Abstract
A transmitter (TX) for a WDM optical link includes a light source (CS) generating a plurality of discrete lines (EL) with different frequencies (f), a plurality of modulators (FSM, RRM, MZM), each modulator (FSM, RRM, MZM) being configured to modulate one of the discrete lines (EL) according to a data stream (c.sub.1-c.sub.4), at least one optical amplifier (SOA) configured to simultaneously amplify multiple lines (EL), wherein only a subset of the generated lines (EL) is routed to the optical amplifier (SOA) resp. to each one of the optical amplifiers (SOA). A receiver (RX) for an optical link adapted to work together with the transmitter (TX) is also described. An optical link including the transmitter (TX) and/or the receiver (RX), and a method to operate said link are also described.
Claims
1. A transmitter for an optical link, comprising a light source generating a plurality of discrete lines with different frequencies routed to a common input bus waveguide, further comprising a plurality of frequency selective modulators, each frequency selective modulator having an input port, a through port, an add port and a drop port and being configured to: modulate at least one of the discrete lines provided on the input port according to a data stream, output the modulated line on the drop port, pass all other light frequencies from the input port to the through port, and wherein the transmitter further comprises at least one drop bus waveguide and at least one optical amplifier configured to simultaneously amplify multiple lines, wherein the light source is a comb source, each of the at least one optical amplifiers is a semiconductor optical amplifier, and the frequency selective modulators are arranged in a drop configuration to share the common input bus waveguide as their input bus and to share the at least one drop bus waveguide as their drop bus, wherein the drop bus is routed to a respective semiconductor optical amplifier, so that only generated lines that are modulated by at least one frequency selective modulator are routed to the respective semiconductor optical amplifier.
2. The transmitter of claim 1, wherein the plurality of frequency selective modulators comprises at least two subsets of frequency selective modulators, wherein each subset of frequency selective modulators share a common drop bus waveguide.
3. The transmitter of claim 2, wherein each of the at least one drop bus waveguide is routed to an individual optical amplifier.
4. The transmitter of claim 3, wherein the outputs of at least two optical amplifiers are recombined.
5. The transmitter of claim 4, wherein the outputs of two optical amplifiers are recombined with an interleaver.
6. The transmitter of claim 5, wherein every second discrete line is referred to as an even discrete line, wherein every other discrete line is referred to as an odd discrete line, and wherein a first subset of frequency selective modulators sharing a first of the at least one drop bus waveguide modulate even discrete lines and a second subset of frequency selective modulators sharing a second of the at least one drop bus waveguide modulate odd discrete lines.
7. A transmitter according to claim 1, wherein at least one frequency selective modulator is a resonant ring modulator.
8. The transmitter of claim 1, wherein a frequency selective modulator comprises a modulator and a first add-drop multiplexer routing at least one discrete line from an input bus to the input of the modulator.
9. The transmitter of claim 8, wherein the frequency selective modulator further comprises a second add-drop multiplexer routing at least one modulated discrete line to a drop bus.
10. An optical link, comprising a transmitter according to claim 1.
11. A receiver for an optical link configured to decode at least one data stream from an optical signal transmitted by the transmitter according to claim 1, comprising a polarization splitting element with one input port and two output ports, at least one resonant add-drop multiplexer tuned to at least one line modulated according to a data stream, and at least one detector to convert a light intensity into an electrical signal, wherein light from the two output ports of the polarization splitting element is coupled into the resonant add-drop multiplexer in opposite directions, and that light is coupled from the resonant add-drop-multiplexer to the at least one detector in two opposite directions.
12. The receiver of claim 11, wherein optical path lengths between the polarization splitting element and the detector are substantially equal for both channel components of one and the same channel, wherein the channel corresponds to an optical carrier frequency and the channel components correspond to different polarization components.
13. The receiver according to claim 11, wherein multiple add-drop multiplexers are coupled to a common waveguide that forms part of an optical path between the two output ports of the polarization splitting element.
14. The receiver according to claim 13, wherein light from multiple add-drop multiplexers is coupled to the same detector or set of detectors.
