Compact efficient and robust ultraviolet

Abstract

A compact and efficient ultraviolet laser source based on a optically-pumped solid-state or fiber laser that produces near-infrared output light suitable for nonlinear frequency conversion. The infrared laser output is frequency tripled or quadrupled to produce light in the ultraviolet wavelength range (200 nm to 400 nm). The novel technology is the use of highly efficient periodically poled nonlinear crystals, such as stoichiometric and MgO-doped lithium tantalate and lithium niobate. As opposed to conventional frequency-converted UV laser sources, which have high power consumption, high cost, and low efficiency, the laser sources of this invention utilize high efficiency nonlinear conversion provided by periodically poled materials and allow lower-cost architectures without additional focusing lenses, high power pump diodes, etc.

Description

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1. Ultraviolet, pulsed laser source with passively Q-switched, infrared, solid-state, microchip laser, external second-harmonic generation using a periodically poled nonlinear crystal, and external third harmonic generation using a periodically poled nonlinear crystal. The two crystals may be integrated in a single optical chip with two poling sections.

FIG. 2. Ultraviolet, pulsed laser source with passively Q-switched, infrared, solid-state, microchip laser, external second-harmonic generation using a periodically poled nonlinear crystal, and external third or fourth harmonic generation using a bulk nonlinear crystal.

FIG. 3. Ultraviolet, cw laser source with a cw, infrared, solid-state, microchip laser with intracavity second-harmonic generation using a periodically poled nonlinear crystal, and external third harmonic generation a periodically poled nonlinear crystal.

FIG. 4. Ultraviolet, cw laser source with a cw, infrared, solid-state, microchip laser with intracavity second-harmonic generation using a periodically poled nonlinear crystal, and external third or fourth harmonic generation a bulk nonlinear crystal.

FIG. 5 Ultraviolet cw laser source with cw infrared, solid-state, microchip laser with both intracavity second and third harmonic generation using the single nonlinear element with two periodically poled sections.

DETAILED DESCRIPTION OF THE INVENTION

The technological challenges in the periodic poling of MgO or ZnO-doped and stoichiometric LiNbO₃ and LiTaO₃ have recently been overcome by the inventors and these new materials have proven to be readily manufacturable. Short poling period crystals suitable for laser conversion into visible and near-UV wavelength ranges have been produced and technology for such production process has been described in copending, commonly assigned (Published US Patent Application 2005/0,133,477). The teaching of this patent creates an opportunity to provide an efficient and compact UV laser source platform based on solid-state lasers.

The present invention discloses compact and low-cost architectures and method for building UV laser sources based on periodically poled lithium niobate and lithium tantalate that contain dopants such as MgO or ZnO or have a specified degree of stoichiometry that ensures high reliability of these materials. The following solid-state laser platforms provide efficient and low-cost UV output:

• • 1. UV laser source based on the compact, passively Q-switched microchip solid-state laser platform, externally converted into the visible and UV wavelength radiation
  • 2. UV laser source based on the cw microchip solid-state laser platform, converted into the visible light by intracavity SHG and externally (single pass SHG) converted into UV light.

The family of highly efficient, reliable, and manufacturable periodically poled materials as described in the above-indicated pending application is a new and enabling feature of the present invention. These materials includes PPMgOLN (periodically poled MgO-doped lithium niobate), PPMgOLT (periodically poled MgO-doped lithium tantalate), PPZnOLN (periodically poled ZnO-doped lithium niobate), PPZnOLT (periodically poled ZnO-doped lithium niobate), PPSLN (periodically poled stoichiometric or near-stoichiometric lithium niobate), and PPSLT (periodically poled stoichiometric or near-stoichiometric lithium niobate).

The preferred embodiments for the compact UV laser sources of this invention are illustrated in FIGS. 1-5. It should be understood that these figures provide the detail necessary to illustrate more general concepts and reasonable deviations from the configurations shown are also within the scope of the present invention.

FIG. 1 illustrates an ultraviolet, pulsed laser source with a passively Q-switched, infrared, solid-state, microchip laser, external second-harmonic generation using a periodically poled nonlinear crystal, and external third harmonic generation using a periodically poled nonlinear crystal. The two crystals may be integrated in a single optical chip with two poling sections.

