Intracavity frequency-tripled CW laser with traveling-wave ring-resonator Number:7,130,321 from the United States Patent and Trademark Office (PTO) owispatent A traveling-wave ring laser resonator includes one or more gain-elements for generating fundamental radiation and three optically nonlinear crystals. A portion of the fundamental radiation is converted to second-harmonic radiation in a first of the crystals. Remaining fundamental radiation and the second-harmonic radiation traverse a second of the optically nonlinear crystals where a portion of each is converted to third-harmonic radiation. Fundamental and second-harmonic radiation pass through the third of the optically nonlinear crystals where most of the second-harmonic radiation is converted back to fundamental radiation. The third-harmonic radiation can be delivered from the resonator as output radiation or mixed with the fundamental radiation in a fourth optically nonlinear crystal to generate fourth harmonic radiation. An optical parametric oscillator arrangement is also disclosed.<H Intracavity, resonator, Intracavity, fre, 7130321.html, Patent, pto, 7130321

Title: Intracavity frequency-tripled CW laser with traveling-wave ring-resonator

Abstract: A traveling-wave ring laser resonator includes one or more gain-elements for generating fundamental radiation and three optically nonlinear crystals. A portion of the fundamental radiation is converted to second-harmonic radiation in a first of the crystals. Remaining fundamental radiation and the second-harmonic radiation traverse a second of the optically nonlinear crystals where a portion of each is converted to third-harmonic radiation. Fundamental and second-harmonic radiation pass through the third of the optically nonlinear crystals where most of the second-harmonic radiation is converted back to fundamental radiation. The third-harmonic radiation can be delivered from the resonator as output radiation or mixed with the fundamental radiation in a fourth optically nonlinear crystal to generate fourth harmonic radiation. An optical parametric oscillator arrangement is also disclosed.

Patent Number: 7,130,321 Issued on 10/31/2006 to Spinelli, et al.

Inventors: Spinelli; Luis A. (Sunnyvale, CA), Caprara; Andrea (Menlo Park, CA)
Assignee: Coherent, Inc. (Santa Clara, CA)

Appl. No.: 10/826,835

Filed: April 16, 2004

Current U.S. Class: 372/22 ; 372/21; 372/23

Current International Class: H01S 3/083 (20060101); H01S 3/10 (20060101)

Field of Search: 372/9,21,22,23,25,92,93,94 359/326,328,329

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Foreign Patent Documents


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Primary Examiner: Harvey; Minsun O.
Assistant Examiner: Sayadian; Hrayr A.
Attorney, Agent or Firm: Stallman & Pollock LLP

Parent Case Text

PRIORITY CLAIM

The present application claims priority to U.S. Provisional Patent Application Ser. No. 60/509,990, filed Oct. 9, 2003, the disclosure of which is incorporated in this document by reference.

Claims

What is claimed is:

1. A laser, comprising: a traveling wave ring laser resonator; first, second, and third optically nonlinear crystals located in said ring resonator; said ring resonator including at least one gain-element generating fundamental radiation in said ring resonator, said fundamental radiation circulating through said first second and third optically nonlinear crystals in the sequence listed; said first optically nonlinear crystal arranged to convert a portion of said fundamental radiation to second-harmonic radiation; said second optically nonlinear crystal arranged to convert a portion of said second-harmonic radiation from said first optically nonlinear crystal into radiation having a converted frequency different from the frequency of said second-harmonic radiation; and wherein said third optically nonlinear crystal is arranged to convert at least a portion of a remaining portion of said second-harmonic radiation from said second optically nonlinear crystal back to fundamental radiation.

2. The laser of claim 1, wherein said converted-frequency radiation is third-harmonic radiation generated by mixing said portion of said second-harmonic radiation with a portion of said fundamental radiation.

3. The laser of claim 2, wherein said third-harmonic radiation is delivered from said resonator as output radiation.

4. The laser of claim 2, further including a fourth optically nonlinear crystal located in said ring resonator between said second and third optically nonlinear crystals, said fourth optically nonlinear crystal arranged to mix said third-harmonic radiation with said fundamental radiation thereby providing fourth-harmonic radiation.

5. The laser of claim 4, wherein said fourth-harmonic radiation is delivered from said ring resonator as output radiation.

