Characterization of scraps produced by the industrial laser cutting of steels

We have studied some properties relevant for technological applications of the debris produced by industrial laser cutting of steels. The investigated material is made up of spheroidal particles, hollow and solid, which get oxidized over the cutting process, and that we have reduced afterwards in a H₂ atmosphere. The samples, before and after the reduction, were characterized by X-ray diffraction, scanning electron microscopy, specific surface area and Mössbauer spectroscopy. We have found that, after the reduction treatment, the shape remains unchanged but that the chemical composition and the physical properties of the external and internal surface structures are modified. In particular, the specific surface area of the material increased by one order of magnitude.     

Lumped parameter model for multimode laser cutting

A lumped parameter mathematical model is developed to relate the cut depth to the laser cutting parameters and material properties. The model takes into account the threshold power of the incident laser beam for the initiation of cutting and modifies an earlier cutting model so that it applies to a wide set of process parameters ranging from low to high laser powers and slow to fast cutting speeds. Plain steel is taken as an example to show the effects of various process parameters such as the laser power, spot size and cutting speed on the cut depth. Special emphasis is given to the effect of laser mode structure on its cutting capability.     

An efficient model for laser surface hardening of hypo-eutectoid steels

This paper presents a model able to predict the austenization of hypo-eutectoid steels during very fast heat cycle such as laser hardening. Laser surface hardening is a process highly suitable for hypo-eutectoid carbon steels with carbon content below 0.6% or for low alloy steels where the critical cooling rate is reached by means of the thermal inertia of the bulk. As proposed by many authors, the severe heat cycle occurring in laser hardening leads to the pearlite to austenite microstructures transformation happening to a temperature much higher than the eutectoid temperature A_c1 and, afterwards, all the austenite predicted during the heating phase become martensite during quenching. Anyway, all these models usually generate a predicted hardness profile into the material depth with an on-off behavior or very complicated and time consumed software simulators. In this paper, a new austenization model for fast heating processes based on the austenite transformation time parameter I_p→a is proposed. By means of the I_p→a parameter it is possible to predict the typical hardness transition from the treated surface to the base material. At the same time, this new austenization model also reduces the calculation time. I_p→a was determined by experimental tests and it was postulated to be constant for low-medium carbon steels. Several experimental examples are proposed to validate the assumptions and to show the accuracy of the model.     

Tuning characteristics of a cw dual-cavity KTP optical parametric oscillator

Received: 21 April 1997/Revised version: 15 June 1997     

Tuning the high-order harmonic lines of a Nd:Glass laser for soft X-ray laser seeding

We report here more than 50% coverage of the XUV spectral range between 18 nm and 35 nm by tuning the high-order harmonics generated by a fixed frequency Nd:Glass laser system. The tuning range achieved is suitable to seed Ni-like Y, Zr and Mo soft X-ray lasers.     

Tuning and stability of a continuous-wave mid-infrared high-power single resonant optical parametric oscillator

A 2.2-W continuous-wave, continuously tunable, single-frequency OPO has been developed in the 3.0–3.8 μm wavelength range for the detection of molecular trace gasses. The oscillation threshold, output power and stability of the single resonant OPO were improved by optimizing pump beam waist and OPO cavity length. Both air-spaced and solid etalons were tested to frequency stabilize and tune the OPO, from which the solid etalon gave a better performance. Temperature oscillations in the PPLN crystal caused oscillations in the idler wavelength of less than 200 MHz over 300 s; the short-term stability was less than 3 MHz over 1 s. The high laser power, in combination with photoacoustic spectroscopy, achieved a detection limit of 10 parts-per-trillion for ethane in nitrogen. Received: 9 April 2002 / Revised version: 14 June 2002 / Published online: 2 September 2002 RID="*" ID="*"Corresponding author. Fax:+31-24/3653311, E-Mail: maartenh@sci.kun.nl     

Tuning and modulation of a laser by acoustic wave

The possibility is suggested of tuning and modulation of an ordinary (not DFB) laser by the acoustic wave of relatively low frequency. The results of a calculation for the simplest case are presented and corresponding estimates are made for the laser of GaAlAs type.     

Tuning a conventional quantum well laser by nonresonant laser field dressing of the active layer

Tunable semiconductor lasers may be considered as a critical technology for optical communications. We investigate the theoretical feasibility of tuning a conventional GaAs/Al_0.2Ga_0.8As quantum well laser emitting at 825 nm by non-resonant laser-dressing of the active layer. Conduction and valence subbands are sensitive to the intense dressing field and this effect can be used to blueshift the active interband transition. The laser-dressed electron and hole states are calculated in the effective mass approximation by using the finite difference method. Emitted wavelength, threshold current and characteristic temperature are discussed as functions of the dressing laser parameter and cavity length.     

A degenerate three-level laser with a parametric amplifier

The aim of this paper is to study the squeezing and statistical properties of the light produced by a degenerate three-level laser whose cavity contains a degenerate parametric amplifier. In this quantum optical system the top and bottom levels of the three-level atoms injected into the laser cavity are coupled by the pump mode emerging from the parametric amplifier. For a linear gain coefficient of 100 and for a cavity damping constant of 0.8, the maximum intracavity squeezing is found at steady state and at threshold to be 93%.     

