Tunable light source for coherent anti-Stokes Raman scattering microspectroscopy based on the soliton self-frequency shift

We present a photonic crystal fiber (PCF)-based light source for generating tunable excitation pulses (pump and Stokes) that are applicable to coherent anti-Stokes Raman scattering (CARS) microspectroscopy. The laser employed is an unamplified Ti:sapphire femtosecond laser oscillator. The CARS pump pulse is generated by spectral compression of a laser pulse in a PCF. The Stokes pulse is generated by redshifting a laser pulse in a PCF through the soliton self-frequency shift. This setup allows for probing up to 4000 cm(-1) with a spectral resolution of approximately 25 cm(-1). We characterize the stability and robustness of CARS microspectroscopy employing this light source.     

Ultrabroadband multiplex CARS microspectroscopy and imaging using a subnanosecond supercontinuum light source in the deep near infrared

Subnanosecond supercontinuum (SC) has been generated by a 1,064 nm microchip laser combined with a photonic crystal fiber. The ultrabroadband (>2,000 cm(-1)) SC has facilitated multiplex coherent anti-Stokes Raman scattering (CARS) microspectroscopy in the spectral range from 1,000 to 3,000 cm(-1) with lateral and depth spatial resolution of 0.9 and 4.6 microm, respectively. A clear CARS image of a Nicotiana tabacum L. cv. Bright Yellow 2 cell has been obtained with high vibrational contrast.     

Fourier-transform coherent anti-Stokes Raman scattering microscopy

We report a novel Fourier-transform-based implementation of coherent anti-Stokes Raman scattering (CARS) microscopy. The method employs a single femtosecond laser source and a Michelson interferometer to create two pulse replicas that are fed into a scanning multiphoton microscope. By varying the time delay between the pulses, we time-resolve the CARS signal, permitting easy removal of the nonresonant background while providing high resolution, spectrally resolved images of CARS modes over the laser bandwidth (approximately 1500 cm(-1)). We demonstrate the method by imaging polystyrene beads in solvent.     

可调空间分辨USEDCARS技术研究

相干反斯托克斯拉曼散射（CARS）技术是一种非常重要的燃烧诊断技术,该技术具有非常强的抗干扰能力和非常高的测量精度。但空间分辨力不足会使CARS技术产生很强的空间平均效应,引起成CARS光谱畸变,进而造成CARS光谱分析困难,无法通过CARS光谱反演燃烧场参数。针对非稳腔空间增强探测CARS（USEDCARS）技术存在的空间分辨不足以及空间分辨力不易改变的特点,分析了影响USEDCARS技术测量空间分辨力的各种因素,采用一组轴棱锥对USEDCARS系统中的泵浦激光进行环状光束整形,并通过调节轴棱锥之间的距离获得了不同直径的环状光束,在此基础上,建立了空间分辨可调USEDCARS诊断系统。开展了空间分辨力分析实验,获得了CARS信号强度随空间位置的分布数据,以CARS信号总强度95%包含的空间区域代表CARS的纵向空间分辨力,以此计算得到了CARS系统空间分辨力为1.7～6.5 mm连续可调。其中,高分辨力情况,达到了现有BOXCARS技术的空间分辨力。利用所建立的空间分辨可调USEDCARS诊断系统测量了酒精/空气预混火焰温度参数,获得了不同空间分辨条件下的CARS光谱。空间分辨力为1.7 mm时,获得了高质量CARS光谱,通过光谱拟合给出了所测火焰的温度信息。分辨力分别为4.9和6.5 mm时获得了较强的CARS信号,但存在光谱畸变。结果显示,空间分辨力对CARS信号的强度和空间平均效应有很大地影响,提高测量的空间分辨力可以有效消除空间平均效应,获得准确的CARS光谱,增强光谱拟合精度,同时空间分辨可调的特性使该系统能够更好地适应不同实验条件下的诊断工作。     

Ultrabroadband (>2000 cm-1) multiplex coherent anti-Stokes Raman scattering spectroscopy using a subnanosecond supercontinuum light source

We have developed ultrabroadband (>2000 cm(-1)) multiplex coherent anti-Stokes Raman scattering (CARS) spectroscopy using a subnanosecond (sub-ns) microchip laser source. A photonic crystal fiber specifically designed for sub-ns supercontinuum (SC) generation has been used for obtaining ultrabroadband Stokes radiation, which enables us to achieve simultaneous vibrational excitation in the range from 800 to 3000 cm(-1). We have successfully obtained multiplex CARS spectra for several molecular liquids. Since the CARS system using the sub-ns SC is simple and compact, it can be easily applied to ultrabroadband multiplex CARS microspectroscopy.     