6. The laser of claim 2, wherein said back conversion of second-harmonic radiation is optimized when there is a particular phase relationship between said fundamental and second-harmonic radiations on entering said third optically nonlinear crystal, and wherein the laser further includes an optical arrangement for causing an optical path difference between said fundamental and third-harmonic radiations before the radiations enter the crystal, thereby causing the phase relationship between said fundamental and second-harmonic radiations entering said third optically nonlinear crystal to vary periodically with frequency of said fundamental radiation, said periodic variation being such that there is at least one possible fundamental radiation frequency of said laser resonator for which said particular phase relationship exists at said third nonlinear crystal.

7. The laser of claim 6, wherein said resonator has a mode-spacing, said gain-element has a gain bandwidth, and said periodically varying phase relationship has a period less than said gain bandwidth and greater than said mode spacing.

8. The laser of claim 7, wherein said fundamental radiation has a frequency of about 217 THz, said mode spacing is about 300 MHz, said gain bandwidth is about 30 GHz and said phase relationship variation period is about 3 GHz.

9. The laser of claim 6, wherein said path difference between said fundamental and second-harmonic radiations is adjustable.

10. The laser of claim 6, wherein second optically nonlinear crystal is located between first and second pairs of pairs of spaced apart mirrors, each pair of mirrors including a first mirror highly reflective for said fundamental radiation and highly transmissive for said second-harmonic radiation, and wherein fundamental and second-harmonic radiation generated by said first optically nonlinear crystal follows a first common path to said first one of said first pair of mirrors whereat fundamental radiation is reflected along a first independent path and said second-harmonic radiation is transmitted to said second one of said first pair of mirrors whereat said second-harmonic radiation is reflected along a second independent path through said first one of said first pair of mirrors, said first and second ones of said first pair of mirrors being spaced and aligned such that said first and second independent paths converge at an entrance face of said second optically nonlinear crystal, and wherein said second optically nonlinear crystal is configured and aligned with respect to said first and second independent paths such that said fundamental and second-harmonic radiations travel along a second common path in said second optically nonlinear crystal and leave said second optically nonlinear crystal at an exit face thereof along respectively third and fourth independent paths diverging one from the other.

11. The laser of claim 10, wherein said fundamental and second-harmonic radiations on said third and fourth independent paths are respectively reflected by and transmitted by said first mirror of said second pair of mirrors, said transmitted second-harmonic radiation being reflected by said second mirror of said second pair of mirrors and re-transmitted through said second mirror, said first and second ones of said second pair of mirrors being spaced and aligned such that said reflected fundamental radiation and said transmitted, reflected and re-transmitted second-harmonic radiation follow a third common path to said third optically nonlinear crystal.

12. The laser of claim 11, wherein spacing between mirrors of one of said first and second pairs of mirrors is variable for adjusting said phase relationship of said fundamental and first harmonic radiations at said third optically nonlinear crystal.

13. The laser claim 1, wherein said gain-element is a Nd:YVO4 gain-element and said fundamental radiation has a wavelength of 1064 nanometers.

14. The laser of claim 1, wherein said gain-element is a semiconductor multilayer gain-structure and said fundamental radiation has a wavelength of 976 nanometers.

15. The laser of claim 1, wherein said first and third optically nonlinear crystals are LBO crystals and said second optically nonlinear crystal is a BBO crystal.

16. The laser of claim 1, wherein said first and third optically nonlinear crystals are LBO crystals and said second optically nonlinear crystal is a CLBO crystal.

17. The laser of claim 1, wherein said resonator is formed from a plurality of optical components and said optical components are spaced-apart and configured such that circulating fundamental radiation is focused to a reduced diameter at first, second and third beam waist positions, and wherein said first, second, and third, optically nonlinear crystals are located in said beam at respectively said first, second, and third waist positions.

18. The laser of claim 1, wherein said converted-frequency radiation has a non-integer relationship with said second harmonic radiation and generated by an optical parametric interaction process in said second optically nonlinear crystal.

19. The laser of claim 18, wherein said second optically nonlinear crystal is collocated in a second resonator arranged such that said converted-frequency radiation circulates therein.

20. The laser of claim 19, wherein said second resonator is a standing-wave resonator.

21. The laser of claim 20, wherein said traveling-wave resonator and said second resonator are partially coaxial and said second optically nonlinear crystal is located in said coaxial part of said resonators.

22. The laser of claim 19, wherein said converted-frequency radiation is delivered from said second resonator as output radiation.

