Effect Of Group Velocity Dispersion Research Articles

Influence of Gain Dispersion on the Propagation Characteristics of Ultrashort Optical Pulses in Distributed Fiber Amplifiers

The physical model for the optical pulse propagating in the amplifier has been established. Within the normal dispersion region of the fiber, the evolution of the optical pulse in the amplifier has been simulated numerically, and the influence of gain dispersion on the propagation characteristics of the pulse has been discussed emphatically. The results show that, during the propagation of the ultrashort optical pulse in a distributed fiber amplifier, group velocity dispersion and Kerr nonlinear effect will affect the pulse profile and spectrum. For the ultrashort optical pulse with a wideband spectrum, the influence of gain spectrum on the propagation characteristics of the optical pulse, in essence, is that of gain dispersion, which is equivalent to a loss mechanism and will generate gain narrowing effect, so it should be considered in the design of an amplifier system.

Third-order dispersion effects generated by non-ideal achromatic doublets on sub-20 femtosecond pulses

Gaussian temporal envelope pulses with initial durations of 10 fs, 15 fs and 20 fs and a carrier wavelength of 810 nm were analyzed at the paraxial focal plane of non-ideal achromatic doublet lenses for well-collimated incoming pulses parallel to the optical axis. The wave vector is expanded up to third order, to investigate the effect of third-order group velocity dispersion on the pulse and the results are compared to those obtained when the wave number is expanded up to second order. The propagation time difference and the primary spherical aberration were included in the calculations using the thin lens approximation theory. Results are presented for a homogenous illumination beam.

Effects of chirp and pulse shape on high harmonic generation and absorption in overdense plasmas

AbstractUsing particle-in-cell simulations, we investigated the effect of group velocity dispersion (GVD) and third order dispersion (TOD) in the laser pulse on high-order harmonic generation and laser absorption in overdense plasmas. A 1020 W/cm−2, 35-fs transform-limited Gaussian pulse was stretched to 160 fs through chirping. When including GVD alone, the temporal pulse shape remains symmetric and no difference was seen in the harmonic spectra for opposite signs of GVD. However, when adding TOD to GVD, the pulse is no longer symmetric and noticeable differences in harmonics intensity were observed for opposite signs of TOD. We show that the higher harmonic intensity obtained with positive TOD is connected with a steeper front edge of the pulse and the appearance of strong modulations in the harmonic spectrum. The chirp broadens and shifts the harmonics. Laser energy absorption is also mostly affected by the pulse shape. Simple estimates indicate that, in the main example considered in this paper, about half the laser energy absorption (10%) is due to vacuum heating.

Femtosecond surface plasmon pulse propagation

We analyze ultrafast surface plasmon-polariton pulse reshaping effects and nonlinear propagation modes for metal/dielectric plasmon waveguides. It is found that group velocity and loss dispersion effects can substantially modify both pulse duration (broadening/narrowing) and intensity decay (acceleration/retardation) by as much as several tens of percentage points in the short-pulse regime and that metallic nonlinearities alone may support soliton, self-focusing, and self-compressing modes.

GVD effect and nonlinear pulse propagation in 40Gbit/s optical fiber communication systems

In this paper, group velocity dispersion (GVD) and second-order GVD effects are shortly discussed and then the limitations on the bit rate induced by dispersion or second-order GVD are estimated. For relative higher pulse energy and shorter pulse width in 40Gbit/s systems, self-phase modulation(SPM) is significant. The combined effect of GVD and SPM on the propagation pulses are analyzed through Nonlinear Schrödinger Equation(NLSE).

Pulsed- and continuous-wave difference-frequency generation in AlGaAs Bragg reflection waveguides

Pulsed- and continuous-wave type-I and type-II difference-frequency generations (DFGs) in monolithic AlGaAs Bragg reflection waveguides were comparatively investigated. Phase matching bandwidth of exceeding 40 nm was observed in all the processes. Highest difference-frequency (DF) power of 2.45 nW was obtained in continuous-wave type-II interaction with the average external pump and signal powers of 62.9 and 2.9 mW, respectively. The corresponding nonlinear conversion efficiency is about 1.3×10−3% W−1 for a sample with a length of 1.5 mm. Using split-step Fourier method, the impacts of third-order nonlinearities including two-photon absorption and self-phase modulation on the efficiency of the DFG are numerically investigated. Furthermore, the adverse effects of group velocity mismatch and group velocity dispersion of the interacting frequencies on the efficiency of the pulsed nonlinear process are theoretically studied. Simulations indicate that the dominant factors in limiting the efficiency of the pulsed interaction are group velocity mismatch between pump and DF signal and two-photon absorption of the interacting waves.

