Compensation of phase-to-amplitude modulation conversion in a complete frequency conversion system with an all-fiber system

We report on an experimental demonstration of linear precompensation of nonlinear phase and amplitude transfer functions. We show the effective compensation with a linear all-fiber system of phase-to-amplitude modulation conversion due to a complete frequency conversion system including plane gratings and a nonlinear crystal.     

Calculation, characterization, and application of the time shift function in wavelength-division-multiplexed return-to-zero systems

We calculate the time shift function for collisions of pairs of pulses in different channels in a prototypical return-to-zero wavelength-division-multiplexed system with dispersion management and precompensation and postcompensation. Once the time shift function is known, the impairments that are due to collision-induced timing jitter can be rapidly determined. We characterize the shape of this function and determine how it scales with the initial pulse separation in time and with channel separation in wavelength. Finally, we apply it to the calculation of the worst-case time shift.     

Symmetrical dispersion compensation for standard monomode-fiber-based communication systems with large amplifier spacing

Optical 10-Gbit / s return-to-zero pulse transmission in cascaded communication systems using dispersion compensation of the standard monomode fiber with large amplifier spacing is examined. It is shown that pulse distortions that are due to Kerr nonlinearity are significantly diminished by symmetrical ordering of the compensation sections when the total number of precompensation and postcompensation sections is equal. Repositioning of these sections is not critical.     

Characterization of transition shifts in perpendicular magnetic recording using 3D FEM

Transition shift can create serious timing problems for high density recording environments. However, it is possible to use techniques such as write precompensation to offset the timing error introduced if the magnitude and the behaviour of the transition shift are properly understood. This paper introduces a 3D finite element method (FEM) to analyse the transition shift in perpendicular magnetic recording (PMR) quantitatively, allowing the combinational effects of the transition shifts from non-linear transition shift (NLTS), HTS, and neighbourhood-induced transition shift (NITS) to be examined. The consideration of all contributing factors to the final written transition position is important in determining the precompensation scheme of magnetic data recording, and 3D FEM can facilitate such quantitative analysis.     

预啁啾10GHz、38km色散管理孤子的传输实验

本文利用5GHz增益开关分布反馈(DFB)激光器结合1×2×1时分复用耦合器获得了10GHz带有预啁啾的孤子脉冲串,利用此脉冲串进行了10GHz、38km的色散管理孤子的传输实验.实验结果表明,对于小功率的色散管理(DM)孤子,尽管有较大的初始啁啾,仍然可以实现孤子的传输,这反映了DM孤子比经典孤子的优越性.对具有负初始啁啾DM孤子传输,前补偿方案要优于后补偿方案.     

Femtosecond-pulse sequence compression by Gires-Tournois interferometers

We demonstrate the compression of femtosecond-pulse sequences by phase-modulating resonators, such as Gires-Tournois interferometers. The experiments are based on the precompensation of the complex phase response of the resonator by a high-resolution liquid-crystal pulse shaper. This method can be utilized for lowering peak intensities at critical points in optical setups, as well as for encryption or decryption of ultra-short pulses.     

Adaptive dispersion compensation for remote fiber delivery of near-infrared femtosecond pulses

We report on remote delivery of 25-pJ broadband near-infrared femtosecond light pulses from a Ti:sapphire laser through 150 m of single-mode optical fiber. Pulse distortion caused by dispersion is overcome with precompensation by adaptive pulse shaping techniques, while nonlinearities are mitigated by use of an SF10 glass rod for the final stage of pulse compression. A near-transform-limited pulse duration of 130 fs was measured after the final compression.     

Characterization of sub-10-fs pulse focusing with high-numerical-aperture microscope objectives

Dispersion precompensation with a prism sequence and a third-order dispersion mirror resulted in negligible broadening of sub-10-fs pulses at subwavelength spot sizes when the pulses were focused with microscope objectives and moderate apertures. At larger apertures, lens chromaticity and spherical aberration led to an effective pulse broadening of up to 1.3x , depending on the aperture size and the detector position. The data suggest that intensities exceeding 10(14) W/cm(2) can be produced directly from femtosecond pulse oscillators.     

