Effect of dispersion on the operation of free-electron lasers driven by short electron bunches

The effect of dispersion on the lasing of waveguide free-electron lasers driven by short periodic electron bunches is considered. In the high-Q cavity approximation, the generation of electromagnetic pulses is described in terms of a parabolic equation. For the linear stage of interaction, starting lasing conditions are found analytically and the spatio-temporal structure of supermodes, which represent a set of phase-locked eigenmodes of the cold cavity, is determined. Dispersion is shown to allow the free-electron laser to operate at both positive and negative mismatches between the period of electron bunch injection and the time taken for the electromagnetic pulse to circulate over the cavity. The simulation of the nonlinear lasing conditions with allowance for dispersion spread makes it possible to find the stationary profile of radiated pulses and the optimum values of the group and time detunings that provide a maximum efficiency of the waveguide free-electron laser. It is also shown that, when the length of interaction is much longer than the starting value, the laser operates under the conditions of periodic or chaotic pulse profile self-modulation.     

Sub-Poissonian radiation of a one-atom two-level laser with incoherent pumping

The quantum statistical properties of the radiation of a one-atom two-level laser with incoherent pumping are analyzed. Solution of the Liouville equation for the density operator in the basis of Fock states shows that stationary radiation from a single-mode laser with incoherent pumping can be in a squeezed (sub-Poissonian) stationary state if the rate of spontaneous decay is lower than the rate of cavity losses and the pump rate. Inside the cavity the Fano factor reaches F=0.85 (15% squeezing). Multiple squeezing (F=0.19) is possible in the transient lasing regime. Significant squeezing obtains at the cavity output; the spectral Fano factor at zero frequency is 0.36 under optimal conditions. Zh. éksp. Teor. Fiz. 115, 1210–1220 (April 1999)     

On phase-modulational bifurcation during passive mode locking in lasers

We have found that phase-modulation instability of a single pulse due to nonlinearity of the refractive index of an in-cavity medium can cause the pulse to break up into several stable identical pulses of smaller amplitude. We show that if the refractive-index nonlinearity is greater than a critical value, then stabilization of the regime with several stationary pulses does not occur, and pulse multiplication accompanied by chaotic variation of the intensity continues until the cavity is completely filled with radiation. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 4, 261–264 (25 February 1998)     

Self-induced transparency,compression, and stopping of electromagnetic pulses interacting with beams of unexcited classical oscillators

The self-induced transparency effects that emerge when short (on the relaxation time scale) light pulses propagate in a two-level noninverted medium are well known in optics. The interaction of microwave pulses with an initially rectilinear electron beam under cyclotron resonance conditions can serve as a classical analog of the described effects. In this case, at a certain intensity of the input signal, the cyclotron absorption is replaced by self-induced transparency when the input pulse propagates almost without any change of its profile, forming a soliton whose amplitude and duration are rigidly related to its velocity. In a certain domain of parameters, this process is accompanied by significant two- or threefold compression of the initial pulse, which is of practical interest for the generation of multigigawatt picosecond microwave pulses. Since the soliton velocity lies between the unperturbed group velocity of the radiation and the translational velocity of the particles, another nontrivial effect in the case of interaction with a counterpropagating electron beam is the possibility of a significant deceleration or full stopping of the electromagnetic pulse.     

Passive mode locking and formation of dissipative solitons in electron oscillators with a bleaching absorber in the feedback loop

The mechanisms of passive mode locking and formation of ultrashort pulses in microwave electron oscillators with a bleaching absorber in the feedback loop have been analyzed. It is shown that in the group synchronism regime in which the translational velocity of particles coincides with the group velocity of the electromagnetic wave, the pulse formation can be described by the equations known in the theory of dissipative solitons. At the same time, the regimes in which the translational velocity of electrons differs from the group velocity and the soliton being formed and moving along the electron beam consecutively (cumulatively) receives energy from various electron fractions are optimal for generating pulses with the maximal peak amplitudes.     

Splitting of an electromagnetic pulse on resonant reflection from a plasma film

It is found theoretically that the temporal profile of a quasimonochromatic electromagnetic pulse is strongly distorted on reflection from a thin (on the wavelength scale) film of a plasma-like (semiconductor, metallic) medium under plasma-resonance conditions. It is shown that an incident Gaussian pulse splits with time (completely or partially) into two reflected pulses, whose amplitudes can be controlled by varying the relationships among the parameters of the incident pulse and the film. Zh. Tekh. Fiz. 67, 65–68 (June 1997)     

Second-harmonic generation taking into account dispersion of nonlinear susceptibility

Second-harmonic generation in the field of an ultrashort pulse and the propagation of extremely short pulses in a medium with quadratic nonlinearity are analyzed. Second-harmonic generation is analyzed taking into account the effect of second-and third-order group velocity dispersion and dispersion of nonlinear susceptibility up to the second order. Corrections, whose order of smallness is determined by the parameter (ω_L t _p)^?1, where t _p is the pulse duration and ω_L is the carrier frequency of the pump wave, are obtained. For a large phase mismatch, two new solutions are found that describe the stationary evolution of solitary pump and second-harmonic waves in the regions of both anomalous and normal group velocity dispersions.     

