CA2146275C - Wavelength division optical multiplexing elements - Google Patents
Wavelength division optical multiplexing elementsInfo
- Publication number
- CA2146275C CA2146275C CA002146275A CA2146275A CA2146275C CA 2146275 C CA2146275 C CA 2146275C CA 002146275 A CA002146275 A CA 002146275A CA 2146275 A CA2146275 A CA 2146275A CA 2146275 C CA2146275 C CA 2146275C
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- optical
- waveguides
- waveguide
- output
- filtering elements
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Classifications
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
- G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
- G02B6/29304—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by diffraction, e.g. grating
- G02B6/29316—Light guides comprising a diffractive element, e.g. grating in or on the light guide such that diffracted light is confined in the light guide
- G02B6/29317—Light guides of the optical fibre type
- G02B6/29319—With a cascade of diffractive elements or of diffraction operations
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
- G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
- G02B6/29379—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device
- G02B6/2938—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device for multiplexing or demultiplexing, i.e. combining or separating wavelengths, e.g. 1xN, NxM
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04J—MULTIPLEX COMMUNICATION
- H04J14/00—Optical multiplex systems
- H04J14/02—Wavelength-division multiplex systems
Abstract
The present invention relates to methods and apparatus for wavelength division multiplexing (WDM) using arrays of optical filtering elements to create a desired wavelength passband in an optical filter. In one aspect of the invention, an optical demultiplexer is formed using an input waveguide, such as a planar waveguide or an optical fiber, coupled to plural output waveguides. Each output waveguide includes a wavelength selective configuration of optical filtering elements formed within a contiguous portion of the waveguide forming an optical channel-selective filter having spectral regions having an optical transmission passband and spectral regions of low transmissivity. Exemplary optical filtering elements areBragg gratings formed into an optical filter which transmits a characteristic wavelength band.
Description
21~6275 ",~ 1 --WAVELENGTH DIVISION OPI ICAL MULTIPLEXING ELEMENTS
Back~round of the I~ ..lion 1. Field of ~e Invenffon The invention relates to wavelength division optical multiplexing and, 5 more particularly, to wavelength division demultiplexers and multiplexers including arrays of optical filtering elements such as Bragg gratings, that form optical filters having characteristic wavelength passbands.
Back~round of the I~ ..lion 1. Field of ~e Invenffon The invention relates to wavelength division optical multiplexing and, 5 more particularly, to wavelength division demultiplexers and multiplexers including arrays of optical filtering elements such as Bragg gratings, that form optical filters having characteristic wavelength passbands.
2. Descripffon of the R~l~t~ Art Wavelength-division multiplexing is a valuable technique for increasing 10 the information-carrying capacity of an optical communication network. In a multiplexed network, it is essential to provide wavelength-selective components that can separate a particular wavelength channel from a group of multiplexed chS~nnel.e.
Bragg gratings, elements comprising regions of refractive index pe~ l,ations which reflect particular wavelengths, are advantageously used to provide spectral 15 selectivity. They are readily made with passbands that match desired channel widths, and they are readily integrated with optical waveguides such as optical fibers, planar, and channel waveguides. Current ch~nn~l-selective filters using Bragg gratings can suffer from relatively high coupling losses; others are difficult to manufacture because they must be made to extremely close tolerances.
For example, one known filter configuration includes an input fiber coupled to a fiber-based Bragg grating through a directional optical coupler. Light reflected from the grating is coupled to an output waveguide through the same coupler. In this manner, the narrow reflection band of the grating is effectively transformed into a trancmi~cion band for the filter as a whole. However, this 25 arrangement suffers 6 dB of coupling loss, to which is added the intrinsic loss in the directional coupler.
Another example is a filter described in K.O. Hill et al., "Narrow-Bandwidth Optical Waveguide Tran.cmi~sion Filters," Electronics Letters 23 (1987) pp. 465-466. This filter consists of a loop of single-mode optical fiber which 30 includes a fused-taper directional coupler at the point where the loop branches off from the straight part of the fiber. A Bragg grating is situated symmetrically midway in the fiber loop. This filter can be made to have relatively low loss. However, this filter is difficult to manufacture because exact placement of the Bragg grating is required in order to achieve the desired trancmicsion charac~eristics. Bilodeau et al., 35 "High-Return-Loss Narrowbond All-Fiber Bandpass Bragg Trancmic~ion Filter,"
21~6275 ,.,~ "
, _ - 2 -IEEE Photonics Technology Letters, Vol. 6, No. 1, (1994), pp. 80-82 describes a design modification of the Hill et al. filter.
Compound Bragg reflection filters made in planar waveguides are described in C.H. Henry et al., "Compound Bragg Reflection Filters Made by Spatial S Frequency Doubling Lithography," J. Li~htwave Technol. _ (1989) pp. 1379-1385.As reported therein, a filter having desirable spectral properties can be made by forming a Bragg grating having many, e.g., 15, sections, each with a different spatial peAod. To avoid phase shifts near 90~ between section~, the secdons are made contiguous, resulting in a grating period that is piecewise constant, but 10 discontinuously varied.
Accordingly, there is a need in the art for wavelength division multiplexing elements which are simple to m~mlf~cture and which are readily formed from optical filtering elements which separate a particular wavelength channel from a group of multiplexed channels. There is a further need in the art for 15 wavelength division optical multiplexing elements which are in~cen~citive to the polarization of incident radiation.
S~ ry of the Invention The present invention relates to methods and apparatus for wavelength division multiplexing (WDM) using arrays of optical filtering elem~ntc to create a 20 desired wavelength passband in an optical filter. In one aspect of the invention, an optical demultiplexer is formed using an input waveguide, such as a planar waveguide or an optical fiber, coupled to plural output waveguides. Each output waveguide includes a wavelength selective configuration of optical filtering elements formed within a contiguous portion of the waveguide. The configuration forms an 25 optical channel-selective filter with spectral regions having an optical tr~n.~mi~ion passband and spectral regions of low tr~n~mis.~ivity.
In one embodiment, the invention relates to an optical demultiplexer having an input waveguide and N output waveguides, N being a whole number greater than or equal to 2. Each output waveguide includes a wavelength-selective 30 configuration of at least (N-l) optical filtering elements such a~s Bragg gratings, each of the Bragg gratings having at lea~st one wavelength band of low tr~ncmi~sivity, ~,~
where each x is a number from 1 to N. The configuration of optical fil~ring elements creates an opdcal filter which transmit~s a characterisdc wavelength band, i.e., the wavelength band i~n of the Bragg grating absent from the configuradon of 35 optical filtering elements in the waveguide. A connecdng element such as a lxN
coupler is posidoned between the input and output waveguides to divide opdcal 3 ~ 2 ~ ~
tr~n~mi~sions from the input waveguide among the output waveguides.
In another aspect, the present invention provides an optical demultiplexer having an input waveguide and N output waveguides. Y branching waveguides are positioned between the input waveguide and the output waveguides, Y being a whole number S greater than or equal to 2 and less than or equal to N-1. As in the previous embodiment, each of the output waveguides transmits a characteristic wavelength band, ~ where n is a whole number from 1 to N. Positioned within the output waveguides and said branching waveguides are a plurality of optical filtering elements each optical filtering element having a different wavelength band of low transmissivity ~x, where x is a whole number from 1 to N and is not equal to n, the number corresponding to the channel passband. The plurality of optical filtering elements is configured such that within the optical path of the tr~n~mi~sion emitted from each output waveguide are positioned at least N-l optical filtering elements, each of the N-1 optical filtering elements having a different wavelength band of low tr~n.smi.~ivity. Thus, the combination of the optical filtering elements is such that the combination of the low tr~n~mi~sivity wavelength bands of each of the N-l optical filtering elements results in tr~n~mission of the characteristic wavelength, ~n. In an exemplary embodiment, the branching waveguide includes all of the optical filtering elements for the wavelengthbands which are not to be transmitted by any of the output waveguides connected to that branching waveguide.