Inscription high-fringe visibility Fabry-Perot etalon in fiber with a high numerical aperture objective and femtosecond laser

In this paper, a femtosecond laser micromachining system to fabricate a microfiber Fabry-Perot (F-P) cavity, and a real-time system to monitor interferometric intensity of F-P cavity have been developed. The relationships of fringe visibility of an F-P cavity on parallelism and surface roughness are analyzed and discussed. In order to eliminate the slope of the cutting section coming from the focusing optics, precompensation and double machining are introduced. The fringe visibility of a F-P interferometric sensor which exceeds 15 dB has been realized.     

Hollow fiber for flexible sub-20-fs pulse delivery

Delivery of sub-20-fs pulses through 80 cm of a chirped photonic crystal fiber with chirped-mirror precompensation is experimentally demonstrated. The pulses out of the fiber are within 15% of the bandwidth limit and exhibit a pulse energy of 1 nJ. Peak powers amount to 50 kW directly out of the fiber. Measurements indicate a good pulse contrast with little satellite content. The combination of a specially designed hollow fiber with chirped mirrors sets a new record for the pulse durations directly delivered through a fiber.     

High quality beam shaping by square soft-edge diaphragm combined with liquid crystal spatial light modulator

<正>The square soft-edge diaphragm with round angle is designed by Matlab,and is sent to a liquid crystal spatial light modulator by the computer.In order to obtain precompensation for the following laser system, local diaphragm transmission can be adjusted by feedback signals of surface-channel charge-coupled device (SCCD).This method can reduce the diffraction effect and realize no modulation,high stability,high homogeneity,and large scale laser beam.Several parameters of soft-edge diaphragms which affect the laser beam quality are studied systematically,and the optimized values are obtained.The method can avoid the serious modulation of hard edges and provide soft-edge diaphragms of different shapes in a fast and convenient way for the large scale laser beam system.     

宽锥角光束对分色合色膜色再现性能的影响

考虑了在CIEL*α*b*3D色空间中以色立体作为综合评价分色合色膜色再现性能的标准，分析了宽锥角光束入射时分色合色膜空间角度特性对图像色再现质量的影响，从优化色立体体积出发提出了用预补偿的方法来优化膜系设计，提高了宽锥角光束入射下膜系的色再现性能。     

光传输中RZ-ODPASK调制格式的SPM效应及其补偿研究

研究了单信道40 Gbps 归零八进制差分相位幅度调制格式（Return-to-zero Octal Phase-amplitude-shift Keying,RZ-ODPASK）光传输系统中,自相位调制(Self-phase Modulation,SPM）效应对于相位支路差分解调性能的影响.理论推导了相邻两个不同幅度的码元由于SPM效应,产生的非线性相移差表达式.提出了一种非线性相移差预补偿方法.仿真结果表明,该方法能够有效降低相位支路解调信号的眼张开代价,提高RZ-ODPASK长距离通信系统的传输性能.     

Engineering the nonlinear phase shift

We treat the nonlinear phase shift response in the weak perturbation limit as a linear digital filter that can be synthesized into the values of its poles and zeros and mapped onto an optical architecture. This procedure results in a significant enhancement in the nonlinear sensitivity with a response that is robust to frequency changes within the filter passband. A precompensation technique can be used to reduce distortions under strongly driven nonlinear operation to achieve a larger phase shift. We also show that nonlinear sensitivity improves with increasing filter group delay and can be increased within constant linear bandwidth by use of higher-order filters.     

超相对论激光和稠密等离子体作用产生阿秒脉冲的优化

利用一维粒子模拟程序研究了超相对论激光脉冲与稠密等离子体相互作用得到的阿秒脉冲.从超相对论近似的角度分析了电子运动行为和高次谐波的产生,发现当等离子体密度一定时,随着无量纲相似参数S的减小,阿秒脉冲的转换效率呈先增大后减小的趋势,因此选择适当的光强就可以得到转换效率较高的阿秒脉冲.当S一定时,随着等离子体密度的增加,阿秒脉冲转换效率有增大的趋势.这说明用适当的光强照射更稠密度的等离子体靶面,可以产生更强的阿秒脉冲.     

