Nonlinear dynamics of free electron terahertz lasers with bragg mirrors based on coupling of traveling and quasi-critical waves

Combination of various modifications of Bragg structures makes it possible to control electromagnetic fluxes in the interaction space, thus ensuring spatial coherence of radiation for high superdimensionality in two transverse coordinates required, which is for the operation of free electron lasers with distributed feedback in the submillimeter range. We propose that coupling between traveling and quasi-critical waves in the input Bragg mirror be used for mode selection in the “narrow” transverse coordinate directed along the normal to the conductors forming a planar waveguide. A traditional Bragg structure coupling copropagating and counterpropagating wave flows can be used as the output mirror.     

Stimulated Cherenkov radiation of a relativistic electron beam moving over a periodically corrugated surface (quasi-optical theory)

The propagation of waves over periodically corrugated surfaces and their excitation by relativistic electron beams are investigated within the framework of a quasi-optical approach. The dispersion equation is derived for normal waves under the assumption of a small (in the scale of the period and wavelength) corrugation depth, based on which two limiting cases are identified. In the first limiting case, the wave frequency is far from the Bragg resonance, and the propagation of waves can be described in terms of the impedance approximation, in which the fundamental spatial harmonic slows down. In the second limiting case realized at frequencies close to the Bragg resonance, the field is represented as two counterpropagating quasi-optical wave beams coupled on a corrugated surface and forming a normal surface wave. When interacting with an electron beam, convective instability, which can be used to realize amplifier regimes, corresponds to the first case, and absolute one, which is applied in surface-wave oscillators, corresponds to the second case. The developed theory is used to determine basic characteristics of amplifier and oscillator schemes: the growth rates, the energy exchange efficiency, and the formation of a self-consistent spatial structure of the radiated field. The practical realization of relativistic submillimeter amplifiers and surface-wave oscillators is shown to hold promise.     

Forces exerted on atoms by stochastic laser fields

We present analytical results and numerical simulations for the force exerted on moving atoms in the fields of two counterpropagating waves whose amplitudes or phases are described as stochastic processes. We assume that one field repeats the other with some delay, as would occur when the two fields derive from a common source through a beam splitter and mirrors. We show that, just as with the force exerted by the field of two counterpropagating sequences of π-pulses, or two counterpropagating bichromatic or frequency-modulated waves, the force on an atom in counterpropagating stochastic waves may considerably exceed the force exerted by the field of a single running wave. For comparison we also discuss the interaction of an atom with two counterpropagating waves when one of them is monochromatic and the other one has a stochastic phase. In this case the force substantially exceeds the force exerted by the field of a single running wave but appreciably smaller then the radiative force in the counterpropagating waves in which one field repeats the other with some delay.     

Counterpropagating waves on the sea surface (Results of Natural Microwave Experiment)

This paper is devoted to the analysis of the results of the natural experiment on the study of nonlinear dynamics of surface waves by microwave methods. During the experiment, waves traveling in opposition to the wind and the dominant wave system were detected by high-resolution microwave radar methods. An analysis of the results of microwave sounding made it possible to obtain a space-frequency spectrum of counterpropagating waves, to estimate their intensity, and to relate the fact of counterpropagating wave generation to experimental conditions. It was shown that the generation of counterpropagating waves under conditions of this experiment can be interpreted on the basis of the mechanism of the cubic interaction of surface gravity waves.     

Short-wave sectioned free-electron masers with Bragg resonators based on the traveling and quasi-critical wave coupling

We consider the possibility of implementation of a free-electron maser with a two-mirror resonator composed of modified and conventional Bragg mirrors, operated in the short-wave part of the millimeter-wave range. The use of a modified Bragg mirror based on the traveling and quasicritical wave coupling at the input of the interaction space permits the transverse-index selection of modes. Amplification of the synchronous co-propagating wave by an electron beam is reached mainly in the regular part of the resonator. Even slight reflections from the conventional output Bragg cavity, which directly couples the co- and counter-propagating traveling waves, turn out to be sufficient for generation of self-excited oscillations. It is shown that the new scheme of a free electron maser ensures the oscillation frequency stabilization with respect to the electron-energy variation. With the optimal choice of the parameters, the oscillation frequency is close to the cutoff frequency of a quasi-critical wave excited in the modified Bragg structure.     

Manifestation of Hamiltonian monodromy in nonlinear wave systems

We show that the concept of dynamical monodromy plays a natural fundamental role in the spatiotemporal dynamics of counterpropagating nonlinear wave systems. By means of an adiabatic change of the boundary conditions imposed to the wave system, we show that Hamiltonian monodromy manifests itself through the spontaneous formation of a topological phase singularity (2π- or π-phase defect) in the nonlinear waves. This manifestation of dynamical Hamiltonian monodromy is illustrated by generic nonlinear wave models. In particular, we predict that its measurement can be realized in a direct way in the framework of a nonlinear optics experiment.     

