WO2007134638A1 - Séparateur/mélangeur de bande optique et appareil comprenant celui-ci - Google Patents

Séparateur/mélangeur de bande optique et appareil comprenant celui-ci Download PDF

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Publication number
WO2007134638A1
WO2007134638A1 PCT/EP2006/062500 EP2006062500W WO2007134638A1 WO 2007134638 A1 WO2007134638 A1 WO 2007134638A1 EP 2006062500 W EP2006062500 W EP 2006062500W WO 2007134638 A1 WO2007134638 A1 WO 2007134638A1
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WO
WIPO (PCT)
Prior art keywords
optical
band
wavelength
output port
splitter
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PCT/EP2006/062500
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English (en)
Inventor
Marco Piazza
Marco Zanatta
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Pirelli & C. S.P.A.
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Priority to PCT/EP2006/062500 priority Critical patent/WO2007134638A1/fr
Publication of WO2007134638A1 publication Critical patent/WO2007134638A1/fr

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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
    • G02B6/10—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
    • 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
    • G02B6/10—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
    • G02B6/122—Basic optical elements, e.g. light-guiding paths
    • G02B6/125—Bends, branchings or intersections
    • 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
    • G02B6/29346—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
    • 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
    • G02B6/29382—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 including at least adding or dropping a signal, i.e. passing the majority of signals
    • 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
    • G02B6/10—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
    • G02B2006/12083—Constructional arrangements
    • G02B2006/12109—Filter
    • 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
    • G02B6/10—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
    • G02B2006/12133—Functions
    • G02B2006/12159—Interferometer

Definitions

  • the present invention generally relates to the field of optical telecommunications, and to devices employed in optical telecommunications networks. More particularly, the invention concerns optical wavelength band splitters/combiners, and it specifically relates to Fiber-To-The- Premises (FTTP) network equipments comprising optical wavelength band splitters/combiners.
  • FTTP Fiber-To-The- Premises
  • FTTP networks include Gigabit Passive Optical Networks (GPONs) for broadband delivery of voice, video and high-speed data directly to the home or broader community through optical fibers.
  • Converged voice, video and data services networks are also known as "triple play networks". These networks support two signals in downstream direction (from a central station to the user) and one signal in upstream direction.
  • a first downstream signal delivers analog television and a second downstream signal delivers digital voice and data services, such as for example telephone and/or Internet connection.
  • the upstream signal is typically a digital signal delivering voice and data from the user to the central station or the service provider.
  • FTTP systems deliver voice, video and data over a PON using the ITU-T GPON standard.
  • the system supports Radio Frequency (RF) analog video delivery using a 1555 nm wavelength overlay.
  • RF Radio Frequency
  • High-quality video on a PON is achieved with a high-power 1555 nm signal, and the power requirement at 1555 nm is greatly demanding.
  • the second downstream signal (for digital voice and data) uses a 1490 nm wavelength.
  • the upstream digital signal is typically allocated at 1310 nm wavelength.
  • FTTP networks as well as in many other applications, a key aspect is signal splitting and/or combining. Signal multi/demultiplexing has to fulfill very demanding requirements such as, among the others, wide bandwidths and small cross-talk over a wide temperature range (typically from -4O 0 C to +85 0 C).
  • WDM Wavelength Division Multiplexing
  • guided-wave type optical devices e.g., splitters/combiners
  • PLC Planar Lightwave Circuit
  • IOC Integrated Optical Circuit
  • an optical band splitter/combiner comprising five cascaded splitting devices and four differential phase delay devices alternated with the splitting devices, as depicted in Figure 1, provides a high on-chip- density PLC device, particularly suitable for FTTP networks, having wide flattened continuous bands and strong tolerance to fabrication errors.
  • a splitter/combiner for a first and a second continuous optical bands wider than 10 nm may be obtained by designing the five splitting devices and the four differential delay devices so as to send the power input at an operating wavelength within a first optical band substantially in one of the two output ports, and the power input at an operating wavelength within a second optical band substantially in the other of the two output ports.
  • a preferred choice of the splitting coefficients of the five splitting devices may result in a splitter/combiner of a first and a second continuous optical bands wider than 10 nm having particularly wide flattened bands and very strong tolerance to fabrication errors.
  • Figure 1 shows a symbolic diagram of the optical device disclosed in WO 2005/124412.
  • the device globally denoted as 100, comprises a first input port 101, and a first and a second output ports 102, 103 optically coupled to the first input port 101.
  • the output port that is on the same waveguide as the input port is denoted “bar” output port, or shortly “bar” port
  • the output port that is on the other waveguide is denoted "cross” output port, or, shortly, "cross” port.
  • the device 100 is made up of two waveguides
  • the output port 102 is the bar output port
  • the output port 103 is the cross output port.
  • the first input port 101 is adapted to receive an input optical radiation Pj n .
  • the device 100 may optionally comprise a second input port 104. With respect to the second input port 104, bar and cross designations are reversed.
  • the optical device 100 is adapted to split the input optical radiation Pj n received at the input port 101 into two output optical radiations Pbar and Pcross, outputting respectively from the bar port 102 and the cross port 103 and having each a respective optical power spectrum.
  • a first, a second, a third, a fourth and a fifth optical splitting devices represented respectively by blocks 106, 107, 108, 109 and 110, are optically coupled in cascade, and a first, a second, a third and a fourth optical differential delay devices, represented respectively by blocks 111, 112, 113 and 114, are optically coupled and intercalated to the optical splitting devices 106, 107, 108, 109 and 110.
  • Figure 2 is a diagram showing a simulated transmittance spectral response of an optical splitter/combiner designed according to the teachings of WO 2005/124412, in an attempt to meet the ITU-T specification in terms of ripple and isolation in the two prescribed wavelength bands.
  • Curve 205 represents the response at the cross output port 103
  • curve 210 is the response at the bar output port 102.
  • the single device so designed is not suitable to be used in GPONs, because it would be not compliant with the requirements in terms of cross-talk of the ITU-T prescriptions, which specify, for the level of crosstalk of the digital signal, comprised within the wavelength band 1490 nm ⁇ 10 nm, on the analog signal, comprised within the wavelength band 1555 nm ⁇ 5 nm, a value of at least -28 dB, and a value of at least -32 dB for the level of cross-talk of the analog signal on the digital signal.
  • the optical response of the device so designed is substantially symmetric: considering two equally wide optical bands respectively centered at the wavelength of symmetry of the response curve of the respective pass-band output port, the optical characteristics are substantially equal for the two bands. For example, considering two bands respectively centered at approximately 1490 nm and 1558 nm, and 10 nm wide (depicted as shaded areas in Figure 2) the optical splitter/combiner flattens the two pass-bands substantially equally, being the in-band ripples less than 0.1 dB, and isolates the two bands substantially equally, being the isolation value approximately -22 dB for the first band, centered at 1490 nm, and approximately -19 dB for the second band, centered at approximately 1558 nm (the isolation values are denoted IM and I1-2 in the drawing).
  • an optical band splitter device is considered to have a symmetric response when the difference between the isolation of the first band compared to the second band, and of the second band compared to the first band does not exceed approximately 5 db, otherwise the optical band splitter is considered to have an asymmetric response.
  • WO 2005/12441 it was proposed that, in order to comply with the cross-talk requirements of the ITU-T specifications, two or more devices of the type depicted in Figure 1 can be cascaded.
  • a device was proposed consisting of three optical splitters/combiners of the type depicted in Figure 1, in a tree-cascade configuration, featuring a simulated response complying with the above ITUT specifications.
  • the resulting device dimensions are relatively large, due to the necessity of providing five plus five directional couplers, intercalated by four plus four differential delay devices for each of the two optical paths. This contrasts with the trend in reducing the devices' size, so as to achieve higher integration levels. Furthermore, the relatively high number of directional couplers to be provided introduces non-negligible insertion losses, and renders the final device more sensitive to manufacturing errors. The production yield is thus negatively affected.
  • the Applicant has tackled the problem of providing an optical device adapted to be used as a splitter/combiner device in PONs, particularly in GPONs complying with the ITU-T prescriptions, and not, or at least less affected by the drawbacks of the known devices.