In another aspect, the present invention relates to polarization-insensitive wavelength division demultiplexers. The demultiplexer includes an input optical waveguide and plural output waveguides. Each output waveguide has a plurality ofoptical filtering elements integrally formed within a contiguous portion of the waveguide.
The demultiplexers exhibit a polarization sensitivity of about 1 part in 100,000 or less.
In one particularly preferred embodiment the present invention provides an optical demultiplexer comprising: an input optical waveguide; N output optical waveguides, N being a whole number greater than or equal to 2, each output waveguide including a wavelength-selective configuration of optical filtering elements comprising at least (N-l) Bragg gratings, each of said Bragg gratings having at least one wavelength band of low transmissivity, ~x where x is a number between 1 and N, said configuration of optical filtering elements forming an optical filter transmitting a characteristic -- 3a-wavelength band, the characteristic wavelength band being the wavelength band ;~n of the Bragg grating absent from the configuration of optical filtering elements in thewaveguide; and an element positioned between said input waveguide and said output waveguides for splitting optical tr~n~micsions from the input waveguide to the output S waveguides.
Brief Description of the Dr~wi ~.~
FIG. I is a schematic representation of an 8-channel optical demultiplexer according to one embodiment of the present invention.
FIG. 2 is a schematic representation of an 8-channel optical demultiplexer 10 according to a further embodiment of the present invention.
FIG. 3 is a schematic representation of an 8-channel demultiplexer according to a further embodiment of the present invention.
FIG. 4 is a tr~n~mi~cion spectrum of a Bragg grating used as an optical filtering element according to the invention in one embodiment.
,~
!~
21~6275 -~,k ~_ - 4 ~
FIG. S is a trancmiCcion spectrum for one channel of the demultiplexer of FIG. 1.
Detailed Des~ ,lion The invention will now be described with reference to the drawings in 5 detail, in which like reference numerals indicate the same or similar elements in each of the FIGS. While the invention is described in terms of its application to demultiplexing, it will be appreciated by those skilled in the art that the invention applies equally well to multiplexing. A demultiplexing apparatus, operated in the opposite order, i.e., specific wavelength bands added to the trAn~mitted radiation 10 rather than filtered out of a composite trAn.cmicsion, functions as a multiplexing apparatus. The term "demultiplexer" or "demultiplexing" as used herein encompasses any device used to separate a particular wavelength band from a group of multiplexed wavelength bands. This additionally includes devices used to route wavelength bands positioned well before the receivers in a tran.cmi~ion system.
In one embodiment, of the present invention, the demultiplexer includes an input optical waveguide and a plurality of output optical waveguides, each output optical waveguide having an optical filter comprising an array of optical filt~ring elements, such as Bragg diffraction gratings, formed in a condguous portion of the output waveguide.
Bragg gratings are regions of refractive index perturbations within a waveguiding element which create a reflection peak from wavelengths of incident radiation satisfying the Bragg condition:
i~ Bragg = 2 n eff A = 2 n eff 2 where ~ Bragg = ~-a~le~ ~lhs of incident radiation which satisfy the Bragg cQ~ n n eff = effective refiractive index of the waveguiding medium = wavelength of the uv radiâtion used to form the grating oc = half angle of inc~Aenoe of ~e exposing radiation spacing of the ~irla~ ndex pe.~ons (grating period) 30 The optical filter formed from the array of optical filt~ring elements, such as an array of Bragg gratings, transmits a characteristic wavelength band by reflecting the 2146~75 ~.
..~"~.
, ~ - 5 -collective wavelength bands reflected by each individual Bragg grating. The termreflection, as used herein, includes any radiation which is not tran~minP~ by the waveguide, i.e., it includes radiation which is scattered out of the waveguide.
As schematically illustrated in FIG. 1, demuldplexer 220 includes an 5 input waveguide 210 and N output waveguides 220 coupled to waveguide 210 through 1 X N coupler 230. For the demultiplexer shown in FIG. 1, N=8; however Nis any whole number greater than or equal to 2. Each output waveguide transmits a characteristic wavelength band, ~n, where n is a whole number between 1 and N.
For the FIG. 1 demultiplexer, n=8. The wavelength indices which appear at the 10 terminus of each waveguide denote which desired signal wavelength band is passed by that output waveguide. It will be appreciated by those skilled in the art that radiation outside the signal wavelength bands may also be passed to the output waveguides; such radiation does not deleteriously impact system perforrn~n~e Positioned within each of the output waveguides are N- 1 optical filt~pring 15 elemen~ 240. For the illustrated demultiplexer, N=8, therefore there are 8-1='7 optical filtering element.c. Exemplary optical filtering element.c 240 are Bragggratings, each Bragg grating having a wavelength band of low tr~n~missivity, i.e., the wavelength band which the grating attenuates. The combination of the opticalfiltering elements forms an optical filter in which all of the wavelength bands are 20 blocked except the characteAstic wavelength band of the output waveguide. Thus, for the output waveguide labeled ~ 1, the seven Bragg gratings positioned within that waveguide collectively attenuate wavelength bands i~2, ~3, ~4~ ~5~ ~6~ ~7, and ~8 as illustrated in FIG. 1. The wavelength numbers which appear adjacent each grating in the output waveguides indicate the wavelength band blocked by that 25 grating.
Optionally, an additional Bragg grating is positioned in the array. The additional Bragg grating does not correspond to a channel, but instead is used to broaden the channel of the Bragg grating with the shortest wavelength. A furtherBragg grating can also be used to broaden the channel of the Bragg grating with the 30 longest wavelength. The use of an additional Bragg grating on the short and/or long wavelength end of the spectrum permits attenuation of additional undesirable wavelengths, such as those which might result from amplified spontaneous emission noise in an optical amplifier.
Each of the rem~ining output waveguides includes a similar array of 35 filters, i.e., for the 8-channel demultiplexer of FIG. 1, there are seven Bragg gratings in each output waveguide, each Bragg grating blocking a different wavelength band u between ~ 1 and ~8 . Using ~" to represent the characteristic wavelength band of the output waveguide, the seven Bragg gratings block wavelengths from n=l to n=8.
There is no Bragg grating blocking a wavelength for ~ n where n=x, i.e., no Bragg grating is positioned in the output waveguide which blocks the wavelength band to be passed by that output waveguide.
Although the demultiplexer of FIG. 1 employs a Bragg grating for each channel to be blocked, a single Bragg grating can be used to block two or more channels simultaneously when the reflection peak of the grating is sufficiently wide.
For this embodiment, the demultiplexer has N channels but fewer than (N-l) opdcal 10 filtering elPmen~.~ positioned within each output waveguide.
