WO2004106977A2

WO2004106977A2 - Infrared optical fibers

Info

Publication number: WO2004106977A2
Application number: PCT/IL2004/000479
Authority: WO
Inventors: Eran Rave; Abaraham Katzir
Original assignee: Ramot at Tel Aviv University Ltd
Current assignee: Ramot at Tel Aviv University Ltd
Priority date: 2003-06-03
Filing date: 2004-06-03
Publication date: 2004-12-09
Anticipated expiration: 2005-12-03
Also published as: WO2004106977A3

Abstract

An infrared optical fiber (10) including a core consisting of a first optical material with a first refractive index; and a clad substantially surrounding said core including a plurality of clad elements (103), wherein the clad elements (103) include a second optical material of second refractive index, and the first refractive index is greater than the second refractive index. A method for design and production of an infrared optical fiber (10), the method including constructing a parameterized model for the infrared optical fiber (10), simulating propagation of infrared radiation confined within the model of the infrared optical fiber (10); and assembling a preform by stacking longitudinally optical fiber segments based on results of the simulation.

Description

INFRARED OPTICAL FIBERS

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a process for the design and manufacture of a new type of optical fiber and, more particularly, to a process for manufacturing an optical fiber for transmission of light in the infrared spectral range. The fiber includes multiple clad elements and a large effective core.

Silica optical fibers are commonly used for transmitting visible and near infrared light of wavelength 0.4- 2 microns for communications, laser power delivery and sensing. However, silica does not transmit light of wavelength longer than about 2 microns. Therefore, for the middle and far infrared spectral bands, other fiber materials are used including fluoride glass fibers, chalcogenide glass fibers and metal halide crystalline fibers.

Silver halides have been used for the fabrication of IR fibers. For example, Kayashima et al. disclosed in US patents 5,182,790 and 5,575,960 a method for manufacturing an unclad infrared fiber by extruding silver halide polycrystals through a die.

Single crystals of silver halides of composition AgC^Br^ (O≤x≤l) have a wide transparency window in the spectral range 3 - 30 μm . The mechanical and optical properties of the crystals vary with composition, and in particular the refractive index of AgCl_xBr_j.x increases almost linearly with x, from 1.98 for pure AgCl to 2.16 for pure AgBr. Unlike other infrared transparent fibers, fibers of poly crystalline silver halide are flexible, non- toxic and non-hygroscopic.

Conventional optical fibers consist of a core surrounded by a clad. The core material has an index of refraction slightly higher than that of the clad material. The light being transmitted along the fiber is, therefore, confined within the fiber by total internal reflection. The term "core/clad fiber" herein denotes conventional optical fibers. As the core diameter increases in core/clad fibers, light is transmitted in multiple discrete modes. For many applications, e.g. spectroscopy and interferometry, it is desirable to have a fiber that transmits light in a single mode. Such a fiber conventionally includes a very small core diameter, on the order of 10-100 microns depending on wavelength. In such a small core, it is difficult to efficiently couple infrared radiation into the core. Furthermore, for relatively low power, the optical power density in a small core exceeds the damage threshold of the fiber material. Therefore, conventional single mode fibers are generally not suitable for transmitting a high optical power.

For transmission of high optical power in the middle and far infrared spectral bands, hollow fibers including fibers with metallic coatings on the interior surface and Omni Guide Bragg fibers¹ are used. Hollow fibers do not generally have single mode transmission. Hollow fibers suffer from additional disadvantages, compared with core/clad fibers. These disadvantages include a very limited wavelength range and very large losses when bent.

There is thus a need for, and it would be highly advantageous to have, a fiber design and manufacturing process for fibers with a large core diameter that propagate infrared light with minimal optical losses and particularly single mode fiber.

