Optical Network

Abstract

Optical network. The present invention provides an optical network and a method in the optical network. The optical network includes: a first optical transmitter; an optical splitter; a plurality of optical receivers; a first optical fiber connecting the first optical transmitter and the optical splitter; a plurality of second optical fibers, each second optical fiber connecting the optical splitter to a corresponding optical receiver, the first optical transmitter transmitting a first optical signal to each of the plurality of optical receivers via the first optical fiber, the optical splitter, and the corresponding second optical fiber; and a plurality of second optical fiber phase correction units, each second optical fiber phase correction unit being associated with a second optical fiber and comprising a reference optical transmitter, a reference error signal generator, and a reference phase shifter, wherein: each reference optical transmitter transmits a reference optical signal on an associated second optical fiber, each reference error signal generator generates a reference error signal based on reflection of the reference optical signal on the associated second optical fiber, and each reference phase shifter applies a phase shift to the first optical signal based on the reference error signal.

Description

Technical Field

[0001] The present invention relates to an optical network. Background Art

[0002] The traditional optical network architecture includes a single optical line terminal (OLT) and multiple optical network units (ONUs), which are connected in a one-to-many relationship via an optical splitter (and optionally via an aggregation node). The OLT includes a light source for generating an optical signal distributed to each ONU via an optical splitter. One or more ONUs may have specific requirements for one or more characteristics of the optical signal, such as line width and / or signal coherence time requirements. The characteristics of the optical signal at the ONU are a function of the characteristics of the optical signal generated by the OLT and any changes that the optical signal undergoes when it is transmitted to the ONU via optical components (such as an optical splitter and connecting optical fibers between the OLT, the optical splitter and the ONU). These characteristic changes can be achieved in terms of widening the line width and / or reducing the signal coherence time. Summary of the Invention

[0003] According to a first aspect of the present invention, an optical network is provided, comprising: a first optical transmitter; an optical splitter; a plurality of optical receivers; a first optical fiber connecting the first optical transmitter and the optical splitter; a plurality of second optical fibers, each second optical fiber connecting the optical splitter to a corresponding optical receiver among the plurality of optical receivers, wherein the first optical transmitter is configured to transmit a first optical signal to each of the plurality of optical receivers via the first optical fiber, the optical splitter, and a corresponding second optical fiber among the plurality of second optical fibers; and a plurality of second optical fiber phase correction units, each second optical fiber phase correction unit being associated with a second optical fiber among the plurality of second optical fibers, each second optical fiber 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 an associated second optical fiber among the plurality of second optical fibers, each reference error signal generator is configured to generate a reference error signal based on reflection of the reference optical signal on the associated second optical fiber among the plurality of second optical fibers, and each reference phase shifter is configured to apply a phase shift to the first optical signal based on the reference error signal.

[0004] Each optical receiver of the plurality of optical receivers may include an associated second optical fiber phase correction unit of the plurality of second optical fiber phase correction units.

[0005] The reference optical signal may be reflected by the optical splitter to generate a reflection of the reference optical signal.

[0006] The first optical signal may be sent in a downstream direction, and the reference optical signal may be sent in an upstream direction.

[0007] The method comprises the following steps: a first optical fiber phase correction unit is associated with a first optical fiber, the first optical fiber 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 reflection of a first optical signal on the first optical fiber, and the first phase shifter is configured to apply a phase shift to the first optical signal based on the first error signal.

[0008] The first optical transmitter may be configured to transmit a first optical signal at a first wavelength to each of the plurality of optical receivers via the first optical fiber, the optical splitter, and a corresponding second optical fiber among the plurality of second optical fibers, and the reference optical transmitter may be configured to transmit a reference optical signal at a second wavelength on an associated second optical fiber among the plurality of second optical fibers, wherein the first wavelength is different from the second wavelength.

