We formulate a first-principles description of the behavior of a bidirectional ring cavity containing a saturable absorbing medium, and driven by an external coherent field. The setting of interest to this study is an extension of the ordinary optically bistable system, where a ring resonator supports only one of the two possible directions of propagation of the cavity field. In this more general case and in the uniform-field limit, we show that the dynamics of the fields-atoms system is described by an infinite set of coupled equations that can be readily solved by standard numerical means after appropriate truncation of the number of atomic variables. With the help of two different approaches, we find unexpected, long-time stationary solutions such that the forward and backward fields oscillate with different carrier frequencies: the first is based upon the time-dependent equations and the second on a set of nonlinear-algebraic equations describing the steady states. As an added confirmation, we carry out a linear-stability analysis of the unidirectional steady state for the purpose of identifying the conditions under which a backward field can grow from an initial fluctuation. By this approach we also suggest a possible strategy for the experimental observation of stationary, frequency-shifted output fields.
We formulate a first-principles description of the behavior of a bidirectional ring cavity containing a saturable absorbing medium, and driven by an external coherent field. The setting of interest to this study is an extension of the ordinary optically bistable system, where a ring resonator supports only one of the two possible directions of propagation of the cavity field. In this more general case and in the uniform-field limit, we show that the dynamics of the fields-atoms system is described by an infinite set of coupled equations that can be readily solved by standard numerical means after appropriate truncation of the number of atomic variables. With the help of two different approaches, we find unexpected, long-time stationary solutions such that the forward and backward fields oscillate with different carrier frequencies: the first is based upon the time-dependent equations and the second on a set of nonlinear-algebraic equations describing the steady states. As an added confirmation, we carry out a linear-stability analysis of the unidirectional steady state for the purpose of identifying the conditions under which a backward field can grow from an initial fluctuation. By this approach we also suggest a possible strategy for the experimental observation of stationary, frequency-shifted output fields.
We present an examination of the stability and time-dependent behavior of a semiclassical model of the ring laser with a saturable absorber, in the vicinity of multiple steady-state solutions. We consider particularly the case in which the cavity is tuned away from the resonance frequency of the gain medium. The model considers the absorbing and amplifying media to consist of two-level atoms. Only homogeneous spectral broadening and unidirectional propagation of the light are considered. We are able to treat the steady state with an approximate mean field solution and also with its exact longitudinal dependence. The latter is found numerically from an implicit function solution to the Maxwell-Bloch equations in the steady state. There may be as many as four steady-state solutions for a given set of parameter values. When there are four solutions, we find that three solutions are of one intensity class (high or low) and that one is of a second intensity class (low or high). When the absorbing and emitting atoms are in resonance and the cavity is tuned to their line-center frequency, two of the members of the three-solution class have the same intensity. This degeneracy is broken by detuning.
This paper presents new results for the steady states of a detuned ring laser with a saturable absorber. We employ a semiclassical model which assumes homogeneously broadened two-level atoms. We proceed by solving the Maxwell-Bloch equations for the longitudinal dependence of the steady states of this system, and then simplify our solution by use of the uniform-field approximation. We present uniform-field results for squared electric field versus operating frequency, and for each of these versus cavity tuning and laser excitation. Various cavity linewidths and both resonant and nonresonant amplifier and absorber line-center frequencies are considered. The most notable finding is that cavity detuning breaks the degeneracies found in the steady-state solutions of the fully tuned case. This leads to the prediction that an actual system will bifurcate from the zero-intensity solution to a steady-state solution as laser excitation increases from zero, rather than to the small-amplitude pulsations found for the model with exactly resonant tuning of the cavity and the media line centers. Other phenomena suggested by the steady-state results include tuning-dependent hysteresis and bistability, and instability in both intensity and frequency due to the appearance of one or more new steady-state solutions as tuning is varied. These effects of detuning are being tested by a linearized stability analysis whose results will be reported separately.
