EP1754330A1 - Transmetteur/récepteur optique bidirectionnel à deux étages - Google Patents

Transmetteur/récepteur optique bidirectionnel à deux étages

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Publication number
EP1754330A1
EP1754330A1 EP05753050A EP05753050A EP1754330A1 EP 1754330 A1 EP1754330 A1 EP 1754330A1 EP 05753050 A EP05753050 A EP 05753050A EP 05753050 A EP05753050 A EP 05753050A EP 1754330 A1 EP1754330 A1 EP 1754330A1
Authority
EP
European Patent Office
Prior art keywords
optical
diffraction
grating
input
output
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP05753050A
Other languages
German (de)
English (en)
Other versions
EP1754330A4 (fr
Inventor
Ashok Balakrishnan
Serge Bidnyk
Matt Pearson
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Enablence Inc
Original Assignee
Enablence Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Enablence Inc filed Critical Enablence Inc
Publication of EP1754330A1 publication Critical patent/EP1754330A1/fr
Publication of EP1754330A4 publication Critical patent/EP1754330A4/fr
Withdrawn legal-status Critical Current

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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
    • G02B5/00—Optical elements other than lenses
    • G02B5/18—Diffraction gratings
    • G02B5/1861—Reflection gratings characterised by their structure, e.g. step profile, contours of substrate or grooves, pitch variations, materials
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/40—Transceivers
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29304—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by diffraction, e.g. grating
    • G02B6/29316—Light guides comprising a diffractive element, e.g. grating in or on the light guide such that diffracted light is confined in the light guide
    • G02B6/29325—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 of the slab or planar or plate like form, i.e. confinement in a single transverse dimension only
    • G02B6/29328—Diffractive elements operating in reflection
    • 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
    • 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
    • G02B6/42—Coupling light guides with opto-electronic elements
    • G02B6/4201—Packages, e.g. shape, construction, internal or external details
    • G02B6/4204—Packages, e.g. shape, construction, internal or external details the coupling comprising intermediate optical elements, e.g. lenses, holograms

Definitions

  • the present invention relates to a two stage optical filter, and in particular to a planar lightwave c ircuit (PLC) optical b i-directional transceiver for u se in fiber-to-the- home (FTTH) optical networks.
  • PLC planar lightwave c ircuit
  • FTTH fiber-to-the- home
  • a bi-directional transceiver e.g. a triplexer or Voice-Data-Video (VDV) processor, serves as an optical gateway from an FTTH optical network into a subscriber's home.
  • a triplexer is an extremely compact and low-cost access device capable of receiving two high-speed channels (e.g. 1490 nm for telephone & internet, and 1550 nm for video), while simultaneously transmitting on a third channel (e.g. 1310 for information out). All these signals are multiplexed onto a single optical fiber for simple installation.
  • the video channel can be omitted forming a two channel bi-directional transceiver or biplexer.
  • optical architecture requires that a laser, nominally 1310 nm in wavelength, is coupled to a single-mode fiber for transmitting optical signals from the home. In the other direction on that same fiber, light at wavelengths of nominally 1 490 nm and 1550 nm from outside the home are captured, demultiplexed and directed to optical detectors. The difficulty arises due to the operational passbands at these wavelengths.
  • the laser diode operates in a single transverse mode, and the common input/output fiber is a single mode fiber; hence, the path followed by the laser channel must be at all points compatible with single-mode optics. In other words the laser channel's path must be reversible.
  • Prior art devices such as the one disclosed in United States Patent No. 6,493,121 issued December 10, 2002 to Althaus, and illustrated in Figure 1, achieve the functionality of the VDV processor (triplexer 1) using a number of individually crafted thin film filters (TFF) 2a and 2b, placed in specific locations along a collimated beam path.
  • TFFs 2a and 2b are coupled with discrete lasers 3 and photo-detectors 4a and 4b, and packaged in separate transistor-outline (TO) cans 6 and then individually assembled into one component.
  • An incoming signal with the two incoming channels (1490nm and 1550nm) enter the triplexer 1 via an optical fiber 7.
  • the first channel is demultiplexed by the first TFF 2a and directed to the first photo-detector 4a
  • the second channel is demultiplexed by the second TFF 2b and directed to the second photo- detector 4b.
  • the outgoing channel (1310nm) is generated in the laser 3 and output the optical fiber 7 via the first and second TFFs 2a and 2b.
  • a diffraction grating is an array of fine, parallel, equally spaced grooves ("rulings") on a reflecting or transparent substrate, which grooves result in diffractive and mutual interference effects that concentrate reflected or transmitted electromagnetic energy in discrete directions, called “orders, " or “spectral orders. " The groove dimensions and spacings are on the order of the wavelength in question. In the optical regime, in which the use of diffraction gratings is most common, there are many hundreds, or thousands, of grooves per millimeter.
  • Order zero corresponds to direct transmission or specular reflection. Higher orders result in deviation of the incident beam from the direction predicted by geometric (ray) optics. With a normal angle of incidence, the angle ⁇ , the deviation of the diffracted ray from the direction predicted by geometric optics, is given by the following equation, where m is the spectral order, ⁇ is the wavelength, and d is the spacing between corresponding parts of adjacent grooves:
  • a diffraction grating is dispersive , i.e. it separates the incident beam spatially into its constituent wavelength components, producing a spectrum.
