizing element such as a Brewster plate or thin film polarizer. A nonlinear crystal can also be placed within the cavity of the two diode embodiment shown in FIGURE 17. FIGURE 18 shows such a nonlinear crystal 19 placed near the cavity-defining mirror 7. In this case, mirror 7 reflects both the fundamental wavelength ofthe laser and also the wavelength generated by the nonlinear crystal. Nonlinear optical conversion occurs in two passes through the crystal 19, and the wavelength-converted light exits the laser cavity through the dichroic mirror 14, which is transmissive at the nonlinearly-generated wavelength. One especially advantageous construction is shown in FIGURE 18A. In this embodiment a relay lens 13 images the divergent fiber bundle beam 4 through a 45° dichroic mirror 14' onto an un-mirrored end face of the crystal 3, and the cavity is defined between mirrors 7 and 7a. The crystal is preferably a material such as Nd:YVO 4 which lasers in a single polarization, so there are no losses off the turning mirror 14', and the crystal output light travels in two passes through the non linear doubling element 19, reflected by mirror 7, before passing out through mirror 14' as a doubled output beam 16. Since this construction has no mirrored crystal ends, the pump beam does not distort the end faces and degrade the doubled beam. This construction enhances brightness of the intracavity doubled lasers and their efficiency, while maintaining a compact size. As before, relay lens 13 allows spacing to insert elements such ofthe turning mirror 14', while effectively translating the divergent diode output bundle 4 to the crystal face. FIGURE 20 shows another embodiment in which first and second laser rods 3, 3a are pumped by first and second laser diode packages 1, 1a aimed inward in opposite directions. The rods have mirrored pump ends which define the lasing cavity, while a beam splitter 24 placed between the rods reflects a portion of the laser light to output beam 9. Optionally, a beam conditioning element 23 such as a wave plate may be inserted to control the amount of light coupled out ofthe cavity by the splitter 24. FIGURE 21 shows yet another embodiment which simultaneously solves the problem of providing high pump power levels while effectively coupling large amounts of laser power out ofthe cavity. In this embodiment, as in the system of FIGURE 20, a pair of laser diode packages 1, la are butt-coupled to opposed ends ofa laser cavity defined by a mirror surface on the back end 5 of each rod. In this system, however, a nonlinear element 13 such as a frequency doubling crystal is placed between the two rods to receive the laser light generated in the crystals, and an end face 5' of one rod is coated for reflection at the doubled frequency. Face 5' is angled slightly to reflect the doubled output obliquely out ofthe cavity. Thus, butt coupling of a laser diode package to drive a crystal not only produces an unexpectedly effective pumping system but is amenable to diverse arrangements of system elements to increase the output power or provide additional laser frequencies and allow compact spatial layouts ofthe lasing and optical or support elements ofthe system. The foregoing completes a description of basic laser systems embodying the present invention. It will be understood, however, that the invention is capable of being varied and modified to accommodate or be incoφorated in diverse system architectures and improved constructions analogous to ones explored in the prior art, and that variations and modifications thereof will occur to those skilled in the art. All such variations and modifications are considered to lie within the scope ofthe invention, as set forth in the claims appended hereto. What is claimed is:

Claims: Claims: 1. An optically-pumped lasing system, comprising a lasing medium a source of pumping radiation with a plurality of laser diodes which emit diode light and apply it to an output aperture less than 3 mm. in diameter, wherein said aperture is placed adjacent to an end ofthe lasing medium so that the pumping light is transferred through a free space gap of less than one millimeter into the lasing medium without intervening imaging or coupling optics, and a housing supporting said medium and said source of pumping radiation and allowing heat sinking thereof. 2. An optically-pumped lasing system, comprising: a lasing medium a pump source, a plurality of optical fibers, coupling light from the pump source to the lasing medium, said fibers being bundled into a shape matching that of a desired mode within the lasing medium, and wherein a pumping end of said fibers is placed adjacent to an end ofthe lasing medium so that the pumping light is transferred through free space into the lasing medium without intervening imaging or coupling optics, and a housing supporting and allowing heat sinking for all the above elements. 3. The optically-pumped lasing system of claim 1, wherein the medium amplifies an injected pulse or CW laser beam. 4. The optically-pumped lasing system of claim 1, in which the medium is a laser oscillator, surrounded by a front and a back mirror, which provide a laser cavity having optical feedback of radiation to thereby produce a laser output beam. 5. The optically-pumped lasing system of claim 2, wherein said lasing medium is a crystal with a contoured surface polished onto a distal, unpumped end such that an optical path difference induced by said surface effectively cancels the optical path difference imparted to a laser mode by thermally-induced lensing within the laser crystal. 