Design and analysis of 90° image rotating four-mirror non-planar ring resonator based on mid-infrared optical parametric oscillator beam quality optimization
Jilin Key Laboratory of Solid Laser Technology and Application, College of Science,Changchun University of Science and Technology, Changchun 130022, China
Fund Project:Project supported by the Foundation of Education Department of Jilin Province, China (Grant No. JJKH20181105KJ), the Foundation of Jilin Province Science and Technology Department, China (Grant No. 20180101033JC), and the Cooperation Foundation of Changchun Science and Technology Bureau, China (Grant No. 17DY027)
Received Date:11 November 2018
Accepted Date:09 June 2019
Available Online:01 September 2019
Published Online:05 September 2019
Abstract:Mid-infrared optical parametric oscillator (OPO) operating in the mid-infrared transmission window (3—5 μm wavelength range) is one of hot issues in the field of laser system. It has many applications in environmental detection, remote sensing, and medicine. Besides, this laser system is used as a key component of infrared countermeasures. The optical damage limit of nonlinear crystal is a great challenge to the mid-infrared OPO which is pumped by a nanosecond laser source. Therefore, the pump beam diameter should be appropriately increased to avoid damaging the crystal when scaling a nanosecond OPO to high pulse energy. The result of this design is that the Fresnel number in the cavity is increased and the beam quality is deteriorated. In order to improve the beam quality of mid-infrared OPO laser, a 90° image-rotating four-mirror non-planar ring resonator structure is designed. The advantages of this design include the general ring resonators, such as greatly reduced feedback into the pump laser and the avoidance of optical damage caused by standing wave cavity structure. Most importantly, the image rotating cavity can uniform the beam in the cavity and improve the beam quality. In this paper, the equivalent sphere representation of a four-mirror nonplanar ring resonator is established, and the image rotation angle of this special cavity structure is calculated. Based on this method, the parameters related to the 90° image rotating resonator structure suitable for mid-infrared OPO operation are designed. The self-reproduction of the transverse mode in the axially-asymmetric resonator is further established. It is found that the transverse mode in the resonator is gradually uniformed as the rotation angle of the image changes from 0° to 90°. When the rotation angle is 90°, the fundamental mode and the high-order mode both exhibit very good central symmetry. Finally, the mid-infrared ZnGeP2 OPO laser with the 90° image rotating resonator structure is used to verify the improvement of beam quality. The beam quality of $M_X^2=1.81 $ and $M_Y^2=1.61$ are achieved. It can be proved that the 90° rotating four-mirror non-planar ring resonator has a significant effect on the optimization of the output beam quality of the mid-infrared OPO laser system. Keywords:non-planar ring resonator/ image rotation/ beam quality/ optical parametric oscillator
图 4 四镜非平面环形腔的两种等效球体表示方式 (a) 透明等效单位球体; (b) 非透明等效单位球体 Figure4. Two equivalent sphere representations of a four-mirror nonplanar ring resonator: (a) Transparent equivalent unit sphere; (b) non-transparent equivalent unit sphere
式中$\gamma $为上文所说的两镜之间的像旋转角. 由此基于之前所设计的谐振腔结构, 所选总腔长为L = 150 mm, 腔内初始光束直径为d = 700 μm. 根据(10)式—(14)式可以计算得出在不同旋转角下的各支路腔长参数. 由此仿真给出当总旋转角为0°, 5°, 45°, 90°时腔内TEM00, TEM10和TEM01横模光强分布图, 如图6所示. 图 6 不同旋转角下四镜非平面环形腔内横模光强分布 (a) 0°旋转角光强分布; (b) 5°旋转角光强分布; (c) 45°旋转角光强分布; (d) 90°旋转角光强分布 Figure6. The intensity distribution of transverse mode in a four-mirror non-planar ring resonator at different rotation angles: (a) The intensity distribution at 0° rotation angle; (b) the intensity distribution at 5° rotation angle; (c) the intensity distribution at 45° rotation angle; (d) the intensity distribution at 90° rotation angle.
从图6可以看出, 在旋转角为0°时, 即平面环形腔中, 腔内光强分布类似于典型的平平两镜谐振腔中的光强分布, 高阶模的光场分布不为中心对称, 而是在某个方向产生分离. 随着旋转角的增加, 腔内基模在横向和纵向出现轻微椭圆变化, 而高阶模出现逐渐融合的现象, 当旋转角为90°时, 基模以及高阶模都具有非常好的中心对称性. 图7为所设计90°像旋转四镜非平面环形腔腔内横模光强分布三维视图. 可以看出, 其光场均匀化并不是变为一般意义上的高斯光束, 而是变为环形光斑. 考虑到输出激光主要是以基模激光为主, 再由此叠加像旋转均匀化处理之后的高阶模式激光, 可以预见该腔型结构对输出激光光束质量的优化有着非常优良的效果. 图 7 90°像旋转四镜非平面环形腔内横模光强分布 (a) TEM00模; (b) TEM01模; (c) TEM10模 Figure7. The intensity distribution of transverse mode in a 90° image rotating four-mirror non-planar ring resonator: (a) TEM00 mode; (b) TEM01 mode; (c) TEM10 mode