1101208 IEEE JOURNAL OF SELECTED TOPICS INQUANTUM ELECTRONICS, VOL. 19, NO. 4, JULY/AUGUST 2013 A Novel Scheme for Two-Level Stabilization of Semiconductor Mode-Locked Lasers Using Simultaneous Optical Injection and Optical Feedback Ehsan Sooudi, Student Member, IEEE, Cristina de Dios, John G. McInerney, Senior Member, IEEE, Guillaume Huyet, Senior Member, IEEE, Franc ¸ois Lelarge, Kamel Merghem, Ricardo Rosales, Anthony Martinez, Abderrahim Ramdane, and Stephen P. Hegarty, Member, IEEE Abstract—We introduce a novel scheme for the simultaneous reduction of time-bandwidth product (TBP) and RF linewidth of quantum-dash two-section mode-locked lasers using opti- cal injection-locking and filtered optical feedback. The optical injection-locked laser, double-locked with optical feedback showed 2× TBP reduction and RF linewidth reduction by two orders of magnitude. This stabilization technique is implemented in an all- optical arrangement without optical/electrical conversion which is ideal for high-repetition-rate devices and photonic integration. Index Terms—Mode-locked lasers, optical feedback, optical injection-locking, quantum-dash lasers, timing jitter. I. INTRODUCTION P ASSIVELY mode-locked semiconductor lasers are com- pact, efficient sources for high speed (∼1–100 GHz), pi- cosecond pulse generation, with applications in optical com- munication systems such as clock recovery, comb generation, and optical time-division multiplexing, as well as remote sens- ing, optical sampling, and arbitrary waveform generation. While Manuscript received November 1, 2012; revised January 25, 2013; accepted February 10, 2013. Date of publication February 25, 2013; date of current version May 13, 2013. This work was conducted under the framework of the INSPIRE programme, funded by the Irish Government’s PRTLI Cycle 4, Na- tional Development Plan 2007–2013, and by the Science Foundation Ireland (SFI) under Contract 07/IN.1/I929. This work was supported by the EU FP7 Marie Curie Action FP7-PEOPLE-2010-ITN through the PROPHET project under Grant 264687. E. Sooudi and J. G. McInerney are with the Department of Physics, University College Cork, Cork, Ireland, and also with the Tyndall National Institute, Lee Maltings, Cork, Ireland (e-mail: Ehsan.Sooudi@tyndall.ie; mcinerney@ucc.ie). C. de Dios is with the Electronics Technology Department, Universidad Carlos III de Madrid, Legan´ es, Madrid 28911, Spain (e-mail: cdios@ing. uc3m.es). G. Huyet and S. P. Hegarty are with Center of Advanced Photonics and Pro- cess Analysis, (CAPPA) Cork Institute of Technology, Cork, Ireland, and also with the Tyndall National Institute, Lee Maltings, Cork, Ireland (e-mail: guil- laume.huyet@tyndall.ie; Stephen.Hegarty@tyndall.ie). F. Lelarge is with the III-V Lab, a joint Laboratory of “Alcatel Lucent Bell Labs,” “Thales Research & Technology” and “CEA-LETI”, Route de Nozay, 91460, Marcoussis, France (e-mail: francois.lelarge@3-5lab.fr). K. Merghem, R. Rosales, A. Martinez, and A. Ramdane are with the Lab- oratory for Photonics and Nanostructures, CNRS, Marcoussis 91460, France (e-mail: kamel.merghem@lpn.cnrs.fr; ricardo.rosales@lpn.cnrs.fr; Martinez@ lpn.cnrs.fr; abderrahim.ramdane@lpn.cnrs.fr). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSTQE.2013.2249045 picosecond pulse duration from these lasers has been demon- strated routinely, these pulses have significant chirp and poor timing jitter. The latter is usually determined by measurement of the RF linewidth of the repetition-rate peak. Several tech- niques based on external stabilization have been demonstrated to improve these two key characteristics, summarized in Table I. In hybrid mode-locking [1]–[3], the timing of the pulses is synchronized to an external RF source by electrical modula- tion of the absorber section of the laser. While subpicosecond timing jitter with RF linewidth limited by the RF source can be achieved with this technique, the time-bandwidth product (TBP) of the pulses does not improve significantly. Also, the electrical bandwidth is limited for a laser with highrepetition- rate (40 GHz or more) and special care has to be taken in the design of appropriate impedance matching circuits [2], [20]. Dual-mode optical injection, which consists injection of two co- herent continuous wave (CW) lines, has demonstrated both tim- ing jitter and TBP improvement simultaneously [4]–[6]. Also, this technique is not limited by available electrical bandwidth as the injection/synchronization mechanism is all-optical. How- ever, both of these techniques require an external RF source as a reference clock for improvement of timing jitter. Optoelectronic feedback [7], [8] can also be utilized to stabilize timing jitter by conversion of the optical oscillation (using a fast photodetector) to an electrical oscillation for use in a long feedback loop. This technique does not require an RF source, but requires optical to electrical conversion and the repetition-rate of the laser de- termines the electrical bandwidth required. A simpler version of this technique without optical/electrical conversion has been demonstrated to improve timing jitter of the laser in a form of optical feedback [9]–[15]. It should be noted that none of the above feedback-based techniques contribute to the improvement of the TBP. These techniques improve the mutual coherence of the laser modes by reinjecting a part of the pulsation from an auxiliary cavity with higher quality factor. CW optical injection has demonstrated the improvement of TBP [16], [18], and wave- form stabilization [6], [19]. While the timing jitter improvement as a result of CW optical injection has been reported [17] due to super-mode noise reduction, only slight improvement in RF linewidth was observed [18]. This contrasts with feedback-based techniques which achieve RF linewidth improvement by more than one order of magnitude. This is to be expected as CW 1077-260X/$31.00 © 2013 IEEE