United States Patent
`[19]
`pi] Patent Number:
`5,875,272
`
`Kewitseh et al.
`|451 Date of Patent:
`Feb. 23, 1999
`
`U3005875272A
`
`[54| WAVELENGTH SEIECTIVE OPTICAL
`DEVICES
`
`[?5|
`
`Inventors: Anthony S. Kewitseh. Hacienda
`Heights; George A. Rakuljie, Santa
`.‘ _
`_
`.
`_
`.
`gym: “mm" “'1‘“. ban Manna. all
`
`73| Assignee: Arroyo Optics, ll'lc., Santa Monica,
`Calif.
`
`
`
`21| App]. No.1 188,068
`
`22|
`
`Filed:
`
`()et. 25, 1996
`
`Related U S Application Data
`
`(13] Continualion-in-part of 50!. No. 703.351. Aug. 20. [096.
`Pal. No. 5.305.751.
`
`Provisional application No. Oil-TMSHIS. Del. 2?. 1993.
`(DUI
`........... (£023 (n34
`Int. Cl.”
`51|
`52] US. Cl.
`
`335m 385'94
`
`58]
`Field of Search
`385...7.za,31.37,3e.4a
`
`56]
`
`References Cited
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`
`4,4?4.427
`4,?25_.£ltl
`4331007
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`4,802.95“
`4,9tuJ,tI9
`SJIIIWJS
`5.[Il6_.967
`5.104.209
`5.107.360
`5.15174?
`5.181%“
`_
`1215-739
`5.218.555
`135,55”
`5.221.024
`5.237.422
`
`.. 3853123
`1|).r 984 Iliil et al.
`..
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`
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`
`
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`3353?
`
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`385!!!
`
`4f 902 I-luhel
`359.!!24
`
`10;" 992 Atkins cl al
`
`383.8?
`2.! 903 Iluhei
`_
`-- $55?
`
`”-3 993 “1“ “L a ‘
`58::th
`is; 903 thht .
`.. 585339
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`385”}:
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`3&5t124
`.
`
`
`
`
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`
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`
`533?,382
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`......
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`
`_
`.
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`
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`
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`3853?
`(List continued on next page.)
`.
`_
`‘
`.
`y
`‘
`y
`‘
`_
`.
`l-(JRL-[taN PAIL-NI DOCUML-NI'S
`WO 89312243
`12.!198‘) WII’O.
`\vo 05.14940
`0:15:95 WII’O.
`
`OTHER PUBLICAI'IUNS
`
`”All-Fihre Narrowband Reflection Gratings at 1500 nrn",
`Elect. letters. vol. 26. No.
`l 1. May 24. [999. pp. ”330—132.
`R' Kashyap at 3"
`CM. Ragdale, et al., "Integrated Three Channel laser and
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`P-E- ”W. '-'l ah "High Reflectivity Fibre Gratings Produced
`
`'9‘"
`3""
`'
`'
`(List continued on next page.)
`
`"
`
`Primary Examiner—John D. Lee
`Attorney, Agent, or Finn—Jones, Tullar & Cooper, PC
`.
`.
`1. 1
`ABS] RALI
`[57']
`Wavelength selective devices and subsystems having vari-
`_,
`._
`.
`.
`_.
`_
`,
`_,
`_
`3“? ldp'ig“?ll1'1°“’:'d“‘ul'j It‘ddd‘” "INF“ me‘UTILMmT‘J‘”:
`“3““ '
`‘5" mm 3‘” “’sthms “" “WW” 0
`Iii—directional grating assisled mode couplers. The high
`addtdrep efliciencv and low lone; of this coupler enable low
`less wavelength seleclive elements such as optical switches,
`amplifiers, routers, and sources to be fabricated. The grating
`assisted mode coupler can he wavelength tuned by modify—
`ing the optical properties of the coupler interaction region.
`A programmable. wavelength selective router composed of
`.
`_
`.
`.
`..
`,
`,
`1
`..
`.
`_
`multiple grating abfilhled mods. couplers 1!: also disclosed.
`3! Claims, 10 Drawing Sheets
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1026, Page 1
`Exhibit 1026, Page 1
`
`

