Enhancing and suppressing four‐wave mixing in electromagnetically induced transparency window

We demonstrate the enhancement and suppression of four‐wave mixing (FWM) in an electromagnetically induced transparency (EIT) window in Y‐type ⁸⁵Rb atomic system. The generated two‐photon FWM signal can be selectively enhanced and suppressed via an EIT window. The EIT of probe as well as the enhancement and suppression of FWM signals can be modified by the sequential‐cascade double dressing. The influence of different probe polarization configurations is also studied. Different polarization states of the probe laser can select different transitions among Zeeman sublevels and different dressing strengths. Copyright © 2010 John Wiley & Sons, Ltd.     

N5B五能级系统中重复缀饰四波混频研究

从理论上研究了N5B五能级系统中一个激光场重复缀饰四波混频过程.结合能级图分析,从它特殊的Autler-Town分裂峰中可以非常清晰地看出其重复缀饰的作用.还研究了在强Probe场和强耦合场下N5B五能级下缀饰四波混频信号的抑制增强现象.采取独特的处理方法——独立作用法,研究结果表明一个激光场作用于N5B五能级系统时存在两次缀饰,并形成缀饰能级的二重分裂或者三重分裂,不同于多个耦合场对原子的多重缀饰作用.应用于非线性光谱术中对多峰结构的研究. 关键词： 四波混频 电磁感应透明 重复缀饰     

Enhancement and Suppression of Four-Wave Mixing in a Four-Level Atomic System

We report observations of the enhancement and suppression of four-wave mixing （FWM） in an electromagnetically induced transparency window in a Y-type ^85 Rb atomic system. The results show the evolution of the dressed effects （from pure enhancement to partial enhancement/suppression, and finally into pure suppression） in the degenerate-FWM processes. Moreover, we use the perturbation chain method to describe the FWM process. Finally, we observe the polarization dependence of the enhancement and suppression of the FWM signal.     

Enhancement and suppression of spatial splitting four-wave mixing in atomic medium

We experimentally report on the evolution from singly-dressed to doubly-dressed four-wave mixing (FWM) process by controlling the powers of the probe, the pump and the dressing fields respectively. The differences in the enhancement and the suppression of FWM signal between the two-level and cascade three-level atomic systems are observed and explained by the multi-dressed effect theoretically. Both the x direction and the y direction spatial splittings of the degenerate-FWM (DWFM) beams are obtained. We also investigate the switch between the enhancement and the suppression of the DWFM signals and between its spatial splittings in x direction and y direction. The spatial splittings in x direction and y direction can be controlled by the relative position and the intensity of the involved laser beams. Such a study can be useful for optimizing the efficiency of the FWM process and providing potential applications in spatial signal processing.     

受限原子极化相干缀饰四波混频光谱

研究了两电介质面间级联四能级系统原子的缀饰四波混频(FWM)光谱.在缀饰场的作用下FWM信号可产生Autler-Townes (AT)分裂,分裂所产生的峰及凹陷的线型及缀饰场对FWM信号的抑制与增强效应均受原子极化相干及受限原子与光场相互作用瞬态机制的调制.     

Wavelength dependence four-wave mixing spectroscopy in a micrometric atomic vapour

This paper presents a theoretical study of wavelength dependence four-wave-mixing (FWM) spectroscopy in a micrometric thin atomic vapour. It compares three cases termed as mismatched case Ⅰ, matched case and mismatched case Ⅱ for the probe wavelength less, equal and greater than the pump wavelength respectively. It finds that Dickenarrowing can overcome width broadening induced by Doppler effects and polarisation interference of thermal atoms, and high resolution FWM spectra can be achieved both in matched and mismatched wavelength for many cases. It also finds that the magnitude of the FWM signal can be dramatically modified to be suppressed or to be enhanced in comparison with that of matched wavelength in mismatched case Ⅰ or Ⅱ. The width narrowing and the magnitude suppression or enhancement can be demonstrated by considering enhanced contribution of slow atoms induced by atom-wall collision and transient effect of atom-light interaction in a micrometric thin vapour.     

Use of multimode optical fibers for fiber-based coherent anti-Stokes Raman scattering microendoscopy imaging

A multimode fiber (MMF) was used for both delivery of excitation lasers and collection of returned coherent anti-Stokes Raman scattering (CARS) signals in a CARS microendoscopy prototype imaging system. We demonstrated a polarization-based scheme for suppression of four-wave mixing (FWM) signals in delivery fibers. Our experimental results showed that this polarization-based FWM-suppressing scheme can dramatically reduce FWM signals generated in MMFs, and MMFs can be used to produce CARS images in this microendoscopy system. The proposed MMF-based CARS microendoscopy imaging system with the polarization-based FWM-suppressing scheme offers a potential platform for building fiber-based CARS microendoscopes that can effectively suppress FWM background noises.     

