WO2009030822A1 - Protocol-independent regeneration of optical data signals - Google Patents

Abstract

A method for regenerating a first optical data signal (S0,λ1, SA,λ1) transmitted at an optical channel located at a first wavelength (λ-1) comprises: - recovering a first clock signal (SCLK,λCW) from said first optical data signal (S0,λ1, SA,λ1). - providing a low-noise data signal (SB,λ1) by using said first optical data signal (S0,λ1, SA,λ1), and - re-shaping said low-noise data signal (SB,λ1) by using said first recovered clock signal (SCLK,λCW).

Description

PROTOCOL-INDEPENDENT REGENERATION OF OPTICAL DATA SIGNALS

The present invention relates to signal processing in optical communication systems.

BACKGROUND

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.

An all-optical data recovery device has been described for example in an article "Compact all-optical packet clock and data recovery circuit using generic integrated MZI switches", by P.Bakopoulos, D.Tsiokos, O.Zouraraki, H.Avramopoulos, G.Maxwell and A.Poustie, in Optics Express Vol. 13, No. 17, 22 August 2005.

Recovery of a clock signal may be needed for the regeneration of the data signal. 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.

In order to effectively utilize an optical path, e.g. an optical fiber, 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. SUMMARY

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.

According to a first aspect of the invention, there is provided a method for regenerating an optical data signal according to claim 1.

According to a second aspect of the invention, there is provided a device for regenerating a data signal according to claim 21.

According to a third aspect of the invention, there is provided an optical communications system according to claim 39.

In an embodiment, 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.

In an embodiment, 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.

Consequently, 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.

In an embodiment, 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.

In an embodiment, 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.

In an embodiment, there is not need for further equalization of the intensity of the clock pulses, because the optical resonators of the clock recovery unit may have a long time constant. This, in turn, is possible because 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.

In an embodiment, 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 μm. In particular, 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 adjusted to operate at any data rate in the range of 1 Gb/s to 10 Gb/s, in the range of 10 Gb/s to 40 Gb/s, in the range of 40 Gb/s to 100 Gb/s and/or in the range of 100 Gb/s to 160 Gb/s (Gb/s = Gigabits per second).

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.

The embodiments of the invention and their benefits will become more apparent to a person skilled in the art through the description and examples given herein below, and also through the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following examples, the embodiments of the invention are described in more detail with reference to the appended drawings, in which

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, and Fig. 18b shows a clock recovery device for recovering clock frequencies from two optical data signals.

DETAILED DESCRIPTION

Optical communications system

Referring to Fig. 1 , an optical communication system 900 may comprise an optical transmitter 910, an optical path 930, and an optical receiver 920. An optical signal S₀ 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 S₀,λi. S₀,λ2, S₀,λ3. S₀,λ4. sent at different optical channels at different wavelengths λ-i, λ₂, λ₃, λ₄. A first signal S₀,λi may comprise spectral components in the vicinity of the wavelength λi, and the second signal S₀,λ₂ may comprise spectral components in the vicinity of the wavelength X₂.

The data signals S₀,_λi. S₀,_λ2, S₀,_λ3. S₀,_λ4 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,λi 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. The lower curve of Fig. 2a shows a clock signal S_Cι_κ,λi associated with the data signal S_0,λi- The temporal separation between the rising edges of adjacent data pulses is equal to an integer multiple of the clock period T_Cι_κ- The inverse of the clock period T_Cι_κ is the clock frequency vcLK associated with the data signal S₀,_λi- The data signals S₀,_λi. S₀,_λ2, So,λ3. S₀,λ₄ may have the same or different clock frequencies.

The quality of the optical data signals degrade when they are sent via the optical path 930. Fig. 2b shows, by way of example, a degraded data signal.

Signal regenerating device

Referring to Fig. 3, the optical data signals S₀,λi. S₀,λ2_> S₀,λ3. S₀,λ4 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

700. In other words, 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 S₀,_λi. S₀,_λ2, S₀,_λ3. S₀,_λ4-

Each data signal is processed at its wavelength by a separate regenerating unit 600a, 600b, 600c, 60Od to provide regenerated data signals So,REG,λi > SO,REGΛ2> SO,REGΛ3> SO,REGΛ4-

The signal regenerating device 800 may also comprise a spectral demultiplexer DEMUX to spectrally separate the different data signals

S₀,λi. ^so,λ2. S₀,_λ3, S₀,_λ4 from the optical signal S₀. The signal regenerating device 800 may also comprise a spectral multiplexer MUX to combine regenerated data signals SO.REGΛL S₀,_REG,λ2. ^SO,REGΛ3.

S_O,REGΛ₄ in order to provide a single regenerated multi-wavelength optical signal S₀,_REG- Referring to Fig. 4, 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_λCw at a wavelength λ_Cw- The continuous light B_λCw and a data signal S₀,λi are coupled to the modulation format converter 200.

The modulation format converter 200 provides two output signals: a transmitted signal S_A,λi and a converted signal S_A,λcw- The converted signal S_A,λcw is substantially at the wavelength λ_Cw of the continuous light B_λCw and its intensity is modulated by the intensity of the data signal S₀,_λi.

The transmitted signal S_A,λi is substantially at the same wavelength, i.e. at the wavelength λ1 as the data signal S₀,λi- The transmitted signal S_A,λi has substantially the same modulation as the data signal S₀,_λi-

The converted signal S_A,λCw corresponding to a phase-modulated RZ data signal (e.g. DPSK-RZ) exhibits a substantially continuous beat at the clock frequency.

On the other hand, e.g. a RZ-modulated or CS-RZ-modulated data signal S₀,λi may have several consecutive data pulses of low intensity. Thus, the converted signal S_A,λCw corresponding to intensity-modulated RZ data signal may also have long periods of low intensity when the data signal S_0,λi carries several consecutive low-intensity data bits.

The converted signal S_A,_λCw is coupled to a clock recovery unit 300. Regardless of the modulation format of the data signal S_0,λi. the clock recovery unit 300 provides a substantially continuous clock signal S_CLKΛ_CW even when the data signal S₀,λi is at the zero level during several clock cycles. The continuous clock signal S_CLKΛ_CW is substantially at the wavelength of the continuous light B_λCw- The clock signal S_CLKΛ_CW may be at a different wavelength than the transmitted signal S_A,λi- This facilitates the later separation of the clock signal S_CLKΛ_CW from the regenerated data signal.

When the same light source 700 is used for recovering clock signals SC_LKΛCW from several optical data signals S₀,λi. S₀,λ2 (see Fig. 3), the recovered clock signals corresponding to said optical data signals S₀,λi. S₀,λ₂ may be at the same spectral position, i.e. substantially in the vicinity of λ_Cw- In particular, a first recovered clock signal S_CLKΛ_CW corresponding to the first optical data signal S_0,λi and a second recovered clock signal S_CLKΛ_CW corresponding to a second optical data signal S₀,λ₂ may be at the same spectral position.

The original data signal S₀,λi may comprise phase noise which may be reduced in the phase recovery unit 400. The transmitted signal S_A,λi is coupled into the phase recovery unit 400, which provides a filtered signal S_B,λi- The phase recovery unit 400 may also reduce amplitude noise, i.e. random variations in the intensity.

The filtered signal S_B,λi and the clock signal SC_LKΛCW are coupled into the reshaping unit 500 which restores the shape of the data pulses using the clock signal S_CLKΛ_CW to restore precise timing of the data pulses. The reshaping unit 500 provides a regenerated data signal S_O._REGΛ_I which is substantially at the wavelength λ1 , i.e. at the wavelength of the original data signal S₀,λi-

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 S₀,_REGΛ_L Tne signal regeneration unit 600 may comprise one or more further optical amplifiers to provide amplification of the regenerated data signals SO._REGΛ_I and/or to provide amplification of the data signal S₀,λi prior to coupling it into the modulation format converter 200.

