色散补偿微结构光纤拉曼放大特性的研究北大核心CSCD

为了提高光纤拉曼放大器的放大性能,设计了一种高拉曼增益系数和较大负色散的光子晶体光纤。利用全矢量有限元分析方法对包层为正八边形的光子晶体光纤进行数值分析,探究空气孔结构和纤芯掺锗浓度的改变对有效模场面积和拉曼增益系数的影响,最后得到一种小模场面积、高拉曼增益系数和较大负色散的光子晶体光纤。研究结果表明:在泵浦波长为1450 nm且信号波长为1550 nm处包层空气孔直径为1μm、孔间距为1.2μm的掺锗光子晶体光纤结构可获得19.97 W^(-1)·km^(-1)的高拉曼增益系数,同时在1550 nm处可获得-327.6 ps/(nm·km)的较大负色散,该光纤的综合特性对于拉曼放大器放大性能的提高有重要意义。     

基于拉曼放大对色散渐减光纤的绝热压缩效果的优化

数值研究了光纤损耗对色散渐减光纤中基孤子脉冲的绝热压缩的影响,并分析了利用分布式的拉曼放大来提高DDF的绝热压缩效果的方案.结果表明:光纤损耗严重破坏了DDF对基孤子绝热压缩的效果;采用分布式的拉曼放大能对DDF的绝热压缩起到很好的优化效果,获得更高的压缩因子和脉冲能量;当拉曼增益系数选取优化值0.27/LD时,脉宽为10 ps的基孤子脉冲压缩后可得到脉宽仅417fs、脉冲能量增至5.9倍、峰值功率增至136.3倍、基座能量比仅为2.8%的输出脉冲.     

C+L波段色散补偿光纤及模块的研制

波分复用(WDM)传输系统工作波段已经覆盖了C波段和L波段．工作在C L波段的色散补偿模块及色散补偿光纤的研制是通信器件开发的一个重要领域．文章分析了C L波段色散补偿光纤的波导结构,并采用PCVD工艺研制成功了C L波段色散补偿光纤,该光纤可以同时对工作在C和L波段的通信系统的色散进行补偿。     

Novel methods for fast fiber Raman amplifier pump configuration

Traditional heuristic algorithms are time consuming in searching the pump configuration to obtain a wide-band flat-gain fiber Raman amplifier (FRA). In this paper, two methods are proposed to deal with this problem. One is called combined generic algorithm (CGA). By using the quadratic programming algorithm instead of generic algorithm (GA) to solve the pump integral, the search space dimension is reduced by half, and obviously, this method will converge faster. Based on a linear time-invariant (LTI) system model, another method is also presented under the assumption that the effective area of fiber is weakly dependent on stokes wave. Then, the amplifier gain can be viewed as the output of a special LTI system, which characterizes the normalized Raman gain profile as its unit impulse response and the pump power integral impulse sequence in frequency domain as its input. By using the nonnegative constraint least-square error (LSE) and clustering technique, we will solve this problem quickly. Numerical simulations shows that the total computation time will be halved at the cost of a slight deterioration in gain flatness.     

A novel design for dispersion compensating photonic crystal fiber Raman amplifier

This letter presents a novel design for dispersion compensating photonic crystal fiber (DCPCF) which shows inherently flattened high Raman gain of 19 dB (/spl plusmn/1.2-dB gain ripple) over 30-nm bandwidth. The proposed design module has been simulated through an efficient full-vectorial finite element method. The designed DCPCF has a high negative dispersion coefficient (-200 to -250 ps/nm/km) over C-band wavelength (1530-1568 nm). The proposed fiber module of 5.2-km length not only compensates the accumulated dispersion in conventional single-mode fiber (SMF-28) but also compensates for the dispersion slope. Hence, the designed DCPCF module acts as the gain-flattened Raman amplifier and dispersion compensator.     

Model for polarization-dependent gain due to pump depletion in a WDM system with forward-pumped Raman amplification

We study polarization-dependent gain (PDG) due to signal-induced pump depletion (SIPD) in a wavelength-division-multiplexing (WDM) system with forward-pumped Raman amplification. It is found that SIPD can polarize the pump significantly in fiber with very low polarization-mode dispersion (PMD). To quantify the impact of fiber PMD on SIPD-induced PDG for a practical WDM system with many signal channels and multiple Raman pumps, an approximate vector model has been developed. The developed model allows us to directly calculate PDG from both SIPD and signal-signal Raman interaction (SSRI) with greatly reduced computation time. Based on the developed model, detailed numerical investigations for two typical C-band WDM systems are presented. It is shown that significant PDG can be introduced by SIPD when the fiber PMD coefficient is lower than 0.01 ps/km/sup 1/2/ even if the pumps are fully depolarized. It is also shown that PDG due to SIPD and PDG due to SSRI are in phase at shorter wavelength channels but out of phase at longer wavelength channels.     