There has been substantial research and development interest in a special type of optical fiber, called photonic crystal fibers (PCFs). Photonic crystal fibers are usually single material optical fibers, which have a periodic array, e.g. hexagonal or triangular, of cylindrical air holes running along the entire length of the PCF². PCFs can be classified into two families, according to the mechanism of light confinement. One family is based on the principle of the photonic band gap (PBG) effect, and it is therefore called PBG-PCF³. The second family is based on structures where one or more holes at the center are missing. The refractive index of the center is therefore higher than the effective refractive index of the surrounding array. The second family of fibers is based on total internal reflection (TIR) and it is thus called TIR-PCF. Most photonic crystal fibers made of silica glass, offer significant advantages over conventional core/clad optical fibers. In particular, PCFs can be designed to operate as single mode fibers (SMF) for any wavelength or any core size, due to the unusual dispersion properties of the cladding. The core dimension can be designed to be very large⁵ (approximately 50 free-space wavelengths) while maintaining single mode propagation. Therefore, an extremely high power can propagate through the core, without exciting undesirable nonlinear effects. Furthermore, zero dispersion⁶ can be obtained at any wavelength. Another important parameter, is the attenuation per unit length. In silica core/clad fibers the minimal attenuation is 0.2 dB/km at wavelength 1.55 microns. Theoretical research shows that PCF can reach lower losses. Experimentally, the minimal attenuation reported⁷ for TIR-PCF was 0.58 dB/km and for PBG- PCF 13 dB km.

The following three references published by the applicants are incorporated by reference for all purposes as if fully set forth herein: E. Rave, K. Roodenko and A. Katzir, "Infrared photonic crystal fiber", Appl. Phys. Lett, 83(10), 1912-1914, (2003).

E. Rave, P. Ephrat, M. Goldberg, E. Kedmi and A. Katzir, "Silver Halide Photonic Crystal Fibers for the Middle Infrared", Appl. Opt. 43(11), 2236-2241, (2004). E. Rave, P. Ephrat and A. Katzir, "AgClBr photomc crystal fibers for the middle infrared", Proc. SPIE, Vol. 5360-43, (2004).

References

1. B. Temelkuran, S. D. Hart, G. Benoit, J. D. Joannopoulos and Y. Fink, "Wavelength- scalable hollow optical fibers with large photomc bandgaps for CO₂ laser transmission",

Nature 420, 650-653 (2002)

2. J. Broeng, D. Mogilevstev, S. E. Barkou and A. Bjarklev, "Photonic Crystal Fibers: A New Class of Optical Waveguides", Opt. Fiber Tech., 5, 305 (1999)

3. N. M. Litchinitser, A. K. Abeeluck, C. Headley and B. J. Eggleton, "Antiresonant reflecting photonic crystal optical waveguides" Opt. Lett., 27, 1592 (2002)

4. T.A. Birks, J. C. Knight and P. St. J. Russell, "Endlessly single-mode photonic crystal fiber", Opt Lett., 22, 961 (1997)

5. J.C. Knight, T.A. Brinks, R. F. Cregan, P. St. J. Russell and J. P. de Sandro, "Large Mode Area Photonic Crystal Fiber". Electron Lett., 34, 1347 (1998) 6. D. Mogilevtsev, T. A. Birks and P. St. J. Russell, "Group Velocity Dispersion in photonic crystal fibers", Opt. Lett., 23, 1662 (1998) 7. P. Russell, "Photonic Crystal Fibers", Science 299, 358 (2003)

SUMMARY OF THE INVENTION An embodiment of the present invention, analogous to silica TIR-PCF, includes an infrared transmitting matrix material and multiple infrared transmitting clad elements including a material of lower refractive index than the refractive index of the matrix material.

According to the present invention there is provided an infrared optical fiber including a core consisting of a first optical material with a first refractive index; and a clad substantially surrounding the core including a plurality of clad elements, wherein the clad elements include a second optical material of second refractive index, and the first refractive index is greater than the second refractive index. Preferably, the first optical material and the second optical material are selected from optical materials including metal halides, alkali halides, fluoride glasses and chalcogenide glasses. Preferably, the infrared optical fiber includes clad elements that are discretely arranged in concentric rings having a shape of a circle, a triangle, a square, a pentagon, a hexagon, or n-fold polygon. According to the present invention there is provided, a method for producing a preform for manufacture of an infrared optical fiber, including (a) cutting laterally a fiber with a first refractive index into first segments; (b) cutting laterally a second fiber with a second refractive index into a plurality of second segments; the first refractive index being greater than the second refractive index; and (c) stacking longitudinally the first segments and the second segments so that the second segments surround the first segments. Preferably, all the segments are equal in length. According to further features in preferred embodiments of the invention, the stacking is performed inside a form and the preform is produced by fusing the first segments and the second segments.