The data signal S₀,λi and/or the continuous light B_λCw may have an arbitrary state of polarization, because the operation of the units 200, 300, 400, 500 is, in principle, polarization-independent. In particular, the data signal S₀,_λi and/or the continuous light B_λCw may be horizontally or vertically polarized.

However, 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 S₀,REG,λ2. S₀,REG,λ3. and S₀,REG,λ4 which are at different wavelengths λ2, λ3, λ4.

Thus, 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_λCw 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,λi 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. However, 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 π. (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 S₀,λi and the continuous light B_λcw- The signal propagating through the second non-linear unit SOA2 is also substantially the sum of the data signal S₀,λi and the continuous light B_λCw- Each unit SOA1 , SOA2 exhibits nonlinearity at both wavelengths λ_Cw and λ-i. Thus, the intensity of the signal component propagating at the wavelength λi is capable of modulating the intensity of the signal component propagating at the wavelength λ_Cw- Consequently, both non-linear units SOA1 , SOA2 provide modulated light S_A,λcw which is modulated by the intensity of said optical data signal S_Olλi.

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 λ1 is coupled out of the first output 01 , and a majority of light having the second wavelength λ_Cw is coupled out of the second output 02 of the combiner 22.

Ideally, the second output of the combiner 22 provides a modulation format converted signal S_A,λcw substantially only at the wavelength λ_Cw_> and the first output 01 of the combiner 22 provides a transmitted signal S_A,λi substantially only at the wavelength λ-i. Thus, the modulation format converter 200 may separate the signals even without spectral filters.

However, as the wavelengths λi and λ_Cw may be different, the converted signal S_A,λcw and the transmitted signal S_A,_λCi 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,λi and the continuous light B_λCw may take place in the semiconductor optical amplifier SOA1 ,

SOA2 by cross gain modulation (i.e. by a process involving electrons in the semiconductor) and/or by an intensity-induced change in the refractive index in the semiconductor optical amplifier. Consequently, the modulation format converter using semiconductor optical amplifiers may substantially omit phase information. In other words, the wavelength-converted signal S_A,λcw does not comprise the phase information of the original data signal S₀,λi-

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,λi is modulated according to such a modulation format that the spectral decomposition of the data signal S_0,λi does not have sideband components.

For example, 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,λcw 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 S₀,λi-

The modulation format converter may provide the modulation format converted signal S_A,λcw without substantially reducing the optical power of the in-coupled data signal. Thus, the power of the data signal may used in an optimum way.

The transmitted signal S_A,λi which has passed through the non-linear units SOA1 , SOA2 has substantially the same phase information as the original data signal S₀,λi-

The non-linear units SOA1 , SOA2 may amplify the intensity of the pulses of the data signal S_A,λi and/or equalize variations in the intensity of the pulses of the data signal S_A,λi- Nevertheless, when the original data signal S_0,λi has an intensity pulse corresponding to a digital bit "1 ", then the corresponding transmitted signal S_A,λi has also an intensity pulse corresponding to a digital bit "1 ". When the original data signal S_0,λi has a low intensity corresponding to a digital bit "0", then the corresponding transmitted signal S_A,λi has also a low intensity corresponding to the digital bit "0". In that sense the transmitted signal S_A>λi which has passed through the non-linear units SOA1 , SOA2 may be considered to carry the same intensity information as the original data signal S₀,_λi-

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 S₀,λi-

Referring to Fig. 5b, 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.

Clock signal recovery unit

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,_λCw 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_CLκ,λcw- The optical resonators OR1 , OR2 may be ring resonators. The converted signal S_A,λcw 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. Thus, 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,λcw which does not pass through the resonators OR1 , OR2, may coupled to a beam dump (not shown). Alternatively, 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,λi and/or regarding the quality of the continuous light B_λCw_> 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. E.g. the light guide WG2 may be used as the combiner, which is arranged to provide a recovered clock signal

S_CLKΛ_CW by combining the output of the resonator OR1 with substantially continuous light B_λCw- The substantially continuous light

Bcw may be provided e.g. by the light source 700. Also 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_λCw 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,λi having a spectral component at a reference frequency V_REF, and spectral components at a sideband frequencies vsiDE and V-SiDE .wherein the spectral separation V_SIDE - V_REF is equal to the clock frequency v_Cι_κ associated with the data signal S₀,_λi- In this case, the reference frequency V_REF corresponds to the wavelength λ_Cw of the light source 700 (V_REF = c/λ_Cw in vacuum), v denotes optical frequency and dl/dv denotes spectral intensity.

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, and 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 S₀,λi-

Δ_SR,i and Δ_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).

Referring to Fig. 9, 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,λcw is at a low level. The first optical resonator OR1 provides and output signal S_REF, and 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_CLKΛ_CW .

INT denotes intensity, t denotes time, and ENV denotes the envelope curve of the beating clock signal S_CLKΛ_CW ■

In general, the clock signal S_CLKΛ_CW may be recovered by

- providing substantially continuous light B_λCw at a reference peak v_REF of the modulated light S_A,λcw.

- providing a substantially continuous sideband signal S_SIDE at a sideband peak V_SIDE of the modulated light S_A,λcw by storing energy of said modulated light in one or more optical resonators OR1 , OR2, the spectral separation (V_SIDE - v_REF) between said sideband peak (V_SIDE) and said reference peak (V_REF) being equal to the clock frequency (VCLK) of said optical data signal (S₀,λi). and

- providing said clock signal S_CLKΛ_CW as a combination of said sideband signal S_SIDE and said substantially continuous light B_λCw-

Referring back to Fig. 6b, the unit 300 shown in Fig. 6b is an example where the clock signal S_CLKΛ_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.

Referring back to Fig. 6a and Fig. 8, if the spectral separation Δ_SRJ between the passbands PB of the first resonator OR1 substantially matches with the clock frequency v_Cι_κ_> then 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 S₀,λi may require changing the optical length of the resonator OR1.

When a first optical data signal S₀,λi is sent at an optical channel located at a first wavelength λi, a method for recovering a first clock signal SC_LKΛCW from said first optical data signal S₀,λi may comprise:

- modulating continuous light B_λCw by the intensity of said first optical data signal S_0,λi 'ⁿ at least one non-linear unit SOA1 , SOA2 in order to provide first modulated light S_A,λcw at the wavelength λ_Cw of said continuous light B_λCw_>

- separating said first modulated light S_A,λcw from transmitted components S_A>λi of said first optical data signal S₀,λi which have passed through said at least one non-linear unit SOA1 , SOA2,

- coupling said first modulated light S_A,λCw to at least one optical resonator OR1 , OR2 such that a sideband peak V_SIDE of said first modulated light S_A,_λCw is matched with a first passband PB of an optical resonator and a reference peak V_REF of said first modulated light S_A,_λCw is matched with a second passband PB of an optical resonator, the spectral separation V_SIDE - V_REF between said passbands being substantially equal to the clock frequency v_Cι_κ of said first optical data signal S₀,λi. and - coupling said first clock signal S_CLKΛ_CW out of said at least one optical resonator OR1.

