BBO晶体中飞秒脉冲的二次和三次谐波振荡产生

同时考察了群速色散失配、最低阶群速色散和二阶群速色散。研究了在ＢＢＯ晶体中飞秒激光脉冲的谐产生，计算了ＢＢＯ晶体的作为波长函数的晶体的最低阶和二谐群速色散，借助求解改进的耦合波方程的数值计算，得到了飞秒脉冲的二次及三次谐波辐射，分析了最低工阶及二阶ＧＶＤ对于基波脉冲和谐波脉的影响，最后考虑了三次谐波产生中群速失配的补偿。     

Transmission of return-to-zero pulses in an optical split-step system based on reflecting fiber gratings

A new variety of the “soliton management” in heterogeneous optical media is proposed. The system is composed as a periodic chain of nonlinear fibers with negligible intrinsic group-velocity dispersion (GVD), alternating with sections of unchirped fiber Bragg gratings (FBGs) operating in the reflection regime. Losses due to incomplete reflection are compensated by linear amplifiers. The model may apply to fiber-optic telecommunication links with periodically installed FBG modules, and it may be used for the design of laser setups. By means of extended simulations, we identify small regions in the underlying parameter space where this model, featuring the periodic separation of the Kerr nonlinearity and FBG-induced GVD (hence the name of the “split-step” system), supports stable transmission of RZ (return-to-zero) pulses, i.e., quasi-solitons. The effect of nonzero fiber’s GVD on the stable transmission regime is considered too. Moderately unstable (partly usable) transmission regimes are found in larger regions of the parameter space; they may be of two different types, with the average nonlinearity either undercompensating or overcompensating the GVD. Interactions between the stable RZ pulses are also studied, leading to the identification of a minimum separation between them necessary for the suppression of interaction effects.     

Group-velocity-dispersion-compensated femtosecond optical parametric amplifier

A tilted pulse front geometry is known to allow group-velocity-dispersion (GVD) compensated propagation of short pulses of different wavelengths in dispersive media. This scheme is shown to be applicable to an optical parametric amplifier (OPA) pumped at 450nm. A numerical calculation shows that a certain combination of the tilt of the pulse front and the angle between the interacting beams provides optimum GVD compensation for each wavelength. The parameters providing broadband tunability are identified. The tunability of the GVD-compensated OPA pumped with 55fs pulses was experimentally demonstrated between 545nm and 645nm. This revised version was published online in November 2006 with corrections to the Cover Date.     

Characteristics of Wavelength Tunable Femtosecond Soliton Pulse Generation Using Femtosecond Pump Laser and Polarization Maintaining Fiber

We have investigated both experimentally and numerically the characteristics of wavelength tunable femtosecond soliton pulse generation using a pulse width variable fiber laser and two different types of polarization maintaining fibers. The generated soliton shows the feature of the pulsewidth becoming almost constant at - 250 fs under any conditions of the pump pulse for 220m fiber. High conversion efficiency from pump pulse to a generated soliton pulse accounting for as much as 73% can be obtained. This efficiency decreases with increase in the input power or wavelength shift. A superior conversion efficiency and broad wavelength shift can be obtained by using a more shortened pump pulse. In the numerical calculations, it is predicted that under a condition of constant power of pump pulse, the maximum wavelength shift is achieved when the soliton number N is -1.4. The difference of mode field diameter and the group velocity dispersion (GVD) coefficient β₂ affect the wavelength shift and conversion efficiency. Using the fiber with small mode field diameter and small absolute value of GVD coefficient β₂, a high conversion efficiency and large wavelength shift can be obtained.     

