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(11) EP 2 830 239 A1

(12) EUROPEAN PATENT APPLICATION

(43) Date of publication: (51) Int Cl.: 28.01.2015 Bulletin 2015/05 H04B 10/25 (2013.01) H04J 14/02 (2006.01) H04B 10/516 (2013.01) (21) Application number: 13003679.1

(22) Date of filing: 23.07.2013

(84) Designated Contracting States: • Elbers, Jörg-Peter AL AT BE BG CH CY CZ DE DK EE ES FI FR GB 82256 Fürstenfeldbruck (DE) GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO • Griesser, Helmut PL PT RO RS SE SI SK SM TR 86551 Aichach (DE) Designated Extension States: • Eiselt, Michael BA ME 99334 Kirchheim (DE)

(71) Applicant: ADVA AG Optical Networking (74) Representative: Eder, Thomas 98617 Meiningen (DE) Eder & Schieschke Patentanwälte (72) Inventors: Elisabethstrasse 34 • Grobe, Klaus 80796 München (DE) 82152 Planegg (DE)

(54) Method, system and transceiver device for bi- directionally transmitting digital optical signals over an optical transmission link

(57) The invention relates to a method for bi-direc- (S1TX,CHI) received at the second end of the optical trans- tionally transmitting digital optical signals over an optical mission link, the optical wavelength reuse signal being transmission link, wherein a first optical transmit signal modulated according to a second digital signal in such a

(S1TX,CHI) at a predetermined optical wavelength is sup- way that the bit information of the second digital signal plied to a first end of the optical transmission link and is included in second sections of the symbol interval of

transmitted in a first transmission direction to a second the first optical transmit signal (S1TX,CHI) received. Ac- end of the optical transmission link, wherein a second cording to the invention, the first binary digital signal optical transmit signal (S2TX,CHI) at said predetermined (S1TXI) is a non-retum-to-zero signal and the first optical optical wavelength is supplied to the second end of the transmit signal (S1TX,CHI) is an optical bit-interleaved optical transmission link and transmitted in a second op- seeding signal having a symbol rate equal to the bit rate

posite transmission direction to the first end of the optical of the first binary digital signal (S1 TXI), wherein the sym- transmission link, wherein said first optical transmit signal bol interval of the optical bit-interleaved seeding signal (S1TX,CHI) is created according to a first binary digital is divided Into two equally long sub-intervals, the bit In- signal (S1TXI) in such a way that the bit information of the formation of the first binary digital signal (S1TXI) being first binarydigital signal (S1 TXI) isincluded in first sections transported in a first of the two sub- intervals and the sec- of the symbol interval of the first optical transmit signal, ond sub-intervals being set to a seeding level with respect and wherein said second optical transmit signalto the optical power.

(S2TX,CHI) is created by creating an optical wavelength reuse signal using the first optical transmit signal EP 2 830 239 A1

Printed by Jouve, 75001 PARIS (FR) (Cont. next page) EP 2 830 239 A1

2 1 EP 2 830 239 A1 2

Description lowing description, a level "0" or a logical value "0" of an amplitude-modulated optical signal shall be understood [0001] The invention relates to a system and a method in such a way that an optical power of essentially zero or for bi-directionally transmitting digital optical signals over at least significantly below the "1" level, for example 10 an optical transmission link as well as to an optical trans- 5 dB below the "1" level, is transported within the respective ceiver device for a respective system. symbol interval. Therefore, another modulation format [0002] In optical wavelength division multiplex (WDM) than NRZ-OOK must be chosen for the downstream sig- passive optical networks, which are commonly applied nals. However, other modulation formats always Involve to realize fiber-to-the home access structures, a plurality additional effort and costs. of optical network units (ONU) is connected to a central 10 [0007] A known approach for a WDM-PON applying node, also referred to as optical line terminal (OLT), via wavelength-reuse is based on using frequency-shift key- a remote node (RN). Each ONU connects at least one ing (FSK) for the downstream signal, wherein this signal end-user to the RN. Generally, a single fiber is used for reveals an essentially constant envelope, and a standard connecting the RN to the ONUs in order to save fiber OOK modulation format for the upstream signal. Instead usage. Using a single-fiber connection between the OLT 15 of FSK any other modulation format may be used that and the RN further reduces the fiber usage. However, produces a downstream signal having an essentially con- PON configurations with dual-fiber connection between stant envelope, like any phase shift keying (PSK) format. the OLT and the RN are also widely used. Often, a pro- [0008] Further, it has been proposed to use an inverse- tection mechanism, especially a fiber-protection method retum-to-zero (IRZ) OOK modulation format for the and hardware, may be implemented for the transmission 20 downstream signal and an RZ OOK modulation format link between the OLT and the RN. for the related upstream signal. [0003] PONs enable a bi-directional point-to-point con- [0009] However, all these solutions require costly com- nection between each ONU and the OLT using dedicated ponents, e.g. FSK demodulators or IRZ/RZ pulse shap- optical channels, i.e. for each point-to-point connection ing within the ONU, or increase the bandwidth necessary a pair of downstream and upstream signals having pre- 25 to transmit the downstream or even the downstream and determined optical wavelengths is used. In general, the the upstream signals and thus increase the costs for the downstream channel signal transmitted from the OLT to components necessary to process the respective sig- the respective ONU and the upstream channel signal nals. transmitted in the reverse direction may have identical [0010] Based on this known prior art, it is an object of or differing optical wavelengths. The plurality of down- 30 the present invention to provide a system and method stream and upstream optical channel signals is transmit- for bi-directionally transmitting digital optical signals over ted as a combined WDM signal within the transmission an optical transmission link which applies wavelength re- link between the OLT and the RN. use and which can be implemented simply and at low [0004] One of the main challenges when deploying this costs. It is a further object of the invention to provide an PON technology in access networks consists in the35 optical transceiver device for realizing a respective sys- wavelength-assignment problem of WDM transmitters tem. and the costs related therewith. The costs of installation, [0011] The invention achieves these objects with the administration, and maintenance for a PON can be dras- combination of features of claims 1, 10 and 13, respec- tically reduced if at least the ONUs reveal a so-called tively. "colorless", i.e. non-wavelength-specific, design. Typical 40 [0012] According to the present invention, a first optical low-cost solutions for colorless ONUs are based on the transmit signal at a predetermined optical wavelength is use of reflective optical transmitters like semiconductor created and supplied to a first end of an optical transmis- optical amplifiers (RSOA), injection-locked Fabry-Perot sion link and transmitted in a first transmission direction laser diodes (IL-FP-LD) or reflective electro-absorption to a second end of the optical transmission link. This first modulators with integrated semiconductor optical ampli- 45 optical transmit signal is created in such a way that It can fiers (REAM-SOA) as optical transmitter components. be reused at the second end of the transmission link for [0005] In a PON design using such reflective optical creating a second optical transmit signal, preferably by transmitters, the downstream (channel) signals can be using a reflective modulator device. This second optical reused in the ONUs in order to create an upstream signal transmit signal is transmitted in a second transmission having the same wavelength. Thus, such a system for 50 direction to the first end of the optical transmission link. bi-directionally transmitting digital optical signals over an Said first optical transmit signal is created according to optical transmission link is referred to as wavelength-re- a first binary digital signal in such a way that the bit in- use transmission systems. formation of the first binary digital signal is included in [0006] In a WDM-PON with wavelength reuse, the (op- first sections of the symbol interval of the first optical tical) downstream (channel) signals must not use stand- 55 transmit signal, ard non-retum-to-zero (NRZ) on-off keying (OOK) be- [0013] Said second optical transmit signal is created cause for levels "0" sent, upstream modulation is impos- by creating an optical wavelength reuse signal using the sible or subject to severe penalties. Here and in the fol- first optical transmit signal received at the second end of

