Patent Description:
A conventional optical network architecture comprises a single Optical Line Termination (OLT) and a plurality of Optical Network Units (ONUs) connected in a one-to-many relationship via an optical splitter (and optionally an aggregation node). The OLT comprises an optical source for generating an optical signal that is distributed to each ONU via the optical splitter. One or more ONUs may have specific requirements for one or more properties of the optical signal, such as a linewidth and/or signal coherence time requirement. A property of the optical signal at the ONU is a function of the property of the optical signal as generated by the OLT and any changes experienced by the optical signal as it is communicated via the optical components to the ONU (e.g. the optical splitter and the connecting optical fibres between the OLT, optical splitter and ONU). These changes in properties may be realised in linewidth broadening and/or a reduction in signal coherence time. Prior art document <CIT> discloses phase noise cancellation of a modulated optical signal in a point to point system using transmission of a local oscillator.

According to a first aspect of the invention, there is provided an optical network comprising: a first optical transmitter; an optical splitter; a plurality of optical receivers; a first optical fibre connecting the first optical transmitter and the optical splitter; a plurality of second optical fibres, each second optical fibre connecting the optical splitter to a respective optical receiver of the plurality of optical receivers, wherein the first optical transmitter is configured to transmit a first optical signal to each optical receiver of the plurality of optical receivers via the first optical fibre, the optical splitter and a respective second optical fibre of the plurality of second optical fibres; and a plurality of second optical fibre phase correction units, each second optical fibre phase correction unit being associated with a second optical fibre of the plurality of second optical fibres, each second optical fibre phase correction unit comprising a reference optical transmitter, a reference error signal generator and a reference phase shifter, wherein: each reference optical transmitter is configured to transmit a reference optical signal on the associated second optical fibre of the plurality of second optical fibres, each reference error signal generator is configured to generate a reference error signal based on a reflection of the reference optical signal on the associated second optical fibre of the plurality of second optical fibres, and each reference phase shifter is configured to apply a phase shift to the first optical signal based on the reference error signal.

Each optical receiver of the plurality of optical receivers may comprise the associated second optical fibre phase correction unit of the plurality of second optical fibre phase correction units.

The reference optical signal may be reflected by the optical splitter so as to generate the reflection of the reference optical signal.

The first optical signal may be transmitted in a downstream direction and the reference optical signal may be transmitted in an upstream direction.

The method may further comprise the steps of: a first optical fibre phase correction unit associated with the first optical fibre, the first optical fibre phase correction unit comprising a first error signal generator and a first phase shifter, wherein: the first error signal generator is configured to generate a first error signal based on a reflection of the first optical signal on the first optical fibre, and the first phase shifter is configured to apply a phase shift to the first optical signal based on the first error signal.

The first optical transmitter may be configured to transmit the first optical signal at a first wavelength to each optical receiver of the plurality of optical receivers via the first optical fibre, the optical splitter and the respective second optical fibre of the plurality of second optical fibres, and the reference optical transmitter may be configured to transmit the reference optical signal at a second wavelength on the associated second optical fibre of the plurality of second optical fibres, wherein the first wavelength is different to the second wavelength.

The optical splitter may be a first hop optical splitter of a plurality of optical splitters, the plurality of optical splitters may further comprise a first last-hop optical splitter and a second last-hop optical splitter, a first set of the plurality of optical receivers may be connected to the first last-hop optical splitter, a second set of the plurality of optical receivers may be connected to the second last-hop optical splitter, the first optical signal may comprise data for the first set of the plurality of optical receivers and may be transmitted at a first wavelength using wavelength-division-multiplexing via the first-hop optical splitter and the first last-hop optical splitter, and the first optical transmitter may be further configured to transmit a second optical signal at a second wavelength using wavelength-division-multiplexing via the first-hop optical splitter and the second last-hop optical splitter, the second optical signal comprising data for the second set of the plurality of optical receivers, and the optical network may further comprise: a first wavelength-selective reflector associated with the first last-hop optical splitter and configured to reflect the first optical signal at the first wavelength; a second wavelength-selective reflector associated with the second last-hop optical splitter and configured to reflect the second optical signal at the second wavelength; and a first optical fibre phase correction unit associated with the first optical fibre, the first optical fibre phase correction unit comprising a first error signal generator, a first phase shifter, a second error signal generator and a second phase shifter, wherein: the first error signal generator is configured to generate a first error signal based on a reflection of the first optical signal at the first wavelength on the first optical fibre, the first phase shifter is configured to apply a phase shift to the first optical signal at the first wavelength based on the first error signal, the second error signal generator is configured to generate a second error signal based on a reflection of the second optical signal at the second wavelength on the first optical fibre, and the second phase shifter is configured to apply a phase shift to the second optical signal at the second wavelength based on the second error signal.

