`for relative or absolute movement detection, and methods for
`4
`
`fabrication the same
`
`Inventor: Boris Kobrin, Penfield, NY
`
`Field of Search.................... ..356/356,345,351,358,363;
`250/237
`
`385/37,49,50,122,10
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`-1-
`
`Mitchell D.K., Thorburn W.G. “Apparatus for detecting relative movement wherein a detecting
`means is positioned in the region of natural interference” U.S. Patent 5,486,923, Jan.23, 1996
`
`Mitchell D.K., Thorburn W.G. “Relative movement detecting apparatus using a detector
`positioned in a region of natural interference” U.S. Patent 5,646,730, Jul.8,1997
`
`Mitchell D.K. “Apparatus for detecting relative movement” U.S.Patent 5,559,600, Sep.24,l996
`
`Remijan P.W. “Apparatus for position encoding” U.S. Patent 4,542,989, Sep.24,1985
`
`Remijan P.W. “Apparatus for position encoding” U.S. Patent 4,395,124, Jul.26,1983
`
`lshizuka K., Kaneda Y. “Rotary encoder measuring substantially coinsiding phases of
`interference light components” U.S. Patent 5,661,296, Aug.26,l997
`
`Pettigrew R.M. “Displacement measuring apparatus and method” U.S. Patent 4,776,701, Oct.l1,
`1 988
`
`Yeung P.C. “Fiber optic rotation rate encoder”, U.S. Patent 4,767,164, Aug.30,l 988
`
`Lenox T.G. “Cylindrical encoder for use with fiber optics”, U.S. Patent 4,240,006, Dec. 1 6,1980
`
`Urbanik P.J. “Optical position sensor including a specially designed encoder plate”, U.S. Patent
`4,442,423, Apr.10,1984
`
`Senuma T., et. all. “Wavelength-division multiplex digital optical position sensor”, U.S. Patent
`5,498,867, Mar 12,1996
`
`Zimmerman B.D. et.al. “Fiber optic strain gauge patch” U.S. Patent 5,649,035 Jul.l5, 1997
`
`
`
`Confidential and proprietary
`ILLUMINA, INC. EXHIBIT 1023
`
`P3991
`
`Page 1
`
`
`
`Linear, Rotational, and Conformal encoders, based on Fiber Gratings,
`for relative or absolute movement detection, and methods for
`
`fabrication the same
`
`Inventor: Boris Kobrin, Penfield, NY
`
`Bieren K. et. al. “Conformal fiber optic strain sensor” U.S. Patent 5,201,015 Apr.6, 1993
`
`-2-
`
`
`
`
`
`Udd E. et.al. “Sensor systems employing optical fiber gratings” U.S. Patent 5,397,891 Mar.
`
`14,1995
`
`Glenn W. et.al. “Fiber optic sensor arrangement having gratings responsive to particular
`
`wavelengths” U.S. Patent 4,950,883 Aug.2l , 1990
`
`Wanser K., Voss K.F. “Fiber optic displacement sensor for high temperature environment”,
`
`U.S. Patent 5,661,246, Aug.26,1997
`
`Kashyap R., et. al. “Method of forming a refractive index grating in an optical waveguide”
`U.S. Patent 5,377,288, Dec. 27, 1994
`
`Hill K. et.al. “Method of fabricating Bragg gratings using silica glass grating mask and mask used
`
`by same” U.S. Patent 5,367,588 Nov.22, 1994
`
`Bruesselbach H. et.al. “Optical waveguide with diffraction grating and method of forming the
`
`same” U.S.Patent 5,604,829 Feb.l 8, 1997
`
`Hill K. et.al. “Method of creating an index grating in an optical fiber and a mode converter using
`
`the index grating” U.S. Patent 5,104,209 Apr.l4, 1992
`
`Askins c.g. et. al. “Technique to prepare high-reflectance optical fiber Bragg gratings
`
`with single exposure in-line or fiber draw tower” U.S.Patent 5,400,422 Mar. 21, 1995
`
`Macomber, et.al. “Chirped grating surface distributed feedback semiconductor laser”, U.S. Patent
`
`5,241,556 Aug.3l, 1993
`
`Chesnoy, et. al. “Method of forming a dispersing grating in an optical fiber”, U.S. Patent
`
`5,655,040 Aug. 5, 1997
`
`Mizrahi et.al. “Article comprising a spatially varying Bragg grating in an optical fiber”, U.S.
