Signal-to-noise ratio improvement of a supercontinuum continuous-wave optical source using a dispersion-imbalanced nonlinear optical loop mirror

A multiwavelength continuous-wave (cw) optical source is based on supercontinuum (sc) produced in a highly nonlinear fiber. Noise properties of this optical source are studied through numerical simulations based on the nonlinear Schrödinger equation. The numerical simulations show that the amplified spontaneous emission (ASE) generation in an erbium-doped fiber amplifier and the ASE amplification during the sc generation result in a serious degradation of the signal-to-noise ratio (SNR). A novel scheme for improvement of the SNR of the multiwavelength cw optical source based on sc by using a dispersion-imbalanced nonlinear optical loop mirror is presented and the numerical simulation results are given. As a result, the average level of the SNR improvement is 11 dB and the eye-opening penalty is reduced by 0.91 dB due to the SNR improvement. In addition, the scheme can also be applied for improvement of the SNR of a pulse-type multiwavelength light source. PACS 42.50.Lc; 42.65.Re; 42.79.Sz; 02.60.Cb     

Analytical design of nonlinear optical loop mirrors for fiber-optic communication systems

We propose an easy and efficient method for analytically designing nonlinear optical loop mirrors (NOLMs) for fiber-optic communication systems. This analytical design is based on a Taylor series expansion of the transfer function of the NOLM, from which highly stable dynamical regimes can be readily obtained for any desired pulse parameters. We present numerical simulations showing dramatically improved performances in a 160 Gb/s transmission system that incorporates the NOLMs designed analytically.     

Generating a high-extinction-ratio pulse from a phase-modulated optical signal with a dispersion-imbalanced nonlinear loop mirror

We demonstrate a method for generating ultrashort pulses from a phase-modulated optical signal by using a dispersion-imbalanced nonlinear loop mirror instead of the traditional linear dispersion medium. The extinction ratio of the pulses is greatly improved at the same time. By controlling the bandwidth of the phase-modulated signal and the dispersion map of the dispersion-imbalanced nonlinear loop mirror, we can control the pulse width from several picoseconds to hundreds of femtoseconds.     

Autosoliton transmission in dispersion-managed systems guided by in-line nonlinear optical loop mirrors

We examine the feasibility of optical pulse transmission in dispersion-managed fiber systems with in-line nonlinear optical loop mirrors. Applying numerical analysis, we find regimes of stable propagation over long distances in such lines, with a significant increase in the signal-to-noise ratio.     

Experimental observation of autosoliton propagation in a dispersion-managed system guided by nonlinear optical loop mirrors

Observation of autosoliton propagation in a dispersion-managed optical transmission system controlled by in-line nonlinear fiber loop switches is reported for what is believed to be the first time. The system is based on a strong dispersion map with large amplifier spacing. Operation at transmission rates of 10 and 40 Gbits/s is demonstrated.     

All-optical pulse shaping for ultrawideband doublet pulses using nonlinear optical loop mirror with optical parametric amplification

We propose and experimentally demonstrate an alternative photonic scheme for the generation of ultrawideband (UWB) doublet pulses, which is based on an optical fiber-based nonlinear optical loop mirror (NOLM) incorporating the optical parametric amplification (OPA) effect. The proposed scheme uses both cross-phase modulation and OPA within an optical fiber-based NOLM to produce an ideal transfer function for the shaping of input soliton pulses into doublet pulses. Using the proposed scheme, a successful conversion of input soliton pulses into doublet pulses is readily demonstrated. The system performance of UWB doublet pulses is also assessed by propagating the 1.25?Gbit/s doublet pulses over a fiber link. Error-free UWB doublet signal transmission is demonstrated.     

Dynamics of optical pulses in periodic nonlinear fibers

Dynamics of a two-mode wave packet with strong linear and nonlinear (cross-modulation) coupling is investigated. The effect of the initial conditions of the pulse excitation on its further transformation is considered. The possibility to control the dispersion parameters of the wave packet by varying the initial conditions of its input is pointed out. The effective parameters of the dispersion and nonlinearity that govern the dynamics of an optical pulse in a periodic nonlinear fiber light guide are obtained.     