  • the Applicant has found that the asymmetry in the ITU-T requirements in terms of cross- talk between the analog and digital signals may be exploited to design an optical device adapted to be used as a splitter/combiner device in an FTTP, providing better performances than the known devices.
  • an optical device as set forth in appended claim 1 is provided.
  • the optical device is adapted to split/combine a first and a second wavelength bands, the first wavelength band having a first central wavelength, and the second wavelength band having a second central wavelength, wherein:
  • the first and second central wavelengths are in a range of wavelengths from approximately 600 nm to approximately 2000 nm;
  • the first and second wavelength bands are continuous and have equal widths ranging from approximately 5 nm to approximately 40 nm
  • the optical device comprises a first optical band splitter comprising an input port, a first output port and a second output port, and five optical couplers optically coupled in cascade between the input port and the first and second output ports, and intercalated by four optical differential delay devices, wherein the first optical band splitter is adapted to split the first and the second wavelength bands, when input at the input port, and to make them available respectively at the first and second output ports, the first optical band splitter having a respective power response at the first and second output ports wherein the central wavelength of each wavelength band corresponds to the maximum of the power response at the respective output port and an isolation, at the first output port, of the second wavelength band with respect to the first wavelength band that is essentially equal to an isolation, at the second output port, of the first wavelength band with respect to the second wavelength band.
  • a second optical band splitter comprising an input port coupled to the first output port of the first optical band splitter, a first output port and a second output port, and three optical couplers optically coupled in cascade between the input port and the first and second output ports, and intercalated by two optical differential delay devices, wherein the second optical band splitter is adapted to split the first and the second wavelength bands, when input at the input port of the second optical band splitter, and to make them available respectively at the first and second output ports thereof, the second optical band splitter having a respective power response at the first and second output ports thereof, wherein the central wavelength of each wavelength band corresponds to the maximum of the amplitude response at the respective output port of the second optical band splitter, and wherein an isolation, at the first output port of the second optical band splitter, of the second wavelength band with respect to the first wavelength band is higher than an isolation at the second output port of the second optical band splitter of the first wavelength band with respect to the second wavelength band.
  • Figure 1 shows a schematic diagram of a known optical device, disclosed in WO 2005/124412;
  • Figure 2 shows a simulated spectral response of an optical device according to Figure 1;
  • Figure 3 shows, in terms of functional blocks, an optical band splitter/combiner according to an embodiment of the present invention
  • Figure 4 shows a symbolic diagram of an optical coupler used in the construction of the optical band splitter/combiner of Figure 3;
  • Figure 5 shows a general scheme of an optical device adapted to be used in the optical band splitter combiner of Figure 3, according to an embodiment of the present invention
  • Figure 6 schematically shows a possible layout arrangement of an optical device according to the general scheme of Figure 5;
  • Figure 7 is a schematic cross-sectional view of the device of Figure 6, taken along the line VII-VII of Figure 6;
  • Figure 8 schematically shows a layout arrangement of an optical coupler used in the device of Figure 6;
  • Figure 9 schematically shows a layout arrangement of a differential optical delay device of the device of Figure 6;
  • Figures 1OA and 1OB show, respectively, a spectral response, and the isolation between the two channels of a first example of optical device according the general scheme of Figure 5;
  • Figure 11 schematically shows a layout arrangement of the optical band splitter/combiner of Figure 3, in an embodiment of the present invention
  • Figure 12 is a diagram showing an experimental spectral response of the optical band splitter/combiner of Figure 11 ;
  • Figure 13 is a diagram showing the isolation between the two channels of a second example of optical device according to the general scheme of Figure 5;
  • Figure 14 shows, in terms of functional blocks, a second embodiment of a device according to Figure 3;
  • Figure 15 is a diagram showing the spectral response of the device of Figure 14;
  • Figure 16 shows, in terms of functional blocks, a third embodiment of a device according to Figure 3;
  • Figure 17 is a diagram showing the spectral response of the device of Figure 16.
  • Figure 18 shows, in terms of functional blocks, a fourth embodiment of a device according to Figure 3;
  • Figure 19 is a diagram showing the spectral response of the device of Figure 18;
  • Figure 20 shows, in terms of functional blocks, an optical device according to an embodiment of the present invention, including a device according to Figure 3;
  • Figure 21 is a diagram showing a spectral response of a 1300 nm/1500 nm symmetric optical filter included in the device of Figure 20, according to an embodiment of the present invention.
  • FIG. 3 shows a schematic diagram of an exemplary optical device 300 adapted to be used as an optical band splitter/combiner, particularly, albeit not limitatively, in a GPON FTTP.
  • the device 300 comprises a cascade of a first and a second optical devices.
  • the first optical device is the device disclosed in the above cited International application
  • WO 2005/124412 and shown with the reference numeral 100 in Figure 1, i.e. a filter having a symmetric response in respect of the equally-wide bands centered around two prescribed central wavelengths, characterized by a substantially equally flat response at both the bar and the cross output ports 102 and 103, and isolation values of one band with respect to the other at the two ports differing of less than approximately 5 dB.
  • This kind of optical device will be hereinafter also shortly referred to as a "symmetric filter”.
  • the second optical device denoted 310 in the drawing, is a device according to the general scheme of Figure 5, and has an input port 335 optically coupled to the bar output port 102 of the first optical device 100, and an output port 345.
  • an input optical radiation or signal Pi n inputted to the device 300 may be an optical signal comprising a signal Pin,i at a first wavelength ⁇ i and a signal Pin,2 at a second wavelength ⁇ 2 comprised within a first and a second optical bands, respectively.
  • the first and the second wavelength bands are assumed to be continuous and to have equal widths, ranging from approximately 5 nm to approximately 40 nm.
  • the first and the second wavelength bands have respective first and second central wavelengths, both the first and the second central wavelengths being in a range of wavelengths from approximately 600 nm to approximately 2000 nm, and being separated from each other of an amount such that the first and second wavelength bands do not overlap.
  • Each of the first wavelength ⁇ i and the second wavelength ⁇ 2 making up the input optical signal Pi n may be close to the respective central wavelength.
  • the first and the second signal wavelengths ⁇ i, ⁇ 2 are typically widely spaced, for example the spacing between the first and the second wavelengths may be greater than about 10 nm, or even greater than about 30 nm.
  • the optical device 300 is adapted to separate the input optical signal Pi n into two components P ou t,i and P ou t,2, corresponding to the two signals Pin,i and Pin,2, outputted at the output port 345 of the second device 310 and at the cross port 103 of the first device 100.
  • the optical device 100 can be characterized, in terms of its optical behavior, as being adapted to receive, at its input port 101, a relatively broad-band input optical signal Pi n , and being adapted to split a first and a second optical wavelength bands comprised within the input optical signal, making them available respectively at the bar and cross output ports 102 and 103.
  • split a first and a second optical wavelength bands means that, comparing the two output power spectra Pbar and P cro ss normalized at their respective maximum intensity, the optical power in the cross output radiation Pcross is, at each wavelength of one of the two optical bands, greater than the optical power at the same wavelength in the power spectrum of the bar output radiation Pbar.
  • the optical power in the bar output radiation Pbar is, at each wavelength of the other of the two optical bands, greater than the optical power at the same wavelength in the power spectrum of the cross output radiation P cro ss.
  • the optical power outputting from one of the output ports 102, 103 of the device 100 is more than half of the total output power outputting from the output ports of the splitter/combiner at the given wavelength.
  • the device 100 may be used as a wavelength bands combiner, instead of a splitter.
  • cross-talk refers to a relative-power level Xi and X2 (in dB) set for the first and the second optical bands, respectively.
  • the output power spectrum outputting from that of the two output ports 102 and 103 at which the considered band is suppressed exhibits, at each wavelength within said band, a power level below the predetermined level of cross-talk.
  • the power at any wavelength within one of the two bands (called “stop-band”) should be Xi dB below the power of any wavelength within the other of the two bands (called "pass-band”).
