US20010017959A1 - Dense WDM optical multiplexer and demultiplexer - Google Patents
Dense WDM optical multiplexer and demultiplexer Download PDFInfo
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- US20010017959A1 US20010017959A1 US09/770,367 US77036701A US2001017959A1 US 20010017959 A1 US20010017959 A1 US 20010017959A1 US 77036701 A US77036701 A US 77036701A US 2001017959 A1 US2001017959 A1 US 2001017959A1
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- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/12007—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind forming wavelength selective elements, e.g. multiplexer, demultiplexer
- G02B6/12009—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind forming wavelength selective elements, e.g. multiplexer, demultiplexer comprising arrayed waveguide grating [AWG] devices, i.e. with a phased array of waveguides
- G02B6/12019—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind forming wavelength selective elements, e.g. multiplexer, demultiplexer comprising arrayed waveguide grating [AWG] devices, i.e. with a phased array of waveguides characterised by the optical interconnection to or from the AWG devices, e.g. integration or coupling with lasers or photodiodes
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- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
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- 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
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- G02B6/24—Coupling light guides
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- 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
- G02B6/29346—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 wave or beam interference
- G02B6/2935—Mach-Zehnder configuration, i.e. comprising separate splitting and combining means
- G02B6/29352—Mach-Zehnder configuration, i.e. comprising separate splitting and combining means in a light guide
- G02B6/29355—Cascade arrangement of interferometers
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- 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
- G02B6/29346—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 wave or beam interference
- G02B6/29361—Interference filters, e.g. multilayer coatings, thin film filters, dichroic splitters or mirrors based on multilayers, WDM filters
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- G02B6/24—Coupling light guides
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- 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
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- H04J14/02—Wavelength-division multiplex systems
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- H—ELECTRICITY
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- H04J—MULTIPLEX COMMUNICATION
- H04J14/00—Optical multiplex systems
- H04J14/02—Wavelength-division multiplex systems
- H04J14/0201—Add-and-drop multiplexing
- H04J14/0202—Arrangements therefor
- H04J14/0204—Broadcast and select arrangements, e.g. with an optical splitter at the input before adding or dropping
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- H—ELECTRICITY
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- H04J14/0202—Arrangements therefor
- H04J14/0209—Multi-stage arrangements, e.g. by cascading multiplexers or demultiplexers
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- H—ELECTRICITY
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- H04J14/02—Wavelength-division multiplex systems
- H04J14/0201—Add-and-drop multiplexing
- H04J14/0202—Arrangements therefor
- H04J14/0213—Groups of channels or wave bands arrangements
Definitions
- the present invention is directed to an optical demultiplexer for demultiplexing optical signals in a dense wavelength division multiplexed system.
- Optical communication systems are a substantial and fast growing constituent of communication networks.
- the expression “optical communication system,” as used herein, relates to any system which uses optical signals to convey information across an optical waveguiding medium, for example, an optical fiber.
- Such optical systems include but are not limited to telecommunication systems, cable television systems, and local area networks (LANs).
- LANs local area networks
- the many optical communication systems are configured to carry an optical channel of a single wavelength over one or more optical waveguides.
- time-division multiplexing is frequently employed (TDM).
- TDM time-division multiplexing
- a particular time slot is assigned to each signal source, the complete signal being constructed from the portions of the signals collected from each time slot. While this is a useful technique for carrying plural information sources on a single channel, its capacity is limited by fiber dispersion and the need to generate high peak power pulses.
- WDM Wavelength division multiplexing
- DWDM dense WDM
- These systems can include a demultiplexer having a 1 ⁇ 8 optical splitter, which receives the 8 channels on an input fiber, and outputs the channels on each of 8 outputs.
- the power level on each of the outputs is approximately ⁇ fraction (1/8) ⁇ the input power level.
- Optical components are respectively coupled to the outputs of the 1 ⁇ 8 splitter for outputting a corresponding one of the 8 channels, which introduce additional loss.
- an optical amplifier could be used to increase the power on the input of the 1 ⁇ 40 splitter, such an amplifier can be difficult to manufacture, and would fail to provide the requisite optical power per channel at higher channel counts. Moreover, if amplifiers were to be provided at each of the outputs of the 1 ⁇ 40 splitter, the cost of the demultiplexer would be excessive.
- an optical device comprising an optical splitter having an input and first and second outputs.
- the input of the optical splitter is coupled to an input optical path, which carries a plurality of optical channels.
- Each of the optical channels has a respective wavelength.
- the first output of the optical splitter being coupled to a first output optical path, and the second output being coupled to a second output optical path, with the first and second output optical paths each carrying the plurality of optical channels.
- the optical device consistent with the present invention further comprises a first optical filtering element coupled to the first output optical path for selecting a first group of said plurality of optical channels; and a second optical filtering element coupled to the second output optical path for selecting a second group of said plurality of optical channels.
- a first optical demultiplexer is coupled to the first optical filtering element and includes a plurality of outputs. A corresponding one of the first group of channels appearing on a respective one of the plurality of outputs of said first optical demultiplexer.
- a second optical demultiplexer is coupled to the second optical filtering element.
- the second optical demultiplexer comprises a plurality of outputs, a corresponding one of the second group of channels appearing on a respective one of the plurality of outputs of the second optical demultiplexer.
- FIG. 1 illustrates a schematic diagram of an optical demultiplexer in accordance with the present invention
- FIG. 2 illustrates a schematic diagram of an interference filter
- FIG. 3 illustrates transmittance characteristics of filtering elements 125 - 1 , 125 - 2 and 125 - 3 shown in FIG. 1;
- FIG. 4 illustrates transmittance characteristics of filtering elements 130 - 1 and 130 - 2 shown in FIG. 1;
- FIG. 5 illustrates plots of loss vs. channel count for the present invention and a conventional demultiplexer
- FIG. 6 illustrates an additional embodiment of the present invention
- FIG. 7 illustrates an example of a sub-demultiplexer
- FIG. 8 illustrates an additional example of a sub-demultiplexer
- FIG. 9 illustrates a further example of a sub-demultiplexer
- FIG. 10 illustrates a fiber optic coupler demutliplexer for separating two wavelengths
- FIG. 11 illustrates an additional embodiment of the present invention.
