Communication network local equipment, remote equipment and method for power over fiber transmission

Abstract

A local equipment (100) for connection to a single fibre optical link (152), comprising: a laser light source (104) for generating a high power continuous wave, cw, downstream optical signal; and monitoring apparatus (106) for determining, based on an amplitude characteristic of a received upstream optical signal, whether the upstream optical signal is from a remote node or is a reflection from within the optical link, wherein the laser light source is operative to control generation of the downstream optical signal dependent what is determined. A remote equipment (200) for connection to the optical link (152) for receiving a downstream optical signal having an optical power, comprising: an optical splitter (204) for forming a return optical signal; an optical to electrical converter (206) configured to provide electrical power to optical signal conditioning apparatus (210) operative to apply an amplitude characteristic to the return optical signal dependent on the received optical power; and an optical routing element (208).

Description

[0001] COMMUNICATION NETWORK LOCAL EQUIPMENT, REMOTE EQUIPMENT AND METHOD FOR POWER OVER FIBER TRANSMISSION

[0002] Technical Field

[0003] The invention relates to communication network local equipment and to communication network local equipment. The invention further relates to a communication network optical link comprising the local equipment and the remote equipment. The invention further relates to a method of controlling a laser light source at a communication network local equipment.

[0004] Background

[0005] In order to reduce the number of cables to control, transport data and to electrically power remote equipment, Power over Fibre, PoF, techniques are used in which only optical cables are used for control signals, traffic data and for power supply to the remote equipment. PoF techniques suffer from optical safety issues since it uses a powerful laser (or a number of lasers) delivering over 500mW optical power to the remote equipment to power it. ITU-T standard G.664 “Optical safety procedures and requirements for optical transmission systems” addresses optical safety issues in optical communication systems and states the methods and energy to be applied to reduce the risk of injury for the eyes or skin. A problem arises during restart of a remote equipment after a fibre disconnection, since the remote equipment is electrically powered off, so is completely passive. The automatic power reduction, APR, mechanisms proposed in G.664 cannot be easily applied during restart because the restart pulses are not sufficiently long in time (milliseconds to be compliant with the IEC-60825) to give electrical power to the remote equipment for enough time to restart the system, which remains powered off.

[0006] US7813646B2 describes a PoF system having a high power laser at the local equipment, which operates restart based on restart pulses at low power (1 / 4 of the full power of the PoF laser) but enough to power on a microprocessor and an active transmitter (a laser) at the remote equipment which generates a return optical signal. However, for shutdown after a fibre break or fibre disconnection or a failure, the usual solution is to power down the laser at the remote equipment if the traffic or control signals are not received from the local equipment. This method is very complicated to be compliant with the safety standards (IEC- 60825-2) that allows a maximum failure rate of 500 FIT for the safety mechanism; IEC 60825-2 defines FITs as “an indicator of reliability defined as the number of failures per 109h”. Using a dedicated laser to provide the return signal from the remote equipment, other than increasing the FIT of the solution, also increases the total cost and the complexity of the solution. US2019 / 0019912 described a PoF system having a totally passive mechanism at the remote equipment, which permits a generally simpler and potentially more reliable safety system. However, this system needs a second dedicated optical fibre which is used only for the optical safety.

[0007] US11063630B2 describes a PoF system having a low power startup phase, in which an optical feedback signal is sent back from the remote equipment using the data traffic generated. Also in this case, the solution has higher cost because a low power section of the remote equipment must be implemented and used.

[0008] Summary

[0009] It is an object to provide an improved communication network local equipment. It is a further object to provide an improved communication network local equipment. It is a further object to provide an improved communication network optical link comprising the local equipment and the remote equipment. It is a further object to provide an improved method of controlling a laser light source at a communication network local equipment.

[0010] An aspect provides a communication network local equipment comprising an input / output port, a laser light source and monitoring apparatus. The input / output is for connection to a single fibre optical link. The laser light source is operative in an operational mode to generate a high power continuous wave, cw, downstream optical signal, for transmission on the single fibre optical link. The monitoring apparatus is configured to receive an upstream optical signal from the single fibre optical link, the upstream optical signal having an amplitude characteristic. The monitoring apparatus is configured to detect the upstream optical signal amplitude characteristic. The monitoring apparatus is also configured to determine, based on the amplitude characteristic, whether the upstream optical signal is from a remote node connected to the single fibre optical link or the upstream optical signal is a reflection from within the single fibre optical link. The monitoring apparatus is also configured to generate a control signal comprising a first indication indicative that the upstream optical signal is a signal from a remote node or comprising a second indication indicative that the upstream optical signal is a reflection from within the single fibre optical link. The laser light source is operative to receive a said control signal and to control generation of the downstream optical signal dependent on which of the first indication or the second indication the control signal comprises.

