Degenerate four-wave mixing with a tunable excimer laser

We use a simple, forward-geometry degenerate four-wave mixing setup to monitor weak absorptions of various gas-phase molecules with a tunable excimer laser. With this technique, state-selective detection of H(2), CO, H(2)O, and O(2) is demonstrated. The dependence of the forward-geometry degenerate four-wave mixing signals on various experimental parameters (e.g., pressure, laser power) is determined. Several mechanisms for the generation of the observed signals are discussed, including diffraction from density gratings or thermal gratings and two-photon degenerate four-wave mixing.     

Line Raman, Rayleigh, and laser-induced predissociation fluorescence technique for combustion with a tunable KrF excimer laser

We have applied a line UV Raman, Rayleigh, and laser-induced predissociation fluorescence technique for measurement of turbulent hydrocarbon flames. The species concentration of CO(2), O(2), CO, N(2), CH(4), H(2)O, OH, and H(2) and the temperature are measured instantaneously and simultaneously along a line of 11.4 mm, from which the gradients with respect to mixture fraction and spatial direction are obtained. The technique has been successfully tested in a laminar premixed stoichiometric methane flame and a laminar hydrogen diffusion flame. In addition the technique has been tested in a highly turbulent rich premixed methane flame. The data show that the technique can be used to provide instantaneous measurements of local profiles that describe the local flame structure in highly turbulent flames.     

Using tunable lasers for photochromic coherent speckle diagnostics of bulk scattering media and optical fibers

We have observed for the first time the dynamics of a speckle field generated in bulk scattering partly ordered media or low-mode optical fibers probed by a laser diode (λ=654 nm) and a compact diode-pumped YAG:Nd laser (λ=532 μm) in the regime of frequency tuning. The dynamic coherent speckle-photo-chromic effect can be observed provided that the deviation of the probing radiation frequency is comparable with or greater than the effective phase delay difference between modes in the optical fiber or between waves in the random scattering medium. Using the proposed photochromic speckle technique, it is possible to determine the intermode dispersion of fibers with a length on the order of one meter and the dispersion of phase delay in test fluoroplastic structures in the regimes of severalfold scattering (for a sample thickness of 10–20 μm) or multiple scattering (for a thickness of up to ∼2 cm).     

Frequency locking of tunable diode lasers to a rubidium-stabilized ring-cavity resonator

We demonstrate a technique for locking the frequency of a tunable diode laser to a ring-cavity resonator. The resonator is stabilized to a diode laser that is in turn locked to an atomic transition in rubidium, thus giving it absolute frequency calibration. The principal advantage of the ring-cavity design is that there is no feedback destabilization of the laser. The cavity has a free-spectral range of 1.3 GHz and Q of approximately 35, which provides robust locking of the laser. The locked laser is able to track large scans of the cavity.     

Characterization of excimer lasers for application to lenslet array homogenizers

We investigate the best method of characterizing high-divergence lasers, such as excimer lasers, to suppress fine-scale intensity nonuniformity that is due to coherence effects of lenslet homogenizers. We show by a detailed analysis of the lenslet homogenizer that, for highest accuracy, a direct measurement of the value of the autocorrelation function should be made at the separation p of the lenslet elements, identified as the critical spatial period. We show that the commonly used characterization of lasers by the 1/e(2) width of the angular divergence is not the most accurate test and may overstate or understate the effectiveness of a given laser.     

Multipass grating interferometer applied to line narrowing in excimer lasers

The line narrowing of excimer lasers is discussed. The theory for an optical two-effect intracavity line narrowing device, the multipass grating interferometer (MGI), is presented. An MGI contains a grating aligned in its second-order Littrow configuration and a mirror aligned parallel to the grating surface reflecting back the beam normal to the grating corresponding to the first-order diffraction. The Littrow grating is doing the coarse line narrowing, and the mirror aligned parallel to the grating has similar line narrowing properties as tilted intracavity Fabry-Perot etalons. An MGI is applied to a KrF laser cavity to achieve a linewidth of 0.03 cm(-1).     