[0009] The optical splitter may be a first-hop optical splitter among the plurality of optical splitters, the plurality of optical splitters may further include 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 include 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 including data for the second set of the plurality of optical receivers, and the optical network The network may further include: a first wavelength selective reflector associated with the first last-hop optical splitter and configured to reflect a first optical signal at a first wavelength; a second wavelength selective reflector associated with the second last-hop optical splitter and configured to reflect a second optical signal at a second wavelength; and a first fiber phase correction unit associated with the first optical fiber, the first fiber 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 reflection of the first optical signal at the first wavelength on the first optical fiber, 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 reflection of the second optical signal at the second wavelength on the first optical fiber, 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.

[0010] According to a second aspect of the present invention, a method in an optical network is provided, the optical network comprising: a first optical transmitter; an optical splitter; a plurality of optical receivers; a first optical fiber connecting the first optical transmitter and the optical splitter; a plurality of second optical fibers, each second optical fiber connecting the optical splitter to a corresponding optical receiver among the plurality of optical receivers, wherein the first optical transmitter is configured to send a first optical signal to each of the plurality of optical receivers via the first optical fiber, the optical splitter, and a corresponding second optical fiber among the plurality of second optical fibers; and a plurality of second optical fiber phase correction units, each second optical fiber phase correction unit being associated with a second optical fiber among the plurality of second optical fibers, the method comprising the steps of: sending a reference optical signal on an associated second optical fiber among the plurality of second optical fibers; generating a reference error signal based on reflection of the reference optical signal on the associated second optical fiber among the plurality of second optical fibers; and applying a phase shift to the first optical signal based on the reference error signal.

[0011] According to a third aspect of the present invention, there is provided a computer program comprising instructions which, when executed by a computer, cause the computer to perform the steps of the method of the second aspect of the present invention. The computer program may be stored on a computer readable carrier medium. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] For a better understanding of the present invention, embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0013] Figure 1 is a schematic diagram illustrating a first optical network;

[0014] Figure 2 It shows Figure 1 Schematic diagram of the optical line terminal (OLT) and optical splitter of the optical network;

[0015] Figure 3 is a flow chart illustrating a first method;

[0016] Figure 4 It shows Figure 1 Schematic diagram of optical splitter and optical network unit (ONU) of optical network;

[0017] Figure 5 is a flow chart illustrating a second method;

[0018] Figure 6 is a schematic diagram illustrating a second optical network;

[0019] Figure 7 is a schematic diagram illustrating a third optical network; and

[0020] Figure 8 is a flowchart illustrating the third method. DETAILED DESCRIPTION

[0021] Figure 1 A first optical network 100 is shown, which includes an optical line terminal (OLT) 110, an optical splitter 130, and a plurality of optical network units (ONUs) 150. The OLT 110 and the optical splitter 130 are connected by a first optical fiber, referred to as a trunk fiber 120. The optical splitter 130 is connected to each of the plurality of ONUs 150 by a corresponding second optical fiber, referred to as a distribution fiber 140.

[0022] exist Figure 2 1. OLT 110 is shown in greater detail in FIG. OLT 110 includes a light source 111 (e.g., a laser) configured to generate a 1550 nm optical signal. This wavelength is within the C-band for optical telecommunications, so that it experiences relatively low attenuation when transmitted through optical fiber (relative to the attenuation experienced by wavelengths outside the optical telecommunications band). One or more of the plurality of ONUs 150 have specific requirements for the 1550 nm optical signal, such as linewidth requirements and / or signal coherence time requirements.

[0023] The OLT 110 further includes a circulator 113, an error signal generator 115, and a phase shifter 117. The circulator 113, the error signal generator 115, and the phase shifter 117 participate in an active noise cancellation (ANC) mechanism, as described below.

[0024] OLT 110 also includes an optical communication interface 119 with a trunk optical fiber 120, enabling a 1550 nm optical signal (generated by light source 111 of OLT 110) to be transmitted to optical splitter 130 via trunk optical fiber 120. When transmitted from light source 111 toward multiple ONUs 150, the 1550 nm optical signal is hereinafter referred to as a "downstream 1550 nm optical signal."

