Advanced multifunctional optical switch for multimode optical fiber networks

In this work, an advanced multifunctional optical switch based on multimode fibers is proposed. It can work as a 3 × 1 optical multiplexer/combiner, a 2 × 2 optical switch, a variable optical attenuator and a variable optical power splitter. All these functionalities can be developed in the same device without any hardware modification, only by using the proper ports and control electronics.The proposed switch has been developed for being used in the visible and near infrared wavelength range: 450–650 nm for optical fiber automobile applications, 650–850 nm for home broadband applications; and 850–1300 nm for multimode fiber access networks. Up to three different types of twisted nematic liquid crystal cells have been designed and fabricated for fulfilling these different wavelength ranges as part of the proposed device.The multifunctional switch has been implemented and experimentally tested. Crosstalk usually better than ? 15 dB at 532 nm, 660 nm and 850 nm, in any state has been measured. Switching is achieved at voltage levels of 4 Vrms. Fiber to fiber insertion losses when operating as a 2 × 2 optical switch, range from 10 to 15 dB within 200 nm wavelength range; with a non-optimized optics for collimation and coupling.     

Application of multimode fiber optical sensor in optical tomography technology

In this paper, an attempt to use a multimode optical fiber sensor in tomography is made. Two types of lens, one sphere and one cylinderical, are designed for two terminals of the fiber. It solves the problem of insufficient power collection of natural light beam using current fiber with polished terminals. A theoretical analysis of the relationships among the sphere lens radius, the cylinderical lens radius, the refractive index of the fiber is carried out. The relationship between the scanning angles and the number of the optical fibers is experimentally investigated in the air medium.     

Bandwidths of single-mode and multimode optical fibre

An analysis is given of the effect of carrier spectral width on bandwidth of optical fibre transmission lines and a simple generalized expression is deduced relating carrier bandspread and maximum permissible pulse rate. It is shown that for the case of a semiconductor laser carrier source the bandwidth of a multimode fibre can be made to approach that expected from single-mode and graded-index fibres.     

Transmission properties of cabled multimode optical fibres

The study of real multimode fibres suggests a division into two main classes: very-low-dispersion fibres, employed in high-capacity transmission systems, for which a bandwidth limitation occurs in the repeater spacing; and other fibres. Surprisingly, for the fibres in the first class, the cabling process need not be too sophisticated and expensive, provided that no bandwidth limitations arise from material dispersion. In fact, in this case, the bandwidth limitation can be partly relaxed owing to the fibre perturbation.     

Geometrical theory of multimode optical fiber-to-fiber connectors

For optical communication to become a reality, it will be necessary to develop both permanent splices and detachable connectors for optical fibers. In this paper, I develop a simple, one-dimensional, geometrical theory with which to calculate losses introduced at a connection by alignment errors or fiber-end separation. I assume the fibers to be illuminated uniformly across the diameter of the core and within the acceptance angle. The calculation is verified with experiment.For typical fibers, the theory shows, not surprisingly, that radial misalignment of the fiber axes is by far the most severe problem. Even fibers with core diameters as large as 100 μm will have to be aligned more precisely than ± 10 μm.     

Theory of scattering in multimode optical fibres

The problem of scattering in multimode optical fibres is treated in a rigorous way by means of both a ray and a statistical approach. These two methods are interlaced and harmonized. Applications to microbending, ellipticity, core radius and maximum numerical aperture variations, and fluctuations of index profile shape are performed. Useful results about power distribution and fibre attenuation are derived in each case.     

Thermal optical path difference analysis of off-axis lens ray trace foot-print

The optical system lens temperature difference will cause optical aberration increase. This study is applying finite element method (FEM) and optical ray tracing; calculate off axis ray thermal optical path difference (OPD). The optical system temperature distribution is calculated from finite element simulation result and weighting temperature to each optical ray path. The simulation result shows off-axis ray major thermal optical aberrations are tilt and piston at this study.     

