Er:Glass NPRO激光强度噪声的全量子理论分析与实验研究

Abstract: Homodyne coherent optical communication is an important technique for breaking through the bandwidth bottleneck of space communication in the future. This system has high requirements for the intensity noise, linewidth and power of light source. It is easy to get narrow linewidth and high power from nonplanar ring oscillators (NPRO). Benefiting from this virtue, it will be an ideal 1.55 micrometer laser source for space communication application by constructing NPRO with Er:Glass as the gain medium and suppressing its noise. The research contents of this project include: the theory of three-level solid-state laser noise, design of low noise NPRO and laser noise suppressing experiments. By constructing a full quantum theoretical model for three-level solid-state laser system, noise transfer function and output noise spectrum will be deduced, to overcome the inaccurateness of traditional rate equation in quantification. NPRO would be designed and constructed based on medium with low refraction index, and enough loss difference of different eigenpolarizations could still be reached, which breaks through the traditional conception that the refractive index has to be larger than 1.73. High order phase advancing circuits would be used in the design of negative feedback loop, which can compensate phase delay at high frequency more effectively and improve the noise reduction. Relative intensity noise better than -150 dB/Hz and more than 500-mW output power in single frequency would be reached.

零差相干光通信是解决未来天基通信带宽瓶颈的重要途径，该体制对光源的强度噪声、线宽和功率均提出了很高的要求。利用非平面环形激光器（NPRO）易获得窄线宽、高功率输出的优点，采用Er:Glass建立NPRO并抑制其强度噪声，将为天基通信应用提供一种理想的1.55微米波段光源。主要研究内容包括三个方面：三能级固体激光噪声的理论分析，低噪声NPRO设计和激光噪声抑制实验研究。通过建立三能级固体激光噪声的全量子理论分析模型，求解噪声传递函数和输出噪声谱，克服传统速率方程理论定量分析不准确的缺陷。基于低折射率介质设计和研制NPRO，获得足够大的本征偏振态损耗差，实现稳定的单频行波振荡，突破NPRO介质折射率须大于1.73的传统观念。采用高阶相位超前电路设计反馈控制环路，更好的补偿高频相位延迟，改善噪声抑制效果。预期获得相对强度噪声低于-150dB/Hz、功率大于500mW的单频窄线宽激光输出。

低噪声、窄线宽和高功率激光源在激光通信、探测和测量等领域具有重要应用，也是激光技术领域的一个重点研究方向。1.55µm波段的Er:Glass激光能与现有光通信器件和设备很好的兼容，且具有线宽窄的优势，通过进一步抑制其输出强度噪声，将具备良好的应用前景。完成了项目预定研究内容，包括以下三个方面。（1）在理论上建立了三能级固体激光强度噪声的全量子理论分析模型，通过线性微扰处理和傅里叶变换，求解出各噪声源到激光输出噪声的传递函数，得到激光输出强度噪声谱。在全量子理论模型中引入反馈控制机制，求解出加入反馈控制环路后的输出强度噪声谱，为噪声抑制系统设计和实验研究提供更准确的指导。（2）完成了低噪声Er:Glass NPRO激光器设计，包括三能级NPRO激光器的泵浦均匀性设计、低折射率NPRO谐振腔的本征偏振模式的损耗差设计、低噪声输出特性设计、反馈控制电路设计。在反馈控制电路设计中，采用级联相位超前单元的方式，可在不同频率处分别设计相位超前，补偿高频相位延迟，使得环路在高增益下仍具有良好的稳定性，从而获得更好的噪声抑制效果。（3）在实验上，建立了1.55µm Er:Glass NPRO激光器及噪声反馈控制环路，在整个频谱范围内实现了对于激光噪声的大幅度抑制，获得低噪声输出。Er:Glass NPRO激光器各项技术指标达到项目预期指标，如下：（1）激光波长：1.55µm；（2）激光输出功率：514mW；（3）相对强度噪声：-151dB/Hz；（4）激光线宽：0.7kHz。项目执行过程中，发表论文14篇，其中8篇论文被SCI检索，3篇被EI检索；申请专利3项；培养硕士研究生3名。低噪声Er:Glass NPRO激光器可应用于零差相干通信系统，提高系统接收信噪比，延伸通信距离。在激光相干探测系统应用中，可提高接收灵敏度。