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AssigneeCSELT - Centro Studi e Laboratori Telecomunicazioni S.p.A.
InventorBruno Costa
Estimating the term…
Light from a laser, converted into pulses by a chopper, is trained upon an end of an optical fiber--whose refractive-index profile is to be determined--through a first polarizer, a beam splitter, a focusing objective and a 90.degree. phase shifter, the space between the objective and the fiber end being occupied by a fluid whose refractive index is close to that of the fiber. Light reflected from the fiber end, having passed twice through the phase shifter, has a plane of polarization orthogonal to that of the incident light and is passed by a second polarizer, to which it is directed by the beam splitter, onto a photodiode feeding a calculator that also receives a reference signal from another photodiode to which part of the polarized laser pulses are directly reflected by the beam splitter. Spurious reflections from the objective surface, which do not pass through the phase shifter, retain the original polarization and are rejected by the second polarizer.
In an apparatus for determining the refractive-index profile of an optical fiber comprising a source of linearly polarized coherent incident light, an optical objective in the path of said incident light for focusing same on a substantially punctiform area of a confronting end of said fiber, light-guiding means for intercepting light reflected from the confronting fiber end through said objective, fiber-engaging means displaceable relatively to said objective in a plane perpendicular to the optical axis thereof, and evaluation means positioned to receive the reflected light intercepted by said light-guiding means,
Dependencies are inferred from the claim text.
Refractivity; Phase-affecting properties, e.g. optical path length: index profiling of optical fibres
Investigating or analysing materials by determining their chemical or physical properties
Term ended 1 Aug 2000 (estimated)
USPTO file history isn’t available for this patent, so only indexed dates are shown.
My present invention relates to an apparatus for determining the refractive-index profile of optical fibers.
The refractive index of an optical fiber is known to vary, within relatively narrow limits, gradually or in steps throughout its cross-section as a function of radius. A knowledge of this variation, known as the index profile, is important for a determination of some of the basic characteristics of a fiber such as its light-gathering efficiency, its light-guiding properties and its bandwidth.
Several techniques have already been developed for this purpose. Among them is the reflection method according to which a light beam is focused upon a substantially punctiform area (usually on the order of 1.mu. in diameter) of a fiber end by an optical objective through which reflected luminous energy is passed by suitable light-guiding means such as a beam splitter to a photodetector feeding an evaluator. As the projected light spot is displaced along a diametrical line of the fiber end confronting the objective, the evaluator calculates the refractive index from the reflected energy and plots it for different distances from the fiber axis; this calculation can be carried out with the aid of a reference value derived, advantageously via the aforementioned beam splitter, from the output of the light source.
The above and other features of my invention will now be described in detail with reference to the accompanying drawing the sole FIGURE of which schematically shows a representative embodiment.
As shown in the drawing, a laser 1 serving as a source of coherent light illuminates a beam splitter 4 by way of a chopper 2 and a polarizer 3; the latter, of course, may be omitted if the light entering the chopper 2 is already linearly polarized. The beam splitter 4 divides the pulsating light into a reflected reference beam B.sub.O, directed to a photodiode 5, and a transmitted illuminating beam B.sub.1 impinging upon an objective 8 in a housing 14 provided with a support 14' which holds an extremity of an optical fiber 7 to be tested. The space between objective 8 and the confronting end 7' of fiber 7 is filled with a fluid 13, such as oil, whose refractive index substantially matches that of the fiber. The opposite end of fiber 7 is immersed in similar fluid in a container 15 with nonreflecting inner wall surfaces.
Objective 8 focuses the incident beam B.sub.1 upon a small spot on fiber end 7' from which luminous energy is reflected via the same path, and thus by way of objective 8, toward beam splitter 4. Part of the returning light, which includes spurious rays reflected by lens surfaces of the objective, is redirected as a measuring beam B.sub.2 to an evaluator which includes, besides the aforementioned photodiode 5, a second photodiode 11 and a calculator 16. Photodiodes 5 and 11 work by way of respective amplifiers 6 and 12 into respective inputs 6' and 12' of calculator 16 whose output 17 carries a signal representing the refractive index of fiber 7 at the spot of end 7' illuminated by the focused beam B.sub.1. Calculator 16 operates according to the well-known Fresnel formula ##EQU1## where R is the reflected energy and n.sub.r, n.sub.o respectively represent the refractive indices of the fiber 7 and the surrounding medium 13. Support 14' and objective 8 are relatively displaced, in steps synchronized with the operation of chopper 2, in a plane perpendicular to the objective axis as indicated by arrows A. The system so far described is conventional and corresponds essentially to the one discussed in the above-identified paper by B. Sordo and me.
In accordance with my present invention, and for the purpose of eliminating the influence of spurious reflections at objective 8, I insert between that objective and the fiber end 7' a 90.degree. (or .pi./2) phase shifter 9 in the form of a quarter-wavelength plate. The light of incident beam B.sub.1, polarized in one plane, is converted by this phase shifter to circular polarization and is reconverted, after reflection, to linear polarization but in a plane orthogonal to the first one. Since the light reflected at objective 8 undergoes no such double conversion, the desired reflections from the fiber end and the spurious reflections from the objective have polarization planes perpendicular to each other as they arrive at a second polarizer (or analyser) 10 which allows only the former to pass toward photodiode 11.