WO2009154745A2

WO2009154745A2 - Hygroscopic crystal protection

Info

Publication number: WO2009154745A2
Application number: PCT/US2009/003613
Authority: WO
Inventors: Wen-Jui Ray Chia; Ming Ko; Hong Fu
Original assignee: Ams Research Corporation
Priority date: 2008-06-17
Filing date: 2009-06-17
Publication date: 2009-12-23
Also published as: WO2009154745A3

Abstract

Embodiments of the invention are directed to a laser resonator (102) that includes a nonlinear hygroscopic crystal (1 12) having an optical surface (130 or 132) and a coating (134) comprising a fluoropolymer on the optical surface. Another embodiment is directed to a method of protecting a hygroscopic crystal. In the method, a nonlinear hygroscopic crystal (112) having an optical surface (130 or 132) is provided (142). A fluoropolymer is also provided (144). The optical surface of the crystal is coated (146) with the fluoropolymer.

Description

HYGROSCOPIC CRYSTAL PROTECTION

BACKGROUND

Embodiments of the invention generally relate to optical crystals and more particularly the protection of surfaces of hygroscopic crystals.

Laser systems, such as surgical laser systems, utilize crystals that are prone to interact with a surrounding atmosphere to an extent that is sufficient to change the crystal's surface properties, resulting in subsequent degradations in the material's performance. For instance, it is known that optical surfaces on hygroscopic crystals such as are used in laser resonators or light modulators, for example, can deteriorate very quickly when exposed to water vapor from the surrounding environment.

Embodiments described herein provide solutions to these and other problems, and offer other advantages over the prior art.

SUMMARY

Embodiments of the invention are generally directed to the protection of nonlinear crystals utilized in laser resonators to generate a harmonic of a laser beam. More specific embodiments are directed to a laser resonator that includes a nonlinear hygroscopic crystal having an optical surface and a coating comprising a fluoropolymer on the optical surface.

Another embodiment is directed to a method of protecting a hygroscopic crystal. In the method, a nonlinear hygroscopic crystal having an optical surface is provided. A fluoropolymer is also provided. The optical surface of the crystal is coated with the fluoropolymer.

Another embodiment of the invention is directed to a surgical laser system that includes a laser resonator and a laser delivery probe. The laser resonator comprises a laser element and a nonlinear hygroscopic crystal. The laser element is configured to emit laser light at a first frequency responsive to a light input. The nonlinear hygroscopic crystal is in the optical path of the emitted laser light and has an optical surface, on which is a coated comprising a fluoropolymer. The laser delivery probe is configured to discharge the laser light having the second frequency. Other features and benefits that characterize embodiments of the present disclosure will be apparent upon reading the following detailed description and review of the associated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified block diagram of a surgical laser system in accordance with embodiments of the invention.

FIG. 2 is a perspective view of a crystal 112 having optical surfaces that are coated with a fluoropolymer, in accordance with embodiments of the invention. FIG. 3 is a side cross- sectional view of the crystal of FIG. 2 taken generally along line 3-3.

FIG. 4 is a flowchart illustrating a method of protecting a crystal for use in a laser resonator, in accordance with embodiments of the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Embodiments of the present invention relate to the protection of surfaces of optical crystals. More specific embodiments relate to the protection of nonlinear crystals utilized in laser resonators to generate harmonics of a laser beam. Embodiments of the invention can be used with laser systems, such as surgical laser systems, which are commonly utilized to vaporize or ablate tissue, for example, to treat a condition of a patient.

FIG. 1 is a simplified block drawing of an exemplary surgical laser system 100 in accordance with embodiments of the invention. The exemplary system 100 comprises a laser resonator. The exemplary laser resonator 102 may include a first resonator mirror 104, a second resonator mirror 106 and a laser rod or element 108. In one embodiment, the laser element 108 comprises a yttrium-aluminum-garnet crystal rod with neodymium atoms dispersed in the YAG rod to form a Nd: YAG laser element. Other conventional laser elements 108 may also be used.

