Powerful, diffraction-limited semiconductor laser using photorefractive beam coupling

We use semiconductor laser amplifiers and a photorefractive crystal to generate a high-power, diffraction-limited laser beam at 860 nm. Using a single flared amplifier, we obtain 1.09 W in a diffraction-limited beam from 2.2 W of pump power. Using an array of flared amplifiers, we also demonstrate efficient beam coupling, showing that this technique is easily extended to semiconductor amplifier arrays.     

Spatial beam profiles of femtosecond X-ray pulses generated by laser Compton scattering from a low-emittance electron beam

Polarization-dependent spatial beam profiles of femtosecond X-ray pulses generated by a laser Compton scheme were measured. The X-ray pulses were generated by the interaction at an angle of 90° between 100-fs laser light and a 3-ps, 3π-mm mrad electron beam. The polarization of the laser light was linear in two different directions, either parallel or perpendicular to the electron beam axis. The measured profiles showed good agreement with theoretical results. Received: 5 July 2002 / Revised version: 17 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +81-424/684477, E-mail: msf_yorozu@shi.co.jp     

非线性增益介质中强激光热像的演化规律

高功率激光系统中的热像效应可能导致光束的峰值功率剧烈增加,增益非线性介质会使这种光强增幅更为强烈。基于菲涅尔-基尔霍夫衍射理论和非线性近轴波动方程,对强激光在增益克尔介质工作在饱和区时的热像产生过程进行理论分析,将光束传输方程中增益饱和部分进行麦克劳林展开,取其近似,经过推导得出了介质薄近似时热像强度解析式和热像位置。通过数值模拟对解析结论预测的热像强度和位置进行验证。仿真结果表明,热像的位置在衍射物相对于介质对称处,热像强度解析结果与模拟结果相符,在薄介质时,解析解与模拟结果拟合较好。热像强度随非线性介质内非线性效应增强而停止增加,此外,讨论了热像强度随调制类型的变化。     

Diode-pumped passively mode-locked Nd:YAG laser with 10-W average power in a diffraction-limited beam

We present a passively mode-locked Nd:YAG laser with 10.7-W average output power in a diffraction-limited beam. Stable self-starting mode locking with a pulse duration of 16 ps and a pulse energy of 120 nJ is obtained with a semiconductor saturable-absorber mirror. The laser is directly side pumped with two 20-W diode bars. Single-pass frequency doubling in an external 5-mm-thick KTP crystal yields 3.2-W average power at 532 nm.     

Investigations on laser plasma soft X-ray sources generated with low energy laser systems

Spectra of laser-induced plasmas at low laser energies and intensities (around 100 mJ and 10¹⁰ W cm^–2 respectively) have been recorded in the spectral range of 20 to 100 nm for different target materials, laser intensities at the target and laser wavelengths. For heavy target materials, a broadband spectrum with a spectral maximum typically around 30 nm is obtained. This broadband radiation source is well suited for photoionization processes and the generation of short wavelength inner-shell photoionization lasers. For the cadmium-photoionization laser, the influence of different soft X-ray spectra on the laser energy has been investigated. The potential of laser plasma soft X-ray sources for scientific and technical applications is briefly reviewed.Dedicated to Prof. Dr. Herbert Welling on the occasion of his 60th birthday     

940 nm low beam divergence tapered window laser arrays

The structures of 940 nm low vertical beam divergence semiconductor lasers grown by molecular beam epitaxy are presented. The high power laser consists of an array of closely spaced tapered waveguides giving lower parallel beam divergence and window structure. The emission wavelength is 939 nm. The FWHM of the far field pattern is 8×30°. The maximum continuous wave (CW) output power of 30 W has been achieved. In the aging tests, the laser arrays have been operating for over 3000 h under the CW condition of 25 W.     

Flat-top laser beam generated by coherent beam combining of Gaussian lasers

We present a fiat-top laser beam generation scheme using coherent beam combining of hexagonally arranged Gaussian lasers. To produce a beam with a flat-top profile, we optimize the amplitude and phase of each unit laser using the least-square method. Simulation results show that with 13 unit lasers, a beam with the flatness of less than 1% in the optimizing region can be achieved. The main lobe contains over 95% of the total power. The scheme requires no external beam shaping element and has the potential to be designed for high-power applications.     

Time-resolved nonlinear refraction in femtosecond laser gain media

We present time-resolved measurements of the electronic and the nuclear contributions to the third-order nonlinearities of Ti:sapphire and Cr:LiSGaF. The results suggest that nonlinear refraction in Ti:sapphire is resonantly enhanced relative to that of undoped sapphire. The implications of the nonlinear responses for short-pulse formation are discussed.     

Theory of laser action in scattering gain media

A laser model based on feedback produced by scattering has been developed to explain the narrow linewidth emission and input-output behavior observed in scattering gain media. The model is based on the transient two-level laser equations and includes the detailed spectral properties of the dye gain system. Monte Carlo methods were employed to calculate the threshold gain required for modeling the input-output and linewidth emission characteristics.     

