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Journal of Modern Optics

Mode locking in coherently driven laser systems

VICTOR KOZLOV, Marlan O. Scully

Published Mar 1, 2002• 0 citations
Publisher

Abstract

A new concept of mode locking for a three-level gain medium with two coherent fields is proposed. The strong field applied on the coupling transition produces broadening of the lasing transition. The broadened emission spectrum allows one to generate pulses with durations far beyond limits inherent to a conventional mode-locking technique.

DOI
10.1080/09500340110088632

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A new concept of mode locking for a three-level gain medium with two coherent fields is proposed. The strong field applied on the coupling transition produces broadening of the lasing transition. The broadened emission spectrum allows one to generate pulses with durations far beyond limits inherent to a conventional mode-locking technique.
Generation of single-cycle pulses from a laser passively mode locked by the technique of coherent mode locking is theoretically demonstrated for the case when resonant lines of both the gain and absorbing media are inhomogeneously broadened. In contrast to conventional mode-locked lasers, for which the inhomogeneous nature of line broadening sets a severe limitation on their performance, here, the stable single-cycle pulse operation is shown to persist even when the spectral widths of the inhomogeneously broadened lines of the amplifier and the absorber become nearly as wide as the central resonance frequency of their two-level transitions. This study is aimed at showing the potential for application of the coherent-mode-locking technique for quantum-dot lasers, which are typically characterized by a great amount of inhomogeneous broadening.
We consider a single-mode unidirectional ring laser containing a homogeneously broadened two-level medium whose atoms are injected in a coherent superposition of states. Provided that appropriate phase-matching conditions are met, the injected coherence contributes directly to the polarization of the medium and hence to the laser gain. We assume that the field is classical and perfectly phase matched with the injected coherence. We find that laser oscillation can occur below the usual laser threshold, and even in the absence of an inversion, with a frequency locked to the atomic line center. Mode locking is unstable for sufficiently strong relative excitation.
We consider a single-mode unidirectional ring laser containing a homogeneously broadened two-level medium whose atoms are injected in a coherent superposition of states. Provided appropriate phase matching conditions are met, the injected coherence contributes directly to the polarization of the medium and hence to the laser gain. We suppose the field is classical and perfectly phase matched with the injected coherence. We find that laser oscillation can occur below the usual laser threshold and even in the absence of an inversion. In these cases the steady state laser frequency is locked to the atomic line center. As the excitation increases, the ability of the injected atomic coherence to lock the laser frequency decreases. Slightly above the usual laser threshold, the system unlocks and produces an output weakly modulated in time.
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.
Summary form only given. We experimentally demonstrate field-enhanced lasing, i.e., lasing made possible through an excitation-field-mediated boost in the gain normally realizable on a weakly inverted transition. The excitation field responsible for the boost in gain, the helper field couples the lasing transition to another transition in a cascade-type three-level atomic system. The gain boosting operative in our driven three-level-atom experiments derives from the same essential physics that gives rise to the gain that has recently attracted so much attention. In terms of achieving laser operation in new regimes, gain boosting may play much the same role as inversionless gain.
Noninversion lasing is investigated in \ensuremath{\Lambda}-type three-level atomic systems that are coupled to two modes of the electromagnetic field. As a result of an external field driving the transition between the lower two closely spaced levels, laser action can be achieved in both modes simultaneously at arbitrarily small initial upper-level population (lasing without inversion) provided the pumping rate exceeds a certain threshold. The relative phase of the two modes is locked to a particular value controlled by the phase of the external field. In addition, new regimes of the laser operation arise when initial coherences between various atomic levels are also injected into the resonator. Firstly, laser action can be achieved in both modes at arbitrarily small pumping (no threshold). Secondly, the injected atomic coherence can control competition between the modes, and result in a variety of multistable steady-state behavior illustrating the very sensitive dynamics of the system.