`
`Patent 5,636,304 Jun. 3, 1997
`
`Epworth R.E., et.al “Chirped distributed Bragg Grating optical fibre filters”, U.S. Patent
`
`5,602,949 Feb.l 1, 1997
`
`Confidential and proprietary
`
`Page2
`
`Page 2
`
`
`
`Linear, Rotational, and Conformal encoders, based on Fiber Gratings,
`
`for relative or absolute movement detection, and methods for
`
`fabrication the same
`
`Inventor: Boris Kobrin, Penfield, NY
`
`/
`
`E1-Sherif M. “Process for producing a grating on an optical fiber”, U.S. Patent 4,842,405
`
`Jun.27, 1989
`
`Drummond T. et.al. “Direct write with microelectronic circuit fabrication”, U.S. Patent 5,132,248
`
`Jul.2 1 , 1 992
`
`Gerber J. et.al. “Method of forming a photomask for a printing plate with an ink jet”, U.S.Patent
`
`5,495,803 Mar 5,1996
`
`Haxell J. et.a1 “Dispersed pigmented hot melt ink” U.S. Patent 5,053,079 Oct. 1,1991
`
`Matsutoshi M. et.al. “Apparatus for lithography or intaglio printing”, U.S. Patent 4,454,810,
`
`Jun 1 9,1 984
`
`OTHER PUBLICATIONS
`
` 1. Model LS 106, J.Heidenhain GmbH, Traunreut, Germany
`
`
`2. Model L-104, Canon USA Inc.,New York, NY. 1 1042
`
`3. I Bennion et al. “UV —written in-fibre Bragg Gratings”, Tutorial Review, Optical and Quantum
`
`Electronics, 28,93-135, 1996
`
`4. R. Kashyap “Photosensitive Optical Fibers: Devices and Applications” Optical Fiber
`
`Technology 1,17-34 (1994)
`
`5. K.O. Hill et.al. Electronics Letters,26, 1270 (1990)
`
`6. C.G. Askins, et.al. “Fiber Bragg reflectors prepared by a single excimer pulse”,
`
`Optics Letters, 17(1 1), 833-835 (1992)
`
`7. J.L. Archambault, et.al. “High reflectivity and narrow bandwidth fibre gratings written by
`
`single excimer pulse”, Electronics Letters, 29(1), 28-29 (1993)
`
`8. RJ. Lemaire et.al. Electronics Letters, 29 1191, 1993
`
`Confidential and proprietary
`
`Page3
`
`Page 3
`
`
`
`Linear, Rotational, and Conformal encoders, based on Fiber Gratings,
`for relative or absolute movement detection, and methods for
`fabrication the same
`
`Inventor: Boris Kobrin, Penfield, NY
`
`-4-
`
`9. B. Horwitz “Diffractive technique improve encoder performance” Laser Focus World, Oct.
`1996
`
`10. P.A. Krug et.al. “Measurement of index modulation along an optical fiber Bragg grating”
`
`Optics Letters, 20(l7),1767 (1995)
`
`ll. Y.Xia et.al. J.Am.Chem.Soc. 117,3274,l994
`
`_
`
`ya,/
`
`12. Rogers J. et.al. “Using microcontact printing to generate amplitude photomask on the surface
`of optical fibers: A method for producing in-fiber gratings”, Appl.Phys.Lett., 70(l),6, Jan 1997
`
`FIELD AND SUMMARY OF THE INVENTION
`
`The Field of the present invention is relative and absolute position detection or movement
`detection devices (encoders). Linear, rotational, and conformal Fiber encoders with very long
`
`length or very big diameter are suggested. These devices use Fiber Gratings of different types to
`produce certain orders, interference fringes from which are detected and analyzed by poly—phase
`periodic detector. A number of embodiments of Fiber Grating Encoders are suggested, such as
`one using Fiber Bragg Grating (FBG) with refractive index modulation, Surface Relief Fiber
`Gratings, and Amplitude (or Amplitude-Phase) Fiber Grating. Different embodiments can use
`Uniform or Chirped period size along the length of the Grating. Different embodiments can use
`
`transmission, reflection, or Bragg angle blaze reflection schemes.
