High peak power operation and harmonic generation of a single-polarization, Yb-doped photonic crystal fiber amplifier

We report on the use of a single-polarization, 41 μm core-diameter, intrinsically single-mode photonic crystal fiber (PCF) to obtain high peak power (up to 800 kW), 1 ns-duration pulses in a 100:1 linearly polarized, intrinsically single-mode (M²  1.2) output. By transmitting the PCF output through nonlinear crystals, we also obtained efficient second, third, and fourth harmonic generation resulting in peak power >400 kW in the visible (green, 531 nm) and 200 kW in the UV (265.5 nm). To our knowledge these results represent the highest peak power obtained in a linearly polarized output from a fiber and the highest peak power in the visible and UV obtained through harmonic generation of the direct fiber output.     

Refractive-index dispersion measurement of bulk optical materials using a fiber raman laser widely tunable in the visible and near-infrared

We propose a simple, highly sensitive fiber-optic autocollimation method for refractive-index dispersion measurement of solid-state and liquid bulk optical materials using a double-pass fiber Raman laser with Littrow-prism-tuned emission. The optical fiber is a key element of the scheme and serves simultaneously as a point laser source for the test, as a highly sensitive point receiver (or spatial filter) of the autocollimation backreflectance signal and as a medium for nonlinear frequency conversion and generation of a broadband continuum spectrum. When the Raman medium is a graded-index multimode fiber with powerful pumping (over 100 kW) using the second harmonic of a Q-switched Nd:YAG laser (λ_p=532nm), we obtain widely tunable (0.54-1.01 μm) generation in both the visible and near-IR ranges. The results obtained in the refractive-index dispersion measurements are fitted to the Sellmeier dispersion equation and the standard deviation of the experimental data from the analytical curve does not exceed 5x10^-5.     

Generation of green light in a thermally poled silica fiber by quasi-phase-matched second harmonic generation

Thermal poling is an attractive method to induce a nonlinear coefficient in silica materials. We have investigated potential silica materials with respect to their suitability for thermal poling and analyzed the achievable depletion thickness in the poling process. Based on this investigation, a fiber was designed and prepared for second harmonic generation. The generation of light in the visible range at a wavelength of 532 nm was successfully demonstrated with a poled fiber and with quasi-phase matching using a pump wavelength of 1064 nm.     

Internal resonantly enhanced frequency doubling of continuous-wave fiber lasers

In this Letter we describe a simple method for frequency doubling in high-power CW fiber lasers that offers the prospect of very high conversion efficiency and high power in the visible wavelength regime. Our approach is based on second harmonic generation in an enhancement resonator within the fiber laser cavity and does not require active cavity length stabilization. This technique has been applied to a cladding-pumped Yb-doped fiber laser to generate 19 W of linearly polarized CW green output at 540 nm, with excellent temporal stability for 90 W of absorbed diode pump power at 975 nm. The prospects for further improvement in performance with respect to conversion efficiency and output power are considered.     

Generation of optical waveforms in 1.3-μm SOA-based fiber lasers

Passive optical waveform generation is obtained in fiber lasers using a 1.3-μm semiconductor optical amplifier (SOA) as the gain medium. Various waveforms, including square wave, staircase wave, triangular wave, pulse, and dark pulse are generated in SOA-based fiber lasers by adjusting intracavity polarization controllers. The passive waveform generation might be attributed to the SOA gain dynamics and the enhanced nonlinear interaction at the 1.3-μm zero dispersion wavelength of traditional single-mode fiber (SMF), as well as the interference effect between the two sub-cavities of fiber laser. With figure-8 cavity configuration, 1250th-order harmonic pulses have been successfully demonstrated. We have also obtained a free-running SOA-based fiber laser with 3-dB spectral width of 16 nm, and the center wavelength can be tuned over 45 nm range.     

Efficient second-harmonic generation of continuous-wave Yb fiber lasers coupled with an external resonant cavity

Efficient second-harmonic generation of continuous-wave Yb fiber lasers is reported. A simple bow-tie external resonant cavity incorporating a type I LBO nonlinear optical crystal was employed for second harmonic frequency conversion of a multi-longitudinal-mode Yb fiber laser. It is shown that strong coupling was formed between the Yb fiber laser and the external cavity and, as a result, the laser produced 9.1 W of green output at 535 nm for 43?W of absorbed diode pump power at 975 nm corresponding to an optical conversion efficiency of 21?% with respect to absorbed diode pump power. The prospects for further improvement are discussed.     

