Field Of The Invention

[0001] The present invention relates to transmission of signals in fiber optic networks. Background Of The Invention

[0002] A state-of-the-art passive optical network will be described, with reference to Figure 1 . Figure 1 shows a passive optical network ("PON") and illustrates some concepts associated with it. In fiber-optic access networks, used for example in fiber-to-the-home or fiber-to-the-premises broadband communi- cation, a Passive Optical Network (PON) has various advantages, as network cost is shared between a number of customers. Current PON standards include IEEE 802.3ah GEPON and ITU-T G.984 GPON. PON has a point-to- multipoint topology, passively connecting service provider site to several customer sites with optical signals. In service-provider-to customers, or down- stream, direction, an optical signal is passively split among several fibers, such that each customer site is served by a fiber. In upstream direction, signals are passively combined to same fiber. Downstream and upstream directions usually employ the same fibers. A PON provides several advantages. For example, the absence of active electronic or electrical equipment makes the PON re- sistant to harsh outside environment. Compared with an electronic network, a PON requires no electrical power, is more reliable, and avoids risks for safety or signal interference from electrical equipment. Furthermore, a PON can support several different optical signal formats and bit rates, and the fiber bandwidth can be shared by multiple optical signals in downstream as well as up- stream directions.

[0003] In a basic PON, downstream traffic to multiple customers is combined into single optical signal from one transmitter, and distributed using one or more passive power splitters, which divide the optical signal among the multiple fibers. At each customer site, an Optical Network Unit ("ONU") receives an identical copy of the signal, and uses electronic means to extract traffic addressed to that particular customer from the optical signal. Security techniques, such as cryptography, is frequently used to prevent customers from accessing traffic destined for other customers.

[0004] Upstream traffic typically uses the same fiber paths as downstream, but at a different wavelength. Wavelengths may be separated by spectral filters at endpoints of the optical network, for example. As upstream traffic is transmitted from multiple customers and combined to a single common receiver, each customer site may be assigned a different timeslots, so as to avoid collision of signals at the receiver. Thus at any given time, a signal from only one customer is allowed to arrive at a receiver. The upstream receiver needs to be able to handle such burst-mode traffic, which is discontinuous, and with optical power levels and clock frequencies unsynchronized between signal bursts.

[0005] Referring to Figure 1 , a service provider's interface to a PON is called an Optical Line Termination ("OLT"), denoted by reference numeral 1 -2. Refer- ence numeral 1 -8 denotes a set of remote units. It is conventional to refer to the remote units as Optical Network Units ("ONUs") if the final connection from there to customer site is made using copper line, or Optical Network Terminals (ONTs) if the final connection is via fiber. For general considerations relating to a PON, it is not necessary to make this difference, which is why in the present context, the remote units are called ONUs, unless specifically stated otherwise.

[0006] PONs are currently mainly limited by available optical power in terms of a tolerated attenuation range during transmission. A power budget refers to the difference in the signal power launched at a transmitter on one hand, and the receiver sensitivity level at a required bit-error-rate on the other hand. The power budget needs to accommodate for both fiber transmission losses and power losses due to the splitting of the optical signal among multiple ONUs, and also possible penalties due to deterioration of signal quality. While it may not be immediately apparent, upstream signals will experience similar splitting losses in a PON splitter tree as downstream signals do.

[0007] Power budget considerations frequently necessitate resorting to reach extenders, to compensate for losses caused by transmission fiber lengths and high splitting ratios. Reach extension has been standardized in ITU-T G984.6. Reach extension involves including an active element, boosting signal powers to higher levels at mid-span, between the splitting PON tree and a common optical trunk feeder fiber, which connects the splitters to the OLT. In such cases, the optical network between the OLT and ONUs will not be fully passive. Reach extension can be based on regenerators or optical amplifiers. Regenerators include a receiver for converting the signal to the electrical domain, and a transmitter for sending it forward at a higher optical power. The optical amplifi- er is typically a semiconductor optical amplifier (SOA) having an optical gain within the used spectral bandwidth. Reference numeral 1 -4 denotes an exemplary reach extender.

[0008] Current PON systems may have bit rates in the range of several gigabits per second, while transmission distances may be as long as 10 to 20 km, and the number of ONUs may be up to 32. Next-generation PONs are expected to have data rates up to 10 Gb/s, first asymmetrically in only downstream direction, and later also symmetrically, in upstream direction. Other expected developments are increased transmission lengths, especially using long trunk feeder (up to around 100 km) before splitting, and the number of ONUs may extend up to 512 or even 1024. Reference numeral 1 -6 denotes one of several splitters used in the PON. Specifically, the splitter 1 -6 is the first splitting point of a downstream signal or the last combination point of an upstream signal.

[0009] A general description of state-of-art PONs is given by Reference docu- ment 1 (Shumate). ASK modulation is the predominant modulation in the PONs, in view of the fact that the customer-site devices, ONUs, are very cost- sensitive. The costs of the OLT equipment and the possible reach extender are shared by multiple ONUs, and thus are not as critical.

[0010] As described above, reach extenders for PONs are mainly of two varie- ties. Regenerators receive and retransmit the signal in the original format. Optical amplifiers amplify the signal, otherwise maintaining it in original form, except for some impairments related to amplification process (adding optical noise and signal waveform distortion). Particularly, semiconductor optical amplifiers must generally operate below their saturated output power, to avoid signal distortion, and this limits the output power of reach extenders.

[0011] An object of the present invention is to alleviate one or more problems associated with prior art optical networks. Specifically, the object of the invention is to improve one or more parameters of signal transmission in optical networks, wherein the improved parameters include one or more of the follow- ing: number of supported receivers, resistance to network faults, quality of transmitted signal, equipment complexity or cost.

Summary Of The Invention

[0012] The object of the invention is achieved by a method and apparatus as defined in the attached independent claims. The dependent claims as well as the present patent specification and drawings provide specific embodiments which provide additional features and/or solve additional problems.

[0013] An aspect of the invention is an optical system comprising:

- an optical signal filter comprising at least one input port, a first output port and a second output port, wherein the optical signal filter is configured to receive a differential phase shift keyed ["DPSK"] input signal from the at least one input port and to divide the DPSK input signal into to a first and second amplitude shift keyed ["ASK"] output signals which are respectively outputted from the first output port and a second output port;

- wherein each of the first ASK output signals exhibits a spectral response to the DPSK input signal, the spectral response being periodic within a predetermined signal bandwidth;

- wherein the first and second ASK output signals have identical data but opposite polarities with respect to each other; and

- the spectral responses of the first and second ASK output signals are complementary to each other;

wherein the optical system further comprises:

- one or more optical amplifiers operatively coupled to the optical signal filter and configured to amplify the DPSK input signal and/or the ASK output sig- nals;

- means for operatively coupling the first and second ASK output signals respectively to a first set and second set of optical receiving network elements via different portions of an optical network.

