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`MN
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`MWWWWWMMWM
`USOO5604829A
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`
`United States Patent
`
`[191
`
`[11] Patent Number:
`
`5,604,829
`
`Bruesselbach
`
`[45] Date of Patent:
`
`Feb. 18, 1997
`
`Saleh, Bahaa E. A., and Teich, Malvin Carl, Fundamentals
`of Phototonics, 1991, John Wiley & Sons, Inc., p.801.
`Considine, Douglas M., and Considine, Glenn D., editors,
`Van Nostra_nd’s Scientific Encyclopedia, Seventh Edition,
`1989, Van Nostrand Reinhold, p. 424.
`
`Primary Examiner—Rodney B. Bovernick
`Assistant Examiner—Hemang Sanghavi
`Attorney, Agent, or Finn—V. D. Duraiswamy; W. K. Den-
`son-Low
`—
`
`[57]
`
`V ABSTRACT
`
`A method for forming an index grating in an optical
`waveguide, such as an optical fiber, with precise control over
`the grating’s period, cross-sectional shape and length. A
`single writing beam is passed through an optical grating
`mask, such as a phase mask. A photosensitive waveguide is
`spaced from the optical grating by a distance that corre-
`sponds to an integer fraction of the Talbot self-imaging
`distance, so that the optical grating (or a desired transfor-
`mation of it) is imaged in the waveguide core. The grating
`image has substantially the same cross-sectional shape,
`period and length as the portion of the optical grating that is
`illuminated by the writing beam. Thus, an index grating that
`substantially replicates the cross-sectional shape, period and
`length of the optical grating mask, which preferably has a
`substantially square-wave shaped cross-section, is written in
`the waveguide core. The substantially square-wave shaped
`cross-section results in higher reflectivity per unit length
`than prior waveguide gratings with sinusoidal cross-sec-
`tions. As a result, a high order waveguide index grating may
`be formed over a waveguide length that is shorter than
`would previously be required. Alternatively, a low order
`grating may be formed that has higher reflectivity than a low
`order sinusoidal grating.
`
`6 Claims, 2 Drawing Sheets
`
`46
`
`[54] OPTICAL WAVEGUIDE WITH
`DIFFRACTION GRATING AND METHOD OF
`FORMING THE SAME
`
`[75]
`
`Inventor: Hans Bruesselbach, Calabasas, Calif.
`
`[73] Assignee: Hughes Aircraft Company, Los
`Angeles, Calif.
`
`[21] Appl. No.: 423,070
`
`[22]
`
`Filed:
`
`Apr. 17, 1995
`
`Int. Cl.“ ..................................................... .. G02B 6/34
`[51]
`[52] U.S. Cl.
`............................................. .. 385/37; 359/569
`[58] Field of Search ................................ .. 385/28, 37, 33,
`'
`385/10, 123; 359/566, 569, 573
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`............................ 385/123
`2/1989 Glenn et al.
`4,807,950
`4,974,930 12/1990 Blyler, Jr. et al.
`385/28
`5,066,133
`11/1991 Brienza .......... ..
`. 385/37 X
`5,104,209
`4/1992 Hill et al.
`. 385/37 X
`5,307,437
`4/1994 Facq et al.
`. 385/37 X
`5,313,538
`5/1994 Sansonetti . ... . . .
`. ... .. 385/28
`5,327,515
`7/1994 Anderson et al.
`385/123
`5,351,321
`9/1994 Snitzer et al.
`385/37
`5,367,588
`11/1994 Hill et al. ................................ .. 385/37
`
`
`
`OTHER PUBLICATIONS
`
`Meltz et al., “Formation of Bragg gratings in optical fibers
`by a transverse holographic method”, Optics Letters, vol. 14,
`No. 15, Aug. 1989, pp. 823-825.
`——
`Anderson et al., “Phase—Mask Method for Volume Manu-
`facturing of Fiber Phase Gratings”, Proceedings of the
`Optical Fiber Conference, Feb. 1993, papger PDl.6—l, pp.
`68-70.
`
`Leger et al., “Eflicient array illuminator using binary—optics
`phase plates at fractional—Talbot planes”, Optics Letters,
`vol. 15, No. 5, Marcy 1990, pp. 288-290.