[0025] The optical splitter 130 is a 32-way optical splitter that is used to split and distribute the downstream 1550nm optical signal to the multiple ONUs 150 (although other optical splitters, such as a 16-way optical splitter, may be used). The optical splitter 130 also reflects a portion of the downstream 1550nm optical signal (hereinafter referred to as "reflection of the downstream 1550nm optical signal") back along the trunk optical fiber 120 toward the OLT 110. The OLT 110 in the ANC mechanism can use the reflection of the downstream 1550nm optical signal to eliminate phase noise on the downstream 1550nm optical signal, thereby helping the downstream 1550nm optical signal meet the specific requirements of one or more of the multiple ONUs 150 (such as linewidth requirements and / or signal coherence time requirements).

[0026] The optical splitter 130 also includes a multiplexer-demultiplexer 131 for multiplexing the downstream 1550 nm optical signal onto a plurality of distribution optical fibers 140 (described in more detail below).

[0027] The filter 129 is located on the trunk optical fiber 120 between the OLT 110 and the optical splitter 130 .

[0028] Now refer to Figure 2 Schematic diagram of OLT 110 and optical splitter 130 and Figure 3 The flowchart below describes the ANC mechanism of OLT110. Figure 3 In the first step (S101), the light source 111 generates a downstream 1550nm optical signal. The downstream 1550nm optical signal is sent in two paths, a first path to the circulator 113 (which transmits the downstream 1550nm optical signal to the phase shifter 117) and a second path to the error signal generator 115.

[0029] After the first path of the downstream 1550nm optical signal, the circulator 113 transmits the downstream 1550nm optical signal to the phase shifter 117. The phase shifter 117 is configured to apply a phase shift to the downstream 1550nm optical signal to minimize the error signal generated by the error signal generator 115 (described in more detail below). The downstream 1550nm optical signal, phase-shifted by the phase shifter 117, is then transmitted to the optical splitter 130 via the optical communication interface 119 and the trunk optical fiber 120. As described above, the optical splitter 130 reflects the downstream 1550nm optical signal, and the reflection of the downstream 1550nm optical signal is received at the optical communication interface 119.

[0030] The reflection of the downstream 1550nm optical signal is transmitted from optical communication interface 119 to phase shifter 117, which applies a phase shift to the reflection of the downstream 1550nm optical signal. The reflection of the downstream 1550nm optical signal is then transmitted to circulator 113, which transmits the reflection of the downstream 1550nm optical signal to error signal generator 115.

[0031] Therefore, error signal generator 115 receives the downstream 1550nm optical signal from light source 111 (via the second path of the downstream 1550nm optical signal, which has not been phase-shifted by phase shifter 117) and the reflection of the downstream 1550nm optical signal from circulator 113 (which has been phase-shifted twice by phase shifter 117). In step S103, error signal generator 115 generates an error signal proportional to the coherence between the downstream 1550nm optical signal and the reflection of the downstream 1550nm optical signal. Error signal generator 115 can generate this error signal by acting as a mixer that mixes the downstream 1550nm optical signal and the reflection of the downstream 1550nm optical signal, wherein the error signal generated by mixing the two optical signals is proportional to the coherence between the two optical signals.

[0032] The error signal generator 115 transmits the generated error signal to the phase shifter 117. In step S105, the phase shifter 117 applies a phase shift to the downstream 1550 nm optical signal to minimize the error signal. The phase shifter 117 includes a controller configured to control the phase shift value applied by the phase shifter 117 to the downstream 1550 nm optical signal. Thus, the controller changes the phase shift applied to the downstream 1550 nm optical signal until the error signal is minimized (e.g., in a negative feedback loop) or at least reduced, such that the downstream 1550 nm optical signal meets the requirements of one or more of the plurality of ONUs 150 (e.g., the linewidth of the downstream 1550 nm optical signal remains within the linewidth requirement and / or the signal coherence time of the downstream 1550 nm optical signal remains within the signal coherence time requirement).