`
`BACKGROUND OF THE INVENTION
`
`Most commercially available position encoders are based on a glass scales which are
`transilluminated with an array of secondary gratings. The shadow of the gratings, forming Moire
`effect is analyzed with photodiodes giving information about relative position. For example such
`an encoder [1] provides a resolution down to 0.5 mm. Many systems, like proposed by Ishizuka
`[5,661,296] and Pettigrew [4,776,70l] involve separation of diffractive orders obtained from first
`diffraction grating which are then brought together and interfered giving fringe pattern which
`analyzed by photodetector. More advanced high resolution encoders, for example [2], are based
`on the diffraction of an illuminating beam at a grating and detection of the interference pattern of
`
`selected diffraction orders. Position encoder of Remijan [4,3 95,124] and [4,542,989] detects a
`fringe pattern, created by diffraction of zero, plus first and minus first orders from phase
`diffraction grating. Light from He-Ne laser is collimated and focused at a focal point at a distance
`
`from grating. The spherical wave illuminates the grating, designed and fabricated to diffiact equal
`
`Confidential and proprietary
`
`Page4
`
`
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`Page 4
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`
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`Linear, Rotational, and Conformal encoders, based on Fiber Gratings,
`for relative or absolute movement detection, and methods for
`
`fabrication the same
`
`Inventor: Boris Kobrin, Penfield, NY
`
`
`
`intensity 0 and +/-1 orders. Zero order cone interferes with plus and minus cones separately
`
`giving fringe patterns. If the grating is moved in a plane that is perpendicular to the direction of
`the fringes, all the fringes appear to slide in the plane of photodetector, providing a possibility to
`
`encode a position. Mitchel [5,486,923], [5,646,730] and [5,559,600] uses phase grating with
`
`minimized zero order. A poly-phase periodic detector is spaced close to the grating so that each
`
`detector phase or element responds principally to interference between the positive and negative
`first orders diffracted from the grating without intermediate reflection or magnification.
`
`-5-
`
`Three types of optical encoders, which use fibers, are known. The first type uses fibers only as
`a light delivery device. The advantage of using optical fibers in existing encoders was that fibers
`
`allow the electrical elements of the light source and decoding circuitry to be remotely located from
`
`the code plate. For example, Yeung [4,767,l64] disclosed rotational fiber optic encoder. The
`
`system utilizes optical fiber to transmit light to a pair of interrupter disks, and to collect light
`reflected from disks. Both disks are provided with alternating reflected and transmissive parts.
`Decoding circuitry is provided to convert the modulated component of the light signal to an
`
`electrical signal that represents the rotational speed of the wheel. Lenox [4,240,066] described
`the cylindrical encoder for use with fiber optics, which contains of transmitting head of fibers,
`receiving head of fibers and code plate with windows that can slide between the heads.This
`
`enables the optical signals to be picked up reliably, but still requires bundle of individual fibers
`
`for transmitting head and bundle of fibers for receiving head, that makes system very
`complicated. Urbanik [4,442,423] describes fiber optic position sensor, which employs 4 fibers
`
`for transmitting and 4 for receiving light passed through code plate with windows. Senuma
`
`[5,498,867] suggested wavelength and time-division multiplexing to distribute light pulses to
`
`different collimators in encoder, and thus employ only one light source and one optical fiber. The
`limitation of position resolution (tens of micron) is due to code plate technique.
`
`
`
`The second type of fiber encoders uses fibers for measuring strain in material. Zimmerman B.D
`
`et. al [5,649,035] disclosed fiber optic sensor with the two reflective markers. This fiber is
`
`attached or embedded into the structure of material. An optical signal is input into the fiber and
`
`reflected at reflective markers at predetermined positions in the fiber. The time delay of the
`
`signals received back is analyzed to calculate strain in the structure. Bieren K. et.al [5,20l,015]
`
`suggested conformal fiber optic strain sensor, where fiber is attached between two points of
`
`connection under tension. An interferometer is formed in the tensioned portion of the fiber. The
`sensor is mounted to a surface and changes in interference patterns output by the interferometer
`are monitored to measure strain in the surface.