3 3 EP 2 830 239 A1 4 the optical transmission link. The optical wavelength re- coding the pre-coded binary bit-interleaved digital signal use signal is modulated according to a second digital and modulating an optical light source having the prede- signal in such a way that the bit information of the second termined optical wavelength using the pre-coded and en- digital signal is included in second sections of the symbol coded binary bit-interleaved digital signal as modulation interval of the first optical transmit signal received. 5 signal. [0014] According to the invention, a non-return-to-zero [0019] Although this is a rather simple method to im- (NRZ) signal is used as first binary digital signal and the plement the method according to the invention, it requires first optical transmit signal is an optical bit-interleaved hardware that is fast enough to effect the required signal seeding signal having a symbol rate equal to the bit rate manipulation. of the NRZ signal, wherein the symbol Interval of the10 [0020] According to another, preferred embodiment of optical bit-interleaved seeding signal is divided into two the invention, the duobinary optical bit-interleaved seed- equally long sub-intervals, the bit information of the first ing signal is created by pre-coding the first binary digital binary digital signal being transported in a first of the two signal and duobinary encoding the pre-coded first binary sub-intervals and the second sub-intervals being set to digital signal, low-pass filtering the pre-coded and encod- seeding level with respect to the optical power. The seed- 15 ed first binary digital signal in such a way that the filtered ing level may of course be equal to the maximum (optical) pre-coded and encoded first binary digital signal as- power level of the digital optical bit-interleaved seeding sumes in each transition between its extreme values a signal. value which essentially equals the average value of the [0015] Thus, no pulse shaping is necessary at the OLT extreme values at times which essentially are equal to a when creating the first optical transmit signal, so that the 20 quarter of the bit interval, and modulating an optical light method can be implemented at low cost. source having the predetermined optical wavelength us- [0016] According to a preferred embodiment, the opti- ing the low-pass filtered pre-coded and encoded first bi- cal bit-interleaved seeding signal is a duobinary coded nary digital signal as modulation signal. optical bit-interleaved seeding signal. Using a duobinary [0021] With this specific low-pass filtering, the same coding leads to a reduction of the bandwidth of the first 25 effect is reached as with the insertion of the additional optical transmit signal that would otherwise be caused logical 1 bits as described above. However, no signal by dividing the symbol interval of the first optical transmit manipulation apart from simple low-pass filtering is re- signal (and as a result also the second optical transmit quired. signal) into two subintervals and merely using one of the [0022] According to a further embodiment, the duobi- subintervals for the data transmission in each direction. 30 nary optical bit-interleaved seeding signal may be creat- Thus, a decisive reduction of costs for implementing the ed in such a way that it is a ternary optical signal with transmission method is achieved. respect to the electric field and a binary digital signal with [0017] The invention is especially applicable for WDM- respect to the optical power. This can be achieved by PON access networks providing bit-rates of 10 Gbit/s for using an optical modulator device, e.g. a Mach-zehnder each channel signal in the downstream and upstream 35 Modulator or a dual electro-obsorption-modulator, which direction. Using duobinary coding leads to the effect that converts the duobinary modulating signal into the duob- the duobinary optical bit-interleaved seeding signal has inary optical bit-interleaved seeding signal in such a way a bandwidth less than the bandwidth of the first binary that the extreme values of the ternary modulation signal digital signal, i.e. in case of a 10 Gbit/s first binary digital correspond to +E and -E and the intermediate value of signal the bandwidth is less than 10 GHz. This is a dear 40 the ternary modulation signal corresponds to 0, wherein advantage over the known method of using an IRZ optical +E, 0 and -E designates the amplitude of the electric channel signal in the downstream direction and an OOK vector of the optical bit-interleaved seeding signal. RZ scheme in the upstream direction. The invention re- [0023] This reduces the intersymbol interference (ISI) quires a Mach-Zehnder modulator (MZM) or a dual elec- as the duobinary coded signal reveals the property that tro-absorption modulator (dual EAM) in the OLT for cre- 45 the high bits in high-zero-high sequences of the optical ating the individual channel signals, but reduces the op- signal reveal a 180 degrees phase shift in the optical tical (components) bandwidth and enhances the toler- frequency that leads to a respective cancellation of over- ance against chromatic dispersion. lapping areas of the high bits/pulses due to optical dis- [0018] According to an embodiment of the invention, persion effects. Thus, the dispersion tolerance of the first the duobinary optical bit-interleaved seeding signal is 50 optical transmit signal is increased if duobinary encoding created by converting the first binary digital signal into a is used. binary bit-interleaved digital signal having twice the bit [0024] According to an embodiment of the invention, rate of the first binary digital signal by dividing each bit the optical bit-interleaved seeding signal received at the interval into two equally long sub-intervals, setting one second end of the transmission link is divided, with re- sub-interval to the signal value of the first binary digital 55 spect to the optical power of the signal, into an optical signal and setting the respective other sub-interval to the receive signal and an optical branch-off signal, and the logical 1 value of the first binary digital signal, pre-coding optical receive signal is used for receiving the first binary the binary bit-interleaved digital signal and duobinary en- digital signal by detecting the optical power during the