[0011] The communication network local equipment may enable safe PoF supply to a remote equipment over a single fibre optical link. The local equipment is advantageously able to determine, from the amplitude characteristic of a received upstream optical signal, whether a single fibre optical link to which it is connected is closed, i.e. connected to a remote equipment at its other end, or is open, i.e. has a discontinuity, such as a fibre break or disconnection, so that it is not connected to the remote equipment. The local equipment is advantageously able to control the laser light source generation of a downstream optical signal dependent on whether the single fibre optical link to the remote equipment is closed or open.

[0012] In an embodiment, the laser light source is additionally operative in a restart mode to generate a pulsed downstream optical signal of low power restart optical pulses for transmission on the single fibre optical link. The laser light source is configured to continue to operate in the restart mode when the control signal comprises a said second indication. The laser light source is configured to switch to operating in the operational mode when the control signal comprises a said first indication. This advantageously enables the local equipment to transmit safe restart optical pulses to determine whether a single fibre optical link to which it is connected is open or has been restored, i.e. is now closed, and it is safe to recommence transmitting a high power cw downstream optical signal.

[0013] In an embodiment, the low power restart optical pulses have a peak pulse power not exceeding an accessible emission limit, AEL, of the laser light source and less than a nonlinear threshold of the single fibre optical link. In an embodiment, the low power restart optical pulses have a peak pulse power of up to 5 mW (= 7 dBm), such as up to 1 mW (= 0 dBm). The restart optical pulses are advantageously eye safe, so it is safe to transmit restart optical pulses to determine both whether the optical link is closed or is open.

[0014] In an embodiment, restart optical pulses have a first pulse shape and the upstream optical signal comprises upstream optical pulses having a second pulse shape. The monitoring apparatus is configured in the restart mode to detect upstream optical pulses and determine the second pulse shape. The monitoring apparatus is also configured in the restart mode to compare the second pulse shape to the first pulse shape. The monitoring apparatus is also configured in the restart mode to determine that upstream optical pulses are reflections from within the single fibre optical link responsive to determining that the second pulse shape is substantially the same as the first pulse shape. The monitoring apparatus is also configured in the restart mode to determine that upstream optical pulses are received from a remote node responsive to determining that the second pulse shape is different to the first pulse shape. In this way, the local equipment is advantageously able to determine whether the optical link is closed or is open.

[0015] In an embodiment, the restart optical pulses have a pulse duration. The monitoring apparatus is configured in the restart mode to compare a total received optical power of an upstream optical pulse with a total transmitted optical power of a restart optical pulse. The monitoring apparatus is also configured in the restart mode to determine that upstream optical pulses are reflections from within the single fibre optical link responsive to determining a constant difference across said pulse duration between the total transmitted optical power of a restart optical pulse and the total received optical power of an upstream optical pulse. The monitoring apparatus is also configured in the restart mode to determine that upstream optical pulses are received from a remote node responsive to determining an increasing difference across said pulse duration between the total transmitted optical power of a restart optical pulse and the total received optical power of an upstream optical pulse. This advantageously enables a difference in the pulse shape of the upstream optical pulses as compared to the restart optical pulses to be determined by a simple comparison of total optical power.

[0016] In an embodiment, the increasing difference increases nonlinearly across at least part of said pulse duration. This may enable a clear determination of a difference in pulse shape by a simple comparison of total optical power.

[0017] In an embodiment, the amplitude characteristic is one of an amplitude modulation or a constant amplitude. The monitoring apparatus is configured in the operational mode to determine that the upstream optical signal is from a remote node responsive to detecting an amplitude modulation. The monitoring apparatus is also configured in the operational mode to determine that the upstream optical signal is a reflection from within the single fibre optical link responsive to detecting a constant amplitude. The laser light source is configured in the operational mode to automatically shut down when the control signal comprises a said second indication. This may enable the local equipment to rapidly and automatically shut down the laser light source when a single fibre optical link to which it is connected has a discontinuity, ensuring that the high power cw downstream optical signal is not transmitted when the optical link is broken or disconnected and an engineer may be working on the optical link.

[0018] Corresponding embodiments and advantages apply to the communication network optical link and the method described below.

