CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part of application Ser. No. 08/112,558, filed 26 Aug. 1993, now abandoned. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     This invention relates generally to lasers, and more specifically to diode-pumped solid-state blue microlasers which employ second harmonic generation. 
     2. Discussion of the Related Art 
     Many developments have been made concerning the generation of harmonic radiation from solid-state laser devices. These advances relate both to the efficiency with which such devices operate and to the output wavelengths which have been obtained. For applications requiring small-to-modest levels of optical power, an attractive form of solid-state laser has been that of the microlaser. Such a device comprises a monolithic or composite resonant cavity wherein a diode laser is used to pump a short element of solid-state gain medium, the latter being formed by reflective surfaces on opposite ends of the cavity. 
     The solid-state gain medium consists of a rare-earth ion, such as trivalent neodymium (Nd 3+ ), doped into a suitable host material. The most well studied gain medium is Nd 3+  -doped yttrium aluminum garnet (Nd:YAG) which has been diode laser-pumped and has been made to lase at either 1342 nm, 1064 nm or 946 nm. 
     In Zayhowski, &#34;Microchip Lasers,&#34; The Lincoln Laboratory Journal, Vol. 3, No. 3, pp. 427-445 (1990), the demonstration of single-frequency microchip lasers which use a miniature, monolithic, flat-flat, solid-state cavity whose longitudinal mode spacing is greater than the gain bandwidth of the gain medium, is reported. These microchip lasers are longitudinally pumped with the close-coupled, unfocused output of a laser diode to generate near-infrared radiation. Mooradian has disclosed in U.S. Pat. No. 4,860,304 a microlaser employing a gain medium made from Nd:YAG having a cavity length that is less than 700 μm. 
     Nonlinear optical crystals can be used to convert near-infrared radiation to the visible portion of the spectrum via second harmonic generation (SHG) (sometimes termed frequency doubling). This process generates a harmonic wavelength which is one-half of the fundamental wavelength. Since the SHG conversion efficiency is a function of the fundamental laser beam intensity, the nonlinear crystal is often placed inside the cavity of a low power continuous wave laser to benefit from the high intracavity fundamental beam intensity. This technique is well known and is discussed by Mooradian in U.S. Pat. No. 4,953,166 where a solid-state gain material is bonded to a SHG nonlinear crystal. Dielectric reflective coatings are deposited directly to the gain and nonlinear material surfaces to form a composite cavity intracavity doubled laser. However, the main intent of this reference is to teach configurations which give rise to single frequency operation by selecting the cavity length such that the gain bandwidth is less than or substantially equal to the frequency separation of the cavity modes. This is not the case with the present disclosure. 
     A typical SHG method is intracavity doubling using KTP (potassium-titanyl-phosphate, or KTiOPO 4 ) as a nonlinear crystal. Radiation at 532-nm with as much as a few watts of power has been obtained in this way with a longitudinally oriented, diode laser pumped Nd:YAG laser. Additionally, 4 mW at 473 nm has been achieved using KNbO 3  (potassium niobate) as an intracavity SHG crystal in combination with Nd:YAG lasing at 946 nm [Risk et al., Appl. Phys. Lett. 54 (17), 1625 (1989)]. However, diode laser pumped blue-green lasers based on intracavity SHG of Nd:YAG are limited in efficiency and stability due to the unpolarized emission, and the relatively weak, narrow diode absorption features of Nd:YAG. The conversion of optical radiation at one frequency into optical radiation of another frequency by integration with a nonlinear optical material within an optical cavity is known. Byer et al., in U.S. Pat. Nos. 4,739,507 and 4,731,787, disclose a diode-pumped laser having a harmonic generator. In U.S. Pat. No. 4,809,291, Byer describes a diode-pumped solid-state laser which is frequency doubled to produce blue light. Byer also discusses the same subject in the article &#34;Diode Laser-Pumped Solid-State Lasers&#34; Science, Vol. 239, p. 745 (1988). In these documents there is no mention of a requirement for polarized emission and/or broad absorption lines. 
