High efficiency laser system for third harmonic generation

Abstract

A frequency conversion laser system is configured with a single mode (SM) laser source outputting a pulsed pump beam at a fundamental frequency and a nonlinear optical system operating to convert the fundamental frequency sequentially to a second harmonic (SH) and then third harmonic (TH). The nonlinear optical system includes an elongated SHG crystal traversed by the SM pulsed pump beam which generates the SH beam. The SHG crystal has an output surface inclined relative to a longitudinal axis of the SHG crystal at a first wedge angle different from a right angle. The nonlinear optical system further has an elongated THG crystal with an input surface which is impinged upon by a remainder of the pump and SHG beams which propagate through the THG crystal at a walk-off angle therebetween to generate a third harmonic (TH) beam, the input surface of the THG crystal being inclined to a longitudinal axis of the THG crystal at a second wedge angle. The output and input surfaces of respective SHG and THG crystals are inclined so as to minimize the walk-off angle between SH and IR pointing vectors in the THG crystal thereby improving the conversion efficiency and TH output beam's ellipticity.

Claims

1. A nonlinear optical system, comprising: an elongated second harmonic generation (SHG) crystal having a longitudinal axis and traversed by a pulsed pump beam at a fundamental frequency which generates a second harmonic (SH) beam, SHG crystal having an output surface inclined relative to the longitudinal axis of the SHG crystal at a first angle α different from a right angle; an elongated third harmonic generation (THG) crystal having a longitudinal axis, which is parallel to the longitudinal axis of the SHG crystal, and an input surface, a remainder of the pump beam and the SH beam being collinear and coaxial with one another upon impinging the input surfaces of the THG crystal and propagating through the THG crystal at a walk-off angle therebetween to generate a third harmonic (TH) beam, the input surface of the THG crystal being inclined to the longitudinal axis of the THG crystal at a second angle δ different from the right angle.

2. The nonlinear optical system of claim 1, wherein the SHG crystal is a type I non-critically phase-matched LBO, whereas the THG crystal is configured as a type II critically phase-matched LBO.

3. The nonlinear optical system of claim 1, wherein the output and input surfaces of respective SHG and THG crystals extend in respective planes which are either parallel to or non-parallel to one another.

4. The nonlinear optical system of claim 1, wherein the SHG and THG crystals are either coplanar or offset in a direction perpendicular to the longitudinal dimension of the crystals at a distance which does not exceed 1 mm.

5. The nonlinear optical system of claim 1, wherein the SHG and THG crystals are spaced from and in optical communication with one another over free space, the free space being void of optical components interposed between the SHG and THG crystals.

6. The nonlinear optical system of claim 1, wherein the output and input surfaces of respective SHG and THG crystals are next to one another or spaced apart along a light path at a distance of up to 20 mm.

7. The nonlinear optical system of claim 1 further comprising a focusing lens upstream from the SHG crystal, the focusing lens and the SHG crystal being controllably displaced relative to one another to provide the pump beam with a waist formed within the SIG crystal, wherein the pump beam expands along the light path so that as the pump beam propagates through the THG crystal it is confined within the THG crystal.

8. The nonlinear optical system of claim 1 further comprising a corrective optical scheme downstream from the THG crystal, the corrective optical scheme including a reflective element with a concave surface reflecting the THG beam and a collimator receiving the reflected THG beam to provide the latter with astigmatism of about 0.95 D and ellipticity of about 0.99, wherein a maximum conversion efficiency of a fundamental frequency of the pump beam, which is a single transverse mode (SM) beam, to the third harmonic in the THG crystal reaches 60% for the SM pump beam of up to 30 W and 70% for the SM pump beam above 30 W.

9. A frequency conversion laser system comprising: a single mode (SM) pulsed laser light source configured to output a puked pump light beam at a fundamental frequency; and the nonlinear optical system of claim 1.

