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WO2009030822A1 - Régénération de signaux de données optiques indépendante d'un protocole - Google Patents

Régénération de signaux de données optiques indépendante d'un protocole Download PDF

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Publication number
WO2009030822A1
WO2009030822A1 PCT/FI2008/050497 FI2008050497W WO2009030822A1 WO 2009030822 A1 WO2009030822 A1 WO 2009030822A1 FI 2008050497 W FI2008050497 W FI 2008050497W WO 2009030822 A1 WO2009030822 A1 WO 2009030822A1
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Prior art keywords
signal
optical
data signal
optical data
clock signal
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English (en)
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Tuomo Lerber
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Luxdyne Oy
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Luxdyne Oy
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L7/00—Arrangements for synchronising receiver with transmitter
    • H04L7/0075—Arrangements for synchronising receiver with transmitter with photonic or optical means
    • 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
    • H04B10/29—Repeaters
    • H04B10/291—Repeaters in which processing or amplification is carried out without conversion of the main signal from optical form
    • H04B10/299—Signal waveform processing, e.g. reshaping or retiming

Definitions

  • the present invention relates to signal processing in optical communication systems.
  • An optical data signal may be degraded when sent over long distances.
  • An optical communications system may comprise one or more regenerating devices, e.g. repeaters, to correct the distortion of the data signal.
  • the pulses of the signal may be re-amplified, re-shaped, and/or re-timed in order to restore the quality of the signal. If all three steps are performed, the process is called "3R regeneration".
  • All-optical processing of optical data signals is expected to be advantageous especially when the modulation frequency exceeds 10 GHz.
  • Optical clock recovery has been described e.g. in an article "Optical Tank Circuits Used for All-Optical Timing Recovery", by M.Jinno and T.Matsumoto, IEEE Journal of Quantum Electronics Vol. 28, No. 4 April 1992, pp. 895-900.
  • an optical signal transmitted through the fiber may comprise a plurality of optical data signals sent at different wavelength channels (wavelength multiplexing).
  • the signals may be modulated according to different formats, and/or they may have different data rates.
  • An object of the present invention is to provide a method for regenerating an optical data signal.
  • An object of the present invention is to provide a device for carrying out said method.
  • An object of the present invention is to provide an optical communications system comprising said device.
  • a further object of the present invention is to provide a method for regenerating several optical data signals sent at different wavelength channels.
  • An object of the present invention is to provide a method for recovering the clock signal of an optical data signal.
  • An object of the present invention is to provide a device for carrying out said method.
  • An object of the present invention is to provide an optical communications system comprising said device.
  • a further object of the present invention is to provide a method for recovering clock signal of several optical data signals sent at different wavelength channels.
  • An object of the present invention is to provide a method for re-shaping and re-timing an optical data signal.
  • An object of the present invention is to provide a device for carrying out said method.
  • An object of the present invention is to provide an optical communications system comprising said device.
  • a device for regenerating a data signal according to claim 21 According to a second aspect of the invention, there is provided a device for regenerating a data signal according to claim 21.
  • an optical communications system according to claim 39.
  • the recovered clock signal may be provided at a different wavelength than the data signal.
  • the clock signal may be provided by modulating continuous light of a light source by the rapidly varying intensity of the data signal in a non-linear medium. Providing recovered clock signals at a different wavelength than the data signal may enable cost effective regeneration of several data signals.
  • a signal regenerating device may comprise two or more signal regenerating units for regenerating two or more optical data signals sent at two or more optical channels. Two or more clock signals corresponding to said different optical data signals may be recovered by using the same light source for providing continuous light.
  • the recovered clock signals corresponding to said different optical data signals may be at the same spectral position, which is different from the spectral positions of the optical data signals.
  • the modulated continuous light is separated from the modulating data signal by using an interferometric arrangement. Consequently, no spectral filters are needed for separating the modulated continuous light from the modulating data signal.
  • the signal regenerating device may comprise several signal regenerating units which are used in parallel to regenerate optical data signals sent at different wavelength channels.
  • the signal regenerating device may comprise only one common stabilized narrowband light source to provide substantially continuous light for the clock recovery units of the signal regenerating units. This is expected to reduce manufacturing costs and/or to increase the stability of the overall system.
  • the optical resonators of the clock recovery unit may have a long time constant.
  • the light source may be selected to have a narrow bandwidth.
  • the light source is common to the regenerating units operating at different data channels, and therefore higher costs of a stable narrowband light source may be tolerated.
  • the data signal is re-shaped and re-timed using an interferometric arrangement. This allows clock signal recovery by using passive optical resonators, without a need for further equalization of the intensity of the recovered clock pulses.
  • the signal regenerating device and/or a modulation format converter of a signal regenerating device is capable of processing optical data signals modulated according to RZ-type formats, including at least the RZ format (Return to Zero), CS-RZ format (Carrier Suppressed Return to Zero), DPSK-RZ format (Differential Phase Shift Keyed Return to Zero), and/or DQPSK-RZ format (Differential Quadrature Phase Shift- Keyed Retum-to-Zero).
  • the signal regenerating device and/or the modulation format converter may be adapted process different modulation formats. However, a change of the modulation format may require re-tuning of optical resonators.
  • the signal regenerating device may be arranged to regenerate data signals sent at any wavelength e.g. in the range of 300 nm to 10 ⁇ m.
  • the device may be arranged to regenerate data signals sent at the Original band (O, 1260 - 1360 nm), Extended band (E, 1360 - 1460 nm), Short wavelength band (S, 1460 - 1530 nm), Central band (C, 1530 - 1565 nm), Long, wavelength band (L, 1565-1625 nm) and/or Ultra long wavelength band (U, 1625-1675 nm)
  • the signal regenerating device is data rate agnostic.
  • the signal regenerating device may be applied to provide all-optical full regeneration at arbitrary data rates.
  • the signal regenerating device may be adapted to handle different data rates e.g. by tuning optical resonators and/or delay lines. A change in the data rate does not typically require replacement of components of the signal regenerating device.
  • the optical splitters and combiners of the signal regenerating device may be substantially identical, which is expected to provide cost savings.
  • the signal regenerating device may also be applied to regenerate polarization diversity modulated signals.
  • Fig. 1 shows an optical communication system
  • Fig. 2a shows, by way of example, an optical data signal, and a clock signal associated with the optical data signal
  • Fig. 2b shows, by way of example, a degraded data signal
  • Fig. 3 shows an optical communication system comprising a signal regenerating device, wherein said regenerating device comprises a plurality of signal regenerating units,
  • Fig. 4 shows a signal regenerating unit comprising a modulation format converter unit, a clock recovery unit, a phase recovery unit, and a reshaping unit,
  • Fig. 5a shows a modulation format converter unit
  • Fig. 5b shows a modulation format converter unit comprising a fiber optic splitter and a fiber optic combiner
  • Fig. 6a shows a clock recovery unit
  • Fig. 6b shows a clock recovery unit
  • Fig. 7 shows schematically a spectral decomposition of a modulation converted signal
  • Fig. 8 shows schematically matching of optical resonators with the spectral peaks of the modulation converted signal
  • Fig. 9 shows schematically the temporal behavior of a reference component, a sideband component, and a beat signal corresponding to the data signal of Fig. 2b,
  • Fig. 10 shows a phase recovery unit based on a finite impulse response filter
  • Fig. 11 illustrates reduction of phase noise by the phase recovery unit
  • Fig. 12 shows a phase recovery unit comprising optical resonators
  • Fig. 13 shows a reshaping unit
  • Fig. 14 shows a spectral filter
  • Fig. 15 illustrates processing of optical signals by the reshaping unit
  • Fig. 16 shows a modulation format converter and a clock recovery unit
  • Fig. 17 shows a signal reshaping unit based on a Sagnac interferometer
  • Fig. 18a shows a clock recovery device
  • Fig. 18b shows a clock recovery device for recovering clock frequencies from two optical data signals.
  • an optical communication system 900 may comprise an optical transmitter 910, an optical path 930, and an optical receiver 920.
  • An optical signal S 0 sent by the transmitter 910 is transmitted via the path 930 and received by the receiver 920.
  • the path 930 may be e.g. an optical fiber.
