WO2011123822A2

WO2011123822A2 - Apparatus and method for generating continuous wave ultraviolet light

Info

Publication number: WO2011123822A2
Application number: PCT/US2011/031001
Authority: WO
Inventors: Alan Petersen; James Kafka
Original assignee: Newport Corporation
Priority date: 2010-04-02
Filing date: 2011-04-01
Publication date: 2011-10-06
Also published as: WO2011123822A3

Abstract

The present application is directed to various novel systems and methods for practical and efficient generating continuous wave ultraviolet light. More specifically, the present application discloses a continuous wave UV laser system and includes at least one laser source outputting an input signal, at least one intracavity harmonic generator in communication with the laser source and configured to output at least one harmonic signal of the input signal, at least one resonator having at least one Ti:sapphire gain cell configure to output at least one resonator signal, and at least one mixing assembly configured to receive the harmonic signal and the resonator signal and output an output signal.

Description

APPARATUS AND METHOD FOR GENERATING CONTINUOUS WAVE ULTRAVIOLET LIGHT

CROSS-REFERENCE TO RELATED APPLICAITONS

[0001 ] The present application claims priority to United States Provisional Patent Application Serial Number 61/341,696, filed April 2, 2010, and United States Provisional Patent Application Serial Number 61/427,771, filed December 28, 2010, the contents of which are incorporated by reference in their entirety herein

BACKGROUND

[0002] Many ultraviolet laser applications at wavelengths below 300nm, including, for example, semiconductor inspection applications, require a laser system that outputs continuous wave (CW) rather than pulsed beam. Some of these applications additionally require that the UV beam provided by the laser system be single frequency, i.e. a single transverse and longitudinal lasing mode. The best developed, most robust and electrically efficient CW lasers at the shortest wavelength are diode- pumped solid state (DPSS) lasers operating near l OOOnm. The most common of these are lasers operate on Nd+3 ion transitions at 1064nm. To generate an output light beam at wavelength below 300nm, the fourth harmonic of the fundamental 1064nm laser must be generated. Often, two cascaded second harmonic generation (SHG) processes are used to generate the fourth harmonic at a wavelength below 300nm. Harmonic conversion is more effective when the intensity of the beam to be converted is high. In cases where the harmonic conversion does not deplete the fundamental beam, second harmonic generation (hereinafter SHG) conversion is quadratic with the intensity of the fundamental beam. It is therefore advantageous to perform nonlinear harmonic and mixing processes within a laser resonator, where the circulating fundamental intensity can be high. [0003] Commercial CW DPSS lasers which produce their output via a SHG crystal within their resonant cavities are well developed. These intracavity second harmonic generation (ICSHG) lasers offer typical output powers of about 10W to about 20W at the second harmonic wavelength of 532nm The available output powers from these devices continue to increase with new designs. This output power is a significant fraction of the diode pump power, often about 25%. Thus, CW ICSHG DPSS lasers are efficient converters of the basic diode pump power to 532nm.

[0004] Currently there are a number of techniques used to generate the fourth harmonic output of 266nm from the second harmonic wavelength of 532nm. For example, sequential nonlinear mixing between the harmonic beams and a 1064nm beam may be employed to initially produce the third harmonic at 355nm and subsequently the fourth harmonic at 266nm. These mixing processes will typically have the highest efficiency when conducted within a laser resonant cavity at 1064nm and may employ one or more additional, free-running lasers coupled together to accomplish this mixing process. While this mixing technique has proven useful in the past, a number of shortcomings have been identified For example, although mixing with a high power 1064nm signal enhances the overall conversion from the second to fourth harmonic, systems employing this mixing scheme are quite complicated and less desirable for industrial use.

[0005] In response thereto, alternate methods of generating fourth harmonic optical signals at wavelengths below 300nm have been developed. For example, an alternate method for fourth harmonic generation uses a passive, external build-up cavity, containing another harmonic crystal to resonate the 532nm beam. By locking the resonant frequency of the build-up cavity to that of the 532nm beam, the circulating power in the build-up cavity can be greatly increased, resulting in efficient conversion to the fourth harmonic. ^"While this technique overcomes some of the problems associated with previous detailed mixing scheme, a number of shortcomings of the build-up cavity technique have been identified. Like the previous mixing scheme, the build-up cavity technique is also complex due to the fact that a high-speed electronic feedback loop must maintain lock between the two cavities. Mechanical and thermal perturbations resulting in than less than a wavelength shift can destroy the resonance and the harmonic conversion efficiency. As such, commercial products based on the build-up cavity scheme have not achieved success in industrial use.

[0006] Thus, in light of the foregoing, there is an ongoing need for an apparatus and method for practical and efficient production of continuous wave ultraviolet light.

SUMMARY

[0007] The present application is directed to various novel systems and methods for practical and efficient generating continuous wave ultraviolet light. In one embodiment, the present application is discloses a continuous wave UV laser system and includes at least one laser source outputting an input signal, at least one intracavity harmonic generator in communication with the laser source and configured to output at least one harmonic signal of the input signal, at least one resonator having at least one Rb gain cell configure to output at least one resonator signal, and at least one mixing assembly configured to receive the harmonic signal and the resonator signal and output an output signal.

[0008] In another embodiment, the present application is directed to a continuous wave UV laser system and includes at least one laser source outputting an input signal, at least one intracavity harmonic generator in communication with the laser source and configured to output at least one harmonic signal of the input signal, at least one resonator having at least one Ti:sapphire gain cell configured to output at least one resonator signal, and at least one mixing assembly configured to receive the harmonic signal and the resonator signal and output an output signal.

