Patent Description:
A passive optical network, PON, is a telecommunications technology for providing fiber to end consumers. A PON comprises commonly a point-to-multipoint set-up whereby unpowered fiber optic splitters are used to provide a single optical fiber to serve multiple end-points.

A PON further comprises an optical line terminal, OLT, which serves as the service provider endpoint, and a number of optical network terminals, ONTs, near end users.

A PON using a gigabit passive optical network, GPON, is able to provide a bitrate of <NUM>. 5Gbps, thereby providing subscriber rates from 30Mbps to 600mbps. In other standards, such as XGSPON and 10GEPON, higher bitrates per wavelength are offered thereby providing bitrate speeds up to 10Gbps. In a NGPON2 variant several 10Gbps wavelengths are multiplexed thereby delivering 40Gbps in the same optical fiber.

<NPL>, describes the principles of different colourless-light-source schemes and the architectures of the corresponding TWDM and WDM PON systems for wavelength control at the optical network unit, ONU, for the development of a low-cost and plug-and-play ONUs. Four types of wavelength control methods for light source based on tuneable laser diodes are described and analysed, one of which is optical-beat-noise-based wavelength control method.

<NPL>, describes a remotely pumped EDFA and tunable transmitter-based <NUM>-Gb/s/λ downstream and <NUM>-Gb/s/λ upstream transmissions long-reach WDM-PON scheme and an optical beat noise-based automatic wavelength control method for tunable transmitter.

The higher the optical bitrate, the more expensive optical components need to be used. These components are, among others, the transmitting laser and the receiving photodiode. Further, to support bidirectionality, PON systems use two different wavelengths for transmitting and receiving respectively.

To support bidirectionality by simultaneous transmission and reception in a single wavelength, an isolation between transmitting and receiving paths is provided at each end of the fiber. This isolation is achieved using optical circulators. Optical circulators nevertheless do not provide a perfect isolation and a leakage from a transmitting port to an undesired receiving port occurs. This increases a noise in the PON thereby significantly reducing the performance thereof.

Amongst others, it is an object of embodiments of the present disclosure to provide a solution that improves the supporting of bidirectional transmitting and receiving signals in a PON.

This object is achieved, according to a first example aspect of the present disclosure, by a controller circuitry as defined by claim <NUM>, for controlling an optical transceiver of an optical line terminal, OLT, in a passive optical network, PON, operating in an in-band full-duplex mode, the optical transceiver comprising an optical transmitter and optical receiver and configured to simultaneously exchange signals in the PON, the controller circuitry comprising means for performing:.

The optical transceivers of the OLT and the ONT are configured to simultaneously exchange signals in the PON. The optical transceivers comprise each an optical transmitter and optical receiver, such as for example a laser and a photodiode, whereby through in-band full duplex, this is the simultaneous exchange from and to the OLT and from and to the ONT, the bandwidth is doubled per wavelength.

When transmitting and receiving the optical signals, the signals pass through optical circulators which exhibit undesired optical leakage. This leakage may further be combined with reflections of transmitted signals when they cross splitters in the PON. Furthermore, discontinuities may be present in the PON as well, which may be crossed by the optical signals before they reach the far end. These phenomena produce the OBI effect at the receiving photodiode of an optical transceiver.

More specifically, the OBI effect originates from the detection process of the optical receiver such as a receiving photodiode, i.e. the opto-electrical conversion, having an inherent quadratic function. The quadratic function output comprises the desired signal combined with an electrical signal with a frequency equal to the frequency difference of the two optical signals present at the receiving photodiode. If transmitters at both ends of the PON use a same wavelength, or wavelengths close to each other, for example when the frequency difference between the two optical signals is lower than the data bandwidth, the OBI results in a significant performance degradation.

The level of OBI at a photodiode of the optical transceiver from the OLT is derived. This may be performed by monitoring a signal received at the optical receivers such as a photodiode. The signal is a received upstream optical signal, whereby upstream means that the stream originates from the ONT in the PON towards the OLT.

Next, based on the derived level of OBI, a laser of the optical transceiver of the OLT is controlled by setting the wavelength of its transmitting signals. This wavelength is set based on the derived level of OBI such that the wavelength is forced to differ from the wavelength of the upstream optical signal. In other words, the downstream optical signal, thus from the OLT towards the ONT, comprises a wavelength which will be different from the wavelength of the upstream optical signal, whereby the difference is based on the level of OBI.

