Image-rotating cavity designs for improved beam quality in nanosecond optical parametric oscillators

Abstract

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...

Extracavity Parametric Oscillation with Continuous and/or Discontinuous Frequency Tuning

“…Simultaneously the similar detector (5) receives stray radiation of the second channel. Then the electric signals run from the detectors to two inputs of ADC (4) which is connected to PC (10). The PC display shows a vibration rotation absorption spectrum υ 3 of the methane band, which central branch Q is a benchmark.…”

“…However, such cavities are extra sensitive to disalignment and are suitable only to provide oscillation at a stable wavelength. Basic research of OPO cavities [9] and their computer simulation [10] show that image-rotation cavities ensure high quality of the output beam.…”

High-energy mid-IR source based on two-stage conversion from 1.06 µm

“…The problem is that the resonator must be short to keep the build-up time small, and if the beam width is scaled up in order to avoid optical damage, the resulting large Fresnel number of the cavity leads to oscillation of multiple transverse modes. High beam quality can be maintained by use of unstable [9,10] or image rotating resonators [8,11].Even with a short resonator, the build-up time can limit the efficiency of all pulsed OPOs because the leading part of the pump pulse is not converted efficiently. This problem is particulary relevant with short pump pulses, but the build-up time can be reduced by injection seeding the OPO.…”

“…However, maintaining high beam quality while scaling an OPO to high pulse energy is a challenge [8]. The problem is that the resonator must be short to keep the build-up time small, and if the beam width is scaled up in order to avoid optical damage, the resulting large Fresnel number of the cavity leads to oscillation of multiple transverse modes.…”

“…The problem is that the resonator must be short to keep the build-up time small, and if the beam width is scaled up in order to avoid optical damage, the resulting large Fresnel number of the cavity leads to oscillation of multiple transverse modes. High beam quality can be maintained by use of unstable [9,10] or image rotating resonators [8,11].…”