WO2002071038A1 - Optical waveguide monitoring - Google Patents
Optical waveguide monitoring Download PDFInfo
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- WO2002071038A1 WO2002071038A1 PCT/US2002/006601 US0206601W WO02071038A1 WO 2002071038 A1 WO2002071038 A1 WO 2002071038A1 US 0206601 W US0206601 W US 0206601W WO 02071038 A1 WO02071038 A1 WO 02071038A1
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- photonic crystal
- crystal fiber
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/02—Optical fibres with cladding with or without a coating
- G02B6/02295—Microstructured optical fibre
- G02B6/023—Microstructured optical fibre having different index layers arranged around the core for guiding light by reflection, i.e. 1D crystal, e.g. omniguide
- G02B6/02304—Core having lower refractive index than cladding, e.g. air filled, hollow core
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- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
- C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
- C03B37/01—Manufacture of glass fibres or filaments
- C03B37/02—Manufacture of glass fibres or filaments by drawing or extruding, e.g. direct drawing of molten glass from nozzles; Cooling fins therefor
- C03B37/025—Manufacture of glass fibres or filaments by drawing or extruding, e.g. direct drawing of molten glass from nozzles; Cooling fins therefor from reheated softened tubes, rods, fibres or filaments, e.g. drawing fibres from preforms
- C03B37/0253—Controlling or regulating
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
- G01M11/30—Testing of optical devices, constituted by fibre optics or optical waveguides
- G01M11/37—Testing of optical devices, constituted by fibre optics or optical waveguides in which light is projected perpendicularly to the axis of the fibre or waveguide for monitoring a section thereof
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- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
- C03B2203/00—Fibre product details, e.g. structure, shape
- C03B2203/42—Photonic crystal fibres, e.g. fibres using the photonic bandgap PBG effect, microstructured or holey optical fibres
Definitions
- This invention relates to optical waveguides, and more particularly to monitoring optical waveguides.
- Optical waveguides guide optical signals to propagate along a prefe ⁇ ed path or paths. Accordingly, they can be used to carry optical signal information between different locations and thus they form the basis of optical telecommunication networks.
- the most prevalent type of optical waveguide is an optical fiber based on index guiding. Such fibers include a core region extending along a waveguide axis and a cladding region su ⁇ ounding the core about the waveguide axis and having a refractive index less than that of the core region. Because of the index-contrast, optical rays propagating substantially along the waveguide axis in the higher- index core can undergo total internal reflection (TIR) from the core-cladding interface.
- TIR total internal reflection
- the optical fiber guides one or more modes of electromagnetic (EM) radiation to propagate in the core along the waveguide axis.
- the number of such guided modes increases with core diameter.
- the index-guiding mechanism precludes the presence of any cladding modes lying below the lowest-frequency guided mode.
- Almost all index-guided optical fibers in use commercially are silica-based in which one or both of the core and cladding are doped with impurities to produce the index contrast and generate the core-cladding interface.
- silica optical fibers have indices of about 1.45 and index contrasts of up to about 2-3% for wavelengths in the range of 1.5 microns.
- a Bragg fiber which includes multiple dielectric layers surrounding a core about a waveguide axis.
- the multiple layers form a cylindrical minor that confines light to the core over a range of frequencies.
- the multiple layers form what is known as a photonic crystal, and the Bragg fiber is an example of a photonic crystal fiber.
- An important characteristic of an optical waveguide is the transmission loss, or attenuation, of the waveguide. Transmission loss can be described as a logarithmic relationship between the optical output power and the optical input power in a waveguide system. It is a measure of the decay of signal strength, or loss of light power, that occurs as light pulses propagate through the length of a waveguide.
- Transmission loss can be caused by several intrinsic and extrinsic factors.
- intrinsic factors include scattering and absorption.
- Extrinsic causes of attenuation include cable-manufacturing stresses, environmental effects, and physical bends in the fiber.
- the primary mechanism for transmission loss is Rayleigh scattering in the solid fiber core. Any structural and physical defect, such as voids in the core and/or cladding, or fiber eccentricity, significantly enhance light scattering and/or increase the fiber transmission loss. In addition to the increased transmission loss, structural flaws in the fiber can also lead to reduced mechanical strength and consequently a higher probability of fiber failure in the field. Therefore, it is important to detect the presence of defects in the fiber during the fiber manufacturing process. A number of methods have been developed to monitor the quality of the fiber in real time during fiber drawing.
- One such method involves measuring the diameter of an optical fiber and detecting the presence of holes or voids in the fiber.
- the fiber is transversely illuminated with monochromatic light and an interference pattern is produced in the far field due to the superposition of light reflected from the fiber surface and light refracted through the fiber.
- the interference fringe pattern depends on the fiber core and cladding diameters and their respective refractive indexes. The number of interference fringes is related to the fiber diameter. However, the presence of a hole results in missing fringes. Thus changes in the interference pattern can be used to detect holes in the fiber.
- Another method utilizing the far field interference pattern created by illuminating an optical fiber with monochromatic light, determines a spatial frequency spectrum from the interference pattern using a Fast Fourier transform.