[0014] In the present context, a spectral response within a DPSK signal band- width consists of one or more pass bands periodically separated in frequency by a frequency difference that corresponds to the DPSK signal's symbol rate, wherein the pass band has a width which is about half of this spectral period, and wherein the pass bands are separated by stop bands also having widths about half of this spectral period. The spectra for the two filter outputs are complementary, such that a pass band for one output is a stop band for the other output. The optical signal filter acts as a demodulator as it demodulates a DPSK signal into a pair of ASK signals. As used herein, the term "DPSK" also includes multi-level variations of the DPSK format, such as DBPSK, DQPSK or D8PSK, wherein the B, Q and 8 stand for binary-level, quaternary-level and 8- level signal, respectively. In an illustrative but non-restrictive embodiment, with a DBPSK input signal, the first ASK output signal is in a duobinary format and the second ASK output signal is in an alternate-mark inverted format.

[0015] A benefit of the invention is that utilization of signal energy is optimized in the sense that both of the first and second ASK output signals are used to serve, respectively, a first set and second set of optical receiving network elements via different portions of an optical network.

[0016] For downstream operation the first and second set of optical receivers comprise optical network units ("ONUs"). For upstream operation the first and second set of optical receivers comprise optical line termination units ("OLTs"). The first set and second set of receivers may be mutually exclusive sets, whereby the optical system provides increased coverage in downstream operation, or it may provide equipment redundancy and protection in upstream operation. Alternatively, the first set and second set of receivers may share at least some of the receivers, whereby the optical system provides protection to the shared receivers, by providing the first and second ASK output signals to the receiver sites via different optical networks or portions thereof.

[0017] In some embodiments of the invention, at least one optical amplifier is configured to operate in a saturating region wherein the amplifier exhibits a gain which is a decreasing function of an input power to the amplifier. The sat- urating amplifier may be a semiconductor optical amplifier (SOA). Operating the optical amplifier(s) in saturation is somewhat contrary to established teaching in the art, for several reasons. For instance, the output of a saturating amplifier depends on certain time-variant effects, such as a finite gain recovery time, which in turn gives rise to a so-called "pattern effect," wherein the ampli- fier output varies according to the input power signal shape, thus creating a pattern. It turns out, however, that in some implementations of the present invention, the saturating amplifier provides certain benefits that outweigh its disadvantages. While an amplifier configured to operate in a linear operating region provides more gain, a saturating amplifier emits maximal output power, which can be used to serve a larger number of receivers than can be served by a linear amplifier. Alternatively, the higher emitted output power of the saturating amplifier can be utilized to relax some of the required specifications of the receivers. Furthermore, operation of the amplifier in saturation helps mitigate optical power variations of the optical signals. This is particularly benefi- cial in upstream signal processing, wherein the different ONUs may transmit at different power levels, or they may be connected via different PONs to the in- ventive optical system. Yet further, a saturating amplifier emits less amplified stimulated emission ("ASE") noise than a linear amplifier does, whereby the saturating amplifier has less need for a subsequent bandpass filter.

[0018] Saturating amplification provides particular benefits in some implemen- tations of the present invention, wherein the saturating amplifier simultaneously amplifies two symbols of opposite polarity from the two ASK output signals corresponding to the same symbol from the DPSK input signal. The two ASK output signals are coupled from the two outputs of the optical signal filter to mutually opposite ends of a two-ended optical amplifier, which is configured to op- erate in saturation. The simultaneous amplification of the two ASK signals of opposite polarity tends to keep the combined power level of the ASK signals at a constant level, thus reducing the effects of input power level variations. In the case of very weak input signals it may be beneficial to precede the optical signal filter with at least one amplifier configured to operate in a linear region wherein the amplifier exhibits a constant gain, which is typically higher than the gain of the saturating amplifier.

[0019] The optical signal filter is preferably a delay interferometer, in which case the delay interferometer may be configured to receive the DPSK input signal either to its first or second input port. Such a configuration provides pro- tection (redundancy) to a trunk line providing the DPSK input signal. Alternatively, the optical signal filter may be realized using other types of optical filters, such as multilayer thin film filters, fiber Bragg gratings, or the like.

Brief Description Of The Drawings

[0020] Specific embodiments of the present invention will be described in con- nection with the attached drawings, in which:

Figure 1 shows a passive optical network ("PON") and illustrates some key concepts;

Figure 2 illustrates division of a differential phase shift keyed ("DPSK") input signal into to a first and second amplitude shift keyed ("ASK") output signals, which are of opposite polarity to each other;

Figures 3 and 4 illustrate alternative locations for an optical amplifier;

Figures 5, 6 and 7 illustrate operation of an optical amplifier operating in a saturating region;

Figure 8 shows an embodiment in which a saturating amplifier stage is located after the optical signal filter (eg a delay interferometer); Figure 9 shows a variation of the embodiment shown in Figure 8;

Figure 10 shows an embodiment of the invention as adapted for use in a wavelength-division multiplexed ("WDM") PON;

Figure 1 1 shows a PON network, as complemented with an optical system ac- cording to the present invention;

Figure 12 shows a use case in which the inventive optical system is located at a point other than the first downstream splitter/last upstream combiner;

Figure 13 illustrates a use case in which the first and second sets of receivers share some or all of the receivers, whereby the optical system provides protec- tion to the shared receivers;

Figure 14 illustrates a use case in which the first and second inputs of the optical system are both used for the purpose of providing protection to the downstream feeder trunk line;

Figure 15 illustrates an alternative for providing protection to the downstream feeder trunk line;

Figure 16 shows a use case in which the two outputs of the inventive optical system server OLT equipment;

Figure 17 shows an alternative use case to the one shown in Figure 16;

Figure 18 shows a conventional bi-directional amplifier for a PON, called a PON extender;

Figure 19 shows how an optical system according to the invention can be used to improve the PON extender shown in Figure 18;

Figure 20 shows an enhanced version of the embodiment shown in Figure 19, Figure 21 shows an embodiment in which the bi-directional amplifier shown in Figure 20 can be simplified by omitting one of the two delay interferometer; Figure 22 shows a variation of the embodiment shown in Figure 21 ;

Figure 23 shows an embodiment of the invention in connection with a multi- wavelength optical amplifier;

Figure 24 clarifies certain issues relating to a spectral response of the optical signal filter and bandwidth; and

Figure 25 illustrates how the invention can be used with variations of the DPSK format, such as DBPSK, DQPSK or D8PSK.

