EXTERNAL FREQUENCY-QUADRUPED 1064 NM MODE-LOCKED LASER
The output of a mode-locked solid-state NIR laser having a pulse duration less than 50 picoseconds at a pulse-repetition frequency is frequency doubled in a nonlinear crystal to provide green radiation. The green radiation is type-I frequency doubled in a BBO crystal to provide UV radiation. The green radiation is focused into an elliptical spot in the BBO crystal with the major axis of the spot in the walk-off plane of the crystal. The length of the crystal is chosen to be much less than the Rayleigh range of the green radiation in the walk-off plane of the BBO crystal.
1 . Optical apparatus, comprising:
a mode-locked laser arranged to deliver repeated pulses of near infrared (NIR) radiation having a duration about equal to or less than 50 picoseconds at a pulse-repetition frequency (PRF) about equal to or greater than 20 megahertz;
a first optically nonlinear crystal arranged for non-resonant frequency-doubling of the NIR radiation to provide corresponding pulses of green radiation;
a second optically nonlinear crystal of beta barium borate (BBO) having a predetermined length and arranged for non-resonant type-I frequency-doubling of the green radiation to provide corresponding pulses of ultraviolet (UV) radiation, the BBO crystal being characterized as having a walk-off plane and a non walk-off plane perpendicular to the walk-off plane; and
wherein an optical arrangement is provided for focusing the green radiation into the BBO crystal for the frequency doubling such that the focused beam has an elliptical cross-section in the BBO crystal with a major axis in the walk-off plane and a minor axis in the non walk-off plane, and the focused green radiation has a Rayleigh range in the walk-off plane greater than about 10 times the length of the BBO crystal.
2 . The apparatus of claim 1 , wherein the NIR radiation has a wavelength of about 1064 nm.
3 . The apparatus of claim 1 , wherein an optical arrangement is provided for focusing the green radiation in BBO crystal is arranged such that the green radiation is about collimated in the walk-off plane of the BBO crystal.
4 . The apparatus of claim 1 , wherein the pulse duration of the IR radiation is about 15 picoseconds and the PRF is about 120 MHz.
5 . The apparatus of claim 1 , wherein the average power of UV radiation is greater than 1% of the average power of green radiation.
6 . The apparatus of claim 1 , wherein the UV radiation has a beam-quality M 2 less than about 1.2 measured in each of the walk-of and non-walk off planes.
7 . Optical apparatus, comprising:
a mode-locked laser arranged to deliver repeated pulses of near infrared (NIR) radiation having a duration about equal to or less than 50 picoseconds at a pulse-repetition frequency (PRF) about equal to or greater than 20 megahertz;
a first optically nonlinear crystal arranged for non-resonant frequency-doubling of the NIR radiation to provide corresponding pulses of green radiation;
a second optically nonlinear crystal of beat barium borate (BBO) having a predetermined length and arranged for non-resonant type-I frequency-doubling of the green radiation to provide corresponding pulses of ultraviolet (UV) radiation, the BBO crystal being characterized as having a walk-off plane and a non walk-off plane perpendicular to the walk-off plane;
wherein an optical arrangement is provided for focusing the green radiation into the BBO crystal for the frequency doubling such that the focused beam has an elliptical cross-section in the BBO crystal with a major axis in the walk-off plane and a minor axis in the non walk-off plane, and the focused green radiation has a Rayleigh range in the walk-off plane greater than about 10 times the length of the BBO crystal; and
wherein the average power of UV radiation is greater than 1% of the average power of green radiation, and the UV radiation has a beam-quality, M 2 , less than about 1.2 measured in each of the walk-of and non-walk off planes.
8 . The apparatus of claim 7 , wherein the NIR radiation has a wavelength of about 1064 nm.
9 . The apparatus of claim 7 , wherein an optical arrangement is provided for focusing the green radiation in BBO crystal is arranged such that the green radiation is about collimated in the walk-off plane of the BBO crystal.
10 . The apparatus of claim 7 , wherein the pulse duration of the IR radiation is about 15 picoseconds and the PRF is about 120 MHz.