WO2021211195A1

WO2021211195A1 - Modification of internally clad sapphire fiber to attenuate cladding modes at high temperatures

Info

Publication number: WO2021211195A1
Application number: PCT/US2021/017534
Authority: WO
Inventors: Joshua T. Jones; Steven Derek ROUNTREE; Thomas E. BLUE; Daniel Kominsky; Osgar John Ohanian Iii
Original assignee: Luna Innovations Incorporated
Priority date: 2020-04-14
Filing date: 2021-02-11
Publication date: 2021-10-21

Abstract

A sapphire optical waveguide includes a sapphire core, at least one layer of internal cladding within the sapphire optical waveguide, and a mode stripping material, structure, or geometry that reduces a power of light traveling in the internal cladding at high temperatures. The mode stripping material, structure, or geometry strips higher order modes so that the low order modes of light are propagated along the waveguide. Methods for making a sapphire optical waveguide with various mode stripping materials, structures, or geometries are described.

Description

MODIFICATION OF INTERNALLY CLAD SAPPHIRE FIBER TO ATTENUATE CLADDING MODES AT HIGH TEMPERATURES

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority from U.S. provisional patent application serial number 63/009,493, filed on April 14, 2020, the contents of which are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with US Government support under Contract No. ’s

DE-SC0019834 and DE-SC0018767, awarded by the United States Department of Energy. The Government has certain rights to the invention.

TECHNICAL FIELD

[0003] The technology described in this application relates to treatment of sapphire optical devices that contain an internally clad sapphire waveguide.

INTRODUCTION

[0004] Because of their low cost, small mass, low signal attenuation over long lengths, immunity to electromagnetic interference and potential for high bandwidth data transfer, guided wave optical devices (e.g., fiber optics, waveguides formed in planar dielectric materials, etc.) have been developed for sensing applications. Optically-based sensors may be used for measuring, for example, temperature, strain, pressure and acceleration with applications varying from nuclear energy instrumentation and controls, to smart structures, to the oil and gas industry, and more. Intrinsic optical sensors use the optical fiber itself as the sensing element. Examples of intrinsic optically based sensors are fiber Bragg grating (FBG) sensors, intrinsic Fabry-Perot interferometric sensors, and inherent Rayleigh scatter sensors.

[0005] These sensors function well for single mode fiber, but function less well, or not at all, if the fiber is multimodal. The presence of multiple transverse modes of electromagnetic waves, each of which may propagate with a different effective group index of refraction, causes light propagating in the fiber to suffer from time-of-flight dispersion and, for FBGs, Fabry-Perot interferometers, and Rayleigh scatter, a blurring of the spectral characteristics of the sensor reflection spectrum. The fiber Rayleigh scatter signature may exhibit mode dependence, as modes may have differing intensity in physically separate sections of the available waveguide cross section, and thus respond to scattering centers located in different parts of the waveguide. Furthermore, the mode population may not be stable in time or with distance down the fiber, making correcting for temporal or spectral dispersion difficult.

[0006] Single crystal sapphire optical fiber can potentially operate at higher temperatures than silica optical fiber, which is attractive for many very high temperature sensing applications. But while silica optical fiber can be manufactured such that it propagates a single optical mode, conventionally manufactured single crystal sapphire optical fiber is highly multimodal, due to its relatively large diameter, lack of a well-defined core, and the large change in the index of refraction in passing from the sapphire fiber to the air that surrounds the fiber, when the fiber is used in air.

[0007] What is needed is a waveguide sensor that can operate in a high temperature environment with improved modal characteristics.

SUMMARY

[0008] The following presents a simplified summary of the innovation in order to provide a basic understanding of some aspects of the innovation. This summary is not an extensive overview of the innovation. It is not intended to identify key/critical elements of the innovation or to delineate the scope of the innovation. Its sole purpose is to present some concepts of the innovation in a simplified form as a prelude to the more detailed description that is presented later.

[0009] A sapphire optical waveguide structure includes a sapphire core and at least one layer of cladding. The sapphire optical waveguide structure also includes a mode stripping material, geometry, or structure configured to reduce the power of light traveling in the cladding. The core and cladding are capable of surviving and operating at temperatures in excess of 800°C.

[0010] A sapphire optical waveguide structure includes a sapphire core and at least one layer of cladding. The sapphire optical waveguide structure also includes a mode stripping material, geometry, or structure configured to reduce the power of light traveling in the cladding. The core and cladding are capable of surviving and operating in chemically harsh environments of molten salt, molten metal, acids, or high temperature steam. [0011] In some embodiments, the core and cladding are capable of surviving and operating at temperatures in excess of 800°C and in chemically harsh environments of molten salt, molten metal, acids, or high temperature steam.

[0012] In some embodiments, the mode stripping material, geometry, or structure includes an absorptive mode stripping material or structure.

[0013] In some embodiments, the mode stripping material, geometry, or structure includes a dispersive mode stripping material or structure.

[0014] In some embodiments, the mode stripping material, geometry, or structure includes an absorptive and dispersive mode stripping material or structure.

[0015] In some embodiments, the mode stripping material, geometry, or structure is a different composition than the outermost layer of cladding and is grown or formed on the outermost layer of the optical waveguide cladding under controlled conditions. One specific example embodiment provides a layer of aluminum oxyhydroxide, AIO(OH), which is formed on a surface of the sapphire optical waveguide.

[0016] In some embodiments, the mode stripping material, geometry, or structure includes one or more layers of deposited material on a surface of the sapphire optical waveguide.

[0017] In some embodiments, the mode stripping material, geometry, or structure is produced by etching the surface of the sapphire optical waveguide with boric acid, or another etchant.

[0018] In some embodiments, the mode stripping material, geometry, or structure includes one or more layers of a non-etched additive layer or dye on the surface of the sapphire optical waveguide.

[0019] In some embodiments, the mode stripping material, geometry, or structure includes impurity elements, or dopants, below a surface of the sapphire optical waveguide. [0020] In some embodiments, the mode stripping material, geometry, or structure is fabricated with a femtosecond (fs) laser treatment of the sapphire optical waveguide.

[0021] In some embodiments, the mode stripping material, geometry, or structure includes a material with an index of refraction that matches or is greater than the index of refraction of sapphire or of the cladding.

[0022] In some embodiments, the mode stripping material, geometry, or structure includes a modulated surface of the sapphire optical waveguide.

[0023] In some embodiments, the mode stripping material, geometry, or structure provides one or more of: a) stability at high temperatures above 800°C, b) an index of refraction comparable to or greater than that of sapphire, c) chemical inertness, d) a coefficient of thermal expansion near that of sapphire, or e) a capability of bonding with sapphire.