  • the spectral orders produced by diffraction gratings may overlap, depending on the spectral content of the incident beam and the number of grooves per unit distance on the grating. The higher the spectral order, the greater the overlap into the next-lower order.
  • Diffraction gratings are often used in monochromators and other optical instruments. By controlling the cross-sectional shape of the grooves, it is possible to concentrate most of the diffracted energy in the order of interest. This technique is called "blazing.
  • a planar waveguide reflective diffraction grating includes an array of facets arranged in a regular sequence.
  • the performance of a simple diffraction grating is illustrated with reference to Figure 3.
  • the optical beam is then angularly dispersed at an angle ⁇ out depending upon wavelength and the order, in accordance with the grating equation:
  • the quantity d0Nout d ⁇ is the change of the diffraction angle ⁇ Nout corresponding to a small change of wavelength ⁇ , which is known as the angular dispersion of the diffraction grating.
  • the angular dispersion increases as the order m increases, as the grading pitch A decreases, and as the diffraction angle 0 NOUt increases.
  • the linear dispersion of a diffraction grating is the product of this term and the effective focal length of the system. Since light of different wavelengths ⁇ N are diffracted at different angles ⁇ MOU , each order m is drawn out into a spectrum.
  • the number of orders that can be produced by a given diffraction grating is limited by the grating pitch ⁇ , because ⁇ 0u t cannot exceed
  • the free spectral range (FSR) of a diffraction grating is defined as the largest bandwidth in a given order which does not overlap the same bandwidth in an adjacent order.
  • the order m is important in determining the free spectral range over which continuous dispersion is obtained. For a given input-grating-output configuration, with the grating operation at a preferred diffraction order m for a preferred wavelength ⁇ , other wavelengths w ill follow t he s ame p ath a t o ther d iffraction o rders. The first o verlap of orders occurs when
  • a blazed grating is one in which the grooves of the diffraction grating are controlled to form right triangles with a blaze angle w, as shown in Figure 3.
  • the selection of the blaze angle w offers an opportunity to optimize the overall efficiency profile of the diffraction grating, particularly for a given wavelength.
  • Planar waveguide diffraction based devices provide excellent performance in the near-IR (1550 nm) region for Dense Wavelength Division Multiplexing (DWDM).
  • DWDM Dense Wavelength Division Multiplexing
  • Echelle gratings which usually operate at high diffraction orders (40 to 80), high angles of incidence (approx 60°) and large grading pitches, have lead to large phase differences between interfering paths.
  • An optical signal propagating through an optical fiber has an indeterminate polarization state requiring that the (de)multiplexer be substantially polarization insensitive so as to minimize polarization dependent losses.
  • a reflection grating used near Littrow condition, and blazed near Littrow condition light of both polarizations reflects equally well from the reflecting facets (F in Fig. 3).
  • the metalized sidewall facet S introduces a boundary condition preventing light with polarization parallel to the surface (TM) from existing near the surface.
  • light of one polarization will be preferentially absorbed by the metal on the sidewall S, as compared to light of the other polarization.
  • the presence of sidewall metal manifests itself in the device performance as polarization-dependent loss (PDL).
  • the free spectral range of gratings is proportional to the size of the grating facets. It has long been thought that gratings with a small diffraction order could not be formed reliably by means of photolithographic etching, because low order often implies steps smaller or comparable to the photolithographic resolution. The photolithographic resolution and subsequent processing steps blur and substantially degrade the grating performance. Therefore, the field of etched gratings has for practical reasons limited itself to reasonably large diffraction orders typically in excess of order 10. Devices with orders ranging close to order 1 have long been thought to be impractical to realize.
  • isolation of close to 50 dB is sometimes required between the laser source at 1310 nm and the receiver channels at 1490 and 1550 nm.
  • the main source of background light arises from scattering from defects on the facet profile.
  • the facets themselves are arranged to create phase coherent interference to disperse and focus light in a wavelength specific manner. Corner rounding between the reflective facet and the non-reflective sidewall will also be periodic, and therefore spatially coherent, but with an inappropriate phase, leading to periodic ghost images with low intensity. Facet roughness will be spatially incoherent, leading to random low-level background light.
  • the receiver channels will have a strong background contributed from the laser, at a level typically 30 dB below the strength of the laser. Isolation of- 50 dB is closer to the requirement for a practical VDV processor.
  • An object of the present invention is to overcome the shortcomings of the prior art by providing a two-stage optical filter planar lightwave circuit bi-directional transceiver with high isolation and low insertion loss.
  • the spectrometer output angles are selected to maximize the throughput of the intended wavelengths to the intended locations. Little consideration is given to Littrow radiation that may be quite intense, almost as intense as the intended output emission. In the realm of optical telecommunications, light that returns along an input path can be disastrous to the overall performance of an optical system. Accordingly, reflective grating-based devices may introduce problems to telecommunications systems. As a result, nearly all components for telecommunications have a specification for maximum "Return Loss", or "Back- reflection”, which has been particularly difficult to achieve using reflective grating technology, in which the device has a fundamental layout that is, by design, optimized for reflecting high intensities of light directly back towards the input fiber.
  • wavelength separating devices used in optical telecommunications systems are ultimately transmissive in nature, e.g. employing arrayed waveguide gratings or thin-film filters, in which there are no strong interferences caused by light rebounding directly backwards from the component.