- the spatial frequency spectrum contains a frequency component co ⁇ esponding to the fiber diameter. If there are voids or holes present in the fiber, the spatial frequency spectrum will contain additional components. Detecting these additional components and observing their behavior over time enables determining the extent of the fiber defects as well as their growth with time.
- the total power of the interference pattern is measured, which is affected by the size of a hole. Analysis of the total power in the interference pattern together with the intensity of the components in the spatial frequency spectrum compensates for fluctuations in the light source intensity and enables determining void size.
- the invention features techniques for monitoring the quality (e.g., optical and mechanical properties, including the presence of defects) in optical waveguides (e.g., photonic crystal fibers).
- optical waveguides e.g., photonic crystal fibers
- the inventors have recognized that the spectral composition of light reflected from the side (e.g., light incident on the outside of the waveguide non-parallel to the waveguide axis, such as having an angle of incidence from about - 85 degrees to + 85 degrees) of certain optical waveguides (e.g., photonic crystal fibers) depends on the structure and composition of the waveguide.
- the spectral composition of light reflected from the side of the waveguide and comparing the measured spectrum to a reference spectrum, one can evaluate the quality of the fiber.
- the invention features a method for monitoring the quality of a photonic crystal fiber.
- the method includes directing test light toward a side of a photonic crystal fiber and detecting measurement light emerging from the photonic crystal fiber in response to the test light.
- the method also includes monitoring the quality of the photonic crystal fiber based on the measurement light.
- Embodiments may include one or more of the following.
- the emerging light can include reflected light.
- Monitoring the quality of the photonic crystal fiber can include determining a measurement spectrum of the measurement light.
- the measurement spectrum can be related to the bandgap of the photonic crystal fiber.
- Monitoring the quality of the photonic crystal fiber further can include determining an error signal that is based on a function of the measurement spectrum.
- the function of the measurement spectrum can also be a function of a reference spectrum (e.g., an empirically or theoretically determined reference spectrum).
- the function can be related to a difference (e.g., a weighted difference) between the measurement spectrum and the reference spectrum.
- the method can further include drawing a photonic crystal fiber preform into the photonic crystal fiber while the measurement light is detected. Moreover, the method can include adjusting draw parameters based on the photonic crystal fiber quality.
- the photonic crystal fiber can be a Bragg fiber.
- the photonic crystal fiber can be designed to guide light having a wavelength between 1.2 microns and 1.7 microns, or a wavelength between 0.7 microns and 1.0 microns.
- the measurement light can be detected over a range of angles.
- the detection of measurement light can include collecting the measurement light with light collecting optics.
- Monitoring the quality of the photonic crystal fiber can include detecting structural and/or compositional defects in the photonic crystal fiber.
- the detection of structural and/or compositional defects is based on a spectrum of the measurement light.
- Monitoring the quality of the photonic crystal fiber can also include detecting differences between a measurement spectrum based on the measurement light and a reference spectrum.
- Directing the test light can include directing (e.g., simultaneously directing) the test light to different regions of the photonic crystal fiber. Directing the test light can include focusing the test light onto the side of the photonic crystal fiber.
- Detecting the measurement light can include detecting the measurement light emerging from the regions of the photonic crystal fiber. Detecting the measurement light can also include gathering the measurement light scattered from the side of the photonic crystal fiber. A single optical component can perform the focusing and gathering.
- Monitoring the quality of the photonic crystal fiber can include determining a measurement spectrum of each region of the photonic crystal fiber based on the measurement light.
- the invention features a method for monitoring the quality of an optical waveguide, which includes directing broadband test light to a side of an optical waveguide and detecting measurement light reflected from the optical waveguide in response to the test light.
- the method also includes determining the measurement light intensity at a plurality of wavelengths and monitoring the quality of the optical waveguide based on a measurement spectrum of the measurement light.
- Embodiments can include one or more of the following features.
- Monitoring the quality of the optical waveguide can include comparing the measurement spectrum to a reference spectrum.
- Monitoring the quality of the optical waveguide can include detecting structural and/or compositional defects in the optical fiber.
- the method can also include drawing an optical waveguide preform into the optical waveguide, wherein detecting the measurement light occurs during the drawing.
- the method can further include adjusting a draw parameter for the drawing based on the optical waveguide quality.
- the optical waveguide can be a photonic crystal fiber (e.g., a Bragg fiber)
- the invention features an apparatus for monitoring a photonic crystal fiber, which includes a mount for supporting the photonic crystal fiber, an illumination system which during operation directs test light to a side of the photonic crystal fiber, and a detection system which during operation detects measurement light emerging from the photonic crystal fiber in response to the test light.
- Embodiments can include one or more of the following.
- the apparatus can also include a controller, which during operation causes the illumination system to direct the test light and receive information based on the measurement light detected by the detection system. During operation the controller can determine a measurement light spectrum. The controller can also detect structural and/or compositional defects in the photonic crystal fiber based on the measurement light spectrum.