  • the power at any wavelength within the other of the two bands should be X2 dB below the power of any wavelength within the one of the two bands. It is observed that, in a similar way, cross-talk values can be defined for each of the devices making up the optical device 300, such as the device 310, as well as for the device 300 considered in its entirety. In the first optical device 100 of the band splitter 300, each of the optical splitting devices
  • each of the optical splitting devices 106, 107, 108, 109 or 110 may be a multi-layer beam splitter or a Fabry-Perot cavity or any mirror (e.g. a Brewster angle window).
  • the generic optical splitting device may be an MZI (Mach-Zehnder Interferometer) splitter/combiner or an optical coupler, such as for example an MMI coupler or a PLC directional coupler, particularly a single 2-port PLC directional coupler.
  • the optical splitting device 106 may also be a Y-branch coupler, with one input and two outputs.
  • each of the optical splitting devices 106, 107, 108, 109 and 110 may be viewed as an optical device comprising a respective first input port 407, a first (bar) output port 408 and a second (cross) output port 406; the optical splitting devices 107, 108, 109 and 110 also have a second input port 405.
  • the power coupling ratio C of a generic optical coupler is defined according to the following equation (1):
  • the power coupling ratio C of an optical coupler may be expressed by the following equation (2):
  • the first and fifth optical splitting devices 106 and 110 have substantially the same associated coupling angle, equal to a first coupling angle value QA(K).
  • the second and fourth optical splitting devices 107 and 109 have substantially the same associated coupling angle, equal to a second coupling angle value ⁇ e( ⁇ ).
  • the third optical splitting device 108 has an associated coupling angle equal to a third coupling angle value Qc(K), and may comprise two distinct optical splitting devices having substantially the same associated coupling angle, equal to about QA(K), and optically connected to each other.
  • the overall coupling angle value Qc(K) is substantially equal to about twice QA(K).
  • a generic optical splitting device will be regarded, for the purpose of the present invention, as having an associated generic coupling angle of value ⁇ ( ⁇ ) if its actual coupling angle does not depart from the exact coupling angle value ⁇ ( ⁇ ) by more than
  • the first and fifth optical splitting devices 106, 110 will be regarded as having the same associated coupling angle value ⁇ A ( ⁇ ) even when their actual coupling angles deviate from each other by at most 2%.
  • the associated coupling angle value ⁇ A ( ⁇ ) is the average of the two values.
  • the differential delay changes when changing the wavelength of the optical radiation propagating therethrough.
  • a generic differential optical path delay typically the longer the wavelength, the smaller the corresponding absolute value of the differential delay, as expressed by the following equation (3):
  • a generic optical differential delay device 111, 112, 113 or 114 may be viewed as an optical device comprising a first input port and a first output port optically connected through the first optical branch, and having a second input port and a second output port optically connected through the second optical branch.
  • the optical differential delay devices 111, 112, 113 or 114 may comprise a pair of planar waveguides having different optical paths.
  • the differential delay can be obtained, for example, by adiabatically changing the waveguide shape (e.g. enlarging or narrowing its width) on one arm or by providing an extra length on one arm or by thermally heating one arm with respect to the other. By placing the physical modification of the waveguide on the opposite arm, the actual differential delay changes the sign.
  • a "positive" differential delay will correspond to an optical path of the second branch longer than that of the first branch, while a "negative" differential delay means the opposite.
  • the optical splitting device 111 determines the sign of the respective differential delay ⁇ A ( ⁇ .thfc first and the second branch of the remaining optical differential delay devices 112, 113 and 114 are consequently determined by the optical connection between said differential delay devices 112, 113 and 114 and the first optical differential delay device 111. More in detail, the optical splitting device
  • the subsequently cascaded second optical differential delay device 112 has its first input port optically coupled to the first (bar) output port of the preceding splitting device 107 and its second input port optically coupled to the second (cross) output port of the preceding splitting device 107. Hence the first and the second optical branches of the second optical differential delay device 112 are determined.
  • the subsequently cascaded third optical splitting device 108 has its first input port optically coupled to the first output port of the preceding differential delay device 112 and its second input port optically coupled to the second output port of the preceding differential delay device 112.
  • the subsequently cascaded third optical differential delay device 113 has its first input port optically coupled to the first (bar) output port of the preceding splitting device 108 and its second input port optically coupled to the second (cross) output port of the preceding splitting device 108.
  • the subsequently cascaded fourth optical splitting device 109 has its first input port optically coupled to the first output port of the preceding differential delay device 113 and its second input port optically coupled to the second output port of the preceding differential delay device 113.
  • the subsequently cascaded fourth optical differential delay device 114 has its first input port optically coupled to the first (bar) output port of the preceding splitting device 109 and its second input port optically coupled to the second (cross) output port of the preceding splitting device 109.
  • the subsequently cascaded fifth optical splitting device 110 has its first input port optically coupled to the first output port of the preceding differential delay device 114 and its second input port optically coupled to the second output port of the preceding differential delay device 114.
  • the absolute values of the differential delays of the differential delay devices 111, 112, 113 and 114 may be substantially the same (
  • the third differential delay device 113 may have an associated differential delay ⁇ c( ⁇ ) which is opposite in sign with respect to the differential delay ⁇ ( ⁇ ) of the second differential delay device 112.
  • the fourth differential delay device 114 may have an associated differential delay ⁇ o( ⁇ ) which is opposite in sign with respect to the differential delay ⁇ A( ⁇ ) of the first differential delay device 111.
  • first and the third differential delay devices 111 and 113 may have substantially the same differential delay
  • the second and the fourth differential delay devices 112 and 114 may have substantially the same respective differential delay, opposite in sign to the differential delay of the first and the third differential delay devices 111 and 113.
  • the first and the second differential delay devices 111 and 112 may have substantially the same differential delay
  • the third and the fourth differential delay devices 113 and 114 may have substantially the same respective differential delay, opposite in sign to the differential delay of the first and the second differential delay devices.
  • a difference between two optical devices due to the unavoidable fabrication errors does not depart from the optical devices being substantially identical. Accordingly, a deviation of the actual value of a generic differential delay from a respective nominal value ⁇ by at most 1% does not depart, for the purpose of the present invention, the actual differential delay device from having an associated differential delay equal to about the nominal value ⁇ .
  • the optical splitting devices 106, 107, 108, 109 and 110 have respective associated coupling angles 0A( ⁇ ), ⁇ ( ⁇ ) and ⁇ c( ⁇ ) adapted to direct, in combination with above described differential delay devices 111, 112, 113 and 114, more than half of the total power of an optical radiation (like the signal Pi n ,i in Figure 3) inputting at the first input port 101 and having any wavelength within the first optical band to one of the first and second output ports 102, 103 (e.g., to the bar port 102), and more than half of the total power of an optical radiation (like the signal Pin,2 in Figure 3) inputting at the first input port 101 and having any wavelength within the second optical band to the other of the first and second output ports 102, 103 (e.g., to the cross port 103).
  • an optical radiation like the signal Pi n ,i in Figure 3
  • the coefficients 0A( ⁇ ), ⁇ ( ⁇ ) and ⁇ c( ⁇ ) comply with the following relations at least at a third wavelength ⁇ 3 0p within the first optical band and at least at a fourth wavelength ⁇ 4 0p within the second optical band (relations (5)): 2 ⁇ ,( ⁇ 4o J+ ⁇ c ( ⁇ 4o J-2 ⁇ ⁇ ( ⁇ 4o J« ⁇ + ⁇
  • the first wavelength ⁇ i op coincides with the third wavelength ⁇ 3 0p and the second wavelength ⁇ 2 0p coincides with the fourth wavelength ⁇ 4 0p , but wavelength deviations may occur because of the fine tuning of the overall structure.
  • the designer needs to make the pass-band and the stop- band of each output port of the device to fit with the desired optical bands.
  • the designer needs to take into account the specific desired level of cross-talk of each stop-band, which in general may vary with the first and the second optical band. As a consequence, a fine-tuning process is usually required to achieve the desired specifications and it is generally driven by the optical band having the most stringent specification for the cross-talk.
