Transmission characteristics of graded-index fibers

The use of graded-index fibers for optical communication systems is now under consideration for numerous civil and military applications, and in these either LED or laser sources can be developed. The bandwidth of the system depends on the source linewidth, the dispersion of refractive index in the fiber, the strength of excitation of the modes by the source, and the extent of mode mixing caused, by example, by microbending. In fibers with refractive-index profiles that have a nearly parabolic dependence upon radius, pulse broadening is a slight, but small departures from an optimum profile can cause a dramatic decrease in bandwidth. It is of considerable importance to predict the bandwidth of fibers from a knowledge of the refractive-index profile and source geometry. The purpose of the first part of this paper is to compare two computational methods by which the propagation characteristics of a fiber, hence the impulse response and system bandwidth, may be determined. In the second part, we shall describe a computer program that determines the excitation of modes in an arbitrarily graded refractive index fiber for a model of laser source.     

Poly(styrene)-based graded-index plastic optical fiber for home networks

We investigated poly(styrene) (PSt)-based graded-index plastic optical fiber (GI-POF) with low loss and high bandwidth for home networks. To install the GI-POF in home networks, the attenuation must be below 200 dB/km at a 670-680 nm wavelength, and the bandwidth must be over 2.0 GHz for the 50 m fiber. In this study, we selected a dibenzothiophene (DBT) as a dopant to PSt, and we fabricated PSt-DBT-based GI-POF. We confirmed that the PSt-DBT-based GI-POF has high bandwidth (4.4 GHz) for 50 m fiber and low loss (166-193 dB/km) at a 670-680 nm wavelength and obtained the GI-POF that satisfied the requirements for home networks.     

Mode excitation in circularly symmetric graded-index optical fibers by focused gaussian beams

Field distributions and propagation constants of modes in weakly guiding circularly symmetric optical waveguides having arbitrary radial refractive index distribution are determinable from a variational analysis, which uses the solution of the scalar wave equation for the infinitely extended parabolic refractive index profile as a reference (Laguerre-Gaussian functions). The power coupling coefficients of the power, which is transferred from a focused Gaussian beam to an LP_vp-mode, depend on frequency and on four normalized launching parameters. Once the field-describing matrix equation has been solved numerically, closed-form expressions are obtained. The condition for optimal matching of the fundamental mode is given, and it is found that the maximum power excitation coefficient may be close to 100% even for substantially disturbed profiles.     

Mode excitation in circularly symmetric graded-index optical fibers by focused gaussian beams

Field distributions and propagation constants of modes in weakly guiding circularly symmetric optical waveguides having arbitrary radial refractive index distribution are determinable from a variational analysis, which uses the solution of the scalar wave equation for the infinitely extended parabolic refractive index profile as a reference (Laguerre-Gaussian functions). The power coupling coefficients of the power, which is transferred from a focused Gaussian beam to an LP_vp-mode, depend on frequency and on four normalized launching parameters. Once the field-describing matrix equation has been solved numerically, closed-form expressions are obtained. The condition for optimal matching of the fundamental mode is given, and it is found that the maximum power excitation coefficient may be close to 100% even for substantially disturbed profiles.     

Reversible UV degradation of PMMA plastic optical fibers

Laser-induced fluorescence, Raman and absorption spectroscopy are used to investigate reversible degradation of transmission in PMMA optical fibers. When exposed to 254 nm UV light, optical transmission of PMMA plastic optical fiber in 400-800 nm range shows a significant increase in attenuation for shorter wavelengths. Over a period of 10 days following UV exposure, the transmittance of the plastic fiber recovers to a significant fraction of its pre-exposure value. UV-exposed fiber exhibits strong laser-induced fluorescence with 488 nm argon-ion laser. This fluorescence spans a spectral region between 450 nm and 750 nm with a peak around 580 nm. The fluorescence intensity decreases over several days following UV exposure. Likewise, Raman is also used to investigate degradation process. Freshly UV-exposed fiber shows total absence of Raman spectrum of PMMA. Following UV exposure, recovery of Raman signal over several days is correlated to the recovery of fiber transmittance as well as the decay of laser-induced fluorescence. A widely believed plausible explanation for UV-induced increase of attenuation involves formation of different macro radicals which recombine progressively after UV is stopped. Laser-induced fluorescence over several days is reported here providing direct evidence for molecular-level deterioration and recovery of PMMA.     