15. The receiver according to claim 13, wherein the optical path between each of the output ports of the polarization splitting element and the add-drop multiplexers comprises at least one phase splitting element.
16. An optical link comprising a receiver according to claim 11.
Description
DESCRIPTION OF THE DRAWINGS
(1) Further advantageous embodiments will now be illustrated using drawings without limiting the scope of the invention. The Figures show:
(2) FIGS. 1a-1f: Examples of frequency selective modulators operated in through and drop configurations.
(3) FIGS. 2a-2e: Examples of configurations of cascaded frequency selective modulators.
(4) FIGS. 3a-3b: Filtering of undesired comb lines with a CROW filter.
(5) FIG. 4: Filtering of undesired comb lines with cascaded frequency selective modulators in drop configuration.
(6) FIG. 5: Filtering using an optical filter with periodic stop bands.
(7) FIG. 6: Diagrams of CROW filters in through and drop configuration.
(8) FIG. 7: Transmitter with optical filter and two interleavers.
(9) FIG. 8: Transmitter with two drop buses and one interleaver.
(10) FIG. 9: Receiver with closed loop between the two output ports of the polarization splitting element.
(11) FIG. 10: Receiver with two add-drop multiplexers per carrier frequency.
(12) FIG. 11: Receiver for DPSK encoded signals.
(13) FIG. 12: Electrical reconfiguration of frequency selective modulators in a transmitter.
(14) FIG. 13: Optical reconfiguration of frequency selective modulators in a transmitter.
(15) FIG. 14: Optical reconfiguration of a receiver.
(16) FIG. 15: Example of unidirectional coupling between two resonators.
(17) FIG. 1 illustrates examples of frequency selective modulators operated in through and drop configurations.
(18) FIG. 1a is a block diagram of a frequency selective modulator FSM operated in through configuration. The frequency selective modulator FSM has an input port I and a through port T. A component of the input signal that matches the line to which the frequency selective modulator FSM is tuned is modulated according to a data stream c.sub.i-c.sub.4, which is not shown for clarity. The remainder of the input signal remains untouched. The output at the through port T is a superposition of the modulated and unmodulated signals.
(19) FIG. 1c shows a realization of the configuration shown in FIG. 1a using a resonant ring modulator RRM. This resonant ring modulator RRM is connected to the input port I and to the through port T by means of a bus waveguide B. The arrows illustrate the direction in which signals propagate.
(20) FIG. 1e shows another realization of the configuration shown in FIG. 1a using a Mach-Zehnder modulator MZM. The frequency selectivity sits in the add-drop multiplexer (ADM). Out of the input signal delivered at port I, the add-drop multiplexer ADM picks the one component that matches the line (carrier) to which this add-drop multiplexer ADM is tuned. This component is coupled into the modulation loop ML. After passing the Mach-Zehnder modulator MZM and being modulated according to the data stream, this component reaches the add-drop multiplexer ADM again and is coupled out to through port T. This frequency selective modulator can be implemented with a single resonant add-drop multiplexer ADM comprising a single resonator. The input port I and the through port T are implemented and coupled to the add-drop multiplexer ADM by means of a bus waveguide B.
(21) FIG. 1b shows a frequency selective modulator FSM in drop configuration. Out of the signal delivered at input port I, the component that matches the line to which the frequency selective modulator FSM is tuned is modulated according to a data stream and output to the drop port D. The part of the input signal that is not modulated is passed through to the through port T. A signal delivered at the add port A that does not match the line to which the frequency selective modulator FSM is tuned is added to the output signal at the drop port D.
(22) FIG. 1d shows a realization of the configuration shown in FIG. 1b with a resonant ring modulator RRM. The input port I and through port T are implemented and coupled to the resonant ring modulator RRM by means of an input bus waveguide IB. The drop port D and the add port A are implemented and coupled to the resonant ring modulator RRM by means of a drop bus waveguide DB.