  • ⁇ i op is equal to ⁇ 3 0p and is at the center of the desired optical band
  • ⁇ 2 0p is equal to ⁇ 4 0p and is at the center of the respective desired optical band.
  • ⁇ i op is equal to ⁇ 3 0p and is at the center of the desired optical band
  • ⁇ 2 0p is equal to ⁇ 4 0p and is at the center of the respective desired optical band.
  • the above combination of the optical splitting devices and differential delay devices of Figure 1 does not direct an optical radiation inputting at the input port 101 and having a specific wavelength within the first optical band totally at one of the first and second output port 102, 103 and an optical radiation inputting at the input port 101 and having a specific wavelength within the second optical band totally in the other of the first and second output ports 102, 103.
  • This is because of the deviation of ⁇ i op from ⁇ 3 0p and ⁇ 2 0p from ⁇ 4 0p at the end of the fine tuning process described above.
  • the optical device 100 is adapted to direct an optical radiation inputting at the input port 101 and having a wavelength within the first optical band (like the signal Pin,i) substantially at one (e.g., the bar port 102) of the first and second output ports 102, 103, and an optical radiation inputting at the first input port 101 and having a wavelength within the second optical band (like the signal Pin,i) substantially in the other (e.g., the cross port 103) of the first and second output ports 102, 103, wherein "substantially” means at least the 85% of the total power outputting from the device 100 at the respective wavelength.
  • the deviation of ⁇ i op from ⁇ 3 0p and ⁇ 2 0p from ⁇ 4 0p at the end of the fine-tuning process causes the response of the first optical device 100 not to be exactly symmetric in the first and second bands, but only approximately symmetric, as shown in Figure 2.
  • the deviation from exact symmetry in the response of the optical device 100 due to the deviation of ⁇ i op from ⁇ 3 0p and ⁇ 2 0p from ⁇ 4 0p are nonetheless neglected, and the response is considered symmetric even in presence of such deviations.
  • Table 1 below shows some of the possible configurations corresponding to the lower-order choices of indexes t, m and k in accordance to the rules (6):
  • the last column of Table 1 shows the respective sum of all the coupling angles of the band splitter/combiner 100 at the third wavelength ⁇ 3 0p within the first optical band, according to the second relation of equations (6).
  • the sum is a good indication of the total length of the optical splitters 106, 107, 108, 109 and 110 in the particular case of PLC optical splitters.
  • the last column is advantageously sorted in ascending order.
  • the last column is particularly useful when optimizing the splitter/combiner 100, in that it shows how a choice of t, m and k corresponding to a given value of 2 ⁇ A + ⁇ c+2 ⁇ B is preferred over a choice of t, m and k corresponding to a higher value of 2 ⁇ A + ⁇ c +2 ⁇ B .
  • a PLC splitter/combiner comprising longer optical couplers will occupy more space on the substrate of fabrication, thus reducing the yield of the manufacturing process.
  • longer optical couplers will generally lead to narrower rejection bands and to higher sensitivity of the overall spectral response of the respective splitter/combiner with respect to small fluctuations of the structural parameters arising from the manufacturing process. This is especially true in the context of high index contrast PLC splitters/combiners, wherein the tolerance of the optical devices to manufacturing errors worsen with the increasing of the refractive index contrast.
  • the coupling angles ⁇ A, ⁇ B and ⁇ c satisfy the relation 2 ⁇ ⁇ (A 30 ⁇ )+ ⁇ c (A 30 ⁇ ) +2 ⁇ B (A 30 ⁇ ) ⁇ 2 ⁇ at least at a third wavelength ⁇ 3 0p within the first optical band.
  • m should be advantageously chosen smaller than or equal to INT[(2+t)/2], corresponding to the first six rows of Table 1, even more preferably m should be smaller than or equal to INT[(1+t)/2], corresponding to the first three rows of Table 1.
  • the coupling angles ⁇ A, ⁇ B and ⁇ c more preferably satisfy the relation 2 ' ⁇ A ( ⁇ 3op )+ ⁇ c ( ⁇ 3op )+ 2' ⁇ B ( ⁇ 3op ) ⁇ 3/2 ⁇ , even more preferably they satisfy the relation 2B A ( ⁇ 3op )+ ⁇ c ( ⁇ 3op )+ 2 ⁇ B ( ⁇ 3op ) ⁇ ⁇ , at least at a third wavelength ⁇ 3 0p within the first optical band.
  • the first optical device 100 may be a filter adapted to perform a first, relatively coarse splitting of the input optical signal Pin,i into two components at the first wavelength ⁇ i and at the second wavelength ⁇ 2, corresponding to the signals Pin,i and Pin,2.
  • relatively coarse splitting there is meant that the separation of the first and second optical bands, operated by the first optical device 100 is typically not perfect, and a certain crosstalk between the two bands exists at both of the two output ports 102 and 103 of the first device 100.
  • the signal outputted therefrom is fed to the second optical device 310, which further filters out the undesired signal component, thereby providing, at the output port 345 thereof, a signal P ou t,i, corresponding to the input signal Pin,i in the first optical band, which is less affected by cross-talk with the signal Pin,2 in the second optical band.
  • the second device 310 in Figure 3 is, as mentioned in the foregoing, an optical device according to the general scheme of Figure 5.
  • the optical device according to the general scheme of Figure 5, globally denoted 500, comprises at least a first input port 501 and at least a first and a second output ports 502 and 503 optically coupled to the first input port 501.
  • the output port 501 is the bar output port
  • the output port 503 is the cross output port.
  • the optical device 500 may optionally comprise a second input port 504. With respect to the second input port 504, bar and cross designations are reversed.
  • the first input port 501 is adapted to receive an input optical radiation, denoted P'in in the drawing, including two optical bands
  • the optical device 500 is adapted to split the input optical radiation P'in into two output optical radiations PW (bar output radiation) and P'cross (cross output radiation), outputting respectively from the bar port 502 and the cross port 503, and having each a respective optical power spectrum.
  • the optical device 500 is in particular adapted to split a first and a second optical wavelength bands as defined in the foregoing, i.e.
  • two continuous wavelength bands having equal widths, ranging from approximately 5 nm to approximately 40 nm, having respective first and second central wavelengths in a range of wavelengths from approximately 600 nm to approximately 2000 nm and being separated of an amount such that the first and second wavelength bands do not overlap.
  • first and second bands that are split by the first optical device 100 be identical to the first and second bands that are split by the second optical device 310; deviations (e.g. of about ⁇ 5 nm) in the respective central wavelengths, separation therebetween and width may be envisaged, for example in order to optimize the design of the two devices.
  • the optical device 500 comprises a first, a second and a third optical splitting devices, represented respectively by blocks 506, 507 and 508, optically coupled in cascade, and a first and a second optical differential delay devices, represented respectively by blocks 511 and 512, optically coupled, and intercalated to the optical splitting devices 506, 507 and 508.
  • the optical splitting devices 506, 507 and 508 may be similar in structure to the optical splitting devices 106, 107, 108, 109 and 110 of the device depicted in Figure 1, and the same considerations made above hold; likewise, the differential delay devices 511 and 512 may be similar in structure to the differential delay devices 111, 112, 113 and 114 of the device of Figure 1, and the same considerations made above hold.
  • the optical splitting device 506 has an input port optically connected to the first input port 501 of the device 500.
  • the optical splitting device 508 has a first output port optically connected to the first output port 502 of device 500 and a second output port optically connected to the second output port 503 of device 500.
  • the first and third optical splitting devices 506 and 508 have substantially the same associated coupling angle equal to about 0A'( ⁇ ).
  • the second optical splitting device 507 has an associated coupling angle equal to about ⁇ B'( ⁇ ), as shown in Figure 5.
  • the optical splitting device 507 has a first input port optically coupled to the first output port of the first differential delay device 511 and a second input port optically coupled to the second output port of the first differential delay device 511.
  • the second optical differential delay device 512 has its first input port optically coupled to the first (bar) output port of the preceding optical splitting device 507 and its second input port optically coupled to the second (cross) output port of the preceding optical splitting device 507. Hence the first and the second optical branches of the second optical differential delay device 512 are determined.