一种面向声音变换的参数化模型

在源滤波器模型的基础上,利用统计学习方法,建立了一种面向声音变换的混合参数化模型。该模型包括浊音声学模型、清音声学模型和韵律补偿模型三部分。基于线性预测分析和mel倒谱分析的浊音声学模型,刻画了说话人声腔的共振特性。基于线性预测分析和噪声源分析的清音声学模型,反映了说话人发清音的特点。基于统计学习方法的韵律补偿模型描述了音高、能量与时长等分布特性。在该混合参数化模型的基础上,提出了一个声音变换算法,并将其应用到汉语音节的变换问题上。实验结果表明,对清浊音和韵律特性分别建模的变换算法能够提高重建语音的清晰度和可懂度,缩小重建语音与目标语音之间的感知距离,使重建语音具有目标说话人的韵律特征.     

A parametric model for dark energy

Determining the mechanism behind the current cosmic acceleration constitutes a major question nowadays in theoretical physics. If the dark energy route is taken, this problem may potentially bring to light new insights not only in cosmology but also in high energy physics theories. Following this approach, we explore in this Letter some cosmological consequences of a new time-dependent parameterization for the dark energy equation of state (EoS), which is a well behaved function of the redshift z   over the entire cosmological evolution, i.e., z∈[−1,∞)$z\in [-1,\infty )$. This parameterization allows us to divide the parametric plane (w₀,w₁)$(w_0,w_1)$ in defined regions associated to distinct classes of dark energy models that can be confirmed or excluded from a confrontation with current observational data. By assuming a flat universe, a statistical analysis involving the most recent observations from type Ia supernovae, baryon acoustic oscillation peak, Cosmic Microwave Background shift parameter and Hubble evolution H(z)$H(z)$ is performed to check the observational viability of the EoS parameterization here proposed.     

Temporal growth of a parametric excitation by a self-focused laser beam

The effect of self-focusing of the pump laser beam on the temporal growth of a parametric excitation has been investigated in the paraxial region. The two equations for the signal and idler modes have been decoupled by assuming the near self-trapping condition and a linearly varying phase mismatch. By employing the WKBJ approximation, it is found that the growth rate is a strong function of the radial intensity inhomogeneity of the pump laser beam. The condition for validity of the first-order approximate theory employed here has been derived.     

Exact region of stability for a linear orthogonal cutting model

In this work, stability of chatter in orthogonal cutting is investigated in order to identify the critical spindle speeds. The process is modeled by a second-order linear delay-differential equation and the characteristic equation is analyzed in formulating the continuous stability boundaries, explicitly for any given range of the spindle speeds. Moreover, a simple algorithm is introduced for checking whether the system is in the stable zone without determining the whole stability diagram. Numerical simulations and comparison with some other methods are presented to justify the theoretical results.     

Ultrafast laser system based on noncollinear optical parametric amplification for laser spectroscopy

We report the experimental demonstration of transform-limited sub-6 fs pulses at an optimal central wavelength by a tunable noncollinear optical parametric amplification(NOPA) source. Meanwhile, a white light continuum in the near-infrared(NIR) range from 900 to 1100 nm is also successfully generated by focusing the unconverted800 nm beam during NOPA generation on a sapphire rod. Both visible-pump/visible-probe and visible-pump/NIR-probe experiments are realized using the same laser system. As examples, ultrafast photo-induced exciton dynamics inside two kinds of materials are investigated by the visible-pump/visible-probe and visible-pump/NIR-probe spectroscopy, respectively.     

Surface temperature field survey for laser heating of steels

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Process map for laser heat treatment of carbon steels

We propose a process map for diode-laser heat treatment of carbon steels. After first identifying a heat treatable region in terms of laser intensity and interaction time using a heat conduction model, two most important factors in heat treatment, carbon diffusion time in austenite and cooling time, are calculated and plotted in the heat treatable region. Because overall characteristics of laser heat treatment for a given steel type can be graphically visualized on a map, this map can be used in the determination of optimal process parameters. Hardening depth is also calculated using the concept of the critical effective carbon diffusion time. For demonstration and validation purposes, we have systematically conducted laser heat treatment on AISI 1020 and 1035 steel specimens using a 3 kW diode laser and measured surface hardness and hardening depth. The experimental results are in agreement with the calculated process map.     

Tuning the detection sensitivity: a model for axial backfocal plane interferometric tracking

Backfocal plane (BFP) interferometry is a single particle tracking technique that allows one to measure minute displacements of a microscopic particle from the center of a beam's focus in three dimensions. In this Letter, we present a Fourier optics model to describe the interference effects that allow one to track the position of a particle moving along the optical axis. A detection numerical aperture is derived theoretically and confirmed experimentally, within which the interference intensity has a positive correlation with the axial position of the scatterer. For larger detection angles, the correlation is negative. The model helps to understand previously reported measurements and to optimize BFP interferometric tracking.     

Tuning of ring laser by varying the Gouy phase

Tuning a ring laser by using the Gouy phase is explored. We find a sensitive capability by moving a lens in the ring cavity. Application of the effect to ring laser gyro is discussed.