[0019] An aspect provides a communication network remote equipment comprising an input / output port, an optical splitter, an optical to electrical converter, optical signal conditioning apparatus and an optical routing element. The input / output port is for connection to a single fibre optical link for receiving a downstream optical signal, having a received optical power, from the single fibre optical link. The optical splitter is in communication with the input / output port and is configured to split off a portion of the downstream optical signal to form a return optical signal having a return optical power. The optical to electrical converter is configured to receive the downstream optical signal from the optical splitter, convert the downstream optical signal into electrical power and to provide electrical power to optical signal conditioning apparatus. The optical signal conditioning apparatus is operative to apply an amplitude characteristic to the return optical signal to form a modified return optical signal, wherein the applied amplitude characteristic is dependent on the received optical power. The optical routing element is configured to direct the modified return optical signal back to the input / output port for upstream transmission on the single fibre optical link. The communication network remote equipment is advantageously enabled to be powered by PoF and to provide a return optical signal to a local equipment, at the other end of a single fibre optical link to which it is connected, indicative that the optical link is closed. The remote equipment advantageously provides a passive, low optical power, restart mechanism to enable safe restart of a high power cw optical signal at the local equipment following a discontinuity in the single fibre optical link. The remote equipment advantageously enables a local equipment to determine whether the single fibre optical link is open or closed based on the amplitude characteristic applied to the return optical signal.

[0020] In an embodiment, the downstream optical signal is one of a high power continuous wave, cw, optical signal or low power optical pulses. The optical signal conditioning apparatus comprises an optical attenuator and an optical limiter. The optical attenuator is configured to apply an optical attenuation to the return optical signal when electrically powered and is configured to transmit the return optical signal without applying optical attenuation when not electrically powered. The optical limiter is configured to prevent transmission of light above a pre-configured optical power threshold. The optical splitter and the optical attenuator are configured to produce an attenuated return optical signal by attenuating the return optical signal to have an optical power lower than the optical power threshold and to produce an unattenuated return optical signal having a total optical power higher than the optical power threshold, such that amplitude shaping is applied to an unattenuated return optical signal. This advantageously enables the remote equipment to apply an amplitude characteristic to the return optical signal that is dependent on whether the optical attenuator is electrically powered, which is dependent on the optical power of a received downstream optical signal; the return optical signal is thus indicative of whether a high power cw downstream optical signal or low power optical pulses are received.

[0021] In an embodiment, the optical power threshold is configured to apply amplitude shaping to low power optical pulses. This advantageously applies an amplitude characteristic to return low power optical pulses that enables a local equipment to determine that the amplitude shaped optical pulses are sent from the remote equipment and are not reflections from a discontinuity in the single fibre optical link, since reflections will not have the amplitude shaping.

[0022] In an embodiment, the single fibre optical link has a nonlinear threshold above which fibre nonlinearities occur and the optical power threshold is configured to limit a peak pulse optical power of a return optical signal of optical pulses to less than the nonlinear threshold. This advantageously ensures that return optical pulses only have amplitude shaping as applied by the remote equipment and do not acquire any additional amplitude shaping due to fibre nonlinearities on their return trip to the local equipment.

[0023] In an embodiment, the optical limiter is a nonlinear absorber having an absorption threshold in the range -23 dBm to -16 dBm. In an embodiment, the optical attenuator is additionally operative to apply an amplitude modulation when electrical power is received. The remote equipment is advantageously configured to provide a return optical signal that serves as an optical safety signal, indicating that the optical link is closed.

[0024] In an embodiment, the downstream optical signal is a high power continuous wave, cw, optical signal. The optical signal conditioning apparatus comprises an optical modulator operative when electrical power is received to apply an optical modulation to the return optical signal to form an amplitude modulated return optical signal. The remote equipment is advantageously configured to provide a return optical signal that serves as an optical safety signal, indicating that the optical link is closed.

[0025] Corresponding embodiments and advantages apply to the communication network optical link described below.