According to the present invention there is provided, a method for design and production of an infrared optical fiber, the method including: (a) constructing a parameterized model for the infrared optical fiber, the model including as parameters a matrix material refractive index, and a clad refractive index of clad elements, wherein the clad refractive index is less than the matrix material refractive index; (b) simulating propagation of infrared radiation confined within the model of the infrared optical fiber; and (c) assembling a preform by stacking longitudinally optical fiber segments based on results of the simulation. Preferably, the model further includes as parameters, the distance between the clad elements, and/or a diameter of the infrared optical fiber. Preferably, values of parameters are set so that the propagation is in a single mode. Preferably, the infrared optical fiber is produced by (d) extruding the preform under pressure through a die or alternatively by pulling the preform while heating.

According to the present invention there is provided, a method of manufacturing an infrared optical fiber, including (a) stacking longitudinally optical fiber segments; and (b) extruding the fiber segments under pressure through a die or (b) pulling the fiber segments while heating. Preferably, the optical fiber segments include core fiber segments and clad fiber segments, wherein the refractive index of the core fiber segments is greater than a refractive index of the clad fiber segments. According to the present invention there is provided, an infrared optical waveguide including: (a) a core consisting of a first optical material with a first refractive index; and (b) a clad substantially surrounding the core including a plurality of clad elements, wherein the clad elements include a second optical material of second refractive index, wherein the first refractive index is greater than the second refractive index; wherein the clad elements are discretely arranged in substantially concentric rings; wherein the rings substantially have a shape selected from the group of shapes consisting of circle, triangle, square, pentagon, hexagon, and n-order polygon.

According to the present invention there is provided an infrared optical fiber or waveguide manufactured by any of the methods included herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: FIG. 1 is a cross sectional view of an infrared optical fiber with multiple clad elements according to an embodiment of the present invention;

FIG. 2 is an illustration of simulated optical intensities in cross section of lower order modes, according to an embodiment of the present invention;

FIG. 3 a is a simplified flow chart of a manufacturing method of an infrared optical fiber, according to an embodiment of the present invention;

FIG. 3b is a simplified flow chart of a manufacturing method of an infrared optical fiber, according to another embodiment of the present invention;

FIG. 4 is a simplified cross sectional drawing of a fiber segment organization used to make a preform for manufacturing an infrared optical fiber, according to an embodiment of the present invention;

FIG. 5 is a schematic drawing of a system used to verify performance of an infrared optical fiber;

FIG. 6 is a graph of output intensity profile test results of an infrared optical fiber of the present invention; FIG. 7 is a thermal image of output intensity from an infrared optical fiber of the present invention; FIG. 8 is a schematic drawing of an infrared spectroscopic system used to verify performance of an infrared optical fiber;

FIG. 9 is a graph of spectroscopic test results of an infrared optical fiber of the present invention; FIG. 10 is a schematic drawing of a system used to verify performance of an infrared optical fiber;

FIG. 11 is a graph of test results of an infrared optical fiber of the present invention;

FIG. 12 is a schematic drawing of a system for measuring numerical aperture of an infrared optical fiber; FIG. 13 is a graph of sample measurement results of numerical aperture of an infrared optical fiber of the present invention;

FIG. 14 is a cross sectional view of an infrared single mode optical fiber with multiple clad elements according to an embodiment of the present invention

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is of an optical fiber for transmission of light in the infrared, the fiber including multiple clad elements and a large effective core.

Specifically, the present invention can be used to design, simulate, manufacture and test an optical fiber with multiple clad elements. The principles and operation of the present invention according to the present invention may be better understood with reference to the drawings and the accompanying description.

It should be noted, that although the discussion herein relates to design, simulate, manufacture and test with silver halides as the predominant optical materials, the present invention may, by non-limiting example, alternatively be configured with other infrared transmitting optical materials, particularly metal halides, crystalline alkali halides, fluoride glasses and chalcogenide glasses.

