Simplified back-propagation for fibre nonlinearity compensation employing multi-band OFDM signals

In this paper we investigate the efficacy of applying the coupled field back-propagation algorithm as a post-compensation method for nonlinearity mitigation in a coherently detected fibre optic long-haul system using multi-band orthogonal frequency division multiplexed signals. Specifically, we analyze the impact of varying the band-spacing and the number of bands. We find that its efficacy is higher for largely spaced bands and a small number of bands. Additionally, we propose a method to include the Four Wave Mixing compensating term within the coupled field method to provide simplified means of multichannel compensation. We conclude that this method is more efficient in improving the performance especially for small band spacings. The coupled field method proves to be an interesting choice for the implementation of receiver-based real-time digital signal processing.     

FWM conversion with pump suppression for filterless optical packet switching applications

The use of a Pump-Suppressed Four Wave Mixing PS-FWM wavelength converter in a wavelength conversion and selection switching scheme is examined and demonstrated. The scheme requires a single 2 × 1 Mach Zenhder Interferometer (MZI) and offers substantial pump suppression and efficiency improvement in terms of optical signal-to-noise ratio. Performance measurements on the combination of PS-FWM wavelength converter with an Arrayed Waveguide Grating (AWG) for passive routing, shows the potentially successful use of the scheme in an ultra-fast wavelength routed optical packet switching scheme where the use of optical filters for the separation of the converted signal from the pump is avoided.     

Influence of complexation and solid environment on the vibrational coherence of DCl

The vibrational dynamics of DCl molecules embedded in different cryogenic matrices is studied by time resolved infrared Degenerate Four Wave Mixing experiments using the infrared free electron laser CLIO. The coherence behaviour of the D-Cl stretch in pure argon as well as in mixed argon/nitrogen van der Waals solids is investigated. Different interactions between the excited molecules and their environment are probed: the van der Waals interaction between the trapped molecules and the solid lattice and the interaction binding complexes trapped in the matrix (either the vdW interaction between DCl and nitrogen or weak H bonds in (DCl)_n clusters). The dependence of the dephasing time on the solid and on specific interactions is clearly observed.     

Investigation on modified FWM suppression methods in DWDM optical communication system

The focus of this paper is to investigate the methods for Four Wave Mixing (FWM) suppression. Modified techniques equal and unequal-channel spacing with polarization, equal channel spacing with alternate channel delay, optical coupling and varied laser power have been proposed to reduce the impact of FWM on Dense Wave Length Division Multiplexing (DWDM) optical communication system. Further the comparison of reduction of FWM for existing and proposed techniques has been discussed by varying the dispersion of fiber from 0 to 16 ps/nm/km. It has been observed that the suggested techniques are simpler to design optical communication system and superior to the existing methods.     

High intensity coherent transient in multiple quantum wells

In this paper we report Degenerate Four Wave Mixing (DFWM) experiments on the excitons of GaAs/AlGaAs Multiple Quantum Well (MQW) at high incident intensities. These experiments provide a clear manifestation of Many-Body Interactions (MBI) among the excitons, in the form of a multi-photon (intensity-dependant) resonance at the low energy side of the heavy-hole exciton of the first subband of the MQW. The temporal resolution of the DFWM response provides a demonstration of the multi-photon nature of this resonance and, hence, of its MBI origin.     

Exploitation of the Fiber Capacity in Optical Networks Using Time, Frequency, and Space Division Multiple Accesses