Different advantages are identified. Firstly, there is no need for composing or producing additional signals for injecting in the PON, for example controlling signals, which could aversely influence the functioning of the PON. Characteristics of existing signals, and in particular the level of OBI therefrom, may be used directly as a parameter to control the PON.

Secondly, by forcing the wavelengths to differ from each other, the level of OBI will be reduced by the low pass filtering effect of the channel. At a random spectral location of receiving and transmitting lasers of an optical transceiver that are very close to each other, the OBI is produced since the detecting optical receiver or photodiode is performing an equivalent quadratic function. This quadratic function causes a beat of the optical signals yielding an electrical component that relates to the wavelength difference between the wavelengths of the received upstream optical signal and the sent downstream optical signal. The closer the two wavelengths, the higher the impact of OBI produced.

Thus, thirdly, by using the existing signal in the PON and deriving therefrom the level of OBI, combined with the use of the level of OBI itself to reduce it through pushing the wavelengths such that they differ, the effect is reduced based on a measurement therefrom. In other words, there is a direct link between the interference that may be present in the PON and the way that it is reduced. The reduction means that it is either eliminated, either significantly reduced.

Fourthly, optical in-band full duplex is performed by the existing components of the optical system. This is achieved by doubling the bandwidth per wavelength using the normal bandwidth of the optical devices. There is thus no need on increasing the bandwidth of the transceiver, for example lasers and photodiodes, which would lead to a significant increase in costs of the components.

Fifthly, since these components, like transmitting and receiving lasers, operate in their respective wavelength with a certain degree of tolerance, the forcing of the difference between the wavelengths is achieved by letting the laser operate at both ends within their tolerance margins, although opposite to each other. This way, they still operate around their respective nominal wavelength, but such a difference is enough to shift the OBI totally or partially outside the received interest spectrum.

To derive the level of OBI of the received upstream optical signal, the means are configured as follows.

Firstly, a monitored signal indicative for the received upstream optical signal is obtained. For example, at the receiving photodiode of the optical transceiver of the OLT, an electrical signal representative for the received upstream optical signal is measured and nominated as the monitored signal.

Next, by amplifying the monitored signal, for example by a transconductance amplifier, an amplified signal is obtained. This amplified signal is filtered, for example by a high-pass filter. This way most of the OBI signal is taken or identified from the amplified signal. Then an electrical envelope detector detects an envelope of the filtered signal as the enveloped signal and this enveloped signal is integrated by an integrator or, alternatively, by a low pass filter to remove any carrier frequency from the OBI electrical signal. As a result, an integrated signal is obtained that represents only the power of energy level of the OBI signal. This integrated signal is then representative or indicative of the level of OBI. In other words, the amount of OBI energy present in the PON is determined.

Subsequently, the integrated signal is used to set the wavelength of the downstream optical signal. The setting is, for example, performed by controlling a laser bias the produces a shift in wavelengths. In other words, a control loop at the OLT arises. This control loop can be implemented at the infrastructure side, this is without a need of intervening at the ONT side.

According to example embodiments, the means are further configured to perform:.

and wherein the monitored signal is the responding optical signal.

The control loop can either operate through continuous tracking thereby continuously adjusting and setting the wavelength such that the level of OBI is reduced. Alternatively, at start-up, this is before data is transmitted in the PON, the means are configured to instruct the optical transceiver at the OLT to produce an initializing optical signal. The initializing optical signal is, for example, a continuous optical wave configured to instruct the ONT to send in reply a responding optical signal. The responding optical signal may also be a continuous optical wave. This way the control loop may be initiated whereby the responding optical signal from the ONT serves as the monitored signal for the loop.

According to example embodiments, the initializing optical signal is further configured to instruct a plurality of optical transceivers of ONTs to sequentially send in reply a respective responding optical signal; and wherein the deriving further comprises deriving the level of OBI based on a maximum value of the respective responding optical signals.

In the occurrence that the OLT serves a plurality of ONTs, for example through an optical splitter, the initialization process is performed by transmitting the initializing optical signal to the ONTs. The initializing optical signal is then further configured such that the ONTs each sequentially reply by sending a respective responding signal.