  • the optical signal SO may comprise one or more data signals S 0 , ⁇ i. S 0 , ⁇ 2, S 0 , ⁇ 3. S 0 , ⁇ 4. sent at different optical channels at different wavelengths ⁇ -i, ⁇ 2 , ⁇ 3 , ⁇ 4 .
  • a first signal S 0 , ⁇ i may comprise spectral components in the vicinity of the wavelength ⁇ i
  • the second signal S 0 , ⁇ 2 may comprise spectral components in the vicinity of the wavelength X 2 .
  • S 0 , ⁇ 4 may be modulated, for example, according to the RZ format (Return to Zero), CS-RZ format (Carrier Suppressed Return to Zero), DPSK-RZ format (Differential Phase Shift Keyed Return to Zero), or DQPSK-RZ format (Differential Quadrature Phase Shift-Keyed Retum-to-Zero).
  • the data signals may simultaneously be modulated according to the same format or according to different formats.
  • the upper curve of Fig. 2a shows, by way of example, an optical data signal S 0, ⁇ i modulated according to the RZ format.
  • INT denotes intensity and t denotes time.
  • Signal values below a limit LO1 may be interpreted to represent logical state "0", i.e. zero.
  • Signal values above a limit HM may be interpreted to represent logical state "1 ", i.e. one.
  • FIG. 2a shows a clock signal S C ⁇ _ ⁇ , ⁇ i associated with the data signal S 0, ⁇ i-
  • the temporal separation between the rising edges of adjacent data pulses is equal to an integer multiple of the clock period T C ⁇ _ ⁇ -
  • the inverse of the clock period T C ⁇ _ ⁇ is the clock frequency vcLK associated with the data signal S 0 , ⁇ i-
  • S 0 , ⁇ 4 may have the same or different clock frequencies.
  • Fig. 2b shows, by way of example, a degraded data signal.
  • S 0 , ⁇ 4 may be restored by using a multichannel signal regenerating device 800.
  • the signal regenerating device 800 comprises one or more signal regenerating units 600a, 600b, 600c, 60Od, and a common light source
  • a light source 700 may be common to several signal regenerating units 600a, 600b, 600c, 60Od which are similar to the unit 600.
  • the regenerating units 600a, 600b, 600c, 60Od provide at least retiming and re-shaping of the data signals S 0 , ⁇ i. S 0 , ⁇ 2, S 0 , ⁇ 3. S 0 , ⁇ 4-
  • Each data signal is processed at its wavelength by a separate regenerating unit 600a, 600b, 600c, 60Od to provide regenerated data signals So,REG, ⁇ i > SO,REG ⁇ 2> SO,REG ⁇ 3> SO,REG ⁇ 4-
  • the signal regenerating device 800 may also comprise a spectral demultiplexer DEMUX to spectrally separate the different data signals
  • the signal regenerating device 800 may also comprise a spectral multiplexer MUX to combine regenerated data signals SO.REG ⁇ L S 0 , RE G, ⁇ 2. S O,REG ⁇ 3.
  • a signal regenerating unit 600 for a single wavelength channel may comprise a modulation format converter unit 200, a clock signal recovery unit 300, a phase recovery unit 400, and a signal re-shaping unit 500.
  • the signal regenerating unit 600 may also comprise the light source 700.
  • the light source 700 provides substantially continuous light B ⁇ C w at a wavelength ⁇ C w-
  • the continuous light B ⁇ C w and a data signal S 0 , ⁇ i are coupled to the modulation format converter 200.
  • the modulation format converter 200 provides two output signals: a transmitted signal S A , ⁇ i and a converted signal S A , ⁇ cw-
  • the converted signal S A , ⁇ cw is substantially at the wavelength ⁇ C w of the continuous light B ⁇ C w and its intensity is modulated by the intensity of the data signal S 0 , ⁇ i.
  • the transmitted signal S A , ⁇ i is substantially at the same wavelength, i.e. at the wavelength ⁇ 1 as the data signal S 0 , ⁇ i-
  • the transmitted signal S A , ⁇ i has substantially the same modulation as the data signal S 0 , ⁇ i-
  • the converted signal S A, ⁇ C w corresponding to a phase-modulated RZ data signal exhibits a substantially continuous beat at the clock frequency.
  • a RZ-modulated or CS-RZ-modulated data signal S 0 , ⁇ i may have several consecutive data pulses of low intensity.
  • the converted signal S A, ⁇ C w corresponding to intensity-modulated RZ data signal may also have long periods of low intensity when the data signal S 0, ⁇ i carries several consecutive low-intensity data bits.
  • the converted signal S A , ⁇ C w is coupled to a clock recovery unit 300. Regardless of the modulation format of the data signal S 0, ⁇ i. the clock recovery unit 300 provides a substantially continuous clock signal S CLK ⁇ CW even when the data signal S 0 , ⁇ i is at the zero level during several clock cycles.
  • the continuous clock signal S CLK ⁇ CW is substantially at the wavelength of the continuous light B ⁇ C w-
  • the clock signal S CLK ⁇ CW may be at a different wavelength than the transmitted signal S A, ⁇ i- This facilitates the later separation of the clock signal S CLK ⁇ CW from the regenerated data signal.
  • the recovered clock signals corresponding to said optical data signals S 0 , ⁇ i. S 0 , ⁇ 2 may be at the same spectral position, i.e. substantially in the vicinity of ⁇ C w-
  • a first recovered clock signal S CLK ⁇ CW corresponding to the first optical data signal S 0, ⁇ i and a second recovered clock signal S CLK ⁇ CW corresponding to a second optical data signal S 0 , ⁇ 2 may be at the same spectral position.
  • the original data signal S 0 , ⁇ i may comprise phase noise which may be reduced in the phase recovery unit 400.
  • the transmitted signal S A , ⁇ i is coupled into the phase recovery unit 400, which provides a filtered signal S B , ⁇ i-
  • the phase recovery unit 400 may also reduce amplitude noise, i.e. random variations in the intensity.
  • the filtered signal S B , ⁇ i and the clock signal SC LK ⁇ CW are coupled into the reshaping unit 500 which restores the shape of the data pulses using the clock signal S CLK ⁇ CW to restore precise timing of the data pulses.
  • the reshaping unit 500 provides a regenerated data signal S O . REG ⁇ I which is substantially at the wavelength ⁇ 1 , i.e. at the wavelength of the original data signal S 0 , ⁇ i-
  • the modulation format converter 200 and/or the phase recovery unit 400 and/or reshaping unit 500 may also be adapted to provide amplification of the regenerated data signal S 0
  • RE G ⁇ L Tne signal regeneration unit 600 may comprise one or more further optical amplifiers to provide amplification of the regenerated data signals SO.
  • the data signal S 0 , ⁇ i and/or the continuous light B ⁇ C w may have an arbitrary state of polarization, because the operation of the units 200, 300, 400, 500 is, in principle, polarization-independent.
  • the data signal S 0 , ⁇ i and/or the continuous light B ⁇ C w may be horizontally or vertically polarized.
  • the regenerating device 800 or a unit 600 may further comprise polarization controllers, thermal phase shifters, optical isolators and/or further components to further stabilize and/or to enhance the operation of the regenerating device 800 or unit 600.
  • Other signal regeneration units 600b, 600c, 60Od which may use the same light source 700, may provide further regenerated data signals S 0 ,REG, ⁇ 2.
  • S 0 ,REG, ⁇ 3. and S 0 ,REG, ⁇ 4 which are at different wavelengths ⁇ 2, ⁇ 3, ⁇ 4.
  • a multi-channel regenerating device 800 may be implemented by using only one stabilized narrowband light source 700.
  • Fig. 5a shows a modulation format converter 200.
  • the converter 200 comprises a splitter 21 , two non-linear units SOA1 , SOA2, and a combiner 22.
  • the non-linear units SOA1 , SOA2 may be, e.g. semiconductor optical amplifiers.
  • the splitter 21 and the combiner 22 may be e.g. semitransparent interfaces.
  • the splitter 21 and the combiner 22 may be e.g. cube beamsplitters which have two inputs and two outputs with a splitting ratio of 50% to 50%.
  • the splitter 21 and the combiner 22 may be semitransparent reflectors.
  • the splitter 21 divides the continuous light B ⁇ C w into parts and couples the parts to propagate through the first non-linear unit SOA1 and through the second non-linear unit SOA2. Also the data signal S 0, ⁇ i is divided into parts which are coupled to propagate through the first non- linear unit SOA1 and through the second non-linear unit SOA2.
