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Physical Review A

Experimental and theoretical study of the anisotropy induced in a gas laser by a saturating field

Guy Michel Stéphan, Roger Le Naour, A. Le Floch

Published Feb 1, 1978• 22 citations
Publisher

Abstract

The optical anisotropy induced by a saturating stationary field in a gas laser which is initially isotropic is described theoretically. The calculation of the polarizability tensor shows that linear or circular anisotropies can be induced depending upon the electromagnetic field polarization. The Jones matrix formalism is extended to the nonlinear case. The experimental verification of the predicted linear gain anisotropy in a He-Ne laser is obtained at 3.39 \ensuremath{\mu}m. The Jones matrix for a probe field in this particular case is also calculated. The corresponding linear anisotropy is attributed to Zeeman coherences between sublevels. Moreover, we find that the maximum of this anisotropy is slightly shifted from the center of the line.

DOI
10.1103/physreva.17.733

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Magnetic field and pressure effects in a saturated gas laser amplifier

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Polarization Properties of a Single-Mode Operating Gas Laser in a Small Axial Magnetic Field

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Comment on "Experimental and theoretical study of the anisotropy induced in a gas laser by a saturating field"

H. de Lang

Misunderstandings concerning earlier work are clarified. The crucial role attributed to noncoincidence of the polarization states of the counterpropagating waves in a saturated gas laser is questioned.

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Polarization switching in an anisotropic cavity coherently pumped J=1→J′=0 laser

Carles Serrat, Ramon A. Vilaseca, Germán J. de Valcárcel +1 more

We show that, in an optically pumped resonant $J=1\ensuremath{\rightarrow}{J}^{\ensuremath{'}}=0$ laser, a gain anisotropy is induced by a linearly polarized coherent pump beam that can be balanced or countered by $x$-$y$ cavity-loss anisotropies, leading to switching between the $x$ and $y$ linearly polarized states of the generated laser field as the laser intensity is increased. This switching is not brought about by selection of the mode with the maximum emission intensity. A domain of bistability between the two linearly polarized states is found, whose width depends on the value of the decay rate of the two photon coherence induced among the sublevels of the $J=1$ level manifold. Differently from the incoherently pumped $J=1\ensuremath{\rightarrow}{J}^{\ensuremath{'}}=0$ laser, the decay rate for the magnetic dipole in that manifold has no influence on the polarization of the emission.

