Femtosecond broadband stimulated Raman: a new approach for high-performance vibrational spectroscopy

Femtosecond stimulated Raman spectroscopy (FSRS) is a new technique that produces high-quality vibrational spectra free from background fluorescence. FSRS combines a narrow-bandwidth picosecond Raman pump pulse with an approximately 80 fs continuum probe pulse to produce stimulated Raman spectra from the pump-induced gain in the probe spectrum. The high intensity of the Raman pump combined with the broad bandwidth of the probe produces high signal-to-noise vibrational spectra with very short data acquisition times. FSRS spectra of standard solutions and solvents such as aqueous Na2SO4, aqueous KNO3, methanol, isopropanol, and cyclohexane are collected in seconds. Furthermore, stimulated Raman spectra can be obtained using just a single pump-probe pulse pair that illuminates the sample for only approximately 1 ps. Fluorescence rejection is demonstrated by collecting FSRS spectra of dyes (rhodamine 6G, chlorophyll a, and DTTCI) with varying degrees of fluorescence background and resonance enhancement. The high signal-to-noise, short data acquisition time, fluorescence rejection, and high spectral and temporal resolution of femtosecond stimulated Raman spectroscopy make it a valuable new vibrational spectroscopic technique.     

Q-switched mode-locking of an erbium-doped fiber laser using cavity modulation frequency detuning

We present the results of an investigation regarding a Q-switched mode-locked fiber laser scheme based on a cavity modulation frequency detuning technique. The approach is based on undamped laser relaxation oscillations occurring due to frequency detuning in the fundamental cavity resonance frequency. Through a range of experiments with an erbium-doped, fiber-based, ring-cavity laser, this approach has been shown to be capable of generating high-quality Q-switched mode-locked pulses from an optical fiber-based laser. The maximum frequency detuning range for a stable Q-switched mode-locking operation has been observed to vary depending on the pump power used. We found that the highest pulse peak power was obtained at the frequency detuning threshold at which the operation changed from the mode-locking to the Q-switched mode-locking regime.     

Airborne detection of oceanic turbidity cell structure using depth-resolved laser-induced water Raman backscatter

Airborne depth-resolved laser-induced sea-water Raman-backscatter waveforms have been obtained along a flight line extending westward from a point approximately 30 km seaward of Assateague Island to a point where the beach was intersected at latitude 38.1 degrees N and longitude 75.2 degrees W. Pulses from a 337.1-nm nitrogen laser were repetitively transmitted vertically downward into the water column. The laser-induced water Raman backscatter pulse at 381-nm wavelength was depth (or time) resolved into forty bins having widths of -25 cm each. When converted to along-track profiles, the waveforms reveal cells of decreased Raman backscatter superimposed on an overall trend of monotonically decreasing water column optical transmission. This airborne lidar technique shows potential for (1) rapid, quantitative, synoptic study of the homogeneity of the oceanic water column and (2) measurement of the horizontal spatial distribution of the optical transmission of the upper mixed layer of the ocean. A multiple convolution model of a Gaussian transmitted pulse, Gaussian sea surface height, and slope probability density, together with an exponential-decay water-column impulse response, is shown to qualitatively account for the observed pulse shape.     

9Cr2凹模淬火裂纹的搭桥修复

9Cr2凹模淬火裂纹经脉冲放电止裂和电火花熔焊的搭桥修复后，裂纹尖端纯化，模具表面的淬火裂纹被焊合，模具的承载能力和表面光洁度可达到使用要求。     

How to drive CARS in reverse

Remote chemically specific detection of trace impurities in the atmosphere from distances on the order of kilometers is an important problem from both an environmental and a national defense viewpoint. A new scheme is discussed consisting of the remote generation of a backward propagating stimulated Raman pulse. This pulse is then used to drive a coherent anti-Stokes Raman scattering scheme, resulting in a strong chemically specific signal propagating back to the detector.     

Pulsed Raman fiber laser and multispectral imaging in three dimensions

Raman scattering in single-mode optical fibers is exploited to generate multispectral light from a green nanolaser with high pulse repetition rate. Each pulse triggers a picosecond camera and measures the distance by time-of-flight in each of the 0.5 Mpixels. Three-dimensional images are then constructed with submillimeter accuracy for all visible colors. The generation of a series of Stokes peaks by Raman scattering in a Si fiber is discussed in detail and the laser radar technique is demonstrated. The data recording takes only a few seconds, and the high accuracy 3D color imaging works at ranges up to approximately 200 m. Applications for optical tomography in highly scattering media such as water and human tissue are mentioned.     