6. The optically-pumped lasing system of claim 4, further comprising a nonlinear optical means included within the laser cavity for converting fundamental laser output to a converted wavelength. 7. The optically-pumped lasing system of claim 4, wherein the front and back mirrors provide feedback at more than one lasing wavelength ofthe laser crystal, so as to allow simultaneous oscillation of more than one wavelength. 8. The optically-pumped lasing system of claim 7, further comprising a nonlinear crystal placed within the laser cavity for inducing optical mixing of a plurality of wavelengths to produce laser output at a different wavelength. 9. The optically-pumped lasing system of claim 8, wherein longitudinal mode separation is greater than 500 MHz, and induces passive stabilization ofthe laser output at said different wavelength. 10. The optically-pumped lasing system of claim 8, wherein the cavity length is less than 100 mm., for inducing passive stabilization ofthe different wavelength output. 1 1. The optically-pumped lasing system of claim 1, wherein length ofthe lasing medium in the pumping direction is sufficiently short that pumping light exits the crystal before it diverges out of a TEM 00 mode volume ofthe laser cavity, thereby achieving single-mode operation. 12. A single transverse mode laser comprising, a solid-state laser crystal mirror means for providing optical feedback to said laser crystal, an array of laser diodes coupled to a plurality of optical fibers, the fibers being bundled together so their output faces form a cross-sectional shape matching that ofthe TEM 00 lasing mode within the laser crystal, the output faces of said fibers being placed adjacent to an end ofthe lasing medium so that the pumping light transferred directly into the lasing medium without intervening imaging or coupling optics excites said TEM 00 mode. 13. An optically-pumped lasing system according to claim 1, further comprising means for pulsing the output of the source of pumping radiation, so that the lasing medium has a pulsed output. 14. An optically-pumped lasing system according to claim 13, further comprising a nonlinear optical crystal for frequency converting radiation from the lasing medium to produce a pulsed frequency-converted output. 15. An optically-pumped lasing system according to claim 14, further comprising heater means for heating the nonlinear crystal to a steady-state CW operating temperature to eliminate crystal warm-up time transients during pulsed operation. 16. A solid-state laser system comprising: a compact laser head including a lasing medium a laser power supply excluding a laser diode pumping source, a laser diode-pumping source contained within the compact laser head, and a plurality of fibers optically coupled to diodes ofthe laser diode source and having fiber ends butt-coupled to an end of the lasing medium. 17. An optically-pumped laser comprising: an optical resonator cavity, a laser medium in said resonator cavity, said laser medium having an optical axis and two ends, one end at each of two opposite sides of said medium intersected by said optical axis, an array of laser diodes positioned in an orientation unrelated to direction of pumping and direction ofthe axis of said resonator cavity, and means for causing the pump distribution from said array of laser diodes to be concentrated inside the lasing mode volume of said medium, said means being insensitive to change in orientation of the diode emitters. 18. The optically-pumped laser of claim 17, wherein the means for causing the pump distribution from said array of laser diodes to be concentrated inside the lasing mode volume of said medium is a light guide, light "duct", or light "pipe". 19. The optically-pumped laser of claim 17, wherein the means for causing the pump distribution from said array of laser diodes to be concentrated inside the lasing mode volume of said medium is a plurality of optical fibers. 20. The optically-pumped laser of claim 17, wherein the means for causing the pump distribution from said array of laser diodes to be concentrated inside the lasing mode volume of said medium includes means for reflecting pump radiation by walls of the laser medium. 21. A laser diode-pumped laser comprising: a laser medium supported in a housing, said medium having an absoφtion coefficient sufficiently low that the characteristic absoφtion depth of pumping radiation within said medium is greater than 500 μm, a back mirror, a laser diode supported in said housing behind the laser rod with the diode emitter in near contact and directed at the back mirror ofthe laser crystal, front mirror means forming an optical cavity with the back mirror, so that laser output from the diode is selectively maintained in said laser medium to induce lasing thereof. 