Ulllted States Patent [19]
`Kewitsch et al.
`
`US005 875272A
`[11] Patent Number:
`[45] Date 0f Patent:
`
`5,875,272
`Feb. 23, 1999
`
`[54] WAVELENGTH SELECTIVE OPTICAL
`DEVICES
`
`[75] Inventors: Anthony S_ Kewitsch, Hacienda
`
`5,309,260
`5,327,515
`5,337,382
`5,351,321
`
`5/1994 MiZrahi et al. ........................... .. 359/3
`7/1994 Anderson et al. ..
`. 385/123
`8/1994 Mizrahi ......... ..
`385/37
`9/1994 SnitZeret al. .......................... .. 385/10
`
`
`
`George AI Rakuljic, Santa Monica Amnon Yariv San Marino all
`
`
`
`MlZfahl Ct 8.1. ....................... .. 5,367,588 11/1994 H111 et al. ...... ..
`
`385/37
`
`, ’
`of Calif.
`
`’
`
`’
`
`5,377,288 12/1994 Kashyap et al. ........................ .. 385/37
`
`[73] Assignee: Arroyo Optics, Inc., Santa Monica,
`Calif
`
`[21] Appl. No.: 738,068
`
`[22]
`
`Filed:
`
`Oct. 25, 1996
`
`-
`-
`t Dt
`RltdU.S.A 1
`pp ‘ca 10“ a a
`e a 9
`[63] Continuation-in-part of Ser. No. 703,357, Aug. 26, 1996,
`Pat. No. 5,805,751.
`
`[60] Provisional application N°~ 60/005,915, 00L 27, 1995-
`
`[51]
`Int. Cl? ..................................................... .. G02B 6/34
`[52] US. Cl. ............................................... .. 385/37; 385/24
`.
`[58] Field of Search ................................ .. 385/14—16, 24,
`385/27, 28, 31, 37, 39, 48
`
`[56]
`
`References Cited
`
`U'S' PATENT DOCUMENTS
`
`4,474,427 10/1984 Hill et al. .............................. .. 385/123
`4,725,110
`2/1988 Glenn et a1. .............................. .. 359/3
`477377007
`4/1988 Alfemess et a1‘
`" 385/30
`4’737’607
`4/1988 Bernard et a1‘
`218/23
`4,807,950
`2/1989 Glenn et al. .... ..
`385/123
`4 900 119 M1990 Hill et al
`385/27
`5:007:705
`4/1991 Morey et'
`385/12
`570167967
`5/1991 MeltZ et a1_ __
`385/37
`5,104,209
`4/1992 Hill et a1, _ _ _ _ _ _
`_ _ _ __ 385/27
`5,107,360
`4/1992 Huber .......... ..
`359/124
`5,157,747 10/1992 Atkins et a1- -
`385/37
`5482760 2/1993 Hflber ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
`~ ~ ~ ~ ~~ 385/37
`572167739
`6/1993 Hm ct fal'
`385/123
`5’218’655
`6/1993 Mmahl ' ' ' ' ' ' ' ' '
`' ' ' ' " 385/39
`5,235,659
`8/1993 Atkins et al.
`385/124
`5,271,024 12/1993 Huber ...... ..
`372/6
`5,287,427
`2/1994 Atkins et al. ......................... .. 385/124
`
`(List continued on next page.)
`
`FOREIGN PATENT DOCUMENTS
`
`WO 89/12243 12/1989 WIPO .
`WO 95/14946
`6/1995 WIPO .
`
`OTHER PUBLICATIONS
`
`“All—Fibre NarroWband Re?ection Gratin s at 1500 nm”
`g
`n
`Elect. Letters, V01. 26, No. 11, May 24, 1990, pp. 730—732,
`R‘ Kashyap et a1‘
`C.M. Ragdale, et al., “Integrated Three Channel Laser and
`Optical Multiplexer for NarroWband Wavelength Division
`Multiplexing”, Elect. Ltrs., vol. 30, No. 11, May 26, 1994,
`
`PP- 897—898~
`“ _
`_
`_
`_
`_
`P~E~ Dye“ et a1” Hlgh Re?ecnvlty Flbre Gratmgs pjoduced
`by Incubated Damage Using a 193 nm ArF Laser , Elect.
`Ltrs V0 30 NO 11 Ma 26 1994
`860_862
`"
`'
`’
`'
`’
`y
`’
`’pp'
`'
`(List continued on next page.)
`
`Primary Examiner—John D. Lee
`Attorney, Agent, or Firm—Jones, Tullar & Cooper, PC
`
`ABSTRACT
`[57]
`Wavelength selective devices and subsystems having vari
`ous a lications in the ?eld of o tical communications are
`.
`pp
`.
`p
`disclosed. These devices and subsystems are composed of
`bi-directional ‘grating assisted mode‘ couplers. The high
`add/drop ef?ciency and loW loss of this coupler enable loW
`loss Wavelength selective elements such as optical switches,
`ampli?ers, routers, and sources to be fabricated. The grating
`assisted mode coupler can be Wavelength tuned by modify
`ing the optical properties of the coupler interaction region.
`A programmable, Wavelength selective router composed of
`multiple grating assisted mode couplers is also disclosed
`'
`
`31 Claims, 10 Drawing Sheets
`
`4
`
`14
`
`Petitioner Ciena Corp. et al.
`Exhibit 1026-1
`
`

