基于掺Er3+玻璃回音壁模谐振腔的中红外微激光器和光学频率梳的研究

Mid-infrared(IR) coherent light sources have significant applications in the fields of atmospheric and space optical communication, air pollution monitoring, and precision spectral analysis and measurements, etc. This project aims at developing miniatured mid-IR coherent light sources that possess high performance and low power consumption. Mid-IR microlasers and optical frequency combs (OFCs) with low thresholds and high conversion efficiencies will be developed by employing high Q whispering gallery mode (WGM) micro-resonator and waveguide coupled systems. The project mainly includes: 1) high-Q Er3+doped glass (fluoride and sulfide glass) mm-sized WGM resonators will be coupled with mid-IR fused fiber tapers or angle polished fibers for generating mid-IR lasers with lasing wavelength centered at 3.5 μm pumped by a double laser source of 980 nm and 1970 nm; 2) Based on the tailoring of the geometrical and modal dispersion in the mid-IR range of the micro-resonator and optimization of coupling between resonator and waveguide, laser power can be increased and mid-IR OFCs with center wavelength of 3.5 μm will be produced by pumping the active resonator itself with newly generated laser from 1), as parametric oscillations and cascaded four wave mixing are realized in the micro-resonator. Most importantly, the mid-infrared OFCs based on active cavities will eliminate the dependence of mid-IR pump sources with narrow-linewidth and tunable single frequency, which are urgently needed by passive-cavity-based mid-IR OFCs.

中红外相干光源在大气与星际光通信、大气污染监测、精密光谱分析与测量等领域有着重要的应用前景。本项目以发展高性能、低功耗的微型中红外相干光源为动机，采用高Q值回音壁模微腔与波导耦合系统，实现低阈值、高转换效率的中红外微激光源与光学频率梳。具体研究内容为：1）利用掺Er3+氟/硫化物玻璃棒制备毫米尺寸的高Q值微盘，采用熔锥或角抛的中红外光纤作为耦合波导，在980 nm和1970 nm双光泵浦下实现3.5 μm波段的中红外激射，同时得到相对较高的激射功率；2）在此基础上，通过调控微腔在中红外波段的结构和模式色散、调节波导与微腔的耦合、提高腔内功率等手段，在掺Er3+增益补偿材料吸收损耗的情况下，利用新产生的激光泵浦微腔自身实现有源微腔中的参量振荡和级联四波混频，从而实现以3.5 μm 微腔激光为中心波长的中红外光频梳，同时解决当前基于无源微腔产生光频梳对窄线宽、可调谐单频中红外泵浦源的依赖。

中红外激光在生物分子传感、光电对抗、精密光谱学测量等研究领域有着非常重要的应用。本项目原计划采用掺Er3+氟/硫化物玻璃棒制备高Q值回音壁模微盘腔，并实现中红外微激器和光学频率梳。但由于氟/硫化物玻璃存在熔点低、空气中易氧化、机械性能易脆等特性，因此受限于现有的制备工艺而无法制备高Q值氟/硫化物玻璃微盘腔。为此，本项目采用了另外两种方案进行解决：1）基于掺杂稀土离子微球腔的有源微激光器；2）基于高Q回音壁模微腔反馈的光纤激光器。在方案1）中，在理论上对基于掺杂稀土离子微球腔的有源微激光器进行速率方程研究；在实验上分别制备了高Q值掺Er3+和的Tm3+石英微球腔，实现了极低阈值的1.5 μm和2 μm波段的掺杂有源微激光器，并可用于相关光学传感应用。在方案2）中，在理论上对基于高Q回音壁模微腔反馈的光纤激光器进行速率方程研究；在实验上分别将高Q值石英微球腔与掺Pr3+氟化物（Pr:ZBLAN）光纤、掺Yb3+石英光纤、掺Er3+石英光纤以及掺Tm3+石英光纤结合，实现了可见光波段（~0.635 μm）、近红外波段（1.06 μm和1.55 μm）以及2 μm波段的高Q石英微球腔反馈光纤激光器，并利用高Q石英微球腔具有压缩激光线宽和腔内的自身强度增强作用，分别实现了1 μm、1.55 μm和2 μm波段的窄线宽激光以及激发产生了包括受激布里渊散射、受激拉曼散射、克尔频率梳在内的三阶非线性光学效应。特别是，在高Q石英微球腔反馈的2 μm光纤激光器实验中，利用高Q石英微球腔内的参量振荡和级联四波混频非线性光学效应，实现了从近红外（1600 nm）扩展到中红外（> 2400 nm）波段的光学频率梳。