Detailed Description Of Specific Embodiments

[0021] The specific embodiments described in connection with the attached drawings are intended to illustrate the present invention and not to restrict it. Throughout the drawings, like reference numerals and reference signs denote similar elements, and the beginning of the reference numeral or sign indicates the drawing in which the associated element is first described. For instance, reference numeral 2-10 denotes an element which first appears in Figure 2, and a repetitive description of the same element in connection with further drawings is omitted.

[0022] Figure 2 illustrates division of a differential phase shift keyed ("DPSK") input signal into to a first and second amplitude shift keyed ("ASK") output signals, which are of opposite polarity to each other.

[0023] Reference numeral 2-10 denotes a delay interferometer, which is an illustrative example of an optical signal filter capable of dividing a DPSK input signal to two ASK output signals of opposite polarity. In a binary ASK signal, data is coded into two digital levels by the amplitude level of symbols, which is either at a high or low level. The mutually opposite polarity of the two signals means that when the amplitude for a symbol in one signal is at a high level, the amplitude for the symbol in the other signal is at a low level. The two signals of opposite polarity have complete redundancy with respect to each other in the information they carry.

[0024] A DPSK input signal from a transmitting entity Tx (not shown) 2-20 en- ters a first input port 2-1 1 of the delay interferometer 2-10, while its second input port 2-12 is left uncoupled in the configuration shown in Figure 2. The delay interferometer 2-10 comprises a first and second optical couplers 2-13, 2-16, which may be 3 dB couplers. The two optical couplers 2-13, 2-16 are connected by two optical paths 2-15, 2-14. The first and second optical paths 2-15, 2-14 have different optical path lengths, and the difference is inversely proportional to the received symbol rate, such that the phase modulated DPSK signal is transformed into the two ASK output signals 2-31 , 2-32 at the first and second outputs 2-17, 2-18 of the delay interferometer 2-10. In effect, the optical path length difference equals the distance travelled by the optical signal in an optical medium in a time corresponding to one symbol period. As shown in Figure 2, input signal 2-20 is DBPSK modulated, which means that information is carried in differential phase shifts between consecutive pulses, shown as {π, π, 0}. In typical demodulation, one of the ASK output signals, eg, the first ASK output signal 2-31 , is in a duobinary format and the other ASK output signal, eg, the second ASK output signal 2-32, which is of opposite polarity to the first ASK output signal 2-31 , is in an alternate-mark inverted (AMI) format. Or, the first ASK output signal 2-31 may be in an AMI format and the second ASK output signal 2-32 may be in a duobinary format. By adjusting the relative phase difference between the two arms 2-14 and 2-15 of the delay interferometer 2- 10 by π (±η ^* 2π) radians, it is possible to switch the duobinary and AMI formats between the two output signals 2-31 and 2-32. A relative phase shift of π (±η ^* 2π) radians corresponds to a λ/2 (±η*λ) difference in optical path length, wherein λ means the wavelength of the optical signal. In case the phase difference is detuned to differ from these values, or for DPSK formats other than DBPSK, the output ASK formats are not strictly duobinary and alternate-mark- inverted, but they are still of opposite polarity to each other.

[0025] Had the input signal 2-20 been coupled to the second input port 2-12 instead of the first input port 2-1 1 , the delay interferometer 2-10 would have provided the same output signals but from different output ports. In other words, output port 2-17 would provide output signal 2-32 and output port 2-18 would provide output signal 2-31 . Embodiments utilizing both input ports for input protection will be described later, in connection with Figures 14 and 15.

[0026] According to the invention, the optical system is capable of coupling the first and second ASK output signals 2-31 , 2-32, respectively to a first and second set of optical receiving network elements via different portions of an opti- cal network. The two sets of optical receiving network elements are denoted by reference signs Rx1 , Rx2, although the sets themselves are not shown in Figure 2.

[0027] It should be noted that the two amplitude modulated output signals 2-31 and 2-32 carry identical information but in mutually opposite signal polarities, shown as {0, 1 , 0} and {1 , 0, 1}. The receivers in the sets Rx1 , Rx2 must be able to take the signal polarity in account, and they must adjust their operations accordingly. For example, the receivers of one set, say Rx2, may need to flip (reverse) the opposite polarity bits to the correct polarity. Such polarity reversal may be achieved by hardware, such as electronics, or by software. The polarity may be known by the receiver by its location in the network, which means that the polarity is indicated to the receiver beforehand. Alternatively or additionally, the receivers can receive a polarity configuration signal from the transmitter, and recognize the polarity of the signal and automatically adjust their operation accordingly. For such automatic detection the receivers may use an a-priori known sequence of overhead bytes, for example. [0028] Figure 3 illustrates three alternative locations for an optical amplifier. Reference numeral 3-10 denotes an optical amplifier positioned in front of the optical signal filter (shown as delay interferometer) 2-10. The optical amplifier 2-10 is configured to amplify the DPSK input signal 2-20. Reference numerals 3-12 and 3-14 each denote a pair of optical amplifiers configured to amplify the DPSK signals inside the delay interferometer and the ASK output signals, respectively. The optical amplifiers 3-12 reside within the delay interferometer 2- 10, while optical amplifiers 3-14 are located after it.

[0029] Figure 4 shows an embodiment in which an amplifier stage 4-10 is con- figured to simultaneously amplify two symbols of opposite polarity from the two ASK output signals corresponding to the same input symbol. This configuration is advantageous in connection with a saturating amplifier. As will be explained in connection with Figure 5, the use of a saturating amplifier in any of the configurations 3-10, 3-12 or 3-14 shown in Figure 3 will reduce or eliminate power level variations of high-level pulses. A saturating amplifier in any of the configurations has little or no effect on power level variations of low-level pulses, however, because low-level pulses do not drive the amplifier into saturation.

[0030] In the embodiment shown in Figure 4, an amplifier stage 4-10 comprising an optical amplifier 4-12 between two circulators 4-14, or other types of optical couplers, is located after the optical signal filter 2-10 (shown as delay interferometer), such that the two optical couplers 4-14 are located at the two output ports of the optical system of the present embodiment. Provided that the optical amplifier 4-12 is capable of operating in saturation at the power levels being used, the simultaneous amplification of a high-level pulse and a low-level pulse drives the amplifier into saturation, which reduces power level variations of all pulses, including high-level pulses and low-level pulses.