`
`The third type of fiber encoders, for example one has been disclosed by Udd E. et. al.
`
`[5,397,891] uses fiber gratings to sense strain that can vary the spacing between the lines of the
`grating to vary center wavelength of the reflection or transmission spectra. Strain can also change
`
`Confidential and proprietary
`
`Page5
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`Page 5
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`
`
`Linear, Rotational, and Conformal encoders, based on Fiber Gratings,
`for relative or absolute movement detection, and methods for
`
`fabrication the same
`
`Inventor: Boris Kobrin, Penfield, NY
`
`
`
`the relative distance between two gratings written in one fiber, each at a different end, and thus
`change the resonance build up of light at certain wavelengths. The last approach was
`
`demonstrated by Glenn W. et. al. [4,950,883].
`All abovementioned types of fiber encoders have a limitation of position encoding length.
`At some extend more advanced approach has been chose by Wanser [5,66l ,246], which used
`
`bending characteristics of fiber. His Fiber Optic Displacement sensor measures the distance
`between a fixed point and movable location using light loss characteristics of bent optical fiber.
`
`The sensor take advantage of the specific shape that the fiber assumes upon changing the distance
`
`between the two attachment points. Reproducibility of this method requires well defined
`
`boundary conditions of the holders of the fibers. The effect is highly non-linear with the largest
`contributions in the regions of smallest bend radii and fastest change of bend radii. The sensitivity
`
`of this method is about 1 mdB/um and since values like 10 mdB can be measured, position
`
`resolution of this method is not exceeds 10 um. Moreover, it is obvious, that this method will not
`
`work for very long length of fiber.
`
`To date, all diffraction-based linear or rotary optical encoders have been made on flat plates (glass
`
`or other material) and thus:
`
`1. Their sizes are limited by substrate size availability (maximum about 20”).
`
`2. The price of such substrates grows almost exponentially with their sizes.
`
`3. The surface of the motion controlled elements (we don’t consider strain control, but motion)
`
`has to be plane (non-conformal).
`
`
`
`
`OBJECTS AND ADVANTAGES OF THE INVENTION
`
`In accordance with the present invention, for the first time a Fiber Grating is suggested as a
`
`diffractive element of encoder. Depend on the variety of proposed embodiments this Fiber
`
`Grating could mean:
`
`1. Fiber Bragg Grating (refractive index modulation grating)
`
`2. Fiber Surface Relief Phase Grating,
`
`Confidential and proprietary
`
`Page6
`
`Page 6
`
`
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`for relative or absolute movement detection, and methods for
`fabrication the same
`
`,/
`
`Linear, Rotational, and Conformal encoders, based on Fiber Gratings,
`4
`
`Inventor: Boris Kobrin, Penfield, NY
`
`3. Fiber Amplitude or Amplitude-Phase Grating
`
`All above listed Fiber Gratings may have Uniform or Chirped period.
`
`All above listed Fiber grating encoders can be implemented using transmission, reflection, or
`
`Bragg angle reflection schemes.
`
`An object of the invention is to provide an encoder without reasonable limitation of it’s
`sizes.
`
`Since Fiber, which is suggested to use as a substrate for diffractive element, is manufactured on
`
`continuos bases by drawing it from preform into multi-kilometer length wheels, there is no
`
`reasonable limitation of the length of linear fiber-based encoders. As soon as Fiber Grating can be
`
`mounted on circular symmetric figure with any large diameter, there is no limitation on the size of
`
`rotary encoder either.
`
`A further object of the invention is to provide encoder for conformal parts motion
`control.
`
`As soon as fiber can be mounted on any shape figures, it can be used for conformal parts motion
`encoders.
`
`A principal advantage of the invention is that cost of large size diffraction-based
`
`encoders can be significantly reduced.
`
`Optical polished substrates cost grow almost exponentially with it’s area or length, whereas the
`cost of optical fiber (per linear unit) is constant or even decrease with the total length.
`
`
`
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Fig.1 is a schematic representation of the transmission mode Fiber Grating linear encoder a)
`
`and spatial distribution of different diffraction orders b) .
