# US 10,120,262 B2 · Power-scalable nonlinear optical wavelength converter

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- Number: US 10,120,262 B2
- Document ID: US10120262B2
- Status: Granted
- Kind: B2
- Application number: 15/809,016
- Filed: 2017-11-10
- Published: 2018-03-08
- Granted: 2018-11-06
- Earliest claimed: 2015-07-01
- Assignees: KLA-Tencor Corporation
- Applicants: Kla-Tencor Corporation
- Inventors: Dirk Woll
- Web page: https://patentgrep.com/patent/US10120262B2

## CPC

- G02F 1/353: {Frequency conversion, i.e. wherein a light beam is generated with frequency components different from those of the incident light beams}
- G02B 27/09: Beam shaping, e.g. changing the cross-sectional area, not otherwise provided for {(adapting the beam shape of a laser diode G02B19/0052; adapting the beam shape of an LED G02B19/0061; coupling into light guides using intermediate optical elements G02B6/4204; beam shaping specially adapted for lasers H01S3/005)}
- G02F 1/3501: {Constructional details or arrangements of non-linear optical devices, e.g. shape of non-linear crystals}
- G02F 1/3551: {Crystals}
- G02F 1/3503: {Structural association of optical elements, e.g. lenses, with the non-linear optical device}
- G02F 1/3505: {Coatings; Housings; Supports}
- H01S 3/005: {Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping (shaping laser beam for working metal or other materials B23K26/06; optical elements, systems or apparatus in general G02B)}
- H01S 3/0092: {Nonlinear frequency conversion, e.g. second harmonic generation [SHG] or sum- or difference-frequency generation outside the laser cavity (nonlinear frequency conversion per se G02F1/35)}
- G02F: OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- H01S: DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL

## Abstract

A system includes a nonlinear crystal positioned such that a focus of a laser beam is outside the nonlinear crystal in at least one plane perpendicular to a beam propagation direction of the laser beam. The nonlinear crystal is disposed in a crystal mount assembly. A laser beam may be directed at the nonlinear crystal for wavelength conversion. The system may be used as a deep-UV wavelength converter.

## Claims

### Claim 1

A method comprising:
generating a laser beam;
directing the laser beam at a nonlinear crystal configured for wavelength conversion, wherein the nonlinear crystal is positioned such that a focus of the laser beam is outside the nonlinear crystal in at least one plane perpendicular to a beam propagation direction of the laser beam, wherein the nonlinear crystal is disposed on a crystal mount assembly, wherein the crystal mount assembly provides an angle stability of greater than 0% and less than or equal to 27% of an angular acceptance range during operation, wherein the crystal mount assembly includes a plurality of mounting features at different distances from a laser source in the beam propagation direction, wherein each of the mounting features includes a hole that the crystal mount assembly is fastened into, and wherein a beam size of the laser beam in the nonlinear crystal is provided by selecting one of the mounting features; and
nonlinearly converting the laser beam from a first wavelength to a second wavelength with the nonlinear crystal.

### Claim 2 (depends on 1)

The method of claim 1, wherein the nonlinearly converting is one of second harmonic generation, sum-frequency generation, or difference frequency generation.

### Claim 3 (depends on 1)

The method of claim 1, further comprising directing the laser beam through beam shaping optics disposed between a laser source and the nonlinear crystal and disposed downstream of the nonlinear crystal in the beam propagation direction.

### Claim 4 (depends on 1)

The method of claim 1, further comprising adjusting a beam size of the laser beam in the nonlinear crystal with the crystal mount assembly by adjusting a distance between a center of the nonlinear crystal and the focus.

### Claim 5 (depends on 1)

The method of claim 1, wherein the nonlinear crystal is positioned such that the focus of the laser beam is outside the nonlinear crystal in the at least one plane perpendicular to the beam propagation direction of the laser beam with a Rayleigh range configured such that time averaged fundamental optical power density or harmonic optical power density at the spatial peak of the beam profile on or in at least one optical component in the system is limited to below 1 MW/cm2.

### Claim 6 (depends on 1)

The method of claim 1, wherein the laser beam is a pulsed laser beam, and wherein the nonlinear crystal is positioned such that the focus of the pulsed laser beam is outside the nonlinear crystal in the at least one plane perpendicular to the beam propagation direction of the pulsed laser beam with a Rayleigh range configured such that fundamental optical fluence power density or harmonic optical fluence on or in at least one optical component of the system is limited to below 10 J/cm2.

### Claim 7 (depends on 1)

The method of claim 1, wherein the crystal mount assembly is adjustable, and wherein the nonlinear crystal is positioned such that the focus of the laser beam is outside the nonlinear crystal in the at least one plane perpendicular to the beam propagation direction of the laser beam with a Rayleigh range configured such that time-averaged fundamental optical power density or harmonic optical power density at the spatial peak of the beam profile on or in at least one optical component in the system is limited to below 1 MW/cm2.

### Claim 8 (depends on 1)

The method of claim 1, wherein the laser beam is a pulsed laser beam, wherein the crystal mount assembly is adjustable, and wherein the nonlinear crystal is positioned such that the focus of a pulsed laser beam is outside the nonlinear crystal in the at least one plane perpendicular to the beam propagation direction of the pulsed laser beam with a Rayleigh range configured such that fundamental optical fluence or harmonic optical fluence on or in at least one optical component in the system is limited to below 10 J/cm2.

### Claim 9 (depends on 1)

The method of claim 1, wherein the focus is at least one of circular, elliptical, or astigmatic.

### Claim 10 (depends on 1)

The method of claim 1, wherein the focus is elliptical, and wherein the focus in a plane parallel to a walk-off is larger than the focus in a plane perpendicular to the walk-off.

### Claim 11 (depends on 1)

The method of claim 1, wherein the focus is astigmatic, and wherein the focus in one plane is inside the nonlinear crystal and the focus in another plane is outside the nonlinear crystal.

### Claim 12 (depends on 1)

The method of claim 1, wherein the focus is astigmatic and elliptical, wherein the focus in one plane is inside the nonlinear crystal and the focus in another plane is outside the nonlinear crystal, and wherein the focus inside the nonlinear crystal has a larger width than the focus outside the nonlinear crystal.

### Claim 13 (depends on 1)

The method of claim 1, further comprising fastening the crystal mount assembly into one of the mounting features.

### Claim 14 (depends on 13)

The method of claim 13, wherein the fastening comprises screwing the crystal mount assembly into one of the mounting features.