10. The frequency conversion laser system of claim 9, wherein the SM pulsed light source has a SM diode laser and a fiber amplifier assembled as a master oscillator power fiber amplifier scheme and operating at a 300 kHz pulsed repetition rate.

11. The nonlinear optical system of claim 1, wherein the first angle α is within a range of 20°±10°, and wherein the second angle δ is derived from equations (1) to (4)
β=arcsin(sin(α)×n  (1)
k=β−α  (2)
γ=k+δ  (3)
sin(δ)×η=sin(k+δ)  (4), wherein n is the refraction index of the THG crystal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above-disclosed aspects are illustrated in the following drawings, in which:

(2) FIG. 1 is the known prior art illustrated the walk-off phenomenon.

(3) FIG. 2 is a schematic of the walk off phenomenon in a nonlinear crystal.

(4) FIG. 3 is a diagrammatic view of an optical system of the known prior art configured to minimize the walk-off effect.

(5) FIG. 4 is a diagrammatic view of a laser system provided with the inventive frequency conversion scheme which is designed to minimize the adverse walk-off phenomenon in a third harmonic nonlinear crystal.

(6) FIG. 5 is a diagrammatic view of the inventive beam-shape corrective optical system configured to improve astigmatism and ellipticity of the output beam of system of FIG. 4.

(7) FIGS. 6 and 7 are computer generated images of the output beam emitted by the inventive system of FIG. 4 assembled without and with the corrective system of FIG. 5, respectively.

(8) FIGS. 8 and 9 are computer generated graphs indicating experimental data obtained with the use of the inventive system of FIG. 4.

SPECIFIC DESCRIPTION

(9) The disclosed system 25 has a simple, easily manufacturable LBO-based structure including fewer components than the known prior art. The laser system 25 is configured with two frequency conversion stages aligned with one another in a simple manner and operative to convert a fundamental wavelength λ.sub.f of single mode (SM) pump beam to a ⅓ λ.sub.f wavelength of substantially circular TH beam with the conversion efficiency of up to 70%.

(10) Referring to FIG. 4, inventive laser system 25 is configured with pump 20, such as a SM or substantially SM fiber or any other suitable laser outputting a SM pump beam at a fundamental wavelength, such as a 1064 nm, at a 300 kHz pulsed repetition rate along a light path. The pump beam 20 is incident on a first elongated LBO crystal 24 having a longitudinal axis 26 which extends along the light path. Preferably, first crystal 24 is a type I non-critically phase-matched LBO for SHG process. As pump beam 20 propagates through first crystal 24 parallel to axis 26, its energy transfers to a generated SH beam often referred to as a signal beam 28 at a 532 nm wavelength. The SH beam 28 does not walk off pump beam 22 due to the nature of crystal 24, i.e., beams 22 and 28 are coaxial and collinear. Both beams—reminder of pump beam 22 and generated SH beam 28—exit through an output surface 30 into free space, which is void of any obstacles, remaining collinear and coaxial, but have respective polarization planes o and e orthogonal to one another.

(11) The second LBO crystal 32 is a type II critically phase matched (PM) LBO cut at angle θ to a positive principle axis A for SFM at the interacting 1064 and 532 nm wavelengths to generate a third 355 nm wavelength of output TH beam. The crystal 32 is also elongated with a longitudinal axis 34 which is parallel to axis 26 of first crystal 24. The first and second crystals 24 and 32 are spaced apart along the light path at a distance varying between 0 and 20 mm. Also, in a plane perpendicular to both longitudinal axes, the crystals are spaced at distance between zero (0), which makes axes 26 and 34 collinear, and 1 mm.

(12) Optimization of the frequency conversion efficiency in a SFM process of beams 22 and 28 resulting in a greater interaction length in second crystal 32 between these beams is based on the wave vector double refraction effect. The realization of the latter includes optimization of a wedge angle α, at which an input surface 38 of second crystal 32 is slanted and an incident angle β between ordinary pump and extraordinary beams 22, 28 and the normal N2 to input surface 38.