According to a second aspect of the invention, there is provided a method in an optical network, the optical network comprising: a first optical transmitter; an optical splitter; a plurality of optical receivers; a first optical fibre connecting the first optical transmitter and the optical splitter; a plurality of second optical fibres, each second optical fibre connecting the optical splitter to a respective optical receiver of the plurality of optical receivers, wherein the first optical transmitter is configured to transmit a first optical signal to each optical receiver of the plurality of optical receivers via the first optical fibre, the optical splitter and a respective second optical fibre of the plurality of second optical fibres; and a plurality of second optical fibre phase correction units, each second optical fibre phase correction unit being associated with a second optical fibre of the plurality of second optical fibres, the method comprising the steps of: transmitting a reference optical signal on the associated second optical fibre of the plurality of second optical fibres; generating a reference error signal based on a reflection of the reference optical signal on the associated second optical fibre of the plurality of second optical fibres; and applying a phase shift to the first optical signal based on the reference error signal.

According to a third aspect of the invention, there is provided a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method of the second aspect of the invention. The computer program may be stored on a computer readable carrier medium.

<FIG> illustrates a first optical network <NUM> comprising an Optical Line Termination (OLT) <NUM>, an optical splitter <NUM>, and a plurality of Optical Network Units (ONUs) <NUM>. The OLT <NUM> and optical splitter <NUM> are connected by a first optical fibre known as a spine fibre <NUM>. The optical splitter <NUM> is connected to each ONU <NUM> of the plurality of ONUs <NUM> by a respective second optical fibre known as a distribution fibre <NUM>.

The OLT <NUM> is shown in more detail in <FIG>. The OLT <NUM> includes an optical source <NUM> (e.g. a laser) configured to generate a <NUM> optical signal. This wavelength is within the C-band of optical telecommunications such that it experiences relatively low attenuation when transmitted over optical fibre (relative to the attenuation experienced by wavelengths outside the optical telecommunications bands). One or more ONUs of the plurality of ONUs <NUM> have specific requirements for the <NUM> optical signal, such as a linewidth requirement and/or signal coherence time requirement.

The OLT <NUM> also includes a circulator <NUM>, an error signal generator <NUM> and a phase shifter <NUM>. The circulator <NUM>, error signal generator <NUM> and phase shifter <NUM> participate in an Active Noise Cancellation (ANC) mechanism, described below.

The OLT <NUM> also includes an optical communications interface <NUM> with the spine fibre <NUM>, enabling communication of the <NUM> optical signal (generated by the optical source <NUM> of the OLT <NUM>) to the optical splitter <NUM> via the spine fibre <NUM>. The <NUM> optical signal, when being transmitted from the optical source <NUM> towards the plurality of ONUs <NUM>, is hereinafter known as the "downstream <NUM> optical signal".

The optical splitter <NUM> is a <NUM>-way optical splitter for splitting and distributing the downstream <NUM> optical signal to the plurality of ONUs <NUM> (although other optical splitters, such as a <NUM>-way optical splitter, may be used). The optical splitter <NUM> also reflects (via a passive reflection) a portion of the downstream <NUM> optical signal (hereinafter, the "reflection of the downstream <NUM> optical signal") back towards the OLT <NUM> along the spine fibre <NUM>. The reflection of the downstream <NUM> optical signal may be used by the OLT <NUM> in an ANC mechanism to cancel phase noise on the downstream <NUM> optical signal and therefore contribute to the downstream <NUM> optical signal satisfying the specific requirements of one or more ONUs of the plurality of ONUs <NUM> (such as a linewidth requirement and/or signal coherence time requirement).

