Nonlinear electromagnetic propagation in plasma filled cylindrical waveguides

Using an appropriate expression for the field-dependent dielectric constant of a collisionless plasma, we have studied the field distribution of the TM₀₁ mode in a cylindrical waveguide. The wave equation has been solved by a Runge Kutta technique. It is seen that on account of redistribution of carriers from the high-field regions to low-field regions a beam of frequency lower than the average plasma frequency can be transmitted if its power is high. Work supported by NSF     

Generation of millimeter wave Gaussian beams and their propagation in a dielectric waveguide

Linearly polarized fundamental mode Gaussian beams were generated and coupled to a hollow circular oversized dielectric waveguide by placing the waist of the beam at the guide entrance. The transmission properties of the waveguide were characterized as a function of frequency for a variety of coupling conditions. These conditions included changes in the input beam waist radius, angle of incidence, and displacement perpendicular and parallel to the guide axis. It has been found that: 1.) power transmission is maximized when the waist of the input beam is centered at the guide input, injected normally, and has a radius of 0.43 times the waveguide radius, 2.) power transmission decreases rapidly with increasing angle of incidence and the rate of that loss increases with frequency, 3.) the waveguide preserves the linear polarization of the input beam, 4.) power transmission in the fundamental waveguide mode is not greatly affected by moderate displacements in the input beam position, and 5.) upon exit from the waveguide the launched EH₁₁ mode propagates as a fundamental mode Gaussian beam in the quasi-far field. The results compare favorably to the transmission theory of Belland and Crenn and approximately to the near and far field mode pattern theory of Degnan.     

Self-focusing of a laser beam in a strongly ionized plasma

In the presence of a strong Gaussian laser beam, the non-linearity in the dielectric constant of a strongly ionized plasma has been investigated. The non-linearity arises due to the heating and redistribution of the electrons; the loss of electron energy gained from the field has been assumed to be due to thermal conduction. This self-induced non-linearity causes a self-focusing and oscillatory waveguide propagation of the beam even when the non-linear dielectric constant does not fall in the saturating range. In a typical case of a 10¹⁰ W laser, the enhancement of axial intensity by a factor of 25 has been predicted in a length of 0.6 cm.Works supported by N.S.F. (USA).On leave from Malviya Regional Engeeniring College, Jaipur-4, India.     

Nonlinear gain of a dipole antenna immersed in a collisionless plasma

This paper presents an investigation of the effect of collisionless plasma on the equatorial gain of a high power dipole antenna. The plasma experiences a ponderomotive force in the non-uniform radiation field and the electron density gets redistributed; the plasma gets depleted in the centre and the near equatorial zone of the antenna. This nonuniformity in the electron density (hence in the dielectric constant) tends to converge the electromagnetic power to the equatorial zone. For typical parameters a 20% (i.e., ∼1 dB) enhancement in the gain of the antenna has been predicted within the limits of the perturbation theory. Work partially supported by NSF (USA)     

Hybrid output mirror for optically pumped far infrared lasers

A hybrid metallic mesh multilayer dielectric coating mirror has been developed for use as an output mirror for optically pumped far infrared lasers. The metallic mesh provides a high reflectance in the far infrared while the multilayer dielectric coating is chosen to provide a maximum reflectivity at the pump wavelength (10 μm). This hybrid mirror has increased the output power of a CH₃F waveguide laser at 496 μm by a factor of 350 over that obtained with a hole coupling mirror. In addition, this mirror results in a far infrared output beam which has a minimum angular divergence limited only by the particular oscillating transverse waveguide mode (the EH₁₁ mode for this experiment).     

A subpicosecond dye laser pumped by a xenon ion laser

A pulsed xenon ion laser has been used to pump a rhodamine 6G dye laser utilizing a ring resonator. The dye laser has been passively mode-locked; a pulsewidth of 0.5 ps and a 50 kW peak power have been obtained.Work partly supported by Italian C.N.R.     

Highly Efficient Frequency Doubling with a KNbO3 Semi-Monolithic Resonator

A semi-monolithic standing-wave resonator has been constructed and employed for frequency doubling of a Ti:sapphire laser. The resonator consists of a 5 mm long KNbO₃ crystal with dielectric coatings on both end faces and a spherical mirror with 20 mm radius of curvature. With 31.2 mW input power at 861.4 nm second harmonic power of 11.4 mW has been generated corresponding to the conversion efficiency of 36.5%.     