`
`Fig.2 shows a Uniform Fiber Bragg grating structure
`
`Fig. 3 is a schematic representation of the reflection mode Fiber Grating linear encoder a) and
`
`spatial distribution of different diffraction orders b).
`
`Confidential and proprietary
`
`Page?
`
`,
`
`Page 7
`
`
`
`Linear, Rotational, and Conformal encoders, based on Fiber Gratings,
`for relative or absolute movement detection, and methods for
`
`
`
`fabrication the same
`
`Inventor: Boris Kobrin, Penfield, NY
`
`Fig.4 shows a Chirped Fiber Bragg Grating structure
`
`Fig.5 is a schematic representation of the Bragg reflection mode Fiber Grating encoder in
`transmission scheme.
`
`Fig.6 shows a Surface relief Uniform Fiber Grating structure (one side and entire surface
`patterned)
`
`Fig.7 shows a Surface relief Chirped Fiber Grating structure (one side and entire surface
`
`patterned)
`
`Fig.8 is a representation of the optical configuration for the Surface relief Fiber Grating
`fabrication by laser ablation of fiber protective layer..
`
`Fig.9 is s representation of the two ink-jet surface relief fiber grating fabrication methods.
`
`Fig.l0 is a representation of the two microcontact lithography techniques for fabrication of
`surface relief fiber gratings
`
`Fig. 11 is a representation of the example of fiber fixturing for Rotational Fiber Grating
`encoder
`
`Fig.12 is a representation of the example of fiber fixturing for Conformal Fiber Grating
`encoder.
`
`
`
`DETAILED DESCRIPTION
`
`The main idea of the present invention is to use Fiber Grating as a diffractive element for
`encoding a relative or absolute position. It gives a possibility of disign and manufacturing a very
`long lengths linear encoders or very big diameter rotational encoders, or arbitrary shaped surface
`encoders. Number of implementations of such an encoder can be used».
`
`Fiber Bragg Grating with uniform period of index modulation
`(transmission or reflection schemes).
`
`The optical configuration of the encoder, that employs Fiber Bragg grating, is shown on Fig.la.
`Coherent light with the wavelength 9». from the light source 1 is collimated by the lens 2 onto of
`the fiber 3. The fiber 3 is stretched and fixed into the movable carrier 4. The Fiber Bragg Grating
`
`P
`
`8 Confidential and proprietary
`
`399 .
`
`Page 8
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`
`
`Linear, Rotational, and Conformal encoders, based on Fiber Gratings,
`for relative or absolute movement detection, and methods for
`
`fabrication the same
`
`Inventor: Boris Kobrin, Penfield, NY
`
`
`
`
`
`(Fig.2, Where 5 is a core, 9- cladding layer, and 8- polymer protective layer-jacket) with
`
`refractive index modulation An is written inside a core of the fiber. Light collimated on fiber is
`
`diffracted by the grating, different diffraction orders come out of the fiber. The light scattered on
`
`the cylindrical surface of the cladding is collimated on the fiber axis plane by the cylindrical lens
`
`6. The milti-phase detector 7 is placed on the distance DO, where the natural interference fringe
`
`pattern is created. Fig. lb demonstrates a relation between spot size S, period of the grating P ,
`
`and the region of the best fringe contrast cl < D0< D, where D = S/2tan 6 is the maximum
`
`distance from the grating for plus and minus first order interference, d = S/2tan(p is the maximum
`
`distance from the grating for plus and minus third orders interference.
`
`For example for the 0.78p.m laser diode source and period of the grating P=50um, G is equal to
`
`0.89°, and (p is equal to 2.68°. Using 0.5 mm spot size we have got D= 16.1 mm and d= 5.3
`
`mm. The period size of the fringe pattern will be 25 um, so poly-phase detector should have a
`
`pitch of 25 um, which is available on the market.
`
`The Fiber Bragg grating parameters An and period P can be implemented in order to minimize
`
`Zero order from the grating in order to get high contrast fringe pattern. It means providing
`
`Optical Path Difference (OPD) equal to M2 . OPD for Fiber Bragg grating can be estimated fiom
`
`the following equation OPD= dcx An , where do is a core diameter, An is a refractive index
`modulation.