动作电位编码光学二次谐波快速检测神经纤维膜电位动力学

神经纤维膜电位动力学特性与神经信息的传导和编码密切关联。目前,传统电生理测量方法无法同时对膜上多个部位的动作电位进行快速检测。利用非线性光学二次谐波方法,通过数学建模,研究了髓鞘神经纤维动作电位编码的二次谐波信号特征及其检测灵敏性,并将其用于分析由压力引起的神经纤维形态改变,包括轴突直径和髓鞘厚度的改变,实现神经传导信息变化的快速检测。发现神经纤维膜电位的变化可以通过光学二次谐波信号的特征来表征。当神经纤维严重脱鞘时,其上的动作电位会产生明显的传输阻滞。结果表明光学二次谐波技术有望成为神经纤维损伤状态快速检测的一种有力手段。     

Broadband second harmonic generation of an optical frequency comb produced by four-wave mixing in highly nonlinear fibers

We demonstrate broadband second harmonic generation of low-energy pulses produced by injecting two single-frequency lasers into a highly nonlinear fiber. Full nonlinear conversion of the corresponding spectra, consisting of broadband (∼200 nm) optical frequency combs at ∼1580 nm, were obtained by using conventional birefringence phase-matching in two BIBO crystals (2-mm and 100-μm long) with a normal incidence configuration. The crystals were not tilted and the pulses were not compressed. This broadband conversion results from the large phase-matching bandwidth of the nonlinear BIBO crystals at ∼1550 nm, but also seems to be a consequence of a fundamental comb with small spectral phase variation.     

Highly efficient second-harmonic and sum-frequency generation of nanosecond pulses in a cascaded erbium-doped fiber:periodically poled lithium niobate source

By combining erbium-doped fiber sources based on a large mode-area design and periodically poled lithium niobate, we have obtained single-pass conversion efficiencies of as much as 83% (energy efficiency) for second-harmonic generation into the near IR (768 nm) and of 34% for sum-frequency generation into the green (512 nm) for nanosecond pulses, using first-order quasi-phase matching. Pulse energies in excess of 80microJ of second harmonic have been obtained from systems pumped by a single laser diode.     

参变过程中的群速延迟对晶体匹配长度的影响

基于能量守恒和三波耦合波方程,建立了超短脉冲在参变过程中二次谐波产生时的I类和II类相位匹配条件、基波与谐波之间的群速延迟时间、以及群速失配对晶体长度限制的理论基础。以负单轴非线性光学晶体CsLiB6O10为例,分析和数值计算了超短脉冲宽度为100fs时,谐波的群速匹配长度随基波波长变化的规律。研究结果表明在I类相位匹配条件下,基波波长为642nm时,群速延迟最小,相应的群速匹配晶体长度最长为19.1mm;在II类相位匹配条件下,基波波长为767nm,群速延迟最小,群速匹配长度最长为0.89mm。     

基于预啁啾控制的极宽带光参量放大

在以飞秒钛宝石放大系统的倍频光为抽运光和超连续白光为信号光的光参量放大中，针对抽运光的宽带特点，分析了一种新的极宽带相位匹配方法.结果表明，10nm的抽运光带宽可得到近400nm的相位匹配带宽，若抽运光带宽达到20nm,相位匹配带宽就能达到近600nm.零色散波长为800nm的光子晶体光纤产生的超连续谱经光纤传输后为二次啁啾，宽带抽运光经棱镜对展宽具有线性啁啾，满足了极宽带相位匹配方法所需要的光谱分布.理论计算了对输入脉冲进行预啁啾控制应选择的光纤长度和棱镜对在光路中的插入量，为实现极宽带光参量放大提供了     

基于预啁啾控制的极宽带光参量放大

在以飞秒钛宝石放大系统的倍频光为抽运光和超连续白光为信号光的光参量放大中,针对抽运光的宽带特点,分析了一种新的极宽带相位匹配方法.结果表明,10nm的抽运光带宽可得到近400nm的相位匹配带宽,若抽运光带宽达到20nm,相位匹配带宽就能达到近600nm.零色散波长为800nm的光子晶体光纤产生的超连续谱经光纤传输后为二次啁啾,宽带抽运光经棱镜对展宽具有线性啁啾,满足了极宽带相位匹配方法所需要的光谱分布.理论计算了对输入脉冲进行预啁啾控制应选择的光纤长度和棱镜对在光路中的插入量,为实现极宽带光参量放大提供了 关键词： 啁啾 极宽带相位匹配 光参量放大 光子晶体光纤     

Experimental study of actively mode-locked semiconductor laser with grating external cavity

In this paper,complete mode-locking optical pulses have been obtained froman actively mode-locked GaAs/GaAlAs semiconductor laser with grating external cavity.Theshortest optical pulse is 7.3 ps measured by second harmonic generation(SHG)autocorrelator.The repetition rate is 995.12 MHz and the central wavelength is 798.4 nm.The effect ofmodulation frequency,modulation current,and bias current on the optical pulses width areinvestigated.     