[0009] In still another embodiment, the present application is directed to a method of generating a continuous wave UV signal and discloses generating an input signal having a wavelength of about 532nm, generating a harmonic signal having a wavelength of about 266nm with a harmonic generator irradiated with the input signal, generating a resonator signal with a Rb laser resonator, the resonator signal having a wavelength of about 795nm, mixing the harmonic signal and resonator signal via sum frequency mixing with a mixing assembly, and outputting an output signal having a wavelength of about 1 9nm.

[001 0] In another embodiment, the present application discloses a method of generating a continuous wave UV signal and discloses generating an input signal having a wavelength of about 532nm, generating a harmonic signal having a wavelength of about 266nm with a harmonic generator irradiated with the input signal, forming a pump signal by splitter s portion of the harmonic signal with a beam splitter, generating a resonator signal with a Ti. sapphire laser resonator pumped with the pump signal, the resonator signal having a wavelength of about 707nm, mixing the harmonic signal and resonator signal via sum frequency mixing with a mixing assembly, and outputting an output signal having a wavelength of about 193nm.

[001 1 ] In addition, the present application discloses a wavelength sum frequency mixing assembly for with a laser system which includes at least one first mixing body formed from a nonlinear optical material having a first crystal orientation, and at least one second mixing body formed from a nonlinear optical material having a second crystal orientation, the second orientation of the second mixing body differing from the first orientation of the first mixing body, the first and second mixing bodies positioned in alternating pairs.

[001 2] Finally, the present application is directed to a method of manufacturing a sum frequency mixing assembly for use in a laser system and discloses providing a nonlinear optical material having a crystal orientation, cleaving the nonlinear optical material to form at least one first mixing body and at least one second mixing body, orientating the second mixing body such that the crystal orientation differs from the crystal orientation of the first mixing body, positioning alternating pairs of first mixing bodies and second mixing bodies within the path of at least collinear optical signal, each optical signal having a different wavelength, and correcting for walkoff of a desired optical signal while sum frequency mixing the two optical inputs signal having different wavelengths to produce an output signal having an output wavelength.

[001 3] Other features and advantages of the embodiments of the continuous wave ultraviolet laser systems disclosed herein will become apparent from a consideration of the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The various embodiments of the continuous wave ultraviolet laser systems described herein will be explained in more detail by way of the accompanying drawings, wherein:

[001 5] Figure l shows a block diagram of an embodiment of a laser system capable of generating continuous wave ultrav iolet laser light;

[001 6] Figure 2 shows a schematic diagram of an embodiment of a laser system for generating continuous wave ultraviolet light using a Rb gain cell;

[001 7] Figure 3 shows a schematic diagram of an embodiment of a laser system for generating continuous wave ultraviolet light using a Ti:sapphire gain cell;

[001 8] Figure 4 shows a schematic of an embodiment of a mixing assembly for use in a laser system for generating continuous wave ultraviolet light;

[001 9] Figure 5 shows a schematic of an embodiment of an external mixing assembly for use with a laser system for generating continuous wave ultraviolet light;

[0020] Figure 6 shows a schematic diagram of an embodiment of a mixing assembly for use in a continuous wave ultraviolet laser system, the mixing assembly having a mixing device formed from first mixing bodies and second mixing bodies;

[0021 ] Figure 7 shows an elevated perspective view of an embodiment of a mixing assembly for use in a continuous wave ultraviolet laser system, the mixing assembly having a mixing device formed from first mixing bodies and second mixing bodies separated by a distance Z ; and

[0022] Figure 8 shows an elevated perspective view of an embodiment of a mixing body for use in a mixing assembly of a continuous wave ultraviolet laser system.

DETAILED DESCRIPTION

[0023] The present application describes an apparatus and method for efficiently producing continuous wave ultraviolet light. Figure I shows a block diagram of an embodiment of a laser system capable of generating continuous wave ultraviolet laser light. As shown, the laser system 10 comprises at least one laser source 12 in optical communication with at least one resonator system 40. The resonator system 40 is configured to output a continuous wave ultraviolet signal 30 to a target..

[0024] Figure 2 shows a more detailed schematic of an embodiment of the laser system 10 shown in Figure 1. As shown, the laser system 10 includes at least a first laser source 12 configured to generate a first pump signal 14. In one embodiment, the first laser source 12 comprises an intracavity doubled Nd laser. In another embodiment, the first laser source 12 comprises a diode pumped solid state laser system. Optionally, the first laser source 12 may comprise any variety of laser systems.

[0025] Referring again to Figure 2, the first laser source 12 may be configured to output a first pump signal 14 to at least one harmonic generator 16 in optical communication with the laser source 12. In response thereto, the harmonic generator 16 may be configured to receive the pump signal 14 and output a harmonic pump signal 18. For example, in one embodiment, the laser source 12 is configured to output a pump signal 14 having a wavelength of about 532nm to the harmonic generator 16 may be configured to output at least one harmonic pump signal 18 having a wavelength of about 266nm, assuming that the harmonic generator 16 is configured to output second harmonic signals of an input signal. As such, in one embodiment, the laser system 10 may comprise a CW ICSHG laser device. Optionally, those skilled in the art will appreciate that the harmonic generator 16 may be configured to output any variety of harmonic signals. For example, the harmonic generator 16 may be configured to output second, third, fourth, and/or fifth harmonic signals of an input signal. In the illustrated embodiment, the laser source 12 and harmonic generator 16 comprise individual devices. Optionally, the harmonic generator 16 may be positioned within or otherwise included within the housing of the laser source 12. [0026] As shown in Figure 2, at least one beam splitter or beam director 20 may be used in the laser system 10. As shown, the beam director 20 may be positioned within a resonator cavity 40 and configured to receive the harmonic signal 18 and direct a portion of the signal to the lens, lens system, or optical suite 22. In one embodiment, the beam director 20 comprises a mirror, such as a dichroic mirror. In another embodiment, the beam director 20 comprises a beam splitter. Optionally, the beam director 20 may comprise one or more optical devices or elements configured to divide the harmonic signal 18 as desired. As such, those skilled in the art will appreciate that any variety of devices may be used as a beam splitter or beam director 20, including, without limitations, mirrors, gratings, holographic elements, and the like.

