Abstract

DELLA PICCA, Fabricio; SLEZAK, Verónica  and  PEURIOT, Alejandro. Characterization of second harmonic generation and optical spontaneous parametric down-conversion using periodically poled LiNbO₃ (PPLN) crystals. An. AFA [online]. 2015, vol.26, n.1, pp.22-29. ISSN 0327-358X.

Non-linear optical conversion phenomena can be classified into two different groups: up-conversion, like second harmonic and sum-frequency generation, and down-conversion like spontaneous parametric or difference-frequency conversion. The system developed in our lab consists of a magnesium oxide-doped periodically poled lithium niobate crystal manufactured by Covesion, enclosed in an oven with precise temperature control and a pulsed laser at 1064 nm, maximum power 7 W at 10 kHz (Coherent mod. Matrix 7-10). In order to determine the beam diameter a beam profiler was built based on a chopper, where a rotating support around the laser beam axis and an acquisition program allow measuring in a quick, practical and economical way. The first task was the characterization of the laser source, its output power, polarization, spatial and time intensity distribution, where astigmatism and beating of several longitudinal modes appeared. The process of frequency conversion is generated by phase matching and, in our case, quasi phase-matching in a 1cm long PPLN crystal was chosen. The second harmonic generation process was characterized measuring the average efficiency for different power values and repetition rates (around 1 W and 2 kHz respectively), as well as the time shape of the infrared input beam and of the green and infrared output beams from the crystal. At last, the input to output beam size ratio for different input power values was measured. All the results were compared with theory based on numerical simulations and analytical calculations. Parametric spontaneous conversion was obtained with another crystal 4 cm long with different periodicity and consisted in two waves in the mid-infrared originating from a single passage through the crystal. Moreover, visible light emission was observed, which presents diverse red lines, a green line due to second harmonic generation of the pump laser and UV corresponding to the pump frequency triplicate. The occurrence of red lines is due to the sum of the pump frequency and two IR lines (around 1.6 y 3.2 µm) and the UV line is due to the sum with the green wave. This phenomenon was characterized by varying the crystal temperature.

Keywords : PPLN; SHG; Parametric up-conversion; Laser beam measurement.

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