  • the splitter 21 and the combiner 22 are coupled to the nonlinear units SOA1 , SOA2 in a similar way as in a Mach-Zehnder interferometer.
  • the phase difference caused the difference between the optical path length between the upper arm ARM1 and the lower arm ARM2 is advantageously adjusted to be all the time substantially equal to zero or all the time substantially equal to ⁇ . (For comparison, the phase shift is varied in a typical Mach-Zehnder interferometer arrangement).
  • the signal propagating through the first non-linear unit SOA1 is substantially the sum of the data signal S 0 , ⁇ i and the continuous light B ⁇ cw-
  • the signal propagating through the second non-linear unit SOA2 is also substantially the sum of the data signal S 0 , ⁇ i and the continuous light B ⁇ C w-
  • Each unit SOA1 , SOA2 exhibits nonlinearity at both wavelengths ⁇ C w and ⁇ -i.
  • the intensity of the signal component propagating at the wavelength ⁇ i is capable of modulating the intensity of the signal component propagating at the wavelength ⁇ C w- Consequently, both non-linear units SOA1 , SOA2 provide modulated light S A, ⁇ cw which is modulated by the intensity of said optical data signal S Ol ⁇ i.
  • the combiner combines coherently the signal components which have propagated through the first non-linear unit SOA1 with the signal components which have propagated through the second non-linear unit SOA2.
  • the signals propagating through the non-linear units SOA1 , SOA2 are substantially identical, and the phase shift caused by them is substantially constant. Thanks to constructive and destructive inference at the combiner 22, a majority of light having the first wavelength ⁇ 1 is coupled out of the first output 01 , and a majority of light having the second wavelength ⁇ C w is coupled out of the second output 02 of the combiner 22.
  • the second output of the combiner 22 provides a modulation format converted signal S A , ⁇ cw substantially only at the wavelength ⁇ C w > and the first output 01 of the combiner 22 provides a transmitted signal S A, ⁇ i substantially only at the wavelength ⁇ -i.
  • the modulation format converter 200 may separate the signals even without spectral filters.
  • the converted signal S A , ⁇ cw and the transmitted signal S A , ⁇ C i may also be separated by a spectral filter instead of or in addition to the interferometric arrangement described above.
  • the interaction between the data signal S 0, ⁇ i and the continuous light B ⁇ C w may take place in the semiconductor optical amplifier SOA1 ,
  • the modulation format converter using semiconductor optical amplifiers may substantially omit phase information.
  • the wavelength-converted signal S A , ⁇ cw does not comprise the phase information of the original data signal S 0 , ⁇ i-
  • the non-linear interaction may also take place in an optical fiber at high intensity levels, in particular in a silicon waveguide or in an Indium Phosphide waveguide.
  • the interaction may be based e.g. on cross- gain modulation, Kerr effect, and/or optically induced transparency.
  • the modulation format conversion is advantageous especially when the data signal S 0, ⁇ i is modulated according to such a modulation format that the spectral decomposition of the data signal S 0, ⁇ i does not have sideband components.
  • the spectral decomposition of a DPSK-RZ modulated data signal does not have sideband components directly suitable for clock recovery, but after the phase information has been substantially removed in the modulation format conversion, the respective converted signal S A , ⁇ cw has spectral components, e.g. a sideband component and a component at the carrier frequency, which may be effectively utilized in the clock recovery.
  • the modulation format conversion using semiconductor optical amplifiers may also be substantially insensitive to the polarization state of the data signal S 0 , ⁇ i-
  • the modulation format converter may provide the modulation format converted signal S A, ⁇ cw without substantially reducing the optical power of the in-coupled data signal.
  • the power of the data signal may used in an optimum way.
  • the transmitted signal S A, ⁇ i which has passed through the non-linear units SOA1 , SOA2 has substantially the same phase information as the original data signal S 0 , ⁇ i-
  • the non-linear units SOA1 , SOA2 may amplify the intensity of the pulses of the data signal S A, ⁇ i and/or equalize variations in the intensity of the pulses of the data signal S A , ⁇ i- Nevertheless, when the original data signal S 0, ⁇ i has an intensity pulse corresponding to a digital bit "1 ", then the corresponding transmitted signal S A, ⁇ i has also an intensity pulse corresponding to a digital bit "1 ". When the original data signal S 0, ⁇ i has a low intensity corresponding to a digital bit "0", then the corresponding transmitted signal S A, ⁇ i has also a low intensity corresponding to the digital bit "0". In that sense the transmitted signal S A> ⁇ i which has passed through the non-linear units SOA1 , SOA2 may be considered to carry the same intensity information as the original data signal S 0 , ⁇ i-
  • the non-linear units SOA1 , SOA2 may be arranged to operate near the saturation level of the units SOA1 , SOA2 so as to equalize variations in the intensity of the pulses of the first optical data signal S 0 , ⁇ i-
  • the splitter 21 and/or the combiner 22 may also be fiber optic splitters/combiners.
  • the splitter 21 and/or the combiner 22 may also be implemented by integrated optics.
  • Fig. 6a shows a clock signal recovery unit 300.
  • the clock signal recovery unit 300 may comprise two optical resonators OR1 , OR2.
  • the converted signal S A , ⁇ C w is coupled to the first optical resonator OR1 and to the second optical resonator OR2, and the signals provided by the optical resonators OR1 , OR2 are combined to provide a recovered clock signal S CL ⁇ , ⁇ cw-
  • the optical resonators OR1 , OR2 may be ring resonators.
  • the converted signal S A , ⁇ cw may be coupled to a first end of a first light guide WG1.
  • the first light guide WG1 may be adapted to couple light into the first ring resonator OR1 and to the second ring resonator OR2 by directly by evanescent coupling.
  • Light may be coupled to a second light guide WG2 also directly by evanescent coupling.
  • the first light guide WG1 may also act as a beam splitter and the second light guide WG2 may also act as a beam combiner.
  • the resonators OR1 , OR2 may also be e.g. Fabry-Perot resonators comprising an optical cavity between two reflectors.
  • a residual signal provided by a second end of the first light guide WG1 i.e. that part of the converted signal S A, ⁇ cw which does not pass through the resonators OR1 , OR2, may coupled to a beam dump (not shown).
  • the residual signal may be analyzed in a monitoring device MON 1 in order to provide information regarding the quality of the data signal S 0, ⁇ i and/or regarding the quality of the continuous light B ⁇ C w > in order to provide information regarding the operation of the modulation format converter 200, the clock signal recovery unit 300 and/or the light source 700 .
  • Fig. 6b shows a clock recovery unit 300 comprising a first optical resonator OR1 and a combiner.
  • the light guide WG2 may be used as the combiner, which is arranged to provide a recovered clock signal
  • Bcw may be provided e.g. by the light source 700.
  • another type of combiner may be used, e.g. a bifurcated fiber or a cube beamsplitter.
  • the resonator OR1 may also be e.g. a Fabry-Perot resonator.
  • the clock recovery unit 300 may comprise one or more optical attenuators to match the intensity of the substantially continuous light B ⁇ C w with a sideband signal provided by the resonator OR1.
  • Fig. 7 shows, by way of example, the spectral decomposition of the converted signal S A, ⁇ i having a spectral component at a reference frequency V REF , and spectral components at a sideband frequencies vsiDE and V-SiDE .
  • the spectral separation V SIDE - V REF is equal to the clock frequency v C ⁇ _ ⁇ associated with the data signal S 0 , ⁇ i-
  • Fig. 8 shows the matching of the optical resonators OR1 , OR2 with the spectral components.
  • the spectral position of a passband PB of the first optical resonator OR1 is matched with the reference frequency v REF
  • the spectral position of a passband PB of the second optical resonator OR2 is matched with the sideband frequency V SIDE such that their spectral separation is substantially equal to the clock frequency vcLK associated with the data signal S 0 , ⁇ i-
  • ⁇ SR, i and ⁇ SR,2 denote separation ranges, i.e. the spectral separation between adjacent passbands PB.
  • the separation range is also known as the free spectral range (FSR).
  • the optical resonators OR1 , OR2 store optical energy, and they may provide a continuous optical output even during the periods when the intensity of the converted signal S A , ⁇ cw is at a low level.