[0026] An aspect provides a communication network optical link comprising a communication network local equipment, a communication network remote equipment and a single fibre optical link connecting the communication network local equipment and the communication network remote equipment. The communication network local equipment comprises an input / output port, a laser light source and monitoring apparatus. The input / output is for connection to a single fibre optical link. The laser light source is operative in an operational mode to generate a high power continuous wave, cw, downstream optical signal, for transmission on the single fibre optical link. The monitoring apparatus is configured to receive an upstream optical signal from the single fibre optical link, the upstream optical signal having an amplitude characteristic. The monitoring apparatus is configured to detect the upstream optical signal amplitude characteristic. The monitoring apparatus is also configured to determine, based on the amplitude characteristic, whether the upstream optical signal is from a remote node connected to the single fibre optical link or the upstream optical signal is a reflection from within the single fibre optical link. The monitoring apparatus is also configured to generate a control signal comprising a first indication indicative that the upstream optical signal is a signal from a remote node or comprising a second indication indicative that the upstream optical signal is a reflection from within the single fibre optical link. The laser light source is operative to receive a said control signal and to control generation of the downstream optical signal dependent on which of the first indication or the second indication the control signal comprises. The communication network remote equipment comprises an input / output port, an optical splitter, an optical to electrical converter, optical signal conditioning apparatus and an optical routing element. The input / output port is for connection to a single fibre optical link for receiving a downstream optical signal, having a received optical power, from the single fibre optical link. The optical splitter is in communication with the input / output port and is configured to split off a portion of the downstream optical signal to form a return optical signal having a return optical power. The optical to electrical converter is configured to receive the downstream optical signal from the optical splitter, convert the downstream optical signal into electrical power and to provide electrical power to optical signal conditioning apparatus. The optical signal conditioning apparatus is operative to apply an amplitude characteristic to the return optical signal to form a modified return optical signal, wherein the applied amplitude characteristic is dependent on the received optical power. The optical routing element is configured to direct the modified return optical signal back to the input / output port for upstream transmission on the single fibre optical link.

[0027] In an embodiment, the optical link is a power over fibre optical link.

[0028] An aspect provides a method of controlling a laser light source at a communication network local equipment. The method comprises the following steps. In an operational mode, generating a high power continuous wave, cw, downstream optical signal, for transmission on an single fibre optical link. Receiving an upstream optical signal from the single fibre optical link, the upstream optical signal having an amplitude characteristic. Detecting the upstream optical signal amplitude characteristic. Determining, based on the amplitude characteristic, whether the upstream optical signal is from a remote node connected to the single fibre optical link or the upstream optical signal is a reflection from within the single fibre optical link. Generating a control signal comprising a first indication indicative that the upstream optical signal is a signal from a remote node or comprising a second indication indicative that the upstream optical signal is a reflection from within the single fibre optical link. Receiving a said control signal and controlling generation of the downstream optical signal dependent on which of the first indication or the second indication the control signal comprises.

[0029] In an embodiment, the method further comprises the following steps, in a restart mode, generating a pulsed downstream optical signal of low power restart optical pulses for transmission on the single fibre optical link. Continuing to operate in the restart mode when the control signal comprises a said second indication. Switching to operating in the operational mode when the control signal comprises a said first indication.

[0030] In an embodiment, the method further comprises the following steps. Determining that the upstream optical signal is from a remote node responsive to detecting an amplitude modulation and determining that the upstream optical signal is a reflection from within the single fibre optical link responsive to detecting a constant amplitude. Automatically shutting down the laser light source when the control signal comprises a said second indication.

[0031] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings.

[0032] Brief Description of the drawings

[0033] Figures 1 and 2 are block diagrams illustrating embodiments of a communication network local equipment; Figures 3 to 7 are block diagrams illustrating embodiments of a communication network remote equipment;

[0034] Figure 8 is a block diagram illustrating an embodiment of a communication network link;

[0035] Figure 9 shows plots illustrating the shapes of transmitted restart optical pulses and received optical pulses at a communication network local equipment of the communication network link of Figure 8; and

[0036] Figure 10 is a flowchart illustrating embodiments of method steps.

[0037] Detailed description

[0038] The same reference numbers are used for corresponding features in different embodiments.

[0039] The present disclosure relates to a power over fibre, PoF, system for powering a communication network remote equipment. The present disclosure provides a passive, low optical power, restart mechanism at the remote equipment and an active mechanism at a local equipment hosting the PoF high power laser. The proposed solution can also be used during the switching down phase (for example for a fibre break or fibre disconnection), powering the safety mechanism of the remote equipment.

[0040] After a fibre break or fibre disconnection or a failure, the local high-power laser sends restart pulses at very low optical power (so to be compliant with the G.664 and IEC-60825). These restart pulses are reflected-back to the local equipment from the fibre (for example, due to Rayleigh scattering) or from optical connectors (15 dB) or from a restart mechanism at the remote equipment. Using a suitable algorithm and monitoring this back-reflected optical power, the present disclosure allows the local equipment to understand if the optical path is closed or open and thus whether or not the high-power laser can transmit high power light to the remote equipment.