Nonlinear images of scatterers in chirped pulsed laser beams

The bandwidth and the duration of incident pulsed beam are proved to play important roles in modifying the nonlinear image of amplitude-type scatterer. It is found that the initially positive chirp-type bandwidth can suppress the nonlinear image, while the negative one can enhance it, and that both effects are inversely proportional to the incident pulse duration. Numerical simulations further demonstrate that the location of nonlinear image is at the conjugate plane of the scatterer and that, for negatively pre-chirped pulsed beam, the nonlinear image peak intensity can be higher than that in the corresponding monochromatic case under certain conditions. Moreover the effect of group velocity dispersion on nonlinear image is found to be similar to that of chirp-type bandwidth.

Distribution of IR-UWB Signals With PAM Modulation in Long-Reach FTTH Networks

Fiber-to-the-home (FTTH) networks have been proposed as a means to extend the application of ultrawideband (UWB) radio signals in future wireless personal area networks to the delivery of high-definition content. In this paper, we analyze, using numerical simulation, the feasibility of the distribution of impulse-radio UWB signals with pulse amplitude modulation in long-reach FTTH networks. The simulation results show a severe performance degradation of the transmitted signals in FTTH networks, both in terms of the channel error probability and in terms of the temporal and spectral distortion factors. This degradation is mainly due to the effect of the fiber group velocity dispersion.

Modeling of mode locking in a laser with spatially separate gain media

We present a novel laser mode locking scheme and discuss its unusual properties and feasibility using a theoretical model. A large set of single-frequency continuous-wave lasers oscillate by amplification in spatially separated gain media. They are mutually phase-locked by nonlinear feedback from a common saturable absorber. As a result, ultra-short pulses are generated. The new scheme offers three significant benefits: the light that is amplified in each medium is continuous-wave, thereby avoiding issues related to group-velocity dispersion and nonlinear effects that can perturb the pulse shape. The set of frequencies on which the laser oscillates, and therefore the pulse repetition rate, is controlled by the geometry of resonator-internal optical elements, not by the cavity length. Finally, the bandwidth of the laser can be controlled by switching gain modules on and off. This scheme offers a route to mode-locked lasers with high average output power, repetition rates that can be scaled into the THz range, and a bandwidth that can be dynamically controlled. The approach is particularly suited for implementation using semiconductor diode laser arrays.

Testing ultrafast two-photon spectral amplitudes via optical fibres

We test two-dimensional TPSA of biphoton light emitted via ultrafast spontaneous parametric down-conversion (SPDC) using the effect of group-velocity dispersion in optical fibres. Further, we apply this technique to demonstrate the engineering of biphoton spectral properties by acting on the pump pulse shape.

Comparison of Orthogonal Frequency-Division Multiplexing and On-Off Keying in Amplified Direct-Detection Single-Mode Fiber Systems

We discuss the use of orthogonal frequency-division multiplexing (OFDM) for combating group-velocity dispersion (GVD) effects in amplified direct-detection (DD) systems using single-mode fiber. We review known OFDM techniques, including asymmetrically clipped optical OFDM (ACO-OFDM), DC-clipped OFDM (DC-OFDM) and single-sideband OFDM (SSB-OFDM), and derive a linearized channel model for each technique. We present an iterative procedure to achieve optimum power allocation for each OFDM technique, since there is no closed-form solution for amplified DD systems. For each technique, we minimize the optical power required to transmit at a given bit rate and normalized GVD by iteratively adjusting the bias and optimizing the power allocation among the subcarriers. We verify that SSB-OFDM has the best optical power efficiency among the different OFDM techniques. We compare these OFDM techniques to on-off keying (OOK) with maximum-likelihood sequence detection (MLSD) and show that SSB-OFDM can achieve the same optical power efficiency as OOK with MLSD, but at the cost of requiring twice the electrical bandwidth and also a complex quadrature modulator. We compare the computational complexity of the different techniques and show that SSB-OFDM requires fewer operations per bit than OOK with MLSD.