4 5 EP 2 830 239 A1 6 first of the two equally long sub-intervals. e.g. by using channels, each optical channel using a dedicated optical an integrate-and-dump receiver, channel bandwidth. A WDM port 9 of the OLT 3 is con- [0025] Thus, direct detection can be used which reduc- nected to a WDM port 11 of the RN 5 via a single optical es the costs of the ONU components as compared to fiber 13. Each of the ONUs 7 is connected to a channel other modulation formats used for the first or downstream 5 port 14, (1≤i≤n) of the RN at a remote port 15 by a single optical transmit signal. optical fiber 17. [0026] The optical branch-off signal can be used to cre- [0031] The OLT comprises a number n of transceiver ate the optical wavelength reuse signal using a reflective devices 19, each of which receives a first digital transmit modulator device, e.g. a reflective semiconductor optical signal S1TXi (1≤i≤n), which is supplied to a respective 10 amplifier, an injection-locked Fabry-Perot laser diode or local input port 21i (1≤i≤n) of the OLT 3. The first digital a reflective electro-absorption modulator with an integrat- transmit signal S1 TXi may e.g. be a 10 Gbit/s NRZ signal. ed semiconductor optical amplifier. This is a simple and Each transceiver device 19 further creates a first digital cheap method to design a colorless transceiver, receive signal S1RXi (1≤i≤n) and supplies this signal to a [0027] Especially an NRZ or RZ coded signal can be local output port 23i (1≤i≤n). 15 used as second digital signal for creating the second op- [0032] Of course, the first digital transmit signals S1 TXi tical transmit signal using the optical bit-interleaved seed- and the first digital receive signal S1RXi may be optical ing signal so that no complex and/or expensive pulse signals or electrical signals. The transceiver devices 19 shaping is required in an ONU which is adapted to im- may convert optical signals S1 TXi into respective electri- plement the method according to the invention. cal signals, as generally electrical signals S1TX are re- [0028] Further embodiments of the invention are ap- 20 quired to perform the signal processing within the trans- parent from the dependent claims. ceiver devices 19. Likewise, signals S1RXi. which are [0029] In the following, the invention will be described generally created as electrical signals, may be converted with reference to the embodiments apparent from the into respective optical signals, if required. drawing. In the drawing, the Figures show: [0033] Each transceiver device 19 creates a first opti- 25 cal transmit signal or first optical channel transmit signal Fig. 1 a schematic block diagram of a WDM-PON ap- S1TX,Chi (1≤i≤n) which transports the information includ- plying the wavelength reuse method according ed in the respective first digital signals S1 TXi and supplies to the invention; the signal S1TX,Chi via an output port of the transceiver device 19, to a dedicated channel port of an optical multi- Fig. 2 a schematic block diagram of an ONU applying 30 plexer device 25, which may be realized as arrayed the wavelength reuse method according to the waveguide grating (AWG). A combined first optical WDM invention using a reflective modulator device; transmit signal S1TXWDM is output at a WDM port of the optical multiplexer device 25 and supplied to an optical Fig. 3 a schematic block diagram of an ONU applying circulator 27, which directs the first optical WDM transmit 35 the wavelength reuse method according to the signal S1TX,WDM to the WDM port 9 of the OLT 3. invention using a non-reflective modulator de- [0034] The WDM transmit signal S1 TX,WDM is transmit- vice; ted via the optical fiber 13 to the WDM port 11 of the RN 5, which demultiplexes the WDM transmit signal Fig. 4 a schematic diagram showing various steps for S1TX,WDM and outputs the optical channel transmit sig- 40 converting a first NRZ digital signal into a duo- nals S1TX,Chi to the respective ONU at the respective binary coded modulating signal according to a channel port 14. first embodiment of the method according to the [0035] Each ONU 7, which may be regarded as trans- present invention, ceiver device, receives the dedicated optical channel transmit signal S1 TX,Chi at its remote port 15 and creates 45 Fig. 5 a diagram similar to Fig. 4 additionally showing a second digital receive signal S2RXi (1≤i≤n), which is the time relation between the first optical trans- output at a local output port 31, (1 ≤i≤n). Provided that no mit signal received and the second optical transmission errors occur and that the same coding is

transmit signal created at the second end of the used, thesecond digitalreceive signal S2 RXiis, of course, transmission link: and identical with the first digital transmit signal S1TXi. 50 [0036] Further, each ONU 7 receives a second digital Fig. 6 a schematic diagram similar to Fig, 5 for a sec- transmit signal S2TXi (1≤i≤n) at a local input port 33 and ond embodiment of the method according to uses this signal to create a second optical transmit signal

the present invention. or second optical channel transmit signal S2 TX,Chi (1≤i≤n) that is output at the remote port 15 of the ONU. The n 55 [0030] Fig. 1 shows a schematic diagram of a WDM- second optical channel transmit signals S2 TX,Chi (1≤i≤n) PON 1 comprising an OLT 3, a remote node 5 and a are multiplexed by the RN 5 into a second optical WDM plurality of ONUs 7. For simplicity, only a single ONU 7 transmit signal S2TX,WDM and transmitted, via the WDM is displayed, The WDM-PON uses a number n of optical port 11 of the RN 5, to the WDM port 9 of the OLT 3. The