  • An object of the present invention is to overcome the shortcomings of the prior art by providing a multiplexer/demultiplexer with input and output ports optimally positioned in accordance with the grating facet diffraction envelope to minimized back reflection to the input ports and maximize output light collected from different diffraction orders.
  • MUX/DEMUX systems perform consistently in spite of small fluctuations in laser wavelength, which requires that the MUX/DEMUX be designed with flat passbands in the frequency domain.
  • an object of the present invention is to overcome the shortcomings of the prior art by providing a MUX/DEMUX including a pair of gratings to be used in sequence such that the emission from the first grating achieves a cyclic offset of incidence angle into the second grating of the system.
  • the present invention relates to a two stage optical filter planar lightwave circuit device for receiving first and second input channels from a system waveguide and for transmitting an output channel onto the system waveguide comprising:
  • a laser transmitter for transmitting the output channel
  • a non-diffractive filter having a first passband for multiplexing the output channel onto the system waveguide, and for separating the first and second input channels from the output channel;
  • a diffraction grating filter for demultiplexing the first and second input channels, each of the first ands second input channels having a second passband n arrower than the first passband.
  • the diffraction grating filter comprising an input port for receiving the first and second input channels; a diffraction grating receiving the first and second input channels at an incident angle; and first and second output ports for outputting the first and second input channels from the diffraction grating filter, respectively.
  • the two stage optical filter planar lightwave circuit device further comprising first and second output waveguides optically coupled to the first and second ports, respectively for transmitting the first and second input channels, respectively;
  • first and second photo-detectors optically coupled to the first and s econd output ports, respectively, for converting the input channels into electrical signals.
  • the present i nvention relates to a planar waveguide optical device comprising: an input port for launching an input optical signal, which is comprised of a plurality of optical channels; a reflective waveguide diffraction grating for dispersing the optical signal into a diffraction envelope having a principle diffraction maximum, a plurality of higher order diffraction maxima, and a plurality of diffraction minima therebetween; and a first plurality of output ports outputting said optical channels; wherein said input port is positioned at one of said diffraction minima to limit the amount of light reflected from the reflective waveguide diffraction grating from re- entering the input port.
  • a planar waveguide optical device comprising: an input port for launching an input optical signal, which is comprised of a plurality of optical channels; a reflective waveguide diffraction grating for dispersing the optical signal into a diffraction envelope having a principle diffraction maximum, a plurality of higher order diffraction maxima, and a plurality of diffraction minima therebetween; and a first plurality of output ports positioned along the principle diffraction maximum for outputting said optical channels.
  • a planar waveguide optical device comprising: an input port for launching an input optical signal, which is comprised of a plurality of optical channels; a reflective waveguide diffraction grating for dispersing the optical signal into a diffraction envelope having a principle diffraction maximum, a plurality of higher order diffraction maxima, and a plurality of diffraction minima therebetween; a first plurality of output ports for outputting said optical channels; and second plurality of output ports positioned along one of the higher order diffraction maxima for outputting light therefrom.
  • the present invention relates to an optical channel demultiplexer device for separating an input optical signal into a plurality of output channel bands with a given channel spacing
  • an input port for launching an input optical signal including a plurality of optical channel bands at the given channel spacing
  • a first optical grating having a first order a first F SR s ubstantially equal to the given channel spacing, for dispersing each optical channel band over substantially a same range of output angles
  • a second optical grating having a second order and a second FSR for receiving the optical channel bands from the first reflective grating, for directing each wavelength in one of the optical channel bands at a same output angle, and for directing each optical channel band at a different output angle
  • a plurality of output ports for outputting a respective one of the plurality of optical channel bands.
  • an optical channel multiplexer device for combining a plurality of input channel bands with a given channel spacing into a single output signal comprising: a plurality of input ports for inputting a respective one of the plurality of optical channel bands; a first reflective grating having a first FSR and a first order for receiving each of the optical channel bands at different input angles from their respective input ports, and for directing each optical channel band over substantially a same range of output angles; a second reflective grating having a second order and a second FSR substantially equal to the given channel spacing for combining each optical channel band into the output signal; and an output port for outputting the output signal.
  • Figure 1 illustrates a conventional thin film filter based triplexer
  • Figure 2 illustrates a conventional thin film filter based triplexer utilizing a semiconductor substrate
  • Figure 3 illustrates a conventional reflective diffraction grating
  • Figure 4 illustrates a diffraction grating according to the present invention
  • Figure 5 illustrates a reflective concave diffraction grating PLC filter a ccording to the present invention
  • Figure 6 illustrates a two-stage optical filter according to the present invention
  • Figure 7 illustrates an output spectrum from a second stage of the optical filter of Fig. 6
  • Figure 8 illustrates an output spectrum from a first stage of the optical filter of Fig. 6.
  • Figure 9 is a plot of output angle vs. frequency for a reflective diffraction grating
  • Figure 10 is a top view of a double-grating subtractive-dispersion MUX/DEMUX according to the present invention.