In this work we investigate how to obtain very high capacity transmissions in optical networks taking into account the limitations due to the physical channel. We consider both the case in which all the users are connected by a star coupler and the case in which the users are directly connected by the network topology. As a reference, we consider a ring network and a Shuffle Multihop Network (SMN). The use of optical systems to implement high-capacity networks is numerically investi gated by means of numerical simulations taking into consideration the channel limitations due to the chromatic dispersion, the Kerr effect, and the amplified spontaneous emission (ASE) noise of the optical amplifiers. In our model, we consider that the signal, during the routing process that is performed at the user position, undergoes only an attenuation. We suppose the use of intensity modulated signals and receivers with direct detection. Packet switching and digital transmission are assumed with soliton and conventional nonreturn to zero signals. Both wave length and time division multiple accesses are considered. The results show that, in the case of the Time Division Multiple Access (TDMA) technique, the use of a star coupler to connect the users reduces the capacity of a network with respect to the case in which a direct connection of the users is used. This is due to the strong power fluctuations that are present during the signal propagation and to the large quantity of accumulated ASE noise. On the other hand, the use of a star coupler shows the advantage to being easily reconfigurable. The Wavelength Divison Multiple Access (WDMA) technique permits us to achieve higher capacities with respect to the TDMA. This is due to the fact that in the propagation conditions, due to the presence of a star coupler, high bit rate signals are strongly degraded. On the other hand, several low bit rate signals operating at different wavelengths can propagate with a low power level, avoiding strong degradation due to the Four Wave Mixing (FWM) effect. Among the topologies considered in this work, the SMN is the one that generally permits us to reach the highest throughput because in the SMN the signal hops in a limited number of Network Interface Units (NIUs) before reaching the final destination.     

Exploitation of the Fiber Capacity in Optical Networks Using Time,Frequency, and Space Division Multiple Accesses

In this work we investigate how to obtain very high capacity transmissions in optical networks taking into account the limitations due to the physical channel. We consider both the case in which all the users are connected by a star coupler and the case in which the users are directly connected by the network topology. As a reference, we consider a ring network and a Shuffle Multihop Network (SMN). The use of optical systems to implement high-capacity networks is numerically investi gated by means of numerical simulations taking into consideration the channel limitations due to the chromatic dispersion, the Kerr effect, and the amplified spontaneous emission (ASE) noise of the optical amplifiers. In our model, we consider that the signal, during the routing process that is performed at the user position, undergoes only an attenuation. We suppose the use of intensity modulated signals and receivers with direct detection. Packet switching and digital transmission are assumed with soliton and conventional nonreturn to zero signals. Both wave length and time division multiple accesses are considered. The results show that, in the case of the Time Division Multiple Access (TDMA) technique, the use of a star coupler to connect the users reduces the capacity of a network with respect to the case in which a direct connection of the users is used. This is due to the strong power fluctuations that are present during the signal propagation and to the large quantity of accumulated ASE noise. On the other hand, the use of a star coupler shows the advantage to being easily reconfigurable. The Wavelength Divison Multiple Access (WDMA) technique permits us to achieve higher capacities with respect to the TDMA. This is due to the fact that in the propagation conditions, due to the presence of a star coupler, high bit rate signals are strongly degraded. On the other hand, several low bit rate signals operating at different wavelengths can propagate with a low power level, avoiding strong degradation due to the Four Wave Mixing (FWM) effect. Among the topologies considered in this work, the SMN is the one that generally permits us to reach the highest throughput because in the SMN the signal hops in a limited number of Network Interface Units (NIUs) before reaching the final destination.     

Performance analysis of dual-pump optical parametric amplifiers in silicon waveguide

Nondegenerate Four Wave Mixing (FWM) process in a silicon waveguide is influenced by two photon absorption (TPA), TPA induced free carrier absorption (FCA) besides self-phase modulation (SPM), cross-phase modulation (XPM), SRS, GVD and linear scattering losses. Influences of pump parameters of the dual-pump parametric amplifier (OPA) in the silicon waveguide on gain spectrum and noise figure are numerically investigated. Compared with single-pump configuration, dual-pump silicon OPA indicates a flat gain and noise figure. Then impact of waveguide dispersion on bandwidth and ripples for dual-pump silicon optical parametric amplifiers is discussed. Optimal pump configurations are evaluated. The results show that polarization independent net gain of silicon OPA can be achieved by using circular and orthogonally polarized pump.     

Analysis of 2.5 Gbit/s GPON downlink optical-receiver performance

In the gigabit-capable passive optical network (GPON) optical communication system, the selection of fiber and system performance-analysis is the key links for the realization of system function. Especially the characteristic budget and parameter setting of downlink receiver are topped the list. The analysis of receiver power penalty can reduce the influence on receiver sensitivity and bit error rate (BER) that caused by waveguide dispersion and pulse widening. This article will simulate the performance of GPON downlink receiver, then analyze typical characteristics such as Four Wave Mixing (FWM), Erbium-Doped Fiber Amplifier (EDFA), system eye pattern and Q factor and so on, so as to validate the feasibility of the optical downlink.     