Propagation of optical pulses in periodic nonlinear fibers

We study the conditions of formation and propagation of soliton pulses in a periodic two-mode nonlinear fiber with allowance for dispersion of the fiber material and intermode dispersion of interacting modes under the condition of their phase synchronism. State University, Ul’yanovsk, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 8, pp. 1032–1042, August, 1998.     

Picosecond soliton transmission using concatenated gain-distributed nonlinear amplifying fiber loop mirrors

Stable picosecond soliton transmission is demonstrated numerically by use of concatenated gain-distributed nonlinear amplifying fiber loop mirrors (NALMs). We show that, as compared with previous soliton transmission schemes that use conventional NALMs or nonlinear optical loop mirror (NOLM) and amplifier combinations, the present scheme permits significant increase of loop-mirror (amplifier) spacing. The broad switching window of the present device and the high quality pulses switched from it provide a reasonable stability range for soliton transmission. Soliton-soliton interactions can be reduced efficiently by using lowly dispersive fibers.     

飞秒激光在晶体中传输的级联非线性过程

系统地回顾了有关级联过程导致的各种非线性效应的工作,指出了级联效应可导致不同光谱成分间出现能量转移现象,且二阶级联过程中产生的白光还能够提供种子光进而得到基于参量放大过程的宽带锥形辐射。在此基础上,进一步研究了入射的基频波长对于超连续白光能量转移效果及产生倍频效率的影响以及二阶级联过程引起的偏振调制现象,指出当入射波长越靠近相位匹配波长时倍频效率越高,发现二阶级联效应还可以引起显著的基频光偏振调制。     

飞秒激光在晶体中传输的级联非线性过程

系统地回顾了有关级联过程导致的各种非线性效应的工作,指出了级联效应可导致不同光谱成分间出现能量转移现象,且二阶级联过程中产生的白光还能够提供种子光进而得到基于参量放大过程的宽带锥形辐射。在此基础上,进一步研究了入射的基频波长对于超连续白光能量转移效果及产生倍频效率的影响以及二阶级联过程引起的偏振调制现象,指出当入射波长越靠近相位匹配波长时倍频效率越高,发现二阶级联效应还可以引起显著的基频光偏振调制。     

Short optical pulse profile characterization using a nonlinear optical loop mirror

We propose in this work a technique for short pulse profile retrieval based on the Kerr effect in a fiber nonlinear optical loop mirror (NOLM). Under some assumptions, the profile can be determined from the energy transfer characteristic of the pulses through the NOLM, which can be measured with a low-frequency detection setup. Two numerical approaches are considered, both relying on the resolution of a system of nonlinear algebraic equations, and which differ in the way that the profiles are discretized. We show numerically that both approaches allow proper profile retrieval for a wide variety of pulse shapes. The two techniques are compared, and their advantages and drawbacks are discussed. The effect of the amplitude noise of the pulses is assessed, as well as the impact of an inaccurate knowledge of the NOLM transfer function, or of the energy transfer characteristic. The technique is demonstrated in the frame of the characterization of both ns and ps pulses.     

非线性光纤环形镜的脉冲透过特性研究

研究了光纤反常色散区非线性环形镜（NOLM）的脉冲透过特性，得到NOLM周期性透过率函数 第一极大值处透过率、压缩比和对应的孤子阶数与环长之间的函数关系图.通过比较长环和 短环NOLM对无啁啾、啁啾脉冲的透过率和压缩比特性，得出了长环有利于脉冲整形而短环有 利于脉冲压缩的结论. 关键词： 非线性环形镜 透过率 压缩比 孤子阶数     

Bit-error-rate analysis for a 100-Gbit/s nonlinear optical loop mirror demultiplexer employing soliton control and signal pulses

On the basis of a previously derived mathematical model, the power penalty associated with a nonlinear optical loop mirror (NOLM) demultiplexer was calculated, with timing jitter noise, bit-to-bit cross talk, and channel cross talk taken into consideration. Simulation results show that a 3-dBm improvement in the receiver sensitivity at a bit error rate of 10(-9) can be achieved by choice of the appropriate walk-off time between the control and the signal pulses within the NOLM demultiplexer.     