  • Figure 11 illustrates a plot of input angle vs. frequency and a plot of output angle vs. frequency for the second diffraction grating of the device of Fig. 10;
  • Figure 12 is a plot of the angle error vs. frequency for the second diffraction grating of the device of Fig. 10;
  • Figure 13 illustrates an alternative embodiment of an optical device incorporating the planar waveguide reflective diffraction grating according to the present invention with the input waveguide positioned at a minimum of the diffraction envelope;
  • Figure 14 illustrates a diffraction envelope from the central facet for the device of Figure 13;
  • Figure 15 illustrates an alternative embodiment of an optical device incorporating the planar waveguide reflective diffraction grating according to the present invention with the input waveguide positioned at a minimum of the diffraction envelope and the first and second sets of output waveguides positioned at maximums of the diffraction envelope;
  • Figure 16 illustrates a spectrum for a situation in which an input waveguide is located physically near an output waveguides
  • Figure 1 7 illustrates a spectrum for a situation in which an input waveguide h as b een located at a third diffraction envelope minimum.
  • Equation (1) simplifies to
  • Telecommunications networks have evolved from DWDM to CWDM and FTTH networks.
  • the latter two network architectures have channels spanning large wavelength ranges, from - 1250 nm to ⁇ 1630 nm. These wide ranges cannot be served by a high- diffraction order device, and often require orders as low as 1.
  • Practitioners of the prior art have not been aware of, or taken advantage of equation (8). At low diffraction orders m and operating angles ⁇ j n and ⁇ out of 45° to 65° the resulting facet size F for a planar waveguide diffraction grating would be too small to be practically manufacturable.
  • Existing planar waveguide diffraction based devices include AWGs and echelle gratings.
  • the present invention encompasses all planar waveguide diffraction grating designs with the ratio of reflecting to non-reflecting facets ( or sidewalls) of at least 3.
  • planar w aveguide m aterials include silica, silicon oxynitride, s ilicon n itride, silicon on insulator, or indium phosphide
  • the amount of PDL is strongly dependent on the aspect ratio F/S and the length of the non-reflecting facet S.
  • Conventional echelle designs have an aspect ratio of ⁇ 1, and are strongly subjected to sidewall dependent PDL; however, for F/S in excess of 3, the non- reflecting facets make substantially smaller contribution to the PDL.
  • F/S it is possible to design manufacturable facets with the non-reflecting grating facet sizes S at or smaller than the wavelength of the reflected light, e.g. S ⁇ 3000nm, preferably ⁇ 2500nm, even more preferably ⁇ 2000nm, and ultimately preferably ⁇ 1550nm.
  • the interaction length of light with the metallized sidewall is so small that PDL- free operation of the device becomes possible.
  • the pitch increases with F, not S.
  • the present inventors recognize this fact and can increase the aspect ratio, i.e. decrease S/F, shown in equation (9) without risk of affecting the pitch.
  • the fidelity of the grating reproduction is limited not by photolithography but by the accuracy of the features on the mask itself. This limit is several orders of magnitude (100-fold) smaller than the photolithographic resolution.
  • a dispersive PLC optical filter 19 includes a concave reflective diffraction grating 20 is formed at an edge of a slab waveguide 21 provided in chip 22.
  • An input port is defined by an end of a waveguide 23, which extends from an edge of the chip 22 to the slab waveguide 21 for transmitting an input wavelength division multiplexed (WDM) signal, comprising a plurality of wavelength channels ( ⁇ j, ⁇ 2 , ⁇ 3 ...), thereto.
  • WDM wavelength division multiplexed
  • the diffraction grating 20, as defined above with reference to Figure 4, has an aspect ratio (F/S) greater than 5, and a sidewall length S less than or equal to the average wavelength of the wavelength channels ( ⁇ j, ⁇ 2 , ⁇ 3 ).
  • the input waveguide 23 is positioned to ensure that the incident angle ⁇ ; n is less than 45°, preferably less than 30°,and more preferably less than 15°, and the grating pitch ⁇ is selected to ensure that the grating 20 provides diffraction in an order of 5 or less.
  • the diffraction grating 20 disperses the input signal into constituent wavelengths and focuses each wavelength channel on a separate output port in the form of an output waveguide 25, the ends of which are disposed along a focal line 26 of the grating 20 defined by a Rowland circle, for transmission back to the edge of the chip 22.
  • the illustrated device could also be used to multiplex several wavelength channels, input the waveguides 25, into a single output signal transmitted out to the edge of the chip 22 via the input waveguide 23.
  • the input and output ports represent positions on the slab waveguide 21 at which light can be launched or captured; however, the ports can be optically coupled with other transmitting devices or simply blocked off.
  • a concave reflective diffraction grating 10 is formed at an edge of a slab waveguide 11 provided in chip 12.
  • An input port is defined by an end of a waveguide 13, which extends from an edge of the chip 12 to the slab waveguide 11 for transmitting an input wavelength division multiplexed (WDM) signal, comprising a plurality of wavelength channels ( ⁇ i, ⁇ 2 , ⁇ 3 ...), thereto.
  • the diffraction grating 10, as defined above with reference to Figure 2 has an aspect ratio (F/S) greater than 5, and a sidewall length S less than or equal to the average wavelength of the wavelength channels ( ⁇ ], ⁇ 2 , ⁇ 3 ).
  • the input waveguide 13 is positioned to ensure that the incident angle ⁇ j n is less than 30°, and the grating pitch ⁇ is selected to ensure that the grating 10 provides diffraction in an order of 5 or less.