(23) FIG. 1f shows a realization of the configuration shown in FIG. 1b with two add-drop multiplexers ADM and one Mach-Zehnder modulator MZM. Out of the input signal on the input bus waveguide IB, the add-drop multiplexer ADM picks the component that corresponds to the line to which it is tuned. This component is coupled into the Mach-Zehnder modulator MZM and modulated according to the data stream. The outcome of this is coupled onto the drop port waveguide DB by means of a second add-drop multiplexer ADM that is tuned to the same line as the first one.
(24) FIG. 2a shows a cascade of two frequency selective modulators FSM1 and FSM2 in through configuration. Out of the signal at the input port I1, the FSM1 picks the component that corresponds to the line to which it is tuned. This component is modulated according to a first data stream. The rest of the input signal remains unmodulated. Both the modulated and the unmodulated signal are output as a superposition on through port T1, which is coupled to the input port I2 of the second frequency selective modulator FSM2.
(25) The second frequency selective modulator FSM2 picks a second signal component that corresponds to the line to which it is tuned. This component is modulated according to a second data stream. The component that was modulated in the first modulator FSM1 is not modulated further. Signal components that match neither of the lines to which FSM1 and FSM2 are tuned are not modulated at all. The superposition of the two modulated components and the unmodulated signal is output at the through port T2.
(26) FIG. 2b shows a cascade of two frequency selective modulators FSM1 and FSM2 in drop configuration. This configuration differs from the one shown in FIG. 2a in that FSM1 outputs its modulated signal onto drop port D1 and passes the unmodulated part of the input signal to through port T1. The drop port D1 of FSM1 is coupled to the add port A2 of FSM2. The through port T1 of FSM1 is coupled to the input port I2 of FSM2.
(27) Out of the input signal at port I2 that FSM1 has left unmodulated, FSM2 picks the component that corresponds to the line to which it is tuned. After this component has been modulated with the second data stream, it is output on drop port D2 together with the signal that had previously been modulated by FSM1; since this signal does not match the line to which FSM2 is tuned, it is just added to the component modulated by FSM2 at the drop port D2.
(28) The net effect of this cascade is that D2 now carries only signal components that have been modulated either by FSM1 or FSM2, while the through port T2 only carries components that have not been modulated at all. According to an embodiment of the invention, only D2 is coupled to the optical amplifier SOA, so that its available power is used solely to amplify carriers that have been actually modulated with a data stream. The signal from through port T2 is discarded into a beam dump.
(29) FIG. 2c shows a realization of the configuration shown in FIG. 2a with four resonant ring modulators RRM. They share a common bus waveguide B.
(30) FIG. 2d shows a realization of the configuration shown in FIG. 2b with four Mach-Zehnder modulators MZM that are each coupled to an input bus waveguide IB by means of an individual add-drop multiplexer ADM. Each of the modulators picks a signal component corresponding to the line to which it is tuned, and outputs it onto one of the two drop buses DBA and DBB. Drop bus DBA begins with a first add port AA and ends in a first drop port DA. Drop bus DBB begins with a second add port AB and ends in a second drop port DB. Unmodulated signal components pass along the entire input bus waveguide to through port T and are discarded. Each one of the drop buses DBA and DBB carry only carriers (lines) that have been modulated according to a data stream. Both drop ports DA and DB can be connected to separate optical amplifiers SOA, so that their power can be combined to amplify the signals modulated by all four Mach-Zehnder modulators MZM.
(31) FIG. 2e shows a cascade of resonant ring modulators (RRM) disposed between a common input bus waveguide IB and drop bus waveguide DB in drop configuration. The main difference compared with a cascade of frequency selective modulators according to FIG. 2d is that the add and drop ports have been reversed. Consequently, the direction of propagation along the drop bus waveguide DB has been reversed as well.