The optical splitter <NUM> also comprises a multiplexer-demultiplexer <NUM> for multiplexing the downstream <NUM> optical signal onto the plurality of distribution fibres <NUM> (this is described in more detail below).

A filter <NUM> is positioned on the spine fibre <NUM> between the OLT <NUM> and optical splitter <NUM>.

The OLT <NUM> ANC mechanism will now be described with reference to the schematic diagram of the OLT <NUM> and optical splitter <NUM> in <FIG> and the flow diagram of <FIG>. In a first step (S101) of <FIG>, the optical source <NUM> generates the downstream <NUM> optical signal. The downstream <NUM> optical signal is transmitted in two paths - a first path to the circulator <NUM> (which passes the downstream <NUM> optical signal to the phase shifter <NUM>) and a second path to the error signal generator <NUM>.

Following the first path of the downstream <NUM> optical signal, the circulator <NUM> passes the downstream <NUM> optical signal to the phase shifter <NUM>. The phase shifter <NUM> is configured to apply a phase shift to the downstream <NUM> optical signal so as to minimise an error signal generated by the error signal generator <NUM> (described in more detail below). The downstream <NUM> optical signal, as phase shifted by the phase shifter <NUM>, is then communicated via the optical communications interface <NUM> and spine fibre <NUM> to the optical splitter <NUM>. As noted above, the optical splitter <NUM> reflects the downstream <NUM> optical signal and the reflection of the downstream <NUM> optical signal is received at the optical communications interface <NUM>.

The reflection of the downstream <NUM> optical signal is communicated from the optical communications interface <NUM> to the phase shifter <NUM> which applies a phase shift to the reflection of the downstream <NUM> optical signal. The reflection of the downstream <NUM> optical signal is then communicated to the circulator <NUM>, which passes the reflection of the downstream <NUM> optical signal to the error signal generator <NUM>.

The error signal generator <NUM> therefore receives both the downstream <NUM> optical signal from the optical source <NUM> (via the second path of the downstream <NUM> optical signal which has not been phase shifted by the phase shifter <NUM>) and the reflection of the downstream <NUM> optical signal from the circulator <NUM> (which has been twice phase shifted by the phase shifter <NUM>). In step S103, the error signal generator <NUM> generates an error signal that is proportional to the interference between the downstream <NUM> optical signal and the reflection of the downstream <NUM> optical signal. The error signal generator <NUM> may generate this error signal by acting as a mixer that mixes the downstream <NUM> optical signal and the reflection of the downstream <NUM> optical signal, wherein the error signal produced from the mixing of these two optical signals is proportional to the interference between the two optical signals.

The error signal generator <NUM> communicates the generated error signal to the phase shifter <NUM>. In step S105, the phase shifter <NUM> applies a phase shift to the downstream <NUM> optical signal so as to minimise the error signal. The phase shifter <NUM> includes a controller configured to control a value of the phase shift applied by the phase shifter <NUM> to the downstream <NUM> optical signal. The controller therefore varies the phase shift applied to the downstream <NUM> optical signal until the error signal is minimised (e.g. in a negative feedback loop) or at least reduced such that the downstream <NUM> optical signal satisfies the requirement(s) of the one of more ONUs of the plurality of ONUs <NUM> (e.g. the linewidth of the downstream <NUM> optical signal remains within the linewidth requirement and/or the signal coherence time of the downstream <NUM> optical signal remains within the signal coherence time requirement).

The downstream <NUM> optical signal is then communicated, via the optical communications interface <NUM> and spine fibre <NUM> to the optical splitter <NUM>.

The above ANC mechanism is able to correct phase noise on the point-to-point link of the spine fibre <NUM> between the OLT <NUM> and optical splitter <NUM>. However, this technique is not directly applicable to correct phase noise on a point-to-multipoint architecture such as the OLT <NUM> to plurality of ONU <NUM> architecture of <FIG>. That is, the amount of phase noise experienced by a particular ONU of the plurality of ONUs <NUM> may differ from the phase noise experienced by another ONU of the plurality of ONUs <NUM> (due to, for example, the phase noise at each respective port of the optical splitter <NUM> imparting a different amount of phase noise and/or each respective distribution fibre <NUM> imparting a different amount of phase noise). Accordingly, a single phase shift cannot be applied by the OLT <NUM> to correct the different amounts of phase noise experienced by each ONU of the plurality of ONUs <NUM>.