From the sequentially received responding optical signals from the ONTs, the controller circuitry derives the level of OBI based on the maximum value among the respective responding optical signals. This way, the control loop adapts itself to a worst-case scenario, this is, to a highest level of OBI that may be present in the PON originating from an ONT that transmits a strongest signal from a power level point of view.

According to example embodiments, the initializing optical signal is further configured to instruct a plurality of optical transceivers of ONTs to sequentially send a respective responding optical signal in reply; and wherein the deriving further comprises sequentially deriving the level of OBI for the respective responding optical signals; and where the means are further configured to perform:.

Alternatively, the circuitry may derive the level of OBI for each of the replying ONTs. These levels are then stored such that an overview is obtained of the whole PON with each of the ONTs and related level of OBI. The storing may, for example, be stored in a bias table, whereby the storing media is either incorporated in the circuitry, or accessible through exchanging means by the circuitry.

and wherein the setting is based on the selected level of OBI.

In other words, when the OLT start communicating with one of the ONTs, the ONT is identified such that the circuitry can adapt the setting of the wavelength for exchanging data between the OLT and the ONT as addressee based on the expected level of OBI. This way, the presence of OBI is reduced or eliminated prior to the exchange of data such that this may be performed efficiently.

According to example embodiments, the means are further configured to:.

The transmitting laser of the optical transceiver of the OLT may be controlled by a laser bias. The control loop, and in particular the integrated signal thereof, then instructs the laser bias to shift the wavelength of the transmitting laser such that it differs from the wavelength of the monitored received upstream optical signal.

At an initialization phase, the laser bias may, for example, be set at a maximum or minimum position, and during tracking gradually be adapted if needed.

According to a second aspect, an OLT is disclosed comprising a controller circuitry according to the first aspect.

In other words, the controller circuity for controlling the OLT may be incorporated in the OLT itself. This way, the OLT comprises the functionality to control the wavelengths of the optical transceiver such that it may operate in an efficient manner, without a need to implement the circuitry afterwards.

According to an embodiment, the OLT comprises a laser bias configured to control a transmitting laser for producing a downstream optical signal at the optical transceiver of the OLT; and the controller circuitry according to the first aspect is further configured to perform the instructing of the laser bias.

According to a third aspect, a method is disclosed comprising the steps of:.

According to a fourth aspect, a computer program product is disclosed comprising computer-executable instructions for performing the following steps when the program is run on a computer:.

According to a fifth aspect, computer readable storage medium comprising computer-executable instructions for performing the following steps when the program is run on a computer:.

In <FIG> an example embodiment of a passive optical network (PON) <NUM> is illustrated. The PON <NUM> comprises an optical line terminal (OLT) <NUM> and an optical network terminal (ONT) <NUM>. The OLT <NUM> and ONT <NUM> are configured such that they can mutually exchange data over a fiber <NUM>. The optical fiber <NUM> can comprise several kilometres, illustrated by loop <NUM>. The ONT <NUM> is further configured to serve an end-user, and whereby the OLT <NUM> is considered as the infrastructure side of the PON <NUM>.

Between the OLT <NUM> and the ONT <NUM> data is exchanged using optical signals. For this end, both the OLT <NUM> and ONT <NUM> comprise each an optical transceiver. An optical transceiver further comprises a transmitting laser and a receiving photodiode. As for the OLT <NUM> the transmitting laser is laser <NUM> and the receiving photodiode is diode <NUM>. As for the ONT <NUM> the transmitting laser is laser <NUM> and the receiving photodiode is diode <NUM>.

A signal produced by the laser <NUM> of the OLT <NUM> and transmitted to the ONT <NUM> is nominated as a downstream optical signal, while a signal produced by the laser <NUM> of the ONT <NUM> and transmitted to the OLT <NUM> is nominated as an upstream optical signal. Further, the downstream optical signal comprises a wavelength λ' <NUM>, while the upstream optical signal comprises a wavelength λ <NUM>. The photodiode <NUM> is configured to receive the upstream optical wavelength with wavelength λ <NUM>, while the photodiode <NUM> is configured to receive the downstream optical signal with wavelength λ' <NUM>.

The different components of the optical transceivers may further be connected by an electrical cable whereby the data exchanged by the optical signal can be transformed into an electrical signal. For example, laser <NUM> is connected through wire <NUM>, photodiode <NUM> is connected through wire <NUM>, laser <NUM> is connected through wire <NUM>, and photodiode <NUM> is connected through wire <NUM>.

