Kerr-lens Mode-locked Solid-state Lasers Research Articles

Continuous-wave to pulse regimes for a family of passively mode-locked lasers with saturable nonlinearity

The transition dynamics from continuous-wave to pulse regimes of operation for a generic model of passively mode-locked lasers with saturable absorbers, characterized by an active medium with non-Kerr nonlinearity, are investigated analytically and numerically. The system is described by a complex Ginzburg–Landau equation with a general m:n saturable nonlinearity (i.e , where I is the field intensity and m and n are two positive numbers), coupled to a two-level gain equation. An analysis of stability of continuous waves, following the modulational instability approach, provides a global picture of the self-starting dynamics in the system. The analysis reveals two distinct routes depending on values of the couple (m, n), and on the dispersion regime: in the normal dispersion regime, when m = 2 and n is arbitrary, the self-starting requires positive values of the fast saturable absorber and nonlinearity coefficients, but negative values of these two parameters for the family with m = 0. However, when the spectral filter is negative, the laser can self-start for certain values of the input field and the nonlinearity saturation coefficient Γ. The present work provides a general map for the self-starting mechanisms of rare-earth doped figure-eight fiber lasers, as well as Kerr-lens mode-locked solid-state lasers.

A Review of Cavity Design for Kerr Lens Mode-Locked Solid-State Lasers

We provide a critical review of the fundamental concepts of Kerr lens mode-locking (KLM), along with a detailed description of the experimental considerations involved in the realization of a mode-locked oscillator. In addition, we review recent developments that overcome inherent limitations and disadvantages in standard KLM lasers. Our review is aimed mainly at readers who wish to realize/maintain such an oscillator or for those who wish to better understand this major experimental tool.

Multipulse operation and limits of the Kerr-lens mode-locking stability

Numerical analysis in combination with experimental data for Cr/sup 2+/:ZnSe and Ti:sapphire lasers reveal the following main mechanisms of multiple-pulse generation for Kerr-lens mode-locked solid-state lasers: 1) continuum amplification due to a spectral loss growth for ultrashort or chirped pulses and 2) a bounded perturbation rise for high-energy pulses. The role of such laser parameters as gain saturation and relaxation, saturable and unsaturable loss, self-phase modulation, Kerr-lensing, and pump intensity is analyzed. This analysis provides basic directions for single-pulse stability enhancement and for multiple-pulse generation control.

Theory of sub-10 fs-generation in Kerr-lens mode-locked solid-state lasers with a coherent semiconductor absorber

The results of the study of ultra-short pulse generation in continuous-wave Kerr-lens mode-locked (KLM) solid-state lasers with semiconductor saturable absorbers are presented. The issues of extremely short pulse generation are addressed in the frames of the theory that accounts for the coherent nature of the absorber–pulse interaction. We developed an analytical model that bases on the coupled generalized Landau–Ginzburg laser equation and Bloch equations for a coherent absorber. We showed, that in the absence of KLM semiconductor absorber produces 2 π-non-sech-pulses of self-induced transparency, while the KLM provides an extremely short sech-shaped pulse generation. 2 π- and π-sech-shaped solutions and variable-area chirped pulses have been found. It was shown, that the presence of KLM removes the limitation on the minimal modulation depth in absorber. An automodulational stability and self-starting ability were analyzed, too.

Spatiotemporal model of femtosecond pulse generation in Kerr-lens mode-locked solid-state lasers

A spatiotemporal model for the evolution process of the pulse and beam shapes in femtosecond Kerr-lens mode-locked solid-state lasers is presented. For different cavity configurations we comprehensively studied the dependence of the pulse and beam parameters on laser-control parameters such as pump rate, linear phase dispersion up to fourth order, self-phase modulation, and self-amplitude modulation owing to nonlinear resonator transmission. We determine the conditions for the ultimate shortest pulse duration. The influence of third- and fourth-order dispersion results in spectral sidebands, which are phase matched with the peak of the principal spectrum. Excessive fourth-order dispersion yields a steady-state multipulse operating regime with constant peak separation.

Higher-order phase dispersion in femtosecond Kerr-lens mode-locked solid-state lasers: sideband generation and pulse splitting

The influence of higher-order phase dispersion on the pulse generation in femtosecond Kerr-lens mode-locked lasers for small net group-velocity dispersion is numerically analyzed. Depending on the third- and the fourth-order dispersion, we obtain the formation of spectral sidebands phase matched with the principal spectrum. At a relatively large amount of fourth-order dispersion pulse splitting arises, which leads to a quasi-steady-state multipulse operation regime of the Kerr-lens mode-locked laser.

Kerr-lens mode locking without dispersion compensation

We propose and theoretically investigate a novel operating regime of femtosecond Kerr-lens mode-locked solidstate lasers that avoids group-velocity dispersion compensation by use of a nonresonant semiconductor plate in the focused resonator section that provides an overall negative nonlinear refractive index per round trip. The saturable loss of the laser resonator with an effective self-defocusing nonlinearity is derived from a generalized ABCD matrix formalism, and the correspondingly calculated steady-state pulse parameters show that a Kerrlens mode-locked laser with an overall negative nonlinear refractive index generates stable femtosecond pulses without any dispersion compensation.

Starting dynamic, self-starting condition and mode-locking threshold in passive, coupled-cavity or Kerr-lens mode-locked solid-state lasers

The starting dynamic of mode-locked solid-state lasers is analyzed taking into account the gain reduction by the total random radiation. A simple self-starting condition is derived, depending on the pump-rate, the gain cross section, the nonlinear coefficient, the gain bandwidth and the resonator length. It is shown that an additional slow saturable absorber can greatly improve the self-starting conditions. Experimental results are in good agreement with the predictions of the theory.

Cavity design for optimum nonlinear absorption in Kerr-lens mode-locked solid-state lasers

Resonators with an internal Kerr-lens for mode locking representing self-focusing effects in the laser rod and an aperture acting analog as a fast saturable absorber are analyzed. The cavity design for optimum nonlinear absorption and the nonlinear coefficient of the aperture combined with the Kerr-lens are calculated.