Cascaded optical super lattice helps raise mid-infrared laser output
An engineered synchronously pumped picosecond optical parametric oscillator.
Ultrafast laser radiation with picosecond (ps) duration in the 2-5 μm mid-infrared region plays a key role in many fields including material processing, time-domain spectroscopy, and biomedical applications.
An optical parametric oscillator converts an input laser wave (called "pump") into two output waves of lower frequency by means of second-order nonlinear optical interaction. The two output waves are called "signal" and "idler", where the output wave with higher frequency is called signal. A synchronously pumped optical parametric oscillator (SPOPO) combined using a quasi-phase-matching (QPM) technique is a versatile method of generating tunable mid-infrared picosecond laser pulses. However, the pump-to-idler conversion efficiency decreases when the idler wavelength increases. The transmission of the light above 3.8 μm also decreases with the increase of the wavelength. The two factors stated above restrict the pump-to-idler conversion efficiency in the longer idler wavelength region.
So as to increase the pump-to-idler efficiency, the research group led by Prof. Shining Zhu at Nanjing University, designed a tandem optical parametric oscillation and amplification (OPO-OPA) process and demonstrated an efficiency-enhanced ps mid-infrared radiation via optical parametric down conversion for the first time. The details of the study are reported in Chinese Optics Letters, Vol. 14, No. 4, 2016 (S.F. Li et al., Efficiency-enhanced picosecond mid-infrared optical parametric downconversion based on a cascaded optical superlattice).
In this study, the quantum efficiency of the cascaded process can reach 200% in theory. Therefore, the energy of the pump light could be converted to the idler light in order to achieve a maximum conversion efficiency as high as 100%. This scheme can break though the quantum efficiency limit and have a wide field of applications where high power mid-infrared lasers in the ps region are urgently needed.
In this experiment, only one certain wavelength can be further amplified in the OPA process. If a muti-grating or fanout grating structure is used in the OPA process, muti-wavelength amplification or wide bandwidth amplification will be achieved.
利用级联光学超晶格提高皮秒中红外激光功率
An engineered synchronously pumped picosecond optical parametric oscillator.
光参量振荡是指通过二阶非线性光学作用将入射激光（称作“抽运光”）转换为两束频率较低的出射激光（分别被称作“信号光”和“闲频光”，其中频率较高的为“信号光”）。同步抽运的光学参量振荡器是产生中红外可调谐皮秒脉冲的重要途径。然而，随着波长增加，非线性晶体对激光的吸收增强，并且抽运光到闲频光的转换效率降低。
为解决这一问题，南京大学的祝世宁教授团队首次在一块光学超晶格上实现了皮秒级联光参量振荡与光参量放大过程，获得了高效率的中红外皮秒激光输出。相关研究结果发表在Chinese Optics Letters 2016 年第4 期上（S.F. Li et al., Efficiency-enhanced picosecond mid-infrared optical parametric downconversion based on a cascaded optical superlattice）。
此方案可将抽运光能量最大限度地转化到闲频光，理论上，级联过程量子效率可达200%甚至更高，能量转换效率最高可达100%。这为突破非线性过程量子转换效率极限，实现更高功率的中红外激光输出提供了一种可行方案。
目前，该团队只能对某一特定波长的激光进行光参量放大，但后续通过将光参量放大过程的单周期结构设计成多周期结构或扇形周期结构，就可实现多波长或宽带的光参量放大。