United States Patent
`
`[19]
`
`4,807,950
`
`Glenn et al.
`[45] Date of Patent:
`Feb. 28, 1989
`
`[11] Patent Number:
`
`[54] METHOD FOR IMPRESSING GRATINGS
`WITHIN FIBER OPHCS
`Inventors; Wflljam H_ Glenn, Vernon; Gerald
`Mimi AV°“’ "°‘h °f C°““'i EH”
`Smtzer, Wellesley, Mass.
`
`[75]
`
`[73] Assignee: United Technologies Corporation
`Hartford’ Conn‘
`‘
`
`’
`
`[21] Appl. No.: 122,830
`-
`,
`[22] Flled'
`
`Now 19’ 1987
`
`[60]
`
`Related U.S. Application Data
`Division of Ser. No. 925,512, Oct. 27, 1986, Pat. No.
`4,725,110, which is a continuation of Ser. No. 640,489,
`Aug 13, 1934, aba,,doned_
`
`[51]
`Int. Cl.4 ........
`.................................... .. G02F 1/01
`
`P2]
`350/3,61; 350/96,19
`58] Field of Search .................. .. 350/3.61, 3.7, 96.11,
`350/96.19, 3.65
`
`[56]
`
`References Cited
`U.s. PATENT DOCUMENTS
`4,400,056
`8/1983 Cielo .............................. .. 350/96.19
`FOREIGN’ PATENT DOCUMENTS
`
`55-110207
`
`8/1980’ Japan .
`
`OTHE_R PUBLICATIONS
`Kawasaki et al., “Narrow—Band Bragg Reflectors in
`Optical Fibers”, Optics Letters, vol. 3, No. 2, Aug. 1978.
`Primary Examiner-—,—Bruce Y. Arnold
`Assistant Exami'ner4Martin Lerner
`Attorney, Agent, or Firm—Peter R. Ruzek
`
`57]
`
`ABSTRACI‘
`_
`.
`_
`,
`,
`,
`[
`3 d1°]°_°m° PFn9d1° mdex Offefrac‘
`A3“ Optlcal fiber
`tion phase grating established in its core by intense
`angled application of several transverse beams of ultra-
`violet light, enabling the establishment of a distributed,
`spatially resolving optical fiber strain gauge.
`5 Claims, 3 Drawing Sheets
`
`UV
`SOURCE
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`UV
`SOURCE
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`Page 1
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`ILLUMINA, INC. EXHIBIT 1032
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`Page 1
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`

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`U.S. Patent
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`Feb. 28, 1989
`
`Sheet 1 of 3
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`4,807,950 ~
`
` BEAMSPLITTER
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`
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`SPECTROMETER
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`FVG/
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`LIGHT/S
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`Page 2
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`Page 2
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`

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`U.S. Patent
`
`Feb. 28, 1989
`
`Sheet 2 of3
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`4,8()7,950
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`F/G.2/1
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`/9
`
`<——-—RAA
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`
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`F/6.26‘
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`/9
`/5x
`/ix
`T
`j——-HE---W—/
`<——%RAC
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`7 é
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`F/6.3
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`REFLECTED
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`INTENSITY
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`Page 3
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`Page 3
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`

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`U.S. Patent
`
`Feb. 23, 1989
`
`Sheet 3 of 3
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`4,807,950
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`L:HHll|
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`/W'
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`UV
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`/0/’
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`UV SOURCE
` %”flLLl"
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`FIG.4
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`Page 4
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`Page 4
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`