- FIG. 12 illustrates an optical multiplexer in accordance with the present invention.
- optical channels to be demultiplexed are supplied to first and second optical fibers via an optical splitter.
- Low loss interference filters for example, coupled to the first and second optical fibers, select respective groups of channels.
- Each group of channels is next demultiplexed with sub-demultiplexers into individual channels, each of which is then sensed with a corresponding photodetector.
- the optical splitter introduces an optical power loss at the input to the demultiplexer, the interference filters and sub-demultiplexers create little additional loss.
- the total power loss associated with the present invention is significantly less than that obtained with a conventional n channel demultiplexer based on a 1 ⁇ n splitter. Accordingly, large numbers of channels, e.g., in excess of forty can be readily demultiplexed and detected.
- FIG. 1 illustrates optical demultiplexer 100 consistent with the present invention.
- the exemplary demultiplexer illustrated in FIG. 1 demultiplexes 40 channels.
- the present invention is not limited to this number of channels. Rather, the present invention is applicable to demultiplexers that can separate any appropriate number of optical channels.
- optical demultiplexer 100 receives, for example, a plurality of multiplexed optical channels ⁇ 1 - ⁇ 40 on input optical fiber 105 .
- the multiplexed channels are emitted by laser transmitters (not shown) coupled to optical fiber 105 .
- the multiplexed channels are supplied to the input of optical splitter 110 , which supplies channels ⁇ 1 - ⁇ 40 to first and second output fibers 115 and 120 , respectively.
- optical amplifier 101 may be provided at the input of coupler 110 in order to offset any loss introduced by splitter 110 .
- the loss imposed by splitter 110 can be compensated by coupling optical amplifiers to output fibers 115 and 120 .
- first optical filtering elements 125 - 1 to 125 - 3 are coupled to first output fiber 120
- second filtering elements 130 - 1 and 130 - 2 are coupled to second output fiber 115 .
- Each of these filtering elements typically includes an interference filter commercially available from JDS Fitel or DiCon, for example.
- FIG. 2 A simplified diagram of filtering element 125 - 1 incorporating an interference filter, for example, is illustrated in FIG. 2.
- Filtering element 125 - 1 receives channels ⁇ 1 - ⁇ 40 on input fiber 210 having an end portion spaced from graded refractive index (GRIN) lens 215 .
- GRIN graded refractive index
- Channels ⁇ 1 - ⁇ 40 are collimated by lens 215 and directed toward dielectric thin film filter 220 .
- Thin film filter 220 includes a substrate coated with a plurality of thin dielectric films of appropriate thickness and number to pass, in this case, channels ⁇ 1 - ⁇ 8 , and reflect channels ⁇ 17 - ⁇ 40 .
- Channels ⁇ 1 - ⁇ 8 are thus transmitted to GRIN lens 218 where they are focused onto an end portion of output fiber 235 and transmitted to a corresponding sub-demultiplexer via output port 240 of filtering element 125 - 1 .
- Channels ⁇ 9 - ⁇ 40 are reflected back to GRIN lens 215 , focused onto fiber 225 and output through port 230 to filtering element 125 - 2 , which along with filtering element 125 - 3 , typically has a similar construction as filtering element 125 - 1 .
- Transmittance band 325 - 1 of filtering element 125 - 1 has a relatively high transmittance over a range of wavelengths including the first eight channels, ⁇ 1 - ⁇ 8 . As noted above, therefore, these channels are transmitted toward an output port of filtering element 125 - 1 . It should be noted, however, that channels ⁇ 9 - ⁇ 11 , for example, are also transmitted, but only partially, since they lie in the roll-off region 324 of transmittance band 325 - 1 .
- Remaining channels ⁇ 12 - ⁇ 40 are reflected to filtering element 125 - 2 having high transmittance band 325 - 2 .
- channels ⁇ 17 - ⁇ 24 are transmitted toward the output port of filtering element 125 - 2 .
- channels ⁇ 25 - ⁇ 27 for example, adjacent ⁇ 17 - ⁇ 24 are also transmitted, but to a lesser extent.
- Remaining channels ⁇ 28 - ⁇ 40 are reflected and output to filtering element 125 - 3 of which channels ⁇ 33 - ⁇ 40 fall within high transmittance band 325 - 3 and are thus passed to an output port of filtering element 125 - 3 .
- Channels ⁇ 28 - ⁇ 31 are partially transmitted.
- channel groups ⁇ 1 - ⁇ 8 , ⁇ 17 - ⁇ 24 , and ⁇ 33 - ⁇ 40 are respectively directed to sub-demultiplexers 135 - 1 , 135 - 2 and 135 - 3 where further demultiplexing is performed to output individual channels on corresponding output ports. These output ports, in turn, are coupled to respective ones of photodiodes 150 where the channels are converted into electrical signals for processing by appropriate circuitry (not shown).
- the term sub-demultiplexer is used herein to describe an optical apparatus that demutliplexes the groups of channels respectively output from the filtering elements. Sub-demultiplexers 135 - 1 , 135 - 2 and 135 - 3 are discussed in greater detail below.
- channels lying outside the high transmittance bands of filtering elements 125 - 1 , 125 - 2 and 125 - 3 are only partially reflected due to the relatively broad roll-off regions of the transmittance characteristics of interference filters, for example. Accordingly, channels adjacent the intended channel sub-groups incur a significant amount of power loss and cannot be effectively demultiplexed. These channels are therefore supplied by filtering elements 130 - 1 and 130 - 2 coupled to second output fiber 115 , as discussed in greater detail below with reference to FIG. 4.
- filtering element 130 - 1 has high transmittance band 430 - 1 extending over wavelengths ⁇ 9 - ⁇ 16 .
- Filtering element 130 - 1 typically has a similar construction to filtering element 125 - 1 , and therefore transmits channels ⁇ 9 - ⁇ 16 for output to corresponding sub-demultiplexer 140 - 1 and further demultiplexing.
- Remaining channels ⁇ 1 - ⁇ 8 and ⁇ 17 - ⁇ 40 are reflected to filtering element 130 - 2 having high transmittance band 430 - 2 encompassing channels ⁇ 25 - ⁇ 32 . Accordingly, channels ⁇ 25 - ⁇ 32 are transmitted through filtering element 130 - 2 and directed toward sub-demultiplexer 140 - 2 for further demultiplexing into individual channels.
