WO1994029937A2

WO1994029937A2 - Blue microlaser

Info

Publication number: WO1994029937A2
Application number: PCT/US1994/006590
Authority: WO
Inventors: David E. Hargis
Original assignee: Laser Power Corporation
Priority date: 1993-06-11
Filing date: 1994-06-10
Publication date: 1994-12-22
Also published as: WO1994029937A3

Abstract

A microlaser formed from a self-doubling crystal which has two opposite dielectrically coated faces and which is positioned in close proximity to a diode laser pump source for intracavity second harmonic generation of blue light. By employing the proper doping concentrations of lasant material and pumping the gain medium which has a specific crystalline orientation, the desired efficient blue microlaser is achieved.

Description

DESCRIPTION BLUE MICROLASER

TECHNICAL FIELD

This invention relates generally to lasers, and more specifically to diode- pumped blue microlasers which employ second harmonic generation.

BACKGROUND 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 laser resonator 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 (Nd³⁺), doped into a suitable host material. The most well studied gain medium is Nd³⁺-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, "Microchip Lasers," 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. patent 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 frequency doubling or second harmonic generation (SHG). 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. patent 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 the above disclosure 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. Byer et al., in U.S. patent 4,739,507, disclose a diode- pumped solid-state laser having a harmonic generator. Byer also discusses the same subject in the article "Diode Laser-Pumped Solid-State Lasers," Science. Vol. 239, p. 745 (1988).

An alternative technique to intracavity doubled, composite cavity microlasers is to use microlasers made from rare-earth doped nonlinear materials. Materials of this type act as the laser gain medium and the nonlinear crystal. In other words, upon pump excitation the material lases at the fundamental frequency and self-frequency doubles to the harmonic frequency. Examples of such self- doubling crystals include Nd.MgO: LiNbO₃ (NMLN) [Fan et al., J* Opt. So. Am. £B), 3, 140 (1980)], Nd:LaBGeO₄ [Kaminski et al., Phys. Stat. Sol. (a) 125, 671 (1991)], Cr.KTP, and neodymium-doped yttrium aluminum borate, Nd_xY₁._xAl₃(BO₃)₄ (Nd.YAB or NYAB) [Dorozhkin et al., Sov* Phvs. Lett* 7, 555 (1981), and Lin, Lasers and Optronics. 8 (7), 61 (1989)]. Self-doubling of the ⁴F_3/2 → \_m 1320 nm transition to red light at 660 nm has been reported using lamp pumping of an NYAB laser [Dimitriev et al., "Stimulated Emission at the Fundamental Frequency and the Second Harmonic in an Active Nonlinear Medium: Neodymium-Doped Lithium Metaniobate," Soviet Technical Phvsics Letters. Vol. 5 (11), p. 590 (1979)]. NYAB lasers which self- double the ⁴F_3/2 -→ %_m 1062 nm transition to green light at 531 nm have been reported extensively in the literature [Lu et al., Chinese Phvsics Letters. 3:413 (1986), and Lin, Double Jeopardy: The Blue-Green Race's New Players," Laser and Optronics, p. 34 (1990), Wang et al., Topical Meeting on Advanced Lasers. Session TuB4, Mar. 6, 1990, and Schutz et al., "Self-Frequency Doubling Nd: YAB Laser Pumped by a Diode Laser," paper CWC4, CLEO-90. Anaheim, Calif., May 23, 1990].

Dixon (U.S. patent 5,070,505) describes a self-doubled microlaser which employs NYAB in the production of green second-harmonic radiation at 532 nm. However, this patent nowhere references the possibility of using the ⁴F_3/2 -→ ⁴I_9/2 -900 nm transition in NYAB to achieve a self-doubled output in the blue at ~450 nm. Also, it emphasizes harmonic resonance as an essential feature of efficient green light production. This latter feature stems from its teaching of high lasant ion doping concentrations (~6%) due to the fact that short crystal lengths are required to avoid significant absorption at 532 nm.

Byer et al. (U.S. patent 4,809,291), although making no direct reference to microlaser geometries, describes a diode-pumped solid-state laser which is frequency doubled to produce blue light. However, this patent does not address self-doubling. Also, as in the above noted Dixon patent, the requisite ion doping concentrations are much higher than that presently described for an efficient self- doubling blue device. Additionally, 4mW of blue light at 473 nm has been achieved using KNbO₃ (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)].

