1. Nonlinear generation of radiation by periodically poled LiTaO 3 crystals Single pass UV generation Intracavity Second Harmonic Generation HWP: half wave plate PBS: polarizing beam splitter OSA:Opticcal Spectrum Analyzer L 1,L 2 : collimation lenses 1064 nm 532 nm 355 nm SHG, SFG: non linear crystal DM: dichroic mirror PD: photodiode IF: interference filter Bow-tie cavity PZT OI: Optical Isolator L 1,L 2 : collimation lenses SM: steering mirror M1: input/output cavity mirror (R= 200mm) M4: concave cavity mirror (R=200mm) M2, M3: plane cavity mirror BS: beam splitter PBS: polarizing beam splitter QWP: quarter wave plate PD1,2: photodiode PZT : piezoelectric actuator I. Ricciardi, M. De Rosa, A. Rocco, P. Ferraro, P. De Natale CNR-INOA, Istituto Nazionale di ottica Applicata, Sezione di Napoli A. Vannucci, P. Spano Andromedra s.r.l In the field of nonlinear frequency generation Lithium tantalate (LiTaO 3 ) represents an interesting and suitable choice for high power applications in the visible and IR range: its nonlinear coeffcient is comparable to KTP one, its transparency window extends in the UV region, it is less sensitive to absorption respect to other similar crystals and shows a high resistence against photorefractive damage. Moreover quasi phase-matching (QPM) in periodically poled ferroelectric crystals has become a well assesed and versatile technique for efficient nonlinear generation. By the use of periodically poled Lithium Tantalate we have first realized a mW level CW laser source by Sum Frequency Generation at 355 nm in single pass configuration and then tested a cavity enhanced Second Harmonic Generation with conversion efficiencies from 50% to 80%. The cavity is made by four mirrors in a bow-tie configuration, with two curved mirrors M1 and M4, 200 mm radius of curvature, and two plane mirrors M2 and M3. Relevant parameters:  total cavity length : 1.22 m  free spectral range : 245 MHz  waist in the crystal: 60 μm  locking scheme: Hänsch–Couillaud  input mirror M1 reflectivities: 88%, 94%, 97%  mirrors M2, M3, M4 reflectivities: 99.9 % Bow tie cavity Second harmonic power as a function of the pump power for the three different input cavity mirrors Photo of the cavity for second harmonic generation. Fig. 1: Schematic for non linear generation at 355 nm: SHG represents the stage for Second Harmonic while SFG respresents the stage for Sum Frequency Generation We tested an experimental configuration where SHG is generated in a first crystal, starting from a fiber laser emitting up to 10 W at 1064 nm; then the resulting radiation at 532 nm and the remaining radiation at the fundamental frequency are sent to a second crystal for sum frequency generation (SFG) at 355 nm. The experimental set up including both non linear stages is shown below. We performed single pass SHG using two different periodically poled crystals:  z-cut stochiometric LiTaO 3 crystal, 1% MgO-doped, 30 mm long, 1 mm thick, with a 7.97 μm period.  z-cut congruent LiNbO 3, 5% MgO doped, 30 mm long, 0.5 mm thick, with a 6.92 µm period.  Single pass Second Harmonic Generation  Single pass Sum Frequency Generation CrystalMaximum pump power (W) Maximum second harmonic (W) Efficiency ( % W -1 cm -1 ) PPMgOLiTaO 3 8.51.40.65 PPMgOLiNbO 3 8.52.61.2 Second harmonic power at 532 nm as a function of the pump power for LiNbO 3 and LiTaO 3 crystals Sum frequency power at 355 nm as a function of the fundamental pump power We used a z-cut stochiometric LiTaO 3 crystal, 1% MgO-doped, 15 mm long, 0.5 mm thick, with a 6.48 μm period (third order quasi phase matching ). Second harmonic conversion efficiencies as a function of the pump power for the three different input cavity mirrors  Thermal effects The cavity suffers from thermal effects due to the heating of the mirrors and the crystal, which in the highest finesse configurations affect the stability of the cavity dynamics. A detailed study of these effects is still in progress. Distorsion of green peak ? Photothermal effect