`
`5,875,272
`Page 2
`
`US. PATENT DOCUMENTS
`
`5,400,166
`5,416,866
`
`3/1995 Huber .................................... .. 359/173
`5/1995 Sahlén
`.... .. 385/37
`
`5,420,948
`
`5/1995 Byron . . . . . . . . . . . .
`
`. . . . .. 385/37
`
`385/24
`6/1995 Dragone et al.
`5,425,116
`385/28
`8/1995 Kim et al.
`5,444,803
`.... .. 385/37
`9/1995 Dragone
`5,450,511
`5,457,758 10/1995 SnitZer .................................... .. 385/30
`5,459,801 10/1995 SnitZer .................................... .. 385/30
`5,495,548
`2/1996 Bilodeau et al. .
`385/123
`5,506,925
`4/1996 Greene et al.
`385/129
`
`5,517,589
`5/1996 Takeuchi . . . . . . .
`. . . . .. 385/24
`5,574,807 11/1996 SnitZer .................................... .. 385/24
`5,581,642 12/1996 Beacon et al. .......................... .. 385/15
`
`OTHER PUBLICATIONS
`
`V. MiZrahi, et al., “Four Channel Fibre Grating Demulti
`plexer”, Elect. Ltrs., vol. 30, No. 10, May 12, 1994, pp.
`780—781.
`B. Malo et al., “Point—by—Point Fabrication of Micro—Bragg
`Gratings in Photosensitive Fibre Using Single Excimer
`Pulse Refractive Index Modi?cation Techniques”, Elect.
`Ltrs., vol. 29, No. 18, Sep. 2, 1993, pp. 1668—1669.
`Francois Ouellette, et al., “Enhancement of Second—Har
`monic Generation in Optical Fibers byAHydrogen and Heat
`Treatment”, Appl. Phys. Lett. 54(12), Mar. 20, 1989, pp.
`1086—1088.
`P.J. Lemaire, et al., “High Pressure H2 Loading as A Tech
`nique for Achieving Ultrahigh UV Photosensitivity and
`Thermal Sensitivity in GeO2 Dodped Optical Fibers”, IEE
`1993, Apr. 23, 1993, pp. 1191—1193.
`R.M. Atkins, et al., “Mechanisms of Enhanced UV Photo
`sensitivity Via Hydrogen Loading in Gerrnanosilicate
`Glasses”, IEE 1993, May 11, 1993, 2 pp.
`B. Malo, et al., “Effective Index Drift From Molecular
`Hydrogen Diffusion in Hydrogen—Loaded Optical Fibres
`and Its Effect on Bragg Grating Fabrication”, Elect. Ltrs.,
`vol. 30, No. 5, Mar. 3, 1994, pp. 442—443.
`G.D. MaxWell, et al., “UV Written 13 dB Re?ection Filters
`in Hydrogenated LoW Loss Planar Silica Waveguides”, No
`Journal Name/Date.
`“Ef?cient Mode Conversion in Telecommunication Fibre
`Using Externally Written Gratings”, Elect. Ltrs. vol. 26, No.
`16, Aug. 1990, pp. 1270—1272.
`O. Okamoto, et al., “16—Channel Optical Add/Drop Multi
`plexer Using Silica—Based Arrayed—Waveguide Gratings”,
`IEE 1995, Mar. 1995, 2 pp.
`“32 Wavelength Tunable
`Y. TachikaWa, et
`al.,
`Arrayed—Waveguide Grating Laser Based on Special Input/
`Output Arrangement”, Elect. Ltrs., vol. 31, No. 19, Sep.
`1995, pp. 1665—1666.
`L. Dong, et al., “Ultraviolet Absorption in Modi?ed Chemi
`cal Vapor Deposition Preforms”, J. Opt. Soc. Am. B/Vol. 11,
`No. 10, Oct. 1994, pp. 2106—2111.
`D.L. Williams, et al., “Accelerated Lifetime Tests on UV
`Written IntraCore Gratings in Boron Germania Codoped
`Silica Fibre”, Elect. Ltrs., vol. 31, No. 24, Nov. 1995, pp.
`2120—2121.
`M.S. Yataki, et al., “All—Fibre Wavelength Filters Using
`Concatenated Fused—Taper Couplers”, Elect. Ltrs., vol. 21,
`No. 6, Mar. 1985, pp. 248—249.
`M.C. Farries, et al., “Very Broad Re?ection BandWidth (44
`nm) Chirped Fibre Gratings and NarroW Bandpass Filters
`Produced by the Use of an Amplitude Mas ”, Elect. Ltrs.,
`vol. 30, No. 11, May 1994, pp. 891—892.