Ultrabroadband background-free coherent anti-Stokes Raman scattering microscopy based on a compact Er:fiber laser system

We demonstrate a scheme for efficient coherent anti-Stokes Raman scattering (CARS) microscopy free of nonresonant background. Our method is based on a compact Er:fiber laser source. Impulsive excitation of molecular resonances is achieved by an 11 fs pulse at 1210 nm. Broadband excitation gives access to molecular resonances from 0 cm(-1) up to 4000 cm(-1). Time-delayed narrowband probing at 775 nm enables sensitive and high-speed spectral detection of the CARS signal free of nonresonant background with a resolution of 10 cm(-1).     

Laser surface profiler

By accurate measurement of the angle of reflection of a laser beam that is incident upon a reflective surface with a position-sensitive detector, changes in the surface-normal direction (the slope or the derivative of the surface) can be determined directly. An instrument has been built that makes repeated measurements over the surface and uses the collected data to produce a gray-scale image of the slope. The resolution of this system for detecting changes in the surface-normal direction is found to be better than 0.01 degrees . By focusing of the laser beam to achieve a lateral resolution of 5 mum , the resolvable surface-height change owing to a variation in slope is estimated to be <1 nm .     

Two-dimensional synthetic aperture imaging in the optical domain

In scan-mode synthetic aperture imaging radar, spatial resolution in a range is given by a frequency-swept waveform, whereas resolution in the orthogonal direction is derived from the record of phase as the beam footprint executes linear motion over the object. We demonstrate here what is to our knowledge the first two-dimensional imaging that uses exactly this process in the optical domain for a 1 cm x 1 cm object with 90 mumx170 mum resolution.     

NO气体的空间交叉CARS

采用具有高空间分辨率的空间交叉BOX CARS相位匹配方法,完成了对NO气休室温下、Q支(v=0→v=1)振转CARS谱的测量,研究了它与NO气体压强及激光线宽的关系.从CARS基本理论出发,考虑到激光线宽及不同线型,计算出理论曲线,与实验谱线进行了比较,洛伦兹线型的理论模拟和实验结果符合较好.     

Simple approach to one-laser, broadband coherent anti-Stokes Raman scattering microscopy

Coherent anti-Stokes Raman scattering (CARS) microscopy is emerging as a powerful method for imaging materials and biological systems, partly because of its noninvasiveness and selective chemical sensitivity. However, its full potential for species-selective imaging is limited by a restricted spectral bandwidth. Recent increases in bandwidth are promising but still are not sufficient for the level of robust component discrimination that would be needed in a chemically complex milieu found, for example, in intracellular and extracellular environments. We demonstrate a truly broadband CARS imaging instrument that we use to acquire hyperspectral images with vibrational spectra over a bandwidth of 2500 cm(-1) with a resolution of 13 cm(-1).     

Tip-enhanced coherent anti-stokes Raman scattering for vibrational nanoimaging

An electric field enhanced by a metallic nanoprobe has locally induced coherent anti-Stokes Raman scattering (CARS) of adenine molecules in a nanometric DNA network structure. Owing to the third-order nonlinearity, the excitation of the CARS polarization is extremely confined to the end of the tip apex, resulting in a spatial resolution far beyond the diffraction limit of light. Our tip-enhanced CARS microscope visualized the DNA network structure at a specific vibrational frequency (approximately 1337 cm(-1)) corresponding to the ring-breathing mode of diazole of adenine molecules.     

利用CARS技术研究激发态Rb2和H2的碰撞转移

利用相干反斯托克斯拉曼光谱(coherent anti-stokes Raman spectroscopy,CARS)探测技术,研究了激发态Rb2与H<,2>间的电子-振转能级的碰撞转移.扫描CARS谱确认了H<,2>分子仅在v=1,J=1,2及v=2,J=0,1,2能级上有布居,用n1,n2,n3,n4,n5分别表示...     

Optical guidance of terrawatt laser pulses by the implosion phase of a fast Z-pinch discharge in a gas-filled capillary

A new method of optical guidance by the implosion phase of a fast Z-pinch discharge in a gas-filled capillary is proposed. An imploding plasma column has a concave electron-density profile in the radial direction, just before a stagnation phase driven by a converging current sheet and a shock wave. The feasibility of optical guidance of a high-intensity (>1 x 10(17) W/cm(2)) Ti:sapphire laser pulse by use of this method over a distance of 2 cm, corresponding to 12.5 times the Rayleigh length, has been experimentally demonstrated. The guiding-channel formation process was directly probed with a He-Ne laser beam. The electron density in the fully ionized channel was estimated to be 2.0 x 10(17) cm(-3) on the axis and 7.0 x 10(17) cm(-3) on the peaks of the channel edge, with a diameter of 70 mum, as indicated by the experimental results, which were corroborated by a magnetohydrodynamics simulation.     