[0024] In some embodiments, the mode stripping material, geometry, or structure is configured to reduce a power of light traveling in the internal cladding by at least 3dB over a length of the sapphire optical waveguide.

[0025] In some embodiments, the mode stripping material, geometry, or structure is configured to reduce a power of light traveling in the internal cladding by at least 20dB over a portion of a length of the sapphire optical waveguide.

[0026] In some embodiments, the cladding is a refractive cladding, the sapphire optical fiber is a graded index sapphire optical fiber, and the mode stripping material, geometry, or structure does not attenuate light confined to the sapphire core by more than ldB over a length of the sapphire optical waveguide.

[0027] In some embodiments, the cladding is a refractive cladding, the sapphire optical fiber is a step index sapphire optical fiber, and the mode stripping material, geometry, or structure does not attenuate light confined to the sapphire core by more than ldB over a length of the sapphire optical waveguide.

[0028] In some embodiments, the sapphire optical waveguide is a fiber inscribed with at least one fiber Bragg grating.

[0029] In some embodiments, the sapphire optical waveguide incorporates at least one region of refractive index variation that produces enhanced scatter points.

[0030] In some embodiments, the sapphire optical waveguide incorporates at least one intrinsic Fabry -Perot interferometer.

[0031] . In some embodiments, the sapphire optical waveguide includes at least one Extrinsic Fabry -Perot interferometer.

[0032] In some embodiments, the sapphire optical waveguide is part of a distributed measurement system.

[0033] In some embodiments, the sapphire optical waveguide is capable of operating and sensing at temperatures exceeding those attainable in silica fiber.

In some embodiments, the sapphire optical waveguide is capable of operating and sensing in chemical environments which cannot be sensed in by silica fiber.

[0034] A further aspect includes a sensor having a low-noise, mode-stripped, internally-clad sapphire optical waveguide. That sensor may measure for example one or more of temperature, strain, pressure, vibration, or acceleration. The sensor is configured to be interrogated using Optical Frequency Domain Reflectometry (OFDR). [0035] A further aspect includes methods of fabricating a sapphire optical waveguide according to one or more of the example embodiments. One example method of fabricating a sapphire optical waveguide includes: obtaining a sapphire core; incorporating at least one layer of cladding; and incorporating a mode stripping material, geometry, or structure configured to reduce the power of light traveling in the cladding. The core and cladding are capable of surviving and operating in one or both of the following conditions: (i) at temperatures in excess of 800°C and/or (ii) in chemically harsh environments of molten salt, molten metal, acids, or high temperature steam.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG. 1 shows far field images taken using a dual scanning slit beam profiler with light coming from a fiber-coupled laser source of a sapphire fiber produced in accordance with the techniques described in U.S. Patent 10,436,978.

[0037] FIG. 2 shows the behavior of light traveling in an internally clad sapphire fiber produced with the techniques described in US Patent 10,436,978.

[0038] FIG. 3 shows the same image as FIG. 1 but with a drop of high index oil added near the end of the fiber closest to the beam profiler to strip any light traveling within the fiber cladding.

[0039] FIG. 4a shows the addition of a layer at the surface of the fiber, region c, that strips the cladding modes out of the fiber.

[0040] FIG. 4b shows the addition of a Fiber Bragg Grating.

[0041] FIG. 5 illustrates the addition of structures within the cladding of the fiber to absorb and/or disperse cladding modes.

[0042] FIG. 6 illustrates the bonding of a high temperature high index of refraction material to the sapphire fiber in an annular structure.

[0043] FIG. 7 illustrates the bonding of a high temperature high index of refraction material to the sapphire fiber.

[0044] FIG. 8 illustrates having a ribbed surface to strip the cladding modes, by greatly varying the angle of the fiber surface, the chances that light impacts the surface normal to the surface is greatly increased.

[0045] FIG. 9a illustrates a sapphire optical waveguide with an Intrinsic Fabry-Perot interferometer.

[0046] FIG. 9b illustrates a sapphire optical waveguide with an Extrinsic Fabry-Perot interferometer. [0047] FIG. 10 illustrates an example of using the sapphire optical waveguide, a, with an optical interrogator, b, as a sensor to make distributed measurements at sensing locations c, d, and e, in a test environment restricted to area i. The sapphire optical waveguide is connected to the sensor interrogator though an optical fiber lead h, which connects to the sensor interrogator at f and the sapphire waveguide at g.

[0048] FIG. 11 illustrates the use of a sapphire optical waveguide, a, configured to be measured with a sensor interrogator b which employs the OFDR interrogation technique.

DETAILED DESCRIPTION

[0049] In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the subject innovation. It may be evident, however, that the innovation can be practiced without these specific details. Certain illustrative aspects of the innovation are described herein in connection with the following description and the drawings. These aspects are indicative, however, of but a few of the various ways in which the principles of the innovation can be employed and the subject innovation is intended to include all such aspects and their equivalents. Other advantages and novel features of the innovation will become apparent from the following detailed description of the innovation when considered in conjunction with the drawings.

[0050] The description applies to optical waveguides. An example optical waveguide is an optical fiber. The following description refers to optical fibers for ease of description; however, it should be understood that this description applies to optical waveguides in general.

[0051] Optical fiber-based sensors are capable of distributed temperature and/or strain measurements with high accuracy and sub-cm spatial resolution. Optical fibers have a small diameter («100 pm), a small mass (and therefore fast response time), and are fabricated from high temperature tolerant, radiation-hard materials. Optical fiber sensing is used in the petroleum industry to measure the temperature and pressure profiles in down-hole applications. Examples of other harsh environments in which optical fiber sensing is used include coal gasifiers and jet engines.

[0052] There is an interest in using fiber optics for advanced instrumentation in high temperature power plants. Example applications include monitoring fuel performance during harsh test environments, embedding fibers in commercial reactor components or structures, and others. Single crystal sapphire (e.g., al pha-AhCri) optical fiber has a melting temperature in excess of 2000 °C, which makes single crystal sapphire attractive for extremely high temperature sensing of, for example, fuel centerline temperature, during irradiation testing of metallic fuels and some oxide fuels. Unfortunately, conventionally manufactured sapphire optical fiber exhibits highly multimodal light transmission and is therefore not ideal for most optical sensing techniques.

[0053] There are at least four different sensing techniques that can be applied to optical fibers to produce distributed sensing along the fiber: Optical Frequency Domain Reflectometry (OFDR), Optical Time Domain Reflectometry (OTDR), Stimulated Brillouin Scattering, and Bragg Grating Sensing. With the exception of Raman-based distributed sensing, a type of OTDR sensing that has been demonstrated in sapphire fiber up to temperatures up to 1200°C, most sensing techniques require nearly single mode light transmission in an optical fiber because the measurement is made by either interferometric or time of flight methods. For both methods, the measurements can be distorted by additional light modes in the fiber.