`
`Victor MiZrahi, et al., “Optical Properties of Photosensitive
`Fiber Phase Gratings”, J. of LightWave Tech., vol. 11, No.
`10, Oct. 93, pp. 1513—1517.
`Jocelyn LauZon, et al., “Numerical Analysis of the Optimal
`Length and Pro?le of a Linearly Chirped Fiber Bragg
`Grating for Dispersion Compensation”, Optics Ltrs., vol. 20,
`No. 6, Mar. 1995, pp. 647—649.
`Francois Ouellette, “All—Fiber Filter for Ef?cient Dispersion
`Compensation”, Optics Ltrs., vol. 16, No. 5, Mar. 1991, pp.
`303—305.
`F. Ouelette, et al., “Broadband and WDM Dispersion Com
`pensation Using Chirped Sampled Fibre Bragg Gratings”,
`Elect. Ltrs., vol. 31, No. 11, May 1995, pp. 899—901.
`Philip St. J. Russell, et al., “Fibre Gratings”, Physics World,
`Oct. 1993, 6 pp.
`D.L. Williams, et al., “Enhanced UV Photosensitivity in
`BoronCodoped Gerrnanosilicate Fibres”, Elect. Ltrs., vol.
`29, No. 1, Jan. 1993, pp. 45—47.
`R. Kashyap, et al., “Measurement of Ultra—Steep Edge,
`High Rejection Fibre Bragg Grating Filters”, Elect. Ltrs.,
`vol. 31, No. 15, pp. 1282—1283, Jul. 1995.
`Vikram Bhatia et al, “Optical Fiber Long—Period Grating
`Sensors” Optics Ltrs., vol. 21, No. 9, May 1996, pp.
`692—694.
`Richard L. Laming, et al., “Fibre Bragg Gratings; Applica
`tion to Lasers and Ampli?ers”, Optoelect. Res. Ctr, 31 pp. no
`date.
`L. Dong, et al., “Single Pulse Bragg Gratings Written During
`Fibre DraWings”, Elect. Ltrs., vol. 29, No. 17, Aug. 1993,
`pp. 1577—1578.
`J .L. Archambaualt, et al., “High Re?ectivity and NarroW
`BandWidth Fibre Gratings Written by Single Excimer
`Pulse”, 2 pp., *no Journal name/date.
`F. Bilodeau, et al., “An All—Fiber Dense—Wavelength—Di
`vision Multiplexer/Demultiplexer Using Photoimprinted
`Bragg Gratings”, IEEE Photo. Tech. Ltrs., vol. 7, No. 4, Apr.
`1995, pp. 388—390.
`F. Bilodeau, et al., “PhotosensitiZation of Optical Fiber and
`Silica—on—Silicon/Silica Waveguides”, Optics Ltrs., vol. 18,
`No. 12, Jun. 1993, pp. 953—955.
`paul J. Lemaire, “Reliability of Optical Fibers Exposed to
`Hydrogen: Prediction of Long—Term Loss Increases”, Optic.
`Eng., vol. 30, No. 6, Jun. 1991, pp. 780—789.
`James F.f. Shackelford, et al., “Solubility of Gases in Glass.
`II. He, Ne, and H2 in Fused Silica”, J. Appl. Phys., vol. 43,
`No. 4, Apr. 1972, pp. 1619—1626.
`L. Dong, et al., “Enhanced Photosensitivity in Tin—Codoped
`Gerrnanosilicate Optical Fibers”, IEEE Photo. Tech. Ltrs.,
`vol. 7, No. 9, Sep. 1995, pp. 1048—1050.
`K.O. Hill, et al., “Photosensitivity in Optical Fibers”, Ann.
`Rev. Mater Sci. 1993, 125—157. (No month).
`KP. Jones, et al., “Optical Wavelength Add—Drop Multi
`plexer in Installed Submarine WDM Network”, Elect. Ltrs.,
`vol. 31, No. 24, Nov. 1995, pp. 2117—2118.
`T.A. Birks, et al., “2x2 Single—Mode Fiber Routing SWitch”,
`Optics Ltrs., vol. 21, No. 10, May 1996, pp. 722—724.
`T.A. Birks, et al., “LoW PoWer Acousto—Optic Device Based
`on a Tapered Single—Mode Fiber”, IEEE Photo. Tech. Ltrs.,
`vol. 6, No. 6, Jun. 1994, pp. 725—727.
`D.O. Culverhouse, et al., “Four Port Fused Taper Acous
`to—Optic Devices Using Standard Singlemode Telecommu
`nications Fibre”, Elect. Ltrs., vol. 31, No. 15, Jul. 1995, pp.
`1279—1280.
`
`Petitioner Ciena Corp. et al.
`Exhibit 1026-2
`
`