(32) In each frequency selective modulator, a first resonant ADM couples target frequencies from the input bus waveguide to a second waveguide connected to the input of the MZM. A second resonant ADM couples target frequencies coinciding with the target frequencies of the first resonant ADM from a third waveguide connected to the output port of the MZM to a drop bus waveguide of the frequency selective modulator. It should be noted that each of the ADMs also have an input waveguide and a drop waveguide, wherein the input waveguide of the first ADM is the input bus waveguide of the frequency selective modulator, the drop waveguide of the first ADM is the second waveguide connected to the input port of the MZM, the input waveguide of the second ADM is the third waveguide connected to the output port of the MZM, and the drop waveguide of the second ADM is the drop bus waveguide of the frequency selective modulator. Two subsets of frequency selective modulators are shown, wherein a first subset shares a first drop bus waveguide and a second subset shares a second drop bus waveguide.
(33) FIG. 3 shows how the configuration shown in FIG. 2c can be upgraded to get rid of unmodulated signal components on the through port T. FIG. 3a shows a configuration where the optical CROW filter F comprising coupled resonators R1, R2, R3 and R4 is placed before the frequency selective modulators RRM modulating comb lines with frequencies f closest to the edges of the stop bands of the optical filter. FIG. 3b shows a configuration where the optical filter is placed after said frequency selective modulators. The optical filter is exemplarily shown as a CROW based optical filter F and four modulated channels are being exemplarily shown to fall in between adjacent stop bands of the CROW filter (equivalently, to fall in a pass-band of the CROW filter F). Insets (i) to (vi) show optical spectra along the bus waveguide B, where PSD stands for power spectral density. EL denotes emitted lines from the comb source in general, while SL denotes unwanted, unmodulated sidelines that are filtered. There are two relevant CROW filter F stop bands here: One immediately below the frequency of the lowest frequency carrier (further referred to as the lower stop band) and one immediately above the frequency of the highest frequency carrier (further referred to as the upper stop band). In (i), the two vertical dashed lines show the frequency range in which the lower edge of the upper stop band has to fall for configuration (a). This allows filtering out the two highest frequency comb lines shown in the diagrams that are undesired since their power is too low, without impacting the highest frequency optical carrier. A similar tolerance holds for the upper edge of the lower stop band. The two vertical dashed lines in (v) show the frequency range in which the lower edge of the upper stop band has to fall for configuration (b). This frequency range is smaller than in (i), because the adjacent optical carrier has already been modulated, and thus its spectrum has been broadened in the frequency domain. A similarly reduced tolerance also holds for the upper edge of the lower stop band. Vertical arrows in the optical spectra represent Dirac peaks and symbolize unmodulated optical carriers. Modulated optical carriers are represented with widened distributions.
(34) FIG. 4 shows an embodiment where undesired comb lines are filtered in a configuration in which cascaded frequency selective modulators are operated in drop configuration. Each frequency selective modulator consists of a Mach-Zehnder modulator MZM that is connected to the input bus waveguide IB and to the drop bus waveguide DB by one add-drop multiplexer ADM each. The insets show the optical spectra at different locations in the structure. Since only modulated optical carriers are routed to the drop bus, filtering occurs naturally in this structure without the need to add an additional optical filter. This can be seen in that the lower power unmodulated comb lines remain in the input bus waveguide and are routed to the through port, which can for example be further connected to a waveguide termination/beam dumb device suppressing back-reflections or to a monitor photodiode for system monitoring or for a control system. Vertical arrows in the optical spectra represent Dirac peaks and symbolize unmodulated optical carriers. Modulated optical carriers are represented with widened distributions.
(35) FIG. 5 shows an overlay between the spectrum of the comb source and the transmission function TF of an optical filter F with periodically spaced stop bands SB separated by a free spectral range FSR. The comb source is exemplarily shown as having four high power comb lines used as optical carriers. Six additional undesirable comb lines, three on either side of the optical carriers, are also represented. The high transmission regions of the optical filter transfer function correspond to pass bands, while the low transmission regions of the optical filter transfer function correspond to stop bands. The horizontal arrows in (i) represent the frequency regions in which the edges of the stop bands have to fall. The horizontal arrow in (ii) represents the FSR of the optical filter. This FSR has to be larger than the difference between the frequencies of the highest frequency and the lowest frequency optical carriers.
