Microlaser-Pumped Optical Parametric Generator Is Small, Tunable IR Source
Just as a high-gain laser can function in a superradiant mode -- i.e., without a resonator -- a high-gain optical parametric oscillator can forgo its resonator and still operate effectively. In such a case, the device is called an optical parametric generator. Now researchers from Laboratoire de Spectrométrie Physique and Laboratoire de Cristallographie at Université Joseph Fourier in Saint Martin d'Heres and Grenoble, respectively, and from JDS Uniphase Commercial Lasers in Meylan, all in France, have developed an optical parametric generator that is pumped with a commercial Nd:YAG microlaser. The combined package is a compact, efficient source of continuously tunable infrared radiation in the 1.4- to 1.7- and the 2.8- to 4.2-µm regions.
Researchers have developed a combined laser and optical parametric generator that offers a compact, efficient source of tunable IR radiation. The data plot shows the tuning curve of the LiNbO3 optical parametric generator, pumped with an Nd:YAG microlaser. The solid lines are the theoretical values, and the points are the experimental results.
The dual output is due to the nature of the optical parametric interaction. The energy of the 1.06-µm pump photon is divided between two output photons, one in each of the output wavelength regions. The nonlinear crystal -- in this case, a cylindrical, periodically poled, 40-mm-long LiNbO3 crystal -- is angle-tuned to quasi phase match a given pair of output wavelengths with the pump wavelength. The output wavelengths, as a function of crystal angle, are shown in the adjacent figure. At any angle, the energy in the two output photons is equal to the energy in the pump photon.
The solid lines in the figure are the theoretical values, and the points are the experimental values observed when the LiNbO3 was pumped with 430-ps, 85-µJ Gaussian pulses. The spectral tuning is truly continuous, with none of the mode hopping associated with an oscillator. The maximum conversion of input pump energy to wavelength-converted output energy was 27 percent.
When the optical parametric generator operated at low power, the output beams exhibited the Gaussian profile of the pump beam. Even as the crystal was rotated to quasi phase match the output, the Gaussian shape was retained because of the cylindrical geometry of the crystal. At higher powers, however, back conversion depleted energy from the most intense regions of the output, producing a ringlike structure in the output beam. The same effect was apparent temporally, as the single-peak pulse shape at low power broadened into a dual-peak structure at higher powers.
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