飞秒激光在双折射微结构光纤中模式控制的四波混频效应的实验研究

利用飞秒激光脉冲在长度为10cm，包层具有大空气比的双折射微结构光纤中通过高阶模相位 匹配的四波混频获得了波长可调谐的反斯托克斯波.实验中脉冲宽度为35fs，中心波长820nm ，单脉冲能量4nJ的飞秒激光脉冲耦合到长轴直径为5μm，短轴为46μm的双折射微结构光 纤中.在高阶模传输情况下，通过调制耦合光的偏振方向，获得了具有不同中心波长的反斯 托克斯波.通过对比分析，讨论了输入光的偏振态对双折射微结构光纤中高阶模式下四波混 频效应的影响情况.理论计算分析很好的解释了实验结果. 关键词： 微结构光纤 飞秒脉冲激光 四波混频     

Generation of femtosecond anti-stokes pulses through phase-matched parametric four-wave mixing in a photonic crystal fiber

Phase-matched parametric four-wave mixing in higher-order guided modes of a photonic crystal fiber is shown to result in an efficient decay of 40-fs 800-nm Ti:sapphire laser pump pulses into an anti-Stokes signal with a central wavelength around 590-600 nm and a Stokes signal centered at 1.25 microm. The photonic crystal fiber is designed in such a way as to minimize the group-velocity dispersion at the pump wavelength, phase match the parametric four-wave-mixing process, and reduce the group delay between the pump and the anti-Stokes pulses. The duration of the anti-Stokes pulse under these conditions, as shown by cross-correlation frequency-resolved optical gating measurements, is less than 200 fs.     

Anti-Stokes Frequency Shift and Evolution in Polarization-Maintaining Photonic Crystal Fiber with Two-Zero Dispersion Wavelengths

Using the tunable pump pulses with about lOO fs pulse duration and 1064 nm central wavelength; the polarization-, wavelength- and power-dependent anti-Stokes lines are generated and modulated simultaneously in a polarization-maintaining photonie crystal fiber （PM-PCF） with two zero-dispersion wavelengths. By accurately controlling the polarization directions, the wavelength and the power of the pump pulse in the fiber anomalous region close to the second zero-dispersion wavelength of the PM-PCF, the output anti-Stokes pulse spectra can be tuned between 563 nm and 603 nm, which is in good agreement with the theoretical simulation. The color conversion of the mode image from yellow to orange is also observed with the different polarization pump pulses. These results can be attributed to the combined interaction between the fiber birefringence （including linear- and nonlinear- birefringence） and dispersion, and are attributed to phase-matching parametric four-wave mixing.     

双折射色散阶跃光纤中拉曼效应和参量放大增益谱

在拉曼效应和参量放大共同作用下,当激光脉冲在双折射光纤中传输时,根据所遵循的耦合模方程,通过引入平行拉曼增益的洛伦兹模型,导出了当输入泵浦波偏振方向同双折射轴成45°角时,在双折射色散阶跃光纤中拉曼效应和参量放大共同作用所导致的增益,讨论并分析了在不同色散区增益谱随相关参量的变化。结果表明:由于拉曼效应、参量放大、双折射和色散的相互作用,导致增益谱的斯托克斯波与反斯托克斯波彼此不对称;在反常色散区,产生的增益以反斯托克斯波为主,在正常色散区则以斯托克斯波为主;当表征距离的级数m发生变化时,增益谱也随之发生变化,可以利用色散阶跃光纤在适当的级数m位置提取T频率脉冲。     