The pump diode laser 1 emits a beam 2, preferably at a wavelength between 800 and 900 nm, such as ˜808 nm or 885 nm for efficient absorption into the gain material 8. The beam 2 is usually astigmatic and beam-shaping optics 3 is used to convert the pump beam 2 into beam 4 so that beam 4 can form a circular cross-section of the desired diameter on surface 9 of the gain crystal 5. This type of pumping scheme can efficiently overlap the pump area in the gain crystal with the intracavity circulating beam, which should be a single-spatial mode (or TEM₀₀) for efficient nonlinear frequency doubling. A typical diameter for the pump spot on the gain crystal 5 is in the range of 100 to 300 microns. The output power of laser diode 1 is in the range of several Watts. The beam-shaping optics can be a micro-lens, a gradient-index lens, or a combination of such optical elements. When efficiency can be sacrificed in favor of simplicity and compactness, the beam-shaping optics 3 can be eliminated. Another part of assembly 3 may be a volume Bragg grating used to narrow down the spectral emission of the diode laser 1. Narrowing down the spectral output of the pump laser may be beneficial for the efficiency of the solid-state laser. Methods to achieve such spectral narrowing have been described, e.g., in the paper by L. Glebov. “Optimizing and Stabilizing Diode Laser Spectral Parameters,” Photonics Spectra, January 2005.

The gain medium 5 is preferably a Nd-doped crystal with a higher gain in one axis, such as Nd:YVO₄ or Nd:GdVO₄ so that the element 5 provides both gain and polarization control for the laser cavity. Alternatively, a gain crystal such as Nd:YAG with equal gain for both crystalline axes can be used. One advantage of using Nd:YAG is that it can provide a lasing wavelength of 946 nm that can be frequency doubled into 473 nm and then tripled to provide a 237 nm output beam. In case of using Nd:YAG, a method of polarization control for the solid-state laser is necessary. These may include adding extra polarizing elements such as Brewster surfaces, waveplates, or using a polarization sensitive saturable absorber element as described below to provide higher loss to the undesired polarization. These methods are known in the art of making infrared microchip solid-state lasers and can be implemented by skilled laser designers and will not be discussed further. Further discussion on this topic can be found, for example, in the paper by H. Liu, O. Homia, Y. C. Chen, and S.-H. Zhou, “Single-frequency Q-switched Cr—Nd:YAG laser operating at 946-nm wavelength,” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 3, p. 26 (1997) and references cited therein.

The level of Nd doping for maximizing laser efficiency in our invention will typically be in the range of 1% to 3% atm (atomic percent). The crystal 5 also provides the transverse mode control in the otherwise flat-flat laser cavity through gain-guiding and thermal lensing effects. Element 6 is a saturable absorber that is used for passive Q-switching of the laser. In a preferred embodiment, the saturable absorber 6 is optically bonded or otherwise attached to the gain crystal 5 without the need for high-cost active alignment and forms a monolithic, alignment-free laser resonator defined by the end mirrors 9 and 11. The saturable absorber 6 may be based on either a solid-state or semiconductor material. The well-known example of solid-state based saturable absorber is Cr:YAG. These crystals are commercially available from several commercial suppliers, e.g. from Casix Semiconductor-based saturable absorbers, also known as semiconductor saturable absorber mirrors (SESAM) have also been known for some time. They can be based on such material systems as InGaAsP and are also available from several commercial suppliers, e.g. from Del Mar Photonics. The use of solid-state-based saturable absorbers in a microchip laser is described in the literature, e.g. by J. J. Zayhowski and C. Dill Ill, “Diode-pumped passively Q-switched picosecond microchip lasers,” Optics Letters, vol. 19, p. 1427 (1994). The use of SESAM is described, e.g. in a paper by R. Fluck, B. Braun, E. Gini, H. Melchior, and U. Keller, “Passively Q-switched 1.34-micron Nd:YVO₄ microchip laser with semiconductor saturable-absorber mirrors,” Optics Letters, vol. 22, p. 991 (1997). The laser cavity mirrors 9 and 11, which are essentially optical coatings or semiconductor epitaxial layers, are designed to provide high reflectivity at the target infrared wavelength. In most Nd-based solid-state gain elements 6 (including Nd:YAG, Nd:YVO₄, Nd:GdVO₄, Nd:YLF, etc.), the most efficient lasing wavelength is approximately 1064 nm. However, lasing at other infrared wavelengths such as 946 nm, 914 nm, 1340 nm, etc. can also be obtained by providing high reflectivity end mirrors at these wavelengths and ensuring that the reflectivity of these mirrors at the dominant 1064 nm wavelength is low enough to suppress lasing at that wavelength. Thus, a passively Q-switched operation is established at the fundamental infrared wavelength and the intracavity fundamental wavelength beam 12 (illustrated by a two-sided arrow to show the circulating nature of that beam) is outcoupled into the beam 13.

