United States Patent [19]
`Kobrin
`
`111111
`
`1111111111111111111111111111111111111111111111111111111111111
`US006087655A
`[11] Patent Number:
`[45] Date of Patent:
`
`6,087,655
`Jui. 11,2000
`
`[54] FIBER GRATING ENCODERS AND
`METHODS FOR FABRICATING THE SAME
`
`[76]
`
`Inventor: Boris Kobrin, 186 Willow Pond Way,
`Penfield, N.Y. 14526
`
`[21] Appl. No.: 09/081,286
`
`[22] Filed:
`
`May 19, 1998
`
`Related U.S. Application Data
`[60] Provisional application No. 60/061,860, Oct. 15,1997.
`
`Int. CI? .................................. HOlJ 3/14; FOlD 5/34
`[51]
`[52] U.S. CI. .................................. 250/237 G; 250/231.18
`[58] Field of Search ......................... 250/231.18,231.14,
`250/231.13, 227.21, 237 G
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`5,760,391
`5,869,835
`5,892,582
`
`6/1998 Narendran .......................... 250/227.14
`2/1999 Udd .................................... 250/227.18
`4/1999 Bao ......................................... 356/345
`
`Primary Examiner-Edward P. Westin
`Assistant Examiner---Glenn T Kinnear
`Attorney, Agent, or Firm---Harter, Secrest & Emery LLP;
`Stephen B. Salai, Esq.; Brian B. Shaw, Esq.
`
`[57]
`
`ABSTRACT
`
`Very long linear large diameter rotational, and arbitrary
`shape conformal fiber encoders are suggested. These devices
`are based on detection of non-zeroth diffraction order or
`interference pattern of selected diffraction orders from the
`fiber grating. Relative and absolute position detection or
`movement detection can be realized. Depending on the
`variety of disclosed configurations of fiber encoders, fiber
`grating could be either a fiber Bragg grating (refractive
`index modulation grating), a fiber surface-relief phase
`grating, or fiber amplitude or amplitude phase grating. Each
`of these fiber gratings may have a uniform or chirped period,
`and the fiber grating encoders may be implemented using
`transmission, reflection, or Bragg angle reflection schemes.
`An optical fiber may be manufactured on a continuous basis
`by drawing it from preform. Consequently, there is really no
`limitation to the length of the linear fiber based encoders. In
`addition, since fiber grating can be mounted on a circularly
`symmetric figure of arbitrary diameter, there is therefore no
`limitation on the manufacturable size of a rotary encoder.
`Due to flexibility of fibers, the proposed gratings can be
`mounted on a surface of arbitrary shape, thereby enabling
`optical motion encoders to be fabricated for conformal
`surfaces. Some manufacturing techniques are disclosed.
`
`46 Claims, 16 Drawing Sheets
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`2
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`6
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`Page 1
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`Sheet 1 of 16
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`FIG.IA
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`26 7
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`Page 2
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`Sheet 2 of 16
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`FIG 2
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`Page 3
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`Sheet 3 of 16
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`FIG 3A
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`Page 4
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`Sheet 4 of 16
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`FIG. 4
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`Page 5
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`Sheet 5 of 16
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`3
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`4
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`FIG. 5
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`Page 6
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`Sheet 6 of 16
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`FIG. 6A
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`FIG 68
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`Page 7
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`Sheet 7 of 16
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`FIG.7A
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`FIG 78
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`Page 8
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`Sheet 8 of 16
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`14
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`Page 9
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`Sheet 9 of 16
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`29
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`28
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`FIG. 9A
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`FIG. 98
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`Page 10
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`Sheet 10 of 16
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`37
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`FIG. IDA
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`FIG. lOB
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`Page 11
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`Sheet 11 of 16
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`39
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`FIG.IIA
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`Page 12
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`Sheet 12 of 16
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`FIGI2A
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`Page 13
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`Sheet 13 of 16
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`FIG. /3A
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`Page 14
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`Sheet 14 of 16
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`14
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`Page 15
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`Sheet 15 of 16
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`FIG 15A
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`Page 16
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`Sheet 16 of 16
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`6,087,655
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`25
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`36
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`FIG. 16A
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`FIG. 168
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`Page 17
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`6,087,655
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`1
`FIBER GRATING ENCODERS AND
`METHODS FOR FABRICATING THE SAME
`
`This application claims benefit of Provisional Applica(cid:173)
`tion Ser. No. 60/061,860, filed Oct. 15, 1997.