23. A laser, comprising: a traveling wave ring laser resonator; first, second, and third optically nonlinear crystals located in said ring resonator; said ring resonator including at least one gain-element generating fundamental radiation in said ring resonator, said fundamental radiation circulating through said first second and third optically nonlinear crystals in the sequence listed; said first optically nonlinear crystal arranged to convert a portion of said fundamental radiation to second-harmonic radiation; said second optically nonlinear crystal arranged to convert portions of said fundamental and second-harmonic radiations from said first optically nonlinear crystal into third-harmonic radiation; and wherein said third optically nonlinear crystal is arranged to convert at least a portion of a remaining portion of said second-harmonic radiation from said second optically nonlinear crystal back to fundamental radiation; and wherein said laser resonator is formed by a plurality of optical components into first and second superposed planes, with said optically nonlinear crystals located in said resonator in said first plane thereof and said at least one gain-element and any other gain-elements being located in said second plane thereof.

24. The laser of claim 23, wherein said resonator includes two gain-elements said plurality of optical components includes first and second prisms each thereof including two spaced-apart total internal reflecting surfaces, and first, second, third, and fourth mirrors, wherein said first and fourth mirrors are in said second plane, said second and third mirrors are in said first plane, said first and second reflecting surfaces of said first prism are in said second and first planes respectively, and said first and second reflecting surfaces of said second prism are in said first and second planes respectively, and wherein fundamental radiation generated by said gain elements is reflected from said first mirror to said first prism, sequentially reflected from said first and second surfaces of said first prism, transmitted through said first optically nonlinear crystal to said second mirror, reflected by said second mirror through said second optically nonlinear crystal to said third mirror, reflected by said third mirror to said second prism, sequentially reflected from said first and second surfaces of said second prism, and reflected by said fourth mirror through said gain-elements.

25. The laser of claim 24, further including a fifth mirror spaced apart from said second mirror and a sixth mirror spaced apart from said third mirror, said second and third mirrors being highly reflective for said fundamental radiation and highly transmissive for said second-harmonic radiation, and wherein fundamental and second-harmonic radiation generated by said first optically nonlinear crystal follows a first common path to said second mirror whereat fundamental radiation is reflected along a first independent path and said second-harmonic radiation is transmitted to said fifth mirror whereat said second-harmonic radiation is reflected along a second independent path through said second mirror, said second and fifth mirrors being spaced and aligned such that said first and second independent paths converge at an entrance face of said second optically nonlinear crystal, and wherein said second optically nonlinear crystal is configured and aligned with respect to said first and second independent paths such that said fundamental and second-harmonic radiations travel along a second common path in said second optically nonlinear crystal and leave said second optically nonlinear crystal at an exit face thereof along respectively third and fourth independent paths diverging one from the other.

26. The laser of claim 25, wherein said fundamental and second-harmonic radiations on said third and fourth independent paths are respectively reflected by and transmitted by said third mirror, said transmitted second-harmonic radiation being reflected by said sixth mirror and re-transmitted through said third mirror, said third and sixth mirrors being spaced and aligned such that said reflected fundamental radiation and said transmitted, reflected and re-transmitted second-harmonic radiation follow a third common path to said third optically nonlinear crystal.

27. The laser of claim 26, wherein spacing between said third and sixth mirrors is variable for adjusting said phase relationship of said fundamental and first harmonic radiations at said third optically nonlinear crystal.

28. The laser of claim 26, wherein said back conversion of second-harmonic radiation is optimized when there is a particular phase relationship between said fundamental and second-harmonic radiations on entering said third optically nonlinear crystal, and wherein the spacing between said second and fifth and said third and sixth mirrors creates an optical path difference between said fundamental and third-harmonic radiations before the radiations enter said third optically nonlinear crystal thereby causing the phase relationship between said fundamental and second-harmonic radiations entering said third optically nonlinear crystal to vary periodically with frequency of said fundamental radiation, said periodic variation being such that there is at least one possible fundamental radiation frequency of said laser resonator for which said particular phase relationship exists at said third nonlinear crystal.

29. A method of generating third-harmonic radiation in a ring laser resonator, the ring laser resonator including one or more gain-elements generating fundamental laser radiation therein, the method comprising the steps of: (a) providing first, second, and third optically nonlinear crystal located in the resonator; (b) configuring the laser resonator such the laser radiation circulates in one direction only therein through said first, second, and third optically nonlinear crystals in the sequence listed; (c) converting a portion of the fundamental radiation to second-harmonic radiation in said first optically nonlinear crystal; (d) converting a portion of unconverted fundamental radiation and a portion of the second-harmonic radiation from step (c) to third-harmonic radiation in the second optically nonlinear crystal; and (e) converting unconverted second-harmonic radiation from step (d) to fundamental radiation.