Raman spectroscopy of carbon-nanotube-based composites

Recent developments in the application of Raman spectroscopy to carbon-nanotube-based composite materials are reviewed. This technique may be used to identify carbon nanotubes, access their dispersion in polymers, evaluate nanotube/matrix interactions and detect polymer phase transitions. The Raman spectra of nanotubes can also be used to quantify the strain or stress transferred to nanotubes from the surrounding environment and to investigate local stresses and strains in polymers and composites. A polarized Raman technique was developed to detect the stress or strain in a matrix using randomly dispersed single-walled nanotubes. This technique has been used to detect and map stress fields in model fibre-polymer composites. The stress distributions around fibre breaks were mapped and compared with classical load transfer models.     

Time-resolved beam-quality characterization of copper-vapor lasers with unstable resonators

Beam quality (BQ) of a 4-cm copper-vapor laser (CVL) with unstable resonators of different magnifications was characterized based on time-resolved far-field measurement. It was found that the BQ improvement after each round trip of the cavity cannot be predicted correctly from resonator theory. With a cavity Fresnel number of ~ 300, the achievable CVL BQ at the later part of the pulse was limited to approximately 4 times diffraction limited (×DL), even with a cavity magnification of 130. A pronounced temporal BQ oscillation, which is synchronized with the temporal pulse modulation, was also observed throughout the entire pulse. Examination of the temporal evolution of the far-field spot with use of a gated camera revealed that the strong presence of amplified spontaneous emission (ASE) in the cavity during the entire laser pulse severely limited the achievable BQ because of consecutive cavity feedback that included this highly divergent ASE. BQ deterioration caused by intense ASE throughout the pulse was reduced when a cavity with a smaller Fresnel number was used.     

Generation of single short tunable UV pulses using a simple short-cavity dye laser

Generation of single, short, tunable UV pulses using a short-cavity dye laser (SCDL) with a novel simple cavity structure is described. A single 80-ps pulse has been obtained at the 616-nm wavelength from a rhodamine 640 dye laser pumped with a 2-ns N2 laser pulse. By amplifying and subsequent frequency-doubling, a 90-ps 308-nm pulse was generated, which can be used as a short-duration XeCl excimer laser source. The temporal characteristics coupled rate equations.     

集成锁相环在激光脉冲调制系统中的应用

锁相环由鉴相器、环路滤波器及压控震荡晶体组成,是一个能跟踪输入信号频率和相位的闭环自动控制系统。研制的激光脉冲调制系统采用锁相技术,以单片、集成锁相环代替分立元件,实现了片内鉴频和鉴相的功能。研制的腔倒空驱动器能够输出 4MHz, 800kHz, 400kHz, 80kHz,40kHz, 8kHz, 4kHz, 800Hz, 400Hz 等不同重复频率的脉冲信号,输出功率达到瓦级,满足了声光布拉格池的要求。该激光脉冲调制系统已经应用在皮秒时间相关单光子计数光谱仪系统中,取得了比较理想的效果。     

Tropospheric ozone differential-absorption lidar using stimulated Raman scattering in carbon dioxide

A UV ozone differential-absorption lidar (DIAL) utilizing a Nd:YAG laser and a single Raman cell filled with carbon dioxide (CO(2)) is designed, developed, and evaluated. The generated wavelengths are 276, 287, and 299 nm, comprising the first to third Stokes lines of the stimulated Raman scattering technique. The correction terms originated from the aerosol extinction, the backscatter, and the absorption by other gases are estimated using a model atmosphere. The experimental results demonstrate that the emitted output energies were 13 mJ/pulse at 276 nm and 287 nm and 5 mJ/pulse at 299 nm, with pump energy of 91 mJ/pulse and a CO(2) pressure of 0.7 MPa. The three Stokes lines account for 44.0% of the available energy. The use of argon or helium as a buffer gas in the Raman cell was also investigated, but this leads to a dramatic decrease in the third Stokes line, which makes this wavelength practically unusable. Our observations confirmed that 30 min of integration were sufficient to observe ozone concentration profiles up to 10 km. Aerosol extinction and backscatter correction are estimated and applied. The aerosol backscatter correction profile using 287 and 299 nm as reference wavelengths is compared with that using 355 nm. The estimated statistical error is less than 5% at 1.5 km and 10% at 2.6 km. Comparisons with the operational carbon-iodine type chemical ozonesondes demonstrate 20% overestimation of the ozone profiles by the DIAL technique.     