Bile enterogastric reflux sensor using plastic optical fibers

A portable optical-fiber sensor for ambulatory assessment of the enterogastric reflux, based on the optical properties of the bile, was developed. It makes use of two light-emitting diodes and of an appropriate electronic circuit that processes the signals and provides the readout on a display. A suitable probe for in vivo measurements was designed. Plastic fiber bundles are used in order to obtain probe flexibility and low price. In vivo measurements performed on different patients show the superiority of this technique over existing methods.     

Ultrahigh-resolution plastic graded-index fused image plates

Fiber-optic imaging systems such as the medical endoscope, the boroscope, the fused-image faceplate, and the image conduit are now made from glass step-index (SI) fibers, and the image resolution of the SI fiber-optic imaging systems is limited to ~5 microm. Ultrahigh-resolution fiber-optic fused-image plates with fiber diameter sizes of 5 and 2.8 microm were fabricated with plastic graded-index (GRIN) fibers. The measured image resolutions of the 5-microm SI and GRIN-based guides were comparable, and the resolution of the plastic GRIN image guides improved as the fiber diameter was reduced from 5 to 2.8 microm.     

Bile enterogastric reflux sensor using plastic optical fibers

Abstract

A portable optical-fiber sensor for ambulatory assessment of the enterogastric reflux, based on the optical properties of the bile, was developed. It makes use of two light-emitting diodes and of an appropriate electronic circuit that processes the signals and provides the readout on a display. A suitable probe for in vivo measurements was designed. Plastic fiber bundles are used in order to obtain probe flexibility and low price. In vivo measurements performed on different patients show the superiority of this technique over existing methods.     

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.     

High intensity picosecond transmission through optical fibers

The spectra of picosecond pulses with peak powers of up to 1 kW transmitted through a 12 m singlemode fiber have been investigated. Asymmetric broadening and the creation of an intense peak at shorter wavelengths was observed. Furthermore it was found that the symmetry of the spectrum was critically dependent on the dye-laser alignment.     

Low-loss coupling between two single-mode optical fibers with different mode-field diameters using a graded-index multimode optical fiber

We present a method for ultra-low-loss coupling between two single-mode optical fibers with different mode-field diameters using multimode interference in a graded-index multimode optical fiber. We perform a detailed analysis of the interference effects and show that the graded-index fiber can also be used as a beam expander or condenser. The results are important for devices in which optical fibers with different mode-field diameters are coupled in series, such as in ultra-short-pulse fiber ring lasers, or in optical fiber communication links.     

Loss measurements in graded-index compound-glass fibers

The optical fiber scattering loss coefficient is measured directly in a scattering sphere or deduced indirectly from total loss measurements. The results show agreement for graded-index silica-based fibers, but they seem conflicting for graded-index compound-glass fibers. This is explained from the diffusion-controlled refractive index profile and the ensuing mode-dependent scattering and absorption loss due to the different optical properties of core and cladding glass. Using this model the two-lengths total loss measurement method is discussed. A detailed experiment is described that convincingly illustrates the mode of operation of the scattering sphere as used in daily practice. The wavelength-independent term in the total loss, different for fibers drawn from the same glass, is explained as being scattering partly due to 1-mode mixing of modes with the same β by imperfections that affect high-1-modes predominantly. The total loss of the glasses to be investigated can be measured using low NA excited silicone-clad fibers and safely can be decomposed into scattering and absorption contributions.     

