US-12620768-B2 - Non-collinearly phase-matched frequency mixing

Abstract

A laser apparatus with non-collinearly phase-matched frequency mixing includes a nonlinear crystal generating an output laser beam from non-collinearly phase-matched frequency mixing of first and second input laser beams. The output laser beam is subject to walk-off in a walk-off plane. The second input laser beam is less powerful than the first input laser beam. The first input laser beam is directed to more closely align its Poynting vector to the output beam Poynting vector than in collinear phase matching. To achieve good spatial overlap in this phase matching scheme, the second input laser beam is elongated in the walk-off plane, such that the second input laser beam has a greater transverse size than the first input laser beam in the walk-off plane. This non-collinear phase matching scheme is capable of achieving an improved beam quality of the output beam, as compared to collinear phase matching with circular input beams.

Inventors

• Christian Hagemann
• Jens SCHÜTTLER

Assignees

• COHERENT LASERSYSTEMS GMBH & CO. KG

Dates

Publication Date

20260505

Application Date

20221027

Claims (20)

1. 1 . A laser apparatus with non-collinearly phase-matched frequency mixing, comprising: a first laser source configured to generate a first input laser beam; a second laser source configured to generate a second input laser beam; and a first nonlinear crystal arranged to generate an output laser beam from non-collinearly phase-matched frequency mixing of the first and second input laser beams, wherein: (a) the output laser beam is subject to walk-off in a walk-off plane in the first nonlinear crystal, (b) wave vectors of the first and second input laser beam are non-collinear and intersecting in the first nonlinear crystal, (c) the second input laser beam has a lower power than the first input laser beam in the first nonlinear crystal, (d) in the first nonlinear crystal, an angle between a Poynting vector of the first input laser beam and a Poynting vector of the output laser beam is smaller than an angle between a Poynting vector of the second input laser beam and the Poynting vector of the output laser beam, and (e) the second input laser beam has a greater transverse size than the first input laser beam in the walk-off plane in the first nonlinear crystal.
2. 2 . The laser apparatus of claim 1 , wherein the second input laser beam has an oblong transverse intensity distribution that is elongated in the walk-off plane.
3. 3 . The laser apparatus of claim 1 , wherein the second input laser beam is ultraviolet.
4. 4 . The laser apparatus of claim 3 , wherein the first input laser beam is near-infrared and the non-collinearly phase-matched frequency mixing is sum-frequency mixing.
5. 5 . The laser apparatus of claim 1 , wherein the non-collinearly phase-matched frequency mixing is type-I sum-frequency mixing.
6. 6 . The laser apparatus of claim 1 , wherein the second laser beam is an elliptical Gaussian beam, or the oblong transverse intensity distribution has a flat-top or super-Gaussian profile in the walk-off plane.
7. 7 . The laser apparatus of claim 1 , wherein the power of the second input laser beam is at most ten percent of the power of the first input laser beam when incident on the first nonlinear crystal.
8. 8 . The laser apparatus of claim 1 , wherein the second laser source includes a second nonlinear crystal configured to generate the second input laser beam by frequency converting a third laser beam with collinear phase matching, the oblong transverse intensity distribution being caused by walk-off of the second input laser beam in the second nonlinear crystal.
9. 9 . The laser apparatus of claim 8 , wherein the first and second input laser beams are continuous-wave laser beams, the apparatus further comprising an imaging module configured to image the second input laser beam from the second nonlinear crystal to the first nonlinear crystal.
10. 10 . The laser apparatus of claim 8 , wherein the second nonlinear crystal is configured to form the second input laser beam as a second harmonic of the third laser beam.
11. 11 . The laser apparatus of claim 1 , wherein the first laser source includes a laser resonator, the first nonlinear crystal being positioned in the laser resonator.
12. 12 . The laser apparatus of claim 1 , further comprising a resonant enhancement cavity arranged to receive the first input laser beam from the first laser source, the first nonlinear crystal being positioned in the resonant enhancement cavity.
13. 13 . The laser apparatus of claim 1 , wherein a crossing point between respective transverse centers of the first and second input laser beams is inside a middle third of a propagation path of the second input laser beam through the first nonlinear crystal.
14. 14 . The laser apparatus of claim 1 , wherein, in the first nonlinear crystal, the output laser beam has a walk-off angle, and the Poynting vector of the first input laser beam is oriented within half the walk-off angle of the output laser beam.
15. 15 . The laser apparatus of claim 1 , wherein the transverse size of the second input laser beam exceeds the transverse size of the first input laser beam by at least 75 percent.
16. 16 . The laser apparatus of claim 1 , wherein the second input laser beam is a fourth harmonic of the first input laser beam, and the output laser beam is a fifth harmonic of the first input laser beam.
17. 17 . A method for non-collinearly phase-matched frequency mixing of laser beams, comprising steps of: generating first and second input laser beams; and directing the first and second input laser beams into a first nonlinear crystal such that (i) wave vectors of the first and second input laser beams are non-collinear and intersect in the first nonlinear crystal, and (ii) the first and second wave vectors cooperate with the orientation of the first nonlinear crystal to promote non-collinearly phase-matched frequency mixing of the first and second input laser beams resulting in generation of an output laser beam, the output laser beam is subject to walk-off in a walk-off plane in the first nonlinear crystal; wherein: the first input laser beam is more powerful than the second input laser beam, the transverse size of the second input laser beam, in the walk-off plane, exceeds a corresponding transverse size of the first input laser beam, and in the first nonlinear crystal, an angle between a Poynting vector of the first input laser beam and a Poynting vector of the output laser beam is smaller than an angle between a Poynting vector of the second input laser beam and the Poynting vector of the output laser beam.
18. 18 . The method of claim 17 , wherein the second input laser beam has an oblong transverse intensity distribution that is elongated in the walk-off plane.
19. 19 . The method of claim 17 , wherein at least 25 percent of power in the second laser beam is transferred to the output laser beam.
20. 20 . The method of claim 17 , wherein the non-collinearly phase-matched frequency mixing is type-I sum-frequency mixing.