Physical Review A•Sep 1, 1997•11 citations
The optical anisotropy induced by a saturating stationary field in a gas laser which is initially isotropic is described theoretically. The calculation of the polarizability tensor shows that linear or circular anisotropies can be induced depending upon the electromagnetic field polarization. The Jones matrix formalism is extended to the nonlinear case. The experimental verification of the predicted linear gain anisotropy in a He-Ne laser is obtained at 3.39 \ensuremath{\mu}m. The Jones matrix for a probe field in this particular case is also calculated. The corresponding linear anisotropy is attributed to Zeeman coherences between sublevels. Moreover, we find that the maximum of this anisotropy is slightly shifted from the center of the line.
Theoretical and experimental studies of a gas laser amplifier are presented, assuming the amplifier is operating with a saturating optical frequency signal. The analysis is primarily concerned with the effects of the gas pressure and the presence of an axial magnetic field on the characteristics of the amplifying medium. Semiclassical radiation theory is used, along with a density matrix description of the atomic medium which relates the motion of single atoms to the macroscopic observables. A two-level description of the atom, using phenomenological source rates and decay rates, forms the basis of our analysis of the gas laser medium. Pressure effects are taken into account to a large extent through suitable choices of decay rate parameters. Two methods for calculating the induced polarization of the atomic medium are used. The first method utilizes a perturbation expansion which is valid for signal intensities which barely reach saturation strength, and it is quite general in applicability. The second method is valid for arbitrarily strong signals, but it yields tractable solutions only for zero magnetic field or for axial magnetic fields large enough such that the Zeeman splitting is much larger than the power broadened homogeneous linewidth of the laser transition. The effects of pressure broadening of the homogeneous spectral linewidth are included in both the weak-signal and strong-signal theories; however the effects of Zeeman sublevel-mixing collisions are taken into account only in the weak-signal theory. The behavior of a He-Ne gas laser amplifier in the presence of an axial magnetic field has been studied experimentally by measuring gain and Faraday rotation of linearly polarized resonant laser signals for various values of input signal intensity, and by measuring nonlinearity - induced anisotropy for elliptically polarized resonant laser signals of various input intensities. Two high-gain transitions in the 3.39-μ region were used for study: a J = 1 to J = 2 (3s2 → 3p4) transition and a J = 1 to J = 1 (3s2 → 3p2) transition. The input signals were tuned to the centers of their respective resonant gain lines. The experimental results agree quite well with corresponding theoretical expressions which have been developed to include the nonlinear effects of saturation strength signals. The experimental results clearly show saturation of Faraday rotation, and for the J = 1 t o J = 1 transition a Faraday rotation reversal and a traveling wave gain dip are seen for small values of axial magnetic field. The nonlinearity induced anisotropy shows a marked dependence on the gas pressure in the amplifier tube for the J = 1 to J = 2 transition; this dependence agrees with the predictions of the general perturbational or weak signal theory when allowances are made for the effects of Zeeman sublevel-mixing collisions. The results provide a method for measuring the upper (neon 3s2) level quadrupole moment decay rate, the dipole moment decay rates for the 3s2 → 3p4 and 3s2 → 3p2 transitions, and the effects of various types of collision processes on these decay rates.
The polarization properties of a single-mode operating gas laser in a small axial magnetic field (field splitting\ensuremath{\ll}natural linewidth) with an initial cavity anisotropy are considered in detail. Expressions are given for the effects on the polarization parameters due to the active medium, the external field, and the initial anisotropy of the cavity. The cooperation of these three effects is discussed. Existing theories are in this respect shown to be either incomplete or inadequate. Specializations are made to several types of atomic transitions and cavity anisotropies. A group of low-field polarization phenomena observed on the He-Ne 1.153-\ensuremath{\mu} mode is theoretically discussed. In particular, it is shown that the observed polarization flip by tuning through the line center and the hysteresis effect, observed by Kannelaud and Culshaw, can be completely understood assuming a cavity whose main anisotropy is a linear phase anisotropy. At zero or very small magnetic field the theory predicts a preference for linear or circular polarization, depending on the type of atomic transition.
The basic mechanisms of the polarization switching in vectorial bistable lasers are studied both theoretically and experimentally versus internal parameters. In particular, by scanning the frequency of a gas laser with two linearly polarized eigenstates, two quite different flipping processes with hysteresis occur according to the linear-phase-anisotropy value of the cavity. Indeed, when the anisotropy increases, rotation and inhibition mechanisms appear successively. The variations of the hysteresis loop for different anisotropy and excitation values are theoretically found to be opposed in the two processes. Additional evolutions of the polarization, such as multiple switchings in the inhibition mechanism and ``hybrid'' hysteresis loops, are predicted. The experiments, essentially performed on a monomode 3.39-\ensuremath{\mu}m $^{3}\mathrm{\ensuremath{-}}^{20}$Ne laser containing an adjustable linear phase anisotropy, confirm the existence of the two processes and their corresponding properties. These processes may also occur in other quasi-isotropic lasers versus other internal parameters, such as, for instance, the injection current in semiconductor lasers. In this case the TE and TM modes flip only in the inhibition mechanism when the bistability conditions are satisfied. The knowledge of the basic flipping mechanisms in the laser itself enables us to understand and realize the external optical-polarization control by an anisotropic feedback (one-frequency systems). Induced-rotation and induced-inhibition mechanisms are then theoretically predicted and experimentally verified. Sensitive optical gates are realized by varying the feedback phase in both processes.
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 show that, in an optically pumped resonant $J=1\ensuremath{\rightarrow}{J}^{\ensuremath{'}}=0$ laser, a gain anisotropy is induced by a linearly polarized coherent pump beam that can be balanced or countered by xxx-yyy cavity-loss anisotropies, leading to switching between the xxx and yyy linearly polarized states of the generated laser field as the laser intensity is increased. This switching is not brought about by selection of the mode with the maximum emission intensity. A domain of bistability between the two linearly polarized states is found, whose width depends on the value of the decay rate of the two photon coherence induced among the sublevels of the J=1J=1J=1 level manifold. Differently from the incoherently pumped $J=1\ensuremath{\rightarrow}{J}^{\ensuremath{'}}=0$ laser, the decay rate for the magnetic dipole in that manifold has no influence on the polarization of the emission.