Free electron laser spectral dynamics with a modulation of electron beam energy

Recent experimental and theoretical works on free electron laser spectral dynamics have pointed out the difficulty to obtain a narrow and stable spectrum operation. This goal can only be achieved by avoiding the sideband generation leading to a broadband and unstable spectrum. Tapered wiggler and two-frequency wiggler are well suited for combining sharp spectrum and high efficiency but are not really compatible with a wide tunability of laser light. Filtering sidebands is a good way for lower power experiments but it seems to be difficult to conceive wideband filters, specially in the far-infrared region. Modulation of electron energy is a new potential soft way for controlling the spectral dynamics of longpulse free electron laser. Spectral dynamics under the modulation is investigated in the linear and non-linear regimes in the far-infrared region. Simulations show that a pulsed and sharp spectrum behavior can be obtained by optimizing the modulation parameters. The interest of such a method for the far-infrared experiments is discussed.     

Flow measurement: using a chopper together with a CW laser to realize a frame straddling technique

A simple method that uses a chopper together with a CW laser to realize a frame straddling technique for flow measurement is introduced, in which a chopper with two windows is used to generate double pulses for exposure. By changing the size and position of each window, the time and interval of exposure can be adjusted easily. The light source is a CW semiconductor laser. The advantage of this method is in its being simple, cheap and compact. The method has been validated on a hydrodynamic flow involving a von Kármán Vortex Street.     

Magneto-optical modulation of a triple-mirror laser beam

Based on calculations and experiments, a He–Ne laser has been developed with effective intracavity magnetooptical modulation at λ = 1.15 μm. A polarization-anisotropic element — a magnetic diffraction grating made of yttrium iron garnet — is mounted in a matched passive arm of the triple-mirror cavity of the laser and serves as the modulator element. Modulation is produced using the Faraday effect, which simultaneously leads to efficient diffraction of the laser radiation on the asymmetric strip domain structure of a magnetic diffraction grating and to rotation of the E vector. The Jones matrix method is used to calculate the frequency and polarization characteristics. These, along with the modulation characteristics, were also studied experimentally. It is shown that amplitude modulation at frequencies up to 180 kHz takes place for small single-pass Faraday rotation angles (θ_F). The threshold frequency is determined by the response time of the magnetizing Helmholtz coil. The depth of modulation is 50%. For θ_F > θ_t (where θ_t is the threshold), frequency modulation takes place with a deviation of up to several megahertz. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 2, pp. 188–193, March–April, 2009.     

Conditions for the harmonic-like and efficient amplitude modulation of the cw gaussian laser beam by means of a mechanical chopper

The values of the chopping disc slot and mark space widths relative to the radius of the Gaussian beam are found, which achieve optimum harmonic-like and efficient amplitude modulation of the cw laser beam. The simple approximation for the waveform of the modulated laser power valid for these optimum or near optimum values of the chopping disc parameters is presented.     

Free carrier confinement for infrared modulation in a semiconductor sheet

Some relations between the normalized propagation constants of TE and TH waves in a complex multilayer structure are derived. The possibility of making a carrier confinement useful for infrared modulation is demonstrated theoretically for a p-Ge sheet.     

Deceleration of a continuous-wave （CW） molecular beam with a single quasi-CW semi-Gaussian laser beam

We propose a promising scheme to decelerate a CW molecular beam by using a red-detuned quasi-cw semi-Gaussian laser beam （SGB）. We study the dynamical process of the deceleration for a CW deuterated ammonia （ND3） molecular beam by Monte-Carlo simulation method. Our study shows that we can obtain a ND3 molecular beam with a relative average kinetic energy loss of about 10% and a relative output molecular number of more than 90% by using a single quasi-cw SGB with a power of 1.5kW and a maximum optical well depth of 7.33mK.     

Sinusoidal modulation of a CW GaAs laser from 9 MHz to 1.1 GHz

Sinusoidal modulation and its detection of a room-temperature cw GaAs laser from 9 Mhz to 1.1 GHz is reported. A 25 dB signal-to-noise ratio with 20 dB harmonic rejection is obtained at 35 mA bias above threshold with a useful power output for optical fibre measurements. Possible extension of the frequency range and the application to sweep-frequency measurements in optical fibre waveguides are discussed.     

Plasma production in a Cs vapor by a weak CW laser beam at 6010 Å

When a dye laser beam at 6010.5 Å (which excites the $\text{Cs 6}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{→8D}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ transition), of power 0.1 W, is focused into a Cs vapor of density ? 10¹⁷ cm^-3, we observe a bright localized white glow near the laser focus. Our experiments indicate that this glow is due to the production of a Cs plasma, which exhibits many characteristics of a discharge plasma, like emission of numerous atomic lines and of recombination continua, plasma-broadening of atomic lines, and hysterisis. This laser-produced plasma is in Saha equilibrium with a typical electron density of 3 × 10¹⁴ cm^-3, and a typical electron temperature of 0.2 eV.     