[0031] Figures 5, 6 and 7 further illustrate operation of an optical amplifier operating in a saturating region. An optical amplifier is usually configured to operate in a linear region, in which the amplifier has an essentially constant gain, that is, the gain is independent from input power. When the input power is increased, the output power ceases to follow variations in input power, and the amplifier saturates. In the example shown in Figure 5, which illustrates amplifier gain as a function of input power, when the amplifier operation is in the saturating region, the amplifier exhibits a constant output power, which means that variations in input power have negligible effect on output power. The amplifier gain is thus a decreasing function of input power. [0032] Saturation is normally considered an undesirable effect for several reasons. For instance, with high-speed optical signals saturation means that the amplifier output depends on certain time-variant effects, such as a finite gain recovery time, which in turn gives rise to a so-called "pattern effect," wherein the amplifier output varies according to the input power signal shape, thus creating a pattern. Therefore, in a typical access network the semiconductor optical amplifiers ("SOAs") are almost invariably used in the linear operating region.

[0033] Some embodiments of the present invention can utilize a saturating optical amplifier without the disturbing pattern effect. This is because the optical power that is injected into the saturating amplifier is kept substantially constant over time. Even if the phase-modulated signal bears an additional amplitude modulation, the pulse-to-pulse peak power remains more or less at a constant level.

[0034] Reference document 2 discloses applications of an optical amplifier configured to operate in a saturating region. Surprisingly, noise suppression may be improved by operating the optical amplifier in a saturating region, in which the amplifier's gain is a generally decreasing function of input power. As stated in reference document 2, saturation may be used for regeneration of an amplitude-modulated optical signal. One or more saturating amplifier(s) may be placed before, inside and/or after the optical signal filter 2-10, as shown in Figure 3.

[0035] Figures 6 and 7 illustrate an embodiment in which a saturating optical amplifier 6-10 is located before the optical signal filter 2-10.

[0036] Now, because the input signal is phase modulated (DPSK), the peak- to-peak power of the signal from a single source remains almost constant from one symbol to another. Any power variation in the train of the symbols is converted into a phase distortion, which in turn will be converted back to an amplitude distortion after the optical signal filter 2-10. Thus, although the saturated SOA regenerates the amplitude modulation, this benefit is normally lost after the demodulator. However, if the optical signal-to-noise ratio of the input signal is high, which is the case in most access network applications, the possible increase in signal noise is acceptable.

[0037] Figure 6 depicts a scenario, in which power differences occur from one symbol to the next within an input pulse stream 6-32. Reference sign 6-34 denotes the output pulse stream of the saturating optical amplifier 6-10, wherein variations in input power have caused phase errors φ _βΙΓΐ and <> _en2 between consecutive pulses of the output pulse stream 6-34. Processing of the phase- modulated pulse stream 6-34 in the delay interferometer 2-10 results in a first amplitude shift-keyed ("ASK") output stream 6-37 at a first output port 2-17 and a second ASK output stream 6-38, which is of opposite polarity to the first ASK output stream 6-37, at a second output port 2-18. The phase errors in the phase-modulated pulse stream 6-34 cause amplitude errors in the ASK output streams 6-37 and 6-38, wherein amplitudes of the pulses may deviate from nominal "0" or "1 " levels.

[0038] Figure 7 depicts differences of transmitted power levels among several ONUs. In upstream communications within a PON, such power differences of symbol bursts from different ONUs are an important source of variation. Such variations among the transmitted power levels among several ONUs may be caused by different power losses within different portions of the PONs. Alterna- tively or additionally the ONUs may be transmitting at different power levels. In Figure 7, reference numeral 7-32 denotes a set of three upstream data bursts 7-32i through 7-32 ₃ , sent to the OLT by three different ONUs. Each of the ONUs, called ONU1 through ONU3, sends its upstream data burst 7-32i through 7-32 ₃ to the OLT in its respective time slot. Within Figure 7, each of the bursts includes a large number of symbols. There are typically some gaps between bursts, and interference between consecutive symbols only occurs within a burst. In the scenario shown in Figure 7, the data burst 7-32i from ONU1 has the lowest power level, while the data burst 7-32 ₂ from ONU2 has the highest power level at the amplification point, which is represented by am- plifier 6-10.

[0039] Reference numeral 7-34 denotes a corresponding set of three upstream data bursts 7-34i through 7-34 ₃ , after propagation through the saturating amplifier 6-10, which evens out some of the differences. Finally, reference numeral 7-36 denotes the set of three upstream data bursts, after propagation through the optical signal filter (shown as delay interferometer) 2-10. The two outputs 2-17, 2-18 of the optical signal filter 2-10 have equal power envelopes but the optical signals on the outputs 2-17, 2-18 have opposite polarities with respect to each other.

[0040] During operation, the saturating amplifier 6-10 reduces or eliminates amplitude variation between the bursts, which means that all symbols within a burst experience virtually equal phase distortion, which remains essentially constant. The optical signal filter (delay interferometer) 2-10 compares two consecutive symbols to each other within the same burst, as a result of which demodulation is performed correctly and the power level variations between the bursts have little or no effect on information content.

[0041] Figure 8 shows an embodiment in which a saturating amplifier stage 8- 10 is located after the optical signal filter (shown as delay interferometer) 2-10. The component layout of the saturating amplifier stage 8-10, which comprises a saturating amplifier 8-12 and a pair of optical couplers (shown as optical circulators) 4-14 was generally described in connection with Figure 4, although in that embodiment, the corresponding amplifier 4-12 was not necessarily a saturating amplifier.

[0042] As described in connection with Figure 6, reference numeral 6-32 denotes a DPSK input pulse stream, wherein the pulses have different power levels. As usual, the DPSK input pulse stream 6-32 is divided into a first ASK output stream 8-34 and a second ASK output stream 8-35 in the lower arm. In the first ASK output stream 8-34, the dashed line depicts a nominal power level, while the solid line depicts the actual power level. Reference numerals 8-37 and 8-38 denote two pulse streams at the two outputs of the embodiment shown in Figure 8, denoted by reference numerals 8-27 and 8-28. Saturation of the amplifier 8-12 has levelled out the power level variations, which were present in the input pulse stream 6-32.

[0043] In the embodiment shown in Figure 8, the saturating amplifier stage 8- 10 is configured to simultaneously amplify two symbols of opposite polarity from the two ASK output signals corresponding to the same input symbol. This means that high- and low-level signals "collide" in the saturating amplifier 8-12, and the saturation of the amplifier maintains the symbol-to-symbol peak power at a substantially constant level, as a result of which the saturating amplifier 8- 12 is virtually free from the pattern effect.