(13) The methodology of determining the angle α is known to one skilled in the art of lasers and well explained in U.S. Pat. No. 7,292,387 fully incorporated here by reference. Typically the conversion efficiency depending on this angle selection is a result of compromise between efficiency of conversion, phase-matching conditions, such as temperature and polarization Sλ.sub.2 and wave vector kλ.sub.2 of SH signal beam 28 and spatial overlap requirements among three interacting beams 22, 28 and SM TH output beam ordinarily polarized beam 40. In the disclosed system, desired wedge angle α and orientation of cut angle θ is selected so that pump beam 22 upon refraction is collinear with a longitudinal axis 34 of crystal 32. With the selected wedge angle α and cut angle θ, pointing vector Sλ.sub.2 and wave vector kλ.sub.2 of SH signal beam 28, shown in dash lines and corresponding to the refraction of SH signal beam 28 in crystal 32 with a perfect rectangular cross-section, are angularly displaced to respective positions of vectors S′λ.sub.2 and k′λ.sub.2 As shown, vectors S′λ.sub.2 and k′λ.sub.2 straddle vectors kλ.sub.1, Sλ.sub.1 of pump beam 22 because of the wedge angle α and appropriate selection of cut angle θ. As a result, the direction of wave vector kλ.sub.3 of output TH beam 40 is determined by the vector phase-matching conditions k.sub.3=k.sub.1+k.sub.2. The output TH beam 40 is decoupled from crystal 32 through its output surface in a plane parallel to that of input pump beam 22.

(14) Although the walk-off angle p in system 25 characterized by parameters, which are disclosed below, is not fully compensated, the right balance is struck between the frequency conversion efficiency and ellipticity of output TH beam 40. The range of angle α is preferably within a 20°±10° range.

(15) A further salient structural feature of this invention explaining the compactness and simplicity of disclosed system 25 includes a slanted output surface 36 of first crystal 24 at a wedge angle δ with respect to normal N1 to output surface 36. The selection of angle δ here is such that beams 22 and 28 are incident on input surface 38 of second crystal 32 at angle β with pump beam 22 propagating in crystal 32 collinearly with longitudinal axis 34. The wedge angle δ of output surface 36 of the first crystal 24 is determined in accordance with the following methodology.

(16) First, max a angle to be determined based on max angle between wave vectors of respective pump and SH signal beams 22, 28 to preserve phase matching for a given length of the crystal. Second, orientation of the cut angle should be chosen so as to make the wave vector and pointing vector of signal beam 28 form a fan around the wave vector of pump beam 22. Knowing wedge angle α and based on Snell's law, an angle of incidence β (with respect to normal N.sub.2) of both beams 22 and 28 on input surface 38 of second crystal 32 is determined as
β=arcsin(sin(α)×n.sub.2(1064)  (1)
In the concrete example, with angle α equal to 17.5θ and refraction index n2=1.564 of second crystal 32 phase-matched at 100° C. for pump beam at 1064 nm, incidence angle β=28.05°. Next an angle k between incident and wedge/refraction angle α will be determined as
k=β−α  (2)
In the given example, k=10.55°. Now an angle γ between output beams 22 and 28 from first crystal 24 and normal N1 to output face 36 can be determined as
γ=k+δ  (3)
where δ is the wedge angle of output surface 36. Based on Snell's law
sin(δ)×n.sub.1(1064)=sin(γ)  (4)
Based on 3, 4 may be rewritten as
sin(δ)×n1(1064)=sin(k+δ)  (5)
For the given example, refraction index of first crystal 24 phase-matched at 150° C. for the pump beam at 1064 nm is 1.605. With the latter,
sin(δ)×1.605°=sin(10.55+δ)  (6)
From 6, wedge angle δ is equal to 16.4°.