Reduction of fiber-nonlinearity-enhanced amplifier noise by means of phase-sensitive amplifiers

In optical fiber transmission systems near the zero-dispersion wavelength that use in-line erbium-doped fiber amplifiers (EDFA's), the enhancement of optical amplifier noise caused by four-wave mixing (FWM) in transmission fibers degrades signal-to-noise ratio (SNR) excessively. We theoretically show that the enhancement of amplifier noise by the FWM in transmission fibers can be effectively eliminated by implementing in-line phase-sensitive amplifiers (PSA's). Small-signal analysis of the nonlinear Schr?dinger equation shows that the transmission distance limited by the SNR of an in-line PSA system is expanded four times more than that of an in-line EDFA system.     

Characterization of the pairwise correlations in different quantum networks consisting of four-wave mixers and beamsplitters

We investigate the performances of the pairwise correlations(PCs) in different quantum networks consisting of fourwave mixers(FWMs) and beamsplitters(BSs). PCs with quantum correlation in different quantum networks can be verified by calculating the degree of relative intensity squeezing for any pair of all the output fields. More interestingly, the quantum correlation recovery and enhancement are present in the FWM+BS network and the repulsion effect phenomena(signal(idler)-frequency mode cannot be quantum correlated with the other two idler(signal)-frequency modes simultaneously)between the PCs with quantum correlation are predicted in the FWM + FWM and FWM + FWM + FWM networks. Our results presented here pave the way for the manipulation of the quantum correlation in quantum networks.     

Coherent control of dressed images of four-wave mixing

In two-level as well as V-type three-level atomic systems, we study probe transmission, four-wave mixing (FWM) and fluorescence signals with dressing effect experimentally and theoretically. We find both the hyperfine structure (at the same energy level) and the transition dipole moment (at different energy levels) can affect the dressing effect. We also experimentally investigate that angle-control dynamics in the nonlinear propagation of the images of the probe and generated FWM in two-level atomic systems, and find that the focusing and defocusing of probe beam and FWM signals can be greatly affected by the angles between dressing fields.     

Spatial Splitting and Intensity Suppression of Four-Wave Mixing in V-Type Three-Level Atomic System

We illustrate our experimental observation of coexisting the controllable spatial splitting and intensity suppression of four-wave mixing （FWM） beam in a V-type three-level atomic system. The peak number and separation distance of the FWM beam are controlled by the intensities and frequencies of the laser beams, as well as atomic density.     

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.     

Detailed theoretical investigation on relative intensity noise reduction enhancement based on reflective SOAs

The enhanced performance for relative intensity noise (RIN) reduction based on reflective semiconductor optical amplifiers (R-SOA) has been investigated theoretically by comparison with conventional transmission SOA. The results show that, by selecting appropriate input optical power, as large as >20 dB RIN suppression improvement can be achieved for R-SOA, without sacrificing the noise rejection bandwidth. With increased injection current, the optimized input signal power is decreased and the operation region is extended for the best RIN reduction. For RIN suppression in WDM spectrum slicing, the bandwidth optimization of receiver filter should be performed to avoid the spectral broadening induced by self-phase modulation (SPM) and four wave mixing (FWM). Our derived result is helpful for designing and optimizing the R-SOA in application of noise suppression enhancement.     

Influence of degenerate four-wave mixing on the performance of supercontinuum-based multiwavelength optical source

The influence of degenerate four-wave mixing (FWM) on the performance of supercontinuum-based multiwavelength optical source has been investigated in detail experimentally and theoretically. Numerical simulation results show that the degenerate FWM effect has a deteriorative influence on the spectral uniformity and the optical signal-to-noise ratio (OSNR) of supercontinuum-based optical source, and by suppressing degenerate FWM effect the performance enhancement of the supercontinuum can be achieved successfully. These results are also confirmed by our experiments. Experimentally, by suppressing degenerate FWM the crosstalk of adjacent channels to the filtered channel can be reduced by as much as 15 dB, and consequently the measured receiver sensitivity at 10 Gbit/s for the filtered optical source is improved from $-1.7$ to $-17.8$~dBm.     

Performance investigation of suppression of four wave mixing using optical phase conjugation with different modulation format in DWDM soliton communication system

The performance of dense wavelength division multiplexed (DWDM) soliton transmission system for returnto-zero (RZ) and non-return-to-zero (NRZ) modulation formats have been investigated. The main aim of this paper is to estimate and mitigate the four wave mixing (FWM) power by using in-line optical phase conjugator (OPC). The effect of FWM has been estimated using real fiber link having nonlinear and attenuation losses. The FWM power is strongly suppressed by introducing destructive interference between the first and second halves of in-line OPC. It has been indicated that RZ with OPC yields the better performance with FWM power suppression (more than 20 dBm in certain cases) with reasonable bit error rate and Q-factor.     