Before explaining embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details of design, simulation, manufacture and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. As such, those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, and methods for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. Specifically, although the discussion herein relates to "optical fibers" that generally have circular cross sections, embodiments of the present invention include optical waveguides that have square, rectangular or other cross sectional symmetry. The term "optical fiber" as used herein refers to any optical waveguide of circular or non-circular cross sectional symmetry. By way of introduction, principal intentions of the present invention are to: (1) provide a set of appropriate design guidelines and a parameterized model system for an optical fiber including a matrix material and multiple clad elements; (2) simulate the propagation of infrared radiation along the model optical fiber; (3) manufacture optical fibers with a matrix and multiple clad elements based on results from the simulation. The method of manufacture of infrared optical fiber with a matrix and multiple clad elements, according to the present invention depends strongly on the nature of the materials in use. According to an embodiment of the present invention, fibers from some infrared transmitting glass materials, e.g. fluoride glasses or chalcogenide glass are preferably manufactured by controlled pulling of a hot preform, a method similar to silica fiber production. According to other embodiments of the present invention, infrared optical fiber are of crystalline or polycrystallme optical materials, e.g. metal halides, alkali halides and are preferably manufactured by extrusion techniques.

The ensuing description further includes a number of test methods and results that verify that the manufactured optical fiber, according to embodiments of the present invention, behaves according to simulation results. A model design, according to an embodiment of the present invention, for an infrared optical fiber analogous to a TIR-PCF silica fiber includes a matrix of higher refractive index and multiple clad elements including a material of lower refractive index. The design guidelines are:

(a) The diameter of the fiber has to be less than ~lmm. to insure flexibility, for instance, for silver halide materials.

(b) For high power delivery, the effective area of the optical fiber core should be large to decrease power density within the core. (c) The number of fiber optic elements arranged around the core is preferably relatively large, to operate effectively as a cladding layer.

(d) A small number of modes or single mode behavior is preferred.

Based on these guidelines, a model structure was designed. Referring now to the drawings, Figure 1 illustrates a cross sectional view of a model infrared optical fiber 10 with multiple clad elements according to an embodiment of the present invention. A material 101, is a matrix material for infrared fiber 10 of diameter D. Clad elements 103 are preferably chosen including a material with a lower refractive index than matrix material 101. Other adjustable parameters of the model include diameter B of clad elements 103, and distances C between adjacent clad elements.

Simulation of a model infrared optical fiber

Model infrared optical fiber 10 was analyzed in the time-domain, using the Finite

Difference Method for solving Maxwell's equations, based on the work of M. Qiu "Analysis of guided modes in photomc crystal fibers using the finite-difference time-domain method", published in Microwave and Optical Tech. Lett. 30, 327-330 (2001) which is incorporated by reference for all purposes as if fully set forth herein.

Model infrared optical fiber 10 is assumed to consist of dielectric materials, only with no free charges or currents. According to an embodiment of the present invention, the simulation method includes solving for the magnetic field, since all magnetic field components are continuous at the boundaries due to the uniformity of the magnetic permeability μ .

Assuming, as usual, harmonic time dependence, and a expriβz) dependence of the longitudinal field components, a pair of coupled equations are obtained for the transverse components of the magnetic field:

In these equations, n is the refractive index, ω is the angular frequency and β is the propagation constant, space was divided into a grid of 200 by 200 points. It was found subsequently, that further refinement of the grid generally did not alter the results significantly. Using the finite difference method, simulations of the model infrared optical fiber 10 are performed, in order to determine a model structure that show properties elucidated in the design guidelines above: core-clad behavior, a small number of modes, mechanical flexibility, and large core area. According to an embodiment of the present invention, clad elements 103 are situated on a hexagonal array, a structure suitable for manufacture by extrusion methods. Parameters varied during the course of simulation include the number of clad element rings, distance C between fiber optic elements in each ring, clad element diameter B, and overall diameter D.

Model infrared optical fiber 10 was simulated, according to an embodiment of the present invention, with clad element diameter B of 50 microns, clad inter-element distance C of 110 microns, a total number of 30 clad elements 103 arranged discretely in two concentric hexagonal rings, and outer diameter D of 1mm. The term "discretely arranged" as defined herein means that clad elements 103 are not continuous concentric rings and clad elements 103 are individually placed with given clad inter-element distances C along the concentric rings. Equations (1) were solved for the bound modes that exist in model infrared optical fiber 10 propagation constant β of each mode were found. The condition on β was that k^_FSM < β < kn_core • Here k is the wave-number, ncore^.lό is the refractive index of the AgBr core, and n_FSM is the effective index of the fundamental space-filling mode, which is the fundamental mode of the infinite clad if the core volume were filled with similarly with clad elements. The difference in refractive index between matrix material 101 and the material of clad elements 103 is expected to influence the number of modes. However, preliminary simulations showed that the number of modes is relatively insensitive to the exact refractive index difference between matrix material 101 and the material of clad elements 103. Consequently, a preferred embodiment of the present invention includes a matrix material 101 of pure AgCl and clad element 103 material of pure AgBr, since the pure materials AgCl and AgBr are softer and more easily extruded than ternary materials AgCl_xBr_j._x (0 < x < 1) .