Photoresist Bragg gratings as beam splitters and beam reflectors for integrated optical systems

Based on the theory of coupled waves the diffraction efficiency (DE) of Bragg reflectors on planar glass waveguides is described for the case of obliquely incident waves. The coupling coefficients of the different types of mode conversion are determined both by a perturbational analysis and by fitting of the theoretical DE curves to the experimental data. Results on the fabrication and use of photoresist gratings on monomode glass waveguides as beam splitters and beam reflectors are given.     

A tunable optical diode based on gyrotropic metamaterials in the field of ultrasonic waves

The modified Ambartsumyan method of addition of layers is used for solving the problem of oblique light propagation through a layer of gyrotropic metamaterial in the field of two counterpropagating ultrasonic waves. It is shown that the studied system can operate as a tunable optical diode. The possibility of controlling the system parameters by changing the parameters of ultrasonic waves is studied.     

Two-level model of a superradiant laser with distributed feedback

Numerical one-dimensional simulation of the superradiance phenomenon in a sample of a two-level medium with cw pumping under the conditions of resonant Bragg backscattering of counterpropagating waves has been performed. A range of parameters is determined in which a superradiant laser generating a chaotic sequence of ultrashort high-power pulses can be implemented, and the properties of these pulses are studied.     

Oscillatory energy exchange between waves coupled by a dynamic artificial crystal

We describe a general mechanism of controllable energy exchange between waves propagating in a dynamic artificial crystal. We show that if a spatial periodicity is temporarily imposed on the transmission properties of a wave-carrying medium while a wave is inside, this wave is coupled to a secondary counterpropagating wave and energy oscillates between the two. The oscillation frequency is determined by the width of the spectral band gap created by the periodicity and the frequency difference between the coupled waves. The effect is demonstrated with spin waves in a dynamic magnonic crystal.     

Length scales and polarization properties of annular standing waves in circular broad-area vertical-cavity surface-emitting lasers

It is shown that the ‘flower-like’ emission patterns along the perimeter of broad-area circular vertical-cavity surface-emitting lasers can be interpreted in a first approximation as annular standing waves. Their modulation period follows the dispersion relation of tilted plane waves for a plano-planar cavity. For high enough divergence angle, they show a very peculiar polarization behavior of radial polarization in near field and azimuthal polarization in far field. The polarization selection is qualitatively explained by the breaking of the isotropy between s- and p-components for waves propagating off axis in the Bragg reflectors.     

Collective spontaneous emission in a distributed feedback laser with an inhomogeneously broadened active medium

Peculiarities of the implementation of a superradiant generation mode in a laser with a distributed feedback of the waves and an inhomogeneously broadened spectral line of the active medium are investigated. Based on a numerical solution of the semiclassical Maxwell-Bloch equations for the amplitudes of counterpropagating electromagnetic waves, polarization, and inversion of a two-level medium population, it is shown that a sequence of coherent high-power pulses is generated by the Bragg structure in a wide range of the laser parameters as a result of the selected mode and the cw pumping.     

准周期加隔板有限长圆柱壳声散射

研究准周期加隔板有限长圆柱壳在水中的声散射特性,隔板位置存在小的随机偏差.首先给出理论推导,通过计算周期加隔板情况验证理论公式的正确性.然后以角度-频率谱形式给出准周期加隔板圆柱壳声散射计算结果.计算表明隔板的准周期性导致Bloch-Floquet弯曲波和散射声场背景出现扩散和增强现象,而近乎平行于横轴的由隔板共振引起的亮条纹被散射声场背景所掩盖.最后讨论了随机因子、隔板个数以及隔板间距对Bragg散射的影响.计算表明随机因子越大Bragg散射条纹的频率范围越宽扩散越明显,隔板个数越多Bragg散射条纹的频率范围越窄能量越集中,隔板间距变大时Bragg散射条纹增多而且越高阶次的Bragg散射条纹扩散越严重.根据Bragg散射的几何特征导出的近似估算公式可以较准确预报Bragg散射在频谱图上的位置,也可以大致预报隔板准周期排列时Bragg散射的扩散现象.     

Fiber optic liquid leak detection technique with an ultrasonic actuator and a fiber Bragg grating

We present a technique for liquid leak detection in which ultrasonic and optical waves are introduced into a fiber simultaneously. The system is based on an ultrasonic technique using an ultrasonic actuator and a fiber Bragg grating receiver. A fiber-guided ultrasonic wave is utilized to stress the fiber Bragg grating, which is remote from the ultrasonic transmitter. When the traveling ultrasonic wave encounters a liquid, part of the wave will leak out from the fiber, which results in an ultrasonic strain decrease in the fiber Bragg grating. The ultrasonic wave and its attenuation are detected by the light variation of a narrowband laser source reflected and transmitted from the fiber Bragg grating, and the amplitude variation of the ultrasound can eventually be correlated with the fiber area coupled with the liquid.     

Frequency stabilization in free-electron masers with 2D and 1D distributed feedback

We analyze the dynamics of free-electron masers (FEMs) with two-mirror hybrid Bragg resonators using the coupling between running and quasi-critical modes in the input mirror. Such a mirror may have the form of a 2D Bragg structure with coaxial geometry or a segment of a cylindrical waveguide with axisymmetric corrugation having a period close to the wavelength. The output mirror has the traditional Bragg structure coupling two counterpropagating running waves (the corrugation period is close to half the wavelength). It is shown that a stable unimodal lasing with a radiation frequency close to the cutoff frequency of the quasicritical mode excited in the input mirror can be attained using this scheme under optimal conditions. Such a regime is insensitive to variations of the electron beam parameters. Simulation of experimentally implemented FEMs and those being developed is carried out.     