[0044] Figure 9 shows a variation of the embodiment shown in Figure 8. In cases wherein a single amplifier stage fails to provide a sufficient gain or output power, more amplifier stages can be provided, as shown in Figure 9. In the present embodiment, the first saturating amplifier stage 8-10 is followed by a second saturating amplifier stage 9-10. Reference numerals 9-37 and 9-38 denote two pulse streams at the two outputs of the embodiment shown in Fig- ure 9. [0045] Figure 10 shows an embodiment of the invention as adapted for use in a wavelength-division multiplexed ("WDM") PON. A WDM PON is an alternative type of PON, in which traffic to different customer sites is transmitted at different wavelengths. Each wavelength reserves a spectral band from the total spectrum of optical fiber used in the network. A WDM PON can be realized, for example, by sharing the part of the fiber path to a wavelength demultiplexer (WDM filter), which separates the different wavelengths to different fibers which are connecting to the customers. Each wavelength can also serve more than one customer in a power-splitting PON.

[0046] The embodiment shown in Figure 10 combines linear and saturating amplification. Especially in connection with a low-power DPSK input signal, it may be advantageous to amplify the input signal linearly before demodulation. In the present embodiment, the optical signal filter, such as a delay interferometer, 2-10 is preceded by a linear optical amplifier 10-2. The linear optical am- plifier 10-2 and the optical signal filter 2-10 process a wavelength-division multiplexed ("WDM") optical composite signal, which comprises signals from several distinct sources, each signal being transmitted at individual wavelengths λι through λ _η . The individual signals, each at its own individual wavelength λι through λ _η , are separated at a WDM demultiplexer 10-4. In the present embod- iment, each of the individual wavelength signals is further amplified at a wavelength-specific amplifier stage, which may be similar to the amplifier stage 4-10 described in connection with Figure 4. Or, provided that the wavelength- specific amplifier stages comprise saturating amplifiers, the wavelength- specific amplifier stages may be similar to elements 8-10 and 9-10 described in connection with Figures 8 and 9, respectively. It should be understood that the difference between a saturating amplifier and a non-saturating one does not necessarily depend only on the actual amplifiers but also on the input power levels to the amplifiers.

[0047] A combination of a linear amplifier 10-2 and a saturating amplifier 8-10 or 9-10 is particularly beneficial in cases wherein the power level of the input signal is low. In such cases the linear amplifier 10-2 can be used to amplify the input signal to a power level that is sufficiently high for the saturating amplifier

8- 10 or 9-10 to operate in its saturating region. The saturating amplifier 8-10 or

9- 10 can support single-ended reception with a tolerance to ASE noise from the first stage, which is similar to the tolerance provided by a balanced receiver. [0048] The linear optical amplifier 10-2 that precedes the demodulator is not restricted to any particular technology. Illustrative but non-restrictive examples of applicable technologies include semiconductor optical amplifiers ("SOA") and fiber amplifiers. An SOA can operate basically at any wavelength band used in single-mode fiber communication. Fiber amplifiers may be implemented as erbium-doped fiber amplifiers, which are frequently used to amplify a number of closely-spaced wavelengths carrying WDM signals. The WDM demodulator 2-10 may then be shared by several individual signals having different wavelengths λι through λ _η . These signals are separated by a wavelength demultiplexer 10-4, and each of these signals can then be further amplified by a saturated amplifier 4-10. Alternatively, signals at different wavelengths can be separated, each to their own demodulator. The two demodulated outputs at each wavelength can also have separate optical amplifiers to amplify to sufficient power levels, before connection to PONs. It should be understood that the present embodiment requires a WDM demodulator having a broad spectral range, such that for a signal of any wavelength, the WDM demodulator has the required spectral response over the optical bandwidth of the signal.

[0049] For each of the different wavelengths λι through λ _η , the two outputs of each amplifier stage 4-10 may be coupled to two mutually exclusive sets of receivers (shown as Rx1 , Rx2 in Figures 1 , 1 1 and 12), whereby the optical system provides increased coverage by utilizing both of the ASK output signals of each amplifier stage 4-10. Wavelength division multiplexing (WDM) means that a single fiber is used to carry more than one optical signals such that each signal is carried at a distinct wavelength. A WDM PON means that down- stream and/or upstream signals at several wavelengths are used between the OLTs and the ONUs. In a WDM PON, single wavelengths may not be branched to multiple ONUs, as each ONU may have a dedicated wavelength of its own. In such a case, the embodiment shown in Figure 10 can be used for wavelength-level protection purposes, such that the two outputs of each ampli- fier stage 4-10 may be coupled to two the same sets of receivers via different PON networks, as will be discussed in connection with Figure 13, whereby the optical system provides protection against PON failures.

[0050] Figures 1 1 through 17 relate to different use cases of the invention and its embodiments.

[0051] Figure 1 1 shows a PON network, as complemented with an optical system 1 1 -2 according to the present invention. The inventive optical system 1 1 -2 comprises a delay interferometer or other type of optical signal filter, denoted by reference numeral 2-10, and an optical amplifier stage, which may be similar to the amplifier stage 4-10 described in connection with Figure 4. In order to provide the benefits of the invention and its embodiments to maximal number of customer premises, it is generally advantageous to install the inventive optical system 1 1 -2 at the first splitting point of a downstream signal or the last combination point of an upstream signal, which point was denoted by reference numeral 1 -6 in Figure 1 . Improved noise suppression may be achieved by implementing the amplifier stage 4-10 as a saturating amplification stage 8- 10.

[0052] In downstream direction, demodulation is carried out before splitting, so that the demodulator is shared by multiple ONUs. In the embodiment shown in Figure 1 1 , the two outputs of the delay interferometer 2-10, which carry the first and second ASK output signals, are operatively coupled to a first and second set of optical receiving network elements via different portions of an optical network. The first and second set of optical receiving network elements are denoted by reference signs Rx1 and Rx2. In some use cases the first and second sets of receivers Rx1 and Rx2 are mutually exclusive sets, whereby the inventive optical system 1 1 -2 provides increased coverage by utilizing both of the ASK output signals. Alternatively, the first and second set of receivers Rx1 and Rx2 may share at least some of the receivers, whereby the inventive optical system 1 1 -2 provides protection (redundancy) to the shared receivers. Apart from the requirement for the ONUs to take into account the polarity of the ASK signal received by them, the ONUs need only to have simple low-cost ASK receivers, and the cost of the delay interferometer 2-10 is shared by several ONUs.