Analysis of broadly tunable coupled-cavity semiconductor lasers

We report on the spectral properties for above-threshold operation of broadly tunable, asymmetric multiple quantum well (AMQW), coupled-cavity InGaAsP/InP semiconductor diode lasers. We developed a traveling wave model to understand the mode selection that the lasers exhibit. We find that a weak, short external cavity (SXC) can be used to obtain single frequency operation on each longitudinal mode over the ～100 nm tuning range of the uncoated AMQW coupled-cavity lasers. We measured the spectral properties of AMQW coupled-cavity lasers with and without an SXC. In a synthesized optical coherent optical tomography application, the use of an SXC with an AMQW coupled-cavity laser reduces the coherence length and hence enhances the performance of the AMQW coupled-cavity laser for optical coherence tomography applications.     

Output coupler design of unstable cavities for excimer lasers

We tested the performance of a XeCl laser with unstable resonators using as an output coupler a phase unifying (PU) mirror, a super-Gaussian mirror, and a hard-edge mirror. The quantitative impact of the output coupler design on the energy extraction efficiency, near-field profile, far-field energy distribution, and spatial coherence time evolution has been investigated. Laser beams of larger brightness have been obtained with the PU unstable cavity. A faster growth of the laser beam spatial coherence has been observed with the PU cavity by time-resolved, far-field measurements.     

小波变换在可调谐半导体激光器模跳变检测中的应用

利用小波变换的局部化性质分析出拍频信号的奇异点,从而找出半导体激光器电流调制时的模跳变位置,简化了半导体激光器参数测量.     

The diversity of combustion synthesis processing: a review

The harnessing of heat emanating from powder-based exothermic reactions to produce advanced materials has been around for many decades, and is manifested in the process of combustion synthesis (CS). A plethora of work has been published on the topic covering fundamental aspects of the process for a large number of material systems. Over time, CS has been combined with other processes and effects to potentially improve on conventionally produced CS products and alleviate some of the inherent disadvantages of CS. This article discusses processing aspects of CS, and provides a review of CS-related hybrid processes with the intent to exemplify the diversity of CS processing. Approaches such as reactant microstructural design, reactive bulk deformation/compaction processes, reactive casting, laser-assisted CS, activation techniques (field/current, mechanical, microwave), and unconventional heat treatments are discussed together with other methods.     

Coloring linens with excimer lasers to simulate the body image of the Turin Shroud

The body image of the Turin Shroud has not yet been explained by traditional science; so a great interest in a possible mechanism of image formation still exists. We present preliminary results of excimer laser irradiation (wavelength of 308 nm) of a raw linen fabric and of a linen cloth. The permanent coloration of both linens is a threshold effect of the laser beam intensity, and it can be achieved only in a narrow range of irradiation parameters, which are strongly dependent on the pulse width and time sequence of laser shots. We also obtained the first direct evidence of latent images impressed on linen that appear in a relatively long period (one year) after laser irradiation that at first did not generate a clear image. The results are compared with the characteristics of the Turin Shroud, reflecting the possibility that a burst of directional ultraviolet radiation may have played a role in the formation of the Shroud image.     

Application of tunable diode laser diagnostics for temperature and species concentration profiles of inhibited low-pressure flames

We have employed tunable diode laser absorption spectroscopy (TDLAS) to characterize low-pressure premixed CH(4)/O(2)/Ar flames inhibited with Halon 1301 (CF(3)Br) and the candidate Halon alternative compounds FE-13 (CF(3)H) and HFC-125 (C(2)F(5)H). This work is part of a larger program designed to help identify replacement fire-suppression compounds for the currently used Halon 1301. We have used CO two-line thermometry to profile the temperature in low-pressure laminar flames and have determined concentration profiles for a large number of flame species, including reactive intermediates. To date, we have detected 12 flame species by using TDLAS in our laboratory and report on seven of them here: CH(4), H(2)O, CO, CF(2)O, CF(2)H(2), CF(3)H, and CF(4). To the best of our knowledge, this is the first time the last four species have been observed in flame by the use of TDLAS. Our data are important for validating the detailed kinetic mechanisms of chemical flame inhibition. Our results indicate that TDLAS is a versatile and powerful diagnostic technique for studying combustion processes.     