- the apparatus can also include a fiber drawing system, which during operation draws a photonic crystal fiber preform into the photonic crystal fiber. The controller can also adjust a draw parameter of the fiber drawing system based on the measurement light spectrum.
- FIG. lA is a schematic view of an embodiment of an optical waveguide monitoring system
- FIG. IB is a schematic view of an embodiment of an optical waveguide monitoring system
- FIG 2 is a cross-sectional view of an embodiment of optics for illuminating the side of an optical waveguide and collecting light reflected from the optical waveguide;
- FIG 3 is a cross-sectional view of an embodiment of a photonic crystal fiber
- FIG. 4 is the reflectance spectrum of a defect-free photonic crystal fiber showing the primary and secondary photonic band gap
- FIG. 5 A are the reflectance spectra of the defect free photonic crystal fiber and a photonic crystal fiber with a void in the multilayer structure located close to the core, and the difference spectrum in the vicinity of the primary photonic band gap;
- FIG 5B are the reflectance spectra of the defect free photonic crystal fiber and a photonic crystal fiber with a void in the multilayer structure located close to the core, and the difference spectrum in the vicinity of the secondary photonic band gap;
- FIG. 6A are the reflectance spectra of the defect free photonic crystal fiber and a photonic crystal fiber with a void in the multilayer structure located close to the cladding, and the difference spectrum in the vicinity of the primary photonic band gap;
- FIG. 6B are the reflectance spectra of the defect free photonic crystal fiber and a photonic crystal fiber with a void in the multilayer structure located close to the cladding, and the difference spectrum in the vicinity of the secondary photonic band gap;
- FIG 7 A are the reflectance spectra of the defect free photonic crystal fiber and a photonic crystal fiber with a large void in the multilayer structure, and the difference spectrum in the vicinity of the primary photonic band gap;
- FIG. 7B are the reflectance spectra of the defect free photonic crystal fiber and a photonic crystal fiber with a large void in the multilayer structure, and the difference spectrum in the vicinity of the secondary photonic band gap;
- FIG. 8 A are the reflectance spectra of the defect free photonic crystal fiber and a photonic crystal fiber with 5% layer thickness fluctuations, and the difference spectrum in the vicinity of the primary photonic band gap;
- FIG. 8B are the reflectance spectra of the defect free photonic crystal fiber and a photonic crystal fiber with 5% layer thickness fluctuations, and the difference spectrum in the vicinity of the secondary photonic band gap;
- FIG. 9 is a plan view of an embodiment of light launching and collecting optics for illuminating and collecting light reflected by an optical waveguide.
- the invention features methods and apparatus for monitoring the quality in optical waveguides (e.g., photonic crystal fibers).
- the method is based on directing test light to the side of a waveguide (e.g., at a non-zero angle relative to the waveguide axis), and measuring the spectrum of light emerging (e.g., reflected) from the side of a waveguide in response to the test light.
- Measuring the spectrum includes measuring the intensity of light for a known wavelength or band of wavelengths (e.g., measuring an intensity for each wavelength or wavelength band in a range of wavelengths).
- Monitoring waveguide quality can include detecting structural defects (e.g., deviations in the core radius or cladding layer thickness, or the presence of air bubbles or contaminant particles) and compositional defects (e.g., variations in the refractive index of the core or cladding layers from their designed values) in the waveguide.
- the method can be implemented while the waveguide is being made (e.g., drawn from a waveguide preform).
- the structure of some optical waveguides, such as photonic crystal fibers can exhibit reflectance spectra that are strongly dependent on their structure. For example, photonic crystal fibers exhibit photonic bandgaps (PBGs), which are wavelength bands for which the fibers reflect substantially all incident light.
- PBGs photonic bandgaps
- analysis of the reflection spectra of waveguides can provide information about the waveguide's structure. Analysis of the reflection spectra can include comparing the measured spectra to a reference spectrum, e.g., a theoretical reflection spectrum for a defect-free waveguide. Moreover, when the optical waveguide is monitored during drawing, the information about the waveguide structure can be used to control the draw parameters to adjust for any deviations from the defect-free structure.
- a reference spectrum e.g., a theoretical reflection spectrum for a defect-free waveguide.
- photonic crystal fiber monitoring system 101 monitors a newly drawn photonic crystal fiber 20 for defects before a protective coating is applied to the fiber by a coating system 30.
- photonic crystal fiber monitoring system 101 includes an illumination system 10 and a detection system 12. Both illumination system 10 and detection system 12 are in communication with a controller 120, which performs data acquisition and spectral analysis. Controller 120 is also in communication with a photonic crystal fiber drawing system 15, which draws photonic crystal fiber 20 from a photonic crystal fiber preform (not shown).
- a fiber mount 18 positions photonic crystal fiber 20 appropriately relative to illumination system 10 and detection system 12.
- the illumination system directs test light to the side of photonic crystal fiber 20.
- Test light reflects from photonic crystal fiber 20, and a portion of the reflected light is detected by detection system 12.