  • the first optical resonator OR1 provides and output signal S REF
  • the second optical resonator OR2 provides an output signal S SIDE combining of the output signals provides a substantially continuous beat signal, i.e. the recovered clock signal S CLK ⁇ CW .
  • INT denotes intensity
  • t denotes time
  • ENV denotes the envelope curve of the beating clock signal S CLK ⁇ CW ⁇
  • the clock signal S CLK ⁇ CW may be recovered by
  • the unit 300 shown in Fig. 6b is an example where the clock signal S CLK ⁇ CW is recovered by combining the sideband signal S SIDE provided by the optical resonator OR1 with light provided e.g. by the light source 700 of Fig. 4.
  • the unit 300 of Fig. 6 is very simple and stable.
  • the clock signal recovery unit 300 may also be implemented using only one optical resonator OR1. In that case both output signals S REF and S SIDE may be provided by the same resonator OR1.
  • the first resonator OR1 may be arranged to provide the clock signal as a combination of the signals S REF and Ssi DE out of the first optical resonator OR1.
  • a change in the data rate of the data signal S 0 , ⁇ i may require changing the optical length of the resonator OR1.
  • a method for recovering a first clock signal SC LK ⁇ CW from said first optical data signal S 0 , ⁇ i may comprise:
  • the separation of the modulation converted signal S A, ⁇ cw may be carried out using the interferometric modulation conversion unit 200 shown in Fig. 5a or 5b, said unit 200 comprising a splitter 21 , a first non-linear unit SOA1 , a second non-linear unit SOA2, and a combiner 22, said splitter 21 having a first splitter input, a second splitter input, a first splitter output, and a second splitter output, said combiner 22 having a first combiner input, a second combiner input, a first combiner output 01 , and a second combiner output 02.
  • the continuous light B ⁇ C w is coupled to the first splitter input, and the first optical data signal S 0, ⁇ i is coupled to the second splitter input.
  • the first splitter output is coupled to the first non-linear unit SOA1.
  • the second splitter output is coupled to the second non-linear unit SOA2.
  • the output of the first non-linear unit SOA1 is coupled to the first combiner input.
  • the output of the second non-linear unit SOA2 is coupled to the second combiner input.
  • the first arm ARM1 of the interferometer 200 comprises the first splitter output, the first non-linear unit SOA1 , and the first combiner input.
  • the second arm ARM2 of the interferometer 200 comprises the second splitter output, the second non-linear unit SOA2, and the second combiner input.
  • the continuous light B ⁇ C w is split into a first part B1 which propagates via the first arm ARM1 of the interferometer 200, and into a second part
  • the phase shift between said first B1 and second parts B2 may be arranged to be substantially equal to zero at the second combiner output 02, wherein most of the optical power of the combination of said first B1 and second B2 parts may be coupled out of the second combiner output 02.
  • the second combiner output is adapted to provide the modulation converted signal S A , ⁇ cw-
  • the first optical data signal S 0, ⁇ i is split into a first portion B3 which propagates via the first arm ARM1 of the interferometer 200, and into a second portion B4 which propagates via the second arm ARM2 of the interferometer 200.
  • the phase shift between said first portion B3 and said second portion B4 may be arranged to be substantially equal to zero at the first combiner output 01 , wherein most of the optical power of the combination of first B3 and second B4 portions may be coupled out of the first combiner output 01.
  • the optical path length difference between the first arm ARM1 and the second arm ARM2 is maintained substantially constant.
  • the difference between the optical path lengths of the first arm ARM1 and the second arm ARM2 may be e.g. zero or ⁇ .
  • the optical path length of one or both arms may be adjustable in order to maintain said path length difference.
  • the optical path length may be adjusted e.g. changing the operating temperature of at least one optical component in at least one path. The adjustment may be based e.g. on maximizing the intensity of the modulation-converted signal S A, ⁇ cw and/or the intensity of the transmitted signal S A , ⁇ i-
  • a method for recovering a clock signal S CLK ⁇ CW from an optical data signal S 0 , ⁇ i may comprise:
  • the combination B1 +B2 of the first part B1 and the second part B2 may be coupled to the second combiner output 02 when the phase difference between the first part B1 and the second part B2 is substantially equal to zero at said second combiner output 02 and substantially equal to ⁇ at said first combiner output 01.
  • the combination B3+B4 of the first portion B3 and the second portion B4 may be coupled to the first combiner output 01 when the phase difference between the first portion B3 and the second portion B4 is substantially equal to zero at said first combiner output 01 and substantially equal to ⁇ at said second combiner output 02, thanks to the constructive and destructive interference at said outputs 01 , 02.
  • the combination B1 +B2 of the first part B1 and the second part B2 may be coupled to the second combiner output 02 when the phase difference between the first part B1 and the second part B2 is substantially equal to zero at said second combiner output 02 and substantially equal to ⁇ at said first combiner output 01.
  • the combination B3+B4 of the first portion B3 and the second portion B4 may be coupled to the first combiner output 01 when the phase difference between the first portion B3 and the second portion B4 is substantially equal to zero at said first combiner output 01 and substantially equal to ⁇ at said second combiner input 02.
  • a phase difference ⁇ + n x 2 ⁇ is interpreted to mean the phase difference ⁇ , wherein n is an integer.
  • a phase difference 3 ⁇ leads to the same destructive interference as the phase difference ⁇ .
  • the light source 700 may be selected such that the wavelength ⁇ C w of the continuous light B ⁇ C w does not overlap the wavelengths ⁇ -i, ⁇ 2 , ⁇ 3 , ⁇ 4 of the data signals.
  • the wavelength ⁇ C w is advantageously stable, and the bandwidth of the light provided at ⁇ C w is advantageously less than 100 MHz.
  • the optical resonators OR1 , OR2 of the clock recovery unit 300 can have a narrow passband PB, and a long time constant which ensures a high number of recovered zero bits (for SDH or SONET data traffic) without pulse equalization.
  • SDH means the synchronous digital hierarchy standard developed by the International Telecommunication Union (ITU), documented in a standard G.707 and its extension G.708.
  • SONET means the synchronous optical networking standard as defined in the standard GR-253-CORE by Telcordia Technologies.
  • Additional equalization could be carried out e.g. in a further semiconductor optical amplifier, which equalizes the maximum intensities of the data pulses.
  • the clock period T C ⁇ _ ⁇ corresponding to a data rate of 40 Gb/s (gigabits/second) is 25 ps.
  • the time constant of the optical resonators OR1 , OR2 of the clock recovery unit 300 may be selected to be greater than 3 times the required time, e.g. 2.4 ns - 3 x 0.8 ns.
  • the time constant ⁇ of the optical resonators OR1 , OR2 may be selected to be greater than or equal to 2.4 ns, which corresponds to a bandwidth of 66 MHz.
  • This may be implemented e.g. by using a ring resonator whose diameter is 1.5 mm, whose refractive index is 1.5 mm, and whose coupling efficiency, i.e. single-pass transmittance is 1 %.
  • the separation range ⁇ SR,I i.e. free spectral range of such a resonator is 31 GHz, which is smaller than the data rate 40 GHz.
  • the separation range ⁇ SR,I is smaller than the modulation bandwidth, the resonator may also pass undesired frequency components. A better rejection of unwanted spectral components may be attained by using the Vernier effect: two or more optical resonators having slightly different separation ranges ⁇ SR,I may be coupled optically in series to implement a substantially broader separation range.
  • optical resonators e.g. Fabry-Perot cavities or Bragg gratings.
  • the correct spectral position of the passbands PB may be adjusted e.g. by adjusting the temperature of the resonators.
  • the units 200, 300, 400, 500 may comprise remote-controlled heating elements.
  • the bandwidth of the continuous light B ⁇ C w is advantageously narrower than or equal to the bandwidth of the resonators OR1 , OR2.
  • the optical resonators OR1 , OR2 of the clock recovery unit 300 may have a shorter time constant than in the above-mentioned case.
  • the pulse equalization may be performed using a semiconductor optical amplifier operating close to saturation, using a Mamyshev regenerator, or using a non-symmetrical Mach-Zehnder interferometer.
  • the light source 700 may be e.g. a stabilized single-mode Helium- Neon laser arranged to provide continuous light B ⁇ C w at 632.8 nm, 1152.3 nm, or 3391.3 nm.