5 7 EP 2 830 239 A1 8 circulator 27 directs the second optical WDM transmit nected via a first fiber to a dedicated channel port of a signal S2TX,WDM to a WDM port of a demultiplexer device first one of two separate demultiplexer/multiplexer devic- 35, which may be realized as an AWG. The demultiplexer es in the RN5 and via a second fiber to a dedicated chan- device 35 demultiplexes the second optical WDM trans- nel port of the second demultiplexer/multiplexer device 5 mit signal S2TX,WDM into the individual second optical in the RN5. If a single demultiplexer/multiplexer device channel transmit signals S2 TX,Chi and outputs these sig- is provided in the RN5, only, the necessary separating nals at dedicated channel ports, which are connected to and combining of the downstream and upstream trans- respective input ports of the dedicated transceiver devic- mission paths may be achieved by suitable means like es 19. Each transceiver device 19 extracts the informa- optical separators, wherein the channel port of the single tion includedwithin the respective second opticalchannel 10 demultiplexer/multiplexer device is coupled to a first com- transmit signal S2 TX,Chi and creates a corresponding first mon port of the path separation/combination means and digital receive signal S1RXi. eachof thetwo connecting fibers guiding thedownstream [0037] Of course, provided that no transmission errors and upstream channel transmit signals TX,Chi S1 and occur and that the same coding is used, the first digital S2TX,Chi are connected to a downstream and an up- 15 receive signal S1RXi is identical to the second digital stream port of the dedicated port of the path separa- transmit signal S2TXi tion/combination means. [0038] Further, the second digital transmit signals [0042] Fig. 2 shows a schematic diagram of a first em- S2TXi and the second digital receive signal S2 RXi may be bodiment of an ONU 5 that is configured to realize the optical signals or electrical signals. The ONUs 7 may method according to the invention described below. The 20 convertoptical signals S2 TXi into respective electrical sig- ONU 5 comprises a directional coupler 37 adapted to nals, as generally electrical signals are required In order split the optical channel transmit signals S1TX,Chi that is to perform the signal processing within the ONUS. Sig- received at the remote port 15 with respect to its optical nals S2RXi, which are created as electrical signals, may power. A first portion of the optical power represents an beconverted by theONUS into respective opticalsignals, optical receive signal which is supplied to an opto-elec- if required. 25 trical converter device 39, e.g. a photodiode. The elec- [0039] The RN 5 shown in Fig. 1, which merely com- trical signal output by the opto-electrical conversion de- prises a demultiplexing device, is, of course, to be un- vice 39 is supplied to a clock and data recovery (CDR) derstood as an example, only. Any configuration may be device 41 which extracts the information transported in used that performs the demultiplexing of the WDM trans- the signal received and creates a corresponding electri- 30 mit signal 51TX,WDM and the distribution of the individual cal digital signal which Is supplied to a low-pass filter 43 optical channel transmit signals S1 TX,Chi to the dedicated for pulse shaping purposes. The filtered electrical signal ONUs 7. Each connection between the output port and represents the second digital receive signal S2RXi. the input port of a transceiver device 19 and the remote [0043] The CDR device 41 is adapted to recover the port 15 of an ONU 7 represents an optical transmission clock information of the digital signal received. The clock link for bi-directionallytransmitting the respective first and 35 information is used to synchronize the sampling or re- second optical channel transmit signals S1TX,Chi and ceiving mechanism with the signal received in order to S2TX,Chi in the corresponding first and second transmis- correctly detect the bit information within each symbol sion directions. That is, the transmission method accord- interval. As apparent from the description below, the op- ing to the invention is not restricted to WDM-PONs but tical channel transmit signals S1TX,Chi received at each may also be applied for arbitrary optical point-to-point 40 ONU is a binary digital signal, at least with respect to the connections. optical power of the signal. Thus, for detecting the bit [0040] Instead of single-fiber connections between the information included in the signal received, the CDR de- OLT 3, the RN 5 and the ONUs 7, also dual-fiber con- vice 41 may comprise a simple integrate-and-dump re- nections may be used without any impact on the nature ceiver. However, the CDR device 41 restricts the detec- of theoptical signals used forthe bi-directionaldata trans- 45 tion process to a selected section of the symbol interval, mission. Of course, in case of dual-fiber connections, the namely, to the first or second half of the symbol interval, RN 5 may comprise two separate demultiplexer/multi- as described below. plexer devices which are used for the upstream and [0044] The CDR device 41 supplies the recovered downstream transmission direction, respectively. As the clock signal S CK also to a driver circuit 45 which receives 50 downstream and upstream channel transmit signals the second digital transmit signal S2TX that is supplied S1TX,Chi and S2 TX,Chi use the same optical channels also to the local input port 33 of the ONU 7. The second digital in case of a dual-fiber connection between the OLT 3 and transmit signal S2TXi may be a 10 Gbit/s binary signal the RN 5, a single demultiplexer/multiplexer device may including the information to be transmitted to the OLT 3 be used in connection with suitable means, e.g. an optical in the upstream direction. circulator, adapted to combine and separate the trans- 55 [0045] The driver circuit 45 creates a second modulat- mission paths. ing signal S2MODi supplied to a reflective modulator de- [0041] If dual-fiber connections are used for connect- vice 47 such as a reflective semiconductor optical am- ing the RN 5 and the ONUs 7, each ONU 7 may be con- plifier (RSOA), an injection-locked Fabry-Perot laser di-

6 9 EP 2 830 239 A1 10 ode (IL-FP LD) or a reflective electro-absorption modu- section of equal length. Of course, if the signal S1TXi is lator with an integrated semiconductor optical amplifier not a binary NRZ signal, the transceiver device 19 may (REAM-SOA). The reflective modulator device 47 is able be configured to convert this signal into a respective (in- to reflect or reflect and amplify the second signal portion ternal) NRZ signal. The transceiver device creates a bit- 5 of the first optical channel transmit signal S1 TX,Chi which interleaved signal having twice the bit rate of the signal is split-off by the directional optical coupler 37 and sup- S1TXi, wherein each first section of each bit interval is set plied to an input/output port of the reflective modulator to the value of the respective bit of the signal S1TXi and device 47. This branch-off signal serves for creating the wherein each second section of each bit interval is set second optical channel transmit signal S2 TX,Chi which is to logical 1 (or "high"). This (intemal) signal is shown in 10 modulated according to the modulatlng signal S2MODi Fig. 4b. and which has the same optical wavelength as the [0050] This signal is duobinary pre-coded, i.e. the bit branch-off signal and thus as the first optical channel value of the duobinary pre-coded signal in Fig. 4c is ob- transmit signal S1TX,Chi. tained by inverting the bit-interleaved signal in Fig. 4b [0046] As shown in Fig. 3, in another embodiment of and applying an exlusive or (EXOR) operation on the an ONU realization for the invention, instead of a reflec- 15 inverted signal and the resulting signal (i.e. the signal tive modulator device a non-reflective modulator device which results from the EXOR operation) which is delayed 49 is used for creating the second optical channel trans- by one bit interval. For simplicity, the resulting pre-coded mit signal S2TX,CHi. The non-reflective modulator device signal, which is shown in Fig, 4c, reveals a logical 0 bit 49 may consist of an optical amplifier device 49b and an as start value for the EXOR operation. However, even if optical modulator device 49a, wherein these components 20 a logical 1 value is used as start value, this does not are preferably integrated in a single unit or component. change the resulting optical first channel transmit signal In this alternative, the optical branch-off signal is supplied STX,Chi. Merely, the phase of the E-vector of the electrical to the optical amplifier device 49b which optically ampli- field is shifted by 180 degrees, which has no influence fies thebranch-off signal created by theoptical directional on the optical power of the signal that is detected in the coupler and supplies the amplified signal to an optical 25 ONU using a direct detection method. modulator device 49a which modulates the amplified sig- [0051] As known, the pre-coding using inversion and nal according to the modulating signal S MODi supplied by EXOR operation of the delayed resulting signal is difficult the driver circuit 45. to perform at high bit rates. Thus, an equivalent pre-cod- [0047] As the modulator device 49 is non-reflective, an ing method has been developed which applies an AND optical circulator 51 is necessary for separating/combin- 30 operation for the inverted input signal (the signal to be ing the receive path comprising the components for cre- precoded) and a clock signal having the same bit rate, ating the second optical channel receive signal S1 TX,Chi the resulting AND signal being supplied to a T flip-flop in the respective ONU 7 and the transmit path comprising which realizes a modulo 2 counter. It has been shown the components for creating the second optical channel that this simplified method which does not require a delay 35 transmit signal S2 TX,Chi. As apparent from Fig, 3, the op- of the resulting signal by one bit interval leads to the same tical circulator 51 receives the first optical channel trans- results (W. Kaiser et al, "Reduced Complexity Optical mit signal S1TX,Chi and supplies it to the directional cou- Duobinary 10Gb/s Transmitter Setup Resulting in an In- pler 37. Further, the optical circulator 51 receives the creased Transmission Distance". Photonics Technology second optical channel receive signal S2 TX,Chi output by Letters, August 2001, which is incorporated herein by the non-reflective modulator device 49. 40 reference). [0048] The signal processing which is necessary within [0052] In a next step, the transceiver device 19 duob- the transceiver devices 19 of the OLT 3 and within the inary encodes the pre-coded signal according to Fig. 4c ONUS 7 in order to realize the data transmission method which results in the signal as shown in Fig. 4d. The du- according to the invention will now be described in detail obinary encoding, which includes an adding operation of with reference to Figures 4 to 6. For this purpose, the 45 the bit value of the actual bit interval and the bit value of communication and data processing necessary for a the previous bit interval, leads to the ternary signal shown point-to-point connection between a selected transceiver in Fig. 4d. device 19 within the OLT 3 and a dedicated ONU 7 is [0053] The signal processing including the pre-coding considered which uses a dedicated optical channel of and encoding operations is performed in a signal the WDM-PON. 50 processing device 52 comprised by a transmitter section [0049] As shown in Fig. 4, the selected transceiver de- of each transceiver device 19. The signal processing de- vice 19 receives the first digital transmit signal S TXi, which vice 52 performs all operations necessary to create a first may be a 10 Gbit binary NRZ signal represented by the modulating signal S1 MODi using the respective first trans- curve according to Fig. 4a. In order to create a first optical mit signal S1TXi. The modulating signal S1MODi is sup- 55 channel transmit signal S1TX,Chi whick can be reused to plied to an optical modulator device 53 of the transceiver create the respective second optical channel transmit device 19 or the transmitter section 19a of the transceiver signal S2TX,Chi in the dedicated ONU, the bit interval of device, respectively. Of course, the signal processing de- the NRZ signal S1TXi is divided into a first and a second vice 52 may be combined with a further signal processing