[0041] In addition, during normal operation of the equipment, the present disclosure provides a safe mechanism (based on few hardware components for which is very simple to be below 500FIT required by the safety standards) to understand if the optical path is broken. This mechanism is based on a tone, generated by the safety mechanism of the remote equipment, and transmitted to the local equipment. This tone is generated only when the remote equipment is powered ON: so, as soon as the optical path is broken, the remote equipment is not powered, the tone is not received by the local equipment that can switch down the high-power laser.

[0042] The present disclosure solves optical safety issues in systems electrically powered by high- power lasers, for example, as Raman laser, using PoF techniques. High-power lasers are forbidden in telecommunication systems, except if suitable automatic power reduction, APR, systems are put in place to reduce the risk of injury of the personnel. The APR systems, usually adopted as described in the G.664 recommendation, are based on remote equipment that is always electrically powered during all the phases (optical path interruption and normal operation). This is not the case of optical system powered by a PoF techniques. The present disclosure provides a new APR system, based on a simple, hardware and totally passive “reverse” optical path, so that it can operate even if the remote equipment is not electrically powered; this “reversed” optical power can be used to establish the safety of the optical path.

[0043] The present disclosure offers several advantages. For instance, it does not require a dedicated optical fibre for the optical safety signal. Additionally, the solution is based on low power optical restart pulses, ensuring compliance with safety standards. The present disclosure also addresses both the powering down of the optical power after a fibre disconnection and the subsequent restart, reducing costs and complexity of the system.

[0044] The present disclosure provides a totally passive restart mechanism (at the remote equipment side) with the high power source laser based at the local equipment. Furthermore, since the restart mechanism of the remote equipment is totally passive, during the restart phase, it does not need to be powered from the local equipment by PoF techniques, and the optical power of the restart pulses can be very low, and therefore very safe, in Class 1 . As a consequence, not only can laser at the local equipment remain ON at all times since it emits in Class 1 (as defined in G.664 standard), but also that back scattering within the single optical fibre link is not stimulated (and there is no possibility of mistakes). So, the proposed restart mechanism is very safe, very low cost and the request of 500 FIT can be easily accomplished with standard hardware components.

[0045] Unlike the system of US2019 / 0019912, the present disclosure does not require a dedicated optical fibre for the optical safety signal, thanks to the usage of a non-linear absorbed and an algorithm to detect reflections (for example, from open optical connectors). In addition, the present disclosure uses low power optical restart pulses (< 10 dBm), therefore the reliability of the non-linear absorber is not compromised.

[0046] The present disclosure enables powering down of the optical power after a fibre discontinuity in a PoF system and the subsequent restart. The present disclosure advantageously reduces costs (since no supervisory channels or interactions with control signals or traffic are required) remaining independent from other system functions, so reducing the complexity and the cost of the whole system (local and remote).

[0047] Referring to Figure 1 , an embodiment of a communication network local equipment 100 is illustrated. The communication network local equipment 100 comprises an input / output port 102, a laser light source 104 and monitoring apparatus 106. The laser light source and the monitoring apparatus are coupled to the input / output port via an optical circulator 108.

[0048] The input / output port 102 is for connection to a single fibre optical link. The laser light source 104, is operative in an operational mode to generate a high power continuous wave, cw, downstream optical signal, for transmission on the single fibre optical link. The optical circulator 108 is configured to receive cw downstream optical signals from the laser light source at port 2 and to route the downstream optical signals to port 3, connected to the input / output port 102. The optical circulator 108 is configured to receive upstream optical signals from the input / output port at port 3 and to route the upstream optical signals to port 1 , connected to the monitoring apparatus 106. It will be understood that the circulator could be replaced by a splitter or any other optical component able to separate the upstream signals from the downstream signals.

[0049] The monitoring apparatus 106 is configured to receive an upstream optical signal from the single fibre optical link. The upstream optical signal has an amplitude characteristic. The monitoring apparatus 106 is configured to detect the upstream optical signal amplitude characteristic. The monitoring apparatus 106 is configured to determine, based on the amplitude characteristic, whether the upstream optical signal is from a remote node connected to the single fibre optical link or the upstream optical signal is a reflection from within the single fibre optical link. The monitoring apparatus 106 is also configured to generate a control signal to be sent to the laser light source. The control signal either comprises a first indication indicative that the upstream optical signal is a signal from a remote node or comprises a second indication indicative that the upstream optical signal is a reflection from within the single fibre optical link, depending on what the monitoring apparatus has determined to be the source of the upstream optical signal.