In particular, the separation of the modulation converted signal S_A,λcw 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_λCw is coupled to the first splitter input, and the first optical data signal S_0,λi 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. Thus, 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_λCw is split into a first part B1 which propagates via the first arm ARM1 of the interferometer 200, and into a second part

B2 which propagates via the second arm ARM2 of the interferometer

200. 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. Thus, the second combiner output is adapted to provide the modulation converted signal S_A,λcw- The first optical data signal S_0,λi 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. In particular, the difference between the optical path lengths of the first arm ARM1 and the second arm ARM2 may be e.g. zero or π. 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,λcw and/or the intensity of the transmitted signal S_A,λi-

Thus, a method for recovering a clock signal S_CLKΛ_CW from an optical data signal S₀,λi may comprise:

- coupling continuous light B_λCw to said first splitter input in order to distribute said continuous light B_λCw to said first non-linear unit SOA1 and to said second non-linear unit SOA2,

- coupling said first optical data signal S_0,λi to said second splitter input in order to distribute a first portion B3 of said optical data signal S_0,λi to said first non-linear unit SOA1 and to distribute a second portion B4 of said optical data signal S_0,λi to said second non-linear unit SOA2, - modulating said continuous light B_λCw by the first portion B3 of the first optical data signal S_0,λi in said first non-linear unit SOA1 to provide a first part B1 of modulated light,

- modulating said continuous light B_λCw by the second portion B4 of the first optical data signal S_0,λi in said second non-linear unit SOA2 to provide a second part B2 of modulated light,

- combining said first part B1 and said second part B2 by coupling said first part B1 to said first combiner input, and by coupling said second part B2 to said second combiner input such that the combination B1 +B2 of said first B1 and second B2 parts is coupled to said second combiner output 02, and such that the combination B3+B4 of said first portion B3 and said second portion B4 is coupled to said first combiner output 01 , and

- coupling the modulated light S_A,λcw into a clock recovery unit 300.

Referring to the known theory of Mach-Zehnder interferometers, 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 π 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 π at said second combiner output 02, thanks to the constructive and destructive interference at said outputs 01 , 02.

Alternatively, 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 π 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 π at said second combiner input 02.

When determining constructive and destructive interference, a phase difference φ + n x 2π is interpreted to mean the phase difference φ, wherein n is an integer. For example, a phase difference 3π leads to the same destructive interference as the phase difference π. The light source 700 may be selected such that the wavelength λ_Cw of the continuous light B_λCw does not overlap the wavelengths λ-i, λ₂, λ₃, λ₄ of the data signals. The wavelength λ_Cw is advantageously stable, and the bandwidth of the light provided at λ_Cw is advantageously less than 100 MHz. Thus, 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.

For example, the clock period T_Cι_κ corresponding to a data rate of 40 Gb/s (gigabits/second) is 25 ps. If the data signal S₀,_λi has 32 consecutive zero bits, the intensity of the data signal S₀,λi is at the low or zero value during 0.8 ns (32 x 25 ps = 800 ps). In order to limit the decrease in the intensity of the recovered clock signal S_CLKΛ_CW, 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. In other words, the time constant τ 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 Δ_SR,I (i.e. free spectral range) of such a resonator is 31 GHz, which is smaller than the data rate 40 GHz. However, if the separation range Δ_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 Δ_SR,I may be coupled optically in series to implement a substantially broader separation range.

Also other types of optical resonators may be used, 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_λCw is advantageously narrower than or equal to the bandwidth of the resonators OR1 , OR2.

In case of burst-switched data traffic, the optical resonators OR1 , OR2 of the clock recovery unit 300 may have a shorter time constant than in the above-mentioned case. In case of burst-switched data traffic, 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_λCw 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.

Clock signal recovery is also described in PCT/FI2006/050080, herein incorporated by reference. Phase recovery unit

The degraded data signal S₀,λi. and consequently also the transmitted signal S_A,λi comprise phase noise. The modulation format converter 200 may add more phase noise to the transmitted signal S_A,λi. in addition to the phase noise of the degraded data signal S₀,_λi- The phase noise and/or amplitude noise is reduced by using the phase recovery unit 400.

Referring to Fig. 10, the phase recovery unit 400 may comprise an optical finite impulse response (FIR) filter. The transmitted signal S_A,λi 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,λi- The phase recovery unit 400 provides a filtered signal S_B,λi which is substantially at the wavelength λ1.

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.

Referring to Fig. 11 , 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,λi- The third curve from the top shows the phase of said consecutive pulses, φ denotes phase. In this example, the transmitted signal S_A>λi 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,λi- The filtered pulses may be broader than the original pulses. In addition, the filtered pulses may be time- shifted with respect to the original pulses. However, 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,λi- The phase noise of the filtered signal S_B,λi ^maY ^be substantially reduced with respect to the phase noise of the transmitted signal S_A,λi-

Times t₃, U, t₅, t₆, t₇, t₈, t₉, t₁₀ are associated with the peaks of the pulses.

Referring to Fig. 12, the phase recovery unit 400 may also comprise one or more optical infinite impulse response (NR) filters, such as optical resonators OR4, ORδ.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.

Signal re-shaping unit

The signal reshaping unit 500 performs at least the retiming and reshaping of the pulses of the filtered signal S_B,λi-

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,λi 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_PAss,i- A second part is coupled to a second phase shifting unit SOA4 in order to provide a second passed signal S_PAss,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_CLKΛ_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,λi and the recovered clock signal S_CLKΛ_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_PAss,i, S_PAss,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_PAss,i, S_PAss,2- The passed signals S_PASs,i_> S_PASs,2 are coupled to the first port when the phase difference Δφ between S_PASs,i and S_PASs,2 is substantially equal to π. The passed signals S_PASs,i_> S_PASs,2 are coupled to the second port when the phase shift if substantially equal to zero. The phase of the passed signals S_PASs,i_> S_PASs,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,λi and/or the operation of the units 200, 300, 400, 500.

Switched out-coupling of the signals S_PASs,i, S_PASs,₂ from the first port provides two signals: a data signal S_Ouτ,λi which is substantially at the wavelength λi and a clock signal S_Ouτ_,λcw which is at the wavelength λcw-

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. For example, the delay may be e.g. 10 - 20 ps (picoseconds) in case of a 10 GHz data signal. In other words, the phase shifting units SOA3, SOA4 are adapted to cause a controllable phase difference Δφ between the first passed signal S_PAss,i and the second passed signal S_PAss,2 such that the combined passed signals S_PAss,i, S_PAss,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.

Thus an optical data signal S₀,λi. S_B,λi ^may be re-shaped and re-timed by using a Mach-Zehnder interferometer 500, said method comprising:

- splitting said optical data signal S₀,λi. S_B,λi into a first part and a second part,

- coupling said first part to pass through a first phase shift unit SOA3 to provide a first passed signal S_PASs,i, - coupling said second part to pass through a second phase shift unit SOA4 to provide a second passed signal S_PASs,₂,

- coupling a first delayed clock signal CLK1 to said first phase shift unit SOA3 to control the phase of said first passed signal S_PASs,i,

- coupling a second delayed clock signal CLK2 to said second phase shift unit SOA4 to control the phase of said second passed signal

S_PASS,2.

- combining said passed signals S_PASs,i, S_PASs,2, such that the combined passed signals S_PASs,i_> S_PASs,2 are directed to a predetermined output of said interferometer 500 only when only one of the first clock signal CLK1 and the second clock signal CLK2 is above a predetermined limit LIM3, LIM4,

- coupling a re-shaped data signal S_Ouτ,λi and a re-shaped clock signal SOU_TΛCW out of said output, and

- separating said re-shaped data signal S_Ouτ,λi f^rom said re-shaped clock signal SQU_TΛCW- 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. The saturation level of a semiconductor optical amplifier may be lowered simply by adjusting the operating current. Thus, 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_CLKΛ_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. Despite the further amplifier, 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.