  • the diffraction grating 10 disperses the input signal into constituent wavelengths and focuses each wavelength channel o n a s eparate o utput p ort i n the form o f a n output w aveguide 1 5, the ends of which are disposed along a focal line 16 of the grating 10 defined by a Rowland circle, for transmission back to the edge of the chip 12.
  • the illustrated device could also be used to multiplex several wavelength channels, input the waveguides 15, into a single output signal transmitted out to the edge of the chip 12 via the input waveguide 13.
  • the input and output ports represent positions on the slab waveguide 11 at which light can be launched or captured; however, the ports can be optically coupled with other transmitting devices or simply blocked off. Specific examples for operating the aforementioned optical device are:
  • the relevant passbands are 100 nm for the laser, and ⁇ 20 nm for the detector channels.
  • Such a device would be impractical to implement with a single diffractive structure because the various channels would share a common physical dispersion.
  • a spectrometer slab region has been chosen such that the smallest reasonable guiding waveguide widths handle the 20 nm passbands at the grating output.
  • the waveguide width necessary for the 100 nm passband channel would be so wide as to support innumerable modes, creating a device with high sensitivity to fabrication tolerances if a reversible path is necessary for this channel.
  • the two-stage optical filter includes a non-dispersive filter 31, a dispersive filter 32, a laser source 33, and first and second photo-detectors 34 and 35 formed in a planar lightwave circuit (PLC) chip 36.
  • a single photo-detector 34 can be provided, when one of the detector channels is omitted.
  • the non-dispersive filter 31 is a wavelength selective directional coupler, i.e. two parallel waveguides of specific width, spacing and coupling length, which separates the receiver channels from the laser channel.
  • the non-dispersive filter 31 can be a wavelength d ependent modal interference (MMI) filter o r a phase d ependent wavelength splitter, e.g. a Mach Zehnder interferometer designed for splitting wavelength bands.
  • MMI wavelength d ependent modal interference
  • a multi-stage coupler or MMI can be used, which provides flatter passbands than those commonly produced by s ingle-stage filters, which slightly improves the insertion loss at the outer edges of the channels, where the passbands from the single-stage filters begin to roll off.
  • the laser source 33 transmits the data channel along waveguide 41 to the non-dispersive filter 31, which multiplexes the data channel onto output waveguide 42.
  • a system waveguide 43 e.g. an optical fiber, is optically coupled to the output waveguide 43 at the edge of the PLC chip 36.
  • a monitor photodiode 46 can be positioned proximate the back facet of the laser source 33; however, the structure of the present invention enables the monitor photodiode 46 to be positioned upstream of the laser source 33 optically coupled thereto via a tap coupler 47, which separates a small portion (2%) of the laser light.
  • Back facet monitors measure the light produced by the laser, but not what is actually coupled to the waveguide 41, i.e. into the PLC chip 36; however, the downstream photodiode 46 is able to directly measure what light has been coupled in the waveguide 41.
  • the detector channels must pass through both stages of the filter, i.e. the non-dispersive filter 31 and the dispersive filter 32, and are processed by the grating-based dispersive filter 32.
  • the dispersive filter 32 is similar the dispersive filter 19, as disclosed with reference to Figure 5, including a concave reflective diffraction grating 50 with a focal line 56, preferably defined by a Rowland circle.
  • a launch waveguide 53 extending between the non-dispersive filter 31 and the dispersive filter 32 is positioned to ensure that the incident angle ⁇ j n is less than 45°, preferably less than 30°,and more preferably less than 15°.
  • the diffraction grating 50 has a pitch ⁇ selected to ensure that the diffraction grating 50 provides diffraction in an order of 5 or less.
  • the present invention incorporates multi-mode output waveguides 51 and 52 at output ports along the focal line 56.
  • the multi-mode waveguides 51 and 52 support an innumerable collection of modes, which serves to flatten the spectral response of the grating output, as shown in Figure 7.
  • the first and second output waveguides 51 and 52 include a multimode section adjacent to the first and second ports, respectively, and a single mode section remote therefrom for providing the diffraction grating filter 31 with a flattened spectral response.
  • the waveguides 51 and 52 direct the light from the output ports to the first and second photo-detectors 34 and 35, respectively.
  • the present invention achieves the varying passbands for the detector and signal channels by incorporating a dual-stage filter, in which the laser channel is separated from the detector channels, ' which are further demultiplexed with a dispersive element of higher resolution.
  • the passband of the laser channel is therefore determined by the first stage of the filter, e.g. the wavelength-selective directional coupler 31 , while the passband of the detector channels is determined predominantly by the second stage of the filter, e.g. grating-based dispersive element 32.
  • the directional coupler 31 can be designed to easily cover a passband of lOOnm, as shown in Figure 8.
  • the detector channels undergo further processing by the grating.
  • narrow transmission passbands are achieved for detector channels, whereas the laser channel is quite broad.
  • the detector channels at 1490 and 1552 nm encounter both stages of the filter, and they are dispersed into narrow bands by the dispersive filter 32.
  • the output waveguides 51 and 52 used in the dispersive filter 32 enable the passbands to be extremely flat and wide across the whole range of interest.
