Backscattering of an intense laser beam by an electron

We present a novel, simple asymptotic expansion for the spectrum of radiation that is backscattered from a laser by a counterpropagating (or copropagating) electron. The solutions are presented in such a way that they explicitly show the relative merit of using an intense laser and of an energetic electron beam in x-ray production in the single particle regime. Simple scaling laws are given.     

An intermittent frequency offset lock of a transverse Zeeman laser to an iodine stabilized He-Ne laser

A single-mode frequency stabilized laser with modulation-free and moderate power is desired as a light source for an ultra-high resolution interferometer system and/or a rapid laser calibration system. For this purpose, we developed a new stabilized laser system that utilizes intermittent control of a 2 mW transverse Zeeman stabilized He-Ne laser (Zeeman laser) with an iodine stabilized He-Ne laser (I2 stabilized laser). Because of the intermittent control, working time of the I2 stabilized laser is reduced. The Zeeman laser has two operational modes: independent and slave mode. In the independent mode, the Zeeman laser is stabilized through control of Zeeman beat frequency. Temperature dependent drift of the oscillation frequency during the independent mode is periodically corrected by the slave operation utilizing frequency offset locking to the I2 stabilized laser. Frequency instability of the Zeeman laser in independent and slave modes is 7.7X10-11 and 2.0X10-11, respectively, at the sampling time of 100 s.     

Coaxial laser cladding on an inclined substrate

This paper describes an experimental and theoretical study of the cladding mode of coaxial laser cladding on an inclined substrate. Based on the image analysis of the powder stream and clad profile measurements in coaxial laser cladding, it was found that irregular clad profiles always formed on an inclined surface and the location of the peak profile shifted away from the clad center. This phenomenon is caused by uneven distributions of powder concentration and laser beam intensity. A modified Gaussian mode for powder stream and laser beam was proposed to estimate the clad profiles on an inclined plane under laser beam irradiation. The effects of the inclined steel substrate on the CO₂ laser beam absorption and stainless-steel powder catchment were examined experimentally. The results show that both the laser absorption and the powder catchment on the mild steel decrease with increasing the cladding angle. From the analysis of laser beam mode, the clad width is equivalent to the beam spot size on the inclined substrate. However, the clad height correlates well with the distribution of the powder concentration. The results show that the Gaussian cladding mode could be adopted in various laser cladding applications such as rapid prototyping and butt welding to predict the clad profiles precisely.     

Optimization of an AO Q-switched solid-state Raman laser

An extension of the rate equation models for solid-state Raman laser is proposed and investigated with accounting for the dynamics of an acousto-optically (AO) Q-switch modulator and the effects of thermal lens provided by the pump beam. The improved model is aimed at outlining optimization strategies for wonderful laser performance with short pulse width and high peak power. A comparison between traditional intra-cavity and new couple-cavity configurations is made. The simulation results indicate that it is possible to realize an efficient sub-nanosecond coupled-cavity Raman laser by optimizing several important parameters.     

分子的飞秒强光电离

文章以一个实验者的角度，介绍了分子的飞秒强光电离的研究现状。文章从对比飞秒激光电离质谱与纳秒激光电离质谱开始，接着介绍分子在激光场作用下的可能电离机理，着重描述了几个处理分子场致电离的理论模型和实验验证，最后对飞秒激光导致的分子在激光脉冲作用后取向研究进行了简单介绍。     

All-optical realization of an atom laser

We demonstrate an atom laser using all-optical techniques. A Bose-Einstein condensate of rubidium atoms is created by direct evaporative cooling in a quasistatic dipole trap realized with a single, tightly focused CO2-laser beam. An applied magnetic field gradient allows the formation of the condensate in a field-insensitive m(F)=0 spin projection only, which suppresses fluctuations of the chemical potential from stray magnetic fields. A collimated and monoenergetic beam of atoms is extracted from the Bose-Einstein condensate by continuously lowering the dipole trapping potential in a controlled way to form a novel type of atom laser.     

