Abstract
Wavelength tunability from the visible to near IR using nonlinear optical materials extends the applications of lasers in various fields, such as medical diagnosis, laser displays, and scientific research. The accomplishment of this work is mainly to employ the optical parametric generation (OPG) for obtaining a yellow-orange spectrum including the sodium yellow wavelength, which is an important light source for astronomy. The nonlinear optical material used for this parametric generation is periodically poled lithium niobate (PPLN) fabricated by the electric field poling technology. The PPLN has demonstrated high frequency-conversion efficiency. With adequate phase matching schemes, one obtains the desirable wavelengths via appropriate nonlinear processes. We used two strategies to obtain the visible light source.In the first method, the pumping source is a Q-switched Nd:YAG laser that has a 5ns-pulse width, 10 Hz-repetition-rate, variable energy control, and is frequency doubled to 532nm by a KTP crystal. The PPLN crystal is 50mm-long, 6mm-wide, with five different grating periods from 11.00μm to 12.00μm in 0.25μm increments. With adequate focusing, the signal and idler waves generated via PPLN by OPG process are ~600nm and ~3.7μm under the phase matching condition. The pumping threshold varied as the signal wavelength is temperature-tuned; for a shorter wavelength, the pumping threshold is higher. The minimum pumping threshold is approximately 35μJ at 597nm with a 12μm-grating period PPLN. The temperature change results in continuous wavelength tuning from 589nm to 621nm. At 130℃, when the signal wave is 583.8nm, the idler wavelength approaches 6μm, which is beyond the theoretical predictions based on the published Sellmeier equation.The second method involves a cascade SHG+OPG process in PPLN. In the SHG process, the PPLN chip is first a 10mm long 20.4μm-grating period section for generating a third-order second harmonic wave from the fundamental pump wave. When Pumped by a diode-pumped passively Q-switched Nd:YAG microchip laser at 1.064μm with 7μJ-pulse-energy and 500ps-pulsewidth, the section converts the 1.064μm wave into 532nm with a narrowed pulse width. The phasematching temperature is 40.6℃; the conversion efficiency is estimated to be >70%. The 532 nm laser from the SHG wave pumps the subsequent section of the PPLN via OPG process to generate visible laser in the orange-yellow spectrum. The grating periods of the second stage are from 11.00μm to 12.00μm with 0.25μm increments. The configuration demonstrated a novel compact device capable of tuning wavelength discretely. The overall conversion efficiency of this device is ~10%.