(36) FIG. 6 shows diagrams of CROW filters in (a) through and (b) drop configuration. The insets respectively show the transfer functions from the input port to the through port and from the input port to the drop port. Note that CROW filter in through configuration refers to the fact that the data channel path passes through the through port, i.e., the through port is directly or indirectly routed to a SOA (e.g., with interposed frequency selective modulators). A CROW filter in through configuration may or may not have a drop port, wherein an optional drop port can be used for other purposes, such as for example connection to a monitor photodiode for monitoring the system, providing a feedback signal for a control system, or connecting to a waveguide termination/beam-dump that suppresses back-reflections. As shown in (b), the signal arriving at the drop port is typically attenuated even inside the pass-bands of the drop port, making the through configuration more desirable.
(37) FIG. 7 shows an embodiment of a complete transmitter TX. The light source CS is a comb source. Undesired lines are filtered out by a filter F, which is implemented as a CROW filter with coupled resonators R1, R2, R3, R4. The remaining lines at frequencies f.sub.1, f.sub.2, f.sub.3 and f.sub.4 are each modulated according to different data streams with resonant ring modulators RRM. A first interleaver INT1 forwards odd lines f.sub.1, f.sub.3 to a first semiconductor optical amplifier SOA and even lines f.sub.2, f.sub.4 to a second semiconductor optical amplifier SOA, so that each of these amplifiers has to take only half the power of the modulated signal. The outputs of both amplifiers SOA are then recombined into one output O that is fed into an optical fiber.
(38) FIG. 8 shows another embodiment of a transmitter TX wherein frequency selective modulators FSM are cascaded in drop configuration, and wherein two groups of frequency selective modulators FSM each share their own drop bus DBA, DBB. The two drop busses DBA, DBB are each connected to an individual SOA. The outputs of the SOAs are recombined with an interleaver INT. The first group of frequency selective modulators FSM sharing a first drop bus DBA modulate odd optical carriers with center frequencies f.sub.1 and f.sub.3, while the second group of frequency selective modulators FSM sharing a second drop bus modulate even optical carriers with center frequencies f.sub.2 and f.sub.4. This partitioning into even and odd carriers allows subsequent recombination of the carriers transported by the two drop bus waveguides DBA, DBB into a single output waveguide O by means of an interleaver INT.
(39) FIG. 9 shows an embodiment of a receiver RX in which light from each channel is split into two channel components by a polarization splitting grating coupler PSC according to the polarization of the light inside the fiber that delivers the input optical signal IN to the polarization splitting grating coupler PSC. The two channel components are each routed to individual output ports of the polarization splitting grating coupler PSC. The two output ports are connected to a waveguide forming a loop PL connected to frequency selective add-drop multiplexers (ADMs) tuned to different carrier frequencies f.sub.1-f.sub.5. The ADMs each couple both components of a same channel to an individual photodiode PD, wherein the photodiode PD may have two input ports, each receiving one channel component. The lengths of the two waveguides between the ADM and the photodiode (PD) are generally different from each other and vary from channel to channel, so that for each channel f.sub.1-f.sub.5 the optical path lengths between the polarization splitting grating coupler and the photodiode input ports are substantially equal for both channel components. The optical paths followed by the two channel components of the channel with optical carrier frequency f.sub.1 are respectively shown by dashed and continuous arrows and are designed to be substantially of equal length.
(40) FIG. 10 shows a receiver RX in which light from each channel IN is split into two channel components by a polarization splitting grating coupler PSC according to the polarization of the light inside the fiber connected to the polarization splitting grating coupler PSC. The two channel components are each routed to individual output ports of the polarization splitting grating coupler. The two output ports are connected to two waveguides each connected to frequency selective add-drop multiplexers (ADMs). The ADMs each couple a component of a channel to an individual channel-specific photodiode, wherein the photodiode may have two input ports, each receiving one channel component. The lengths of the two waveguides between the channel specific ADMs and the photodiode (PD) are chosen such that the optical path lengths between the polarization splitting grating coupler and the photodiode input ports are substantially equal for both channel components.
