A nanosecond optical parametric oscillator in the mid-IR region with double-pass pump

An optical parametric oscillator (OPO) with double-pass pump based on MgO:PPLN and PPLN periodic structures is described. A compact nanosecond Nd:YLF laser has been used as a pump source at 1.053 μm (the pumping pulse duration is 5–7 ns at a maximum pulse energy of 300 μJ at frequencies of 1–7 kHz). The oscillation threshold of the OPO based on MgO:PPLN was varied in a range of 11–28 μJ at wavelength of 2.1–4.3 μm. The conversion efficiency from the pump wave to an idler wave decreased from 8.6 to 2.5% in the range of 2.0–4.3 μm. For PPLN-OPO the measured threshold was 36 μJ at 4.2 μm and 49 μJ at 4.7 μm. The conversion efficiency of the pump energy into the energy of an idler wave was 3.3μ-0.4% at wavelengths of 4.2–4.7 μm.     

三镜直腔结构MgO∶PPLN高效连续光参量振荡器

为了优化MgO:PPLN连续光参量振荡器(OPO)的输出特性,对三镜直腔结构的内腔式OPO系统进行腔结构设计,对其同时输出高效的信号光和闲频光进行研究。采用半导体激光端面抽运Nd:YVO4晶体实现连续的1064 nm激光为基频光。对比分析了基频激光腔和OPO腔各腔镜分别采用平面镜或平凹镜的三种腔型结构的激光输出特性。基于30.5 μm的极化周期和12.4 W入射抽运功率时,获得了最高输出功率3.92 W（信号光2.6 W和闲频光1.32 W）,转化效率31.6%的激光输出,对应的信号光和闲频光的中心波长分别为1549 nm和3394 nm。结果表明三个腔镜均采用平凹镜时,可有效的压缩基频激光腔在MgO:PPLN晶体上的光斑,提升基频激光的功率密度,而且基频激光腔和OPO腔的基模光斑在MgO:PPLN晶体上更好的匹配,从而提升变频效率。     

A continuous-wave optical parametric oscillator around 5-μm wavelength for high-resolution spectroscopy

We present a continuous-wave optical parametric oscillator (OPO) capable of high resolution spectroscopy at wavelengths between 4.8 μm and 5.4 μm. It is based on periodically poled lithium niobate (PPLN) and is singly resonant for the signal radiation around 1.35 μm. Because of the strong absorption of PPLN at wavelengths longer than 4.5 μm, the OPO threshold rises to the scale of several watts, while it produces idler powers of more than 1 mW and offers continuous tuning over 15 GHz. A supersonic jet spectrometer is used in combination with the OPO to perform measurements of the transient linear molecule Si(2)C(3) at 1968.2 cm(-1). Fifty rovibrational transition frequencies of the ν(3) antisymmetric stretching mode have been determined with an accuracy on the order of 10(-4) cm(-1), and molecular parameters for the ground and the v(3) = 1 state have been determined most precisely.     

Infrared focal plane array based on GaAs/AlGaAs quantum-well multilayer structures

An infrared 320 × 256 focal plane array (FPA) for 7.5–10 μm was designed, fabricated and investigated. The photodetector matrix based on GaAs/AlGaAs quantum-well multilayer structures was grown by hydride epitaxy from organometallic compounds. The sensitivity peak was achieved at a wavelength of 9.3 μm and equaled 340 mA/W. The hybrid FPA was fabricated by cold welding indium bumps of the photodetector and silicon multiplexer. The obtained NETD was 29.4 mK at 68 K.     

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.     

Registration of picosecond CO<Subscript>2</Subscript> laser pulses by means of two-stage parametric transformation in nonlinear crystals

A technique for registering the temporal structure of picosecond pulses of CO₂ laser radiation with an energy of 1.5–4.5 μJ at a wavelength of 10.27 μm using two-stage parametric transformation of IR radiation frequency into visible light under pumping of nonlinear crystals by Nd:YAG-laser radiation in a Q-switched mode is described. A GaSe nonlinear crystal was used at the first stage of transformation (10.27 μm + 1.064 μm → 0.964 μm). Radiation was further transformed (1.064 μm + 0.960 μm → 0.506 μm) by using the same pumping in an α-HIO₃ nonlinear crystal. For the first time, no additional optical elements were present between the stages of the frequency transformer in the proposed optical scheme. The transformed radiation was registered with a Hamamatsu Temporal Disperser C1587 streak camera in a region of the photocathode maximum spectral sensitivity of ～0.5 μm with a temporal resolution of up to 2 ps. The minimum recorded pulse duration of the CO₂ laser was ～45 ps.     