Spectral broadening of femtosecond pulses in an nonlinear optical fiber amplifier

A promising method for the generation of a supercontinuum with a high spectral power density based on the spectral broadening of ultrashort pulses in a fiber amplifier is considered. The advantage of the method, as compared to the conventional way of the supercontinuum generation in a microstructure fiber, is a lower pulse spectral broadening rate, which allows one to achieve higher SC spectral power densities. The initial stage of the supercontinuum generation in an ytterbium fiber amplifier (a fiber core diameter of 7 μm) with side pumping from an array of laser diodes with a total power of 8 W at a wavelength of 976 nm is experimentally studied. Yb:KYW laser pulses with a duration of 250 fs, a central wavelength of 1046 nm, and an average power of 150 mW have been supplied to the input of the amplifier. In this case spectrally broadened radiation with an average spectral power density of higher than 65 mW/nm and a spectrum width of 50 nm has been obtained.     

Soliton squeezing in a highly transmissive nonlinear optical loop mirror

A perturbation approach is used to study the quantum noise of optical solitons in an asymmetric fiber Sagnac interferometer (a highly transmissive nonlinear optical loop mirror). Analytical expressions for the three second-order quadrature correlators are derived and used to predict the amount of detectable amplitude squeezing along with the optimum power-splitting ratio of the Sagnac interferometer. We find that it is the number-phase correlation owing to the Kerr nonlinearity that is primarily responsible for the observable noise reduction. The group-velocity dispersion affecting the field in the nonsoliton arm of the fiber interferometer is shown to limit the minimum achievable Fano factor.     

Third-order nonlinear optical response in transparent solids using ultrashort laser pulses

The third-order optical nonlinearity, χ ⁽³⁾, is measured in transparent glasses (BK7 and fused silica) and crystals (BaF₂ and quartz) using 36-fs, 800-nm laser pulses and the optical Kerr gate (OKE) technique; values are found to lie in the range 1.3–1.7×10^-14 esu, in accordance with theoretical estimates. We probe the purely electronic response to the incident ultrashort laser pulse in fused silica and BK7 glass. In BaF₂ and quartz, apart from the electronic response we also observe contribution from the nuclear response to the incident ultrashort pulses. We observe oscillatory modulations that persist for ～400 fs. The response of the media (glasses and crystals) to ultrashort pulses is also measured using two-beam self-diffraction; the diffraction efficiency in the first-order grating is measured to be in the range of 0.06–0.13 %. Third harmonic generation due to self-phase matching in the transient grating geometry is measured as a function of temporal delay between the two incident ultrashort pulses, yielding the autocorrelation signal.     

Waves in nonlinear lattices: ultrashort optical pulses and Bose-Einstein condensates

The nonlinear Schr?dinger equation i (partial differential)(z)A(z,x,t)+(inverted Delta)(2)(x,t)A+[1+m(kappax)]|A|2A=0 models the propagation of ultrashort laser pulses in a planar waveguide for which the Kerr nonlinearity varies along the transverse coordinate x, and also the evolution of 2D Bose-Einstein condensates in which the scattering length varies in one dimension. Stability of bound states depends on the value of kappa=beamwidth/lattice period. Wide (kappa>1) and kappa=O(1) bound states centered at a maximum of m(x) are unstable, as they violate the slope condition. Bound states centered at a minimum of m(x) violate the spectral condition, resulting in a drift instability. Thus, a nonlinear lattice can only stabilize narrow bound states centered at a maximum of m(x). Even in that case, the stability region is so small that these bound states are "mathematically stable" but "physically unstable."