  • the light source 700 may also be e.g. a diode laser combined with a beam-narrowing optical cavity.
  • the clock recovery unit may be implemented using optical resonators which are not polarization independent.
  • the optical resonators may be e.g. ring resonators implemented with integrated optics.
  • the ring resonators may be race track ring resonators, i.e. the form of the ring may resemble the oval form of a race track or a stadium.
  • the degraded data signal S 0 , ⁇ i. and consequently also the transmitted signal S A, ⁇ i comprise phase noise.
  • the modulation format converter 200 may add more phase noise to the transmitted signal S A , ⁇ i. in addition to the phase noise of the degraded data signal S 0 , ⁇ i-
  • the phase noise and/or amplitude noise is reduced by using the phase recovery unit 400.
  • the phase recovery unit 400 may comprise an optical finite impulse response (FIR) filter.
  • the transmitted signal S A, ⁇ i is distributed to one or more delay lines D1 , D2, D3 by splitters 41.
  • the delayed signals are combiner together with combiners 42.
  • the delayed signal or signals may also be combined with the original transmitted signal S A , ⁇ i-
  • the phase recovery unit 400 provides a filtered signal S B , ⁇ i which is substantially at the wavelength ⁇ 1.
  • the delays of the delay lines D1 , D2, D3 and/or the splitting ratios of the splitters 41 may be adjustable to provide optimum impulse response for different data rates and modulation formats.
  • the phase recovery unit 400 acts as a filter which reduces phase noise.
  • the uppermost curve of Fig. 11 shows, by way of example, the intensity of consecutive pulses of the transmitted signal S A , ⁇ i-
  • the third curve from the top shows the phase of said consecutive pulses, ⁇ denotes phase.
  • the transmitted signal S A> ⁇ i has only phase noise but not amplitude noise.
  • the second curve from the top shows the intensity of consecutive pulses of the filtered signal S B , ⁇ i-
  • the filtered pulses may be broader than the original pulses.
  • the filtered pulses may be time- shifted with respect to the original pulses.
  • the time constant of the phase recovery unit 400 is selected so that the tail of a preceding pulse does not substantially overlap the rising edge of a following pulse.
  • the lowermost curve shows the phase of the pulses of the filtered signal S B , ⁇ i-
  • the phase noise of the filtered signal S B , ⁇ i ma Y be substantially reduced with respect to the phase noise of the transmitted signal S A , ⁇ i-
  • Times t 3 , U, t 5 , t 6 , t 7 , t 8 , t 9 , t 10 are associated with the peaks of the pulses.
  • the phase recovery unit 400 may also comprise one or more optical infinite impulse response (NR) filters, such as optical resonators OR4, OR ⁇ .
  • NR optical infinite impulse response
  • the phase recovery unit 400 may comprise a combination of NR and FIR filters.
  • the use of the optical resonators optical infinite impulse response (NR) filters may provide more degrees of freedom to implement a desired complex impulse response (i.e. the transmission function of the amplitude and phase in the frequency domain) than when using a pure FIR filter.
  • the signal reshaping unit 500 performs at least the retiming and reshaping of the pulses of the filtered signal S B , ⁇ i-
  • Fig. 13 shows a signal re-shaping unit 500.
  • the signal re-shaping unit 500 may comprise a Mach-Zehnder interferometer implemented by a splitter 51 , two phase shifting units SOA3, SOA4, and a combiner 52.
  • the filtered signal S B , ⁇ i is divided into two parts by the splitter 51.
  • a first part is coupled to a first phase shifting unit SOA3 in order to provide a first passed signal S PA ss,i-
  • a second part is coupled to a second phase shifting unit SOA4 in order to provide a second passed signal S PA ss, 2 -
  • the passed signals are combined in a combiner 52 which has a first port and a second port.
  • the output ports of the combiner 52 act as the output port of the Mach- Zehnder interferometer.
  • the clock signal S CLK ⁇ CW is also split into two parts by a splitter 53.
  • the first part CLK1 is coupled to the first phase shifting unit SOA3 by a combiner 54 to control the phase shift of the unit SOA3.
  • the second part CLK2 is delayed by a delay line D5 and coupled to the second phase shifting unit SOA4 by a combiner 56 to control the phase shift of the unit SOA4.
  • the filtered signal S B , ⁇ i and the recovered clock signal S CLK ⁇ CW may be synchronized by using a delay line D4.
  • the time delay of the delay lines D4 and D5 may be adjustable.
  • the Mach-Zehnder interferometer operates as an optical switch which couples the passed signals S PA ss,i, S PA ss, 2 either to the upper output port or to the lower output port of the combiner 52, depending on the phase difference between the passed signals S PA ss,i, S PA ss,2-
  • the passed signals S PAS s,i > S PAS s,2 are coupled to the first port when the phase difference ⁇ between S PAS s,i and S PAS s,2 is substantially equal to ⁇ .
  • the passed signals S PAS s,i > S PAS s,2 are coupled to the second port when the phase shift if substantially equal to zero.
  • the phase of the passed signals S PAS s,i > S PAS s,2 is controlled by adjusting the optical length of the optical paths between the splitter 51 and the combiner 52, the first path being via the first phase shifting unit SOA3, and the second path being via the second phase shifting unit SOA4.
  • the second port may be coupled to a beam dump (not shown) or to an optical signal monitor MON2 in order to analyze the data signal S 0, ⁇ i and/or the operation of the units 200, 300, 400, 500.
  • Switched out-coupling of the signals S PAS s,i, S PAS s, 2 from the first port provides two signals: a data signal S O u ⁇ , ⁇ i which is substantially at the wavelength ⁇ i and a clock signal S O u ⁇ , ⁇ cw which is at the wavelength ⁇ cw-
  • the Mach-Zehnder interferometer acts as an optical exclusive-or-gate (XOR) which directs the degraded data signal to the first output port only during a short time interval when the first CLK1 and the second clock CLK2 signals are in different logical states.
  • the length of said time interval is defined by the delay of the second clock signal CLK2 with respect to the first clock signal CLK1.
  • the delay may be e.g. 10- 20% of the unit interval of the data signal.
  • the delay may be e.g. 10 - 20 ps (picoseconds) in case of a 10 GHz data signal.
  • phase shifting units SOA3, SOA4 are adapted to cause a controllable phase difference ⁇ between the first passed signal S PA ss,i and the second passed signal S PA ss,2 such that the combined passed signals S PA ss,i, S PA ss, 2 are directed to the first output of the interferometer only when only one of the clock signals CLK1 , CLK2 is above a predetermined level LIM3, LIM4 (see Fig. 15).
  • the operation of the signal reshaping unit 500 could be described so that the signal reshaping unit 500 carves the desired form of the data pulses from the broadened pulses provided by the phase recovery unit 400.
  • an optical data signal S 0 , ⁇ i. S B , ⁇ i m ay be re-shaped and re-timed by using a Mach-Zehnder interferometer 500, said method comprising:
  • - separating said re-shaped data signal S O u ⁇ , ⁇ i f rom said re-shaped clock signal SQU T ⁇ CW- Said separation may also be carried out in the spectral multiplexer MUX (Fig. 3) or after said spectral multiplexer MUX.
  • the phase shifting units may be e.g. Semiconductor Optical Amplifiers (SOA) which may be operated near saturation or in saturation.
  • SOA Semiconductor Optical Amplifiers
  • the saturation level of a semiconductor optical amplifier may be lowered simply by adjusting the operating current.
  • the saturation level in the phase shifting unit may be adjusted to be so low, that also a non- amplified clock signal may be sufficient to induce a significant change in the phase shift.
  • the clock signal S CLK ⁇ CW provided by the passive optical resonators of the unit 300 do not necessarily need further amplification prior to coupling into the phase shifting units, and a respective amplifier stage may be eliminated from the system.
  • the regenerating device provides retiming and shaping of the signal pulses.
  • a further optical amplifier may be needed to implement full 3R regeneration.
  • a reduction in the number of the optical amplifiers may be attained in multi-channel 3R regeneration of optical signals, as a single further amplifier may be adapted to amplify a plurality of data signals simultaneously at different wavelengths.
  • a non-amplified clock signal recovered by a passive optical resonator may be utilized, and
  • Further amplifiers may be used to provide amplification of the signals prior to the retiming and reshaping and/or after the retiming and reshaping, in order to implement full 3R regeneration.