- Channels lying adjacent the high transmitttance bands 430 - 1 and 430 - 2 are partially transmitted by filtering elements 130 - 1 and 130 - 2 and are thus difficult to detect after further demultiplexing. These channels, however, lie within the high transmittance bands of filtering elements 125 - 1 to 125 - 3 and are thus demultiplexed through sub-demultiplexers 135 - 1 to 135 - 3 , respectively, as noted above.
- first output fiber 120 , filtering elements 125 - 1 to 125 - 3 and corresponding sub-demultiplexers 135 - 1 to 135 - 3 serve to demultiplex channels ⁇ 1 - ⁇ 8 , ⁇ 17 - ⁇ 24 and ⁇ 33 - ⁇ 40 , respectively.
- second output fiber 120 , filtering elements 130 - 1 and 130 - 2 and corresponding sub-demultiplexers 140 - 1 and 140 - 2 serve to demultiplex channels ⁇ 9 - ⁇ 16 and ⁇ 25 - ⁇ 32 , respectively. Accordingly, alternating groups of channels are respectively selected by filtering elements 125 - 1 to 125 - 3 and 135 - 1 to 135 - 3 .
- filtering elements 130 - 2 and 125 - 3 can be omitted and sub-demultipelxers 140 - 2 and 135 - 3 can be coupled directly to the outputs of filtering elements 130 - 1 and 125 - 2 , respectively.
- Filtering elements 125 - 1 to 125 - 3 , 130 - 1 and 130 - 2 introduce little power loss to the selected sub-groups of channels.
- the power loss associated with the channels reflected by the filtering elements is typically about 0.5 dB, and the loss associated with the channels transmitted through the filtering elements is about 0.7 dB.
- the overall loss of the demultiplexer in accordance with the present invention is significantly reduced.
- FIG. 5 illustrates loss L as a function of channel count n in accordance with the present invention (curve 910 ) and a conventional 1 ⁇ n splitter based demultiplexer (curve 920 ).
- the loss associated with the present invention is higher than the conventional demultiplexer. This is due to the loss introduced by the 1 ⁇ 2 splitter 110 , which imposes a minimum loss in the demultiplexer.
- the loss associated with the conventional demultiplexer increases linearly with n (curve 920 ).
- the loss associated with the present invention (curve 910 ) is stepped and increases at a much more gradual rate due, at least in part, to the low loss of the filtering elements. Therefore, at higher channel counts, channels demultiplexed in accordance with the present invention experience less loss than those demultiplexed with a conventional 1 ⁇ n splitter.
- FIG. 6 illustrates an alternative embodiment in which each filtering element constitutes a combination of a circulator and grating.
- filtering element 130 - 1 includes a circulator 830 - 1 receiving channels ⁇ 1 - ⁇ 40 at a first circulator port 831 - 1 coupled to output fiber 115 . These channels are circulated to port 831 - 2 and output to in-fiber Bragg grating 832 - 1 , which is configured to have a high reflectivity band that coincides with the band of channels ⁇ 9 - ⁇ 16 . Accordingly, channels ⁇ 9 - ⁇ 16 are reflected back to port 831 - 2 while the remaining channels are transmitted to circulator 830 - 2 .
- Channels ⁇ 9 - ⁇ 16 are then circulated to output port 831 - 3 and supplied to a corresponding sub-demultiplexer.
- circulators 830 - 2 , 825 - 1 , 825 - 2 and 825 - 3 output channel groups ⁇ 25 - ⁇ 32 , ⁇ 1 - ⁇ 8 , ⁇ 17 - ⁇ 24 and ⁇ 33 - ⁇ 40 in conjunction with in-fiber Bragg gratings 832 - 2 , 832 - 3 , 832 - 4 , and 832 - 5 , respectively.
- in-fiber Bragg gratings 832 - 3 , 832 - 4 and 832 - 5 reflect successively lower bands or wavelengths, such that the wavelengths in channel group ⁇ 1 - ⁇ 8 are greater than the wavelengths in channel group ⁇ 17 - ⁇ 24 , and the wavelengths in channel group ⁇ 17 - ⁇ 24 are greater than the wavelengths in channel group ⁇ 33 - ⁇ 40 .
- in-fiber Bragg gratings 831 - 1 and 832 - 2 reflect successively lower bands of wavelengths. As a result, the demultiplexer shown in FIG. 6 avoids cladding or radiation mode loss.
- sub-demultiplexers will next be described with reference to FIGS. 7 - 10 .
- exemplary configurations are discussed in detail with respect to sub-demultiplexer 133 - 1 , it is understood that the remaining sub-demultiplexers described above can have a similar structure.
- the present invention is not limited to these specific sub-demultiplexer configurations nor the specific number of inputs and outputs disclosed. Rather, it is understood that any optically demultiplexing component capable of demultiplexing a group of channels can be used.
- FIG. 7 illustrates a first exemplary construction of sub-demutliplexer 133 - 1 .
- sub-demultiplexer 133 - 1 receives channels ⁇ 1 - ⁇ 8 at an input 525 of a 1 ⁇ 8 splitter 515 , commercially available from IOT, for example.
- Splitter 515 has eight outputs, each of which supplying channels ⁇ 1 - ⁇ 8 to a corresponding one of eight optical selectors 530 .
- Splitter 515 may include a waveguide doped with an optically active material, such as erbium. Such a waveguide may further be optically pumped so that splitter 515 has reduced loss or provides optical gain.
- in-fiber Bragg grating 520 has a reflectance characteristic as a function of wavelength such that a reflectivity maximum is obtained at a desired channel or wavelength, e.g., ⁇ 1 .
- channel ⁇ 1 is reflected back to second port 510 - 2 and circulated to third circulator port 510 - 3 , which outputs channel ⁇ 1 to one of photodiodes 150 for subsequent detection and further processing.
- the in-fiber Bragg gratings in the remaining selectors have reflectivity maxima that respectively reflect a corresponding one of each of remaining wavelengths ⁇ 2 - ⁇ 8 for output to one of photodiodes 150 .
- FIG. 8 illustrates an alternative construction for sub-demutliplexer 133 - 1 .