[0053] Figure 12 shows a use case in which the inventive optical system is located at a point other than the first downstream splitter/last upstream combiner, which point is shown as item 1 -6 in Figure 1 . In the present use case, the first downstream splitter/last upstream combiner 1 -6 serves a highly asymmetrical network, wherein one branch serves a very small set of ONUs, denoted by reference sign Rx3, while the other branch serves a far larger number of ONUs, shown as sets Rx1 and Rx2. In such use cases it may be beneficial to install an optical system according to the invention, denoted by reference numeral 12-2, to a splitting point downstream of the point 1 -6. Apart from the different placement, the optical system 12-2 may be similar to element 1 1 -2 described in connection with Figure 1 1 .

[0054] Figures 1 1 and 12 illustrate use cases in which the first and second sets of receivers, denoted by reference signs Rx1 and Rx2, are mutually ex- elusive sets, whereby the optical system 1 1 -2 or 12-2 provides increased coverage by utilizing ASK output signals provided by both outputs 8-27 and 8-28 of the optical system.

[0055] Figure 13 illustrates a use case in which the first and second sets of receivers, denoted by reference signs Rx1 ' and Rx2', share some or all of the receivers 1 -8, whereby the optical system, denoted by reference numeral 13-2 provides protection (also called resilience or redundancy) to the shared receivers 1 -8. Other than that, the optical system 13-2 may be similar to the optical systems 1 1 -2 and 12-2. In the use case shown in Figure 13, each of the receivers 1 -8 is served by one passive optical network (PON) 13-3 from the first output 8-27 of the optical system 13-1 and by another PON 13-4 from the second output 8-28 of the optical system 13-2. The receivers 1 -8 are immune to failures of a single PON 13-3, 13-4. The two PONs 13-3, 13-4 can be coupled to each of the receivers 1 -8 via an optical 2-to-1 switch (not shown) that precedes each receiver. Alternatively, two separate optical receivers may be in- stalled at each customer site, and the switching between currently serving PONs may be implemented in the electrical domain. It should be kept in mind, however, that the two PONs 13-3, 13-4 carry signals with opposite polarities, which is why the receivers 1 -8 must be able to detect the proper signal polarity and adapt to it as needed.

[0056] Figure 14 illustrates a use case in which the first and second inputs 2- 1 1 and 2-12 of the optical system, denoted herein by reference numeral 14-2, are both used for the purpose of providing protection to the downstream feeder trunk line. In the present use case, the OLT 1 -2 is provided with an optical 1 -to- 2 switch 14-4, by means of which the OLT 1 -2 can be coupled to either of the two inputs 2-1 1 , 2-12 of the inventive optical system, herein denoted by reference numeral 14-2. As an alternative to the optical 1 -to-2 switch 14-4, the OLT 1 -2 may be provided with two separate OLT interfaces (not shown), which implementation provides protection against OLT equipment failures.

[0057] Figure 15 illustrates a third alternative for providing protection to the downstream feeder trunk line. In the use case shown in Figure 15, two separate OLTs, denoted by reference numerals 1 -2 and 1 -2' are each coupled to a respective input 2-1 1 , 2-12 of the inventive optical system, herein denoted by reference numeral 15-2. Only one of the two OLT interfaces is transmitting at any given time, to avoid signal collision. When a transmitting path fails, the signal is switched through the other interface This implementation provides complete protection against OLT equipment failure. Again, when the input of the delay interferometer 2-10 is changed, the optical polarity between the two outputs 8-27, 8-28 is also changed. As an alternative to handling the polarity change at the ONUs, as described in connection with Figure 13, the polarity change can be handled by adjusting the phase shift in the delay interferometer 2-10 so that a proper output polarity is maintained. By way of example, the phase shift can be adjusted by providing a temperature difference between the two optical paths of the delay interferometer, shown as elements 2-14 and 2- 15 in Figure 2. For instance, the optical fiber may be locally heated in one of the optical paths. The heating alters the index of refraction, which in turn alters the optical path length of the electromagnetic wave travelling in the heated optical path. Any phase shift control may take place virtually anywhere along the optical path, or the phase shift control may take place in a distributed manner. Any phase shift of a given sign (plus or minus) in one of the optical paths may be replaced by a phase shift of the opposite sign (minus or plus) in the other optical path. As a yet further alternative, the secondary OLT 1 -2' may be provided with means for reversing the transmitted polarity in the electrical domain (not shown).

[0058] Figures 1 1 through 15 relate to use cases wherein the two outputs 8- 27, 8-28 of the inventive optical system drive ONU equipment. Alternatively or additionally, the inventive optical system can be used in the upstream direction, for driving OLT equipment, as shown in Figure 16.

[0059] A particular reason to use inventive optical system in upstream direction is that commonly available optical fibers exhibit a relatively high attenuation at about 1300 nm wavelength, which is usually reserved for the upstream direc- tion.

[0060] In the use case shown in Figure 16, two PONs, denoted by PON1 and PON2, are each coupled to an input 2-1 1 , 2-12 of the inventive optical system, herein denoted by reference numeral 16-2. In this case the delay interferometer 2-10 is similar in character to a 2-by-2 splitter, in the sense that it has no fundamental loss, but the delay interferometer 2-10 converts the signal from the two PONs to an amplitude-modulated (ASK) format, wherein the two outputs 8-27, 8-28 of the optical system 16-2 have opposite polarities with respect to each other. In a TDMA (Time Division Multiple Access) PON, all upstream traffic from each ONU within a PON is combined to a single fiber. As described in connection with Figure 7, each transmitting element has been as- signed a unique time slot for transmission, whereby collision is avoided. In connection with the present use case, the transmitting element is an ONU.

[0061] Signal polarity can be taken into account by configuring the transmitting element (ONU) to transmit in the polarity required by the currently receiving element (OLT), or by configuring the receiving element (OLT) to detect the po- larity of the transmitting element (ONU) that utilizes the current time slot.

[0062] Figure 17 shows an alternative use case to the one shown in Figure 16. As shown in Figure 17, the two outputs 8-27, 8-28 of the optical system 17-2 are coupled to a single OLT via an optical switch 17-4. Otherwise the optical system 17-2 is similar to the optical system 16-2.

[0063] The use cases shown in Figures 1 1 through 17 have been described in connection with an optical amplifier arrangement described in connection with Figures 4 and 8, in the sense that a semiconductor optical amplifier (SOA) is located after the optical signal filter, such as the delay interferometer. The invention is not restricted to this arrangement, and any of the amplifier arrange- ments shown described in connection with Figure 3 can be used instead.

[0064] Figures 18 through 23 illustrate some specific embodiments in more detail.