Optical characterization of tunable solid-state laser gain media

The transition metal ions are optical centres that promise tunable laser operation at room temperature over much of the visible and near-infrared regions. This review presents the essential characteristics used in optical assessment of possible laser-active solids and some of the theory required to understand the experimental results. Also presented is a survey of the main features of the optical absorption-emission cycle, luminescence decay profiles, optical bandshapes and excited-state absorption for Ti³⁺, Cr³⁺ and Co²⁺ ions in (essentially) octahedral environments. Finally, materials characteristics (for example, radiative against non-radiative decay, excited-state absorption and disorder) that limit laser performance are discussed.     

Interaction of a high-power laser beam with low-density porous media

We have experimentally investigated the processes of laser light absorption and energy transfer in porous targets made of “agar-agar” (C₁₄H₁₈O₇) with an average density of 1–4 mg/cm³ illuminated by the focused beam of a neodymium laser with an intensity of 10¹⁴ W/cm² within a pulse of duration 2.5 ns. Many important scientific and technical problems, e.g., inertial-confinement thermonuclear fusion, the creation of lasers in the x-ray regime, and the modeling of astrophysical phenomena under laboratory conditions, can be successfully addressed by using low-density porous media as components of such targets. In our experiments with porous targets of variable density and thickness we used optical and x-ray diagnostic methods, which ensured that our measurements were made with high temporal and spatial resolution. We show that a region forms within the porous target consisting of a dense high-temperature plasma which effectively absorbs the laser radiation. Energy is transferred from the absorption region to the surrounding layer of porous material at up to 2×10⁷ cm/s. Experimental data are in good agreement with the predictions of our theoretical model, which takes into account the specific features of absorption of laser radiation in a porous material and is based on representing the energy transfer within the material as a hydrothermal wave. Zh. éksp. Teor. Fiz. 111, 903–918 (March 1997)     

一种大功率宽波段激光束散角的校准测试方法

在大功率激光系统的评价与分析中,激光器的光束品质是系统光束品质的决定性因素,也是激光器验收、鉴定的重要指标,其中束散角是判别激光光束质量的重要参数。本系统测试激光波长的范围比较宽,一般在0.532μm～10.6μm之间,没有合适的探测器能够覆盖整个波段,所以采用了一种新的方法来解决宽波段束散角的测量问题。选用CCD成像和扫描狭缝相结合的方法来实现宽波段激光光束束散角的测量,可见光和近红外波段（0.532μm～1.2μm）激光光束采用CCD法测量激光束散角,中红外波段（1.2μm～10.6μm）激光光束采用扫描狭缝法测量激光束散角。两种方法的结合可以较为精确地测量出不同波段的激光束散角。     

A Nd: YAG laser with a flat-top beam profile and constant divergence

An active resonator designed especially for Nd: YAG lasers is proposed. The expected performance of this resonator is the generation of a laser beam with a flat-topped distribution and a divergence invariant under the thermal lensing effect, such that it could be applied to surface treatment without special optical or mechanical elements. The beam quality and efficiency of such a resonator are presented and discussed in comparison with a parallel-plane resonator as adopted by most industrial Nd: YAG lasers.     

A free-electron laser generated gamma-ray beam for nuclear physics

The High Intensity Gamma Ray Source (HIγS), a collaborative project between TUNL and the Duke Free Electron Laser Laboratory, is described. An initial experiment and plans for a future research program are discussed briefly.     

A novel technique for X-ray laser beam characterization

The use of a novel diagnostic based on the Moiré effect, known as Moiré deflectometry, to characterize X-ray laser beams is described. This technique is purely geometric in nature and does not require coherent radiation for its operation. The unique feature of Moiré deflectometry is that it provides a fully quantitative laser beam diagnostic which can be operated in any wavelength region of the spectrum. This technique allows the number of transverse modes present in the beam to be determined from the inherent beam divergence. The latter is measured from the degree of reduction in the contrast of the Moiré fringes. From the same deflectogram the geometrical-beam behaviour can be determined. This includes both geometrical-beam divergence and ray-aberrations which lead to wavefront distortions.     

Influence of unstable cavity optical component aberrations on laser beam divergence

Based on the geometrical optics approximation, the influences of some optical component aberrations of confocal unstable resonators on the laser beam divergence are discussed. The explicit dependence of the laser beam divergence on the cavity magnification, the structure of the mirror defects, the mirror curvature radii and the tilting angle of intracavity windows are given for both positive- and negative-branch resonator configurations.ENEA guest and with Electronic Engineering (EL.EN), Florence, Italy.     

A novel automatic measurement method of laser beam divergence angle based on two linear array charge coupled devices

We propose a scanning imaging measurement method using two linear array charge coupled devices (CCD) to evaluate the transmission performance of the linear laser beam of 90° × 2°. The two linear array CCDs are placed at two positions in the far field separated by a definite angle. The beam divergence angles in the view direction are measured by linear fitting because the beam width conforms to the linear relationship in the far field. This method has the ability for real-time testing and is appropriate for the complicated beam shapes with a wide measurement range.