30. The method of claim 29, wherein said conversion of second-harmonic radiation in step (e) is optimized when there is a particular phase relationship between said fundamental and second-harmonic radiations on entering said third optically nonlinear crystal, and wherein the method further includes the step of (f) creating an optical path difference between said fundamental and third-harmonic radiations before the radiations enter said third optically nonlinear crystal, thereby causing the phase relationship between said fundamental and second-harmonic radiations entering said third optically nonlinear crystal to vary periodically with frequency of said fundamental radiation, said path difference being sufficiently great that said periodic variation is such that there is at least one possible fundamental radiation frequency of said laser resonator for which said particular phase relationship exists at said third nonlinear crystal.

31. A method of generating third harmonic radiation in a traveling wave ring laser resonator having a gain medium therein generating fundamental radiation comprising the steps of: converting the frequency of the fundamental radiation into a second harmonic thereof; converting a combination of the fundamental and second harmonic radiation into a third harmonic thereof; and down converting a portion of the unconverted second harmonic radiation into fundamental radiation, said down converted fundamental radiation being recirculated and used to create additional second and third harmonic radiation.

Description

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to harmonic generation in lasers. The invention relates in particular to intracavity generation of third and higher harmonics in a continuous wave (CW) laser.

DISCUSSION OF BACKGROUND ART

Prior art arrangements for intracavity third-harmonic generation in CW lasers result in relatively inefficient conversion of fundamental radiation to the third harmonic. By way of example, ultraviolet (UV) radiation having a wavelength of 355 nanometers (nm) can be generated by frequency doubling fundamental 1064 nm (infrared) radiation in a first optically nonlinear crystal to provide second-harmonic radiation having a wavelength of 532 nm (green radiation), then focusing the 532 nm radiation and the fundamental radiation into a second optically nonlinear crystal to generate the 355 nm radiation. The generated UV power can be estimated, for appropriate focusing of the beams and appropriate choice of propagation direction into the crystal, by an equation: P.sub.355=.alpha.P.sub.1064P.sub.532 (1)

Where .alpha. has dimensions of Watts.times.10.sup.-1 and has dimensions for typical crystals of between about 10.sup.-5 and 10.sup.-3 and where P.sub.355, P.sub.1064, and P.sub.532 are the powers for the UV, infrared (IR) and green radiations respectively. In a Coherent.RTM. Verdi.TM.V10.TM., intracavity frequency-doubled, diode-pumped, Nd:YVO.sub.4 laser, about 350 Watts (W) of IR radiation having a wavelength of about 1064 nm are circulating in a ring-resonator, and about 10 W of green (532 nm) radiation are generated by frequency-doubling the IR radiation in an optically nonlinear crystal of lithium borate (LBO). If a second optically nonlinear crystal of LBO having a length of 20.0 millimeters (mm) were included in the ring-resonator and the IR and green radiation were focused into that crystal, a value of a of 3.times.10.sup.-4 can be achieved and equation (1) predicts that about 1 W of ultraviolet radiation at 355 nm would be generated. This represents a conversion efficiency of pump-power to third harmonic of only about 2.5%. There is a need for an improvement in efficiency for third-harmonic generation in a CW laser.

SUMMARY OF THE INVENTION

In a laser in accordance with the present invention efficiency of third harmonic conversion in an intracavity frequency tripled CW traveling wave laser is increased by converting second-harmonic radiation that is not converted to third-harmonic radiation back into fundamental radiation and using that fundamental radiation for further harmonic conversion. This improves the third harmonic generating efficiency by a factor of about two or greater.

One aspect the inventive laser comprises a traveling-wave ring laser resonator having first, second, and third optically nonlinear crystals therein. The resonator includes at least one gain-element for generating fundamental radiation therein. The fundamental radiation circulates through the first, second and third optically nonlinear crystals in the sequence listed. The first optically nonlinear crystal is arranged to convert a portion of the fundamental radiation to second-harmonic radiation. The second optically nonlinear crystal is arranged to convert a portion of the second-harmonic radiation from the first optically nonlinear crystal into radiation having a frequency different from that of the second-harmonic radiation. The third optically nonlinear crystal is arranged to convert at least a portion of any remaining portion of the second-harmonic radiation back to fundamental radiation.

The fundamental radiation from the re-conversion process is re-circulated in the resonator and added to newly generated fundamental radiation. This improves the efficiency of generating the second-harmonic radiation and increases the amount of second-harmonic radiation available for conversion to radiation of other wavelengths. The converted-frequency radiation from second optically nonlinear crystal may be a third-harmonic frequency resulting from mixing fundamental radiation with the second-harmonic radiation. The converted-frequency radiation from second optically nonlinear crystal may also have a non-integer relationship with the second harmonic radiation as a result of parametric decomposition of the second-harmonic radiation in the second optically nonlinear crystal.

