A compact free electron laser investigation

An electron beam source based on pseudospark discharge was successful in operation at the beam voltage of 200keV and beam current of 2kA. The detailed design of a compact free electron laser using an electron beam by a pseudospark discharge is described. The compact free electron laser consists of a smaller Marx generator with 6 capacitors and switches, a water capacitance of 6nF and a beam source with a high brightness of 3×10¹¹ A/(m rad)². The computer simulation shows that an output power of 101MW is expected at a frequency of 38GHz with a beam energy of 300keV, a current of 2kA and a beam emittance of 48mm mraa.     

M 2-factor of a partially coherent Lorentz–Gauss beam in a turbulent atmosphere

Based on the second-order moments of partially coherent beams in atmospheric turbulence, the analytical beam propagation factor of a partially coherent Lorentz–Gauss beam in a turbulent atmosphere is derived. As a special case of the general formula, the beam propagation factor of a partially coherent Lorentz beam in a turbulent atmosphere is also presented. By using the derived analytical expression, the beam propagation factor is illustrated and analyzed with numerical examples, and the influences of the beam parameters a _j and w _0j , the coherent parameter τ _j , the axial propagation distance z, the structure constant C_n²C_{n}^{2}, and the inner scale turbulence l ₀ on the beam propagation factor are also discussed in detail.     

部分相干平顶光束通过湍流大气传输的等效曲率半径

推导出了部分相干平顶光束在湍流大气中传输的等效曲率半径的解析表达式,详细地研究了湍流对其等效曲率半径的影响.研究表明,湍流使得等效曲率半径R减小,但是只有当湍流足够强时等效曲率半径极小值出现的位置z_min才会改变.在弱湍流中,R随着光束相干长度β的增大而增大;但是在强湍流中,R随着β的增大而减小.R随着光束阶数M(N)的增大而缓慢减小.若β越大、 关键词： 等效曲率半径 湍流 部分相干平顶光束     

Propagation factors of Hermite–Gaussian beams in turbulent atmosphere

Based on the extended Huygens–Fresnel integral and the second-order moments of the Wigner distribution function, an analytical formulae for the propagation factors (M²-factors) of coherent and partially coherent one-dimensional Hermite–Gaussian beams in a turbulent atmosphere are derived. Evolution properties of the M²-factor of the Hermite–Gaussian beam in a turbulent atmosphere are studied numerically in detail. Our results show that the M²-factor of the Hermite–Gaussian beam increases upon propagation in a turbulent atmosphere. The M²-factor of the Hermite–Gaussian beam with larger beam order (or lower coherence) increases slower that of the Hermite–Gaussian beam with smaller beam order (or higher coherence) in a turbulent atmosphere, which means that the Hermite–Gaussian beam with a larger beam order and lower coherence is less affected by a turbulent atmosphere. Our results will be useful in long-distance free-space optical communications.     

Beam propagation of a higher-order cosh-Gaussian beam

On the basis of the second-order moment, the beam propagation of a higher-order cosh-Gaussian beam has been investigated. Two analytical expressions of the M² factor have been derived, which corresponds to the parity of the beam order n. The M² factor is determined by the beam order n and the parameter δ. The influences of the beam order n and the parameter δ on the M² factor are illustrated with numerical examples. The analytical kurtosis parameter has also been presented. The analytical formula is further simplified in the source plane and the far field, respectively. With given appropriate values of n and δ, the kurtosis parameter in the far field is smaller than 3. This research is helpful to the practical applications of a higher-order cosh-Gaussian beam.     

大气湍流中光束束宽扩展和角扩展的比较研究

以厄米-双曲余弦-高斯（H-ChG）光束为例,对H-ChG光束通过大气湍流传输时的束宽扩展和角扩展做了详细研究.用相对束宽和相对角扩展代替束宽和角扩展来研究湍流对光束影响的灵敏程度.研究表明,折射率结构常数C²_n越小,光束束宽扩展和角扩展越小.有较大阶数m,n,较小参数Ω₀和束腰宽度w₀ H-ChG光束的角扩展受湍流影响较小.当传输距离足够远时,这一结论对H-ChG光束的束宽扩展也成立.当传输距离不长时,对H-ChG光束相对束宽随Ω₀和w₀的变化规律做了分析.用数值计算例做了说明,并对结果的正确性做了物理解释.厄米-高斯,双曲余弦高斯和高斯光束在大气湍流中的扩展可作为H-ChG光束的特例来处理. 关键词： 束宽扩展和角扩展 大气湍流 厄米-双曲余弦-高斯光束     

Measurement of beam quality degradation due to spherical aberration in a simple lens

We have measured the effect on laser beam quality factorM ² of spherical aberration in a simple planoconvex lens. Theory predicts that the laser beam quality should remain essentially unchanged until the incident beam widthw exceeds a critical widthw _q above which the beam quality will degrade rapidly, with theM ² value increasing as (w/w _q ⁴). We find good agreement between this theory and our experimental data.     