The laser element 108 is pumped by a light input 110 from an optical pump source, such as a Kr arc lamp or other conventional optical pump source, to produce laser light or beam 111 at a first frequency. The laser resonator 102 also includes a nonlinear (NLC) crystal 112 for generating a second harmonic of the laser beam 111 emitted by the laser element 108. The laser beam 111 inside the resonator 102 bounces back and forth between the first and second resonator mirrors 104 and 106, reflects off a folding mirror 116 and propagates through the laser element 108 and nonlinear crystal 112. The laser element 108 has optical gain at a certain wavelength and this determines the wavelength of the laser beam 111 inside the resonator 102. This wavelength is also referred to as the fundamental wavelength. For the Nd: YAG laser element 108, the fundamental wavelength is 1064 nm.

When the laser beam 111 inside the resonator 102 propagates through the nonlinear crystal 112 in a direction away from the folding mirror 116 and toward the second resonator mirror 106, a beam 118 of electromagnetic radiation at the second harmonic wavelength output from the crystal 112. The second resonator mirror 106 is highly reflective at both the fundamental and second harmonic wavelengths and both beams 111 and 118 propagate back through the nonlinear crystal 112. On this second pass, more beams 118 at the second harmonic wavelength are produced. For example, the nonlinear crystal 112 can produce a laser beam 118 having a wavelength of approximately 532 nm (green) when a Nd: YAG rod is used as the laser element 108. One advantage of the 532 nm wavelength is that it is strongly absorbed by hemoglobin in blood and, therefore, is useful for cutting, vaporizing and coagulating vascular tissue. The folding mirror 1 16 is highly reflective at the fundamental wavelength and is highly transmissive at the second harmonic wavelength and hence the laser beam 118 at the second harmonic passes through the folding mirror 116 and produces a second harmonic laser beam 118 outside the optical resonator 102. An optical coupler 120 is connected to a waveguide, such as a fiber optic cable 122, to deliver the laser beam 118 to a laser delivery probe 124, such as a side-fire optical fiber, that delivers the beam 118 to desired tissue for treating a condition of the patient.

The laser beam 111 inside the resonator 102 at the fundamental wavelength continues through the laser element 108 and reflects off the first resonator mirror 104 which is highly reflective at the fundamental wavelength. A Q-switch 126 may be used in the resonator 102 to change the laser beam 111 to a train of short pulses with high peak power. These short pulses increase the conversion efficiency of the second harmonic laser beam 118 and increase the average power of the laser beam 118 outside the resonator 102.

In one embodiment, the crystal 112 is a hygroscopic crystal. Exemplary hygroscopic crystals include borate crystals, such as lithium borate LBO (LiB₃O₄), BBO (P-BaB₂O₄), and CLBO (CsLiB₆O₁₀). Other examples of known hygroscopic crystals are the so-called ADP-isomorphs, such as KD₂PO₄ (KD*P), NH₄D₂ PO₄ (deuterated ADP, or AD*P) and CsD₂AsO₄ (CD*A), as well as others. These crystals are commonly used in electro-optic light modulators or in nonlinear frequency conversion of laser light to shorter wavelengths (i.e., higher frequency). Unfortunately, these crystals may suffer deterioration in performance upon mere exposure to ambient environment, such as air. This is because they can chemically react with absorbed water molecules. Such reactions can cause undesirable alterations in the crystals' optical and physical properties and are highly detrimental to reliable and long term use and operation of the crystals in high power laser systems. Embodiments of the invention generally relate to the protection of one or more optical surfaces of the nonlinear crystal 112, which may be configured for use in a laser resonator, such as the exemplary laser resonator 102 described above. FIG. 2 is a perspective view of the crystal 112 and FIG. 3 is a side cross- sectional view of the crystal 112 illustrated in FIG. 2 taken generally along line 3-3.

The crystal 112 may be in the form of a rectangular prism, a rod or other conventional shape. In one embodiment, the crystal 112 includes first and second optical surfaces 130 and 132. In one embodiment, the optical surfaces 130 and 132 is the face 131 of the crystal 112. In accordance with another embodiment, the optical surfaces 130 and/or 132 comprise the face 131 of the crystal 112 and an optical coating 133 on face 131, as shown in FIGS. 2 and 3. Exemplary embodiments of the optical coating 133 include an anti-reflective coating(s) (i.e., coating or film), a highly reflective coating(s) (i.e., coating or film), and/or other conventional coatings or films.