In one preferred embodiment of the inventive laser back conversation of second-harmonic radiation is optimized when there is a particular phase relationship between said fundamental and second-harmonic radiations on entering the third optically nonlinear crystal. An optical path difference is introduced between the fundamental and third-harmonic radiations before the radiations enter the third optically nonlinear crystal. This path difference causes the phase relationship between the fundamental and second-harmonic radiations entering the third optically nonlinear crystal to vary periodically with frequency of the fundamental radiation. The periodic variation is such that there is at least one possible fundamental radiation frequency of the laser resonator for which said particular (optimum) phase relationship at the third nonlinear crystal exists.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the present invention.

FIG. 1 schematically illustrates a preferred embodiment of an intracavity frequency-tripled, CW laser in accordance with the present invention, including a ring-resonator formed by two concave mirrors and two plane mirrors, the resonator including a gain-medium located between the plane mirrors and three optically nonlinear crystals located between the two concave mirrors.

FIG. 2 schematically illustrates another preferred embodiment of an intracavity frequency-tripled, CW laser in accordance with the present invention, including a ring-resonator formed by four concave mirrors and two plane mirrors, the resonator including a gain-medium located between the plane mirrors and three optically nonlinear crystals, each thereof separately located between a pair of the four concave mirrors.

FIG. 3 schematically illustrates a preferred embodiment of an intracavity frequency-quadrupled, CW laser in accordance with the present invention, including a ring-resonator formed by two concave mirrors and two plane mirrors, the resonator including a gain-medium located between the plane mirrors and four optically nonlinear crystals located between the two concave mirrors.

FIG. 4 schematically illustrates a further preferred embodiment of an intracavity frequency-tripled, CW laser in accordance with the present invention, including a ring-resonator formed by four concave mirrors, a plane mirror, and the mirror-structure of an OPS-structure, the resonator including a gain-structure of the OPS-structure, and three optically nonlinear crystals, each thereof separately located between a pair of the four concave mirrors.

FIG. 5 schematically illustrates details of cut angles and electric-field orientations in one example of an optically nonlinear crystal of LBO arranged for second-harmonic generation and back-conversion in the frequency-multiplying method of the present invention.

FIG. 6 is a table providing exemplary parameters of the crystal of FIG. 5 for two different fundamental-radiation wavelengths and two different crystal temperatures for each fundamental wavelength.

FIG. 7 schematically illustrates details of cut angles and electric-field orientations in one example of an optically nonlinear crystal of LBO arranged for third-harmonic generation in the frequency-multiplying method of the present invention.

FIG. 8 is a table providing exemplary parameters of the crystal of FIG. 7 for two different fundamental-radiation wavelengths.

FIG. 9 schematically illustrates details of cut angles and electric-field orientations in one example of an optically nonlinear crystal of CBO arranged for third-harmonic generation in the frequency-multiplying method of the present invention.

FIG. 10 is table providing exemplary parameters of the crystal of FIG. 9.

FIG. 11 schematically illustrates details of cut angles and electric-field orientations in one example of an optically nonlinear crystal of BBO arranged for third-harmonic generation in the frequency-multiplying method of the present invention.

FIG. 12 is a table providing exemplary parameters of the crystal of FIG. 11 for two different fundamental-radiation wavelengths.

FIG. 13A is an elevation view schematically illustrating details of cut angles and electric-field orientations in one example of an optically nonlinear crystal of CBO arranged for fourth-harmonic generation in the frequency-multiplying method of the present invention.

FIG. 13B is a plan view from below schematically illustrating further details of the BBO crystal of FIG. 13A.

FIG. 14 is a table providing exemplary parameters of the crystal of FIGS. 13A and 13B for two different fundamental-radiation wavelengths.

FIG. 15A is an elevation view schematically illustrating details of cut angles and electric-field orientations in another example of an optically nonlinear crystal of BBO arranged for third-harmonic generation in the frequency-multiplying method of the present invention.

FIG. 15B is a plan view from below schematically illustrating further details of the BBO crystal of FIG. 15A.

FIG. 16 is a table providing exemplary parameters of the crystal of FIGS. 15A and 15B for a fundamental wavelength of 1064 nm.