Semianalytical approach to the gain ripple minimization in multiple pump fiber Raman amplifiers

A simplified approach to the search of Raman pump powers and wavelengths minimizing the gain ripple in a Raman multiple-pump amplifier is presented. The first step is to evaluate the pump path average powers and the wavelengths through a minimization algorithm. Then, an approximated analytical derivation of the initial pump powers, that yields an approximated path average power, is given. The results of the algorithm are in excellent agreement with the full numerical calculations.     

S+C bands amplification in a distributed Er-doped Raman fiber

A novel hybrid optical amplifier covering entire S- and C-bands has been proposed. A silica fiber configured with an erbium (Er)-doped cladding and a germanium-doped core was used. Amplification was achieved by stimulated Raman scattering along with the Er ions' radiative transition in a fiber. A numerical simulation has been performed to analyze the amplification characteristics. Fiber parameters such as optical loss, Er concentration, fiber length, and pump power were taken into consideration to calculate the optimum pump power and fiber length for a flat gain characteristic over the entire S- and C-bands.     

Crosstalk in fiber Raman amplification for WDM systems

The crosstalk between channels in Raman amplification for two-channel WDM system is calculated. Theory shows that severe crosstalk can occur even in the linear amplification (or pump undepletion) region. To ensure small crosstalk, the signal gain and injected pump power should be limited to values well below the threshold of Raman amplification. As a numerical example, a 30-dB gain penalty and 3-dB pump power penalty occur when a 30-dB signal-to-interference ratio (SIR) is required. Thus, the conversion efficiency of Raman amplification in WDM systems is very low     

多泵浦喇曼放大器简化模型设计及泵浦优化

对多泵浦光纤喇曼放大器(FRA)功率传输方程进行了合理简化,得到密集波分复用系统(DWDM)信号合成喇曼增益的数学模型,分析了受激喇曼散射(SRS)对信号喇曼增益的影响。并在该简化模型基础上介绍了一种对泵浦波长分布和输入功率进行优化计算的方法,实现了超宽带平坦的喇曼增益谱,为多泵浦FRA在DWDM系统中的应用提供了有价值的参考。     

Amplified spontaneous Raman scattering and gain in fiber Raman amplifiers

The spectrum of amplified spontaneous Raman scattering and gain in a fiber Raman amplifer has been calculated analytically as a function of distance and pump power. The model used makes no assumptions on the magnitude of the gain and considers the pump nondepletion region. From the results, the on/off ratio has been calculated and is found to have a variation with length that depends on frequency detuning. An enhancement in on/off ratio is also found for small fiber lengths but at limited gain. The use of a narrow-bandpass optical fiber leads to quite different behavior of the on/off ratio than for the unfiltered case.     

Complete dispersion compensation for 400-fs pulse transmission over10-km fiber link using dispersion compensating fiber and spectral phaseequalizer

We have demonstrated essentially complete dispersion compensation for 400-fs pulses over a 10-km fiber link using dispersion compensating fiber and a programmable femtosecond pulse shaper functioning as a spectral phase equalizer. The pulse shaper impresses adjustable quadratic and cubic phases onto the spectrum and removes all the residual dispersion and dispersion slope in the dispersion compensated fiber link. Our work shows that the pulse shaper technique provides a powerful and convenient tool for programmable fiber dispersion compensation over broad optical bandwidth. This allows distortion-free femtosecond pulse transmission over a fiber link in excess of 10 km without requiring the exact trimming of the dispersion-compensating fiber     

Purely phase-sampled fiber Bragg gratings for broad-band dispersion and dispersion slope compensation

We demonstrated numerically that both the chromatic dispersion and the dispersion slope can be compensated by using purely phase-sampled superstructure fiber Bragg gratings provided both the grating period and the sampling period are chirped linearly along the grating. Adjusting the refractive index modulation and the chirp of sampling function, they can be designed to compensate dispersion of a large number of wavelength-division-multiplexing channels.     