In one embodiment, at least one of the first and second optical surfaces 130 and 132 includes a coating, that includes a fluoropolymer (hereinafter "fluoropolymer coating), which will generally be referred to as fluoropolymer coating 134. Thus, embodiments include a fluoropolymer coating 134A on the first optical surface and/or a fluoropolymer coating 134B on the second optical surface 132, as shown in FIG. 2. When the surface 130 and/or 132 includes the optical coating 133, the fluoropolymer coating 134 is preferably applied over the optical coating 133. Alternatively, the coating 134 can be applied directly to the face 131 of the surface 130 and/or 132 with the optical coating 133 applied over the coating 134. When the surface 130 and/or 132 does not include the optical coating 133, the fluoropolymer coating 134 is preferably applied directly to the face 131 of the optical surface 130 and/or 132 of the crystal 112.

In one embodiment, the crystal 112 and the fluoropolymer in the coating 134 are selected to be substantially transparent to the fundamental laser beam 111 generated by the laser element 108. Most fluoropolymers are regarded as being transparent to most laser beams having a wavelength of greater than 157 nm. Accordingly, fluoropolymers are generally suitable for use with the Nd: YAG laser element and other laser elements used in surgical laser systems. Accordingly, the fundamental laser beam 111 is configured to pass through the fluoropolymer coating 134 A (if present), the first optical surface 130, the body 138 of the crystal 112, the second optical surface 132 and the second fluoropolymer coating 134B (if present). At least a portion of the beam 111 is output from the second optical surface 132 as the second harmonic laser beam 118, as illustrated in FIG. 2.

In one embodiment, the fluoropolymer coating 134 covers the entire first optical surface 130 and/or the second optical surface 132. It is not essential to polish the non-optical surfaces of the remaining portion of the body 138 of the crystal 112, however, in one embodiment, the entire crystal 112 is coated in the fluoropolymer coating 134.

In one embodiment the fluoropolymer coating 134 has a thickness 140 in the range of 3-500,000 angstroms. In one embodiment, the fluoropolymer coating has a thickness in the range of 3-100 angstroms. This thickness may be most suitable to provide protection of the surface 130 and/or 132 when applied over the optical coating 133, since it will have a negligible effect on the optical properties of the optical surface. In one embodiment, the fluoropolymer coating can be used as an anti-reflective coating on the face 131 of the optical surface 130 and/or 132. This generally requires a thicker coating, such as in the range of 1-10 microns. The fluoropolymer coating 134 can take on many different forms. In one embodiment, the fluoropolymer coating 134 is in the form of a lubricant. In another embodiment, the fluoropolymer coating 134 is in the form of a gel or paste. In another embodiment, the fluoropolymer coating 134 is in the form of a liquid. In another embodiment, the fluoropolymer coating 134 is in the form of a solid.

In accordance with one embodiment, the fluoropolymer used to form the coating 134 comprises perfluoropolyether (PFPE), such as Fomblin® perfluoropolyether. Other suitable fluoropolymers are marketed under the trade names Krytox®, Fomblin® Z DOL, Cytop™ and others.

The fluoropolymer coating 134 is hydrophobic and repels moisture and, thus, the fluoropolymer coated optical surface or surfaces of the crystal 112 are protected from water vapor that may be present in the environment while maintaining the substantial transparency of the crystal 112 to the wavelengths of laser light utilized in surgical laser systems. This provides particularly useful protection of hygroscopic crystals. The fluoropolymer coated surfaces of the crystal 112 are also protected from extreme temperatures in the range of -100 to 200 degrees Celsius. The fluoropolymer coating 134 is non-stick and also are resistant to most chemicals and body fluids.

FIG. 4 is a flowchart illustrating a method of protecting the crystal 112 in accordance with embodiments of the invention. At 142, the crystal 112 is provided having an optical surface, such as optical surface 130 or 132, shown in FIGS. 3 and 4. In one embodiment, the crystal 112 is a hygroscopic crystal, as discussed above. At 144, a fluoropolymer is provided. The fluoropolymer can take on any of the exemplary forms described above including perfluoropolyether, a gel, a paste, a liquid and a solid.