`

`5,875,272
`Page 2
`
`5,400,161:
`5,416,866
`5,420,948
`5,425.116
`5,444,803
`5,450,511
`5.4571358
`5,459,801
`5,495,548
`5,506,925
`5,517.589
`5,5?4,807
`5,581,642
`
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`
`411996 Greene cl at.
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`
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`571996 Takeuchi
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`
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`PJ. Lemaire, el al.. "High Pressure I12 Loading asA rl‘cch-
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`R.M. Atkins, et al., "Mechanisms of Enhanced UV Photo—
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`OD. Maxwell, et al., "UV Written 13 dB Reflection Filters
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`0. Okamoto, et a1., "lo—Channel Optical AdeDrop Mulli—
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`et
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`L. Dong, cl al., "Ultraviolet Absorption in Modified Chemi—
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`111.. Williams, et al., “Accelerated Lifetime ‘l‘esls on UV
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`MS. Yataki, et al., "All—Fibre Wavelength Filters Using
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`No. 6, Mar. 1985, pp. 248-349.
`M.C. l-‘arries, et al., "Very Broad Reflection Bandwidth (44
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`vol. 30, No. 11, May 1994, pp. 891—892.
`
`Victor Mi'zrahi, et al., "Optical Properties. of Photoscnsitive
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`I 1, No.
`10, Oct. 93, pp. 1513—1517.
`Jocelyn Lanzon, et al., “Numerical Analysis of the Optimal
`Length and Profile of a Linearly (Thirped Fiber Bragg
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`No. 6, Mar. 1995, pp. 647-649.
`Francois Oucllette. "All—Fiber Filter for Efficient Dispersion
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`tion to Lasers and Amplifiers", Optoelect. Res. Ctr, 31 pp. no
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`pp. 1571-1578.
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`Bandwidth Fibre Gratings Written by Single Excirner
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`vision MultiplexerfDemultiplexer Using Photoimprinted
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`No. 12, Jun. 1993, pp. 953—955.
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`James Ff. Shackelford, cl al., "Solubility of Gases in Glass.
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`No. 4, Apr. 1972, pp. 1619—1626.
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`Optics l.lrs., vol. 21, No. 10, May 1996, pp. 722—324.
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`on a Tapered SingIe—Mode Fiber", IEEE Photo. Tech. Llrs.,
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`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1026, Page 2
`Exhibit 1026, Page 2
`
`

`

`5,875,272
`Page 3
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`TA. Birks. c1 :1]., “Four-Purl Fiber Frequency Shiflcr Wilh
`a Null ‘l‘apcr Coupler", Optics [.lrs.. v01. 1‘). No. 33. 138:.
`1994, pp. watt—mm.
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`
`Y. Imus, c1 3].. "Silica-Based Arraycd—Wavuguidc Graling
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`
`l-Iirushi Yasaka. a! £11., "Multiwuvclcnglh Lighl Sourct: With
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`
`(Thuang, el al., "Enhanced Wavelenglh Tuning in
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`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1026, Page 3
`Exhibit 1026, Page 3
`
`

`

`US. Patent
`
`Feb. 23, 1999
`
`Sheet 1 of 10
`
`5,875,272
`
`
`
`FIG. 1
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1026, Page 4
`Exhibit 1026, Page 4
`
`

`

`US. Patent
`
`Feb. 23, 1999
`
`Sheet 2 of 10
`
`5,875,272
`
`
`
`FIG. 2
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1026, Page 5
`Exhibit 1026, Page 5
`
`

`

`US. Patent
`
`Feb. 23, 1999
`
`Sheet 3 of 10
`
`5,875,272
`
`
`
`FIG. 3
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1026, Page 6
`Exhibit 1026, Page 6
`
`

`

`US. Patent
`
`Feb. 23, 1999
`
`Sheet 4 of 10
`
`5,875,272
`
`
`
`FIG. 4
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1026, Page 7
`Exhibit 1026, Page 7
`
`

`

`US. Patent
`
`Feb. 23, 1999
`
`Sheet 5 of 10
`
`5,875,272
`
`
`
`FIG. 5
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1026, Page 8
`Exhibit 1026, Page 8
`
`