This problem is solved in the optical network <NUM> by implementing the ANC mechanism at each ONU <NUM> of the plurality of ONUs <NUM>. The ONU <NUM> ANC mechanism will now be described with reference to the schematic diagram of an ONU <NUM> of the plurality of ONUs <NUM> and optical splitter <NUM> in <FIG> and the flow diagram of <FIG>. The ONU <NUM> comprises an optical source <NUM> configured to generate a reference optical signal. The reference optical signal may have a different wavelength to the optical signal generated by the optical source <NUM> of the OLT <NUM>. In an example, the reference optical signal has as wavelength of <NUM>. The ONU <NUM> further comprises a circulator <NUM>, an error signal generator <NUM>, a multiplexer-demultiplexer <NUM> and a phase shifter <NUM>.

The circulator <NUM>, error signal generator <NUM> and phase shifter <NUM> participate in an ANC mechanism, described below.

The ONU <NUM> also includes an optical communications interface <NUM> with its respective distribution fibre <NUM>, enabling communication of the reference optical signal (generated by the optical source <NUM> of the ONU <NUM>) to the optical splitter <NUM> via the respective distribution fibre <NUM>. The reference optical signal, when being transmitted from the ONU <NUM> towards the optical splitter <NUM>, is hereinafter known as the "upstream reference optical signal".

The optical splitter <NUM> reflects a portion of the upstream reference optical signal (hereinafter, the "reflection of the upstream reference optical signal") back towards the ONU <NUM> along the respective distribution fibre <NUM>. The reflection of the upstream reference optical signal may be used by the ONU <NUM> in an ANC mechanism to cancel phase noise on the downstream <NUM> optical signal and therefore contribute to the downstream <NUM> optical signal satisfying the specific requirements of one or more ONUs of the plurality of ONUs <NUM> (such as a linewidth requirement and/or signal coherence time requirement).

As noted above, the optical splitter <NUM> also comprises a multiplexer-demultiplexer <NUM> which multiplexes the downstream <NUM> optical signal (received from the OLT <NUM> from the spine fibre <NUM>) onto the distribution fibre <NUM>. The multiplexed downstream <NUM> optical signal is communicated from the optical splitter <NUM> to the multiplexer-demultiplexer <NUM> of the ONU <NUM> via the distribution fibre <NUM>, optical communications interface <NUM> and phase shifter <NUM>. The phase shifter <NUM> applies a phase shift to the multiplexed downstream <NUM> optical signal so as to minimise (or at least reduce) an error signal generated by the error signal generator <NUM>, as described below.

In a first step (S201) of the flow diagram of <FIG>, the optical source <NUM> generates the upstream reference optical signal. The upstream reference optical signal is transmitted in two paths - a first path to the circulator <NUM> (which passes the upstream reference optical signal to the multiplexer-demultiplexer <NUM>) and a second path to the error signal generator <NUM>.

Following the first path of the upstream reference optical signal, the circulator <NUM> passes the upstream reference optical signal to the multiplexer-demultiplexer <NUM>. The multiplexer-demultiplexer <NUM> multiplexes the upstream reference optical signal with the downstream <NUM> optical signal. The multiplexed upstream reference optical signal is communicated to the phase shifter <NUM>.

The phase shifter <NUM> is configured to apply a phase shift to the multiplexed upstream reference optical signal and multiplexed downstream <NUM> optical signal based on an error signal generated by the error signal generator <NUM> (described in more detail below). The multiplexed upstream reference optical signal, as phase shifted by the phase shifter <NUM>, is then communicated via the optical communications interface <NUM> and distribution fibre <NUM> to the optical splitter <NUM>.