- Controller 120 acquires data related to the detected measurement light from the detection system and analyzes the data to determine the quality of photonic crystal fiber 20. Based on the quality of the fiber, controller 120 optionally sends a signal to photonic crystal fiber drawing system 15 to adjust various draw parameters to co ⁇ ect for deviations of the fiber from the desired fiber structure. For example, if the measured spectrum indicates that the fiber is thicker than desired, controller 120 sends a signal to drawing system 15 causing drawing system 15 to decrease the fiber diameter (e.g., by changing the preform temperature and/or adjusting the drawing speed).
- Information from the controller can be constantly fed back to drawing system 15 to ensure the fiber is drawn to conform to predetermined specifications. Furthermore, the controller can halt the drawing process when the controller detects a defect potentially catastrophic to the fibers performance. The controller can also record the position of the defective areas in the fiber for later follow-up and offline quality control.
- the illumination system includes two light sources 80A and 80B, two optical fibers 60 A and 60B, and two sets of illumination and light collection optics 40 A and 40B.
- Optical fibers 60A and 60B guide test light from light sources 80A and 80B to illumination and light collection optics 40 A and 40B, which focus test light onto the side of photonic crystal fiber 20.
- Illumination and light collection optics 40A and 40B are mounted on alignment stages 50A and 50B, respectively. Alignment stages 50A and 50B position illumination and light collection optics 40A and 40B appropriately with respect to photonic crystal fiber 20.
- a portion of the test light is reflected back to the illumination and light collection optics 40 A and 40B, which now function as part of the detection system by gathering measurement light and coupling it back into optical fibers 60 A and 60B.
- Directional couplers 70 A and 70B separate measurement light propagating in optical fibers 60A and 60B from counter propagating test light.
- Directional couplers 70 A and 70B direct the measurement light toward detector 100.
- fiber optic coupler 90 Prior to impinging on detector 100, fiber optic coupler 90 combines measurement light in fibers 60A and 60B.
- Detector 100 measures the spectral intensity of the measurement light, yielding a reflection spectrum R( ⁇ ).
- the spectral data from detector 100 is recorded by controller 120 (e.g., a computer).
- controller 120 e.g., a computer
- measurement light from fibers 60A and 60B are combined by a coupler 90 before being delivered to detector 100.
- detector 100 measures the spectral content of measurement light from opposite sides of fiber 20.
- each fiber can direct light to an individual detector, each detector measuring a co ⁇ esponding reflection spectrum.
- Detector 100 includes a diffraction grating 105 and an a ⁇ ay photodetector 110.
- a diffraction grating 105 For spectral resolution of about 5 ⁇ m to 10 ⁇ m, an inexpensive, short focal length fixed grating monochromators can be used (e.g., Oriel Instruments MS 125) with diffraction gratings having line density 300 lines/mm and a blaze wavelength in the wavelength range utilized for the spectral measurements. Also, depending on the wavelength range, different types of detectors can be used. Cooled InGaAs photodiode linear a ⁇ ay detectors may be employed for spectral measurements in the vicinity of the primary PBG, e.g., in the range of 1.2 -2.5 ⁇ m.
- Cooled CCD linear a ⁇ ays can be used for measurements in the vicinity of the secondary PBG, e.g., in the range of 0.7 - 1.1 ⁇ m.
- Array detectors and fixed-grating monochromators offer reasonable speed of operation.
- the a ⁇ ay detectors can be operated up to 1 MHz scanning rates and thus a 1024 element a ⁇ ay can be scanned in about 1 to 2 ms. It is the minimal exposure time that determines the speed of spectral data acquisition.
- illumination light collection optics 40A includes a holder 340 and a focusing lens 330.
- Focusing lens 330 focuses divergent test light emitted from optical fiber 60 onto the side of fiber 20. Focusing lens 330 also gathers measurement light reflected from fiber 20 and couples the measurement light into optical fiber 60.
- a holder 340 positions focusing lens 330 relative to fiber 60. Holder 340 is mounted on an alignment stage (not shown), which positions the focusing lens and fiber relative to fiber 20.
- the cross-section 20 A of photonic crystal fiber 20 is shown in FIG. 3, and includes a dielectric core 320 extending along a waveguide axis and a dielectric confinement region 310 su ⁇ ounding the core.
- confinement region 310 is shown to include alternating layers 330 and 340 of dielectric materials having different refractive indices.
- One set of layers, e.g., layers 340 define a high-index set of layers having an index «/,, and a thickness dht
- the other set of layers, e.g., layers 330 define a low-index set of layers having an index 0 and a thickness d ⁇ ⁇ , where nu > 0 .
- confinement region 310 may include many more layers (e.g., twenty or more layers).
- Photonic crystal fiber 20 has a circular cross-section, with core 320 having a circular cross-section and region 310 (and layers therein) having an annular cross-section.
- the waveguide and its constituent regions may have different geometric cross-section such as a rectangular or a hexagonal cross-section.
- core and confinement regions 320 and 310 may include multiple dielectric materials having different refractive indices.