  • the subsequently cascaded third optical splitting device 508 has its first input port optically coupled to the first output port of the preceding differential delay device 512 and its second input port optically coupled to the second output port of the preceding differential delay device 512.
  • the length difference ⁇ L
  • L2 - L11 should be according to the following expression:
  • the absolute values of the differential delays of the differential delay devices 511 and 512 are substantially the same (
  • the optical splitting devices 506, 507 and 508 have respective associated coupling angles 0A'( ⁇ ) and ⁇ '( ⁇ ) adapted to direct, in combination with the above described differential delay devices 511 and 512, more than half of the total output power of an optical radiation inputting at the first input port 501 and having any wavelength within the first optical band to one of the first and second output ports 502, 503, and more than half of the total output power of an optical radiation inputting at the first input port 501 and having any wavelength within the second optical band to the other of the first and second output ports 502, 503.
  • the coefficients 0A'( ⁇ ) and ⁇ '( ⁇ ) should substantially comply with the following relations, at least at a third wavelength ⁇ '3 0p within the first optical band and at least at a fourth wavelength ⁇ '4 0p within the second optical band (relations (7)):
  • k' is an integer and m' is a non-negative integer.
  • k' and m' are selected in order to have 0A'( ⁇ ) and ⁇ '( ⁇ ) positive.
  • the first wavelength ⁇ 'i O p coincides with the third wavelength ⁇ '3 0p and the second wavelength ⁇ '2 0p coincides with the fourth wavelength ⁇ '4 0p , but wavelength deviations may occur because of the fine tuning of the overall structure, as discussed above.
  • an optical wavelength band splitter/combiner in accordance with the present invention, including, as shown in Figure 3, a device of the type shown in Figure 1, i.e. a symmetric filter, coupled to a device according to the general scheme of Figure 5, one needs to make the pass-band and the stop-band of each output port of the device to fit with the desired optical bands.
  • the designer needs to take into account the specific target level of crosstalk of each stop-band, which in general may be different for the first and the second optical bands. As a consequence, a fine tuning process is required to achieve the desired specifications, and it is driven by the optical band having the most stringent specification for the cross-talk.
  • ⁇ 'i op is equal to ⁇ '3 0p and is at the center of the desired optical band
  • ⁇ '2 0p is equal to ⁇ '4 0p and is at the center of the respective desired optical band.
  • ⁇ 'i op is equal to ⁇ '3 0p and is at the center of the desired optical band
  • ⁇ '4 0p is at the center of the respective desired optical band.
  • the combination of the optical splitting devices 506, 507 and 508 and of differential delay devices 511 and 512 to form the device of Figure 5 does not direct an optical radiation inputting at the first input port 501 and having a specific wavelength within the first optical band totally at one of the first and second output ports 502, 503 and an optical radiation inputting at the first input port 501 and having a specific wavelength within the second optical band totally in the other of the first and second output port 502, 503. This is because of the deviation of ⁇ 'i op from ⁇ '3o P and ⁇ '2o P from ⁇ '4 0p at the end of the fine tuning process described above.
  • the optical device 500 is adapted to direct an optical radiation inputting at the first input port 501 and having a wavelength within the first optical band substantially at one of the first and second output ports 502, 503, and an optical radiation inputting at the first input port 501 and having a wavelength within the second optical band substantially in the other of the first and second output ports 502, 503, wherein "substantially” means at least the 85% of the total power outputting from the device 400 at the respective wavelength. It can be appreciated that in relations (7) account is taken of the wavelength dependence of the splitting devices in the optical bands of interest.
  • ⁇ ( ⁇ ) is the wavelength-dependent coupling per unit length in the straight part of the coupler and ⁇ l_( ⁇ ) is an equivalent effective interaction length accounting for the wavelength- dependent coupling contribution of the coupler input and output curved sections
  • the lengths of the splitting devices 506, 507 and 508 are as low as possible; this allows reducing the wavelength dependence of the coupling angle itself.
  • the reduced wavelength dependence of the splitting devices is particularly critical in high index contrast technology.
  • reducing the lengths of the optical splitters 506, 507 and 508 is particularly advantageous in PLC technology; in fact, a PLC splitter/combiner comprising longer optical couplers occupies more substrate space, thus reducing the yield of the manufacturing process.
  • longer optical couplers generally lead to narrower rejection bands and to higher sensitivity of the overall spectral response of the respective splitter/combiner with respect to small fluctuations of the structural parameters arising from the manufacturing process. This is especially true in the context of high index contrast PLC splitters/combiners, wherein the tolerance of the optical devices to manufacturing errors worsen with the increasing of the refractive index contrast.
  • an input optical signal P'm including signals at a first and second wavelengths ⁇ i and ⁇ i respectively within the first and the second optical bands is fed to the input port 501 of the optical device 500.
  • the optical device 500 splits the optical radiation into two optical radiations PW and P'cross outputting respectively from the first output port 502 and the second output port 503. More particularly, at the bar port 502 the output signal PW comprises most of the optical power at one of the first and second wavelengths ⁇ i and ⁇ i, whereas at the cross port 503 the output signal P'cross comprises most of the optical power at the other of the first and second wavelengths ⁇ i and ⁇ i. In case a predetermined level of cross-talk is set, the output signal at the respective output port comprises an optical power at the considered wavelength below the specified cross-talk level.
  • the first device 100 and the second device 310 of Figure 3 can be realized in the form of PLC optical devices.
  • a possible layout of the device of Figure 1 realized as a PLC device is disclosed in the cited International application, and is also depicted in Figure 11.
  • the optical devices 100 and 500 may advantageously be formed on a (preferably common) substrate (520 in Figure 5), such as for example a silicon or oxide substrate.
  • the optical devices 100 and 500 are PLC optical devices, more preferably PLC optical devices each comprising a pair of optical waveguides, even more preferably high density PLC optical devices.
  • the core-to-cladding refractive index contrast of the waveguides is greater than about 1%, preferably equal to or greater than about 2%, preferably equal to approximately 2.5 % (at 1550 nm).
  • the refractive index contrast of the waveguides is lower than or equal to about 4.5%, preferably lower than or equal to about 3%.
  • FIG. 6 is a schematic top-plan view of an exemplary PLC optical device 600 in accordance with the general scheme of Figure 5 described in the foregoing, in an embodiment of the present invention.
  • the optical device 600 comprises two substantially identical planar optical waveguides
  • optical couplers 606, 607 and 608, which are advantageously put in close proximity to each other at three different locations, in order to form three optical couplers 606, 607 and 608, corresponding to the optical splitting devices 506, 507 and 508 of Figure 5.
  • Two optical differential delay devices 611 and 612, corresponding to the devices 511 and 512 of Figure 5, are intercalated to the couplers 606, 607 and 608.
  • the two optical-path lengths of each of the optical waveguides 621 and 622 in a region between two successive couplers 606, 607 and 608 are made different from each other in order to form the optical differential delay devices 611 and 612.
  • the first optical differential delay device 611 comprises a first and a second (arbitrarily selected) optical arms 611' and 611", formed on the waveguides 621 and 622, respectively.
  • the second optical arm 611" is, in the shown example, longer than the first optical arm 611' and hence, according to the convention introduced above, the associated differential delay ⁇ is positive in sign.
  • the sign of the differential delays associated with the remaining optical differential delay device 612 is determined by its connections, as described in greater detail above.
  • the optical arm of the optical differential delay device 612 laying on the first optical waveguide 621 is regarded as the first optical arm
  • the optical arm of the optical differential delay device 612 laying on the second optical waveguide 622 is regarded as the second optical arm.
  • the lower arm 612" of the second optical differential delay device 612 is regarded as the second arm, being optically connected to the second output port of the optical splitter 607, which in turn is the cross output port of the optical splitter 607 connected to the second arm 611" of the first differential delay device 611.
  • the first and the second optical differential delay devices 611 and 612 have respective associated differential delays ⁇ and ⁇ 'B which are equal and opposite in sign, the first optical differential delay device 611 having a positive differential delay ⁇ , whereas the second optical differential delay device 612 has a negative differential delay - ⁇ .