`
`FIELD 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 diameters are suggested.
`These devices use Fiber Gratings of different types to
`produce diffraction orders, which are detected and analyzed
`by light 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. Differ(cid:173)
`ent embodiments can use transmission, reflection, or Bragg
`angle blaze reflection schemes.
`
`BACKGROUND OF THE INVENTION
`
`Most commercially available position encoders are based 25
`on glass scales which are transilluminated with an array of
`secondary gratings. The shadow of the gratings, forming
`Moire effect is analyzed with photodiodes giving informa(cid:173)
`tion about relative position. For example such an encoder
`[Model LS 106,1. Heidenhain GmbH, Traunreut, Germany] 30
`provides a resolution down to 0.5 mm. Many systems, like
`proposed by Ishizuka [U.S. Pat. No. 5,661,296] and Petti(cid:173)
`grew [U.S. Pat. No. 4,776,701] involve separation of dif(cid:173)
`fractive orders obtained from first diffraction grating which
`are then brought together and interfered giving fringe pattern 35
`which analyzed by photodetector. More advanced high
`resolution encoders, for example [Model L-104, Canon USa
`Inc., New York, N.Y. 11042], are based on the diffraction of
`an illuminating beam at a grating and detection of the
`interference pattern of selected diffraction orders. Position 40
`encoder of Remijan [U.S. Pat. No. 4,395,124] and [U.S. Pat.
`No. 4,542,989] detects a fringe pattern, created by diffrac(cid:173)
`tion 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 45
`spherical wave illuminates the grating, designed and fabri(cid:173)
`cated to diffract equal 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 50
`appear to slide in the plane of photodetector, providing a
`possibility to encode a position. Mitchell [U.S. Pat. Nos.
`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 55
`element responds principally to interference between the
`positive and negative first orders diffracted from the grating
`without intermediate reflection or magnification.
`Three types of optical encoders, which use fibers, are
`known. The first type uses fibers only as a light delivery 60
`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 [U.S. Pat. No.
`4,767,164] disclosed rotational fiber optic encoder. The 65
`system utilizes optical fiber to transmit light to a pair of
`interrupter disks, and to collect light reflected from disks.
`
`2
`Both disks are provided with alternating reflected and trans(cid:173)
`missive 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.
`5 Lenox [U.S. Pat. No. 4,240,066] described the cylindrical
`encoder for use with fiber optics, which contains of trans(cid:173)
`mitting 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
`10 requires bundle of individual fibers for transmitting head and
`bundle of fibers for receiving head, that makes system very
`complicated. Urbanik [U.S. Pat. No. 4,442,423] describes
`fiber optic position sensor, which employs 4 fibers for
`transmitting and 4 for receiving light passed through code
`15 plate with windows. Senuma [U.S. Pat. No. 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
`20 code plate technique.
`The second type of fiber encoders uses fibers for measur(cid:173)
`ing strain in material. Zimmerman B. D et. al [U.S. Pat. No.
`5,649,035] disclosed fiber optic sensor with the two reflec(cid:173)
`tive 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 posi-
`tions in the fiber. The time delay of the signals received back
`is analyzed to calculate strain in the structure. Bieren K. et
`al [U.S. Pat. No. 5,201,015] 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 sur(cid:173)
`face.
`The third type of fiber encoders, for example one has been
`disclosed by Udd E. et. al. [U.S. Pat. No. 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
`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. [U.S. Pat. No. 4,950,
`883].
`All above-mentioned types of fiber encoders have a
`limitation of position encoding length.
`At some extent more advanced approach has been chosen
`by Wanser [U.S. Pat. No. 5,661,246], which uses 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 takes 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/
`mm and since values like 10 mdB can be measured, position
`resolution of this method does not exceed 10 mm. Moreover,
`it is obvious, that this method will not work for very long
`length of fiber.
