Coherent Mode-locking Research Articles

Observation of operation states and wavelength-switching in spatiotemporal mode-locked lasers

The construction and investigation of spatiotemporal mode-locked (STML) lasers offer us an opportunity to comprehend and analyze exceedingly intricate high-dimensional nonlinear physical phenomena that underlie diverse behaviors. While the exploration of coherent mode-locking, such as multimode solitons, has been progressing in depth, there remains a dearth of understanding regarding random pulses or extreme events within STML lasers. In this study, an STML laser was constructed and implemented with two operating regimes. The laser demonstrated mutual switching between noise-like pulses and dissipative soliton states, accompanied by corresponding changes in the output spectrum. The transition of the laser's output spectrum from a single wavelength to a synchronous dual-wavelength at noise-like pulses operation was observed for the first time, as far as we all know. These findings contribute to a deeper understanding of spatiotemporal mode-locked lasers.

Self-starting coherent mode locking in a two-section laser with identical gain and absorber media

Coherent mode-locking (CML) represents a method for ultrashort pulse generation in lasers based on the self-induced transparency (SIT) phenomena, which allows us to overcome the principal limitations on the pulse duration imposed by the active medium's gain bandwidth in standard passively mode-locked lasers. So far, this regime was only studied in lasers with transition dipole moments in the absorber medium being twice larger than in the gain medium. This fact ensures that the $2\ensuremath{\pi}$-SIT-soliton in the absorber also forms the stable $\ensuremath{\pi}$-soliton in the gain but can be barely realized in experiments. In this paper we demonstrate theoretically that the self-starting stable coherent mode-locking regime can also arise if the same medium is used both in the gain and absorber laser sections, i.e., the transition dipole moments are equal. These results are both obtained analytically using the area theorem and confirmed by the numerical solution of the Maxwell-Bloch equations. Our findings can enable new opportunities in the area of ultrafast lasers.

Single-cycle-pulse generation in a coherently mode-locked laser with an ultrashort cavity

We theoretically investigate the possibility of a passive mode-locking regime in a two-section laser with an ultrashort cavity of the length around ten wavelengths of the medium resonant transition or even less. It is shown that such a regime is enabled through the coherent mode-locking phenomena arising due to the self-induced transparency when the gain and absorption bandwidths do not impose any restrictions on the duration of the generated pulse. Under such conditions, a laser with an ultrashort cavity could produce output pulses with the terahertz (THz) repetition rate and the duration up to a single optical cycle. Moreover, several scaling rules were derived, which allow for further shortening of the output pulse duration with the respective scaling of the media parameters.

Stable coherent mode-locking based on pi pulse formation in single-section lasers

Here we consider coherent mode-locking (CML) regimes in single-section cavity lasers, taking place for pulse durations less than atomic population and phase relaxation times, which arise due to coherent Rabi oscillations of the atomic inversion. Typically, CML is introduced for lasers with two sections, the gain and absorber ones. Here we show that, for certain combination of the cavity length and relaxation parameters, a very stable CML in a laser, containing only gain section, may arise. The mode-locking is unconditionally self-starting and appears due to balance of intra-pulse de-excitation and slow interpulse-scale pump-induced relaxation processes. We also discuss the scaling of the system to shorter pulse durations, showing a possibility of mode-locking for few-cycle pulses.

Self-induced transparency mode locking, and area theorem.

Self-induced transparency mode locking (or coherent mode locking, CML), which is based on intracavity self-induced transparency soliton dynamics, potentially allows achievement of nearly single-cycle intracavity pulse durations, much below the phase relaxation time T2 in a laser which, despite having great promise, has not yet been realized experimentally. We develop a diagram technique which allows us to predict the main features of CML regimes in a generic two-section laser far from the single-cycle limit. We show that CML can arise directly at the first laser threshold if the phase relaxation time is large enough. Furthermore, we discuss the stability of the corresponding mapping. We also predict the existence of "super-CML regimes," with a pulse coupled to several Rabi oscillations in the nonlinear medium.

Self-starting stable coherent mode-locking in a two-section laser

Coherent mode-locking (CML) uses self-induced transparency (SIT) soliton formation to achieve, in contrast to conventional schemes based on absorption saturation, the pulse durations below the limit allowed by the gain line width. Despite the great promise it is difficult to realize it experimentally because a complicated setup is required. In all previous theoretical considerations CML is believed to be non-self-starting. In this paper we show that if the cavity length is selected properly, a very stable (CML) regime can be realized in an elementary two-section ring-cavity geometry, and this regime is self-developing from the non-lasing state. The stability of the pulsed regime is the result of a dynamical stabilization mechanism arising due to finite-cavity-size effects.

On coherent mode-locking in a two-section laser

Currently existing lasers with passive mode-locking are two-section systems including an amplifier section and a saturable absorber section. The mechanism of generation of short pulses in them is based on the effects of saturation of the gain in the amplifier and absorption in the absorber, which prevent obtaining pulses with durations shorter than the relaxation time of the polarization in the amplifying and absorbing media. In this work, the new possibility of the generation of ultrashort pulses in a laser with passive mode-locking owing to the coherent character of the interaction of light with matter in the amplifying and absorbing media in ring and linear cavities has been theoretically analyzed. A practically interesting case where the amplifying and absorbing media are separated in the space of the cavity, rather than being homogeneously mixed in its volume, as was previously considered for such type of lasers, has been studied. It has been shown that the width of the spectrum of generated pulses can be much larger than the width of the amplification line and can vary depending on the parameters of the amplifying and absorbing media of the laser. The existence of nonsoliton scenarios of the regime of coherent passive mode synchronization, as well as the possibility of generation without injection of an auxiliary pulse, has been demonstrated.

The optical spectrum of a coherently mode-locked laser

The spectrum of pulses from a mode-locked cavity dumped argon laser operated in the “coherent mode-locking” regime has been studied and found to exhibit a characteristic double-peaked structure is interpreted as arising from the coherent ringing of the electric field envelope of the pulse.