FIG. 2 illustrates a further embodiment of a demultiplexer according to the present invention. In the demultiplexer of FIG. 2, branching waveguides 260 are positioned between the input waveguide 210 and N output waveguides 220. These branching waveguides have optical filtering elements 270, such as Bragg gratin~
formed therein. Advantageously, the optical filtering element.~ formed within the br~nching waveguides elimin~te the need to repeat the branching waveguide filtering elements within the output waveguides. Thus, the wavelength bands which are to be blocked by all output waveguide within a set of output waveguides have the corresponding optical filtering elements formed within the branching waveguide, 20 elimin~ting the need to duplicate filtering element formation within each of the output waveguides.
The input waveguide 210 is connected to the branching waveguides 260 through 1 x Y coupler 230, as in the previous embodiment, where Y is the number of branching waveguides which connect to the input waveguide. In general, Y is a whole number greater than or equal to 2 and less than or equal to N- 1. In the demultiplexer of FIG. 2, N is equal to eight and Y is equal to two. A coupler 250is positioned between branching waveguides 260 and output waveguides 220. In FIG. 2,1x 4 couplers are used to connect the branching and output waveguides.
For the demultiplexer configuration of FIG. 2, the output waveguides passing wavelength bands ~ 4 all must block wavelength bands ~5 - ~ 8.
Therefore, the branching waveguide which transmits radiation into these four output waveguides includes filtering elements which block the tr~ncmic~ion of wavelength bands ~5-~8. As compared to the demultiplexer configuration of FIG. 1, twenty-four fewer optical filtering elements are included in the output waveguides due to the addition of the branching waveguides having filtering elements blocking wavelength bands common to the connecting output waveguides.
214~7S
. ~ ..
An exemplary optical filtering element array for the br~nrl~ing configuration depicted in FIG. 2 is illustrated in FIG. 3. In this embodiment, the selection of optical filtering elements is staggered such that the array of optical filtering elements in the output waveguides does not include optical filtçring S elements which block wavelength bands adjacent to the pass wavelength band of the output waveguide. The wavelength passband for the four output waveguides which connect to the first branching waveguide are selected to be ~ 1, A, 3, ~ 5, and ~ 7 .
Thus, none of the output waveguides connected to the first branching waveguide will transmit wavelength bands ~2. ~4~ ~6. and ~8 For this reason, filtering elemçnt.c 10 which block the tran.emi.ccion of ~2. ;~4. ~6. and ~8 are positioned within the first branching waveguide. The rem~ining wavelength bands to be blocked in order to transmit the desired wavelength band are positioned in the output waveguide. For the output waveguide which tr~ncmite wavelength band ~ 1, the rem~ining wavelength bands to be blocked are ~3, ~5, and ~7. Filtering elçm~nt.e such as Bragg gr?~tinge 15 which block these wavelength bands are positioned within the output waveguide as shown in FIG. 3. This configuratdon of filtçring elements advantageously permitsrelaxed tolerances on gratdng positdoning in the output waveguides since twice the channel spacing is produced in the output waveguides through the blocking of alternate wavelength bands by the gratings positioned in the br~n~hing waveguides.
An i~ç~1i7Pd tranemi.esion spectrum for a single Bragg grating is depicted in FIG. 4. In spectrum 10, region 20 is a wavelength band having a tr~n.emi.e.eion minimum which corresponds to incident radiatdon reflected or scattered by the gradng. On the short wavelength side of tr~nemi.eeion minimum 20 is dip 30.
Dip 30 results from radiatdon loss and is separated from tr~nemi.eeion minimum 20 25 by relatdvely high tr~nemi.seivity region 40. The expression "radiation loss"describes any light which is scattered out of the core of the waveguide, including r~ tion scattered in the cladding of the waveguide.
The short-wavelength edge of tr~n~emieeion minimum 20 is i~gragg, i.e., the Bragg wavelength in the limit of a very weak grating. In this limit, the 30 long-wavelength edge of the dip occurs at the wavelength ~Bragg~;~offset where ~offset is given by:
o Bragg ( 1 llCI ) where ~" - 8 -n cl = cladding refractive index neff = effective lcirla~ index of the waveguide medium It is usually advantageous to reduce the depth of region 30, minimi7ing radiation loss. However, as grating strength increases, measured by the widening of s tr~ncmi.c.cion minimum region 20, radiation loss increases. On a log tr~ncmi.c~sion plot, radiation loss increases quadratically for a given increase in grating strength, whereas widening of region 20 increases only linearly for strong gratings. Radiation loss can interfere with tr~ncmiscion when it occupies a spectral region for a channel passband.
Several techniques are used to elimin~te or reduce the impact of radiation loss on an optical filter formed from plural Bragg gratings. One technique minimi7~s radiation loss through the use of waveguiding media having large effective refractive indices. Alternatively, gratings may be formed in waveguiding media having weak radiation loss and fused to tr~n.cmi.c.cion waveguiding media.15 Peak radiation loss can further be reduced by recoating the grating region of the waveguide to improve tr~ncmiscion characteristics. See, for example, Mi_rahi andSipe, J. of Light Tech., Vol. 11 (October 1993) pp. 1513-1517, especially Figs. 3(a) and 4, the disclosure of which is incorporated herein by reference.
The design of the optical filter also minimi7~s or reduces the effects of 20 radiation loss. For filters having a small number of channels, the grating configuration positions channel passbands in spectral regions where there is no radiation loss region from other gratings in the filter.
For filters having larger numbers of channels, it is often not possible to avoid a passband for a particular channel falling within a spectral region of radiation 2s loss for another grating. In such situations, plural weaker gratings can be used to pass a single channel since weaker gratings exhibit proportionally less radiadonmode loss, as discussed above. Weaker gratings are forrned by controlling the amount of actinic radiation exposure, and hydrogenation level and gerrnanium doping levels in hydrogen-treated germanosilicate waveguides.
The waveguides employed in the demultiplexers of FIGS. 1-3 are exemplarily single-mode optical fibers, as are typically employed in optical tr~ncmiccion systems, while the lxN coupler is exemplarily a fused fiber coupler.
However, waveguides for the above demultiplexers are selected from any waveguiding media which transmit optical signals including, but not limited to, 3s multiple-mode fiber and planar waveguides. Additionally, fibers and planar waveguides can be incorporated into a single device, e.g., input and/or output fibers coupled to a planar waveguide in which Bragg gratings are formed.
Various coupler combinations can provide the desired I xN coupling. For example, the configuration illustrated in FIG. 2 employs one I x2 coupler and two I x4 5 couplers to create an array with 1 input waveguide and 8 output waveguides. Numerous other combinations can be used to achieve similar results.
Radiation losses may be minimi7ed by using a section of multiple-mode fiber having a small number of modes since multiple-mode fiber generally has better mode-field confinement for the fundamental mode than single-mode fiber. By carefully injecting radiation from a single-mode fiber into a multiple-mode fiber at a wavelength near cutoff, it is possible to largely avoid exciting any but the fundamental mode of the multiple-mode fiber. A section of multiple mode fiber can be inserted through fusion splicing of multiple-mode and single-mode fibers.
When using Bragg diffraction gratings as optical filtering elements, a preferredmethod for grating formation is to optically induce refractive index perturbations. A
photosensitive optical waveguide, such as an optical fiber, is exposed to an interference pattern of actinic, typically ultraviolet, radiation to form refractive index perturbations at least partially in the fiber core. Alternatively, the waveguide is exposed to aninterference pattern created by impinging a single actinic beam on a phase mask. The phase mask is a patterned, diffractive medium that modulates the relative phase of the diffracted wavefronts. Phase masks are typically manufactured by reactive ion etching of a fused-quartz substrate through a chromium mask patterned by electron-beam lithography. However, various other methods may be used to form phase masks, such as holographic patterning techniques. The use of phase masks to fabricate Bragg gratings is described in C~n:~-lian Patent Application Serial No. 2,111,808, filed December 17, 1993, by D. Z. Anderson et al.
