A hologram of large field depth

A hologram of an object with field depth greatly exceeding the coherence length has been made with an ordinary gas laser, taking account of the reoccurence of the temporal coherence and its stability dependence on operating time. Only one object beam is used for a number of seperate exposures, so that the full output power is used.     

A magnetically polarized He-Ne laser

The performance of an internal mirror He-Ne laser that has a fraction of its capillary immersed in a strong transverse magnetic field will be described. The field is produced by permanent magnets which are built into the laser's housing. The laser produces a light output which has less than 0.1% of its power in the modes polarized orthogonal to the magnetic field. The attainment of such a high degree of polarization requires that the magnetic field be oriented relative to naturally occuring anisotropies of the laser cavity. Misalignment not only lowers the degree of polarization, but also decreases the laser's output.     

High brightness 735 nm tapered lasers – optimisation of the laser geometry

High brightness tapered laser diodes with different resonator geometries were fabricated and analysed. The devices consist of an index-guided straight section and a gain-guided tapered section. Lasers with a total length L = 2 and 4 mm and different length of the ridge waveguide L _RW (500 μm ≤ L _RW ≤ 1250 μm for L = 2 mm and 500 μm ≤ L _RW ≤ 2000 μm for L = 4 mm) were processed to study the influence of the straight section on the spatial mode filtering. The power–voltage–current-characteristics, the beam waist, the far field, and the beam propagation factor M ² were measured. From the experiments, it can be stated that the lasers with a small L _RW reach higher output powers compared to those with larger L _RW. Concerning the beam quality the length L _RW should exceed a minimal value to guarantee efficient spatial mode filtering. Devices optimised concerning maximum output power and excellent beam quality reach a beam propagation factor smaller than 2.1 at an output power P = 2 W.     

Use of induced spatial incoherence for uniform illumination of laser fusion targets

We report a technique to achieve very uniform illumination of laser irradiated targets by inducing a controlled amount of transverse spatial incoherence in the output beam of a broadband laser. The spatial incoherence is created by imposing different optical delays upon different transverse sections of the beam, and choosing the delay increments to be larger than the laser coherence time t_c=1/δν. This technique may allow easy attainment of the pellet illumination uniformity required for laser fusion.     

激光空间相干性对照明均匀性的影响

 研究了激光相干性对照明均匀性的影响，为照明激光器的选择提供了理论参考。其中，部分相干高斯光束分解为模式间相互独立的厄米-高斯光束的迭加。采用相位屏的近似处理方法对激光通过大气湍流的传输进行计算模拟。数值模拟的结果表明：当照明光束的空间相干性降低时，其照明均匀度逐步提高。因此对于照明激光器而言，选择空间相干性较差的激光器对其照明均匀度更加有益。     

Laser-based volumetric colour-coded three-dimensional particle velocimetry

We present a method of three-dimensional particle velocimetry with a single digital colour camera using multiple colour illumination to encode image depth over a large volume. A copper vapour laser operating at 511 nm is used to pump an optical fibre producing a multiple-wavelength beam via multiple order stimulated Raman scattering. The beam is dispersed and formed into a stack of thin sheets to illuminate a volume of space. The spatial co-ordinates of particles imaged within the illuminated volume are obtained from their imaged x,y positions with depth discerned from particle hue (set by the wavelength of illumination). The method exhibits an RMS depth error of 3% in relation to the thickness of the illuminated region. This paper reports a proof-of-principle of three-dimensional particle imaging using a multi-wavelength laser source with a view to 3D-3C particle velocimetry.     

Determination of the nonlinear optical coefficient and SHG coherence length for crystal single zinc selenide

Zinc selenide is characterized by very low infrared absorption and a high threshold for optical damage making its use as high power laser windows almost universal. Since the meterial is a member of the $\text{4}$3 m (zinc-blende) crystal class, it possesses a second order electric susceptibility tensor. We have measured the absolute value of the tensor components by the method of second harmonic generation of 10.6 micrometer radiation using a properly oriented single crystal wedge. Furthermore, the coherence length for the second harmonic process was accurately determined for various CO₂ laser lines. The results of the susceptibility measurement are compared to the results of Miller's phenomenological analysis while the coherence length measurements are compared to that quality derived from previously available refractive index data.     

