We measure and model parametric gain and oscillation for two crystals arranged for walkoff compensation. We show how the orientation of the crystals determines the relative sign of the nonlinear mixing coefficient in the two crystals. This sign dramatically influences small signal gain and oscillator performance, and we show how to determine the correct crystal orientation from parametric-gain measurements. The performance of two-crystal oscillators is examined with particular attention to beam tilts, conversion efficiency, and beam quality. We find reduced efficiency and increased oscillation threshold when the coefficients have opposite signs in a two-crystal ring oscillator. Sign reversal seems to have little influence on spectral purity or far-field beam profiles when the oscillator is seeded.
We present three new methods for modeling broad-bandwidth, nanosecond optical parametric acillators in the plane-wave approximation. Each accounts for the group-velocity differences that determine the operating linewidth of unseeded optical parametric oscillators, and each allows the signal and idler waves to develop from quantum noise. The first two methods are liased on split-step integration methods in which nonlinear mixing and propagation are calculated separately on alternate steps. One method relies on Fourier transforming the fields between t and w to handle propagation, wiih mixing integrated over a AZ step: the other transforms between z and IC, in the propagation step, with mixing integrated over At. The third method is based on expansion of the three optical fields in terms of their respective longitudinal empty cavity modes, taking into account the cavity boundary conditions. Equations describing the time development of the mode amplitudes are solved to yield the time dependence of the three output fields. These plane-wave models exclude diffractive effects, but can he readily extended to include them.
We show that if two waves are incident on a quadratically nonlinear crystal, with the third wave generated entirely within the crystal, a phase-velocity mismatch ͑Dk fi 0͒ leads to intensity-dependent phase shifts of the generated wave only if there is walk-off, linear absorption, or significant diffraction of at least one of the waves as well as significant energy exchange among the waves. The result is frequency broadening and wave-front distortion of the generated wave. Although the induced phase distortions are usually quite small, they may be significant in applications that require high spectral resolution or pointing accuracy.
The anisotropic potentials of He–N2, Ne–N2, and Ar–N2 are predicted using the Tang–Toennies potential model. This model damps the long-range ab initio dispersion terms individually using a universal damping function and adds to this a simple Born–Mayer repulsive term. The Born–Mayer parameters for the three systems were derived from SCF calculations. The dispersion coefficients were estimated from established combining rules using an effective multipole spectrum for the N2 molecule computed by Visser and Wormer from the time-dependent coupled Hartree–Fock approximation. The resulting potentials were used to predict the second interaction virial coefficients for each system, and they are found to be in excellent agreement with experiment. It is concluded that the spherical symmetric potentials are within 2%–3% of the true potentials. Some discrepancies with recent molecular beam experiments appear to be present, however, for the anisotropies especially in the case of He–N2. Finally, it is found that the law of corresponding states for anisotropic systems, which predicts that the reduced shapes of the potentials for a given geometrical configuration are identical, also holds for the highly anisotropic rare gas–N2 systems.
Abstract:We show by computer simulation that high beam quaMy can be achieved in high energy, nanosecond optical parametric oscillators by using image-rotating resonators. OCIS : 190.4970 Parametric oscillators and amplifiers, 190.4420 Nontinear optics, transverse effects in One of the greatest challenges in scaling nanosecond optical parametric oscillators (OPO'S) to high energy is obtaining good beam quality along with high efficiency.Because of constraints imposed by crystal nonlinearities and damage thresholds, scaling an OPO from low to high energy implies increasing the beam diameters so the fluences, crystal lengths, and cavity length can remain relatively unchanged. The result is a high-Fresnel-number cavity that can support many transverse modes, often resulting in poor beam quality.We have applied numerical models to examine the effectiveness of various image-rotating resonator designs in improving signal and idler beam quality for high energy, nanosecond OPO'S. The numerical model for monochromatic OPO'S was described in an earlier paperl. We demonstrated that this model accurately predicted beam quality for a low-Fresnel-number, three-mirror-ring, KTP OPO for pump levels 2.5 times threshold and M2's of 4. The model includes diffraction and birefiingent walk off but not group velocity effects. The input pump and signal waves are assumed to be perfect spatial Gaussian beams. We believe the fact that the initial signal beam is a perfect Gaussian does not invalidate the model for the purpose of beam quality studies because in the amplification process the beam is greatly gain narrowed due to the nonuniform pump profile. The signal wave reaches threshold fist at the center of the pump beam so at the time of turn on it has a small diameter and thus populates many transverse plane-wave components. Additionally, after turn on back conversion distorts the signal and idler waves, and this also populates high order transverse components.Beam clean up must thus occur after turn on so the starting profile is relatively unimportant.Image rotating cavities have long been used to improve the beam quality of lasersz. On successive passes each portion of the laser beam samples different gain regions, averaging to some extent the gain and refractive index inhomogeneities~This mechanism can also be effective in OPO'S where inhomogeneities of the pump light can be averaged.However, the principal clean-up mechanism in OPO'S is not gain averaging but the establishment of phase and amplitude correlations across the signal and idler beams due to lateral walk off between them as they propagate through the crystal. For type II mixing, the birefringent walk off in a single pass of a critically phase matched crystal is typically 0.1-1 mm. This can be a significant fraction of the beam diameter. By its nature, parametric gain tends to establish local phase and amplitude correlations between the signal and idler beams, and also within the signal beti and within the idler beam, over stripes of length equal to the walk off, and or...
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