- the channel group ⁇ 1 - ⁇ 8 is supplied to a planar arrayed waveguide grating (AWG) 610 or dielectric thin film demultiplexer, which supplies a respective channel on each of outputs 610 - 1 to 610 - 8 .
- AWG 610 can introduce an unacceptably high level of undesired cross-talk. Accordingly, additional filtering may be required.
- a selector 630 is further illustrated in FIG. 8 to isolate a single channel, e.g., ⁇ 1 , and remove any cross-talk.
- selector 630 includes a coupler 655 receiving substantially channel ⁇ 1 at a first port 655 - 1 .
- the input light is next supplied to in-fiber Bragg grating 640 through second port 655 - 2 .
- In-fiber Bragg grating 640 substantially reflects only channel ⁇ 1 , while passing other wavelengths.
- channel ⁇ 1 is reflected back to second port 655 - 2 and output to one of photodiodes 150 via third output port 655 - 3 of coupler 655 .
- feedback circuit 670 is optionally provided to insure that grating 640 reflects at the intended wavelengths.
- an in-fiber Bragg grating typically includes a periodic variation of index of refraction, with the spacing between refractive index peaks defining, in part, the wavelength to be reflected by the grating. Variations in the wavelength of light reflected by grating 640 can be sensed with photodiode 660 , which supplies an electrical signal to a feedback circuit 670 .
- Feedback circuit 670 in turn, generates an appropriate voltage signal to adjust a temperature regulator coupled to grating 640 . Since the grating refractive index spacing changes with temperature, the temperature of the grating is set by feedback circuit 670 to alter the grating spacing so that grating 640 reflects a desired wavelength.
- FIG. 9 illustrates a further exemplary construction of sub-demultiplexer 133 - 1 .
- sub-demultiplexer 133 - 1 comprises a plurality of Mach-Zehnder interferometers. It is known that Mach-Zehnder interferometers, which include optical waveguides of varying lengths, can be used to separate wavelengths (see for example, published European Patent Application EP0482461). Accordingly, as shown in FIG. 9, Mach-Zehnder interferometers can be cascaded to separate a group of wavelengths.
- Mach-Zehnder interferometer 710 separates input channels ⁇ 1 - ⁇ 8 into sub-groups of channels ⁇ 1, 3, 5, 7 and ⁇ 2, 4, 6, 8 , respectively.
- Channel sub-group ⁇ 1, 3, 5, 7 is supplied to Mach-Zehnder interferometer 715 and channel sub-group ⁇ 2, 4, 6, 8 is supplied to Mach-Zehnder interferometer 720 .
- Channel sub-group ⁇ 1, 3, 5, 7 is supplied to Mach-Zehnder interferometer 715 and channel sub-group ⁇ 2, 4, 6, 8 is supplied to Mach-Zehnder interferometer 720 .
- Mach-Zehnder interferometers 715 and 720 further break down these channel sub-groups to channel pairs ⁇ 1,5 , ⁇ 3,7 , ⁇ 2,6 , and ⁇ 4,8 , which are further demultiplexed into individual channels by Mach-Zehnder interferometers 725 , 730 , 735 and 740 , respectively.
- Fiber optical couplers can also be used to demultiplex the groups of channels supplied by the filtering elements.
- Fiber optic coupler 1000 is illustrated in FIG. 10 in which two wavelengths ⁇ i and ⁇ j , are separated and supplied on respective outputs 1010 and 1020 .
- a plurality of such fiber optic couplers can be cascaded in a manner similar to that shown in FIG. 9 to thereby demultiplex a plurality of wavelengths within a given group of channels.
- selectors 530 or 630 may be provided at the outputs of Mach-Zehnder interferometers 725 , 730 , 735 and 740 of FIG. 7, or at the outputs of the cascaded fiber optic couplers 1000 , in order to reject any extraneous wavelengths, e.g., cross-talk.
- FIG. 11 shows an alternative embodiment of the present invention, whereby splitter 110 and filtering elements 125 - 1 to 125 - 3 , 135 - 1 and 135 - 2 are replaced by five port circulator 1110 and in-fiber Bragg gratings 1111 to 1114 .
- Channels ⁇ 1-40 are supplied to a first port 1110 - 1 of circulator 1110 and are output through port 1110 - 2 .
- In-fiber Bragg grating 1114 is configured to transmit channels ⁇ 1-8 and reflect channels ⁇ 9-40 .
- channels ⁇ 1-8 are supplied to sub-demultiplexer 1120 - 1 for further demultiplexing while channels ⁇ 9-40 are reflected back to second port 1110 - 2 and output to port 1110 - 3 .
- In-fiber Bragg grating 1113 which is coupled to second port 1110 - 3 , is configured to reflect channels ⁇ 17-40 and transmit channels ⁇ 9-16 .
- channels ⁇ 9-16 are passed to sub-demultiplexer 1120 - 2 for further demultiplexing and channels ⁇ 17-40 are reflected back to port 1110 - 3 and output through port 1110 - 4 .
- In-fiber Bragg gratings 1112 and 1111 coupled to corresponding ports 1110 - 4 and 1110 - 5 , are configured to transmit channel groups ⁇ 17-24 and ⁇ 25-32 , respectively, and reflect the remaining channels.
- channel groups ⁇ 17-24 and ⁇ 25-32 are selected in a manner similar to that described above, and supplied to corresponding sub-demultiplexers 1120 - 3 and 1120 - 4 for further demultiplexing.
- channels ⁇ 33-40 are output through port 1110 - 6 and supplied to sub-demultiplexer 1120 - 5 for further demultiplexing.
- sub-demultiplexers 1120 - 1 to 1120 - 5 have a structure similar to that shown in FIG. 7.
- FIG. 12 illustrates optical multiplexer 1200 in accordance with the present invention for multiplexing forty channels onto a single fiber 1205 .
- each channel input to multiplexer 1200 is supplied on a respective input optical communication path or line to one of optical combiners 1211 to 1215 .
- Optical combiners 1211 to 1215 are similar to optical splitter 515 , but the inputs and outputs are reversed.
- combiner 1211 receives channels ⁇ 1 - ⁇ 8 on respective input lines and combines these channels as a channel group onto a single output line 1231 .