`
`
`
`Wavelength
`
`44
`
`37
`
`Page 1
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`ILLUMINA, INC. EXHIBIT ‘I034
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`Page 1
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`U.S. Patent
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`Feb. 18, 1997
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`Sheet 1 of 2
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`5,604,829
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`20
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`Transmitted
`Signal
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`Page 3
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`5,604,829
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`1
`OPTICAL WAVEGUIDE WITH
`DIFFRACTION GRATING AND IVIETHOD OF
`FORNIING THE SAME
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`This invention relates to optical diffraction gratings, and
`more specifically to a method for forming refractive index
`gratings in photosensitive optical fibers, and the resulting
`fibers.
`
`2. Description of the Related Art
`Optical fiber diffraction gratings are useful for optical
`communications devices such as single-mode fiber lasers,
`hybrid serniconductor-fiber lasers, mode converters, optical
`filters and fiber sensors.
`
`Fiber gratings typically consist of a refractive index
`grating written in the core of a photosensitive fiber, as
`described in G. Meltz et al., “Formation of Bragg gratings in
`optical fibers by a transverse holographic method”, Optics
`Letters, vol. 14, no. 15, August 1989, pages 823-825. The
`photosensitive fiber is typically a silicon fiber with a core
`that
`is doped with a material
`that makes its index of
`refraction sensitive to its history of exposure to optical
`radiation of a given wavelength. For example, a silicon fiber
`doped with germanium exhibits an intense 35 nm wide
`absorption band centered at 244 nm.
`In the transverse process described by Meltz, the diifrac-
`tion grating is written in the core of the fiber by exposing it
`to a two-beam interference pattern. The wavelength of the
`two beams are chosen to coincide with the absorption band
`in the fiber (for example, 244 nm) and the beams illuminate
`the core from the side of the fiber. The two interfering beams
`create sinusoidal light and dark interference fringes in the
`fiber, which cause a corresponding sinusoidal variation in
`the refractive index of the fiber core (an index grating). The
`sinusoidal index grating has the same period as the optical
`interference fringes. The period of the interference fringes,
`and hence the period of the resulting index grating,
`is
`dependent on the writing angle between the two optically
`interfering beams and their wavelength. Since there is typi-
`cally a constraint on the wavelength of the writing beams (it
`must coincide with the absorption band in the fiber), the
`index grating period is typically controlled by varying the
`angle between the two writing beams.
`Another method of forming index gratings in fibers is
`described in Dana Z. Anderson et al., “Phase-Mask Method
`for Volume Manufacturing of Fiber Phase Gratings”, Pro-
`ceedings of the Optical Fiber Conference, February 1993,
`paper PDl6-1, pages 68-70. In this method a single source
`beam is passed through a phase mask (a phase grating),
`which difiracts the beam into multiple dilfraction orders.
`The fiber is positioned in close proxirrrity to (but not in direct
`contact with) the phase mask. The diifracted orders, which
`have the same function as the writing beams in the Meltz
`method, interfere in the fiber core and produce a sinusoidal
`index grating with a period that is equal to the phase mask
`grating period. With this method, the index grating period
`will always be equal
`to the phase mask grating period,
`regardless of the angle that the source beam makes with the
`phase mask.
`Regardless of which exposure method is used, the fiber
`index grating will reflect light at its Bragg wavelength,
`which can be expressed with the equation 2.3 =2nA/N, where
`2.3 is the Bragg wavelength (the wavelength reflected by the
`
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`30
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`2
`
`grating), n is the index of refraction of the fiber, A is the
`index grating period and N is the grating order. The sinu-
`soidal gratings produced by the above described method are
`low order gratings with a grating period that is no larger than
`twice the primary design wavelength RD (the primary wave-
`length that the grating is designed to reflect). These low
`order gratings reflect light at the fundamental order or, at
`most, the second order. As the grating period is increased
`relative to the primary design wavelength (resulting in a
`higher order grating),
`less light is reflected at both the
`fundamental order and at the higher orders for a given
`grating length.
`One can compensate for this phenomenon by increasing
`the total length of the fiber grating as the grating period is
`increased. However, some applications impose lirrrits on the
`fiber length that can be used. In addition, for the interfero-
`metric method described by Meltz, costly large aperture
`precision optical elements would have to be used to form
`long high order fiber gratings. In the phase mask method
`described by Anderson, the length of the grating is limited
`by the length over which the diifracted orders overlap in the
`fiber and the size of the mask. Chirp-free masks more than
`a few inches long are very diflicult to obtain.
`As a result of these limitations, prior fiber grating forming
`methods have only been used to form gratings that are low
`order with respect to the design wavelength (the grating
`period is no larger than twice the primary design wave-
`length). These gratings exhibit eflicient reflectivity at no
`more than two grating orders (eflicient reflectivity at two
`grating orders is very rare).
