Systems and methods for enhancing backscatter in hermetic optical fibers

Abstract

Described herein are systems, methods, and articles of manufacture for high backscattering waveguides (e.g., optical fibers) and sensors employing high backscattering optical fibers. Briefly, one embodiment includes a high backscattering fiber, or enhanced scattering fiber or "ESF," characterized by a resistance specification that remains intact over a length of fiber greater than 1 m, or preferably greater than 100 m, or preferably greater than 1 km, the reflectivity of the ESF can be precisely tuned within a range of -100 dB / mm to -70 dB / mm, and the enhanced scattering may be spatially continuous or may be at discrete locations spaced by 100 microns to 10 m.

Description

[Technical Field]

[0001] [Reference to Related Applications] This application claims the benefit of U.S. Provisional Application No. 63 / 148,927 (filed February 12, 2021), which is incorporated herein by reference.

[0002] The present specification relates to systems, methods, and articles of manufacture for enhancing backscatter in hermetic optical fibers. [Background technology]

[0003] Backscattered light in optical fibers is used for distributed acoustic sensing. This has significant applications, for example, in downhole applications in oil and gas exploration. However, the high temperatures and high hydrogen environments in such wells cause rapid degradation of conventional optical fibers. The addition of a gas-tight carbon coating and the use of a germanium-free (Ge-free) core have improved fiber life in such harsh environments.

[0004] To increase sensitivity to acoustic events, refractive index perturbations may be introduced along the optical fiber, increasing the amount of backscattered light. While this is well known for conventional fibers, introducing refractive index perturbations into carbon-coated and Ge-free fibers is problematic for a number of reasons. In particular, actinic radiation, for example at UV wavelengths, is not effective in Ge-free fibers and requires the use of femtosecond pulse writing. Furthermore, actinic radiation pulses (e.g., femtosecond laser pulses) damage the silica glass structure, causing increased optical loss. While this may be acceptable for short gratings for certain applications, distributed sensing over several meters results in unacceptable loss. Finally, actinic radiation exposure is known to damage carbon coatings, reducing their hermeticity and mechanical reliability.

[0005] Thus, there remains a problem in the art of fabricating long lengths of actinic pulse incidence gratings in Ge-free fibers with hermetic coatings. Summary of the Invention

[0006] The present disclosure provides high backscattering waveguides (e.g., optical fibers) and sensors employing high backscattering optical fibers. Briefly, one embodiment includes a high backscattering fiber, or enhanced scattering fiber or "ESF," characterized by a resistivity specification that remains intact over a length of fiber greater than 1 meter, or preferably >100 meters, or preferably >1 km, where the reflectivity of the ESF can be precisely tuned within a range of -100 dB / mm to -70 dB / mm, and the enhanced scattering may be spatially continuous or, alternatively, at discrete locations spaced 100 microns to >10 meters apart.

[0007] Other systems, devices, methods, features, and advantages will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of this disclosure, and be protected by the accompanying claims. [Brief explanation of the drawings]

[0008] [Figure 1A] FIG. 1 is a schematic diagram of an FBG writing setup according to one embodiment of the present invention. [Figure 1B] FIG. 10 illustrates the projection of a focused beam onto the center of the core during the FBG writing process according to one embodiment of the present invention. [Figure 1C] 1 shows a cross-sectional view of a hermetically coated optical fiber according to one embodiment of the present invention. [Figure 1D] FIG. 1 shows an FBG inscribed along the length of a hermetically coated fiber according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0009] Exemplary embodiments described herein relate to enhanced backscattering in hermetic optical fibers. More specifically, exemplary embodiments relate to actinic pulse-written gratings in optical fibers with hermetic (carbon) coatings, where the relative intensities in the coating and core are adjusted so that the writing process does not degrade the aging characteristics of the optical or mechanical properties when the fiber is exposed to "harsh" environments. The resulting fiber devices, which can range in length from a few millimeters to several kilometers, exhibit total transmission losses of <2 dB / km, backscattering greater than that of native Rayleigh scattering (enhanced scattering fiber or "ESF"), and a scattering figure of merit (FOM) >1 over at least one range of optical frequencies (U.S. Patent 9,766,396). It is caused by spatial modulation of the refractive index (Δn) in at least a portion of the core of the fiber waveguide, allowing stable and non-destructive operation at elevated temperatures (>30°C) and other harsh environmental conditions (humidity levels >50%, and / or hydrogen exposure at partial pressures >0.1 psi, and / or strains >0.5%) for more than 50 hours without significant increase in optical attenuation.