[0050] The laser light source 104 is operative to receive the control signal from the monitoring apparatus and to control generation of the downstream optical signal dependent on which of the first indication or the second indication the control signal comprises.

[0051] In certain embodiments, the laser light source 104 is additionally operative in a restart mode to generate a pulsed downstream optical signal of low power restart optical pulses for transmission on the single fibre optical link. The laser light source 104 is configured to continue to operate in the restart mode when the control signal comprises a second indication, indicative that the upstream optical signal is a reflection from within the single fibre optical link. This indicates that the single optical fibre link is still has some discontinuity, such as a fibre break or disconnection.

[0052] The laser light source 104 is also configured to switch to operating in the operational mode when the control signal comprises a first indication, indicative that the upstream optical signal is a signal from a remote node. This indicates that the single optical fibre link is restored and it is therefore safe for the local equipment to power up the laser light source to transmit a high power cw optical signal to a remote node, to power it.

[0053] In certain embodiments, the low power restart optical pulses have a peak pulse power not exceeding an accessible emission limit, AEL, of the laser light source 104 and less than a nonlinear threshold of the single fibre optical link. This may ensure that only low power optical pulses are transmitted, thereby ensuring that the light transmitted is ‘eye safe’ and thus preventing potential damage to the eyes of operators.

[0054] The low power restart optical pulses may have a peak pulse power of up to 5 mW, such as up to 1 mW. The AEL for Hazard Level 1 equipment in the telecomms C-band is 10 mW, that is to say, the laser can remain always ON at this power level. However, to mitigate Brillouin scattering, which is the worst nonlinear effect at low power and has a threshold of 5 mW, peak pulse power should be less than 5 mW, potentially 1 mW in the worst case scenario.

[0055] In certain embodiments, the restart optical pulses have a first pulse shape and the upstream optical signal comprises upstream optical pulses having a second pulse shape. The monitoring apparatus 106 is configured in the restart mode to detect upstream optical pulses and to determine their shape, i.e. to determine the second pulse shape.

[0056] The monitoring apparatus 106 is also configured to compare the second pulse shape to the first pulse shape. The monitoring apparatus 106 determines whether the second pulse shape is substantially the same as the first pulse shape. If the pulse shapes are substantially the same, the monitoring apparatus determines that the upstream optical pulses are reflections from within the single fibre optical link. If the pulse shapes are different, the monitoring apparatus determines that the upstream optical pulses are received from a remote node.

[0057] In certain embodiments, illustrated in Figure 9, the restart optical pulses, 902, have a pulse duration, AT. The monitoring apparatus 106 is configured in the restart mode to compare a total received optical power, P, of an upstream optical pulse with a total transmitted optical power of a restart optical pulse. The monitoring apparatus determines that upstream optical pulses are reflections from within the single fibre optical link responsive to determining a constant difference, APi, across the pulse duration between the total transmitted optical power of a restart optical pulse, 902, and the total received optical power of an upstream optical pulse, 904. The monitoring apparatus determines that upstream optical pulses are received from a remote node responsive to determining an increasing difference, AP2, across the pulse duration between the total transmitted optical power of a restart optical pulse, 902, and the total received optical power of an upstream optical pulse, 906.

[0058] In certain embodiments, as illustrated in Figure 9, the increasing difference increases nonlinearly across at least part of the pulse duration, AT.

[0059] In certain embodiments, the upstream optical signal has an amplitude characteristic which is either an amplitude modulation or a constant amplitude. The monitoring apparatus 106 is configured in the operational mode to detect whether the upstream optical signal has an amplitude modulation or a constant amplitude. The monitoring apparatus 106 is configured to determine that the upstream optical signal is from a remote node responsive to detecting an amplitude modulation. The monitoring apparatus 106 is configured to determine that the upstream optical signal is a reflection from within the single fibre optical link responsive to detecting a constant amplitude.

[0060] The laser light source 104 is configured in the operational mode to automatically shut down when the control signal comprises a second indication, i.e. a control signal indicative that the upstream optical signal is a reflection from within the single fibre optical link. This indicates that the single optical fibre link has some discontinuity, such as a fibre break or disconnection, and it is therefore not safe to continue to transmit a high power cw optical signal. The local equipment thus performs automatic laser shutdown, ALS. This may ensure that the high power laser is switched off in the event of a fibre break or fibre disconnection, thereby preventing potential damage to the eyes of operators.