`

`US. Patent
`
`Feb. 23, 1999
`
`Sheet 6 of 10
`
`5,875,272
`
`
`
`FIG. 6
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1026, Page 9
`Exhibit 1026, Page 9
`
`

`

`US. Patent
`
`Feb. 23, 1999
`
`Sheet 7' of 10
`
`5,875,272
`
`
`
`3. 1 + 5
`
`FIG . 7 ( b )
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1026, Page 10
`Exhibit 1026, Page 10
`
`5":
`
`:
`
`.
`
`._._.._
`
`_.i
`
`In-
`
`: E
`
`GQ
`
`,
`Dd
`
`

`

`US. Patent
`
`Feb. 23, 1999
`
`Sheet 8 of 10
`
`5,875,272
`
`
`
`
`
`FIG. 8
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1026, Page 11
`Exhibit 1026, Page 11
`
`

`

`US. Patent
`
`Feb. 23, 1999
`
`Sheet 9 of 10
`
`5,875,272
`
`
`
`Effimency
`
`(add/drop#1)
`
`Effimency
`
`(add/drop#2)
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1026, Page 12
`Exhibit 1026, Page 12
`
`

`

`US. Patent
`
`Feb. 23, 1999
`
`Sheet 10 0f 10
`
`5,875,272
`
`2.05
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1026, Page 13
`Exhibit 1026, Page 13
`
`