22. A laser diode-pumped, solid-state laser comprising: a laser crystal supported in a housing, with an absoφtion coefficient sufficiently low that the characteristic absoφtion depth of pumping radiation within said medium is greater than 500 μm, a back mirror coated onto one end ofthe laser crystal, and front mirror means for forming an optical cavity with the back mirror, a laser diode supported in said housing behind the laser rod with the diode emitter in near contact (i.e., "butt-coupled") with the back mirror ofthe laser crystal, a nonlinear optical crystal within said optical cavity, for converting the fundamental laser radiation to its second harmonic wavelength, wherein the front mirror means is spaced not more than approximately 100 mm. from the back mirror, so as to passively stabilize the doubled radiation generated by the nonlinear crystal by making the longitudinal mode separation greater than 500 MHz. 23. A laser diode-pumped, solid-state laser comprising: a laser crystal supported in a housing, a back mirror coated onto one end ofthe laser crystal, a source of pumping radiation including a plurality of laser diodes which collectively emit diode light from an aperture less than 3 mm. in diameter, wherein said aperture is placed adjacent to an end ofthe lasing medium so that the pumping light is transferred through a free space gap of less than one millimeter into the lasing medium without intervening imaging and coupling optics, and front mirror means for output coupling of fundamental laser light. 24. A laser diode-pumped laser, comprising: a lasing medium supported in a housing, a source of pumping radiation including a plurality of laser diodes arranged to emit diode light from a stub with an aperture less than three mm. in diameter, a relay lens for collecting light emitted from the stub and directing it onto the end ofthe lasing medium with 1 :1 imaging, back mirror means through which the diode light enters the medium, and front mirror means, together forming an optical cavity with the back mirror means, and said front mirror means allowing output coupling of laser light generated within said cavity, and a housing containing and heat sinking all said elements. 25. A diode-pumped light amplifier, comprising: a lasing medium with two ends, a source of pumping radiation with a plurality of laser diodes which emit diode light from an aperture less than three mm. in diameter, relay lens means for collecting diode light emitted from the aperture and directing it onto an end ofthe lasing medium with 1 :1 imaging, a turning mirror located between the relay lens and pumped end ofthe laser medium, which transmits pump wavelength but reflects laser wavelength, a housing containing and heat sinking all said elements. 26. The diode-pumped light amplifier of claim 25, wherein said aperture is a fiber bundle ending, and the relay lens means is a microlens array aligned with the fiber bundle. 27. The diode-pumped light amplifier of claim 25, wherein the relay lens means includes a convex surface on the end of each fiber making up the fiber bundle. 28. The light amplifier of claim 25, comprising back mirror means opposite the end of the lasing medium through which the diode light enters, and front mirror means, for together forming an optical cavity with the back mirror means, and wherein said front mirror means output couples laser light from said cavity. 29. A laser pumped by two laser-diodes, comprising: a lasing medium with two ends, a first source of pumping radiation with a plurality of laser diodes which emit diode light from an aperture less than three mm. in diameter, a relay lens for collecting diode light emitted from the aperture and directing it onto an end ofthe lasing medium with 1 : 1 imaging, a turning mirror located between the relay lens and pumped end ofthe lasing medium, which transmits the pump wavelength but reflects the laser wavelength, a second lasing medium with two ends, placed along the same optical axis as the first lasing medium, a second source of laser diode pumping radiation identical to the first, with an output stub butt-coupled to an end of the second crystal opposite the pumped end of the first lasing medium, back mirror means deposited directly onto the pumped end ofthe second lasing medium, front mirror means which forms an optical cavity with the back mirror means and output couples the laser light from said cavity, and a housing containing and heat sinking said elements. 30. The laser of claim 29, further comprising a rotator placed between the first and second lasing media for causing a 90-degree phase shift between laser beams passing through the first and second lasing media. 31. The laser of claim 30, further comprising a polarizing element placed within the optical cavity to induce polarized output from the laser. 32. The laser of claim 29, further comprising a nonlinear optical crystal placed between the output coupling mirror and the turning mirror for converting fundamental wavelength laser light to a converted wavelength, the output coupling mirror being totally reflecting at fundamental and at the converted wavelength for directing light of both wavelengths back into the laser cavity, and the turning mirror is transmissive at the converted wavelength.

Inventors: MARSHALL LARRY R, MARSHALL, LARRY, R.
Applicants: LIGHT SOLUTIONS CORP, MARSHALL LARRY R, LIGHT SOLUTIONS CORPORATION, MARSHALL, LARRY, R.
Application year: 1996
Main classification: H01S3
IPC codes: G02B6, H01S3