[0033] Then, the downstream 1550 nm optical signal is transmitted to the optical splitter 130 via the optical communication interface 119 and the trunk optical fiber 120 .

[0034] The above-described ANC mechanism can correct phase noise on a point-to-point link of the trunk fiber 120 between the OLT 110 and the optical splitter 130. However, this technique is not directly applicable to correcting Figure 1 Phase noise on a point-to-multipoint architecture (such as an OLT 110 to multiple ONUs 150 architecture) can be corrected. That is, the amount of phase noise experienced by a particular ONU in the multiple ONUs 150 can be different from the phase noise experienced by another ONU in the multiple ONUs 150 (e.g., due to the phase noise at each respective port of the optical splitter 130 imparting a different amount of phase noise and / or each respective distribution fiber 140 imparting a different amount of phase noise). Therefore, the OLT 110 cannot apply a single phase shift to correct for the different amounts of phase noise experienced by each of the multiple ONUs 150.

[0035] This problem is solved in the optical network 100 by implementing an ANC mechanism at each of the plurality of ONUs 150. Figure 4 Schematic diagram of the ONU 150 and the optical splitter 130 in the plurality of ONUs 150 and Figure 5 The flowchart of FIG1 is used to describe the ANC mechanism of ONU 150. ONU 150 includes a light source 151 configured to generate a reference optical signal. The reference optical signal can have a different wavelength than the optical signal generated by light source 111 of OLT 110. In one example, the reference optical signal has a wavelength of 1551 nm. ONU 150 also includes a circulator 153, an error signal generator 155, a multiplexer-demultiplexer 156, and a phase shifter 157. Circulator 153, error signal generator 155, and phase shifter 157 participate in the ANC mechanism, as described below.

[0036] The ONU 150 also includes an optical communication interface 159 with its corresponding distribution optical fiber 140, enabling the reference optical signal (generated by the optical source 151 of the ONU 150) to be transmitted to the optical splitter 130 via the corresponding distribution optical fiber 140. When transmitted from the ONU 150 toward the optical splitter 130, the reference optical signal is hereinafter referred to as an "upstream reference optical signal."

[0037] The optical splitter 130 reflects a portion of the upstream reference optical signal (hereinafter referred to as "reflection of the upstream reference optical signal") back to the ONU 150 along the corresponding distribution optical fiber 140. In the ANC mechanism, the ONU 150 can use the reflection of the upstream reference optical signal to eliminate phase noise on the downstream 1550nm optical signal, thereby helping the downstream 1550nm optical signal meet specific requirements (such as linewidth requirements and / or signal coherence time requirements) of one or more ONUs 150.

[0038] As described above, optical splitter 130 also includes a multiplexer-demultiplexer 131 that multiplexes the downstream 1550 nm optical signal (received from OLT 110 on trunk optical fiber 120) onto distribution optical fiber 140. The multiplexed downstream 1550 nm optical signal is transmitted from optical splitter 130 to multiplexer-demultiplexer 156 of ONU 150 via distribution optical fiber 140, optical communication interface 159, and phase shifter 157. Phase shifter 157 applies a phase shift to the multiplexed downstream 1550 nm optical signal to minimize (or at least reduce) the error signal generated by error signal generator 153, as described below.

[0039] exist Figure 5In the first step (S201) of the flowchart, the light source 151 generates an uplink reference optical signal. The uplink reference optical signal is sent in two paths, a first path to the circulator 153 (which passes the uplink reference optical signal to the multiplexer-demultiplexer 156) and a second path to the error signal generator 155.

[0040] After the first path of the uplink reference optical signal, the circulator 153 transmits the uplink reference optical signal to the multiplexer-demultiplexer 156. The multiplexer-demultiplexer 156 multiplexes the uplink reference optical signal with the downlink 1550 nm optical signal. The multiplexed uplink reference optical signal is transmitted to the phase shifter 157.