`
`If the carrier 4 moves slightly along the axis 2, fringe pattern is also shifting in the plane of the
`
`photodetector 7, which enable the electronic circuit to analyze and give information about the
`
`relative movement. This method of detecting and analyzing of interference fringes can provide
`
`resolution up to 0.6 nm [9]. Moreover, one can build a very long encoder due to the fact that fiber
`
`grating is available in a Very long lengths.
`
`Fabrication of such a long fiber gratings is possible by a few known techniques. One can use free-
`space holographic exposure [3,4], prizm-based interferometric exposure by method proposed by
`Kashyap [5,3 84,884], or phase mask near field holography, proposed by Hill [5,367,588] and
`
`Bruesselbach [5,604,829]. Long Fiber grating can be written by few (or many) consecutive
`
`exposures with the fiber feeding system between these exposures (linear translation or wheel to
`
`wheel). Special attention has to be paid to stitching between the consecutive feeding steps. More
`
`suitable method for fabricating a very long Fiber Gratings is probably the point-by-point method.
`
`This method was demonstrated by Hill et.al. [5,l04,209] and [5]. Fiber was exposed to the image
`
`of a slit produced by the excimer laser pulses, and the fiber was translated between pulses. Fiber
`
`Bragg Gratings 1.7 m long with period size of 155 um have been written [5,lO4,209]. Linewidths
`
`achievable by this method could be decreased further down to l um [5]. For better control of
`
`grating period one can use writing method with fiber translation with a constant speed, as
`
`described by Askins et.a1.[5 ,400,422] and [6] and Archambault et. al.[7]. The last methods use
`
`so called Fiber Gratings Type-II fabrication by the single excimer pulse. The total exposure time
`
`P
`
`age
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`9 Confidential and proprietary
`
`Page 9
`
`
`
`Linear, Rotational, and Conformal encoders, based on Fiber Gratings,
`for relative or absolute movement detection, and methods for
`
`fabrication the same
`
`Inventor: Boris Kobrin, Penfield, NY
`
`
`
`-1 0-
`
`is about 20 ns (the duration of the pulse). In this short time, a fiber feeding at speed 1 m/s is
`displaced by only 0.02 mm, which is small compared to a grating period. This technique allow
`writing Bragg Gratings in a fiber as it is being drawn from its preform, so enables to fabricate a
`fiber grating length in a kilometers range. Since the exposure technique proposed in [5,400,422] is
`the Free-space holographic, the drawback of this technique is that only Fiber grating with the
`length equal to interference pattern can be written on fiber.
`From this point of View the best method could be a single pulse Type-II point-to-point
`technique with the constant speed fiber feeding as a method for writing a Very long Fiber Bragg
`Gratings.
`Among the advantages of this approach are the follows:
`1. Better control of grating period: It is more easy to keep constant feeding speed than to get high
`accuracy of step-and-repeat positioning;
`2. More robust gratings, which do not degrade at high temperatures [3,4]. It is due to the fact that
`Type II gratings, produced by single excimer pulse, are the result of physical damage onto the
`core of the fiber, that is permanent. Contrary, Type I gratings produced by low power multi-shot
`holographic exposure, have been shown to decay being exposed to elevated temperatures[3,4]
`
`In order to get a high contrast interference fringe pattern grating should be written with the
`maximum refractive index modulation available. For the special optical communication fibers with
`
`core codoped with boron and GeO2 the values An up to 7xl0—4 were achieved. Lemare et al. [8]
`described a Very effective method of sensitization of fibers by soaking it in high-pressure
`hydrogen in the range of 20-750 atm at a temperatures in the range 20-75° C for a period of
`several days. The values of An easily exceeding 0.01 have been reported so far. So the fibers for
`Fiber Bragg encoders should be hydrogen sensitized before writing a grating in it.
`The high contrast of the interference fringes, created by the plus and minus first orders, can be
`achieved when zero order is suppressed efficiently. For Zero order suppression the phase grating
`has to be written with the optical path differences of M2, which for the diode laser source of 0.78
`
`mm gives the value 0.36 um. This optical path difference can be created by writing a Fiber Bragg
`
`grating with index of refraction modulation of 0.01 within a core diameter of 36 um.
