Comprehensive research on self phase modulation based optical delay systems

This paper comprehensively investigates the properties of self phase modulation based optical delay systems consisting of dispersion compensation fibre and highly nonlinear fibres.It researches into the impacts of power level launched into highly nonlinear fibres,conversion wavelength,dispersion slope,modulation format and optical filter bandwidth on the overall performance of optical delay systems.The results reveal that,if the power launched into highly nonlinear fibres is fixed,the time delay generally varies linearly with the conversion wavelength,but jumps intermittently at some conversion wavelengths.However,the time delay varies semi-periodically with the power launched into highly nonlinear fibres.The dispersion slope of highly nonlinear fibres has significant influence on the time delay,especially for the negative dispersion slope.The time delay differs with modulation formats due to the different combined interaction of nonlinearity and dispersion in fibres.The bandwidth of the optical filters also greatly affects the time delay because it determines the bandwidth of the passed signal in the self phase modulation based time delay systems.The output signal quality of the overall time delay systems depends on the conversion wavelength and input power level.The optimisation of the power level and conversion wavelength to provide the best output signal quality is made at the end of this paper.     

Temperature-stable polished fibre-optic coupler

One possible way to prepare a compact and temperature stable polished-type optical fibre coupler (OFC) from different kinds of single-mode fibres with different profiles is demonstrated. The proposed technology enables one to prepare an OFC with high reproducibility and stability of its parameters. The experimental characteristics of the glued OFC are presented and compared with theoretical ones.     

Velocity measurement by the laser speckle method using optical fibres

An optical device for in-plane velocity measurement is described. Laser light is focused by the use of an optical fibre onto the object, and the device utilizes the dynamic sparkle effect for velocity determination. The influence of different optical fibres on the speckle formation is investigated with special respect to fibre movement, and as the diameter of the aperture used for speckle detection also affects the measuring signal, an optimum value is found. A simple electronic circuit processes the measuring value proportional to the object velocity.     

Fundamental studies on laser radiation therapy: dispersion of a laser beam passing through optical fibres

For laser internal therapy in medical applications, optical fibres are required to propagate the laser beam. The far-field pattern of the laser beam at the exit face of the optical fibre is studied using a multimode step-index fibre and a graded index Selfoc fibre. The degree of dispersion, or the output angular distribution, of the beam passing through the Selfoc fibre is much smaller than that passing through the multimode step-index fibre. Consequently, the energy density in the far-field pattern of the beam passing through the Selfoc fibre is very much larger than that for the beam passing through the multimode step-index fibre. It is concluded that the Selfoc fibre is more effective in laser internal therapy.     

Remote actuation by optical power transmission in fibres

Some preliminary experiments on the remote actuation of a servo-valve are described in which the power transmission medium is an optical fibre. Using a semiconductor laser or a Nd:YAG laser as the power source and silicon cells as the opto-electrical transducer, switching of the servo-valve has been observed at power levels of a few milliwatts over 800 m of fibre.     

Design and fabrication of Fourier plane diffractive optical elements for high-power fibre-coupling applications

In industrial processes using fibre beam delivery of high peak power laser light, diffractive optical elements are a very useful and flexible tool in maximising the amount of light reaching the work surface. This is due to the ability of diffractive optical elements to accurately couple light into multiple fibres, while conditioning the light in order to maximise the throughput for each fibre. We discuss the design techniques for diffractive optical elements and the application of these techniques to fibre-coupling problems. The flexibility of diffractive optical elements is demonstrated by their application to several fibre-coupling geometries, including a fibre bundle, a linear array of fibres, and a rotationally symmetric fibre connector. The diffraction efficiencies for the elements approach 90% with uniformity errors of less than 5%.     

Automatic fringe analysis of the induced anisotropy of bent optical fibres

A new theoretical model considering the refraction of the incident light beam by the fibre is suggested to determine the refractive index profile of bent optical fibres. This new model (slabs model) considering the cross section of the bent optical fibre consists of large number of slabs. The slabs model bases on the refraction of the incident beam by the fibre. The refractive index profile of the optical fibre cladding before bending obtained using the automated Fizeau interferometer with the aid of suggested model is compared with other models such as, the homogenous model and the multilayer model to verify the ability of this slab model. The refractive index profile of the bent optical fibre cladding is investigated using this suggested model. In addition, the new model is used to obtain the induced birefringence and the guiding parameters. The bending radius is recommended to be greater than 7.1 mm for the used optical fibre. The consideration of the refraction increases the accuracy of the results.     