`

`5 ,875 ,272
`
`1
`WAVELENGTH SELECTIVE OPTICAL
`DEVICES
`
`REFERENCE TO REL/WED APPLICATIONS
`
`This application is a continuation-in-part of US. appli-
`cation Ser. No. tI8flfl3,357, filed on Aug. 26, 19%, now Us.
`Pat. No. 5,805,751,
`issued Sep. 8, 1998, and claims the
`benefit of Provisional Application No. 60i005,915, filed Oct.
`27, 1995.
`
`FIELD OF THE lNVEN’l‘lON
`
`The present invention relates to the communication of
`signals via optical fibers, and particularly to an optical fiber
`couplcr and methods for making the same. More
`particularly.
`the invention relates to optical devices and
`subsystems using a wavelength selective optical coupler.
`
`DESCRIPI'ION 0F RELATED ART
`
`Low loss, wavelength selective couplers are important
`componen Ls for optical fiber communication networks based
`on wavelength division multiplexing (WUM). WDM
`enables an individual optical fiber to transmit several chan—
`nels simultaneously.
`the channels being distinguished by
`their center wavelengths. An objective is to provide a precise
`wavelength selective coupler that is readily manufactured
`and possesses high efficiency and low loss. ()ne technology
`to fabricate wavelength selective elements is based on
`recording an index of refraction grating in the core of an
`optical fiber. See. for instance. Hill et aL. US. Pat. No.
`4,474,427 {1984) and Glenn el al., US. Pat. No. 4.72:3.l ll]
`([988). The currently preferred method of recording an
`in-linc grating in optical fiber is to subject a photosensitive
`core to the interference pattern between two beams of actinie
`(typically UV) radiation passing through the photoinsensi-
`tive cladding.
`Optical fiber gratings reported in the prior art almost
`universally operate in the reflection mode. To gain access to
`this reflected mode in a power efficient manner is diflicult,
`because the wave is reflected backwards within the same
`fiber. A first method to access this reflected light is to insert
`a 3 dB coupler before the grating, which introduces a net 6
`dB loss on the backwards reflected and outcoupled light. A
`second method is to insert an optical circulator before the
`grating to redirect
`the backwards propagating mode into
`another fiber. This circulator introduces an insertion loss of
`1 dB or more and involves complicated hulk optic compo-
`nents A method to combine the filtering function of a fiber
`grating with the splitting function of a coupler in a low loss
`and elegantly packaged manner would be highly desirable
`[or WDM communication networks.
`
`Another method well known in the prior art uses direc—
`tional coupling to transfer energy from one waveguide to
`another by evanescent coupling (D. Marcuse, "Theory of
`Dielectric Waveguides,” Academic Press 1991 and A. Yariv,
`"Optical Electronics," Saunders College Publishing, 1991].
`This evanescent coupling arises from the overlap of the
`exponential
`tails of the modes of two closely adjacent
`waveguides, and is the typical mode of operation for direc-
`tional coupler based devices. In contrast, non—evanescent
`coupling occurs when the entire optical modes substantially
`overlap, as is the case when the two wave-guides are merged
`into a single waveguide. Devices that rely on evanescent
`coupling (e.g., directional couplers} in contrast
`to non-
`evancscent coupling have inherently weaker
`interaction
`strengths.
`
`10
`
`15
`
`3t}
`
`40
`
`45
`
`SE}
`
`$5
`
`of]
`
`65
`
`2
`One realization of a directional coupling based device
`uses gratings recorded in a coupler composed of two iden-
`tical polished fibers placed longitudinally adjacent to one
`another (J.—[.. Archambault et al., Optics Letters, Vb]. 19, p.
`180 (1994)]. Since the two waveguides are identical in the
`coupling region, both waveguides possess the same propa-
`gation constant and energy is transferred between them. This
`results in poor isolation of the optical signals traveling
`through the two waveguides, because optical power leaks
`from one fiber to the other. Another device also based on
`evanescent coupling was patented by [5. Snitzer. US. Pat.
`No. 5,459,80l (Oct. [7. 1995]. 'lltis device consists of two
`identical single mode fibers whose cores are brought close
`together by [using and elongating the fibers. The length of
`the coupling region should be precisely equal to an oven or
`odd multiple of the mode interaction length for the output
`light to emerge entirely in one of the two output porLs. A
`precisely positioned Bragg grating is then UV recorded in
`the cores of the waist region.
`An alternative grating assisted directional coupler design
`reported by R. Atferncss ct al.. US. Pat. No. 4,737,007 and
`M. S. Whalen et al., Electronics letters. Vol. 22, p. 681
`(1986) uses locally dissimilar optical fibers. 'lhe resulting
`asymmetry of the two fibers improves the isolation of the
`optical signals within the two fibers. However, this device
`used a reflection grating etched in a thin surface layer on one
`of the polished fibers. dramatically reducing, the coupling
`strength 01‘
`the grating.
`It also is based on evanescent
`coupling. A serious drawback of this device is that
`the
`wavelength for which light is backwards coupled into the
`adjacent fiber is very close to the wavelength for which light