Finally, at 146, the optical surface of the crystal 112 is coated with the fluoropolymer to form the embodiments of the crystal 112 described above. That is, the fluoropolymer is applied to the one or more of the optical surfaces of the crystal 112. In accordance with additional embodiments, the crystal 112 includes both first and second optical surfaces 130 and 132, and the fluoropolymer coating 134 is applied to both of the surfaces 130 and 132, as illustrated in FIGS. 2 and 3. In one embodiment, the fluoropolymer coating 134 on one or both of the optical surfaces 130 and 132 is 3-5,000 angstroms thick.

The fluoropolymer coating or applying step 146 can be carried out using a variety of different techniques. In one embodiment of step 146, the fluoropolymer is wiped on the optical surfaces 130 or 132 using a cloth, or other suitable medium. In another embodiment, the optical surface 130 and/or optical surface 132 may be coated with the fluoropolymer by dipping the surface 130 and/or 132 into the fluoropolymer. In accordance with another embodiment, the fluoropolymer is sprayed on the surface 130 and/or surface 132. In accordance with these embodiments, the fluoropolymer is preferably in the form of a lubricant, or mixed with appropriate solvents. Exemplary solvents include fluorocarbon compounds, such as Fluorinert™ produced by 3M™, Asahiklin® AK-225 family and other fluorocarbon and hydrogenated fluorocarbons. The fluoropolymer can also be in the form of a gel. In accordance with another embodiment of step 146, the fluoropolymer is deposited on to the optical surface 130 and/or the optical surface 132 through a chemical vapor deposition (CVD) or physical vapor deposition (PVD) technique. When the fluoropolymers to be deposited on the surface 130 and/or surface 132 of the crystal 112 using chemical vapor deposition, the fluoropolymer is provided in a gaseous state. For the physical vapor deposition process, the fluoropolymer is provided in a solid or liquid phase and is deposited to the surface 130 or the surface 132 of the crystal 112 in a substantially dry state. In accordance with another embodiment of step 146, the fluoropolymer is applied or coated to the surface 130 and/or the surface 132 of the crystal 112 through a conventional ambient vapor deposition process.

In accordance with another embodiment, the fluoropolymer is printed onto the surface 130 and/or 132 in step 146. This generally involves a contact printing or contact coating process. Prior to or after the fluoropolymer is coated or applied to the optical surface 130 and/or the optical surface 132 of the crystal 112 in step 146, the crystal 112 and/or the fluoropolymer may be heated to assist in the application of the fluoropolymer to the desired surface of the crystal 112 or to dry the fluoropolymer on the surface. For instance, the coated crystal 112 may be heated from 50-200°C for a period of 1-500 minutes.

In accordance with one embodiment, the fluoropolymer has a functional group, such as carboxylic, hydroxyl, amino, amide, vinyl, acetylenic bonding, to allow further chemical reactions of adhesion promoting, cross-linking and inter- and intra-molecular polymerizations via free radical or condensation reactions to promote adhesion of the fluoropolymer to the optical surface 130 and/or the optical surface 132 of the crystal 112.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A laser resonator comprising: a nonlinear hygroscopic crystal (112) having an optical surface (130 or 132); and a coating (134) comprising a fiuoropolymer (hereinafter

"fluoropolymer coating") on the optical surface.

2. The laser resonator of claim 1, wherein the optical surface comprises an optical coating (133) selected from the group consisting of an anti- reflective coating and a highly reflective coating.

3. The laser resonator of claim 1 , wherein: the optical surface is a first optical surface (130); the fluoropolymer coating is a first fluoropolymer coating (134A); the hygroscopic crystal comprises a second optical surface (132); and the laser resonator comprises a second coating (134B) comprising a fluoropolymer (hereinafter "second fluoropolymer coating") on the second optical surface.

4 The laser resonator of claim 3, further comprising a laser element (108) configured to emit laser light (111) at a first frequency responsive to a light input (110), wherein the emitted laser light passes through the first and second fluoropolymer coatings and the first and second optical surfaces, and is output (118) from the hygroscopic crystal having a second frequency, which is higher than the first frequency.