Remote Raman spectroscopic detection of minerals and organics under illuminated conditions from a distance of 10 m using a single 532 nm laser pulse

Raman spectra of several minerals and organics were obtained from a small portable instrument at a distance of 10 m in a well-illuminated laboratory with a single 532 nm laser pulse with energy of 35 mJ/pulse. Remote Raman spectra of common minerals (dolomite, calcite, marble, barite, gypsum, quartz, anatase, fluorapatite, etc.) obtained in a short period of time (1.1 mus) clearly show Raman features that can be used as fingerprints for mineral identification. Raman features of organics (benzene, cyclohexane, 2-propanol, naphthalene, etc.) and other chemicals such as oxides, silicates, sulfates, nitrates, phosphates, and carbonates were also easily detected. The ability to identify minerals from their Raman spectra obtained from a single laser pulse has promise for future space missions where power consumption is critical. Such a system could be reduced in size by minimizing the cooling requirements for the laser unit. The remote Raman system is also capable of performing time-resolved measurements. Data indicate that further improvement in the performance of the system is possible by reducing the gate width of the detector (ICCD) from 1.1 mus to approximately 20 ns, which would significantly reduce the background signal from daylight or a well-illuminated laboratory. The 1.1 mus signal gating was effective in removing nearly all background fluorescence with 532 nm excitation, indicating that the fluorescence in most minerals is probably from long-lifetime inorganic phosphorescence.     

Non-linear switching based on dual-core non-linear optical fiber couplers with XPM and Raman intrapulse applied to femtosecond pulse propagation

In this work, we investigated the optical switching process for three shapes of femtosecond pulses (soliton, Gaussian and super-Gaussian) propagating inside a symmetrical dual-core non-linear directional coupler by simulating their propagation via the coupled non-linear Schrödinger equations. In all simulations, we considered the dispersive effects of second and third order, besides the self-phase modulation and self-steepening non-linear effects. We studied three scenarios for each of the three pulse shapes under investigation. In the first scenario, we added only cross-phase modulation (XPM); in the second approach, we added only Raman scattering; in the third one, we combined both. The study was performed for distinct polarization modes and for different values of the Raman factor, with power range varying from 1 to 300 W. We noted that the XPM non-linear effect results in a decrease in the critical power threshold, whereas the Raman scattering causes an increase. For the first scenario (only XPM effect), the critical power threshold reduced from 113.72 to 104.69 W for the soliton pulse, from 111.49 to 100.77 W for the Gaussian and from 92.79 to 80.47 W for the Super-Gaussian pulse shape. For the second scenario (only Raman scattering), the critical power increased for a Raman factor varying from 1 to 10 fs, and the three pulse shapes reached thresholds above 150 W from a 5 fs factor, reaching more than 200 W for the super-Gaussian pulse as the Raman factor increased. For the third scenario (with both effects combined), we highlight that for a fixed XPM factor of 2, the critical power remained unchanged with the variation of the Raman factor. Hence, we observed that the Super-Gaussian pulse reached lower values for critical power when compared to the other pulse shapes.     

An evaluation of specific heat measurement methods using the laser flash technique

Different methods for adapting the laser flash technique to measure simultaneously specific heat have been proposed in the literature. Among them are the coating method, the absorbing disk method, the double-specimen method, the pulse heating-cooling method, and the cavity method. These methods are briefly reviewed, and their merits and demerits are evaluated.     

Complete characterization of a self-mode-locked ti:sapphire laser in the vicinity of zero group-delay dispersion by frequency-resolved optical gating

The intensity and the phase of ultrashort pulses from a self-mode-locked Ti:sapphire laser operating in the vicinity of zero group-delay dispersion (GDD) have been completely characterized by the technique of frequency-resolved optical gating (FROG). For small values of negative GDD, the appearance of a dispersive wave in the pulse spectrum is manifested in the measured FROG trace, and pulse retrieval directly shows its association with a broad leading-edge pedestal. For positive GDD, we confirm previous experimental observations of picosecond pulses with large positive chirp and report a new operating regime in which the output pulses are of picosecond duration but are intensity modulated at 20 THz. The physical origin of this modulation is discussed by analogy with similar effects observed during pulse propagation in optical fibers, and the experimental results are compared with a model of intracavity four-wave mixing about the cavity zero GDD wavelength.     