To produce strong gratings, i.e., those gratings having wide spectral reflectionpeaks, waveguides are sensitized by exposure to hydrogen gas prior to patterning with actinic radiation. This technique is described in U.S. Patent No. 5,235,659 issued on August 10, 1993 by R. M. Atkins et al. and U.S. PatentNo. 5,287,427 issued on February 15, 1994 by R. M. Atkins et al. Grating strength is controlled through actinic radiation exposure for a given photosensitive medium. When using hydrogen-treated germanosilicate glass waveguides, the hydrogenation levels in the waveguide core also controls grating strength. Further description of Bragg grating ~i~
2 ~ 7 5 .". ~
1 o -fabrication to form optical filters is found in U.S. Patent Application Serial No.
08/078,992 filed June 17, 1993 to Mizrahi and assigned to the present acsign~e, the disclosure of which is incorporated herein by reference.
FIG. S is a transmiccion spectrum, i.e., tran~mission as a function of 5 wavelength, for the channel corresponding to ~ 2 of the 8-channel demultiplexer of FIG. 1. In this FIG., region 100 represents the tran~miccion minimum resnlting from gratings which block wavelength bands ~ 3. ;~4. ~5- ~6. ;~7. and ;~8. Note that the noise which appears in region 100 is instrumentation-induced and not representative of the attenuation characteristics of the demultiplexer. Channel 2 10 (~2), shown as region 110, is passed by this output waveguide of the demultiplexer.
The approximate locations of the wavelengths corresponding to each channel are indicated by the dashed lines in FIG. 5.
Advantageously, demultiplexers formed according to the present invention are insensitive to the polarization of the light which passes through them.
15 By polarization insensitivity, it is meant that the tr~n.cmi~ion spectrum of the demultiplexer does not shift for radiation of different polarizations. Thus, a device having a polarization sensitivity of 1 part in 100,000 exhibits a worst-case shift of O O
0.15 A or less at a wavelength of 15,000 A. Since optical fibers often do not maintain polarization over long distances, polarization insensitivity of the 20 demultiplexers results in effective signal filtering regardless of the polarization of the incident radiation. In particular, the demultiplexers of the present invention can be formed with a polarization sensitivity of less than 1 part in 100,000.
Demultiplexers formed according to the present invention are suitable for incorporation in terrestrial and underseas optical communication systems. Such 25 optical communications systems generally include an optical source such as a laser, a mod--lating member to create a desired optical signal, a transmi.~sic)n member such as a fiber or planar waveguide, conventional or optical amplifiers, e.g., erbium-doped fiber amplifiers (EDFAs), and receivers. Advantageously, the use of wavelength division demultiplexers increases the capacity of a single-mode optical fiber 30 tr~ncmi.ccion system by perrnitting a single fiber to carry plurality wavelength bands.
Such systems typically incorporate optical isolators in the optical amplifiers to prevent tran.cmi~.cion of light reflected by the optical filters back down the trarl~mi~.~ion member. When incorporated in a terrestrial or underseas communication system, it is advantageous to package the demultiplexer to assure 35 thermal stability of the device. Heating and/or cooling elements permit a desired temperature range to be selected, ensuring the accuracy and reliability of the 21~27~
demultiplexer.
The dem~ iplexers of the present invention can also be employed in optical s~lems other than the above-named optical telecommnni~ations systems.
Such optical systems include, but are not limited to, local area network (LAN) and S CATV systems.
While the invention has been described with respect to the foregoing embodimen~, it will be appreciated that various changes and modifications may bemade. Accordingly, changes and modifications such as those suggested above, but not limited thereto, are within the scope of the claimed invention.
Bragg gratings, elements comprising regions of refractive index pe~ l,ations which reflect particular wavelengths, are advantageously used to provide spectral 15 selectivity. They are readily made with passbands that match desired channel widths, and they are readily integrated with optical waveguides such as optical fibers, planar, and channel waveguides. Current ch~nn~l-selective filters using Bragg gratings can suffer from relatively high coupling losses; others are difficult to manufacture because they must be made to extremely close tolerances.
For example, one known filter configuration includes an input fiber coupled to a fiber-based Bragg grating through a directional optical coupler. Light reflected from the grating is coupled to an output waveguide through the same coupler. In this manner, the narrow reflection band of the grating is effectively transformed into a trancmi~cion band for the filter as a whole. However, this 25 arrangement suffers 6 dB of coupling loss, to which is added the intrinsic loss in the directional coupler.
Another example is a filter described in K.O. Hill et al., "Narrow-Bandwidth Optical Waveguide Tran.cmi~sion Filters," Electronics Letters 23 (1987) pp. 465-466. This filter consists of a loop of single-mode optical fiber which 30 includes a fused-taper directional coupler at the point where the loop branches off from the straight part of the fiber. A Bragg grating is situated symmetrically midway in the fiber loop. This filter can be made to have relatively low loss. However, this filter is difficult to manufacture because exact placement of the Bragg grating is required in order to achieve the desired trancmicsion charac~eristics. Bilodeau et al., 35 "High-Return-Loss Narrowbond All-Fiber Bandpass Bragg Trancmic~ion Filter,"
21~6275 ,.,~ "
, _ - 2 -IEEE Photonics Technology Letters, Vol. 6, No. 1, (1994), pp. 80-82 describes a design modification of the Hill et al. filter.
Compound Bragg reflection filters made in planar waveguides are described in C.H. Henry et al., "Compound Bragg Reflection Filters Made by Spatial S Frequency Doubling Lithography," J. Li~htwave Technol. _ (1989) pp. 1379-1385.As reported therein, a filter having desirable spectral properties can be made by forming a Bragg grating having many, e.g., 15, sections, each with a different spatial peAod. To avoid phase shifts near 90~ between section~, the secdons are made contiguous, resulting in a grating period that is piecewise constant, but 10 discontinuously varied.
Accordingly, there is a need in the art for wavelength division multiplexing elements which are simple to m~mlf~cture and which are readily formed from optical filtering elements which separate a particular wavelength channel from a group of multiplexed channels. There is a further need in the art for 15 wavelength division optical multiplexing elements which are in~cen~citive to the polarization of incident radiation.
S~ ry of the Invention The present invention relates to methods and apparatus for wavelength division multiplexing (WDM) using arrays of optical filtering elem~ntc to create a 20 desired wavelength passband in an optical filter. In one aspect of the invention, an optical demultiplexer is formed using an input waveguide, such as a planar waveguide or an optical fiber, coupled to plural output waveguides. Each output waveguide includes a wavelength selective configuration of optical filtering elements formed within a contiguous portion of the waveguide. The configuration forms an 25 optical channel-selective filter with spectral regions having an optical tr~n.~mi~ion passband and spectral regions of low tr~n~mis.~ivity.