Pump wavelength independent gain with bidirectionally pumped alumino-silicate erbium-doped fiber amplifiers in the 800 nm pump band

Using a comprehensive computer model, it was shown that the effects of excited state absorption (ESA), in the 800 nm pump band, in alumino-phosphate-silicate (APS) erbium-doped fiber amplifiers (EDFAs) can be reduced significantly by using bidirectional pumping. If a band optimum length (BOL) is selected, a near optimum small signal gain (SSG) can be constant over broad pump wavelength regions due to the combined effects of ESA and ground state absorption (GSA). Hence, multilongitudinal mode, high power, reliable and inexpensive GaAlAs laser diodes can be used as pumping sources for EDFAs having SSG > 40 dB, noise figures of <4 dB, and output signal powers approaching 100 mW. These fibre amplifiers could be used as line repeaters, preamplifiers, and power amplifiers in different lightwave systems.<>     

Configurable three-wavelength Raman fiber laser for Ramanamplification and dynamic gain flattening

A three-wavelength Raman fiber laser is presented that is suitable for the generation of a flattened Raman gain curve over a broad range of extended telecommunications bandwidths. This laser utilizes an 1100-nm Yb-doped cladding pumped fiber laser and a cascaded Raman resonator to generate output radiation at 1427, 1454, and 1480 nm. The slope efficiency for conversion from the 1100- to 14xx-nm radiation is 0.38, the total output power is 1.1 W, and the fiber laser "wall-plug" efficiency is 0.10. The output power at 1454 and 1480 nm is adjusted by varying the reflectivity of the fiber grating output couplers and the suppression of lower order spectral components is between 15 and 20 dB. Low frequency relative intensity noise (RIN) measurements indicate RIN values less than -90 dB/Hz below 100 kHz     

Exact compensation for both chromatic dispersion and Kerr effect ina transmission fiber using optical phase conjugation

We propose a new method to compensate exactly for both chromatic dispersion and self-phase modulation in a transmission fiber, where the light intensity changes due to fiber loss and amplifier gain. This method utilizes optical phase conjugation (OPC). The pulse shape is precompensated before OPC by transmission through a fiber with large dispersion. A computer simulation demonstrates effective compensation for waveform distortion in a 40 Gb/s NRZ intensity-modulated light transmission     

宽带可调谐光纤光栅色散补偿器系统设计

文章提出一种在光纤光栅自身热膨胀效应产生啁啾的基础上,利用铝片热膨胀系数比较高的特点产生应力来增强光纤光栅啁啾,从而实现了宽带、大范围色散调谐的新型光纤光栅色散补偿器。该色散补偿器能够分别对群速度色散及中心波长独立调谐。实验结果表明,在中心波长为1 551.25nm处,能够实现>1.5nm的色散补偿带宽,-350～-690ps/nm的群时延色散调谐范围;在色散为-660ps/nm情况下,能够实现中心波长1nm的偏移。     

A method for progressive optimization of pump power and wavelengths for Raman amplification

We present a method for progressive optimization of pump power and wavelengths for Raman amplification, which allows for selective compensation of the relevant nonlinear interactions. This method first allows for a rapid one-step assessment of gain performance for a given set of wavelengths without determining pump powers in a novel approach which completely avoids the numeric solution of the Raman equations, thereby enabling ideal performance prediction while significantly reducing computational effort. The method then provides a technique to selectively compensate for pump-pump, pump-signal, and amplified signal-signal interactions, enabling one to control how much computational effort is expended determining pump powers based on the level of accuracy desired, which varies widely in practice. Lastly, the method is independent of solution techniques for the Raman equations, thereby eliminating the need to recast the Raman equations with separate, typically restrictive assumptions (such as ignoring amplified spontaneous emission and Rayleigh backscattering).     

Analysis of fiber Bragg gratings for dispersion compensation inreflective and transmissive geometries

Numerical analysis of the dispersion-compensating properties of fiber Bragg gratings (FBGs) in both reflective and transmissive modes is presented. First, the sensitivity of chirped, reflective gratings to the grating chirp parameter, index modulation, and grating length is examined, showing that apodization provides lower sensitivity to variations in these parameters. Second, we introduce a new transmissive geometry for grating-based dispersion compensation that utilizes the dispersive properties of a uniform Bragg grating in transmission     

Grating compensation of third-order material dispersion in thenormal dispersion regime: Sub-100-fs chirped-pulse amplification using afiber stretcher and grating-pair compressor

We describe a method for compensating large amounts of second- and third-order material dispersion, and we present two simple, compact and robust stretcher-compressor systems for microjoule-and millijoule-level chirped-pulse amplification. These systems, which use dispersive material to stretch and a modified grating pair to compress the pulse, provide expansion and compression with full cubic-phase compensation. Unlike previous fiber-stretcher systems which were limited to picosecond pulse durations, these systems can be used effectively with 50-fs microjoule pulses or 100-fs millijoule pulses. The results of our model are described, including the quartic-phase limitations for both systems. We discuss other applications of this grating pair to other areas of ultrafast optics, including intracavity dispersion compensation for femtosecond oscillators