偶极声波实现钻前地质探测的方法研究

针对随钻地震技术在声波长距离传播导致的衰减和信号混叠等问题,本文提出采用井下发射和接收声波的方式,利用偶极声波实现前方地质情况探测。结合数值模拟结果,分析了偶极辐射声场和反射声场特征,重点讨论了反射波信号与钻头前方反射面之间的关系。结果表明,偶极横波在低倾角反射面的反射强度较大,尤其是SH波具有振幅强、倾角覆盖范围大的特点,可以用于前方地质情况探测。偶极横波还表现出了较高的方位灵敏度,有助于识别前方反射体的方位。     

On the theory of self-modulation instability in an FEL amplifier due to stimulated scattering of counterpropagating waves

We present the results of numerical simulation of the self-modulation processes in an amplifier based on the effect of stimulated scattering of two counterpropagating transverse electromagnetic waves by a relativistic electron beam (FEL amplifier). Two models of the studied system are considered. One model allows for the effects of overbunching of electrons and is based on the modified method of macroparticles. The other is a simplified wave model obtained in the approximation that the amplitude of a combination wave is small if the nonlinearity of the electron-beam processes is negligibly small. The mechanisms of self-modulation are studied. The scenarios of transition to chaos, observed with increase in the input-signal intensity and system length, are examined. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 6, pp. 471–484, June 2007.     

Light guidance in a photonic bandgap slab waveguide consisting of two different Bragg reflectors

By means of a zigzag-wave model, we perform a comprehensive analysis of the guided modes of a photonic bandgap slab waveguide that consists of a low-index guiding layer placed between two different Bragg reflectors. We show that the guided modes of the waveguide operate inside the overlapped bandgaps of the two Bragg reflectors. We label each mode with three mode orders, which reflect the features of the mode-field distribution in the guiding layer and the two Bragg reflectors, respectively. We discuss in detail the transmission characteristics of the guided modes, including the dispersion curves, the cutoff conditions, and the confinement factors, and highlight the mode-selection characteristics of the waveguide. We also study the effects of truncating the Bragg reflectors by deriving an analytical expression to show the dependence of the leakage losses of the modes on the numbers of the periods in the truncated Bragg reflectors.     

激波聚焦反射的实验和数值研究

 采用高速摄影技术和数值模拟方法,对入射激波在两种不同形状的抛物形反射器表面聚焦和反射的过程进行了研究,得到了激波聚焦反射过程的波系结构的实验阴影照片和数值计算结果,两者符合得很好。对激波聚焦形成的气体动力学焦点的特性进行了分析,结果表明,入射激波在两种反射器反射后聚焦所形成的气体动力学焦点均是由三波点在轴心处的会聚所导致的,气体动力学焦点位于相应的反射器壁面的几何焦点附近。不同的反射器中,激波聚焦前后的波系结构也不同,对较浅的反射器,入射激波反射前在反射器壁面形成了弓形激波,反射之后需要相对较长的时间完成聚焦,形成聚焦反射激波之后弓形激波仍未相交;对较深的反射器,入射激波反射后在更短的时间内聚焦,聚焦时弓形激波已经相交,聚焦反射激波之后的流场波系结构更加复杂。     

Lateral field instability and six-wave mixing in a diode laser with broad active area

The electromagnetic field inside a nonlinear active medium of a laser is considered as a system of counterpropagating waves. Such an approach changes radically an earlier studied behavior of the lateral field instability due to self-deformaion (or self-focusing). In our calculations we used an expression for a laser field in the form of two “strong” counterpropagating waves whose complex amplitudes have weak perturbations. Amplitude perturbations of each of the “strong” waves can be presented by two spatial harmonics corresponding to two weak perturbation waves with wave vectors making some tilted angle ±φ with the cavity axis. Thus six waves would participate in the interaction: two counterpropagating strong waves and two pairs of weak waves. Using this approach, we have developed a theory for the propagation of four “weak” perturbation waves in a nonlinear amplifying medium in the presence of two counterpropagating “strong” waves. It is shown that perturbation waves with tilted angle φ⋍0.5–1.2° inside the active region, and respecively, with the side lobes of the far-field pattern at ∼1.7–4°, have the greatest growth increment. These perturbation waves produce lateral intensity modulation with period 10–30 μm for the 0.85 μm lasing wavelength. The appearance of such waves corresponds to the instability threshold of a homogeneous lateral distribution of optical power in a diode laser. The present theory makes it possible to investigate the stability of the homogeneous lateral optical intensity distribution in a diode laser of any design. This allows one to choose a suitable design of a laser with a homogeneous lateral distribution at high radiation power. Translated from Preprint No. 43 (1992) of the Lebedev Physics Institute, Russian Academy of Sciences.