- the boundary between region 320 and 310 is defined by a change in index. The change may be caused by the interface of two different dielectric materials or by different dopant concentrations in the same dielectric material (e.g., different dopant concentrations in silica).
- Dielectric confinement region 310 guides EM radiation in a first range of wavelengths to propagate in dielectric core 320 along the photonic crystal fiber axis.
- the confinement mechanism is based on a photonic crystal structure in region 310 that forms a bandgap including the first range of wavelengths. Because the confinement mechanism is not index-guiding, it is not necessary for the core to have a higher index than that of the portion of the confinement region immediately adjacent the core. To the contrary, core 320 may have a lower average index than that of confinement region 310.
- core 320 may be air, some other gas, such as nitrogen, or substantially evacuated.
- core 320 may include a porous dielectric material to provide some structural support for the su ⁇ ounding confinement region while still defining a core that is largely air. Accordingly, core 320 need not have a uniform index profile.
- the alternating layers 330 and 340 of confinement region 310 form what is known as a Bragg fiber.
- the alternating layers are analogous to the alternating layers of a planar dielectric stack reflector (which is also known as a Bragg minor).
- the annular layers of confinement region 310 and the alternating planar layers of a dielectric stack reflector are both examples of a photonic crystal structure. Photonic crystal structures are described generally in Photonic Crystals by John D. Joannopoulos et al. (Princeton University Press, Princeton NJ, 1995).
- a photonic crystal is a dielectric structure with a refractive index modulation that produces a photonic bandgap in the photonic crystal.
- a photonic bandgap is a range of wavelengths (or inversely, frequencies) in which there are no accessible extended (i.e., propagating, non-localized) states in the dielectric structure.
- the structure is a periodic dielectric structure, but it may also include, e.g., more complex "quasi- crystals.”
- the bandgap can be used to confine, guide, and/or localize light by combining the photonic crystal with "defect" regions that deviate from the bandgap structure.
- accessible states means those states with which coupling is not already forbidden by some symmetry or conservation law of the system. For example, in two-dimensional systems, polarization is conserved, so only states of a similar polarization need to be excluded from the bandgap. In a waveguide with uniform cross-section (such as a typical fiber), the wavevector ⁇ is conserved, so only states with a given ⁇ need to excluded from the bandgap to support photonic crystal guided modes.
- the "angular momentum" index m is conserved, so only modes with the same m need to be excluded from the bandgap.
- the dielectric stack reflector is highly reflective in the photonic bandgap because EM radiation cannot propagate through the stack.
- the annular layers in confinement region 310 provide confinement because they are highly reflective for incident rays in the bandgap. Strictly speaking, a photonic crystal is only completely reflective in the bandgap when the index modulation in the photonic crystal has an infinite extent.
- incident radiation can "tunnel" through the photonic crystal via an evanescent mode that couples propagating modes on either side of the photonic crystal.
- rate of such tunneling decreases exponentially with photonic crystal thickness (e.g., the number of alternating layers). It also decreases with the magnitude of the index contrast in the confinement region.
- a photonic bandgap may extend over only a relatively small region of propagation vectors.
- a dielectric stack may be highly reflective for a normally incident ray and yet only partially reflective for an obliquely incident ray.
- a "complete photonic bandgap" is a bandgap that extends over all possible wavevectors and all polarizations.
- a complete photonic bandgap is only associated with a photonic crystal having index modulations along three dimensions.
- an omnidirectional photonic bandgap which is a photonic bandgap for all possible wavevectors and polarizations for which the adjacent dielectric material supports propagating EM modes.
- an omnidirectional photonic bandgap can be defined as a photonic band gap for all EM modes above the light line, wherein the light line defines the lowest frequency propagating mode supported by the material adjacent the photonic crystal. For example, in air the light line is approximately given by ⁇ - c ⁇ , where ⁇ is the angular frequency of the radiation, ?is the wavevector, and c is the speed of light.
- an omnidirectional photonic bandgap may co ⁇ espond to a reflection in a planar geometry of at least 95 % for all angles of incidence ranging from 0° to 80° and for all polarizations of EM radiation having frequency in the omnidirectional bandgap.
- photonic crystal fiber 30 may still support a strongly guided mode, e.g., a mode with radiation losses of less than 0.1 dB/km for a range of frequencies in the bandgap.
- a strongly guided mode e.g., a mode with radiation losses of less than 0.1 dB/km for a range of frequencies in the bandgap.
- whether or not the bandgap is omnidirectional will depend on the size of the bandgap produced by the alternating layer (which generally scales with index contrast of the two layers) and the lowest-index constituent of the photonic crystal.
- the dielectric confinement region may include photonic crystal structures different from a multilayer Bragg configuration.
- the confinement region may be selected to form, for example, a two- dimensionally periodic photonic crystal (in the planar limit), such as an index modulation co ⁇ esponding to a honeycomb structure. See, for example, R.F. Cregan et al., Science 285:1537-1539, 1999.
- the high-index layers may vary in index and thickness, and/or the low-index layers may vary in index and thickness.