5. The laser resonator of claim 1, wherein the fluoropolymer coating comprises perfluoropolyether.

6. The laser resonator of claim 5, wherein the hygroscopic crystal comprises a crystal selected from the group consisting of a LBO crystal (LiB₃O₄), a BBO (β-BaB₂O₄) crystal, a CLBO (CsLiB₆O₁₀) crystal, a KD*P (KD₂PO₄) crystal, an AD*P (NH₄D₂ PO₄) crystal and a CD*A (CsD₂AsO₄) crystal.

7. The laser resonator of claim 1, wherein the fluoropolymer coating is in a form selected from the group consisting of a gel, a paste, a liquid and a solid.

8. A method of protecting a hygroscopic crystal comprising: providing (142) a nonlinear hydroscopic crystal (112) having an optical surface (130 or 132); providing (144) a fluoropolymer; and coating (146) the surface with the fluoropolymer.

9. The method of claim 8, wherein providing a fluoropolymer comprises providing a coating comprising perfluoropolyether.

10. The method of claim 8, wherein coating the surface with the fluoropolymer comprises coating the entire optical surface with the fluoropolymer.

11. The method of claim 8, wherein the optical surface comprises an optical coating (133) selected from the group consisting of an anti-reflective coating and a highly reflective coating.

12. The method of claim 8, wherein coating the surface with the fluoropolymer comprises applying the fluoropolymer to the surface using a technique selected from the groups consisting of wiping the fluoropolymer on the surface, spraying the fluoropolymer on the surface, dipping the surface into the fluoropolymer, printing the fluoropolymer on the surface, depositing the fluoropolymer on the surface through chemical vapor deposition and depositing the fluoropolymer on the surface through physical vapor deposition.

13. The method of claim 12, wherein providing a fluoropolymer comprises providing a fluoropolymer in a form selected from the group consisting of a gel, a paste, a liquid and a solid.

14. The method of claim 8, wherein: the optical surface of the hygroscopic crystal is a first optical surface

(130); the hygroscopic crystal comprises a second optical surface (132); and the method comprises coating the second optical surface with the fluoropolymer.

15. The method of claim 8, wherein providing a nonlinear hygroscopic crystal having an optical surface comprises providing a hygroscopic crystal comprising a crystal selected from the group consisting of a LBO crystal (LiB₃O₄), a BBO (β-BaB₂O₄) crystal, a CLBO (CsLiB₆O₁₀) crystal, a KD*? (KD₂PO₄) crystal, an AD*P (NH₄D₂ PO₄) crystal and a CD*A (CsD₂AsO₄) crystal.

16. A surgical laser system (100) comprising: a laser resonator (102) comprising: a laser element (108) configured to emit laser light (111) at a first frequency responsive to a light input (110); a nonlinear hygroscopic crystal (112) in the optical path of the emitted laser light, the hygroscopic crystal comprising an optical surface (130 or 132); and a coating (134) comprising a fluoropolymer (hereinafter "fluoropolymer coating") on the optical surface; wherein the emitted laser light passes through the fluoropolymer coating and the optical surface and is output (118) from the hygroscopic crystal having a second frequency, which is higher than the first frequency; and a laser delivery probe (124) configured to discharge the laser light having the second frequency.

17. The system of claim 16, wherein the fluoropolymer coating comprises perfluoropolyether.

18. The system of claim 16, wherein: the optical surface is a first optical surface (130); the fluoropolymer coating is a first fluoropolymer coating (134A); the hygroscopic crystal comprises a second optical surface (132); and the laser resonator comprises a second coating (134B) comprising a fluoropolymer (hereinafter "second fluoropolymer coating") on the second optical surface; wherein the emitted laser light passes through the first and second fluoropolymer coatings and the first and second optical surfaces, and is output (118) from the hygroscopic crystal at the second frequency.

19. The system of claim 18, wherein the hygroscopic crystal is selected from the group consisting of a LBO crystal (LiB₃O₄), a BBO (β-BaB₂O₄) crystal, a CLBO (CsLiB₆O₁₀) crystal, a KD*P (KD₂PO₄) crystal, an AD*P (NH₄D₂ PO₄) crystal and a CD*A (CsD₂AsO₄) crystal.

20. The system of claim 16, wherein the optical surface of the hygroscopic crystal comprises an optical coating (133) selected from the group consisting of an anti-reflective coating and a highly reflective coating.