Model infrared optical fiber 10, with the parameter values as presented above, was found to support 12 modes, and in Fig. 2, are illustrated optical intensities in cross section of lower order modes, specifically, mode 1 201, mode 3 203, mode 7207 and mode 9209. The intensity profile of the fundamental mode was calculated, the 1/e width was found to be 370μm . This value is similar to the geometrical width of the core, as viewed by an optical microscope. Other parameters were calculated such as the relative index

difference Δn = — — £_* = 0.035%, and the numerical aperture ft Core

Manufacture of an infrared optical fiber

Figure 3 a illustrates a method of manufacturing an infrared optical fiber, according to an embodiment of the present invention. Several meters of core/clad AgCl/AgBr fiber, of outer diameter 1mm, are extruded (step 301) preferably using a "rod in tube" method described in "Properties of silver halide core-clad fibers and the use of fiber bundle for thermal imaging" by I. Paiss, F. Moser and A. Katzir, and published in Fiber and Integrated Opt. 10, 275-290 (1991), which is incorporated by reference for all purposes as if fully set forth herein. Unclad AgBr fiber is extruded, using a system and method such as described in US patents 5,182,790 and 5,575,960. US patents 5,182,790 and 5,575,960 which are incorporated by reference for all purposes as if fully set forth herein. The AgCl AgBr fiber is cut into segments (step 305) each of similar length, e.g. 5 cm. The unclad AgBr is also cut into segments (step 307) each of similar length, e.g. 5 cm. Since silver halides are soft materials, the fiber cutting is performed with a glass knife, similar to that used in a biological microtome. The segments are organized (step 309) to form a preform for extrusion (step 311) through a die of diameter 1mm. This method according to an embodiment of the present invention, produces infrared optical fibers that have diameter 1mm and length about 1 meter.

Figure 3b illustrates a method 31 of manufacturing an infrared optical fiber, according to another embodiment of the present invention, appropriate for manufacturing methods similar to those used for conventional silica glass optical fibers. According to method 31, infrared fiber is manufactured (step 323) from an infrared glass material appropriate for a core material and similarly an infrared fiber is manufactured (step 321) including an infrared glass material appropriate for a clad material. Clad segments are cleaved (step 325) and core segments are similar cleaved (327) and are subsequently organized into a form with the core segments in the center surrounded by the clad segments. Referring now to Figure 4, illustrating in cross section a fiber segment organization used to make a preform 40 for manufacturing, e.g. extruding, an infrared optical fiber, according to some embodiments of the present invention. Figure 4 illustrates core segments 401 e.g. six unclad AgBr fiber segments arranged in the center and clad fiber segments 403, e.g. thirty AgCl/AgBr fiber segments arranged in two concentric hexagonal rings around the center. According to other embodiments of the present invention as shown in method 31, core fiber segments 401 and clad fiber segments 403 are cleaved from infrared glass fibers of infrared materials, e.g. fluoride or chalcogenide glass appropriately chosen based on simulation results of infrared propagation as discussed in the previous section. Referring back to Figure 3b, infrared glass core fiber segments 401 and infrared glass clad fiber segments 403 are organized and assembled (step 329) in a form. A preform is manufactured, for instance, by raising the temperature of the form to above the glass transition temperature, and/or under pressure thereby fusing (step 331) glass core fiber segments 401 and glass clad fiber segments 403 into a single preform for optical fiber manufacture. As discussed above silver halide fibers are "cut" with a knife. Conventional glass fibers that are hard and brittle, e.g. silica are "cut" by "cleaving", a process that includes scratching the fiber surface laterally such as with a diamond scribe and breaking along the scratch. The term "cutting" is used hereinafter to refer also to a "cleaving" process or any other process used to generate fiber segments from a continuous optical fiber. Measurements and Optical Characterization

A number of techniques may be used to verify performance of an infrared optical fiber, according to the present invention. Performance verification is an integral part of the production process. Examples of techniques including power distribution measurement, core- clad characterization and transmission quality measurement, appropriate for process verification are shown in the embodiments below.