In one embodiment, the invention relates to an optical demultiplexer having an input waveguide and N output waveguides, N being a whole number greater than or equal to 2. Each output waveguide includes a wavelength-selective 30 configuration of at least (N-l) optical filtering elements such a~s Bragg gratings, each of the Bragg gratings having at lea~st one wavelength band of low tr~ncmi~sivity, ~,~
where each x is a number from 1 to N. The configuration of optical fil~ring elements creates an opdcal filter which transmit~s a characterisdc wavelength band, i.e., the wavelength band i~n of the Bragg grating absent from the configuradon of 35 optical filtering elements in the waveguide. A connecdng element such as a lxN
coupler is posidoned between the input and output waveguides to divide opdcal 3 ~ 2 ~ ~
tr~n~mi~sions from the input waveguide among the output waveguides.
In another aspect, the present invention provides an optical demultiplexer having an input waveguide and N output waveguides. Y branching waveguides are positioned between the input waveguide and the output waveguides, Y being a whole number S greater than or equal to 2 and less than or equal to N-1. As in the previous embodiment, each of the output waveguides transmits a characteristic wavelength band, ~ where n is a whole number from 1 to N. Positioned within the output waveguides and said branching waveguides are a plurality of optical filtering elements each optical filtering element having a different wavelength band of low transmissivity ~x, where x is a whole number from 1 to N and is not equal to n, the number corresponding to the channel passband. The plurality of optical filtering elements is configured such that within the optical path of the tr~n~mi~sion emitted from each output waveguide are positioned at least N-l optical filtering elements, each of the N-1 optical filtering elements having a different wavelength band of low tr~n.smi.~ivity. Thus, the combination of the optical filtering elements is such that the combination of the low tr~n~mi~sivity wavelength bands of each of the N-l optical filtering elements results in tr~n~mission of the characteristic wavelength, ~n. In an exemplary embodiment, the branching waveguide includes all of the optical filtering elements for the wavelengthbands which are not to be transmitted by any of the output waveguides connected to that branching waveguide.
In another aspect, the present invention relates to polarization-insensitive wavelength division demultiplexers. The demultiplexer includes an input optical waveguide and plural output waveguides. Each output waveguide has a plurality ofoptical filtering elements integrally formed within a contiguous portion of the waveguide.
The demultiplexers exhibit a polarization sensitivity of about 1 part in 100,000 or less.
In one particularly preferred embodiment the present invention provides an optical demultiplexer comprising: an input optical waveguide; N output optical waveguides, N being a whole number greater than or equal to 2, each output waveguide including a wavelength-selective configuration of optical filtering elements comprising at least (N-l) Bragg gratings, each of said Bragg gratings having at least one wavelength band of low transmissivity, ~x where x is a number between 1 and N, said configuration of optical filtering elements forming an optical filter transmitting a characteristic -- 3a-wavelength band, the characteristic wavelength band being the wavelength band ;~n of the Bragg grating absent from the configuration of optical filtering elements in thewaveguide; and an element positioned between said input waveguide and said output waveguides for splitting optical tr~n~micsions from the input waveguide to the output S waveguides.
Brief Description of the Dr~wi ~.~
FIG. I is a schematic representation of an 8-channel optical demultiplexer according to one embodiment of the present invention.
FIG. 2 is a schematic representation of an 8-channel optical demultiplexer 10 according to a further embodiment of the present invention.
FIG. 3 is a schematic representation of an 8-channel demultiplexer according to a further embodiment of the present invention.
FIG. 4 is a tr~n~mi~cion spectrum of a Bragg grating used as an optical filtering element according to the invention in one embodiment.
,~
!~
21~6275 -~,k ~_ - 4 ~
FIG. S is a trancmiCcion spectrum for one channel of the demultiplexer of FIG. 1.
Detailed Des~ ,lion The invention will now be described with reference to the drawings in 5 detail, in which like reference numerals indicate the same or similar elements in each of the FIGS. While the invention is described in terms of its application to demultiplexing, it will be appreciated by those skilled in the art that the invention applies equally well to multiplexing. A demultiplexing apparatus, operated in the opposite order, i.e., specific wavelength bands added to the trAn~mitted radiation 10 rather than filtered out of a composite trAn.cmicsion, functions as a multiplexing apparatus. The term "demultiplexer" or "demultiplexing" as used herein encompasses any device used to separate a particular wavelength band from a group of multiplexed wavelength bands. This additionally includes devices used to route wavelength bands positioned well before the receivers in a tran.cmi~ion system.
In one embodiment, of the present invention, the demultiplexer includes an input optical waveguide and a plurality of output optical waveguides, each output optical waveguide having an optical filter comprising an array of optical filt~ring elements, such as Bragg diffraction gratings, formed in a condguous portion of the output waveguide.
Bragg gratings are regions of refractive index perturbations within a waveguiding element which create a reflection peak from wavelengths of incident radiation satisfying the Bragg condition:
i~ Bragg = 2 n eff A = 2 n eff 2 where ~ Bragg = ~-a~le~ ~lhs of incident radiation which satisfy the Bragg cQ~ n n eff = effective refiractive index of the waveguiding medium = wavelength of the uv radiâtion used to form the grating oc = half angle of inc~Aenoe of ~e exposing radiation spacing of the ~irla~ ndex pe.~ons (grating period) 30 The optical filter formed from the array of optical filt~ring elements, such as an array of Bragg gratings, transmits a characteristic wavelength band by reflecting the 2146~75 ~.
..~"~.
, ~ - 5 -collective wavelength bands reflected by each individual Bragg grating. The termreflection, as used herein, includes any radiation which is not tran~minP~ by the waveguide, i.e., it includes radiation which is scattered out of the waveguide.
As schematically illustrated in FIG. 1, demuldplexer 220 includes an 5 input waveguide 210 and N output waveguides 220 coupled to waveguide 210 through 1 X N coupler 230. For the demultiplexer shown in FIG. 1, N=8; however Nis any whole number greater than or equal to 2. Each output waveguide transmits a characteristic wavelength band, ~n, where n is a whole number between 1 and N.
For the FIG. 1 demultiplexer, n=8. The wavelength indices which appear at the 10 terminus of each waveguide denote which desired signal wavelength band is passed by that output waveguide. It will be appreciated by those skilled in the art that radiation outside the signal wavelength bands may also be passed to the output waveguides; such radiation does not deleteriously impact system perforrn~n~e Positioned within each of the output waveguides are N- 1 optical filt~pring 15 elemen~ 240. For the illustrated demultiplexer, N=8, therefore there are 8-1='7 optical filtering element.c. Exemplary optical filtering element.c 240 are Bragggratings, each Bragg grating having a wavelength band of low tr~n~missivity, i.e., the wavelength band which the grating attenuates. The combination of the opticalfiltering elements forms an optical filter in which all of the wavelength bands are 20 blocked except the characteAstic wavelength band of the output waveguide. Thus, for the output waveguide labeled ~ 1, the seven Bragg gratings positioned within that waveguide collectively attenuate wavelength bands i~2, ~3, ~4~ ~5~ ~6~ ~7, and ~8 as illustrated in FIG. 1. The wavelength numbers which appear adjacent each grating in the output waveguides indicate the wavelength band blocked by that 25 grating.
Optionally, an additional Bragg grating is positioned in the array. The additional Bragg grating does not correspond to a channel, but instead is used to broaden the channel of the Bragg grating with the shortest wavelength. A furtherBragg grating can also be used to broaden the channel of the Bragg grating with the 30 longest wavelength. The use of an additional Bragg grating on the short and/or long wavelength end of the spectrum permits attenuation of additional undesirable wavelengths, such as those which might result from amplified spontaneous emission noise in an optical amplifier.