Absolute frequency measurement of an acetylene-stabilized laser at 1542 nm

The absolute frequency of an acetylene-stabilized laser at 1542 nm is measured at its second harmonic (771 nm) by use of a femtosecond optical comb based on a mode-locked Ti:sapphire laser. Frequency stability and reproducibility of the acetylene-stabilized laser are evaluated by the femtosecond comb with a H maser as a frequency reference. The absolute frequency of a laser diode stabilized on the P(16) transition of 13C2H2 is determined to be 194 369 569 383.6(1.3) kHz. The acetylene-stabilized laser serves as an important optical frequency standard for telecommunication applications.     

Lasing from an electron-beam-excited nonchain HF laser

The lasing parameters of a nonchain chemical HF laser (an active volume of 20 l) excited by a fast electron beam are studied at different energies delivered to the gas. It is shown that the laser operates efficiently when the energy deposited is below 30 J/l. Above specific excitation energies of 60–70 J/l, both the lasing efficiency and the laser output decrease.     

Intermittent frequency offset lock of a symmetric three-mode stabilized He-Ne laser to an iodine stabilized He-Ne laser

There is a need for an intense, unmodulated single-frequency stabilized laser light that guarantees absolute optical frequency in a rapid laser calibration or an ultra-high resolution interferometer. To obtain such a light, we developed a new laser system that uses an intermittent frequency offset lock of a symmetric three-mode stabilized He-Ne laser to an iodine stabilized He-Ne laser. The proposed laser system provides two operational modes: (1) independent and (2) slave mode. In the independent mode, frequency of the three-mode laser is stabilized via control of frequency difference between two intermode beats. The resultant output is a single longitudinal mode light of maximum intensity that locates at the top of the gain curve. Frequency instability of 8X10^-12 (at a sampling time of 100 s) which is better than conventional stabilized lasers is attained in the independent mode. Slow optical frequency drift during the independent mode is periodically corrected by the offset lock to the iodine stabilized laser (slave mode), resulting in accurate reset of the frequency drift. After reset of the frequency deviation, the three-mode laser is again operated in the independent mode. Due to such intermittent offset lock, duty factor of the iodine stabilized laser was reduced to a few % of continuous operation.     

Switchable mode-locking states in an all-fiber all-normal-dispersion ytterbium-doped laser

Multi-operation state in an all-normal-dispersion ytterbium-doped fiber laser is obtained by introducing nonlinear polarization or hybrid mode-locking mechanism in the laser cavity. In addition to operating the laser at continuous-wave mode-locking, various operation states including continuous-wave, Q-switched mode-locking, pulse-splitting, and harmonic mode-locking can be achieved by tuning the laser pump power or an intracavity polarization controller. Insertion of a semiconductor saturable absorber mirror in the cavity helps to shorten the output pulsewidth of the hybrid mode-locked laser. When the pump power is rapidly adjusted, a strong asymmetric hysteresis behavior can be observed during the manipulation between different orders of harmonic mode-locking.     

Automatic feedback control of an Er-doped fiber laser with an intracavity loss modulator

Suppression of Q-switching instabilities with an actively controlled intracavity loss modulator is demonstrated in an Er-doped waveguide laser that is mode locked with a slow saturable absorber at repetition rates of as much as 100 MHz. By automatic gain control in the feedback loop, stable mode locking is achieved over the entire parameter range of the laser. This approach renders laser stabilization independent of the characteristics of the gain medium and intracavity power. The pulse-shaping dynamics is not affected by the presence of the intracavity loss modulator.     

强激光脉冲在部分离化等离子体中的传播特性

采用变分法给出了强激光脉冲在部分离化等离子体中传播的参数演化方程,其中考虑了相对论自聚焦、有质动力激发尾波场以及部分离化非线性极化强度的影响。通过非线性动力学分析的方法,得到了强激光脉冲以恒定焦斑半径传播时的激光、等离子体参数匹配条件。研究表明:在部分离化等离子体中,激光自聚焦随着电离程度的增大而增强;有质动力激发的尾波场进一步增强激光脉冲的自聚焦;相对于等离子体密度而言,激光强度对激光脉冲的自聚焦更有影响。     