At the optical splitter <NUM>, the multiplexed upstream reference optical signal is demultiplexed at the multiplexer-demultiplexer <NUM>. The upstream reference optical signal is then reflected at the optical splitter <NUM>. The reflection of the upstream reference signal is then multiplexed by the multiplexer-demultiplexer <NUM> (together with the downstream <NUM> optical signal) onto the distribution fibre <NUM> and received at the optical communications interface <NUM> of the ONU <NUM>.

The demultiplexing of the upstream reference optical signal at the optical splitter <NUM> may be imperfect. To avoid any portion of the upstream reference optical signal being communicated along the spine fibre to the OLT <NUM>, the filter <NUM> (as shown in <FIG> and <FIG>) acts to remove the upstream reference optical signal from the spine fibre <NUM>.

Returning to <FIG> and <FIG>, the multiplexed reflection of the upstream reference optical signal and multiplexed downstream <NUM> optical signal is then communicated from the optical communications interface <NUM> to the phase shifter <NUM> which applies a phase shift to the multiplexed upstream reference optical signal and multiplexed downstream <NUM> optical signal based on an error signal generated by the error signal generator <NUM>. It is noted that the downstream <NUM> optical signal is phase shifted once by the phase shifter <NUM> but the reflection of the upstream reference signal is phase shifted twice (a first phase shift as it is communicated from the ONU <NUM> to the optical splitter <NUM> prior to reflection, and a second phase shift as it is communicated from the optical splitter <NUM> to the ONU <NUM> after reflection).

The multiplexer-demultiplexer <NUM> demultiplexes the downstream <NUM> optical signal and the reflection of the upstream reference optical signal. Following the path of upstream reference optical signal, the reflection of the upstream reference optical signal is communicated to the circulator <NUM>, which passes the reflection of the upstream reference optical signal to the error signal generator <NUM>.

The error signal generator <NUM> therefore receives both the upstream reference optical signal from the optical source <NUM> (via the second path of the upstream reference optical signal which has not been phase shifted by the phase shifter <NUM>) and the reflection of the upstream reference optical signal from the circulator <NUM> (which has been twice phase shifted by the phase shifter <NUM>). In step S203, the error signal generator <NUM> generates an error signal that is proportional to the interference between the upstream reference optical signal and the reflection of the upstream reference optical signal. The error signal generator <NUM> may generate this error signal by acting as a mixer that mixes the upstream reference optical signal and the reflection of the upstream reference optical signal, wherein the error signal produced from the mixing of these two optical signals is proportional to the interference between the two optical signals.

The error signal generator <NUM> communicates the generated error signal to the phase shifter <NUM>. In step S205, the phase shifter <NUM> applies a phase shift to both the multiplexed upstream reference optical signal and the multiplexed downstream <NUM> optical signal so as to minimise the error signal. The phase shifter <NUM> includes a controller configured to control a value of the phase shift applied by the phase shifter <NUM> to the multiplexed upstream reference optical signal and the multiplexed downstream <NUM> optical signal. The controller therefore varies the phase shift applied to the multiplexed upstream reference optical signal and the multiplexed downstream <NUM> optical signal until the error signal generated by the error signal generator <NUM> is minimised (e.g. in a negative feedback loop) or at least reduced such that the downstream <NUM> optical signal satisfies the requirement(s) of the one or more ONUs of the plurality of ONUs <NUM> (such as a linewidth requirement and/or signal coherence time requirement).

The multiplexed downstream <NUM> optical signal is therefore communicated, with a corrected phase, to the multiplexer-demultiplexer <NUM>. As noted above, the multiplexer-demultiplexer <NUM> demultiplexes the downstream <NUM> optical signal and (optionally) communicates the downstream <NUM> optical signal to one or more network nodes (not shown) downstream of the ONU <NUM>.

The combination of the OLT ANC mechanism and the ONU ANC mechanism therefore enables the downstream <NUM> optical signal to be generated by a single optical source <NUM> and distributed to the plurality of ONUs <NUM> (and any downstream network nodes) with cancelled phase noise so as to satisfy requirements for the downstream <NUM> optical signal at the plurality of ONUs <NUM>.