`It would be advantageous to have high order fiber gratings
`with relatively high reflectivity at multiple orders. The
`ability to reflect light at multiple wavelengths using a single
`fiber grating would expand the flexibility and usefulness of
`fiber grating devices.
`
`SUMMARY OF THE INVENTION
`
`In view of the above problems, the present invention
`provides a method for forming a high order index grating in
`an optical waveguide, such as an optical fiber, with precise
`control over the grating’s period, cross-sectional shape and
`length. This is accomplished by passing a single writing
`beam through an optical grating mask, either an amplitude
`or phase mask. A photosensitive optical fiber is spaced from
`the optical grating by a distance that corresponds to the
`Talbot self-imaging distance, or an integer fraction thereof,
`so that the optical grating is imaged in the photosensitive
`fiber core. The grating image has the same cross-sectional
`shape, period and length as the portion of the grating mask
`that is illuminated by the writing beam. Thus, an index
`grating that
`substantially replicates the cross-sectional
`shape, period and length of the grating mask is written in the
`fiber core.
`
`Since the index grating that is formed in the fiber has the
`same length as the portion of the grating mask that is
`illuminated by the writing beam, a long fiber index grating
`may be written without the use of the large aperture preci-
`sion optical elements that would be required if one used the
`Meltz method. Unlike the Meltz method, the length of the
`fiber grating formed with the present method is not lirrrited
`by the overlap of two or more diverging interfering beams.
`The cross-sectional shape of the index grating can be
`easily adjusted by adjusting the cross-sectional shape of the
`grating mask. In contrast, the sinusoidal cross-section al
`shape of the index gratings formed by prior methods cannot
`
`Page 4
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`4
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`5,604,829
`
`be adjusted, resulting in the reflectivity problems discussed
`above.
`
`A fiber index grating with a substantially square-wave
`shaped cross-section is also provided using the present fiber
`grating fomiing method. The square-wave shaped cross-
`section results in higher reflectivity per unit length than prior
`fiber gratings with sinusoidal cross-sections. As a result, a
`high order fiber index grating (one with a grating period that
`is greater than twice the primary design wavelength) may be
`formed over a fiber length that is shorter than would be
`required with prior sinusoidal gratings.
`These and other features and advantages of the invention
`will be apparent to those skilled in the art from the following
`detailed description of preferred embodiments,
`taken
`together with the accompanying drawings, in which:
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a perspective view schematic diagram illustrat-
`ing the present method for forming an index grating in a
`fiber.
`
`20
`
`FIG. 2 is a cross-sectional view of the phase mask grating
`and optical fiber of FIG. 1.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`FIG. 1 illustrates the present method for forming an index
`grating in an optical waveguide, preferably the core of an
`optical fiber 10. A coherent optical source 12, such as a laser,
`is used to generate an optical beam 14. The optical beam 14
`must have a wavelength that corresponds to an absorption
`peak in the fiber 10. The fiber 10 is preferably a single-mode
`telecommunications optical fiber with a fiber core that is
`doped with germanium, such as ComingTM SMF-28 fiber.
`However, the fiber 10 may be made of any photosensitive
`material whose index of refraction is alterable by exposure
`to light. As explained above, a fused silica fiber doped with
`germanium exhibits an intense 35 nm wide absorption band
`centered at 244 nm. Therefore, the optical beam 14 prefer-
`ably has a wavelength that lies within this absorption band.
`This wavelength requirement is preferably satisfied by using
`a pulsed excimer laser that emits at 248 nm. However, any
`other laser source, pulsed or con- tinuous-wave, that emits
`within the desired wavelength range may be used.
`The optical beam 14 is expanded and collimated by lenses
`16 and 18, and the expanded beam 20 is passed through an
`optical grating, preferably a phase mask 24. Phase masks are
`common and are commercially available. Although a phase
`mask 24 is used in the preferred embodiment, an amplitude
`grating or any other type of grating that operates in trans-
`mission mode may be used. The grating 25 on the phase
`mask 24 typically consists of ridges 27 that are etched into
`the phase mask material to a depth that is approximately
`equal to ‘/2 the writing beam 20 wavelength. The phase mask
`24 is preferably formed from a material that transmits the
`wavelength of the writing beam 20. The eventual fiber index
`grating will have a period and cross-sectional shape that is
`substantially identical
`to the period and cross-sectional
`shape of the grating 25 on the phase mask 24.