`is backreflectcd within the original fiber (about I nrn}. This
`leads to undesirable pass—band characteristics that are ill
`suited for addrdrop filter devices designed to add or drop
`only one wavelength. For optical communications applicam
`tions in the Lir doped fiber amplifier (EDFA) gain window
`(1520 to 1560 nm). this hackrefiection should occur at a
`wavelength outside this window to prevent undesirable
`crosstalk. The separation between the backretleetcd and
`backwards coupled wavelengths is impractically small for
`the all-fiber, grating assisted directional coupler approaches
`of the prior art.
`Alternatively, F. Bilodeau et al., IEEE Photonics Tech—
`nology Letters, Vol. 7, p. 388 (1995) fabricated a Mach-
`Zender interferometer which served as a wavelength selec-
`tive coupler. This device relies on the precisely controlled
`phase dill'erencc between two interferometer arms and is
`highly sensitive to environmental fluctuations and manu l'ac-
`turing variations.
`In addition, a significant fraction of the
`input signal
`is back reflected. Therefore,
`it
`is uncertain
`whether this device will be able to meet
`the demanding
`reliability requirements for telecommunications compo-
`nents.
`
`The conventional grating assisted directional coupler suf—
`l'ers from both a relatively low coupling strength and small
`wavelength separation of back-reflected and backwards
`coupled light. These problems arise because the two coupled
`optical waveguides remain physically separate and the light
`remains guided primarily in the original cores. Only the
`evanescent tails of the modes in each of the two waveguides
`overlap. corresponding to evanescent coupling.
`Two locally dissimilar optical fibers can instead be [used
`and elongated locally to form a single merged waveguide
`core of much smaller diameter, forming a mode coupler. The
`resulting optical mode propagation characteristics are effec-
`tively those of a multimode silica coret’air cladding
`waveguide. The two waveguides are merged such that the
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1026, Page 14
`Exhibit 1026, Page 14
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`5 ,8?5 ,272
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`3
`energy in the original optical modes of the separate
`waveguides interact in a substantially non-evanescent man-
`ner in the merged region. 'lhe index profile of the optical
`waveguide varies sufficiently slowly in the longitudinal
`direction such that light entering the adiabatic taper region
`in a single eigenrnode of the waveguide evolves into a single
`local superrnode upon propagating through the adiabatic
`transition region. By merging the waveguides into a single
`wave propagation region, the wavelength selective coupling
`achieved upon the subsequent recording of an index of
`refraction grating in the waist of the coupler can be sub-
`stantially increased. This device is called a grating assisted
`mode coupler. and is described at length in the US and PCI'
`patent application PCTIUS96I13481.
`GLOSSARY
`
`An "active" optical device is a device whose optical
`properties change in response to an electrical input;
`A "passive" optical device is a device lacking, an elec—
`trical input which elTecLs a change in optical properties;
`An "optical fiber" herein is an elongated structure of
`nominally circular cross section comprised of a "core" of
`relatively high refractive index material surrounded by a
`"cladding” of lower refractive index material, adapted for
`transmitting an optical mode in the longitudinal direction;
`A "waveguide" herein is an elongated structure comprised
`of an optical guiding region of relatively high refractive
`index transparent material (the core) surrounded by a mate-
`rial of lower refractive index (the cladding), the refractive
`indices being selected for transmitting an optical mode in the
`longitudinal direction. This structure includes optical fiber
`and planar waveguides;
`An "addr‘drop liller” is an optical device which directs
`optical energy at a particular set of wavelengths From one
`waveguide into another waveguide;
`A "grating" herein is a region wherein the refractive index
`varies as a function of distance in the medium. The variation
`typically, but not necessarily,
`is such that
`the distance
`between adjacent indcx maxima is constant;
`The “bandwidth" of a grating is the wavelength separation
`between those two points for which the reflectivity of the
`grating is 50% of the peak reflectivity of the grating;
`A ”coupler" herein is a waveguide composed of two or
`more fibers placed in close proximity of one another, the
`proximity being such that the mode fields of the adjacent
`waveguides overlap to Some degree;
`A "waist" herein refers to that portion of an elongated
`waveguide with minimum cross sectional area;
`An “asymmetric coupler" herein is a structure composed
`of two or more waveguides that are dissimilar in the region
`longitudinally adjacent to the coupling region;
`A "transversely asymmetric" grating is an index of refrac-
`tion grating in which the index variation as a function of
`distance from the central axis of the waveguide along a
`direction perpendicular to the longitudinal axis is not iden~