Microanalytical nonlinear single-beam spectroscopy combining an unamplified femtosecond fibre laser,pulse shaping and interferometry

Nonlinear vibrational spectroscopy using a single beam of femtosecond pulses from an unamplified fibre laser oscillator is demonstrated. To achieve high spectral resolution and sensitive signal detection with the picojoule pulse energies available, pulse shaping and integrated interferometric detection is employed. The spectroscopic technique used is coherent anti-Stokes Raman scattering (CARS), which yields well-resolved spectra of molecular vibrations in the 100–350 cm^-1 domain of halomethane samples in the liquid phase. We explore the implications of phase control for the interferometric detection of weak signals. The presented combination of a fiber laser, pulse shaping and CARS microspectroscopy is a first example of simplified schemes for compact and robust nonlinear spectroscopic detection and sensing, which is demonstrated exemplarily by on-line monitoring of the chemical composition in a microfluidic flow cell. PACS 42.55.Wd; 42.62.Fi; 78.47.Fg; 42.65.Dr; 82.80.Gk; 92.20.cn     

Coherent anti-Stokes Raman scattering imaging with a laser source delivered by a photonic crystal fiber

We demonstrate laser-scanning coherent anti-Stokes Raman scattering (CARS) imaging with two excitation laser beams delivered by a large-mode-area photonic crystal fiber. The group-velocity dispersion and self-phase modulation effects are largely suppressed due to the large mode area of the fiber and the use of picosecond pulses. The fiber delivery preserves the signal level and image spatial resolution well. High-quality images of live spinal cord tissues are acquired using the fiber-delivered laser source. Our method provides a basic platform for developing a flexible and compact CARS imaging system.     

Room-temperature operation of a multiwatt Tm:YAG laser pumped by a 1-mum Nd:YAG laser

We present what is to our knowledge the first demonstration of a 4.7-W cw Tm:YAG. This proof-of-principle experiment clearly demonstrates the possibility of using a pump absorption that is 2 orders of magnitude (~0.0078 cm(-1)) less than that of the conventional pump absorption (typically >1 cm(-1)). This Tm:YAG laser is pumped intracavity within a Nd:YAG laser for multiple-pass absorption. The maximum conversion efficiency of 2.02 mum is 20%, with a slope efficiency of 35% with respect to the absorbed 1.064-mum power.     

Highly sensitive single-beam heterodyne coherent anti-Stokes Raman scattering

Single-beam coherent anti-Stokes Raman-scattering (CARS) microspectroscopy achieves a complete CARS scheme with a femtosecond laser. Here, we introduce heterodyne detection in a simple experimental extension: the optical fields driving the CARS process and the local oscillator used for heterodyning are derived from a single beam of ultrashort laser pulses by pulse shaping. The heterodyne signal is amplified by more than 3 orders of magnitude and is linearly dependent on the concentration of Raman scatterers. This dramatically increases the sensitivity of chemically selective detection at microscopic resolution while maintaining the simplicity of the single-beam setup.     

Combined coherent anti-Stokes Raman spectroscopy and linear Raman spectroscopy for simultaneous temperature and multiple species measurements

The simultaneous application of pure rotational coherent anti-Stokes Raman spectroscopy (CARS) and vibrational linear Raman spectroscopy (LRS) for the measurement of temperature and species concentrations in combustion systems is demonstrated. In addition to the standard rotational CARS experimental setup, only one detection system (spectrometer and intensified CCD camera) for the collection of the LRS signals was applied. The emission of the broadband dye laser used for CARS was shifted to the deep red to avoid interferences with the LRS signals located in the visible region. First experimental results from a vaporizing propane spray using an engine injection system are shown.     

In situ observation of photoinduced refractive-index changes in filaments formed in glasses by femtosecond laser pulses

We investigated the relationship between the formation of filaments and local refractive-index changes induced by femtosecond laser pulses in silica glass. In situ observation revealed that the location of a filament coincided with that of the refractive-index change. Observation also showed that the region of refractive-index change was elongated toward the upstream direction of the laser pulses with increasing exposure time. The region of refractive-index change was several hundred micrometers long, and its diameter was smaller than 2 mum. The refractive-index change was confirmed by two of three different methods to be as large as 0.8 x 10(-2).     

基于超连续光谱激发的时间分辨相干反斯托克斯拉曼散射方法与实验研究

采用钛宝石飞秒激光器输出的一部分光抽运光子晶体光纤以产生超连续光谱,作为抽运光和斯托克斯光,另一部分飞秒激光作为探测光,并结合时间延迟方法,建立超连续光谱激发时间分辨相干反斯托克斯拉曼散射(CARS)实验系统,测试了具有较宽拉曼光谱的二甲基亚砜样品.实验结果表明,所建立的实验系统能有效抑制非共振背景噪声,并且通过一次测量,即可获得二甲基亚砜在690—3200cm-1范围内的CARS光谱信息,获得的二甲基亚砜CARS光谱范围达到2500cm-1.同时给出了所采用的光子晶体光纤光谱展宽的实验结果.