用于去除非共振背景的圆偏振宽带CARS光谱研究

相干反斯托克斯拉曼散射(CARS)是一种受激拉曼现象,在显微成像时,存在非共振背景,会导致光谱产生峰位偏移和谱线变形。本文利用飞秒激光作为光源,通过光栅滤波系统产生窄带泵浦光,飞秒激光激发光子晶体光纤产生超连续谱作为斯托克斯光,两束光被调制为圆偏振光后同时激发样品产生CARS光谱。通过模拟计算说明圆偏振光可以有效去除各向异性材料CARS光谱中的非共振背景,从而使CARS光谱具有和自发拉曼相似的谱线形状。聚苯乙烯微球和液晶样品的CARS光谱实验结果与模拟计算基本相符,说明圆偏振是一种有效去除CARS光谱非共振背景的方法。     

高功能可调谐激光器——光学参量振荡器的新进展

本文简要阐述OPO的工作原理 ,并对最新的几种OPO系统作简单介绍和评述。     

分布式光纤温度传感器及其应用

本文介绍了近年来发展的分布式光纤温度传感器的工作原理和应用实例     

基于分束镜的两路光纤激光器相干合成研究

设计了利用一块45°半透半反分束镜作为干涉元件,使两路光纤激光器在输出端相干相长,从而实现相干合成输出的方案。详细分析了此方案的锁相机理,并在实验中成功实现了两路光纤激光器的相位锁定,获得了功率约360mW的相干合成激光输出,功率合成效率约为73%,合成输出后的激光束在轴上光强获得了大幅度提高。本方案结构简单,所有元件均可承受高功率,因此可实现更高功率的相干合成激光输出,是一种有前途的高功率光纤激光器相干合成方案。     

分布式光纤传感器多点温度测量的研究

基于光时域反射技术(OTDR)的原理,建立了拉曼散射分布式光纤多点温度测量系统,提出了一种新型的解调方法,即循环解调方法,用最近已知OTDR温度曲线解调下一个待测点OTDR温度曲线。通过实验验证了理论分析的正确性,该系统能够抑制温漂噪声的积累,防止瑞利背向散射光窜扰反斯托克斯背向散射光,其空间分辨力小于3m,温度分辨力约为±3℃,时间分辨力不大于3min。     

周期色散补偿光纤参量放大器平坦性的研究

针对单泵浦光纤参量放大器（FOPA）增益平坦性较差的情况,研究了双泵浦FOPA的准相位匹配问题。将高非线性光纤（HNLF）均匀地分成多段,在其中分别插入色散补偿光纤（DCF）,对下一段高非线性光纤中的色散进行补偿,从而实现了准相位匹配,在较宽频带内得到了较平坦的增益。研究结果表明：通过周期插入色散补偿光纤,增益起伏得到了明显的改善;光纤参量放大器的增益与泵浦光功率有关,当增加泵浦光功率时,增益变大,但平坦性较差。插入色散补偿光纤的段数增加时,能解决增益起伏的问题。     

光子晶体光纤传感器的研究进展

简要介绍了光子晶体光纤的基本结构、导光原理和主要特性.依据传感机理将光子晶体光纤传感器分为4类:吸收型光子晶体光纤传感器、荧光光子晶体光纤传感器、干涉型光子晶体光纤传感器、光子晶体光纤光栅传感器,分别对其工作原理、结构、特点和研究进展进行了讨论.     

纳秒近红外KTP光学参量振荡器的理论设计

对纳秒近红外KTP光学参量振荡器(KTP-OPO)进行了全面系统的理论设计。讨论了KTP-OPO的相位匹配,计算了KTP的走离角、允许角和有效非线性系数,数值模拟了角度调谐曲线和增益曲线,从而确定了调谐范围为1.35~2.0 μm,晶体切割角为59.6°。同时,分析了晶体长度、走离角以及输出镜耦合率对OPO阈值的影响,总结了降低参量阈值的几种方法。     