To illustrate the laser conversion into ultraviolet, we consider the specific case of a Nd:YAG or Nd:YVO₄ microchip laser with Cr:YAG saturable absorber, producing passively Q-switched, 1064 nm wavelength output. For a properly optimized laser system, one can approach ˜50% conversion efficiency from 808 nm pump to 1064 nm in TEM₀₀ spatial mode. Thus, ˜2W pump diode can deliver ˜1W average power output at 1064 nm. The repetition rate will be in the tens of kHz range and pulse duration will be nanoseconds and sub-nanosecond. The diameter of the output infrared beam 12 is suitably in the range 100 to 300 microns. In continuous-wave operation, one would need strong focusing to achieve SHG conversion into only several milliwatts of green (532 nm) light. However, pulsed (passively Q-switched) output allows obtaining high peak power levels that significantly increase conversion efficiency. For a Cr:YAG saturable absorber, the typical pulse duration is on the order of 1 ns and the repetition rate will be on the order of several tens of kHz. The high conversion efficiency can be appreciated from the analysis of Eq. (6) where all power variables for the pulsed operation correspond to the instantaneous power in the pulse due to the substantially instantaneous nature of nonlinear processes. For the laser system of this invention, Eq.(6) is not perfectly applicable since this equation is obtained under the assumption of a non-depleted fundamental beam. Thus, most of the 1064 nm infrared beam can be converted into the 532 nm green beam via SHG.

The second-harmonic crystal 7 shown in FIG. 1 is suitably PPMgOLN, PPZnOLN, or PPSLN since these crystals have highest available nonlinearity d_eff˜15-16 pm/V. However, lithium tantalate based periodically poled materials with a nonlinearity ˜10 pm/V are also efficient enough to provide >80% conversion into the green wavelength. FIG. 1 illustrates the second-harmonic beam 14 generated in the periodically-poled crystal 7. The lateral separation of the fundamental beam 13 and the second-harmonic beam 14 in FIG. 1 is for illustration purposes. In actual practice, these beams are necessarily spatially overlapped.

Further, the remaining fundamental beam 13 and the second-harmonic beam 14 are mixed in periodically poled crystal 8 to generate third-harmonic, ultraviolet light. The crystal 8 is suitably lithium-tantalate based crystal: PPSLT, (periodically poled stoichiometric lithium tantalate) PPMgOLT, (periodically poled MgO doped lithium tantalate) or PPZnOLT (periodically poled ZnO doped lithium tantalite). The reason why lithium tantalate is preferred over periodically poled lithium niobate crystals is that lithium tantalate crystals have a lower short-wavelength absorption edge ˜280 nm compared to ˜320 nm for lithium niobate. This allows using lithium tantalate as an efficient nonlinear material for third-harmonic generation into the ultraviolet. Using PPMgOLN, PPZnOLN, and PPSLN is also within the scope of this invention but the efficiency of these materials is lower due to higher absorption.

To obtain the maximum third-harmonic output, it is important not to deplete the fundamental infrared beam to the level that there is not enough power left to mix it with the second harmonic for the third harmonic generation. This can be achieved by using a short crystal or via temperature detuning of the second harmonic crystal 7 to reduce the converted second-harmonic power to the desired level. The third-harmonic crystal 8 is then temperature tuned to produce the maximum UV output. The UV output beam is illustrated by the arrow 15, which overlaps with beams 13 and 14 but is laterally separated from them in FIG. 1 for illustration. The beams 13 and 14 can then be blocked or deflected with a filter or coated optics to leave the ultraviolet beam as the only laser source output. In the example we are considering for a 1064 nm fundamental infrared laser, several tens of milliwatts of average ultraviolet-beam (355 nm) power can be obtained. The required nonlinear crystal lengths are only 1-3 mm.