(41) FIG. 11 shows a receiver RX for DPSK encoded signals. The two outputs of a polarization splitting grating coupler PSC are each connected to a DPSK demodulator as phase splitting element PSE, each DPSK demodulator PSE having two output ports. Corresponding output ports, labeled by PCA or PCB in the diagram, are respectively connected to each other with a first and a second waveguide loop. Resonant add-drop multiplexers ADM couple both channel components of a given channel and demodulator output port type to an individual photodiode PD, wherein a first pair of waveguides each routing one channel component from the corresponding ADM coupled to the first waveguide loop to the first channel specific photodiode PD are sized in such a manner as to substantially equalize the optical path lengths between the polarization splitting coupler PSC and the first channel specific photodiode PD for the two channel components. Furthermore, a second pair of waveguides each routing one channel component of the same channel from the corresponding ADM connected to the second waveguide loop to a second channel specific photodiode PD are sized in such a manner as to substantially equalize the optical path lengths between the polarization splitting coupler PSC and the second channel specific photodiode PD to each other as well as to the optical path lengths between the polarization splitting coupler PSC and the first channel specific photodiode PD. The DPSK demodulators PSE are constituted by Mach-Zehnder interferometers with an additional waveguide length applied to one of the interferometer arms. The output stage of the interferometer consists in a directional coupler.
(42) FIG. 12 shows six frequency selective modulators with nominal target frequencies f.sub.1 to f.sub.6 used to transport data according to four electrical data streams c.sub.1 to c.sub.4. During the start-up phase, a subset of frequency selective modulators RRM (here 4 out of 6) is selected based on the ease of aligning them to the actual optical carrier frequencies with an active control system. An electrical connectivity matrix is reconfigured to electrically connect the four electrical data streams c.sub.1 to c.sub.4 to the four frequency selective modulators that have been selected and activated. The boxes inside the connectivity matrix represent electrical switches allowing the selective connectivity of the data stream to one out of three possible frequency selective modulators.
(43) FIG. 13 shows an embodiment of a transmitter TX with reconfigurability in the optical domain. ADMs in a subset of frequency selective modulators are each replaced by two ADMs targeted towards nominally different optical carrier frequencies EL. Here, the two leftmost frequency selective modulators are exemplarily implemented with such redundant ADMs, wherein both the ADM coupling the input bus to a MZM and the ADM coupling the MZM to the drop bus are replaced by two redundant ADMs. In the first frequency selective modulator (counting from the left), the ADMs are replaced by two ADMs nominally targeted towards f.sub.1 and f.sub.5, and in the second frequency selective modulator the ADMs are replaced by two ADMs nominally targeted towards f.sub.2 and f.sub.6. The nominally targeted frequencies are ordered as f.sub.1<f.sub.2<f.sub.3<f.sub.4<f.sub.5<f.sub.6 and are spaced by one comb source FSR. The transmitter can be exemplarily configured in one out of three states, utilizing ADMs with nominal target frequencies f.sub.1<f.sub.2<f.sub.3<f.sub.4, f.sub.2<f.sub.3<f.sub.4<f.sub.5 or f.sub.3<f.sub.4<f.sub.5<f.sub.6 depending on which subset is easier to tune to the actual optical carrier frequencies (including for which subset it is easier to maintain the spectral alignment to the optical carrier frequencies over expected temperature variations). In each of these configurations, each frequency selective modulator is only effectively modulating one optical carrier with a frequency corresponding to a target drop frequency of one of the ADMs from within a pair of redundant parallel ADMs. The other ADM of the ADM pair is either detuned relative to any other comb line, tuned to a comb line that is filtered out by an upstream or downstream optical filter or tuned to a comb line that is not coupled to a photodiode inside the receiver. The redundant ADMs for each of the two leftmost frequency selective modulators each have targeted optical carrier frequencies separated by 4 comb source FSRs, while the actively utilized spectrum consists in 4 comb lines covering only 3 comb source FSRs. Thus, an optical filter with a pass band spanning slightly more than 3 comb source FSRs can be implemented, filtering out undesired comb lines. Alternatively, the nominally unused ADMs after start-up selection can be detuned so as not to let a comb line through, in which case the aforementioned filter function is not necessary.