In summary, potential advantages of the signal re-shaping unit 400 are:

- a non-amplified clock signal recovered by a passive optical resonator may be utilized, and

- only two semiconductor optical amplifiers are needed for retiming and reshaping.

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_Ouτ,λcw and S_Ouτ,λi propagate along the same path but they have different wavelengths λi and λ_Cw- The regenerated data signal S_Ouτ,λi ^maY ^be spectrally separated from the reshaped clock signal S_Ouτ,λicw e.g. by a spectral filter FIL1.

Alternatively, 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.

Consequently, the clock signals CLK1 , CLK2 propagate through the phase shift units SOA3, SOA4 in a different direction than the passed signals S_PAss,i, S_PAss,2- Thus, there is no need to separate the clock signals CLK1 , CLK2 from the output of the Mach-Zehnder interferometer. Coupling of the clock signals CLK1 , CLK2 to the phase recovery unit 400 may be prevented by using an optical isolator.

Referring to Fig. 14, the filter FIL1 may comprise a ring resonator OR3. The signals S_Ouτ,λcw and S_Ouτ,λi ^are coupled into a first end of a first light guide WG3. The ring resonator OR3 may be substantially matched with the wavelength λ_Cw of the reshaped clock signal S_OUTΛ_CW so that only the reshaped clock signal S_OUTΛ_CW is coupled via the ring resonator OR3 to a second light guide WG4. The reshaped clock signal S_OUTΛ_CW may be coupled into a beam dump (not shown) or to a monitor MON3 in order to analyze the data signal S_0,λi and/or the operation of the units 200, 300, 400, 500. The regenerated data signal S_Ouτ,λi ^may 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 (Fig. 3) may comprise a filter FIL1 to separate least one reshaped clock signal S_OUTΛ_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_OUTΛ_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,λi. 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, and 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,λi- 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,λi- 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. For example, 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 φ caused by the phase shifting unit SOA3 depends on the in-coupled intensity. The fourth curve from the top shows the phase shift φ1 caused by the first phase shifting unit SOA3. The fifth curve from the top shows the phase shift φ2 caused by the second phase shifting unit SOA4.

The sixth curve from the top shows the phase difference Δφ = φ1 - φ2 between a signal S_PAss_,i passed through the first phase shift unit SOA3 and a signal S_PAss_,2 passed through the second phase shift unit SOA4. The seventh curve from the top shows the intensity of the passed signals S_PAss,i and S_PAss,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,λi- Furthermore, the function describing the relationship between the clock signal level and the phase shift is continuous, i.e. switching from zero to π does not take place infinitely sharp.

Referring to the lowermost curve of Fig. 15, the passed signals S_PAss_,i and S_PAss_,2 are coupled to the first port of the combiner 52 (Fig. 13) only when the phase difference Δφ is substantially equal to π. Thus, the temporal position of the pulses of the signal provided by the first port at the wavelength λi is precisely controlled by the recovered clock signal

S_CLKΛ_CW and the delay line D4 shown in Fig. 13. The temporal duration of the regenerated pulses is controlled by temporal delay between the pulses CLK1 , CLK2, i.e. by the delay line D5.

Further embodiments

Fig. 16 shows an alternative embodiment of the modulation format converter 200. The data signal S₀,_λi and the continuous light B_λCw may be combined. The combining may be carried out e.g. by a ring resonator RR1. The continuous light B_λCw 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 S₀,_λi may be coupled to a first end of a second light guide WG6. The combined data signal S₀,λi and the continuous light B_λCw 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 λ_Cw and at the different wavelengths λ1. Thanks to the non-linearity, the variations in the intensity of the data signal S_0,λi modulate the passed signal provided at the wavelength λ_Cw- The passed signal provided at the wavelength λ_Cw is the modulation converted signal S_A,λcw- The passed signals at the different wavelengths λ1 and λ_Cw may be separated from each other by a spectral filter, which may comprise a second ring resonator RR2 and light guides WG7, WG8. Thus, a spectrally separated signal at the wavelength λ_Cw may be coupled into a clock recovery unit 300. Alternatively, the data signal S₀,λi may be coupled to the first light guide WG5 and the continuous light B_λCw may be coupled to the second light guide WG6, in order to combine them.

Alternatively, the data signal S₀,λi may be combined with the continuous light B_λCw by using a beam splitter as the combiner. In particular, a polarising beam splitter may be used to combine the data signal S₀,λi with the continuous light B_λCw if their polarization state is stable and known. The data signal S₀,λi and the continuous light B_λCw 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_CLKΛ_CW from the components at the wavelength λi may be carried out after the clock recovery unit 300. The passed signals provided by the non-linear unit SOA1 at the wavelength λ_Cw and at the wavelength λi may be coupled into a clock recovery unit 300 before separating them spectrally from each other.

If the spectral separation Δ_SR,I of the passbands PB of the ring resonator substantially matches with the clock frequency v_Cι_κ of the data signal S_0,λi. then the ring resonator RR2 may simultaneously act as a clock recovery unit which directly provides the recovered clock signal SC_LKΛCW and separates it from the transmitted signal S_A,λi- ^|n that case the unit 300 shown in Fig. 16 may be omitted.

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 S₀,λi-

In general, 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_CLKΛ_CW from disturbing components at other wavelengths.

Thus, when a first optical data signal S₀,λi is transmitted at an optical channel located at a first wavelength λi, a method for recovering a first clock signal S_CLKΛ_CW from said first optical data signal S₀,λi may comprise:

- modulating continuous light B_λCw by the intensity of said first optical data signal S_0,λi in at least one non-linear unit SOA1 , SOA2 in order to provide first modulated light S_A,λcw at the wavelength λ_Cw of said continuous light B_λCw_> wherein the wavelength λ_Cw of said continuous light is substantially different from said first wavelength λi

- coupling said first modulated light S_A,λcw to at least one optical resonator OR1 , OR2 such that a sideband peak V_SIDE of said first modulated light S_A,λcw is matched with a first passband PB of an optical resonator and a reference peak V_REF of said first modulated light S_A,λcw is matched with a second passband PB of an optical resonator, the spectral separation V_SIDE - V_REF between said passbands being substantially equal to the clock frequency v_Cι_κ of said first optical data signal S₀,λi.

- coupling said first clock signal S_CLKΛ_CW out of said at least one optical resonator OR1 , and

- separating first clock signal S_CLKΛ_CW from light is which substantially at said first wavelength λ-i.

The phase recovery unit 400 may also be implemented using four wave mixing. A pump beam and the data signal S₀,λi ^may 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,_λi-

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. 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.

Referring to Fig. 17, 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.

Thus, an optical data signal S₀,_λi. S_B,λi ^may ^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 S₀,λi. S_B,λi by a splitter-combiner 62 into a first part and a second part,

- coupling said first part to pass clockwise through a phase shift unit SOA3 to provide a first passed signal S_PAss,i,

- coupling said second part to pass counterclockwise through said phase shift unit SOA3 to provide a second passed signal S_PASs,₂,

- coupling a clock signal S_CLKΛ_CW into said phase shift unit SOA3 in order to modulate the phases of said passed signals S_PAss,i, S_PAss,2 by the intensity of said clock signal S_CLKΛ_CW,

- combining said passed signals S_PASs,i, S_PASs,₂ by the splitter-combiner 62 having two outputs, the first output providing the clock signal

SCLKΛCW and a re-shaped data signal S_Ouτ,λi> and

- separating the re-shaped data signal S_Ouτ,λi from the clock signal SCLKΛCW -

The clock signal S_CLKΛ_CW changes the phases of the passed signals S_PAss,i_> S_PAss,2 substantially simultaneously in the phase shift unit SOA3. However, as the optical distances from the phase shift unit SOA3 to the combiner 62 are different for the passed signals S_PAss,i, Sp_Ass,_2> the phase-shifted signal S_PAss,i arrives at the splitter-combiner 62 earlier or later than the phase-shifted signal S_PAss,₂- Thus, the Sagnac interferometer can act as an optical gate which couples the signals S_PAss,i and S_PAss,₂ to the first output during the short period when the S_PASs,i and S_PASs,₂ arriving at the combiner 62 are in different phases. During the remaining time, the signals S_PASs,i and S_PASs,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 signal

-

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_Ouτ,λi may be separated from the clock signal S_CLKΛ_CW e-9- by a spectral filter FIL1 (Fig. 14).