Loss measurements in graded-index compound-glass fibers

The optical fiber scattering loss coefficient is measured directly in a scattering sphere or deduced indirectly from total loss measurements. The results show agreement for graded-index silica-based fibers, but they seem conflicting for graded-index compound-glass fibers. This is explained from the diffusion-controlled refractive index profile and the ensuing mode-dependent scattering and absorption loss due to the different optical properties of core and cladding glass. Using this model the two-lengths total loss measurement method is discussed. A detailed experiment is described that convincingly illustrates the mode of operation of the scattering sphere as used in daily practice. The wavelength-independent term in the total loss, different for fibers drawn from the same glass, is explained as being scattering partly due to 1-mode mixing of modes with the same β by imperfections that affect high-1-modes predominantly. The total loss of the glasses to be investigated can be measured using low NA excited silicone-clad fibers and safely can be decomposed into scattering and absorption contributions.     

Single-mode low-loss optical fibers for long-wave infrared transmission

In this Letter, we report single-mode fibers made of chalcogenide glasses with low loss in the 5-12μm range. Glasses from the Ge-As-Te-Se system were optimized to prevent nucleation and to exhibit low density of charge carriers. Single-mode fibers were obtained through the rod-in-tube method by substituting 2% Te/Se between the core and cladding glasses. The resulting single-mode fibers had a core diameter of 30μm and exhibited losses of ~6 dB/m at 10.6μm, and as low as 3-4dB/m in the 6-10μm range.     

Optical transmission of bulk plastic material and plastic lightguides at high optical powers

Optical transmission measurements were performed with bulk plastic materials and plastic fibres regarding the possible application as a transmission media for high optical powers. Transmission losses of PMMA bulk material and PS/PMMA core-cladding combination optical fibres were determined to be 0.757 dB m^-1 and 1.41 dB m^-1 at 514.5 nm, respectively. The core material of the investigated fibres withstands a power density of 3.8 x 10³ W mm^-2 when the light is suitably launched. With the light source available a transmitted cw output power of 2.1 W could be attained with a 1.5 m long fibre.     

A variational analysis of single mode graded-index fibers

A variational method is presented to obtain the transverse electric field and the propagation constant of the fundamental mode of weakly guiding graded-index fibers. A comparison has also been made with results obtained by using other approximate methods and it has been shown that the present method gives extremely accurate results (in comparison to other methods) in the practical region of single mode operation and does not involve much computational effort.     

On the transmission by liquid crystal tapered optical fibers

A three-layer liquid crystal tapered optical fiber (LCTF) is investigated with the emphasis on the power confinements by the low order TE and TM modes sustained in the different sections of LCTF. The outermost clad section is considered to be made of liquid crystal with radial anisotropy whereas the core and the inner clad are dielectric regions. Rigorous field expressions in the different LCTF sections are deduced, and the plots of power confinement factors (or the relative distributions) are ultimately made considering different fiber dimensions. The results reveal that the TE modes confine maximum amount of power in the outermost liquid crystal region, which is attributed to the radial anisotropy of the section. Such features of LCTFs attract their usefulness in the area of field coupling devices and optical sensing where evanescent field technique is primarily implemented.     

Mid-infrared radiation transmission through fluoride glass multimode optical fibers

Measurement of the transmission laser radiation from Q-switched and free-running Er:YAG lasers operating at 2.94 μm through fluoride glass multimode optical fibers of 448, 465 and 620 μm core diameter were performed. Attenuation measurements were obtained as a function of the laser energy input, of the bending angle and of the radius of curvature. The fibers exhibited low attenuation under straight and bent conditions. The output beam quality was also studied using the appropriate beam profiler. The output beam profiles showed a central multi-spiked energy distribution.     

Mode dispersion characteristics of graded-index optical waveguides

A method to compute the propagating modes of graded index waveguides with an arbitrary refractive index distribution is proposed, based on the variational method applied to the integral wave equation. The method is applied to two refractive index distributions: a Gaussian index distribution and a complementary error-function index distribution. The results are compared with the predictions of perturbation analysis and are experimentally verified by measuring the mode spectrum of a Ti-diffused LiNbO₃ waveguide.