     In contrast to the above mentioned blue-green solid-state lasers based on Nd:YAG, the uniaxial crystal Nd:YVO 4  (neodymium-doped yttrium orthovanadate) has polarized emission and strong, wide absorption transitions [Yaney et al., J. Opt. Soc. Am., 66, 1405 (1976)]. Diode laser pumped operation of Nd:YVO 4  lasing at 1064 nm with intracavity SHG to 532 nm utilizing KTP has been demonstrated to be more stable and efficient than Nd:YAG based systems [see Sasaki et al., Optics Letters, Vol, 16(21), 1665 (1991) and Tatsumo et al., &#34;Highly Efficient and Stable Green Microlaser Consisting of Nd:YVO 4  with Intracavity KTP for Optical Storage,&#34; paper CWQS, CLEO-92, Anaheim, Calif. (May 1992)]. 
     Kintz et al. in U.S. Pat. No. 4,942,582 disclose a technique for generating single frequency output from Nd:YVO 4  based lasers. Additionally, intracavity SHG utilizing KTP is claimed but no specific wavelengths or transitions are discussed. This patent specifically teaches the use of external, separate mirrors for the output coupler which does not correlate with the currently described invention. 
     Consequently, although in separate references the concept of intracavity doubled microlasers and of blue light production are recognized as practical, nowhere is there a teaching relative to blue microlasers which employ the  4  F 3/2  → 4  I 9/2  914-nm emission line of Nd:YVO 4  to produce blue radiation at 457 nm via second harmonic generation in BBO or KNbO 3 . Nor is there any recognition that low lasant ion concentrations, on the order of less than 2.0%, permit the efficient production of blue light to occur. Additionally, there is no requirement or suggestion in the known prior art for either polarized emission or broad absorption bands of the laser material. 
     SUMMARY OF THE INVENTION 
     A basic objective of the invention is to provide a miniaturized composite cavity blue microlaser which operates efficiently at room temperature. This objective is accomplished by a laser comprising an active gain medium comprising a rare earth doped microlaser crystal which has two opposite faces, said gain medium having a polarized emission at about 900 nm when optically pumped; a nonlinear frequency doubling crystal oriented to phase match at about 900 nm, said crystal having two opposite faces, said gain medium and doubling crystal being bonded together to form a composite cavity; and optical pumping means in operative relationship with said composite cavity for pumping said composite cavity in which frequency-doubling to the blue of its fundamental frequency occurs, the output of said composite cavity being polarized. 
     Specifically, the invention relates to a microlaser device which is based upon fundamental laser action on the  4  F 3/2  → 4  I 9/2  transition in Nd:YVO 4  (about 914 nm) or in Nd:GdVO 4  (neodymium-doped gadolinium orthovanadate at about 912 nm), or a combination of both, and second harmonic generation in BBO (beta barium borate) or KNbO 3  (potassium niobate) so as to provide a doubled output at about 457 nm or about 456 nm. Also, this microlaser device achieves a room-temperature Type-I phase match for the 914-nm line by employing a specifically oriented doubling crystal such as BBO or KNbO 3 . 
     Important features of the invention include the novel use of the vanadates as the lasing crystals. Another aspect is that this is uniaxial crystal gain material with polarized emission, which provides better properties for frequency doubling. Additionally, the vanadate gain material has broad absorption bands, with attendant advantages for this invention. 
     Finally, the invention employs special dichroic coatings on the crystals. Specifically, these combine low reflectivity at 1064 nm and 1340 nm and high reflectivity at 914 nm on both external crystal faces. The crystal surface facing the diode laser pump has low reflectivity at 809 nm and high reflectivity at 457 nm for the Nd:YVO 4  embodiment. 
