Dual-mode Fiber Research Articles

Mode-holding capability of dual-mode fibers

Standard single-mode communication fibers have been characterized with respect to their mode-preservation capability in the dual-mode regime. For a 1300-nm single-mode fiber working at 830 nm, the equivalent h parameter for its LP(01) and LP(11) modes is shown to be as good as that for the best polarization-maintaining fibers, i.e., better than -20 dB-km.

Propagation characteristics of dual-mode elliptical-core optical fibers

Propagation characteristics of dual-mode elliptical-core fibers are obtained by using a perturbation approach. It is found that for very high values of core aspect ratios (a/b > 2.3), the maximum possible value of the V parameter in such fibers is decided by the cutoff of the LP(21)(even) mode instead of the LP(11)(odd) mode. Further, as the core ellipticity increases, unlike the polarization birefringence, the birefringence between the LP(01) and the LP(21)(even) modes decreases and the V value, at which the LP(01) and LP(21)(even) modes have the same group velocities, moves toward the LP(21)(even) mode cutoff.

Theoretical analysis of acoustic-optic frequency shifter by dual mode fiber

The frequency shifting between two modes in an optical fiber resulting from acoustic wave propagation has been studied theoretically. Our theoretical results confirm the earlier experimental work. The polarization effect introduced by acoustic waves has been examined as well. The analysis shows that the polarization characteristic and symmetry property of the modes are not preserved in the coupling process of acoustic-optic frequency shifting for arbitrary ratio of the fiber cladding radius to the acoustic wavelengthd/λa, in contrast to stable mechanical microbending, and that for the givend/λa = 0.397, the polarized light signal at frequencyω2, shifted from that at frequencyω1, is quasi-linearly polarized for any acoustic-optic interaction length. However, the polarization effect can be neglected when the fiber cladding radius is much smaller than the acoustic wavelength.

Bipolar optical modulation and demodulation using dual-mode fiber and a fast diffusion-driven photodetector

A bipolar fiber-optic signal processing and photodetection concept which combines a dual-mode optical fiber with a fast diffusion-driven photodetector is demonstrated. A discussion of the concept is given, and the results of a simple experiment are given. Various permutations of the ideas described can permit different kinds of coherent and incoherent optical-signal processing and optical control of pulsed photodetection.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Design considerations of dispersion-free dual-mode optical fibers: 1.55 µm wavelength operation

The refractive-index profile of dual-mode optical fibers which makes possible dispersion-free operation at 1.55 μm wavelength is proposed. The refractive-index profile is synthetized using methods of quantum mechanics and numerical procedure. Optical fibers with this refractive-index profile have a core diameter of about 23 μm that is much larger than the core diameter of conventional single-mode fibers. Obtained results can be used for design of broad-band optical fibers.

Measurement of intermodal delay in a dual-mode optical fibre

A novel measurement technique for intermodal dispersion between the LP01 and LP11 modes of a dual-mode optical fibre is described. A frequency-modulated optical source produces a beat frequency, proportional to intermodal delay, by interference of the two modes at the fibre output. Experimental results agree with theoretical predictions.

Dual-mode optical-fibre interferometric sensor

The letter describes how a dual-mode optical fibre can be used as an interferometric temperature sensor either by fringe counting or, more importantly, by analysis of the change in chromatic transmission spectrum with temperature. Such a sensor has the geometric flexibility of an all-fibre interferometric sensor as well as the advantages of a wavelength modulating device, i.e. high accuracy, noise immunity, absolute measurement capability and no need for intensity calibration.

Novel technique for measurement of wavelength of zero relative delay between modes of a dual-mode optical fibre

A simple method of determining the wavelength of zero relative time-delay between the LP01 and LP11 modes of short lengths of a dual-mode optical fibre is described. The measurement allows easy comparison of short and long lengths of fibre. The technique may prove to be more convenient and reliable than those presently used for characterising monomode fibres.

Analytical approximations for the propagation characteristics of dual-mode fibers

In this paper we present accurate analytical approximations for the modal fields of dual-mode optical fibers with power law profiles based on a scalar variational analysis. We propose single parameter and two parameter trial functions and use these to study the dispersion characteristics and estimate the value of the normalized frequency corresponding to zero intermodal dispersion, which defines the operating point for such fibers. Our results show that the relatively simple single parameter field for the LP 11 mode gives a good fit to the field inside the core and estimates the propagation constant fairly well, but is inadequate to calculate the dispersion characteristics. On the other hand, the two parameter field estimates all these characteristics with a high degree of accuracy and enables one to accurately compute the normalized frequency for zero intermodal dispersion as well as dispersion tolerance around this value.

Spatial technique for measuring modal delay differences in a dual-mode optical fiber

A simple spatial method using a coherent light source and an interferometer is proposed for measuring the group delay time difference between the LP(01) and LP(11) modes in a dual-mode optical fiber. Experimental results obtained by this method are in good agreement with those obtained by the swept-frequency and pulse methods. Moreover, a group delay time difference of 4 psec is measured. Results suggest that this spatial. method can be applied to a short fiber less than 1 m in length and/or small time differences around 1 psec by using a laser diode with a short coherence length.

Single mode optical fiber characterisation by the LP11 mode radiation pattern

We describe a method for determining the V-value from the far-field radiation pattern of dual mode optical fibers (this method is an extension of the work by Gambling et al. for the LP11 modes). The method involves measurements of the maximum radiation angle and the angle of the first minimum. Curves are presented which can be used with dual mode fibers, determining the core diameter, the refractive index difference between core and cladding, as well as the V-value in the single mode regime for longer wavelength operation with the help of dispersion curves of the dopant materials.

Splice loss evaluation for optical fibers with arbitrary-index profile

A method is presented for calculating offset and tilt losses for fiber splices with axially symmetric arbitrary-index profiles by approximating the profile with a staircase function. This method is applied to a large-core dual-mode fiber with zero intermodal dispersion as well as to single-mode fibers with step- and parabolic-index profiles. When a splice loss of 0.2 dB is permitted, the normalized offset misalignment is found to be D(N) = 0.635 for the dual-mode fiber at normalized frequency v = 4.605 and a power-law exponent alpha = 4.5. The D(N) value compares favorably with the values 0.560 and 0.614 for conventional step- and parabolic-index single-mode fibers, respectively. The dual-mode fiber is superior to the step- and parabolic-index fibers with respect to permissible splice offset tolerances.

Design Considerations of Broadband Dual-Mode Optical Fibers

We propose and present design data for a new type of graded index fiber which has a profile and radius such that only two mode groups (LP/sub 01/ and LP/sub 11/) propagate and both propagate with virtually identical group velocities. This dual-mode fiber has a core diameter approximately twice that of a conventional step index single-mode fiber. For example, a core diameter of 16.3 mu m is attainable with relative index difference Delta=0.3 percent at 1.25-mu m wavelength. Fabrication tolerances securing a group delay difference below 100 ps/km are given by a power-law profile parameter alpha=4.85/spl plusmn/0.25 and a normalized frequency upsilon=4.45/spl plusmn/0.11. The allowable upsilon-value deviation range to keep the group delay difference within 100 ps/km is about five times as large as that of a step-index fiber, in which group delays of two mode groups are matched. Comparison with a multimode graded-index fiber, with respect to group delay characteristics and bending loss of the dual-mode fiber, are also discussed.