`The fiber 10 is spaced from the phase mask grating 25 by
`a distance 23 that corresponds to 1/2 the Talbot distance. The
`Talbot efl"ect, described in J. R. Leger et al., “Eflicient array
`illurninator using binary-optics phase plates at fractional-
`Talbot planes”, Optics Letters, vol. 15, no. 5, March 1990,
`pages 288-290, is a known method of imaging a periodic
`structure via free-space diffraction. When a periodic object,
`
`25
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`30
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`35
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`45
`
`50
`
`55
`
`60
`
`65
`
`such as the phase mask grating 25, is illuminated with a
`coherent optical beanr 20, amplitude patterns occur at inte-
`ger fractions of the Talbot distance. The Talbot distance is
`equal to
`2A1
`A
`
`,
`
`where A is the period of the optical grating 24 and 7.. is
`wavelength of the optical beam 20 If a phase mask with a
`50% fill factor is used, the preferred distance 23 is 1/2 the
`Talbot distance
`
`A
`( “‘
`
`.
`
`)
`
`For illustration, if a 248 nm optical beam is used with a
`phase mask grating 25 that has a period of 10 pm, the
`preferred distance 23 is approximately equal to 400 um.
`Thus, the fiber would be spaced so that its core 30 is 400 pm
`(or 400 um plus an integer fraction of the Talbot distance)
`from the phase mask grating 25.
`The fiber 10 is preferably oriented so that its propagation
`axis 26 is perpendicular to the phase mask grating direction
`28 so that the phase mask grating 25 is imaged in the fiber
`core with the grating direction 28 perpendicular to the fiber
`propagation direction 26. The phase mask grating image in
`the fiber core 30 is a substantial reproduction of the phase
`mask grating 25, with substantially the same period and
`cross-sectional shape. The periodic structure of the phase
`mask grating 25 is reproduced in the fiber core 30 as
`alternating areas of light 34 and dark 36. The index of
`refraction in the fiber core 30 is altered at the locations
`where light 34 is present, resulting in an index grating 37.
`The length of the index grating 37 is substantially the same
`as the diameter of the expanded beam 20 or the width of the
`phase mask grating 25 (measured along the fiber propaga-
`tion direction 26), whichever is smaller.
`The amount of optical energy that is required to expose
`the fiber index grating 37 depends primarily on how highly
`absorbing the fiber 10 is at the writing beam 20 wavelength,
`which is usually dependent on the amount and type of
`doping in the fiber 10. For illustration, if a 300 millijoule per
`square centimeter pulsed writing beam at 248 nm is used to
`expose an index grating in a gerrnanium-doped fiber, one to
`several thousand pulses could be required depending on the
`amount of germanium doping. In the preferred embodiment,
`the grating strength is monitored during exposure by using
`a broadband light source 38 to launch broadband light 40
`into the fiber 10 as the fiber grating 37 is being formed. The
`transmitted light 42 is monitored by a spectrum analyzer 44.
`As the fiber grating 37 forms,
`the spectrum 46 of the
`transmitted light 42 changes as discrete wavelengths are
`reflected by the fiber grating 37. The fiber grating formation
`is stopped when the grating 37 achieves a predetermined
`strength.
`The present method may be used to form a high order
`index grating 48 in which the grating period 50 is greater
`than twice the primary design wavelength RD, as illustrated
`in FIG. 2. For example, if one desires a high order grating
`that will reflect light at a primary design wavelength of 1.6
`pm, then a phase mask grating 25 with a grating period 50
`greater than 3.2 microns is preferably used. As explained
`above, the resulting fiber index grating 48 will have the same
`period 52 as the phase mask grating 25 and reflect light at
`A.B=2nA/N, where 2.3 is the Bragg wavelength (the wave-
`length reflected by the grating 48), n is the index of
`refraction of the fiber 10, A is the fiber index grating period
`
`Page 5
`
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`
`
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`5
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`6
`
`5,604,829
`
`52 and N is the grating order. As an illustration, if an index
`grating period 52 of 10 pm is chosen and the fiber index of
`refraction is 1.45, the resulting fiber index grating 48 will
`reflect at 29.0 urn, 14.5 pm, 9.66 pm, 7.25 pm, 5.8 pm, 4.83
`um, 4.14 pm, 3.62 pm, 3.22 pm, 2.9 pm, 2.64 um, 2.4 pm,
`2.23 pm, 2.1 um, 1.93 pm, 1.8 pm, 1.7 um, 1.6 pm, 1.53 pm,
`1.45 pm, 1.38 pm, 1.32 pm, 1.26 pm, 1.21 pm,
`.