     As an alternative embodiment, when Nd:GdVO 4  is employed as the lasing crystal, its fundamental  4  F 3/2  → 4  I 9/2  lasing wavelength is about 912 nm and the doubled wavelength is about 456 nm. As a further alternative, a combination of Nd:YVO 4  and Nd:GdVO 4  are both used in a microlaser configuration where the Nd:YVO 4  is the lasing medium and the Nd:GdVO 4  acts as a spectral filter narrowing the effective linewidth of the  4  F 3/2  → 4  I 9/2  Nd:YVO 4  emission transition. This embodiment improves laser performance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The objects, advantages and features of this invention will be more clearly perceived from the following detailed description, when read in conjunction with the accompanying drawing, in which: 
     FIG. 1 is a schematic line diagram of the composite cavity intracavity doubled blue microlaser of this invention; 
     FIG. 2 is a schematic diagram of a composite cavity blue microlaser wherein pump radiation is coupled to the device through an intermediate focusing lens; 
     FIG. 3 is an alternative embodiment similar to FIG. 1 showing a dielectric spacer employed for coupling the two crystals together; 
     FIG. 4 shows a conceptual representation of the lasing action within the composite crystal cavity; 
     FIG. 5 is an alternative embodiment of FIG. 3 showing a convex input face; 
     FIG. 6 is an alternative embodiment of FIG. 3 showing a convex output face; 
     FIG. 7 is an alternative embodiment of FIG. 1 showing the use of two laser materials operating together, one acting as the laser material and the second operating as a spectral filter; 
     FIG. 8 is an alternative embodiment of FIG. 7 with a different placement of the second crystal; 
     FIG. 9A represents the  4  F 3/2  → 4  I 9/2  emission spectrum of a Nd:YVO 4  crystal; 
     FIG. 9B represents the  4  F 3/2  → 4  I 9/2  absorption spectrum of a Nd:GdVO 4  crystal; and 
     FIG. 9C is the net  4  F 3/2  → 4  I 9/2  emission spectrum when the crystals of FIGS. 9A and 9B are employed together in the FIG. 7 or FIG. 8 embodiments. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While this invention is susceptible of having many different forms, described herein are specific exemplary embodiments of the invention. 
     Referring now to the drawing, and more particularly to FIG. 1 thereof, laser 11 comprises gain medium 12 in the form of a thin etalon which is made from a rare earth doped crystal material. The etalon is bonded, such as by epoxy 19, to nonlinear crystal 16 to form a composite cavity. That cavity is optically pumped by source 13. 
     A highly suitable optical pumping source consists of at least one gallium aluminum arsenide (GaAlAs) laser diode which emits light having a wavelength of about 800 nm, preferably about 809 nm, and which is attached to a heat sink. The heat sink can be passive in character. However, the heat sink can also comprise a thermoelectric cooler or other temperature regulation means to help maintain the laser diode pumping source at a constant temperature and thereby ensure optimal operation of a laser diode at a constant wavelength. This is only an example of a suitable pumping source. 
     For efficient operation, the pumping radiation emitted by optical pumping means or source 13 is desirably matched with a suitable absorption band of the lasant material. In addition, the diode laser polarization is oriented with respect to the crystal axes of the laser medium for maximum absorption. The term &#34;thin&#34; as used herein means an etalon no more than 2 mm thick. The term &#34;etalon&#34; generally refers to a crystal with parallel faces. Since it is not necessary that the crystal faces be parallel in every embodiment, the term &#34;crystal&#34; will be used in some instances to refer to the element more broadly. 
     In the preferred embodiment of FIG. 1, gain material 12 is made from an etalon of neodymium-doped yttrium orthovanadate (Nd:YVO 4 ) and has two opposite, flat, parallel polished faces 14 and 17. It is oriented with respect to nonlinear crystal 16, having parallel polished faces 15 and 18, such that the propagation axis is phase-matched in BBO or KNbO 3  of blue light wavelength of ˜457 nm. The distance between exterior faces 14 and 15 ranges between about 0.2 mm and 12 mm. At the present time, an element 12 would not likely function as desired if it were thinner than 0.1 mm. Frequency doubling crystal 16 would normally be about 0.1 to 11.9 mm long with the gain crystal being 0.1 to 2.0 mm between flat faces. It is advantageous that the laser of this invention employs separate crystals for lasing (gain material 12) and for wavelength conversion (doubling crystal 16). 
     Preferably, the output facet of semiconductor light source 13 is placed in butt-coupled relationship to input face 14 without the use of a focusing means or lens. As used herein, &#34;butt-coupled&#34; is defined to mean a coupling which is sufficiently close such that a divergent beam of optical pumping radiation emanating from semiconductor light source 13 will optically pump a mode volume within a lasant material with a sufficiently small transverse cross-sectional area so as to support essentially only single transverse mode laser operation (i.e., TEM 00  mode operation) in etalon 12. Optical pumping radiation is expected to be delivered to the lasant etalon in a direction which is substantially along a longitudinal optical path. The result is a miniature all-solid-state device having a blue output. 