“BEST” Homonuclear Adiabatic Decoupling for C- and N-Double-Labeled Proteins

The cyclic irradiation sidebands appearing in homonuclear adiabatic decoupling are calculated in detail, which reveals the origin of the antisymmetric sidebands. The sidebands can be inverted by inserting an initial decoupling with a different period, but the same f_1rms as the main decoupling that is required for Bloch–Siegert shift compensation. The sidebands can be eliminated in a broad decoupling range by adding spectra of opposite sidebands. Based on this scheme, an offset-independent double-adiabatic decoupling, named Bloch–Siegert Shift Eliminated and Cyclic Sideband Trimmed Double-Adiabatic Decoupling, or “BEST” decoupling for short, is constructed, which not only compensates the Bloch–Siegert shift as shown earlier by Zhang and Gorenstein (1998) but also eliminates residual sidebands effectively.     

A performance study of a logical gate using PPM optical pulse modulation for TDM systems

In this paper we propose the operation of an all-optical logical gate based in an acousto optical tunable filter (AOTF) operating with a pulse position modulation (PPM). The performance of an AOTF realizing two-input AND/OR logical functions, which can be applied in transmission and processing of signals in all-optical form in TDM systems, was examined. This novel integrated acousto optical logical gate operates with two ultrashort soliton light pulses (2 ps), which are modulated in agreement with the technique of pulse position modulation (PPM). Initially, we evaluated the effect resulting of an increment in the acousto optical interaction length, for the temporal position of the output pulse, considering the anomalous group velocity dispersion (GVD), nonlinear self phase modulation (SPM) and without loss propagation regime of TE and TM input pulses not modulated. We have analyzed the four possible situations for the two-input logical gate, modulating the TE and TM input pulses through temporal displacement and allowing a variation in the coding parameter offset (ε). We can conclude that is possible to get AND/OR logical operations for the modes TM or TE, without to insert PPM error, since a phase control (Δ?) exists (applied on the TM input pulse) in agreement with each situation in the truth table. Finally we defined the truth table, considering the adequate phase difference and coding parameter offset for the operation of the AND/OR logical gate based in the AOTF.     

Analysis of an optical logic gate using a symmetric coupler operating with pulse position modulation (PPM)

In this paper we propose the operation of an all-optical logical gate based in a symmetric nonlinear directional coupler (NLDC) operating with a pulse position modulation (PPM). The performance of a symmetric NLDC realizing two-input AND/OR logical functions, which can be applied in transmission and processing of signals in all-optical form in TDM systems, is examined. This integrated symmetric NLDC logical gate operates with two ultrashort soliton light pulses (2 ps), which are modulated in agreement with the technique of pulse position modulation (PPM). Initially, we evaluate the effect resulting of an increment in the PPM coding parameter offset (ε), for the temporal position of the output pulse, considering the anomalous group-velocity dispersion (GVD), nonlinear self phase modulation (SPM) and without loss propagation regime of input pulses, in the cores 1 and 2 of the NLDC. In this situation, we analyze the four possible situations for the two-input logical gate, modulating the 1 and 2 input pulses through temporal displacement and allowing a variation in the coding parameter offset. We can conclude that is possible to get AND/OR logical operations for the cores 1 or 2, without to insert PPM error, since a phase control (Δ?) exists applied in agreement with the logical level of the input pulse in the core 1. Finally we define the truth table, considering the adequate phase difference and coding parameter offset for the stable operation of the AND/OR logical gate based in the symmetric NLDC.     

Finite-size effects in a field-theoretic model with long-range exchange interaction

We present a systematic approach to the calculation of finite-size (FS) effects for anO(n) field-theoretic model with both short-range (SR) and long-range (LR) exchange interactions. The LR exchange interaction decays at large distances as 1/r ^d+2–2,0⁺,0⁺. Renormalization group calculations ind=d _u – are performed for a system with a fully finite (block) geometry under periodic boundary conditions. We calculate the FS shift of the critical temperature and the FS renormalized coupling constant of the model to one-loop order. The universal scaling variable is obtained and the FS scaling hypothesis is verified.     