`.
`. etc.
`Unlike prior methods, the present method allows one to
`easily increase the length of the fiber grating 48 to compen-
`sate for the lower reflectivity that results at higher orders.
`This is done, as explained above, by increasing the diameter
`54 of the writing beam 20 as long as the phase mask grating
`25 is large enough to accommodate the larger writing beam
`20. Alternatively, a long fiber grating may be fabricated by
`using a smaller diameter writing beam 20 and exposing
`multiple shorter gratings in the fiber core 30. This is pref-
`erably done by translating the fiber 10 along the fiber
`propagation direction 26 after each short grating exposure.
`The fiber translation is preferably accomplished by mount-
`ing the fiber on a translation stage (not shown).
`Another feature of the present method is the ability to
`control the cross-sectional shape of the fiber index grating
`48 by controlling the cross-sectional shape of the phase or
`amplitude mask grating 25. In the preferred embodiment, a
`phase mask grating 25 with a substantially square-wave
`shaped cross-section is used. The resulting fiber index
`grating 48 exhibits sharp index transitions which are more
`eflicient at reflecting light than the more gradual
`index
`transitions exhibited by prior sinusoidal index gratings. As a
`result,
`the high order substantially square-wave shaped
`index grating 48 may be formed over a shorter length of fiber
`than would be required if sinusoidal gratings were used.
`Alternatively, a low order grating may be formed that has
`higher reflectivity than a low order sinusoidal grating.
`While several illustrative embodiments of the invention
`have been shown and described, numerous variations and
`alternate embodiments will occur to those skilled in the art.
`For example, the present method may be used to expose
`index gratings in any optical waveguide whose index of
`refraction is alterable by exposure to light. Any coherent
`optical source may be used as long as the wavelength of the
`writing beam is matched to an absorption peak in the optical
`waveguide. In addition, a key feature of the present method
`is the ability to define the cross-sectional shape, period and
`length of the fiber index grating with the phase mask grating.
`Although the formation of a high order fiber index grating
`with a substantially square-wave shaped cross-section was
`described as an illustrative example,
`the cross-sectional
`shape and period of the fiber index grating may be custom-
`ized for a particular application by making appropriate
`changes to the phase mask grating shape, period and length,
`and adjusting its distance from the fiber. Such variations and
`
`10
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`alternate embodiments are contemplated, and can be made
`without departing from the spirit and scope of the appended
`claims.
`.
`I claim:
`
`1. A method of forming a high-order diffraction grating in
`a photosensitive optical waveguide for reflecting a first
`optical beam at a design wavelength and at other wave-
`lengths, comprising the steps of:
`generating a second optical beam having a wavelength
`that corresponds to a wavelength to which said
`waveguide is photosensitive,
`passing said second beam through an optical phase grat-
`ing with a non-sinusoidal cross-sectional shape, said
`grating having a period that is more than twice the
`design wavelength, and that is an integer multiple of the
`design wavelength,
`directing said second optical beam to said photosensitive
`waveguide,
`spacing said waveguide from said phase grating by a
`distance that corresponds to an integer fraction of the
`Talbot self-imaging distance so that said optical grating
`is imaged in said waveguide as alternating light and
`dark regions that have substantially the same period
`and cross-sectional shape as said phase grating, said
`alternating light and dark regions establishing a refrac-
`tive index grating in said waveguide with substantially
`the same period and cross-sectional shape as said phase
`grating.
`2. The method of claim 1, wherein said phase grating has
`a substantially square-wave shaped cross-section so that the
`cross-section of the alternating light and dark regions in said
`waveguide is substantially square-wave shaped.
`3. The method of claim 2, wherein said design wavelength
`is approximately 1.6 microns and said phase grating has a
`period of approximately 10 microns.
`4. The method of claim 1, wherein said waveguide is
`spaced from said phase grating by an amount approximately
`equal to
`AZ
`A
`
`s
`
`where A is the period of said phase grating and 7.. is the
`wavelength of said second optical beam.
`5. The method of claim 1, wherein said second optical
`beam is directed to a photosensitive optical fiber that is
`single-mode with respect the wavelength of said first beam.
`6. The method of claim 5, wherein said index grating is
`established at a core of said fiber.
`
`*
`
`*
`
`*
`
`*
`
`*
`
`Page 6
`
`Page 6
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