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`1
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`4,807,950
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`2
`FIGS. 2A through 2C shows the establishment of
`different wavelength gratings 16 corresponding to re-
`spective locations on core 19.
`Each of selected gratings 16 is formed by transverse
`irradiation with a particular wavelength of light in the
`ultraviolet absorption band of the core material associ-
`ated with a position in a structural component 22. This
`procedure establishes a first order absorption process by
`which gratings 16 each characterized by a specific spac-
`ing and wavelength can be formed by illuminating core
`19 from the side with two coplanar, coherent beams
`incident at selected and complementary angles thereto
`with respect to the axis of core 19. The grating period is
`selected by varying the selected angles of incidence.
`Thus, a permanent change in the refractive index is
`induced in a predetermined region of core 19, in effect
`creating a phase grating effective for affecting light in
`core 19 at selected wavelengths.
`As indicated in FIG. 1 the optical waveguide 15 and
`core 19 are attached or embedded in a section of struc-
`tural component 22, in particular a plate for example.
`Core 19 contains characteristic periodic refractive
`index perturbations or gratings 16 in regions A, B and C
`thereof. A broadband light source 33 or tunable laser is
`focused through lens 33’ onto the exposed end of core
`19. A beam splitter 34 serves to direct the return beam
`from core 19 toward a suitable readout or spectrometer
`37 for analysis. Alternatively, a transmitted beam pass-
`ing out of the end 19’ of core 19 could be analyzed.
`The spectrum of the reflected light intensities from
`strain gauge 13 is shown in FIG. 3. A complementary
`transmitted spectrum is also established passing out of
`the end 19’ of core 19. The spectrum contains three
`narrowband output lines centered at respective wave-
`lengths: lambdaA, lambdag and lambdac. These output
`signals arise by Bragg reflection or diffraction from the
`phase gratings 16 at respective regions A, B and C. In
`this example, regions A and C of structural component
`22 have been strained by deformation, causing a com-
`pression and/or dilation of the periodic perturbations in
`the fiber core 19.
`lines are
`As a result,
`the corresponding spectral
`shifted as shown in FIG. 3 to the dotted lines indicated.
`The respective wavelength differences delta lambda,.;
`and delta lambdac are proportional to strain in respec-
`tive regions A and C.
`FIG. 4 illustrates the formation of periodic perturba-
`tions or gratings 16 in a region of fiber core 19 in re-
`sponse to exposure of core 19 to intense transverse
`ultraviolet radiation. Grating spacings Aa and Ac are
`controlled by the incidence angle of incident interfering
`beams 99 and beam 101. As can be seen, the angles of
`incidence of beams 99 are complements (i.e. their sum
`equals 180 degrees) to each other with respect to the
`axis of core 19. The incident pair of beams 99 can be
`derived from a single incident beam 101 passing in part
`through a beam splitter 103 and reflecting from spaced
`parallel reflectors 105. By increasing the separation
`between reflectors 105 and correspondingly varying the
`angles of incidence of beam 101, the angles of incidence
`of beams 99 upon core 19 can be controlled. Accord-
`ingly, the fringe spacing in grating 16 is varied as de-
`sired along the length of core 19, to permit a determina-
`tion of strain or temperature corresponding to location
`along gauge 13.
`Several spacings can be superimposed or colocated
`by this technique for the response set forth below.
`
`METHOD FOR IMPRESSING GRATINGS WITHIN
`FIBER OPTICS
`
`This is a division of application Ser. No. 925,512 filed
`on Oct. 27, 1986 which was a continuation application
`of U.S. Ser. No. 640,489, filed Aug. 13, 1984, now aban-
`doned.
`
`TECHNICAL FIELD
`
`This invention relates to impressing, establishing,
`printing or writing phase gratings in optical fibers or
`waveguides and the optical detection and measurement
`of strain distributions with multi-wavelength light pro-
`vided to said phase gratings.
`BACKGROUND OF THE INVENTION
`
`is known to determine the distribution of axial
`It
`strain or temperature along the length of a fiber optic
`sensor according to the technique described by S. K.
`Yao et al. in 21 Applied Optics (1982) pages 3059-3060.
`According to this technique, very small deformations at
`the,interface between an optical core and its cladding
`will cause light measurably to couple from core to clad-
`ding modes. This permits measurements by time-
`domain reflectometry or a series of cladding taps to
`determine transmission loss and the distribution of ap-
`plied perturbations.
`DISCLOSURE OF INVENTION
`
`According to the invention, phase gratings are im-
`pressed along the core of an optical waveguide by the
`application of intense beams of ultraviolet light trans-
`verse to the axis of the core at selected angles of inci-
`dence and the complements thereto.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a schematic drawing of the spatially resolv-
`ing optical fiber strain gauge according to the invention
`addressed herein;
`FIGS. 2A through 2C are partial schematics of se-
`lected sections of the optical waveguide including its
`cores, indicating grating patterns of varying spacing
`corresponding to selected regions A, B and C in a me-
`chanical structure being monitored for strain;
`FIG. 3 is a graph of the intensity spectrum of the
`reflected light produced by injecting broadband light
`into the core of the waveguide with shifts in the spectral
`lines indicating strain at specific stations; and
`FIG. 4 shows a schematic illustration of a technique
`for establishing a grating pattern of variable spacing at
`selected positions along the length of the optical wave-
`guide.
`BEST MODE FOR CARRYING OUT THE
`INVENTION
`
`FIG. 1 shows a schematic diagram of the spatially
`resolving optical fiber strain gauge 13. The gauge 13
`includes an optical waveguide 15 or fiber operative to
`transmit a single or lowest order mode of injected light.
`The core 19 of waveguide 15 is preferably a germani-
`um-doped silica or glass filament. The core 15 contains
`a series of variable spacing Bragg reflection gratings 16
`written, impressed or otherwise applied by application
`of a variable two-beam ultraviolet (less than 300 nano-
`meter) interference pattern. These periodic gratings 16
`or refractive index perturbations are permanently in-
`duced by exposure to intense radiation.
`
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`Page 5
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`Page 5
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`