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.     

Tight focusing of circularly polarized beam over high NA lens axicon with a diffractive optical element

We propose to use diffractive optical element in combination with high NA lens axicon to achieve a high depth of focus when illuminated by a circularly polarized beam. With this kind of system, the focal depth is increased to 12.816λ and the magnetic spot size is reduced to 0.3764λ. However, in the conventional lens with same NA, the FWHM of the magnetic spot is found to be 0.4308λ and its corresponding magnetization depth is only 0.888λ. The author expects that such a high focal depth strong longitudinal magnetic field with large magnetization depth can be widely used in high density magneto optic recording, laser machining, laser cutting and the scanning near-field magnetic microscope.     

Thermal fields in laser–multi-layer structures interaction

In the present paper, we are dealing with the thermal fields for laser–periodic multilayer structures interaction. Our point of view is originally that we consider any order transverse laser beams, like heating sources. We consider that the laser beam acts in IR (Nd:YAG or CO₂ laser beam) and it is in one transverse mode or in only a few decoupled modes. In order to solve this problem, we will use the Green function method. Specific results are presented for a laser beam (Nd:YAG laser) operating in the mode TEM₀₁ and a two-layer structure.     

On the environmental decoherence and spin interference in mesoscopic loop structures

Mechanisms of ‘environmental decoherence’ such as surface scattering, Elliot–Yafet process and precession mechanisms, as well as their influence on the spin phase relaxation are considered and compared. It is shown that the ‘spin ballistic’ regime is possible, when the phase relaxation length for the spin part of the wave function (L^(s)) is much greater than the phase relaxation length for the ‘orbital part’ (L^(e)). In the presence of an additional magnetic field, the spin part of the electron's wave function (WF) acquires a phase shift due to additional spin precession about that field. If the structure length L is chosen to be L^(s)>L>L^(e), it is possible to ‘wash out’ the quantum interference related to the phase coherence of the ‘orbital part’ of the WF, retaining at the same time that related to the phase coherence of the spin part and, hence, to reveal corresponding conductance oscillations.     

Strain field measurements with dual-beam digital speckle photography

A non-contact strain field measurement technique called dual-beam digital speckle photography is presented. The system facilitates strain measurements in rough environments and at elevated temperatures and can be used on materials where instrumentation of standard strain gauges are difficult. The system uses two symmetrically oriented laser illumination beams and an imaging system oriented normal to the object surface. The laser speckle field for each illumination beam is recorded simultaneously before and after object deformation. From these data sets it is possible to extract one strain component, e.g. _xx, without the need of numerical differentiation. It is shown that the most important criteria is normal viewing but also that deviation from perfect symmetry in the illumination will affect the results. A dynamic strain field, acquired at 15 Hz, illustrates the system performance. The effect of oblique viewing and unstable lasers are also shown.     

Case hardening of steel by a CO2 laser beam

Case hardening of a carbon steel, Armco Iron, an alloyed steel and a high-speed steel was carried out with a CO₂ laser beam. The indispensibility of an absorptive coating in the laser heat treatment was established. Four coatings capable of absorbing CO₂ laser radiation of a 10.6 μm wave length were examined. The influence of spot size, power and scanning speed on the depth of hardening were studied. Processing at optimal conditions yielded a hardening depth of up to 0.7 mm and a surface hardness of up to 220 H_v.     

Beam divergence and refractive power of directly coated solid state lasers

A relation between the far field divergence angle of a laser output beam and the elements of the laser resonator matrix is derived. This relation is applied to the resonator matrix of directly coated laser rods and yields an expression for the far field angle θ_m of the output beam in terms of the pump light induced refractive power D of the rod. By means of this expression D versus the average pumping power is calculated from the experimental values of θ_m for several laser rods and is found to be in good agreement with previously published values.     

de Haas–van Alphen effect in anisotropic superconductor well below Hc2

We study a semi-classical approach to the determination of the oscillatory magnetization of an anisotropic superconductor in a magnetic field. The field is such that the spacing d between flux tubes is large compared to the coherence length ξ₀ and is small compared to the penetration depth. The low temperature de Haas van Alphen effect is enhanced by the Dingle temperature being reduced by a factor of d/ξ₀ for the case of line of zeros of the gap parameter. For point zeros a smaller effect is obtained.     