In Chapter 9 we discussed the case of a nonlinear cavity containing an active medium. In this framework, the outstanding phenomenon which arises is laser emission, and we calculated the stationary solutions for the ring laser. In this chapter, instead, we turn our attention to the passive nonlinear cavity and in this framework the outstanding phenomenon is optical bistability , i.e. the existence of two distinct stable stationary states that coexist for the same fixed values of the parameters in play. This phenomenon can arise also in the case of an active system contained in a cavity, as we have already noted in Section 9.1 and as we will discuss further in connection with the laser with injected signal, but it became popular from the studies of nonlinear passive optical cavities in the 1970s and 1980s. In contrast to the laser case, in the passive configuration we use an input coherent field, which is injected into the cavity. We discussed this case in Chapter 8 and derived a general equation for the transmission of the cavity (Eq. (8.23)), which we used for an empty cavity and for a linear cavity in Sections 8.4 and 8.5, respectively. In this chapter we will discuss the full nonlinear configuration. It is customary to identify two extreme cases of optical bistability called absorptive optical bistability and dispersive or refractive optical bistability . In both cases an essential ingredient to obtain the bistable behavior is the feedback action exerted by the mirrors of the cavity. In the purely absorptive configuration, in which there is exact resonance among the frequencies of the input field, of the atoms and of the cavity, the other essential ingredient is the saturable nonlinearity of the absorption. In the dispersive case, instead, the main feature is the nonlinearity of the refractive index. These two cases will be illustrated in Sections 11.1 and 11.2, respectively, both “exactly” and in the low-transmission limit.
With the help of a microscopic model we investigate the effects of collisions and atomic recoil on the behavior of absorbing atoms placed inside a bidirectional resonator and driven by an external injected field. This model complements and generalizes earlier studies of the collective atomic recoil laser and of optical bistability. According to our model, even in the presence of collisions, the resonator can support bidirectional propagation. In particular, for appropriate selection of the parameters, we predict the existence of stationary solutions such that the cavity field that copropagates with the injected signal is locked in frequency with the external source, while the counterpropagating field is frequency shifted from both, even in steady state. The early stage of growth of the counterpropagating field is accompanied by a spatial modulation (grating structure) in the density of the medium, but this modulation decays away in a time roughly of the order of the average interval between collisions.
In this paper we analyze the steady-state and stability properties of bistable optical systems, lasers with an injected signal, and ordinary free-running lasers. The starting point of our study is a unified model of a ring cavity containing a finite-size cylindrical sample of homogeneously broadened two-level atoms, and capable of supporting a single field mode with a Gaussian transverse profile. After solving the equations of motion in the steady state, we carry out a linear stability analysis of the stationary solutions, identify the domains of unstable operation of each of the three systems, and compare in detail the results of this work with those of earlier plane-wave uniform-field calculations. In the case of the laser with an injected signal, we find a significant enhancement of the instability domain relative to the plane-wave limit. A similar conclusion holds for mixed absorptive and dispersive optical bistability, although the enhancement of the domain of instability is less pronounced. Unstable behavior is predicted to occur for experimentally accessible values of the bistability parameter, and to be favored by the absence of bistability and by the selection of atomic and cavity detunings having opposite signs. This configuration ensures that the instability will be responsible for the emergence of undamped output pulsations of the type that will make the bistable system behave as an optical clock.
This paper contains a description of the behavior of a multimode unidirectional ring laser with a homogeneously broadened active medium. Our formulation is based on the conventional Maxwell-Bloch (MB) equations, but is distinguished from other treatments by the inclusion of a finite mirror reflectivity and an arbitrary value of the gain parameter. We review the steady-state behavior of the system and analyze the longitudinal profile of the field and of the atomic variables. With an appropriate transformation of variables, we transform the boundary conditions of the ring cavity into standard periodicity type, even in the presence of a finite reflectivity, and derive an infinite hierarchy of coupled mode equations. We analyze exactly the linear stability of the system, and investigate the dependence of the instability domain on the reflectivity and gain parameters. A numerical study of the full MB equations for a parameter range of the type explored in the recent experiments by Hillman et al. [Phys. Rev. Lett. 52, 1605 (1984)] reveals similarities, but also considerable differences between the results of the theory and the main experimental signatures of their instability. However, the injection of numerical noise shows the presence of numerous coexisting basins of attraction which are likely to play a significant role in the dynamics of a noisy laser.