Device for Producing and Investigating an Electron Beam Plasma Discharge

A survey on the properties and applications of the beam plasma discharge is given. A device for the experimental investigation of this discharge is described in which a magnetically guided (H = 0.05?0.1 T), ribbonlike electron beam (eU = 0.5–5 KeV, I = 10^?3?1 A) passes through a working chamber (p = 0.1–10 Pa; l = 10 cm, ø = 22 cm). The beam plasma discharge is sustained by collective beam plasma interaction. First results concerning the wall current and the ignition of the discharge as dependent on beam parameters are presented.     

Propagation of the off-axis superposition of partially coherent beams through atmospheric turbulence

The propagation properties of the off-axis superposition of partially coherent beams through atmospheric turbulence and their beam quality in terms of the mean-squared beam width w(z) and the power in the bucket (PIB) are studied in detail, where the effects of partial coherence, off-axis beam superposition and atmospheric turbulence are considered. The analytical expressions for the intensity, the beam width and the PIB are derived, and illustrative examples are given numerically. It is shown that the maximum intensity I_max and the PIB decrease and w(z) increases as the refraction index structure constant C_n² increases. Therefore, the turbulence results in a degradation of the beam quality. However, the resulting partially coherent beam with a smaller value of spatial correlation parameter γ and larger values of separate distance x_d and beam number M is less affected by the turbulence than that with a larger value of γ and smaller values of x_d and M. The main results obtained in this paper are explained physically.     

Zur nichtlinearen Theorie der Strahl-Plasma-Wechselwirkung IV. Numerische Ergebnisse: Elektronenschwingungen

The spatial variations of the electric field E and the electron distribution function f_a for the case of beam plasma amplification are computed on the basis of a nonlinear approximation developed in the earlier parts of the present paper [1–3]. The essential results are: With increasing distance x from the point, where the beam enters the plasma, the amplitude Ã₁ of the first harmonic increases, reaches a maximum value at some point x_m and than decreases monotonically without reaching a further maximum. In the region around x_m we find a large increase of the higher harmonics, i.e. increasing nonlinearity, and also an increasing phase velocity, which soon approches the beam velocity. The non-oscillating part of the distribution function of the beam electrons deformes, but not so much as the quasilinear theory predicts, so that amplitude settling must be interpreted as a nonlinear but not as a quasilinear effect. The results show a good agreement with some typical experiments on beam plasma interaction.     

On the Plasma Beam Instability of the Space Limited Electron Beam in a Magnetic Field

The present paper considers the plasma-beam instability of a space limited electron beam in a strong magnetic field (ω_H ? ω_o). The form of the beam was taken to be both cylindrical and plane. The threshold of the non-linear regime turned out to be weakly dependent on the beam form and independent of the magnetic field intensity. It is defined by the current and the directed velocity of the beam particles.     

Common-path double-pass optical interferometry using a wire-grid polarizer as a reference mirror

A novel common-path double-pass optical interferometer that employs a wire-grid polarizer (WGP) as a reference mirror is presented. When a laser beam polarized at 45° relative to the wire grid is incident on the WGP, the polarization component parallel to the grid direction (s-polarized beam) is reflected and is used as a reference beam. The perpendicular component (p-polarized beam) passes through the WGP coupled with a quarter-wave plate and serves as a probe beam, with its polarization transformed as p, right-circular, s, left-circular, and p, to irradiate the sample surface twice in order to double the phase change due to displacement of the sample. This beam is then retransmitted through the WGP, where it recombines and interferes with the reference beam. Preliminary experiments demonstrate that the WGP performs successfully as a reference mirror, and that the interferometer has a potential displacement sensitivity as low as 0.1 nm.     

Interaction of a modulated electron beam with a magnetoactive plasma

Experimental results concerning the interaction of a modulated electron beam with a magnetoactive plasma in the whistler frequency range are reported. It was shown experimentally that when a beam is injected into the plasma, waves can be generated by two possible mechanisms: Cherenkov emission of whistlers by the modulated beam, and transition radiation from the beam injection point. In the case of weak beam currents (N _b/N ₀)≪⁻⁴) the Cherenkov resonance radiation is more than an order of magnitude stronger than the transition radiation; the Cherenkov emission efficiency decreases at high beam currents. The transformation of the distribution function of the beam is investigated for the case of weak beam currents. It is shown that in the case of the Cherenkov interaction with whistlers the beam is retarded and the beam distribution function becomes wider and acquires a plateau region. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 6, 378–382 (25 March 1998)     

Beam wander of J 0- and I 0-Bessel Gaussian beams propagating in turbulent atmosphere

Root mean square (rms) beam wander of J ₀-Bessel Gaussian and I ₀-Bessel Gaussian beams, normalized by the rms beam wander of the fundamental Gaussian beam, is evaluated in atmospheric turbulence. Our formulation is based on the first and the second statistical moments obtained from the Rytov series. It is found that after propagating in atmospheric turbulence, the collimated J ₀-Bessel Gaussian and the I ₀-Bessel Gaussian beams have smaller rms beam wander than that of the Gaussian beam, regardless of the choice of Bessel width parameter. However, the extent of such an advantage depends on the chosen width parameter, Gaussian source size, propagation distance and the wavelength. Focusing at finite distances of the considered beams causes the rms beam wander to decrease sharply at the propagation distances equal to the focusing parameter.     