[0061] Referring to Figure 2, an embodiment of a communication network local equipment 150 is illustrated. The communication network local equipment 150 comprises traffic / control signals management 154 and a multiplexer, MUX, 156, in addition to the elements described in Figure 1 . The traffic / control signals management is configured to manage the traffic and control signals within the communication network local equipment 150. The MUX is configured to combine the traffic and control signals with the high power cw downstream optical signal for transmission on the single fibre optical link 152 via the input / output port 102. The MUX is also configured to route upstream optical signals from the input / output port 102 to port 3 of the optical circulator 108.

[0062] Referring now to Figure 3, an embodiment of a communication network remote equipment 200 is illustrated. The communication network remote equipment 200 comprises an input / output port 202, an optical splitter 204, an optical to electrical converter 206, optical signal conditioning apparatus 210 and an optical routing element 208.

[0063] The input / output port 202 is for connection to a single fibre optical link and is configured to receive a downstream optical signal from the single fibre optical link. The downstream optical signal is either a high power continuous wave, cw, optical signal or low power optical pulses, e.g. low power restart optical pulses The downstream optical signal has a received optical power.

[0064] The optical routing element is an optical circulator 208. The optical circulator 208 is configured to route a downstream optical signal from port 1 to port 2, which is connected to the optical splitter. The optical circulator 208 is configured to route optical signals received at port 3 to port 1 , which is connected to the input / output port.

[0065] The optical splitter 204 is connected to the input / output port 202 via the optical circulator 208. The optical splitter 204 is configured to split off a portion of the downstream optical signal to form a return optical signal having a return optical power.

[0066] The optical to electrical, O-E, converter 206 is configured to receive the remaining part of the downstream optical signal from the optical splitter 204. The optical to electrical converter 206 is configured to convert the downstream optical signal into electrical power and to provide electrical power to the optical signal conditioning apparatus 210.

[0067] The optical signal conditioning apparatus 210 is operative to apply an amplitude characteristic to the return optical signal to form a modified return optical signal. The applied amplitude characteristic is dependent on the received optical power of the downstream optical signal.

[0068] In this embodiment, the optical signal conditioning apparatus 210 comprises an optical attenuator 212, driven by a controller 216, and an optical limiter 214.

[0069] The optical limiter 214 is configured to prevent transmission of light above a pre-configured optical power threshold.

[0070] When a high power cw downstream optical signal is received, the O-E converter 206 converts the downstream optical signal into electrical power to power the optical attenuator 212, via the controller 216. The optical attenuator 212 is configured to apply an optical attenuation to the return optical signal when it is electrically powered, to form an attenuated return optical signal.

[0071] The optical splitter 204 and the optical attenuator 212 are configured to produce an attenuated return optical signal by attenuating the return optical signal to have an optical power lower than the optical power threshold of the optical limited 214.

[0072] When a downstream optical signal of low power optical pulses is received, the O-E converter 206 does not supply electrical power to the optical attenuator 212; the O-E converter does not generate sufficient electrical power to supply any part of the remote equipment. The optical attenuator 212 is configured to transmit the return optical signal without applying optical attenuation when it is not electrically powered, the return low power optical pulses are simply transmitted. When the optical attenuator is not electrically powered, it does not apply any optical attenuation, although it may suffer a small amount of insertion loss.

[0073] The optical splitter 204 and the optical attenuator 212 are configured to produce return low power optical pulses having a total optical power higher than the optical power threshold of the optical limiter 214. Amplitude shaping is therefore applied to the return low power optical pulses on transmission through the optical limiter 204.

[0074] The optical circulator 208 is configured to route a modified return optical signal back to the input / output port 202 for upstream transmission on the single fibre optical link.

[0075] In certain embodiments, the optical limiter’s optical power threshold is configured to apply amplitude shaping to the return low power optical pulses.

[0076] In certain embodiments, the single fibre optical link has a nonlinear threshold above which fibre nonlinearities occur. The optical limiter’s optical power threshold is configured to limit a peak pulse optical power of the return low power optical pulses to less than the nonlinear threshold of the optical link. In certain embodiments, the optical limiter is a nonlinear absorber 214 having an absorption threshold in the range -23 dBm to -16 dBm, transmitting low power optical pulses with an optical power of up to 0 dBm (= 1 mw). The non-linear absorber absorbs all the light exceeding the threshold.