分数自成像平面波导的光束组束

研究了激光束在分数自成像平面波导中的传输特性,提出了利用分数自成像平面波导进行光束组束的技术方案,讨论了激光束之间相位失配对光束组束的影响,并对两个输出平均功率为300W的板条激光器的光束组束进行了数值实验模拟.结果表明,分数自成像平面波导的光束组束可以有效地改善多个激光器组束后的光束质量因子M²,从而提高激光输出的亮度;而且这种组束方法对激光束之间相位失配的敏感度明显降低. 关键词： 分数自成像 平面波导 组束 光束质量     

Conditions for single-line and single-mode tuning of a CO2 waveguide laser

Single-mode tuning over 500 MHz in 90 lines, and 700 MHz in 70 lines is reported for a pyrex waveguide CO₂ laser in long-pulse operation. The peak power exceeds 100 W in the strongest lines, and the beam profile is diffraction limited. These features make the laser well suited as pump source for optically pumped molecular lasers.Work supported by the Danish National Science Research Council under Grant No. 11-3389     

High-pressure,high-repetition-rate RF CO2 laser

Summary In this work we report the physical properties of a radiofrequency supplied waveguide CO₂ laser operating with different standard mixtures at high pressures up to 1.5 bar, with about 10% electrical efficiency. We describe some problems to obtain a complete RF matching in the discharge and we analyse the conditions for a better efficiency, gain and peak power in different mixtures, both to obtain a larger mode tuning and to eliminate the he contents so that the laser running costs are largely reduced. Work partially supported by the Progetto Finalizzato Optoelettronica of the Italian National Research Council (CNR). The authors of this paper have agreed to not receive the proofs for correction.     

CW HCN laser gain in a hollow dielectric rectangular cross-section discharge tube

This paper describes a 1 m-long CW HCN discharge laser with a hollow dielectric rectangular discharge tube of 5×20 cm² cross-section. For optimum working conditions, the characteristics of the amplifying medium have been made constant over the cross-section, by using an additional magnetic field. Then, the unsaturated gain for the 337 μm line is 6.8% m⁻¹. It is shown to be dependent on the smaller dimension of the cross-section rather than on the other dimension. By using the tube as a waveguide, laser output power of 1 Watt might be obtained with a discharge length of 4 m, instead of the 12m-long discharge required to get the same output power from a cylindrical waveguide laser.     

Induced focusing and conversion of a Gaussian beam into an elliptic Gaussian beam

We have presented an investigation of the induced focusing in Kerr media of two laser beams, the pump beam and the probe beam, which could be either Gaussian or elliptic Gaussian or a combination of the two. We have used variational formalism to derive relevant beam-width equations. Among several important findings, the finding that a very week probe beam can be guided and focused when power of both beams are well below their individual threshold for self-focusing, is a noteworthy one. It has been found that induced focusing is not possible for laser beams of any wavelength and beam radius. In case both beams are elliptic Gaussian, we have shown that when power of both beams is above a certain threshold value then the effective radius of both beams collapses and collapse distance depends on power. Moreover, it has been found that induced focusing can be employed to convert a circular Gaussian beam into an elliptic Gaussian beam.     

Amplification and saturation in a CO2 waveguide amplifier

An ir CO₂, dc current pumped, optical waveguide (WG) amplifier has been built, and its active medium optical parameters measured for several CO₂ emission lines, and their dependence from active medium total pressure, discharge current and temperature was investigated.High gain is found which, coupled with relatively high saturation power in the WG fundamental mode and ease of fabrication with this technology in long (up to 1.5 m) lengths, indicates promising use to efficiently amplify high spectral and spatial purity output of a short, highly tunable WG laser up to power levels suited for nonlinear spectroscopy and optical pumping. The dependence of the small signal gain coefficient and of the saturation parameter for individual rotational lines on the radiation intensity was computed using experimentally known parameters of the discharge plasma. The computation was carried out using the two mode rate equation approach for CO₂–N₂–He gas mixtures. A satisfactory agreement between theoretical and experimental results was obtained.Work supported by G.N.S.M.-C.N.R. and M.P.I.     