[0041] Phase shifter 157 is configured to apply a phase shift to the multiplexed upstream reference optical signal and the multiplexed downstream 1550 nm optical signal based on an error signal generated by error signal generator 155 (described in more detail below). The multiplexed upstream reference optical signal, phase-shifted by phase shifter 157, is then transmitted to optical splitter 130 via optical communication interface 159 and distribution fiber 140.

[0042] At the optical splitter 130, the multiplexed upstream reference optical signal is demultiplexed at the multiplexer-demultiplexer 131. The upstream reference optical signal is then reflected at the optical splitter 130. The reflection of the upstream reference signal is then multiplexed by the multiplexer-demultiplexer 131 (together with the downstream 1550 nm optical signal) onto the distribution optical fiber 140 and received at the optical communication interface 159 of the ONU 150.

[0043] The demultiplexing of the uplink reference optical signal at the optical splitter 130 may be imperfect. To prevent any portion of the uplink reference optical signal from being transmitted along the trunk optical fiber to the OLT 110, the filter 129 (e.g. Figure 1 and Figure 2 ) is used to remove the uplink reference optical signal from the trunk optical fiber 120.

[0044] Back to Figure 4 and Figure 5 The multiplexed reflection of the upstream reference optical signal and the multiplexed downstream 1550nm optical signal are then transmitted from the optical communication interface 159 to the phase shifter 157, which applies a phase shift to the multiplexed upstream reference optical signal and the multiplexed downstream 1550nm optical signal based on the error signal generated by the error signal generator 155. Note that the downstream 1550nm optical signal is phase-shifted once by the phase shifter 157, but the reflection of the upstream reference signal is phase-shifted twice (the first phase shift because it is transmitted from the ONU 150 to the optical splitter 130 before reflection, and the second phase shift because it is transmitted from the optical splitter 130 to the ONU 150 after reflection).

[0045] The multiplexer-demultiplexer 156 demultiplexes the downstream 1550 nm optical signal and the reflection of the upstream reference optical signal. After the path of the upstream reference optical signal, the reflection of the upstream reference optical signal is transmitted to the circulator 153, which transmits the reflection of the upstream reference optical signal to the error signal generator 155.

[0046] Therefore, error signal generator 155 receives the uplink reference optical signal from light source 151 (via the second path of the uplink reference optical signal, which has not been phase-shifted by phase shifter 157) and the reflection of the uplink reference optical signal from circulator 153 (which has been phase-shifted twice by phase shifter 157). In step S203, error signal generator 155 generates an error signal proportional to the coherence between the uplink reference optical signal and the reflection of the uplink reference optical signal. Error signal generator 155 can generate this error signal by acting as a mixer that mixes the uplink reference optical signal and the reflection of the uplink reference optical signal, wherein the error signal generated by mixing the two optical signals is proportional to the coherence between the two optical signals.

[0047] Error signal generator 155 transmits the generated error signal to phase shifter 157. In step S205, phase shifter 157 applies a phase shift to both the multiplexed upstream reference optical signal and the multiplexed downstream 1550 nm optical signal to minimize the error signal. Phase shifter 157 includes a controller configured to control the phase shift value applied by phase shifter 157 to the multiplexed upstream reference optical signal and the multiplexed downstream 1550 nm optical signal. Thus, the controller changes the phase shift applied to the multiplexed upstream reference optical signal and the multiplexed downstream 1550 nm optical signal until the error signal generated by error signal generator 155 is minimized (e.g., in a negative feedback loop) or at least reduced, such that the downstream 1550 nm optical signal meets the requirements of one or more of the multiple ONUs 150 (e.g., linewidth requirements and / or signal coherence time requirements).

[0048] Therefore, the multiplexed downstream 1550nm optical signal is transmitted with the corrected phase to the multiplexer-demultiplexer 153. As described above, the multiplexer-demultiplexer 153 demultiplexes the downstream 1550nm optical signal and (optionally) transmits the downstream 1550nm optical signal to one or more network nodes (not shown) downstream of the ONU 150.