`
`If the cylindrical lens has a focal length of 10 mm and focus is positioned approximatly at 37 mm
`in front of the fiber, the beam will be slightly divergent as it reach the air—glass interface.
`The curved surface of the cladding refocuses the beam. This focusing is optimum because it
`
`yields a well-defined beam at the fiber core.
`
`It is apparent that the same device can work in a reflection mode (Fig.3a). In the last case optical
`path difference of M4 has to be obtained. It means for our example, M4 = 0.18 um for 0.78 mm
`
`P
`
`age
`
`10 Confidential and proprietary
`
`
`
`Page 10
`
`
`
`Linear, Rotational, and Conformal encoders, based on Fiber Gratings,
`for relative or absolute movement detection, and methods for
`
`fabrication the same
`
`Inventor: Boris Kobrin, Penfield, NY
`
`
`
`laser source wavelength. Such an OPD can be obtained with the index of refraction modulation of
`
`0.01, written in a core of 18 um diameter.
`
`-11-
`
`Chirped Fiber Bragg Grating
`
`(transmission or reflection schemes)
`
`Another embodiment of the present invention uses Chirped Fiber Bragg grating for encoding a
`
`position (Fig.4). Chirped grating l0 is a non-uniform grating with the period changing along the
`length gradually or step—wise. In this case, interference fringe pattern not only moves in detector
`
`plane with the movement of the grating relative to laser beam, but also changes it’s frequency.
`
`Thus, an absolute position can be deduced from the fringe pattern frequency data.
`
`Chirped Fiber Bragg gratings can be manufactured by certain adjustments of holographic set-up
`as per Macomber [5,238,53 1]. Limitation of length of the grating (less than 150 mm) is reported.
`
`Alternatively, chirped Bragg grating can be written with phase mask scheme as per Chesnoy,
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`et.al. [5,655,040. Interference fringe pattern, created by diffraction of laser beam on phase mask,
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`is delivered to fiber by optical system with variable focal length. Changing a focal length of the
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`system cause chirp in Bragg grating period size. Mizrahi et. al. [5,636,304] described a method of
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`fabrication a chirp by changing a wavelength of the laser during a writing process with phase
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`mask or holographic.The chirped grating length is limited by the length of the phase mask or
`holographic interference pattern. Epworth R.E.et.al [5,602,949] suggested a method of chirped
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`grating fabrication based on applying a non—uniform strain on uniformly written fiber grating,
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`which changes the period of grating locally.
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`All abovementioned methods of fabricating a chirped fiber grating can not help with
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`manufacturing a very long fiber grating. It can be done by changing a fiber feed rate gradually
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`during a single exposure, point-to-point writing.
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`
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`Chirped Fiber Bragg grating
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`(Bragg angle blazing scheme)
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`Further embodiment of the present invention uses a Bragg angle blazing from Chirped Fiber
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`Bragg grating (Fig.5).The probe beam is directed at an incident angle 6; onto the surface of the
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`fiber. In the case of the first order Bragg reflection the transmitted and the reflected beams leave
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`the fiber at the opposite sides of the fiber at equal angles +/-0,. These angles are related to index
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`modulation and retroreflected wavelength by the following equation [10]:
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`sin 6, = sin 9, = 7»/2P= NB 7»/KB
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`Confidential and proprietary
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`Page 11
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`Page 11
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`Linear, Rotational, and Conformal encoders, based on Fiber Gratings,
`for relative or absolute movement detection, and methods for
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`fabrication the same
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`Inventor: Boris Kobrin, Penfield, NY
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`-12-
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`N3 is the effective index of the fiber at a retroreflected Bragg wavelength KB.
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`7» is a probe beam wavelength, P— grating period.