Self-doubling also is addressed by Byer et al., in U.S. patent 4,739,507. However, this patent addresses only the production of green light and does not teach the use of the low doping concentrations which facilitate efficient self- doubling without resort to cavities that also resonate at a harmonic wavelength. Also, this patent is not directed toward devices which have the characteristic dimensions of microlasers.

Consequently, although in separate references the concept of self-doubling microlasers and of blue light production are recognized as practical, nowhere is there a teaching relative to self-doubled blue microlasers which employ the ⁴F_3/2 — ► ⁴I₉₂ transition at -900 nm of NYAB to produce blue radiation at -450 nm. Nor is there any recognition that low lasant ion concentrations, on the order of 1%, permit the efficient production of blue light to occur in the absence of second- harmonic resonance when a NYAB microlaser of specific crystalline orientation is employed to allow for room temperature phase matched SHG to -450 nm.

DISCLOSURE OF INVENTION

A basic objective of the invention is to provide a compact self-doubling blue microlaser which operates efficiently at room temperature in the absence of second-harmonic resonant enhancement. This objective is accomplished by employing a laser comprising an active gain medium comprising a self-doubling microlaser crystal which has two opposite faces, and optical pumping means in operative relationship with said crystal for pumping said crystal in which self- frequency-doubling to the blue of its fundamental frequency occurs in the absence of resonant enhancement at the doubled frequency.

Specifically, the invention relates to a microlaser device which is based upon fundamental laser action on the ⁴F_3/2 — > %_a (-900 nm) in NYAB so as to provide a self-doubled output at -450 nm. Also, this microlaser device achieves a room-temperature Type-I phase match for the -900 nm line by employing a specifically oriented crystalline structure. An important aspect of the invention is that the laser employs a low concentration (-1%) for the Nd³⁺ ion, thereby allowing greater crystal length. This is essential in that it permits efficient self- doubling without requiring a harmonic resonance phase match at a low pumping power threshold.

Finally, the invention employs special dielectric coatings which are directly deposited on the crystal. Specifically, these combine high reflectivity at the fundamental wavelength of -900 nm on both surfaces, low reflectivity at 1062 nm and 1338 nm on both surfaces, high reflectivity for the second harmonic wavelength at -450 nm on the surface closest to the diode laser pump source, low reflectivity at -450 nm on the crystal surface which is opposite that which is coupled to the diode laser, and low reflectivity at the diode pump wavelength at 804 nm on the input crystal face.

BRIEF DESCRIPTION OF DRAWING

The objects, advantages and features of this invention will be more readily appreciated from the following detailed description, when read in conjunction with the accompanying drawing, in which: Fig. 1 is a schematic line diagram of a monolithic self-doubling blue microlaser that is the subject of this invention;

Fig. 1 A is an alternate embodiment of the microlaser of Fig. 1, showing a convex output face;

Fig. 2 is a schematic diagram of a monolithic self-doubling blue microlaser wherein pump radiation is coupled to the device via an intermediate focusing lens;

Fig. 2A is an alternate embodiment of the microlaser of Fig. 2, showing a convex input face; Fig. 3 illustrates an alternative embodiment wherein the monolithic self- doubling crystal is replaced by a two-part composite structure comprising a solid- state gain medium bonded directly to a nonlinear frequency doubling crystal;

Fig. 4 is an alternative embodiment similar to Fig. 3 showing a spacer employed for coupling the two crystals together;

Fig. 5 is an alternative embodiment of Fig. 3, showing a convex input face;

Fig. 6 is an alternative embodiment of Fig. 3, showing convex input and output faces; and

Fig. 7 is an alternative embodiment of Fig. 3, showing a convex output face.

BEST MODES FOR CARRYING OUT THE INVENTION

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 nonlinear self-doubling material and which is optically pumped by source 13, which is preferably a close-coupled source which emits at a near-infrared wavelength which is strongly absorbed by the gain medium. Gain medium 12 may be referred to as a doped nonlinear crystal. Suitable optical pumping means or sources include, but are not limited to, laser diodes and laser diode arrays, together with any conventional packaging or structures such as heat sinks, and thermoelectric coolers. 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. The term "thin" as used herein means an etalon no more than 2 mm thick.

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 804 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.

