WO2003062865A2 - Systeme et procede de transmission de signaux optiques - Google Patents

Systeme et procede de transmission de signaux optiques Download PDF

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Publication number
WO2003062865A2
WO2003062865A2 PCT/US2002/041285 US0241285W WO03062865A2 WO 2003062865 A2 WO2003062865 A2 WO 2003062865A2 US 0241285 W US0241285 W US 0241285W WO 03062865 A2 WO03062865 A2 WO 03062865A2
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WO
WIPO (PCT)
Prior art keywords
optical
pulse
signals
signal
filter
Prior art date
Application number
PCT/US2002/041285
Other languages
English (en)
Other versions
WO2003062865A3 (fr
Inventor
Farhad Hakimi
Hosain Hakimi
Original Assignee
Teraphase Technologies, 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
Priority claimed from US10/052,868 external-priority patent/US20030133650A1/en
Priority claimed from US10/050,749 external-priority patent/US20030133649A1/en
Application filed by Teraphase Technologies, Inc. filed Critical Teraphase Technologies, Inc.
Priority to AU2002364229A priority Critical patent/AU2002364229A1/en
Publication of WO2003062865A2 publication Critical patent/WO2003062865A2/fr
Publication of WO2003062865A3 publication Critical patent/WO2003062865A3/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
    • G02B6/24—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
    • G02B6/29395—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 configurable, e.g. tunable or reconfigurable
    • 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
    • G02B6/12004—Combinations of two or more optical elements
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29304—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by diffraction, e.g. grating
    • G02B6/29316—Light guides comprising a diffractive element, e.g. grating in or on the light guide such that diffracted light is confined in the light guide
    • G02B6/29317—Light guides of the optical fibre type
    • G02B6/29319—With a cascade of diffractive elements or of diffraction operations
    • G02B6/2932—With a cascade of diffractive elements or of diffraction operations comprising a directional router, e.g. directional coupler, circulator
    • 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
    • G02B6/29349—Michelson or Michelson/Gires-Tournois configuration, i.e. based on splitting and interferometrically combining relatively delayed signals at a single beamsplitter
    • 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
    • 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
    • G02B6/29358—Multiple beam interferometer external to a light guide, e.g. Fabry-Pérot, etalon, VIPA plate, OTDL plate, continuous interferometer, parallel plate resonator
    • 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
    • G02B6/29361—Interference filters, e.g. multilayer coatings, thin film filters, dichroic splitters or mirrors based on multilayers, WDM filters
    • 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
    • H04B10/50—Transmitters
    • H04B10/508—Pulse generation, e.g. generation of solitons

Definitions

  • This invention relates to improved systems and methods of transmitting optical signals.
  • Dense Wavelength Division Multiplexing allows a large number of information channels of optical signals to be transmitted onto a single strand of single- mode fiber.
  • current channel spacing is about 25 GHz (0.2 nm @ 1550 nm), meaning that the carrier frequency of one channel is separated from the carrier frequency of an adjacent channel by about 25 GHz.
  • EDFA Erbium Doped Fiber Amplifier
  • linear and non-linear distortions inhibit the ability to send information at higher rates or over longer distances. As data rates increase the known problems will become more acute. In material part, linear distortions include the following:
  • Non-linear problems include the following:
  • DCFs dispersion compensating fibers
  • the invention provides apparatus and methods for transmitting optical signals that are more tolerant to various forms of distortion inherent in transmitting optical signals over fiber.
  • a tunable LTR filter receives optical signals and provides filtered optical signals.
  • the tunable IJJR filter has a predefined pass band spectral width and a center frequency that can be adjusted in response to a control signal.
  • a decision circuit providing a control signal to the tunable IJR filter in response to the filtered optical signals.
  • the optical signals include an optical carrier and associated left and right side band spectral components.
  • Each side band spectral component is separated from the optical carrier by a spectral distance.
  • the optical carrier and the left and right side band spectral components each have at least two associated data side bands.
  • the predefined pass band spectral width of the LTR filter is wide enough to capture at least the optical carrier and one of the left and right side band spectral components and is narrow enough to exclude the other of the left and right side band spectral components and its associated data side bands.
  • the spectral width of the LTR filter is narrow enough to exclude one of the data side bands associated with the optical carrier and one of the data side bands associated with the one side band spectral component.
  • the optical carrier has an associated frequency that can wander and the decision circuit provides the control signal to adjust the center frequency of the LTR filter so that the spectral width of the filter may move to track a wandering frequency of the optical carrier.
  • an optical signal transmission apparatus includes a tunable FIR filter having an input link for receiving an optical signal.
  • the optical signal may be characterized by at least one optical pulse.
  • the filter also includes an output link for providing a filtered optical signal thereon.
  • the filtered optical signal may be characterized by the one optical pulse and a time-delayed replicated version thereof.
  • the one optical pulse and the replicated version thereof are further characterized by a phase-shift therebetween, and the amount of phase-shift is adjustable in response to a control signal.
  • the apparatus also includes a decision circuit, responsive to the filtered optical signal on the output link, and it provides a control signal to the tunable FIR filter.
  • the one optical pulse is characterized by a full width half max (FWHM) pulse width and the time delay between the one optical pulse and the time-delayed replication version is about one half the FWHM pulse width.
  • FWHM full width half max
  • the one optical pulse is characterized by a full width half max (FWHM) pulse width and the time delay between the one optical pulse and the time-delayed replication version is about one FWHM pulse width.