Cl+HN3产生NCl(a1Δ)和NCl(b1∑)的实验研究

对利用微波放电直接解离Cl2生成Cl, Cl与HN3反应生成NCl(a1Δ )和NCl(b1∑)的过程进行了实验研究。得到了较强的NCl(a1Δ 和NCl(b1∑)自发辐射光谱,考察了Cl2流量和He/Cl2配比对NCl(a1Δ 和NCl(b1∑)生成的影响。发现对于一定的He流量,Cl2流量对NCl(a1Δ 和NCl(b1∑)生成的影响存在一最佳范围,而最佳He/Cl2配比不是一定值,而是随He流量升高而变大,在实验所考察的He流量范围（5~40 L/min）内,最佳He/Cl 2配比在30∶1~100∶1之间。     

Amplitude modulation of the cw laser light in the TEM mn mode by means of a mechanical chopper

Amplitude modulation of the laser beam in a TEM _mn mode by means of a mechanical chopper is investigated on the basis of the chopping model represented by a moving system of infinitely long, parallel slots and mark spaces. The cases of rectangular and axial symmetry of the laser beam are both treated. The explicit expressions for the waveform of the modulated normalized transmitted laser power are deduced and their consequences investigated. It is found that in the case of rectangular symmetry, unlike the case of axial symmetry, the TEM _mn modes give, for a constant value of the mode numbern and for any value ofm, the same time dependence of the amplitude modulated laser power. The notion of the equivalent modulation widths is introduced and conditions for the efficient amplitude modulation are found.     

具有波纹调制激光光束的准聚焦特性

采用光波的衍射积分理论,对具有波纹调制激光束的准聚焦特性进行了理论分析。针对中频率段畸变激光束的准聚焦特性,数值模拟研究了准聚焦光斑能量集中度、焦斑光强分布与光束波前调制周期和离焦距离的关系,并得出适当离焦距离有利于控制焦斑大小,增大激光束波前的调制周期,可增加准聚焦光斑能量集中度,降低焦斑光强的调制起伏。     

High power and high beam quality CW green beam generated by diode-side-pumped intracavity frequency doubled Nd:YAG laser

We report a stable high power and high beam quality diode-side-pumped CW green laser from intracavity frequency doubled Nd:YAG laser with LBO crystal. By using a advanced resonator, a large fundamental mode size in the laser crystal and a tight focus in the nonlinear crystal could be obtained simultaneously, which are favorable for high power and high beam quality CW green laser generation. The green laser delivered a maximum 532 nm output power of 40 W. The corresponding optical-to-optical conversion efficiency and electrical-to-optical conversion efficiency were 8.6% and 5.0%, respectively. Under 532 nm output power of 34 W, the beam quality factor was measured to be 1.6.     

Initial performance of a CW chemical laser

The performance of a continuous HF chemical laser is presented in this paper. Population inversion was obtained by diffusion of H₂ into a supersonic jet containing F atoms [H₂+F HF(v)+H₁ H=–31.7 kcal/mole]. A peak power of 630 W was obtained with an F atom flow rate of 0.040 mole/sec, and the efficiency of conversion of chemical energy to laser energy was 12%. The performance of a corresponding DF laser is also given. Major laser output is from 2-1 and 1-0 transitions for both lasers. Radiation is at 2.6 to 2.9m and 3.6 to 4.0m for the HF and DF lasers, respectively. The ratio of DF to HF laser power is 0.7 under similar flow conditions.Work conducted under US Air Force Contract F04701-69-C-0066.     

A CW CO chemical laser based on a hollow-cathode discharge

We report on a small-scale CW chemical CO laser using a hollow-cathode discharge for atomic oxygen production. This kind of discharge exhibits several unique properties which make it an ideal source of atomic oxygen. The construction and operation are simple and the design very compact. A specific laser energy of 100 J g^-1 and laser outputs of 9 W were obtained with an active length of 7.5 cm.     

A mirror-free cavity configuration for a CW ring dye laser

A new cavity configuration without mirrors is described for a CW ring dye laser. The cavity design is based on a new type of focusing prism which uses the Brewster angle of incidence and total internal reflection. Special roof prisms allow wavelength tuning. All of these elements are made from uncoated quartz thus allowing dye laser operation over a large range of wavelengths. CW operation as well as synchronously pumped mode locked operation with intracavity doubling is reported using the Rhodamine 6G dye.     

Thermocapillary self-focusing of a laser beam: A theoretical analysis

A gaussian laser beam reflected from a thermocapillary liquid surface self-focuses at a distance L from the sample surface at a time-instant t_s after the beginning of the irradiation. The theoretical relation t_s(L) is calculated as a function of the liquid physical properties and the laser power.     

808 nm激光二极管阵列波长光束组合20 W输出

基于多波长光束组合技术,利用光栅的衍射和外腔的反馈,将激光二极管阵列(LDA)发光单元锁定在不同的波长上,以近似平行光束沿光栅的-1级衍射方向组合输出,改善LDA输出光束质量。实验采用发光单元宽度为100 μm、周期为500 μm,由19个单元构成的1 cm普通商用LDA,在连续运行最大注入电流60.6 A时,自由运转输出功率49.8 W时,获得功率为20.1 W的组合光束稳定输出,其光谱宽度为15 nm,对应的远场发散角由约70 mrad变为1.66 mrad,改善后光束质量因子约为32,其值与单个发光单元的光束质量相当。     

Dynamics of self-focusing and self-phase modulation of elliptic Gaussian laser beam in a Kerr-medium

Using a direct variational technique involving elliptic Gaussian laser beam trial function, the combined effect of non-linearity and diffraction on wave propagation of optical beam in a homogeneous bulk Kerr-medium is presented. Particular emphasis is put on the variation of beam width and longitudinal phase delay with the distance of propagation. It is observed that no stationary self-trapping is possible. The regularized phase is also seen to be always negative.     

用He-Ne激光声光器件进行声光幅度调制和声光偏转实验

描述了用He－Ne激光声的声光器件进行的声光调制和声光偏转实验。调制曲线上有一个拐点，在拐点附近的一段曲线可看成直线，利用这段曲线进行调制可减少失真。