Each of the rem~ining output waveguides includes a similar array of 35 filters, i.e., for the 8-channel demultiplexer of FIG. 1, there are seven Bragg gratings in each output waveguide, each Bragg grating blocking a different wavelength band u between ~ 1 and ~8 . Using ~" to represent the characteristic wavelength band of the output waveguide, the seven Bragg gratings block wavelengths from n=l to n=8.
There is no Bragg grating blocking a wavelength for ~ n where n=x, i.e., no Bragg grating is positioned in the output waveguide which blocks the wavelength band to be passed by that output waveguide.
Although the demultiplexer of FIG. 1 employs a Bragg grating for each channel to be blocked, a single Bragg grating can be used to block two or more channels simultaneously when the reflection peak of the grating is sufficiently wide.
For this embodiment, the demultiplexer has N channels but fewer than (N-l) opdcal 10 filtering elPmen~.~ positioned within each output waveguide.
FIG. 2 illustrates a further embodiment of a demultiplexer according to the present invention. In the demultiplexer of FIG. 2, branching waveguides 260 are positioned between the input waveguide 210 and N output waveguides 220. These branching waveguides have optical filtering elements 270, such as Bragg gratin~
formed therein. Advantageously, the optical filtering element.~ formed within the br~nching waveguides elimin~te the need to repeat the branching waveguide filtering elements within the output waveguides. Thus, the wavelength bands which are to be blocked by all output waveguide within a set of output waveguides have the corresponding optical filtering elements formed within the branching waveguide, 20 elimin~ting the need to duplicate filtering element formation within each of the output waveguides.
The input waveguide 210 is connected to the branching waveguides 260 through 1 x Y coupler 230, as in the previous embodiment, where Y is the number of branching waveguides which connect to the input waveguide. In general, Y is a whole number greater than or equal to 2 and less than or equal to N- 1. In the demultiplexer of FIG. 2, N is equal to eight and Y is equal to two. A coupler 250is positioned between branching waveguides 260 and output waveguides 220. In FIG. 2,1x 4 couplers are used to connect the branching and output waveguides.
For the demultiplexer configuration of FIG. 2, the output waveguides passing wavelength bands ~ 4 all must block wavelength bands ~5 - ~ 8.
Therefore, the branching waveguide which transmits radiation into these four output waveguides includes filtering elements which block the tr~ncmic~ion of wavelength bands ~5-~8. As compared to the demultiplexer configuration of FIG. 1, twenty-four fewer optical filtering elements are included in the output waveguides due to the addition of the branching waveguides having filtering elements blocking wavelength bands common to the connecting output waveguides.
214~7S
. ~ ..
An exemplary optical filtering element array for the br~nrl~ing configuration depicted in FIG. 2 is illustrated in FIG. 3. In this embodiment, the selection of optical filtering elements is staggered such that the array of optical filtering elements in the output waveguides does not include optical filtçring S elements which block wavelength bands adjacent to the pass wavelength band of the output waveguide. The wavelength passband for the four output waveguides which connect to the first branching waveguide are selected to be ~ 1, A, 3, ~ 5, and ~ 7 .
Thus, none of the output waveguides connected to the first branching waveguide will transmit wavelength bands ~2. ~4~ ~6. and ~8 For this reason, filtering elemçnt.c 10 which block the tran.emi.ccion of ~2. ;~4. ~6. and ~8 are positioned within the first branching waveguide. The rem~ining wavelength bands to be blocked in order to transmit the desired wavelength band are positioned in the output waveguide. For the output waveguide which tr~ncmite wavelength band ~ 1, the rem~ining wavelength bands to be blocked are ~3, ~5, and ~7. Filtering elçm~nt.e such as Bragg gr?~tinge 15 which block these wavelength bands are positioned within the output waveguide as shown in FIG. 3. This configuratdon of filtçring elements advantageously permitsrelaxed tolerances on gratdng positdoning in the output waveguides since twice the channel spacing is produced in the output waveguides through the blocking of alternate wavelength bands by the gratings positioned in the br~n~hing waveguides.
An i~ç~1i7Pd tranemi.esion spectrum for a single Bragg grating is depicted in FIG. 4. In spectrum 10, region 20 is a wavelength band having a tr~n.emi.e.eion minimum which corresponds to incident radiatdon reflected or scattered by the gradng. On the short wavelength side of tr~nemi.eeion minimum 20 is dip 30.
Dip 30 results from radiatdon loss and is separated from tr~nemi.eeion minimum 20 25 by relatdvely high tr~nemi.seivity region 40. The expression "radiation loss"describes any light which is scattered out of the core of the waveguide, including r~ tion scattered in the cladding of the waveguide.
The short-wavelength edge of tr~n~emieeion minimum 20 is i~gragg, i.e., the Bragg wavelength in the limit of a very weak grating. In this limit, the 30 long-wavelength edge of the dip occurs at the wavelength ~Bragg~;~offset where ~offset is given by:
o Bragg ( 1 llCI ) where ~" - 8 -n cl = cladding refractive index neff = effective lcirla~ index of the waveguide medium It is usually advantageous to reduce the depth of region 30, minimi7ing radiation loss. However, as grating strength increases, measured by the widening of s tr~ncmi.c.cion minimum region 20, radiation loss increases. On a log tr~ncmi.c~sion plot, radiation loss increases quadratically for a given increase in grating strength, whereas widening of region 20 increases only linearly for strong gratings. Radiation loss can interfere with tr~ncmiscion when it occupies a spectral region for a channel passband.
Several techniques are used to elimin~te or reduce the impact of radiation loss on an optical filter formed from plural Bragg gratings. One technique minimi7~s radiation loss through the use of waveguiding media having large effective refractive indices. Alternatively, gratings may be formed in waveguiding media having weak radiation loss and fused to tr~n.cmi.c.cion waveguiding media.15 Peak radiation loss can further be reduced by recoating the grating region of the waveguide to improve tr~ncmiscion characteristics. See, for example, Mi_rahi andSipe, J. of Light Tech., Vol. 11 (October 1993) pp. 1513-1517, especially Figs. 3(a) and 4, the disclosure of which is incorporated herein by reference.
The design of the optical filter also minimi7~s or reduces the effects of 20 radiation loss. For filters having a small number of channels, the grating configuration positions channel passbands in spectral regions where there is no radiation loss region from other gratings in the filter.
For filters having larger numbers of channels, it is often not possible to avoid a passband for a particular channel falling within a spectral region of radiation 2s loss for another grating. In such situations, plural weaker gratings can be used to pass a single channel since weaker gratings exhibit proportionally less radiadonmode loss, as discussed above. Weaker gratings are forrned by controlling the amount of actinic radiation exposure, and hydrogenation level and gerrnanium doping levels in hydrogen-treated germanosilicate waveguides.
The waveguides employed in the demultiplexers of FIGS. 1-3 are exemplarily single-mode optical fibers, as are typically employed in optical tr~ncmiccion systems, while the lxN coupler is exemplarily a fused fiber coupler.
However, waveguides for the above demultiplexers are selected from any waveguiding media which transmit optical signals including, but not limited to, 3s multiple-mode fiber and planar waveguides. Additionally, fibers and planar waveguides can be incorporated into a single device, e.g., input and/or output fibers coupled to a planar waveguide in which Bragg gratings are formed.