  • FWHM full width half max
  • the one optical pulse is characterized by a full width half max (FWHM) pulse width and the time delay between the one optical pulse and the time-delayed replication version is between about one half the FWHM pulse width and one FWHM pulse width.
  • the phase-shift between the one optical pulse and the replicated version thereof is about ⁇ ⁇ /2.
  • the phase-shift between the one optical pulse and the replicated version thereof is about ⁇ ⁇ .
  • the optical signals received by the tunable FIR filter has an associated optical carrier and an associated optical carrier frequency that can wander and the decision circuit provides the control signal to adjust the phase shift of the FIR filter so that the filter may track a wandering optical carrier frequency.
  • an optical signal transmission apparatus includes a tunable filter block that receives optical signals and provides filtered optical signals.
  • the tunable filter block includes an HR filter and a FIR filter, at least one of which is tunable in response to a filtered signal, such as the output signal of the apparatus.
  • the tunable filter block includes a tunable LTR filter and a tunable FIR filter.
  • the tunable DR filter receives optical signals and creates LTR filtered signals therefrom.
  • the received optical signals may be characterized in the frequency domain by an optical carrier having associated left and right side band spectral components. Each side band spectral component is separated from the optical carrier by a spectral distance.
  • the optical carrier and the left and right side band spectral components each have at least two associated data side bands.
  • the tunable HR filter may be characterized by a predefined pass band spectral width and a center frequency, in which the center frequency is adjustable in response to a control signal.
  • the tunable FIR filter receives time domain optical pulses and creates FIR filtered signals therefrom.
  • Each FIR filtered signals includes the received time domain optical pulse and a time-delayed replicated version thereof.
  • the time domain optical pulse and the replicated version thereof have a relative phase-shift therebetween, and the amount of phase-shift created by the FIR filter is adjustable in response to a control signal.
  • the LTR filter precedes the FLR filter.
  • an optical transmitter receives control signals as feedback from an impulse response filter.
  • the impulse response filter is an LTR filter, in others an FIR filter, and in others a combination of the two.
  • the impulse response filters may be passive, or tunable.
  • an optical signal transmission system includes an optical signal transmitter for transmitting optical signals.
  • the optical signals may be characterized in the frequency domain by an optical carrier having associated left and right side band spectral components. Each side band spectral component is separated from the optical carrier by a spectral distance, and the optical carrier and the left and right side band spectral components each have at least two associated data side bands.
  • the optical signal transmitter has a control input for receiving a control signal to adjust the frequency of the optical carrier.
  • An IIR filter receives optical signals from the optical signal transmitter and provides filtered optical signals.
  • the LTR filter has a predefined pass band spectral width wide enough to capture at least the optical carrier and one of the left and right side band spectral components and narrow enough to exclude the other of the left and right side band spectral components and its associated data side bands.
  • a decision circuit responsive to the filtered signals, is in communication with the control input of the optical signal transmitter.
  • the HJR. filter is tunable and also in communication with the decision circuit.
  • a control signal may adjust the center frequency of the HR filter.
  • an optical transmitter receives control signals as feedback from an impulse response filter.
  • the impulse response filter is an HR filter, in others an FIR filter, and in others a combination of the two.
  • the impulse response filters may be passive, or tunable.
  • an optical signal transmission system includes an optical signal transmitter for transmitting optical signals characterized in the time domain as a train of optical pulses and in the frequency domain by an optical carrier having associated left and right side band spectral components.
  • the optical carrier and the left and right side band spectral components each have at least two associated data side bands.
  • the optical signal transmitter has a control input for receiving a control signal to adjust the frequency of the optical carrier.
  • a filter block receives optical signals and provides filtered optical signals.
  • the filter block includes an LTR filter that receives optical signals and creates ILR filtered signals therefrom.
  • the HR filter characterized by a predefined pass band spectral width and a center frequency.
  • An FIR filter receives time domain optical pulses and creates FIR filtered signals therefrom.
  • Each FIR filtered signals includes the received time domain optical pulse and a time-delayed replicated version thereof.
  • the time domain optical pulse and the replicated version thereof have a relative phase- shift therebetween.
  • a decision circuit responsive to the filtered signals, is in communication with the control input of the optical signal transmitter.
  • the HR filter includes a control input in communication with the decision circuit and the center frequency of the HR filter adjusts in response to signals on the control input.
  • the FIR filter includes a control input in communication with the decision circuit and the amount of phase shift of the FIR filter adjusts in response to signals on the control input.
  • Figs. 1 A-B are system diagrams of exemplary transmitter apparatus that provide DR filtration according to certain embodiments of the invention
  • Figs. 2A-C are diagrams of spectral components for an optical carrier, modulated pulse sidebands, and data side bands in the frequency domain;
  • Figs. 3-5 are system diagram of certain embodiments of transmission apparatus that provide HR filtration according to certain embodiments of the invention.
  • Figs. 6A-C are system diagrams of an exemplary transmitter apparatus that provides FIR filtration according to certain embodiments of the invention.
  • Fig. 7 is a diagram illustrating pulse reshaping according to certain embodiments of the invention
  • Figs. 8-9 are system diagrams of certain embodiments of transmission apparatus that provide FIR filtration according to certain embodiments of the invention
  • Figs. 10-17 are system diagrams of certain embodiments of transmission apparatus that provide HJR. and FIR filtration according to certain embodiments of the invention.