Propagation of short soliton pulses through a parabolic index fiber with dispersion decreasing along length

A parabolic index dispersion decreasing fiber (DDF) has been designed and optimized to produce high capacity soliton communication system. Variation of different fiber parameters such as core radius, effective core area and GVD factor along the 25 km of DDF length has been carried out to optimize a best possible DDF which can sustain the propagation of fundamental soliton. The variation of non-linearity with length along with the conventional power and GVD factor variation has been included in the generalized non-linear Schrodinger equation (NLSE). This NLSE has been solved numerically by split step Fourier method for shorter pulse propagation, incorporating the term for third order dispersion and intrapulse Raman scattering. Stable soliton pulses in transmission system have been achieved by our simulation, when a correction factor due to Raman induced soliton mean frequency shift is incorporated to the GVD profile predicted by the fundamental soliton condition. The interaction of neighboring soliton pulse pair through the proposed fiber has also been studied.     

Soliton stability against polarization-mode-dispersion in dispersion-managed systems

We study the dynamics of two-component solitons in a dispersion-managed (DM) system, built as a periodic concatenation of segments of optical fibers with anomalous and normal group-velocity dispersion (GVD). The model includes, in addition to the usual GVD and nonlinear terms, birefringence and polarization-mode-dispersion (PMD), in the form of the polarization scrambling (random rotation of the polarization) taking place at randomly distributed defects. We propose a numerical algorithm for finding optimized DM solitons in such a system, which secure stable transmission over a large distance. The analysis includes effects of the PMD-induced noise, together with the noise due to the spontaneous amplifier emission, and the input-source noise. It is concluded that, if the group-velocity birefringence is not excessively large, the use of the optimized solitons makes it possible to tolerate the PMD effects in the long-haul DM link.     

GVD compensation schemes with considering PMD

Three group velocity dispersion (GVD) compensation schemes, i.e., the post-compensation, pre-compensation and hybrid-compensation schemes, are discussed with considering polarization mode dispersion (PMD). In the 10- and 40-Gbit/s non-return-zero (NRZ) on-off-key (OOK) systems, three physical factors, Kerr effect, GVD and PMD are considered. The numerical results show that, when the impact of PMD is taken into account, the GVD pre-compensation scheme performs best with more than 1 dB better of average eye-opening penalty (EOP) when input power is up to 10 dBm in the 10-Gbit/s system. However the GVD post-compensation scheme performs best for the case of 40 Gbit/s with input power less than 13 dBm, and GVD pre-compensation will be better if the input power increased beyond this range. The results are different from those already reported under the assumption that the impact of PMD is neglected. Therefore, the research in this paper provide a different insight into the system optimization when PMD, Kerr e     

Long-Period Structures in Noble-Metal Alloys: New Aspects

Proceeding from precise calculation of the electronic structure and the generalized electronic susceptibility, a systematic study of the relationship between the nesting properties of the Fermi surface (FS) and the character of long-period ordered superstructures (LPSs) in noble-metal alloys, Cu–Au, Cu–Pd, and Ag–Mg, is performed. An answer is provided as to why in Cu–Au, Cu–Pd, and Cu–Pt alloys so-called incommensurate LPSs, characterized by an irrational period and smeared antiphase boundaries, are formed. The reasons for occurrence of such LPSs within a short temperature range only are clarified. The dependence of the long period M on the long-range order parameter in Cu–Au alloys is accounted for. The causes for formation of two-dimensional LPSs in Au₃Cu and Cu₃Pd alloys are found. It is shown that their stability may be explained for by energy gaps opening on coinciding patches of FS in two mutually perpendicular directions. Arguments are drawn in favor of the fact that among quasicrystalline substances, phases with incommensurate LPSs occupy an intermediate position between incommensurate systems and quasicrystals. An attempt is made to identify the reasons for qualitatively different behavior of LPSs with large and small superperiods. It is shown that the dependence of LPS's behavior on the value of 2M is controlled by the quality of nesting on FS and the nesting vector sensitivity (2k _F) to variations in the long-range order parameter .     