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。     

掺铒单模光纤飞秒脉冲激光器和放大器

为了获得一种被动锁模掺铒光纤振荡器及功率放大器,数值模拟出超短脉冲在光纤中的传输和演化过程,并基于此搭建了一种被动锁模掺铒光纤飞秒振荡器及功率放大器。实验获得了中心波长1560 nm、重复频率100 MHz、输出功率30 mW、脉冲宽度85 fs超短脉冲。通过采用PPLN晶体进行倍频,进一步获得了输出功率5 mW,中心波长780 nm的飞秒脉冲。该光纤激光器为全保偏光纤结构,具有体积小巧、可靠性高、稳定性好的特点。     

A setup for measuring the refractive index of a plane-parallel ceramic plate using the optical beam shift method

A setup for measuring the refractive indices of transparent solid samples of optical ceramics using shifts of a beam by a plane-parallel plate in a wavelength range of 400–1200 nm was developed. The minimum cross-sectional sizes of the investigated objects are 5–12 mm, and their thicknesses are 0.3–1.0 mm. A standard sample and a precise system for forming and recording optical signals equipped with a stepping motor with a step discreteness of <1 μm were used to improve the measurement accuracy of the refractive index. The cross-sectional size of the measuring beam is ?1 mm. The accuracy in determining the refractive index is ±0.004.     

An acoustooptical frequency shifter of speckle-containing optical radiation

The fundamental possibility of using the acoustooptic (AO) Bragg diffraction in problems of a frequency shift of a speckle-containing optical field both with and without field filtering for removing fine-grain background has been demonstrated. Two regimes of anisotropic diffraction observed in a TeO₂ uniaxial crystal, when optical rays propagate close to the crystal optical axis (I) and far from it (II), have been studied. Experiments performed on the basis of an acoustooptical cell made of TeO₂, in which speckle-containing optical radiation at a wavelength of 0.63 μm and an average speckle size of 50–70 μm diffracted at a slow traveling acoustic wave at a frequency of 44 MHz, demonstrated the absence of radiation filtering in diffraction regime I and enlargement of the average speckle size to 220–280 μm in regime II resulting from diffraction.     

Compact Amplitude Electro-Optic Modulator Based on Chromophore-Containing Polyimides

This paper describes the development and investigation of compact waveguide electrooptic modulators using original synthesized chromophore-containing polyimides with covalently attached commercial DR-13 dye. Fully polymer planar electro-optic structures with phase-polarization and amplitude modulators in the form of a Mach–Zehnder interferometer were developed and implemented. The characteristics of the developed modulators at a wavelength of 1.3 μm were investigated, and a half-wave voltage of 24 V was achieved with a 1.3 cm length of the active part of the modulator.     

Structuring of titanium using a nanosecond-pulsed Nd:YVO4 laser at 1064 nm

We demonstrate the manufacture of organized microstructures on titanium substrates in an air atmosphere utilizing a pulsed Nd:YVO₄ laser with pulse length of 8 ns and repetition rate of 30 kHz at 1064 nm. The ablation threshold of titanium for irradiation at this wavelength was measured to be in the range of 1.7–1.8 J/cm². For structuring of the metal, we used maximum laser energy fluence above the ablation threshold. This led to the generation of arrays of organized microstructures with average periods ranging from ~40 to ~90 μm. The mechanism for formation of the microstructures is discussed. Formation of such organized structures on titanium could find applications in sensing and biocompatibility.     

晶体已加工表面频率特征对亚表层温度场的影响

为了更加准确地分析表面形貌对KH2PO4(KDP)晶体元件激光损伤和使用性能的影响，通过功率谱密度和连续小波变换对KDP晶体已加工表面存在的实际频率特征进行提取和重构。利用波动光学理论分析经入射波长1.064 μm、功率20 MW/μm2的激光束照射1 ns后，表面频率特征对KDP晶体亚表层光场及温度场的影响。结果表明，当表面频率特征的波长越接近入射光波长1.064 μm，KDP晶体亚表层的光场畸变现象越严重，会造成局部聚焦，温度越高；当波长超过20 μm时，在振幅不变的情况下，最高温度随着波长的增加基本不变。通过切削实验获得的KDP晶体已加工表面上明显存在的波长分别为14 μm、50 μm和140 μm，对KDP晶体亚表层造成的温升分别为56 K、22 K和12 K。当波长相同时，KDP晶体的最高温度与表面频率幅值成线性关系。随着表面频率波长的增加，温度最高点的位置向KDP晶体内部延伸。     