  • Figs. 18-20 are system diagrams of an exemplary transmitter apparatus according to certain embodiments of the invention in which the transmitter operates as a slave to filtered optical signals;
  • Figs. 21-23 are system diagrams of an exemplary transmitter apparatus according to certain embodiments of the invention in which the transmitter operates as a slave to filtered optical signals and in which filtration components are also tunable;
  • Figs. 24-25 are system diagrams of an exemplary transmitter apparatus according to certain embodiments of the invention in which one but not all of the filtration components are tunable.
  • Fig. 26 is a system diagram of an exemplary transmitter apparatus according to certain embodiments of the invention in which filtration components are passive.
  • the present invention provides improved systems and methods of transmitting optical signals.
  • preferred embodiments address linear and nonlinear distortions and improve spectral efficiency by reshaping optical pulses (whether in RZ or NRZ format) in the frequency and/or time domains so that the pulses are more tolerant to dispersion and nonlinear distortions.
  • the reshaped optical pulses helps suppress the dispersion slope (TOD) and channel cross-talk. Additionally, the shape of the pulse makes it more resilient to effects of PMD.
  • Certain embodiments filter optical signals with a tunable LTR filter.
  • Other embodiments filter optical signals with an FIR filter.
  • Still other embodiments filter optical signals with a combination of LTR and FIR filtration.
  • Some embodiments have the filters act as slaves to the filtered optical signals, for example, to tune the filtration to a potentially wandering center frequency.
  • Some embodiments have the transmitter act as a slave to the filtered optical signal.
  • Figure 1 A shows a relevant portion of an optical signal transmission apparatus according to certain embodiments of the invention.
  • the transmission apparatus 114 follows a conventional transmitter Tx 102, and the apparatus 114 filters the signal from the transmitter 102 to reshape the spectrum of the pulses emitted therefrom and to provide the filtered signals on optical link or fiber 120.
  • the apparatus operates as a slave to the filtered, transmitted optical signal, as will be explained below.
  • Figure IB illustrates an arrangement in which the apparatus 114' operates analogously to that of figure 1 A but which also considers the unfiltered signal from Tx 102 on link 106.
  • the apparatus, or filter block, 114 includes a tunable LTR filter 104 in optical communication with transmitter (Tx) 102 via optical link (or fiber) 106.
  • Tx 102 is one of the transmitters in a WDM or DWDM system.
  • Tx 102 includes an optical source (not shown) such as a laser or LED and modulation circuitry (not shown) to form optical signals and data.
  • the filter 104 provides spectrally reshaped optical signals on optical link 120. Though only one LTR filter 104 is shown, preferred embodiments can include multiple LTR filters per transmitter (e.g., in cascaded arrangement), and can include multiple transmitters for each fiber (e.g., one transmitter for each frequency).
  • optical signals from filter 104 are fed back on optical link 110 to an optical to electrical converter (O/E) 109.
  • O/E optical to electrical converter
  • the optical converter produces an electrical version of the optical signal received on link 110 and produces the electrical version of the signal on electrical link 118.
  • a decision circuit 108 receives the electrical signal, processes it accordingly, and uses it to produce a control signal that is transmitted to the filter 104 via electrical link 112. As will be explained below, the control signal is used to tune the IIR filter 104.
  • LTR filter 104 and decision circuit 108 (along with corresponding links and other components) form an active tunable filter block 114.
  • the active filter block 114 filters the optical signals received from Tx to allow signals only within a certain pass band to be transmitted on link 120. More specifically, the filter block 114 reshapes the optical spectrum of the data pulses received by link 106 by removing side-band spectral components. This pass band and reshaping are discussed in more detail below in conjunction with the description of figures 2A-C.
  • the filter block 114 tunes to the emitted signal on link 120 so that the filter may constantly track the center frequency of the optical source in Tx 102. Thus, if the center frequency wanders, the filter tunes accordingly.
  • Figure 2A illustrates the frequency spectrum of signals emitted by a conventional optical transmitter, such as Tx 102.
  • This spectrum is formed when an optical carrier (OC) 200 is pulse modulated to create modulated spectral side bands 210 and 220 and then later modulated with data to create data side bands 211,212, 201, 202, 221, and 222.
  • the separation between optical carrier 200 and modulated spectral side bands depends on the communication system; for example, the separation between pulse 210 and OC 200 may be about 10 GHz for a lOGbits/s system.
  • the number of channels on a fiber is a function of the individual channel data rate and the overall system design.
  • the filter block 114 removes one of the side bands; that is, either left side band (LSB), including side bands 210-212, or right side band (RSB), including side bands 220-222.
  • LSB left side band
  • RSB right side band
  • figure 2B illustrates how one embodiment would remove the RSB.
  • the filter block 114 removes more side band spectral components.
  • Figure 2C illustrates how one preferred embodiment removes some of the side band spectral components of the optical carrier 200 (namely pulse 202) and of LSB (namely pulse 211).
  • the tunable filter block 114 is configured to have an associated spectrum width, represented by reference numeral 230. If the OC wanders, the filter block 114 will track and tune to the wandering frequency to filter out certain of the side bands, as illustrated. The remaining signals, e.g., as shown in figure 2C, are all that are needed at a conventional receiver for data recovery. The receiver does not need the side bands that were filtered out at the transmitter, and in fact, the receiver does not see a significant change in the time domain signal. Without loss of generality, the filter could instead remove spectral components from the LSB and data spectral components 201.