  • the output signals S O u ⁇ , ⁇ cw and S O u ⁇ , ⁇ i propagate along the same path but they have different wavelengths ⁇ i and ⁇ C w-
  • the regenerated data signal S O u ⁇ , ⁇ i ma Y be spectrally separated from the reshaped clock signal S O u ⁇ , ⁇ icw e.g. by a spectral filter FIL1.
  • the combiner 54 may be positioned between the first phase unit SOA3 and the combiner 52.
  • the combiner 56 may be positioned between the second phase unit SOA4 and the combiner 52.
  • the clock signals CLK1 , CLK2 propagate through the phase shift units SOA3, SOA4 in a different direction than the passed signals S PA ss,i, S PA ss,2-
  • Coupling of the clock signals CLK1 , CLK2 to the phase recovery unit 400 may be prevented by using an optical isolator.
  • the filter FIL1 may comprise a ring resonator OR3.
  • the signals S O u ⁇ , ⁇ cw and S O u ⁇ , ⁇ i are coupled into a first end of a first light guide WG3.
  • the ring resonator OR3 may be substantially matched with the wavelength ⁇ C w of the reshaped clock signal S OUT ⁇ CW so that only the reshaped clock signal S OUT ⁇ CW is coupled via the ring resonator OR3 to a second light guide WG4.
  • the reshaped clock signal S OUT ⁇ CW may be coupled into a beam dump (not shown) or to a monitor MON3 in order to analyze the data signal S 0, ⁇ i and/or the operation of the units 200, 300, 400, 500.
  • the regenerated data signal S O u ⁇ , ⁇ i ma y be coupled out of a second end of the first light guide WG3.
  • the filter FIL1 may also be implemented by using e.g. a Bragg grating or a spectral filter based on dielectric multilayer coatings.
  • the spectral multiplexer MUX may comprise a filter FIL1 to separate least one reshaped clock signal S OUT ⁇ CW from the regenerated data signals.
  • a filter FIL1 may be positioned after the spectral multiplexer MUX to separate least one reshaped clock signal S OUT ⁇ CW from the regenerated data signals.
  • Fig. 15 shows schematically operation of the signal re-shaping unit 500.
  • the uppermost curve of Fig. 15 shows temporal evolution of the intensity of the filtered signal S B , ⁇ i. as provided by the phase recovery unit 400.
  • a. u. denotes arbitrary units.
  • the second curve from the top shows the clock signal CLK1 coupled into the first phase shifting unit SOA3
  • the third curve from the top shows the clock signal CLK2 coupled into the second phase shifting unit SOA4.
  • the intensity coupled to the first phase shifting unit SOA3 is the sum of the intensity of the clock signal CLK1 and a part of the filtered signal filtered signal S B, ⁇ i-
  • the intensity coupled to the second phase shifting unit SOA4 is the sum of the intensity of the clock signal CLK2 and a part of the filtered signal filtered signal S B , ⁇ i- CLK2 is slightly time-shifted with respect to CLK1 , by the delay line D5 (Fig. 13). Consequently, the intensity coupled into the first phase shifting unit SOA3 rises before the intensity which is coupled into the second phase shifting unit SOA4.
  • the first clock signal CLK1 reaches a predetermined intensity level LIM3 at a time t1 and the second clock signal CLK2 reaches the predetermined intensity level LIM4 slight later, at a time t2.
  • the levels LIM3 and LIM4 may be substantially equal.
  • the phase shift ⁇ caused by the phase shifting unit SOA3 depends on the in-coupled intensity.
  • the fourth curve from the top shows the phase shift ⁇ 1 caused by the first phase shifting unit SOA3.
  • the fifth curve from the top shows the phase shift ⁇ 2 caused by the second phase shifting unit SOA4.
  • the seventh curve from the top shows the intensity of the passed signals S PA ss,i and S PA ss, 2 - Their intensity is substantially identical but their phases are changed according to the instantaneous intensity of the clock pulses CLK1 , CLK2.
  • the predetermined limits LIM3, LIM4 depend also on the instantaneous level of the filtered signal S B , ⁇ i- Furthermore, the function describing the relationship between the clock signal level and the phase shift is continuous, i.e. switching from zero to ⁇ does not take place infinitely sharp.
  • the passed signals S PA ss , i and S PA ss ,2 are coupled to the first port of the combiner 52 (Fig. 13) only when the phase difference ⁇ is substantially equal to ⁇ .
  • the temporal position of the pulses of the signal provided by the first port at the wavelength ⁇ i is precisely controlled by the recovered clock signal
  • the temporal duration of the regenerated pulses is controlled by temporal delay between the pulses CLK1 , CLK2, i.e. by the delay line D5.
  • Fig. 16 shows an alternative embodiment of the modulation format converter 200.
  • the data signal S 0 , ⁇ i and the continuous light B ⁇ C w may be combined.
  • the combining may be carried out e.g. by a ring resonator RR1.
  • the continuous light B ⁇ C w may be coupled into a first end of a first light guide WG5, and subsequently via the ring resonator to a second light guide WG6.
  • the data signal S 0 , ⁇ i may be coupled to a first end of a second light guide WG6.
  • the combined data signal S 0 , ⁇ i and the continuous light B ⁇ C w may be coupled out of a second end of the second light guide WG6.
  • the combined signal is coupled to a non- linear unit SOA1.
  • the non-linear unit SOA1 provides passed signals at the wavelength ⁇ C w and at the different wavelengths ⁇ 1. Thanks to the non-linearity, the variations in the intensity of the data signal S 0, ⁇ i modulate the passed signal provided at the wavelength ⁇ C w-
  • the passed signal provided at the wavelength ⁇ C w is the modulation converted signal S A , ⁇ cw-
  • the passed signals at the different wavelengths ⁇ 1 and ⁇ C w may be separated from each other by a spectral filter, which may comprise a second ring resonator RR2 and light guides WG7, WG8.
  • the data signal S 0 , ⁇ i may be combined with the continuous light B ⁇ C w by using a beam splitter as the combiner.
  • a polarising beam splitter may be used to combine the data signal S 0 , ⁇ i with the continuous light B ⁇ C w if their polarization state is stable and known.
  • the data signal S 0 , ⁇ i and the continuous light B ⁇ C w may be combined also by using a Bragg grating.
  • the passband of the spectral filter should be broad enough to transmit both the reference frequency V REF and the sideband frequency V SIDE (see Figs. 7a and 7b).
  • the spectral separation of the clock signal S CLK ⁇ CW from the components at the wavelength ⁇ i may be carried out after the clock recovery unit 300.
  • the passed signals provided by the non-linear unit SOA1 at the wavelength ⁇ C w and at the wavelength ⁇ i may be coupled into a clock recovery unit 300 before separating them spectrally from each other.
  • the ring resonator RR2 may simultaneously act as a clock recovery unit which directly provides the recovered clock signal SC LK ⁇ CW and separates it from the transmitted signal S A , ⁇ i-
  • the non-linear units SOA1 may be arranged to operate near the saturation level of the unit SOA1 so as to equalize variations in the intensity of the pulses of the first optical data signal S 0 , ⁇ i-
  • the output of the non-linear unit SOA1 shown in Fig. 16 may be coupled to a clock recovery unit 300 comprising one or more optical resonators.
  • the output of said non-linear unit SOA1 or the output of the clock recovery unit 300 may comprise a spectral filter to separate the recovered clock signal S CLK ⁇ CW from disturbing components at other wavelengths.
  • a method for recovering a first clock signal S CLK ⁇ CW from said first optical data signal S 0 , ⁇ i may comprise:
  • the phase recovery unit 400 may also be implemented using four wave mixing.
  • a pump beam and the data signal S 0 , ⁇ i m ay be simultaneously coupled into a nonlinear material.
  • the four wave mixing provides a reduced pump beam, and amplified data signal, and idler beams.
  • the reduced pump beam and the idler beams may be separated from the amplified data signal e.g. by spectral filtering.
  • the resulting amplified data signal has less phase noise than the data signal So, ⁇ i-
  • the phase recovery unit 400 may also be implemented using a section of a multi-mode fiber, wherein the filtering is based on modal dispersion in the fiber.
  • the phase recovery unit 400 may also be implemented using multimode interference (MMI) one or more couplers.