FIG. 17 is a three-dimensional view schematically illustrating an additional embodiment of an intracavity frequency-tripled, CW laser in accordance with the present invention, including a bi-planar traveling wave ring-resonator formed by two Dove prisms, two lenses and six concave mirrors the resonators including two solid-state gain elements, and three optically nonlinear crystals and wherein four of the concave mirrors are arranged as two spaced apart pairs thereof configured and aligned to provide different paths for fundamental and second harmonic reflection between the mirrors.

FIG. 18 is a plan view from above view schematically illustrating detail of polarization orientations of fundamental and second-harmonic radiation in the resonator of FIG. 17.

FIG. 19 is a graph schematically illustrating calculated mode sizes in the ring resonator of FIG. 17.

FIG. 20 is a graph schematically illustrating a period variation of resonator losses as a function of laser radiation frequency resulting from the path difference between fundamental and second-harmonic radiation in the laser of FIG. 17.

FIG. 21 schematically illustrates another additional embodiment of an intracavity frequency-tripled, CW laser in accordance with the present invention, similar to the laser of FIG. 4, but including a polarization selective optical arrangement configured to create a path difference between fundamental and second-harmonic radiation for causing the frequency dependent variation of resonator losses of the laser of FIG. 17.

FIG. 22 schematically illustrates another additional embodiment of an intracavity frequency-tripled, CW laser in accordance with the present invention, similar to the laser of FIG. 4, but including a wavelength selective optical arrangement configured to create a path difference between fundamental and second-harmonic radiation for causing the frequency dependent variation of resonator losses of the laser of FIG. 17.

FIG. 23 schematically illustrates one preferred embodiment of an optical parametric oscillator non-collinearly pumped by second-harmonic radiation generated in a traveling wave resonator in accordance with the present invention including an OPS structure for generating fundamental radiation, a first optically nonlinear crystal for generating second-harmonic pump radiation from the fundamental radiation, a second optically nonlinear crystal for parametrically converting a portion of the pump radiation to radiation having a different frequency from that of the pump radiation, and a third optically nonlinear crystal for re-converting unconverted second-harmonic pump radiation to fundamental radiation.

FIG. 24 schematically illustrates another preferred embodiment of an optical parametric oscillator similar to the oscillator of FIG. 23 but wherein the second optically nonlinear crystal is collinearly pumped.

FIG. 25 schematically illustrates another preferred embodiment of an optical parametric oscillator non-collinearly pumped by third-harmonic radiation generated in a traveling wave resonator in accordance with the present invention including a solid-state gain-element for generating fundamental radiation, a first optically nonlinear crystal for generating second-harmonic pump radiation from the fundamental radiation, a second optically nonlinear crystal for generating the third-harmonic pump radiation from a portion of the fundamental and second-harmonic radiations, a third optically nonlinear crystal for parametrically converting a portion of the pump radiation to radiation having a different frequency from that of the pump radiation, and a fourth optically nonlinear crystal for re-converting an unconverted portion of the second-harmonic radiation to fundamental radiation.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, wherein like components are designated by like reference numerals, FIG. 1 schematically illustrates a preferred embodiment 20 of an intracavity-tripled, CW laser in accordance with the present invention. Laser 20 includes a ring-resonator (traveling-wave resonator) 22 formed between concave mirrors 24 and 26 and plane mirrors 28 and 30. In practice it may be found advantageous to configure mirrors 28 and 30 as shallow concave mirrors. Resonator 22 includes a solid-state gain-medium 32. It is assumed for the purposes of this description that gain-medium 32 is neodymium orthovanadate (Nd:YVO.sub.4) providing fundamental (IR) radiation having a wavelength of about 1064 nm when optically pumped. Fundamental radiation circulates in the resonator around a resonator axis, in one direction only, as indicated by single arrows F. Arrangements for optically pumping such a gain-medium and light-valve arrangements for causing radiation to circulate in one direction only are well known in the art and are not necessary for understanding principles of the present invention. Accordingly, such arrangements are not depicted in FIG. 1 for simplicity of illustration, and are not described in detail herein.

The radius of curvature and separation of concave mirrors 24 and 26 is preferably selected such that the circulating fundamental radiation is focused to a narrow beam-waist (not explicitly shown) at a position about midway between the mirrors. Located about the beam-waist position are optically nonlinear crystals 34, 36, and 38 arranged such that the circulating fundamental radiation traverses the crystals in turn. One preferred material for these optically nonlinear crystals is lithium borate (LBO), but this should not be construed as limiting the present invention.