`To date, all diffraction-based linear or rotational optical
`encoders have been made on flat plates (glass or other
`material) and thus:
`
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`6,087,655
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`3
`1. Their sizes are limited by substrate SIze availability
`(maximum about 20").
`2. The price of such substrates grows almost exponen(cid:173)
`tially with their sizes.
`3. The surface of the motion controlled elements (we 5
`don't consider strain control, but motion) has to be
`plane (non-conformal).
`Few known optical encoders, based on flexible metal
`tapes, can't provide high positioning accuracy due to non(cid:173)
`flat surface and strong temperature dependence.
`
`10
`
`4
`FIGS. 4A and 4B are schematic representations of the
`reflection mode Fiber Grating linear encoders with off-axis
`position of detector: embodiment V (4A) and VI (4B);
`FIG. 5 is a schematic representation of Absolute position
`Fiber Grating encoder, which uses chirped fiber grating,
`embodiment VII;
`FIG. 6 is a schematic representation of Absolute position
`Fiber Grating encoder, which uses Bragg reflection scheme,
`embodiment VIII
`FIGS. 7A, 7B and 7C are schematic representations of
`Rotational Fiber Grating encoder: side view (7 A) and top
`view (7B) and mounting option (7C)
`FIGS. SeA) and (B) are schematic representations of
`15 Conformal Fiber Grating encoders: rotational elliptical (SA)
`and translational/rotational conical (SB).
`FIGS. 9(A) and (B) are schematic representations of
`Conformal Fiber Grating encoder (complicated motion).
`FIG. 10 is a schematic representation of Linear Fiber
`20 encoder with improved positioning accuracy.
`FIGS. llA and lIB are schematic representations of
`Uniform (llA) and chirped (lIB) Fiber Bragg grating
`structure
`FIGS. 12A and 12B are schematic representations of
`Uniform (12A) Surface relief Fiber grating, one side pat(cid:173)
`terned (12A) and entire surface patterned (12B);
`FIGS. 13A and 13B are schematic representations of
`Chirped Surface relief Fiber grating, one side patterned
`(13A) and entire surface patterned (13B);
`FIGS. 14A and 14B) are a representation of the optical
`configuration for the Surface relief Fiber Grating fabrication
`by laser ablation of fiber protective layer.
`FIGS. 15A and 15B are a representions of the two ink-jet
`surface relief fiber grating fabrication methods.
`FIGS. 16A and 16B are representations of the two micro(cid:173)
`contact lithography techniques for fabrication of surface
`relief fiber gratings
`
`SUMMARY OF THE INVENTION
`In accordance with the present invention, for the first time
`a Fiber Grating is suggested as a diffractive element (scale)
`of optical encoder. Depending on the variety of proposed
`embodiments this Fiber Grating means:
`1. Fiber Bragg Grating-phase grating-refractive index
`modulation grating encrypted in the core of the fiber;
`2. Fiber Surface Relief Grating-phase grating, created
`on a layer of transparent material, deposited on the fiber
`jacket or cladding, or
`on fiber polymer jacket, or
`on fiber cladding layer, or
`on fiber core.
`3. Fiber Amplitude or Amplitude-Phase Grating- 25
`Grating, made with opaque (metal) pattern layers on
`fiber jacket, fiber cladding layer, or fiber core surfaces.
`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 30
`angle reflection schemes.
`An object of the invention is to provide an encoder
`without reasonable limitation of its sizes.
`Since Fiber, which is suggested to use as a substrate for
`diffractive element, is manufactured on continuous basis by 35
`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 diameter of rotary 40
`encoder either.
`A further object of the invention is to provide encoder for
`conformal parts motion control.
`Since fiber can be mounted on figures, having arbitrary
`shape, it can be used for conformal parts motion control. 45
`Flexibility of fiber mounting can be used for design of an
`encoder for conformal type of motion.
`A principal advantage of the invention is that cost of large
`size diffraction-based encoders can be significantly reduced.
`Optical polished substrates cost grows almost exponen- 50
`tially with the area or length, whereas the cost of optical fiber
`(per linear unit) is constant or even decrease with the total
`length.
