[OPTICAL REVIEW Vol. 18, No. 1 (2011) 13-18]
© 2011 The Japan Society of Applied Physics

Generation of Flat Optical Frequency Comb by Fiber Loop Modulation

Masaki HIRANO* and Akihiro MORIMOTO

Department of Photonics, College of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan

(Received October 12, 2010; Accepted November 30, 2010)

We propose a novel flat optical frequency comb generation system that employs fiber loop modulation and experimentally demonstrate its operation. Generally, when an external laser beam is injected into a fiber loop system including an optical modulator and an amplifying medium, multiple sidebands are generated exponentially. On the other hand, the fiber loop starts mode-locked oscillation at a center frequency to maximize the round-trip gain in the fiber loop from the seeds of the injected laser sidebands. By locking the sidebands from fiber loop modulation with the mode-locked oscillation in the fiber loop, the synchronous spectrum between the mode-locked oscillation and the sidebands can be shaped into a flat spectrum. Thus, a flat optical frequency comb with a flatness of about ± 1 dB is obtained in the spectral bandwidth of about 1 THz.

Key words: optical frequency comb, fiber loop modulation, mode locking, optical modulation, fiber ring laser

*E-mail address: ro008000@ed.ritsumei.ac.jp