[0027] Referring again to Figure 2, the laser system 10 includes at least one laser resonator 40 defined by mirrors 28 and 48. In the illustrated embodiment, the laser resonator 40 comprises at least one resonator pump source 42 configured to provide one or more resonator pump signals 44 to at least one resonator gain cell 46. In one embodiment, the resonator pump source 42 comprises a diode laser, although those skilled in the art will appreciate that any variety of laser pump sources may be used. Optionally, a single laser source may be used to provide a pump signal to the harmonic generator 16 and the resonator gain cell 46. For example, a single laser source configured to output a wavelength configured to be frequency doubled to a wavelength of about 500nm to about 600nm may be provided. In the illustrated embodiment, the resonator gain cell 46 includes at least one b gain medium, such as Rb vapor. Optionally, any variety of gain cells may be used with the laser system 10. In the illustrated embodiment, the resonator gain cell 46 is configured to output at least one resonator signal 50 having a wavelength of about 794.8nm. Optionally, the resonator gain cell 46 may be configured to output a resonator gain signal 50 having any desired wavelength. As shown, the resonator 40 may be configured to direct the resonator signal 50 to the beam director 20.

[0028] As stated above, the laser resonator 40 may be defined by reflecting devices 28 and 48. Exemplary reflecting devices 28, 48 include, without limitations, mirrors, prisms, grisms, and similar devices. Optionally, at least one of the reflecting devices 28, 48 may comprise multiple optical elements. In the illustrated embodiment reflecting device 48 comprises is configured to reflect 99%+ of an incident signal at numerous wavelengths. In contrast, reflecting device 28 may comprise an output coupler. As such, reflecting device 28 may be configured to reflect less than 99% of an optical signal at a emission wavelength, while reflecting 99%+ of at least one signal at one or more secondary wavelengths.

[0029] Referring again to Figure 2, at least one focusing device or system 22 is positioned within the laser resonator 40. In the illustrated embodiment, the focusing system 22 is positioned proximate to the mirror 28, although in alternate embodiments the focusing system 22 is positioned anywhere within the resonator 40. Optionally, the focusing system 22 may be located external of the resonator 40. The focusing system 22 may be configured to focus or otherwise condition the harmonic output 18 and resonator signal 50 received from the beam director 20 prior to mixing the two signals in the mixing assembly 24. Exemplary focusing systems 22 include, without limitations, lenses, lens systems, mirrors, filters, gratings, and the like.

[0030] As shown in Figure 2, the mixing assembly 24 may be configured to combine the harmonic output 18 and the resonator signal 50 to produce an output signal 30 at a desired wavelength. As such, the overlapping incident harmonic output 18 and the resonator signal 50 maybe combined via sum frequency mixing to produce at least one output signal 30 at a desired wavelength. In one embodiment, the resonator 40 forms a high Q optical cavity, although those skilled in the art will appreciate that the resonator 40 may be formed any variety of resonator configurations. In one embodiment, the mixing assembly 24 includes at least one nonlinear optical material therein. A more detailed description of the mixing assembly 24 and the materials is provided in later sections of the application.

[0031 ] In one embodiment, the first output 14 from the laser source 12 is about 532nm. As a result, the harmonic output 18 of the harmonic generator 16 is about 266nm. If a resonator gain cell 46 comprises at least one Rb gain cell, the resonator signal 50 will have a wavelength from about 650nm to about 825nm. In another embodiment, the resonator signal 50 will have a wavelength of about 794.8nm. As such, the mixing assembly 24 will be simultaneously irradiated with a harmonic output 18 having a wavelength of about 266nm and a resonator signal 50 having a wavelength of about 794.8nm, resulting in an output signal 30 having a wavelength of about 199.3nm. As such, mirror 28 may be configured to transmit at least a portion of an output signal 30 having a wavelength of about 199.3nm, while reflecting substantially all other wavelengths.

[0032] Figure 3 shows a block diagram of an alternate embodiment of system for generating continuous wave ultraviolet laser light Like the previous system, the laser system 110 shown in Fig. 3 includes at least one laser source 112 configured to generate at least one signal 114. In one embodiment, the laser source 1 12 comprises an intracavity doubled Nd laser. In another embodiment, the laser source 112 comprises a diode pumped solid state laser system. Optionally, the laser source 112 may comprise any variety of laser systems.

[0033] Referring again to Figure 3, the laser source 112 outputs a signal 114 to at least one harmonic generator 116. The harmonic generator 1 16 may be configured to receive the signal 114 and output at least a harmonic signal 1 18 in response thereto. For example, in one embodiment, the laser source 1 12 is configured to output a signal 114 having a wavelength of about 532nm. In response thereto, the harmonic generator 116 may be configured to output a harmonic signal 1 18 having a wavelength of about 266nm, assuming that the harmonic generator 116 is configured to output second harmonic signals of an input signal. Typically, the harmonic generator 116 will convert a portion of the 532nm input signal to 266nm and output both a 532nm signal and a 266nm, harmonic signal. Those skilled in the art will appreciate that the harmonic generator 1 16 may be configured to output any variety of harmonic signals. For example, the harmonic generator 116 may be configured to output second, third, fourth, and/or fifth harmonic signals of the input signal. The harmonic signal 118 is incident on at least one beam splitter or beam separation device 132 configured to direct at least one signal 152 to the resonator gain cell 144 and to direct a portion of the harmonic signal 118 to at least one beam director 120. For example, the beam separation device 132 may be configured to transmit the 266nm signal therethrough while directing the 532nm signal to the resonator gain cell 144.