  • the 1 310 nm radiation is extracted following only the first stage of the filter, e .g. the wavelength-selective directional coupler, with extremely low loss. The loss for the laser channel is therefore far superior to other Triplexer filters in which the laser channel must pass through one or several grating-based elements.
  • the present two-stage configuration ensures that there is no direct path from the laser source 33 to the first and second photo- detectors 34 and 35, and the two channels are always counter-propagating, resulting in extremely high isolation of the laser source 33 from the first and second photo-detectors 34 and 35.
  • the level of isolation is significantly improved from the typical level of 30 dB from a standard grating, and can exceed the 50 dB specification required by some customers.
  • Equation 11 it can be seen from Equation 11 that for a given optical frequency, the output angle can be made to vary by changing the input angle. In fact this is an element of coarse/fine refractive index error correction for standard echelle-grating based optical DEMUX's and OCM/OPMs. Also, from Equation 1, for a given (fixed) output angle, the optical frequency (or wavelength) can be made to vary with the input angle.
  • the output angle of the light would vary (as in Figure 9), and the light would sweep past the output waveguide.
  • the input angle could be made to vary in a complementary direction, i.e. introduce some frequency insensitivity, then the output angle could be held fixed in place.
  • the light must image onto the next output waveguide, with the same insensitivity to frequency variation over the new passband.
  • an angular dependence versus frequency is introduced, as in Figure 9, but with a pattern that repeats with a controlled period, e.g. every 1 00 GHz as with the ITU grid spacing.
  • a second diffraction grating is inserted prior to the first diffraction grating of Figure 9, having a Free-Spectral- Range (FSR) of the required period, e.g. 100 GHz, with a geometry chosen to achieve the required angular variations.
  • FSR Free-Spectral- Range
  • the grating facet size will scale as the order.
  • standard DEMUX's in low diffraction orders m ⁇ 20
  • the high order grating will have a facet of ⁇ 1 mm in size.
  • the output angle of the high-order spectrometer is chosen to be the same as the input angle used in the standard order (m ⁇ 20) design.
  • the gratings and the input to the high-order spectrometer are arranged such that the coupling of light from the high-order spectrometer to the standard spectrometer i s optimum.
  • the choice of input and output angles, and the grating geometries are for convenience of calculation only.
  • a WDM optical signal comprising a plurality of optical channel bands is input an optical waveguide 109 at the edge of a planar lightwave circuit chip 110 and enters a first slab waveguide 111 at input port 112.
  • a first concave reflective grating 113 has a relatively high order, e.g. greater than 1000, preferably greater than 1500, and even more preferably greater than 1800, and a relatively small FSR, e.g. substantially the same as the channel spacing of the optical channel bands to be output. Due to the small FSR, the first grating 113 disperses each channel band over the same small range of output angles through an aperture 114 into a second slab waveguide 116.
  • a second concave reflective grating 117 is positioned at one side of the second slab waveguide 116 opposite the first reflective grating 113 in a face-to face relationship.
  • the first and second reflective gratings 113 and 117 have optical power and focus the light along the same line defined by a Rowland Circle 118.
  • the second reflective grating 117 has a much lower order than the first reflective grating 113, e.g. less than 100, preferably less than 50 and even more preferably less than 25, a much higher FSR, e.g.
  • each wavelength in the band of wavelengths in a single channel will be directed at exactly the same spot on an output port, e.g. output port 119a, corresponding to an output waveguide, e.g. output waveguide 120a.
  • an output port e.g. output port 119a
  • an output waveguide e.g. output waveguide 120a.
  • the new output angle from the second grating 117 will remain fixed for all wavelengths in the new channel band, which is output a second output port, e.g. output port 119b.
  • Other waveguides, such as optical fibers, are attached to the edge of the planar lightwave circuit chip 110 for transmitting the optical signals.
  • the device can also be used in a reciprocal fashion for multiplexing a plurality of input optical channel bands into a single output signal.
  • the second reflective grating 117 r eceives e ach c hannel b and at a d ifferent i nput angle, w hich the s econd reflective grating 117 converts into the same small range of output angles for transmission through the aperture 114.
  • the first reflective grating 113 then converts the small range of input angles into a single output angle, thereby combining all of the channels onto a single output waveguide 109.
  • the output angle of the first spectrometer will vary in a cyclic pattern according to a desired channel spacing of the second grating, i.e. the input signal and the output signals. If the geometry and facet spacing of the first spectrometer are chosen properly, the pattern will repeat every 100 GHz (or other desired channel spacing), with a variation in output angle that becomes a variation in input angle to the second spectrometer.
  • the input angle variation with optical frequency provides a constant output angle for all wavelengths in the band of wavelengths in each channel, which can nearly exactly pin the output image to the designated output waveguide.
  • the second grating 117 is designed so that the change in input angle ⁇ j n compensates for the change in frequency f providing a constant output angle ⁇ out over the given range of wavelengths in the channel band.
  • the frequency keeps increasing, but the input angle ⁇ j n reverts back to the lower end of the repeating range, which results in a new ⁇ out for the next channel.
  • the output from the first spectrometer 113 is not exactly cyclic at a 100 GHz period resulting in a gradual drift in the output angle as the frequency is tuned over the entire ITU grid.
  • the walk-off can be partially compensated for by re-positioning the output ports 119a and 119b of the second spectrometer 117 relative to their usual positions for a fixed input aperture location.
