# US 5,351,332 A · Waveguide arrays and method for contrast enhancement

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- Number: US 5,351,332 A
- Document ID: US5351332A
- Status: Granted
- Kind: A
- Application number: 08/018,576
- Filed: 1993-02-17
- Granted: 1994-09-27
- Earliest claimed: 1992-03-18
- Assignees: Galileo Electro-Optics Corp.
- Inventors: Lee M. Cook
- Web page: https://patentgrep.com/patent/US5351332A

## CPC

- G02B 6/06: the relative position of the fibres being the same at both ends, e.g. for transporting images
- G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS

## Abstract

Optical waveguide array imaging devices have a darkened surface layer in the cladding portion. The imaging devices of the invention have high contrast which is generally independent of pathlength. A method for contrast enhancement in such arrays is disclosed.

## Claims

### Claim 1

A fiber optic array for transferring an image with increased contrast comprising:
a plurality of optical waveguides disposed in a coherent array of a selected length, said array being free of extramural absorbers (EMA), each waveguide having a core and cladding, said array having opposite optically finished, fused together end portions exposing corresponding end surface portions of the core and the cladding, at least one of the end portions of the array having a treated darkened cladding surface layer to a depth sufficient to absorb incident radiation directed at the cladding without producing secondary emission to thereby render the exposed portions of the cladding substantially opaque to said incident radiation, and said array having, as a result of such treatment, at least one of a contrast characteristic independent of array length and wavelength; and a transmission characteristic greater than an untreated array and an array with EMA.

### Claim 2 (depends on 1)

The array of claim 1, wherein the cladding contains reducible cations and the core is substantially free of said cations.

### Claim 3 (depends on 2)

The array of claim 2, wherein the cations are selected from the group consisting of Pb, Bi, As and Sb.

### Claim 4 (depends on 1)

The array of claim 1, wherein the core has a substantially higher glass transition temperature is exposed to a reducing atmosphere at a temperature not to exceed the softening point of the lowest viscosity glass to produce said darkened surface layer on the exposed portions of the cladding only without deformation of the shape of the array.

### Claim 5 (depends on 1)

The array of claim 1, wherein the transmission of incident light through the surface of the cladding is less than 1%.

### Claim 6 (depends on 1)

The array of claim 1, wherein said darkened surface layer is a substantially nonconductive dielectric layer.

### Claim 7 (depends on 1)

The array of claim 1, wherein said darkened surface layer is substantially nonfluorescent.

### Claim 8 (depends on 1)

The array of claim 1, wherein the core surface portions exposed at said at least one end portion thereof are recessed relative to the cladding portions thereby forming an intagliated surface having cavities with wall portions extending into said array formed of exposed cladding, and the darkened layer extends into said channels along said wall portions.

### Claim 9 (depends on 8)

The array of claim 8, wherein each of said optical waveguides has an effective numerical aperture.

### Claim 10 (depends on 8)

The array of claim 8, wherein the waveguides have a nominal optical numerical aperture (NA) and cavities have a selected aspect ratio d/l where d is the diameter of the core and l is the length of the cavity extending into said array thereby resulting is an effective NA.

### Claim 11 (depends on 10)

The array of claim 10, wherein the effective NA is less than the optical NA.

### Claim 12 (depends on 1)

The array of claim 1, wherein the surface layer is <15.mu..

### Claim 13 (depends on 12)

The array of claim 12, wherein the surface layer is <4.mu..

### Claim 14 (depends on 12)

The array of claim 12, wherein the surface layer is <1.mu..

### Claim 15 (depends on 1)

The array of claim 1, wherein the core and cladding have respective glass transition temperatures T.sub.g and the T.sub.g of the core is greater than the T.sub.g of the cladding.

### Claim 16 (depends on 1)

The array of claim 1, wherein the cladding has a glass transition temperature T.sub.g and reduction occurs at a temperature of about 50.degree. C. below the T.sub.g of the cladding.

### Claim 17 (depends on 1)

The array of claim 1, having a contrast characteristic substantially independent of path length.

### Claim 18 (depends on 1)

The array of claim 1 wherein the cladding is preferentially reducible relative to the core.

### Claim 19 (depends on 1)

The array of claim 1 wherein the surface treatment rendering the at least one end portion of the cladding results in an increase in contrast in comparison to an untreated array.

### Claim 20 (depends on 1)

The array of claim 1 wherein the increase in contrast resulting from said surface treatment of the cladding is independent of path length in the waveguides.