  • MMI multimode interference
  • the couplers may be implemented by integrated optics.
  • the reshaping unit 500 may also be based on an optical gate based on e.g. four wave mixing or optical bistability.
  • the signal reshaping unit may also be based on a Sagnac interferometer 501 having a phase shift unit SOA3.
  • the phase shift unit SOA3 is located asymmetrically in a Sagnac loop, i.e. the phase shift unit SOA3 is located asymmetrically with respect to the center CPS of the Sagnac loop.
  • an optical data signal S 0 , ⁇ i. S B , ⁇ i ma y be re-shaped and re-timed by using a Sagnac interferometer 501 , said Sagnac interferometer comprising a phase shift unit SOA3 positioned asymmetrically in the loop of said Sagnac interferometer, said method comprising: - splitting said optical data signal S 0 , ⁇ i. S B , ⁇ i by a splitter-combiner 62 into a first part and a second part,
  • the clock signal S CLK ⁇ CW changes the phases of the passed signals S PA ss,i > S PA ss,2 substantially simultaneously in the phase shift unit SOA3.
  • the Sagnac interferometer can act as an optical gate which couples the signals S PA ss,i and S PA ss, 2 to the first output during the short period when the S PAS s,i and S PAS s, 2 arriving at the combiner 62 are in different phases.
  • the signals S PAS s,i and S PAS s, 2 are coupled backwards from the second output of the splitter-combiner 62, i.e. in a direction opposite the direction of the in-coming data
  • the backwards-propagating light may be separated by an optical isolator 63.
  • the clock signal Scuucw may be coupled into the phase shift unit SOA3 by a combiner 54.
  • the re-shaped data signal S O u ⁇ , ⁇ i may be separated from the clock signal S CLK ⁇ CW e-9- by a spectral filter FIL1 (Fig. 14).
  • the clock signal S CLK ⁇ CW may be split into clockwise and counterclockwise propagating portions, and the data signal S 0 , ⁇ i. S B , ⁇ i may be coupled into the phase shift unit SOA3 in order to chance the phase of the clockwise and counterclockwise propagating portions.
  • the re-shaped signal is at the wavelength ⁇ C w of the clock signal SCLK ⁇ CW-
  • the signal regenerating device 800 and/or the units 200, 300, 400, 500, 600 may be used in combination with optical data receivers, repeaters, transponders, add-drop multiplexers, or other types of devices used in fiber optic networks.
  • the units 200, 300, 400, 500, 600 may also be used in combination with optical data receivers, repeaters, transponders, add-drop multiplexers, or other types of devices used in optical communication systems operating in free air or in space.
  • the system 900 may be a point-to-point communication system.
  • the transmitter 910 or the receiver 920 may be a part of a network node in a communication system 900.
  • the transmitter 910 or the receiver 920 may be a of a part of a signal repeater, which is adapted to restore the optical signal signals in long distance communication systems.
  • the transmitter 910 or the receiver 920 may be a part of a cross-connect, i.e. a circuit switch operating in the electrical domain or in the optical domain.
  • the transmitter 910 and/or the receiver 920 may be a part of a router adapted to forward data packets in the electrical or optical domain.
  • the transmitter 910 and/or the receiver 920 may be a part of a time division add-drop- multiplexer operating in the electrical or optical domain.
  • the optical path 930 may be a fiber network, a light transmissive material, liquid, gas or vacuum.
  • the path 930 may be used for one- directional or two-directional communication.
  • the units 200, 300, 400, 500 may be implemented by methods of integrated optics on a solid-state substrate using miniaturized components. Indium phosphide based (InP), Silicon (Si) based, glass- on-silicon based, or silicon-on-insulator (SOI) based components or integrated structures may be used.
  • the units may also be implemented using fiber optic components.
  • the units may also be implemented using separate free-space optical components.
  • the optical resonators, the semiconductor optical amplifiers and/or further optical components may be implemented on the same substrate.
  • the units 200, 300, 400, 500 may further comprise light-amplifying means to amplify the optical input signals and/or output signals.
  • the light amplifying means may be implemented by e.g. rare-earth doped materials or waveguides.
  • the light amplifying means may be a semiconductor optical amplifier.
  • the non-linear units SOA1 , SOA2 and/or the phase shift units SOA3, SOA4 may be implemented using conventional optical fiber, dispersion-shifted fiber, photonic crystal fiber.
  • the non-linear units SOA1 , SOA2 and/or the phase shift units SOA3, SOA4 may be implemented using an optical substrate (e.g. silicon or fused silica), which has photonic crystal structures or which has light-guiding structures having their width in the micrometer or nanometer regime.
  • Generating the modulation-converted signal S A , ⁇ cw may comprise modulating the intensity, phase, polarization and/or wavelength of the continuous light B ⁇ C w by the varying intensity of the data signal S 0 , ⁇ i-
  • the phase may be modulated by using a Kerr cell.
  • One or more of the optical resonators OR1 , OR2, OR3, OR4, OR5 may comprise two or more resonators which are optically coupled in series.
  • One or more of the optical resonators OR1 , OR2, OR3, OR4, OR5 may be race track ring resonators.
  • the transmitter's emission wavelength do not typically require re-adjusting of the units, 200, 300, 4000, 500, because the clock frequency is recovered at the stable wavelength ⁇ C w of the light source 700. Excessive variations the transmitter's emission wavelength may be handled by adjusting optical resonators.
  • clock recovery devices may be used to recover the clock signal from the modulation converted signal S A , ⁇ cw-
  • the modulation converted signal S A , ⁇ cw may be provided by the interferometric arrangement of Figs. 5a or 5b.
  • the modulation converted signal S A, ⁇ cw may be coupled into a clock recovery unit based on a self-pulsating distributed feedback (DBF) laser, in order to provide the recovered clock signal.
  • the DBF laser may be gain-coupled or index-coupled.
  • the clock recovery may be performed in the incoherent mode, where clock recovery is mediated by carrier modulations in the laser cavity by the intensity associated with the optical clock component of the data signal, such as described e.g.
  • the clock recovery may be performed in the coherent mode, wherein two optical spectral components associated with the clock of the data signal directly injection-lock the two spectral components associated with self-pulsation.
  • the modulation converted signal S A , ⁇ cw may also be coupled into a clock recovery unit 300 based on an actively mode-locked fiber laser or passively mode-locked semiconductor laser.
  • an optical communications system comprising one or more signal regenerating units 600, modulation format conversion units 200, clock signal recovery units 300, phase recovery units 400 and/or reshaping units.
  • a clock recovery device 610 may comprise a combination of the modulation format converter 200 and a clock signal recovery unit 300. For certain applications, only a recovered clock signal, or several recovered clock signals may be needed.
  • the light source 700 may also be an external component.
  • the clock recovery device 610 may also comprise an input 701 to receive substantially continuous light B ⁇ C w-
  • the device 610 for recovering a first clock signal S CLK ⁇ CW from a first optical data signal S 0 , ⁇ i may comprise:
  • the clock signal recovery unit 300 may comprise one or more optical resonators OR1 , OR2 arranged to store optical energy of said first modulated light S A , ⁇ C w, and to provide a substantially continuous sideband signal S SIDE at a sideband peak V SIDE of said first modulated light (S A , ⁇ cw).
  • the clock recovery device is arranged to provide a first clock signal S CLK ⁇ CW as a combination of said sideband signal S SIDE and substantially continuous light B ⁇ C w ⁇ wherein the substantially continuous light B ⁇ C w is at a reference peak V REF of said first modulated light S A , ⁇ cw-
  • the substantially continuous light B ⁇ C w may be provided by the same light source 700, which is arranged to provide continuous light B ⁇ C w for the modulation format converter 200.
  • the substantially continuous light B ⁇ C w may be coupled to the device 610 via one or more inputs 701 , and distributed to the converter 200 and to the clock recovery unit 300.
  • the clock recovery device 610 may comprise two or more modulation format converters 200a, 200b, two or more clock recovery units 300a, 300b, and a common light source 700 to recover clock frequencies from several optical data signals S 0 , ⁇ i. S 0 , ⁇ 2 -
  • the clock recovery units 300a, 300b may be e.g. as shown in Fig. 6b.
  • the modulation format converters 200a, 200b may be e.g. as shown in Figs. 5a, 5b.