A fraction of the fundamental radiation traversing crystal 34 is converted to second-harmonic (2H or green) radiation as indicated by double arrow 2H. By way of example, between about 3% and 10% of the fundamental radiation may be converted to second-harmonic radiation. The wavelength of the second-harmonic radiation is about 532 nm. Optically nonlinear crystal 36 is arranged such that some fraction of the 2H-radiation and some fraction of fundamental radiation traversing the crystal are sum-frequency mixed to provide third-harmonic (3H or ultraviolet) radiation as indicated by triple arrows 3H. The 3H-radiation can be directed out resonator 22, for example, through mirror 26 or by a separate dichroic mirror (not shown). It should be noted here that the paths of fundamental, 2H and 3H-radiations are shown as being widely separate in FIG. 1 merely for convenience of illustration. In practice the paths are either collinear or very narrowly diverging.

Optically nonlinear crystal 38 is arranged with respect to crystal 36 such that 2H-radiation entering crystal 38, together with fundamental radiation, has a phase relationship with the fundamental radiation that causes the 2H-radiation to be converted back to fundamental radiation. The phase relationship and the back conversion mechanism are described briefly as follows.

In conventional second-harmonic generation in an optically nonlinear crystal, the generated 2H-radiation lags in phase by 90.degree. the nonlinear polarization induced by the fundamental radiation. If 2H-radiation is presented at the input of an optically nonlinear crystal together with fundamental radiation, and if the 2H-radiation leads (rather than lags) in phase by 90.degree. the nonlinear polarization induced by the fundamental, then the 2H-radiation generated in the crystal from the fundamental is in opposition of phase with respect to the 2H-radiation entering the crystal. Because of this, cancellation of the input 2H-radiation field takes place with concurrent transfer of the 2H-energy to the fundamental field.

One possible means of achieving the required phase relationship is to use optically nonlinear crystal 36 as a variable phase-retarder. Third-harmonic generation in the crystal is optimum at a particular phase-matching angle that is different for different temperatures of the crystal. Accordingly, there is an infinitely variable range of pairs of temperature and phase-matching angles that will provide optimum third-harmonic generation. The phase relationship of unconverted fundamental and 2H-radiation leaving the crystal will be different for different pairs of temperature and phase-matching angle. Crystal 36 can be located in a temperature controlled oven (not shown) and the temperature and phase-matching angle varied until third-harmonic generation in optically nonlinear crystal 36 and back conversion of 2H-radiation to fundamental radiation in optically nonlinear crystal 38 are optimized. By way of example, in a BBO crystal having a length of about 15.0 mm the phase relationship between fundamental and 2H radiation exiting the crystal varies by about .pi./20 radians per degree Celsius (.degree. C.).

Back-conversion of 2H-radiation in crystal 38 considerably reduces overall losses in resonator 22 seen by the fundamental radiation. This allows circulating fundamental radiation to grow to substantially higher intensity levels than are possible in the absence of back conversion. The higher fundamental-radiation intensity provides that a higher 2H-radiation intensity is generated by optically nonlinear crystal 34. The higher fundamental and 2H-radiation intensities in optically nonlinear crystal 36 provide for higher third-harmonic-radiation intensity than in prior-art lasers in which second-harmonic back-conversion does not occur. Power is coupled out of resonator 22 essentially only as 3H-radiation. The term "essentially", here, meaning that unavoidable parasitic linear losses in the resonator are discounted. Numerical simulations predict that as much as about 10.0 W or more of UV (3H) radiation can be generated in a Nd:YVO.sub.4 ring-laser wherein the gain-medium is diode-pumped with about 60.0 Watts of 808 nm pump radiation.

A shortcoming of laser 20 is that not all of the optically nonlinear crystals 34, 36, and 38 can be optimally positioned at the fundamental beam-waist between mirrors 24 and 26 because of the length and spacing of the crystals relative to the Rayleigh range of the beam-waist. FIG. 2 schematically depicts another embodiment 40 of a laser in accordance with the present invention that remedies this shortcoming by forming a ring-resonator 42 having a separate beam-waist for each of the optically nonlinear crystals. Here again, optical pumping and light valve arrangements are omitted from the drawing for convenience of illustration.