光子晶体光纤中高效的反斯托克斯信号产生

基于四波混频的反斯托克斯变换, 被广泛应用于短波辐射高分辨率成像以及直接激发分子的电子跃迁等方面. 为了实现更加高效的反斯托克斯变换, 利用中心波长为810 nm脉冲宽度为120 fs的钛蓝宝石(Ti: sapphire)飞秒激光器作为抽运光源, 在长度为0.5 m和3 m的光子晶体光纤中分别实现了高阶模和基膜的简并四波混频. 实验中, 采用的光子晶体光纤的零色散波长在820 nm附近. 在基模相位匹配条件下, 在560 nm附近实现了高效地反斯托克斯信号的产生, 反斯托克斯信号与残余抽运信号的最大功率比为33:1; 反斯托克斯信号和斯托克斯信号的最大功率比25:1; 反斯托克斯信号最大功率转换效率P_a/P_p0为34%. 抽运波长从790 nm逐渐增加到810 nm过程中, 在长为3 m的光子晶体光纤中相位从不匹配状态转化为高阶模匹配状态后, 再转化为基模匹配状态. 通过实验研究得出了相位匹配程度随抽运功率、波长和光纤长度的变化规律, 同时分析了造成理论计算与实验结果存在差异的主要因素. 本文为研究在光子晶体光纤基模中实现相位匹配和产生高效反斯托克斯信号提供了理论和实验依据.     

Polarization-dependent two-color dispersion wave generation and evolution in polarization-maintaining photonic crystal fiber

We investigate the effect of the polarization state of the input pulses on the visible emissions in the anomalous dispersion region of polarization-maintaining photonic crystal fiber (PM-PCF), by using ～100 fs pump pulses whose central wavelength (1064 nm) is close to the second zero dispersion wavelength (1100 nm) of the fiber, where the soliton fission mechanisms play an important role. The experimental results show that the phase-matching two-color dispersive wave emission, one at 582 nm and the other at 600 nm, is polarization-dependent and frequency shift results from the different dispersion characteristics along the two orthogonal principal axes of PM-PCF. Furthermore, it is observed for the first time that the variation of the linear input polarization angles in 45° region almost has no influence on the output spectral profiles, and the break variation of the output spectrum exists when the angle between the polarization of the linear incident pulse and the fast-axis or the slow-axis of PM-PCF is 45°, which are attributed to the coupling between the two polarization modes in high birefringent PM-PCF.     

Frequency conversion of subnanojoule femtosecond pulses in microstructure fibers

We experimentally demonstrate highly efficient multiplex frequency conversion of unamplified subnanojoule femtosecond pulses of Ti:sapphire laser radiation in fused silica microstructure fibers. Nonlinear optical spectral transformation of femtosecond pulses in an array of fused silica threadlike channels in these microstructure fibers results in the generation of isolated anti-Stokes spectral components within the wavelength range of 400–500 nm. An efficiency of frequency conversion of about 20% is achieved for 800-nm pump pulses with an energy of 0.7 nJ and a pulse duration of 70 fs.     

Frequency-shifted megawatt soliton output of a hollow photonic-crystal fiber for time-resolved coherent anti-Stokes Raman scattering microspectroscopy

Hollow-core photonic-crystal fibers (PCFs) provide soliton delivery and frequency shifting of 2.8 MW femtosecond pulses with an input central wavelength of 618 nm. The frequency-shifted megawatt soliton output of the hollow PCF is used as a high-peak-power Stokes field for coherent anti-Stokes Raman scattering (CARS) microspectroscopy, providing a dynamic range of nearly four decades for anti-Stokes signal detection, thus enabling time-resolved CARS studies of ultrafast relaxation processes on time scales from tens of femtoseconds up to tens of picoseconds.     

非均匀微结构光纤中双折射现象的研究

报道了利用飞秒脉冲激光与非均匀微结构光纤相互作用中产生超连续光谱后在非均匀微结构光纤传输中双折射拍频现象的研究.利用35?fs的飞秒激光脉冲在高双折射微结构光纤中的传输过程中直接观察到了拍频现象.并利用有限元方法对该光纤进行了模拟计算分析，计算得出在600?nm处拍频长度为毫米量级.所得结果与实验一致. 关键词： 双折射效应 微结构光纤 超连续光谱 有限元法     

