大模场光子晶体光纤中超连续谱的产生与控制

探索利用大模场光子晶体光纤产生大功率、高光束质量的超连续谱。采用分步傅里叶方法求解广义非线性薛定谔方程(GNLSE), 模拟了光脉冲在大模场光子晶体光纤中非线性传输和超连续谱的产生过程。着重分析了光子晶体光纤长度和抽运脉冲的峰值功率、啁啾等对超连续谱产生的影响, 讨论了大模场光子晶体光纤中光谱的非线性展宽机制。发现可将超连续谱产生过程分为初始展宽、剧烈展宽和饱和展宽三个阶段。合理选择光纤长度, 使产生的超连续谱处于剧烈展宽阶段时输出, 既能够得到较宽的光谱, 又能够保证较高的效率。抽运峰值功率对超连续谱的产生有重要影响, 当输入功率较小时, 脉冲的频谱成对称展宽, 仅有自相位调制(SPM)效应起作用, 其他高阶效应的影响都很弱。随着脉冲功率的增加, 频谱短波方向变化较小, 光谱向长波方向展宽。同时, 脉冲时域出现振荡调制, 振荡的起因与光波分裂现象有关。抽运光初始啁啾对超连续谱的产生也有重要影响。当啁啾为正时, 啁啾量的大小对产生超连续谱的影响较小, 蓝移方向基本没有影响, 而红移部分能量随着啁啾量的增大向长波方向转移, 超连续谱总的宽度变化较小; 当啁啾为负且满足一定条件时, 其中的非线性作用得到增强, 有利于光谱展宽。     

飞秒脉冲在高非线性光纤中产生超连续谱的特性研究

采用分步傅里叶方法,对飞秒脉冲在高非线性光纤中超连续谱的形成过程进行了研究,分析了脉冲峰值功率,脉冲宽度及初始频率啁啾对超连续谱特性的影响.结果表明,脉冲峰值功率、脉冲宽度以及初始啁啾对超连续谱的形成有着极其重要的影响;当脉冲峰值功率逐渐增大,超连续谱随之愈宽(-20dB处谱宽从170nm展宽到400nm以上),平坦度愈好.与之相反,脉冲宽度逐渐增大,超连续谱展宽范围减小,其平坦度也逐渐劣化;初始负啁啾有利于脉冲SC谱的展宽而正啁啾使脉冲SC谱的展宽受到一定抑制.进一步研究表明,适当的峰值功率超短啁啾脉冲在高非线性光纤传输时,得到没有泵浦成份残余的超宽且平坦超连续谱.     

光子晶体光纤反常色散区超连续谱产生机理研究

用实验和数值模拟两种方法研究了在反常色散区抽运光子品体光纤(PCF)中飞秒激光脉冲的传输特性和超连续谱的产生机理,给出了抽运脉冲在不同功率情况下输出光谱展宽并形成超连续谱的实际测镀及理论模拟结果.研究表明:在反常色散区抽运时,光谱展宽的初期以自相位调制为主,随后根据抽运功率的不同,孤子自频移、高阶光孤子的裂变和叫波混频效应会逐渐增强,进而成为光谱展宽的主要原因;初始激光脉冲的峰值功率和脉冲初始啁啾对光子晶体光纤反常色散区产生超连续谱形状和带宽是有影响的.     

初始啁啾对高斯脉冲形成光孤子的影响

本文通过解薛定谔方程分别研究了线性光纤中色散导致具有初始频率啁啾的高斯脉冲展宽的详细物理过程和非线性光纤中自相位调制导致光脉冲频谱展宽的详细物理过程 ,得到高斯脉冲在光纤中群速度色散所导致的频率啁啾在反常色散区是下啁啾 (负啁啾 ) ,与脉宽的关系是线性的 ,自相位调制所产生的频率啁啾是上啁啾 (正啁啾 ) ,在脉冲的中心部分近似为线性 ,当CPβ2 <0时 ,脉冲有一个初始窄化的阶段等结论。最后用计算机模拟光孤子传输估算出形成孤子的初始啁啾范围值。     

色散平坦渐减光纤中自相似脉冲演化特性研究

从脉冲在光纤中传输所满足的非线性薛定谔方程出发,采用分布傅里叶方法对脉冲在色散平坦渐减光纤中自相似脉冲演化特性进行了研究。结果表明：双曲正割脉冲在不同初始能量和初始脉冲宽度的情况下,色散平坦渐减光纤可以产生带有线性啁啾的自相似脉冲。进一步研究表明,随着初始啁啾增加,脉冲逐渐展宽且展宽速度逐渐加快,而演化特性的劣化程度随啁啾参量的增加而增大;当初始啁啾为正时自相似脉冲演化特性劣于负啁啾时的特性。     

光子晶体光纤中超连续谱的产生

光谱超连续展宽在光通信中有重要的应用,用分步傅立叶法研究和分析了在光子晶体光纤中飞秒激光脉冲的传输特性和超连续谱的产生机理.结果表明:在光子晶体光纤中,超连续谱的产生是自相位调制、四波混频、交叉相位调制效应综合作用的结果,其中自相位调制效应起先导作用;光脉冲的峰功率、初始啁啾及光纤长度对超连续谱的形状和带宽有影响.     