  • the filtering out of one of the side bands reduces the effective bandwidth or spectral width of a channel.
  • ⁇ V f spectral width in frequency domain, typically measured in Hz
  • I intensity (optical power/unit area) in Watts/cm 2
  • k the wave number in cm "1
  • L is fiber's length in cm
  • ⁇ V f and ⁇ vi are the final and initial spectral widths respectively in Hz.
  • FIG 3 shows one embodiment of the filter block 114 that utilizes a bulk optics approach (i.e., a free space beam propagation technique).
  • the filter block 114 uses a high finesse Fabry perot etalon 302 (e.g., finesse greater than 10) disposed between a first collimator 304 and a second collimator 306.
  • Optical signals are received by the first collimator 304 on link 106 and emitted toward and through etalon 302.
  • the Fabry-Perot etalon 302 is responsible for "chopping off the side band spectral components and establishing the pass band, allowing only certain frequencies of light to pass, centered about a "center frequency" of the positioned etalon.
  • the filter width 230 (see figure 2C) is substantially fixed, as a result of the filter design, e.g., the thickness of the etalon, the optical properties of the material used, etc.
  • the allowed band pass width is dictated by the data rates. For example, if data is sent at 10 Gb/s, then the filter width (pass band) is about 12.5 GHz; if data is sent at 40 Gb/s, the pass band is about 50 GHz (data rate* 1.25).
  • the center frequency of the filter can shift and be adjusted, by rotating the etalon. By rotating the etalon, the effective thickness of the etalon through which light passes changes causing the "center frequency" of the filter to shift and allowing different frequencies to pass through the etalon.
  • a multi-mirror etalon is used. Such an etalon may be used to creates a more rectangular spectral window.
  • Collimator 306 receives the passed optical signals from the etalon and provides them on link 308 to optical tap 310.
  • Optical tap 310 e.g., a beam splitter
  • the output signal once tuned, is represented by figure 2B or figure 2C.
  • the feedback signal on link 314 is received by an optical-to-electrical converter 316, such as a photo diode detector, which then provides an electrical version of the signal to a decision circuit 318.
  • the O/E converter may generates an electric current which is converted to a voltage through use of a transimpedance amplifier.
  • the decision circuit 318 is responsible for tuning the filter by causing the etalon 302 to rotate. Rotation may be achieved in a variety of manners, such as by a mechanical rotation stage or a stepper/gear motor for instance, controlled by a microprocessor.
  • the control signal 320 is used to cause the etalon 302 to rotate accordingly.
  • the decision circuit 318 may establish tuning in many ways.
  • the circuit 318 may detect the energy or power of the feedback signal.
  • the amount of power will be maximum when the filter is tuned as shown in figure 2C to capture the OC and as much of the side band spectral components as will fit within the pass band of the filter (naturally, the filter could also tune to capture the right side band spectral components, instead of the LSB spectral components.
  • FIG. 4 illustrates another embodiment of the filter block 114.
  • This embodiment utilizes an electronically tunable liquid crystal Fabry-Perot 401.
  • Optical signals of arbitrary polarization are received on link 106 by first collimator 402, which then transmit the signals to first polarization beam splitter (PBS) 408 which divides the light into two paths 404, 406.
  • PBS first polarization beam splitter
  • Light on path 406 passes through first half wave plate 410 so that light on path 404 and 407 have states of polarization that are aligned to the optical axis of the liquid crystal cell 401. Since the liquid crystal Fabry-Perot filter 401 is a polarization sensitive element, aligning the light allows it to be tuned by the filter.
  • the filter light is emitted as paths 414, 416 which are recombined into the output fiber using a second half wave plate 417, second PBS 418 and second collimator 420.
  • the optical tap 422 receives the optical signal from collimator 420 and provides the output signal on link 120 and provides a feedback signal on optical link 424.
  • the O E 426, the decision circuit 428, and other components operate analogously to those described above.
  • Electrical stimulus on control line 430 causes the filter 401 to change its filtration properties and thus allows the filter to track the wandering center frequency of the signals on link 106. For example, the index of refraction of the liquid crystal 401 changes in response to electrical stimulus.
  • FIG. 5 shows another embodiment of the filter block 114.
  • the filter block 114 is made using a fiber-based approach.
  • Light is received on link 106 encounters circulator 504, which directs the received light to Fiber Brag Grating (FBG) 508, as suggested by arrow A.
  • the grating 508 allows certain frequencies to pass and others to reflect thus acting as a pass band filter.
  • the frequencies that pass or reflect are a function of the materials and spacing of the grating 508.
  • Reflected light passes up link 506 and through the circulator 504, as suggested by arrow B.
  • When light is received in this direction by the circulator 504 it is directed on link 510 to tap 512.
  • Tap 512 provides the tuned signal to output link 120 and to link 516, as a feedback signal.
  • O/E 518 receives the feedback signal and decision circuit 520 operates analogously to those described above.
  • the control signal 522 from the decision circuit 520 may be used to stretch or heat the grating 508 to change the spacings and thereby cause the center frequency of the grating 508 to shift accordingly.
  • the arrangement of figure 5 could be accomplished on an integrated optics chip as well. In this case, a coupler can replace the optical circulator 504.