Determining the opto-mechanical and geometrical properties of high-density polyethylene fibres

A method is described to determine the opto-mechanical and geometrical properties of high-density polyethylene, HDPE, fibres at the same time for the same region from the fibre. Variations of the transverse sectional shapes, areas, optical and mechanical properties of fibres during the cold drawing process were investigated. A rotator-mechanical drawing device was used for rotating the fibre around its longitudinal axis during the cold drawing process. The Pluta polarizing two-beam interference microscope was attached with the rotator-mechanical drawing device for detecting these variations under the interferometer. The rotator-mechanical drawing device permits determining the tensile stress–strain curve, Poisson's ratio and strain optical coefficient for HDPE fibres. Also, it permits determining the refractive index profile of the drawn fibres taking into considering the transverse sectional area and the enclosed area under the fringe shift. The optical orientation factor and orientation angle was calculated for HDPE fibre to clarify the orientation of molecules during the cold drawing process. Microinterferograms are utilized for illustrations.     

The inter-modes mixing effects in Mode Group Diversity Multiplexing

Multiplexing technology in indoor optical networks with multimode fibres allows the integration of several services to the end user. The Mode Group Diversity Multiplexing technique (MGDM) in Graded-Index Multi-Mode (GI-MMF) fibre makes the system less expensive with simpler transmitters and receivers, keeping the same information capacity as other multiplexing techniques. The capacity conservation and transmission quality obtained using MGDM technique depends on the transmission conditions and the state of fibre. Bending of the fibre can affect the system by changing modes excited for the different propagation channels in the fibre. In this paper an analytical modelling method for MGDM will be presented. Improvements in system sending and receiving conditions are studied. Modelling of the fibre curvature as well as the effects of coupling patterns on the MGDM are discussed.     

LEDs and photodetectors for wavelength-division-multiplexed light-wave systems

The very wide transmission window, from 0.8 to 1.6 μm wavelength, in present low-loss optical fibres offers opportunities for wavelength-division-multiplexing (WDM) in light-wave systems. This paper reviews light emitting diode (LED) light sources and photodetectors capable of meeting the power (distance) and bandwidth (bit rate) requirements of such systems. Particular attention will be paid to the new multiplexing dual wavelength LEDs and demultiplexing photodetectors which point a way toward simpler, perhaps more economical, WDM light-wave systems in wide-band loop and inter-office trunking applications.     

Active fibres and optical amplifiers

The incorporation of rare-earth ions into glass fibres to form fibre lasers and amplifiers is not a recent development. In fact the first glass laser ever demonstrated [l] was flashpumped in the form of an optical fibre, a configuration that was used to overcome the difficulties of obtaining high-quality glass in bulk form. Apart from a report [2] in 1974 of laser operation in an Nd³⁺-doped silica multimode fibre, the idea of guided-wave glass lasers attracted little attention for the next 24 years. The idea resurfaced [3] in 1985 because both optical fibre and laser-diode technologies had advanced to a stage where low-loss, rare-earth-doped, single-mode fibres could be made and high-power semiconductor sources were available to pump them. In addition, low-cost fibre components (couplers, polarizers, filters) were available, which allowed construction of complex, all-fibre ring and Fabry-Perot resonators [4] to form a unique and powerful new fibrelaser technology. Even so, it was only the announcement in 1987 of a high-gain, erbiumdoped fibre amplifier (EDFA) [5] operating in the third telecommunications wave length window at 1.54 μm that sparked widespread interest in rare-earth-doped fibres in the optical telecommunications community. From that moment, frenzied worldwide activity has brought numerous new fibre amplifier developments and in 1990 resulted in several commercial products appearing, a time-lag of only three years after the first research announcement.     