Referring now to Figure 5, showing an apparatus 50 used to measure the power distribution of infrared radiation exiting from an infrared optical fiber, according to the present invention. The optical radiative output of lOμm wavelength, preferably from a CO₂ laser 501 is directed longitudinally into core at entrance face 503a of an infrared optical fiber 505. According to an embodiment of the present invention, directing infrared radiation into the core at entrance face 503a is performed using a silver halide fiber 507 that has a taper 509. Preferably, taper 509 is coated with a thin opaque layer, e.g. a silver coating, but uncoated at the apex of taper 509 leaving an aperture 511 of 5 Oμm diameter. Further details regarding this technique are found in: P. Ephrat, K. Roodenko, L. Nagli and A. Katzir, "Scanning near- field infrared microscopy based on tapered silver-halide probes", Appl. Phys. Lett. 84, 637- 639, 2004, which is incorporated by reference for all purposes as if fully set forth herein. Infrared radiation is collected and measured at exit face 503b of infrared optical fiber 505 preferably with a detector 513 of infrared radiation, preferably a nitrogen-cooled mercury cadmium telluride (MCT) detector 513, e.g. Model HGT-90 manufactured by Infrared Associates, Stuart, FL. USA. Optionally, an infrared collecting fiber 515, e.g. a 900μm unclad silver halide fiber, is used to collect power emitted from exit face 503b and transmit the optical power to detector 513. A knife-edge 517 is scanned laterally between exit face 503b and collecting fiber 515. Optical power measurements are taken as knife-edge 517 is scanned laterally along exit face 503b of infrared optical fiber 505.

An example of measurement results using apparatus 50 is shown in graph 60 of Fig. 6.

Horizontal axis of graph 60 indicates knife-edge 517 position and vertical axis indicates relative measured intensity in arbitrary units. The optical power measured (open circle) at detector 513 was fit to a sigmoidal function 605 and differentiated analytically (open square) to yield a Gaussian distribution 607. The calculated 1/e width of the Gaussian distribution was 41 Oμm , slightly higher than the simulated value 370μm for the fundamental mode.

Measurement results of graph 60 were confirmed with a thermal image of shown in Figure 7 of the exit face 503 b of infrared optical fiber 503 b. Coupling of infrared radiation was performed as shown in apparatus 50. An imaging camera Model 600L (Inframetrics Inc., Billerica, MA.) is used in place of detector 513. The image shown in Fig. 6b shows that although some optical power is transmitted through the clad, there is nevertheless optical power confinement closely corresponding to the measured 41 Oμm . Another measurement technique used for performance verification of infrared optical fiber 505 is fiber optic evanescent wave spectroscopy (FEWS) illustrated schematically in Figure 8. FEWS is preferably performed using an infrared spectrophotometer 801 e.g. Fourier Transform Infrared (FTIR) spectrophotometer Model Vector 22 (Bruker Optics, Ettingen, Germany). Infrared radiation of varying wavelength is focused onto the core of an infrared fiber segment 505 under test using an infrared lens 807, e.g. ZnSe lens of focal length 37mm and f/# =1. Infrared fiber segment 505 under test is immersed in a cell 803 containing a solvent, e.g. dimethyl sulfoxide with a known absorbance spectrum in the infrared with numerous absorption bands in the mid- infrared corresponding to stretch and bend modes of CH₂, CH₃ etc. After propagating through the cell, infrared absorbance is measured with an infrared detector 805, preferably of mercury-cadmium telluride. FEWS is a measure of absorption due to interactions between the evanescent field outside the fiber segment and the solvent, the absorption spectrum being that of the solvent. However, if the mode field is confined within the clad then the evanescent field decays rapidly within the clad and does not reach the solvent.

Sample results of a FEWS measurement of an infrared optical fiber, according to an embodiment of the present invention are shown in the graph of Figure 9. The vertical axis indicates absorbance in arbitrary units and the horizontal axis indicates wavelength in microns. Trace 907 indicates FEWS absorbance of an infrared fiber segment 505 according to the present invention. Trace 905 indicates FEWS absorbance of an unclad silver halide fiber used as a control.