`
`5,875,272
`Page 3
`
`TA. Birks, et al., “Four—Port Fiber Frequency Shifter With
`a Null Taper Coupler”, Optics Ltrs., vol. 19, No. 23, Dec.
`1994, pp. 1964—1966.
`T.A. Birks, et al., “All—Fiber PolariZer Based on a Null Taper
`Coupler”, Optics Ltrs., vol. 20, No. 12, Jun. 1995, pp.
`1371—1373.
`H. Bissessur, et al., “16 Channel Phased Array Wavelength
`DernultipleXer on InP With LoW Polarization Sensitivity”,
`IEEE 1994, Dec. 1993, 2 pp.
`M.S. Whalen, et al., “In—Line Optical—Fibre Filter For
`Wavelength Multiplexing”, Elect. Ltrs., vol. 21, No. 17,
`Aug. 1985, pp. 724—725.
`
`Y. Inoue, et al., “Silica—Based Arrayed—Waveguide Grating
`Circuit as Optical Splitter/Router”, IEEE, Mar. 1995, 2 pp.
`
`Hiroshi Yasaka, et al., “MultiWavelength Light Source With
`Precise Frequency Spacing Using Mode—Locked Sernicon
`ductor Laser and Arrayed Waveguide Grating Filter”, OFC
`’96 Tech. Digest, pp. 299—300, 1996. (No Month).
`
`Z.M. Chuang, et al., “Enhanced Wavelength Tuning in
`Grating—Assisted Codirectional Coupler Filter”, IEEE
`Photo. Tech. Ltrs.,vol. 5, No. 10, Oct. 1993, pp. 1219—1221.
`
`Petitioner Ciena Corp. et al.
`Exhibit 1026-3
`
`

