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IEEE Journal of Quantum Electronics

Polarization states of a single-mode (microchip) Nd/sup 3+/:YAG laser. I. Theory

R. Dalgliesh, A. D. May, Guy Michel Stéphan

Published Jan 1, 1998• 16 citations
Publisher

Abstract

We present a microscopic mathematical model for the polarization states of a single-frequency Nd/sup 3+/:YAG laser. It is a plane wave, mean field, vector model carried to all orders in the laser field. The crystal is assumed to be optically pumped longitudinally with a laser of specified polarization. For D/sub 2/ site symmetry and an odd number of electrons, we establish the phase relationships between the components of the electric dipole matrix elements between the Kramers states. These relationships are central in determining the site-specific coupling between both, the pump and laser fields to the Nd/sup 3+/ ions. The laser cavity is assumed to be linear and quasi-isotropic. The residual optical anisotropies are included using a round-trip Jones matrix formalism.

DOI
10.1109/3.704348

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Apr 29, 1996•0 citations
We present a microscopic mathematical model for the polarization states of a single-frequency Nd/sup 3+/:YAG laser. It is a plane wave, mean field, vector model carried to all orders in the laser field. The crystal is assumed to be optically pumped longitudinally with a laser of specified polarization. For D/sub 2/ site symmetry and an odd number of electrons, we establish the phase relationships between the components of the electric dipole matrix elements between the Kramers states. These relationships are central in determining the site-specific coupling between both, the pump and laser fields to the Nd/sup 3+/ ions. The laser cavity is assumed to be linear and quasi-isotropic. The residual optical anisotropies are included using a round-trip Jones matrix formalism.
The field intensity and polarization behaviour of an optically pumped laser is investigated in different operating conditions. For a linearly polarized pump field, a strong gain anisotropy is induced which favours generation of light with a polarization parallel to that of the pump field. Thus gain anisotropy can be counterbalanced by cavity-loss anisotropy only at low pumping field intensities, and the interplay between both types of anisotropy leads to polarization switching phenomena. In contrast to the case of the incoherently pumped laser, the decay rate for the magnetic dipole induced on the J = 1 level plays a minor role in determining the polarization dynamics. The influence of a longitudinal magnetic field or of modulation of the pump-field polarization orientation leads to new features and complex dynamic behaviour which includes full polarization (or vector ) chaos.
A mathematical model describing the dynamic emission of intracavity polarized isotropic Cr 4+ :YAG solid--state saturable absorber was developed.This model considers double pumping laser pulses to simulate the actions of Cr 4+ :YAG as a dual Q-switched crystal (1.06 µm) and lasing medium (1.4 µm).The model describes the time evolution of interaction between the pumping laser pulse, partial polarizer and the polarized saturable absorber.The analysis of the polarization process is based on the assumption that at each moment of lasing evolution, the state of polarization represented by an eigenstate corresponds to the lowest radiation losses state.The model offers a simple mechanism for studying the kinetics of the pulsed lasers and the influence of the variations of the pumping laser power and nonlinear anisotropy parameter on the characteristics of the output laser pulses 1.06 µm and 1.4 µm.The angular rotation of the passive switch Cr 4+ :YAG reveals that the transmission of the polarized 1.06 µm laser radiation is strongly anisotropic in the saturation regime.The suggested model estimates the transmission of pumping laser density of the polarized light 1.06 µm radiation as a function of nonlinear anisotropy parameter, temporal behavior of the relative population inversion of Nd-laser, population inversion of the different polarized levels of Cr 4+ :YAG pulsed laser and the output laser pulse densities under impact of different values of the nonlinear anisotropy which is due to the self-induced anisotropy of its saturated absorption.
We study theoretically the dynamic behavior of a J′=1→J″=0 laser (J is the angular momentum quantum number) optically pumped by means of a linearly polarized coherent field (coupled to an adjacent transition J=0→J′=1) when the pump field polarization is rotated at a constant angular velocity Ω and the laser field polarization either is fixed by the cavity or is free (isotropic cavity). Because of a strong pump-induced gain anisotropy, the dynamic behavior is completely different in each case. In the case of fixed laser field polarization, rich amplitude dynamics, which depend on the pump field strength, are found. At slow modulation frequencies (with respect to the molecular and the cavity relaxation rates), the system does not always follow the sequence of stationary and dynamic solutions that correspond to the autonomous laser as a function of the relative orientation angle between the polarizations of the pump and the laser fields. Phenomena such as delayed switching and suppression of chaos and stabilization of unstable steady states are found. In the case of an isotropic cavity, the pump field vector rotation is transferred to the laser field vector and amplitude unstable regimes are also strongly inhibited.
We study a Nd-doped microchip glass laser that emits in two polarizations and in many longitudinal modes. Saturation of the inversion by standing waves causes spatial inhomogeneity of both the longitudinal and azimuthal distributions of the laser gain. These nonlinear inhomogeneities couple the modes and result in low-frequency oscillations (10--500 kHz) of the light flux in the individual laser modes. These oscillations are steadily driven by quantum noise and appear as 15% fluctuations of the power in each mode. In-phase fluctuations in the laser modes appear at the frequency of the main relaxation oscillation of the total laser power. Antiphase fluctuations appear at other frequencies in individual longitudinal and polarization laser modes only. The dominant frequency of these fluctuations is determined by the light power in the mode. Numerical simulations of rate equations, including Langevin forces, satisfactorily reproduce the experimental results. These phenomena must be taken into account when lasers are applied as stable coherent optical light sources, and also with sensitive absorption measurements in the laser cavity.
A polarising interferometer was used to investigate changes in the refractive index of an Nd:YAG laser crystal associated with changes in the populations of the electronic energy levels characterised by different polarisabilities. The study was carried out on diode-pumped crystals as well as on crystals which were flashlamp-pumped under conditions of saturation of the gain experienced by the optical beam. The changes in the refractive index depended nonlinearly on the pump power. The ratio of the real and imaginary parts of the nonlinear susceptibility of an Nd:YAG laser amplifier was determined: this ratio rose from 0.2 to 1.2 when the gain increased from 0.2 to 0.45 cm -1 .
We have carried out a series of numerical experiments on the dynamic behavior of J=0 yields J=1 yields J=0 lasers optically pumped by a linearly polarized laser field and operating either: (1) with a strongly anisotropic cavity yielding fixed linear laser field polarization at any angle (theta) with that of the pump beam, or (2) with an isotropic cavity and an axial magnetic field. In the first case (1) we show that by changing (theta) the steady-states experience a smooth monotonic variation, but the dynamic states undergo a complex sequence of bifurcations. In case (2) we find that the laser field is always linearly polarized, but for some parameter settings we report the first observation that the plane of polarization together with the laser intensity change chaotically.