[0034] As shown in Figure 3, at least one beam director 120 may be used in the laser system 1 10. As shown, the beam director 120 may be positioned within the resonator 140 and configured to receive the harmonic signal 118 and direct the signal to the focusing device 122. In one embodiment, the beam director 120 comprises a mirror, such as a dichroic mirror. However, those skilled in the art will appreciate that any variety of devices may be used as a beam director 120, including, without limitations, mirrors, gratings, holographic elements, splitters, and the like.

[0035] As shown in Figure 3 the laser system 110 includes a laser resonator 140 defined by reflecting devices 128 and 148. In the illustrated embodiment, the laser resonator 140 comprises a resonator gain cell 144 configured to receive a portion of the pump signal 152 from the pump laser 112 via the beam separation device 132 and a reflecting device 134. In the illustrated embodiment, the resonator gain cell 144 comprises a Ti:saphhire crystal. Optionally, any variety of gain cells may be used with the laser system 110. In the illustrated embodiment, the resonator gain cell 144 is configured to output at least one resonator signal 150 having a wavelength of about 800nm. Optionally, the resonator gain cell 144 may be configured to output a resonator gam signal 150 having any desired wavelength. iTie resonator 140 may be configured to direct the resonator signal 150 to the beam director 120.

[0036] Referring again to Figure 3, optionally the laser resonator 140 may include one or more additional optical elements 146 located anywhere therein. For example, in the illustrated embodiment, the laser resonator 140 includes at least one wavelength selection device, although those skilled in the art will appreciate that any variety of optical elements may be included within the laser resonator 140. Exemplary optical elements 146 include, without limitations, prisms, gratings, holographic gratings or elements, birefringent filters, optical filters, polarizers, etalons, and the like. For example, the laser resonator 140 may include one or more bandwidth narrowing devices therein.

[0037] As stated above, the laser resonator 140 is defined by reflecting devices 128 and 148. In the illustrated embodiment reflecting device 148 comprises a high reflectance mirror configured to reflect 99%+ of an incident signal at numerous wavelengths. In contrast, reflecting device 128 may comprise an output coupler. As such, the reflecting device 128 may be configured to reflect less than 99% of an optical signal at a emission wavelength, while reflecting 99%+ of at least one signal at one or more secondary wavelengths. In one embodiment, at least one of the reflecting devices 128 and 148 comprise a prism, mirror, grism, and the like, although those skilled in the art will appreciate that any variety of devices may be used to form reflecting devices 128 and 148.

[0038] Referring again to Figure 3, at least one focusing device or system 122 is positioned within the laser resonator 140. In the illustrated embodiment, the focusing device 122 is positioned proximate to the reflecting device 128, although in alternate embodiments the focusing device 122 is positioned anywhere within the resonator 140. The focusing device 122 may be configured to focus or otherwise condition the harmonic output 1 18 and resonator signal 150 received from the beam director 120 prior to mixing the two signals in the mixing assembly 124. The mixing assembly 124 may be configured to combine the harmonic output 118 and the resonator signal 150 to produce an output signal 130 at a desired wavelength. Optionally, the focusing device 122 may be positioned within or outside the resonator 140.

[0039] Like the previous embodiment, the mixing assembly 124 contains at least one nonlinear optical material, thereby enabling sum frequency mixing of the harmonic output 118 and the resonator signal 150. For example, in one embodiment, the first output 114 from the laser source 112 is about 532nm. As a result, the harmonic output 1 18 of the harmonic generator 116 is about 266nm. If a resonator gain cell 146 comprises a Ti.saphirre gain device, the resonator signal 150 will have a wavelength of about 800nm. HoweveT, the output of a Ti:sapphire laser may be tunable form about 650nm to over 800nm. As such, a Ti. sapphire laser may be configured to output a signal at about 707nm. In another embodiment, the resonator gain cell 146 may be configured to output a signal at about 806nm or less. As such, the mixing assembly 124 will be simultaneously irradiated with a harmonic output 118 having a wavelength of about 266nm and a resonator signal 150 having a wavelength of about 707nm, resulting in an output signal 130 having a wavelength of about 193.3nm. As such, the mirror or output coupler 128 may be configured to transmit at least a portion of an output signal 130 having a wavelength of about 193.3nm, while reflecting substantially all otheT wavelengths. [0040] As discussed above, the laser system 10 of Figure 2 and the laser system 110 shown in Figure 3 each include at least one harmonic generation assembly, 16 and 116, respectively and at least one mixing assembly 24 and 124, respectively. In the discussion which follows, harmonic generation is considered a special case of frequency mixing, in which the two input signals to be mixed are the same. Thus, the phenomena and techniques described apply to both mixing and harmonic generation regardless of which process is explicitly cited. Typical nonlinear optical processes, including harmonic generation and sum frequency mixing are often accomplished by means of overlapping light beams within birefringent or nonlinear optical crystals. To be efficient, such a process requires a sufficiently large nonlinear coupling coefficient, precise phase matching, high optical intensity and long interaction length. Such conditions can be achieved by selecting a material with appropriate nonlinear properties, fabricating a sample with its crystal axes oriented for birefringent phase matching and focusing one or more optical beams within the crystal along the appropriate direction. The incident and generated beams exhibit ordinary (o) or extraordinary (e) polarization with respect to the plane defined by the unique optic axis of the crystal and the wavefront normal. The required phase matching between the beams occurs via the birefringence of the material.