  • the clock recovery device 610 may comprise: - a first modulation format converter 200a arranged to provide a first wavelength-converted signal S A , ⁇ cw based on a first optical data signal
  • a first clock signal recovery unit 300a arranged to provide the first clock signal S CLK ⁇ CW based on said first wavelength-converted signal ⁇
  • Said clock recovery device 610 may further comprise:
  • a second modulation format converter 200b arranged to provide a second wavelength-converted signal S A2 , ⁇ cw based on a second optical data signal S 0 , ⁇ 2, and - a second clock signal recovery unit 300b arranged to provide a second clock signal S C ⁇ _ ⁇ 2, ⁇ cw based on said second wavelength- converted signal S A , ⁇ C w-
  • Said clock recovery device 610 may further comprise a common light source 700 or at least one input 701 to receive substantially continuous light B ⁇ C w-
  • the clock recovery device 610 may be arranged to distribute the substantially continuous light B ⁇ C w to the converters 200a, 200b and to the clock recovery units 300a, 300b.
  • Example 1 A method for recovering a first clock signal (S CLK ⁇ CW ) from a first optical data signal (S 0 , ⁇ i). said first optical data signal (S 0 , ⁇ i) being transmitted at an optical channel located at a first wavelength ( ⁇ i), said method comprising:
  • Example 2 The method of example 1 further comprising separating said first modulated light (S A , ⁇ C w) from transmitted components (S A , ⁇ 1 ) of said first optical data signal (S 0 , ⁇ i) which have passed through said at least one non-linear unit (SOA1 , SOA2),
  • Example 3 The method of example 1 or 2 wherein the wavelength ( ⁇ cw) of said continuous light (B ⁇ C w) is substantially different from said first wavelength ( ⁇ -i)
  • Example 4 The method of example 3 comprising separating said first modulated light (S A , ⁇ cw) from transmitted components (S A , ⁇ i) of said first optical data signal (S 0 , ⁇ i) by using a spectral filter (RR2).
  • Example 5 The method of example 2 or 3 wherein said modulating and said separating are performed by using an interferometric unit (200) comprising a splitter (21 ), a first non-linear unit (SOA1 ), a second non-linear unit (SOA2), and a combiner (22), said splitter (21 ) having a first splitter input, a second splitter input, a first splitter output, and a second splitter output, said combiner (22) having a first combiner input, a second combiner input, a first combiner output (01 ), and a second combiner output (02), said method comprising:
  • Example 6 A method for recovering a first clock signal (S CLK ⁇ CW ) from a first optical data signal (S 0 , ⁇ i). said first optical data signal (S 0 , ⁇ i) being transmitted at an optical channel located at a first wavelength ( ⁇ -i), said method comprising:
  • Example 7 The method according to any of the preceding examples 1 to 6 comprising recovering a second clock signal from a second optical data signal (S 0 , ⁇ 2) by
  • Example 8 A method for re-shaping an optical data signal (S 0 , ⁇ i. S B , ⁇ - ⁇ ) by using a Mach-Zehnder interferometer (500), said method comprising:
  • Example 9 The method of example 8 comprising reducing phase noise and/or amplitude noise of said degenerated optical data signal (S 0 , ⁇ i. S B ⁇ - I ) prior to coupling said optical data signal (S 0 , ⁇ i. S B , ⁇ - ⁇ ) to said interferometer by using an optical finite impulse response filter (FIR) or an optical infinite impulse response filter (NR) or a combination of an optical finite impulse response filter (FIR) and an optical infinite impulse response filter (MR).
  • FIR optical finite impulse response filter
  • NR optical infinite impulse response filter
  • MR optical infinite impulse response filter
  • Example 10 The method of example 8 or 9 comprising recovering the clock signal associated with said optical data signal (S 0 , ⁇ i) by using at least one passive optical resonator (OR1 , OR2), wherein said first clock signal (CLK1 ) and said second clock signal (CLK2) are provided by delaying a recovered clock signal (S CLK ⁇ CW ) obtained from said at least one passive optical resonator (OR1 , OR2) without further equalization of the intensity of the recovered clock pulses,
  • Example 11 A method for re-shaping an optical data signal (S 0 , ⁇ i. S B ⁇ I ) by usin 9 a Sagnac interferometer (501 ), said method comprising:
  • Example 12 The method of example 11 comprising reducing phase noise and/or amplitude noise of said degenerated optical data signal
  • FIR optical finite impulse response filter
  • NR optical infinite impulse response filter
  • I I R optical infinite impulse response filter
  • Example 13 A device adapted to carry out a method according to any of the examples 1 to 12.
  • Example 14 An optical communications system comprising a device according to example 13.
  • a device according to the above-mentioned example 13 there is provided a system according to the above-mentioned example 13.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Optics & Photonics (AREA)
  • Electromagnetism (AREA)
  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
  • Optical Communication System (AREA)

Abstract

L'invention concerne un procédé de régénération d'un premier signal de données optique (S0,λ1, SA,ë1) transmis par un canal optique situé à une première longueur d'onde (λ-1). Le procédé consiste à récupérer un premier signal d'horloge (SCLK,λCW) à partir du premier signal de données optique (S0,λ1, SA,λ1); à fournir un signal de données à faible bruit (SB,λ1) en utilisant le premier signal de données optique (S0,λ1, SA,λ1); et là remettre en forme le signal de données à faible bruit (SB,λ1) en utilisant le premier signal d'horloge récupéré (SCLK,ëCW).
PCT/FI2008/050497 2007-09-05 2008-09-05 Régénération de signaux de données optiques indépendante d'un protocole Ceased WO2009030822A1 (fr)

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US93588807P 2007-09-05 2007-09-05
US60/935,888 2007-09-05

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WO2009030822A1 true WO2009030822A1 (fr) 2009-03-12

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Citations (7)

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Publication number Priority date Publication date Assignee Title
US5754325A (en) * 1995-03-31 1998-05-19 Nec Corporation Optical regenerating circuit
US5764396A (en) * 1995-03-24 1998-06-09 Nec Corporation Optical regenerative circuit
WO2002029981A2 (fr) * 2000-10-06 2002-04-11 Alphion Corporation Circuit tout-optique insensible au debit binaire et au format pour la remise en forme, la regeneration et le reajustement du rythme de trains d'impulsions optiques
US20020080453A1 (en) * 2000-12-22 2002-06-27 Juerg Leuthold 3R optical signal regeneration
US20050053377A1 (en) * 2003-09-04 2005-03-10 Yoo Sung-Joo Ben Reconfigurable multi-channel all-optical regenerators
US20050078350A1 (en) * 2002-12-10 2005-04-14 The Trustees Of Princeton University All-optical, 3R regeneration using the sagnac and mach-zehnder versions of the terahertz optical asymmetric demultiplexer (TOAD)
WO2007096455A1 (fr) * 2006-02-24 2007-08-30 Luxdyne Oy Dispositif de récupération d'horloge tout optique compact

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US5764396A (en) * 1995-03-24 1998-06-09 Nec Corporation Optical regenerative circuit
US5754325A (en) * 1995-03-31 1998-05-19 Nec Corporation Optical regenerating circuit
WO2002029981A2 (fr) * 2000-10-06 2002-04-11 Alphion Corporation Circuit tout-optique insensible au debit binaire et au format pour la remise en forme, la regeneration et le reajustement du rythme de trains d'impulsions optiques
US20020080453A1 (en) * 2000-12-22 2002-06-27 Juerg Leuthold 3R optical signal regeneration
US20050078350A1 (en) * 2002-12-10 2005-04-14 The Trustees Of Princeton University All-optical, 3R regeneration using the sagnac and mach-zehnder versions of the terahertz optical asymmetric demultiplexer (TOAD)
US20050053377A1 (en) * 2003-09-04 2005-03-10 Yoo Sung-Joo Ben Reconfigurable multi-channel all-optical regenerators
WO2007096455A1 (fr) * 2006-02-24 2007-08-30 Luxdyne Oy Dispositif de récupération d'horloge tout optique compact

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ZHU, Z. ET AL.: "High-performance optical 3R regeneration for scalable fiber transmission system applications", JOURNAL OF LIGHTWAVE TECHNOLOGY, vol. 25, no. 2, February 2007 (2007-02-01), pages 504 - 511, XP011176268 *

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