Raman spectrum of optical fiber waveguide — Effect of cladding

The spontaneous Raman spectrum of multimode glass core-plastic cladding fibers (core diameter ≈ 100 μm) was measured and compared with that of the bulk core and cladding materials. The fiber spectrum is dominated by the broad peaks of the glass core but also contains two sharp lines at 2909 and 2971 cm^-1, which correspond to the most intense lines of the polymer cladding in bulk form. These spectral features are generated by Raman scattering in the evanescent field region of the cladding with the resulting radiation weakly coupled to the propagating modes of the fiber; a measurement of relative core/cladding peak intensities is in agreement with the predictions of a theory based on this model. It is proposed that this measurement and analysis could be developed into a method for estimating the quality of the contact between the glass and plastic.     

Multifiber optical catheter with bending control of distal end: Applications of Raman biospectroscopy

This work demonstrates a multifiber optical catheter with bending control of the distal end, whose main use is to collect Raman signal for biospectroscopy. Also, a method for fiber background subtraction is presented, allowing removal of fiber emission peaks from gross Raman spectrum. The fiber optic catheter with seven fiber optics wrapped up in a resin and a biocompatible flexible teflon tube has been modified to incorporate a mechanical device with gauntlet, which allows bending of the distal extremity to access a desired location obeying the anatomy of the organ. The mechanical device (gauntlet) was built halfway from the catheter proximal excitation tip, allowing the bending of the distal end of about 38 mm diameter. Dispersive Raman spectra at 785 nm excitation wavelength from samples were measured without and with bending (from 120 mm decreasing to 14 mm) of the distal end of the catheter. An optical catheter without the gauntlet was used as a reference. Results demonstrated that both catheters (with bending mechanism and standard one) had about the same Raman signal when the device was bent semicircularly to 38 mm diameter (bending limit for the mechanism), with negligible implication to the Raman signal-to-noise relation (SNR) from sample. When both catheters were bent in a full turn, the Raman signal decreased to about 40% when bending reached 14 mm diameter, mainly due to losses introduced by changes in the fiber total internal reflection. The background removal was efficient, with minimum residual from fiber peaks. This optical catherer could be very useful in laser diagnosis and clinical applications, introducing a way to control the fiber tip position and angle on the tissue or organ. Published in Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 97–102, January–February, 2007.     

激光泵浦碲化物产生中红外超连续谱数值模拟

对2 m波段脉冲激光泵浦碲化物光子晶体光纤产生中红外超连续谱进行了数值研究。通过材料的拉曼增益谱间接求得了对应的拉曼响应函数;由光子晶体光纤的材料折射率和波导结构,通过COMSOL软件获得了碲化物光子晶体光纤中基模等效折射率,计算了相应的色散曲线和限制损耗 ;利用自适应的分步傅里叶算法,模拟了中心波长为1.96m、峰值功率为20 kW的50 fs脉冲光泵浦碲化物光子晶体光纤时超连续谱的产生,当光纤长度为6 cm时,产生的中红外超连续谱波长范围为1.0~4.5 m。     

Absolute backscatter coefficient estimates of tissue-mimicking phantoms in the 5-50 MHz frequency range

Absolute backscatter coefficients in tissue-mimicking phantoms were experimentally determined in the 5-50 MHz frequency range using a broadband technique. A focused broadband transducer from a commercial research system, the VisualSonics Vevo 770, was used with two tissue-mimicking phantoms. The phantoms differed regarding the thin layers covering their surfaces to prevent desiccation and regarding glass bead concentrations and diameter distributions. Ultrasound scanning of these phantoms was performed through the thin layer. To avoid signal saturation, the power spectra obtained from the backscattered radio frequency signals were calibrated by using the signal from a liquid planar reflector, a water-brominated hydrocarbon interface with acoustic impedance close to that of water. Experimental values of absolute backscatter coefficients were compared with those predicted by the Faran scattering model over the frequency range 5-50 MHz. The mean percent difference and standard deviation was 54%?±?45% for the phantom with a mean glass bead diameter of 5.40 μm and was 47%?±?28% for the phantom with 5.16 μm mean diameter beads.     