[0049] Therefore, the combination of the OLT ANC mechanism and the ONU ANC mechanism enables a downstream 1550 nm optical signal to be generated by a single optical source 111 and distributed to multiple ONUs 150 (and any downstream network nodes) with phase noise eliminated, so as to meet the requirements for downstream 1550 nm optical signals at multiple ONUs 150.

[0050] Thus, the first optical network 100 provides a point-to-multipoint architecture with ANC applied to the corresponding connections between the transmitter (OLT 110) and each receiver (ONU 150). As described above, this is not possible when applying ANC at the OLT 110 alone to the downstream signal, but is achieved in the optical network 100 by applying ANC at each ONU 150 to the downstream signal based on analysis of the upstream signal. As a result, the first optical network 100 may suffer less phase noise than traditional optical networks, enabling the first optical network 100 to meet performance requirements such as linewidth requirements and / or signal coherence time requirements. Therefore, the first optical network 100 may be more useful in applications with relatively strict linewidth and / or signal coherence requirements, such as quantum key distribution (QKD), Rydberg atom-based technologies (such as Rydberg atom-based electromagnetic field detectors, Rydberg atom-based atoms, etc.), and distributed massive MIMO (to provide phase coherence between physically separated antennas).

[0051] The first optical network 100 includes a single optical splitter 130. However, as Figure 6 As shown in the second optical network 200 in FIG, the optical splitter can be one of a plurality of optical splitters 230, and the second optical network 200 can further include an aggregation node 270 between the OLT 210 and the plurality of optical splitters 230. In this case, the second optical network includes a plurality of trunk optical fibers 220, wherein each trunk optical fiber connects the OLT 210 to a corresponding optical splitter in the plurality of optical splitters 230 via the aggregation node 270, and the OLT 210 generates a different optical signal for each trunk optical fiber. Then, based on the reflection of the different optical signals by each optical splitter, the OLT ANC mechanism (as described above for the single optical splitter case) is independently performed on each trunk optical fiber.

[0052] Figure 7FIG3 shows a third optical network 300, which further includes a plurality of optical splitters 330. In the third optical network 300, the plurality of optical splitters 330 are arranged in a hierarchical structure, such that the OLT 310 is connected to each of the plurality of ONUs via a series of optical splitters. In this case, the OLT 310 can utilize wavelength division multiplexing to multiplex the plurality of optical signals to communicate with the plurality of ONUs. Each downstream optical signal in the plurality of optical signals has a wavelength dedicated to a (separate) set of the plurality of ONUs, wherein the (separate) set of the plurality of ONUs are directly connected to a specific last-hop optical splitter in the plurality of optical splitters 330. A fiber Bragg grating 380 can be located at each last-hop optical splitter to reflect the downstream optical signal at a wavelength dedicated to the (separate) set of the plurality of ONUs connected to the last-hop optical splitter. Thus, the OLT 310 can implement the aforementioned OLT ANC mechanism for downstream optical signals from the OLT 110 to each last-hop optical splitter by demultiplexing reflections of downstream optical signals at wavelengths dedicated to corresponding (separate) sets of multiple ONUs and using this to generate an error signal (as described above). This scenario may be relevant when different (separate) sets of multiple ONUs require optical signals at different wavelengths and those sets of multiple ONUs are connected to separate last-hop optical splitters in a cascaded optical splitter architecture.

[0053] The optical network utilizes an upstream reference signal having a wavelength of 1551 nm. 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 when transmitted over the distribution fiber is the same or substantially the same as the phase noise imparted to the downstream signal when transmitted over the distribution fiber. The threshold can be, for example, 0.1%, 1%, 5%, or 10%. The threshold can be determined through a calibration phase in which phase noise (and / or one or more requirements related to phase noise) is measured at one or more ONUs when using upstream reference signals of different wavelengths, and a wavelength for the upstream reference signal is identified that satisfactorily corrects the phase noise (and / or meets one or more requirements related to phase noise) for the ONU ANC mechanism. The acceptable level of phase noise (and / or one or more requirements) can vary for different applications, and thus the wavelengths identified for the upstream reference signal can be application-specific.