Various coupler combinations can provide the desired I xN coupling. For example, the configuration illustrated in FIG. 2 employs one I x2 coupler and two I x4 5 couplers to create an array with 1 input waveguide and 8 output waveguides. Numerous other combinations can be used to achieve similar results.
Radiation losses may be minimi7ed by using a section of multiple-mode fiber having a small number of modes since multiple-mode fiber generally has better mode-field confinement for the fundamental mode than single-mode fiber. By carefully injecting radiation from a single-mode fiber into a multiple-mode fiber at a wavelength near cutoff, it is possible to largely avoid exciting any but the fundamental mode of the multiple-mode fiber. A section of multiple mode fiber can be inserted through fusion splicing of multiple-mode and single-mode fibers.
When using Bragg diffraction gratings as optical filtering elements, a preferredmethod for grating formation is to optically induce refractive index perturbations. A
photosensitive optical waveguide, such as an optical fiber, is exposed to an interference pattern of actinic, typically ultraviolet, radiation to form refractive index perturbations at least partially in the fiber core. Alternatively, the waveguide is exposed to aninterference pattern created by impinging a single actinic beam on a phase mask. The phase mask is a patterned, diffractive medium that modulates the relative phase of the diffracted wavefronts. Phase masks are typically manufactured by reactive ion etching of a fused-quartz substrate through a chromium mask patterned by electron-beam lithography. However, various other methods may be used to form phase masks, such as holographic patterning techniques. The use of phase masks to fabricate Bragg gratings is described in C~n:~-lian Patent Application Serial No. 2,111,808, filed December 17, 1993, by D. Z. Anderson et al.
To produce strong gratings, i.e., those gratings having wide spectral reflectionpeaks, waveguides are sensitized by exposure to hydrogen gas prior to patterning with actinic radiation. This technique is described in U.S. Patent No. 5,235,659 issued on August 10, 1993 by R. M. Atkins et al. and U.S. PatentNo. 5,287,427 issued on February 15, 1994 by R. M. Atkins et al. Grating strength is controlled through actinic radiation exposure for a given photosensitive medium. When using hydrogen-treated germanosilicate glass waveguides, the hydrogenation levels in the waveguide core also controls grating strength. Further description of Bragg grating ~i~
2 ~ 7 5 .". ~
1 o -fabrication to form optical filters is found in U.S. Patent Application Serial No.
08/078,992 filed June 17, 1993 to Mizrahi and assigned to the present acsign~e, the disclosure of which is incorporated herein by reference.
FIG. S is a transmiccion spectrum, i.e., tran~mission as a function of 5 wavelength, for the channel corresponding to ~ 2 of the 8-channel demultiplexer of FIG. 1. In this FIG., region 100 represents the tran~miccion minimum resnlting from gratings which block wavelength bands ~ 3. ;~4. ~5- ~6. ;~7. and ;~8. Note that the noise which appears in region 100 is instrumentation-induced and not representative of the attenuation characteristics of the demultiplexer. Channel 2 10 (~2), shown as region 110, is passed by this output waveguide of the demultiplexer.
The approximate locations of the wavelengths corresponding to each channel are indicated by the dashed lines in FIG. 5.
Advantageously, demultiplexers formed according to the present invention are insensitive to the polarization of the light which passes through them.
15 By polarization insensitivity, it is meant that the tr~n.cmi~ion spectrum of the demultiplexer does not shift for radiation of different polarizations. Thus, a device having a polarization sensitivity of 1 part in 100,000 exhibits a worst-case shift of O O
0.15 A or less at a wavelength of 15,000 A. Since optical fibers often do not maintain polarization over long distances, polarization insensitivity of the 20 demultiplexers results in effective signal filtering regardless of the polarization of the incident radiation. In particular, the demultiplexers of the present invention can be formed with a polarization sensitivity of less than 1 part in 100,000.
Demultiplexers formed according to the present invention are suitable for incorporation in terrestrial and underseas optical communication systems. Such 25 optical communications systems generally include an optical source such as a laser, a mod--lating member to create a desired optical signal, a transmi.~sic)n member such as a fiber or planar waveguide, conventional or optical amplifiers, e.g., erbium-doped fiber amplifiers (EDFAs), and receivers. Advantageously, the use of wavelength division demultiplexers increases the capacity of a single-mode optical fiber 30 tr~ncmi.ccion system by perrnitting a single fiber to carry plurality wavelength bands.
Such systems typically incorporate optical isolators in the optical amplifiers to prevent tran.cmi~.cion of light reflected by the optical filters back down the trarl~mi~.~ion member. When incorporated in a terrestrial or underseas communication system, it is advantageous to package the demultiplexer to assure 35 thermal stability of the device. Heating and/or cooling elements permit a desired temperature range to be selected, ensuring the accuracy and reliability of the 21~27~
demultiplexer.
The dem~ iplexers of the present invention can also be employed in optical s~lems other than the above-named optical telecommnni~ations systems.
Such optical systems include, but are not limited to, local area network (LAN) and S CATV systems.
While the invention has been described with respect to the foregoing embodimen~, it will be appreciated that various changes and modifications may bemade. Accordingly, changes and modifications such as those suggested above, but not limited thereto, are within the scope of the claimed invention.
Claims (9)
1. An optical demultiplexer comprising:
an input optical waveguide;
N output optical waveguides, N being a whole number greater than or equal to
an input optical waveguide;
N output optical waveguides, N being a whole number greater than or equal to
2, each output waveguide including a wavelength-selective configuration of optical filtering elements comprising at least (N-1) Bragg gratings, each of said Bragg gratings having at least one wavelength band of low transmissivity, .lambda.x where x is a number between 1 and N, said configuration of optical filtering elements forming an optical filter transmitting a characteristic wavelength band, the characteristic wavelength band being the wavelength band .lambda.n of the Bragg grating absent from the configuration of optical filtering elements in the waveguide; and an element positioned between said input waveguide and said output waveguides for splitting optical transmissions from the input waveguide to the output waveguides.
2. An optical demultiplexer according to claim 1 wherein said waveguides are optical fibers having cladding and core layers.
2. An optical demultiplexer according to claim 1 wherein said waveguides are optical fibers having cladding and core layers.
3. An optical demultiplexer according to claim 2 where the Bragg gratings are at least partially formed in the waveguide core.
4. An optical demultiplexer comprising:
an input optical waveguide;
N output optical waveguides, where N is a whole number greater than or equal to 3, each of said output waveguides transmitting a characteristic wavelength band, .lambda.n, where n is a whole number from 1 to N;
Y branching waveguides positioned between said input waveguide and said output waveguides where Y is a whole number greater than or equal to 2 and less than or equal to N-1;
positioned within said output waveguides and said branching waveguides are a plurality of optical filtering elements, each optical filtering element having a wavelength band of low transmissivity .lambda.x, where x is a whole number from 1 to N and is not equal to n, said plurality of optical filtering elements configured such that within the optical path of the transmission emitted from each output waveguide are positioned at least N-1 optical filtering elements, each of the at least N-1 optical filtering elements having a different wavelength band of low transmissivity, the combination of the optical filtering elements being such that the combination of the low transmissivity wavelength bands of each of the at least N-1 optical filtering elements results in the transmission of the characteristic wavelength, .lambda.n;
an element for splitting optical transmission from the input waveguide to the branching waveguides positioned between the input waveguide and the branching waveguides; and elements positioned between the branching waveguides and the output waveguides for splitting optical transmission from the branching waveguides among the output waveguides.