External cavity tunable quantum cascade lasers and their applications to trace gas monitoring

Since the first quantum cascade laser (QCL) was demonstrated approximately 16 years ago, we have witnessed an explosion of interesting developments in QCL technology and QCL-based trace gas sensors. QCLs operate in the mid-IR region (3-24?μm) and can directly access the rotational vibrational bands of most molecular species and, therefore, are ideally suited for trace gas detection with high specificity and sensitivity. These sensors have applications in a wide range of fields, including environmental monitoring, atmospheric chemistry, medical diagnostics, homeland security, detection of explosive compounds, and industrial process control, to name a few. Tunable external cavity (EC)-QCLs in particular offer narrow linewidths, wide ranges of tunability, and stable power outputs, which open up new possibilities for sensor development. These features allow for the simultaneous detection of multiple species and the study of large molecules, free radicals, ions, and reaction kinetics. In this article, we review the current status of EC-QCLs and sensor developments based on them and speculate on possible future developments.     

Numerical modeling of short-pulse excimer lasers with negative branch unstable cavities

A one-dimensional code for the numerical simulation of negative branch unstable resonators with an intracavity aperture that are applied to high-gain, short-pulse XeCl lasers is described. The model predicts near- and far-field performance of the output laser beams. The intracavity aperture size is shown as an important parameter for control of the output beam energy and divergence. A comparison with experimental measurements is presented.     

Two-color femtosecond experiments by use of two independently tunable Ti:sapphire lasers with a sample-and-hold switch

We performed femtosecond two-color experiments (four-wave mixing and pump probe) using two independently tunable, partially synchronized femtosecond lasers. Despite the fact that the jitter is of the order of 5-10 ps, the time resolution is limited only by the pulse width when a homemade sample-and-hold switch is used.     

宽可调谐SG-DBR激光器波长测试控制系统

采用传统设备结合虚拟仪器的方法,研制了一套宽可调谐取样光栅DBR(SampledGratingDBR,SG-DBR)激光器的波长测试控制系统。系统能自动向SG-DBR激光器提供电流扫描,同时对激光器输出的光谱数据进行采集、处理,生成SG-DBR激光器的“波长-电流”数据查询表;通过快速查找数据查询表数据,并对激光器提供相应驱动电流,准确控制SG-DBR激光器输出波长。系统数据处理采用了一种新颖的算法,利用输出光边模抑制比(SMSR)来筛选激光器模式稳定工作点,确保数据查询表中的工作点数据均不会发生模式跳变。实验证明,本系统稳定、可靠,波长控制精度高,波长控制误差不超过±0.02nm。     

High-sensitivity detection of tritiated water vapour using tunable diode lasers

A tunable lead-salt diode laser and a multipass optical cell are used to detect traces of tritiated water vapour in air. The present system has a sensitivity of ∼ 700 μCi/m³ if the air sample directly enters the cell, and an enhanced sensitivity of 35 μCi/m³ if the water vapour is precondensed from the air sample. The instrument is free from interference from HT or other radioactive species which may be present in the sample.     

Theory and design of a simple tunable Sagnac loop filter for multiwavelength fiber lasers

We present a novel design for a Sagnac loop filter (SLF) whose periodic output spectrum can be fully controlled by cascading a small-birefringence loop (SBL) with a high-birefringence loop (HBL). We develop a detailed theoretical analysis, using the Jones vector method, to calculate the transfer function of the tunable SLF. It has been quantitatively shown that both the transmission coefficient and the phase of the SLF can be controlled by adjusting the orientation angle of the birefringent axes of the SBL in the plane of polarization. The theory has been confirmed by experimental results where a stress-induced SBL was mechanically rotated to tune both the wavelength and the power of a periodic output spectrum. Such a tunable SLF is used in the design of a multiwavelength erbium-doped fiber laser in the C band.     

激光燃烧诊断技术及应用研究进展

介绍了用于燃烧场温度、组分、火焰构造和流场速度等参数测量的相干反斯托克斯喇曼散射、自发振动喇曼散射、激光诱导荧光和OH示踪测速实验系统,给出了在预混火焰、高能固体推进剂瞬态燃烧场和超声速高温流场测量的部分实验结果,并分析了激光作用区域燃烧场温度、主要组分及流场速度的分布和火焰构造。