Lin Gong-Ru, Kang Jung-Jui, Lee Chao-Kuei
Graduate Institute of Photonics and Optoelectronics, and Department of Electrical Engineering, National Taiwan University, No.1 Roosevelt Rd. Sec. 4, Taipei 10617, Taiwan.
Opt Express. 2010 Apr 26;18(9):9570-9. doi: 10.1364/OE.18.009570.
The 40-GHz rational harmonic mode-locking (RHML) and pulse-amplitude equalization of a semiconductor optical amplifier based fiber-ring laser (SOAFL) is demonstrated by the injection of a reshaped 10-GHz gain-switching FPLD pulse. A nonlinearly biased Mach-Zehnder modulator (MZM) is employed to detune the shape of the double-peak pulse before injecting the SOA, such that a pulse-amplitude equalized 4th-order RHML-SOAFL can be achieved by reshaping the SOA gain within one modulation period. An optical injection mode-locking model is constructed to simulate the compensation of uneven amplitudes between adjacent RHML pulse peaks before and after pulse-amplitude equalization. The indirect gain compensation technique greatly suppresses the clock amplitude jitter from 45% to 3.5% when achieving 4th-order RHML, and the amplitude fluctuation of sub-rational harmonic modulating envelope is attenuated by 45 dB. After pulse-amplitude equalization, the pulsewidth of the optical-injection RHML-SOAFL is 8 ps, which still obeys the trend predicted by the inverse square root of repetition rate. The phase noise contributed by the residual ASE noise of the RHML-SOAFL is significantly decreased from -84 to -90 dBc/Hz after initiating the pulse-amplitude equalization, corresponding to the timing jitter reduction from 0.5 to 0.28 ps.
通过注入整形后的10 GHz增益开关FPLD脉冲，演示了基于半导体光放大器的光纤环形激光器（SOAFL）的40 GHz有理谐波锁模（RHML）和脉冲幅度均衡。在注入SOA之前，采用非线性偏置马赫曾德尔调制器（MZM）对双峰脉冲的形状进行失谐，从而通过在一个调制周期内对SOA增益进行整形来实现脉冲幅度均衡的四阶RHML-SOAFL。构建了光注入锁模模型，以模拟脉冲幅度均衡前后相邻RHML脉冲峰值之间幅度不均匀性的补偿。间接增益补偿技术在实现四阶RHML时，将时钟幅度抖动从45%大幅抑制到3.5%，亚有理谐波调制包络的幅度波动衰减了45 dB。脉冲幅度均衡后，光注入RHML-SOAFL的脉冲宽度为8 ps，仍遵循重复率平方根倒数预测的趋势。在启动脉冲幅度均衡后，RHML-SOAFL的残余ASE噪声贡献的相位噪声从-84显著降低到-90 dBc/Hz，对应于定时抖动从0.5 ps降低到0.28 ps。