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`The grating is written with the gradient of period from P0 to P0 + AP within the length of
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`moving part L. The detector collects the signal of Bragg reflected beam at the angle 9,,
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`correspondent to the period PB=P0+AP/2. Index modulation An is obtained to maximize the
`intensity of the first order Bragg reflection for this period. When the laser beam is reflected from
`eather left or right parts of the Chirped fiber Bragg grating (with the period that is differ fromPB) ,
`the signal of the reflected light is dropped by a certain value, and thus absolute position of grating
`can be obtained. For example, Chirped Fiber Bragg grating written with the
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`period P= 445 +/- 50 nm has the central Bragg Wavelength value 7tB=l290.5 nm. Since
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`NB=l .45 the incidence angle for Bragg reflection of He-Ne laser beam is 9;=45.3. The lens
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`collimates the Bragg reflection beam which comes out of the grating with angle 6, = 45.3
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`The signal has a maximum only for the period of grating P0=445 nm. As the fiber is moving
`against the beam, period of illuminated part of the grating is changing and the signal is dropped.
`The absolute position can be obtained from the precalibrated curve.
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`Surface Relief (uniform or chirped) Fiber Grating
`(transmission, reflection , or Bragg angle blazing schemes)
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`Further embodiment of the present invention employs Surface relief Fiber grating (Fig. 6).
`The configurations of the device in transmission (Fig. 1), reflection (Fig.3) or Bragg angle blazing
`(Fig. 5) can be used with the Uniform (Fig.2) or Chirped (Fig.7) period. The required depth of
`relief and period size of grating can be obtained by applying a lithography methods, including
`resist coating, holographic exposure and development of resist pattern. The requirements of
`temperature stability and robustness of the encoder can be achieved by etching of the grating
`into the cladding layer (or core, as per El-Sherif et.a1. [4,842,405]).
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`Only one side patterning (Fig.6,7a) or entire fiber cylindrical surface patterning(Fig.6,7b) can be
`performed by holographic methods [4,842,405]. Limitation of the length of such a grating is
`obvious due to holographic type of exposure.
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`I suggest three different methods for Surface Fiber grating fabrication.
`The first method (Fig.8) use ablation of existing polymer jacket in order to form protective mask
`for further wet or dry etching into the cladding. A beam of Excimer laser 14 is reflected by mirror
`15 and focused by cylindrical lens 16 on fiber 17. Fiber is feeding with the constant or variable
`speed by means of synchronized rotational system 18 (Wheel to wheel scheme) or high quality
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`P
`
`age
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`12 Confidential and proprietary
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`Page 12
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`
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`Linear, Rotational, and Conformal encoders, based on Fiber Gratings,
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`for relative or absolute movement detection, and methods for
`fabrication the same
`
`_
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`
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`/
`
`Inventor: Boris Kobrin, Penfield, NY
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`-13-
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`translational system 19. Excimer pulse energy is chosed above ablation threshold for fiber jacket
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`material. The result is the fiber, patterned with polymer jacket grating. Then the grating is etched
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`into the cladding or core layer through the polymer jacket mask pattern by means of wet chemical
`etching or plasma etching. Finally, the jacket layer is stripped in a chemical stripper or in an
`oxygen plasma asher.
`The method of fabricating Surface Fiber Chirped grating is similar to the uniform, but the fiber
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`feeding speed in varied gradually from the one end of the fiber to another. One can manufacture
`Very long (length is limited to capacity of the wheel) gratings with explained method.
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`
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`The second proposed method for Surface Relief Grating fabrication (Fig.9 a and b) use ink jet
`printing technique. Description of this technique can be find in application to microelectronic
`circuit fabrication, Drummond et.al. [5,l32,248] and to printing plates fabrication, Gerber et.al.
`
`[5,495,803]. In the first method the material is deposited by providing a colloidal suspension of
`the material and directly writing the suspension onto the substrate surface using orifice printhead
`and piezoelectric driven jet system. Following the printing step, the deposited material is dryed
`by lamps and thermally, preferably laser, armealed. Then excess material can be removed from the
`substrate by washing. The process is suitable for depositing any material which can be formed as
`a colloid either mechanically or chemically. Thus, pure metals, such as Au, Cu, Pt, and Ag, as
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`well as refractory metals, such as W, Ta, M0 or SiC can be deposited by this method.
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`It was deminstrated. The width of the resulted lines in the pattern can be varied by varying
`particle size of the colloidal suspension, orifice size of the ink jet printing nozzle, as well as laser
`armealing step (laser focussing and energy level). Combination of such ink jet printing with laser
`annealing technique gives a possibility to obtain high resolution structures, down to submicron
`sizes.
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`According to another modification of ink