  • a modification can be made to the diffraction order of the first grating 7 in order to tune its period to the required value.
  • Figure 11 illustrates the near-cyclic behavior of the input angle ⁇ ikie to the second spectrometer 117, i.e. the output angle ⁇ out of the first spectrometer 113, versus frequency, as well as the stepped behavior of the output angle ⁇ out of the second spectrometer 117 versus optical frequency.
  • Figure 11 graphically relates the input angles to the second grating 117, and the output angles from the second grating 117, as a function of optical frequency, illustrating the cyclic nature of the input angles, and the resulting stepped response of the output angle.
  • the slight wavelength dependence of the refractive index (of the silica waveguides) leads to a barely perceptible shift in mean input angle to the second spectrometer 117 over the wide frequency range of the C-band; however, in general, the output angles of the second spectrometer 117 do show the expected stepped performance, i.e. over large fractions of each ITU grid the steps show little slope.
  • the angular content of typical waveguide modes in a silica-on-silicon design will have a magnitude on the order of a few degrees. If the angle of coupling into these output guides can be held fixed to a small fraction of this mode angular content, the coupling should remain unchanged.
  • the graph in Figure 12 illustrates the deviation of the output angles from their mean position across the grid. As can be seen from the figure, the output angles are indeed pinned to their required mean position to within 2 m illi-degrees.
  • the physical spacing between the output guide is approximately 15 ⁇ m, so the physical error in the output position from the second grating will correspond to ⁇ 0.3 ⁇ m.
  • the double-grating subtractive-dispersion design according to the present invention has benefits in the time domain as well as the frequency domain. However, in a standard single-grating design with well optimized sharp (Gaussian) passbands, improved performance will be limited when transformed between the time and frequency domains.
  • a temporal impulse broadening arises because the optical path from the input to any output v ia the n ear e dge o f t he g rating v ersus the p ath v ia t he far e dge o f the g rating differs by a non-zero length, which is indicative of the impulse broadening.
  • flat-top passbands are usually obtained by introducing aberrations to the grating or by increasing the input or output apertures; however, none of these solutions reduces the spread in time for different paths across the grating, i.e. the standard flat-top design does not narrow the temporal response.
  • a ray which follows a short path off the first grating 113 to the input of the second grating 117 will then follow a long path off the second grating 117 to the output port 119a of the second grating. The reverse holds for rays initially taking a long path from the first grating 113.
  • a subtractive dispersion double-grating device can be utilized at significantly higher data bit rates than a standard design flat passband device.
  • the device illustrated in Figure 10 with two gratings each operating in a Rowland circle geometry was a first embodiment provided as an example for simplicity of calculation; however, there are a few other options that may be more favorable.
  • One option is to design the first grating with a shape that is more appropriate for imaging along a chord at the second grating's Rowland circle centered for the input to the second grating.
  • a second option is to create the first grating to collimate its diffracted light, i.e. imaging to infinity, and the second grating would be shaped in the same manner to re-focus its diffracted light.
  • the output of the first grating will need to be collected efficiently by the second grating.
  • some form of aperturing will be needed between the first and second gratings because there will be light from multiple orders emanating from the first grating near the intended input to the second grating.
  • Much of the aperturing will be accomplished simply by the fact that the large-order grating facets are physically quite large, leading to a narrowing of the diffraction envelope from the first grating. If blazed properly, only the intended diffraction orders should arrive with any reasonable intensity onto the second grating. In order to prevent order overlap from confounding the spectrum of the second grating, aperturing would also be needed to restrict the angular range, which enters the second grating from the first one.
  • a subtractive dispersion spectrometer pair can also be designed using AWGs; however, in this case the high-FSR first spectrometer will have many drawbacks in terms of phase control.
  • facet shaping is a parameter that has no direct analog for AWGs, i.e. straight, circular, parabolic, elliptical, or other facet shapes can be implemented to control the phase of the radiation as it emerges from the high-FSR first grating.
  • the overall transmission of the double grating device i.e. the height of any passband
  • Diffraction gratings that are designed to be astigmatic over limited angular regions and blazed for that region can be as efficient as ⁇ 0.5 dB excess loss.
  • a theoretical insertion loss of- ldB is not unexpected for the grating-pair device.
  • Traditional channel flattening techniques often require over twice that loss to achieve much less-optimal performance.
  • the sharpness of the passband i.e. the steepness of the band walls, can be increased by narrowing the frequency span covered by a given optical waveguide mode width.
  • the width of the passband will be limited only by the aperturing that has just been mentioned. For a 40 channel, 100 GHz design, widths of ⁇ 40 - 50 GHz should be achievable. Depending on the steepness of the walls, these numbers could represent -0.5 dB, 1 dB and -3 dB widths all within a few GHz of each other.
  • the performance of the double-grating configuration is expected to be near transform- limited, thereby providing good optical performance at higher bit-rates than standard flat- top designs allow.
  • the present invention can be used to create high-transmission, ultra-flat ultra-sharp passband, high bit rate compatible MUX/DEMUX' s.
  • the present invention could be applied to DWDM, CWDM, 1310/1550 nm splitters, comb filters or optical channel monitors, all by proper choice of diffraction order for the first and second gratings.
  • the efficiency of the diffraction from a grating is a coherent superposition of the diffraction envelope from individual facets.