Movement of Polymer Segments by Exciplex Emission of Pyrene and N,N-Dimethylaniline at the Polymer–Silica Interface

Intermolecular interaction between pyrenyl units linked on the silica surface and N,N-dimethylaniline (DMA) units of polymer segments coated on the silica was investigated. Exciplex emission between these units was observed. The spectral shape and the intensity of the exciplex emission depended on the sample preparative conditions. Emission decay curves had no rise components, which showed that preformed ground-state complexes between pyrene and DMA existed and there were various microenvironmental sites around the preformed complexes just after sample preparation. Annealing of the samples made the exciplex emission wavelength shift and its intensity increase, which meant that polymer segments that contacted with the silica surface moved to make the stable conformations during annealing. It was revealed that DMA units of polymer segments are more stable when they face to the polymer–silica interface than when they become part of the polymer bulk. After the samples were kept at room temperature for 1 month, the spectra of all the samples became the same, which showed that the most stable conformations of the polymer segments were at the polymer–silica interface.     

Optimization of NRZ Data Transmission at 10 Gbit/s over G.652 Without In-Line DFAs

Performance limits of NRZ data transmission at 10 Gbit/s over standard single mode fiber (SSMF, G.652) without the deployment of in-line erbium-doped fiber amplifiers (EDFAs) are evaluated by means of numerical simulation. The fiber system was modeled using the standard split-step Fourier algorithm for the solution of nonlinear Schroedinger equation. The effect of group velocity dispersion (GVD) compensation scheme, degree of GVD compensation of the SSMF, and input optical powers to SSMF and to the dispersion compensating fiber (DCF) have been investigated with the aim to maximize the distance between transmitter and receiver. We have found that a post-compensation scheme performs better than the pre-compensation scheme and that, by careful selection of input powers and degree of GVD compensation, it should be possible to keep the bit-error-ratio (BER) below 10 upper index = 15 for an SSMF length of 270 km.     

Kerr-lens mode-locking of Nd:glass laser

We report what is to our best knowledge the first Kerr-lens mode-locking of a Nd:silicate glass laser. Pulses as short as 64 fs were generated. For a broadband inhomogeneously broadened laser, the formation of the soliton-like pulses requires a minimum amount of negative group velocity dispersion (GVD), and more negative GVD is needed to have stable, self-sustained mode locking.     

Spatio-temporal evolution scenarios of femtosecond laser pulse filamentation in fused silica

We investigated the evolution of femtosecond laser pulses at different wavelengths corresponding to normal, zero, and anomalous regimes of group velocity dispersion (GVD) in fused silica. The laser pulse filamentation in different GVD regimes under the same similarity parameters was first considered. It was established numerically that the scenario of the pulse filamentation depends both on temporal factors, which are determined by pulse GVD and self-phase modulation, and spatial factors associated with Kerr self-focusing and plasma defocusing. In presence of strong normal GVD the dispersive stretching causes, a pulse power decrease followed by lowering of the intensity in filament, electron density reduction in plasma channel, and suppressing of the refocusing. For zero GVD the multipeak regime of radiation propagation is realized in the filament as a result of recurring self-focusings of powerful pulse tail, which was defocused in laser plasma. When GVD is anomalous a sequence of ??light bullets?? with duration about 10 fs forms in the filament. And the peak intensity in ??light bullet?? stays the same ?? 5 × 10¹³ W/cm². In the regime of anomalous GVD power is transferred from the pulse edges to its center, where the repeated self-focusings occur and form a ??light bullet?? sequence.     

Optimization of NRZ Data Transmission at 10 Gbit/s over G.652 Without In-Line DFAs

Performance limits of NRZ data transmission at 10 Gbit/s over standard single mode fiber (SSMF, G.652) without the deployment of in-line erbium-doped fiber amplifiers (EDFAs) are evaluated by means of numerical simulation. The fiber system was modeled using the standard split-step Fourier algorithm for the solution of nonlinear Schroedinger equation. The effect of group velocity dispersion (GVD) compensation scheme, degree of GVD compensation of the SSMF, and input optical powers to SSMF and to the dispersion compensating fiber (DCF) have been investigated with the aim to maximize the distance between transmitter and receiver. We have found that a post-compensation scheme performs better than the pre-compensation scheme and that, by careful selection of input powers and degree of GVD compensation, it should be possible to keep the bit-error-ratio (BER) below 10 upper index = 15 for an SSMF length of 270 km.     