Alternatively, the clock signal S_CLKΛ_CW may be split into clockwise and counterclockwise propagating portions, and the data signal S₀,_λi. S_B,λi may be coupled into the phase shift unit SOA3 in order to chance the phase of the clockwise and counterclockwise propagating portions. In that case the re-shaped signal is at the wavelength λ_Cw of the clock signal SCLKΛCW-

Signal regeneration by using an asymmetric Sagnac loop has been described also in an article "Tunable all-optical signal regenerator with a semiconductor optical amplifier and a Sagnac loop: principles of operation", by E.Granot, R.Zaibel, N.Narkiss, S.Ben-Ezra, H.Chayet, N.Shahar, S.Stemklar, S.Tsadka, and P.Prucnal, in J.Opt.Soc.Am B, Vol. 22 No. 12 December 2005, pp. 2534-2541.

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.

Referring back to Fig. 1 , 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,λcw may comprise modulating the intensity, phase, polarization and/or wavelength of the continuous light B_λCw by the varying intensity of the data signal S₀,λi- For example, 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.

Small variations in 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 λ_Cw of the light source 700. Excessive variations the transmitter's emission wavelength may be handled by adjusting optical resonators.

Also other types of clock recovery devices than passive resonators may be used to recover the clock signal from the modulation converted signal S_A,λcw- In particular, the modulation converted signal S_A,λcw may be provided by the interferometric arrangement of Figs. 5a or 5b.

For example, the modulation converted signal S_A,λcw 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. in an article "Wavelength and Polarization Insensitive All-Optical Clock Recovery from 96 Gb/s Data by Using a Two-Section Gain-coupled DFB laser", by Y.Li, C.Kim, K.Li, Y.Kaneko, R.Jungerman, and O.Buccafusca, in IEEE Photonics Technology Letters, VoI 15 No. 4 April 2003, pp. 590-592.

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,λcw may also be coupled into a clock recovery unit 300 based on an actively mode-locked fiber laser or passively mode-locked semiconductor laser.

According to an aspect of the present invention, there is provided 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.

Referring to Fig. 18a, 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. Instead of the light source 700, the clock recovery device 610 may also comprise an input 701 to receive substantially continuous light B_λCw-

The device 610 for recovering a first clock signal S_CLKΛ_CW from a first optical data signal S₀,_λi may comprise:

- a modulation format converter 200 arranged to provide a first wavelength-converted signal S_A,λcw based on said first optical data signal S_0,λi_, and - a clock signal recovery unit 300 arranged to provide the first clock signal SC_LKΛCW based on said first wavelength-converted signal S_A,λcw- In particular, 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,_λCw, and to provide a substantially continuous sideband signal S_SIDE at a sideband peak V_SIDE of said first modulated light (S_A,λcw). wherein the clock recovery device is arranged to provide a first clock signal S_CLKΛ_CW as a combination of said sideband signal S_SIDE and substantially continuous light B_λCw ■ wherein the substantially continuous light B_λCw is at a reference peak V_REF of said first modulated light S_A,λcw- The substantially continuous light B_λCw may be provided by the same light source 700, which is arranged to provide continuous light B_λCw for the modulation format converter 200. The substantially continuous light B_λCw 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.

Referring to Fig. 18b, 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 S₀,_λi. S₀,_λ2- 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,λcw based on a first optical data signal

- a first clock signal recovery unit 300a arranged to provide the first clock signal S_CLKΛ_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,λcw based on a second optical data signal S₀,_λ2, and - a second clock signal recovery unit 300b arranged to provide a second clock signal S_Cι_κ_2,λcw based on said second wavelength- converted signal S_A,_λCw-

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_λCw- The clock recovery device 610 may be arranged to distribute the substantially continuous light B_λCw to the converters 200a, 200b and to the clock recovery units 300a, 300b.

Yet, further advantageous embodiments of the invention are defined by the following examples:

Example 1 : A method for recovering a first clock signal (S_CLKΛ_CW) from a first optical data signal (S₀,_λi). said first optical data signal (S₀,_λi) being transmitted at an optical channel located at a first wavelength (λi), said method comprising:

- modulating continuous light (B_λCw) by the intensity of said first optical data signal (S_0,λi) in at least one non-linear unit (SOA1 , SOA2) in order to provide first modulated light (S_A,λcw) at the wavelength (λ_Cw) of said continuous light (B_λCw).

- coupling said first modulated light (S_A,λcw) to at least one optical resonator (OR1 , OR2) such that a sideband peak (V_SIDE) of said first modulated light (S_A,λcw) is matched with a first passband (PB) of an optical resonator and a reference peak (V_REF) of said first modulated light (S_A,_λCw) is matched with a second passband (PB) of an optical resonator, the spectral separation (V_SIDE - V_REF) between said passbands being substantially equal to the clock frequency (v_Cι_κ) of said first optical data signal (S₀,_λi). and - coupling said first clock signal (S_CLKΛ_CW) out of one of said optical resonators (OR1 ) or forming said first clock signal (S_CLKΛ_CW) by combining outputs of two of said optical resonators (OR1 , OR2),

Example 2: The method of example 1 further comprising separating said first modulated light (S_A,_λCw) from transmitted components (S_A,_λ1) of said first optical data signal (S₀,λi) 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 (λcw) of said continuous light (B_λCw) is substantially different from said first wavelength (λ-i)

Example 4: The method of example 3 comprising separating said first modulated light (S_A,λcw) from transmitted components (S_A,λi) of said first optical data signal (S₀,λi) 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:

- coupling said continuous light (B_λCw) to said first splitter input in order to distribute said continuous light (B_λCw) to said first non-linear unit

(SOA1 ) and to said second non-linear unit (SOA2),

- coupling said first optical data signal (S₀,λi) to said second splitter input in order to distribute a first portion (B3) of said first optical data signal (S₀,λi) to said first non-linear unit (SOA1 ) and to distribute a second portion (B4) of said first optical data signal (S₀,_λi) to said second non-linear unit (SOA2),

- modulating said continuous light (B_λCw) by said first portion (B3) of the first optical data signal (S₀,λi) in said first non-linear unit (SOA1 ) to provide a first part (B1 ) of modulated light, - modulating said continuous light (B_λCw) by said second portion (B4) of the first optical data signal (S₀,λi) in said second non-linear unit (SOA2) to provide a second part (B2) of modulated light,

- combining said first part (B1 ) and said second part (B2) by coupling said first part (B1 ) to said first combiner input, and by coupling said second part (B2) to said second combiner input such that the combination (B1 +B2) of said first (B1 ) and second (B2) parts is coupled to said second combiner output (02), and such that the combination (B3+B4) of said first portion (B3) and said second portion (B4) is coupled to said first combiner output (01 ), and

