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OUTPUT STABILITY OF LASERS

HO YU-PING

Published Aug 5, 2005• 0 citations
OpenAlex

Abstract

The semiclassical equations which describe the behaviours of three-level lasers were derived. The method of nonlinear mechanics was used to treat the electromagnetic oscillations coupled with the dipole moments of molecules. It is shown that when the pumping power is larger than the critical value, the amplitude of output oscillations is stable (corresponding to a stable equilibrium point in phase space), only if the homogeneous line width of molecules (1/T 2 ) is larger than the damping width of the resonan-tor (q). The appearance of instability of output oscillations is due to the effect of coherence. Furthermore, a stable limit cycle was shown to appear when the above mentioned equilibrium point becomes unstable as qT 2 increases. Therefore, it seems true that there is no limit cycle near the equilibrium point when it is stable. These results may account for the instability of amplitude of output oscillations observed by some authors in ruby lasers, and should be checked in detail with gas lasers.

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Journal of the Optical Society of America B•Mar 1, 1988•10 citations
The semiclassical equations which describe the behaviours of three-level lasers were derived. The method of nonlinear mechanics was used to treat the electromagnetic oscillations coupled with the dipole moments of molecules. It is shown that when the pumping power is larger than the critical value, the amplitude of output oscillations is stable (corresponding to a stable equilibrium point in phase space), only if the homogeneous line width of molecules (1/T 2 ) is larger than the damping width of the resonan-tor (q). The appearance of instability of output oscillations is due to the effect of coherence. Furthermore, a stable limit cycle was shown to appear when the above mentioned equilibrium point becomes unstable as qT 2 increases. Therefore, it seems true that there is no limit cycle near the equilibrium point when it is stable. These results may account for the instability of amplitude of output oscillations observed by some authors in ruby lasers, and should be checked in detail with gas lasers.
Interaction of two coherent fields of arbitrary intensity with a general homogeneously broadened three-level molecular system for both an amplifier as well as a laser oscillator under general conditions of pumping is discussed. The amplifier solution is obtained through the time-averaged density-matrix formalism with two coherent fields treated classically. A detailed consideration of both real and imaginary parts of the electric susceptibility at pump and emission frequencies, corresponding to gain and dispersion in the active medium has been taken into account. The results obtained are used to study a number of situations for weak and strong pumping in the laser amplifier. Subsequently these are applied to a mode analysis of a unidirectional single-mode laser oscillator to determine the output characteristics of the laser and to establish conditions under which emission becomes dynamically unstable resulting in sustained oscillatory emission. The results obtained are found to be in good agreement with those of a full numerical analysis of the atomic and field coupled equations describing the system.
We discuss the influence of injected atomic coherence on laser dynamics. The nonlinear equations of motion for the intensity and phase give the dependence on external parameters (inversion, amplitude of the atomic coherence, and cavity-field detuning). Depending on the inversion, there can be one or three steady-state solutions corresponding to a given value of the injected coherence. The situation is analogous to optical bistability, the atomic coherence playing the role of the injected field. For zero detuning the upper, laser-like branch starts at zero value of the injected coherence and a numerical stability analysis indicates that it is the only stable solution, the lower branch being everywhere unstable. For finite detuning the upper branch starts at a finite threshold value of the injected coherence. For coherences smaller than this threshold the system exhibits oscillatory behavior that corresponds to a stable limit cycle in the phase plane of the laser quadrature variables. The intensity fluctuations of the laser exhibit antibunching for sub-Poissonian pumping statistics.
A semiclassical description for dye lasers is proposed, where the energy-level diagram of the dye molecule is assumed to consist of a continuous bandlike ground state and an excited singlet state. Unidirectional single-frequency (s.f.) operation is discussed. The linear-stability analysis for this operation reveals a very low threshold instability, which may appear generally in practical lasers. The ratio of the instability threshold to the lasing threshold may be of any value greater than 1, depending mainly on the bandwidth and the distribution of the dipole moments on the band, but it is independent of the cavity loss. This instability may account for that observed in recent experiments by Hillman, Krasinki, Boyd, and Stroud [Phys. Rev. Lett. 52, 1605 (1984)]. A general approach to analyzing the linear stability of the s.f. operation of the Maxwell-Bloch equations is also proposed, which states that only the eigenvalues of a 2\ifmmode\times\else\texttimes\fi{}2 matrix are relevant: one concerns the stability of the s.f. operation near the lasing threshold, the other determines the instability threshold of this operation.
The influence of injected atomic coherence on a laser's operation is discussed by analyzing the semiclassical equations of motion of this system. Both stationary-steady-state and time-dependent regimes of operation are investigated. We present the dependence of the stationary laser intensity and phase on the external parameters (population inversion, amplitude of atomic coherence, and detuning). For small cavity-field detuning and large atomic coherence, both frequency and phase locking occurs. The laser frequency is locked to the atomic frequency, and the phase is locked to a value determined primarily by the phase of the atomic coherence. Under certain conditions, in the inverted regime, the output intensity is an ssS-shaped function of the atomic coherence, leading to the possibility of a bistable behavior. However, only a portion of the bistable curve gives stable stationary operation, and in critical points (the location of which depends crucially on the external parameters) time-dependent instabilities branch away. When the detuning is larger than a critical value there is neither phase nor frequency locking. We find that the time-dependent behavior of the laser intensity in this case is oscillatory (quasiperiodic), and show that there is a stable limit cycle in the phase plane of the quadratures. There is a small parameter region where a stationary steady state and an oscillatory state may coexist. We also consider the nonlinear quantum theory of a laser with injected atomic coherence, and include the effect of pumping statistics. We derive the Fokker-Planck equation for the P representation, and express the noise in terms of moments. We find that in the steady state the intensity noise can be suppressed below the shot-noise limit but that the phase fluctuations are not affected by pump regularity. For nonzero detuning, we find that transient squeezing of the phase fluctuations is possible.
In a previous paper we considered the linear problem of a one-dimensional laser having output coupling. In this paper we derive the nonlinear equation for the same model of the laser and solve it for steady state. The output coupling is exactly taken into account and the field is treated as having a continuous spectrum composed of the normal modes of the universe. Damping terms for the atomic polarization and for the population inversion are introduced phenomenologically. The fluctuating forces due to these damping terms and to the initial field are ignored. The resultant field inside the cavity consists of two oppositely travelling waves whose amplitudes are functions of the distance from the coupling surface. Outside the cavity we have a monochromatic, outgoing wave whose amplitude does not change with distance. The threshold inversion and the oscillation frequency are the same as those derived in the linear theory.
The phase-conjugate reflectivity produced by degenerate four-wave mixing in a Kerr medium in the Bragg scattering regime is shown to be unstable when the intensity of the external pump beams is greater than a threshold value that can be predicted analytically. Below this threshold, a single branch of the reflectivity curve is stable, eliminating any possibility for bistability. A numerical simulation indicates that the unstable regime is characterized by fast oscillations with a period related to the transit time in the nonlinear medium. A finite relaxation time of the nonlinearity eliminates the high-frequency oscillations but can increase the amplitude of the low-frequency components. The insertion of a phase-conjugate mirror into an optical cavity leads to an instability whose threshold can be evaluated analytically. The temporal evolution of the field intensity in the cavity is shown to change from a stable regime to periodic oscillations and then to quasi-periodic oscillations as the pump intensity is increased.