二极管面阵侧泵浦Nd:YAG双板条激光器

 叙述了二极管面阵侧泵浦双板条耦合单横模 Q 开关激光器的研制。泵浦源为二极管面阵激光器,使用梯形波导板耦合系统,耦合效率达90%,谐振腔为凸凹腔,全反射镜的曲率半径为 3.0m的凹面镜,输出镜为曲率半径3.0m的可变反射率超高斯镜。激光器在 100Hz 重复频率下,输出单脉冲能量18.23mJ,脉宽为15.25ns,光束质量M²<1.5具有平顶分布的超高斯光束。     

Low loss smart hollow waveguides with new polymer coating material

A new kind of cyclic olefin polymer COP-E48R has been selected as the dielectric material for a silver hollow glass tube. Owing to its lower extinction coefficient at the wavelength of 10.6 μm, transmission losses for the CO₂ laser light has been reduced significantly in the COP-E48R-coated silver (COP-E48R/Ag) hollow glass waveguide. By properly selecting the film thickness of COP, Er:YAG and CO₂ laser light are shown to be transmitted with low loss simultaneously or independently. Delivery properties of red and green pilot beams were also evaluated.     

Hot carrier diffusion induced instabilities of electromagnetic waves in plasmas and semiconductors

The stability of a plasma/semiconductor for small fluctuations in the intensity of plane electromagnetic beam has been studied when a non-linearity in the dielectric constant appears on account of the diffusion of non-uniformly heated carriers (due to fluctuations in the intensity of the beam). Fluctuations of optimum size and long duration have been found to grow at moderate powers. The effect of absorption is, however, to suppress this effect.Work supported by CSIR (India) and NSF (USA)     

电子束空间极限电流的非线性理论研究

从电子束一维稳态传输的电势非线性Poisson方程出发,推导了圆柱波导内实心束和环形束空间极限电流与电子入射电势的依赖关系,给出了数值求解方法和解的特征,分析比较了数值计算与现有解析公式及粒子模拟的结果.考虑电子的横向运动,对数值方法进行了二维修正,计算结果与粒子模拟结果具有很好的一致性.说明利用数值方法计算电子电势非线性微分方程能够得到更精确的电子束在圆柱波导内的空间极限电流;另外,对其他形状的波导,尤其是难以得到解析式的情况,根据实际几何结构设置边界条件,数值方法可以方便地给出束流传输特性,对设计新型结构的高功率微波器件提供理论指导. 关键词： 相对论电子束 圆柱波导 空间极限电流 束流传输     

A DC excited waveguide multibeam CO2 laser using high frequency pre-ionization technique

High power industrial multibeam CO₂ lasers consist of a large number of closely packed parallel glass discharge tubes sharing a common plane parallel resonator. Every discharge tube forms an independent resonator. When discharge tubes of smaller diameter are used and the Fresnel numberN ≪ 1 for all resonators, they operate in waveguide mode. Waveguide modes have excellent discrimination of higher order modes. A DC excited waveguide multibeam CO₂ laser is reported having six glass discharge tubes. Simultaneous excitation of DC discharge in all sections is achieved by producing pre-ionization using an auxiliary high frequency pulsed discharge along with its other advantages. Maximum 170 W output power is obtained with all beams operating in EH₁₁ waveguide mode. The specific power of 28 W/m is much higher as compared to similar AC excited waveguide multibeam CO₂ lasers. Theoretical analysis shows that all resonators of this laser will support only EH₁₁ mode. This laser is successfully used for woodcutting     

新型反馈式轴向同轴虚阴极振荡器

提出一种利用低磁场导引环形电子束在同轴波导内轴向激励虚阴极振荡的新型虚阴极振荡器.利用反馈式结构,使虚阴极在由反馈环、阳极箔和同轴波导构成的高品质因子准谐振腔内形成.作用腔内环形电子束激励同轴波导TM₀₁模式,在相对低品质因子的同轴提取区转化为TEM主模输出.用25维KARAT粒子模拟软件研究得到束波功率转换效率12%,输出微波平均27 GW,中心频率38 GHz. 关键词： 环形电子束 轴向导引磁场 同轴波导 反馈环     

新型反馈式轴向同轴虚阴极振荡器

从电子束一维稳态传输的电势非线性Poisson方程出发,推导了圆柱波导内实心束和环形束空间极限电流与电子入射电势的依赖关系,给出了数值求解方法和解的特征,分析比较了数值计算与现有解析公式及粒子模拟的结果.考虑电子的横向运动,对数值方法进行了二维修正,计算结果与粒子模拟结果具有很好的一致性.说明利用数值方法计算电子电势非线性微分方程能够得到更精确的电子束在圆柱波导内的空间极限电流;另外,对其他形状的波导,尤其是难以得到解析式的情况,根据实际几何结构设置边界条件,数值方法可以方便地给出束流传输特性,对设计新型结构的高功率微波器件提供理论指导.