The first optical fibre network <NUM> therefore provides a point-to-multipoint architecture having ANC applied to the respective connection between the transmitter (the OLT <NUM>) and each receiver (the ONU <NUM>). As noted above, this is not possible when applying ANC at the OLT <NUM> alone to the downstream signal, but is achieved in the optical fibre network <NUM> by applying ANC at each ONU <NUM> to the downstream signal based on the analysis of an upstream signal. The first optical network <NUM> may therefore suffer less phase noise than conventional optical fibre networks, enabling the first optical network <NUM> to meet performance requirements such as a linewidth requirement and/or a signal coherence time requirement. The first optical network <NUM> may therefore be more useful in applications having relatively strict linewidth and/or signal coherence requirements, such as in Quantum Key Distribution (QKD), Rydberg-atom based technologies (such as a Rydberg-atom based electromagnetic field detector, a Rydberg-atom based atomic clock, etc.), and distributed massive MIMO (to provide phase coherence between physically separated antennas).

The first optical network <NUM> comprises a single optical splitter <NUM>. However, as shown in the second optical network <NUM> in <FIG>, the optical splitter may be one of a plurality of optical splitters <NUM>, and the second optical network <NUM> may further comprise an aggregation node <NUM> between the OLT <NUM> and the plurality of optical splitters <NUM>. In this scenario, the second optical network comprises a plurality of spine fibres <NUM>, wherein each spine fibre connects the OLT <NUM> to a respective optical splitter of the plurality of optical splitters <NUM> via the aggregation node <NUM>, and the OLT <NUM> generates a distinct optical signal for each spine fibre. The OLT ANC mechanism is then performed independently to each spine fibre based on the reflection of the distinct optical signal by the respective optical splitter (as described above in relation to the single optical splitter scenario).

A third optical network <NUM> is shown in <FIG>, the third optical network <NUM> also comprising a plurality of optical splitters <NUM>. In the third optical network <NUM>, the plurality of optical splitters <NUM> are arranged in a hierarchy so the OLT <NUM> connects to each ONU of the plurality of ONUs by a series of optical splitters. In this scenario, the OLT <NUM> may utilise wavelength division multiplexing to multiplex a plurality of optical signals for communication with the plurality of ONUs. Each downstream optical signal of the plurality of optical signals has a wavelength dedicated to a (disjoint) set of the plurality of ONUs, wherein the (disjoint) set of the plurality of ONUs are all directly connected to a particular last-hop optical splitter of the plurality of optical splitters <NUM>. A fibre Bragg grating <NUM> may be positioned at each last-hop optical splitter so as to reflect the downstream optical signal at the wavelength dedicated to the (disjoint) set of the plurality of ONUs connected to that last-hop optical splitter. The OLT <NUM> may therefore implement the above OLT ANC mechanism to the downstream optical signal from the OLT <NUM> to each last-hop optical splitter by demultiplexing the reflection of the downstream optical signal at the wavelength dedicated to that respective (disjoint) set of the plurality of ONUs and using it to generate an error signal (as described above). This scenario may be relevant when different (disjoint) sets of the plurality of ONUs require optical signals at different wavelengths and those sets of the plurality of ONUs are connected to separate last-hop optical splitters in a concatenated optical splitter architecture.

The optical networks described above utilise an upstream reference signal that may have a wavelength of <NUM>. The difference between the wavelength of the upstream reference signal and the wavelength of the downstream signal is within a threshold such that the phase noise imparted to the upstream reference signal as it is communicated over the distribution fibre is the same as or substantially the same as the phase noise imparted to the downstream signal as it is communicated over the distribution fibre. This threshold may be, for example, <NUM>%, <NUM>%, <NUM>% or <NUM>%. The threshold may be determined by a calibration phase in which the phase noise (and/or one or more requirements related to phase noise) at one or more ONUs is measured when using upstream reference signals at different wavelengths, and identifying the wavelengths for the upstream reference signal that enable the ONU ANC mechanism to satisfactorily correct the phase noise (and/or satisfy the one or more requirements related to phase noise). The acceptable level of phase noise (and/or the one or more requirements) may differ for different applications, so these identified wavelengths for the upstream reference signal may be application-specific.