- channel groups ⁇ 9 - ⁇ 16 , ⁇ 17 - ⁇ 24 , ⁇ 25 - ⁇ 32 , and ⁇ 33 - ⁇ 40 are output from combiners 1212 , 1213 , 1214 and 1215 on lines 1232 , 1233 , 1234 and 1235 , respectively.
- Lines 1231 and 1232 are fed to filtering element 1221 , commercially available from JDS Fitel or E-Tek, for example, for combining channel groups ⁇ 1 - ⁇ 8 and ⁇ 9-16 onto line 1241 .
- Lines 1234 and 1235 are coupled to filtering element 1223 , which combines ⁇ 25 - ⁇ 32 and ⁇ 33 - ⁇ 40 onto a single line 1236 , and lines 1233 and 1236 are fed to filtering element 1222 , which combines channel groups ⁇ 17 - ⁇ 24 , ⁇ 25 - ⁇ 32 , ⁇ 33 - ⁇ 40 onto a single line 1242 .
- Lines 1241 and 1242 are connected to coupler 1231 , similar in construction to splitter 110 but having the inputs and outputs reversed, which combines channel groups ⁇ 1 - ⁇ 8 , ⁇ 9 - ⁇ 16 , ⁇ 17 - ⁇ 24 , ⁇ 25 - ⁇ 32 , and ⁇ 33 - ⁇ 40 onto line 1205 .
- Optical multiplexer 1200 is readily expandable by adding additional combiners and filtering elements.
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Abstract
Description
- The present invention is directed to an optical demultiplexer for demultiplexing optical signals in a dense wavelength division multiplexed system.
- Optical communication systems are a substantial and fast growing constituent of communication networks. The expression “optical communication system,” as used herein, relates to any system which uses optical signals to convey information across an optical waveguiding medium, for example, an optical fiber. Such optical systems include but are not limited to telecommunication systems, cable television systems, and local area networks (LANs). Currently, the many optical communication systems are configured to carry an optical channel of a single wavelength over one or more optical waveguides. To convey information from plural sources, time-division multiplexing is frequently employed (TDM). In time-division multiplexing, a particular time slot is assigned to each signal source, the complete signal being constructed from the portions of the signals collected from each time slot. While this is a useful technique for carrying plural information sources on a single channel, its capacity is limited by fiber dispersion and the need to generate high peak power pulses.
- While the need for communication services increases, the current capacity of existing waveguiding media is limited. Although capacity may be expanded e.g., by laying more fiber optic cables, the cost of such expansion is prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of existing optical waveguides.
- Wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of existing fiber optic networks. WDM systems typically include a plurality of transmitters, each respectively transmitting signals on a designated one of a plurality of channels or wavelengths. The channels are combined by a multiplexer at one end terminal and transmitted on a single fiber to a demultiplexer at another end terminal where they are separated and supplied to respective receivers.
- Recently, dense WDM (DWDM) systems transmitting 8 channels on a single fiber have been proposed. These systems can include a demultiplexer having a 1×8 optical splitter, which receives the 8 channels on an input fiber, and outputs the channels on each of 8 outputs. The power level on each of the outputs, however, is approximately {fraction (1/8)} the input power level. Optical components are respectively coupled to the outputs of the 1×8 splitter for outputting a corresponding one of the 8 channels, which introduce additional loss.
- Although 8 channel WDM systems provide improved capacity, the need for additional capacity has increased with growing internet traffic and demand for multimedia services. Thus, DWDM systems having higher channel counts are currently being developed. In high channel count systems, however, it is difficult to multiplex and demultiplex a large number of optical channels. For example, in a 40 channel DWDM system, a 1×40 splitter would be inadequate to demultiplex each of the channels because the power level at each output of such a splitter would be insufficient to maintain an adequate signal to noise ratio. As a result, the transmitted channels cannot be adequately detected. On the other hand, although an optical amplifier could be used to increase the power on the input of the 1×40 splitter, such an amplifier can be difficult to manufacture, and would fail to provide the requisite optical power per channel at higher channel counts. Moreover, if amplifiers were to be provided at each of the outputs of the 1×40 splitter, the cost of the demultiplexer would be excessive.
- Thus, there is a need for a multiplexer and demultiplexer suitable for incorporation into a high channel count DWDM system which minimizes power loss and enables adequate detection of the transmitted channels. There is also a need for a scaleable DWDM system which can readily accommodate additional channels with minimal expense.
- Consistent with the present invention, an optical device is provided comprising an optical splitter having an input and first and second outputs. The input of the optical splitter is coupled to an input optical path, which carries a plurality of optical channels. Each of the optical channels has a respective wavelength. The first output of the optical splitter being coupled to a first output optical path, and the second output being coupled to a second output optical path, with the first and second output optical paths each carrying the plurality of optical channels.
- The optical device consistent with the present invention further comprises a first optical filtering element coupled to the first output optical path for selecting a first group of said plurality of optical channels; and a second optical filtering element coupled to the second output optical path for selecting a second group of said plurality of optical channels. A first optical demultiplexer is coupled to the first optical filtering element and includes a plurality of outputs. A corresponding one of the first group of channels appearing on a respective one of the plurality of outputs of said first optical demultiplexer. Further, a second optical demultiplexer is coupled to the second optical filtering element. The second optical demultiplexer comprises a plurality of outputs, a corresponding one of the second group of channels appearing on a respective one of the plurality of outputs of the second optical demultiplexer.
- Advantages of the present invention will be apparent from the following detailed description of the presently preferred embodiments thereof, which description should be considered in conjunction with the accompanying drawings in which:
- FIG. 1 illustrates a schematic diagram of an optical demultiplexer in accordance with the present invention;
- FIG. 2 illustrates a schematic diagram of an interference filter;
- FIG. 3 illustrates transmittance characteristics of filtering elements125-1, 125-2 and 125-3 shown in FIG. 1;
- FIG. 4 illustrates transmittance characteristics of filtering elements130-1 and 130-2 shown in FIG. 1;
- FIG. 5 illustrates plots of loss vs. channel count for the present invention and a conventional demultiplexer;
- FIG. 6 illustrates an additional embodiment of the present invention;
- FIG. 7 illustrates an example of a sub-demultiplexer;
- FIG. 8 illustrates an additional example of a sub-demultiplexer;
- FIG. 9 illustrates a further example of a sub-demultiplexer;
- FIG. 10 illustrates a fiber optic coupler demutliplexer for separating two wavelengths;
- FIG. 11 illustrates an additional embodiment of the present invention; and
- FIG. 12 illustrates an optical multiplexer in accordance with the present invention.