基于自适应免疫算法重叠拉曼谱峰的解析

拉曼光谱测量速度快,可以实现原位实时测量,现已成为过程控制中物料检测的一种重要手段。但由于环境的复杂性以及拉曼光谱信号特点,目前在线检测时难免会出现一些重叠谱峰。基于免疫算法特点,将该方法用于芳烃重叠拉曼谱峰信号的解析中,提取混合物质中单个组分拉曼谱峰信息进行分析,结果表明该方法解析快速、定量准确,相对误差低于1%,是解析重叠拉曼光谱信号的有效方法。针对现场样品检测中出现的重叠谱峰伴随荧光背景信号,提出了结合独立成分分析的自适应免疫算法,有效地解析出荧光背景信号,为复杂样品的拉曼光谱检测分析提供了新的手段。     

掺铒硫系玻璃的制备及其微结构光纤的中红外信号放大特性研究

为进一步揭示硫系玻璃基掺Er³⁺微结构光纤对于中红外波段信号的放大特性, 采用熔融淬火法研制了Er³⁺离子掺杂的Ga₅Ge₂₀Sb₁₀S₆₅硫系玻璃, 测试了玻璃样品的吸收光谱和2.7 μm波段荧光光谱, 利用Judd-Ofelt和Futchbauer-Ladenburg理论分别计算得到了Er³⁺离子的辐射跃迁概率、辐射寿命以及2.7 μm波段受激发射截面. 在此基础上, 建立了一个980 nm抽运下该玻璃基掺Er³⁺微结构光纤2.7 μm波段中红外信号的放大模型, 理论上研究了其作为2.7 μm波段中红外信号增益介质时的光放大特性. 结果显示, 硫系玻璃基掺Er³⁺微结构光纤具有优异的高增益和宽带放大品性. 在200 mW抽运功率激励下的100 cm光纤长度上, 最大小信号增益超过了40 dB, 高于30 dB信号增益的放大带宽达到了120 nm (2696—2816 nm). 研究表明, Ga₅Ge₂₀Sb₁₀S₆₅硫系玻璃基掺Er³⁺微结构光纤是一种理想的可应用于2.7 μm波段中红外宽带放大器的增益介质.     

变纤芯直径传能光纤中受激拉曼散射传输特性的理论研究

锥形光纤纤芯直径沿着光纤长度方向均匀增大,在抑制非线性效应方面有着独特的优势。理论对比了单横模高斯光束输入时,输入纤芯直径均为50 μm、纤芯直径沿着光纤长度方向恒定、线性增大和非线性增大的变纤芯直径传能光纤的输出光谱演化和拉曼光特性。在相同条件下,输入功率10 kW时,恒定型传能光纤的输出光谱中,拉曼抑制比（定义为输出光谱中信号光峰值强度分贝值与拉曼光峰值强度分贝值之差）为33.1 dB,线性增大和非线性增大型的输出拉曼抑制比分别为47.0,48.6 dB,分别优于恒定型传能光纤13.9,15.5 dB;输入功率达17.5 kW时,恒定型传能光纤中有81.6%的输入能量被耗散或转移到其他波长,线性增大和非线性增大型仅不到2%,其输出的信号光波段能量占总输入能量的98.1%,98.9%。结果表明,使用线性增大型或非线性增大型传能光纤代替恒定型常规光纤,可以有效提高受激拉曼散射阈值,相关研究可以为大功率光纤合束器和光纤端帽的设计提供有益参考。     

后向泵浦高功率掺镱光纤放大器的非弹性散射效应

 在考虑受激布里渊散射和受激喇曼散射效应的基础上,数值求解了双包层掺镱光纤放大器的速率-传输方程,分析了后向泵浦高功率掺镱光纤放大器在不同信号光线宽、光纤长度和纤芯直径下的输出特性,研究了受激布里渊散射和受激喇曼散射的抑制条件。计算结果表明:在后向泵浦方式下,若掺镱光纤长度小于0.68 m,纤芯直径大于26 μm,信号光线宽大于0.04 nm,则放大过程中的受激布里渊散射和受激喇曼散射能同时得到有效抑制。     

Near-infrared sources in the 1–1.3 μm region by efficient stimulated Raman emission in glass fibers

Low-loss glass fiber waveguides are found to be excellent media for Raman lasers and amplifiers in the near-infrared region of the spectrum. Multiwavelength emission in the 1–1.3 μm range is readily obtained by efficient stimulated Raman scattering in single-mode silica fibers. With a 1.064 μm pulsed pump of 250 W in a 175-m, 6-μm diameter single-mode silica fiber we observed four orders of Stokes radiation at 1.12 μm, 1.18 μm, 1.23 μm and 1.3 μm, respectively. Our results imply that pulsed tunable stimulated Raman emission in this wavelength region is possible using kW tunable infrared dye lasers near 1 μm as pumps. These sources are useful for studying the dispersion of glass fibers as well as for other spectroscopic applications.     