It is also non-essential that the upstream reference signal and downstream signal use different wavelengths. That is, the downstream signal and upstream reference signal may have the same wavelength and are time-multiplexed onto the optical fibres. It is also non-essential that one or both of the downstream and upstream optical signals are within the C-band of optical telecommunications. One or both of the downstream and upstream optical signals may be within any one of the communication bands of optical telecommunications, or outside these communication bands.

In the above first optical network <NUM>, the error signal was generated in both the OLT ANC mechanism and the ONU ANC mechanism by mixing two optical signals. The error signal generators may be improved by applying a known constant frequency offset to one of the signals (that is, one of the downstream <NUM> optical signal or reflection of the downstream <NUM> optical signal in the OLT ANC mechanism, or one of the upstream reference optical signal or reflection of the upstream reference optical signal in the ONU ANC mechanism) prior to mixing. The frequency offset signal may be generated by a Radio Frequency (RF) signal generator. Mixing the shifted and unshifted optical signals produces a resulting beat note (centred at the frequency offset). The frequency offset may then be removed to derive the error signal that may then be used by the phase shifter. This technique may improve the accuracy of the error signal.

In the optical networks described above, the ONU ANC mechanism was applied by modules present in the ONU. However, this is non-essential and one or more separate nodes may cooperate with each ONU to implement the ONU ANC mechanism.

The skilled person will also understand that the OLT ANC mechanism is non-essential, such as when the spine fibre has optical transmission characteristics (such as a sufficiently short length) such that the one or more requirements of the one or more ONUs of the plurality of ONUs are satisfied without the OLT ANC mechanism.

In the optical networks described above, the phase shifters apply a phase shift to the respective optical signals to minimise (or at least reduce) the phase noise using a negative feedback loop based on an error signal. The skilled person will understand that the use of a negative feedback loop is non-essential and other techniques for reducing the phase noise based on the generated optical signal and its reflection may be used (such as a calibrated table between the error signal and a corresponding phase shift).

The optical networks described above are Passive Optical Networks (PONs). However, the skilled person will understand that the methods described above apply to any form of point-to-multipoint optical network.

<FIG> illustrates a method in an optical network, the optical network comprising the optical network comprising: a first optical transmitter; an optical splitter; a plurality of optical receivers; a first optical fibre connecting the first optical transmitter and the optical splitter; a plurality of second optical fibres, each second optical fibre connecting the optical splitter to a respective optical receiver of the plurality of optical receivers, wherein the first optical transmitter is configured to transmit a first optical signal to each optical receiver of the plurality of optical receivers via the first optical fibre, the optical splitter and a respective second optical fibre of the plurality of second optical fibres; and a plurality of second optical fibre phase correction units, each second optical fibre phase correction unit being associated with a second optical fibre of the plurality of second optical fibres, the method comprising the steps of: transmitting (step S301) a reference optical signal on the associated second optical fibre of the plurality of second optical fibres; generating (step S303) a reference error signal based on a reflection of the reference optical signal on the associated second optical fibre of the plurality of second optical fibres; and applying (step S305) a phase shift to the first optical signal based on the reference error signal.

Claim 1:
An optical network comprising:
a first optical transmitter;
an optical splitter;
a plurality of optical receivers;
a first optical fibre connecting the first optical transmitter and the optical splitter;
a plurality of second optical fibres, each second optical fibre connecting the optical splitter to a respective optical receiver of the plurality of optical receivers, wherein the first optical transmitter is configured to transmit a first optical signal to each optical receiver of the plurality of optical receivers via the first optical fibre, the optical splitter and a respective second optical fibre of the plurality of second optical fibres; and
a plurality of second optical fibre phase correction units, each second optical fibre phase correction unit being associated with a second optical fibre of the plurality of second optical fibres, each second optical fibre phase correction unit comprising a reference optical transmitter, a reference error signal generator and a reference phase shifter, wherein:
each reference optical transmitter is configured to transmit a reference optical signal on the associated second optical fibre of the plurality of second optical fibres,
each reference error signal generator is configured to generate a reference error signal based on a reflection of the reference optical signal transmitted on the associated second optical fibre of the plurality of second optical fibres, and
each reference phase shifter is configured to apply a phase shift to the first optical signal based on the reference error signal.