Micro-displacement or bending measurement using a long-period fibre grating in a self-referenced fibre optic intensity sensor

The measurement of micro-displacement or bending using a long-period fibre grating (LPG) in a self-referenced fibre optic intensity sensor is presented. The sensing head is based on a LPG whose attenuation peak changes as a function of its micro-displacement or bending and the fibre optic intensity sensor is referenced in frequency. Two other LPGs were used as rejection filters, located at the output of the optical source to shorten its spectral width. The implemented experimental set-up is described and, the results are presented being considered the measurement range and the intrinsic resolution.     

Performance comparison of various modulation schemes over free space optical (FSO) link employing Gamma–Gamma fading model

Free Space Optics (FSO) is an emerging line-of-sight technology intending to provide last-mile solution to the network problem where fiber technology is not feasible. The use of Wavelength Division Multiplexing (WDM) technology for FSO is inspired due to the demand for broadband communication. This technique has brought a revolution because the system data capacity is enhanced by simply adding more number of channels and reducing the channel spacing without having the need of more than one FSO link. By reducing the channel spacing to an appropriate level, Dense Wavelength Division Multiplexing (DWDM) based FSO systems are also be attained and are reported by various research works. FSO finds applications in vast areas like backhaul networks for cellular communication, disaster recovery, LAN–LAN connectivity, high-definition TV, MAN-extension, video transmission, medicine industry and surveillance. However, its usage is limited due to the serious challenges of link vulnerability to weather and atmospheric turbulence-induced fading. This paper is based on a WDM-FSO system. An 8-channel WDM based FSO system is proposed and performance is evaluated on widely accepted modulation schemes under weak, moderate and strong turbulence conditions. Gamma–Gamma fading model is employed for atmospheric turbulence modelling. The system is simulated on OptiSystem 14.0.     

Performance enhanced down-stream signalling for next generation long-reach 10 Gbit/s passive optical networks

In this paper, we have analyzed the signal processing methods both in digital and optical domain to enhance the transmission performance of downstream signalling in long reach passive optical networks (LR-PONs). The impact of non-linear (NL) equalization through signal processing, i.e. Volterra Equalization (VE), Digital Backpropagation (BP) and Optical Phase Conjugation with Non-linearity Module (OPC-NM) is investigated, in 10 Gbit/s (XG) DP-QPSK long-reach wavelength division multiplexed (WDM) PONs without midspan repeaters over 120 km standard single mode fibre (SMF) link for down-stream signals. Due to the compensation of optical Kerr effects, the sensitivity penalty is reduced to 2 dB by BP algorithm, 1.5 dB by VE algorithm and 2.69 dB by OPC-NM. Moreover, with the implementation of NL equalization technique we are able to get the transmission distance of 126.6 km SMF for the 1:1024 split-ratio at 5 GHz channel spacing in the non-linear region. Furthermore, the concept of super passive optical network (S-PON) is also evaluated, which involves a repeater stage consisting of optical amplifiers, to study the feasibility for receiver side signal processing and simplification.     

Pulsed interferometric optical fibre sensor detecting wiretapping in long transmission lines

A modified optical fibre based Mach-Zehnder interferometer was applied as a sensor to detect wiretapping in long transmission optical fibre lines. The signal consisting of short pulses (around 1?ns) was launched to the input of the interferometer based on the polarization maintaining fibres and polarization elements. When the sensing line was undisturbed, detectors registered only a single pulse. The additional two side pulses appear, if the wiretapping attempt took place. For robust detection of any alarm situation we proposed two-criteria algorithm to minimize false alarm rate. Moreover, slow environmental fluctuations were continuously monitored and compensated by polarization controllers. We measured frequency characteristics of the sensor and performed a hundred wiretapping attempts, which proved high performance of the sensor.     

Evolution of optical solitons in nonlinear dispersive lossy fibres

A variational technique has been applied to the evolution of optical solitons in a nonlinear dispersive lossy fibre. An analytical model has been developed to describe the interaction of two co-propagating orthogonally polarized optical pulses in a lossy fibre, governed by a pair of coupled nonlinear Schroedinger equations, and the threshold amplitude at which the solitons form a bound state has been obtained.     