  • the positioning of the multitude of facets dominates the mode shape of the emissions from the gratings at specific wavelengths, while the size of the individual facets dominates the relative intensity of different modes at different angles/wavelengths.
  • This diffraction envelope is essentially a (sin(x)/x) 2 intensity distribution.
  • a simple optical demultiplexer is designed with one input channel 221 and four output channels 222a to 222d for Coarse Wavelength Division Multiplexing (CWDM).
  • An optical signal with a plurality of optical channels, defined by center wavelengths ⁇ j to ⁇ - t , is launched via input channel 221 into a slab waveguide region 223 to be incident on a grating 224.
  • the grating 224 disperses the optical channels according to wavelength, whereby each optical channel ⁇ j to ⁇ 4 is captured by one of the output channels 222a to 222d.
  • a diffraction envelope from the central facet for the device of Figure 13 is displayed in Figure 14. Note the high principle maximum 231 and the multiple higher-order maxima 232 with minima 233 between them. By moving the input guide 221 to a minimum 233 of the diffraction envelope the return light intensity is greatly reduced. Moreover, by moving the output channels 222a t o 222d to the principle maximum 231 or at least a higher order maximum 232 the transmitted light is maximized. Obviously, positioning both the input guide 221 and the output guides 222a to 222d in minima 233 and maximum 231 is preferred.
  • the design of a demultiplexer device 240 is an iterative process starting with the design of the grating 241 to generally provide a diffraction envelope with a sufficient amount of higher order minima and maxima.
  • the grating 241 is a concave reflective grating, as disclosed above with reference to Figures 4 and 5 , with a focal line along a Rowland circle 243. Next, an initial trial position for the input port 242 is selected, and the resulting diffraction envelope is examined.
  • the primary output ports 244 e.g. Order n
  • all of the output ports 244 and 246 are positioned along the focal line 243 of the grating 241, defined by a Rowland Circle.
  • Figure 16 illustrates a spectrum for a situation in which the input port 242 is located physically near the output ports 244, such that the input port 242 falls somewhere within the principle diffraction maximum, which is very typical for demultiplexers based on Echelle Gratings.
  • the intensity 251 of the return light signal is comparable to the intensity 252 of the main output signals, and can result in very high Return Loss that is unacceptable for telecommunications-grade optical components.
  • a similar spectrum is calculated and illustrated in Figure 17, in which the input port 242 has been located at the third diffraction envelope minimum. Note the nearly 240 dB reduction o f the intensity 253 of light returning along the input channel.
  • the secondary set of output ports 246 are located at higher-order diffraction envelope maxima 232, see Fig. 14, to capture duplicate signals in parallel with the capture of light into the primary output waveguides 244a via primary output ports 244, which is useful, inter alia, as an integrated Demultiplexer/Optical Channel Monitor.
  • the primary Demultiplexer output ports 244 would fit in the region of the principle diffraction envelope maximum 231, while the secondary output ports 246, e.g. channel monitor guides, for the same wavelengths but at a different diffraction order off the grating 241, would fit in the region of a secondary or higher diffraction envelope maximum 232.
  • monitoring of the optical power in each channel ⁇ j to ⁇ 4 of the Demultiplexer can be performed by measurement of the light coupled into a different order, instead of through the insertion of a tap coupler and the subsequent demultiplexing/monitoring of that light signal.

Abstract

L’invention concerne un circuit plan d’onde de lumière incluant un filtre optique à deux étages à utiliser dans un transmetteur/récepteur bidirectionnel. Un premier étage comprend un filtre optique non-dispersant qui autorise la lumière d’une certaine plage de longueurs d’onde, en l’occurrence un canal signal d’une source laser, à être projetée sur un guide d’onde d’entrée/sortie, alors que de la lumière d’une autre plage de longueurs d’onde, en l’occurrence un ou plusieurs canaux de détecteur, sera dirigée du guide d’onde d’entrée/sortie vers un deuxième étage. Le deuxième étage comprend un réseau de diffraction réfléchissant ayant une résolution supérieure au premier étage, fournissant une bande passante de 2 à 5 fois plus étroite que le premier étage.
EP05753050A 2004-06-04 2005-06-02 Transmetteur/récepteur optique bidirectionnel à deux étages Withdrawn EP1754330A4 (fr)

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JP2011107705A (ja) * 2009-11-16 2011-06-02 Tessera North America Inc 光ファィバ用トリプレクサ、これを含むパッケージ、および関連方法
US8737846B2 (en) * 2012-01-30 2014-05-27 Oracle International Corporation Dynamic-grid comb optical source
JPWO2018235200A1 (ja) * 2017-06-21 2019-06-27 三菱電機株式会社 光導波路、光回路および半導体レーザ
US10547408B2 (en) * 2018-05-03 2020-01-28 Juniper Networks, Inc. Methods and apparatus for improving the skew tolerance of a coherent optical transponder in an optical communication system
US10862610B1 (en) 2019-11-11 2020-12-08 X Development Llc Multi-channel integrated photonic wavelength demultiplexer
US11187854B2 (en) * 2019-11-15 2021-11-30 X Development Llc Two-channel integrated photonic wavelength demultiplexer

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NO20066082L (no) 2007-01-04
JP2008501987A (ja) 2008-01-24
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