自相位调制及群速色散对超短脉冲在准周期LiNbO3 Fibonacci超晶格中传输的影响

从LiNbO₃ Fibonacci超晶格的结构特点出发,数值模拟了超短脉冲在LiNbO₃ Fibonacci超晶格内传输过程中,自相位调制及高阶群速色散效应对脉冲形状和频谱的影响．结果表明,超短脉冲在Fibonacci超晶格中传输时,由于自相位调制(SPM)与群速色散(GVD)耦合效应的影响,脉冲形状畸变和频谱展宽均是不对称的,随着晶格长度的增加,脉冲将会发生分裂,以至成为两个独立的子脉冲,同时也会出现脉冲频谱上的分裂. 关键词：     

Impacts of polarization mode dispersion on the pulse-width with considering initial pulse chirp and fiber GVD

The impacts of polarization mode dispersion (PMD) on the pulse-width in linear systems have been investigated. The analytical solutions, including the effects of initial frequency chirp and group velocity dispersion (GVD), are derived. Analyses show that the pulse broadening effects induced by the second-order PMD depend on GVD and chirp parameter, which are different from those induced by the first-order PMD. An initially chirped Gaussian pulse is taken as an example, upon which analytical solutions of rms pulse-width are derived before and after the first-order PMD compensation. The first-order PMD compensator is also evaluated based on these solutions. The results show that the pulse broadening effects will be resisted efficiently by choosing appropriate GVD and chirp parameter; in general, the post-transmission compensation method will be less efficient than the PSP-transmission method.     

Does the Aharonov–Bohm Effect Exist?

We draw a distinction between the Aharonov–Bohm phase shift and the Aharonov–Bohm effect. Although the Aharonov–Bohm phase shift occurring when an electron beam passes around a magnetic solenoid is well-verified experimentally, it is not clear whether this phase shift occurs because of classical forces or because of a topological effect occurring in the absence of classical forces as claimed by Aharonov and Bohm. The mathematics of the Schroedinger equation itself does not reveal the physical basis for the effect. However, the experimentally observed Aharonov–Bohm phase shift is of the same form as the shift observed due to electrostatic forces for which the consensus view accepts the role of the classical forces. The Aharonov–Bohm phase shift may well arise from classical electromagnetic forces which are simply more subtle in the magnetic case since they involve relativistic effects of the order v ² /c ² . Here we first review the experimentally observable differences between phenomena arising from classical forces and phenomena arising from the quantum topological effect suggested by Aharonov and Bohm. Second we point out that most discussions of the classical electromagnetic forces involved when a charged particle passes a solenoid are inaccurate because they omit the Faraday induction terms. The subtleties of the relativisitic v ² /c ² classical electromagnetic forces between a point charge and a solenoid have been explored by Coleman and Van Vleck in their analysis of the Shockley–James paradox; indeed, we point out that an analysis exactly parallel to that of Coleman and Van Vleck suggests that the Aharonov–Bohm phase shift is actually due to classical electromagnetic forces. Finally we note that electromagnetic velocity fields penetrate even excellent conductors in a form which is unfamiliar to many physicists. An ohmic conductor surrounding a solenoid does not screen out the magnetic field of the passing charge, but rather the time-integral of the magnetic field is an invariant; this time integral is precisely what is involved in the classical explanation of the Aharonov–Bohm phase shift. Thus the persistence of the Aharonov–Bohm phase shift when the solenoid is surrounded by a conductor does not exclude a classical force-based explanation for the phase shift. At present there is no experimental evidence for the Aharonov–Bohm effect.