Relationship between coherence length and output power of a multi-longitudinal-mode Ar+ laser

The relationship between the temporal coherence (i.e., the coherence length) and the output power of a multi-longitudinal-mode Ar⁺ laser operated near the threshold current is studied. The experimental measurements show that the coherence length is a hyperbola as a function of the output power. A simple model of the multimode equal-amplitude power spectrum is employed in the theoretical analysis. Good agreement between theory and experiment is obtained. It is shown that for a multi-longitudinal-mode Ar⁺ laser, the product of coherence length and the output power is almost a constant. Mailing address: Department of Physics, Suzhou University, Suzhou, Jiangsu 215006, People's Republic of China     

Optoacoustic imaging with synthetic aperture focusing and coherence weighting

Optoacoustic imaging takes advantage of high optical contrast and low acoustic scattering and has found several biomedical applications. In the common backward mode a laser beam illuminates the image object, and an acoustic transducer located on the same side as the laser beam detects the optoacoustic signal produced by thermoelastic effects. A cross-sectional image is formed by laterally scanning the laser beam and the transducer. Although the laser beam width is generally narrow to provide good lateral resolution, strong optical scattering in tissue broadens the optical illumination pattern and thus degrades the lateral resolution. To solve this problem, a combination of the synthetic aperture focusing technique with coherence weighting is proposed. This method synthesizes a large aperture by summing properly delayed signals received at different positions. The focusing quality is further improved by using the signal coherence as an image quality index. A phantom comprising hair threads in a 1% milk solution was imaged with an optoacoustic imaging system. The results show that the proposed technique improved lateral resolution by 400-800% and the signal-to-noise ratio by 7-23 dB over conventional techniques.     

Study of influences of laser power fluctuation on thermogram of the material

The effects of the laser beam instability on the temperature of the material are studied in this paper. The fluctuation of the temperature not only relates to the laser beam instability, but also correlates with the thermal conductivity, the thermal diffusivity, the emissivity, the laser–solid interaction time, and the laser spot diameter. The temperature fluctuation of the aluminum, the polymethylmethacrylate, the epoxylite, and the low carbon steel is measured by the infrared thermal imagers (HWRX··) to determine validity of the theory prediction. It is found that the theoretical prediction is in good agreement with the experimental results. Besides, the maximum amplitude of the temperature fluctuation is calculated and how to control the temperature fluctuation is discussed. The results show that the temperature fluctuation in the laser–solid interaction is of importance in practice.     

多子光束相干发射小目标探测研究

提出了一种多子光束相干发射方案以提高对远距离小目标的探测能力.将激光器发出的光束等功率地分为若干子光束,在空间按一定的排列平行发射.子光束来自同一激光源,具有很好的相干性,将在目标面上产生干涉.计算结果表明:子光束在目标面上产生干涉主峰,理想情况下,其峰值为单光束直接发射光强的N(N为子光束数目)倍,用此主峰照射目标实现探测;目标越小,此方案相对于直接发射的优势越突出.实际中应适当压缩光束发散角,以增大照射到目标上的总功率.发射镜的位置和角度抖动使目标面上光强闪烁且光斑畸变,为获得稳定、且峰值为单光束直接发射光强N倍的干涉主峰,发射镜的位移控制精度需达到0.1λ(λ为光波长),角度控制精度需达到0.2θ(θ为光束发散角).     

Laser–diode-pumped passively Q-switched Nd:NaY(WO4)2 laser with GaAs saturable absorber

A laser–diode-pumped passively Q-switched new type crystal Nd³⁺:NaY(WO₄)₂ (known as Nd:NYW) laser with GaAs semiconductor saturable absorber has been realized. The dependence of pulse repetition rate, pulse energy, pulse width, and peak power on pump power for different output coupler reflectivities are measured. The coupled rate equations are used to simulate the Q-switched process of laser, and the numerical solutions agree with the experimental results.