高功率单频光纤放大器强度噪声控制的研究

Intensity noise is one of the most important parameters to charactrize the performance of laser noise. The strict standard is established for the intensity noise performance of laser sources to meet the requirements of various applications including gravitational-wave detection, LIDAR, remote telecommunication with fiber link, and coherent manipulation of qunantum system. High-power unltralow intensity noise single-frequency lasers are in great demand to improve the strain sensitivity of the interferometric gravitational-wave detector. Robust, compact all-fiber amplifier represents one of the most promising technologies to be used as laser source for the next-generation gravational-wave detector. In order to develop a high power, low intensity noise all fiberized single-frequency fiber amplifier with long-term reliability, we propose the use of large-mode-area Yb-doped photonic crystal fiber as the gain medium of main amplifier for stimulated Brillouin scattering (SBS) suppression to achieve single-frequency high power output. The self-made mode-field adapter will be designed to realize all fiber intergration of the power amplification system. The intensity noise of fiber amplifier is proposed to be suppressed by direct pump current control of the pump diode, which aims to achieve near shot-noise limit performance at high power output. The goal of the research is laying a solid foundation for the potential laser source development used in gravitational-wave detector.

激光强度噪声是衡量激光噪声特性的重要参数之一。对于引力波探测、激光雷达、远程光纤通信、量子体系的相干控制等应用，它们都对所使用激光光源的强度噪声有着严格的技术指标要求。为了提高引力波探测系统的应力灵敏度，往往需要使用高功率、超低强度噪声的单频激光光源。具有稳定可靠、紧凑结构的全光纤功率放大器，在作为下一代引力波探测器的光源方面，已展现出极好的潜力。为了发展出可长期稳定运行、具有低强度噪声输出的高功率全光纤单频激光放大器，本项目提出利用大模场面积双包层掺镱光子晶体光纤作为主放大功率单元的增益介质，抑制受激布里渊散射散射效应，实现高功率单频激光输出。研发低损耗光纤模场适配器，实现功率放大系统的全光纤集成。通过对半导体泵浦源进行直接电流反馈控制的方式，来抑制光纤放大系统的强度噪声，达到高功率激光输出条件下近散粒噪声极限的超低强度噪声运行，进而为下一步引力波探测器光源研发奠定坚实的基础。

项目研制了面向星载引力波探测应用的单频激光光源。采用非平面环形腔结构设计的种子光源实现直接输出功率~80 mW，线宽1.3 kHz。经过小型化实现的工程原理样机尺寸为61×61×37 mm3，总重量是0.29 kg。通过双包层光纤放大技术，实现稳定超过4 W的连续激光输出，功率抖动噪声抑制到小于1×10-3/√Hz的水平。采用PDH技术对激光频率控制，结合自制光胶FP腔，将激光内环频率稳定度控制到2×10-17@ 1 s水平。在锁定带宽以内，激光器频率噪声可达到小于10 Hz/√Hz的水平。这些技术参数初步满足天琴引力波的应用需求。