Research On Beam Coherent Synthesis Of Optical Parametric Conversion Process

Optical parametric oscillator(OPO)is one of the most widely applications of nonlinear optics.The principle of OPO is the second-order nonlinear effect.Because of its wide range of wavelength tuning,simple structure and high power output,the optical parametric oscillator(OPO)has been widely used in gas monitoring,remote sensing,medical environmental protection,free space optical communications and so on,attracting the attention of many researchers.But due to the damage of the nonlinear crystal and of film,the average power from OPOs has been limited to the level of tens of watts.So improving the power of OPO is an important research direction of infrared laser development.This paper gives a possible way to improve the OPOs’ average power.Coherent beam combining is believed to be another effective way to improve laser power with excellent beam quality.Coherent beam combining is implemented by forcing all elements of an array to operate with the same frequency and the same phase so that the beam coherently superposed at far-filed.Therefore,the coherent beam combining peak intensity at the far-field is N2 times of the single laser beam and N times of incoherent combination.So far,coherent beam combining has demonstrated for the high power output of fiber laser,slab laser and semiconductor laser.Therefore,a possible method of coherent beam combining for optical parametric process is proposed in this paper.As we all know,there are two difficulties in the OPOs for basic requirement of coherent beam combining:the phase-locking and the single longitudinal mode operation.The phase-locking is the primary problem because the phase adjustment devices cannot work for high power laser.So the phase evolution law in optical parametric amplifier for both cases of phase matching and quasi-phase matching is studied theoretically.The results indicate that,under the condition of strong pumping,higher intensity injection signal can lock its phase to be its initial value,while for weaker injection signal the phases of the signal and idler waves exhibit chaos-like evolution.These results are further proved by numerical analysis.But the conversion efficiency is very low for the OPA process,although the injection signal is very high,which is unfavorable for the phase adjustment devices in the experiment.Based on this understanding,we propose a ring configuration for the OPO with the appropriate intensity seed injection.The numerical studies indicate this OPO configuration can achieve the phase-locking and high power output simultaneously.And 0.9W/cm2 of the injection signal seed can meet the condition that the large difference between the signal and idler to lock the phase of signal.This inspires a possible design to achieve coherent beam combining of OPOs.In addition to the realization of phase-locking,this paper also analyzes the main factors of broadening of the parameter light during optical parametric process.Specifically,the factors of linewidth,higher gain,and divergence angle of pumping will result in the linewidth of parameter wave.So that several technologies have been proposed to generate single-longitudinal-mode output in OPOs.Compared with phase-locking,the frequency-locking of OPO is a technique,rather than physical problem.Further study shows that the ring OPO configuration can achieve the frequency-locking through injection single-mode seed.Based on the conclusions,the basic structure of coherent beam combining is designed,and the feasibility of the structure is proved.