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`4,807,950
`
`3
`Sensitivity to external perturbations upon structural
`component 22 and thus also upon core 19 depends upon
`the Bragg condition for reflected wavelength. In partic-
`ular, the fractional change in wavelength due to me-
`chanical strain or temperature change is:
`
`4
`spectral lines are established at each point of measure-
`ment. Strain will affect both lines equally; temperature
`will not. Thus, sufficient
`information is available to
`permit determination of the magnitude of strain and the
`temperature difference.
`The information above is likely to cause others skilled
`in the art to conceive of other variations in carrying out
`the invention addressed herein, which nonetheless are
`within the scope of the invention. Accordingly, refer-
`ence to the claims which follow is urged, as those spec-
`ify with particularly the metes and bounds of the inven-
`tion.
`We claim:
`
`1. An optical fiber having a single core including at
`least one grating with a predetermined grating spacing
`permanently provided in at least one grating region of
`the core by exposing the core to an interference pattern
`resulting from mutual
`interference of two beams of
`ultraviolet radiation simultaneously directed at the fiber
`at such different acute angles of incidence relative to the
`longitudinal axis of the core that the interference pat-
`tern has fringes situated at said predetermined grating
`spacing from each other and propagates transversely
`through the core with attendant permanent change in
`the index of refraction of the core in correspondence
`with the interference pattern.
`2. The optical fiber according to claim 1, wherein said
`different acute angles of incidence complement one
`another to 180° with respect to the longitudinal axis of
`the core.
`
`3. The optical fiber according to claim 2, wherein the
`core includes at least one additional grating similar to
`and provided in the same manner as said one grating but
`having a predetermined grating spacing different from
`that of said one grating.
`4. The optical fiber according to claim 3, wherein said
`additional grating is provided at said grating region.
`5. The optical fiber according to claim 3, wherein said
`additional grating is provided in an additional grating
`region that is longitudinally spaced from said one grat-
`ing region.
`It
`'0!
`II!
`It
`10!
`
`d(1ambda,~)/lambda;
`
`=
`=+
`
`(q + a)AT + (1 + [an/Be]/n)e
`s X no-6/°c.
`+ 8 X I0‘7/microstrain, where:
`
`q is the thermooptic coefficient, which is wavelength
`dependent;
`ais the expansion coefficient;
`eis the axial or longitudinal strain;
`lambda; is the wavelength reflected by the grating at
`location i along the core 19;
`n is the refractive index of the optical waveguide; and
`AT is the change in temperature.
`This relationship suggests a way to compensate for
`temperature changes along the length of the fiber sen-
`sor. In particular, if superimposed gratings of different
`spacings are provided, each of the two gratings will be
`subject to the same level of strain, but the fractional
`change in wavelength of each grating will be different
`because q is wavelength dependent.
`Accordingly, each pair of superimposed gratings will
`display a corresponding pair of peaks of reflected or
`transmitted intensity. Accordingly, the shifts of these
`peaks due to a combination of temperature and strain
`can be subtracted. The shiftslin these peaks due to strain
`will be the same in magnitude. Accordingly, any re-
`maining shift after subtraction is temperature related.
`Thus, when it is desired to know the strain difference as
`between several locations possibly subject to a tempera-
`ture difference, the temperature factor can be compen-
`sated.
`The relationship therefore permits compensation for
`temperature variation during measurement, ‘since the
`photoelastic and thermoptic effects are wavelength
`dependent. In other words, by superimposing two or
`more gratings at each location of interest, two or more
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`Page 6
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`Page 6
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`

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`UNITED STATES PATENT AND TRADEMARK OFFICE‘
`
`CERTIFICATE OF CORRECTION
`
`PATENT NU.
`
`: 4,807,950
`
`DATED
`
`: February 28, 1989
`
`INVENTURKS)
`
`2 William H. Glenn et al
`
`It is certified that error appears in the above-identified patent and that said Letters Patent is hereby
`corrected as shown below:
`
`On the title page:
`
`Item (54)
`
`"METHOD FOR I:-IPRESSING GRATINGS
`
`WITHIN FIBER OPTICS" should be
`
`'
`
`«[514] OPTICAL FIBER WITH IMPRESSED
`
`REFLECTION GRATINGS--
`
`Column 1, Lines 1 and 2
`
`"METHOD FOR IMPRESSING GRATINGS WITHIN FIBER
`
`OPTICS" should be ——OPTICAL FIBER WITH IMRESSED
`
`REFLECTION GRATINGS—-
`
`Signed and Sealed this
`
`Tenth Day of March, 1992
`
`Anun
`
`Arresting Ojficer
`
`Commissioner of Parents and Trademarks
`
`HARRY F. MANBECK, JR.
`
`Page 7
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`Page 7