[0010] Modification of the refractive index of selected regions in optical materials enables optical materials with advanced functionality through control of the behavior of emitted light, resulting in useful photonic structures. Backscattering fibers are one such photonic structure that rely on spatial variations in the refractive index of a material to redirect a portion of propagating light, typically in the opposite direction to the original propagation direction of the light wave, by satisfying the phase-matching condition between the wave vector of the light wave and the vector corresponding to the spatial frequency of the physical variation in the refractive index of the medium.

[0011] Several existing and future applications of ESFs require their ability to provide stable and robust long-term operation under harsh environmental conditions, such as high temperatures, high humidity, and exposure to highly corrosive chemicals or gases. Such harsh environments are known to increase fiber transmission loss and impair ESF performance, thereby sacrificing the long-term reliability and operation of photonic devices and entire systems. Germanium co-doping of the fiber core to sensitize silica glass for ESF inscription using UV radiation is known to substantially increase transmission loss in hydrogen-rich environments. This makes such photonic systems prone to failure and therefore impractical for reliable long-term operation. To circumvent these challenges, coating materials have been designed to protect the fiber and maintain its long-term operation and enhanced scattering. Coating techniques relying on polymeric materials such as carbon and polyimide have been applied to ESFs intended for operation under harsh conditions by avoiding increased loss and degradation of mechanical strength.

[0012] Furthermore, to avoid the use of germanium co-doping of the fiber core, "pure-core" (germanium-undoped) fibers have been developed for their greater resistance to degradation in hydrogen-rich environments. The use of pure-core fibers and the application of coatings that can increase protection against harsh environments necessitate the use of alternative methods for ESF fabrication. One approach is the use of lasers that emit short pulses with durations on the femtosecond to picosecond scale. Such laser pulses can be operated at wavelengths that pass through the coating with reduced loss of radiation intensity and are focused directly into the core region of the fiber to inscribe ESFs of the desired period and configuration. The refractive index modulation induced by such lasers relies on nonlinear absorption by the glass matrix and is independent of the presence of germanium content, thus enabling the inscription of ESFs in pure-core fibers. ESFs can be inscribed "off-tower" on the fiber after the fiber has been coated with carbon and polymer layers. This allows for the mechanical strength of the fiber after the ESF inscription. The amount of refractive index change and the length of the ESF are selected based on the desired reflectivity and its spatial distribution and can be tailored to the target application. The resulting ESF-inscribed fibers can be used as sensors and reflectors for use in transportation, energy exploration, nuclear reactors, telecommunications, and traffic monitoring networks, as well as for monitoring critical infrastructure.

[0013] Exemplary embodiments described herein relate to optical fibers having hermetic coatings, such as carbon and / or polyimide polymer coatings, as well as systems and methods for fabricating ESFs in undoped-core optical fibers. Importantly, the fibers have specific resistance to hydrogen diffusion and humidity, as well as other chemical resistances, specific heat resistance, and specific strain resistance, and the hydrogen, humidity, and other chemical resistances remain intact after processing steps that produce enhanced scattering. Notably, these stringent resistance specifications are maintained over fiber lengths of more than 1 meter, or preferably >100 meters, or preferably >1 km. The reflectivity of the ESF can be precisely tuned from -100 dB / mm to -70 dB / mm. Enhanced scattering can be spatially continuous or can be at discrete locations spaced by 100 microns to 10 meters.

[0014] Examples of ESFs include: 1) continuous periodic or quasi-periodic refractive index perturbations; and 2) isolated refractive index perturbations located along the fiber at intervals anywhere from 100 μm to 10 m. Isolated The scattering centers can have a spatial extent anywhere from 1 micron to 10 cm. Isolated The scattering centers may preferably have a spectral reflection bandwidth of <10 nm or preferably <30 nm or preferably <100 nm. Center of is 1500 to 1700n m The scattering can also be very broadband: it can be present at all wavelengths where Rayleigh scattering occurs.

[0015] Such ESFs have the same hermeticity both before and after treatment to create the perturbation. Alternatively, the hermeticity remains above a certain level, which is still two or preferably ten times greater than that of a fiber without a hermetic seal. For example, the attenuation resulting from the reduced hermeticity is only two to ten times greater than the attenuation in an untreated fiber for the same partial pressure of hydrogen or water vapor, temperature, and exposure time.