Femtosecond second-harmonic generation in AlGaAs Bragg reflection waveguides: theory and experiment

The impact of third-order nonlinearities including self-phase modulation and two-photon absorption on the efficiency of the second-harmonic generation is numerically investigated using the split-step Fourier method in phase-matched Bragg reflection waveguides. Also using the same technique, the adverse effects of group velocity mismatch and group velocity dispersion of the interacting frequencies on the efficiency of the nonlinear process are examined and contrasted for optimal sample design. Using an optimized structure, we report efficient femtosecond second-harmonic generation in monolithic AlGaAs Bragg reflection waveguides for a type II nonlinear interaction. For a 190 fs pulsed pump around 1555 nm with an average power of 3.3 mW, a peak second-harmonic power of 25.5 μW is measured in a sample with a length of 1.1 mm. The normalized conversion efficiency of the process is estimated to be 2.0×104% W−1 cm−2. Pump depletion is clearly observed when operating at the phase-matching wavelength.

Spectral phase shifts in femtosecond parametric down-conversion with media dispersion

We theoretically study the spectral phase shift of a midinfrared (MIR) femtosecond pulse generated through parametric frequency down-conversion in dispersive media, with an emphasis on the effect of group-velocity dispersion (GVD), with the aim of obtaining guidelines for dispersion compensation and pulse compression in such systems. It is found that the spectral phase of the generated MIR pulse differs significantly from that of due to its linear propagation. We consider two common ways of MIR femtosecond pulse generation: difference frequency generation (DFG) and optical parametric amplification (OPA). The resultant spectral phase is determined by the initial injection condition, and the transfer of the spectral phase accumulated by the higher-frequency fields. Our results show that in DFG, pump depletion has little effect on the spectral phase, while in OPA, the combined effect of linear GVD and high parametric gain leads to a notable dependence of spectral phase on gain---the higher the gain, the less the spectral phase. Quantitatively, under conditions where the MIR pulse is generated through injection of two higher-frequency fields, its spectral phase is close to only half of that accumulated through linear propagation in DFG, while it is even smaller in OPA. To obtain the shortest MIR femtosecond pulse, these results determine the amount of dispersion that needs compensation.

紫外超短脉冲激光放大过程中脉宽压缩的方案设计

The discharge-pumped KrF excimer was used to amplify ultraviolet ultrashort pulse laser with wavelength of 248.4 nm.The laser energy was amplified from 0.7 mJ to 15 mJ,and the pulse duration was broadened from 550 fs to 1 200 fs, with the beam diameter of 10 mm unchanged. To compress the output pulse duration, the effects of group velocity dispersion and self phase modulation were analyzed,and experimental schemes with four different prism pair were used to introduce negative frequency chirp into the pulse laser in order to compensate the positive frequency chirp introduced by group velocity dispersion and self phase modulation in the CaF2 windows of the amplifer.The results show that if the prism pair is put before the amplifier, the pulse duration can be compressed to 370 fs when the le

Chirp effects on impulsive vibrational spectroscopy: a multimode perspective

The well-documented propensity of negatively-chirped pulses to enhance resonant impulsive Raman scattering has been rationalized in terms of a one pulse pump-dump sequence which "follows" the evolution of the excited molecules and dumps them back at highly displaced configurations. The aim of this study was to extend the understanding of this effect to molecules with many displaced vibrational modes in the presence of condensed surroundings. In particular, to define an optimally chirped pulse, to investigate what exactly it "follows" and to discover how this depends on the molecule under study. To this end, linear chirp effects on vibrational coherences in poly-atomics are investigated experimentally and theoretically. Chirped pump-impulsive probe experiments are reported for Sulforhodamine-B ("Kiton Red"), Betaine-30 and Oxazine-1 in ethanol solutions with <10 fs resolution. Numerical simulations, including numerous displaced modes and electronic dephasing, are conducted to reproduce experimental results. Through semi-quantitative reproduction of experimental results in all three systems we show that the effect of group velocity dispersion (GVD) on the buildup of ground state wave-packets depends on the pulse spectrum, on the displacements of vibrational modes upon excitation, on the detuning of the excitation pulses from resonance, and on electronic dephasing rates. Akin to scenarios described for frequency-domain resonance Raman, within the small-displacement regime each mode responds to excitation chirp independently and the optimal GVD is mode-specific. Highly-displaced modes entangle the dynamics of excitation in different modes, requiring a multi-dimensional description of the response. Rapid photochemistry and ultrafast electronic dephasing narrow the window of opportunity for coherent manipulations, leading to a reduced and similar optimal chirp for different modes. Finally, non-intuitive coherent aspects of chirp "following" are predicted in the small-displacement and slow-dephasing regime, which remain to be observed in experiment.