  • figure IB shows an arrangement that operates analogously to that of figure 1 A, but which further considers the unfiltered output of Tx 102. Embodiments such as those shown in figures 3-5 may thus be modified to operate like that of figure IB.
  • the decision circuit 108 may consider the unfiltered output from the Tx 102 for several reasons. For example, if the power from Tx 102 fluctuates or changes, the power of the optical circuit (i.e., the feedback signal) may change even though the center frequency of the OC has not changed. If the decision circuit did not consider this change in power from Tx, it might try to tune the filter even though the center frequency has not changed or wandered.
  • the decision circuit may determine that a change in power at the feedback signal does not warrant tracking of the OC (i.e., that the change in power is due to changes in Tx power not due to changes in OC wandering).
  • Figure 6A shows a relevant portion of an optical signal transmission apparatus according to other embodiments of the invention.
  • the transmission apparatus 613 follows a conventional transmitter Tx 102, and the apparatus 614 filters the signal from the transmitter 102 to reshape the pulses emitted therefrom on optical link 120.
  • the apparatus operates as a slave to the filtered, transmitted optical signal, as will be explained below.
  • Figure 6B illustrates an arrangement that operates analogously to that of figure 6a but which further considers a feedback signal 624 (e.g., an error signal) from a receiver or the like.
  • Figure 6C illustrates an arrangement in which the apparatus 614" operates analogously to that of figure 6A but which also considers the unfiltered signal from Tx 102 on link 106.
  • Tx 102 is one of the transmitters in a WDM or DWDM system like those described above.
  • Tunable FIR filter 604 is in optical communication with Tx 102 via link 606 and is in optical communication with decision circuit 608 via link 610 and optical to electrical converter 612.
  • the decision circuit 608 is in electrical communication with FIR filter 604 via electrical link 613.
  • the control signal carried on link 613 is used to tune the FIR filter 604.
  • FIR filter 604 may utilize a plurality of such filters (e.g., cascaded) per transmitter.
  • each fiber 120 may be associated with multiple transmitters (e.g., one transmitter for each frequency).
  • the tunable FIR filter 604 of the filter block 614 reshapes the individual pulses received on link 606 by replicating the received pulse and phase shifting it accordingly.
  • a received pulse 702 is replicated into two pulses 706 and 708.
  • the one pulse 708 is phase shifted relative to the other.
  • the phase shifting is about ⁇ /2.
  • Phase shifting by ⁇ /2 is believed to be desirable to improve tolerance to non-linearities.
  • Phase shifting by ⁇ is believed to be desirable to improve tolerance to PMD.
  • Other amounts of phase shifting may also be beneficial.
  • the delay 712 between these crest pulses could be as long as one half to one full width half max pulse width (FWHM).
  • the time delay reduces the spectral width of the pulse while the phase attenuates the data side bands. This improves the pulse's dispersive and nonlinear tolerances.
  • the resulting pulse is effectively a wider or longer pulse in the time domain. By being longer in the time domain, the same pulse is narrower in the frequency domain.
  • the pulses are reshaped so that there is overlap between the pulse pairs. Due to interference, as the phase changes the output power emerging from the FIR filter may go through minima and maxima.
  • the decision circuit may tune the phase. For example, in certain embodiments, the decision circuit attempts to maximize the power in the monitored signal by adjusting the phase.
  • Figure 8 shows one embodiment of the filter block 614 following a bulk optics approach.
  • the filter block 614 is implemented with a Michelson interferometer and includes a first collimator 802, beam splitter 804, fixed mirror 806, movable mirror 808, second collimator 810, O/E 818 and decision circuit 820.
  • the first collimator receives the optical signal on link 802 and transmits a collimated version of the signal to beam splitter (BS) 804.
  • the beam splitter cause the signal to split, with one version proceeding toward fixed mirror 806 and another toward movable mirror 808.
  • Each mirror causes the signal to reflect back toward the beam splitter 804.
  • Each of the reflected versions is caused by the beam splitter to proceed toward second collimator 810.
  • the beam splitting and subsequent recombination creates the replicated version of the input pulse signal.
  • the different displacements of each mirror, relative to the beam splitter causes the time delay 712 of the replicated versions of the signal.
  • the amount of movement of the mirror causes the phase shift of one signal relative to the other.
  • the second collimator provides the received pulses to tap 814 via optical link 812.
  • the tap 814 provides the reshaped pulses on output link or fiber 120, and it also provides another version of the output signal as a feedback signal on optical link 816.
  • the feedback signal is received by O/E converter 818 which provides the electrical version of the signal to decision circuit 820.
  • the decision circuit produces control signal 822 which may cause moving mirror 818 to move in the direction D according to piezoelectric movement or the like.
  • the phase is adjustable by the moving mirror. As the center frequency of the transmitter 102 shifts, the power in the output signal changes. The decision circuit detects these changes and commands the mirror 808 to adjust the phase to compensate the transmitter frequency shift.
  • a filter block 614 may be realized using electronically tunable Liquid crystal FIR filters which are Fabry-Perots of low finesse akin to the architecture shown in figure 4.
  • the finesse of the Fabry-Perot may be about 4 or lower.
  • the electric field applied to the crystal can change the index of refraction of the crystal and cause a corresponding phase shift.
  • Figure 9 shows another embodiment of filter block 614.
  • This embodiment uses a Mach-Zehnder type interferometer.