Throughout the measured spectral range (6-14μm), a highly reduced absorbance in the case of infrared fiber segment 505 compared with the control unclad fiber segment. The ratio of absorbance of infrared fiber segment 505 to that of the unclad fiber segment was 0.3, indicating that the evanescent field outside infrared fiber segment 505 was weaker by 30% than in the case of an unclad fiber segment, Therefore behavior characteristic of core-clad fibers was found for an infrared fiber segment 505 . Figure 10 illustrates an input scan system 51 similar to apparatus 50 of Figure 5. Silver halide fiber 507 with taper 509 and aperture 511 are scanned across entrance face 503a of infrared optical fiber 505. Infrared radiation transmitted through infrared optical fiber 505 is fully collected by infrared collecting fiber 515 and transmitted to detector 513.

Sample results from an input positional scan of an infrared optical fiber according to the present invention are shown in the graph of Figure 11. The vertical axis indicates in arbitrary units transmitted power as measured at detector 513. The horizontal axis indicates the position of the input aperture 511 relative to entrance face 503a of infrared optical fiber 505. A core-clad transition appears at points 117a and 117b about half maximum transmitted power when aperture 511 was scanned across entrance face 503a. The core width was defined at the points where the slope changed abruptly, and its value was 370μm, in good agreement with the simulated value. Figure 12 illustrates a system 52 for measuring numerical aperture of an infrared optical fiber 505, according to an embodiment of the present invention. A pinhole 521 is placed immediately in front of entrance face 503a of infrared optical fiber 505. Infrared light from laser 501 is directed through pinhole 521 into entrance face 503a. Collecting fiber 515 and detector 513 as well as infrared optical fiber and pinhole 521 are all placed on a common optical bench 525 swivelably attached about the core at entrance face 503a. Transmitted intensity is measured with detector 513 as optical bench 525 rotated about an angle of incidence 523.

Figure 13 shows a graph of sample numerical aperture results using system 52. The vertical axis indicates transmittance in arbitrary units and the horizontal axis indicates angle of incidence 523. Figure 13 shows trace 137 for an infrared optical fiber 505 according to the present invention and trace 135 for an unclad AgClBr silver halide fiber of 900 micron diameter. The numerical aperture is defined by the sine of angle θ_c 523, relative to the normal incidence, where the signal reduced to 1/e of its peak value. The value for the unclad AgCIBr fiber was θ_c = 60 and for infrared optical fiber 505 of the present invention θ_c = 31 .

Clearly, infrared optical fiber 505, according to the present invention is showing core/clad behavior. However, the NA is still much larger than the expected from the fundamental mode simulation (θ_c = 3°). This is probably due to multiple low order modes and cladding modes being transmitted at higher angle of incidence. The attenuation of the IR radiation through infrared optical fiber 505 was measured for a different fiber with system 51 using the expression:

-αL

Pi_n is CO₂ laser 501 power launched into infrared optical fiber 505 and P_outis the power measured at its output. R is the Fresnel reflectance coefficient, which is 0.12 per interface in our case. L is infrared optical fiber 505 length in meters, and α is the attenuation coefficient in dB/m . Attenuation coefficient of α = 60dB/m which is the upper limit at λ = 10.6μm . This attenuation is much higher than that of unclad silver halide fibers presumably because additional scattering is present due to two causes: (a) the distance between neighboring fiber optic clad elements 103 during extrusion did not stay constant, and (b) the surface of each fiber optic clad elements 103 was not perfect.

Simulation of Single Mode Infrared Fiber FIG. 14 illustrates a cross sectional view of a model infrared optical fiber 14 with multiple clad elements, according to an embodiment of the present invention. Model Infrared optical fiber 14 includes five concentric hexagonal rings, ninety fiber optic clad elements B of diameter 32 microns, clad inter-element distance C of 64 microns, and fiber diameter D of 700 microns. The matrix material index of refraction is 2.16 corresponding to pure AgBr and the index of refraction of clad elements 103 is 1.98 corresponding to pure AgCl.

A simulation of the propagation of infrared light, according to the discussion above using finite element analysis, shows single mode propagation within model infrared optical fiber 14 with a mode field diameter of 50 microns.