High power optical fibre delivery system for the laser generation of ultrasound

A plastic-clad optical fibre system has been examined as a delivery system of pulsed laser energy for the generation of ultrasound. The onset of significant optical damage caused by the laser has been investigated. By using multimode fibres of 1 m length with 600 μm core size, it was found that laser power densities from the fibre were sufficient to produce ultrasonic waveforms corresponding to both the thermoelastic and plasma generation regimes. Out-of-plane acoustic displacements of >250 pm can be achieved through 2.5 cm thick aluminium test samples, showing that fibre optical delivery systems may be considered in systems using laser-generated ultrasound for non-destructive testing applications.     

Supercontinuum spectra generation in the single-mode optical fibre with concave dispersion profile

In this paper, a new method is proposed to generate broad supercontinuum (SC) spectra in the single-mode optical fibre with concave dispersion profile. We numerically simulate pulse evolutions and discuss physics mechanism in detail for SC spectrum generation in the optical fibre with concave dispersion profile. Furthermore, general criteria are presented for specifying the shape of SC spectrum by introducing normalized parameters, which are related to the fibres and the initial pump pulses. The results show that the flat and broad SC spectra are indeed generated in our proposed optical fibre.     

Proposals for a standard for the safe use of light-emitting diodes, laser diodes, and optical fibre transmission systems

A new safety standard for light emitting diodes, laser diodes and fibre optical transmission systems is needed because of the difficulties in applying existing standards to these products. New proposals, generated by a BSI working group, include a new classification system which takes into account the inherent beam divergence of most of these products, and the need to classify at different locations within an extended optical system such as a fibre-optic link.     

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.     

Single LED based dual wavelength referenced optical fibre sensor system using intensity modulation

An intensity-modulated optical fibre sensor system is described which employs a single LED source to provide the measurand and the reference signals with two separate wavelength bands. The allocated wavelength bands are selected to minimize any differential intensity effects that may arise from LED thermal variations. The sensor system, comprising a transceiver unit connected to an optical displacement sensor using a single optical fibre, provides an output fully referenced for all major common-mode variations that are likely to occur. Performance characteristics for the prototype system are reported which show a linear displacement range of over 20 mm.     

Propagation of chirped nonlinear pulses in a lossy highly birefringent single-mode optical fibre

A new model of the nonlinear propagation of optical solitons with initial linear frequency chirp through lossy highly birefringent fibres, allowing the improvement of the practical results concerning the distortionless signal transmission in ultrahigh-speed communications, is presented within the framework of a system of coupled nonlinear Schroedinger (CNLS) equations. This model demonstrates the existence of bound chirped solitons in a lossy single-mode highly birefringent fibre.     

Geometrical theory of energy launching and pulse distortion in dielectric optical waveguides

In this paper the geometrical theory of pulse distortion and energy launching into multimode optical fibres is generalized when skew rays are taken into account. To this purpose, within the geometrical theory of skew rays, we firstly obtained a new expression for the numerical aperture. From this expression we derived an extensive analysis of the energy launched and the pulse response of a multimode optical fibre. A better launching efficiency and a different pulse form at the output of the fibre together with a larger broadening are obtained in comparison with the results derived from theories only for meridional rays. The theory is also applied to sources having planar geometry (such as LED) and linear geometry (such as semiconductor lasers), and the response to some rectangular-shaped pulses is investigated. Finally some considerations concerning scattering effects are described.     

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.     

Quantum logic operations on two distant atoms trapped in two optical-fibre-connected cavities

Based on the coupling of two distant three-level atoms in two separate optical cavities connected with two optical fibres, schemes on the generation of several two-qubit logic gates are discussed under the conditions of Δ = δ - 2ν cos πk/2 >> g/2 and ν ~ g. Discussion and analysis of the fidelity, gate time and experimental setups show that our schemes are feasible with current optical cavity, atomic trap and optical fibre techniques. Moreover, the atom-cavity- fibre coupling can be used to generate an N-qubit nonlocal entanglement and transfer quantum information among N distant atoms by arranging N atom-cavity assemblages in a line and connecting each two adjacent cavities with two optical fibres.