### Claim 21 (depends on 1)

The array of claim 1 wherein each of the waveguides are disposed such that at least a selected length of each said waveguides in the end portions of the array are lying in a parallel axial arrangement.

### Claim 22 (depends on 21)

The array of claim 21, wherein an intermediate portion between the end portion is flexible.

### Claim 23 (depends on 1)

The array of claim 1 wherein the waveguides are arranged in a fused fiber optic array of parallel waveguide segments.

### Claim 24 (depends on 1)

The array of claim 1 wherein all exposed cladding portions have the darkened surface layer.

### Claim 25 (depends on 1)

The array of claim 1, wherein the contrast characteristic is a measure of a modulation transfer function (MTF) percent, and where said MTF is greater than about 80%, up to at least 60 lp/mm.

### Claim 26 (depends on 1)

The array of claim 1, wherein the contrast characteristic is a measure of a modulation transfer function (MTF) percent, and said MTF is relatively flat at wavelength, greater than 20 lp/mm.

### Claim 27 (depends on 26)

The array of claim 26, wherein the MTF is uniform in the infra red.

### Claim 28 (depends on 1)

The array of claim 1, wherein the array has a transmission characteristic which is uniform.

### Claim 29

A waveguide image transfer array comprising:
a plurality of optical waveguide segments having a core and cladding lying in a parallel axial array of a selected length, said cladding being preferentially reducible relative to the core, said array having opposite optically finished ends exposing portions of the core and the cladding, at least one of the ends of the array having a reduced cladding portion forming a treated darkened surface layer to a depth sufficient to absorb incident radiation directed at the cladding and to render the exposed portions of the cladding opaque resulting in an enhanced, relatively uniform, array length independent enhanced contrast and reduced cross-talk compared with an untreated array.

### Claim 30

A method for treating end surfaces of a waveguide array formed of a plurality of optical waveguide segments of a selected length extending between fused together end surfaces thereof, each of said waveguide segments including a core surrounded by a cladding, said cladding being a glass having reducible cations surrounding the core formed of a glass free of said cations and having a substantially higher glass transition temperature comprising the steps of:
exposing at least one surface of the waveguide to a reducing atmosphere; maintaining the reducing atmosphere for a selected time and at a selected temperature sufficient to produce an absorbent, non-fluorescent surface layer on the exposed surface of the cladding glass to a depth sufficient to result in contrast characteristics for the array independent of wavelength and array length.

### Claim 31

A method for treating end surfaces of a fiber optic image transfer device, formed of a plurality of optical waveguide segments, each of said waveguides including a core surrounded by a cladding and having fused together end portions in a parallel axial array of a selected length, to substantially eliminate cross-talk from light entering surfaces, said cladding being a glass having reducible cations surrounding the core formed of a glass free of said cations and having a substantially higher glass transition temperature comprising the steps of:
exposing at least one surface of the waveguide to a reducing atmosphere for a time and at a temperature sufficient to produce light absorbing, non-fluorescent surface layer on the exposed surface of the cladding glass to a depth sufficient to result in a relatively uniform light transmitting characteristic greater than an unexposed array.

### Claim 32 (depends on 31)

The method of claim 31, wherein the device has optically finished end faces after reduction which are optically equivalent the same surfaces before reduction.

### Claim 33 (depends on 32)

The method of claim 32, further comprising the step of: recessing the core portion exposed at said at least one end surface relative to the cladding portions thereby forming an intagliated surface having cavities with wall portions formed of exposed cladding, and the surface layer extends into said cavities along said wall portions.

### Claim 34

A waveguide comprising:
at least one waveguide segment of a selected length having a core and cladding and extending between optically finished ends exposing portions of the core and the cladding, at least one of the ends of waveguide having a surface layer developed in the cladding to a depth sufficient to absorb incident radiation directed at the cladding without secondary emission and to render the exposed portions of the cladding opaque for enhancing contrast and reducing cross-talk relative to an array without developed surface layers.

### Claim 35

A method for treating at least one end surface of a waveguide formed of a waveguide segment extending between optically finished surfaces each waveguide segment including a core surrounded by a cladding comprising the steps of:
preferentially developing an opaque surface layer in the cladding sufficient to absorb incident radiation directed at the cladding without secondary emission, and to product a contrast characteristic in said waveguide greater than an undeveloped cladding.