[0054] Optical Frequency Domain Reflectometry (OFDR) is a distributed measurement technique that works on the principles of injecting light into an optical fiber and measuring the reflected light off reflection points caused by natural defects or intentionally inscribed defects in the fiber. Initially, a “map” of the reflection points within the fiber is made at a known temperature (e.g., room temperature). As temperature or stress causes the optical path length to lengthen or contract, the optical distance to these reflection points moves. The measurement of their movement relative to the map can be correlated to temperature or strain. OFDR has been successfully used to measure temperature or strain with silica optical fibers.

[0055] The OFDR technique works well if the fiber is nearly single mode. If the optical fiber supports multiple light modes, then the time of flight measurement is distorted relative to the time of flight for the primary mode, because the light can travel in different modes with different effective group index of refraction in the fiber. Presently, silica optical fibers are preferred with the OFDR technique, because they can be made to support only a single light mode and because they have inherent scattering points within them that are due to defects and density changes of the amorphous silica glass structure. Sapphire fiber's multimodal nature, due to its large core and the lack of a cladding, along with the deficiency of defects that produce scattering centers in sapphire due to its crystalline structure, means that the interference-based OFDR sensing technique is typically not used with normal sapphire fiber.

[0056] Silica glass-based optical fiber sensors have a maximum temperature range of approximately 800 to 1100°C, due to the limitations of the silica glass (transmission through the glass decreases and reflection points evolve dramatically around that temperature). Single crystal sapphire optical fibers transmit light at temperatures well above 1100°C. They would potentially provide a sufficiently high operating temperature for temperature measurements to be made for fuel pins, in other high temperature regions in a reactor, and in other high temperature applications using OFDR, if sapphire fiber could be made sufficiently single mode in its transmission characteristics.

[0057] U.S. Patent 10,436,978 describes a method for forming a cladding in a single crystal sapphire optical fiber by reactor irradiation. The reactor irradiation creates ions external to the fiber that enter the fiber, displace atoms in the fiber, and are implanted in the fiber, thus modifying the index of refraction of the fiber near the surface of the fiber and creating a graded cladding in the sapphire fiber. The internal cladding can be created by irradiation of the fiber with ions that were generated in a (neutron, charged particle) reaction that occurred in a radiator material that surrounded the fiber and was external to the fiber. Irradiation of the fiber may be accomplished using any (neutron, charged particle) reaction, including any neutron source, but not limited to ⁶Li(neutron, alpha)³H or ¹⁰B(neutron, alpha)⁷Li reactions with a radiator containing ⁶Li or ¹⁰B using a Reactor as a source of neutrons.

[0058] In one embodiment of U.S. Patent 10,436,978, the cladding is created by irradiating a sapphire optical fiber, which was surrounded by an annulus of Li-6 enriched lithium carbonate (L12CO3) powder. The ⁶Li(n,a)¾ reaction creates high energy alpha particles and tritons that irradiate the fiber simultaneously to a depth of up to 24 microns along the entire periphery of the sapphire fiber, thereby slightly reducing the index of refraction in the fiber's periphery and creating a cladding within the fiber. Transmitted light intensity profiles show that the ion implanted cladding made the fiber's intensity profile nearly single mode. The cladding survived to a highest temperature that was tested (1500 °C). The cladding makes the sapphire fiber sufficiently single mode so that a read out from an optical backscatter reflectometer produced distributed temperature measurements along the length of the fiber.

[0059] The inventors conducted further work that showed varied repeatability of initial experiments regarding the single mode nature of the cladded sapphire fiber and therefore the sensing performance. Although the fiber production method described in U.S. Patent 10,436,978 produces an internal cladding-core structure that supports single (or few) mode transmission in the cladded sapphire fiber, the inventors determined that the cladding and fiber surface in general allow for light to propagate, not only in the sapphire fiber’s core, but also in the cladding resulting in “cladding modes,” which are modes of light traveling in the fiber that are not confined to the core. In particular, it is difficult to launch light from a conventional, fused silica fiber, used in the sensing instrument, into the modified, cladded sapphire fiber, such that only the core mode is populated, because of differences in the fused silica and sapphire fibers core size and numerical aperture. To make an effective sensor, when light is injected into the sensor fiber from a conventional fused silica single mode fiber, of the light that returns, the light propagating in the lowest order core mode of the sensor preferably makes up the majority of returned power. The inventors determined that stripping out light inadvertently launched into the cladding modes is necessary to insure that the lowest order core mode of the sensor makes up the majority of returned power.

[0060] Cladding mode stripping or stripping of cladding modes is the reduction in power of light traveling in the cladding of a waveguide without significantly affecting light confined to the core of the waveguide. For the example sensing applications described here, a cladding mode reduction of 3dB over the sapphire fiber sensor length may be sufficient. Greater cladding mode reduction is achievable and may be preferred, e.g., a reduction of greater than 20dB over one tenth the sapphire fiber sensor length.

[0061] Although the production methods in U.S. Patent 10,436,978 involving the formation of an internal clad inside sapphire optical fiber are referenced in this application to produce a sapphire fiber, other production methods may be used. One other example of such a production method is described in United States Patent 8,852,695, (the disclosure of which is incorporated herein by reference in its entirety), which produces a fiber with a core cladding structure having an internal cladding that is not graded.

[0062] Absent a method to strip the higher order cladding modes, the cladded sapphire fiber described in U.S. Patent 10,436,978 performs very much like an unclad sapphire fiber with the multimode problems described above. The inventors developed a new type of fiber with an internal clad inside sapphire optical fiber, e.g., using reactor irradiation with Lithium-6 carbonate or other suitable method, in which at least some or all of higher order cladding modes are stripped. Stripping at least some of the higher order cladding modes prevents these modes from interfering with the low order mode(s) of light which is(are) confined to the core of the fiber. Reduction of influence of these higher order cladding modes on the low order mode(s) of light which is(are) confined to the core permits use of the modified fiber in applications which require or greatly benefit from single or few mode fiber performance. The core cladding contains a modification via penetrative, additive, subtractive, or chemical modification of the fiber to strip out cladding modes and provide repeatable low mode performance, at very high temperature, of sapphire fibers with a cladding-core structure where light is traveling within the cladding structure. [0063] The creation of a cladding and mechanism to strip cladding modes in a sapphire optical fiber may extend OFDR sensing to the sapphire optical fiber. Various example embodiments that accomplish cladding mode stripping for internally-clad sapphire fibers will be described later in the application.