In the preferred embodiment of Fig. 1, gain material 12 is made from an etalon of neodymium yttrium aluminum borate (NYAB) and has two opposite, flat, parallel polished faces 14 and 15. It is oriented to have faces 14 and 15 perpendicular to the propagation axis for phase-matched second harmonic generation in NYAB of blue light wavelength of about 450 nm. The distance between faces 14 and 15 ranges between about 0.1 mm and 2.0 mm. At the present time, an element 12 would not likely function as desired if it were thinner than 0.1 mm. 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, "butt-coupled" 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., TEMr.₀ mode operation) in etalon 12. Preferably, optical pumping radiation is delivered to the lasant etalon in a direction which is substantially along a longitudinal optical path. The result is a compact all-solid-state device having a blue output. Alternatively, a focusing means or an imaging means can be used to image a laser diode into self-doubling gain material 22. This embodiment is illustrated as laser 21 in Fig. 2. An imaging means, such as lens 26, 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 about 900 nm and at about 450 nm. Opposite face 15 is coated with a dielectric for high reflection at 900 nm, the fundamental wavelength, and for 1% to 20% anti-reflection (AR) at the harmonic wavelength of 450 nm, so as to form an output coupler. Since it is necessary that the higher gain emission of NYAB at 1062 nm and 1338 nm be suppressed so as not to compete with emission at 900 nm, it is essential that the coatings applied to surfaces 14 and 15 exhibit low reflections at 1062 nm and 1338 nm. The terms "about 450 nm" and "about 900 nm" as employed herein are meant to include the ranges of 439.5 to 452.5 nm and 879 to 905 nm, respectively. It has been found that there are several potentially acceptable emission transitions within these wavelength ranges.

The input face of the etalon is also coated for high transmission (HT) at the pumping wavelength (about 800 nm) of source 13 to allow the pumping radiation to reach the self-doubling lasant material or crystal which forms the etalon. Faces 24 and 25 in Fig. 2 are coated similarly.

An alternative embodiment to the structures of Figs. 1 and 2 employs laser 31 having separate gain and frequency doubling media bonded to form a monolithic structure in the manner of Fig. 3. Here lasing medium 32 is bonded directly to frequency doubling crystal 36 at line 37. This bonding can be an optical contact, diffusion bonding, or can use index matching fluid optical epoxies, among others. As in the devices of Figs. 1 and 2, the end faces 34 and 35 are polished so as to be parallel. Pumping source 33 operates as previously described. Also, the reflectance/transmittance properties of the coating applied to surfaces 34 and 35 are arranged to have appropriate values at the diode pump wavelength and at wavelengths of 450 nm, 900 nm, 1062 nm and 1338 nm, as in the embodiments of Figs. 1 and 2. One possibility for this embodiment is to use rare earth ion doped and undoped nonlinear crystals. The Fig. 4 embodiment is structurally and functionally similar. Laser 41 includes the addition of spacer 47 between lasing medium crystal 42 and doubling crystal 46. As shown here, the spacer is annular in shape and creates a small air gap between the crystals. Crystals 42 and 46 may also be made of NYAB and undoped YAB, respectively. Pumping source 43 and end faces 44 and 45 operate the same as described for the other embodiments.

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 at the ⁴F_3/2 -→ ⁴I_9/2 transition in NYAB (-900 nm) for the -450 nm self-doubled output. The doping concentration of neodymium in the crystal is very low compared with the known prior art. The doping concentration ranges between 0.5 and 3.9%, preferably between 1.5 and 2.5%. It has been found that doping concentrations in the 6% range of the prior art would require such a high pumping threshold as to not be practical for the blue laser light of this invention. The low doping concentration is necessary to achieve lasing in the near infrared at -900 nm. It permits efficient self-doubling without requiring a resonant harmonic. Also important is the crystal orientation. This microlaser device achieves a room- temperature Type-I phase match for the -900 nm line by lasing polarized along the y-axis and orienting the fundamental such that it propagates at -36.2° from the z-axis toward the x-axis.

While NYAB is the preferred doped nonlinear crystal gain medium, other self-doubling materials include Nd:LaBGeO₄, Cr:KTP, Nd:KTP, Nd:LiNbO₃, and Nd:MgO:LiNbO₃. The reflectivities of the etalon faces are also important. These range between 99.5% and 99.95% for the 900 nm line, and 10% and 50% for 1062 nm and 1338 nm. This reflectivity arrangement enables the low pumping threshold, self-doubling, and prevents lasing of the 1062 nm and 1338 nm transitions. The term "low pumping power" is intended to mean a power less than 200-mW.