7 11 EP 2 830 239 A1 12 device that is adapted to perform the necessary signal is used with respect to the first and second optical chan- operations for the receiving of the second or upstream nel transmit signal S1 TX,Chi and S2 TX,Chi as these signals channel transmit signals S2TX,Chi by a receiver section are - at least with respect to the optical field vectors - 19b of the transceiver device 19. ternary signals. The symbol interval is defined in such a [0054] In a last step, a asymmetrical modulating signal 5 way that each symbol interval includes the information ("symmetrical" here means symmetrical with respect to of a single bit (or bit interval) of the respective NRZ signal the extreme values of the signal) is created by subtracting S1TXi. a value of one from the asymmetrical ternary signal. This [0061] In order to correctly detect the information in- symmetrical ternary modulating signal is shown in Fig. cluded in the respective signal S1TX,Chi the CDR device 10 4e. Finally, a low-pass filtering step may be applied in 41 of an ONU recovers the clock of the NRZ signal S1 TXi order to cancel high frequency noise from the signal, re- and thus the signal S1 TX,Chi. The CDR device 41, which sulting in a filtered symmetrical duobinary pre-coded and includes a receiver device, uses this recovered clock to encoded modulating signal S1MODi (see Fig. 4f) that is control the receiver device in such a way that only the used in the transceiver device 19 to create the respective respective (first) half section of each symbol interval is 15 first optical channel transmit signal S1 TX,Chi. For this pur- used for the signal detection. Fig. 5e shows this method pose, the modulating signal S1MODi may be supplied to by a curve indicating the time intervals or first half sec- a suitable optical modulator device 53, e.g. a Mach-Zeh- tions I1 of the bit intervals T B in which the bit information nder modulator (MZM) or a dual electro-absorption mod- of the NRZ signal is included. ulator (dual EAM), which is driven between two transmis- [0062] Fig. 5f illustrates the second half sections I 2 of 20 sion maxima including a 180-degree phase change. each symbol interval T B which include the seeding power [0055] Thus, the first optical channel transmit signal or seeding "half-bits". These sections of the symbol in- S1TX,Chi will have the same shape with respect to the tervals of the first optical channel transmit signal S1 TX,Chi envelope of the electrical field vector E as the modulating received at an ONU can be used to create the second signal S1MODi according to Fig. 4f. Of course, the optical optical channel transmit signal S2TX,Chi as described 25 power of the first optical channel transmit signal S1 TX,Chi above. As the (reflective or non-reflective) modulator de- reveals a shape of the envelope corresponding to the vice 47, 49 receives the recovered clock signal S CK, the squared envelope of the E-vector of the signal S1 TX,Chi. driver circuit 45 may use this signal to effect a modulation which is essentially 0 for bit values 0 of the NRZ signal of the optically amplified branch-off signal within the sec- S1TXi and which is unequal to 0 In bit Intervals that cor- ond sections I2 of the symbol interval TB only. This is respond to bit values 1 in the NRZ signal. 30 shown in Fig. 5g illustrating the optical power of a second

[0056] The method thus produces a first optical chan- optical channel transmit signal S2TX,Chi which has been nel transmit signal S1TX,Chi which includes the bit infor- created using a first optical channel transmit signal mation of the first transmit signal S1 TXi in a first half sec- S1TX,Chi that has been received at the ONU. Apparently, tion of the symbol interval of the first optical channel trans- the second optical channel transmit signal S2 TX,Chi is cre- 35 mit signal S1 TX,Chi and which includes an optical seeding ated in such a way that the information included in the power in the second half section of each symbol interval. first half section of the symbol interval of the signal