高荧光及强背景噪声情况下表面增强拉曼散射光谱提取方法研究

试提出一种高荧光及强背景噪声情况下表面增强拉曼散射光谱的提取方法。该方法从分析荧光谱及背景噪声信号的组成入手,构建相应模型对荧光谱及噪声信号进行估计,通过比较估计结果与实际数据的差别识别谱峰信号,并定位谱峰的基底位置。通过在若丹明6G、前列腺特异抗原测量及pH值传感实验中的实际应用检验可知,该方法可提高表面增强拉曼散射光谱信号的分辩率和测量准确度,在微量物质鉴别及物质含量定量测量方面有较好的应用前景。     

Tm~(3+)掺杂Ge-Ga-S玻璃微球-石英光纤锥耦合系统的荧光回廊模特性

以熔融淬冷法自制了Tm~(3+)掺杂Ge-Ga-S硫系玻璃,并以此为基质材料,用漂浮粉料熔融法制备了直径分布为50—200μm的高品质因数(Q10~4)的有源硫系玻璃微球谐振腔.在显微镜下优选出一颗表面质量好、球形度较高、直径为72.84μm的微球,与氢氧焰扫描拉锥法制备的一根腰锥直径为1.93μm的石英光纤锥进行近场耦合.根据基质材料的吸收光谱特性,选用808 nm的半导体激光器作为抽运源.实验测得光纤锥倏逝波场激发出了掺Tm~(3+)硫系玻璃微球在1460 nm附近的荧光回廊模式,其典型共振峰间隔为4.39 nm.实验测得的荧光回廊模式与米氏散射理论计算结果符合度较高(最大误差仅为0.047%),验证了本文提出的掺Tm~(3+)硫系微球制备及耦合工艺的可行性.     

飞秒脉冲作用下光子晶体光纤超连续谱的产生

采用钛宝石超快fs激光器泵浦2m长光子晶体光纤产生超连续谱,重点研究了不同输入功率下350～959nm波长范围内光谱的演变过程,实验中产生的超连续光谱宽度超过550nm分析表明,内脉冲喇曼散射和孤子自频移在开始阶段对光谱长波方向上的展宽起主要作用,这些新的频率成分与随后光谱向短波方向的拓展有密切关系.     

Elastic scattering from a sapphire microsphere placed on a silica optical fiber coupler: Possible applications to biosensing

Elastic light scattering is performed in the original band of optical fiber communication at 1300 nm for a 500 μm sapphire microsphere placed on a silica optical fiber half coupler. The morphology dependent resonances (MDRs) are observed in the transverse magnetically (TM) polarized and transverse electrically (TE) polarized 0^° transmission and 90^° elastic scattering obtained from the sapphire microsphere. The TE and TM MDRs can be detected selectively with the use of a Glan polarizer. The TE and TM polarization selectivity provides the ability to select relative MDR to BG levels. The TM polarization provides higher MDR signal to background ratio (SBR) and is suitable for optical monitoring, biological sensing or any other optoelectronic application that requires a high resolution optical filter. The polar angular mode spacing of 0.36 nm of the resonances correlates well with the optical size of the sapphire microsphere. The autocorrelation of the 90^° elastic scattering spectra also shows peaks at 0.36 nm. The spectral linewidths of the resonances are on the order of 0.1 nm, which corresponds to quality factors on the order of 10⁴. A sapphire sphere with a radius of 500 μm and relative refractive index of 1.31, resonances will red-shift by 1.01 nm (0.077%). This shift is on the order of 10 linewidths, making sapphire biophotonic sensors an interesting alternative to silica biophotonic sensors.     

Ge20Sb15Se65硫系玻璃光子晶体光纤的中红外色散特性

硫系玻璃与石英玻璃相比具有高折射率(2.0～3.5)、低声子能量(<350cm-1)、优良的中远红外透过性能(可至25μm)等特性.本文制备了一种在中红外具有优良透过特性的无As环保型Ge20Sb15Se65硫系玻璃材料,以此为基质材料设计了一种三层空气孔结构光子晶体光纤,利用多极法对光纤的中红外色散特性进行了数值模拟,系统研究了结构参量孔径d、孔间距Λ以及d/Λ对其色散特性的影响.分析表明:通过改变包层空气孔直径d或空气孔间距Λ,可灵活的调节光子晶体光纤的零色散波长向短波或长波方向移动.通过优化结构参量发现,当Λ=3μm,d/Λ=0.35附近变化时,可获得3～5μm色散平坦,且色散值小于5ps.nm-1.km-1的光子晶体光纤.     