Interferometric studies on the influence of temperature on the optical and dispersion parameters of GRIN optical fibre

Opto-thermal device attached to automate Fizeau interferometer is used to investigate the influence of temperature on opto-thermal properties of multimode graded-index (GRIN) optical fibre in the range from 27 to 54 °C. The effect of temperature on the refractive index profile of fibre is studied. The optical parameters and the opto-thermal coefficient of this fibre are determined. Also the variation of oscillation and dispersion energies, zero dispersion wavelengths, coupling efficiency, normalised frequency, number of propagation modes with the temperature and the material dispersion with the wavelength at different temperatures are calculated. Microinterferograms are given for illustration.     

Paper tape data transmission by wavelength multiplexing through a single fibre

Described is an optical system for transmitting paper tape data through a single fibre. It presents an application of wavelength multiplexing in optical fibre transmission. Each channel is coded by its own wavelength and the different wavelengths pass independently through the fibre, transmitting different information. Advantages of such a system are enlarged transmission capacity and a simple coding and decoding technique for varied channels without any moving parts.     

Short-scan interferometric interrogation and multiplexing of fibre Bragg grating sensors

We describe an interferometric technique for the demodulation of serial fibre Bragg grating sensor arrays, yielding absolute measurement of the individual grating mean wavelengths. The composite beam reflected from the array illuminates a scanning Michelson interferometer but, in contrast to spectral measurement by Fourier Transform Spectroscopy, our technique requires an OPD scan far shorter than the coherence length of the grating reflections. The technique is based on the high-resolution measurement of the phases of the complex analytic signals and of the relationship of these phases to interferometric delay. The analytic signals are derived via the Hilbert transform, allowing frequency domain filtering of individual signals within the signal processing. The technique has yielded a resolution of 0.007 nm for an OPD scan of 1.2 mm.     

Intra-group principal modes in graded-index multimode fibers used for mode group division multiplexing transmission

By considering very strong intra-group mixing while neglecting inter-group mixing in mode group division multiplexing (MGDM) transmission, we theoretically introduce and describe the physical model of intra-group principal modes (IGPMs) in graded-index (GI) multimode fibers (MMFs), from the view of fiber transmission matrices for both cases of one- and dual-polarization. Proof-of-concept numerical calculations for an exemplary mode-group-channel with the two lowest-order degenerate mode groups (namely the three lowest-order fiber eigenmodes) show that IGPMs can exhibit potential possibilities of the MGDM channel with minimal mode mixing/dispersion-induced signal distortion over a GI MMF.     

Investigation of the long-term stability of arc-induced gratings heat treated at high temperatures

A long-period grating written in the SMF-28 fibre was heat treated at 1000 °C for 15 days. The spectrum of the grating shifted to longer wavelengths and the resonances depth decreased as a result of structural relaxation. The background loss increased considerably for times longer than 200 h, and this loss is caused by devitrification of the fibre.     

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.     

Influence of mode coupling on mode group diversity multiplexing in multimode fibers

In this paper, the influence of mode coupling on mode group diversity multiplexing (MGDM) in multimode fibers is investigated. It is found experimentally, that in step index fibers mode coupling is very strong among the lowest mode groups. It is confirmed that mode coupling in graded index fibers is negligible. Theoretical and numerical investigation of the system performance in strong mode coupling conditions, indicates need for greater separation of the lowest MGDM channels.     

Fibre optics systems for space applications

Fibre optics offer many advantages over conventional wire systems for satellite communications; they include low weight, large bandwidth capacity, simple architecture for data bussing, electromagnetic interference (EMI) invulnerability, and cost-effectiveness. However, additional development work and reliability data are required for better performance and system implementation. Radiation effects on fibre optics are reviewed, and potential areas of fibre optics space systems application are explored.     

Single-mode fused coupler for undersea optical transmission systems

A single mode 3 dB fused coupler, to be used as a redundant laser combiner in the submerged repeater of an undersea optical fibre transmission system, is described. The fabrication and package design of the component for this high reliability application is discussed, and the results of a wide range of environmental tests are given.     