[0016] For example, a fiber can be fabricated with a Ge-free core, coated with carbon, and then coated with polyimide. This fiber can then be exposed to actinic radiation that penetrates the polyimide and carbon coatings, changing the refractive index of the fiber's core and resulting in backscattering greater than Rayleigh scattering. The refractive index change can create a series of planes across the fiber core, resulting in periodic or quasi-periodic structures within the fiber core. The length of these structures can be anywhere from 1 micron to 10 cm or more in length. The spacing between individual structures can be anywhere from 100 microns to 10 meters. Importantly, actinic radiation enters the fiber, and excess actinic radiation exits the fiber without altering any of the stringent durability specifications. Thus, hermeticity, thermal stability, and strain tolerance remain the same after the refractive index change is imposed on the fiber. For example, this can be achieved if the actinic radiation has a sufficiently low intensity as it passes through the carbon and polyimide coatings so as not to damage these coatings. Alternatively, the damage may be so minor that the fiber's durability is only partially reduced.

[0017] For example, a beam of actinic radiation can be focused so that its peak intensity is below the damage threshold of polymer and carbon coatings. A detailed schematic of the experimental setup is shown in Figure 1A. Furthermore, the focusing of the actinic radiation can be adjusted so that when the beam leaves the fiber, it is also below the damage threshold of the coating on the fiber. One way to achieve this focusing is to adjust the focal point of the actinic radiation to overlap with the fiber core. If the beam waist size is small enough, the actinic radiation will spread away from the core region to such an extent that its intensity is significantly reduced as it passes through the fiber coating.

[0018] 1A shows a schematic 100 of an FBG writing setup in accordance with one or more embodiments of the present invention. Specifically, the schematic 100 shows a pulsed chemical laser 110 emitting a laser beam 115. The laser beam 115 is redirected using multiple alignment mirrors 120a, 120b. The laser beam 115 then passes through a focusing lens 130. The focusing lens 130 focuses the laser beam 115 onto an optical fiber 140, which inscribes an enhanced backscatter grating 150.

[0019] 1B shows a schematic diagram 200 of the projection of a focused laser beam 210 via a lens 220 at the center of a core 231 of an optical fiber 230 during an FBG writing process, in accordance with one or more embodiments of the present invention. According to an exemplary embodiment, the optical fiber 230 may feature a core 231, a cladding 232, a hermetic coating layer 233, and a polymer coating 234.

[0020] 1C shows a schematic diagram 300 of a cross-sectional view of a hermetically coated optical fiber 310 in accordance with one or more embodiments of the present invention. Similar to optical fiber 230 of FIG. 1B, the exemplary hermetically coated optical fiber 310 has a r core a core 311 having a radius of r, a cladding 312, a hermetic coating layer 313, and coating The polymer coating 314 may have a radius of .mu.m.

[0021] FIG. 1D illustrates a hermetically coated fiber 410 along the length of its core 430, in accordance with one or more embodiments of the present invention. engraved A schematic diagram 400 of an FBG 420 is shown.

[0022] Generally, the effect of actinic radiation is controlled by several factors: wavelength, pulse duration, repetition rate, peak intensity at the fiber core, and peak intensities at the entrance and exit facets of the fiber. In accordance with the present invention, these parameters can be adjusted so that the effect on the core is sufficient to provide the desired refractive index modulation, and the effect on the coating is not damaging from the radiation.

[0023] To quantify these ideas, the threshold values ​​of the glass core and fiber coating can be considered as follows: glass index change = the intensity I required to change the index of the fiber core by an amount sufficient to increase the scattering FOM (from U.S. Application No. 15 / 175,656 (issued June 7, 2016; U.S. Patent No. 9,766,396), incorporated herein by reference) by a desired amount. coating damage = strength that would damage the hermetic coating or impair the mechanical strength of the fiber).

[0024] In general, the actinic parameters can then be adjusted to satisfy the following: I beam (r core )>I glass index change I beam (r coating ) coating damage

[0025] Here, I beam (r core ) and I beam (r coating ) are the fiber core (r core ) and coating (r coating ) shows the beam intensity at the outer edge.

