Nonlinear Dynamics of Temporal Optical Soliton Molecules in Lasers

Abstract

Recent experiments demonstrate that fiber laser cavities are able to support various multisoliton complexes, analogous to soliton molecules. These advances, which could have impact on optical information transmission or storage, are guided by the concept of dissipative soliton and supported by numerical simulations. DOI: 10.2529/PIERS060828120520 As passively mode-locked lasers rely strongly on nonlinear dissipation, there is a growing interest in understanding various pulse dynamics in terms of the dynamics of dissipative solitons [1]. In particular, the interaction between dissipative temporal solitons can lead to the formation of stable multi-soliton complexes. The stability of multi-soliton complexes arises from the existence of attractors that are able to bind stably several pulses in a way somehow analogous to the formation of molecules. The simplest of these complexes is the soliton pair, or stable bound state of two solitons. It was predicted in the frame of the Ginzburg-Landau propagation equation model and characterized by a fixed distance, as well as a fixed phase relationship of ±π/2 between the two solitons [2], then found experimentally in a passively mode-locked erbium-doped fiber ring laser [3]. A soliton pair uses the dissipative nonlinear dynamics of the active cavity in order to remain stable for hours without the need of external feed back stabilization. The formation of soliton molecules is not limited to diatomic molecules: triatomic and other polyatomic molecules were also reported [4]. Moreover, the ability to form soliton molecules is not limited to a very specific cavity design, it is rather a general feature that make them potentially observable in most passively mode-locked laser cavity schemes, since it relies mainly on the strength of nonlinear dissipation, which in turn provides attractor sets for soliton molecules formation. For instance, soliton molecules were found with negative and with positive path-averaged chromatic dispersions, as well as with low and high pumping powers [1, 4]. In the following, after having briefly presented the experimental setup used in the experiments, we shall discuss the latest issues discovered with soliton molecules in fiber lasers. They concern the possible control of formation and dissociation of soliton molecules, as well as vibrational motions. Figure 1: Sketch of the fiber laser experimental setup. PIERS ONLINE, VOL. 3, NO. 3, 2007 358 1. FIBER LASER EXPERIMENTAL SETUP As sketched in Fig. 1, we use a dispersion-managed fiber ring laser which series a 1.6-meter-long 1400-ppm erbium-doped fiber (EDF) in the normal dispersion regime (D = −35 ps.nm−1.km−1) which provides laser emission around 1.5μm, a polarization-insensitive optical isolator, a given length of standard telecom fiber (SMF) in the anomalous dispersion regime (D = ±17 ps. nm−1.km−1), and an open-air section that includes retarding wave plates and a polarizing beam splitter cube. Nonlinear polarization evolution that takes place in both fibers, along with polarization discrimination by the polarizing cube, provides the equivalent of an ultrafast saturable absorber. Under a 980-nm pumping power in the 100-300mW range, stable mode locking is achieved with appropriate adjustment of the retarding wave plates that precede the polarizing cube. According to the length of SMF chosen for a given experiment, the averaged cavity dispersion can be varied between anomalous or normal. Under adjustments of pump power and retarding wave plates, pulse durations are in the 100-600 fs range, with intracavity energies in the 50-500 pJ range. Using these adjustments of pump power and retarding wave plates, we are able to switch from single soliton to multiple soliton operation. Part of the multiple soliton operations lead to the formation of stable multisoliton complexes, such as the stable soliton pair [3].