Detrimental effect of self-phase modulation on the performance of Brillouin distributed fiber sensors

We show that the spectral broadening of the pump pulse through self-phase modulation in a time-domain distributed Brillouin sensor has a considerably detrimental effect in the measurement, especially in the case of long distances and high-resolution pulses. Using 30 ns pump pulses with peak power of 276 mW, self-phase modulation leads to a doubling of the effective gain linewidth after some 20 km, which is equivalent to a contrast loss of 2dB in the measurement. The impact is higher for shorter pulses (higher resolution). The theoretical modeling is fully confirmed by experimental results.     

Reversing intrachannel ghost-pulse generation by midspan self-phase modulation

Intrachannel pulse interactions are the dominating nonlinear effects in modern transmission systems with high modulation speeds. Scaled symmetries have proved to be effective in suppressing amplitude and timing jitter of mark pulses due to nonlinearity but not for ghost-pulse generation into the empty slots. A method of using midspan self-phase modulation to reverse the generation of ghost pulses due to intrachannel four-wave mixing is proposed. Computer simulations demonstrate significant improvement of signal quality by a combination of scaled symmetries and midspan self-phase modulation.     

Theoretical and experimental studies of combined self-phase modulation and stimulated Raman-scattering in single-mode fibres

A theoretical model of pulse propagation through single-mode fibres is presented taking into account the combined action of self-phase modulation, stimulated Raman-scattering and the group velocity mismatch between pump and Stokes pulses. Due to the walk-off of Stokes and laser light an asymmetric pump pulse depletion resulting in considerable reshaping of laser pulses and an asymmetric self-phase modulation spectrum can be calculated and experimentally verified. A maximum laser power is found to be transferable through the fibre.     

Multifrequency parametric infrared Raman generation in KGd(WO(4))(2) crystal with biharmonic ultrashort-pulse pumping

Mutlifrequency parametric Raman generation was carried out in a KGd(WO(4))(2) crystal by use of a dual-wavelength Ti:sapphire laser system. It was found that with femtosecond pump pulses the efficiency of Raman generation is low because of the onset of self-phase modulation. The mechanism for suppression of stimulated Raman scattering by self-phase modulation is discussed. Employing 2-ps-long chirped pulses generated four Stokes and one anti-Stokes component.     

基于频域相位共轭技术的交叉相位调制所致失真的复原

理论分析和讨论了基于频域相位共轭技术的交叉相位调制所致信号失真的复原和补偿机理，数值模拟了在交叉相位调制作用下，高斯脉冲在中距相位共轭光纤系统中的传输演化过程.结果表明，频域相位共轭技术能够抑制交叉相位调制对光纤系统中传输信号的损害，复原其所导致的信号失真，并能够同步补偿群速度色散和自相位调制非线性效应所导致的信号失真.合适的初始脉冲时延和初始脉冲啁啾有利于频域相位共轭技术对交叉相位调制所致信号失真的抑制. 关键词： 频域相位共轭 交叉相位调制 色散 自相位调制     

Generation of photon pairs in dispersion shift fibers through spontaneous four wave mixing: Influence of self-phase modulation

Correlated signal and idler photon pairs with small detuning in the telecom band can be generated through spontaneous four-wave mixing in dispersion shift fibers. However, photons originated from other nonlinear processes in optical fibers, such as Raman scattering and self-phase modulation, may contaminate the photon pairs. It has been proved that photons produced by Raman scattering are the background noise of photon pairs. Here we show that photons induced by self-phase modulation of pump pulses are another origin of background noise. After studying the dependence of self-phase modulation induced photons in signal and idler bands, we demonstrate that the quantum correlation of photon pairs can be degraded by the self-phase modulation effect. The investigations are useful for characterizing and optimizing an all fiber source of photon pairs.     

