基于周期极化反转铌酸锂光波导的全光信号处理

利用周期极化反转铌酸锂(PPLN)光波导中和频(SFG)、级联倍频(SHG)和差频(DFG)、级联和频与差频等二阶和级联二阶非线性效应,提出并理论研究了基于单个PPLN光波导实现40 Gbit/s全光半加器、半减器、与门、或门、异或门等多种逻辑功能.提出并理论研究了基于PPLN光波导环形镜结构实现非归零码(NRZ)到归零码(RZ)的全光码型转换.实验验证了10 Gbit/s和20 Gbit/s基于PPLN光波导NRZ到RZ的全光码型转换.     

基于和频效应1.5 μm波段新型高速全光波长转换器

基于周期极化反转铌酸锂(PPLN)光波导的和频(SFG)二阶非线性效应,提出并实验验证了1.5-μm波段信号光到抽运光的高速全光波长转换。输入信号光采用重复频率为40 GHz,脉宽为1.57 ps的皮秒脉冲或是40 Gbit/s的非归零(NRZ)码信号,输入抽运光为连续光,输出抽运光变为脉冲光,并且是输入信号光的反向波长转换。     

基于差频技术的宽谱中红外飞秒激光的产生

飞秒差频产生器(DFG)是一种获得宽谱中红外激光的有力工具。为了利用DFG产生更高瞬时带宽的中红外激光,可以使用窄带泵浦光、宽带信号光结合大信号光相位匹配带宽的非线性晶体或使用宽带泵浦光、窄带信号光结合大泵浦接受带宽的非线性晶体。研究表明,对于PPLN晶体,当泵浦光波长为1050 nm,闲频光波长在3.4μm附近时,非线性晶体具有较大的泵浦接受带宽,仅使用均匀极化周期PPLN晶体即可获得宽谱中红外激光。基于高重复频率的掺镱光纤激光放大器系统,通过引入自相位调制效应,获得了中心波长为1050 nm的宽谱光源,将其作为DFG系统的泵浦源。利用飞秒脉冲在负色散光子晶体光纤中的拉曼效应,产生了中心波长为1525 nm的超短脉冲,将其作为DFG系统的信号源。在长度分别为1 mm和3 mm的PPLN晶体中,都获得了宽谱中红外闲频光输出,其-10 dB光谱覆盖范围分别为2.72～4.15μm和2.87～4.08μm。     

基于色散位移光纤中交叉相位调制的波长转换

为了研究全光波长转换的实现方法,采用色散位移光纤中脉冲光和连续波间的交叉相位调制效应,使得连续波产生频移和展宽,然后利用光纤光栅滤波,得到了重复频率为57.97MHz、脉冲宽度为2ns的转换脉冲,这与抽运脉冲重复频率、脉冲宽度基本相同,而且连续波可调范围是1537nm~1560nm。结果表明,基于色散位移光纤中交叉相位调制效应的波长转换具有较宽的波长转换范围和较快的转换速度,是一种简单、高效和通用的波长转换技术。     

基于周期极化反转铌酸锂光波导的全光波长转换

利用周期极化反转铌酸锂光波导的二阶非线性效应,基于和频提出并实验验证了1.5 μm波段40 Gbit/s的波长转换和逻辑非门.设计了一种新型双环腔结构的波长转换器,无需注入任何外界连续光,基于级联和频与差频实验实现了可变输入到可变输出皮秒脉冲的可调谐波长转换.     

全光波长转换器及其研究进展

实现全光波长转换主要利用四种非线性效应，交叉增益调制（XGM）、交叉相位调制（XPM）、四波混频（FWM）和差频（DFG）效应。根据所用非线性器件不同，分别介绍了基于这四种效应的全光波长转换器的基本原理，系统结构、特点和发展现状，每种波长转换器都有其不足之处，针对这一现实，重点介绍了国内外最新的全光波长转换方案，这些方案在一定程度上改进了原有波长转换器的性能，促进了全光波长转换器的实用化进程。     

周期极化反转铌酸锂光波导的新特性和新应用

周期极化反转铌酸锂(PPLN)光波导以其响应速度超快和无自发辐射噪声影响等独特优点在高速全光信号处理中表现出优越性能并获得广泛应用.传统上主要利用PPLN光波导对比特速率和调制格式透明的特性实现全光波长转换、全光逻辑门和全光码型转换等功能.我们发现PPLN光波导其实也存在其不透明的一面[1].当携带相位信息的信号光置于PPLN光波导准相位匹配(QPM)波长处时,经过级联倍频和差频(cSHG/DFG)信号光中的相位信息在转换空闲光中被擦除.     