One advantage of the efficient UV laser source of FIG. 1 is its compactness and low-cost architecture. When traditional nonlinear materials, such as LBO, are used, one would have to work against a two-order of magnitude disadvantage in conversion efficiency compared to periodically poled materials. Therefore, additional focusing elements (lenses), longer crystals, and a means of increasing peak pulse power would have to be designed to produce the desired performance in the UV. This is not necessary in the design of FIG. 1 and it eliminates costly elements and labor (alignment steps). Furthermore, periodically poled nonlinear crystals 7 and 8 can be integrated and designed as a single periodically poled chip. For the reasons explained above, lithium tantalate based crystals are preferred in this case. The fabrication of such nonlinear crystals can be done with a dual period poling mask so that the resulting crystal has two poling sections optimized for both nonlinear SHG and THG conversions. Another major advantage of periodically poled crystals is the absence of walk-off between the fundamental beam and other harmonics. This factor is a major limiting factor in efficiency and cost for traditional bulk nonlinear materials (including KTP, LBO, BBO, CLBO, KNbO₃, LiNbO₃) and expensive schemes such as using more than one crystal for walk-off compensation or heating or cooling the crystal to extreme temperatures for noncritical (walk-off-free) phase matching have to be used.

While FIG. 1 illustrates an efficient and preferred embodiment for the passively Q-switched ultraviolet laser source, there are instances when traditional bulk nonlinear materials are preferable for conversion into the ultraviolet. This is illustrated in the second embodiment of the present invention shown in FIG. 2. The architecture for the fundamental and second-harmonic beam generation is the same as in FIG. 1 and the efficient visible light generation is again enabled by the efficient periodically poled nonlinear materials selected as described in this invention. However, the conversion into the ultraviolet is done using bulk nonlinear materials such as BBO, CLBO, LBO, or CBO. One case when using these materials is necessary is to obtain shorter ultraviolet wavelengths, such as 266 nm, via fourth-harmonic generation (FHG) process. In this case, the ultraviolet output is obtained via frequency doubling of the second-harmonic beam 14 in the bulk nonlinear crystal 16.

Another case, when the design of FIG. 2 may be advantageous relative to the design of FIG. 1 is the potentially simpler temperature control of this design, in either THG or FHG case. In this case, one can use the critical phase-matching in material such as BBO, CLBO, LBO, or CBO and tune the bulk crystal into phase matching so both the SHG periodically poled crystal 7 and THG (or FHG) bulk crystal 16 are optimized at the same temperature so they can be controlled by the same thermoelectric cooler.

In some applications, such as the detection of fluorescing particles via flow cytometry, a continuous-wave (cw) ultraviolet source may be preferred over the pulsed one. In this case, the external-nonlinear-conversion architectures displayed in FIGS. 1 and 2 are not going to provide enough efficiency to generate more than one milliwatt of the ultraviolet output, even with the high efficiency of periodically poled materials. The architecture we propose in this invention for a cw ultraviolet laser source will take advantage of efficient intracavity frequency doubling of the infrared solid-state laser using a periodically poled nonlinear material. For the example considered above, over 500 mW of the 532 nm green output can be obtained by using PPMgOLN as the intracavity frequency doubler crystal. A more detailed description of such compact and efficient intracavity frequency doubled architectures can be found in copending, commonly assigned U.S. Provisional Patent Application by S. Essaian and A. V. Shchegrov, “Efficient, compact, and reliable solid-state laser with nonlinear frequency conversion in periodically poled materials” Ser. No. 60/795/212 filed Apr. 27, 2006 the teaching of which is incorporated herein by this reference.

Since sufficiently powerful (hundreds of milliwatts to Watts, depending on the pump and lasing wavelength) visible output is generated, it can be used for generating UV externally either through THG or FHG. The architectures for cw ultraviolet laser source are illustrated in FIGS. 3 and 4. The design of FIG. 3 relies on periodically poled nonlinear material for external THG into the UV wavelength region, while the design of FIG. 4 relies on bulk nonlinear material. The UV conversion concepts of FIGS. 3 and 4 are analogous to those shown in FIGS. 1 and 2, respectively. Using efficient periodically poled materials remains the main feature of all these embodiments. It must be noted that for the case of the THG, the outcoupling surface 22 will advantageously be coated for partial transmission at the fundamental wavelength. In the case of FHG, the surface 22 is coated for high reflection at the fundamental wavelength and for high transmission at the second-harmonic (visible) wavelength. For the example of 1064 nm fundamental beam, tens of milliwatts of cw power can be obtained at 355 nm and sub-milliwatt to milliwatt-level output are obtained at 266 nm. These power output levels are sufficient for many applications of compact UV lasers, especially in detecting fluorescence.

