Optical linewidth of a passively mode-locked semiconductor laser

Tatiana Habruseva (1, 2) , Shane O'Donoghue (2, 1) , Natalia Rebrova (2, 1) , Fabien Kéfélian (2, 1) , Steve Hegarty (2, 1) , Guillaume Huyet (2, 1)

1 CIT - Cork Institute of Technology
2 Tyndall National Institute [Cork]

We measured the optical linewidths of a passively mode-locked quantum dot laser and show that, in agree-ment with theoretical predictions, the modal linewidth exhibits a parabolic dependence with the mode op-tical frequency. The minimum linewidth follows a Schawlow–Townes behavior with a rebroadening at high power. In addition, the slope of the parabola is proportional to the RF linewidth of the laser and can there-fore provide a direct measurement of the timing jitter. Such a measurement could be easily applied to mode-locked semiconductor lasers with a fast repetition rate where the RF linewidth cannot be directly measured. Optical frequency combs have become important tools for time and frequency metrology [1]. They have also been identified as potential sources for coherent communications and signal processing [2]. Optical frequency combs are commonly generated by mode-locked lasers (MLLs) that periodically emit short pulses with an optical spectrum composed of a set of equally spaced narrow lines. Some metrology appli-cations require octave spanning combs generated by Ti:sapphire lasers where the pump intensity and the cavity length fluctuations constitute the main sources of noise. Coherent communications and sig-nal processing require a much narrower frequency line set that can be generated by monolithic passively mode-locked semiconductor lasers where the sponta-neous emission constitutes the main source of noise in a large Fourier frequency range. In recent years monolithic quantum dot (QD) MLLs have shown much promise in producing stable pulses, of the order of picoseconds, with high repetition rates and low timing jitter [3–5]. In general, noise influences mainly the amplitude, the central optical frequency, the comb line to comb line frequency spacing, the pulse-to-pulse timing, and the optical phase of mode-locked semiconductor lasers, but the broadening of the comb lines [6] is dominated by the contributions of optical phase noise and pulse-to-pulse timing fluctuations. The quantum limited fluctuations of the optical phase induce a Lorentzian line shape in all the comb lines, similarly to the Schawlow–Townes line shape of single-mode lasers. The optical linewidth is also related to timing jitter fluctuations. The stochastic dynamics of MLLs can be described by a set of Langevin equations [7] capturing the evolution of the power, frequency, tim-ing jitter, and optical phase of the laser. By retaining only the timing jitter and the optical phase fluctua-tions, the optical field is [8] A͑t͒ = ͚

Mots clés: Lasers and laser optics : Mode-locked lasers; Optical devices : Quantum-well; -wire and -dot devices

Domaines: Optique / photonique

Tatiana Habruseva, Shane O'Donoghue, Natalia Rebrova, Fabien Kéfélian, Steve Hegarty, et al.. Optical linewidth of a passively mode-locked semiconductor laser. Optics Letters, 2009, 34, pp.3307-3309. ⟨10.1364/OL.34.003307⟩. ⟨hal-01075182⟩