- the confinement region may be based on any index modulation that creates a photonic bandgap.
- multilayer structure 310 forms a Bragg reflector because it has a periodic index variation with respect to the radial axis.
- the quarter-wave condition becomes:
- this equation may not be exactly optimal because the quarter-wave condition is modified by the cylindrical geometry, which may require the optical thickness of each layer to vary smoothly with its radial coordinate. Nonetheless, we find that this equation provides an excellent guideline for optimizing many desirable properties, especially for core radii larger than the mid-bandgap wavelength.
- spectral reflectance for a photonic crystal fiber is shown calculated for reflection at normal incidence from the side of an exemplary hollow fiber, disclosed in U.S.
- the multilayer cladding has 28 layers, and the core radius is taken to be 15.4 ⁇ m. Theoretical reflectance values were calculated using the modeling methods described by Steven G.
- the spectrum includes two bands in which the reflectance is close to 100%.
- the two bands are the primary and secondary photonic band gaps (PBGs).
- the primary PBG is at longer wavelengths (i.e., centered at about 1.95 ⁇ m) than the designed PBG center wavelength for light propagating along the waveguide axis.
- the wavelength shift in the PBG center wavelength is due to the different angles of incidence for test light (i.e., close to 0 degrees - near normal incidence) and light propagating along the waveguide axis (where the incidence angle is close to 90 degrees). This shift from near normal incidence to near parallel incidence (with respect to the waveguide axis) results in a shift to shorter wavelengths, centering the propagation PBG at about 1.55 ⁇ m.
- the secondary PBG is centered at about 1.0 ⁇ m.
- Perturbations to the photonic crystal structure can result in variations in the reflection spectrum from the defect-free fiber reflection spectrum. Hence, by comparing a measured reflection spectrum to a reference spectrum it is possible to identify regions of the photonic crystal fiber having structural and/or compositional defects.
- Curve A is the reflection spectrum of the fiber with the designed and defect free PBG structure, which is the reference spectrum, R. e/ e. _nce( .)-
- Curve B is the spectrum obtained for the fiber where a void defect is present inside the multilayer structure, located close to the hollow core, and is the measured spectrum Rmeasure JJ).
- the extent of the defect in the direction along the waveguide axis co ⁇ elates to the drawing speed and the spectra acquisition and analysis speed.
- the extent of the defect in the direction along the waveguide axis can range from a few millimeters to several meters. For this calculation it is assumed that the detection system is fast enough to detect a defect at a given drawing speed.
- Curve C shows the absolute value of the difference between the two spectra given by curves A and B, multiplied by a factor of 0.2 for better presentation on the graph.
- This data can be expressed mathematically as
- the void in the multilayer causes the PBG to be offset to slightly higher wavelengths than the defect-free case. Additionally, the void causes increased reflectance a several satellite peaks located at wavelengths both shorter and longer than the PBG wavelengths.
- the difference spectrum thus has a measurable signal at the PBG edges and outside the PBQ but with negligible signal at the PBG wavelengths.
- measurable signal we mean the signal strength of the difference spectrum is large enough to be attributed to the reflectance of the photonic crystal fiber over any noise present in the measurement.
- the difference spectrum, A( ⁇ ) can be integrated over a chosen wavelength range (e.g., over the primary PBG) to provide an e ⁇ or signal value, T, used for defect detection:
- the e ⁇ or signal, T is determined by integrating the difference function from a first wavelength, ⁇ , to a second wavelength, L 2 .
- the e ⁇ or signal can also be averaged over the wavelength range according to:
- the difference spectrum can additionally be convolved with a weighting function, w( ⁇ ), to enhance sensitivity of the error signal to certain parts of the spectrum relative to the other parts.
- w( ⁇ ) a weighting function
- the weighted e ⁇ or signal can be expressed as:
- a target e ⁇ or signal value, T tg t can be specified so that the controller registers a defect when the detected e ⁇ or signal value exceeds the threshold e ⁇ or signal value (i.e., when T > T tg i).
- Monitoring the wavelength region in the vicinity of the secondary PBG may have certain advantages over monitoring the primary PBG in certain implementations since sensitive detectors (e.g., CCD a ⁇ ay detectors) and/or bright light sources (e.g., miniature arc lamps) are more readily available for this region of the electromagnetic spectrum.
- sensitive detectors e.g., CCD a ⁇ ay detectors
- bright light sources e.g., miniature arc lamps
- the calculated reflection spectrum in the vicinity of the secondary PBG for the above-mentioned exemplary defect free fiber (curve A) is compared to the reflection spectrum of the defective fiber described in reference to FIG. 5A (curve B).
- the difference spectrum, curve C is again multiplied by 0.2 to better show the difference spectrums structure on the cu ⁇ ent axes.
- the spectrum of the defective fiber shows reduced reflection near the secondary PBG wavelengths (i.e., the reflectance at about 0.95 ⁇ m falls from 100% for a defect free fiber to about 70% for the defective fiber). This results in a difference spectrum signal at the secondary PBG center wavelengths.