Resonator 42 includes plane mirrors 28 and 30 and concave mirrors 24 and 26 of above discussed resonator 22, and further includes concave mirrors 44 and 46. Fundamental radiation circulates in one direction only as indicated by single arrows F. Radii of curvature and spacing of the concave mirrors is selected such that optically nonlinear crystal 34 can be placed at a fundamental beam-waist between mirrors 44 and 46; optically nonlinear crystal 36 can be placed at a fundamental beam-waist between mirrors 44 and 26; and optically nonlinear crystal 38 can be placed at a fundamental beam-waist between mirrors 24 and 26. Gain-medium 32 is between mirrors 28 and 30. The sequence of harmonic conversion is as described above for laser 20. Fundamental radiation is converted to 2H-radiation in optically nonlinear crystal 34; fundamental radiation and 2H-radiations are converted to 3H-radiation in optically nonlinear crystal 36, and 2H-radiation is converted back to fundamental radiation in optically nonlinear crystal 38.

The intracavity frequency-conversion method of the present invention can be extended to include at least one additional stage of frequency conversion. By way of example third-harmonic radiation generated as discussed above can be mixed in a fourth optically nonlinear crystal to generate fourth-harmonic radiation. FIG. 3 schematically illustrates a laser 54 in accordance with the present invention arranged to generate fourth-harmonic radiation (4H-radiation) by the inventive method.

Laser 54 is similar to laser 20 of FIG. 1, inasmuch as it includes a ring-resonator 22 formed by concave mirrors 24 and 26 and plane mirrors 28 and 30. Gain-medium 32 is located between the plane mirrors. Located between the two concave mirrors are optically linear crystals 34, 36, and 38, arranged to generate 3H-radiation by the inventive method, with optically nonlinear crystal 38 arranged to reconvert residual 2H-radiation back to fundamental radiation F. Additionally, a fourth optically nonlinear crystal 39 is located between concave mirrors 24 and 26. Optically nonlinear crystal 39 is arranged to generate 4H-radiation by sum-frequency mixing fundamental and 3-H radiations exiting optically nonlinear crystal 38. A crystal of P-barium borate (BBO) is preferred for converting 355 nm-radiation and 1064 nm-radiation to 4H-radiation having a wavelength of 266 nm.

The 4H-radiation and residual (unconverted) 3H-radiation are directed out of resonator 22 and can be separated by any well-known means. Here of course the residual 3H-radiation represents a loss in the conversion process. Nevertheless, the improved efficiency of generating the 3H-radiation and the higher circulating fundamental power from the 2H-reconversion makes 4H-generation more efficient than would be the case in prior-art methods that do not include reconverting 2H-radiation to fundamental radiation.

The inventive frequency-tripling method is described embodied in solid-state lasers including a traveling-wave ring-resonator. A ring-resonator is advantageous in solid-state lasers as it eliminates spatial hole burning in the gain-medium and allows operation in single longitudinal mode. Single longitudinal mode operation is preferred in harmonic generation as it minimizes beam amplitude noise. It is important that all resonator mirrors directing fundamental radiation be coated for maximum reflectivity (for example greater than about 99% reflectivity) at the wavelength of fundamental radiation, so that fundamental radiation is trapped inside the resonator. In this way, fundamental-radiation intensity in the resonator is maximized. Any delivery of fundamental radiation from a resonator between the third harmonic generating crystal and the crystal used to convert 2H-radiation back to fundamental radiation will reduce the circulating fundamental-radiation intensity and could cause the resonator to act in a passively-modelocked, pulsed manner, thereby defeating an object of the present invention to deliver only CW radiation.

Embodiments of lasers in accordance with the present invention are not restricted to use with solid-state gain media such as Nd:YVO.sub.4, but may also be configured to include an optically pumped semiconductor OPS lasers. An OPS-laser includes an OPS-structure comprising a mirror-structure surmounted by a multilayer gain structure. The OPS-structure is usually supported, mirror-structure side down, on a thermally-conductive substrate or an active heat-sink. The multilayer gain-structure includes a plurality of very thin (usually less than 200 nm) active or quantum-well (QW) layers spaced apart (by one-half wavelength of the fundamental wavelength) by pump-light-absorbing spacer layers. An OPS laser is usually pumped by pump-light directed through the front of the gain-structure.

FIG. 4 schematically illustrates one embodiment 120 of a traveling-wave ring-laser in accordance with the present invention. Laser 120 includes a traveling-wave laser resonator 43 similar to resonator 42 of laser 40 of FIG. 2, with an exception that solid-state gain-medium 32 of laser 40 is replaced by gain-structure 68 of an OPS-structure 64, and mirror-structure 66 of the OPS-structure replaces plane mirror 28 of laser 40. OPS-structure is energized by pump light directed through the front of gain-structure 68. Resonator 40 includes optically nonlinear crystals 34, 36, and 38, each thereof located at a beam-waist position between a pair of resonator mirrors. 2H-radiation i