拉曼增强型自相位调制再生技术

自相位调制可用于信号脉冲的整形,可实现归零码(RZ)信号的再生。以高非线性光纤(HNLF)为介质,在高非线性光纤中实现了拉曼集总放大和信号脉冲的自相位调制。从理论上分析了同向拉曼泵浦增强信号的自相位调制,大大降低了所需的信号光强度。论证了在色散长度(LD)远大于HNLF长度(L)时由自相位调制(SPM)导致的频谱展宽,它与脉冲的啁啾无关,频谱的功率密度与脉冲的强度无关。因此在高非线性光纤之后加入偏移载波频率的带通滤波器可实现归零码信号的再生。集中拉曼放大可增强再生性能。     

基于全光时域分数阶傅里叶变换的光脉冲最小损伤传输新方法

提出了一种基于全光分数阶傅里叶变换(optical fractional fourier transformation,OFRFT)的关于色散和自相位调制对光脉冲传输影响的分析方法,并基于该方法给出了一种新的光脉冲在光纤中传输最小损伤的方法,通过在发射端将输入的高斯光脉冲做OFRFT然后再送入到光纤中去传输,可以提高光脉冲抵抗色散和非线性的能力,仿真发现当色散占主要作用的时候,负阶数的分数阶傅里叶变换可以很大程度上减小光脉冲的展宽;当自相位调制作用占主要的时候,正阶数的分数阶傅里叶变换可以消除自相位调制作 关键词： 光脉冲传输 分数阶傅里叶变换 色散 自相位调制     

Numerical study of femtosecond travelling-wave amplified spontaneous emission

The generation of pulses from amplified spontaneous emission in dye solutions is studied in the case of femtosecond pump pulses. The effects of ultrafast vibrational relaxation, self-phase modulation and optimum synchronization are discussed.     

Multiphoton endoscopy

Despite widespread use of multiphoton fluorescence microscopy, development of endoscopes for nonlinear optical imaging has been stymied by the degradation of ultrashort excitation pulses that occurs within optical fiber as a result of the combined effects of group-velocity dispersion and self-phase modulation. We introduce microendoscopes (350-1000 microm in diameter) based on gradient-index microlenses that effectively eliminate self-phase modulation within the endoscope. Laser-scanning multiphoton fluorescence endoscopy exhibits micrometer-scale resolution. We used multiphoton endoscopes to image fluorescently labeled neurons and dendrites.     

Compensation of self-phase modulation in fiber-based chirped-pulse amplification systems

We demonstrate a simple, all-fiber technique for removing nonlinear phase due to self-phase modulation in fiber-based chirped-pulse amplification (CPA) systems. Using a LiNbO3 electro-optic phase modulator to emulate a negative nonlinear index of refraction, we are able to remove 1.0 pi rad of self-phase modulation acquired by pulses during amplification and eliminate nearly all pulse distortion. Our technique is high speed, removes nonlinear phase on a pulse-to-pulse basis, and can be readily integrated into existing CPA systems.     

Modeling interaction of ultrashort pulses with ENZ materials

In this work, the split-step Fourier method for beam propagation is used to investigate the interaction of ultra-short pulses with epsilon-near-zero materials. The propagation of pulses is governed by the nonlinear Schrödinger equation (NLSE) containing dispersion, gain-bandwidth, self-phase modulation, self-steepening, and absorption parameters. It is found that the intensity profile of the pulse is broadened and the phase of the pulse is shifted by dispersion phenomena. The gain/loss related to the imaginary part of the refractive index causes an increase or decrease in intensity and pulse edge effects. These effects do not favor the steady propagation of the pulse. The self-phase modulation is not noted to appreciably affect the intensity pulse profile. The self-steepening modifies the phase and energy of the pulse during propagation, as well as absorption, which influences the losses by both the linear and nonlinear effects.     

Mechanism of pulse shaping in femtosecond lasers based on the high-order soliton

The mechanism of pulse shaping in Kerr-lens mode-locking Ti : sapphire lasers is investigated. Considering the balanced interaction between self-phase modulation, introduced by Kerr effect in Ti : sapphire, and group-velocity dispersion provided by prism-pair, pulse-splitting reflected in the intensity autocorrelation trace and pulse-compression display that the pulses outputting from different positions in laser cavity have different durations. We have concluded that the solitonlike pulse shaping that results from the competition between intracavity self-phase modulation within the Ti : S and negative group-velocity dispersion play dominant role in pulse evolution in Kerr-lens mode-locking lasers.     