- coupling the combination (B1 + B2) of said first and second parts out of said second combiner output in order to provide said first modulated light (S_A,λcw)-

Example 6: A method for recovering a first clock signal (S_CLKΛ_CW) from a first optical data signal (S₀,λi). said first optical data signal (S₀,λi) being transmitted at an optical channel located at a first wavelength (λ-i), said method comprising:

- modulating continuous light (B_λCw) by said first optical data signal (So_,λi) in at least one non-linear unit (SOA1 , SOA2) in order to provide first modulated light (S_A,λcw) at the wavelength (λ_Cw) of said continuous light (B_λCw)_> wherein the wavelength (λ_Cw) of said continuous light is substantially different from said first wavelength (λ-i),

- coupling said first modulated light (S_A,λcw) to one or more optical resonators (OR1 , OR2) such that a sideband peak (V_SIDE) of said first modulated light (S_A,λcw) is matched with a first passband (PB) of an optical resonator and a reference peak (V_REF) of said first modulated light (S_A,λcw) is matched with a second passband (PB) of an optical resonator, the spectral separation (V_SIDE - V_REF) between said passbands being substantially equal to the clock frequency (v_Cι_κ) of said first optical data signal (S₀,_λi).

- coupling said first clock signal (S_CLKΛ_CW) out of one of said optical resonators (OR1 ) or forming said first clock signal (S_CLKΛ_CW) by combining outputs of two of said optical resonators (OR1 , OR2), and

- separating first clock signal (S_CLKΛ_CW) from light is which substantially at said first wavelength (λ-i).

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 (S₀,λ2) by

- modulating continuous light (B_λCw) by said second optical data signal (S_0Λ2) in at least one non-linear unit (SOA1 , SOA2) in order to provide second modulated light (S_A,_λCw) at the wavelength (λ_Cw) of said continuous light (B_λCw)_> wherein said second optical data signal (S₀,λ₂) is transmitted at a second optical channel located at a second wavelength (λ₂), said second wavelength (λ₂) being substantially different from said first wavelength (λ-i) and substantially different from said wavelength (λ_Cw) of the continuous light (B_λCw).

- separating said second modulated light (S_A,λcw) from transmitted components (S_A,λ2) of said second optical data signal (S₀,_λi).

- coupling said second modulated light (S_A,λcw) to at least one further optical resonator (OR1 , OR2) such that a sideband peak (V_SIDE) of said modulated light (S_A,λcw) is matched with a first passband (PB) of an optical resonator and a reference peak (V_REF) of said modulated light (S_A,_λCw) is matched with a second passband (PB) of an optical resonator, the spectral separation (V_SIDE - V_REF) between said passbands (PB) being substantially equal to the clock frequency of said second optical data signal (S₀,_λi). and

- coupling said first clock signal (S_CLKΛ_CW) out of one of said optical resonators (OR1 ) or forming said first clock signal (S_CLKΛ_CW) by combining outputs of two of said optical resonators (OR1 , OR2).

Example 8: A method for re-shaping an optical data signal (S₀,λi. S_B,λ-ι) by using a Mach-Zehnder interferometer (500), said method comprising:

- splitting said optical data signal (S₀,_λi. S_B,λi) ^int° ^{a first} P^{art and a} second part,

- coupling said first part to pass through a first phase shift unit (SOA3) to provide a first passed signal (S_PASs,i)_> - coupling said second part to pass through a second phase shift unit (SOA4) to provide a second passed signal (S_PASs,₂)_>

- coupling a first clock signal (CLK1 ) to said first phase shift unit (SOA3) to control the phase of said first passed signal (S_PASs,i)_>

- coupling a second delayed clock signal (CLK2) to said second phase shift unit (SOA4) to control the phase of said second passed signal

(Sp_ASS,2).

- combining said passed signals (S_PAss,i_> S_PASs,2)_> such that the combined passed signals (S_PASs,i, S_PASs,2) ^are directed to a predetermined output of said interferometer (500) only when only one of the first clock signal (CLK1 ) and the second clock signal (CLK2) is above a predetermined limit (LIM3, LIM4), - coupling a re-shaped data signal (S_Ouτ,λi) and a re-shaped clock signal (SOU_TΛCW) out of said output, and

- separating said re-shaped data signal (S_Ouτ,λi) from said re-shaped clock signal (SOUTΛCW)-

Example 9: The method of example 8 comprising reducing phase noise and/or amplitude noise of said degenerated optical data signal (S₀,_λi. S_BΛ-_I) prior to coupling said optical data signal (S₀,λi. S_B,λ-ι) 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).

Example 10: The method of example 8 or 9 comprising recovering the clock signal associated with said optical data signal (S₀,_λi) 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_CLKΛ_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 (S₀,λi. S_BΛ_I) by ^usin9 ^a Sagnac interferometer (501 ), said method comprising:

- splitting said optical data signal (S₀,λi. S_B,λ-ι) by a splitter-combiner (62) into a first part and a second part,

- coupling said first part to pass clockwise through a phase shift unit (SOA3) to provide a first passed signal (S_PAss,i), said phase shift unit (SOA3) being located asymmetrically in the optical loop of said Sagnac interferometer (501 ), - coupling said second part to pass counterclockwise through said phase shift unit (SOA3) to provide a second passed signal (S_PAss,₂),

- coupling a clock signal (S_CLKΛ_CW) into said phase shift unit (SOA3) in order to modulate the phases of said passed signals (S_PAss,i, S_PAss,2) by the intensity of said clock signal (S_CLKΛ_CW), - combining said passed signals (S_PASs,i, S_PASs,2) by the splitter- combiner (62) having two outputs, the first output providing the clock signal (SCLKΛCW) and a re-shaped data signal (S_Ouτ,λi)> and - separating the re-shaped data signal (S_Ouτ,λi) from the clock signal

(SCLKΛCW)-

Example 12: The method of example 11 comprising reducing phase noise and/or amplitude noise of said degenerated optical data signal

(So,λ-ι. S_B,λi) P^r'or to coupling said optical data signal (S₀,λi. S_B,λ-ι) to said interferometer (501 ) 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 (I I R).

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.

According to a further aspect of the invention, there is provided a method for re-shaping and re-timing an optical data signal according to according to the above-mentioned example 8. According to a further aspect of the invention, there is provided a method for re-shaping and re-timing an optical data signal according to the above-mentioned example 11. According to a further aspect of the invention, there is provided a device according to the above-mentioned example 13. According to a further aspect of the invention, there is provided a system according to the above-mentioned example 13.

For a person skilled in the art, it will be clear that modifications and variations of the devices, the methods, and the systems according to the present invention are perceivable. The particular embodiments described above with reference to the accompanying drawings are illustrative only and not meant to limit the scope of the invention, which is defined by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method for regenerating a first optical data signal (S₀,_λi. S_A,λi) transmitted at an optical channel located at a first wavelength (λ-i), said method comprising:

- recovering a first clock signal (S_CLKΛ_CW) from said first optical data signal (S₀,_λi. S_A,_λi).

- providing a low-noise data signal (S_B,λi) by using said first optical data signal (S₀,_λi, S_A,_λ1), and - re-shaping said low-noise data signal (S_B,λi) by using said first recovered clock signal (SC_LKΛCW)-

2. The method of claim 1 wherein the spectral position of said first recovered clock signal (S_CLKΛ_CW) deviates from the spectral position of said first optical data signal (S₀,λi. S_Aiλi).

3. The method of claim 1 or 2 wherein said first clock signal (S_CLKΛ_CW) is recovered by using one or more optical resonators (OR1 , OR2).