- In accordance with the present invention, optical channels to be demultiplexed are supplied to first and second optical fibers via an optical splitter. Low loss interference filters, for example, coupled to the first and second optical fibers, select respective groups of channels. Each group of channels is next demultiplexed with sub-demultiplexers into individual channels, each of which is then sensed with a corresponding photodetector. Although the optical splitter introduces an optical power loss at the input to the demultiplexer, the interference filters and sub-demultiplexers create little additional loss. As a result, the total power loss associated with the present invention is significantly less than that obtained with a conventional n channel demultiplexer based on a 1×n splitter. Accordingly, large numbers of channels, e.g., in excess of forty can be readily demultiplexed and detected.
- Turning to the drawings in which like reference characters indicate the same or similar elements in each of the several views, FIG. 1 illustrates
optical demultiplexer 100 consistent with the present invention. As discussed in greater detail below, the exemplary demultiplexer illustrated in FIG. 1 demultiplexes 40 channels. The present invention, however, is not limited to this number of channels. Rather, the present invention is applicable to demultiplexers that can separate any appropriate number of optical channels. - Returning to FIG. 1,
optical demultiplexer 100 receives, for example, a plurality of multiplexed optical channels λ1-λ40 on inputoptical fiber 105. The multiplexed channels are emitted by laser transmitters (not shown) coupled tooptical fiber 105. The multiplexed channels are supplied to the input ofoptical splitter 110, which supplies channels λ1-λ40 to first andsecond output fibers optical amplifier 101 may be provided at the input ofcoupler 110 in order to offset any loss introduced bysplitter 110. Alternatively, the loss imposed bysplitter 110 can be compensated by coupling optical amplifiers tooutput fibers - As further shown in FIG. 1, first optical filtering elements125-1 to 125-3 are coupled to
first output fiber 120, and second filtering elements 130-1 and 130-2 are coupled tosecond output fiber 115. Each of these filtering elements typically includes an interference filter commercially available from JDS Fitel or DiCon, for example. - A simplified diagram of filtering element125-1 incorporating an interference filter, for example, is illustrated in FIG. 2. Filtering element 125-1 receives channels λ1-λ40 on
input fiber 210 having an end portion spaced from graded refractive index (GRIN)lens 215. As a result, light emitted from the end portion diverges prior to impinging onlens 215. Channels λ1-λ40 are collimated bylens 215 and directed toward dielectricthin film filter 220.Thin film filter 220 includes a substrate coated with a plurality of thin dielectric films of appropriate thickness and number to pass, in this case, channels λ1-λ8, and reflect channels λ17-λ40. Channels λ1-λ8 are thus transmitted to GRIN lens 218 where they are focused onto an end portion of output fiber 235 and transmitted to a corresponding sub-demultiplexer viaoutput port 240 of filtering element 125-1. Channels λ9-λ40, however, are reflected back toGRIN lens 215, focused ontofiber 225 and output throughport 230 to filtering element 125-2, which along with filtering element 125-3, typically has a similar construction as filtering element 125-1. - The transmittance vs. wavelength characteristics of filtering elements125-1 to 125-3 are shown in FIG. 3. Transmittance band 325-1 of filtering element 125-1 has a relatively high transmittance over a range of wavelengths including the first eight channels, λ1-λ8. As noted above, therefore, these channels are transmitted toward an output port of filtering element 125-1. It should be noted, however, that channels λ9-λ11, for example, are also transmitted, but only partially, since they lie in the roll-off
region 324 of transmittance band 325-1. Remaining channels λ12-λ40, as well as channels λ9-λ11 at a substantially reduced power level, are reflected to filtering element 125-2 having high transmittance band 325-2. As a result, channels λ17-λ24 are transmitted toward the output port of filtering element 125-2. As further shown in FIG. 3, however, channels λ25-λ27 for example, adjacent λ17-λ24 are also transmitted, but to a lesser extent. Remaining channels λ28-λ40 are reflected and output to filtering element 125-3 of which channels λ33-λ40 fall within high transmittance band 325-3 and are thus passed to an output port of filtering element 125-3. Channels λ28-λ31, however, are partially transmitted. - As further shown in FIG. 1, channel groups λ1-λ8, λ17-λ24, and λ33-λ40 are respectively directed to sub-demultiplexers 135-1, 135-2 and 135-3 where further demultiplexing is performed to output individual channels on corresponding output ports. These output ports, in turn, are coupled to respective ones of
photodiodes 150 where the channels are converted into electrical signals for processing by appropriate circuitry (not shown). The term sub-demultiplexer is used herein to describe an optical apparatus that demutliplexes the groups of channels respectively output from the filtering elements. Sub-demultiplexers 135-1, 135-2 and 135-3 are discussed in greater detail below. - As noted above, channels lying outside the high transmittance bands of filtering elements125-1, 125-2 and 125-3 are only partially reflected due to the relatively broad roll-off regions of the transmittance characteristics of interference filters, for example. Accordingly, channels adjacent the intended channel sub-groups incur a significant amount of power loss and cannot be effectively demultiplexed. These channels are therefore supplied by filtering elements 130-1 and 130-2 coupled to
second output fiber 115, as discussed in greater detail below with reference to FIG. 4. - As shown in FIG. 4, filtering element130-1 has high transmittance band 430-1 extending over wavelengths λ9-λ16. Filtering element 130-1 typically has a similar construction to filtering element 125-1, and therefore transmits channels λ9-λ16 for output to corresponding sub-demultiplexer 140-1 and further demultiplexing. Remaining channels λ1-λ8 and λ17-λ40 are reflected to filtering element 130-2 having high transmittance band 430-2 encompassing channels λ25-λ32. Accordingly, channels λ25-λ32 are transmitted through filtering element 130-2 and directed toward sub-demultiplexer 140-2 for further demultiplexing into individual channels.