     Alternatively, a focusing means or an imaging means can be used to image a laser diode array into gain material 22 which is coupled to nonlinear crystal 26, typically by means of epoxy 29. This embodiment is illustrated as laser 21 in FIG. 2. An imaging means, such as lens 27, serves to focus the output of a single stripe laser diode or diode array 23 into input face 24. This focusing results in high pumping intensity and an associated high photon-to-photon conversion efficiency in lasant material 22. The focusing means can comprise any conventional device such as a gradient index (i.e., GRIN) lens, a ball lens, an aspheric lens, or a combination of lenses. 
     Face 14 of etalon 12 may be referred to as the input face and is the face which is closest to source 13. Face 14 is coated with a dielectric for high reflection (HR) at ˜914 nm and at ˜457 nm. Opposite face 15 is coated with a dielectric for high reflection at ˜914 nm, the fundamental wavelength, and for 1% to 20% anti-reflection (AR) at the harmonic wavelength of ˜457 nm, so as to form an output coupler. Interior faces 17 and 18 are coated for anti-reflection at ˜914 nm and ˜457 nm. Since it is necessary that the higher gain emission of Nd:YVO 4  at 1064 nm and 1340 nm be suppressed so as not to compete with emission at 914 nm, it is essential that the coatings applied to surfaces 14 and 15 exhibit low reflectance at 1064 nm and 1340 nm. The terms &#34;high&#34; and &#34;low&#34; reflection relate to the specific values involved, as will be seen in Table 1 below. Wavelengths of 914 nm and 457 nm are shown to be highly reflected at faces 14, 24 and 34, for example, and to be much less reflected by faces 17, 27 and 41. 
     The input face of the etalon is also coated for high transmission (HT) at the pumping wavelength (about 809 nm) of source 13 to allow the pumping radiation to reach the lasant material which forms the etalon. Faces 24, 25, 27 and 28 in FIG. 2 are coated similarly. 
     The embodiments of FIGS. 1 and 2 employ lasers 11,21 having separate gain elements (12,22) and frequency doubling media (16,26) bonded by materials 19, 29 to form monolithic structures. 
     The FIG. 3 embodiment is structurally and functionally similar to those of FIGS. 1 and 2. Laser 31 includes the addition of dielectric spacer 37 between lasing medium crystal 32 and doubling crystal 36. As shown here, the spacer is annular in shape and creates a small air gap between the crystals. That air gap would normally be between 25 and 500 μm wide, and preferably would be 50-100 μm wide. Pumping source 33 and exterior end faces 34 and 35 operate the same as described for the other embodiments. Faces 41 and 42 are also coated and their functions are substantially the same as the interior faces described above with respect to the embodiments of FIGS. 1 and 2. 
     By way of further example, certain specifications are set forth here for enhanced understanding of the invention. They are meant to illuminate, and not to limit, the invention. The microlaser device would normally exhibit laser action on the  4  F 3/2  → 4  I 9/2  transition in Nd:YVO 4  (˜914 nm), the frequency doubled output being ˜457 nm. The doping concentration of neodymium in the crystal is low compared with the known prior art. The doping concentration ranges between 0.3 and 2.0%, depending upon the specific crystal lengths. The concentration--length product is chosen to maximize the efficiency of the blue laser device. This is an important point since the  4  F 3/2  → 4  I 9/2  transition is a quasi-three level system. The low doping concentration enables lasing to be achieved in the near infrared at ˜914 nm. Also important is the crystal orientation. Both KNbO 3  and BBO achieve a room-temperature Type-I phase match for the 914-nm line by lasing polarized along the c-axis of the Nd:YVO 4  and orienting the nonlinear crystal such that it is polarized in the a-b plane and propagates at ˜44° from the b-axis toward the a-axis for KNbO 3  and is polarized along the b-axis and propagates at ˜25.7° from the c-axis to the a-axis for BBO. 