`Other objects, features and advantages of the present
`invention will become apparent from the following detailed 55
`description of the preferred embodiments when taken III
`conjunction with the accompanying drawings.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`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 design and manu(cid:173)
`facturing very long length linear encoders or very big
`diameter rotational encoders, or arbitrary shaped encoders
`with complicated motion. Number of implementations of
`such an encoder can be used.
`Linear Fiber Grating Encoders
`FIG. 1 represents embodiment I, where laser 1 directs a
`spherical wave on a Fiber grating 3, mounted on a movable
`platform 4. The parameters of the grating is optimized for
`diffracting of 3 diffraction orders ("-I", "0", and "+1") with
`almost equal intensity. Moreover, diffraction angles are such
`that the zeroth order beam overlaps with both first and minus
`first order beams, while the first order beams don't touch
`each other. Diffraction fringes are produced in the areas of
`overlap. Detector 7 is positioned at least in one of the areas
`of overlap. If fiber is moved along its axis, fringes in the
`60 areas of overlap appear to slide through those areas. This
`fringe movement gives a position signal from the detector.
`FIG. 2 represents embodiment II, where fiber grating
`parameters are optimized for zeroth order suppression, and
`diffraction angles provide overlap between plus and minus
`65 first orders. The lens 2 focuses light on fiber grating. The
`light, scattered by the cylindrical surface of the fiber, is
`collimated on the fiber axis plane by the cylindrical lens 6.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a schematic representation of the transmission
`mode Fiber Grating linear encoder, embodiment I;
`FIG. 2 is a schematic representation of the transmission
`mode Fiber Grating linear encoder, embodiment II;
`FIGS. 3 and 3B are schematic representations of trans(cid:173)
`mission mode Fiber Grating linear encoders, which use
`off-axis position of detector, embodiments III (3A) and IV
`(3B):
`
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`6,087,655
`
`5
`The detector 7 is placed on the distance, where the natural
`interference fringe pattern is created, and detects fringe
`movement.
`Cylindrical shape of the fiber cause light scattering to
`direction perpendicular to the surface of the fiber. As a result 5
`of this scattering long vertical strips of diffraction orders
`beams are created, as shown on FIGS. 3A and 3B. In the
`embodiments III and IV detector is positioned off-axis
`(relative to light source) and detects the interference fringe
`pattern, which is free from undiffracted light from light
`source.
`Embodiments V and VI use reflection scheme of the
`embodiments III and IV, as shown on FIGS. 4A and 4B,
`where detector and laser source are positioned on the one
`side of the fiber.
`The example of an absolute position fiber grating encoder,
`embodiment VII, presented on FIG. 5. Chirped fiber grating
`is manufactured, having period size, varying monotonically
`through the length of the grating. When the fiber moves, first
`order slides through the detector area giving an information
`about a position of the fiber.
`Another example of absolute position fiber grating
`encoder, embodiment VIII, is given on FIG. 6. It uses a
`Bragg angle blazing from Chirped Fiber grating. The laser
`beam is directed at an incident angle 8i onto the surface of
`the fiber. In the case of the first order Bragg reflection the 25
`transmitted and the reflected beams leave the fiber at the
`opposite sides of the fiber at equal angles +/-8 r . These
`angles are related to period size of the grating Po as follows:
`
`When fiber moves period size of the illuminated part of
`the grating changes from Po, thus first order Bragg reflection
`condition is destroyed and signal of the reflected light is
`dropped by a certain value, so absolute position of the fiber
`can be obtained using calibration curve.
`Rotational Fiber Grating Encoders
`FIGS. 7A and B represent an example of rotational fiber
`encoder, embodiment IX, where fiber (3) is coined around a
`transparent hollow cylinder (4), having a given diameter.
`The laser (1) is mounted inside a cylinder, detector (7),
`which is positioned outside a cylinder off-axis (FIG. 7B),
`detects interference fringes from "-1" and "+1" orders, or
`from "-1" and "0" orders, depending on optical scheme
`chosen.