[0057] Of course, the roles of the first and second half S1TX,Chi is deleted (the optical power in the second sec- sections can be changed. The receiving and modulating tions is set to zero) in order to reduce interference. This function carried out in the ONU 7 may be adopted in a can be effected by using an RZ coding for the modulating 40 suitable manner by adjusting the synchronization of the signal S2MODi. However, it would be possible to use any receiving and modulating processes. type of OOK in order to modulate the amplified branch- [0058] It is not mandatory to use the duobinary pre- off signal in an ONU. coding and encoding of the bit-interleaved signal shown [0063] In order to detect the information included in the in Fig. 4b. This signal could also be directly used for cre- second optical channel transmit signal S2TX,Chi at the ating a corresponding optical signal. However, the re- 45 OLT 3, the respective transceiver device is also adapted quired bandwidth would increase by essentially the factor to recover the clock from the signal received and to syn- two. Additionally, the advantage of a duobinary pre-cod- chronize the receiver to the second half sections of the ed and encoded signal with respect to the reduced inter- signal received for recovering the information included symbol interference and increased dispersion tolerance in these second sections of each symbol interval T B. would be given up. 50 [0064] Fig. 6 illustrates signal sequences in connection [0059] As apparent from Fig. 5, which again shows the with a simplified method to create a duobinary pre-coded signal sequences of Figs. 4a, 4b, 4e and 4f in Figs. 5a and encoded first optical channel transmit signal S1 TX,Chi to 5d. the transceiver device in an ONU can recover the in the OLT 3. It is generally known that duobinary encod- information included in the first optical channel transmit ing can be effected by just lowpass filtering the pre-coded 55 signal S1TX,Chi by direct detection (I.e. evaluating the op- signal. This is apparent from a comparison of Figs. 4c tical power of the signal received) applied to the first half and4f. A low-pass filtering applied to the pre-coded signal section of each symbol interval. In Fig. 4c leads to a similar signal revealing a given slope [0060] It shall be noted that the term "symbol" interval at the rising and trailing edges. The low-pass filter can

8 13 EP 2 830 239 A1 14 be designed such that the filtered (symmetrical) signal the creation of the upstream (or second) optical channel crosses the time axis in essentially the same points as transmitsignal S2 TX,Chi,remain unchanged as compared the filtered (symmetrical) signal in Fig. 4f. For creating a to the identical signals shown in Figs. 5e to 5e, 5f and 5g. duobinary encoded signal having the same bit-rate as [0069] The advantage of the filtered coding method as the pre-coded signal,a low-passfilter having a filter band- 5 compared to the direct insertion of interleave bits accord- width of approximately 0.35 times the bit rate of the re- ing to the coding method illustrated with the signals in spective signal is used. Fig. 4 and 5, is that the coder device included in the signal [0065] In an embodiment of a transceiver device 19 processing device 52 of the transmitter section 19a of adapted to realize the data transmission method or cre- the transceiver device 19 must run at the bit-rate of the 10 ating of a first optical channel transmit signal S1 TX,Chi as NRZ signal, only, whereas in the direct insertion method illustrated with the signals in Fig. 6, respectively, the fol- as described above with reference to Figs. 4 and 5 the lowing steps are performed: in a first step, the first trans- coder device must run at twice the bit-rate of the NRZ mit signal S1TXi (Fig. 6a), which is the respective NRZ signal S1 TXi. The optical modulator devices 53 run in both signal, is duobinary pre-coded as described above. Al- cases at around the bandwidth of the NRZ signal due to though the signal sequence in Fig. 6a is identical to the 15 the bandwidth reduction achieved by the duobinary cod- signal sequence in Fig. 4a, the pre-coded signal shown ing. in Fig. 6b (the signal sequence has already been made [0070] Thus, the invention provides a data transmis- symmetrical with respect to the extreme values) is in- sion method applying wavelength reuse of a downstream verse as compared to the corresponding signal in Fig. signal using an NRZ bit-interleaved signal in the down- 4c. This is due to the fact that in Fig. 6b a value of logical 20 stream direction and any arbitrary amplitude modulation 1has been usedas start value for the pre-codingprocess. format to transmit the information included in an up- As explained above, when using this signal as modulat- stream digital signal In the upstream direction, wherein ing signal S1 MODi that is supplied to an optical modulator the signal portion according to the interleave bits included device like a MZM or a dual EAM, a ternary optical signal in the downstream signal, which is received and reused with respect to the electrical field vector envelope is cre- 25 (amplified and modulated) at the second end of the trans- ated, which is a binary optical signal with respect to the mission link to create a corresponding upstream signal, optical power envelope. As a consequence, an inversion is used to transport the information in the upstream di- of the modulator signal leads to the identical optical signal rection. Especially NRZ or RZ pulse shaping may be ap- as far as the optical power envelope is concerned. plied to modulate the interleaved bit portions of the wave- [0066] The decisive difference between the well-30 length reuse signal created at the second end of the known duobinary encoding using a low-pass filter and transmission link. the present method of creating a bit-interleaved modu- [0071] The method can be implemented with or without lating signal S1MODi is that the low-pass filter must be optical duobinary coding of the optical downstream trans- designed in such a way that the zero-crossings of the mit signal. This method can easily and at low costs be 35 filtered duobinary (ternary) modulating signal S1 MODi are implemented in suitable transceiver devices used at both essentially in the center of the first half-section of the ends of the transmission link. Especially when optical symbol interval (which is equal to the bit interval of the duobinarycoding is applied,using a low-pass filter having NRZ signal S1TXi), i.e. the slope of the filtered duobinary an adjusted filter bandwidth to effect the insertion of in- modulating signal S1MODi Is chosen such that the signal terleaved seeding bits (which represent the part of the zero-crossings are essentially at a quarter of the bit or 40 reused downstream signal that can be amplitude-modu- symbol interval (or at three quarters of the bit or symbol lated in order to include the upstream signal information) interval, if the second half section of the symbol interval drastically reduces the effort and costs to implement the is used for transmitting the information of the first transmit method in suitable transceiver devices at both ends of signal S1TXi and the seeding "half-bit" is included within the transmission link. the first half section). Generally, the low-pass filtering is 45 effected using a low-pass filter that reveals a (3dB) filter List of reference signs bandwidth of approximately 0.7 times the bit-rate of the

NRZ first transmit signal S1 TXi. However, defining merely [0072] the filter bandwidth Is not sufficient to characterize a suit- able low-pass filter without defining the type and/or order 50 1 WDM-PON, wavelength division multiplex pas- of the filter. sive optical network [0067] As apparent from Fig. 6, the low-pass filtering of the pre-coded signal in Fig. 6b as described above is 3 OLT, optical line terminal equivalent to the insertion of seeding "half-bits". [0068] This encoding method does therefore change 55 5 RN, remote node nothing as far as the signal processing in the ONUs 7 is concerned. Thus, the signal sequences shown in Figs. 7 ONU optical network unit 6d, 6e and 6f, which illustrate the signal detection and