[0064] Sapphire optical fibers are produced with radii on the order of 38 to 50 micrometers, which is large compared to the radius of the core of a single mode silica fiber, which is on the order of about 4 to 8 micrometers in radius. The multimodality of optical fibers decreases with decreasing fiber core radius. Creating a cladding that is internal to the fiber reduces the radius of the fiber’s core as well as the multimodality of the fiber. This reduction in multimodality is only realized if the higher order modes that escape the core are effectively prevented from propagating in the fiber.

[0065] The multimodality of optical fibers decreases with decreasing refractive index change between the fiber core and cladding, as long as the cladding has a smaller index than the core. Without an internal cladding, the entire sapphire acts as the core of the fiber and the surrounding atmosphere acts as the cladding. Creating an internal cladding with a refractive index close to, but not greater than that of the sapphire, reduces the multimodality of the fiber compared to an unclad fiber. This reduction in multimode performance is realized if the higher order modes are effectively prevented from propagating in the fiber. Stripping at least some of the higher order cladding modes prevents these modes from interfering with the low order mode(s) of light which is(are) confined to the core of the fiber. Reduction of influence of these higher order cladding modes on the low order mode(s) of light which is(are) confined to the core permits use of the modified fiber in applications which require or greatly benefit from single or few mode fiber performance.

[0066] The inventors identified a need for cladding mode stripping via an experiment that included placing high index oil on an internal cladded sapphire fiber produced in accordance to U.S. Patent 10,436,978 while the fiber modal structure was being monitored with a ThorLabs BP209-IR Dual Scanning Slit Beam Profiler. FIG. 1 shows multiple peaks and valleys in the beam profile in both x and y axes prior to the oil drops being added, which indicates that it is not significantly single mode. Reference numeral 1 identifies the convolution of the two axes of the slit beam profiler. Reference numeral 2 identifies the slit beam profiler response along one axis, and reference numeral 3 identifies a gaussian fit to that response.

[0067] FIG. 2 shows the behavior of light traveling in an internally clad sapphire fiber produced according to US Patent 10,436,978. While light travels confined to the fiber core, region a, a substantial amount of light also travels in the cladding, region b. This light traveling in the cladding is referred to as “cladding modes.” These cladding modes need to be mitigated or stripped such that the light propagating in the lowest order core mode of the waveguide makes up the majority of power reaching the photodetectors of an optical network.

[0068] FIG. 3 shows the modal structure after adding a drop of high index oil near the end of the fiber closest to the beam profiler to strip any light traveling within the fiber cladding. Again, reference numeral 1 identifies the convolution of the two axes of the slit beam profiler. Reference numeral 2 identifies the slit beam profiler response along one axis, and reference numeral 3 identifies a gaussian fit to that response. The addition of the high index oil shows that the fiber contains a single (or few) mode core as indicated by the slit beam profiler measurement along the two axis, Fig. 3-2 being nearly gaussian. Fig. 3-3 is a gaussian fit to the response and Fig. 3-1 is the convolution of the two axis measurements to give a 2-diminsional representation. As seen in FIG. 1, without the addition of the high index oil, the transmitted light is significantly multimodal due to the light traveling in the fiber’s cladding as indicated by the slit beam profiler measurement waveform 2 along one of the two axes. Gaussian fit waveform 3 poorly matches waveform 2. The high index oil allows the cladding modes to escape, leaving a few mode profile as seen in FIG. 3. While adding index matching (or exceeding) oil can strip cladding modes, such oils do not exist that would be successful at achieving cladding mode stripping beyond 300°C due to evaporation.

[0069] Multiple mode stripping embodiments described below strip cladding modes from internally-clad sapphire fibers that provide one or more of the following beneficial characteristics: 1) Stability at high temperatures, 2) an index of refraction comparable or greater than that of sapphire, 3) chemical inertness, 4) a coefficient of thermal expansion near that of sapphire, and 5) good bonding with sapphire. The first characteristic is needed because the mode stripping structure must be able to survive high temperatures, e.g., at and above 800 °C, given that sapphire fibers have high temperature capabilities. If the mode stripping structure does not approach sapphire's high temperature capabilities, then the cladded sapphire fiber would be non-competitive in comparison to existing silica fiber, which is cheaper, mass produced in long lengths and more easily implemented. The second characteristic aids in stripping the cladding modes from the fiber because a structure with comparable index of refraction to sapphire’s does not support internal reflection of cladding modes. The third characteristic addresses the desire to produce a sapphire fiber which is able to be applied within a wide variety of harsh environments where silica fiber is not capable of operation, without negative impacts on performance due to removal or degradation of the mode stripping structure(s) or due to stresses caused by the chemical interaction. The fourth characteristic addresses the stress that a mode stripping structure may put on a sapphire fiber due to any non-uniform thermal expansion coefficients between the clad and the fiber. Any stresses caused by a mode stripping embodiment which does not provide benefits three or four might cause the fiber to break.

[0070] A first example embodiment for stripping the cladding modes that is suitable for high-temperature sensing applications intentionally forms a different composition than the rest of the sapphire waveguide on the surface of the sapphire under controlled conditions.

One example of this embodiment is growing of a layer of aluminum oxyhydroxide, AIO(OH) on the surface of the sapphire fiber by heating the sapphire fiber to high temperature in a humid air environment. The intentional formation of the aluminum oxyhydroxide surface defect on the internally-clad sapphire fiber causes the light traveling in the cladding, which uses the surface of the fiber as a propagation boundary, to be absorbed and/or dispersedly ejected from the fiber, i.e., stripped from the fiber.

[0071] A second example embodiment for stripping the cladding modes for an internally-clad sapphire fiber that is suitable for high-temperature sensing applications intentionally coats the fiber with one or more layers of evaporated material during furnace heating. Heating element (molybdenum disilicide) evaporation and deposition on the fibers are believed to attenuate cladding modes post-heating of the sapphire fibers within a high temperature furnace. In practice, intentional deposition of a layer of high-temperature material such as molybdenum or tungsten with relatively low reflectance may also be used. [0072] A third example embodiment for stripping the cladding modes for an internally-clad sapphire fiber that is suitable for high-temperature sensing applications uses boric acid or similar to etch the fiber. Etching the fiber surface does not modify the base fiber material, leaving the fiber in its original, high temperature-compatible structure with a rough surface, avoiding the potential for any new chemical reactions. The rough surface randomizes the angle of incidence of the cladding modes with the fibers surface, thus increasing the likelihood that the light will impact the surface at an angle that will allow it to escape (be stripped from) the fiber.