线性源啁啾对孤子脉冲压缩的影响

通过线性源啁啾对孤子脉冲压缩影响的研究,发现泵浦脉冲和孤子脉冲的源啁啾参量地脉冲压缩产生重要影响,且孤子脉冲压缩存在最佳光纤长度。在泵浦功率一定的条件下,选取负啁啾的泵浦脉冲正啁啾的孤子脉冲可以获得高压缩因子的光脉冲;同时选取负啁啾的泵浦脉冲和正啁啾的孤子脉冲易于产生更短的压缩光脉冲。     

损耗和啁啾对皮秒脉冲孤子效应压缩的影响

在计及光纤损耗的前提下,近似求解非线性薛定谔方程,分析了单模光纤的负群速度色散区群速度色散(GVD)和自相位调制(SPM)对啁啾脉冲的作用,定量计算光纤损耗和初始正啁啾对皮秒脉冲孤子效应压缩的影响。结果表明,初始正啁啾改善了损耗对压缩参量的不良影响,遏制了最佳光纤长度的增加。若选取适当的光纤长度和初始峰值功率,可以实现正啁啾脉冲在单模光纤中的有效压缩。     

初始啁啾对凸形色散平坦光纤超连续谱的影响

采用分步傅里叶方法数值研究了线性频率啁啾对高斯光脉冲在凸形色散分布平坦光纤中产生超连续(SC)谱的影响,并与啁啾双曲正割脉冲产生SC谱的情况进行了比较.结果表明:无啁啾高斯光脉冲的SC谱达到200 nm以上,其特性优于双曲正割脉冲的SC谱;获得最佳SC谱所需光纤长度随啁啾参量C增加而缩短;负啁啾时的SC谱特性劣于正啁啾时的特性,2种情况下的SC谱特性均劣于无啁啾时的SC谱特性,SC谱特性的劣化程度随|C|的增加而增大;在最佳SC谱对应的传输距离附近,高斯脉冲频谱质心随C的增加而红移且红移速度逐渐增加,比双曲正割脉冲的红移大且红移速度增加;高斯脉冲方均根频谱和脉宽随传输距离和随C增加逐渐展宽且展宽速度逐渐加快,比双曲正割脉冲的相应参量展宽得快且宽.     

高非线性光子晶体光纤中超连续谱产生的特性研究

利用数值方法求解广义非线性薛定谔方程,数值模拟了光脉冲在高非线性光子晶体光纤正常色散区超连续谱产生的演化,研究和分析了脉冲参数如峰值功率,脉冲宽度及初始频率啁啾对超连续谱形成的影响.结果表明,当脉冲峰值功率一定时,随着传输距离增大,超连续谱随之愈宽,平坦度愈好;随着脉冲峰值功率逐渐增大,超连续谱随之更宽,平坦度有所劣化.相反,脉冲宽度逐渐增大,超连续谱展宽范围减小,其平坦度也逐渐劣化;具有适当的正负啁啾脉冲,在高非线性光子晶体光纤传输中获得宽而平坦超连续谱.     

反常色散区抽运光子晶体光纤产生的超连续谱

为了研究反常色散区抽运光子晶体光纤产生的超连续谱,采用分步傅里叶方法数值模拟了飞秒激光脉冲在光子晶体光纤反常色散区中的非线性传输和超连续谱产生.结果表明,初始激光脉冲的峰值功率和脉冲初始啁啾对光子晶体光纤反常色散区产生超连续谱形状和带宽是有影响的.这些结论给光子晶体光纤中产生超连续谱提供了参考.     