Stable helical solitons in optical media

We present a review of new results which suggest the existence of fully stable spinning solitons (self-supporting localised objects with an internal vorticity) in optical fibres with self-focusing Kerr (cubic) nonlinearity, and in bulk media featuring a combination of the cubic self-defocusing and quadratic nonlinearities. Their distinctive difference from other optical solitons with an internal vorticity, which were recently studied in various optical media, theoretically and also experimentally, is that all the spinning solitons considered thus far have been found to be unstable against azimuthal perturbations. In the first part of the paper, we consider solitons in a nonlinear optical fibre in a region of parameters where the fibre carries exactly two distinct modes, viz., the fundamental one and the first-order helical mode. From the viewpoint of application to communication systems, this opens the way to doubling the number of channels carried by a fibre. Besides that, these solitons are objects of fundamental interest. To fully examine their stability, it is crucially important to consider collisions between them, and their collisions with fundamental solitons, in (ordinary or hollow) optical fibres. We introduce a system of coupled nonlinear Schrödinger equations for the fundamental and helical modes with nonstandard values of the cross-phase-modulation coupling constants, and show, in analytical and numerical forms, results of collisions between solitons carried by the two modes. In the second part of the paper, we demonstrate that the interaction of the fundamental beam with its second harmonic in bulk media, in the presence of self-defocusing Kerr nonlinearity, gives rise to the first ever example of completely stable spatial ring-shaped solitons with intrinsic vorticity. The stability is demonstrated both by direct simulations and by analysis of linearized equations.     

激光喇曼光谱的光纤采样技术

设计了一种高效光纤喇曼探针，与常规采样方法相比，简化了操作，改善了测量的信噪比，同时，光纤采样实现了远距离测量，使恶劣环境下的现场检测成为可能。     

The preparation of optical fibre nanoprobe and its application in spectral detection

As an important component of scanning near-field optical microscope (SNOM), optical fibre nanoprobe has been applied to many fields extensively. A melt-stretched etching method is proposed to produce optical fibre nanoprobe with low cost. Firstly, optical fibre tip with micrometer-sized diameter is created by the melt-stretched measure. Next, it is dipped into hydrofluoric acid (HF), and a fine optical fibre nanoprobe will be made after a short-time etching. Owing to the taper structure of tip, it can be etched again in acid if a nanoprobe is not constructed when the first etching is completed. In addition, optical fibre nanoprobe is applied to spectral investigation, and the fluorescence spectroscopy of rhodamine B (Rh B) solution is collected by an optical investigation system with a bifurcated fibre.     

Repetition rate multiplication in figure-eight fibre laser with 3 dB couplers

The operation of a passively modelocked figure-eight laser with all fibre repetition rate multiplier is reported. Thirty two times the fundamental repetition rate is achieved with six conventional 3 dB couplers at a repetition rate of 182 MHz. The repetition rate 2^N times multiplication only requires (N + 1) passive fibre couplers and N fibre delay-lines. This method makes it possible to achieve high multiplication and is inexpensive when compared with conventional methods employing sub-ring cavity or special fibre Bragg grating to control repetition rate.     

Use of fibre gratings for bit skew compensation in all optical bit parallel WDM systems

We demonstrate transmission of two wavelength division multiplexed (WDM) bit parallel channels over 100 km of standard single mode fibre link using direct-modulated laser diodes at 10 Gbit/s channel. Step-chirped fibre gratings are used to simultaneously compensate to bit skew and dispersion. Bit skew of approximately 2.5 ns due to the fibre dispersion is completely compensated. Our work show shows that the step-chirped fibre grating technique can eliminate the bit skew for bit parallel WDM systems operating at Gbit rate over hundreds of kilometres of fibre link.     

Using RLS-TEQ to reduce CP length for 20.48 Gb/s direct-detection optical OFDM transmission over 2400 km of SSMF

This paper uses a Recursive Least Squares-Time Domain Equalizer (RLS-TEQ) to reduce the Cyclic Prefix (CP) length in Direct-Detection Optical Orthogonal Frequency Division Multiplexing (O-OFDM) transmission over 2400 km of Standard Single Mode Fiber (SSMF). The RLS-TEQ can cancel the residual Inter Symbol Interference (ISI) caused by both the Group Velocity Dispersion (GVD) and the CP length being shorter than the Channel Impulse Response (CIR). Using RLS-TEQ reduces size of the CP, and consequently leading to system performance improvement.