[0041 ] Often, materials which have sufficient nonlinearity, transparency and birefringence at the relevant wavelengths also exhibit a characteristic known as walkoff. Walkoff is a condition where, for an e-polarized beam, the vector normal to the optical phase front and the energy flow vector are at a small angle to each other. The resuh is that, when phase-matched with an o-polarized beam, this beam "walks off' from the interaction zone. Although this angle is typically small (<5 degrees), in the very tight focus required to achieve high optical intensity, complete walkoff can occur within a short distance (~1 mm). When beams stay overlapped for only such a short distance, the nonlinear interaction must continually restart as the generating beam(s) pass through the medium, which seriously reduces overall nonlinear conversion efficiency. Furthermore, the beam generated by the nonlinear interaction will be elliptical, sometimes severely so. This is a practical disadvantage for many applications. [0042] As a result of walkoff, the two beams may separate as they propagate through the crystal. This phenomenon causes the generated harmonic beam to be elliptical in shape. In the low conversion limit and in the absence of walkoff, the harmonic power generated would be quadratic with nonlinear crystal length. With walkoff, this dependence is no more than linear. To partially overcome these problems, there are a variety of techniques known as walkoff compensation. In general, walkoff compensation occurs when the fundamental and harmonic beams pass through sequential sections of crystal where walkoff is in opposite directions. There is, therefore, less net walkoff over the entire path length, resulting in higher harmonic conversion efficiency and a less elliptical beam. Such compensation can be accomplished by passage through one or more pairs of crystals with rotated crystal axes.

[0043] As stated above, the harmonic generators 16, 116 and mixing assemblies 24, 124, each include at least one birefringent material. Typically, these devices and/or assemblies 16, 116, 24, 124 include one or more nonlinear optical crystals or materials. Presently, there are a limited number of nonlinear crystals which are sufficiently transparent at the required UV wavelengths, can achieve birefringent phase matching, and have adequate nonlinearity. Among these are KDP and its isomorphs, crystals of the KBBF family, RBBF, CLBO and BBO. Of these, BBO has by far the highest nonlinearity. Since all these crystals accomplish harmonic conversion by means of birefringent phase matching along a propagation direction away from the principal crystal axes, walkoff between the input and generated harmonic beam may occur. One beam will be an ordinary wave (o-wave) and the other will be an extraordinary wave (e-wave) with orthogonal polarizations. Although details vary with the particular crystal class and optical interaction, in general there are at least two optic axis orientations which achieve birefringent phase matching. The angles formed by the optic axis and propagation direction in these two cases have equal magnitude but opposite sign and result in walkoff in the opposite direction.

[0044] One technique used to mitigate the effects of walkoff is to replace a single nonlinear crystal of length L with a sequence of 2N short equal lengths, L/2N, which have alternating, optic axis orientations resulting in equal and opposite walkoff. As shown in Figure 4, the mixing assembly 24 may include a mixing device 60 comprised of a first mixing body 62 and at least a second mixing body 64, each mixing body 62, 64 having differing orientations. In the illustrated embodiment, the arrows indicate relative orientation of the relevant optic axis of the first and second mixing bodies 62, 64. The mixing device 60 may be composed of nonlinear pairs 62, 64 formed as separate crystals or a single unit composed of bonded segments. As such, mixing device may be positioned within the laser resonator 40, 140 of Figures 2 and 3, respectively. As such, Figure 4 shows a mixing device 60 wherein N^ml . In contrast, the mixing device 60 may include any number of crystal sections. For example. Figure 5 shows an embodiment of a mixing device 60 wherein N=4. As a result, in the plane wave approximation (near a focus), such a sequence will behave as a single crystal of length L but with a factor of 2N less walkoff, resulting in 2N increased nonlinear conversion efficiency and 2N reduced ellipticity in the generated beam. In the embodiment in Figure 4, the segments 62, 64 are contiguous and have equal length and identical optical properties. In one embodiment, the interfaces between the sections 62, 64 are approximately lossless. Because the optical axes of mixing bodies 62 and 64 are fixed relative to each other, there will be a unique set of fundamental and harmonic wavelengths for which nonlinear interaction occurs in both sections simultaneously with the desired results cited above, namely wavelength mixing with walkoff compensation. Small errors in the crystal orientation of each segment may make this set of wavelengths unequal to the desired wavelengths. Therefore, optionally, small crystal orientation errors can be compensated for in a number of ways. For example, compensation may be accomplished by varying the crystal temperature. Thus, there will be a unique temperature and adjustment angle for the assembly which will optimize the nonlinear interaction. In the embodiment illustrated in Figure 4, the mixing assembly 24 is positioned within the laser resonator 40, 140 (See Figures 2 and 3).

[0045] In the alternative, the mixing assembly 24 may include one or more additional optical components or elements. As such, the mixing assembly 24 may be positioned within or outside the resonator cavity 40, 140 (See Figures 2 and 3). Figure 4 shows an embodiment of a mixing assembly 24 configured to be positioned within a resonator cavity. In the alternative, Figure 5 shows an embodiment of a mixing assembly 24 positioned configured to be positioned outside the resonator cavity 40, 140 (See Figures 2 and 3). As shown, the mixing assembly 24 includes at least one mixing device 70. Like the previous embodiment, any variety of materials may be used to form the mixing body, including birefringent materials, nonlinear optical crystals, and the like. At least one beam splitter or director 72 may be used to direct an input signal 76 into the mixing device 70. Exemplary beam splitters 72 include, without limitations, dichroic mirrors, beam splitters, prisms, gratings and the like. The input signal 72 and a harmonic signal 78 generated by the mixing device 70 are incident on and reflected by a reflector 74. Thereafter, the harmonic signal 78 is separated by and reflected out of the mixing assembly 24 by the beam splitter 72.