The design of FIG. 5 demonstrates the architecture of an extremely compact and efficient ultraviolet laser source of 355 nm, 315 nm and 305 nm using an infrared, solid-state, microchip laser with intracavity second-harmonic generation using the first periodically poled section of a periodically poled nonlinear crystal, and an intracavity third harmonic generation using the second periodically, poled section of a periodically poled nonlinear crystal. For this application PPSLT or PPMgOSLT and PPZnOSLT(with 0.1-1% MgO and ZnO doped) nonlinear crystals are preferable to use due to low optical losses of LiTaO3 material at 280-350 nm UV spectral region.

In FIG. 5, 105 is the outcoupled infrared beam, preferably blocked by the coated surface 103 that defines the end of the infrared laser cavity. The coating 103 is designed for ˜100% reflection of the fundamental infrared beam (101) and high transmission of the second and third harmonics. 101 is the intracavity infrared beam, circulating in the laser cavity. 106 is the second-harmonic beam. For clarity, only one direction of propagation of this beam is shown. The backward generated second harmonic beam (not shown) can be recovered by a suitable coating at surface 9 or at the interface 20. 107 is the third-harmonic beam. Similarly, only one direction of propagation of this beam is shown. No third harmonic is generated in the backward direction. 100 is a periodically poled crystal consisting of two sections: 101 (second-harmonic section) and 102 (third harmonic section).

The designs illustrated in FIGS. 1-5 are general to the efficient generation of ultraviolet light starting from essentially any infrared wavelength available through a solid-state laser platform. Therefore, frequency doubled or quadrupled UV laser sources obtained via conversion of 946 nm, 914 nm, 1340 nm, and other available solid-state laser wavelengths, are also within the scope of this invention.

It must also be stated that the above description of our invention focused on illustrating the novel and enabling designs for UV laser sources based on periodically poled, nonlinear materials. Manufacturing a reliable UV laser source product usually requires additional effort to avoid optics contamination, maintain laser stability via an adequate control algorithm, and prevent optics surface damage by using translation of THG or FHG crystals to spread the UV exposure over extended crystal area. All of these engineering techniques known to persons skilled in the art of ultraviolet laser sources and are also applicable to the practice of the present invention. The teaching of the following references is incorporated herein in their entirety by this reference.

• • 1. D. Dudley, N. Hodgson, H. Hoffman, and O. Mehl, “Diode pumped laser with intracavity harmonics,” U.S. Pat. No. 7,016,389.
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  • 6. P. Georges, F. Balembois, F. Druon, A. Brun, P. J. Devilder, “Sub-nanosecond passively Q-switched microchip laser system,” U.S. Pat. No. 6,373,864.
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  • 8. K. Mizuuchi, A. Morikawa, T. Sugita, K. Yamamoto, N. Pavel, and T. Taira, “Continuous-wave ultraviolet generation at 354 nm in a periodically poled MgO:LiNbO₃ by frequency tripling of a diode end-pumped Nd:GdVO₄ microlaser,” Applied Physics Letters, vol. 85, p. 3959 (2004).
  • 9. T. Volk, N. Rubinina, M. Wöhlecke, “Optical-damage-resistant impurities in lithium niobate,” Journal of the Optical Society of America B, vol. 11, p. 1681 (1994).
  • 10. Y. Furukawa, K. Kitamura, S. Takekawa, K. Niwa, H. Hatano, “Stoichiometric Mg:LiNbO₃ as an effective material for nonlinear optics,” Optics Letters, vol. 23, p. 1892 (1998).
  • 11. Spectralus Corporation Web Site: http://www.spectralus.com
  • 12. S. Essaian, “Method for the fabrication of periodically poled lithium niobate and lithium tantalate nonlinear optical components,” US patent application 2005/0,133,477.
  • 13. L. Glebov, “Optimizing and Stabilizing Diode Laser Spectral Parameters.” Photonics Spectra, January 2005.
  • 14. S. Essaian and A. V. Shchegrov, “Efficient, compact, and reliable solid-state laser with nonlinear frequency conversion in periodically poled materials,” U.S. provisional patent application.
  • 15. H. Liu, O. Hornia, Y. C. Chen, and S.-H. Zhou, “Single-frequency Q-switched Cr—Nd:YAG laser operating at 946-nm wavelength,” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 3, p. 26 (1997).
  • 16. J. J. Zayhowski and C. Dill Ill, “Diode-pumped passively Q-switched picosecond microchip lasers,” Optics Letters, vol. 19, p. 1427 (1994).
  • 17. R. Fluck, B. Braun, E. Gini, H. Melchior, and U. Keller, “Passively Q-switched 1.34-mm Nd:YVO₄ microchip laser with semiconductor saturable-absorber mirrors,” Optics Letters, vol. 22, p. 991 (1997).