[0054] It is not necessary for the uplink reference signal and the downlink signal to use different wavelengths. That is, the downlink signal and the uplink reference signal can have the same wavelength and be time-division multiplexed onto the optical fiber. It is also not necessary for one or both of the downlink optical signal and the uplink optical signal to be within the C-band of optical telecommunications. One or both of the downlink optical signal and the uplink optical signal can be within or outside any of the optical telecommunications communication bands.

[0055] In the first optical network 100 described above, error signals are generated in the OLT ANC mechanism and the ONU ANC mechanism by mixing two optical signals. The error signal generator can be improved by applying a known constant frequency offset to one of the signals (i.e., the downstream 1550nm optical signal or one of its reflections in the OLT ANC mechanism, or the upstream reference optical signal or one of its reflections in the ONU ANC mechanism) before mixing. The frequency offset signal can be generated by a radio frequency (RF) signal generator. Mixing the shifted and unshifted optical signals produces a resulting beat tone (centered around the frequency offset). The frequency offset can then be removed to derive an error signal, which can then be used by a phase shifter. This technique can improve the accuracy of the error signal.

[0056] In the above optical network, the ONU ANC mechanism is applied by a module present in the ONU. However, this is not essential, and one or more separate nodes may cooperate with each ONU to implement the ONU ANC mechanism.

[0057] Those skilled in the art will also understand that an OLT ANC mechanism is not necessary, such as when the trunk fiber has optical transmission characteristics (such as a sufficiently short length) such that one or more requirements of one or more of the multiple ONUs are met without the OLT ANC mechanism.

[0058] In the optical network described above, phase shifters apply phase shifts to individual optical signals using a negative feedback loop based on an error signal to minimize (or at least reduce) phase noise. Those skilled in the art will appreciate that the use of a negative feedback loop is not essential, and that other techniques for reducing phase noise based on the generated optical signals and their reflections (such as a calibration table between an error signal and a corresponding phase shift) may be used.

[0059] The optical network is a Passive Optical Network (PON). However, those skilled in the art will appreciate that the above method is applicable to any form of point-to-multipoint optical network.

[0060] Figure 8A method in an optical network is shown, the optical network comprising: a first optical transmitter; an optical splitter; a plurality of optical receivers; a first optical fiber connecting the first optical transmitter and the optical splitter; a plurality of second optical fibers, each second optical fiber connecting the optical splitter to a corresponding optical receiver in the plurality of optical receivers, wherein the first optical transmitter is configured to transmit a first optical signal to each of the plurality of optical receivers via the first optical fiber, the optical splitter, and a corresponding second optical fiber in the plurality of second optical fibers; and a plurality of second-fiber phase correction units, each second-fiber phase correction unit being associated with a second optical fiber in the plurality of second optical fibers, the method comprising the steps of: transmitting a reference optical signal on an associated second optical fiber in the plurality of second optical fibers (step S301); generating a reference error signal based on reflection of the reference optical signal on the associated second optical fiber in the plurality of second optical fibers (step S303); and applying a phase shift to the first optical signal based on the reference error signal (step S305).

[0061] A person skilled in the art will understand that any combination of features is possible within the scope of the claimed invention.

Claims

1. An optical network, comprising: a first light emitter; Optical splitter; multiple optical receivers; a first optical fiber connecting the first optical transmitter and the optical splitter; a plurality of second optical fibers, each second optical fiber of the plurality of second optical fibers connecting the optical splitter to a corresponding 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 fiber, the optical splitter, and a corresponding second optical fiber of the plurality of second optical fibers; and a plurality of second optical fiber phase correction units, each of the plurality of second optical fiber phase correction units being associated with a second optical fiber among the plurality of second optical fibers, each second optical fiber phase correction unit comprising a reference light 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 an associated second optical fiber of the plurality of second optical fibers, Each reference error signal generator is configured to generate a reference error signal based on reflection of the reference optical signal on an associated second optical fiber of the plurality of second optical fibers, and Each reference phase shifter is configured to apply a phase shift to the first optical signal based on the reference error signal.