[0046] A further limitation on harmonic conversion may occur due to diffraction of the fundamental beam. Even an ideal, diffraction-limited beam can remain focused over a limited distance. This limited focus distance is known as the Rayleigh range and represents an additional restriction on maximum useful nonlinear crystal length, since beyond the Rayleigh range, the fundamental beam will become larger, lose intensity and no longer generate significant harmonic. To overcome this, both the fundamental and harmonic beams must be re foe used periodically to continue the harmonic generation. Refocusing is best accomplished with a curved mirror rather than a lens to minimize dispersion effects between the fundamental and harmonic wavelengths. Even with these techniques, the fundamental and harmonic beams will eventually get out of phase, due to phase shifts in coatings and dispersion in the air. This will destroy the harmonic generation unless an adjustable phase plate is placed in the beams to compensate. An example of a technique for refocusing and phase compensation on multiple passes through a nonlinear crystal is disclosed in U S Patent 6,982,999, the entire contents of which are hereby incorporated by reference herein.

[0047] By combining walkoff compensation along with refocusing and phase correction on multiple passes through one or more harmonic crystals, the conversion efficiency of a harmonic generation assembly may be substantially increased. The external 266 rum harmonic schemes described above are entirely passive and involve no additional lasers or active stabilization. In one embodiment, it has been proffered that about 1% to about 5% of a 20W 532nm input can be converted to 266nm using the various walkoff compensation and multi-pass schemes disclosed herein. The resulting output of about 200mW to about 1 W at a wavelength of from about 250nm to about 270nm will be comparable to CW UV output available from other less electrically efficient or more complex sources.

[0048] As shown in Figure 6, the mixing device 60 may be formed from multiple mixing bodies 62, 64 positioned proximate to one another. In the embodiment illustrated in Figure 6, the mixing bodies 62, 64 are in contact thus forming a monolithic mixing device 60. In another embodiment, at least one thin buffer layer may be interposed between the adjacent mixing bodies 62, 64. This layer may be isotropic and have a refractive index very close to that of the mixing bodies 62, 64 so as to have almost no optical effect and in particular, almost no reflection. In one embodiment, the buffer layer may be configured to reduce mechanical stress at the interface between adjoining mixing bodies 62, 64 resulting from the anisotropic thermal expansion properties of the two mixing bodies.

[0049] In yet another embodiment shown in Figure 7, the mixing device 60 comprises mixing bodies 62 and 64 separated by a distance D. As such, the individual mixing bodies 62, 64 form discreet elements cooperatively forming the mixing device 60. In one embodiment, the mixing bodies 62, 64 are separated by a distance D approximately equal to a half integer number of wavelengths of the fundamental beam. This also corresponds to an integer number of wavelengths at the second harmonic. The result will be approximately zero reflection at each wavelength. As such, this separation between mixing bodies 62, 64 may be configured to provide a airgap between adjacent mixing bodies 62, 64, the airgap configured to provide virtual antireflective properties between the mixing bodies 62, 64. Optionally, at least one optical antireflection coating may be applied to at least one intermediate surface of each mixing body 62, 64. In one embodiment, each coating applied to the mixing bodies 62, 64 is configured to introduce minimal relative phase shift between the transmitted fundamental and harmonic beams and thus maximize the efficiency of the nonlinear interaction. For example, with such an antireflection coating on each intermediate surface of the mixing bodies 62, 64, spacing between the mixing bodies need not be related to the fundamental and harmonic wavelengths. Thus any convenient spacing between the mixing bodies can be used as long as their relative crystal orientation is preserved. Such spacing may be large compared to a wavelength but small compared to the crystal itself. In one embodiment, the faces of the mixing bodies 62, 64 are maintained parallel relative to one another. Optionally, the faces of the mixing bodies 62, 64 need not be maintained parallel relative to each other. For example, the spacing between the faces of adjoining mixing bodies 62, 64 may take the form of a thin wedged spacer with its wedge angle selected to compensate for crystalline axis orientation errors of the individual mixing bodies. Optionally, a sequence of such walkoff-compensated mixing bodies 62, 64 and wedged spacers may be assembled into a single, optimized nonlinear conversion structure. In another embodiment, the mixing bodies 62, 64 are manufactured from the same material. Optionally, at least one of the mixing bodies 62, 64 may be manufactured from different materials. Similarly, the physical properties such as thickness, cleave angles, and the like of the mixing bodies 62, 64 may be the same or different.

[0050] Optionally, at least one mixing body 62, 64 may include at least one or more optical coatings applied to at least one surface. For example, Figure 8 shows an embodiment of a mixing body 62 defining a first surface 84 and at least a second surface 86. In one embodiment, at least one anti-reflective coating may be applied to at least one of the first and second surfaces 84, 86 of the mixing body 62. Optionally, any variety of optical coating or materials may be applied to any surface of the mixing bodies 62, 64. In one embodiment, at least one optical coating is applied to at least one end surface of the mixing device 60

[0051 ] Referring again to Figure 7, the individual mixing bodies 62, 64 may be formed by cutting or otherwise sectioning a single homogenous crystal. Thereafter, the first and second surfaces 84, 86 of each mixing body 62, 64 may optionally be polished plane parallel. As such, each mixing body 62, 64 may be formed to have an equal path length. As stated above, optionally at least one antireflective coating may be applied to the mixing bodies 62, 64. As such, during use, the mixing body pairs 62, 64 cooperatively reduce or eliminate the walkoff between the beams during the mixing process.