4. The method according to any of the preceding claims 1 to 3 comprising modulating continuous light (B_λCw) by the intensity of said first optical data signal (S₀,λi) 'ⁿ at least one non-linear unit (SOA1 , SOA2) in order to provide first modulated light (S_A>λCw) which has a spectral component at the wavelength (λ_Cw) of said continuous light (B_λCw)-

5. The method of claim 4 comprising separating said first modulated light (S_AΛ_CW) from transmitted components (S_AΛ_I) of said first optical data signal (S₀,λi) which have passed through said at least one non- linear unit (SOA1 , SOA2).

6. The method of claim 4 or 5 comprising equalizing variations in the intensity of the pulses of the first optical data signal (S₀,λi. S_Aiλi) by using said at least one non-linear unit (SOA1 , SOA2).

7. The method according to any of the preceding claims 4 to 6 comprising separating said first modulated light (S_AΛ_CW) from transmitted components (S_A,λi) of said first optical data signal (S₀,_λi) by using a spectral filter (RR2).

8. The method according to any of the preceding claims 4 to 6 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:

- coupling said continuous light (B_λCw) to said first splitter input in order to distribute said continuous light (B_λCw) to said first non-linear unit (SOA1 ) and to said second non-linear unit (SOA2),

- coupling said first optical data signal (S₀,λi) to said second splitter input in order to distribute a first portion (B3) of said first optical data signal (S₀,λi) to said first non-linear unit (SOA1 ) and to distribute a second portion (B4) of said first optical data signal (S₀,_λi) to said second non-linear unit (SOA2),

- modulating said continuous light (B_λCw) by said first portion (B3) of the first optical data signal (S₀,λi) in said first non-linear unit (SOA1 ) to provide a first part (B1 ) of modulated light,

- modulating said continuous light (B_λCw) by said second portion (B4) of the first optical data signal (S₀,λi) in said second non-linear unit (SOA2) to provide a second part (B2) of modulated light,

- combining said first part (B1 ) and said second part (B2) by coupling said first part (B1 ) to said first combiner input, and by coupling said second part (B2) to said second combiner input such that the combination (B1 +B2) of said first (B1 ) and second (B2) parts is coupled to said second combiner output (02), and such that the combination (B3+B4) of said first portion (B3) and said second portion (B4) is coupled to said first combiner output (01 ), and

- coupling the combination (B1 + B2) of said first and second parts out of said second combiner output in order to provide said first modulated light (S_A,λcw)-

9. The method according to any of the preceding claims 4 to 8 comprising:

- providing a substantially continuous sideband signal (S_SIDE) at a sideband peak (V_SIDE) of said first modulated light (S_A,λcw) and a substantially continuous reference signal (S_REF) at a reference peak (V_REF) of said first modulated light (S_A,λcw) by storing energy of said modulated light in one or more optical resonators (OR1 , OR2), the spectral separation (V_SIDE - V_REF) between said sideband peak (V_SIDE) and said reference peak (V_REF) being equal to the clock frequency (VCLK) of said optical data signal (S₀,λi). and

- providing a first clock signal (S_CLKΛ_CW) as a combination of said sideband signal (SS_IDE) and said reference signal (S_REF).

10. The method according to any of the preceding claims 4 to 8 comprising:

- providing substantially continuous light (B_λCw) at a reference peak (VREF) of said first modulated light (S_A,λcw).

- providing a substantially continuous sideband signal (S_SIDE) at a sideband peak (V_SIDE) of said first modulated light (S_A,λcw) by storing energy of said modulated light in one or more optical resonators (OR1 , OR2), the spectral separation (V_SIDE - V_REF) between said sideband peak (V_SIDE) and said reference peak (V_REF) being equal to the clock frequency (v_Cι_κ) of said optical data signal (S₀,_λi). and

- providing a first clock signal (S_CLKΛ_CW) as a combination of said sideband signal (SS_IDE) and said substantially continuous light (B_λCw)-

11. The method according to any of the preceding claims 1 to 10 wherein providing of said low-noise data signal (S_B,λi) comprises:

- distributing said first optical data signal (S₀,_λi. S_A,_λ1) to one or more delay lines (D1 ) to form at least one delayed signal, and

- providing a combination of said at least one delayed signal and said first optical data signal (S₀,λi. S_A,λ-ι)-

12. The method according to any of the preceding claims 1 to 11 wherein providing of said low-noise data signal (S_B,λi) comprises filtering said first optical data signal (S₀,λi. S_A>λi) by an optical finite impulse response filter (FIR), or an optical infinite impulse response filter (MR), or a combination of an optical finite impulse response filter (FIR) and an optical infinite impulse response filter (NR).

13. The method according to any of the preceding claims 1 to 12 comprising re-shaping said low noise data signal (S_B,λi) ^bv using ^a

Mach-Zehnder interferometer (500) or a Sagnac interferometer (501 ).

14. The method of claim 13 wherein said re-shaping by using the Mach-Zehnder interferometer (500) comprises: - splitting said optical data signal (S₀,λi. S_B,λ-ι) into a first part and a second part,

- coupling said first part to pass through a first phase shift unit (SOA3) to provide a first passed signal (S_PAss,i),

- coupling said second part to pass through a second phase shift unit (SOA4) to provide a second passed signal (S_PAss,₂),

- coupling a first clock signal (CLK1 ) to said first phase shift unit (SOA3) to control the phase of said first passed signal (S_PAss,i),

- coupling a second delayed clock signal (CLK2) to said second phase shift unit (SOA4) to control the phase of said second passed signal (SpASS,2).

- combining said passed signals (S_PAss,i, S_PASs,2), such that the combined passed signals (S_PASs,i_> S_PASs,2) are directed to a predetermined output of said interferometer (500) only when only one of the first clock signal (CLK1 ) and the second clock signal (CLK2) is above a predetermined limit (LIM3, LIM4), and

- coupling a re-shaped data signal (S_Ouτ,λi) ^0IJt °f said output.

15. The method of claim 13 wherein said re-shaping by using the Sagnac interferometer (501 ) comprises: - splitting said optical data signal (S₀,λi. S_B,λ-ι) by a splitter-combiner (62) into a first part and a second part,

- coupling said first part to pass clockwise through a phase shift unit (SOA3) to provide a first passed signal (S_PASs,i), said phase shift unit (SOA3) being located asymmetrically in the optical loop of said Sagnac interferometer (501 ),

- coupling said second part to pass counterclockwise through said phase shift unit (SOA3) to provide a second passed signal (S_PASs,₂), - coupling a clock signal (S_CLKΛ_CW) into said phase shift unit (SOA3) in order to modulate the phases of said passed signals (S_PAss,i_> S_PAss,2) by the intensity of said clock signal (S_CLKΛ_CW), and

- combining said passed signals (S_PAss,i_> S_PAss,2) by the splitter- combiner (62) having two outputs, the first output providing a reshaped data signal (S_Ouτ,λi)-

16. The method according to any of the preceding claims 1 to 15 comprising recovering a second clock signal from a second optical data signal (So,λ₂)_> wherein the spectral position of said second recovered clock signal deviates from the spectral position of said second optical data signal (S₀^)-

17. The method of claim 16 wherein said first clock signal (S_CLKΛ_CW) and said second clock signal are at the same spectral position (λ_Cw)-

18. The method according to any of the preceding claims 1 to 17 comprising combining two or more regenerated data signals (S₀,_REGΛ_{I >} S_0,REGΛ₂) by a multiplexer (MUX), and separating said first recovered clock signal (SOU_TΛCW) from the regenerated data signals (S₀,_REGΛ_{I >} S_O,REGΛ₂) simultaneously with said combining or after said combining.