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9 WDM port (OLT) n number of optical channels S1TXi first digital transmit signal (1≤i≤n) 11 WDM port (RN) S2TXi second digital transmit signal (1≤i≤n) S1RXi first digital receive signal (1≤i≤n) 5 13 optical filter S2RXi second digital receive signal (1≤i≤n) S1TX,Chi first optical channel transmit signal (1 ≤i≤n) 14 channel port (RN) (1≤i≤n) S2TX,Chi) second optical channel transmit signal (1≤i≤n) 15 remote port (ONU) S1TX,WDM first WDM transmit signal (1≤i≤n) 10 S2TX,WDM second WDM transmit signal (1≤i≤n) 17 optical fiber SCK clock signal S1MODi first modulating signal (1≤i≤n) 19 transceiver device S2MODi second modulating signal (1≤i≤n)

19a transmitter section 15 Claims 19b receiver section 1. Method for bi-directionally transmitting digital optical 21 local input port (1≤i≤n) signals over an optical transmission link, 20 23 local output port (1≤i≤n) (a) wherein a first optical transmit signal (S1TX,CHi) at a predetermined optical wave- 25 optical multiplexer device length is supplied to a first end of the optical transmission link and transmitted in a first trans- 27 optical circulator 25 mission direction to a second end of the optical transmission link, 29 WCM port (OLT) (b) wherein a second optical transmit signal (S2TX,CHi) at said predetermined optical wave- 31 local output port (ONU) length issupplied tothe second end of the optical 30 transmission link and transmitted in a second 33 local input port (ONU) opposite transmission direction to the first end of the optical transmission link, 35 demultiplexer device (c) wherein said first optical transmit signal (S1TX,CHi) is created according to a first binary 35 37 directional coupler digital signal (S1TXi) in such a way that the bit information of the first binary digital signal

39 opto-electrical converter device (S1TXi) is included in first sections of the symbol interval of the first optical transmit signal, and 41 CDR, clock and data recovery device (d) wherein said second optical transmit signal 40 (S2TX,CHi) is created by creating an optical 43 electrical low-pass filter wavelength reuse signal using the first optical transmit signal (S1 TX,CHi)received at thesecond 45 driver circuit end of the optical transmission link, the optical wavelength reuse signal being modulated ac- 47 reflective modulator device 45 cording to a second digital signal in such a way that the bit information of the second digital sig- 49 non-reflective modulator device nal is included in second sections of the symbol interval of the first optical transmit signal

49a optical modulator device (S1TX,CHi) received, 50 characterized in that 49b optical amplifier device (e) the first binary digital signal (S1 TXi) is a non- return-to-zero signal and

51 optical dirculator (f) the first optical transmit signal (S1TX,CHi) is an optical bit-interleaved seeding signal having 52 signal processing device 55 a symbol rate equal to the bit rate of the first binary digital signal (S1 TXi), wherein the symbol 53 optical modulator device interval of the optical bit-interleaved seeding sig- nal is divided into two equally long sub-intervals,

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the bit information of the first binary digital signal into the optical bit-interleaved seeding signal in such (S1TXi) being transported in a first of the two sub- a way that the extreme values of the ternary modu- intervals and the second sub-intervals being set lating signal (S1MODi) correspond to +E and -E and to a seeding level with respect to the optical pow- the intermediate value of the ternary modulating sig- 5 er. nal (S1MODi) corresponds to 0, wherein +E, 0 and -E designates the amplitude of the electric vector of the 2. Method according to claim 1, characterized in that optical bit-interleaved seeding signal. the optical bit-interleaved seeding signal is an optical bit-interleaved duobinary coded seeding signal cre- 5. Method according to one of the preceding claims, ated by 10 characterized in that the optical bit-interleaved seeding signal received at the second end of the (a) converting the first binary digital signal transmission link is divided, with respect to the opti- (S1TXi) into a binary bit-interleaved digital signal cal power of the signal, into an optical receive signal having twice the blt rate of the first binary digital and an optical branch-off signal, and that the optical 15 signal (S1TXi) by dividing each bit interval into receive signal is used for receiving the first binary two equally long sub-intervals, setting one sub- digital signal (S1TXi) by detecting the optical power interval to the signal value of the first binary dig- during the first of the two equally long sub-intervals,

ital signal (S1 TXi) and setting the respective oth- e.g. by using an integrate-and-dump receiver. er sub-interval to the logical 1 value of the first 20 binary digital signal (S1TXi), 6. Method according to claim 5, characterized in that (b) pre-coding the binary bit-interleaved digital the optical branch-off signal is used to create the signal and duobinary encoding the pre-coded optical wavelength reuse signal using a reflective binary bit-interleaved digital signal and modulator device, e.g. a reflective semiconductor (c) modulating an optical light source having the optical amplifier, an injection-locked Fabry-Perot la- predeterminedoptical wavelength using thepre- 25 ser diode or a reflective electro-absorption modula- coded and encoded binary bit-interleaved digital tor with an integrated semiconductor optical amplifi- signal as modulating signal (S1MODi). er.

3. Method according to claim 1, characterized in that 7. Method according to one of the preceding claims, the optical bit-interleaved seeding signal is an optical 30 characterized in that the second sub-intervals of bit-interleaved duobinary coded seeding signal cre- the optical wavelength reuse signal is amplitude- ated by modulated, preferably using OOK.

(a) pre-coding the first binary digital signal 8. Method according to claim 7, characterized in that 35 (S1TXi) and duobinary encoding the pre-coded the second digital optical signal is an NRZ or RZ first binary digital signal, signal. (b)low-pass filtering the pre-coded and encoded first binary digital signal in such a way that the 9. Optical data transmission system for bi-directionally filtered pre-coded and encoded first binary dig- transmitting digital optical signals over an optical ital signal assumes in each transition between 40 transmission link comprising: its extreme values a value which essentially equals the average value of the extreme values (a) a first optical transceiver device (19) config- at points in time which essentially define a quar- ured to be connected to a first end of the optical ter of the bit interval, and transmission link and adapted (c) modulating an optical light source having the 45 predetermined optical wavelength using the (i) to create a first optical transmit signal

low-pass filtered pre-coded and encoded first bi- (S1TX,CHi) at a predetermined optical wave- nary digital signal as modulation signal length which is supplied to the first end of

(S1MODi). the optical transmission link and transmitted 50 in a first transmission direction to a second 4. Method according to one of claims 2 or 3, charac- end of the optical transmission link, terized in that the duobinary optical bit-interleaved (ii) said first optical transmit signal

seeding signal is a ternary optical signal with respect (S1TX,CHi) being created according to a first to the electric field and a binary digital signal with binary digital signal (S1TXi) in such a way respect to the optical power, preferably created by 55 that the bit information of the first binary dig- using an optical modulator device, e.g. a Mach-Zeh- ital signal (S1 TXi) is included in first sections nder modulator or a dual electro-absorption-modu- of the symbol interval of the first optical lator, which converts the modulating signal (S1 MODi) transmit signal (S1TX,CHi), and

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(iii) to receive a second optical transmit sig- (b) pre-coding the first binary digital signal nal (S2TX,CHi) at said predetermined optical (S1TXi) and duobinary encoding the pre-coded wavelength which is supplied to a second binary bit-interleaved digital signal and end of the optical transmission link and (c) modulating an optical light source having the transmitted in a second opposite transmis- 5 predeterminedoptical wavelengthusing the pre- sion direction to the first end of the optical coded and encoded binary bit-interleaved digital

transmission link, signal as modulating signal (S1MODi).