`
`U.S. Patent
`
`Feb. 23, 1999
`
`Sheet 1 0f 10
`
`5,875,272
`
`FIG. 1
`
`Petitioner Ciena Corp. et al.
`Exhibit 1026-4
`
`

`
`U.S. Patent
`
`Feb. 23, 1999
`
`Sheet 2 0f 10
`
`5,875,272
`
`FIG. 2
`
`Petitioner Ciena Corp. et al.
`Exhibit 1026-5
`
`

`
`U.S. Patent
`
`Feb. 23, 1999
`
`Sheet 3 0f 10
`
`5,875,272
`
`Source
`1
`
`Detector
`2
`
`\
`
`89
`
`Detector
`l
`
`Source
`2
`
`49 \
`
`39
`
`FIG. 3
`
`Petitioner Ciena Corp. et al.
`Exhibit 1026-6
`
`

`
`U.S. Patent
`
`Feb. 23, 1999
`
`Sheet 4 0f 10
`
`5,875,272
`
`FIG. 4
`
`Petitioner Ciena Corp. et al.
`Exhibit 1026-7
`
`

`
`Petitioner Ciena Corp. et al.
`Exhibit 1026-8
`
`

`
`Petitioner Ciena Corp. et al.
`Exhibit 1026-9
`
`

`
`U.S. Patent
`
`Feb. 23, 1999
`
`Sheet 7 0f 10
`
`5,875,272
`
`Re?ectivity
`
`V
`
`FIG. 7(1))
`
`Petitioner Ciena Corp. et al.
`Exhibit 1026-10
`
`

`
`U.S. Patent
`
`Feb. 23, 1999
`
`Sheet 8 0f 10
`
`5,875,272
`
`FIG. 8
`
`Petitioner Ciena Corp. et al.
`Exhibit 1026-11
`
`

`
`U.S. Patent
`
`Feb. 23, 1999
`
`Sheet 9 0f 10
`
`5,875,272
`
`Ef?mency (add/drop
`#1)
`Ef?clency
`
`(add/drop #2)
`
`. 9(1))
`
`v
`
`FIG. 9(a)
`
`Petitioner Ciena Corp. et al.
`Exhibit 1026-12
`
`

`
`U.S. Patent
`
`Feb. 23, 1999
`
`Sheet 10 0f 10
`
`5,875,272
`
`2 .UE
`
`Petitioner Ciena Corp. et al.
`Exhibit 1026-13
`
`

`
`5,875,272
`
`1
`WAVELENGTH SELECTIVE OPTICAL
`DEVICES
`
`REFERENCE TO RELATED APPLICATIONS
`
`This application is a continuation-in-part of US. appli
`cation Ser. No. 08/703,357, ?led on Aug. 26, 1996, now US.
`Pat. No. 5,805,751, issued Sep. 8, 1998, and claims the
`bene?t of Provisional Application No. 60/005,915, ?led Oct.
`27, 1995.
`
`FIELD OF THE INVENTION
`
`The present invention relates to the communication of
`signals via optical ?bers, and particularly to an optical ?ber
`coupler and methods for making the same. More
`particularly, the invention relates to optical devices and
`subsystems using a Wavelength selective optical coupler.
`
`10
`
`15
`
`DESCRIPTION OF RELATED ART
`
`2
`One realiZation of a directional coupling based device
`uses gratings recorded in a coupler composed of tWo iden
`tical polished ?bers placed longitudinally adjacent to one
`another (J .-L. Archambault et al., Optics Letters, Vol. 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 ?ber to the other. Another device also based on
`evanescent coupling Was patented by E. SnitZer, US. Pat.
`No. 5,459,801 (Oct. 17, 1995). This device consists of tWo
`identical single mode ?bers Whose cores are brought close
`together by fusing and elongating the ?bers. The length of
`the coupling region should be precisely equal to an even or
`odd multiple of the mode interaction length for the output
`light to emerge entirely in one of the tWo output ports. 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. Alferness et al., US. Pat. No. 4,737,007 and
`M. S. Whalen et al., Electronics Letters, Vol. 22, p. 681
`(1986) uses locally dissimilar optical ?bers. The resulting
`asymmetry of the tWo ?bers improves the isolation of the
`optical signals Within the tWo ?bers. HoWever, this device
`used a re?ection grating etched in a thin surface layer on one
`of the polished ?bers, dramatically reducing, the coupling
`strength of 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 ?ber is very close to the Wavelength for Which light
`is backre?ected Within the original ?ber (about 1 nm). This
`leads to undesirable pass-band characteristics that are ill
`suited for add/drop ?lter devices designed to add or drop
`only one Wavelength. For optical communications applica
`tions in the Er doped ?ber ampli?er (EDFA) gain WindoW
`(1520 to 1560 nm), this backre?ection should occur at a
`Wavelength outside this WindoW to prevent undesirable
`crosstalk. The separation betWeen the backre?ected and
`backWards coupled Wavelengths is impractically small for
`the all-?ber, 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 difference betWeen tWo interferometer arms and is
`highly sensitive to environmental ?uctuations and manufac
`turing variations. In addition, a signi?cant fraction of the
`input signal is back re?ected. 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
`fers from both a relatively loW coupling strength and small
`Wavelength separation of back-re?ected 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 ?bers can instead be fused
`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 core/air cladding
`Waveguide. The tWo Waveguides are merged such that the
`
`20
`
`25
`
`30
`
`35
`
`40
`
`LoW loss, Wavelength selective couplers are important
`components for optical ?ber communication netWorks based
`on Wavelength division multiplexing (WDM). WDM
`enables an individual optical ?ber 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 ef?ciency and loW loss. One technology
`to fabricate Wavelength selective elements is based on
`recording an indeX of refraction grating in the core of an
`optical ?ber. See, for instance, Hill et al., US. Pat. No.
`4,474,427 (1984) and Glenn et al., US. Pat. No. 4,725,110
`(1988). The currently preferred method of recording an
`in-line grating in optical ?ber is to subject a photosensitive
`core to the interference pattern betWeen tWo beams of actinic
`(typically UV) radiation passing through the photoinsensi
`tive cladding.
`Optical ?ber gratings reported in the prior art almost
`universally operate in the re?ection mode. To gain access to
`this re?ected mode in a poWer ef?cient manner is dif?cult,
`because the Wave is re?ected backWards Within the same
`?ber. A ?rst method to access this re?ected light is to insert
`a 3 dB coupler before the grating, Which introduces a net 6
`dB loss on the backWards re?ected and outcoupled light. A
`second method is to insert an optical circulator before the
`45
`grating to redirect the backWards propagating mode into
`another ?ber. This circulator introduces an insertion loss of
`1 dB or more and involves complicated bulk optic compo
`nents. A method to combine the ?ltering function of a ?ber
`grating With the splitting function of a coupler in a loW loss
`and elegantly packaged manner Would be highly desirable
`for 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
`evanescent coupling have inherently Weaker interaction
`strengths.
`
`50
`
`55
`
`60
`
`65
`
`Petitioner Ciena Corp. et al.
`Exhibit 1026-14
`
`