[0052] While particular forms of embodiments have been illustrated and described, it will be apparent that various modifications can be made without departing from the spirit and scope of the embodiments of the invention. Accordingly, it is not intended that the invention be limited by the forgoing detailed description

Claims

What is claimed is: 1. A continuous wave U V laser system, comprising: at least one laser source outputting an input signal; at least one intracavity harmonic generator in communication with the laser source and configured to output at least one harmonic signal of the input signal: at least one resonator having at least one Rb gain cell configure to output at least one resonator signal; and at least one mixing assembly configured to receive the harmonic signal and the resonator signal and output an output signal.

2. The laser system of claim 1 wherein the laser source comprises a Nd laser source.

3. The laser system of claim 1 wherein the laser source comprises a diode pumped solid stated laser system.

4. The laser system of claim 1 wherein the input signal generated by the laser source has a wavelength of about 532nm.

5. The system of claim 1 wherein the harmonic generator is located within the laser source. 6. The system of claim 5 wherein the laser source and harmonic generator cooperatively form an intracavity second harmonic generator.

7. The laser system of claim 6 wherein the harmonic of the intracavity harmonic generator has a wavelength of about 266nm.

8. The laser system of claim 1 wherein the harmonic generator comprises a second harmonic generator.

9. The laser system of claim 1 wherein the harmonic generator comprises a third harmonic generator.

10. The laser system of claim 1 wherein the harmonic generator comprises a fourth harmonic generator. 11. The laser system of claim 1 wherein the resonator further comprises: a first reflector and at least a second reflector, the first and second reflectors defining the laser resonator, at least one of the first and second reflectors forming an output coupler configured to output at least one output signal at a desired wavelength; at least one pump laser configured to provide a pump signal; and at least one Rb gain device positioned within the laser resonator. 12. The laser system of claim 11 wherein the pump laser is positioned outside the resonator. 13. The laser system of claim 11 wherein the pump laser is positioned within the resonator. 14. The laser system of claim I wherein the laser resonator is configured to output a resonator signal having a wavelength between about 650nm and 825nm. 15. The laser system of claim 1 wherein the laser resonator is configured to output a resonator signal having a wavelength of about 794.8nm. 16. The laser system of claim 1 wherein the mixing assembly comprises at least one nonlinear optical material. 17. The laser system of claim 1 wherein the mixing assembly includes at least one material selected from the group consisting of KDP, isomorphs of KDP, KBBF, RBBF, CLBO, and BBO. 18. The laser system of claim 1 wherein the mixing assembly comprises a mixing device formed from a first mixing body and at least a second mixing body, the first and second mixing bodies.

1 . The laser system of claim 18 wherein mixing device is comprised of multiple sets of mixing pairs, each mixing pair formed by a first mixing body and a second mixing body.

20. The laser system of claim 18 wherein the first mixing body has a first orientation and the second mixing body has a second orientation, wherein the first and second orientations are different.

21. The laser system of claim 18 wherein the mixing device comprises a monolithic body formed from alternating first mixing bodies and second mixing bodies, the first and second mixing bodies having different orientations.

22. The laser system of claim 18 wherein the mixing device comprises alternating first mixing bodies and second mixing bodies, the first and second mixing bodies having different orientations, wherein each first and second mixing body is separated by a distance D.

23. The laser system of claim 22 wherein the distance D is equal to a half-integer multiple of the fundamental wavelength.

24. The laser system of claim 18 wherein the at least one of the first and second mixing bodies has at least one optical coating applied thereto.

25. The laser system of claim 24 wherein the optical coating comprises an antireflective coating.

26. The laser system of claim 1 wherein the mixing assembly is configured to receive an harmonic signal having a wavelength of about 266nm and a resonator signal having a wavelength of about 795nm and output an output signal hav ing a wavelength of about 199nm via sum frequency mixing.

27. A continuous wave UV laser system, comprising: at least one laser source outputting an input signal; at least one intracavity harmonic generator in communication with the laser source and configured to output at least one harmonic signal of the input signal; at least one resonator having at least one Ti;sapphire gain cell configured to output at least one resonator signal; and at least one mixing assembly configured to receive the harmonic signal and the resonator signal and output an output signal.

28. The laser system of claim 27 wherein the laser source comprises a Nd laser source.

29. The laser system of claim 27 wherein the laser source comprises a diode pumped solid stated laser system.

30. The laser system of claim 27 wherein the input signal generated by the laser source has a wavelength of about 532nm.

31. The system of claim 27 wherein the harmonic generator is located within the laser source.

32. The system of claim 31 wherein the laser source and harmonic generator cooperatively form an intracavity second harmonic generator.

33. The laser system of claim 32 wherein the harmonic of the intracavity harmonic generator has a wavelength of about 266nm.

34. The laser system of claim 27 wherein the harmonic generator comprises a second harmonic generator.

35. The laser system of claim 27 wherein the harmonic generator comprises a third harmonic generator.

36. The laser system of claim 27 wherein the harmonic generator comprises a fourth harmonic generator.

37. The laser system of claim 27 further comprising at least one beam splitter in optical communication with the laser source and the harmonic generator, the beam splitter configured to transmit the harmonic signal therethrough and direct a portion of the input signal into the Tiisapphire laser resonator as a pump signal.

38. The laser system of claim 27 wherein the resonator further comprises: a first reflector and at least a second reflector, the first and second reflectors defining the laser resonator, at least one of the first and second reflectors forming an output coupler configured to output at least one output signal at a desired wavelength; and at least one Ti.sapphire gain device positioned within the laser resonator, the Ti.sapphire gain device configured to generate a resonator signal when irradiated with the pump signal.