The PON <NUM> is further configured to bidirectional and simultaneous exchange optical signals between the OLT <NUM> and the ONT <NUM>. In particular, the simultaneous transmission and reception is performed in a single wavelength, this is optical in-band full duplex, by isolating the transmitting and receiving path through the use of optical circulators. For the OLT <NUM> this is optical circulator <NUM>, and for the ONT <NUM> this is the optical circulator <NUM>. Thus, as an example, the optical signal <NUM> produced at the laser <NUM> is deviated by the optical circulator <NUM> to the optical fiber <NUM> and further by the optical circulator <NUM> deviated to the photodiode <NUM>.

In real-life situations, the optical circulators, however, don't provide perfect isolation and optical leakage occurs. Because of such a leakage, one transmitting port to another undesired transmitting port occurs. This is illustrated by optical signal <NUM> produced by laser <NUM> from which a part is deviated via optical circulator <NUM> to the photodiode <NUM>. Furthermore, additionally to the leakage, reflections may occur when the signal crosses splitter, other discontinuities in the fiber <NUM> and/or at the optical circulators. This phenomenon is illustrated by produced optical signal <NUM> at the laser <NUM> which is reflected by the optical circulator <NUM> and further deviated to photodiode <NUM> by optical circulator <NUM>.

Thus, at the photodiode <NUM> the desired signal <NUM> is present, together with the undesired signals <NUM> and <NUM>. The undesired optical leakage illustrated by signal <NUM> whether or not combined with reflections of transmitted signals illustrated by signal <NUM>, produces an optical beat interference (OBI) effect at the photodiode <NUM>. It should be further understood that this effect may also be present at the photodiode <NUM>. The OBI further produces an increase in the noise floor at the receiver which reduces the performance of the PON <NUM> as a whole.

Furthermore, to increase in a very convenient way fiber optic capacity, a PON may further comprises a multitude of ONTs served by the OLT <NUM>. In <FIG> such a PON is illustrated whereby the OLT <NUM> and the ONT <NUM> are likewise represented. Between the OLT <NUM> and the ONT <NUM>, and in particular in the fiber <NUM>, splitters <NUM> and <NUM> are presented. The length between the splitters <NUM> and <NUM>, between the splitter <NUM> and the OLT <NUM>, and between the splitter <NUM> and the ONT <NUM> may also be several kilometres. This is illustrated by loops <NUM>, <NUM>, and <NUM>.

The splitters <NUM> and <NUM> are configured such that additional ONTs may be served by the OLT <NUM>. Splitter <NUM> comprises connections <NUM> configured to serve a plurality of ONTs, while splitter <NUM> comprises connections <NUM> to serve other more ONTs.

It should thus be further understood that in a configuration as illustrated by <FIG> the OBI effect may also be present. The combination of desired and undesired signals at the receiver <NUM> may then even be more complicated, due to for example more reflections on different discontinuities, like the optical splitters <NUM> and <NUM>. Thus, at the photodiode <NUM> all these optical signals are combined leading to the production of the OBI effect.

The OBI effect is further illustrated in <FIG>. At a random spectral location <NUM> of receiving, with wavelength λ, and transmitting, with wavelength λ', lasers that are very close, the OBI is produced by the fact that a detecting photodiode is performing an equivalent quadratic function. The quadratic function causes a beat of the two optical signals, yielding an electrical component that relates to the wavelength difference between the transmitting λ' and the receiving λ wavelengths. The closer <NUM> the two wavelengths, the higher <NUM> the amount of interference <NUM> that is produced at the receiver. When the two wavelengths are further away from each other <NUM>, the smaller <NUM> the OBI effect <NUM> will be.

The same observation is further illustrated in <FIG>. In <FIG> the wavelength separation, expressed in GHz <NUM>, is plotted <NUM> against the noise floor, expressed in dBmV <NUM>. As illustrated in <FIG> the OBI effect represented by the noise floor <NUM> is less when the wavelength separation increases.