[0073] A fourth example embodiment for stripping the cladding modes for an internally-clad sapphire fiber that is suitable for high-temperature sensing applications uses non-etching chemical surface treatment such as additive layers and/or dye. In one non- limiting example, the surface treatment is achieved by the adhesion of nanoparticles sized to absorb light in the wavelength range of interest. These nanoparticles then absorb any light which is incident upon them within the fiber and then release the energy non-radiatively, or radiatively in an isotropic fashion, preventing further guidance by the sapphire optical waveguide.

[0074] A fifth example embodiment for stripping the cladding modes for an internally-clad sapphire fiber that is suitable for high-temperature sensing applications diffuses impurity elements below the fiber surface to change its optical properties in the clad. This may be accomplished with common gemology methods of coloring sapphire gems and involves heating the sapphire to high temperature and diffusing elements such as titanium, beryllium, or chromium beneath the sapphire surface. The diffusion embodiment of mode stripping is suitable up to the temperature required to get the impurity elements to diffuse into the sapphire.

[0075] A sixth example embodiment for stripping the cladding modes for an internally-clad sapphire fiber that is suitable for high-temperature sensing applications uses a femtosecond (fs) laser treatment of the cladding to remove clad propagating light at the sensing inlet or outlet of the fiber depending on the treatment method. The laser treatment process creates defects in the sapphire lattice that cause scattering points internal to the cladding that dispersively scatter light out of the cladding. The laser treatment process is similar to inscribing type-II fiber Bragg gratings which have been tested to 1500°C. Thus, the fs laser treatment of the cladding should be suitable to operation in environments in excess of 1500°C.

[0076] A seventh example embodiment for stripping the cladding modes for an internally-clad sapphire fiber that is suitable for high-temperature sensing applications adds index matching (or index greater) material along the fiber’s length, near the sapphire origin, and/or near the sapphire terminus. One, two, or all three may be used depending on the application. Non-limiting examples include diffusion bonding the fiber to a single crystal sapphire wafer/tube, diffusion bonding the fiber to alumina wafer/tube, index matching/exceeding fluid, and a very high index solder glass, rutile, or chalcogenide glass. Another example is other high index glasses (>1.8) that have been developed for augmented reality lenses. In solid material examples (no fluid), a structure such as a tube or wafer is optically coupled to the sapphire fiber where the structure has a high index of refraction comparable to or greater than that of the fiber. The light is allowed to exit the sapphire cladding and enter this structure. The structure is designed such that the efficiency for light entering it from the fiber is much greater than the reverse process of light entering the fiber from the structure. This efficiency is strongly related to the cross-sectional area of the structure compared to the fiber, and the magnitude of the area bonded between the two. In the case of diffusion bonding of a sapphire wafer/tube to a fiber, all the material is sapphire, and thus, high-temperature resistant and thermally compatible (matching coefficient of thermal expansion). Other example solid materials should be selected to be high- temperature, thermally-compatible materials. In the case of fluid material examples, the cladding mode stripping structure may be in a region where the fluid material is sufficiently colder than the region of interest so that the fluid material will not degrade at elevated temperatures. Examples of such fluid include Cargille’s Refractive Index Fluid Series M. [0077] An eighth example embodiment for stripping the cladding modes for an internally-clad sapphire fiber that is suitable for high-temperature sensing applications is a modulated or ribbed fiber surface that, similar to embodiment three, randomizes the angle of incidence of the cladding modes with the fibers surface, thus increasing the likelihood that the light will impact the surface perpendicularly allowing it to escape the fiber. One example implementation creates a modulated or ribbed surface during the fabrication of the sapphire fiber. A seed crystal is lowered into a pool of molten sapphire and pulled vertically using a stepper motor. As the seed is pulled vertically, a column of sapphire (or sapphire fiber) is produced. The discrete motor “steps” generate a fiber with a modulated, slightly non- uniform diameter or ribbed surface resulting in a fiber that will strip clad propagating light.

A sapphire fiber with a modulated, slightly non-uniform diameter or ribbed surface may also be achieved in post-processing with laser micromachining techniques.

[0078] The eight example embodiments described above are surface absorptive and/or dispersive methods that can be, but are not limited to, bulk processes to the entire fiber. One example, cost effective way of applying/producing these surface treatments is to perform them along the length of the fiber for lengths up to one meter. For fiber lengths beyond one meter, processing the interrogator side of the fiber for a small distance may be sufficient.

[0079] As described earlier, FIG. 2 shows fiber core and cladding modes traveling in the fiber prior to such a surface treatment. FIG. 4a shows the absorption and/or dispersion of the cladding modes by a surface layer produced by one or a combination of the first five example mode stripping embodiments described above.

[0080] The sixth example mode stripping embodiment described above inscribes defects within the cladding that absorb and/or diffuse the cladding modes and that may be done by fs-laser processing. FIG. 5 illustrates an example of addition of such structures, regions c, within the cladding of the fiber to absorb and/or disperse cladding modes.

[0081] The seventh example involves the addition of a high-index material to the outside of the fiber. FIG. 6 depicts such a structure made of a high temperature material (e.g., sapphire, alumina, high-temperature solder glass, etc.) as a tube, FIG. 6 region c, bonded to the fiber surface, such that light can enter and be trapped in this annulus with high probability.

[0082] Similarly, FIG.7 depicts two wafers, show as region c, that perform in a similar manner as the structure shown in FIG. 6. The configuration may have a lower probability per unit length of stripping the cladding modes than the tube version of the previous paragraph, but it will also have a smaller probability of allowing light to reenter the fiber. In both the tube and wafer cases, the structure c has a high index of refraction, comparable to or greater than that of sapphire. The light is allowed to exit the sapphire cladding b and enter the structure c. The structure c is designed so that the efficiency for light entering it from the fiber, region b, is much greater than the reverse process of light entering the fiber, region b, from the structure c. This efficiency is strongly related to the cross-sectional area of the structure c compared to the fiber a and b, and the magnitude of the area bonded between the two. In both the tube and wafer cases, the cladding light is more likely to interact with the structure’s surface than reenter the small diameter of the fiber. As the light hits the ends of the structure closest to the origin or terminus of the sensor, the interaction angle of incidence is very small, promoting loss of the light to the environment. [0083] Regarding the eighth example embodiment for stripping the cladding modes using a modulated cladding surface, an example modulated surface d is illustrated in FIG. 8. The modulated surface may, for example, be radially symmetric or have a helical structure. The ribs or waves scatter cladding mode light in varying directions, increasing the probability that the cladding mode light will escape the fiber. Note the illustration’s cladding modes end where the light impinges the surface at normal incidence causing the light to escape.