[0026] For example, consider these values ​​for a standard germanosilicate fiber coated with a thin layer of carbon. The refractive index change occurs near 800 nm and is introduced using a femtosecond laser with a pulse duration on the order of 150 fs. The threshold for modifying the core of this fiber is: I glass index change =1.8±0.4×10 13 W / cm 2 ​

[0027] The threshold for damaging the carbon coating is: I coating damage =1×10 12 W / cm 2

[0028] Therefore, the following equation is obtained from the above equation: l beam (r coating )<1×10 12 W / cm 2 I beam (r core )>1.8±0.4×10 13 W / cm 2

[0029] The formula implies that the beam can be defocused at the surface of the fiber. If we assume that the beam is focused at the center of the fiber, the radial dependence of the beam can be approximated using the Gaussian beam formula:

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[0030] where λ is the pulse wavelength, ω is the beam waist, n is the refractive index of the glass, r is the distance from the core, and ω(r) is the beam washer at r. Using this relationship, the ratio of the beam dimensions can be estimated in terms of the core radius and coating radius. For simplicity, cylindrical focusing may be omitted from the curved surface of the fiber. This effect can be included in the calculation, or it can be eliminated or reduced by immersing the fiber in a material of the desired refractive index, such as index-matching oil, which eliminates tension effects at the fiber surface. For standard fibers with very thin (<1 micron) carbon coating layers, the radius of the glass fiber cladding, which is 62.5 μm for standard fibers, can be used. The ratio of the intensities after cylindrical focusing on an axis perpendicular to the fiber axis is:

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[0031] Therefore, the focusing optics must be set to obtain a beam waist of 0.77 μm or less at the core of the fiber in order to create a refractive index change in the core without damaging or damaging the carbon coating.

[0032] More generally, for a given fiber and coating, this equation will have different parameters determined from studies of the fiber under various actinic radiation exposures. For example, hermeticity can be achieved using materials other than carbon coatings, such as metals. Also, the fiber may contain gettering regions or different compositions and refractive indices to inhibit hydrogen migration into the core. Once the parameters are determined, the beam-focusing optics are adjusted to satisfy this relationship.

[0033] If multiple pulses were required to generate refractive index perturbations in the fiber core, the number of pulses would be included in the calculation for the two thresholds. In this case, the relevant parameter would be the total chemical dose, D, which can be expressed as: D=It pulse N pulses where I is the pulse intensity and t pulse is the pulse duration and N pulse is the number of pulses.

[0034] Various writing beam parameters (λ, ω, t pulse N pulses ) must then be adjusted to satisfy the following equation: D(r coating ) <D coating damage and, D(r core )>I glass index change Or, more explicitly, as follows:

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[0035] Note that in these inequalities, the number of pulses is not necessarily the same in the core and the coating. Such a difference can occur if the beam is larger in the coating than in the core. If a refractive index change in the fiber core requires the writing beam to be moved through the core, a given portion of the coating may receive many pulses from the large writing beam while the beam is moved through the core.

[0036] Generally, dose parameters must be determined for the fiber and imaging system, and beam parameters adjusted accordingly. To determine the extent of decomposition due to actinic radiation, a fiber can be placed in a chamber at 130°C with 75 psi partial pressure of hydrogen for 7 days. Fiber attenuation at 1550 nm can typically increase by up to 2 dB / km after hydrogen exposure. Fibers exposed to actinic radiation of the present invention preferably have an increase in attenuation of 33% or less, i.e., up to 2.7 dB / km, after the same hydrogen exposure. The carbon should be at least 10 nm thick. However, thicker layers, such as 100 nm or 1 micron thick layers, can also be applied. The carbon may be thick enough to remain hermetic even after ablation due to actinic radiation exposure. Thus, I coating damage The value of may be higher than the above.

[0037] Optical backscattering can be increased through many different refractive index perturbations. Scattering from core guided modes from refractive index perturbations can be estimated using the coupled-mode approximation. In this approximation, the amplitude of the scattered electric field (E) is proportional to the overlap integral:

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[0038] where η is the overlap integral and E incident is the transverse dependence of the E-field amplitude of the incident guided light, and E scattered is the transverse dependence of the E-field amplitude of the scattered E-field, and δn(r,θ) is the refractive index perturbation with a well-defined dependence on the cylindrical coordinate transverse to the axis of the optical fiber, with the integral over the transverse area of ​​the fiber.