The Figs. 1 and 2 etalons are preferably less than 2 mm thick. Similarly, lasing medium 32 of Fig. 3 is of similar thickness. Frequency doubling crystal 36 would be less than 10 mm long from bond line 37 to output face 35.

While the preferred embodiment calls for both end faces 14 and 15 to be flat and parallel, that is not necessary for functioning of the invention. The input face may be flat, and the output face could be convex where the radius of curvature is chosen such that the microlaser forms a stable cavity. Alternatively the input face could be convex and the output face flat, with the same curvature radius requirements. Or even both input and output faces could be convex in shape, still having the above radius of curvature requirements. For example, output faces 18 in Fig. 1A and 78 in Fig. 7 are shown with the curvature mentioned. Input face 29 in Fig. 2A, 59 in Fig. 5 and both faces 68 and 69 in Fig. 6 are similarly shaped.

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 ~ etalon 12 can be circular or rectangular in shape. If desired, the etalon can be end-pumped by an optical fiber. Furthermore, etalon 12 can be formed from both organic and inorganic self- doubling lasant materials. 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, variations, etc., 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.

Claims

1. A laser comprising: an active gain medium comprising a self-doubling microlaser crystal which has two opposite faces; and optical pumping means in operative relationship with said crystal for pumping said crystal in which self-frequency-doubling to the blue of its fundamental frequency occurs in the absence of resonant enhancement at the doubled frequency.

2. The laser recited in claim 1, wherein said crystal comprises a gain medium in the form of a thin etalon of nonlinear, self-doubling material.

3. The laser recited in claim 2, wherein said etalon thickness ranges between about 0.1 and 2.0 mm.

4. The laser recited in claim 1, wherein said crystal is formed of neodymium yttrium aluminum borate (NYAB) with a neodymium doping concentration ranging between 0.5 and 3.9%.

5. The laser recited in claim 1, wherein said crystal is made from an etalon of neodymium yttrium aluminum borate (NYAB) formed with two opposite, flat, parallel polished faces which are perpendicular to the propagation axis for phase-matched second harmonic generation of blue light having a wavelength of about 450 nm.

6. The laser recited in claim 5, wherein the distance between said opposite faces ranges between about 0.1 and 2.0 mm.

7. The laser recited in claim 5, and further comprising: a dielectric coating on one of said faces which is highly reflective at the fundamental wavelength at about 900 nm and at the second harmonic blue light wavelength at about 450 nm; a dielectric coating on the other of said faces which is highly reflective at the fundamental wavelength from which the blue light is generated and is highly transmissive at the blue wavelength.

8. The laser recited in claim 7, wherein said other face dielectric coating is 1-20% anti-reflective of the blue wavelength.

9. The laser recited in claim 7, wherein said dielectric coatings on both faces have a low reflectance at 1062 nm and 1338 nm, a high gain emission of NYAB.

10. The laser recited in claim 7, wherein said one face coating has high transrnissivity at the pumping wavelength from said optical pumping source.

11. The laser recited in claim 1, wherein said optical pumping means is closely coupled to said gain medium.

12. The laser recited in claim 1, wherein said optical pumping means is spaced from said crystal, the output light from said optical pumping means being imaged onto said crystal.

13. The laser recited in claim 1, wherein said crystal comprises: an active gain medium element of NYAB adjacent said optical pumping means; and a frequency doubling element of undoped YAB bonded to said gain element to form a monolithic structure.

14. The laser recited in claim 13, wherein said frequency doubling element is bonded to said gain element by means of optically contacting, diffusion bonding or an index matching fluid.

15. The laser recited in claim 1, wherein said crystal is cut to produce Type-I second harmonic generation to the blue.

16. The laser recited in claim 15, wherein said microlaser achieves a Type-I phase match for the fundamental about 900 nm by lasing polarized along the y-axis and orienting the crystal such that the fundamental propagates at about 36.2° from the z-axis toward the x-axis.

17. The laser recited in claim 1, wherein said etalon is formed with one flat face adjacent said optical pumping means, the opposite face being convex.

18. The laser recited in claim 17, wherein said opposite face has a radius of curvature such that the laser forms a stable cavity.