[0084] Regarding cladding mode stripping, some of the example embodiments strip the cladding modes before application of internal cladding to the sapphire, and other example embodiments strip the cladding modes after application of the internal cladding to the sapphire. Yet in other example embodiments, the cladding modes can be stripped before or after application of the internal cladding to the sapphire. Accordingly, the sequences of processing the optical waveguide to apply an internal cladding and a cladding mode stripping mechanism, implied or stated in the described example embodiments, present just a few example ways in which the principles of the innovation can be employed and the subject innovation is intended to include all such aspects and their equivalents. For example, using example embodiment eight to illustrate some equivalent embodiments, although modulation of the fiber surface may be achieved during fabrication using micromachining techniques, the micromachining techniques may be implemented during post processing of the fiber.

Another example equivalent of embodiment eight uses is to perform micromachining as a preprocessing step prior to application of an internal cladding. Moreover, the time in process implied or stated in the described embodiments is not exhaustive or limiting. For example, the micromachining in embodiment 8 could be performed prior to implementation of the internal cladding process.

[0085] An example embodiment of using a sapphire fiber sensor with a fiber optic sensing interrogator is depicted in FIG. 10. The sapphire optical waveguide, a, is connected to an optical interrogator, b, as a sensor to make distributed measurements at sensing locations c, d, and e. The sapphire waveguide a is connected to the optical interrogator b through a fiber optic lead h, with connection points between the lead and the interrogator f, and between the lead and the sapphire waveguide g. The fiber optic lead typically would be a single mode fused silica fiber, similar to the fiber used in the optical interrogator. The connection point g is where the light transits from the fused silica single mode core to the sapphire waveguide (and back again upon reflection from the sapphire waveguide sensor elements) and may include a pair of connectors in a mating sleeve, a mechanical or fusion splice, or a mode conditioning device. The mode conditioning device may include, for example, a segment of fiber with a tapered core, and is preferably designed to match the fiber core properties on either side of the connection, in order to optimize the light launched into the sapphire waveguide core mode and minimize the power of the light launched into the sapphire waveguide cladding modes. The sapphire waveguide is placed or mounted in a sensing environment i, which may experience extreme temperatures and/or harsh chemicals that the sapphire waveguide can withstand but that the fiber lead h or optical interrogator b may not.

[0086] The sensor interrogator may operate based on techniques which include

Optical Frequency Domain Reflectometry (OFDR), Optical Time Domain Reflectometry (OTDR), and various means of monitoring the sensor reflection spectrum. Spectral reflectometer options include sweeping a tunable optical source and recording the reflected amplitude from the sensor with a broadband optical detector or using a broad-spectrum source with an optical detector that is spectrally sensitive. Fiber Bragg Grating sensors, like the one depicted in FIG. 4b, may be used with spectral reflectometers, and multiplexing multiple FBG sensors at multiple distances along the fiber may be accomplished through wavelength division multiplexing. Intrinsic and extrinsic fiber etalon sensors, like those depicted in FIGS. 9a and 9b, respectively, are also commonly used with spectral reflectometers. OTDR based sensor interrogators typically use Rayleigh, Brillouin, or Raman scatter from the sensor waveguide. OFDR based sensor interrogators may make use of Rayleigh scatter, FBGs, or Fabry-Perot interferometric sensors. Rayleigh scatter may be enhanced in the sapphire waveguide by inducing highly localized refractive index variations with an fs-laser, or by induced scatter centers in the sapphire crystal matrix via ionizing radiation or inclusion of dopants.

[0087] Use of a sensor interrogator b based on the OFDR technique, as depicted in

FIG. 11, is an example embodiment that is advantageous because OFDR can use a wide range of sensor types (FBGs, Fabry Perot etalon interferometers, Rayleigh scatter, enhanced Rayleigh scatter, or a combination thereof), and can provide distributed measurements with 1 mm or less spatial resolution over a sensor distance range of 10’s of meters. The OFDR sensor interrogator b depicted in FIG. 11 consists of a tunable laser source TLS, a laser monitor interferometer used to track the tunable laser optical frequency, a measurement interferometer that contains a reference path in transmission and a measurement path in reflection that extends to the sensor, a polarization controller and polarizing beam splitter that constitute a polarization diverse receiver, optical detectors S and P, analog to digital circuitry ADC, and digital processing and control circuitry included in the instrument controller to control the laser sweep, monitor the laser optical frequency, and process the measurement interferometer results.

[0088] Sapphire fibers are advantageous in comparison to the more common silica fibers because they have a very high melting temperature in comparison with silica fibers. Results published in the article “Optical mode confinement and selection in single-crystal sapphire fibers by formation of nanometer scale cavities with hydrogen ion implantation,” by Spratt et al., Journal of Applied Physics 114, 203501 (2013); doi: 10.1063/1.4833240, indicate that internal cladding formed using hydrogen isotope ion implantation may be stable up to, and exceeding, temperatures of 1700° C. Additionally, sapphire fibers are resistant to many harsh chemical environments: sapphire fibers are known to be more likely to survive than fused silica fibers in environments that include many molten salts, many molten metals, most acids, and in high temperature steam for longer durations or at higher temperatures or pressures. A sapphire fiber, with an internal cladding is better than a sapphire fiber with no cladding because it has a core region, which is removed from the impact of surface defects. This allows the lower order modes traveling through the fiber core to propagate unaffected by fiber surface conditions. By modifying an internally-clad sapphire fiber to strip the higher order cladding modes from the fiber cladding, a superior light signal is propagated along the fiber that is not corrupted/made noisy by the presence of the higher order cladding modes, which otherwise propagate in the fiber cladding. The low-noise, mode-stripped, internally- clad sapphire fiber sensor is theoretically capable of operating and sensing up to and exceeding 1700°C. This sensor has advantageous applications for many industries involving harsh conditions in which parameter sensing and control are needed.

[0089] Whenever it is described in this document that a given item is present in

“some embodiments,” “various embodiments,” “certain embodiments,” “certain example embodiments, “some example embodiments,” “an exemplary embodiment,” or whenever any other similar language is used, it should be understood that the given item is present in at least one embodiment, though is not necessarily present in all embodiments. Consistent with the foregoing, whenever it is described in this document that an action “may,” “can,” or “could” be performed, that a feature, element, or component “may,” “can,” or “could” be included in or is applicable to a given context, that a given item “may,” “can,” or “could” possess a given attribute, or whenever any similar phrase involving the term “may,” “can,” or “could” is used, it should be understood that the given action, feature, element, component, attribute, etc. is present in at least one embodiment, though is not necessarily present in all embodiments.