(44) FIG. 14 shows an embodiment of a receiver RX with reconfigurability in the optical domain. A subset of ADMs are each replaced by two ADMs targeted towards nominally different optical carrier frequencies. Here, the two leftmost photodiodes PD are exemplarily coupled to the loop waveguide PL with such redundant ADMs. One photodiode is coupled with two ADMs nominally targeted towards f.sub.1 and f.sub.5, another photodiode is coupled with two ADMs nominally targeted towards f.sub.2 and f.sub.6. The nominally targeted frequencies are ordered as f.sub.1<f.sub.2<f.sub.3<f.sub.4<f.sub.5<f.sub.6 and are spaced by one FSR of the comb source CS. The receiver RX can be exemplarily configured in one out of three states, utilizing ADMs with target carrier frequencies f.sub.1<f.sub.2<f.sub.3<f.sub.4, f.sub.2<f.sub.3<f.sub.4<f.sub.5 or f.sub.3<f.sub.4<f.sub.5<f.sub.6 depending on which subset is easier to tune to the actual optical carrier frequencies (including for which subset said spectral alignment with incoming optical carriers is easier to maintain over expected temperature variations). In every configuration, each photodiode PD is only receiving one optical carrier EL since the frequency difference between the targeted frequencies of the two redundant ADMs exceeds the total spectral width of the utilized portion of the comb source spectrum.
(45) FIG. 15 shows an example of uni-directional coupling between two ring resonators R1 and R2 (non bi-directional coupling). The counterclockwise propagating mode of a 1.sup.st ring resonator R1 can couple to the counterclockwise propagating mode of a 2.sup.nd ring resonator R2, and the clockwise propagating mode of the 2.sup.nd ring resonator R2 can couple to the clockwise propagating mode of the 1.sup.st ring resonator R1. However, the clockwise propagating mode of the 1.sup.st ring resonator cannot couple to the 2.sup.nd ring resonator, and the counterclockwise propagating mode of the 2.sup.nd ring resonator cannot couple to the 1.sup.st ring resonator, thus bi-directional coupling between a mode of the 1.sup.st resonator and a mode of the 2.sup.nd resonator is not given.
LIST OF REFERENCE SIGNS
(46) A, A1, A2 add port
(47) AA add port of drop bus waveguide DBA
(48) AB add port of drop bus waveguide DBB
(49) ADM add-drop multiplexer
(50) B bus waveguide
(51) CS light source
(52) D, D1, D2 drop port
(53) DA drop output port of drop bus waveguide DBA
(54) DB drop output port of drop bus waveguide DBB
(55) DB, DBA, DBB drop bus waveguide
(56) EL lines generated by the light source CS
(57) c.sub.1-c.sub.4 data streams
(58) F optical filter
(59) f, f.sub.1-f.sub.6 frequencies
(60) FSM, FSM1-2 frequency selective modulator
(61) FSR free spectral range
(62) I, I1, I2 input port
(63) IB input bus waveguide
(64) IN input optical signal for receiver RX
(65) INT, INT1-2 interleavers
(66) ML loop containing Mach-Zehnder modulator MZM
(67) MZM Mach-Zehnder modulator
(68) output port
(69) RRM resonant ring modulator
(70) R1-R4 resonators
(71) RX receiver
(72) PCA output port of phase splitting element PSE for phase component A
(73) PCB output port of phase splitting element PSE for phase component B
(74) PD photodiode
(75) PL loop between output ports of polarization splitting element PSC
(76) PSC polarization splitting element
(77) PSD power spectral density
(78) PSE phase splitting element
(79) SB stop band
(80) SL unmodulated side line
(81) SOA semiconductor optical amplifier
(82) T, T1, T2 through port
(83) TF transmission function of optical filter F
(84) TX transmitter