19. A device (600) adapted to carry out a method according to any of the preceding claims 1 to 18.

20. An optical communications system (900) comprising a device (600) according to claim 19.

21. A device (600) for regenerating a first optical data signal (S₀,λi. S_A,λi) transmitted at an optical channel located at a first wavelength

(λi), said device (600) comprising:

- a clock signal recovery unit (300) for recovering a first clock signal (SCLKΛCW) from said first optical data signal (S₀,_λi. S_AΛi),

- a phase recovery unit (400) for providing a low-noise data signal (S_B,λi) by using said first optical data signal (S₀,λi, S_A,_λ1), and

- a signal re-shaping unit (500) for re-shaping said low-noise data signal (S_B,λi) by using said first recovered clock signal (SC_LKΛCW)-

22. The device (600) of claim 21 wherein the spectral position of said first recovered clock signal (S_CLKΛ_CW) is arranged to deviate from the spectral position of said first optical data signal (S₀,λi. S_A,λ-ι)-

23. The device (600) of claim 22 comprising a spectral filter for separating said first clock signal (S_CLKΛ_CW) from light which is substantially at said first wavelength (λ-i).

24. The device (600) according to any of the claims 21 to 23 comprising one or more optical resonators (OR1 , OR2) arranged to recover said first clock signal (S_CLKΛ_CW)-

25. The device (600) according to any of the claims 21 to 24 comprising:

- a light source (700) to provide continuous light (B_λCw) or an input (701 ) to receive continuous light (B_λCw). and

- at least one non-linear unit (SOA1 , SOA2) for modulating continuous light (B_λCw) by the intensity of said first optical data signal (S₀,λi). ⁱⁿ order to provide first modulated light (S_A,λcw) which has a spectral component at the wavelength (λ_Cw) of said continuous light (B_λCw)-

26. The device (600) of claim 25 wherein said at least one non-linear unit (SOA1 , SOA2) is arranged to operate near the saturation level of said at least one non-linear unit (SOA1 , SOA2) so as to equalize variations in the intensity of the pulses of the first optical data signal

27. The device (600) according to claim 25 or 26 comprising an interferometric unit (200), said interferometric unit (200) in turn comprising: - a splitter (21 ),

- a first non-linear unit (SOA1 ),

- a second non-linear unit (SOA2), and - a combiner (22), wherein said splitter (21 ) has a first input to receive said continuous light (B_λCw) and a second input to receive said first optical data signal (S₀,λi). said splitter (21 ) being arranged to distribute said continuous light (B_λCw) to said first non-linear unit (SOA1 ) and to said second nonlinear unit (SOA2), said splitter (21 ) being further arranged to distribute a first portion (B3) of said first optical data signal (S_0,λi) to said first non- linear unit (SOA1 ) and to distribute a second portion (B4) of said first optical data signal (S₀,λi) to said second non-linear unit (SOA2), said first non-linear unit (SOA1 ) being arranged to provide a first part (B1 ) of modulated light by modulating said continuous light (B_λCw) with said first portion (B3) of the first optical data signal (S₀,_λi). said second non- linear unit (SOA2) being arranged to provide a second part (B2) of modulated light by modulating said continuous light (B_λCw) by said second portion (B4) of the first optical data signal (S₀,λi). said combiner (22) being arranged to combine said first part (B1 ) of modulated light with said second part (B2) of modulated light such that a combination (B1 +B2) of said first part (B1 ) of modulated light and second (B2) part of modulated light is coupled to a second combiner output (02), and such that a combination (B3+B4) of said first portion (B3) of said first optical data signal (S₀,λi) with said second portion (B4) of said first optical data signal (S₀,_λi) is coupled to a first combiner output (01 ).

28. The device (600) according to any of the preceding claims 25 to 27 comprising means (RR2) for separating said first modulated light (S_A,λcw) from transmitted components (S_A,λi) of said first optical data signal (S_0,λi) which have passed through said at least one non-linear unit (SOA1 , SOA2).

29. The device (600) according to any of the preceding claims 25 to 28 comprising:

- one or more optical resonators (OR1 , OR2) arranged to store optical energy of said first modulated light (S_A,λcw)_> and to provide a substantially continuous sideband signal (S_SIDE) at a sideband peak

(VS_IDE) of said first modulated light (S_A,λcw) and a substantially continuous reference signal (S_REF) at a reference peak (V_REF) of said first modulated light (S_A,_λCw)_> the spectral separation (V_SIDE - V_REF) between said sideband peak (V_SIDE) and said reference peak (V_REF) being equal to the clock frequency (v_Cι_κ) of said optical data signal

(S₀,λi). wherein said device (600) is arranged to provide a first clock signal (SC_LKΛCW) as a combination of said sideband signal (SS_IDE) and said reference signal (S_REF)-

30. The device (600) of claim 29 comprising a first optical resonator (OR1 ) to provide said sideband signal (S_SIDE). a second optical resonator (OR2) to provide said reference signal (S_REF), and a combining means (WG2) to combine said sideband signal (S_SIDE) and said reference signal (S_REF)-

31. The device (600) of claim 29 comprising a first optical resonator (OR1 ) to provide said sideband signal (S_SIDE) and said reference signal (S_REF), wherein said first optical resonator (OR1 ) is arranged to provide a combination of said sideband signal (S_SIDE) and said reference signal

32. The device (600) according to any of the preceding claims 25 to 28 comprising:

- a light source (700) to provide substantially continuous light (B_λCw) at a reference peak (V_REF) of said first modulated light (S_A,λcw). or an input (701 ) to receive substantially continuous light (B_λCw)_>

- one or more optical resonators (OR1 , OR2) arranged to store optical energy of said first modulated light (S_A,λcw)_> and to provide a substantially continuous sideband signal (S_SIDE) at a sideband peak (VSIDE) of said first modulated light (S_A,λcw)> wherein said device (600) is arranged to provide a first clock signal (S_CLKΛ_CW) as a combination of said sideband signal (S_SIDE) and said substantially continuous light (B_λCw)-

33. The device (600) according to any of the preceding claims 21 to 32 comprising:

- one or more delay lines (D1 ),

- a splitter (41 ) to distribute said first optical data signal (S₀,λi. S_A>λi) to said one or more delay lines (D1 ) to form at least one delayed signal, and - a combiner (42) to combine said at least one delayed signal with said first optical data signal (S₀,λi. S_A>λi) in order to form said low-noise data signal (S_B,_λi)-

34. The device (600) according to any of the preceding claims 21 to 33 comprising 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) for providing said low-noise data signal (S_B,λi)-

35. The device (600) according to any of the preceding claims 21 to 34 comprising a Mach-Zehnder interferometer (500) or a Sagnac interferometer (501 ) arranged to re-shape said low noise data signal

36. The device (600) according to any of the preceding claims 21 to 35 arranged to recover a second clock signal from a second optical data signal (S₀^), wherein the spectral position of said second recovered clock signal deviates from the spectral position of said second optical data signal (S₀^)-

37. The device (600) of claim 36 wherein said first clock signal (S_CLKΛ_CW) and said second clock signal are at the same spectral position (λ_Cw)-

38. The device (600) according to any of the preceding claims 21 to 37 further comprising a multiplexer (MUX) to combine two or more regenerated data signals (SO.REGΛL ^SO,REGΛ2). and a filter (FIL1 ) arranged to separate said first recovered clock signal (S_CLKΛ_CW) from said recovered data signals (S₀,_REG,λi.

39. An optical communications system (900) comprising a device (600) according to any of the preceding claims 21 to 38.