- the PBG edges of the defective fiber spectrum are shifted slightly from the defect-free fiber spectrum. This results in a measurable difference spectrum signal at the PBG edge wavelengths.
- the calculated reflection spectrum in the vicinity of the primary PBG for the above-mentioned exemplary defect free fiber is compared to the reflection spectrum of a similar fiber structure with a small (2.5 ⁇ m in the direction transverse to the fiber) void defect in the multilayer structure (modeled as layers with the refractive index set to 1), located near the outer surface of the fiber (curve B).
- the void located close to the cladding in the defective fiber causes a marginal change in the position of the PBG edges compared to the defect-free fiber.
- the primary PBG is broadened slightly, shifted to a shorter wavelength at the PBG edge located at about 1.6 ⁇ m, and shifting to a longer wavelength at the 2.4 ⁇ m PBG edge.
- the reflectance at wavelengths on either side of the primary PBG changes. This results in minimal difference spectrum signal within the PBG wavelengths, but a measurable difference spectrum signal at the PBG edge wavelengths and outside the PBG.
- the calculated reflection spectrum in the vicinity of the secondary PBG for the above-mentioned exemplary defect free fiber (curve A) is compared to the reflection spectrum of the defective fiber described in reference to FIG. 6A (curve B).
- the affect of this defect on the reflection spectrum is to shift the secondary PBG to slightly higher wavelengths compared to the defect-free fiber spectrum.
- the difference spectrum has a measurable signal at the PBG edge wavelengths.
- the calculated reflection spectrum in the vicinity of the primary PBG for the above-mentioned exemplary defect free fiber (curve A) is compared to the reflection spectrum of a similar fiber structure with a larger (5.1 ⁇ m in the direction transverse to the fiber) void defect in the multilayer structure (modeled as layers with the refractive index set to 1) (curve B).
- the defect causes a substantial change in the reflectance at the PBG wavelengths shown.
- the resulting difference spectrum has a measurable signal both within and outside the PBG.
- both an increase in reflectance (e.g., at about 2.6 ⁇ m) and a decrease in reflectance (e.g., at about 2.8 ⁇ m) of the defective fiber spectrum with respect to the defect- free fiber spectrum results in a measurable difference spectrum signal.
- the calculated reflection spectrum in the vicinity of the secondary PBG for the above-mentioned exemplary defect free fiber (curve A) is compared to the reflection spectrum of the defective fiber described in reference to FIG. 7A (curve B).
- the reflectance of the defective fiber is reduced at the secondary PBG, resulting in a measurable difference spectrum signal at these wavelengths.
- several satellite peaks appear in the defective fiber spectrum at wavelengths longer than the PBG wavelengths. These satellite peaks are mi ⁇ ored as peaks in the difference spectrum.
- the calculated reflection spectrum in the vicinity of the primary PBG for the above-mentioned exemplary defect free fiber (curve A) is compared to the reflection spectrum of a similar fiber structure perturbed by random fluctuations of its layer thickness within 5% of its designed thickness (curve B).
- the PBG reflectance of the defective fiber is virtually unchanged from that of the defect-free fiber.
- variations in the reflectance of the defective fiber at wavelengths outside the PBG give rise to measurable difference spectrum signals at those wavelengths.
- the calculated reflection spectrum in the vicinity of the secondary PBG for the above-mentioned exemplary defect free fiber (curve A) is compared to the reflection spectrum of the defective fiber described in reference to FIG. 8A (curve B).
- the affect of the defect on the reflectance spectrum includes slightly broadening the secondary PBG. This results in a measurable difference spectrum signal at the PBG edge wavelengths.
- the foregoing method includes measuring the spectrum of light reflected from the side of a photonic crystal fiber; calculating the difference between the measured spectrum for the fiber and a reference spectrum for defect free fiber; and integrating the difference spectrum over a range of wavelengths to obtain an e ⁇ or signal.
- An increase in the e ⁇ or signal above a certain threshold value indicates the presence of a defect (e.g., a structural and/or compositional defect in the inspected fiber).
- the illumination system can direct test light to any number of positions.
- the illumination system can direct light to only one position on the side of the fiber.
- the illumination system can direct test light to more than two positions on the fiber.
- FIG. 9 shows an embodiment in which the fiber is illuminated with test light at six positions around its circumference.
- the illumination and light collection optics include six optical waveguides 921- 926, each positioned to direct light to and from a co ⁇ esponding lens 931-936.
- Each fiber and lens directs test light to a portion of the circumference of photonic crystal fiber 950, and collects measurement light reflected from the side of the fiber. Note, that the present a ⁇ angement illuminates approximately the whole circumference of the fiber 950. While six waveguide/lens systems are shown for this a ⁇ angement, in general the number of waveguide/lens systems depends on the fiber diameter and the specific optics of the waveguide/lens systems, and on the specifics of the system cost/performance requirements.
- the light source(s) can be any light source(s) capable of producing light having the desired spectral composition.
- suitable light sources include diodes, arc lamps, and fluorescent lamps.