Line width of an atom laser

We describe the measurement of the line width of an atom laser beam extracted from a Bose–Einstein condensate. Using a novel magnetic resonance imaging technique, we find that the energy width of the atom laser beam is Fourier-limited by the duration of the output-coupling process. Received: 25 July 2002 / Revised version: 28 October 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +41-1/633-1254, E-mail: koehl@iqe.phys.ethz.ch     

Reflection of a Gaussian laser pulse by an active medium

The reflected laser pulse is both temporally and spatially displaced due to spatial and chromatic dispersions when a Gaussian laser pulse is incident onto an active medium. In the case of no chromatic dispersion, the temporal shift is linearly related to the spatial Goos-Hänchen shift. Furthermore, for an incident TE-mode laser pulse, the transmitted laser pulse shifts spatially and temporally by exactly a half of that of the reflected laser pulse. We further point out that, compared to the Goos-Hänchen shift at the critical angle which was conventionally considered the maximum, the Goos-Hänchen shift for the grazing incidence is bigger by one order of magnitude. This can greatly ease the direct experimental study of the Goos-Hänchen shift.     

Sub-Doppler spectroscopy with an external cavity quantum cascade laser

The linewidth of an external cavity quantum cascade laser is studied as a function of injection current and laser scan rate. The laser linewidth is inferred to be ca. 2.5 MHz from Lamb-dip spectra on a low pressure sample of NO and its variation with injection current is well modeled using literature values for the intrinsic material properties of the lasing medium. The laser linewidth measurements are corroborated by polarization spectroscopy studies as well as by analysis of hyperfine structure and cross-over resonances.     

Photoassociation of NaRb with an asymmetric laser pulse

We investigate the photoassociation dynamics of cold NaRb molecule controlled by an asymmetric laser pulse called slowly-turned-on and rapidly-turned-off (STRT) laser pulse. This new shaped laser pulse has a remarkable merit, compared with the typical Gauss-type pulses, so that we can efficiently associate molecules with the state expected instead of going back to the continuum state. Using the three-state model, we solve the quantum mechanical equation with the “split operator-Fourier transform” method under the rotating-wave approximation (RWA) in propagation of the wave packet. By the projection of the obtained wave function onto each vibrational state, we can get the vibrational population of the ground electronic state. The results reveal that, with the STRT laser pulse, an efficient photoassociation process can be achieved and the vibrational distribution in the ground state can be controlled by the laser parameters.     

Modulation instability of intense laser beam in an electron-positron plasma

Modulation instability of an intense right-hand elliptically polarized laser beam propagating through an electron-positron plasma is investigated by a new method. The nonlinear dispersion relation, in which the relativistic and ponderomotive nonlinearities are taken into account, is obtained for the laser radiation in electron-positron plasma by the Lorentz transformation. The Karpman equation is generalized to the case of three dimensions with three field components. When the nonlinear frequency shift of the electromagnetic field in plasma is involved, the nonlinear evolution equation for the slowly varying envelope of the laser field is obtained. Thus, modulation instability of the intense laser beam in electron-positron plasma is studied and the temporal growth rate of the instability is derived. The analysis shows that the growth rate of modulation instability is increased significantly near the critical surface in a laser-plasma.     

掺Er光纤飞秒激光器载波包络位相偏移的探测

本文介绍了基于掺Er光纤飞秒激光器光学频率梳中光学部分的研制. 实验上采用重复频率为230 MHz的掺Er光纤飞秒激光器,通过放大、光谱展宽以及单臂f—2f系统,在优化及分析相关参数影响的基础上,获得了~30 dB信噪比f₀的输出,为光纤光学频率梳的建立奠定了基础. 关键词： 掺Er光纤激光器 光学频率计量 光纤光学频率梳 光谱展宽     

Experimental studies of an unstable confocal resonator alexandrite laser

The effects of excited converging wave in an unstable confocal resonator onthe properties of an alexandrite laser are investigated experimentally.The results show thatexciting the converging wave inside the resonator is an effective method to reduce the diver-gence of the laser beam for low gain laser medium.     

Locking the laser frequency to an atomic transition

A simple method for stabilizing the laser frequency to an atomic transition is described. It makes use of the frequency shift caused by the Doppler effect. With this method the frequency of a dye laser can be kept tuned to an atomic resonance line within 10% of the linewidth for periods of several hours.