498 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 7, APRIL 1, 2007 Self-Stabilization of an Actively Mode-Locked Semiconductor-Based Fiber-Ring Laser for Ultralow Jitter S. Gee, S. Ozharar, F. Quinlan, J. J. Plant, P. W. Juodawlkis, Senior Member, IEEE, and P. J. Delfyett, Fellow, IEEE Abstract—Noise characteristics are studied for a self-stabilized laser utilizing the interplay between the intracavity dispersion and the optical frequency shift. The noise suppression bandwidth of this scheme is from 0 to 100 KHz and showed the reduction of residual timing jitter (integrated from 0.9 Hz to 1 MHz) from 2.2 fs to 660 attosecond which represents, to our knowledge, the lowest timing jitter reported for an actively mode-locked laser. Index Terms—Harmonic mode locking, phase-locked loops (PLL). I. INTRODUCTION O PTICAL frequency stabilized low-noise mode-locked lasers are important for applications such as coherent op- tical waveform synthesis [1] and high resolution spectroscopy [2]. However, for some applications such as microwave signal processing [3] and optical clock distribution [4], low phase noise is more critical than optical frequency stability. Phase noise of mode-locked lasers has a number of sources including spontaneous emission and the cavity length fluctuation [5], [6]. Phase-locked loops (PLL) have been proven effective to sup- press low-frequency phase noise of actively mode-locked lasers [7]. The concept of the PLL can be easily explained with Fig. 1. The laser output pulse train is detected by a photodetector, producing a microwave at the repetition frequency and its harmonics. The phase difference between the microwave signal and the radio-frequency (RF) driving signal is detected by a phase detector. The phase error is then used to control the phase shifter located between the RF source and the mode-locked laser. The ultimate goal of this type of feedback is to reduce the relative phase noise by zeroing the phase difference between two microwave signals. Here we report a different cavity sta- bilization scheme to suppress the phase noise of mode-locked lasers utilizing the interplay between the intracavity dispersion and the optical frequency shift. For actively mode-locked lasers, fluctuations of the laser cavity length will directly cause phase noise. However, this same rule does not apply if there is a large amount of dispersion in the cavity. Because of the intracavity dispersion, optical cavity length becomes strongly dependent on optical frequency. If the Manuscript received September 1, 2006; revised January 9, 2007. This work was supported in part by the Defense Advanced Research Projects Agency (DARPA) AOSP Program under Grant DAAD1702C0097. S. Gee, S. Ozharar, F. Quinlan, and P. J. Delfyett are with the College of Optics, Center for Research and Education in Optics and Lasers, University of Central Florida, Orlando, FL 32816-2700 USA (e-mail: sgee@creol.ucf.edu). J. J. Plant and P. W. Juodawlkis are with Lincoln Laboratory, MIT, Lexington, MA 02420 USA. Digital Object Identifier 10.1109/LPT.2007.892902 Fig. 1. Schematics of the laser setup. I: optical isolator. IM: intensity mod- ulator. OD: optical delay. F: optical bandpass filter. : microwave phase shifter. laser supports enough optical bandwidth so that lasing optical spectrum can shift freely for a certain range, then there will be a subsequent cavity length change. For actively mode-locked lasers, this effect relates the mode-locking repetition rate with the optical lasing frequency. The effect of repetition rate detuning on optical frequency detuning has been studied using a self-con- sistent time-domain model [8], [9]. For a given mode-locking microwave frequency, the optical frequency adjusts itself so that the optical cavity length for that particular optical fre- quency matches the mode-locking rate. Dutta’s work was for steady-state cases; however, a similar action will take place for any transient cavity length fluctuation or RF driving frequency fluctuation. For any fluctuation of the laser, the cavity length can be compensated by an optical frequency shift that results in an effective cavity length change in the opposite direction. In effect, this process plays the same role as the phase noise suppression scheme similar to an electronic PLL. II. EXPERIMENTS Fig. 1 shows the schematic of actively mode-locked laser cavity used in this experiment. We note that the PLL is shown for the descriptive purpose only and was not used in the exper- iment. The gain medium was a high-power 1.5- m slab cou- pled optical waveguide amplifier [10]. The cavity fundamental mode frequency is 10 MHz and the laser is harmonically mode- locked at 10 GHz. The spectral width is 1.5 nm and temporal pulsewidth is 20 ps with down-chirp indicating pulses that are ten times the transform limit. The overall group delay dispersion 1041-1135/$25.00 © 2007 IEEE