`tical to the index variation in the opposite direction. per—
`pendicular to the longitudinal axis. A transversely asymmet-
`ric grating possesses grating vector components at nonzero
`angles to the longitudinal axis or mode propagation direction
`of the waveguide. Orthogonal modes are not efficiently
`coupled by a transversely symmetric grating;
`A"supermode" is the optical eigenmode of the complete,
`composite waveguide structure.
`SUMMARY OF THE INVENTION
`
`Optical devices and subsystems based on grating assisted
`rnode couplers, which redirect optical energy of a particular
`
`It]
`
`15
`
`3t}
`
`an
`
`45
`
`SE}
`
`$5
`
`of]
`
`65
`
`4
`wavelength from one waveguide to another, are described.
`Index of refraction gratings are impressed within the waist
`of an asymmetric coupler and are arranged to redirect in a
`bi—directional manner a selected wavelength along a par—
`ticular path.
`A tunable grating assisted mode coupler can be fabricated
`by varying the optical properties [e.g., index of refraction,
`length) of the coupler interaction region. Altemately, a
`wavelength selective optical switch can be fabricated by
`redirecting light of a particular wavelength through an
`optical switch by using a single grating, assisted mode
`coupler. This same technique can be used to form a wave-
`length selective optical amplifier and a wavelength selective
`optical modulator. Another type of wavelength selective
`optical switch is described, based on tunable, grating
`assisted mode couplers attached to fixed wavelength, grating
`assisted mode couplers. A WDM multi-wavelength trans-
`mitter subsystem. broadly tunable addr’drop filters, and
`reconfigurable, wavelength selective routers are further dis-
`closed. Accordingly, the present invention provides signifi-
`cant advantages i.n optical communications and sensor sys-
`tems that require narrow optical bandwidth tillers in which
`light in a particular waveguide at a particular wavelength
`channel
`is routed in a low loss manner into another
`waveguide.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The present invention will be described with reference to
`the drawings of the following figures:
`FIG. I shows the operation of a grating assisted mode
`coupler tuned to the Bragg wavelength;
`FIG. 2 shows the operation of a grating assisted mode
`coupler deluned from the Bragg wavelength;
`FIG. 3 shows a schematic of a grating assisted mode
`coupler;
`FIG. 4 shows a tunable, grating assisted mode coupler;
`FIG. 5 shows a wavelength selective optical switch;
`FIG. 6 shows a wavelength insensitive optical element
`joined to a grating assisted mode coupler:
`FIG. 7 shows a zero loss, wavelength selective optical
`switch incorporating a tunable grating assisted mode coupler
`in tandem with a non—tunable grating assisted mode coupler
`with nearly the same drop wavelength;
`FIG. 8 shows an eight—channel, multi—wavelength WDM
`source;
`
`1“] (i. 9 shows a broadly tunable addi’drop filter based on
`the optical vcmicr cli‘ect;
`l’lG. III shows an eight channel. programmable WDM
`router.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`Optical fibers carry signals in the form of modulated light
`waves from a source of data, the transmitter, to a recipient
`of data, the receiver. Once light enters this optical liber, it
`travels in isolation unless an optical coupler is inserted at
`some location along the fiber. Optical couplers allow light
`signals to be transferred between normally independent
`optical waveguides.
`If multiple signals at different wavelengths travel down
`the same fiber, it is desirable to transfer a signal at only a
`predetermined set of wavelengths to or from this fiber into
`another liber. These devices are called wavelength selective
`optical couplers. Adcsirahlc attribute of such a wavelength
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1026, Page 15
`Exhibit 1026, Page 15
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`5,836,212
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`5
`selective optical coupler is that it remains transparent to all
`wavelengths other than those to be coupled. 'Ihis transpar-
`ency is quantified by the insertion loss, crosstalk, and
`bandwidth. Wavelength selective couplers of the prior art are
`not adequately transparent for many important applications.
`The grating assisted mode coupler is a fundamentally trans-
`parent device.
`It transfers light signaLs from one Iiber to
`another at only a predefined, precise set of wavelengths. It
`intrinsically is a bi-directional, 4 port device that serves as
`botlt an add and drop filter. This great functionality allows an
`entirely new class of active optical devices and subsystems
`to be built around it.
`The present invention provides wavelength selective opti-
`cal devices and subsystems using one or more grating
`assisted mode coupler.
`In accordance with the present
`invention,
`light
`is coupled between two or more locally
`dissimilar waveguides by an index of refraction grating in
`the shared coupling regiott of the grating assisted mode
`coupler. The grating assisted mode coupler can be fabricated
`by fusing together two optical fibers, or by fabricating the
`structure in a planar waveguide device. FIGS.
`1 and 2
`illustrate the operating principle of this device. The mode