(17) The first and second crystals may be configured in accordance with two modifications each which works perfectly well with each individual structural component of inventive system 25 as well with any suitable combination of these components. In one modification, output and input surfaces 36, 38 of respective crystals 24, 32 are slanted at different angles δ and α. In the 20°±10° range for both wedge angles, the wedge angles differ from one another, based on selected angle α and temperatures at which respective crystals are phase-matched, as little as possible. The angular difference k between the wedge angles not exceeding 2° is acceptable. Alternatively, system 25 may be designed with input and output surfaces 36, 38 being parallel to one another. Depending on spatial relationship between output and input surfaces 36, 38, respectively, first and second crystals 24 and 32 can be brought into physical contact, i.e., have a zero axial distance therebetween. Generally, the axial distance between the crystals does not exceed 20 mm.

(18) The disclosed structure 25 can be also configured with both crystals 24 and 32 located in a common plane which is perpendicular to longitudinal axes 26 and 34. In this configuration, the axes are collinear. Alternatively, first and second crystals 24, 32, respectively, extend in parallel planes spaced apart in a direction perpendicular to the axes at no more than 1 mm.

(19) Each of dimensional alternatives disclosed above can be used in combination with all other individual features of disclosed structure 25 or any possible combination of these components.

(20) In contrast to systems disclosed in the known prior art, ordinary pump beam 22 and extraordinary SH signal beam 28 propagate between crystals 24 and 32 through obstacle-free free space, if the crystals are axially spaced apart. There is no need for a focusing lens typically located between the crystals.

(21) However, inventive system 25 includes a focusing lens 42 located between pump source 20 and first crystal 24. The lens 42 is configured so that SM pump beam 22 has a waste formed in the first crystal. With the selected crystal's length, wedge angles and spacing between crystals, pump beam 22 extends through second crystal 32 without expanding beyond the physical boundaries of this crystal, except of course for the second crystal's output surface. If necessary, lens 42 may be automatically or manually displaced in a plane parallel to axis 26 to provide the optimal frequency conversion efficiency into TH output beam.

(22) Referring to FIGS. 8 and 9, for the length of each crystal 24, 32 varying in a 2-2.5 cm range, wedge angle α of 17.5° and δ angle of 16.4 and IR pump source 20, which outputs single mode (SM) IR beams at a 1064 nm wavelength and average power above 15 W, the inventive system operates at a frequency conversion efficiency into TH output beam 40 at a 355 nm wavelength and pulse duration of 1.5 ns of up 70% (FIG. 8) The frequency conversion efficiency in the inventive system with pump source 20 outing an IR beam at a 1064 fundamental wavelength and average power below 15 W reaches 50%, as shown in FIG. 9. The conversion efficiency could be even higher, but other considerations such astigmatism and ellipticity of TH output beam 40 weight heavily in, as disclosed below.

(23) In particular, with still very high disclosed frequency conversion efficiencies for Gaussian or substantially Gaussian beams with M.sup.2 of up to 1.2, the disclosed wedge angles α and δ, the astigmatism of TH beam is inconsequential and ellipticity may be about 0.8. However, both astigmatism and ellipticity ranges, even without further optical correction, are still very high for focused SM beams having Gaussian intensity profiles. However, these ranges can be improved with the following beam shape optical corrective scheme.

(24) Referring to FIG. 5, disclosed corrective optical scheme 50 includes a reflective element 46 along the light path downstream from second crystal 32. The element 46 is configured with a concave surface reflecting TH output beam 40. Depending on a concrete set-up, reflected expanding TH beam 40 may be incident on a mirror 42. The beam 40 continues to expand along the light path until it impinges upon a collimating lens 48.

(25) The effect of corrective scheme 50 is illustrated in FIGS. 6 and 7. FIG. 6 shows the ellipticity of about 0.78 of TH output beam of inventive system 25 without corrective system 50. FIG. 7 shows the improvement of both the beam's ellipticity reaching 0.99% and astigmatism that can be of 0.95 D.

(26) While the subject invention has been described with reference to a preferred embodiment, various changes and modifications could be made therein by one skilled in the art, without varying from the scope and spirit of the subject invention as defined by the appended claims.