     The reflectivities of the crystal faces are also important. These range between 99.5% and 99.95%, the &#34;high&#34; end, for the 914-nm line for faces 14,24,34 and 15,25,35, and between 0 and 0.25%, the &#34;low&#34; end, for faces 17,18,27,28,41 and 42. The reflectivities for 1064 nm and 1340 nm range between about 10% and 50%, which are &#34;low&#34; for these wavelengths. In this case, &#34;about&#34; means the low end is less than 10% and the upper end is less than 50%. This reflectivity arrangement enables the low pumping threshold and prevents lasing of the 1064 nm and the 1340 nm transitions. 
     More specifically, Table 1 below gives the reflectivities in percentages for each of the coated faces by wavelength. 
     
                       TABLE 1______________________________________REFLECTIVITIES    Faces     Faces     Faces   FacesWavelength    14,24,34  15,25,35  17,27,41                                18,28,42______________________________________809      &lt;5914      ≧99.9              ≧99.9                        ≦0.25                                ≦0.251064     &lt;10                 &lt;101340     &lt;50                 &lt;50457      &gt;99       1-20      ≦1                                ≦1______________________________________ 
    
     From these values, and from FIGS. 1-3, the Conceptual Representation of FIG. 4 can be easily understood. The pumping diode light at 809 nm enters at face 14, causes lasing at ˜914 nm within the composite cavity between faces 14 and 15. The intracavity nonlinear crystal gives rise to frequency doubling to ˜457 nm which, because of the low reflectivity of face 15 for the blue light (which can be as low as 1% to as high as 20%) is transmitted from the composite cavity as useful blue laser light. 
     As mentioned above in the Summary, Nd:YVO 4  may be replaced by Nd:GdVO 4  as lasing crystal gain medium 12 in FIG. 1. It may be similarly substituted for the equivalent element in FIGS. 2-6. The specific differences are reflected in the applicable wavelengths. The Nd:GdVO 4  element has a fundamental wavelength of ˜912 nm, a doubled wavelength of ˜456 nm, and the suppressed wavelength is ˜1062 nm. These are the only changes that would be required in Table 1 to make it applicable to the Nd:GdVO 4  embodiment. 
     An interesting and useful further alternative is a blue microlaser which combines Nd:YVO 4  and Nd:GdVO 4  elements with the appropriate nonlinear crystal such that the microlaser is formed by three elements. Possible embodiments of this alternative are the structures shown in FIGS. 7 and 8. 
     The relative reference numerals used in FIGS. 1-3 have been continued in these figures. In FIG. 7 laser 51 comprises gain medium 52, preferably Nd:YVO 4 . The cavity is optically pumped by source 53. The nonlinear crystal is element 56. However, in this embodiment, etalon 52 is bonded to etalon 57, the Nd:GdVO 4  element, by suitable bonding means 58. This composite gain medium is bonded to crystal 56 by bonding means 59. 
     Similarly, in FIG. 8, gain medium 62 of laser 61 is pumped by source 63 and is appropriately secured to crystal 66 through dielectric spacer 68. At the other end of crystal 66 is another spacer 69 to which is connected Nd:GdVO 4  element 67. Any face securing means, as previously discussed, may be employed. 
     Due to the relatively broad  4  F 3/2  → 4  I 9/2  emission centered at ˜914 nm in Nd:YVO 4  with a full width at half maximum (FWHM) emission bandwidth of ˜3.5 nm it is often difficult to obtain narrow linewidth laser emission and/or single longitudinal mode operation. Ideally a narrow bandwidth spectral filter could be used inside the laser cavity to effectively narrow the emission bandwidth. Glass elations are often used but they tend to be difficult to implement. Here we disclose the use of an intracavity Nd:GdVO 4  element as a spectral filter for the Nd:YVO 4  laser. This is possible since the Nd:GdVO 4   4  F 3/2  → 4  I 9/2  absorption transition is shifted by about 2 nm relative to the Nd:YVO 4  emission transition and is centered at about 912 nm with a FWHM of about 2.8 nm. Thus when the Nd:GdVO 4  is placed inside a Nd:YVO 4  laser cavity the short wavelength portion of the Nd:YVO 4   4  F 3/2  → 4  I 9/2  914 nm emission is absorbed by the Nd:GdVO 4  which gives rise to a narrowing of the emission bandwidth. By specifying the neodymium concentration in the Nd:GdVO 4 , the absorption of the spectral filter can be optimized for laser performance. The narrowing of the net emission bandwidth allows for a more efficient SHG process in the nonlinear crystal due to the finite spectral acceptance of the nonlinear crystal. 