超短光脉冲在双折射光子晶体光纤中产生超连续谱的偏振特性数值分析

采用矢量耦合非线性薛定谔方程描述了超短光脉冲在双折射光子晶体光纤中的传输过程,并利用分步傅里叶方法求解该方程。数值模拟了中心波长为1550nm的超短光脉冲在不同色散参量的双折射光子晶体光纤中超连续谱的产生及其偏振特性。分析了光纤在不同色散区时,高阶色散和非线性效应对超连续谱及其偏振态的影响。结果表明,当超短光脉冲波长位于近光纤零色散点的反常色散区时,比其在光纤正常色散区和远离光纤零色散点的反常色散区更容易产生宽且平坦的超连续谱,所得到的光谱显示出了复杂的偏振态特性。     

Auto-correction method for differential attenuation in a fiber-optic distributed-temperature sensor

A novel method to improve the accuracy of fiber-optic distributed-temperature measurements derived from Raman backscatterings is presented. This method utilizes two light sources with different wavelengths, such that the wavelength of the primary source's return anti-Stokes component overlaps with the incident wavelength of the secondary light source to cancel out the nonidentical attenuations generated by the wavelength differences between Stokes and anti-Stokes. The concept is successfully verified by the experimental results obtained from several sample fibers. The correction can be made automatically and continuously without any interruptions during the whole measurement periods.     

双折射光纤中拉曼效应对参量放大增益谱的影响

根据激光脉冲在光纤中传输时, 所满足的波动方程, 导出了拉曼效应和参量放大共同作用下, 在双折射光纤中所遵循的耦合模方程, 并引入平行拉曼增益的洛伦兹模型, 给出了输入抽运波偏振方向同双折射轴成45^o 时, 拉曼效应和参量放大共同作用所导致的增益. 讨论并分析了拉曼效应在不同色散区对参量放大增益谱的影响. 结果表明, 在考虑拉曼效应后, 使得参量放大斯托克斯波与反斯托克斯波增益谱彼此不对称; 在反常色散区, 产生的增益以反斯托克斯波为主, 正常色散区则以斯托克斯波为主.     

Soliton interaction in birefringent optical fibers: Effects of nonlinear gain devices

This paper presents the influences of polarization mode dispersion (PMD) on the performance of soliton transmission system in birefringent fibers. Dispersive waves generated in single mode fibers due to PMD degrade the soliton transmission system in two aspects. First, solitons continuously lose their energy, thus cause enhancement in pulse width. Second, the dispersive waves interact with neighboring pulses and cause distortion in a sequence of pulses. Both these effects reduce the effective bit-rate and degrade the performance of high-speed optical transmission systems. Optical fibers with large group velocity dispersion (GVD) have less dispersive waves and are relatively robust to pulse broadening, but it enhances the interaction between the adjacent pulses. In this paper, we analyzed these effects of PMD on soliton propagation in birefringent fibers and introduced nonlinear gain devices with perturbation terms proportional to second and fourth power of amplitudes to reduce these effects. We proposed Symmetric Split-Step Fourier Method to solve the coupled nonlinear Schrödinger equations (CNLSE); which yields better results over the existing Split-Step Fourier Method.     

Broadband Terahertz Wave Generation from Monolayer Graphene Driven by Few-Cycle Laser Pulse

We theoretically investigate the characteristics of terahertz(THz) radiation from monolayer graphene exposed to normal incident few-cycle laser pulses, by numerically solving the extended semiconductor Bloch equations. Our simulations show that the THz spectra in low frequency regions are highly dependent on the carrier envelope phase(CEP) of driving laser pulses. Using an optimal CEP of few-cycle laser pulses, we can obtain broadband strong THz waves, due to the symmetry breaking of the laser-graphene system. Our results also show that the strength of the THz spectra depend on both the intensity and central wavelength of the laser pulses. The intensity dependence of the THz wave can be described by the excitation rate of graphene, while wavelength dependence can be traced back to the band velocity and the population of graphene. We find that a near single-cycle THz pulse can be obtained from graphene driven by a mid-infrared laser pulse.     