Dual-Wavelength Pulse Train Generation Based on Silicon-on-Insulator Optical Waveguides

In this paper, we present and numerically study a method with pump-probe configuration to generate ∼4.17-THz dual-wavelength ultrahigh-repetition-rate pulse train from the continuous-waves (CW) based on the submicron silicon-on-insulator (SOI) optical waveguides. An original ∼4.17 THz repetition rate pulsed pump, with femtosecond pulse duration, at 2900 nm (mid-wave infrared wavelength-MWIR) and 2-channel CW lights at 1450 nm and 1650 nm (near infrared wavelengths) are simultaneously launched into the silicon waveguide. Results show that, when both of CW lights with high intensity pulsed pump are co-propagating along the SOI waveguide, the CW lights will, respectively, experience normal and anomalous group velocity dispersion (GVD) regimes, leading to developing into two inverted ∼4.17 THz repetition rate pulse train by the combined effects of GVD and nonlinear frequency chirp induced by pulsed pump inside the waveguide. Moreover, the outcome pulse trains have short pulse duration and high extinction ratio (ER) comparing with the original pulsed pump if the systems parameters such as input power and waveguide length are judiciously adjusted.

Darboux Transformation and Soliton Solutions for Inhomogeneous Coupled Nonlinear Schrödinger Equations with Symbolic Computation

With the aid of computation, we consider the variable-coefficient coupled nonlinear Schrödinger equations with the effects of group-velocity dispersion, self-phase modulation and cross-phase modulation, which have potential applications in the long-distance communication of two-pulse propagation in inhomogeneous optical fibers. Based on the obtained nonisospectral linear eigenvalue problems (i.e. Lax pair), we construct the Darboux transformation for such a model to derive the optical soliton solutions. In addition, through the one- and two-soliton-like solutions, we graphically discuss the features of picosecond solitons in inhomogeneous optical fibers.

Chirped Biphotons and their Compression in Optical Fibers

We show that broadband biphoton wave packets produced via spontaneous parametric down-conversion in crystals with linearly aperiodic poling can be easily compressed in time using the effect of group-velocity dispersion in optical fibers. This result could foster important developments in quantum metrology and lithography.

Analysis of High-Speed Optical Wavelength/Time CDMA Networks Using Pulse-Position Modulation and Forward Error Correction Techniques

This paper presents a comprehensive analysis of an optical code-division multiple access (OCDMA) network based on two distinct modulation formats, namely ON-OFF keying (OOK) and pulse-position modulation (PPM). We also investigate how each of these modulation formats performs under two distinct 2-D coding schemes, i.e., single-pulse per row (SPR) and multiple-pulse per row (MPR). For both cases, we have accounted for the simultaneous effect of many different dispersion and noise mechanisms (including multiple access interference (MAI) that impair the overall system performance. We have included the laser relative intensity noise at the transmitter side, the fiber dispersive effects (group velocity dispersion (GVD), and first-order polarization-mode dispersion (PMD), and beat, avalanche photodiode (APD), and thermal noises at the receiver side. The effect of GVD and PMD, as well as the influence of noises, on the performance of SPR and MPR codes is also investigated. Another effect studied in this paper is the influence of the APD photodetector on the beat noise of an incoherent OCDMA network. To mitigate systems noises and bit error rate (BER), we have adopted a forward error correction (FEC) RS(255, 239) algorithm in both networks investigated here. New expressions for the BER with all noises and dispersion mechanisms were also derived for the SPR and MPR code schemes. Results indicated that OOK and PPM modulation schemes without additional mechanisms to mitigate MAI and other noise effects are not sufficient to accommodate 32 simultaneous users in an error-free environment (BER < 10-12). This occurs due to the already high BER at the FEC input, which severely affects FECs at the receiver side.

Effect of wakefields on the propagation of a short laser pulse in a plasma channel

The effect of electric wakefields (generated inside the pulse) on the propagation of a short, asymmetric laser pulse in a plasma channel is analyzed. A nonlinear wave equation governing the propagation dynamics of a moderately intense laser pulse in a plasma channel along with equations describing the generation of wakefields is set up. The simultaneous evolution describing the pulse length and spot size in the presence of wakefields is derived using a variational technique. Using numerical methods the evolution of spot size, pulse length and intensity is graphically analyzed. The frequency chirp induced due to the combined effects of group velocity dispersion and self-phase modulation is also obtained. A comparative study of the propagation characteristics of two pulses, one having a fast rise and slow fall time while the other with a slow rise and fast fall time, has been presented.