Wavelength dependent propagation in optical multimode fibers

Under various assumptions concerning the wavelength dependence of the refractive index of a fiber whose radial dependence is not an “a priori” known function of r, a method is described to obtain a closed form expression for the flight time of a ray in a multimode fiber. Consequently, the refractive index n(r) is shown to be the solution of a differential equation. Thus, for these selected profiles, we avoid the problem of the high precision required in the calculation of the flight time of a ray, a problem that arises frequently if the impulse response is to be reconstructed from a large number of rays, and transpose it to the determination of a single function of r, namely the refractive index.     

Loss calculations in bent multimode optical waveguides

We present a straightforward geometric optics method to calculate the power attenuation induced by a circular bend in the axis of a multimode optical waveguide. The method involves tracing of rays and use of generalized power transmission coefficients. Both slab and fibre waveguides (with either step-index or parabolic core profiles) have been considered; the source has been assumed to be Lambertian.     

Theory of dispersion in lossless multimode optical fibres

Using the ray propagation model generalized expressions for the impulse response of multimode fibres have been derived. The analysis has been applied to lossless fibres for pulses having both gaussian and lambertian spatial distributions as well as impulse and gaussian temporal distributions. Detailed results are given for output pulse shapes and fibre dispersions for various configurations.     

Theory of power coupling between multimode optical fibres

In this paper power coupling between two multimode optical fibres is investigated. A formalism (based on a geometrical approach) suitable for analysing any coupling configuration between fibres with any index profile and any radiance distribution is presented. The model is applied to obtain coupling losses both in uniformly excited parabolic- and step-index fibres, in the presence of all kinds of coupling errors. Numerical results of extensive utility are shown and useful asymptotic expressions of coupling efficiency, valid for small values of coupling error parameters, are derived together with some practical rules for error combination. Mention is also made of some experimental results we have obtained, which corroborate some previous assertions. The work can supply a useful tool in the design of joints and connectors between single fibres or optical cables.     

Near-field scanning optical microscopy imaging of multimode interference

By use of a near-field scanning optical microscope (NSOM) in collection mode, the intensity distribution along a 2 x 2 multimode interference coupler was directly imaged as a function of wavelength. Although calculations can predict the general trend of wavelength dependence and the approximate positions of multiple images in the coupler, the accuracy is poor because of uncertainties in the waveguide width. We show that direct imaging using a NSOM bypasses calculational uncertainties and proves to be a powerful technique for studying these waveguide devices.     

Studies of radiative losses from multimode optical fibres

We describe measurements in which the radiative losses from a multimode fibre have been resolved into three components. These are the Rayleigh Scatter component from the material, the loss by tunnelling from the modes closest to cut-off into the surrounding material, and the forward scatter loss caused by imperfections in the fibre that are large compared to a wavelength.     

Rigid reed-type routing switch for multimode optical fibers

A mechanical switch developed earlier for routing light in multimode optical fibers has been improved by reducing its cubic volume, insertion loss, switching time, and required switching power. Using an immersion fluid, an insertion loss below 0.5 dB was realized. The switching time is shorter than 13 msec, and the required switching power less than 16 mW.     

Rigid reed-type routing switch for multimode optical fibers

Abstract

A mechanical switch developed earlier for routing light in multimode optical fibers has been improved by reducing its cubic volume, insertion loss, switching time, and required switching power. Using an immersion fluid, an insertion loss below 0.5 dB was realized. The switching time is shorter than 13 msec, and the required switching power less than 16 mW.     

On optical diffusion coefficients for step-index multimode guides

Optical diffusion coefficients, computed from ray-tracing in step-index slab guides with small continuous width perturbations, show an angular dependence which does not in general correspond well with the mechanical deformation spectrum. This contrasts with assumptions of a one-to-one correspondence in earlier reports. The results indicate that, in a fibre having a steep cut-off in the perturbation spectrum at high mechanical frequencies, the corresponding diffusion coefficient will have a significantly less pronounced cut-off. It is also shown that, in a cylindrical guide with diameter perturbations, the effect of skew rays is to further reduce the cut-off rate in the diffusion coefficient.An immediate consequence of these findings is that, in a perturbed fibre without appreciable cladding loss, the loss penalty (due to radiation) may be much higher than would be predicted by the mechanical spectrum alone.As a prelude to the above ray-tracing studies, a derivation of the optical diffusion equation is given from the analogy with Brownian motion.