高功率光纤激光器中自相位调制的实验研究

利用主振荡功率放大(MOPA)结构高功率皮秒脉冲全光纤激光器,对高功率皮秒脉冲放大器中自相位调制(SPM)效应进行了实验研究。激光器种子源是自行搭建的半导体可饱和吸收镜(SESAM)被动锁模光纤激光器。为了抑制非线性效应,使用一个自制重频倍增器把种子脉冲的重频增加到328 MHz 后再放大。放大器部分采用三级放大结构,最终获得了中心波长为1 066.5 nm,3 dB 光谱线宽约为2.5 nm,平均功率为91W 的稳定皮秒脉冲激光输出。实验对光脉冲在放大的过程中自相位调制引起的光谱变化进行了研究。对激光器输出光谱的分析表明,随着功率的增大,高功率光纤激光器中自相位调制效应受到入射脉冲的初始啁啾和脉冲形状的影响程度也随着变大,与此同时还受到自陡峭效应的影响。     

基于半导体光放大器的四波混频效应对正交频分复用光信号进行全光波长变换

实验研究了基于半导体光放大器(SOA)的四波混频(FWM)效应的单抽运光正交频分复用(OFDM)信号的波长变换系统.信号光源和抽运光源分别由两个不同输出波长的可调分布反馈式激光器(DFB-LD)产生.信号光源经2.5 Gb/s OFDM的电信号直接调制后再和抽运光源耦合,经光放大器后在SOA实现波长变换.实验结果显示,耦合信号经SOA四波混频效应后,产生新波长的信号光将携带OFDM信号,且转换效率与信号光和抽运光的功率、波长以及两者的偏振夹角有关.同时也测量了转换的OFDM信号的功率-误码曲线和接收星座图.     

全固态窄线宽钠导星激光器

研制了一台重复频率400 Hz,平均功率1.0 W的激光二极管抽运全固态窄线宽钠导星激光器.利用两路波长分别为1064 nm和1319 nm的声光(AO)调Q激光器作为基频光,在腔外通过三硼酸锂(LBO)晶体和频产生(SFG)589 nm钠导旱激光,和频效率约达20%.每路基频光均采用标准具压窄线宽,输出和频光线宽约1.8 GHz,并调节控制标准具的温度和倾斜角度将中心波长锁定于589.159 nm(偏差±1 pm),实现了激光器谱线与钠原子D2线的精确匹配.     

Reduction of Light Signal Pulse Distortion in Cascaded SFG–DFG Wavelength Conversion: Segmented QPM Interleaved by Short Wavelength Cut Filters

Cascaded sum-frequency-generation (SFG) and difference-frequency-generation (DFG) can implement a wavelength conversion between arbitrary combinations of input and output signal wavelengths. By using a tunable wavelength pump light, the output wavelength can be tuned to a desired wavelength. As in many wavelength conversion devices using the nonlinear optical effect, the group velocity difference between light pulses with different wavelength causes a walk-off effect deforming the output pulse shape. Thus, the device length should be kept short to avoid the walk-off effect resulting in limited conversion efficiency. In this report, we propose a method, for quasi-phase matched device, to maintain the pulse shape of the SFG light pulse along the propagation distance. The output DFG light pulse deformation is suppressed and the conversion efficiency can be increased by extending the device length.      