[0039] From this relationship, E incidentSince δn(r,θ) is primarily confined to the core, it is clear that the desired refractive index perturbation should spatially overlap at least a portion of the optical fiber's light-guiding core. To increase scattering to backward-propagating modes, such perturbations can have minimal variations in the direction perpendicular to the fiber axis. This can be understood from the overlap integral. Varying δn(r,θ) in the r and θ directions can increase the overlap integral with unguided modes because such modes move at an angle relative to the fiber axis and therefore have more transverse variations in their E-field. Because the purpose of the refractive index perturbation is to increase backscattering to guided modes while minimizing scattering to unguided lossy modes, the desired refractive index perturbation δn(r,θ) has little or no dependence on r and θ.

[0040] For the example of a scattering fiber, δn(r,θ) can be thought of as existing only in the core of the fiber, with little or no dependence on r and θ. The only dependence along the fiber axis is the variation of δn. Let the direction along the fiber be the z direction. First, consider the case where the variation of δn is periodic. In this case, if such perturbations persist over a length L and the perturbations are spaced apart by an amount D, the spatially averaged scattering per unit length is:

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[0041] where η is the overlap integral discussed above. This increased scattering has a spectral dependence centered at λs, with a spectral width Δλ approximately BW It has.

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[0042] where n is the effective index of the guided mode, and in general, if such a perturbation has a uniform amplitude δn along its length L, the spectrum will exhibit large sidebands outside this bandwidth. Consequently, the refractive index perturbation must be apodized to reduce scattering outside the main scattering bandwidth.

[0043] Apodization changes δn from a very small value to a maximum near the middle of the length L, and then to a very small value again at the other end of the length L. Such apodization can make the out-of-band scattering less than 10 dB of the in-band scattering.

[0044] Another approach may utilize refractive index perturbations that are not periodic along the fiber axis. A set of such perturbations may increase the scattering bandwidth by varying the local period δn of the perturbations. The simplest example of this may be a set of perturbations whose period increases linearly with the chirp rate Cs. In this case, the bandwidth Δλ of a set of perturbations with length L is BW teeth, Δλ BW =C S L This becomes: An estimate of the average reflection per unit length is:

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[0045] The formula can be used to estimate the required magnitude of the refractive index perturbation for a given set of scattering parameters for the ESF: The scattering parameters are the scattering per unit length R, the central wavelength λs, and the bandwidth Δλ over which this scattering occurs. BW , and the spacing D between the individual perturbations. In one example, the following values ​​may be considered:

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[0046] Such perturbations give rise to "first order scattering." If the perturbations are further apart, then "higher order scattering" can be used. Such higher order scattering arises from higher spatial Fourier components of the periodic pattern of the refractive index perturbation. For Nth order scattering, it can be calculated as follows:

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[0047] where Λ p is the spacing of the perturbation. If the perturbation uses the Nth-order Fourier coefficient, the perturbation that causes scattering at λs in the following equation is δn N is.

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[0048] In most cases, Max{δn(z)}>δn m Note that for a uniform grid, the required grid length L u can be estimated as follows:

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[0049] For a chirp set of perturbations, the chirp rate is The length of the chirp pattern Lc can be estimated by the following formula:

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[0050] These parameters may also be calculated for the case where D=1 m. In this case, L u and L c The value of δn may be the same. However, the exponential perturbation amplitude value is an order of magnitude larger: u =6×10 -6 and δn c =5×10 -7 This becomes:

[0051] Uniform, linear chirp perturbations are two examples of perturbations, but many other patterns of perturbations are possible. For example, it is possible to have nonlinear chirp periods. It is also possible for each subsequent set of perturbations to have a different period. Finally, it is possible to have single or multiple randomly spaced perturbations. It is also possible for the value of D to vary between exposures.

[0052] Desired ESFs have more scattering than Rayleigh scattering, resulting in little additional attenuation being introduced into the fiber waveguide. Therefore, refractive index perturbations can be introduced into the waveguide so that the attenuation of the fiber's guided modes remains unchanged or is very low compared to a fiber without the refractive index perturbation. It is well known that optical attenuation in silica-core fibers is sensitive to drawing tension. Higher tension (e.g., due to high drawing speeds or low temperatures) creates so-called stretch-induced defects in the glass network that absorb light. The effects of higher drawing tension can be mitigated by lowering the viscosity of the core, for example, by doping with chlorine, fluorine, alkalis, and other elements. Similarly, because refractive index changes due to actinic radiation exposure are likely the result of damage to the silica glass network, the optical attenuation induced during grating writing is also sensitive to drawing conditions and the glass chemistry. Therefore, maintaining low optical attenuation in ESFs requires careful attention to the writing conditions, drawing conditions, and glass composition. Furthermore, drawing conditions, such as temperature and speed, must also be suitable for producing an appropriate hermetic coating.