  • the device of Figure 6 is adapted to flatten the response at the cross port 602.
  • the optical waveguides 621 and 622 may be realized on a flat substrate 630.
  • the overall length Lt of device 600 may be for example equal to about 3800 ⁇ m, and the overall width Wt may be for example equal to about 2000 ⁇ m.
  • One end of the optical waveguide 621 forms a first input port 601 , adapted to receive the input optical signal P'in, and the other end of the optical waveguide 621 forms a bar output port 602 with respect to the input port 601, adapted to emit the bar optical signal PW.
  • One end of the optical waveguide 622 forms a cross-output port 603 adapted to emit the cross optical signal P'cross; the other end of the optical waveguide 622 forms an optional, second input port 604. With respect to this second input port, bar and cross designations are reversed.
  • the two waveguides 621 and 622 may be buried, ridge or rib waveguides on a substrate material, or they may be photonic crystal waveguides on a substrate material.
  • the core-to-cladding structure of the two waveguides 621 and 622 may be made of a combination of materials such as SiU2, Ge:Si ⁇ 2, BPSG, BPTEOS, GeBSG, SiON, S ⁇ 3N4, Si, SiGe, Al x Gai -x As, ln x Gai -x AsP, Cd x Zni -x Te, GaN or the like, or polymeric materials such as polyimides, acrylates, polycarbonates, silicones, benzocyclobutene (BCB), epoxy resins or the like.
  • materials such as SiU2, Ge:Si ⁇ 2, BPSG, BPTEOS, GeBSG, SiON, S ⁇ 3N4, Si, SiGe, Al x Gai -x As, ln x Gai -x
  • the optical device 600 may be fabricated using silica on silicon technology, with a waveguide core layer made of germanium-doped silica, formed for example by means of PECVD (Plasma Enhanced Chemical Vapor Deposition) process; the lower cladding may consist of silica, obtained by means of superficial thermal oxidation of a silicon substrate.
  • the upper cladding layer may be a BPTEOS glass (boron and phosphorus doped silica from liquid precursors), deposited chemically at air pressure condition through an APCVD (Atmospheric Pressure CVD) process.
  • FIG 7 is a schematic cross-sectional view of an exemplary configuration of the waveguide 621 taken along the line VII-VII in Figure 6, wherein the same reference numerals are used where appropriate.
  • the planar optical waveguide 621 is for example an optical waveguide buried into a silica (Si ⁇ 2) layer 740 on the silicon substrate 630, such as for example a silicon wafer having a thickness T of 600 ⁇ m ( Figure 7 may not be to scale).
  • Quotes A and B in Figure 7 are for example 5 ⁇ m and 10 ⁇ m, respectively.
  • Figure 8 shows a schematic (not to scale) diagram of an exemplary optical directional coupler 700.
  • ⁇ f ⁇ j also depends on the length Lx of the straight coupling region, on the wall-to-wall distance S2 between the waveguides at the coupling region over the length Lx, on the shape, width and depth of the waveguides, on the refractive index difference between waveguide core and cladding, on the geometry of the input and output curves, etc.
  • optical splitting devices 506, 507 and 508 of Figure 5 are preferably based on the same design scheme as the optical coupler 800 of Figure 8, provided however that a proper length L x is chosen for the respective straight coupling region length.
  • the optical splitting devices 106, 107, 108, 109 and 110 may be based on the design scheme of Figures 7 and 8.
  • the first and the third optical couplers 606 and 608 forming the first and the third optical splitting devices 506 and 508 are advantageously substantially identical and have advantageously the same coupling lengths LA.
  • the second optical coupler 607, forming the second optical splitting device 507 has a coupling length LB.
  • the straight coupling lengths L x , x A, B, the wall-to-wall waveguide separations S1 and
  • the radius R and the angle ⁇ of the input and output curves of the optical couplers are selected in order to direct, in combination with the optical differential delay devices 511 and 512, more than half of the output power of a first optical radiation inputting at the first input port 501 and having any wavelength within the first optical band to either one of the first and second output ports 502, 503, and more than half of the output power of a second optical radiation inputting at the first input port 501 and having any wavelength within the second optical band to the other of said first and second output ports 502, 503.
  • Figure 9 shows a schematic (not to scale) diagram of an exemplary design of the generic optical differential delay device 611 or 612, particularly the optical differential delay device 612.
  • Both of the optical differential delay devices 611 and 612 are preferably based on the same design scheme of Figure 9, provided however that the respective longer optical-path lays on the proper optical waveguide 621 or 622, in order to achieve the proper sign of the respective differential delay.
  • the optical differential delay device 612 of Figure 9 is obtained by shaping the two optical waveguides 621 and 622 in the form of two arcs having the same angle ⁇ and different radii n and r2 in order to provide the first arm 612' with an extra length ⁇ L with respect to the second arm
  • the angle ⁇ and the radii n and Ti are conveniently selected in order to achieve a differential delay which is substantially an even multiple of ⁇ at a first wavelength ( ⁇ 'i O p) within the first optical band and substantially an odd multiple of ⁇ at a second wavelength ( ⁇ '2 0p ) within the second optical band.
  • the angle ⁇ and the radii n and Ti are 20.935°, 1700 ⁇ m and 1732.1 ⁇ m, respectively.
  • the difference ⁇ L between the lengths of the second and the first arm 612", 612' is set equal to about 11.729 ⁇ m in order to achieve a differential delay, in absolute value, of about 22 ⁇ at a wavelength ⁇ 'i op equal to 1556 nm and of about 23 ⁇ at a wavelength ⁇ '2 0p equal to 1490nm.
  • the layout scheme of Figure 6 is used to make an optical device adapted to split two optical bands as defined above, with central wavelengths of approximately 1490 nm and 1560 nm, and featuring a flat response, at the bar output port 603, for a band centered at the 1560 nm wavelength (a device hereinafter referred to as a "1560 nm flat filter”), wherein, for the purposes of the present invention, by "flat" there is meant that the in-band ripple in the 1560 nm band is less than the in-band ripple of the 1490 nm band.
  • the length of the straight coupling region of the first and third optical couplers 606 and 608 is chosen equal to about 88.201 ⁇ m
  • the extra length of the waveguide 622 compared to the waveguide 621 in the differential delay device 611 is chosen equal to approximately 278.131 ⁇ m
  • the extra length of the waveguide 621 compared to the waveguide 622 in the differential delay device 612 is chosen equal to approximately 11.729 ⁇ m
  • the length of the straight coupling region of the second optical coupler 607 is chosen equal to approximately 278.131 ⁇ m.
  • Figure 10A is a diagram showing the ideal simulated response (transmittance in dB - on the ordinate - vs. wavelength in ⁇ m - on the abscissa) of the 1560 nm flat filter discussed above. It can be appreciated that the response at the cross output port 503, represented by the dotted curve R1 (corresponding to the wavelength band centered at 1560 nm) is flattened (the ripple being less than 0.1 dB), whereas the response at the bar output port 502, represented by curve R2 (corresponding to the wavelength band centered on 1490 nm) is scarcely flattened (the ripple is approximately equal to 0.5 dB).
  • Figure 10B is the same diagram as that of Figure 10A with broader ordinate axis.
  • Figure 10B shows the simulated value of isolation between the two bands (centered on 1490 nm and 1560 nm, respectively) at the cross output port 503 (dotted curve R3) and at the bar output port 502 (curve R4).
  • the isolation I2-1 between the two channels at the bar port 502 is significantly higher than the isolation I1-2 at the cross port 503, so the difference between the isolation values exceeds 5 dB, and the device exhibits an asymmetric response.
  • the output port exhibiting the higher in-band ripple, in the considered example the bar port 502 is also the output port at which the higher isolation between the two bands is achieved.
  • the second optical device 310 of Figure 3 consists of a device of the type shown in Figure 5, designed to have a flat response in the band being the stop-band for the output port 102 of the device 100.