Nonlinear optical studies of a newly synthesized copolymer containing oxadiazole and substituted thiophenes

We investigated the third-order nonlinear optical properties of a newly synthesized soluble copolymer containing oxadiazole and thiophene units using Z-scan and Degenerate Four Wave Mixing (DFWM) techniques. The measurements were performed at 532 nm with 7 ns pulses from a Nd:YAG laser. We found a good agreement between the values of χ⁽³⁾ determined from both experiments. Z-scan results indicate a negative nonlinear refractive index, n₂, whose magnitude is of the order of 10⁻¹⁰ esu. The copolymer exhibits strong nonlinear absorption and good optical limiting properties at 532 nm, and hence may be a potential material for optical limiting applications.     

Analysis of bottlenecks in DWDM fiber optic communication system

This paper deals with the limiting factors like Stimulated Brillouin Scattering (SBS), Stimulated Raman Scattering (SRS) and Four Wave Mixing (FWM) in DWDM Fiber Optic Communication. The variation of SBS threshold power level for the spectral width of the source is analyzed and it is found that addition of spectral width in the source lowers the SBS effect. By reducing the SBS effect, we could transmit more optical power in a single channel and maintain high OSNR. Similarly the effect of phase mismatching (dispersion) and wavelength spacing on both FWM and SRS are analyzed. This work aims at finding the optimal value of spectral width, dispersion and channel spacing, thereby exploring the possibilities of increasing the overall capacity of the fiber optic trunk line.     

混合比和燃烧组分浓度测量系统的原理和设计(英文)

研制了一套新型测量系统,系统由YGA激光器、染料激光器、晶体组、CCD摄像机、图像处理系统和附件等组成.与反斯托克斯拉曼散射、激光诱导荧光、平面激光诱导荧光和拉曼散射系统相仿,该系统具有测量燃烧组分浓度的功能.此外,它还可以测量喷嘴氧化剂和燃料的混合比分布,测量参数是给定时间内给定区域的统计值.系统已经成功用于液体火箭发动机气/气喷嘴、气/液喷嘴和液/液喷嘴喷雾场混合比分布特性研究,并已用于CO、CO2、No、NO2、OH和H2O(g)等燃烧产物组分的浓度测量,这种新系统将成为液体火箭发动机工作过程研究的有力分析工具.     

混合比和燃烧组分浓度测量系统的原理和设计

研制了一套新型测量系统，系统由YGA激光器、染料激光器、晶体组、CCD摄像机、图像处理系统和附件等组成．与反斯托克斯拉曼散射、激光诱导荧光、平面激光诱导荧光和拉曼散射系统相仿，该系统具有测量燃烧组分浓度的功能．此外，它还可以测量喷嘴氧化剂和燃料的混合比分布，测量参数是给定时间内给定区域的统计值．系统已经成功用于液体火箭发动机气／气喷嘴、气／液喷嘴和液／液喷嘴喷雾场混合比分布特性研究，并已用于CO、CO2、NO、NO2、OH和H2O（g）等燃烧产物组分的浓度测量，这种新系统将成为液体火箭发动机工作过程研究的有力分析工具．     

Improving the efficiency of an acoustooptic modulator with a two-lobe directivity pattern by correcting the two-frequency electric signal

The feasibility of forming an efficient two-lobe directivity pattern at Bragg acoustooptic diffraction is theoretically substantiated for the first time. It is shown that the efficiency of diffraction into main beams can be increased by mixing the two-frequency signal with additional spectral components that correspond to the closest combination frequencies. Mixing is accomplished using an original electronic circuit designed for polynomial nonlinear correction of the control signal. Bragg diffraction by the corrected signal is studied experimentally in a high-efficiency paratellurite (TeO₂) modulator. Satisfactory agreement with the theory is obtained, and results of applied interest are obtained. In particular, the maximal overall efficiency of diffraction into two main beams is increased from 68 to 79%.     