`coupler consists of a first waveguide ll and a second
`waveguide 21 dissimilar in the vicinity of the coupling
`region 1 wherein an index of refraction grating has been
`impressed. The two waveguides are dissimilar upon entering
`the coupling, region to provide the necessary coupler asym-
`metry. The input mode 31 with propagation vector 3',
`evolves into the coupler waist mode 71 with propagation
`vector |i., and the backwards propagating waist mode 61
`with propagation vector , p: evolves into the output mode 41
`with propagation vector (3'3. The propagation vectors [3, and
`fl: at the waist satisfy the Bragg law for reflection from a
`thick index grating of period As at a particular wavelength,
`say 1,;
`510-.-)-t51{}~.-)-2 no...
`in the first waveguide 11 is
`l..-
`then the optical energy at
`coupled into the backward propagating mode of the second
`waveguide 21 (FIG. I). The spectral response and elliciency
`of this rellective coupling process is dictated by the coupling
`strength and the interaction length ofthe optical modes with
`the grating.
`In FIG. 2, the wavelength of the input mode is deluned,
`say to 1].. so that [i,(i.j)-|i:(l.j)z2 MAX, and the input mode
`31 in the first waveguide travels through the coupler waist
`and reappears as the transmission output [node of the first
`waveguide 51, as seen in FIG. 2,wilh minimal leakage into
`the second waveguide 21. 'l'heret'ore. only a particular wave-
`length l-.,-
`is coupled out of the first waveguide 11, as
`determiner] by the grating period in the coupling region 1.
`The amount of wavelength dctuning required to reduce the
`reflective coupling by 50% is given by the full—width—half-
`maxima (I-WIIM] bandwidth A}. of the grating:
`
`where hell is the effective interaction length of the optical
`beam and the grating, which may be less than the physical
`length l. of the grating for
`large Ir. The bandwidth of
`reflection gratings is narrower than that of transmission
`gratings by typically ten to fifty times because the grating
`period Ag is much shorter for the former. The narrower
`frequency response in the reflection mode is desirable for
`dense WUM applications. Typically, the desired bandpass is
`approximately [Ll nm at 1.55 pm. This dictates that
`the
`length of the reflection grating should be approximately ]
`
`6
`cm. A reflectivity in excess of 90% for a grating thickness L
`of l ctn requires a x1. larger than 2. it should then he 3 crn‘.
`To achieve this coupling. strength in the fused coupler, the
`grating index modulation should be at least 10”. This level
`of index modulation is achieved in silica planar waveguides
`and optical fibers by appropriate preparation of the materials
`and dimensions of the media.
`in addition to backwards coupling of light into the adja—
`cent waveguide. Ihe grating typically reflects some light
`back into the original fiber at a different wavelength given by
`2B,(}-.:)wkx. To ensure that L:
`is outside the wavelength
`operating range of interest. Ihe difl'crcnce between B, and [3:
`is made sufficiently large. The dilIercnce increases as the
`waveguides become more strongly coupled, until the limit-
`ing case is reached, for which the waveguide cores are
`merged into one another. This dificrencc is maximized for
`small coupler waists, in which [11 and [5: correspond to the
`[Pm and LP,1 modes of an air-clad optical waveguide.
`Furthermore, an appropriate transversely asymmetric grat-
`ing substantially reduces the coupling strength for back-
`reflection.
`The grating, assisted mode coupler 9, illustrated in FIG.
`3. radirects optical energy at a particular wavelength from a
`source 79 to the input optical fiber 69 of the coupler. The
`period of the index grating formed within the coupler is
`chosen to redirect only that optical energy within a particular
`wavelength band into the drop port 59 of a second optical
`fiber, which travels to detector 89. All other wavelengths
`propagate through the coupler front the input port 69 to the
`throughput port 19 attached to detector 29. An additional
`source of light 39 at the same wavelength can be attached to
`the add port 49, and will be directed to the throughput port
`19 by the same coupler 9. This device performs both the add
`and drop functions in a single component.
`A new class of active fiber optic components and sub—
`systems arc made economically and practically feasible by
`linking other optical devices to this grating assisted mode
`coupler. This approach enables standard fiber optic compo-
`nents to be rendered wavelength selective by the simple
`addition of a grating assisted mode coupler. A unique
`property of the grating assisted mode coupler 9 is the
`reciprocal property of the inputs and outputs. That is, the
`input 69—throughpul l9 and add 49—drop 59 ports behave
`in a complementary manner. A single grating assisted mode
`coupler enables complete iii-directional exchange of optical
`energy at a particular wavelength from a first waveguide to
`a second waveguide. This alIows important optical devices
`and subsystems that have been impractical
`to implement
`using existing components to be readily achieved with this
`new, bi-directional device. This new class of devices
`includes wavelength selective optical swit