(b) a second optical transceiver device (7) con- 11. Optical data transmission system according to claim figured to be connected to the second end of the 10 9, characterized in that the first optical transceiver optical transmission link and adapted device (19) is configured to create the optical bitin- terleaved seeding signal as a duobinary optical bit- (i) to receive the first optical transmit signal interleaved seeding signal created by

(S1TX,CHi) and to detect the information in- cluded in the first sections of the symbol in- 15 (a) pre-coding the first binary digital signal terval thereof, and (S1TXi) and duobinary encoding the pre-coded (ii) to create the second optical transmit sig- first binary digital signal,

nal (S2TX,CHi) by creating an optical wave- (b)low-pass filtering thepre-coded and encoded length reuse signal using the first optical first binary digital signal in such a way that the 20 transmit signal (S1TX,CHi) received and filtered pre-coded and encoded first binary dig- modulating the optical wavelength reuse ital signal assumes in each transition between signal according to a second digital signal its extreme values a value which essentially in such a way that the bit information of the equals the average value of the extreme values second digital signal is included in second at times which essentially are equal to a quarter sections of the symbol interval of the first 25 of the bit interval, and optical transmit signal (S1TX,CHi) received, (c) modulating an optical light source having the characterized in that predetermined optical wavelength using the low-pass filtered pre-coded and encoded first bi-

(c) the first binary digital signal (S1 TXi) is a non- nary digital signal as modulating signal 30 return-to-zero signal and (S1MODi). (d) the first optical transceiver device is config- ured to create the first optical transmit signal 12. Optical transceiver device for an optical data trans- (S1TX,CHi) as an optical bit-interleaved seeding mission system for bi-directionally transmitting dig- signal having a symbol rate equal to the bit rate ital optical signals over an optical transmission link. 35 of the first binary digital signal (S1TXi), wherein the symbol interval of the optical bit-interleaved (a) the optical transceiver device (19) configured seeding signal Is divided into two equally long to be connected to a first end of the optical trans- sub-intervals, the bit information of the first bi- mission link and adapted nary digital signal (S1 TXi) being transported in a first of the two sub-intervals and the second sub- 40 (i) to create a first optical transmit signal

intervals being set to a seeding level with respect (S1TX,CHi) at a predetermined optical wave- to the optical power. length which is supplied to the first end of the optical 10. Optical data transmission system according to claim transmission link and transmitted in a first 9, characterized in that the first optical transceiver 45 transmission direction to a second end of device (19) is configured to create the optical bitin- the optical transmission link, terleaved seeding signal as a duobinary optical bit- (ii) said first optical transmit signal

interleaved seeding signal created by (S1TX,CHi) being created according to a first binary digital signal (S1TXi) in such a way (a) converting the first binary digital signal50 that the bit information of the first binary dig- (S1TXi) into a binary bit-interleaved digital signal ital signal (S1 TXi) is included in first sections having twice the bit rate of the first binary digital of the symbol interval of the first optical

signalby dividing each bit intervalinto two equal- transmit signal (S1Tx,CHi), and ly long sub-intervals, setting one sub-interval to (iii) to receive a second optical transmit sig- 55 the signal value of the first binary digital signal nal (S2TX,CHi) at said predetermined optical (S1TXi) and setting the respective other sub-in- wavelength which is supplied to a second terval to the logical 1 value of the first binary end of the optical transmission link and digital signal (S1TXi), transmitted in a second opposite transmis-

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sion direction to the first end of the optical of the bit interval, and transmission link, (c) modulating an optical light source having the predetermined optical wavelength using the characterized in that low-pass filtered pre-coded and encoded first bi- 5 (b) the first binary digital signal (S1 TXi) is a non- nary digital signal as modulating signal return-to-zero signal and (S1MODi). (c) the optical transceiver device (19) is config- ured to create the first optical transmit signal (S1TX,CHi) as an optical bit-interleaved seeding signal having a symbol rate equal to the bit rate 10

of the first binary digital signal (S1TXi), wherein the symbol interval of the optical bit-interleaved seeding signal is divided into two equally long sub-intervals, the bit information of the first bi- 15 nary digital signal (S1 TXi) being transported in a first of the two sub-intervals and the second sub- intervals being set to a seeding level with respect to the optical power.

13. Optical transceiver device according to claim 12,20 characterized in that the optical transceiver device (19) Is configured to create the optical bit-interleaved seeding signal as a duobinary optical bit-interleaved seeding signal created by 25 (a) converting the first binary digital signal (S1TXi) into a binary bit-interleaved digital signal having twice the bit rate of the first binary digital

signal (S1TXi) by dividing each bit interval into two equally long sub-intervals, setting one sub- 30 interval to the signal value of the first binary dig- ital signal (S1 TXi) and setting the respective oth- er sub-interval to the logical 1 value of the first binary digital signal (S1TXi), (b) pre-coding the binary bit-interleaved digital 35 signal and duobinary encoding the pre-coded binary bit-interleaved digital signal and (c) modulating an optical light source having the predeterminedoptical wavelength using thepre- coded and encoded binary bit-interleaved digital 40

signal as modulating signal (S1MODi).

14. Optical transceiver device according to claim 1. characterized in that the optical transceiver device (19) is configured to create the optical bit-interleaved 45 seeding signal as an duobinary optical bit-inter- leaved seeding signal created by

(a) pre-coding the first binary digital signal 50 (S1TXi) and duobinary encoding the pre-coded first binary digital signal, (b)low-pass filtering the pre-coded and encoded first binary digital signal in such a way that the filtered pre-coded and encoded first binary dig- ital signal assumes in each transition between 55 its extreme values a value which essentially equals the average value of the extreme values at times which essentially are equal to a quarter

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