Polarization-Dependent Light-Gated Phase Conjugation in Methyl-Orange Doped Polymer Film

We have investigated the polarization state of a gated phase conjugate beam using a methyl-orange doped poly-vinyl alcohol (MO/PVA) film in 16 different combinations of linear and circular polarization states of a probe beam and pump beams. The experimental results agree with the theoretical prediction based on the Jones matrix method. Of the third-order nonlinear susceptibility tensors X_ijkl of the MO/PVA film, the X₁₂₂₁ at the 647 nm wavelength in degenerate four-wave mixing under exposure of a gating 515 nm light takes a very small value compared with X₁₁₁₁ and X₁₁₂₂. On the other hand, in the configuration of a probe beam and pump beams having linearly crossed polarizations and without gating light, the MO/PVA film generates a strong linearly polarized PC beam, so that X₁₂₂₁ takes a large value at 515 nm.     

Beam quality of high-power broad-area visible diode lasers

We characterize the beam quality of visible-wavelength GRIN-SCH-SQW lasers in terms of geometry and driving current. A second-moment-based, free beam measurement of M ² yields values ranging from near unity on the plane parallel to the junction, to near 10 in the perpendicular plane. The M ² values exhibit a dependence on the shape of the L-I characteristic. Higher-order moments are uso quantitatively describe the changes in beam shape as a function of current, and a correlation between the kurtosis and M ² is observed.Current address: Spire Corporation, Bedford, MA 01730, USA.     

Kurtosis parameters of super Lorentzben Gauss beams through a paraxial and real ABCD optical system

Based on the propagation equation of higher-order intensity moments, analytical propagation expressions for the kurtosis parameters of a super Lorentz-Gauss (SLG) SLG₀₁ beam through a paraxial and real ABCD optical system are derived. By replacing the parameters in the expressions of the kurtosis parameters of the SLG₀₁ beam, the kurtosis parameters of the SLG₁₀ and SLG₁₁ beams through a paraxial and real ABCD optical system can be easily obtained. The kurtosis parameters of an SLG₀₁ beam through a paraxial and real ABCD optical system depend on two ratios. One is the ratio of the transfer matrix element B to the product of the transfer matrix element A and the diffraction-free range of the super-Lorentzian part. The other is the ratio of the width parameter of the super-Lorentzian part to the waist of the Gaussian part. As a numerical example, the properties of the kurtosis parameters of an SLG₀₁ beam propagating in free space are illustrated. The influences of different parameters on the kurtosis parameters of an SLG₀₁ beam are analysed in detail.     

Tight focusing of partially polarized vortex beams by binary phase Fresnel zone plates

Based on vectorial Debye theory, the focusing properties of partially polarized vortex beam by high numerical aperture Fresnel zone plate are investigated. The effects of the numerical apertures of and the phase difference of binary phase Fresnel zone plates, the topological charge of vortex beam and the degree of polarization of incident beam on the intensity distribution and degree of coherence in the focal plane are investigated in detail. It is shown that elliptical light spots and the flat top beam can be obtained by selecting certain parameters. Studies of degree of coherence reveal that the degree of coherence between x and y components of the electric field, which is zero in the source plane, is improved in the focal plane for vortex beam, but it is hardly changed for the nonvortex beam. It is also proved that any two of the three electric field components E_x, E_y and E_z are completely coherent everywhere in the focal region if the incident light beam is linearly polarized.     

相位片角向衍射产生拉盖尔-高斯光束的实验研究

设计了一种角向分布的相位片,利用离子束刻蚀技术加工成0和π二级的相位片.利用角向衍射理论对相位片的衍射场分析表明,衍射场为拓扑指数相反的两束拉盖尔-高斯光束的叠加场.用直径为4 mm的近平行光照射相位片,获得径向指数为零,拓扑荷相反的叠加拉盖尔-高斯光场.采用较大孔径的光束入射时,仍为拓扑荷相反的两束光的叠加,但径向指数会发生变化. 关键词： 信息光学 拉盖尔-高斯光束 相位片 离子束刻蚀     

Analysis of TEM00 laser beam quality degradation caused by a birefringent Nd: YAG rod

The effects produced by a birefringent rod on the beam quality factor,M _infr ^sup2 , of a Gaussian TEM₀₀ laser beam are studied. Analytical expressions for the beam parameters at the output plane of a rod amplifier are derived. The theory predicts that the beam quality degrades rapidly with the heat power dissipated in the rod.