Femtosecond spectroscopy of coherent anti-Stokes Raman scattering with frequency-tunable chirped pulses generated in a microstructure fiber

A new scheme of chirped-pulse femtosecond coherent anti-Stokes Raman scattering spectroscopy is proposed and experimentally implemented. A theory of this modification of coherent nonlinear spectroscopy is developed. We use this approach to show that a linear time-frequency mapping defined by linearly chirped pulses allows the spectra of nonlinear response of a medium to be measured by varying the delay time between the pump pulses. Microstructure fibers with a special dispersion profile are at the heart of the experimental implementation of this technique. Such fibers are ideally suited for the generation of frequency-tunable pulses with a smooth envelope and a controlled chirp. We present the results of experimental characterization of the envelope, spectrum, and chirp of anti-Stokes pulses generated in microstructure fibers by femtosecond Cr:forsterite-laser pulses. These frequency-tunable anti-Stokes pulses produced and shaped in microstructure fibers are then employed for coherent anti-Stokes Raman scattering spectroscopy of toluene solution.     

Generation of polarization-shaped ultraviolet femtosecond pulses

We experimentally demonstrate the generation and characterization of polarization-shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 400 nm. Near-infrared laser pulses are first polarization shaped and then frequency doubled in an interferometrically stable setup that employs two perpendicularly oriented nonlinear crystals. A new pulse shaper design involving volume phase holographic gratings reduces losses and hence leads to an increase in pulse energy.     

双折射光纤中三阶色散对孤子脉冲传输的影响

研究了孤子脉冲在双折射光纤零色散点附近的传输特性,应用数值分析方法,讨论了在双折射光纤中,三阶色散对孤子脉冲传输的影响。在输入脉冲两个偏振分量强度相等的情况下,分析发现孤子捕获条件没有变化,但是,脉冲一个偏振分量迅速展宽,另一偏振分量展宽变慢。同时,脉冲的形状、光谱在两个偏振分量中都呈现出不对称性。在一定的条件下,孤子的辐射波主要集中在一个偏振分量中。     

Influence of Polarization and Pulse Shape of Femtosecond Initial Laser Pulses on Spectral Broadening in Microstructure Fibers

We theoretically studied the influence of initial parameters of laser pulses, such as polarization, pulse shape and frequency chirp, on the broadening of spectrum during pulse propagation through microstructure fibers (MSFs). We utilized two coupled-mode equations based on the nonlinear Schrödinger equation using an intermediate-broadening model for a Raman response function, and the dispersion coefficients from 2nd to 7th orders for the slow and fast axes, respectively, of highly birefringent MSFs.     

Multi-wavelength Brillouin fiber laser using dual-cavity configuration

A simple technique for achieving multi-wavelength tunable multi-wavelength Brillouin fiber laser (BFL) based on dual-pass configuration is demonstrated. The BFL uses a piece of 10 km long non-zero dispersion shifted fiber (NZ-DSF) as a Brillouin gain medium to obtain odd-order Brillouin Stokes waves as an output with the line spacing of 0.16 nm (20 GHz) between each two consecutive waves by employing even-order Brillouin Stokes to improve output performance of the laser. With a BP of 15.3 dBm, at least 15 odd-order Brillouin Stokes and anti-Stokes lines are generated. The laser is room temperature stable, tunable over 50 nm wavelength range and has an optical signal to noise ratio of more than 30 dB at 1560 nm region. This Brillouin fiber laser can operate at any wavelength depending on the Brillouin pump (BP) wavelength used.     

A novel polarization splitter based on dual-core hybrid photonic crystal fibers

Highly birefringent dual-core photonic crystal fibers (PCFs) can be used as a polarization splitter because the orthogonal polarization modes with dissimilar coupling lengths are easily separated from each other. Different from the traditional methods achieving high birefringence, a new highly birefringent hybrid PCF that guides light by both index guiding and bandgap guiding is proposed. Firstly, a novel polarization splitter based on this kind of dual-core hybrid PCF is designed. The transmission modes, coupling lengths for the two orthogonal polarizations and performance of the proposed polarization splitter are investigated and numerically analyzed. The results demonstrate that it is possible to obtain a 4.72-mm-long polarization splitter. The splitting ratio is better than −20 dB in a large wavelength range of 1.53-1.72 μm. Its bandwidth is about 190 nm.