- Channels lying adjacent the high transmitttance bands430-1 and 430-2, like those channels discussed previously adjacent high reflectance bands 325-1 to 325-3, are partially transmitted by filtering elements 130-1 and 130-2 and are thus difficult to detect after further demultiplexing. These channels, however, lie within the high transmittance bands of filtering elements 125-1 to 125-3 and are thus demultiplexed through sub-demultiplexers 135-1 to 135-3, respectively, as noted above. Therefore, in the presently described example of the present invention,
first output fiber 120, filtering elements 125-1 to 125-3 and corresponding sub-demultiplexers 135-1 to 135-3, serve to demultiplex channels λ1-λ8, λ17-λ24 and λ33-λ40, respectively. Additionally,second output fiber 120, filtering elements 130-1 and 130-2 and corresponding sub-demultiplexers 140-1 and 140-2 serve to demultiplex channels λ9-λ16 and λ25-λ32, respectively. Accordingly, alternating groups of channels are respectively selected by filtering elements 125-1 to 125-3 and 135-1 to 135-3. - Alternatively, if only 40 channels are to be separated by the demultiplexer in accordance with the present invention, filtering elements130-2 and 125-3 can be omitted and sub-demultipelxers 140-2 and 135-3 can be coupled directly to the outputs of filtering elements 130-1 and 125-2, respectively.
- Filtering elements125-1 to 125-3, 130-1 and 130-2 introduce little power loss to the selected sub-groups of channels. For example, the power loss associated with the channels reflected by the filtering elements is typically about 0.5 dB, and the loss associated with the channels transmitted through the filtering elements is about 0.7 dB. Thus, the overall loss of the demultiplexer in accordance with the present invention is significantly reduced.
- It is noted that as the channels propagate down one of
output fibers output fibers - FIG. 5 illustrates loss L as a function of channel count n in accordance with the present invention (curve910) and a conventional 1×n splitter based demultiplexer (curve 920). As seen in FIG. 5, at low channel counts, the loss associated with the present invention is higher than the conventional demultiplexer. This is due to the loss introduced by the 1×2
splitter 110, which imposes a minimum loss in the demultiplexer. At higher channel counts, however, the loss associated with the conventional demultiplexer increases linearly with n (curve 920). On the other hand, the loss associated with the present invention (curve 910) is stepped and increases at a much more gradual rate due, at least in part, to the low loss of the filtering elements. Therefore, at higher channel counts, channels demultiplexed in accordance with the present invention experience less loss than those demultiplexed with a conventional 1×n splitter. - FIG. 6 illustrates an alternative embodiment in which each filtering element constitutes a combination of a circulator and grating. In particular, filtering element130-1 includes a circulator 830-1 receiving channels λ1-λ40 at a first circulator port 831-1 coupled to
output fiber 115. These channels are circulated to port 831-2 and output to in-fiber Bragg grating 832-1, which is configured to have a high reflectivity band that coincides with the band of channels λ9-λ16. Accordingly, channels λ9-λ16 are reflected back to port 831-2 while the remaining channels are transmitted to circulator 830-2. Channels λ9-λ16 are then circulated to output port 831-3 and supplied to a corresponding sub-demultiplexer. In a similar fashion circulators 830-2, 825-1, 825-2 and 825-3 output channel groups λ25-λ32, λ1-λ8, λ17-λ24 and λ33-λ40 in conjunction with in-fiber Bragg gratings 832-2, 832-3, 832-4, and 832-5, respectively. - Typically, in-fiber Bragg gratings832-3, 832-4 and 832-5 reflect successively lower bands or wavelengths, such that the wavelengths in channel group λ1-λ8 are greater than the wavelengths in channel group λ17-λ24, and the wavelengths in channel group λ17-λ24 are greater than the wavelengths in channel group λ33-λ40. Further, in-fiber Bragg gratings 831-1 and 832-2 reflect successively lower bands of wavelengths. As a result, the demultiplexer shown in FIG. 6 avoids cladding or radiation mode loss.
- Examples of sub-demultiplexers will next be described with reference to FIGS.7-10. Although exemplary configurations are discussed in detail with respect to sub-demultiplexer 133-1, it is understood that the remaining sub-demultiplexers described above can have a similar structure. Moreover, the present invention is not limited to these specific sub-demultiplexer configurations nor the specific number of inputs and outputs disclosed. Rather, it is understood that any optically demultiplexing component capable of demultiplexing a group of channels can be used.
- FIG. 7 illustrates a first exemplary construction of sub-demutliplexer133-1. As shown in FIG. 7, sub-demultiplexer 133-1 receives channels λ1-λ8 at an
input 525 of a 1×8splitter 515, commercially available from IOT, for example.Splitter 515 has eight outputs, each of which supplying channels λ1-λ8 to a corresponding one of eightoptical selectors 530.Splitter 515, as well assplitter 110, may include a waveguide doped with an optically active material, such as erbium. Such a waveguide may further be optically pumped so thatsplitter 515 has reduced loss or provides optical gain. - In FIG. 7, only one of the selectors is shown including a three
port circulator 510 having a first port 510-1 for receiving channels λ1-λ8 from one of the outputs ofsplitter 515. These channels are transmitted to in-fiber Bragg grating 520 via second port 510-2 ofcirculator 510. Typically, in-fiber Bragg grating 520 has a reflectance characteristic as a function of wavelength such that a reflectivity maximum is obtained at a desired channel or wavelength, e.g., λ1. Thus, only channel λ1 is reflected back to second port 510-2 and circulated to third circulator port 510-3, which outputs channel λ1 to one ofphotodiodes 150 for subsequent detection and further processing. Likewise, the in-fiber Bragg gratings in the remaining selectors have reflectivity maxima that respectively reflect a corresponding one of each of remaining wavelengths λ2-λ8 for output to one ofphotodiodes 150. - FIG. 8 illustrates an alternative construction for sub-demutliplexer133-1. Here, the channel group λ1-λ8 is supplied to a planar arrayed waveguide grating (AWG) 610 or dielectric thin film demultiplexer, which supplies a respective channel on each of outputs 610-1 to 610-8. If the spacing between adjacent ones of channels λ1-λ8 is relatively narrow,