单模光纤中喇曼类孤子脉冲理论研究

本文在考虑群速度色散(GVD)、受激喇曼散射(SRS)、自相位调制(SPM)、交叉相位调制(XPM)及光纤损耗同时作用的条件下,建立了关于单模光纤中喇曼类孤子的理论计算模型。通过模拟计算Gouveia-neto等人的实验,得到了84fs的喇曼类孤子脉冲,与其实验结果基本一致,并给出了喇曼类孤子脉冲形成过程的图象和新见解。 关键词：     

Postnonlinearity compensation with data-driven phase modulators in phase-shift keying transmission

A novel scheme for postnonlinearity compensation is proposed to reduce the phase jitter in phase-shift keying transmission. A phase modulator is used to modulate the phase of the data pulses in front of the receiver. The magnitude of the phase modulation is proportional to the detected pulse intensity, and the sign is opposite to that of the nonlinear phase shift caused by self-phase modulation. Thus, the nonlinear phase noise induced by amplitude fluctuation and self-phase modulation is partially compensated for. We show by numerical simulations that a differential phase-shift keying dispersion-managed soliton system at 10Gbits/s with such postnonlinearity compensation can provide greater than 3dB of improvement in ultralong-haul dense wavelength-division multiplexing transmissions.     

Strong self-phase modulation in planar chalcogenide glass waveguides

Single-mode planar waveguides were fabricated from chalcogenide glass compounds with large Kerr nonlinearities. Strong self-phase modulation of subpicosecond pulses along with low linear and nonlinear absorption losses demonstrates the potential for ultrafast, low-power, all-optical processing applications.     

Frequency-resolved optical gating of femtosecond pulses in the extreme ultraviolet

Femtosecond extreme ultraviolet (XUV) pulses were fully characterized for the first time by using a newly developed cross-correlation frequency-resolved optical gating (FROG) technique in the XUV region. This method utilizes laser-assisted two-photon ionization as a nonlinear optical process. Near-infrared pulses characterized by FROG were used as a reference. The amplitude and phase of XUV pulses with a pulse duration of 10 fs were found to be in good agreement with a model analysis, taking into account phase modulation by ionization, self-phase modulation, and the atomic dipole phase.     

Kilowatt-peak-power, single-frequency, pulsed fiber laser near 2 μm

We generated single-frequency pulses at kilowatt peak power from an all-fiber Tm-doped master oscillator power amplifier system, which is the first report of this kind (to the best of our knowledge) of a laser in the 2 μm region. Compared with the laser linewidth of seed pulses, spectral broadening by a factor of 3 was observed with the amplified pulses. This was attributed to self-phase modulation in passive pigtail fibers of the components (isolator and wavelength division multiplexing) that were placed after the fiber amplifier. The short pulse width (~7 ns) of the kilowatt-level pulses prevents an onset of stimulated Brillouin scattering in the long fiber. When launching the pulses into several-meter single-mode fiber, significant nonlinear spectral broadening occurs due to modulation instability in the fiber. This reaction is beneficial for generation of a mid- and long-wavelength IR supercontinuum in nonlinear IR fibers.     

Shorter and bandwidth-limited Nd: YAG laser pulses

Stable generation of 12 ps pulses in a passively mode-locked YAG system is reported using an intracavity etalon. It is also shown that self-phase modulation via the nonlinear index of YAG increases the frequency width of the laser emission; at low intracavity pulse energy < 200 μJ, chirp free pulses are observed.     

Solid-state lasers mode-locked by nonlinear and external linear frequency shift

A new mechanism of the ultrashort pulse formation in solid-state lasers has been investigated. It was shown that linear frequency shift of the pulse from the centre of the luminescence band along with the self-phase modulation creates the conditions for the generation of stable symmetrical ultrashort pulses in the presence of external low-frequency modulation.