高功率飞秒光梳相干合成新方法研究

Optical femtosecond frequency combs with high accuracy and broadband spectral range have many applications in the fields of precision spectroscopy, matter composition analysis and surface topography measurement. Especially for the application of ultraviolet or mid-infrared frequency comb generation, high-power high accurate femtosecond frequency combs are essential to acting as the driving sources. The advanced large-mode-area fiber manufacturing technology and the widespread use of chirped pulse amplification method promote the development of high-power optical femtosecond frequency combs. However, with the increase of output power, the nonlinear effects during amplification will distort the pulse characteristics in time and frequency domain and thus affect the accuracy of high-power femtosecond frequency combs. Based on the proposed coherently combining of optical femtosecond frequency combs, this program has the potential to break through the limitation of single fiber amplifier and generate high-power high accuracy femtosecond frequency combs by the use of self-similar amplification and carrier-envelope phase control methods. The nonlinear effects, especially the self-phase modulation, were used to compensate the gain narrowing in amplification process via pre-chirp management amplification method. We used active phase control method which is based on minimum-searching algorithm to investigate coherently combining of femtosecond frequency combs, and provide a possible solution for high-power high accuracy femtosecond frequency combs both in theory and experiment.

高精度宽谱带的飞秒光梳在时频域精密光谱测量、物质成分检测、表面形貌分析等领域都有着广泛的应用。飞秒光梳光谱范围向紫外、极紫外以及中红外、远红外拓展，都离不开高功率高精度的飞秒光梳作为驱动源。大模场光纤制造工艺的成熟以及啁啾脉冲放大技术的广泛应用，推动了高功率飞秒光梳的发展。然而，随着飞秒光梳功率的提高，光纤中的非线性不断增大，放大器输出脉冲的时频域特性发生畸变，放大过程中累积的附加相位噪声将严重影响高功率飞秒光梳的精度。本项目采用主动式相干合成技术，结合自相似放大技术与超短脉冲载波包络相位控制方法，突破单一放大器的功率极限，实现高功率高精度飞秒光梳。对于单路光纤放大器，自相似放大技术有效利用放大过程中的自相位调制效应，克服增益窄化，有利于提高飞秒光梳控制的精度；对于相干合成方式，采用基于最小值寻找算法的主动式结构，研究合成过程中脉冲时频域特性、偏振态特性对合成效果的影响。

飞秒光梳技术在时频域精密测量、高精度测距、物质成分检测等领域应用广泛。受限于增益介质的发射波段，目前飞秒光梳的应用仍限制在近红外波段。研制高功率飞秒光梳，结合非线性频率变换技术，是实现紫外、极紫外和中远红外光学频率梳的前提条件。通过相干合成的方法提高飞秒光梳的平均功率，能够有效避免增益介质非线性效应和高阶色散对超短脉冲时频域特性造成的畸变，突破单一放大器的功率瓶颈。本项目开发了基于最小值算法的主动式相干合成方法，结合超短脉冲精密时频域控制技术，实现飞秒光梳的相干合成、精密控制和基于光纤超快激光的非线性光谱成像。项目期间，发展了基于单片机的相位锁定技术、偏振合成技术、前馈式载波包络相位锁定技术、自适应双光梳噪声补偿技术、异步采样技术、光纤参量振荡技术、色散滤波技术，实现了高功率飞秒光梳相干合成的新方法，并开展了精密光谱测量和非线性光谱成像的应用研究。在相干合成各支路相位控制方面，克服了温度变化、机械震动、空气抖动等环境干扰，获得了λ/20的相位控制误差，实现91%的合成效率；在光梳载波包络相位控制方面，发展了全光纤全保偏的前馈式控制技术，实现了1 MHz的控制带宽和1 Hz的光梳梳齿线宽；在高精度光梳光谱测量方面，研制自适应双光梳系统，对乙炔和甲烷气体的吸收峰进行测量，获得1.09 GHz的光谱分辨率和0.60 ms的单次采样时间，系统的稳定工作时间达到1200小时；在非线性光谱测量与成像方面，初步开展了基于四波混频效应的光纤参量振荡器，对相位匹配条件进行主动操控，获得781-791 nm和960-1000 nm的双色可调谐超快脉冲，具有时间自同步、空间自重合的特点，并将其用于小鼠耳朵皮下脂肪组织的无标记成像，得到清晰的相干反斯托克斯拉曼散射图像。

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