Super-FM and analogue-to-digital-conversion technologies for next-generation wireless access in optical fibres

Fibre-in-the-loop (FITL) or hybrid fibre coaxial (HFC) networks are the ideal transmission infrastructure for next-generation wireless communication networks, such as digital microcells, personal communication networks (PCNs) and wireless local area networks (LANs). In this paper, the feasibility of two cost-effective optical fibre transmission technologies, super-frequency-modulation (FM) and analogue-to-digital conversion, for wireless access in broadband optical fibres are experimentally demonstrated. Associated circuit and system implementation issues are investigated. Furthermore, by using a statistical model of multiple access in different microcellular environments, we find that both transmission technologies can meet the dynamic range requirements with a significant safe margin.     

Spatial coherence properties of light from optical fibres

The mutual intensity of the optical field in the Fraunhofer diffraction region produced from the end of an optical fibre under illumination of a coherent beam, is theoretically analysed by using an optical fibre-end source model. In this source model, the optical field at the end of the fibre is expressed as an aggregation of independent plane waves with random directions and phases, because the guided modes propagating through the fibre may be regarded as a stochastic process. It is found that the optical field in the diffraction region from the multimode optical fibre is not homogeneous butquasi homogeneous in a statistical sense. The spatial coherence properties in the field are discussed in connection with theV number (the normalized frequency) of the fibre and in comparison with those in the field produced from an incoherent source obeying the van Cittert-Zernike theorem.     

Propagation of partial coherence along optical fibres

The degree of coherence of the field along an optical fibre excited by a spatially coherent source is discussed. The degree of coherence undergoes a transitions as the source becomes less coherent; initially it is determined by the source coherence properties but for highly incoherent source it is controlled by fibre acceptance parameters.     

Development of ultrahigh-precision coherent control and its applications

Coherent control is based on optical manipulation of the amplitudes and phases of wave functions. It is expected to be a key technique to develop novel quantum technologies such as bond-selective chemistry and quantum computing, and to better understand the quantum worldview founded on wave-particle duality. We have developed high-precision coherent control by imprinting optical amplitudes and phases of ultrashort laser pulses on the quantum amplitudes and phases of molecular wave functions. The history and perspective of coherent control and our recent achievements are described.     

四波混频在长距离相干光纤传输中的影响

本文研究光纤非线性现象之一的四波混频对长距离相干多路光纤传输系统的影响.通过对一条长度为550km,采用9个掺铒光纤放大器的频分复用系统的实验,清楚地观察到了四波混频这一光纤非线性.实验的结果符合对四波混频的理论分析与计算.结果表明:对信息容量在每秒10~9bit数量级,采用常规的不归零码作信号方式的多路传输,光纤中的四波混频现象将严重地影响其传输性能.     

Amplification Characteristics of Optical Fibres Doped with Nano Materials of InP

We take an enhanced structure of existing optical fibres to investigate the amplification characteristics. Nano and optical fibre technologies both are employed in our structure, i.e. the so-called nano-film amplification fibre where nano-film is inserted between the core and the inner cladding of the optical fibre. In the developed model, InP is chosen as a doped semiconductor to examine its amplification performance and 69.7nm is chosen as the thickness of the film. From our experimental results, the optical fibre structure shows its significant amplification characteristics for wavelength between 1080nm and 1491 nm. Amplification characteristics are also found in wavelengths 906-1044 nm and 1524-1596nm.     

Spatial coherence of laser light propagating in an optical fibre

The modulus of the complex degree of spatial coherence at the exit face of an optical fibre is determined for a quasi-monochromatic, spatially coherent source. It is found that the contribution of the cross-terms between many modes to the formulus of pointwise correlation is negligible for a highly multimode fibre in which even and odd modes are equally excited, although care must be taken when even or odd modes only are selectively excited in the waveguide. Experimental results obtained by using a Koesters prism in the wavefront reversing interferometer are also presented for a graded-index multimode fibre and a quasi-single mode fibre. Modal contents of the propagating field in the optical fibre can be successfully determined, provided that the coherence time of a quasi-monochromatic source and the path difference of the wavefront reversing system are knowna priori.     