Wavelength conversions in quasi-phase matched LiNbO/sub 3/ waveguide based on double-pass cascaded /spl chi//sup (2)/ SFG+DFG interactions

Based on the cascaded nonlinear interactions (/spl chi//sup (2)/:/spl chi//sup (2)/) of sum- and difference-frequency generation (SFG+DFG), a novel all-optical wavelength conversion scheme is proposed for the first time in periodically poled LiNbO/sub 3/ (PPLN) waveguide, in which a double-pass configuration is introduced. The performance of this scheme is thus different from the previous single-pass SFG+DFG scheme. The concept of the "balance condition" is presented to optimize the power and frequencies of the two pump sources. Under this condition, the energy is transferred irreversibly from the pump waves to the SF wave during the forward propagation. The equations describing the SFG can be solved analytically under this condition. Subsequently, the DFG equations are solved under the assumption that the SF wave would be constant during the backward propagation. Theoretical expressions are derived and are found to be consistent with numerical calculations. Compared with the conventional converter based on the cascaded /spl chi//sup (2)/:/spl chi//sup (2)/ interactions of second-harmonic generation and difference frequency generation SHG+DFG, the same conversion efficiency can be achieved in our scheme by employing two pump sources with lower power, or conversely higher conversion efficiency can be reached using two pump sources similar to that used in SFG+DFG scheme. The profile of the conversion efficiency can be further improved by adjusting the wavelengths of the two pump sources. In addition, compared with the single-pass SFG+DFG scheme, the main advantage of this new scheme rests on the fact that the conversion efficiency can be enhanced significantly. The advantages of the double-pass SHG+DFG scheme and the single-pass SFG+DFG scheme are combined in this new design to a great extent.     

Tunable wavelength converters of picosecond pulses based on periodically poled LiNbO3 waveguides

A tunable wavelength conversion between picosecond pulses is experimentally demonstrated by using cascaded sum- and difference-frequency generation (cSFG/DFG) in a periodically poled LiNbO3 (PPLN)waveguide. The pulsed signal with 40 GHz repetition rate and 1.57 ps pulse width is adopted. When the input signal and the first control wavelengths are kept at 1554.2 and 1532.5 nm, respectively, the output signal wavelength can be tuned from 1536.0 to 1545.2 nm as the second control wavelength varies from 1550.5 to 1541.0 nm. By varying the first control wavelength to satisfy the quasi-phase matching (QPM) condition for sum-frequency generation (SFG) and simultaneously adjusting the second control wavelength, the tunable output signal wavelength can also be obtained when the input signal wavelength is changed. In the experiment,the amplified spontaneous emission (ASE) noise from the erbium-doped fiber amplifier (EDFA) is effectively suppressed by employing two narrow band tunable filters. Therefore, the wavelength down- and up-conversions are simultaneously observed.     

A tunable wavelength conversion and wavelength add/drop scheme based on cascaded second-order nonlinearity with double-pass configuration

A selective and tunable wavelength conversion and wavelength add/drop scheme based on sum- and difference-frequency generation (SFG+DFG) is proposed, in which the concept of "double-pass" is introduced. An arbitrary channel can be dropped from a wavelength division multiplexing (WDM) signal and added to another WDM signal at arbitrary wavelength. The channel to be dropped is selected and depleted (dropped) by adjusting the pump 1 through sum frequency generation (SFG) during the forward propagation. Subsequently, the difference frequency generation (DFG), taking place during the backward propagation, is employed to convert (add) the dropped channel to another channel in another WDM signal by adjusting the pump 2. For the dropped and added channel, the phase matching of SFG and DFG are nearly perfect and the theoretical expressions are derived under the assumption that the two pumps are undepleted. The power of pump 1 is optimized to deplete the dropped channel completely, while that of pump 2 is chosen to maximize the output power of the added channel. Numerical calculations are performed to investigate the propagation of the other channels whose phase is mismatched. To suppress the crosstalk, the spacing of the WDM channels is chosen to be 0.2 nm (25 GHz). We have also compared our scheme with others (such as the single-pass scheme and the double waveguide scheme) and shown that ours possesses several distinct advantages.     