- the light source(s) provide light of sufficient intensity at the wavelengths of interest such that the signal-to-noise ratio of the detected light is not limited by the light source(s).
- the acquisition time of the detection system is limited by the speed of the detector and not the signal strength of the measurement light.
- the light source can include one or more na ⁇ ow band or monochromatic sources.
- the spectral characteristics of such sources can be chosen specific to the PBG wavelengths.
- the light source can include a number of laser diodes having a laser wavelength at or near the PBG edges.
- the light source(s) can be pulsed or continuous, and can include any number of filters and/or other optical components to modify the light source spectrum, pulse width, beam shape, polarization, etc. to provide the desired test light to the fiber being monitored.
- detector 100 is a diffraction grating spectrometer with an array photodetector, however, detector 100 can be any detector capable of spectrally detecting light at the desired wavelengths.
- detector 100 can be a near-IR Fourier transform spectrometer.
- one or several na ⁇ ower wavelength regions either in the vicinity of the primary PBG or other PBGs (e.g., the secondary PBG) can be measured.
- Single element detectors (rather than e.g., detector a ⁇ ays) can also be used.
- the detection system can include one or more wavelength filters to spectrally filter light before the light impinges on the detector(s).
- a consideration in selecting the spectroscopic technique and instrumentation is the speed of spectra acquisition and analysis. The speed of spectra acquisition can determine the minimal extent of the PBG perturbations along the fiber that can be readily detected for a given fiber drawing speed.
- the foregoing techniques can be used to monitor articles other than photonic crystal fibers.
- the methods and apparatus described above can be applied to any optical device, including optical waveguides, whose reflection spectrum is dependent on the devices structure and composition.
- analysis of the measurement light is not limited to comparing the measured reflected light spectrum to a reference spectrum. Analysis of the measured light can simply include studying the measurement spectrum itself, or with respect to some other benchmark. Alternatively, or additionally, analysis can include determining an e ⁇ or signal that is any function of the measured reflectance spectrum:
- the e ⁇ or signal can be determined as the average reflectance of the wavelength range spanned by the PBG.
- the analysis can include determining an e ⁇ or signal that is a function of the measured reflectance spectrum and a reference spectrum:
- This example can include embodiments where the analysis compares the reflectance spectra measured at two different positions on the circumference of the photonic crystal fiber, or at two different times during the fiber drawing.
- the illumination system and detection system need not share common components, as shown in FIG. 1.
- the measurement light can be gathered by independent light collection optics.
- the measurement light need not be light back-reflected by the photonic crystal fiber.
- Measurement light can also be light reflected by the fiber at glancing angles.
- Measurement light can also be light transmitted by the photonic crystal fiber, or light scattered by the fiber. Accordingly, other embodiments are within the scope of the following claims.
Abstract
Description
Claims
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US27359601P | 2001-03-05 | 2001-03-05 | |
US60/273,596 | 2001-03-05 |
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WO2002071038A1 true WO2002071038A1 (en) | 2002-09-12 |
WO2002071038A9 WO2002071038A9 (en) | 2003-02-06 |
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PCT/US2002/006601 WO2002071038A1 (en) | 2001-03-05 | 2002-03-05 | Optical waveguide monitoring |
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US7082242B2 (en) | 2003-01-31 | 2006-07-25 | Corning Incorporated | Multiple core microstructured optical fibers and methods using said fibers |
CN103098387A (en) * | 2010-06-21 | 2013-05-08 | 悉尼大学 | Photonic monitoring for optical signals |
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US7272285B2 (en) * | 2001-07-16 | 2007-09-18 | Massachusetts Institute Of Technology | Fiber waveguides and methods of making the same |
US20040141702A1 (en) * | 2002-11-22 | 2004-07-22 | Vladimir Fuflyigin | Dielectric waveguide and method of making the same |
US7231122B2 (en) * | 2004-04-08 | 2007-06-12 | Omniguide, Inc. | Photonic crystal waveguides and systems using such waveguides |
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US7331954B2 (en) | 2004-04-08 | 2008-02-19 | Omniguide, Inc. | Photonic crystal fibers and medical systems including photonic crystal fibers |
US7310466B2 (en) * | 2004-04-08 | 2007-12-18 | Omniguide, Inc. | Photonic crystal waveguides and systems using such waveguides |
US7349589B2 (en) * | 2004-04-08 | 2008-03-25 | Omniguide, Inc. | Photonic crystal fibers and medical systems including photonic crystal fibers |
US20070147752A1 (en) * | 2005-06-10 | 2007-06-28 | Omniguide, Inc. | Photonic crystal fibers and systems using photonic crystal fibers |
US7352466B2 (en) * | 2005-06-17 | 2008-04-01 | Canon Kabushiki Kaisha | Gas detection and photonic crystal devices design using predicted spectral responses |
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Also Published As
Publication number | Publication date |
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US20020171823A1 (en) | 2002-11-21 |
WO2002071038A9 (en) | 2003-02-06 |
US20050063633A1 (en) | 2005-03-24 |
US6816243B2 (en) | 2004-11-09 |
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