Parametric analysis of four wave mixing in DWDM systems

The developments made over the years in WDM (Wavelength Division Multiplexing), on one side, produced compensation of increasing traffic and channel capacity, but on other side, pertinently caused WDM systems to undergo different linear and non-linear effects thereby degrading the performance as such. To overcome linear effects is relatively convenient and manageable, however, the non-linear effects including four wave mixing (FWM) is not only difficult to eliminate but also highly incompatible from system point of view. In this research, valid parameters which lead to generation and enhancement of FWM, are discussed, simulated and based on exhaustive sets of obtained results, a detailed analysis has been carried out to reduce the FWM. The simulated results obtained by exploiting nine important parameters using Optisys simulator, divulge very incisive information for the removal of nonlinear effects in WDM. The authors are reasonably confident to extend their results as a contribution in the subject area.     

Priority-based parameter optimization strategy for reducing the effects of four-wave mixing on WDM system

In order to meet the ultra high speed and ultra long-haul transmission distance in wavelength division multiplexing (WDM) systems, the nonlinear impairment affecting the overall spectral efficiency and system performance should be minimized. This paper proposes a strategy to mitigate the four-wave mixing (FWM) effect in WDM system. The strategy determines the effect of both single and combined effects of second, third, and fourth optimization priority parameters such as fiber length, input power, dispersion, channel spacing, and effective area on FWM power. A comparison study was made under different types of optical fiber such as single-mode fiber (SMF), dispersion shifted fiber, non-zero dispersion fiber, and non-zero dispersion shifted fiber. In addition, the system performance in term of bit-error-rate was calculated in the case of single priority (impact of effective area) and combined priority (impact of effective area, input power, fiber length and channel spacing). The results show that the FWM effect was reduced based on the transmission parameters order of optimization, i.e., priority selection proposed. Moreover, the results indicated that increasing sequentially the effective area, fiber length; channel spacing and decreasing the input power provide the most significant sequence in suppressing the effects of FWM. This priority sequence brought the suppression ratio to approximately 26.3% in SMF, which suppressed the FWM effects up to −50 dBm. In term of BER; the combined priority introduces improvement in BER of 2.31 × 10⁻²⁵ in comparison with single priority that has value of BER 4 × 10⁻¹⁴. Finally, this work suggests that the proposed priority-based parameter optimization strategy is an ideal solution for optimum performance of WDM system.     

DWDM系统非简并四波混频串扰的分析

考虑到DWDM系统中的比特序列的随机性、信道间脉冲走离效应等多种因素的影响,完善了归零调制格式下的四波混频效率的计算方法,给出了相应的非简并四波混频噪声标准差的计算公式,并利用该公式进行实例分析,计算结果表明,当占空比和信道间隔变小时,不同信道内的随机比特序列的相对初始时延及光纤色散值的取值组合对四波混频效率的影响变得更大,为保证系统的低色散和低四波混频串扰特性,必须对这些量取值组合进行优化. 关键词： 非简并四波混频 脉冲走离 密集波分复用     

Effects of frequency/polarization allocations and the zero-dispersion frequency on FDM lightwave transmission systems

In long-haul frequency-division-multiplexing lightwave transmission systems, transmission characteristics are degraded by four-wave mixing (FWM) generated in optical fibers. To overcome this problem, several FWM suppression techniques have been reported. In this paper, dependence of FWM noises on frequency allocations, polarization allocations, and the zero-dispersion frequency is investigated. It is revealed that FWM noises are drastically decreased in frequency allocations such as equally spaced, repeated unequally spaced (RUS), equally spaced RUS, and unequally spaced RUS allocations by arranging polarization allocations of the channels with an increase in a separation between the channel frequencies and the zero-dispersion frequency.     

Optimized all-optical amplitude reshaping by exploiting nonlinear phase shift in fiber for degenerated FWM

We describe a novel performance optimization method for all-optical amplitude reshaping via degenerated four-wave mixing(FWM) in highly nonlinear optical fiber.The proposed optimization method is achieved by judiciously configuring the FWM operational condition and exploiting the nonlinear phase shift induced by self-and cross-phase modulations to properly influence the FWM conversion efficiency.Through the proposed scheme,fully functional and prevailing reshaping performance,including significant amplitude jitter suppression and extinction ratio improvement,is obtained within a single FWM stage.Results of the present theoretical calculation and numerical simulation verify the feasibility and advantages of the proposed scheme.