Millimeter Wave Radar for detecting the speech signal applications

MilliMeter Wave (MMW) Doppler Radar with grating structures for the applications of detecting speech signals has been discovered in our laboratory. The operating principle of detection the acoustic wave signals based on the Wave Propagation Theory and Wave Equations of The ElectroMagnetic Wave (EMW) and Acoustic Wave (AW) propagating, scattering, reflecting and interacting has been investigated. The experimental and observation results have been provided to verify that MMW CW 40GHz dielectric integrated radar can detect and identify out exactly the existential speech signals in free space from a person speaking. The received sound signal have been reproduced by the DSP and the reproducer.Research project supported financially from the NSFC (National Natural Science Foundaton of China).     

Novel non-linear optical techniques for diagnostics: laser-induced gratings and two-colour four-wave mixing

Electrostriction, population transfer caused by absorption of laser energy and subsequent thermalization are the most important mechanisms responsible for the formation of laser-induced gratings. Applying time resolved light diffraction from electrostrictive gratings allows for the measurements of thermodynamic properties like temperature, gas composition, and flow velocity. Thermal gratings can be employed as a very sensitive spectroscopic tool.The Two-Colour Four-Wave Mixing (TCFWM) signal formed by a non-resonant pump laser and a resonant probe laser in an acetylene/air diffusion flame comprises of contributions due to soot and C₂, the latter is generated by vaporising the soot. Profile measurements across the flame have shown that the signal composition changes within the flame.Intermediate level labelling by Two-Colour Resonant Four-Wave Mixing Spectroscopy (TC-RFWM) is applied to reduce the spectral complexity of molecular spectra. In addition, we observe extra resonances forbidden by a conventional three-level scheme in OH- and NH-containing flames. A detailed investigation shows that the newly observed TC-RFWM resonances are induced by collisional energy transfer within the population, alignment and orientation gratings formed in both electronic states that are coupled to the laser fields.     

Gravitational wave production at the end of inflation

We consider gravitational wave production due to parametric resonance at the end of inflation, or "preheating." This leads to large inhomogeneities that source a stochastic background of gravitational waves at scales inside the comoving Hubble horizon at the end of inflation. We confirm that the present amplitude of these gravitational waves need not depend on the inflationary energy scale. We analyze an explicit model where the inflationary energy scale is approximately 10{9} GeV, yielding a signal close to the sensitivity of Advanced Laser Interferometer Gravitational Wave Observatory and Big Bang Observer. This signal highlights the possibility of a new observational "window" into inflationary physics and provides significant motivation for searches for stochastic backgrounds of gravitational waves in the Hz to GHz range, with an amplitude on the order of Omega_{gw}(k)h{2} approximately 10{-11}.     

少模光纤中信号光的超宽带拉曼放大

本文利用受激拉曼散射和受激四光子混频的联合效应，对少模光纤中波长连续可调的信号光进行放大研究。实验发现，在５６００－６１００的调谐范围内，波长的变化对放大倍数影响不大。文中分析了放大的机理     

Photonic crystal electro-optical switching cell

We investigated the physical mechanism of a photonic crystal (PhC) switching cell based on an optical directional coupler (ODC). This ODC is driven by a low power external electrical command signal, inserted in the central coupling region, which causes the changes in the refractive index.The switching process is based on the change of the bar state to the cross state owing to the external command signal. In our simulations we used the following methods: Plane Wave Expansion by MPB (MIT Photonic-Bands), Finite-Difference Time-Domain by MEEP (MIT Electromagnetic Equation Propagation), Finite Element by COMSOL Multiphysics and our own Binary Propagation Method.     

Behavior of the soliton pulse envelope in a nonlinear medium in the subpicosecond range

The impact of additional higher dispersion and nonlinear terms in the model equation on the properties of short pulses is studied. The analysis is based on two mutually complementary asymptotic approaches. We found explicit expressions describing distortions in the amplitude and phase of the signal envelope. The conditions under which the additional terms compensate for each other are examined. Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation of the Russian Academy of Sciences, Troitsk, Moscow region, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 9, pp. 1117–1124, September, 1998.