`Fiber can be spliced and attached to the part by stretching
`the two-part cylinder 40 with the treaded pin 41 (FIG. 7C).
`The laser source can be mounted outside a solid or opaque
`cylinder, thus reflection mode should be implemented,
`embodiment X.
`Conformal Motion Fiber Grating Encoders
`The fiber can be mounted on any conformal surface and
`work as a conformal surface motion encoder. FIGS. SA and
`SB demonstrates two examples of conformal type fiber
`encoders, rotational elliptical (FIG. SA), embodiment XI,
`and translational/rotational cone (FIG. SB) encoder, embodi(cid:173)
`ment XII. Both encoders shown having reflection mode
`scheme and off-axis detection position.
`Flexibility of fiber allows design of encoder, which is
`capable to follow and detect any given way of motion. Part
`shown on FIG. 9A) performs a combination of linear trans(cid:173)
`lation and 360 degree rotation movements. Fiber is mounted
`on the part according to the trajectory of part's movement.
`Light beam is focused to the point of movement's junction.
`Splicing of pieces of fiber in that point give a reference for
`absolute movement detection.
`Part shown on FIG. 9A) represents a Z-type of motion,
`that is a combination of linear motion and 180 degree
`rotation. Fiber is mounted on the surface of transparent
`movable plate.
`
`6
`Part shown on FIG. 9B) performs a combination of linear
`translation and 360 degree rotation movements.
`Linear Motion Fiber Encoder with Improved Positioning
`Accuracy
`Fiber is coiled around cylinder, cylinder is translating and
`rotating along its axis (embodiment XIII presented on FIG.
`10). This device is capable of translational movement detec(cid:173)
`tion with high accuracy. The accuracy is dependent on the
`pitch of fiber coiling around the cylinder (larger pitch, higher
`10 accuracy), and diameter of the cylinder (larger diameter,
`higher accuracy).
`Fiber gratings for use in fiber encoders, described above,
`can be implemented in a form of Phase gratings-Fiber
`Bragg grating or Surface relief grating, or Amplitude
`gratings-thin film opaque layer on fiber surface.
`15 Fiber Bragg Gratings for Optical Fiber Encoders
`A drawing of a uniform and chirped Fiber Bragg Gratings
`are given on FIGS. HA and HB, accordingly, where 5 is a
`core, 9-cladding layer, and 8-polymer protective layer(cid:173)
`jacket. Phase grating with refractive index modulation ll.ll is
`20 written inside a core of the fiber.
`Fabrication of fiber gratings is possible by few known
`techniques. One can use free-space holographic exposure [I
`Bennion et al. "UV-written in fibre Bragg Gratings", Tutorial
`Review, Optical and Quantum Electronics, 28, 93-135, 1996
`and R. Kashyap "Photosensitive Optical Fibers: Devices and
`Applications" Optical Fiber Technology 1, 17-34, 1994]],
`prism-based interferometric exposure by method proposed
`by Kashyap [U.S. Pat. No. 5,384,884], or phase mask near
`field holography, proposed by Hill [U.S. Pat. No. 5,367,588]
`30 and Bruesselbach [U.S. Pat. No. 5,604,829]. Long Fiber
`grating can be written by few (or many) consecutive expo(cid:173)
`sures 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.
`35 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. [U.S. Pat. No.
`5,104,209]. Fiber was exposed to the image of a slit pro(cid:173)
`duced by the excimer laser pulses, and the fiber was trans-
`40 lated between pulses. Fiber Bragg Gratings 1.7 m long with
`period size of 155 ,urn have been written [U.S. Pat. No.
`5,104,209]. Linewidths achievable by this method could be
`decreased further down to 1 ,urn. For better control of grating
`period one can use writing method with fiber translation
`45 with a constant speed, as described by Askins et al.[U.S. Pat.
`No. 5,400,422] and [CO G. Askins, et al. "Fiber Bragg
`reflectors prepared by a single excimer pulse", Optics
`Letters, 17(11),833-835 (1992)] and Archambault et. al.[J.