  • the optical signal from Tx 102 is received by tap or coupler 902, which provides the signals on optical links 903 and 904.
  • Link 904 is responsive to phase shifter 906 and is longer than link 903.
  • Links 903 and 904 feed coupler 905 which provides output signals on link 120 and a feedback signal on optical link 907.
  • the feedback signal on link 907 is provided to O/E 908, which provides an electrical version thereof to decision circuit 912 via electrical link 910.
  • the decision circuit may consider the power in the feedback signal of link 910 and cause the phase shifter to tune accordingly via control signal 914.
  • phase shifter 906 Any shift in transmitter frequency is detected through the second leg 907, and the decision circuit changes the phase appropriately.
  • the phase change or shifting may be made via a heating element (in glass wave- guides) or an electrode (in Lithium Niobate).
  • figure 6B shows an arrangement which operates analogously to that of figure 6A but which further considers a feedback signal from a receiver.
  • Embodiments such as those shown in figures 8-9 may be modified to operate like that shown in figure 6B.
  • the decision circuit 608 may consider the feedback signal 624 for several reasons. For example, the decision circuit may tune the phase until the receiver reports the least error.
  • the feedback signal 624 may be provided by a low speed or unused channel or by any of a variety of forms of communication, including via software commands to a driver program.
  • figure 6C shows an arrangement that operates analogously to that of figure 6A, but which further considers the unfiltered output of Tx 102.
  • Embodiments such as those shown in figures 8-9 may thus be modified to operate like that of figure 6A.
  • the decision circuit 608 may consider the unfiltered output from the Tx 102 for several reasons. For example, if the power from Tx 102 fluctuates or changes, the power of the optical circuit (i.e., the feedback signal) may change even though the center frequency of the OC has not changed. If the decision circuit did not consider this change in power from Tx, it might try to tune the filter even though the center frequency has not changed or wandered.
  • the decision circuit may determine that a change in power at the feedback signal does not warrant tracking of the OC (i.e., that the change in power is due to changes in Tx power not due to changes in OC wandering).
  • Figure 10 shows a relevant portion of an optical signal transmission apparatus according to other embodiments of the invention.
  • the transmission apparatus 1014 follows a conventional transmitter Tx 102, and the apparatus 1014 filters the signals from the transmitter 102 to reshape the pulses emitted therefrom on optical link 120.
  • the signals are reshaped both with LTR filtration to remove side band spectral components as discussed above, and to reshape the remaining pulses with FIR filtration as discussed above.
  • HR filter 1004 then removes side band spectral components as discussed above in connection with figures 2B and 2C. HR filter 1004 may be tuned in response to control signal 1020 from decision circuit 1016 to track a wandering center frequency of transmitter 102, as discussed above.
  • FIR filter 1008 receives the remaining pulses and causes them to be replicated, delayed and phase shifted as discussed above. It may be tuned in response to control signal 1018 from decision circuit 1016, as discussed above. Though this arrangement and the ones that follow illustrate the LTR filter preceding the FIR filter, the order may be reversed. Moreover, like the embodiments above, each of the filters are shown as one entity but may be realized in a cascaded arrangement as well.
  • Figure 11 shows one embodiment of the filter block 1014.
  • the filter block is arranged first to provide LTR filtration using a tunable etalon like that described in connection with figure 3 and then to provide LTR filtration using Michelson interferometer like that described in connection with figure 8.
  • Optical signals are received on optical link 106 by collimator 1102 which provides a collimated version of the signal to a tunable etalon 1104.
  • the remaining pulses of the optical signal 1106 are then directed toward beam splitter 1108.
  • the signal is split, with a portion being directed to movable mirror 1112 and another portion being directed to fixed mirror 1110.
  • Each mirror reflects the optical signal back to beam splitter 1108 which causes the reflected versions 1114 to be directed to collimator 1116.
  • Optical tap 1120 receives the tuned signals on optical link 1118 and provides the tuned signals on output link 120 as an output signal and on optical link 1122 as a feedback signal.
  • O/E 1124 receives the optical feedback signal, converts the signal accordingly, and provides an electrical version thereof to decision circuit 1126.
  • the decision circuit 1126 then may cause the movable mirror 1128 to move to adjust phase via control signal 1128 and may cause the etalon 1104 to rotate to change the center frequency of the pass band via control signal 1130.
  • Figure 12 illustrates another embodiment of the filter block 1014.
  • a tunable liquid crystal Fabry-Perot is used like that described in connection with figure 4 to provide IIR filtration, and a low finesse Fabry Perot is placed inside the polarization independent section to provide FIR filtration.
  • Optical signals of arbitrary polarization are received on link 106 by first collimator 1202, which then transmit the signals to first polarization beam splitter (PBS) 1204 which divides the light into two paths 1206, 1208.
  • PBS polarization beam splitter
  • Light on path 1206 passes through first half wave plate 1210 so that light on path 1212 and 1208 have states of polarization that are aligned to the optical axis of the liquid crystal cell 1214.
  • the filter light is emitted from Fabry-Perot 1214 and received by low finesse Fabry-Perot 1216.
  • the tuned light is then emitted with the light on path 1212 encountering half wave plate 1218.
  • the light then is received by second PBS 1220, second collimator 1222, and optical tap 1226.
  • the optical tap 1226 provides the output signal on link 120 and provides a feedback signal on optical link 1228.