Semiconductor optical fibres: progress and opportunities

This paper reviews recent progress in the nascent field of semiconductor optical fibres, from the fundamentals through to device demonstration. The incorporation of semiconductor materials into both the step‐index and microstructured fibre geometries provides a route to introducing new optoelectronic functionality into existing glass fibre technologies. Herein, the various fabrication methods that have been developed as of to date are described, and their compatibility with the different semiconductor materials and fibre designs discussed. Results will be presented on the optical transmission properties of several fibre types, with particular attention being paid to the observation of nonlinear propagation in silicon core fibres. Finally, some speculation regarding the future prospects and applications of this new class of fibre will be provided.     

Measurement of polarization fluctuations in installed single-mode optical fibre cables

Measurements were made of the random polarization fluctuations of conventional single-mode fibres of five optical fibre cables installed in the city of Berlin (West). The measurements were performed with the aid of a semiconductor laser, the frequency (f) of which was 227.3 THz (wavelength 1320 nm). Such preliminary studies are very important for optically coherent transmission over cables not maintaining the polarizations. The signal intensity fluctuations as the fibre output can be represented both as a function of time and by the related spectral power density.     

Optical fibre excitation by partially coherent sources

Sources for optical fibre excitation have previously been assumed to be either totally coherent or incoherent. This paper formulates the modal excitation problem for partially coherent sources. The modal excitation coefficients are given in terms of the source complex degree of coherence and numerical results and simple analytical expressions appropriate to multimode step index fibres are presented. The assumption of equal modal power when excitation is by a very incoherent source is examined and the following simple criterion developed: for highly incoherent sources, modes with eigenvaluesU < the reciprocal of the coherence length, measured in units of fibre radius, are approximately equally excited, while the remaining modes carry little power.     

Coherent recombination of laser beams with interferometrical phase control

A HeNe laser beam is split into four component beams which are subsequently transmitted through four polarization-preserving monomode fibres. At the end of the fibres the emerging beams are coherently recombined in pairs in two steps using inverse beamsplitter cubes. The unavoidable phase differences within each pair of beams, which are caused by temperature and pressure fluctuations in the fibre environment, are detected interferometrically at the fibre ends. Using homodyne techniques, these differences are measured and compensated for by lengthening the optical path in the individual fibres with piezo-electric stretching. With appropriate adjustment of the relative phases the emerging beams can be superimposed onto each other so as to re-form one single coherent beam.     

双法布里—珀罗干涉仪传感模型的理论分析

本文提出了一种用发光二极管作为光源、用自聚焦透镜构成法布里-珀罗腔的双法布里-珀罗干涉仪光纤位移传感模型.根据部分相干光的干涉理论,得到了这个传感模型输出光强与两个法布里一珀罗干涉仪腔长之差的函数曲线.     

Enregistrement et selection de mode d'une fibre optique par spectroscopie optique cohérente

Résumé En utilisant des techniques de spectroscopie optique cohérente, on réalise, avec une résolution subpicoseconde, une sélection temporelle des différentes modes qui se propagent dans une fibre optique multimodale. La structure spatiale, en intensité, des modes excités est visualisée par filtrage optique d'un hologramme de Fourier. Une démonstration expérimentale est présentée dans le cas élémentaire d'une fibre bimodale.

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Using coherent optical spectroscopic techniques, we perform a temporal selection of the different modes propagating in a multimode optical fibre, with subpicosecond resolution. The intensity patterns of the excited modes are visualized by optical filtering of a Fourier hologram. An experimental demonstration is presented in the elementary case of a bimodal fibre.