The same OBI effect can also be seen in the time domain by means of eye patterns as illustrated in <FIG>. For a small wavelength separation like, for example, <NUM>,<NUM> <NUM> the eye pattern shows that the signal is not usable <NUM>, while pushing the wavelength to <NUM>,<NUM> <NUM> the quality of the signal is equivalent to not having OBI at all. The eye pattern <NUM> illustrates a wavelength difference of <NUM>,<NUM>.

The OBI effect is reduced, according to an embodiment, by a loop that controls a transmitting laser bias. <FIG> illustrates such a loop <NUM> comprising components <NUM>-<NUM> controlling a transmitting laser bias <NUM> in a PON comprising an OLT <NUM> and an ONT <NUM>. The transmitting laser bias <NUM> controls the transmitting laser <NUM> such that the transmitting optical wavelength λ' is kept in a certain difference with respect to the receiving wavelength λ, received at the photodiode <NUM>. This in turn produces a shift to the OBI spectrum to a frequency position wherein it may be filtered out and therefore reducing the impact on the received signal quality.

The wavelength push has to be high enough to reduce the OBI, while simultaneously low enough for not forcing the transmitting laser <NUM> to operate at current stress. To meet these requirements, the steps performed to push the wavelength will now further be illustrated with reference to <FIG> combined with reference to <FIG>.

In <FIG> the illustrated case relates to a 25Gbps case, wherein the transmitting is performed at 50Gbps by the use of 25Gbps components in in-band full duplex mode. The wavelength separation is performed by the loop <NUM>. Initially, the laser bias <NUM> is set to its maximum or minimum position at start-up. Next, the received signal at the photodiode <NUM> which comprises the desired signal as well as spurious undesired signals is monitored by deriving <NUM> therefrom the level of OBI. As highlighted, the setting <NUM> is initially performed by setting the laser bias <NUM> to its maximum or minimum position at start-up. Next, a signal <NUM> representative for the signal received by the photodiode <NUM> is obtained <NUM>. This signal <NUM> is amplified <NUM> by an amplifier <NUM>, for example a transconductance amplifier. Subsequently, the result is filtered <NUM> by a high-pass filter <NUM> to take most of the OBI signal. Then an electrical envelope detector <NUM> detects <NUM> the envelope of the signal and an integrator <NUM> or low pass filter determines by integrating <NUM> the amount of OBI energy present. Preferably, this value is high since this means that the OBI has been shifted to the upper part of the spectrum where it is more innocuous. The OBI energy value is then used to control the laser bias <NUM> by setting <NUM> it in such a way that it produces a shift in the wavelength.

A similar loop <NUM> can likewise be implemented in a PON comprising a point-to-multipoint system as illustrated in <FIG>. In the illustrative embodiment of <FIG> a plurality of ONTs <NUM>, <NUM>-<NUM> are present and connected via the splitter <NUM> to the same optical fiber with loop <NUM>.

An approach for initialization of the circuitry <NUM> combined with a switch <NUM>, a storage medium <NUM>, and the laser bias <NUM> will now be further discussed. At the early ONTs <NUM>, <NUM>-<NUM> connection phase and without data being transmitted, a continuous optical wave may be transmitted from each of the transmission ends <NUM>, <NUM>-<NUM> sequentially. In a first step, the OLT <NUM> or infrastructure side transmits a continuous optical wave to the first PON termination, this is ONT <NUM>, and ONT <NUM> transmits a continuous optical wave to the OLT <NUM>. During this transmission, the loop at the OLT <NUM> comprising an amplifier <NUM>, a low pass filter <NUM>, an envelope detector <NUM>, and an integrator <NUM>, derives <NUM> the amount of radio frequency energy received as difference between the wavelengths λ and λ'. This energy value is used to set <NUM> the laser bias <NUM> to shift λ' to a value that minimize this energy value. The process is repeated for each of the ONTs <NUM>, <NUM>-<NUM> of the active PON terminations, and the laser bias <NUM> values are stored in the storage medium <NUM> for further use. The loop can, according to an embodiment, digitally be implemented such that an analogue to digital converter is operating before the values are stored in the storing medium <NUM>. Subsequently, a digital to analogue converter to translate the stored values can be used to transform the values into bias currents or voltages to the laser <NUM>. The loop has a switch <NUM> to activate the initialization process during a training phase in such a way that the loop is active only during the continuous optical wave transmission phase.