  • the O/E 1230, the decision circuit 1232, and other components operate analogously to those described above.
  • Electrical stimulus on control line 1234 causes the filter 1214 to change its filtration properties and thus allows the filter to track the wandering center frequency of the signals on link 106. For example, the index of refraction of the liquid crystal changes in response to electrical stimulus.
  • control line 1236 may be used to change the properties of Fabry-Perot 1216 to adjust phase shifts.
  • the thickness of the second Fabry Perot is chosen to cause the replicated, time-delayed pulses to be produced.
  • Figure 13 illustrates another embodiment of the filter block 1014.
  • LTR filtration is implemented using a grating like that described in connection with figure 5
  • FLR filtration is implemented using a Mach Zehnder type interferometer like that described in connection with figure 9.
  • Optical signals are received on link 106 and encounters circulator 1302, which directs the received light to grating 1306 via link 1304, as suggested by arrow A.
  • the grating 1306 allows certain frequencies to pass and others to reflect thus acting as a pass band filter.
  • Reflected light passes up link 1304 and through the circulator 1302, as suggested by arrow B.
  • the tap or coupler 1310 provides the signals on optical links 1312 and 1314.
  • Link 1314 is responsive to phase shifter 1316 and is longer than link 1312.
  • Links 1312 and 1314 feed coupler 1318 which provides output signals on link 120 and a feedback signal on optical link 1320.
  • the feedback signal on link 1320 is provided to O/E 1322, which provides an electrical version thereof to decision circuit 1324.
  • the decision circuit may consider the power in the feedback signal and cause the phase shifter to tune accordingly via control signal 1326. (The time delay between replicated signals is largely fixed as a result of the longer link 1314.)
  • the grating may be tuned via control signal 1328 to cause the center frequency of the pass band to track the OC of the input signal.
  • Figure 14 illustrates another embodiment of the filter block 1014, in this case using integrated planar optics.
  • Optical signals are received on link 106 and provided to coupler or beam splitter 1402.
  • the coupler splits the signal and provides an optical signal on link 1406 and 1408.
  • the signals on link 1406 are received by tunable grating 1410 which operates in reflection mode. Certain frequencies of the signal pass through grating and the frequencies of interest are reflected back on link 1406.
  • the signals on link 1408 are first received by tunable phase shifterl412 and then provided on link 1413 where they are received by a second tunable grating 1414 which also operates in reflection mode.
  • the reflected signal is again passed through the tunable phase shifter 1412.
  • the reflected signals on links 1408 and 1406 are merged by coupler 1402 and provided on link 1420. Since one of the gratings 1414 has a corresponding distance of removal 1416 relative to the other grating 1410, the coupler 1402 provides a merging of two signals, one time delayed relative to the other (the time delay being a function of the distance of separation 1416).
  • the signal on link 1420 is provided to a tap 1422, which feeds output link 120 and feedback link 1424.
  • O/E 1426 converts the optical feedback signal to an electrical form and provides it to decision circuit 1428.
  • the decision circuit may then tune the phase shifter 1412 via control signal 1430, using techniques like those described above, and may tune the gratings 1410, 1414 via control signal 1432, using techniques like those described above.
  • Figure 15 shows another embodiment of filter block 1014.
  • Optical signals are received from link 106 by grating 1502 which operates in transmissive mode to provide LTR filtration. Thus, the frequencies of interest pass through the grating on link 1504.
  • the signal pulses on link 1504 (which has had side band spectral components removed as described above in connection with figures 2B-C) are then received by a second grating 1506 which also operates in transmissive mode but to provide FIR filtration.
  • the grating 1506 is formed or machined to create time-delayed and phase shifted versions of the pulses received on link 1504.
  • the output signals of grating 1506 are received by tap 1508 which provides output signals on link 120 and feedback signals on link 1510.
  • the feedback signal is received by O/E 1512 which provides an electrical version thereof to decision circuit 1514.
  • the decision circuit 1514 may tune the phase shifting caused by grating 1506 via control signal 1516 and it may cause the center frequency of the pass band of grating 1502 to shift via control signal 1518.
  • the second grating includes two reflective gratings with a space between them (e.g., a fiber section). Each reflective grating acts like a mirror surface with a spacing in between the two, and thus forms replicated, time-delayed pulses.
  • Figure 16 shows another embodiment of filter block 1014.
  • Optical signals are received on link 106 by coupler 1602 which splits the beam and feeds links 1604 and 1606.
  • Signals on link 1604 are received by collimator 1608, and signals on link 1606 are received by collimator 1610.
  • the light from each collimator 1608, 1610 then passes through rotatable etalon 1612, which allows only certain frequencies to pass, as described above.
  • the filtered signals are then received by corresponding collimators 1614 and 1616, which are separated relative to one another by a distance D.
  • the signals then pass through respective fibers or links 1618 and 1620.
  • Phase shifter 1622 may heat or stretch the link to introduce the phase shift.
  • the reflected signals are then provided through the phase shifter 1622 and the rotatable etalon 1612 and eventually provided by links 1604 and 1606 to coupler 1602.
  • the merged signals are provided on link 1628 to tap 1630, which provides an output signal on link 120 and a feedback signal on link 1632.
  • the feedback signal is received by O/E 1634 which provides an electrical version thereof to decision circuit 1636.
  • Decision circuit 1636 may cause the etalon 1612 to rotate to track the wandering OC of signals on link 106 via control link 1638 and may cause the phase shifter 1622 to tune phase via control link 1640.