     The above composite cavity discussion is illustrated in FIGS. 9A, 9B and 9C. FIG. 9A shows the  4  F 3/2  → 4  I 9/2  emission transition at ˜914 nm of the Nd:YVO 4  element 52, 62 in FIGS. 7 and 8, respectively. The  4  F 3/2  → 4  I 9/2  absorption transition at ˜912 nm of the Nd:GdVO 4  element 57, 67 is depicted in FIG. 9B. The resulting net  4  F 3/2  → 4  I 9/2  emission of Nd:YVO 4  with intracavity Nd:GdVO 4  is shown in FIG. 9C. The 914 nm emission is shown as a narrowed peak. 
     As a general matter the gain and spectral filter crystals are preferably less than 2 mm thick. The nonlinear frequency doubling crystal would typically be less than ˜10 mm long from face 18,28,42 to output face 15,25,35. The same is true for crystals 56 and 66 in FIGS. 7 and 8. 
     While the preferred embodiment calls for both external end faces 14 and 15 to be flat and parallel, that is not necessary for functioning of the invention. The input face may be convex, as is face 44 in FIG. 5. The output face 45 (FIG. 6) could be convex. Either face could alternatively be concave. In either case the radius of curvature is greater than the distances between the faces of the etalon. 
     There are several features of the invention which are of significance. One is the use of vanadate to produce blue light in a thin crystal microlaser. Another is that the gain medium is made of uniaxial crystal having polarized emission, providing better properties for frequency doubling. Still another important feature is that the output of the composite cavity is polarized. Other operational advantages arise from the fact that the gain medium has strong broad absorption bands, more than 2 nm FWHM. By way of contrast, a Nd:YAG crystal has narrow absorption bands, less than 1.5 nm FWHM. 
     From the foregoing description, it will be observed that numerous variations, alternatives and modifications will be apparent to those skilled in the art. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the manner of carrying out the invention. Various changes may be made, materials substituted and separate features of the invention may be utilized. For example, the precise geometric shape of lasant material 12 can be varied-crystal 12 can be circular or rectangular in shape. If desired, the gain crystal can be end-pumped by an optical fiber. The bonding of one crystal to another can be by optical contact, diffusion bonding, or by use of index matching fluid optical epoxies, among others. The drawing figures are schematic only, intended to show element relationships, but not to depict specific sizes and shapes. Thus, it will be appreciated that various modifications, alternatives, and variations may be made without departing from the spirit and scope of the invention as defined in the appended claims. It is, of course, intended to cover by the appended claims all such modifications involved within the scope of the claims.
A monolithic diode pumped solid-state laser (11) comprising as the laser host neodymium-doped yttrium orthovanadate (Nd:YVO 4) (12, 52) or neodymium-doped gadolinium orthovanadate (Nd:GdVO 4) (57, 67) operating on the 4 F 3/2 - 4 I 9/2 (~914 nm or ~912 nm respectively) transition, to which a suitable nonlinear optic material (16), such as potassium niobate (KNbO 3) or beta barium borate (BBO), is bonded. The nonlinear crystal gives rise to intracavity frequency doubling to ~457 or ~456 nm. The microlaser is a composite cavity formed from a gain medium crystal and a nonlinear frequency doubling material which together have four spaced parallel dielectrically coated faces (14, 17, 18, 15) and which is positioned in close proximity to a diode laser pump source (13) for phase-matched harmonic generation of blue light along an axis of propagation which lies substantially perpendicular to the two faces of the composite cavity. By employing specific doping concentration-lengths products of lasant material and pumping the gain medium which has a specific crystalline orientation the desired efficient blue microlaser is achieved. Alternative embodiments combine the Nd:YVO 4 and Nd:GdVO 4 elements to enhance certain output characteristics of the laser.