`
`5,875,272
`
`3
`energy in the original optical modes of the separate
`Waveguides interact in a substantially non-evanescent man
`ner in the merged region. The index pro?le of the optical
`Waveguide varies suf?ciently sloWly in the longitudinal
`direction such that light entering the adiabatic taper region
`in a single eigenmode of the Waveguide evolves into a single
`local supermode 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 PCT
`patent application PCT/US96/13481.
`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 effects a change in optical properties;
`An “optical ?ber” 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 ?ber
`and planar Waveguides;
`An “add/drop ?lter” 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 indeX maXima is constant;
`The “bandWidth” of a grating is the Wavelength separation
`betWeen those tWo points for Which the re?ectivity of the
`grating is 50% of the peak re?ectivity of the grating;
`A “coupler” herein is a Waveguide composed of tWo or
`more ?bers placed in close proXimity of one another, the
`proXimity being such that the mode ?elds 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. Atransversely 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 ef?ciently
`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
`mode couplers, Which redirect optical energy of a particular
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`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.
`Atunable grating assisted mode coupler can be fabricated
`by varying the optical properties (e.g., indeX of refraction,
`length) of the coupler interaction region. Alternately, 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 ampli?er and a Wavelength selective
`optical modulator. Another type of Wavelength selective
`optical sWitch is described, based on tunable, grating
`assisted mode couplers attached to ?Xed Wavelength, grating
`assisted mode couplers. A WDM multi-Wavelength trans
`mitter subsystem, broadly tunable add/drop ?lters, and
`recon?gurable, Wavelength selective routers are further dis
`closed. Accordingly, the present invention provides signi?
`cant advantages in optical communications and sensor sys
`tems that require narroW optical bandWidth ?lters 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 ?gures:
`FIG. 1 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 detuned 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;
`FIG. 9 shoWs a broadly tunable add/drop ?lter based on
`the optical vernier effect;
`FIG. 10 shoWs an eight channel, programmable WDM
`router.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`Optical ?bers 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 ?ber, it
`travels in isolation unless an optical coupler is inserted at
`some location along the ?ber. Optical couplers alloW light
`signals to be transferred betWeen normally independent
`optical Waveguides.
`If multiple signals at different Wavelengths travel doWn
`the same ?ber, it is desirable to transfer a signal at only a
`predetermined set of Wavelengths to or from this ?ber into
`another ?ber. These devices are called Wavelength selective
`optical couplers. A desirable attribute of such a Wavelength
`
`Petitioner Ciena Corp. et al.
`Exhibit 1026-15
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`