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to walk-off compensation in nonlinear frequency conversion processes where the spatial beam overlap is significantly affected by walk-off, such as in sum-frequency mixing in the ultraviolet, where a long propagation path through a nonlinear crystal is often needed to achieve a desired output power. The present invention relates in particular to the impact of walk-off on the beam quality of the frequency-converted beam. DISCUSSION OF BACKGROUND ART Ultraviolet (UV) laser radiation has a variety of uses. High-power UV laser radiation is used to perform photolithography, laser machining, and eye surgery, for example, while moderate-power UV laser radiation has other applications such as semiconductor inspection, flow cytometry, and confocal microscopy. In microscopy and semiconductor inspection, the short wavelength of ultraviolet laser radiation enables detection of features that are smaller than those detectable with visible radiation. In many situations, solid-state lasers are a preferred laser-source architecture. However, so far, no solid-state laser is capable of directly generating UV laser radiation with high or even moderate power. Instead, high- and moderate-power UV laser radiation is generated from solid-state lasers by frequency conversion of longer-wavelength laser radiation generated in the solid-state-laser gain medium. For example, neodymium-doped yttrium aluminum garnet (Nd:YAG) and neodymium-doped yttrium orthovanadate (Nd:YVO4) crystals very effectively generate both continuous-wave and pulsed laser radiation with a wavelength of 1064 nm, and form the basis of many commonly used UV solid-state laser systems. Multi-stage sum-frequency generation in nonlinear crystals is used to convert 1064 nm laser radiation to ultraviolet laser radiation. Frequency-doubling of 1064 nm laser radiation to generate the second harmonic at 532 nm followed by sum-frequency mixing of 532 nm and 1064 nm laser radiation produces the third harmonic at 355 nm. In another approach, two stages of frequency doubling convert 1064 nm laser radiation to the fourth harmonic at 266 nm. Sum-frequency mixing of 266 nm and 1064 nm laser radiation produces the fifth harmonic at 213 nm. Efficient frequency mixing in a nonlinear crystal relies on the input laser beam (or beams) being phase matched with the frequency-converted output laser beam, such that, as the input and output laser beams propagate through the nonlinear crystal, frequency-converted laser radiation generated at each spatial location interferes constructively with frequency-converted laser radiation generated at preceding spatial locations. This is a nontrivial task since the refractive index of the nonlinear crystal varies with wavelength. Critical phase matching, also known as “angle phase matching”, is the preferred phase matching technique in many scenarios and sometimes the only viable phase matching technique. Critical phase matching utilizes a birefringent nonlinear crystal and takes advantage of the polarization dependence of the refractive index of this birefringent nonlinear crystal. Critical phase matching is generally performed with linearly polarized beams and is most easily understood within the context of uniaxial birefringent crystals. A uniaxial crystal has an optic axis and is characterized by ordinary and extraordinary refractive indices. For any given laser beam, the optic axis of the uniaxial crystal and the wave vector of the laser beam together define a principal plane. A beam is termed an “ordinary” beam when its polarization is normal to the principal plane and an “extraordinary” beam when its polarization is parallel to the principal plane. An ordinary beam always experiences the ordinary refractive index. An extraordinary beam, on the other hand, experiences a refractive index in the range between the ordinary and extraordinary refractive indices, with the value of the refractive index depending on the angle between the wave vector and the optic axis. In critical phase matching, the interacting laser beams include both ordinary and extraordinary beams. For suitable combinations of crystal material and temperature, and wavelengths and polarization directions of the input beams, phase matching is achieved at a particular orientation of the optic axis of the nonlinear crystal relative to the wave vectors of the input beams. Critical phase matching is most often performed with collinear beams. However, one or two of the interacting laser beams is subject to walk-off, i.e., the Poynting vector of each such beam is at a non-zero angle to the wave vector of the beam. In uniaxial crystals, extraordinary beams are subject to walk-off, and the walk-off angle depends on the difference between the ordinary and extraordinary refractive indices and the angle between the wave vector and the optic axis. Walk-off affects the spatial overlap between the interacting beams and limits the effecti