高速线性光采样用被动锁模光纤激光器重复频率优化

线性光采样是一种测量基于先进调制码型的高速光信号的有效手段, 而被动锁模光纤激光器是其实施所需的关键组件. 本文在介绍线性光采样工作原理的基础上, 首次分析得到被动锁模光纤激光器重复频率与待测信号光线宽的约束关系, 对于正交相移键控(QPSK)信号, 当信号光线宽与采样光脉冲重复频率的比值小于1.5×10^-3 时, 高速信号的相位噪声对线性光采样带来的损伤可以忽略不计. 利用95.984 MHz重复频率的被动锁模光纤激光器对线宽为100 kHz速率为28 Gbaud的QPSK信号开展相关实验, 通过标准数字相干接收算法可以得到与传统高速示波器相同的星座图, 理论分析与实验结果完全符合. 这一研究结果有助于线性光采样用被动锁模光纤激光器的优化设计.

Repetition rate optimization of passively mode-locked fiber laser for high-speed linear optical sampling

Optical fiber communication systems are going to adopt the use of advanced modulation formats. It is thus important to develop measurement techniques and solutions capable of quantifying such signals. Linear optical sampling is an effective technique to characterize the quality of an advanced modulation format for high-speed optical signal with high fidelity, while the passively mode-locked fiber laser is an enabling module to implement the linear optical sampling. In this paper, we obtain a trade-off relationship between the repetition rate of passively mode-locked fiber laser and the linewidth of high-speed signal under test, after the introduction of operation principle for linear optical sampling. It is found that, for the quadrature phase shift keying (QPSK) signal, when the ratio of the linewidth of the signal under test to the repetition rate of passively mode-locked fiber laser is less than 1.5×10^-3, the linear optical sampling-induced impairments can be ignored when there occurs phase noise. Therefore, the phase estimation can be successfully made by using the Viterbi-Viterbi algorithm applied to the block of samples corresponding to the modulation format phase states. Next, we use an optical sampling pulse with a repetition rate of 95.984 MHz, and carry out the optical linear sampling to a 28 Gbaud QPSK signal with a linewidth of 100 kHz. The error vector magnitude (EVM) has long been a commonly used parameter for quantifying the quality of advanced modulation signals. Using the standard coherent detection algorithm, we can successfully recover the constellation with the error vector magnitude (EVM) error less than 1%. Theoretical investigations agree well with the experimental characterization. Such a conclusion is helpful to optimize the design of passively mode-locked fiber laser for optical sampling application.