Claims

1. A ultraviolet, pulsed solid-state laser comprising i) a solid state laser cavity defined by two end mirrors,ii) a solid-state gain crystal pumped by a semiconductor diode laser and disposed between said two mirrors,ii) a saturable absorber element for providing passive Q-switchingiii) a first periodically poled nonlinear crystal, for frequency doubling of the fundamental, infrared-wavelength laser beam into the visible-wavelength beam, said crystal having material stoichiometry and/or dopingiv) a second nonlinear crystal for nonlinear conversion of the resulting visible and infrared beam into an ultraviolet beam.

2. The laser of claim 1, in which the solid-state gain crystal is Nd:YVO4, Nd:GdVO4, or Nd:YGdVO4, Nd:YAG, or Nd:YLF.

3. The laser of claim 1, in which the pump laser beam is delivered to the gain crystal via a microlens or a gradient-index lens.

4. The laser of claim 1, in which the pump laser beam is delivered the gain crystal directly, without beam shaping optics.

5. The laser of claim 1, in which the pump laser beam is spectrally narrowed using a volume Bragg grating.

6. The laser s of claim 1, in which the first nonlinear crystal is selected from the group of crystals comprising PPMgOLN (periodically poled MgO-doped lithium niobate), PPMgOLT (periodically poled MgO-doped lithium tantalate), PPZnOLN (periodically poled ZnO-doped lithium niobate), PPZnOLT (periodically poled ZnO-doped lithium niobate), PPSLN (periodically poled stoichiometric or near-stoichiometric lithium niobate), and PPSLT (periodically poled stoichiometric or near-stoichiometric lithium niobate).

7. The laser of claim 1, in which the second nonlinear crystal is selected from the family of lithium niobate and lithium tantalate based crystals that comprises PPMgOLN (periodically poled MgO-doped lithium niobate), PPMgOLT (periodically poled MgO-doped lithium tantalate), PPZnOLN (periodically poled ZnO-doped lithium niobate), PPZnOLT (periodically poled ZnO-doped lithium niobate), PPSLN (periodically poled stoichiometric or near-stoichiometric lithium niobate), and PPSLT (periodically poled stoichiometric or near-stoichiometric lithium niobate).

8. The laser of claim 1, further comprising an external non-linear crystal selected from the family of bulk borate-based nonlinear crystals comprising LBO, BBO, CLBO, or CBO.

9. The laser of claim 1, wherein the fundamental wavelength of the solid state laser is operates at one of the following wavelengths: 1064 nm, 946 nm, 914 nm, and 1340 nm.

10. The laser of claim 1 wherein the ultraviolet wavelength is obtained via third-harmonic generation in the second nonlinear crystal and is emitted at approximately one of the following wavelengths: 355 nm, 315 nm, and 305 nm.

11. The laser of claim 1, in which the saturable absorber comprises a doped solid-state gain crystal.

12. The saturable absorber of claim 11, which comprises a Cr:YAG crystal.

13. The laser of claim 1, in which the saturable absorber comprises an epitaxially grown semiconductor.

14. The laser of claim 1, in which the ultraviolet wavelength is obtained via fourth-harmonic generation in the second nonlinear crystal and is emitted at or near the following wavelengths: 266 nm, 237 nm, 229 nm, and 335 nm.