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`selective optical coupler is that it remains transparent to all
`Wavelengths other than those to be coupled. This transpar
`ency is quanti?ed 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 ?ber to
`another at only a prede?ned, precise set of Wavelengths. It
`intrinsically is a bi-directional, 4 port device that serves as
`both an add and drop ?lter. 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 region of the grating assisted mode
`coupler. The grating assisted mode coupler can be fabricated
`by fusing together tWo optical ?bers, 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 ?rst Waveguide 11 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‘1
`evolves into the coupler Waist mode 71 With propagation
`vector [31, and the backWards propagating Waist mode 61
`With propagation vector , [32 evolves into the output mode 41
`With propagation vector [35. The propagation vectors [31 and
`[32 at the Waist satisfy the Bragg laW for re?ection from a
`thick index grating of period Ag at a particular Wavelength,
`say hi:
`[31(>\'i)_[320\'i)=2 T's/A9
`then the optical energy at k,- in the ?rst Waveguide 11 is
`coupled into the backWard propagating mode of the second
`Waveguide 21 (FIG. I). The spectral response and ef?ciency
`of this re?ective coupling process is dictated by the coupling
`strength and the interaction length of the optical modes With
`the grating.
`In FIG. 2, the Wavelength of the input mode is detuned,
`say to )t], so that [31()\,]-)—[32()\,j)#2 J's/Ag, and the input mode
`31 in the ?rst Waveguide travels through the coupler Waist
`and reappears as the transmission output mode of the ?rst
`Waveguide 51, as seen in FIG. 2, With minimal leakage into
`the second Waveguide 21. Therefore, only a particular Wave
`length k,- is coupled out of the ?rst Waveguide II, as
`determined by the grating period in the coupling region 1.
`The amount of Wavelength detuning required to reduce the
`re?ective coupling by 50% is given by the full-Width-half
`maxima (FWHM) bandWidth A)» of the grating:
`
`Where Leff 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 K. The bandWidth of
`re?ection gratings is narroWer than that of transmission
`gratings by typically ten to ?fty times because the grating
`period Ag is much shorter for the former. The narroWer
`frequency response in the re?ection mode is desirable for
`dense WDM applications. Typically, the desired bandpass is
`approximately 0.1 nm at 1.55 pm. This dictates that the
`length of the re?ection grating should be approximately 1
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`cm. A re?ectivity in excess of 90% for a grating thickness L
`of 1 cm requires a KL larger than 2. K should then be 2 cm‘.
`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 ?bers by appropriate preparation of the materials
`and dimensions of the media.
`In addition to backWards coupling of light into the adja
`cent Waveguide, the grating typically re?ects some light
`back into the original ?ber at a different Wavelength given by
`2B1(>\.2)=kg. To ensure that )»2 is outside the Wavelength
`operating range of interest, the difference betWeen [31 and [32
`is made suf?ciently large. The difference 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 difference is maximiZed for
`small coupler Waists, in Which [31 and [32 correspond to the
`LPO1 and LP11 modes of an air-clad optical Waveguide.
`Furthermore, an appropriate transversely asymmetric grat
`ing substantially reduces the coupling strength for back
`re?ection.
`The grating, assisted mode coupler 9, illustrated in FIG.
`3, redirects optical energy at a particular Wavelength from a
`source 79 to the input optical ?ber 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
`?ber, Which travels to detector 89. All other Wavelengths
`propagate through the coupler from 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 ?ber optic components and sub
`systems are made economically and practically feasible by
`linking other optical devices to this grating assisted mode
`coupler. This approach enables standard ?ber 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—throughput 19 and add 49—drop 59 ports behave
`in a complementary manner. A single gra