2. The optical network according to claim 1, wherein: Each optical receiver of the plurality of optical receivers includes an associated second optical fiber phase correction unit of the plurality of second optical fiber phase correction units.

3. The optical network according to claim 1, wherein: The reference optical signal is reflected by the optical splitter to generate the reflection of the reference optical signal.

4. The optical network according to claim 1, wherein: The first optical signal is transmitted in a downstream direction, and the reference optical signal is transmitted in an upstream direction.

5. The optical network according to claim 1, further comprising: a first optical fiber phase correction unit associated with the first optical fiber, the first optical fiber 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 reflection of the first optical signal on the first optical fiber, and The first phase shifter is configured to apply a phase shift to the first optical signal based on the first error signal.

6. The optical network according to claim 1, wherein: The first optical transmitter is configured to transmit the first optical signal to each of the plurality of optical receivers at a first wavelength via the first optical fiber, the optical splitter, and a corresponding second optical fiber of the plurality of second optical fibers; and The reference optical transmitter is configured to transmit the reference optical signal at a second wavelength on an associated second optical fiber among the plurality of second optical fibers, The first wavelength is different from the second wavelength.

7. The optical network according to claim 1, wherein: The optical splitter is a first-hop optical splitter among a plurality of optical splitters, the plurality of optical splitters further comprising a first-last-hop optical splitter and a second-last-hop optical splitter, a first set of the plurality of optical receivers being connected to the first-last-hop optical splitter, a second set of the plurality of optical receivers being connected to the second-last-hop optical splitter, the first optical signal comprising data for the first set of the plurality of optical receivers and being transmitted at a first wavelength via the first-hop optical splitter and the first-last-hop optical splitter using wavelength division multiplexing, and the first optical transmitter being further configured to transmit a second optical signal at a second wavelength via the first-hop optical splitter and the second-last-hop optical splitter using wavelength division multiplexing, the second optical signal comprising data for the second set of the plurality of optical receivers, and the optical network further comprising: 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 fiber phase correction unit associated with the first optical fiber, the first optical fiber 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 reflection of the first optical signal of the first wavelength on the first optical fiber, the first phase shifter being configured to apply a phase shift to the first optical signal of the first wavelength based on the first error signal, The second error signal generator is configured to generate a second error signal based on reflection of the second optical signal of the second wavelength on the first optical fiber, and The second phase shifter is configured to apply a phase shift to a second optical signal at the second wavelength based on the second error signal.

8. A method in an optical network, the optical network comprising: a first light emitter; Optical splitter; multiple optical receivers; a first optical fiber connecting the first optical transmitter and the optical splitter; a plurality of second optical fibers, each second optical fiber connecting the optical splitter to a corresponding optical receiver among the plurality of optical receivers, wherein the first optical transmitter is configured to transmit a first optical signal to each of the plurality of optical receivers via the first optical fiber, the optical splitter, and a corresponding second optical fiber among the plurality of second optical fibers; and a plurality of second optical fiber phase correction units, each second optical fiber phase correction unit being associated with a second optical fiber of the plurality of second optical fibers, The method comprises the following steps: transmitting a reference optical signal on an associated second optical fiber among the plurality of second optical fibers; generating a reference error signal based on reflection of the reference optical signal on an associated second optical fiber of the plurality of second optical fibers; and A phase shift is applied to the first optical signal based on the reference error signal. 9 . A non-transitory computer-readable storage medium storing a computer program, the computer program comprising instructions that, when the computer program is executed by a computer, cause the computer to perform the method of claim 8 .

10. A signal transmission system, comprising: At least one processor and a memory, the memory storing a computer program which, when executed by the signal transmission system, causes the signal transmission system to perform the method of claim 8.

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