AWG 610 can introduce an unacceptably high level of undesired cross-talk. Accordingly, additional filtering may be required. Thus, aselector 630 is further illustrated in FIG. 8 to isolate a single channel, e.g., λ1, and remove any cross-talk. In this case,selector 630 includes acoupler 655 receiving substantially channel λ1 at a first port 655-1. The input light is next supplied to in-fiber Bragg grating 640 through second port 655-2. In-fiber Bragg grating 640 substantially reflects only channel λ1, while passing other wavelengths. Thus, channel λ1 is reflected back to second port 655-2 and output to one ofphotodiodes 150 via third output port 655-3 ofcoupler 655. - As further shown in FIG. 8,
feedback circuit 670 is optionally provided to insure that grating 640 reflects at the intended wavelengths. As is generally understood, an in-fiber Bragg grating typically includes a periodic variation of index of refraction, with the spacing between refractive index peaks defining, in part, the wavelength to be reflected by the grating. Variations in the wavelength of light reflected by grating 640 can be sensed withphotodiode 660, which supplies an electrical signal to afeedback circuit 670.Feedback circuit 670, in turn, generates an appropriate voltage signal to adjust a temperature regulator coupled to grating 640. Since the grating refractive index spacing changes with temperature, the temperature of the grating is set byfeedback circuit 670 to alter the grating spacing so that grating 640 reflects a desired wavelength. - FIG. 9 illustrates a further exemplary construction of sub-demultiplexer133-1. In this instance, sub-demultiplexer 133-1 comprises a plurality of Mach-Zehnder interferometers. It is known that Mach-Zehnder interferometers, which include optical waveguides of varying lengths, can be used to separate wavelengths (see for example, published European Patent Application EP0482461). Accordingly, as shown in FIG. 9, Mach-Zehnder interferometers can be cascaded to separate a group of wavelengths. For example, Mach-
Zehnder interferometer 710 separates input channels λ1-λ8 into sub-groups of channels λ1, 3, 5, 7 and λ2, 4, 6, 8, respectively. Channel sub-group λ1, 3, 5, 7 is supplied to Mach-Zehnder interferometer 715 and channel sub-group λ2, 4, 6, 8 is supplied to Mach-Zehnder interferometer 720. As further shown in FIG. 9, Mach-Zehnder interferometers 715 and 720 further break down these channel sub-groups to channel pairs λ1,5, λ3,7, λ2,6, and λ4,8, which are further demultiplexed into individual channels by Mach-Zehnder interferometers - Fiber optical couplers, commercially available from Applied Fiber Optics, Inc. and Gould, for example, can also be used to demultiplex the groups of channels supplied by the filtering elements.
Fiber optic coupler 1000 is illustrated in FIG. 10 in which two wavelengths λi and λj, are separated and supplied onrespective outputs selectors Zehnder interferometers fiber optic couplers 1000, in order to reject any extraneous wavelengths, e.g., cross-talk. - FIG. 11 shows an alternative embodiment of the present invention, whereby
splitter 110 and filtering elements 125-1 to 125-3, 135-1 and 135-2 are replaced by fiveport circulator 1110 and in-fiber Bragg gratings 1111 to 1114. Channels λ1-40 are supplied to a first port 1110-1 ofcirculator 1110 and are output through port 1110-2. In-fiber Bragg grating 1114 is configured to transmit channels λ1-8 and reflect channels λ9-40. Accordingly, channels λ1-8 are supplied to sub-demultiplexer 1120-1 for further demultiplexing while channels λ9-40 are reflected back to second port 1110-2 and output to port 1110-3. In-fiber Bragg grating 1113, which is coupled to second port 1110-3, is configured to reflect channels λ17-40 and transmit channels λ9-16. As a result, channels λ9-16 are passed to sub-demultiplexer 1120-2 for further demultiplexing and channels λ17-40 are reflected back to port 1110-3 and output through port 1110-4. - In-
fiber Bragg gratings 1112 and 1111, coupled to corresponding ports 1110-4 and 1110-5, are configured to transmit channel groups λ17-24 and λ25-32, respectively, and reflect the remaining channels. Thus, channel groups λ17-24 and λ25-32 are selected in a manner similar to that described above, and supplied to corresponding sub-demultiplexers 1120-3 and 1120-4 for further demultiplexing. In addition, channels λ33-40 are output through port 1110-6 and supplied to sub-demultiplexer 1120-5 for further demultiplexing. Typically, sub-demultiplexers 1120-1 to 1120-5 have a structure similar to that shown in FIG. 7. - FIG. 12 illustrates
optical multiplexer 1200 in accordance with the present invention for multiplexing forty channels onto asingle fiber 1205. As seen in FIG. 12, each channel input tomultiplexer 1200 is supplied on a respective input optical communication path or line to one ofoptical combiners 1211 to 1215.Optical combiners 1211 to 1215 are similar tooptical splitter 515, but the inputs and outputs are reversed. As further shown in FIG. 12,combiner 1211 receives channels λ1-λ8 on respective input lines and combines these channels as a channel group onto asingle output line 1231. Likewise, channel groups λ9-λ16, λ17-λ24, λ25-λ32, and λ33-λ40 are output fromcombiners lines Lines filtering element 1221, commercially available from JDS Fitel or E-Tek, for example, for combining channel groups λ1-λ8 and λ9-16 ontoline 1241.Lines filtering element 1223, which combines λ25-λ32 and λ33-λ40 onto asingle line 1236, andlines filtering element 1222, which combines channel groups λ17-λ24, λ25-λ32, λ33-λ40 onto asingle line 1242.Lines coupler 1231, similar in construction tosplitter 110 but having the inputs and outputs reversed, which combines channel groups λ1-λ8, λ9-λ16, λ17-λ24, λ25-λ32, and λ33-λ40 ontoline 1205. -
Optical multiplexer 1200 is readily expandable by adding additional combiners and filtering elements. - While the foregoing invention has been described in terms of the embodiments discussed above, numerous variations are possible. Accordingly, modifications and changes such as those suggested above, but not limited thereto, are considered to be within the scope of the following claims.
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Also Published As
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US6404948B2 (en) | 2002-06-11 |
AU9229798A (en) | 1999-03-29 |
US6281997B1 (en) | 2001-08-28 |
WO1999013607A1 (en) | 1999-03-18 |
EP0937347A1 (en) | 1999-08-25 |
US6459516B1 (en) | 2002-10-01 |
WO1999013607A9 (en) | 1999-05-27 |
CA2270053A1 (en) | 1999-03-18 |
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