  • Figure 11 shows schematically a possible physical layout of the cascade of the two devices 100 and 500 of Figures 1 and 5, forming the optical band splitter 300 of Figure 3, particularly designed to form a symmetric filter in respect of two bands centered around 1490 nm and 1555 nm (a "symmetric 1490 nm/1555 nm” filter), and of an asymmetric filter designed to split these two bands, flattening (at the output port 502) the band centered on 1555 nm (a "1555 nm flat” filter).
  • the two devices denoted globally 1105 and 1110, are integrated on a common substrate 1115, and are realized as a PLC optical device, as discussed above.
  • the dimensions (Ls, Ws) of the symmetric 1490 nm/1555 nm filter 1105 are significantly greater than those (L1555, W1555) of the 1555 nm flat filter 1110, due to the fact that only three cascaded optical couplers, and two differential delay devices are used in the latter, compared to the five couplers and three differential delay devices used in the former.
  • the length Ls 5825 mm
  • the width Ws 2020 mm
  • the length L1555 3760 mm
  • the width W1555 630 mm.
  • the black dots (interpolated by curve R12a) relate to the measured TE (Transverse Electric) field
  • the black squares (interpolated by curve R12b) relate to the measured TM (Transverse Magnetic) field in a cascade of two symmetric 1490 nm/1555 nm filters of the type shown in Figure 1, as proposed in the cited International application
  • the empty dots (interpolated by curve R12c) and the empty squares (interpolated by curve R12d) relate respectively to the measured TE and TM fields in a cascade of one symmetric1490 nm/1555 nm filter and one 1555 nm flat filter according to the general scheme of the present invention shown in Figure 3.
  • cascading one symmetric 1490 nm/1555 nm filter and one 1555 nm flat filter seems to represent an excellent trade-off between optical performances and device dimensions, because in addition to featuring a lower on-wafer occupation and similar in-band losses compared to the cascade of two symmetrical filters, the isolation value is even higher.
  • Designing an optical device having the general scheme of Figure 5 and featuring a flat response for a specified wavelength band is a matter of appropriate choice of the device parameters, particularly layout parameters like the bending radii of the couplers, the coupling lengths, the extra lengths for the differential delay devices; for example, like the discussed selection of parameters is adapted to give the 1555 nm flat filter discussed above, a "1490 nm flat" filter can be obtained based on the general scheme of Figure 5, i.e. an optical device adapted to split two optical bands as defined above, with central wavelengths of approximately 1490 nm and 1555 nm, and featuring a flat response at the cross port for the wavelength band around 1490 nm.
  • the diagram of Figure 13 plots (transmittance in dB - on the ordinate - vs. wavelength in ⁇ m - on the abscissa) the isolation between the two channels of one such device; by comparison with the diagrams of Figures 1OA and 1OB, it can be appreciated that although the response at the cross port 503 (dotted curve R5) in the 1490 nm band has a lower ripple compared to the response in the same, 1490 nm band exhibited by the 1555 nm flat filter of Figures 10A and 10B (less than 0.1 dB of in-band ripple), the isolation I2-1 of the band of interest with respect to the other band is worse (approximately -11 dB instead of -28 dB).
  • a third optical device is provided, denoted 1405, having an input port 1410 coupled to the output port 103 of the first device 100, and an output port 1415.
  • the signal made available at the second output port 103 of the first optical device 100 is fed to the third optical device 1405, adapted to further filter out the undesired component, thereby providing, at an output port 1415 thereof, a signal less affected by cross-talk with the signal in the other band.
  • the device 1405 may either be an asymmetric filter according to the general scheme of Figure 5 and designed to flatten the response to the other band (e.g., a
  • the signal outputting from the cross output port 103 of the first device 100 is fed to the input port 1410 of the third device 1405 and is then taken from the output port 1415 thereof (corresponding to either the bar output port 102 of the device of Figure 1, or to the output port 502 of the device of Figure 5).
  • the device 310-1 b is an asymmetric filter according to the general scheme of Figure 5 and designed to split the first and the second bands defined in the foregoing, and featuring (at an output port thereof) a flat response in respect of the second band (e.g., a 1555 nm flat filter)
  • the device 310-2b is an asymmetric filter according to the general scheme of Figure 5, designed to split the first and the second bands defined in the foregoing, and featuring (at an output port thereof) a flat response in respect of the first band (e.g., a 1490 nm flat filter).
  • the input signal Pi n composed of the signals Pin,i and of the signal Pin,2, is fed to the input port 101 of the first device 100; the signal taken from the bar output port 102 of the first device 100 is fed to an input port 335b of the asymmetric filter 310-1 b (corresponding to the input port 501 of Figure 5), and taken from the bar output port 1420 thereof (corresponding to the output port 502 of Figure ⁇ ); the signal is then fed to an input port 1420 of the asymmetric filter 310-2b (corresponding to the input port 501 of Figure 5), and the signal P ou t,i is finally taken from the bar output port 345b of the asymmetric filter 310-2b (corresponding to the output port 302 of Figure 5).
  • This allows increasing the optical isolation of the digital channel with respect to the analog channel, and to increase the on-wafer yield.
  • the curves R7 and R8 plotted in Figure 15 are the results of experimental measures conducted by the Applicant on a device according to Figure 14, designed to split a first and a second band centered on about 1490 nm and 1555 nm. It is observed that the Insertion Losses (IL) include two contributes of coupling losses between waveguide and single-mode fiber and extra propagation losses due to the longer waveguide layout designed for device testing.
  • IL Insertion Losses
  • the curves on the left side of the diagram of Figure 15 relate to the measured response of the device in respect of a third wavelength band, centered on about 1310 nm, and the isolation I1310-1490 and I1310-1555 with respect to the first and second bands centered on 1490 and 1555 are indicated, as will be explained with reference to Fig.20.
  • Figure 16 is similar to that of Figure 14, with two devices 310-1c and 310-2c cascaded to the bar output port 102 of the first device 100, but the asymmetric filter 310-2a of Figure 14 has been replaced by an asymmetric filter 310-2c designed to split the first and the second bands defined in the foregoing, and featuring (at an output port thereof) a flat response in respect of the second band [i.e., in the above considered example, two 1555 nm flat filters have been cascaded along the digital signal path); the signal outputting from the bar output port 102 of the first device 100, fed to the input port 335c of the first asymmetric filter 310-1c, outputted from the bar output port 1605 thereof, fed to the input port 1610 of the second asymmetric filter 310-2c, is finally taken from the bar output port 345c of the second asymmetric filter 310-2c.
  • the curves R9 and R10 plotted in Figure 17 are the results of experimental measures conducted by the Applicant on a device according to Figure 16, designed to split a first and a second bands centered on about 1490 nm and 1555 nm. It can be appreciated that, compared to the embodiment of Figure 14, higher isolation values on the band centered on 1490 nm can be obtained, but the in-band ripple is higher.
  • the curves on the left side of the diagram of Figure 17 relate to the measured response of the device in respect of the third wavelength band, centered on about 1310 nm, and the isolation I1310-1490 and I1310-1555 with respect to the first and second bands centered on 1490 nm and 1555 nm are indicated.
  • FIG 18 a still further embodiment 30Od of the device 300 of Figure 3 is shown, wherein two devices 310-1 d and 310-2d are cascaded (to the bar output port 102) of the first device 100: the device 310-1 d is a symmetric filter like the first device 100 (e.g., a symmetric 1490 nm/1555 nm filter), whereas the device 310-2d is an asymmetric filter according to the general scheme of Figure 5 designed to split the first and the second bands defined in the foregoing, and featuring (at an output port thereof) a flat response in respect of the second band (i.e., in the above considered example, a 1555 nm flat filter).
  • the device 310-1 d is a symmetric filter like the first device 100 (e.g., a symmetric 1490 nm/1555 nm filter)
  • the device 310-2d is an asymmetric filter according to the general scheme of Figure 5 designed to split the first and the second bands defined in the foregoing, and featuring (
  • the signal outputted from the bar output port 102 of the first device 100 is fed to the input port 335d of the symmetric filter 310-1d, and taken from the bar output port 1805 thereof; the signal is then fed to input port 1810 of the asymmetric filter 310-2d, and the signal P ou t,2 is finally taken from the bar output port 345d of the asymmetric filter 310-2d.