Generation of femtosecond laser pulses at 396 nm in K_3B_6O_(10)Cl crystal

K_3B_6O_(10)Cl(KBOC), a new nonlinear optical crystal, shows potential advantages for the generation of deep ultraviolet(UV) light compared with other borate crystals. In this paper we study for the first time the second harmonic generation(SHG) of a femtosecond Ti:sapphire amplifier with this crystal. Laser power is obtained to be as high as 220 m W at the central wavelength of 396 nm with a 1-mm-long crystal, and the maximum SHG conversion efficiency reaches 39.3%. The typical pulse duration is 83 fs. The results show that second harmonic(SH) conversion efficiency has the room to be further improved and that the new nonlinear crystal is very suited to generate the high efficiency deep ultraviolet laser radiation below 266 nm.     

Surface plasmon enhanced ultra-low threshold second harmonic generation in periodically poled whispering gallery resonator

The surface plasmon enhanced ultra-low threshold second harmonic generation is observed, designed and simulated in whispering gallery resonator made of MgO doped periodically poled LiNbO₃. Here the electric field associated with incident optical radiation of picowatt level is amplified to milliwatt level through surface plasmon resonance in Kretschmann geometry which is formed by a BK₇ prism plane, 29 nm thin gold layer and 20 nm thin GaAs layer. This enhanced electric field then coupled to a whispering gallery resonator, which facilitated the generation of second harmonic for an incident laser radiation of picowatt level. In this proposed configuration with an incident optical power of 94.6 pW, generated second harmonic through whispering gallery resonator is found to be 14.6 mW.     

飞秒BBO光参量放大中闲频光二次谐波的产生

由于相位匹配条件和非线性晶体透光范围的限制，400nm蓝光抽运的飞秒β-BaB₂O₄（BBO）光参量放大（OPA）输出的参量光调谐范围有限，很难得到波长小于460nm的蓝光和近紫外光.实验采用1kHz钛宝石九通啁啾脉冲放大器的倍频蓝光作抽运光，超连续白光 作种子光，在Ⅰ类非共线相位匹配条件下，利用宽带的飞秒BBO OPA，在一定的实验参数下 获得了530—810nm放大的信号光，以及810nm—17μm波段范围的闲频光.与此同时 ，还获得了410—700nm连续可调的闲频光的二次谐波，其与闲频光层叠分布，单脉冲能量 为26μJ，转换效率大于5%.仅利用单块晶体的飞秒BBO OPA就可以获得410—810nm连 续可调的飞秒脉冲输出，从而为更多研究和应用的需要提供了重要的光源.对飞秒光参量放 大中闲频光二次谐波产生的条件也进行了理论分析. 关键词： 二次谐波 闲频光 非共线相位匹配 飞秒光参量放大     

Influence of transition metal doping on tris(thiourea)zinc(II) sulphate crystals

Optical, thermal and dielectric properties of Ni(II)-doped tris(thiourea)zinc(II) sulphate single crystals grown by slow evaporation solution growth technique has been investigated. The lattice parameters of the as-grown crystals were obtained by single crystal X-ray diffraction analysis. The modes of vibrations of different functional groups present were identified by FT-IR studies. The surface morphological changes are observed in the doped specimen. The structure and the crystallinity of the material were confirmed by powder X-ray diffraction analysis. The UV–vis optical absorption spectrum shows the lower optical cut-off at ∼284 nm and the crystals are transparent in the entire visible region. The relative second harmonic generation efficiency measurements reveal the enhancement of efficiency by doping with small quantities of Ni(II).     

普通单模石英光纤中频率上转换过程的研究

介绍了在普通通讯用单模ＧｅＯ２／ＳｉＯ２石英光纤中从近红外到可见光的频率上转换过程。利用１．３１３μｍ的调Ｑ锁模Ｎｄ：ＹＬＦ激光脉冲泵浦单模石英光纤，产生了５８８ｎｍ的黄光，峰值功率达２０Ｗ，对应０．０９％的转换效率。     

A new method of assessing the efficiency of second harmonic generation by small samples of organic crystals

The intensity of optical second harmonic generation was measured from pressed KCl disks containing low concentrations (1%) of non-linear optical (NLO) materials. Comparison of the signals at 532 nm, with the known second order NLO coefficients exhibited a monotonic relationship. An estimate was made of the effect of light scattering at 532 nm, on the results.     

Enhanced thermal stability of optical nonlinearity for anilino–silane derived inorganic–organic hybrid thin films

A series of chromophore-bonded inorganic–organic hybrid films were successfully prepared via a sol–gel process using the alkoxysilane dye (ICTES-DR1), anilinomethyltriethoxysilane (AMTES) and tetraethoxysilane (TEOS) as precursors. Second-order NLO properties and their thermal stability of the hybrid films were investigated by in situ second harmonic generation (SHG) measurement. The poled films exhibit large second harmonic coefficient (d₃₃) in the range of 32–41 pm/V at 1064 nm. The aniline group of the AMTES was found to be effective for improving the thermal stability of optical nonlinearity due to the formation of hydrogen bonds between the carbonyl and aniline group, which was confirmed by virtue of FTIR and UV–visible spectra.