     

Maximum repeaterless transmission of lightwave systems imposed by stimulated Brillouin scattering in fibres

To aid in designing high-speed optical networks, the fundamental limitations of lightwave transmission systems are investigated from the viewpoint of fibre nonlinearities, particularly of stimulated Brillouin scattering (SBS). When SBS occurs, the fibre launch power is usually restricted; thus, SBS is detrimental to lightwave systems. The effects of SBS on signals transmitted in fibres are clarified for several modulation schemes, especially coherent modulation schemes. A generalized model based on the maximum power spectrum density in the signals is proposed to estimate the degree of SBS generation. The maximum repeaterless transmissions for various bit rates, laser diode linewidths, and Brillouin gains of optical fibres are presented. The degradation of transmission due to SBS is clarified experimentally, focusing on the bit error rate, fibre input power, and spectra of signals transmitted in fibres. In addition, various techniques for suppressing the SBS effects are proposed. A repeaterless transmission experiment conducted over 364.3 km using optical booster amplifiers is presented to verify the effectiveness of the SBS suppression technique.     

单模光纤布拉格反射滤波器

采用非对称五层光波导模型，分析了光纤布拉格反射滤波器特性，讨论实现窄带和宽带高反射滤波器的结构参数。文中报道关于光纤布拉格反射滤波器的实验研究，包括制作工艺和测试方法。典型的光纤布拉格反射滤波器在１．５μｍ波段的反射率达８０％，带宽１．５ｎｍ。     

New phase and polarization-insensitive receivers for coherent optical fibre communication systems

We propose new phase- and polarisation-insensitive receivers for coherent optical fibre communication systems which have the following characteristic features: (a) insensitivity to LO excess noise in addition to phase- and polarization-insensitivity; (b) absence of optical PLL and polarization control devices; (c) a smaller detector bandwidth requirement than for heterodyne systems; (d) the same source linewidth requirement as for heterodyne systems with non-coherent demodulation; (e) the possibility of a complete optoelectronic integrated circuit (OEIC) version in the future.     

Realization of an All-Fibre Self-Organization Intra-Cavity Coherent Erbium-Doped Fibre Laser

An intra-cavity coherent coupling Michelson Er-doped fibre （EDF） laser （MCEDFL） is proposed and demonstrated. Characteristics of the MCEDFL are investigated. It is found that the MCEDFL with a polarizer can be coherent combined effectively. By the experiment based on fibre Bragg gratings （FBGs） with different reflectivity, we find that the reflectivity of the FBG play a vital role in improving the performance of the MCEDFL. This outcome adequately shows many favourable features, such as high efficiency, easy operation, and simple all-fibre configuration.     

Coherent Beam Combination of Two Polarization Maintaining Ytterbium Fibre Amplifiers

We investigate coherent beam combination of fibre laser beams by phase locking. Phase noise of a polarization maintaining ytterbium fibre amplifier is inspected with a fibre interferometer. In a feed back control loop, two fibre polarization maintaining ytterbium amplifiers are phase locked and coherent combined when the phase noise is properly controlled by a LiNO3 phase modulator.     

光纤激光模场及表征技术进展

在光纤通信、光纤激光器和光纤传感等领域的实际应用中,需要重点关注光纤中的模式问题。模分复用是提高光通信信息容量的有效方法,模间干涉是大多数光纤传感的基本方法,高功率光纤激光的光束质量控制的关键技术之一就是模式控制,因此,对光纤模式理论、模式产生及转换、模式表征技术开展研究具有重要的研究意义和实际应用价值。论文讨论了光纤的模式及光束质量,分析了多种模式发生及转换的方法,将模式表征方法归结为非相干、相干和低相干测量法。光纤模式表征是目前的研究热点,在多种表征方法中,空间和频谱成像法（S2）和双重傅里叶变换法（F2）具有显著的优越性,可不需要提前知道光纤的几何参数,就可获得模场分布、模式功率占比、群时延等特性。研究表明F2法更适合于表征高功率光纤激光的模场特性。