`L. Archambault, et al. "High reflectivity and narrow band-
`50 width fibre gratings written by single excimer pulse", Elec(cid:173)
`tronics Letters, 29(1), 28-29 (1993)]. The last methods use
`so called Fiber Gratings Type-II fabrication by the single
`excimer pulse. The total exposure time is about 20 ns (the
`duration of the pulse). In this short time, a fiber feeding at
`55 speed 1 m/s is displaced by only 0.02 ,urn, which is small
`compared to a grating period. This technique allows writing
`Bragg Gratings in a fiber as it is being drawn from its
`preform, so it enables the fabrication of a fiber grating length
`in a kilometers range. Since the exposure technique pro-
`60 posed in U.S. Pat. No. 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
`65 pulse Type-II point-to-point technique with the constant
`speed fiber feeding as a method for writing a very long Fiber
`Bragg Gratings.
`
`Page 20
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`
`6,087,655
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`5
`
`7
`Among the advantages of this approach are as 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 [I Bennion et al. "UV-written in fibre
`Bragg Gratings", Tutorial Review, Optical and Quan(cid:173)
`tum Electronics, 28, 93-135, 1996 and R. Kashyap
`"Photosensitive Optical Fibers: Devices and Applica(cid:173)
`tions" Optical Fiber Technology 1, 17-34, 1994]]. 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 holo(cid:173)
`graphic exposure, have been shown to decay being 15
`exposed to elevated temperatures [I Bennion et al.
`"UV-written in fibre Bragg Gratings", Tutorial Review,
`Optical and Quantum Electronics, 28, 93-135, 1996
`and R. Kashyap "Photosensitive Optical Fibers:
`Devices and Applications" Optical Fiber Technology 1, 20
`17-34,1994]]
`In order to get a high contrast interference fringe pattern
`grating should be written with the sufficient refractive index
`modulation. For the special optical communication fibers
`with core codoped with boron and Ge0 2 the values ll.ll up to 25
`7xlO-4 were achieved. I. Bennion et al. [I. Bennion et al.
`"UV-written in fibre Bragg Gratings", Tutorial Review,
`Optical and Quantum Electronics, 28, 93-135, 1996]
`describes a very effective method of sensitization of fibers
`by soaking it in high-pressure hydrogen in the range of 30
`20-750 atm at a temperatures in the range 20-75° C. for a
`period of several days. The values of ll.ll 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, which is
`necessary for optical encoder of embodiment II, 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 Al2, which for the diode laser 40
`source of 0.78,um gives the value 0.36 ,urn. 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 ,urn.
`In reflection type encoder, embodiment V, optical path 45
`difference of Al4 has to be obtained for zero order suppres(cid:173)
`sion. It means Al4=0.18 ,urn for 0.78 ,urn laser source
`wavelength. Such an OPD can be obtained with the index of
`refraction modulation of 0.01, written in a core of 18 ,urn
`diameter.
`Chirped Fiber Bragg Grating
`Chirped Fiber Bragg gratings can be manufactured by
`certain adjustments of holographic set-up as per Macomber
`[U.S. Pat. No. 5,238,531]. Limitation oflength of the grating
`(less than 150 mm) is reported.
`Alternatively, chirped Bragg grating can be written with
`phase mask scheme as per Chesnoy, et al. [U.S. Pat. No.
`5,655,040]. Interference fringe pattern, created by diffrac(cid:173)
`tion of laser beam on phase mask, is delivered to fiber by
`optical system with variable focal length. Changing a focal 60
`length of the system cause chirp in Bragg grating period
`size. Mizrahi et. al. [U.S. Pat. No. 5,636,304] describes a
`method of fabrication a chirp by changing a wavelength of
`the laser during a writing process with phase mask or
`holographic. The chirped grating length is limited by the 65
`length of the phase mask or holographic interference pattern.
`Epworth R. E. et al [U.S. Pat. No. 5,602,949] suggested a
`
`8
`method of chirped grating fabrication based on applying a
`non-uniform strain on uniformly written fiber grating, which
`changes the period of grating locally.
`All above-mentioned methods of fabricating a chirped
`fiber grating can not help with manufacturing a very long
`fiber grating. It can be done by changing a fiber feed rate
`gradually during a single exposure, point-to