Analysis of novel cascaded /spl chi//sup (2)/ (SFG+DFG) wavelength conversions in quasi-phase-matched waveguides

A novel cascaded second-order nonlinear interaction (/spl chi//sup (2)/), which is simultaneously based on sum frequency generation (SFG) and difference frequency generation (DFG) processes, is proposed and analyzed in quasi-phase-matched wavelength converters. Analytical expressions with clear physical insights are derived for the converted light. It is shown that the same conversion efficiency can be achieved by employing two pump sources with lower output power (P/sub p1/,P/sub p2/) in this novel scheme as compared with the conventional cascaded wavelength conversion technique based on second-harmonic generation and difference frequency generation (SHG+DFG) with a single higher power pump beam (P/sub p/=P/sub p1/+P/sub p2/). The theoretical results are consistent with the experimental ones. It is found that the pump wavelength difference can be separated by a span as large as 75 nm, while large 3-dB signal conversion bandwidth is retained. The results show that the novel cascaded /spl chi//sup (2)/ wavelength conversion scheme is very attractive for practical applications.     

Experimental realization of 40 Gbit/s single-to-single and single-to-dual channel wavelength conversions in LiNbO3 waveguides with two-pump configuration

An all-optical 40 Gbit/s tunable single-tosingle channel wavelength conversion is experimentally realized based on cascaded sum- and difference-frequency generation (cSFG/DFG) in periodically poled LiNbO3 (PPLN) waveguides. By employing two tunable filters to effectively suppress the amplified spontaneous emission (ASE) noise, both wavelength down- and upconversions are simultaneously observed. We also propose and verify a novel cSFG/DFG-based single-todual channel wavelength conversion by setting two pumps (pumpl, pump2) close to each other or pump2 and the signal close to each other. For the latter, two kinds of cSFG/DFG schemes are both demonstrated The dependence of the conversion efficiencies of two channel idler waves on pumpl wavelength is discussed.The wavelength relationships between two channel idler waves and the three incident waves are investigated in detail theoretically as well as experimentally.     

4～5μm全固化可调谐激光实验研究

报道了采用KTP晶体和LiIO3晶体实现4~5μm可调谐激光输出的光参量振荡器(OPO)至差频产生器(DFG)的全固化结构和相应的实验结果。其中光参量振荡器的抽运源为倍频Nd∶YAG激光,差频产生器的抽运源分别是上述光参量振荡器激光和Nd∶YAG基频激光经KTP倍频晶体后剩余的1.064μm激光。实验中Nd∶YAG基频脉冲激光脉宽12 ns,单脉冲能量300 mJ。观察到最大倍频效率达到66.7%,KTP参量量子转换效率达到50%,差频量子转换效率为1.5%,在4.45μm得到了单脉冲100μJ的激光输出。差频光的调谐范围为4.1~4.5μm,发散角为垂直方向12 mrad,水平方向4 mrad。     

Low crosstalk and variable wavelength conversion using multiple QPM LiNbO3 waveguide module

A multiple quasi-phase matched LiNbO₃ device in a four-port fibre-pigtail module for variable difference frequency generation (DFG) is described. The module structure realises DFG pumped by second-harmonic light that results in wavelength conversion with low crosstalk. The module is also used to demonstrate polarisationinsensitive conversion. This device technology will be useful for future photonic networks that employ a grouped wavelength path.     

Quasi-phase-matched (QPM) difference frequency generation in amirrorless counterpropagating configuration

We present a theoretical analysis of second-order nonlinear difference frequency generation (DFG) in a generalized mirrorless quasi-phase-matching (QPM) frame, aimed at a comparison of counterpropagating DFG configuration (CDFG) to other DFG schemes, in view of all-optical processing applications. Field nonlinear coupling equations have been numerically solved under the hypothesis of phase-matched interaction. The evolution of propagating fields within the material and the wavelength conversion efficiency have been calculated in dependence of operating parameters. The increased complexity in the evolution of amplitude and phase for fields interacting in CDFG with respect to forward-propagating DFG (FDFG) is at the basis of a dramatic increase in the wavelength conversion efficiency under particular settings of device parameters