an input optical waveguide;
N output optical waveguides, where N is a whole number greater than or equal to 3, each of said output waveguides transmitting a characteristic wavelength band, .lambda.n, where n is a whole number from 1 to N;
Y branching waveguides positioned between said input waveguide and said output waveguides where Y is a whole number greater than or equal to 2 and less than or equal to N-1;
positioned within said output waveguides and said branching waveguides are a plurality of optical filtering elements, each optical filtering element having a wavelength band of low transmissivity .lambda.x, where x is a whole number from 1 to N and is not equal to n, said plurality of optical filtering elements configured such that within the optical path of the transmission emitted from each output waveguide are positioned at least N-1 optical filtering elements, each of the at least N-1 optical filtering elements having a different wavelength band of low transmissivity, the combination of the optical filtering elements being such that the combination of the low transmissivity wavelength bands of each of the at least N-1 optical filtering elements results in the transmission of the characteristic wavelength, .lambda.n;
an element for splitting optical transmission from the input waveguide to the branching waveguides positioned between the input waveguide and the branching waveguides; and elements positioned between the branching waveguides and the output waveguides for splitting optical transmission from the branching waveguides among the output waveguides.
5. An optical demultiplexer according to claim 4 wherein N=8 and Y=2.
6. An optical demultiplexer according to claim 4 wherein the waveguides are optical fibers having cladding and core layers.
7. An optical demultiplexer according to claim 6 wherein the optical filtering elements are Bragg gratings at least partially formed in the waveguide core.
8. An optical communications system comprising:
an optical source;
a modulator for modulating light from said optical source to create a modulated optical signal having a plurality of wavelength bands;
a transmission member for transmitting said optical signal; and an optical demultiplexer for separating wavelength bands from said optical signal, said optical demultiplexer comprising:
an input optical waveguide;
N output optical waveguides, N being a whole number greater than or equal to 2, each output waveguide including a wavelength-selective configuration of optical filtering elements comprising at least (N-1) Bragg gratings, each of said Bragg gratings having at least one wavelength band of low transmissivity, .lambda.x where x is a number between 1 and N, said configuration of optical filtering elements forming an optical filter transmitting a characteristic wavelength band, the characteristic wavelength band being the wavelength band .lambda.n of the Bragg grating absent from the configuration of optical filtering elements in the waveguide; and an element positioned between said input waveguide and said output waveguides for splitting optical transmissions from the input waveguide to the output waveguides.
an optical source;
a modulator for modulating light from said optical source to create a modulated optical signal having a plurality of wavelength bands;
a transmission member for transmitting said optical signal; and an optical demultiplexer for separating wavelength bands from said optical signal, said optical demultiplexer comprising:
an input optical waveguide;
N output optical waveguides, N being a whole number greater than or equal to 2, each output waveguide including a wavelength-selective configuration of optical filtering elements comprising at least (N-1) Bragg gratings, each of said Bragg gratings having at least one wavelength band of low transmissivity, .lambda.x where x is a number between 1 and N, said configuration of optical filtering elements forming an optical filter transmitting a characteristic wavelength band, the characteristic wavelength band being the wavelength band .lambda.n of the Bragg grating absent from the configuration of optical filtering elements in the waveguide; and an element positioned between said input waveguide and said output waveguides for splitting optical transmissions from the input waveguide to the output waveguides.
9. An optical communications system comprising:
an optical source;
a modulator for modulating light from said optical source to create a modulated optical signal having a plurality of wavelength bands;
a transmission member for transmitting said optical signal; and an optical demultiplexer for separating wavelength bands from said optical signal, said optical demultiplexer comprising:
an input optical waveguide;
N output optical waveguides, where N is a whole number greater than or equal to 3, each of said output waveguides transmitting a characteristic wavelength band, .lambda.n where n is a whole number from 1 and N;
Y branching waveguides positioned between said input waveguide and said output waveguides where Y is a whole number greater than or equal to 2 and less than or equal to N-1;
positioned within said output waveguides and said branching waveguides are a plurality of optical filtering elements, each optical filtering element having a wavelength band of low transmissivity .lambda.x, where x is a whole number from 1 to N and is not equal to n, said plurality of optical filtering elements configured such that within the optical path of the transmission emitted from each output waveguide are positioned at least N-1 optical filtering elements, each of the at least N-1 optical filtering elements having a different wavelength band of low transmissivity, the combination of the optical filtering elements being such that the combination of the low transmissivity wavelength bands of each of the at least N-1 optical filtering elements results in the transmission of the characteristic wavelength, .lambda.n;
an element for splitting optical transmission from the input waveguide to the branching waveguides positioned between the input waveguide and the branching waveguides; and elements positioned between the branching waveguides and the output waveguides for splitting optical transmission from the branching waveguides among the output waveguides.
an optical source;
a modulator for modulating light from said optical source to create a modulated optical signal having a plurality of wavelength bands;
a transmission member for transmitting said optical signal; and an optical demultiplexer for separating wavelength bands from said optical signal, said optical demultiplexer comprising:
an input optical waveguide;
N output optical waveguides, where N is a whole number greater than or equal to 3, each of said output waveguides transmitting a characteristic wavelength band, .lambda.n where n is a whole number from 1 and N;
Y branching waveguides positioned between said input waveguide and said output waveguides where Y is a whole number greater than or equal to 2 and less than or equal to N-1;
positioned within said output waveguides and said branching waveguides are a plurality of optical filtering elements, each optical filtering element having a wavelength band of low transmissivity .lambda.x, where x is a whole number from 1 to N and is not equal to n, said plurality of optical filtering elements configured such that within the optical path of the transmission emitted from each output waveguide are positioned at least N-1 optical filtering elements, each of the at least N-1 optical filtering elements having a different wavelength band of low transmissivity, the combination of the optical filtering elements being such that the combination of the low transmissivity wavelength bands of each of the at least N-1 optical filtering elements results in the transmission of the characteristic wavelength, .lambda.n;
an element for splitting optical transmission from the input waveguide to the branching waveguides positioned between the input waveguide and the branching waveguides; and elements positioned between the branching waveguides and the output waveguides for splitting optical transmission from the branching waveguides among the output waveguides.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US239,098 | 1981-02-27 | ||
| US08/239,098 US5457760A (en) | 1994-05-06 | 1994-05-06 | Wavelength division optical multiplexing elements |
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| Publication Number | Publication Date |
|---|---|
| CA2146275A1 CA2146275A1 (en) | 1995-11-07 |
| CA2146275C true CA2146275C (en) | 1999-05-04 |
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| CA002146275A Expired - Fee Related CA2146275C (en) | 1994-05-06 | 1995-04-04 | Wavelength division optical multiplexing elements |
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| US (1) | US5457760A (en) |
| EP (1) | EP0684493A3 (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
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-
1994
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-
1995
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- 1995-04-26 EP EP95302819A patent/EP0684493A3/en not_active Withdrawn
- 1995-05-08 JP JP13263295A patent/JP3268166B2/en not_active Expired - Lifetime
Also Published As
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|---|---|
| EP0684493A2 (en) | 1995-11-29 |
| US5457760A (en) | 1995-10-10 |
| JPH07301723A (en) | 1995-11-14 |
| EP0684493A3 (en) | 1996-02-14 |
| CA2146275A1 (en) | 1995-11-07 |
| JP3268166B2 (en) | 2002-03-25 |
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