Switching of the generation of nano/picosecond pulses in Nd:YAG laser with Pockels Q-switch

A method for switching the regime of single-frequency generation of nanosecond pulses to a train of picosecond pulses in the Nd:YAG laser with an electro-optic Pockels Q-switch without changing cavity elements is proposed. The single-frequency regime was provided by aligning an active element which played the role of a longitudinal cavity mode selector. The lasing spectrum was measured using a Fabry-Perot interferometer with simultaneous recording of pulse oscillograms.     

Generation of dual-wavelength domain-wall rectangular-shape pulses in HNLF-based fiber ring laser

The generation of dual-wavelength domain-wall rectangular-shape pulses in a highly nonlinear fiber (HNLF)-based fiber ring laser is experimentally demonstrated. The dual-wavelength lasing operation is realized by employing the intracavity birefringence-induced spectral filtering effect. An 85 m long HNLF is introduced into the fiber ring laser to enhance the nonlinear effect, which is favorable for the cross coupling between the two lasing beams. Experimentally, it was found that the interval of two domain walls in the time domain could be adjusted by simply tuning the linear cavity phase delay, which results in the achievement of different output pulse shapes. By properly rotating the polarization controller (PC), the dual-wavelength rectangular-shape pulses could be efficiently obtained. The proposed fiber laser provides a simple and efficient way to generate rectangular-shape pulse.     

Threshold self-start of laser passive mode-locking under conditions of high-order frequency dispersion of the refractive index

The dependence of the self-start of laser passive mode-locking on the peak intensity of initial pulses under conditions of high-order frequency dispersion of the refractive index is analyzed using numerical simulation and analytical calculations. It is found that frequency dispersion of this type can lead to generation bistability. Specifically, laser passive mode-locking requires the generation of a sufficiently intense seed pulse in the initial noise distribution of the field in the laser cavity; otherwise, no laser passive mode-locking occurs and a steady-state lasing mode with the entire laser cavity filled with radiation is established in the system after the transient process is completed. The revealed threshold dependence of the self-start of laser passive mode-locking on the intensity of the initial seed pulse is one of the possible factors preventing the generation of ultrashort pulses in Kerr-lens lasers.     

High-power single-frequency tunable solid-state lasers

Experimental results are presented on the achievement of single-frequency tunable lasing in ruby, Nd-glass, and Nd:YAG lasers with electrooptic Q switching of the cavity by the injection of an external signal. An optimization of the parameters is carried out for lasers on neodymium ions in yttrium aluminum garnet, lanthanum beryllate, chromium-doped gadolinium scandium gallium garnet, and lanthanum hexaaluminate with passive Q switching of the cavity by means of lithium fluoride shutters containing F ₂ ⁻ color centers. High-power single-frequency generation of giant pulses is achieved, with the output wavelength tunable over the half-width of the gain lines of the active media. Zh. Tekh. Fiz. 68, 74–79 (October 1998)     

Prepulse dependence ofJ=0−1 lasing at 32.6 nm in neon-like titanium

The dependence of the intensity of the 3p—3s,J = 0–1 lasing line at 32.6 nm in neon-like titanium on the prepulse level has been investigated experimentally. Titanium slabs were irradiated with 1.315 µm/450 ps pulses from the Asterix IV iodine laser using a defined prepulse of 5.2 ns before the main pulse. It is found that for pump energies close to the minimum energy for which lasing is observed, a prepulse level of order 0.5% gives the highest XUV laser intensity, whereas a higher prepulse level, of order 10% and more, is required for optimum XUV lasing far above the threshold. For a 2.7 cm long titanium target lasing was observed down to a pump irradiance of 2.5 TW/cm² (50 J/450 ps) for the 0.5% prepulse.     

Electron kinetics in a discharge plasma produced by a focused microwave beam in free space

A study is made of the electron kinetics in a discharge plasma produced by a high-power beam of electromagnetic radiation in the centimeter-wave region under conditions approaching free space, when the dimensions of the chamber are much greater than the wavelength of the microwave radiation. Two regimes of discharge production are investigated: the regime of short microsecond pulses at a repetition rate of 200 Hz, and a single millisecond pulse regime. It is shown that at threshold values of the microwave energy flux density the electron density in the initial stages of discharge formation reaches the critical value, and that the average energy of the electrons is of the order of 1.5–3 eV. Zh. Tekh. Fiz. 67, 10–14 (June 1997)     

LD抽运被动调Q固体激光器的脉冲稳定性

报道了二极管激光抽运Cr,Nd∶YAG被动调Q Nd∶YVO₄ 1064nm激光输出.为了提高被动调Q激光的输出稳定性，谐振腔采用长腔，增益介质位于谐振腔中间位置.实验中获得了稳定的被动调Q激光输出.在最高入射抽运功率为17.5W时，脉冲重复频率达到71.3kHz，脉冲宽度为0.4μs.调Q脉冲幅度不稳定性低于±10%，脉冲时间波动性低于±3.5%.     