[0065] Figure 18 shows a conventional bi-directional amplifier for a PON, denoted by reference numeral 18-2. Such an amplifier is often called a PON ex- tender. For operation in downstream direction, the conventional PON amplifier 18-2 comprises a wavelength division multiplexer (WDM) filter 18-12, a semiconductor optical amplifier (SOA component) 18-14, an optical isolator 18-16 and, optionally, a narrowband ASE filter after the SOA (not shown). For upstream operation the amplifier 18-2 comprises another set of similar elements, respectively denoted by 18-22, 18-24 and 18-26.

[0066] Up- and downstream signals are differentiated from each other on the basis of different wavelengths used for up- and downstream. For example, in a recent physical layer specification for 10G EPON standard (IEEE 802.3av), the up- and downstream wavelength ranges of the 10G transmissions have been specified as 1260-1280 nm and 1575-1580 nm, respectively. The up- and downstream signals reserve these spectral bands for their use. Alternatively, the up- and downstream signals can be separated from each other by a pair of optical circulators (not shown).

[0067] One or more optical systems according to the invention can be used for improving the operation of the PON amplifier/extender 18-2 shown in Figure 18. Figure 19 shows a system in which an optical system according to the invention, denoted by reference numeral 19-2, replaces the optical amplifier (SOA component) 18-14 and optical isolator 18-16 shown in Figure 18. Specific embodiments for implementing the optical system 19-2 have been described in connection with many of the preceding drawings, and particularly Figures 8 and 9 (elements 8-2 and 9-2, respectively). WDM filter 19-12 corresponds to the WDM filter 18-12 shown in Figure 18.

[0068] Utilization of both ASK output signals from the optical system 19-2 is possible by installing two WDM filters 19-18 and 19-22 downstream of the optical system 19-2. Each of the two WDM filters 19-18, 19-22 is coupled to a re- spective set of network elements via two portions of passive optical networks denoted by PON1 and PON2. Compared to Figure 18, The ONUs can be divided into two PON networks PON1 and PON2, whereby one optical splitter stage can be eliminated from the network. Alternatively, the number of ONUs can be increased.

[0069] In the embodiment shown in Figure 19, upstream direction from the two WDM filters 19-18, 19-22 is handled by combining the upstream signals from PON1 and PON2 at a 3 dB coupler 19-23. Elements whose reference numeral begins by 18- have been described in connection with Figure 18, and a repeated description is omitted. Transmission from the OLT must naturally be phase coded, such as DPSK, while the upstream transmission can be amplitude coded.

[0070] Figure 20 shows an enhanced version of the embodiment shown in Figure 19, wherein the optical system 20-2 corresponds to the optical system 19-2 shown in Figure 19 and processes downstream signals. The present em- bodiment also comprises a second optical system 20-3, which processes upstream signals and replaces elements 19-23, 18-24 and 18-26.

[0071] Compared with the embodiment shown in Figure 19, the one shown in Figure 20 utilizes both inputs and both outputs of both optical systems 20-2 and 20-3. One input of the downstream optical system 20-2 is coupled to a working OLT 1 -2 via WDM 20-12, while its other input is coupled to a protection OLT 1 -2' via WDM 20-28. One input of the upstream optical system 20-3 is coupled to PON1 via WDM 20-18, while its other input is coupled to PON2 via WDM 20-22. Apart from a different coupling to the inventive optical systems 20-2 and 20-3, the WDMs 20-18 and 20-22 correspond to their counterparts 19-18 and 19-22 shown in Figure 19. The upstream optical system 20-3 is coupled similarly, such that each of its two inputs is coupled to one port of the two WDMs 20-18 and 20-22, while each of its two outputs is coupled to one port of the two WDMs 20-12 and 20-28. In the present embodiment, both up- and downstream transmission at the inputs to the optical system must be phase coded.

[0072] The present embodiment utilizes the invention to improve a PON extender with DPSK-to-ASK format conversion in both upstream and downstream directions, such that protection for optical trunk line and OLT equipment is provided.

[0073] Figure 21 shows an embodiment in which the bi-directional amplifier shown in Figure 20 can be simplified by omitting the delay interferometer from the second optical system 20-3. The simplified bi-directional amplifier is denoted by reference numeral 21 -2. This simplification provides cost benefits, considering the fact that the delay interferometer is normally the most expensive part of the amplifier. In the embodiment shown in Figure 21 , the downstream signal is amplified by amplifier 21 -16, similarly to the previous example (Figure 20). Structurally and functionally the amplifier 21 -16 is similar to the amplifier 8- 12 described in connection with Figure 8. In other words, the amplifier 21 -16 is configured to simultaneously amplify the two symbols of opposite polarity from the two ASK output signals corresponding to the same input symbol, ie, the high-level symbol and low-level symbol, thus keeping their combined power level substantially constant.

[0074] Upstream signals are processed differently, however. Upstream of the two WDM filters 21 -18 and 21 -22, the upstream signals, symbolized by dashed arrows, are combined by a 3-dB coupler 21 -23. No undesired overlap should occur, because the upstream traffic is time division multiplexed, and a unique time slot for upstream transmission has been assigned to the different ONUs. The combined upstream signals are amplified by an SOA 21 -24 that may or may not operate in saturation. The SOA 21 -24 may optionally be followed by a passband filter (not shown), especially if it is used in linear operation. After the amplifier 21 -24, the upstream signals are coupled into a 4-port circulator 21 -14, which replaces one of the three-port circulators 4-14 described in connection with Figure 4, and directs the amplified upstream signals into the delay interferometer 2-10 and thereafter to the working OLT 1 -2 and protection OLT 1 -2'.

[0075] Figure 22 shows a variation of the embodiment shown in Figure 21 . The embodiment shown in Figure 22 is symmetrical in the sense that upstream signals and downstream signals are processed by identical elements In comparison with the embodiment shown in Figure 21 , the present embodiment omits the amplifier 21 -24, and upstream amplification is provided by amplifier 22-15, the operation of which is similar to the amplifier 8-12 described in connection with Figure 8. 4-port circulators 22-14 are used throughout the system.