基于掺镁周期极化铌酸锂晶体的内腔单共振连续可调谐光参量振荡器

为了用简单、紧凑的谐振腔获得稳定的激光输出,大的调谐范围和转换效率,设计了信号光单共振V型光学参量振荡(OPO)腔,采用内腔式抽运周期极化掺镁铌酸锂晶体(PPMgLN)的光学参量振荡技术获得了连续中红外宽波段调谐激光的输出.用808 nm半导体激光抽运Nd:YVO4晶体产生的1 064 nm激光作为光参量振荡的基频光,通过V型腔灵活控制激光光斑并改变PPMgLN的极化周期和控制温度实现了2 249～3 706 nm中红外的连续宽波段调谐激光输出.在半导体激光抽运功率为10.5W,极化周期为29.98μm,控制温度为411 K的情况下获得了最高650 mW的中红外激光输出,对应的中心波长为3 466 nm,线宽为2.6 nm,具有较好的单色性.在7.5W的入射功率下,最高808 nm抽运光到闲频光的转化效率达7.73％,对应输出功率为580 mW.     

CARS microscopy with folded BoxCARS phasematching

Three-dimensional microscopy based on coherent anti-Stokes Raman scattering (CARS) is a powerful new imaging technique, in which the contrast arises from molecular vibrations. Based on a simple numerical model, it is shown how the CARS interaction volume depends on the focusing parameters and the type of phasematching used. Collinear phasematching yields an ellipsoidal interaction volume, with lateral dimensions that readily cause vignetting of the CARS signal emission at the collection microscope objective. A folded BoxCARS phasematching geometry, on the other hand, results in an almost cylindrical interaction volume — at the cost of a reduced resolution, for which the possible vignetting of the CARS emission is much reduced. In addition, this type of phasematching provides spatial separation of the signal from the input laser beams, permitting simple signal detection of low frequency vibrational modes. Calculations show that when CARS is performed in a microscopic volume, the phasematching restraint on tuning over the vibrational band is strongly relaxed. A first example of CARS imaging using a folded BoxCARS imaging geometry is shown.     

Chemically specific imaging of cryptosporidium oocysts using coherent anti-Stokes Raman scattering (CARS) microscopy

We demonstrate the application of coherent anti-Stokes Raman scattering microscopy for the rapid, label-free chemical imaging of waterborne pathogens. Chemically selective images of cryptosporidium were acquired in just a few seconds using coherent anti-Stokes Raman scattering microscopy, demonstrating its capability for the rapid detection of cryptosporidium at the single oocyst level. We discuss the applicability of such a technique in a near-real time automated water testing system.     

Flexible and stable optical parametric oscillator based laser system for coherent anti‐Stokes Raman scattering microscopy

The characteristics of a stable and flexible laser system based on a synchronously pumped optical parametric oscillator (OPO) is presented. This OPO can offer very stable operation with both ～1 ps and ～300 fs outputs over a broad wavelength range, i.e., 920–1200 nm. Combining the pump tuning with the OPO tuning, a total Raman range of 1900–5500 cm^?1 is accessible. For maximum spectral sensitivity, the CARS microsope based on the ps laser system is presented in detail. The lateral resolution of the microscope is diffraction limited to be about 390 nm. Fast wavelength switching (sub‐second) between two Raman vibrational frequencies, i.e., 2848 cm^?1 for C? H aliphatic vibrations and 3035 cm^?1 for C? H aromatic vibrations is presented as an example, although this also extends to other Raman frequencies. The possibility of obtaining a multimodal imaging system based on the fs laser system is also discussed. Microsc. Res. Tech., 2010. © 2009 Wiley‐Liss, Inc.     

基于光子晶体光纤的轨道角动量模式的产生

为了让光纤中产生轨道角动量(orbital angular momentum,OAM)模式更加集成化,提出了一种利用光子晶体光纤(photonic crystal fiber,PCF)产生轨道角动量模式的方法。通过设计特殊的PCF,使其本征模的奇偶模传输速度不同,在传输一段距离后形成π/2相位差,从而产生OAM模式。该PCF由中心空气孔、二氧化硅环形层和外包层构成,其中外包层由两层圆形空气孔组成,每层圆的数目为2n+2个,对应产生n阶OAM模式。采用有限元法对所设计光纤进行了三维数值仿真分析,成功产生了+2、+3、+4、+5阶OAM模式。该PCF产生OAM模式的方法,满足了未来光纤通信系统集成化、小型化的趋势,在全光纤模分复用系统中有着潜在的应用前景。     