A compact Schwarzschild soft X-ray microscope with a laser-produced plasma source

A compact Schwarzschild soft X-ray microscope using a laser-produced plasma soft X-ray source has been developed. The laser-produced plasma source, which is small but of high brilliance, has made it possible to use the soft X-ray microscope in a small laboratory. The microscope is composed of a Schwarzschild objective and a grazing incidence mirror condenser. Image contrast for biological specimens in soft X-ray regions is investigated briefly. It is possible to observe the fine structures of a thin specimen at a wavelength of 15 nm; at this wavelength high-contrast images of biological specimens have been obtained with a single laser shot of pulse width of 8 ns at a resolution of 0·3 μm. The resolution of the system is limited by the detector.     

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.     

Development of a 6-DoF FBG force–moment sensor for a haptic interface with minimally invasive robotic surgery

A new and precise 6-Degree-of-freedom (DoF) Fiber Bragg grating (FBG) force–moment sensor integrated with a platform frame was proposed for the haptic feedback of loadings at the tip cutting tools of end-effectors of a minimally invasive surgical robot. As the platform deformed during surgery, the attached FBG pretensioned with 2000 μm strain. Strains were calculated by Finite element analyses (FEAs) and related to optical wavelength equations. Experiments integrated with sagacious ways of how to apply forces and moments for the sensor fabricated were conducted to measure the strains and wavelength changes caused in FBGs. Experimental wavelength changes correlated well in 3% to 4% error with the FEA results for all cases. A realistic design of a small 6-DoF FBG force–moment sensor was proposed using the analytic method. Wavelength changes slightly increased as temperature increased in the study of thermal compensation.

     

A promising new wavelength region for three-photon fluorescence microscopy of live cells

We report three-photon laser scanning microscopy (3PLSM) using a bi-directional pumped optical parametric oscillator (OPO) with signal wavelength output at λ= 1500 nm. This novel laser was used to overcome the high optical loss in the infrared spectral region observed in laser scanning microscopes and objective lenses that renders them otherwise difficult to use for imaging. To test our system, we performed 3PLSM auto-fluorescence imaging of live plant cells at λ= 1500 nm, specifically Spirogyra, and compared performance with two-photon excitation (2PLSM) imaging using a femtosecond pulsed Ti:Sapphire laser at λ= 780 nm. Analysis of cell viability based on cytoplasmic organelle streaming and structural changes of cells revealed that at similar peak powers, 2PLSM caused gross cell damage after 5 min but 3PLSM showed little or no interference with cell function after 15 min. The λ= 1500 nm OPO is thus shown to be a practical laser source for live cell imaging.     

可调谐声光调Q CO2激光器

摘要：利用调Q脉冲激光器速率方程计算了自行设计的声光调Q小型CO2激光器的主要技术参数,分析了影响声光调Q CO2激光器输出的主要因素,提出了优化设计的途径和方法,实验结果与理论计算基本一致。经优化设计的激光器脉冲重复频率达1Hz～50kHz, 1kHz时在获得脉宽180ns,峰值功率4062w的激光输出,并利用光栅实现了激光输出波长的连续调谐,波长调谐范围9.2μm～10.8μm,该激光器在激光测距、环境探测及空间通讯等领域具有广泛的用途。     

A tapered parallel plate waveguide for frequency up-conversion of terahertz radiation

A tapered parallel plate waveguide was developed for frequency up-conversion experiments in the terahertz (THz) region by flash ionization. The element at the plasma-source-wave interaction area determines the conversion efficiency. It causes THz pulses to converge to a narrow plate separation, which is smaller than the wavelength. The waveguide exhibited good performance for transmitting p-polarized THz pulses in a 50 μm separation, making it suitable for flash ionization experiments.     

A polarization analyzer of optical radiation on the basis of the light diffraction by sound

A device with an acousto-optic polarization splitter as the basic unit has been developed to record polarization-modulated optical signals. It spatially separates input radiation into two beams with mutually orthogonal polarizations. The difference of their intensities serves as the measure of changes in the input-radiation polarization plane. Experiments were performed using a TeO₂ single crystal as the acousto-optic splitter and showed the high efficiency of splitting optical radiation with a wavelength of 0.63 μm in the range of acoustic frequencies of 10–26 MHz.