  • the mirrors may be separated by relative distances to introduce the necessary time delay.
  • Figure 17 shows another embodiment of filter block 1014, in this case arranged akin to figures IB and 6C.
  • Optical signals are received on link 106 and provided to optical tap 1702.
  • the tap 1702 provides optical signals on links 1703 and 1705.
  • the signal on link 1705 is provided to a collimator 1704 which then provides the collimated light to first etalon 1706.
  • the first etalon provides LTR filtration by cutting off the side band spectral components as described above.
  • the filtered signal is then provided to second etalon 1708 which provides FIR filtration.
  • the light from second etalon 1708 is received by collimator 1710 which provides optical signals on link 1712 to tap 1714.
  • Tap 1714 feeds output link 120 with an output signal and also provides a feedback signal on link 1716.
  • the signals on links 1705 and 1716 are each received by respective O/Es 1720 and 1718, each of which provides electrical versions of its input signal to decision circuit 1722.
  • the decision circuit may then cause the first etalon to rotate via control link 1724 to track the wandering center frequency of Tx 102, and it may cause the second etalon to rotate to tune the phase of the signal vial control link 1726.
  • the above embodiments illustrated a transmission apparatus that operated as a slave to the output signal.
  • the filtration apparatus would tune to the changing OC.
  • These embodiments may thus operate with conventional transmitters.
  • these embodiments may be changed if the transmitters allowed feedback signals.
  • the information used or derived by the decision circuit could be used as a feedback signal to a tunable transmitter Tx.
  • the LTR still filters out side band spectral components and the FIR reshapes pulses.
  • the feedback signal may cause the tunable transmitter to change the frequency of OC.
  • the phase is a function of the optical frequency, the feedback signal may also be used for the Tx to adjust phase.
  • an LTR may be made of a high finesse multi- mirror etalon for example.
  • an LTR may be made of various forms of gratings whether in a bulk optics or integrated approach, e.g., FBG.
  • FIRs may be constructed from various forms of interferometers and etalons discussed above, except that they no longer need movable mirrors or rotatable etalons.
  • the phase shifting components may be removed.
  • the transmission apparatus may be modified in many ways.
  • a tunable LTR may still supply a feedback signal to a transmitter to adjust frequency if such transmitter permitted feedback.
  • both could cooperate.
  • the same arrangement may be used for tunable FIR arrangements and for arrangements having some combination of LTR and FIR, in which at least one is tunable. These arrangements are shown in figures 21-23.
  • the arrangements having an LTR and FLR may employ different arrangements.
  • the embodiments described above had either both the UR and FIR be tunable or passive.
  • a transmission apparatus may have one be tunable and the other passive.
  • FIGS 24-25 These figures suggest certain points in the apparatus for feedback to the decision circuit but others might be employed, though they are not illustrated.
  • the decision circuit may operate off of the feedback from the FIR.
  • these arrangements may also supply feedback to the transmitter in arrangements having transmitters that receive feedback, and their decision circuits may consider the various other signals discussed above, e.g., unfiltered signal from Tx, etc.
  • the arrangement of UR and FIR may be changed.
  • Figure 26 shows another embodiment, particularly useful in certain arrangements.
  • the filter block includes passive LTR and FIR filtration mechanisms as discussed above. Though illustrated with the LTR as a first stage, the order may be changed. This embodiment may be particularly useful for certain forms of transmitters Tx 102' ' having internal feedback to stabilize its center frequency from wandering. For example, transmitter Tx 102" may have a wavelength locker.
  • the illustrated designs were shown with single filters for the most part to avoid clutter.
  • the filters may be implemented as a cascaded arrangement of filters as well.
  • gaining elements may be incorporated into the design to compensate for any insertion loss from various components of the designs.
  • the insertion loss of a device may be compensated by Erbium doped optical fiber amplifiers or the like. These may be placed before, after or within a filter block.
  • the grating could be formed in a fiber, or formed in an integrated optical chip.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Engineering & Computer Science (AREA)
  • Electromagnetism (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Optical Communication System (AREA)

Abstract

La présente invention concerne un appareil et des procédés de transmission de signaux optiques qui sont plus tolérants vis-à-vis de diverses formes de distorsion inhérentes dans la transmission de signaux optiques sur fibre. Des filtres à réponse impulsionnelle finie et à réponse impulsionnelle infinie sont utilisés de manière indépendante ou en combinaison. Dans certains modes de réalisation, les filtres sont accordables et utilisent la rétroaction.
PCT/US2002/041285 2002-01-16 2002-12-19 Systeme et procede de transmission de signaux optiques WO2003062865A2 (fr)

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US5064102A 2002-01-16 2002-01-16
US5075102A 2002-01-16 2002-01-16
US5347802A 2002-01-16 2002-01-16
US5063502A 2002-01-16 2002-01-16
US10/050,749 2002-01-16
US10/052,868 US20030133650A1 (en) 2002-01-16 2002-01-16 System and method of transmitting optical signals using IIR and FIR filtration
US10/050,751 2002-01-16
US10/053,478 2002-01-16
US10/050,635 2002-01-16
US10/052,868 2002-01-16
US10/050,749 US20030133649A1 (en) 2002-01-16 2002-01-16 System and method of transmitting optical signals using IIR filtration
US10/050,641 2002-01-16

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