Pulse compression without chirp control and frequency detuning by high-order coherent Raman scattering in impulsively excited media

It is shown that phase-locked pulses as short as 3 fs can be generated by coherent scattering in impulsively excited Raman media without the necessity of external phase control. The underlying mechanism, temporal characteristics, spectra, phase relations, physical limitations owing to competition processes, and precompensation of dispersion by the hollow waveguide window are studied analytically and numerically without the use of the slowly varying envelope approximation and with a global approach to dispersion. Additionally, the large frequency shifts in both the Stokes and anti-Stokes directions of as much as half the carrier frequency raise the possibility of generating widely tunable ultrashort pulses.     

Experimental demonstration of coherent beam combining over a 7 km propagation path

We demonstrate coherent combining (phase locking) of seven laser beams emerging from an adaptive fiber-collimator array over a 7 km atmospheric propagation path using a target-in-the-loop (TIL) setting. Adaptive control of the piston and the tip and tilt wavefront phase at each fiber-collimator subaperture resulted in automatic focusing of the combined beam onto an unresolved retroreflector target (corner cube) with precompensation of quasi-static and atmospheric turbulence-induced phase aberrations. Both phase locking (piston) and tip-tilt control were performed by maximizing the target-return optical power using iterative stochastic parallel gradient descent (SPGD) techniques. The performance of TIL coherent beam combining and atmospheric mitigation was significantly increased by using an SPGD control variation that accounts for the round-trip propagation delay (delayed SPGD).     

Rotational energy conversion and thermal evolution of neutron stars

Pulsars are rapidly spinning, strongly magnetized neutron stars. Their electromagnetic dipole radiation is usually assumed to be at the expense of the rotational energy. In this work, we consider a new channel through which rotational energy could be radiated away directly via neutrinos. With this new energy conversion channel, we can improve the chemical heating mechanism that originates in the deviation from β equilibrium due to spin-down compression. The improved chemical and thermal evolution equations with different magnetic field strengths are solved numerically. The results show that the new energy conversion channel could raise the surface temperature of neutron stars, especially for weak field stars at later stages of their evolution. Moreover, our results indicate that the new energy conversion channel induced by the non-equilibrium reaction processes should be taken into account in the study of thermal evolution.     

Poling pattern for efficient frequency doubling of Gaussian beams

The commonly used periodic patterning for quasi phase-matching plane-waves is theoretically adequate for completely depleting and transferring a fundamental wave to its second harmonic. However, when working with Gaussian beams the conversion efficiency of such a design lacks due to the inherent Gouy phase shift and the spatially varying beam profile, yielding roughly 88?% conversion efficiency. In this paper, we study the possibility of adding a linear chirp and Gouy-phase shift compensation to the periodic poling. We demonstrate that this poling pattern enables us to achieve near-optimal frequency doubling efficiency of up to 97?%.     

All-Optical Wavelength Conversion Based on Four-Wave Mixing in Dispersion-Engineered Silicon Nanowaveguides

We demonstrate experimentally all-optical wavelength conversion based on four-wave mixing in dispersion-engineered silicon nanowaveguides with a picosecond pulse pump. We find that the conversion efficiency is significantly limited by nonlinear losses induced by the two-photon absorption and freecarrier absorption. Using a picosecond pulse pump centered at 1,550 nm, we show that the input continuous-wave signals can efficiently be converted into a broadband idler pulse in silicon waveguides with various dimensions. Conversion efficiencies versus signal wavelengths are different for silicon waveguides with different dimensions due to the variation in the phase mismatch; we obtain a conversion efficiency of – 32 dB in silicon nanowaveguides with a length of 5.8 mm. Such on-chip optical wavelength converters can find important potential applications in highly-integrated optical circuits for all-optical ultrafast signal processing.