Application of a high power Yb fiber‐based laser compatible with commercial optical parametric oscillator for coherent anti‐stokes raman scattering microscopy

Coherent anti‐Stokes Raman scattering (CARS) microscopy is a powerful tool for chemical analysis at a subcellular level, frequently used for imaging lipid dynamics in living cells. We report a high‐power picosecond fiber‐based laser and its application for optical parametric oscillator (OPO) pumping and CARS microscopy. This fiber‐based laser has been carefully characterized. It produces 5 ps pulses with 0.8 nm spectral width at a 1,030 nm wavelength with more than 10 W of average power at 80 MHz repetition rate; these spectral and temporal properties can be slightly modified. We then study the influence of these modifications on the spectral and temporal properties of the OPO. We find that the OPO system generates a weakly spectrally chirped signal beam constituted of 3 ps pulses with 0.4 nm spectral width tunable from 790 to 930 nm optimal for CARS imaging. The frequency doubling unconverted part is composed of 7–8 ps pulses with 0.75 nm spectral width compatible with CARS imaging. We also study the influence of the fiber laser properties on the CARS signal generated by distilled water. In agreement with theory, we find that shorter temporal pulses allow higher peak powers and thus higher CARS signal, if the spectral widths are less than 10 cm^?1. We demonstrate that this source is suitable for performing CARS imaging of living cells during several hours without photodamages. We finally demonstrate CARS imaging on more complex aquatic organisms called copepods (micro‐crustaceans), on which we distinguish morphological details and lipid reserves. Microsc. Res. Tech. 77:422–430, 2014. © 2014 Wiley Periodicals, Inc.     

Two‐frequency CARS imaging by switching fiber laser excitation

To fully exploit the power of coherent Raman imaging, techniques are needed to image more than one vibrational frequency simultaneously. We describe a method for switching between two vibrational frequencies based on a single fiber‐laser source. Stokes pulses were generated by soliton self‐frequency shifting in a photonic crystal fiber. Pump and Stokes pulses were stretched to enhance vibrational resolution by spectral focusing. Stokes pulses were switched between two wavelengths on the millisecond time scale by a liquid‐crystal retarder. Proof‐of‐principle is demonstrated by coherent anti‐Stokes Raman imaging of polystyrene beads embedded in a poly(methyl methacrylate) (PMMA) matrix. The Stokes shift was switched between 3,050 cm^?1, where polystyrene has a Raman transition, and 2,950 cm^?1, where both polystyrene and PMMA have Raman resonances. The method can be extended to multiple vibrational modes.     

High sensitivity internal refractive index sensor based on a photonic crystal fiber long period grating

A high sensitivity method is reported for the measurement of the internal refractive index using a photonic crystal fiber long period grating. Long period gratings of different lengths were inscribed in photonic crystal fibers, and the air holes of the fiber had varying refractive indices. A side lobe appeared near the resonant dip through the analysis of the characteristics of transmission spectra showing variation in the refractive index. The resonant dip and its side lobe provided varying sensitivities to the internal refractive index values. The sensitivity of the side lobe was as high as 2343?nm/refractive index unit (RIU), which exceeded the value for the resonant dip (2047?nm/RIU) for refractive indices from 1.3333–1.3792. Due to the high resolution of 8.5?×?10^?6 RIU, this method offers promising applications for biological and chemical analysis in which high-precision refractive index measurements are required.     

Optical fiber distributed temperature sensor using short term Fourier transform based simplified signal processing of Raman signals

A simplified technique using short term Fourier transform to reduce the errors in distributed temperature measurement with a Raman scattering based optical fiber sensor system is presented. The two main sources of errors are differential attenuation to anti-Stokes and Stokes signal by fiber and local change in Stokes due to change in temperature. The proposed technique compensates these errors and extracts correct temperature profile in spite of practical difficulties encountered in applying the theoretical concept. Moreover proposed technique is less complex, self-reliant, can tolerate variation in laser power, requires less dead zone and suits automation using embedded solution. Results of measurement carried out, using the system developed at RRCAT, Indore, for two hot zones having spatial width of 1.9 m (kept at 56 °C) and 1.5 m (kept at 78 °C), located at 47 m and 85 m respectively, show that these parameters can be recovered with significantly small errors.