  • the curves R11 and R12 plotted in Figure 19 are the results of experimental measures conducted by the Applicant on a device according to Figure 18, designed to split a first and a second bands centered on about 1490 nm and 1555 nm.
  • the curves on the left side of the diagram of Figure 19 relate to the measured response of the device in respect of the third wavelength band, centered on about 1310 nm, and the isolation I1310-1490 and I1310-1555 with respect to the first and second bands centered on 1490 nm and 1555 nm are indicated.
  • the order of the devices 310-1 b and 310-2b in Figure 15 and of the devices 310-1 d and 310-2d in Figure 19 may be reversed. It is pointed out that, similarly to the path of the signal in the first band, two or more filters can be cascaded along the path of the signal in the other band for achieving the desired performance, e.g. in terms of isolation.
  • the device 1405 may be, as already mentioned, either a symmetric filter, or an asymmetric filter flattening the band stopped at the cross port 103 of the first device 100, or an asymmetric filter flattening the band passed at the cross port 103, or a cascade of two or more of these components.
  • the optical device 300 may be used in a PON, particularly a GPON compliant with the ITU-T specifications, and in this case the first and the second optical wavelength bands contain the wavelengths of 1490 nm and 1555 nm, respectively or in inverse order.
  • the optical device 300 is in this case adapted to receive the input optical signal Pi n comprising a downstream analog TV and a digital data signal, and made up for example of the two channels at approximately 1490 nm (1490 nm ⁇ 10 nm - the signal denoted P1490 in the drawing) and at approximately 1555 nm (1555 ⁇ 5 nm - the signal denoted P1555 in the drawing), and to split the two channels, making them available at the output ports 330 and 345, respectively.
  • FIG 20 a schematic diagram of an optical device 2100 is depicted, making use of the optical device 300 according to the present invention.
  • the optical device 2100 includes a first device 2105, with cascaded the device 300 of Figure 3, realized for example according to any one of the embodiments described above.
  • the device 2100 may be used as an Optical Network Unit (ONU) in a PON, i.e. as a terminal apparatus of an FTTP network, particularly as a triplexer in triple-play networks, to be installed at the customer side. More in detail, the optical device 2100 comprises the optical band splitter/combiner device
  • the optical band splitter/combiner 300 has the input port 101 adapted to receive a signal composed by the P1490 signal and the P1555 signal, to separate the two components and to make each of them available at the output ports 345 and 103 (or 1415), respectively.
  • the optical device 2100 may also comprise a first optical receiver 2110, adapted to receive a first optical wavelength ⁇ i within the first optical band (e.g. near 1490 nm), and optically connected, for example by means of a waveguide, to the output port 345 of the optical band splitter/combiner 300, and a second optical receiver 2115, adapted to receive a second optical wavelength ⁇ 2 (e.g.
  • the optical device 2100 further comprises an additional optical band splitter/combiner 2105, adapted to split/combine a third optical band from/to the first and the second optical band.
  • the device 2105 has an output port 2130 that is connected to the input port 101 of the optical band splitter/combiner 300, for example through an optical waveguide 2120.
  • the optical device 2100 may also comprise an optical transmitter 2125, adapted to emit an optical radiation having a third wavelength ⁇ 3 within the third optical band (e.g.
  • the optical band splitter/combiner 2100 is realized according to the general scheme of Figure 1, i.e. it is a symmetric filter, and is designed in such a way to be adapted to split/combine optical signals in a wavelengths band around 1300 nm and optical signals in a wavelengths band around 1500 nm.
  • the response at the cross port 2135 exhibits a flat high transmittance over a wide band (of about 80 nm) centered at 1520 nm, that is, the downstream pass-band.
  • an optical signal P 1310 having the third wavelength ⁇ 3, emitted by the optical transmitter 2125 is directed into the port 2135 of the device 2105.
  • the signal P 1310 is then mostly directed to the optical port 2140 of the device 2105, in order to be fed into the optical transmission line in an upstream direction (signal P up ).
  • An optical signal P down comprising a signal P 1490 at a first wavelength ⁇ i within the band 1490 ⁇ 10 nm, and a signal P 1555 at a second wavelength ⁇ 2 within the band 1555 ⁇ 5 nm, propagating through the optical transmission line in a downstream direction, is fed to the port 2140 of the device 2105.
  • the signal P down is then mostly directed to the output port 2130 of the device 2105.
  • the optical signal P down is then fed into the input port 101 of the device 300, which splits the optical signal P down into the two component optical signals P 1490 and P 1555 , outputting them from the two output ports 345 and 103 (or 1415), respectively.
  • Each of the two optical radiations is subsequently received by the respective optical receiver 2110 and 2115.
  • optical networks may be triple play networks or passive optical networks or fiber-to-the-premises networks or, more generally, access networks.
  • the services are delivered using at least a first and a second signal having respectively a first and a second wavelength within respectively a first and a second optical band.
  • the optical network comprises a central station, a plurality of terminal stations, and a plurality of optical links connecting each terminal station to the central station.
  • Each optical link may comprise cabled fibers and may include passive or active optical devices apt to branch, add, route, amplify, attenuate or switch the optical signals propagating through the link.
  • the terminal station may be of the kind described in Figure 20.

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  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Engineering & Computer Science (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Optical Integrated Circuits (AREA)

Abstract

La présente invention concerne un dispositif optique (300) adapté pour séparer/combiner une première et une seconde bande de longueur d'onde, comprenant un premier séparateur de bande optique (100) comportant un port d'entrée et deux ports de sortie (102, 103), cinq coupleurs optiques (106-109) couplés optiquement en cascade entre le port d'entrée et les ports de sortie, et intercalés par quatre dispositifs de retard différentiel optiques (111-114). Le dispositif comprend en outre un second séparateur de bande optique (310; 310a; 310-l b; 310-l c; 310-2d) comprenant un port d'entrée (335, 335a-c; 1910) couplé à un port de sortie du premier séparateur de bande optique, deux ports de sortie (345, 345a; 1505; 1705, 345d) et trois coupleurs optiques (506-508) couplés optiquement en cascade entre le port d'entrée et les ports de sortie du second séparateur de bande optique, et intercalés par deux dispositifs de retard différentiel optiques (511, 512). Dans ledit dispositif, une isolation, à un port de sortie du second séparateur de bande optique, de la seconde bande de longueur d'onde par rapport à la première bande de longueur d'onde, est plus élevée qu'une isolation au second port de sortie du second séparateur de bande optique de la première bande de longueur d'onde par rapport à la seconde bande de longueur d'onde.
PCT/EP2006/062500 2006-05-22 2006-05-22 Séparateur/mélangeur de bande optique et appareil comprenant celui-ci WO2007134638A1 (fr)

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0724173A1 (fr) * 1994-12-28 1996-07-31 AT&T Corp. Filtres monolithiques à guides d'ondes optiques conçus selon un développement en série de Fourier
WO2005071453A1 (fr) * 2004-01-26 2005-08-04 The Furukawa Electric Co., Ltd. Filtre de multiplexage et de demultiplexage de longueur d'onde large bande et diviseur optique a fonction de multiplexage et de demultiplexage de signal optique
WO2005124412A1 (fr) * 2004-06-21 2005-12-29 Pirelli & C. S.P.A. Diviseur/combineur de bande optique et appareil dans lequel il est integre

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0724173A1 (fr) * 1994-12-28 1996-07-31 AT&T Corp. Filtres monolithiques à guides d'ondes optiques conçus selon un développement en série de Fourier
WO2005071453A1 (fr) * 2004-01-26 2005-08-04 The Furukawa Electric Co., Ltd. Filtre de multiplexage et de demultiplexage de longueur d'onde large bande et diviseur optique a fonction de multiplexage et de demultiplexage de signal optique
WO2005124412A1 (fr) * 2004-06-21 2005-12-29 Pirelli & C. S.P.A. Diviseur/combineur de bande optique et appareil dans lequel il est integre

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