39. The laser system of claim 27 wherein the laser resonator further includes at least one wavelength selection devices positioned therein.

40. The device of claim 39 wherein the wavelength selection device is selected from the group consisting of prisms, gratings, holographic gratings or elements, birefringent filters, optical filters, polarizers, and etalons.

41. The laser system of claim 27 wherein the laser resonator is configured to output a resonator signal having a wavelength between about 650nm and 825nm.

42. The laser system of claim 27 wherein the laser resonator is configured to output a resonator signal having a wavelength of about 707nm.

43. The laser system of claim 27 wherein the mixing assembly comprises at least one nonlinear optical material.

44. The laser system of claim 27 wherein the mixing assembly includes at least one material selected from the group consisting of KDP, isomorphs of KDP, K.BBF, CLBO, and BBO

45. The laser system of claim 27 wherein the mixing assembly comprises a mixing device formed from a first mixing body and at least a second mixing body, the first and second mixing bodies.

46. The laser system of claim 45 wherein mixing device is comprised of multiple sets of mixing pairs, each mixing pair formed by a first mixing body and a second mixing body.

47. The laser system of claim 45 wherein the first mixing body has a first orientation and the second mixing body has a second orientation, wherein the first and second orientations are different.

48. The laser system of claim 45 wherein the mixing device comprises a monolithic body formed from alternating first mixing bodies and second mixing bodies, the first and second mixing bodies having different orientations.

49. The laser system of claim 45 wherein the mixing device comprises alternating first mixing bodies and second mixing bodies, the first and second mixing bodies having different orientations, wherein each first and second mixing body is separated by a distance D.

50. The laser system of claim 49 wherein the distance /⁾ is equal to a half-integer multiple of the fundamental wavelength.

51. The laser system of claim 45 wherein the at least one of the first and second mixing bodies has at least one optical coating applied thereto.

52. The laser system of claim 51 wherein the optical coating comprises an antireflective coating.

53. The laser system of claim 27 wherein the mixing assembly is configured to receive an harmonic signal having a wavelength of about 266nm and a resonator signal having a wavelength of about 707nm and output an output signal having a wavelength of about 193nm via sum frequency mixing.

54. A method of generating a continuous wave UV signal, comprising. generating an input signal having a wavelength of about 532nm; generating a harmonic signal having a wavelength of about 266nm with a harmonic generator irradiated with the input signal; generating a resonator signal with a Rb laser resonator, the resonator signal having a wavelength of about 795nm; mixing the harmonic signal arid resonator signal via sum frequency mixing with a mixing assembly; and outputting an output signal having a wav elength of about 199nm.

55. A method of generating a continuous wave UV signal, comprising: generating an input signal having a wavelength of about 532nm; generating a harmonic signal having a wavelength of about 266nm with a harmonic generator irradiated with the input signal; forming a pump signal by splitter s portion of the harmonic signal with a beam splitter; generating a resonator signal with a Ti:sapphire laser resonator pumped with the pump signal, the resonator signal having a wavelength of about 707nm; mixing the harmonic signal and resonator signal via sum frequency mixing with a mixing assembly; and outputting an output signal having a wavelength of about 193nm.

56. A wavelength sum frequency mixing assembly for with a laser system, comprising: at least one first mixing body formed from a nonlinear optical material having a first crystal orientation; and at least one second mixing body formed from a nonlinear optical material having a second crystal orientation, the second orientation of the second mixing body differing from the first orientation of the first mixing body, the first and second mixing bodies positioned in alternating pairs.

57. The device of claim 56 wherein the mixing assembly forms a monolithic structure formed from alternating first mixing bodies and second mixing bodies.

58. The device of claim 56 wherein the mixing assembly comprises first mixing bodies and second mixing bodies, each separated by a distance D.

59. The device d of claim 58 wherein distance D is equal to a half-integer multiple of the fundamental wavelength. 60. The device of claim 56 wherein at least one of the first mixing body and second mixing body include at least one optical coating applied to a surface thereof. 61. The device of claim 60 wherein the optical coating comprises an antireflective coating. 62. The device of claim 56 wherein the first mixing body and second mixing body are manufactured from the same nonlinear optical material. 63. The device of claim 56 wherein the first mixing body and second mixing body are manufactured from different nonlinear optical material.64. The device of claim 56 wherein at least one of the first mixing body and second mixing body is manufactured from a material selected from the group consisting of KDP, isomorphs of KDP, KBBF, CLBO, and BBO. 65. The device of claim 56 wherein the first mixing body and second mixing body are manufactured from the same material. 66. The device of claim 56 wherein the first mixing body and second mixing body are manufactured from the different materials. 67. A method of manufacturing a sum frequency mixing assembly for use in a laser system, comprising: providing a nonlinear optical material having a crystal orientation; cleaving the nonlinear optical material to form at least one first mixing body and at least one second mixing body; orientating the second mixing body such that the crystal orientation differs from the crystal orientation of the first mixing body ; positioning alternating pairs of first mixing bodies and second mixing bodies within the path of at least collinear optical signal, each optical signal having a different wavelength; and correcting for walkoff of a desired optical signal while sum frequency mixing the two optical inputs signal having different wavelengths to produce an output signal having an output wavelength. 68. The method of claim 67 further comprising positioning the second mixing body in contact with the first mixing body. 69. The method of claim 67 further comprising forming a monolithic mixing assembly by coupling second mixing bodies to first mixing bodies.

70. The method of claim 67 further comprising separating the second mixing bodies from the first mixing bodies by a distance D.

71. The method of claim 67 further comprising applying at least one optical coating at least one of the first and second mixing bodies.