Next, in normal operation, each time that a PON termination <NUM>, <NUM>-<NUM> is addressed for data transmission, the corresponding laser bias value stored in the storing medium <NUM> is loaded to control the laser <NUM> frequency at the infrastructure side <NUM> for this particular ONT.

Since the laser frequencies may experience drift along time in operation, the switch <NUM> may be closed such that the tracking is continuously performed as illustrated by the control loop of <FIG>.

<FIG> shows a suitable computing system <NUM> enabling to implement embodiments of the method for controlling an optical transceiver of an optical line terminal <NUM>. Computing system <NUM> may in general be formed as a suitable general-purpose computer and comprise a bus <NUM>, a processor <NUM>, a local memory <NUM>, one or more optional input interfaces <NUM>, one or more optional output interfaces <NUM>, a communication interface <NUM>, a storage element interface <NUM>, and one or more storage elements <NUM>. Bus <NUM> may comprise one or more conductors that permit communication among the components of the computing system <NUM>. Processor <NUM> may include any type of conventional processor or microprocessor that interprets and executes programming instructions. Local memory <NUM> may include a random-access memory (RAM) or another type of dynamic storage device that stores information and instructions for execution by processor <NUM> and/or a read only memory (ROM) or another type of static storage device that stores static information and instructions for use by processor <NUM>. Input interface <NUM> may comprise one or more conventional mechanisms that permit an operator or user to input information to the computing device <NUM>, such as a keyboard <NUM>, a mouse <NUM>, a pen, voice recognition and/or biometric mechanisms, a camera, etc. Output interface <NUM> may comprise one or more conventional mechanisms that output information to the operator or user, such as a display <NUM>, etc. Communication interface <NUM> may comprise any transceiver-like mechanism such as for example one or more Ethernet interfaces that enables computing system <NUM> to communicate with other devices and/or systems, for example with the circuitries <NUM>, <NUM>. The communication interface <NUM> of computing system <NUM> may be connected to such another computing system by means of a local area network (LAN) or a wide area network (WAN) such as for example the internet. Storage element interface <NUM> may comprise a storage interface such as for example a Serial Advanced Technology Attachment (SATA) interface or a Small Computer System Interface (SCSI) for connecting bus <NUM> to one or more storage elements <NUM>, such as one or more local disks, for example SATA disk drives, and control the reading and writing of data to and/or from these storage elements <NUM>. Although the storage element(s) <NUM> above is/are described as a local disk, in general any other suitable computer-readable media such as a removable magnetic disk, optical storage media such as a CD or DVD, -ROM disk, solid state drives, flash memory cards,. could be used. Computing system <NUM> could thus correspond to the controller circuitry <NUM> or <NUM> in the embodiments illustrated by <FIG> or <FIG>.

Claim 1:
A controller circuitry (<NUM>, <NUM>) for controlling an optical transceiver of an optical line terminal (<NUM>), OLT, in a passive optical network (<NUM>), PON, operating in an in-band full-duplex mode, the optical transceiver comprising an optical transmitter and optical receiver and configured to simultaneously exchange signals in the PON, the controller circuitry comprising means for performing:
- deriving (<NUM>) a level of optical beat interference, OBI, in a received optical signal (<NUM>) comprising a desired upstream signal originating from an optical transceiver of an optical network terminal (<NUM>), ONT, and comprising an undesired signal originating from signals simultaneously transmitted by the optical transmitter; and
- setting (<NUM>) a wavelength (<NUM>) of a downstream optical signal (<NUM>) based on the level of OBI such that the wavelength (<NUM>) is forced to differ from the upstream optical signal wavelength (<NUM>);
wherein the means are further configured to perform:
- obtaining (<NUM>) a monitored signal (<NUM>) indicative for the received optical signal (<NUM>);
and wherein the deriving of the level of OBI is performed by the steps of:
- amplifying (<NUM>, <NUM>, <NUM>) the monitored signal thereby obtaining an amplified signal; and
- high-pass filtering (<NUM>, <NUM>, <NUM>) the amplified signal thereby obtaining a filtered signal;
- enveloping (<NUM>, <NUM>, <NUM>) the filtered signal thereby obtaining an enveloped signal; and
- integrating (<NUM>, <NUM>, <NUM>) the enveloped signal thereby obtaining an integrated signal; and
whereby the integrated signal is indicative for the level of OBI.