15. A ultraviolet continuous-wave laser comprising: i) a solid state laser cavity defined by two end mirrors,ii) a solid-state gain crystal pumped by a semiconductor diode laser and disposed between the two mirrors,iii) a periodically poled nonlinear crystal designed for intracavity frequency doubling of the infrared laser beam into the visible-wavelength beam said crystal having material stoichiometry or dopingiv) a second nonlinear crystal providing extra-cavity nonlinear conversion of the resulting visible and infrared beam into the ultraviolet beam.

16. The laser of claim 15, wherein the solid-state gain crystal is Nd:YVO4, Nd:GdVO4, or Nd:YGdVO4, Nd:YAG, or Nd:YLF.

17. The laser of claim 15, in which the pump beam is delivered to the gain crystal via a microlens or a gradient-index lens.

18. The laser of claim 15, in which the pump beam is delivered to the gain crystal directly, without beam shaping optics.

19. The laser of claim 15, in which the pump laser is spectrally narrowed using a volume Bragg grating.

20. The laser of claim 15, in which the intracavity nonlinear crystal is selected from the group of crystals comprising PPMgOLN (periodically poled MgO-doped lithium niobate), PPMgOLT (periodically poled MgO-doped lithium tantalate), PPZnOLN (periodically poled ZnO-doped lithium niobate), PPZnOLT (periodically poled ZnO-doped lithium niobate), PPSLN (periodically poled stoichiometric or near-stoichiometric lithium niobate), and PPSLT (periodically poled stoichiometric or near-stoichiometric lithium niobate).

21. The laser of claim 15, in which the external nonlinear crystal is selected from the family of lithium niobate and lithium tantalate based crystals comprising PPMgOLN (periodically poled MgO-doped lithium niobate), PPMgOLT (periodically poled MgO-doped lithium tantalate), PPZnOLN (periodically poled ZnO-doped lithium niobate), PPZnOLT (periodically poled ZnO-doped lithium niobate), PPSLN (periodically poled stoichiometric or near-stoichiometric lithium niobate), and PPSLT (periodically poled stoichiometric or near-stoichiometric lithium niobate).

22. The laser of claim 15, further comprising an external nonlinear crystal is selected from the family of bulk borate-based nonlinear crystals comprising LBO, BBO, CLBO, and CBO.

23. The laser of claim 15, in which the fundamental wavelength of the solid state laser is optimized for operation at or near one of the following wavelengths: 1064 nm, 946 nm, 914 nm, 1340 nm.

24. A continuous-wave ultraviolet laser of claim 15 comprising: i) a solid state microchip laser cavity defined by two end mirrors,ii) a solid-state gain crystal pumped by a semiconductor diode laser and disposed between the two mirrors,iii) a periodically poled nonlinear crystal comprising two sections with different periods providing both intracavity second frequency doubling and third harmonic generation of the infrared laser beam and said crystal having material stoichiometry or doping

25. The laser of claim 24, wherein the ultraviolet wavelength is obtained via intracavity harmonic generation by nonlinear conversion of the resulting visible-second harmonic and infrared fundamental beams into the ultraviolet beam, and is emitted at approximately one of the following wavelengths: 355 nm, 315 nm, or 305 nm.

26. The laser of claim 24 wherein the solid-state gain crystal is Nd:YVO4, Nd:GdVO4, Nd:YGdVO4, Nd:YAG, or Nd:YLF.

27. The laser of claim 24, wherein the pump laser beam is delivered to the gain crystal via a microlens or a gradient-index lens.

28. The laser of claim 24, wherein the pump laser beam is delivered to the gain crystal directly, without the utilization of beam shaping optics.

29. The laser of claim 24, wherein the pump laser is spectrally narrowed using a volume Bragg grating.

30. The laser of claim 24 in which the nonlinear crystal is selected from the family of lithium niobate and lithium tantalate based crystals comprising PPMgOLN (periodically poled MgO-doped lithium niobate), PPMgOLT (periodically poled MgO-doped lithium tantalate), PPZnOLN (periodically poled ZnO-doped lithium niobate), PPZnOLT (periodically poled ZnO-doped lithium tanatelate), PPSLN (periodically poled stoichiometric lithium niobate), and PPSLT (periodically poled stoichiometric lithium tantalte), PPMgOSLN or PPZnOSLN(periodically poled MgO or ZnO doped near stoichometric litium niobat), and PPMgOSLT or PPZnOSLT(periodically poled MgO or ZnO doped near stoichometric litium tantalate).