孔缝对金属腔体强电磁脉冲耦合特性影响研究

核电磁脉冲和高功率微波等强电磁脉冲易造成电子设备功能失效甚至损毁,在实际工程实施中用金属腔体对电子设备进行屏蔽是常用的强电磁脉冲抑制手段。基于电磁仿真计算,对含矩形孔缝金属腔体的强电磁脉冲耦合特性进行了系统研究,阐述了孔缝宽长比、腔体尺寸等因素对多种不同类型强电磁脉冲（核电磁脉冲、宽带高功率微波、窄带高功率微波）作用下腔体内耦合场的影响;并以此为基础,重点分析了强电磁脉冲与含孔缝金属腔体之间的作用机制。研究结果表明:不同类型强电磁脉冲耦合信号差异明显,金属腔体对强电磁脉冲的响应是腔体谐振模式、孔缝谐振频率与强电磁脉冲共同作用的结果;当腔体谐振模式、孔缝谐振频率在强电磁脉冲的带内时,腔体内部的耦合场会出现增强效应;特别地,腔体与孔缝间的相互作用还可造成腔体与缝隙的谐振频率发生偏移。因此,在为电子设备设计金属屏蔽外壳时,应基于不同强电磁脉冲的频带范围,对腔体与孔缝的尺寸进行综合设计,抑制腔体、孔缝谐振及谐振频率偏移,提升其强电磁脉冲防护性能。     

Efficient high-power UV laser generated by an optimized flat–flat actively Q-switched laser with extra-cavity harmonic generations

We investigate the parasitic lasing effect in the high-power actively Q-switched laser with a flat–flat resonator. The parasitic lasing effect in the low-Q stage is found to lead to long tails in the output pulses, corresponding to the peak-power reduction. We experimentally determine the shortest cavity length without the parasitic lasing effect to optimize the performance of the high-power actively Q-switched laser with a flat–flat cavity. We further use the optimized fundamental laser to generate green and UV lasers by extra-cavity harmonic generations. At a pump power of 44 W, the output powers at 355 nm and 532 nm as high as 6.65 W and 8.38 W are obtained at a pulse repetition rate of 40 kHz.     

自由电子激光器的稳定脉冲解

本文用自由电子激光器经典理论的数值解法,给出一组稳定脉冲解。计算表明,这种解只能在特定条件下获得,只要满足这些条件,解的形状与初始脉冲形状无关,对于微小扰动是稳定的,并保持传播不变。 关键词：     

Resonant electromagnetic soliton in a dense plasma

A study is made of the propagation of an electromagnetic wave along a constant magnetic field B₀ in a high-density plasma under cyclotron resonance conditions when the interaction between the electrons and the wave is strongly nonlinear. It is found that the relativistic reduction in the electron gyrofrequency in the field of the electromagnetic pulse stabilizes the wave. A solution is obtained in the form of an envelope soliton and it is shown that its group velocity is considerably reduced.State University, Rostov-on-Don. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 3–7, January, 1994.     

Cyclotron superradiance of a high-current electron bunch under group synchronism conditions

We have theoretically and experimentally investigated the process of induced coherent emission (superradiance) in an electron bunch rotating in a homogeneous magnetic field. We have shown that this process makes it possible to generate ultrashort microwave pulses. In this case, the optimum conditions are found under group synchronism conditions, when the translational velocity of the bunch matches the group velocity of the radiation propagating in the waveguide circuit. For experimental investigation of the superradiance, we used a RADAN accelerator with subnanosecond electron pulse sharpener. In the 35 GHz range, we obtained microwave pulses with record short duration, down to 0.4 nsec for a peak power level up to 200 kW. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 89–97, December, 1996.     

Role of the wall ablation in the operation of a 46.9 nmAr capillary discharge soft x‐ray laser

A capillary discharge pumped soft x‐ray laser operating at 46.9 nm on the 3p–3s transition of the Ne‐like Ar has been realized by pumping the active medium with a relatively slow current pulse (dI/dt ≈ 6 · 10¹¹ A/s). In order to study the role of the ablation in the production of the laser effect, the intensity of the amplified 46.9 nm line has been investigated using the same pumping current pulses in the plastic (polyacetal) and ceramic (Al₂O₃). We showed that the ablation of the capillary walls is unfavorable both for the compression and stability of the plasma and consequently for the soft x‐ray laser production. The amplification and lasing effects are observed only in the ceramic channel. The measurements of the line intensity at 46.9 nm showed the lasing with a gain‐length product of ≈ 9, a laser pulse energy of ≈ 5 μJ, a pulse duration of 1.3 ns and a beam divergence of ≈ 3.5 mrad. In addition, effect of the scaling of the time of lasing with the initial plasma diameter was demonstrated experimentally and compared with a one‐dimensional MHD model.