利用光子晶体光纤产生超连续飞秒激光光谱

将单脉冲能量约为2．5nJ、脉宽为25fs、对应峰值功率为0．1MW的的800nm钛宝石激光耦合到长为10mm，芯径为1．8μm的光子晶体光纤中，产生了耦合效率约为17％、谱宽覆盖可见光及近红外波段的超连续光谱。     

大气边界层白天温度测量用转动拉曼激光雷达

设计了一个转动拉曼激光雷达系统,对大气边界层温度进行全天候高精度测量。系统选用波长355 nm的紫外激光作光源,采用高光谱分辨力光栅,将雷达接收到的散射信号以及太阳背景光从空间上分离,配合边缘反射镜,反射转动拉曼信号,去除大部分米氏-瑞利散射和太阳背景光噪声信号,并用两个窄带干涉滤光片,分离中心波长为353.9 nm和353.1 nm转动拉曼散射信号,同时对噪声信号进行2次高精度剔除,以保证白天测量的需要。对系统进行了分析及数值计算,结果表明在激光脉冲能量300 mJ,望远镜有效口径25 cm,测量时间10 min的条件下,可以在白天太阳背景光辐射度为3×108W/(m2.sr.nm)的边界层内测量高度2.5 km以下的大气温度分布,并在大气散射比低于2.5的条件下,温度测量精度可达到1 K。     

采用光纤光栅级联结构实现光纤延迟线

在使用光纤光栅实现皮秒级别时延的基础上,提出一种光纤光栅与单模光纤相结合的微秒级别级联结构,该结构可以实现中心波长1 550~1 553 nm范围内,间距为1 nm的窄波长反射型时延线,共1,1.5,2和2.5 μs四种不同的时延。将单波长反射的啁啾布拉格光纤光栅与103 m单模光纤连接构成延迟单元,再利用光环形器将4个延迟单元级联并使用内半径为3 cm的光纤绕线盘,将四种延时单元的传输光纤进行整合。借助光纤光栅的反射镜作用,控制不同波长光信号通过不同的传输距离,从而达到时延目的。本文通过对啁啾布拉格光纤光栅的反射谱进行仿真分析,发现相邻反射谱的旁瓣会出现交叠现象,因此使用六个切趾函数对旁瓣滤除。结果显示：不同切趾函数的滤除效果也不同,能够完全滤除旁瓣并且对反射谱包络影响最小的是柯西切趾函数,经柯西切趾后能使不同波长光信号在对应中心波长1 nm范围内反射率达到1,而其他位置均为0。由于使用光纤绕线盘整合延迟单元传输光纤会产生一定损耗,因此对弯曲损耗进行仿真分析,结果表明：弯曲半径相同时,损耗与工作波长成正比;工作波长相同时,弯曲损耗与弯曲半径成反比。当弯曲半径大于2.9 cm时,弯曲损耗曲线变化平缓并趋于0,因此当光纤绕线盘内半径为3 cm时保证了在减小延迟模块体积的同时又不会有过大的损耗。通过TDS784D型示波器对频率为2 000 Hz的信号经不同传输距离后的波形进行测试,结果显示经3 m和5 km传输线后信号的各项参数基本保持不变,经过长距离传输后,依然能保持原信号特性,因此使用103 m传输线可达到延迟目的。使用W-GGL型光功率计对不同频率下的输出功率进行测量,与直光纤的输出功率相比,当弯曲半径为2~3 cm时偏差较大,等于3 cm时偏差为0.18 dBm,大于3 cm时则无限趋近,因此设置绕线盘内半径为3 cm符合光纤延迟线的损耗范围。     

相位匹配下锥形光纤激发出的回廊模谐振

分别计算不同直径下锥形光纤基模和玻璃微球谐振腔内最低阶径向回廊模的传播常量,利用相位匹配条件,作出了锥形光纤与石英玻璃微球腔的直径对应关系曲线.在此基础上,选择锥腰直径2.8 μm左右的低损耗锥形光纤与直径143.1 μm球形度很好的玻璃微球腔进行近场耦合以激发球内的最低阶径向回廊模谐振,在锥形光纤的两端进行通光测试,在输出端获得了等间距分布的窄线宽滤波谱线,其吸收峰位置与利用Mie理论计算的球内最低阶径向回廊模谐振峰位置相一致.     

超低频调制光缔合光谱的实验研究

通过激光冷却技术在磁光阱中俘获原子数约107,温度约200 μK,直径约400 μm的超冷铯原子,利用超冷铯原子光缔合方法制备了激发态的超冷铯分子。实验研究了光缔合光不同扫描速率对铯分子振转光谱分辨率的影响,发现光缔合光扫描速率较慢时,铯分子振转光谱分辨率较高。通过高灵敏的雪崩光电探测器探测冷原子荧光,获得了超冷铯分子第一激发态6S_1/2+6P_3/2离解限0-_g长程态高分辨振转光谱。为了实现受控拉曼光缔合制备超冷基态分子,光缔合激光频率需要锁定在原子-分子共振跃迁线,对超冷原子光缔合光谱进行了超低频波长调制,通过改变调制幅度和调制频率获得最优化的一阶微分信号,将该信号反馈回激光器,实现闭合环路稳频,满足了受控拉曼光缔合制备振转能级可控的基态分子的实验要求,该工作对研究受限空间中的超冷原子分子具有很重要的意义。     

基于共聚焦显微拉曼的真菌菌丝中几丁质的原位检测研究

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