Generation of frequency-tripled laser radiation

1. A method of generating frequency-tripled laser radiation, having the following method steps:

providing a first laser radiation at a fundamental frequency,

coupling the first laser radiation into an optical resonator, which is resonant at the fundamental frequency,

generating a second laser radiation by second-harmonic generation of the first laser radiation in a type-I process in a first nonlinear optical crystal, which is located in the optical resonator, wherein the second laser radiation has a polarization direction,

rotating the polarization direction of the second laser radiation, preferably by an angle of substantially 90°, and

generating a third laser radiation by generating the sum frequency of the first and second laser radiations in a type-I process in a second nonlinear optical crystal, which is likewise located in the optical resonator.

2. The method according to claim 1 , wherein the second laser radiation is coupled out of the resonator behind the first and in front of the second nonlinear optical crystal in the beam direction, wherein the rotation of the polarization direction takes place outside the resonator, after which the second laser radiation is coupled back into the resonator.

3. The method according to claim 2 , wherein the second laser radiation is coupled out of the resonator by reflection at a first dichroic mirror.

4. The method according to claim 3 , wherein the first laser radiation has a polarization direction and the angle of incidence of the first laser radiation on the first dichroic mirror deviates by less than 20°, from the Brewster angle for the first laser radiation.

5. The method according to claim 2 , wherein the second laser radiation is coupled back into the resonator by means of a second dichroic mirror, so that the first laser radiation and the second laser radiation are overlaid in the second nonlinear optical crystal.

6. The method according to claim 1 , wherein the third laser radiation is coupled out of the resonator by means of a third dichroic mirror.

7. The method according to claim 4 , wherein the first laser radiation has a polarization direction and the angle of incidence of the first laser radiation on the third dichroic mirror deviates by less than 10°, from the Brewster angle for the first laser radiation.

8. The method according to claim 1 , wherein the wavelength of the third laser radiation is in the ultraviolet spectral region, preferably in the deep ultraviolet spectral region.

9. A device for generating frequency-tripled laser radiation, having

an optical resonator, which is resonant at the fundamental frequency,

a first nonlinear optical crystal-O, which is located in the optical resonator and converts part of a first laser radiation to a second laser radiation by second-harmonic generation in a type-I process, wherein the second laser radiation has a polarization direction,

a polarization adjusting element, which rotates the polarization direction of the second laser radiation, preferably by an angle of substantially 90°, and

a second nonlinear optical crystal, which is likewise located in the resonator, behind the first nonlinear optical crystal in the beam path, and which converts at least part of the first and second laser radiations to a third laser radiation by sum frequency generation in a further type-I process.

10. The device according to claim 9 , wherein the first nonlinear optical crystal is an LBO crystal and the second nonlinear optical crystal is a BBO crystal.

11. The device according to claim 9 , further comprising a first dichroic mirror arranged behind the first nonlinear optical crystal in the beam path, said first dichroic mirror coupling the second laser radiation out of the resonator, and a second dichroic mirror arranged between the first dichroic mirror and the second nonlinear optical crystal in the beam path, said second dichroic mirror coupling the second laser radiation back into the resonator.

12. The device according to claim 11 , wherein the polarization adjusting element is located between the first and the second dichroic mirror in the beam path of the outcoupled second laser radiation.

13. The device according to claim 9 , wherein the first laser radiation has a wavelength in the red spectral region.

14. The device according to claim 9 , wherein the first laser radiation has an average power of less than 100 W.

15. The device according to claim 9 , wherein the wavelength of the third laser radiation is in the ultraviolet spectral region.

16. The device according to claim 9 , wherein the average power of the third laser radiation is al least 100 mW.

17. The method according to claim 3 , wherein the first laser radiation has a polarization direction and the angle of incidence of the first laser radiation on the first dichroic mirror deviates by less than 10°, from the Brewster angle for the first laser radiation.

18. The method according to claim 7 , wherein the first laser radiation has a polarization direction and the angle of incidence of the first laser radiation on the third dichroic mirror deviates by less than 5°, from the Brewster angle for the first laser radiation.

19. The device according to claim 9 , wherein the wavelength of the third laser radiation is in the deep ultraviolet spectral region.