Chirped Microwave Pulse Generation Based on Optical Spectral Shaping and Wavelength-to-Time Mapping Using a Sagnac Loop Mirror Incorporating a Chirped Fiber Bragg Grating

In this paper, we propose and demonstrate an approach to optically generating chirped microwave pulses with tunable chirp profile based on optical spectral shaping using a Sagnac loop filter incorporating a chirped fiber Bragg grating (CFBG) and linear wavelength-to-time mapping in a dispersive element. In the proposed approach, the optical power spectrum of an ultrashort optical pulse is shaped by a CFBG-incorporated Sagnac loop mirror that has a reflection spectral response with a linearly increasing or decreasing free spectral range. The spectrum-shaped optical pulse is then sent to a dispersive element to perform the linear wavelength-to-time mapping. A chirped microwave pulse with the pulse shape identical to that of the shaped spectrum is obtained at the output of a high-speed photodector. The central frequency and the chirp profile of the generated chirped microwave pulse can be controlled by simply tuning the time delay in the Sagnac loop mirror. A simple mathematical model to describe the chirped microwave pulse generation is developed. Numerical simulations and a proof-of-principle experiment are implemented to verify the proposed approach.      

Optically Preamplified Direct Detection in the Presence of Modulator Chirp

We analyze the impact of modulation induced chirp on optically preamplified direct-detection communication systems transmitting over dispersion-free channels, as found in optical free-space communication links. We show that commonly used quantitative measures of chirp (the small-signal chirp parameter and the effective chirp parameter) can fail to describe changing chirp characteristics under large-signal modulation in a satisfactory manner. We present an extended chirp model that lends itself both to measurement and simulation in a straightforward way. Regardingreceiver performance, we show thatreturn-to-zero (RZ) coded data signals can experience receiver sensitivity degradations of several dB due to spectral broadening of the chirped signal. By optimizing optical and electrical receiver bandwidths the penalty can be kept considerably lower than for receivers optimized for chirp-free signals. For non return-to-zero (NRZ) coding, chirpcan even improve receiver sensitivity by means of a pulse compression effect in the presence of narrow-band optical filters, leading to reduced inter-symbol interference (ISI). In our simulations and for the measurements, we exploit the variable chirp characteristics of a dual-drive electro-optic Mach-Zehnder modulator.     

Generation of monocycle-like optical pulses using induced-phasemodulation between two-color femtosecond pulses with carrier phaselocking

Detailed numerical analysis of monocyclical optical pulse generation using induced-phase modulation between two-color carrier-phase-locked femtosecond pulses is presented. For the generation of a short pulse with a quasi-linear chirped white-continuum spectrum, it is found to be important that the large spectral broadening of each input pulse after fiber propagation is as similar as possible and the total spectral intensity of the synthesized wave at the fiber output is as homogenous as possible. As a result, it is shown that a 2.2-fs 1.3-cycle pulse is generated     

Chirped RF Pulse Generation Based on Optical Spectral Shaping and Wavelength-to-Time Mapping Using a Nonlinearly Chirped Fiber Bragg Grating

Chirped radio-frequency (RF) pulse generation based on optical spectral shaping and nonlinear wavelength-to-time mapping in a nonlinearly chirped fiber Bragg grating (NLCFBG) is investigated. In the proposed approach, the spectrum of a femtosecond pulse generated by a mode-locked fiber laser is shaped by an optical filter that has a sinusoidal frequency response. The spectrum-shaped optical pulse is sent to the NLCFBG, to implement nonlinear wavelength-to-time mapping. A chirped electrical pulse with the central frequency and chirp rate determined respectively by the first- and second-order dispersions of the NLCFBG is then obtained at the output of a high-speed photodetector. An approximate model that describes the chirped RF pulse generation is derived, which is verified by numerical simulations. Chirped pulse generation with a pulse compression ratio as high as 450 is demonstrated. The key device in the chirped RF pulse generation system is the NLCFBG, which is investigated in detail with an emphasis on the influence of its group delay ripples on the performance of the pulse generation system. Techniques to design and fabricate the NLCFBG are also discussed. The proposed approach provides a potential solution for the generation of chirped RF pulse with a high central frequency and large chirp rate for applications in pulse compression radar systems.     

10GHz ps光孤子在G652光纤中的绝热传输实验

利用二次谐波频率分辨光学门(FROG)脉冲分析仪,实验研究了一阶啁啾ps光孤子在普通单模光纤(SMF)中长距离绝热传输时脉冲波形、脉宽和啁啾的变化规律。实验结果表明,在色散较大的标准SMF中,一阶啁啾孤子的稳定传输长度超过1000个色散长度,即很好地保持双曲正割波形不变(啁啾不影响孤子的波形),但啁啾孤子的脉宽展宽速度高于非啁啾孤子绝热传输的微扰结果;采取预加重措施,可以减小孤子脉宽的增大。因此,对于占空比较小的ps孤子脉冲,可以不必采用复杂的消啁啾和色散管理措施,就可以利用绝热方式进行稳定传输。