Wave propagation equations for optical parametric oscillator(OPO) which included pump depletion, phase mismatch, walkoff, diffraction, and the multi-mode generation of signal and idler from the single longitudinal mode of pump were derived, and calculated using the modified Runge-Kutta method with Fourier-space transform. At the end of the nonlinear crystal, pump which was the extraordinary wave was observed to move by the amount of the product of the walkoff angle and the length of the nonlinear crystal. By reducing the backconversion from signal and idler to pump, the walkoff gave the higher conversion efficiency. Higher conversion efficiency was calculated in the multi-mode of signal and idler waves than in the single mode due to lower mode intensity which reduced the backconversion. The transverse widths of the three waves were broadened by the diffraction, which compensated the walkoff. The energy distribution over the cavity modes of signal was calculated to be symmetric around the critical phase matched mode with diffraction ignored. But, the peak mode was moved to the shorter frequency with diffraction, because the diffraction affected the phase of the waves, and had an effect of shortening the wave vectors. The optimized crystal length which could be found by investigating the intensity distribution in nonlinear crystal was determined as the length between the end of the crystal where the pump entered and the starting point of the backcoversion. By eliminating the backcoversion, the higher conversion efficiency could be obtained with optimized crystal length.
In the collinear phase matched OPO, the wavelength tuning was performed in the range of 660-880 nm as the phase matching angle was changed, and the measured values coincided with the calculated results. The conversion efficiency to the signal of 695 nm saturated to 16%. The threshold energies over 680-780 nm was 6-7.5 mJ. The measured values were nearly 3 times higher than the calculated results because the theoretical equation postulated the single longitudinal mode of pump, signal, and idler, but the second harmonics of Nd:YAG laser which was used as a pump source in our experiment was operated in multi-mode. Because the output spectrum of the OPO was too broad to be used in the high resolution spectroscopy, it was necessary to reduce the output linewidth. Of the many ways to reduce the linewidth of OPO output, the injection seeding by diode laser was chosen. The diode laser was composed of the master oscillator with Littman configuration and the tapered amplifier. The linewidth of less than 0.03 nm which was the resolution limit of spectrometer was measured in the injection seeded OPO. The linewidth of the OPO output without injection seeding was 1.3 nm. The peak power of injection seeded OPO was 3 times larger than that of OPO without injection seeding.
To obtain the high efficiency and low threshold operation, the noncollinear phase matched OPO with pump reflection geometry was designed. The walkoff was completely compensated at q=5.6˚, but due to limitation of crystal entrance aperture the above condition was not reached. The experiments were performed at q=3˚ in which the difference angle between the Poynting vector of pump and signal was 1.4˚. The maximum conversion efficiency at the signal wavelength of 765 nm was 22.9%, and the threshold energy was measured to be 4.8 mJ. The maximum conversion efficiency and the threshold energy were 18.9%, and 4.9 mJ, respectively in the collinear phase-matched geometry with pump reflection. In the noncollinear phase-matched geometry without pump reflection, 16.4% of the maximum conversion efficiency and 8.6 mJ of the threshold energy was obtained. The linewidth was 1.5 nm which was the narrow one compared with 7.2 nm in noncollinear phase-matched configuration without pump reflection. Though the exact walkoff compensation was not achieved, the experiment was sufficient to prove that our geometry was more efficient than any other cavity configuration.'