19. The laser recited in claim 1, wherein said etalon is formed with an input face and an output face, at least one of said input face and output face having a convex shape.

20. The laser recited in claim 1, wherein said optical pumping means is close-coupled with said crystal and emits at a near-infrared wavelength which is strongly adsorbed by said gain medium.

21. The laser recited in claim 20, wherein said optical pumping means is selected from the group consisting of laser diodes and laser diode arrays.

22. The laser recited in claim 21, wherein said optical pumping means comprises at least one gallium aluminum arsenide (GaAlAs) laser diode which emits light at a wavelength of about 800 nm and which is attached to a heat sink.

23. The laser recited in claim 5, wherein the blue wavelength is in the range of 439.5 nm to 452.5 nm.

24. The laser recited in claim 16, wherein the fundamental wavelength is in the range of 879 nm to 905 nm.

25. A laser comprising: an active gain medium microlaser comprising a neodymium yttrium aluminum borate (NYAB) thin etalon crystal of nonlinear, self-doubling material to emit blue light in the about 450 nm wavelength range at room temperature in the absence of second-harmonic resonant enhancement, based on fundamental laser action in NYAB at about 900 nm, said crystal having a crystalline structure oriented at a predetermined orientation with respect to the optical pumping energy applied to said crystal; and optical pumping means in operative relationship with said crystal to pump said crystal at its fundamental frequency.

26. The laser recited in claim 25, wherein fundamental laser action at the ⁴F₃₂ -→ ⁴I_9/2 transition in NYAB is such as to provide the self-doubled output at about 450 nm.

27. The laser recited in claim 25, wherein said microlaser achieves a Type-I phase match for the fundamental about 900 nm by lasing polarized along the y-axis and orienting the crystal such that the fundamental propagates at about 36.2° from the z-axis toward the x-axis.

28. The laser recited in claim 25, wherein said thin etalon ranges between 0.1 and 2.0 mm thick.

29. The laser recited in claim 25, wherein the neodymium concentration ranges between 0.5 and 3.9%.

30. The laser recited in claim 25, wherein said etalon is formed with two opposite, flat, parallel polished faces which are perpendicular to the propagation axis for phase matched second harmonic generation of the blue light, said microlaser further comprising: a dielectric coating on a first said face adjacent said optical pumping means which is highly reflective at about 450 nm and at the fundamental of about 900 nm; and a dielectric coating on the second said face which is highly reflective at the fundamental wavelength and is partially transmissive at the blue wavelength.

31. The laser recited in claim 30, wherein said second face dielectric coating is 1-20% anti-reflective to the blue light.

32. The laser recited in claim 30, wherein said first face dielectric coating is highly transmissive at the pumping wavelength.

33. The laser recited in claim 25, wherein said optical pumping means is closely coupled to said gain medium.

34. The laser recited in claim 33, and further comprising imaging means mounted between said crystal and said optical pumping means.

35. The laser recited in claim 25, wherein said crystal comprises: an active gain medium element of NYAB adjacent said optical pumping means; and a frequency doubling element of undoped YAB bonded to said gain element to form a monolithic structure.

36. The laser recited in claim 34, wherein said frequency doubling element is bonded to said gain element by means of optically contacting, diffusion bonding or an index matching fluid.

37. The laser recited in claim 25, wherein said etalon is formed with one flat face adjacent said optical pumping means, the opposite face being curved.

38. The laser recited in claim 25, wherein said crystal is formed with an input face and output face, at least one of said input face and output face having a convex shape.

39. The laser recited in claim 25, wherein said optical pumping means produces optical radiation at about 800 nm, said gain medium has a neodymium doping concentration of about 1%, said microlaser exhibiting a room-temperature

Type-I phase match for the about 900 nm by lasing polarized along the y-axis and orienting the crystal such that the fundamental propagates at about 36.2° from the z-axis toward the x-axis.

40. The laser recited in claim 25, wherein said crystal is formed with first and second opposite faces having dichroic coatings thereon, said coatings on both faces having low reflectivity at about 1062 nm and about 1338 nm, low reflectivity on said first face which is adjacent said optical pumping means at the pumping wavelength.

41. The laser recited in claim 25, wherein the fundamental wavelength ranges between 879 nm and 905 nm.

42. The laser recited in claim 26, wherein the output wavelength ranges between 439.5 nm and 452.5 nm.