[0090] Terms and phrases used in this document, and variations thereof, unless otherwise expressly stated, should be construed as open-ended rather than limiting. As examples of the foregoing: “and/or” includes any and all combinations of one or more of the associated listed items (e.g., a and/or b means a, b, or a and b); the singular forms "a", "an" and "the" should be read as meaning “at least one,” “one or more,” or the like; the term “example” is used provide examples of the subject under discussion, not an exhaustive or limiting list thereof; the terms "comprise” and “include” (and other conjugations and other variations thereol) specify the presence of the associated listed items but do not preclude the presence or addition of one or more other items; and if an item is described as “optional,” such description should not be understood to indicate that other items are also not optional.

In the present application, the words “configured to... ” are used to mean that an element of an apparatus has a configuration able to carry out the defined operation. In this context, a “configuration” means an arrangement or manner of interconnection. “Configured to” does not imply that the apparatus element needs to be changed in any way in order to provide the defined operation. The terms “wherein,” “such that,” etc. indicate structure, requirements of a method, and/or other features to be given patentable weight.

[0091] Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. Where a specific range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is included therein. All smaller subranges are also included. The upper and lower limits of these smaller ranges are also included therein, subject to any specifically excluded limit in the stated range.

[0092] The term “about” or “approximately” means an acceptable error for a particular recited value, which depends in part on how the value is measured or determined. In certain embodiments, “about” can mean 1 or more standard deviations. When the antecedent term "about" is applied to a recited range or value it denotes an approximation within the deviation in the range or value known or expected in the art from the measurements method. For removal of doubt, it shall be understood that any range stated herein that does not specifically recite the term “about” before the range or before any value within the stated range inherently includes such term to encompass the approximation within the deviation noted above.

[0093] It is the express intention of the applicant not to invoke means-plus-function, step-plus-function, or other functional claiming treatment for any claim except for those in which the words "means for" or "step for" explicitly appear together with an associated function in such claim.

[0094] Although various embodiments have been shown and described in detail, the claims are not limited to any particular embodiment or example. None of the above description should be read as implying that any particular element, step, range, or function is essential. All structural and functional equivalents to the elements of the above-described embodiments that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the invention. No embodiment, feature, element, component, or step in this document is intended to be dedicated to the public.

Claims

1. A sapphire optical waveguide structure comprising: a sapphire core; at least one layer of cladding; and a mode stripping material, geometry, or structure configured to reduce the power of light traveling in the cladding, wherein the core and cladding are capable of surviving and operating in one or both of the following conditions:

(i) at temperatures in excess of 800°C,

(ii) in chemically harsh environments of molten salt, molten metal, acids, or high temperature steam.

2. The sapphire optical waveguide in claim 1, wherein the core and cladding are capable of surviving and operating at temperatures in excess of 800°C.

3. The sapphire optical waveguide in claim 2, wherein the mode stripping material, geometry, or structure includes an absorptive mode stripping material or structure.

4. The sapphire optical waveguide in claim 2, wherein the mode stripping material, geometry, or structure includes a dispersive mode stripping material or structure.

5. The sapphire optical waveguide in claim 2, wherein the mode stripping material, geometry, or structure includes an absorptive and dispersive mode stripping material or structure.

6. The sapphire optical waveguide in claim 2, wherein the mode stripping material, geometry, or structure is a different composition than an outermost layer of cladding and is grown or formed on the outermost layer of cladding under controlled conditions.

7. The sapphire optical waveguide in claim 2, wherein the mode stripping material, geometry, or structure includes one or more layers of deposited material on a surface of the sapphire optical waveguide.

8. The sapphire optical waveguide in claim 2, wherein the mode stripping material, geometry, or structure includes etching the surface of the sapphire optical waveguide with boric acid, or another etchant.

9. The sapphire optical waveguide in claim 2, wherein the mode stripping material, geometry, or structure includes one or more layers of a non-etched additive layer or dye on the surface of the sapphire optical waveguide.

10. The sapphire optical waveguide in claim 2, wherein the mode stripping material, geometry, or structure includes impurity elements, or dopants, below a surface of the sapphire optical waveguide.

11. The sapphire optical waveguide in claim 2, wherein the mode stripping material, geometry, or structure is fabricated with a femtosecond (fs) laser treatment of the sapphire optical waveguide.

12. The sapphire optical waveguide in claim 2, wherein the mode stripping material, geometry, or structure includes a material with an index of refraction that matches or is greater than the index of refraction of sapphire or of the cladding.

13. The sapphire optical waveguide in claim 2, wherein the mode stripping material, geometry, or structure includes a modulated surface of the sapphire optical waveguide.

14. The sapphire optical waveguide in claim 2, wherein the mode stripping material, geometry, or structure provides one or more of: a) stability at high temperatures, b) an index of refraction comparable or greater than that of sapphire, c) chemical inertness, d) a coefficient of thermal expansion near that of sapphire, or e) a capability of bonding with sapphire.

15. The sapphire optical waveguide in claim 1, wherein the core and cladding are capable of surviving and operating in chemically harsh environments of molten salt, molten metal, acids, or high temperature steam.

16. The sapphire optical waveguide in claim 15, wherein the mode stripping material, geometry, or structure includes an absorptive mode stripping material or structure.

17. The sapphire optical waveguide in claim 15, wherein the mode stripping material, geometry, or structure includes a dispersive mode stripping material or structure.

18. The sapphire optical waveguide in claim 15, wherein the mode stripping material, geometry, or structure includes an absorptive and dispersive mode stripping material or structure.

19. The sapphire optical waveguide in claim 15, wherein the mode stripping material, geometry, or structure is a different composition than an outermost layer of cladding and is grown or formed on a surface of the outermost layer of cladding under controlled conditions.

20. The sapphire optical waveguide in claim 15, wherein the mode stripping material, geometry, or structure includes one or more layers of deposited material on a surface of the sapphire optical waveguide.

21. The sapphire optical waveguide in claim 15, wherein the mode stripping material, geometry, or structure includes etching the surface of the sapphire optical waveguide with boric acid, or another etchant.

22. The sapphire optical waveguide in claim 15, wherein the mode stripping material, geometry, or structure includes one or more layers of a non-etched additive layer or dye on the surface of the sapphire optical waveguide.

23. The sapphire optical waveguide in claim 15, wherein the mode stripping material, geometry, or structure includes impurity elements, or dopants, below a surface of the sapphire optical waveguide.

24. The sapphire optical waveguide in claim 15, wherein the mode stripping material, geometry, or structure is fabricated with a femtosecond (fs) laser treatment of the sapphire optical waveguide.

25. The sapphire optical waveguide in claim 15, wherein the mode stripping material, geometry, or structure includes a material with an index of refraction that matches or is greater than the index of refraction of sapphire or of the cladding.