Near-infrared surface-enhanced-Raman-scattering-mediated detection of single optically trapped bacterial spores

A novel methodology has been developed for the investigation of bacterial spores. Specifically, this method has been used to probe the spore coat composition of two different Bacillus stearothermophilus variants. This technique may be useful in many applications; most notably, development of novel detection schemes toward potentially harmful bacteria. This method would also be useful as an ancillary environmental monitoring system where sterility is of importance (i.e., food preparation areas as well as invasive and minimally invasive medical applications). This unique detection scheme is based on the near-infrared (NIR) surface-enhanced Raman scattering (SERS) from single, optically trapped, bacterial spores. The SERS spectra of bacterial spores in aqueous media have been measured using SERS substrates based on approximately 60-nm-diameter gold colloids bound to 3-aminopropyltriethoxysilane derivatized glass. The light from a 787-nm laser diode was used to trap and manipulate as well as simultaneously excite the SERS of an individual bacterial spore. The collected SERS spectra were examined for uniqueness and the applicability of this technique for the strain discrimination of Bacillus stearothermophilus spores. Comparison of normal Raman and SERS spectra reveals not only an enhancement of the normal Raman spectral features but also the appearance of spectral features absent in the normal Raman spectrum.     

Remote pulsed Raman spectroscopy of inorganic and organic materials to a radial distance of 100 meters

A portable pulsed remote Raman spectroscopy system has been fabricated and tested to 100 m radial distance. The remote Raman system is based on a directly coupled f/2.2 spectrograph with a small (125 mm diameter) telescope and a frequency-doubled Nd:YAG pulsed laser (20 Hz, 532 nm, 25 mJ/pulse) used as the excitation source in a co-axial geometry. The performance of the Raman system is demonstrated by measuring the gated Raman spectra of calcite, sodium phosphate, acetone, and naphthalene. Raman spectra of these materials were recorded with the 532 nm pulsed laser excitation and accumulating the spectra with 600 laser shots (30 s integration time) at 100 m with good signal-to-background ratio. The remote pulsed Raman system can be used for remotely identifying both inorganic and organic materials during daytime or nighttime. The system will be useful for terrestrial applications such as monitoring environmental pollution and for detecting minerals and organic materials such as polycyclic aromatic hydrocarbons (PAHs) on planetary surfaces such as Mars.     

探测低空大气温度分布的转动拉曼激光雷达

提出了一种新的探测对流层低层大气温度的转动拉曼激光雷达方法,通过测量N2和O2的后向散射的纯转动拉曼谱的强度,计算它们的比值来确定大气温度的垂直分布,并对其性能进行了数值模拟。转动拉曼激光雷达的光源是一个调Q的Nd:YAG激光器,经扩束器后输出能量200mJ;采用双光栅单色仪提取所需要的氮气和氧气的转动拉曼谱;接收机采用光电倍增管和双通道光子计数器,量子效率是10%(48000个脉冲累加)。夜晚它对近地面10.2km高度内的探测信噪比在10:1以上,白天它对近地面3.6km高度内的探测信噪比在10:1以上,计算的温度与模拟用的温度真值阔线相差约0.3K。     

Development of multipoint vibrational coherent anti-Stokes Raman spectroscopy for flame applications

A novel technique for coherent anti-Stokes Raman spectroscopy (CARS) measurements in multiple points is presented. In a multipass cavity the pump and Stokes laser beams are multiply reflected and refocused into a measurement volume with an adjustable number of separated points along a line. This optical arrangement was used in a vibrational CARS setup with planar BOXCARS phase-matching configuration. The CARS spectra from spatially separated points were recorded at different heights on a CCD camera. Measurements of temperature profiles were carried out in the burned gas zone of a premixed one-dimensional flame to demonstrate the applicability of this method for temperature measurements in high-temperature regions. The ability to measure in flames with strong density gradients was demonstrated by simultaneous measurements of Q-branch spectra of N2 and CO in a Wolfhard-Parker burner flame. Interference phenomena found in multipoint spectra are discussed, and possible solutions are proposed. Merits and limitations of the technique are discussed.     

Dynamic behavior of postfilamentation Raman pulses

We report on the postfilamentation behavior of a Stokes pulse created from intense and collimated ultrashort pulses propagating in air. A systematic analysis of the pulse propagation revealed that the redshifted Raman pulse produced during filamentation had a larger divergence than the postfilamentation intense pump pulse. Also, the analysis of the far-field Stokes transverse ring revealed that the intensity in this ionization-free light channel is still sufficiently high to induce stimulated Raman scattering after ionization had ended. This behavior further extends the potential of filamentation to remotely induce third-order nonlinearities.