With respect to the above description then, it is to be realized that the optimum dimensional relationships for the parts of the invention, to include variations in size, materials, shape, form, function and manner of operation, assembly and use, are deemed readily apparent and obvious to one skilled in the art, and all equivalent relationships to those illustrated in the drawings and described in the specification are intended to be encompassed by the present invention. Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention. While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.

Claims

WHAT IS CLAIMED IS

1. An infrared optical fiber comprising:

(a) a core consisting of a first optical material with a first refractive index; and

(b) a clad substantially surrounding said core including a plurality of clad elements, wherein said clad elements include a second optical material of second refractive index, wherein said first refractive index is greater than said second refractive index.

2. The infrared optical fiber, according to claim 1, wherein said first optical material and said second optical material are selected from the group of optical materials consisting of metal halides, alkali halides, fluoride glasses and chalcogenide glasses.

3. The infrared optical fiber, according to claim 1, wherein said clad elements are discretely arranged in substantially concentric rings.

4. The infrared optical fiber, according to claim 3, wherein said rings substantially have a shape selected from the group of shapes consisting of circle, triangle, square, pentagon, hexagon, and n-order polygon.

5. A method for producing a preform for manufacture of an infrared optical fiber, comprising the steps of:

(a) cutting laterally a first fiber consisting of a first optical material with a first refractive index into at least one first segment;

(b) cutting laterally a second fiber including a second optical material with a second refractive index into a plurality of second segments; wherein said first refractive index is greater than said second refractive index; and (c) stacking longitudinally said at least one first segment and said second segments so that said second segments substantially surround said at least one first segment.

6. The method, according to claim 5, wherein said at least one first segment and said second segments are substantially equal in length.

7. The method, according to claim 5, wherein said stacking is performed inside a form.

8. The method, according to claim 5, further comprising the step of:

(d) fusing said first segments and said second segments thereby producing the preform.

9. A method for design and production of an infrared optical fiber, the method comprising the steps of:

(a) constructing a parameterized model for the infrared optical fiber, said model including as parameters a matrix material refractive index, and at least one clad refractive index of a plurality of clad elements, wherein said at least one clad refractive index is less than said matrix material refractive index;

(b) simulating propagation of infrared radiation confined within said model of the infrared optical fiber; and

(c) assembling a preform by stacking longitudinally optical fiber segments based on results of said simulating.

10. The method, according to claim 9, wherein said model further includes at least one of said parameters selected from the group consisting of at least one distance between said clad elements, and a diameter of the infrared optical fiber.

11. The method, according to claim 9, wherein said simulating includes setting a value of at least one of said parameters so that said propagation is in a single mode.

12. The method, according to claim 9, further including the step of:

(d) extruding said preform under pressure through a die thereby manufacturing the infrared optical fiber.

13. The method, according to claim 9, further including the step of:

(d) pulling said preform while heating thereby manufacturing the infrared optical fiber.

14. A method of manufacturing an infrared optical fiber, comprising the steps of:

(a) stacking longitudinally a plurality of optical fiber segments; and

(b) extruding said plurality under pressure through a die.

15. The method, according to claim 14, wherein said plurality of optical fiber segments includes at least one core fiber segment and a plurality of clad fiber segments, wherein a refractive index of said at least one core fiber segment is greater than a refractive index of said clad fiber segments.

16. A method of manufacturing an infrared optical fiber, comprising the steps of:

(a) stacking longitudinally a plurality of optical fiber segments; and

(b) pulling said plurality while heating.

17. The method, according to claim 16, wherein said plurality of optical fiber segments includes at least one core fiber segment and a plurality of clad fiber segments, wherein a refractive index of said at least one core fiber segment is greater than a refractive index of said clad fiber segments.

18. An infrared optical fiber manufactured by the method of claim 5.

19. An infrared optical fiber manufactured by the method of claim 9.

20. An infrared optical fiber manufactured by the method of claim 14.

21. An infrared optical fiber manufactured by the method of claim 16.

22. An infrared optical waveguide comprising:

(a) a core including a first optical material with a first refractive index; and

(b) a clad substantially surrounding said core and including a plurality of clad elements, wherein said clad elements include a second optical material of second refractive index, wherein said first refractive index is greater than said second refractive index; wherein said clad elements are discretely arranged in substantially concentric rings; wherein said rings substantially have a shape selected from the group of shapes consisting of circle, triangle, square, pentagon, hexagon, and n-fold polygon.