26. The sapphire optical waveguide in claim 15, wherein the mode stripping material, geometry, or structure includes a modulated surface of the sapphire optical waveguide.

27. The sapphire optical waveguide in claim 15, wherein the mode stripping material, geometry, or structure provides one or more of: a) stability at high temperatures, b) an index of refraction comparable or greater than that of sapphire, c) chemical inertness, d) a coefficient of thermal expansion near that of sapphire, or e) a capability of bonding with sapphire.

28. The sapphire optical waveguide in claim 1, wherein the mode stripping material, geometry, or structure is configured to reduce a power of light traveling in the internal cladding by at least 3dB over a length of the sapphire optical waveguide.

29. The sapphire optical waveguide in claim 1, wherein the mode stripping material, geometry, or structure is configured to reduce a power of light traveling in the internal cladding by at least 20dB over a portion of a length of the sapphire optical waveguide.

30. The sapphire optical waveguide in claim 1, wherein the cladding is a refractive cladding, the sapphire optical fiber is a graded index sapphire optical fiber, and the mode stripping material, geometry, or structure does not attenuate light confined to the sapphire core by more than ldB over a length of the sapphire optical waveguide.

31. The sapphire optical waveguide in claim 1, wherein the cladding is a refractive cladding, the sapphire optical fiber is a step index sapphire optical fiber, and the mode stripping material, geometry, or structure does not attenuate light confined to the sapphire core by more than ldB over a length of the sapphire optical waveguide.

32. The sapphire optical waveguide in claim 1, wherein the sapphire optical waveguide is a fiber inscribed with at least one fiber Bragg grating.

33. The sapphire optical waveguide in claim 1, wherein the sapphire optical waveguide incorporates at least one region of refractive index variation that produces enhanced scatter points.

34. The sapphire optical waveguide in claim 1, wherein the sapphire optical waveguide incorporates at least one intrinsic Fabry-Perot interferometer.

35. The sapphire optical waveguide in claim 1, wherein the sapphire optical waveguide includes at least one Extrinsic Fabry-Perot interferometer.

36. The sapphire optical waveguide, wherein the sapphire optical waveguide is part of a distributed temperature measurement system.

37. A sensor comprising the sapphire optical waveguide of claim 1.

38. The sensor of claim 37, wherein the sensor measures one or more of temperature, strain, pressure, or acceleration.

39. The sensor of claim 37, wherein the sensor is configured to be interrogated using Optical Frequency Domain Reflectometry (OFDR).

40. A method of manufacturing a sapphire optical waveguide structure comprising: obtaining a sapphire core; incorporating at least one layer of cladding; and incorporating a mode stripping material, geometry, or structure configured to reduce the power of light traveling in the cladding, wherein the core and cladding are capable of surviving and operating in one or both of the following conditions:

(i) at temperatures in excess of 800°C,

41. The method in claim 40, wherein the core and cladding are capable of surviving and operating at temperatures in excess of 800°C.

42. The method in claim 41, wherein the mode stripping material, geometry, or structure includes a dispersive mode stripping material or structure.

43. The method in claim 41, wherein the mode stripping material, geometry, or structure includes an absorptive and dispersive mode stripping material or structure.

44. The method in claim 41, wherein the mode stripping material, geometry, or structure is a different composition than an outermost layer of cladding and is grown or formed on a surface of the outermost layer of cladding under controlled conditions.

45. The method in claim 41, wherein the mode stripping material, geometry, or structure includes one or more layers of deposited material on a surface of the sapphire optical waveguide.

46. The method in claim 41, wherein the mode stripping material, geometry, or structure includes etching the surface of the sapphire optical waveguide with boric acid, or another etchant.

47. The method in claim 41, wherein the mode stripping material, geometry, or structure includes one or more layers of a non-etched additive layer or dye on the surface of the sapphire optical waveguide.

48. The method in claim 41, wherein the mode stripping material, geometry, or structure includes impurity elements, or dopants, below a surface of the sapphire optical waveguide.

49. The method in claim 41, wherein the mode stripping material, geometry, or structure is fabricated with a femtosecond (fs) laser treatment of the sapphire optical waveguide.

50. The method in claim 41, wherein the mode stripping material, geometry, or structure includes a material with an index of refraction that matches or is greater than the index of refraction of sapphire or of the cladding.

51. The method in claim 41, wherein the mode stripping material, geometry, or structure includes a modulated surface of the sapphire optical waveguide.

52. The method in claim 41, wherein the mode stripping material, geometry, or structure provides one or more of: a) stability at high temperatures, b) an index of refraction comparable or greater than that of sapphire, c) chemical inertness, d) a coefficient of thermal expansion near that of sapphire, or e) a capability of bonding with sapphire.

53. The method in claim 40, wherein the core and cladding are capable of surviving and operating in chemically harsh environments of molten salt, molten metal, acids, or high temperature steam.

54. The method in claim 53, wherein the mode stripping material, geometry, or structure includes a dispersive mode stripping material or structure.

55. The method in claim 53, wherein the mode stripping material, geometry, or structure includes an absorptive and dispersive mode stripping material or structure.

56. The method in claim 53, wherein the mode stripping material, geometry, or structure is a different composition than an outermost layer of cladding and is grown or formed on a surface of the outermost layer of cladding under controlled conditions.

57. The method in claim 53, wherein the mode stripping material, geometry, or structure includes one or more layers of deposited material on a surface of the sapphire optical waveguide.

58. The method in claim 53, wherein the mode stripping material, geometry, or structure includes etching the surface of the sapphire optical waveguide with boric acid, or another etchant.

59. The method in claim 53, wherein the mode stripping material, geometry, or structure includes one or more layers of a non-etched additive layer or dye on the surface of the sapphire optical waveguide.

60. The method in claim 53, wherein the mode stripping material, geometry, or structure includes impurity elements, or dopants, below a surface of the sapphire optical waveguide.

61. The method in claim 53, wherein the mode stripping material, geometry, or structure is fabricated with a femtosecond (fs) laser treatment of the sapphire optical waveguide.

62. The method in claim 53, wherein the mode stripping material, geometry, or structure includes a material with an index of refraction that matches or is greater than the index of refraction of sapphire or of the cladding.

63. The method in claim 53, wherein the mode stripping material, geometry, or structure includes a modulated surface of the sapphire optical waveguide.

64. The method in claim 53, wherein the mode stripping material, geometry, or structure provides one or more of: a) stability at high temperatures, b) an index of refraction comparable or greater than that of sapphire, c) chemical inertness, d) a coefficient of thermal expansion near that of sapphire, or e) a capability of bonding with sapphire.