[0076] Figure 23 shows an embodiment of the invention in connection with a multi-wavelength optical amplifier. Operation in downstream direction will be described, in the case of two distinct wavelengths. Wavelengths used for upstream direction may be separated into additional outputs and inputs of WDM filters, and amplified by separate optical amplifiers (not shown). From the work- ing OLT 1 -2, a WDM filter, denoted by reference numeral 23-12 divides DPSK signals at a first wavelength (spectral band) and a second wavelength to one input of a first and a second optical system, respectively denoted by reference numerals 23-2 and 23-3. Optionally, if protection for the input feeder line and/or OLT equipment is desired, another WDM filter in demultiplexing direction, de- noted by reference numeral 23-28, performs a similar wavelength-based signal splitting for the protection OLT 1 -2', and drives the other inputs of the two optical systems 23-2 and 23-3. Each of the two optical systems 23-2 and 23-3 is similar to the optical system 8-2 described in connection with Figure 8. The first and second optical systems 23-2 and 23-3 process signals at the first wave- length (spectral band) and second wavelength, respectively, and convert the DPSK signals at either wavelength to two ASK signals of opposite polarity. The ASK signals at the first and second wavelength are respectively combined by WDM filters 23-18 and 23-22 in the multiplexing direction. The layout and operation of the present embodiment can be generalized to any number of wave- lengths (spectral bands).

[0077] Figure 24 clarifies certain issues relating to the spectral response of a DPSK-to-ASK demodulator filter and bandwidth. Reference numeral 24-10 denotes a pair of transmission-versus-frequency curves, which describe the complementary spectra for the two outputs of a DPSK-to-ASK demodulator filter. Reference numeral 24-1 1 denotes the transmission as a function of frequency for output 1 , while reference numeral 24-12 denotes the corresponding curve for output 2. In this example, the periodic response is maintained over a wide spectral band, as a result of which the demodulator can be used for several different wavelength signals within the spectral band.

[0078] Reference numeral 24-30 in turn denotes a set of transmission-versus- frequency curves of a narrow-band demodulator, the spectral response of which only provides filtering for a bandwidth of a single DPSK signal, wherein reference numerals 24-31 and 24-32 denote the transmission as a function of frequency for outputs 1 and 2, respectively. Reference numeral 24-33 denotes the bandwidth of the single DPSK signal.

[0079] The invention is also applicable to multilevel, phase-modulated variants of DPSK modulation, such as differential quadrature phase shift keyed modulation ("DQPSK"), which is a combination of DPSK modulations at two different quadratures. For embodiments using DQPSK transmission, the signal power is split into two demodulators. Each of the two demodulators is tuned to demodu- late the quadrature of the signal. Both of these demodulators would then have two demodulated ASK outputs to be distributed into respective PONs. The two demodulators would then have different data distributed to their respective PONs, corresponding to the different data transmitted in the two quadratures of DPSK signal, and the ONUs would operate at the half of bit rate of DQPSK signal. Even higher order modulation formats, such as D8PSK can obviously be used.

[0080] Figure 25 illustrates how the invention can be used with variations of the DPSK format, such as DQPSK or D8PSK. In the embodiment shown in Figure 25, a DQPSK signal is divided by an optical coupler 25-10, such as a 3 dB coupler. The DQPSK signal comprises an I channel and a Q channel. In the present example, the I channel is processed by an embodiment of the optical system 25-20, which is very similar to the optical system 4-2 described in connection with Figure 4. The only difference is that there is mutual +π/4 phase shift between the two optical arms of the optical signal filter 25-22, as dis- cussed in connection with Figure 2. Reference numeral 25-24 denotes the means for implementing the mutual phase shift.

[0081] Apart from a different phase shift, the Q channel could be processed by an optical system similar to the one that processes the I channel, but the invention is not restricted to such symmetrical configurations. In the present ex- ample, the Q channel is processed by an optical system as described in connection with Figure 3, in that an optical amplifier 3-10 precedes that optical signal filter 25-32, which is similar to the optical signal filter 25-22 of the I channel, apart from the phase shifter 25-34, which causes a mutual -π/4 phase shift between the two optical arms of the optical signal filter 25-32.

[0082] The two outputs of the I channel convey mutually identical information INFO1 but in mutually opposite polarities. The two outputs of the I channel are coupled to a first and second set of receivers via optical networks PON1 and PON2. Similarly, the two outputs of the Q channel convey mutually identical information INFO2 but in mutually opposite polarities. The two outputs of the Q channel are coupled to a third and fourth set of receivers via optical networks PON3 and PON4. The sets of receivers may be mutually exclusive sets, as described in connection with Figures 1 1 and 12, or they may be wholly or partially overlapping sets, as described in connection with Figure 13.

Practical considerations and further alterations

[0083] Particularly in case the invention is used in connection with multi- wavelength optical networks, some form of wavelength management will be needed. The transmission line has usually some wavelength-dependent components, such as narrowband filters. Also, the optical signal filters, such as delay interferometers, used in the invention are wavelength dependent. For the downstream direction the management of the single wavelength system is relatively simple: the relative phase of the delay interferometer can be tuned. In case of upstream lasers at the ONUs capable of operating at several different emission wavelengths, the lasers should be tuned to stay at the desired optical frequency. This may be accomplished by sending wavelength management information to each ONU, wherein the wavelength management information instructs the device to adjust its emission wavelength to a higher or lower wavelength, as needed.

[0084] In some applications it may be beneficial to provide means for the management of the inventive optical system itself, considering the fact that active amplification devices are involved. Such management may involve perfor- mance monitoring and fault handling. Management information may need to be communicated between the inventive optical system and the OLT equipment. One way of handling this supervisory information is to use an optical link with a separate wavelength dedicated to management purposes. Management information from the OLT equipment can also be transmitted in a downstream sig- nal, part of which is separated and received at the inventive optical system. As regards upstream signalling, specific time slots may be reserved for management information from the inventive optical system to the OLT, such that the management information is combined into the upstream signal received by OLT. From the point of view of the OLT, the management section of the in- ventive optical system thus appears as an extra ONU.

[0085] The above description of the specific embodiments is intended to be illustrative rather than restrictive. It is apparent to those skilled in the art that with advancing technology, the invention can be implemented in various embodiments. Accordingly, the scope of protection is defined by the attached in- dependent claims.

Acronyms

[0086]

ASE: Amplified Stimulated Emission

ASK: Amplitude Shift Keyed

DPSK: Differential Phase Shift Keyed

DQPSK: Differential Quadrature Phase Shift Keyed

OLT: Optical Line Termination

ONU: Optical Network Unit

PON: Passive Optical Network

SOA: Semiconductor Optical Amplifier

WDM: Wavelength-Division Multiplexing

Reference documents

[0087]

1 . Shumate, P.W., "Fiber-to-the-Home: 1977 - 2007"), Journal of Lightwave technology, Volume 26, Issue 9, p. 1093-1 103 (2008);

2. Commonly assigned patent applications Fl 20095288, filed 19 March 2009 and US 61/174053, filed 30 April 2009.