Broad, Tunable and Stable Single-Frequency Erbium Fiber Compound-Ring Lasers Based on Parallel and Series Structures in L-Band Operation oleh Yu-Ting Lai, Lan-Yin Chen, Teng-Yao Yang, Tsu-Hsin Wu, Chien-Hung Yeh, Kuan-Ming Cheng, Chun-Yen Lin, Chi-Wai Chow, Shien-Kuei Liaw