[0053] It is important to note that the glass defects created during stretching and actinic radiation exposure are metastable, i.e., they can anneal over time, with the annealing rate depending on the exposure conditions, the glass chemistry, and the annealing conditions (typically time and temperature). Gratings in ESFs are typically inscribed at higher intensities to allow for some recovery during annealing before or during actual use. Determination of grating strength and optical attenuation typically takes into account changes during annealing.

[0054] According to one embodiment, increased attenuation for the above two examples can be considered. In general, the fiber has an attenuation coefficient α before exposure and e over the length z of the fiber. -αz After exposure, the attenuation coefficient is α e= α + δα(δn), where δα is the change in the attenuation coefficient of the portion of the fiber exposed to the perturbation δn. In addition, there may be discrete loss points that do not depend on the length of the perturbation L, but rather depend only on the refractive index step discontinuities at the beginning and end of the refractive index perturbation, for example. The transmission through such a discrete loss point is:

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[0055] According to one exemplary embodiment, notable elements for testing femto scattering fibers may include: 1) Fiber: with Ge core and carbon / polyimide; 2) Spacing: 10cm; 3) Grid length 0.5mm; 4) Reflection bandwidth 1540nm±6nm; 5) The chirp is approximately linear but not very significant; 6) Reflection intensity of one grating R=-70dB; 7) Length: 200m.

[0056] For example, one can start with several test gratings with various strengths (e.g., from -75 dB to -50 dB for short lengths). Such a process can be used to test shorter lengths of fiber. Other test procedures might test for hydrogen susceptibility, attenuation, mechanical strength, thermal stability at a given temperature (e.g., 15°C), etc.

[0057] The present disclosure has been described with reference to exemplary embodiments thereof. All exemplary embodiments and conditional descriptions disclosed in this disclosure have been set forth with the intention of helping those skilled in the art to understand the principles and concepts of the present disclosure. Therefore, those skilled in the art will understand that the present invention can be modified and implemented without departing from the spirit and scope of the present invention. Although numerous embodiments having various features have been described herein, combinations of such various features in other combinations not discussed herein are contemplated to be within the scope of the embodiments of the present disclosure.

Claims

1. 1. An optical fiber whose refractive index changes upon application of actinic pulsed radiation, comprising: a germanium (Ge)-free core having a core length greater than one meter (1 m); Clad and a hermetic coating layer that is not damaged by actinic radiation; Coating and a refractive index perturbation inscribed by the actinic radiation through the hermetic coating along the length of the core of the optical fiber; The optical fiber, wherein the reflectivity of the refractive index perturbation is in the range of -100 dB / mm to -70 dB / mm.

2. The optical fiber of claim 1 , wherein the refractive index perturbation is inscribed along a spatially continuous length of the core of the optical fiber.

3. 10. The optical fiber of claim 1, wherein the refractive index perturbations are inscribed at discrete locations along the length of the core of the optical fiber, the discrete locations being spaced apart within a range of 100 microns to 10 m.

4. 10. The optical fiber of claim 1, wherein the scattering wavelength of the refractive index perturbation is centered between about 1500 nm and about 1700 nm.

5. 2. The optical fiber according to claim 1, wherein the scattering wavelength center of the refractive index perturbation is located at a wavelength at which Rayleigh scattering occurs.

6. 2. The optical fiber of claim 1, wherein the refractive index perturbation has an isolated scattering center with a spatial extent in the range of 1 micron to 10 cm.

7. 2. The optical fiber of claim 1, wherein the scattering wavelength center of the refractive index perturbation has a spectral reflection bandwidth of less than 10 nm.

8. 2. The optical fiber of claim 1, wherein the scattering wavelength center of the refractive index perturbation has a spectral reflection bandwidth of less than 30 nm.

9. 2. The optical fiber of claim 1, wherein the scattering wavelength of the refractive index perturbation is centered at a spectral reflection bandwidth of less than 100 nm.

10. 2. The optical fiber of claim 1, wherein the hermetic coating layer is carbon.

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