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Physical Review A

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

A. V. Pakhomov, Mikhail V. Arkhipov, Nikolay N. Rosanov, Rostislav M. Arkhipov

Published Jan 17, 2023• 10 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.107.013510

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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\ensuremathπ2\ensuremath{\pi}2\ensuremathπ-SIT-soliton in the absorber also forms the stable \ensuremathπ\ensuremath{\pi}\ensuremathπ-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.
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.
In self-induced-transparency (SIT) mode locking, 2\ensuremath{\pi} SIT solitons with a duration much smaller than the polarization relaxation time T2{T}_{2}T2​ in the gain and absorber are formed in a laser cavity. This is in contrast to standard passive mode-locking schemes based on gain and absorption saturation. Despite the great promise, up to now SIT mode locking with 2\ensuremath{\pi} pulses was mainly studied theoretically. In this paper, a stable self-starting passive mode locking is demonstrated experimentally in a Ti:sapphire laser with a Rb vapor cell. We show that the mode locking indeed appears to be due to SIT in the Rb cell; that is, the pulse in the Rb cell is a 2\ensuremath{\pi} SIT soliton. We also confirm self-starting of the SIT mode locking. Self-starting takes place via a set of intermediate regimes, including the one containing zero-area pulses.
Stable coherent mode-locking based on π\piπ 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.
We propose a simple and convenient analytical model for the description of coherent passive mode locking in a two-section ring-cavity laser based on the generalized area theorem for the coherent pulse propagation in a resonant medium with inhomogeneous line broadening. This model is applied to theoretically analyze the possible regimes of the spatiotemporal dynamics of such a laser together with their stability properties. We examined the laser dynamics, when varying the energy and phase relaxation times in the media, the gain and absorption rate in the respective laser sections, as well as the ratio of the transition dipole moments of the absorber and gain media. As the result, the stability of the coherent mode locking as well as the large flexibility of the arising laser behavior were shown in wide parameter ranges.