[0077] In certain embodiments, the optical attenuator 212 is additionally operative to apply an amplitude modulation to a cw return optical signal, i.e. when the downstream optical signal is a high power cw optical signal meaning that electrical power is supplied to the optical attenuator by the O-E converter. The controller 216 comprises an electrical driver and tone generator.

[0078] In certain embodiments, the proposed solution may be based on low power optical restart pulses, ensuring the reliability of the non-linear absorber is not compromised. The low power optical restart pulses may have a peak pulse power not exceeding an accessible emission limit, AEL, of the laser light source and less than a nonlinear threshold of the single fibre optical link, such as up to 10 dBm, such as up to 0 dBm. This may ensure that the optical safety of the system is maintained.

[0079] Referring now to Figure 4, an embodiment of a communication network remote equipment 400 is illustrated. The communication network remote equipment 400 comprises an input / output port 202, an optical splitter 204, optical signal conditioning apparatus 210 and an O-E converter 206, as in the previous embodiment.

[0080] In this embodiment, the optical routing element 302 is a mirror. The mirror 302 is configured to reflect the modified return optical signal back to the input / output port 202 for upstream transmission.

[0081] In some cases, the communication network remote equipment 400 may also include an optical signal conditioning apparatus as defined in claim 9. The optical signal conditioning apparatus may be operative to apply an amplitude characteristic to the return optical signal to form a modified return optical signal. The applied amplitude characteristic may be dependent on the received optical power.

[0082] In some aspects, the optical signal conditioning apparatus may include an optical attenuator and an optical limiter. The optical attenuator may apply an optical attenuation to the return optical signal when electrically powered and may transmit the return optical signal without applying optical attenuation when not electrically powered. The optical limiter may prevent transmission of light above a pre-configured optical power threshold.

[0083] In some cases, the optical power threshold may be configured to apply amplitude shaping to low power optical pulses. The single fibre optical link may have a nonlinear threshold above which fibre nonlinearities occur and the optical power threshold may be configured to limit a peak pulse optical power of a return optical signal of optical pulses to less than the nonlinear threshold. In some aspects, the optical limiter may be a nonlinear absorber having an absorption threshold in a specific range, such as in the range -23 dBm to -16 dBm. The optical attenuator may be additionally operative to apply an amplitude modulation when electrical power is received.

[0084] In some cases, the proposed solution may address both the initial switching down of the optical power after the fibre disconnection and the subsequent restart. This single solution may reduce costs and complexity of the whole system, as no supervisory channels or interactions with control signals or traffic are demanded. The solution may remain independent from the rest of the functionalities of the systems, further simplifying the overall system design.

[0085] The solution disclosed herein allows for fast switching down of the optical power and a fast restart. A fast switching-off of the laser is important for safety reasons, but in some embodiments the switching on operation may be done at somewhat slower pace if this is necessary for carrying out additional operations.

[0086] Referring now to Figure 5, an embodiment of a communication network remote equipment 500 is illustrated. The communication network remote equipment 500 comprises an input / output port 202, an optical splitter 204, an optical routing element 208 and an O-E converter 206, as in the previous embodiments.

[0087] In this embodiment, the optical signal conditioning apparatus 510 is operative to apply an amplitude characteristic to the return optical signal to form a modified return optical signal. The applied amplitude characteristic is dependent on the received optical power.

[0088] Referring now to Figure 6, an embodiment of a communication network remote equipment 600 is illustrated. The communication network remote equipment 600 comprises an input / output port 202, an optical splitter 204, an optical routing element 208 and an O-E converter 206, as in the previous embodiments.

[0089] In this embodiment, the input / output port 202 is for receiving a high power cw downstream optical signal. The optical signal conditioning apparatus 610 comprises an optical modulator 604 and a controller 602. The O-E converter 206 converts the downstream optical signal into electrical power to power the optical modulator 604, via the controller 602.

[0090] The optical modulator is operative when electrical power is received to apply an optical modulation to the return optical signal to form an amplitude modulated return optical signal.

[0091] In certain embodiments, the optical modulator 604 is a time variant mirror switch driven by the switch controller 602. The switch 604 is configured to chop the cw return optical signal into a pulsed return optical signal, comprising periodic optical pulses having a pulse duration of the order of microseconds. Referring now to Figure 7, an embodiment of a communication network remote equipment 250 is illustrated. The communication network remote equipment 250 comprises traffic / control signals management 254 and a demultiplexer, DEMUX, 252, in addition to the elements described in Figure 3. The traffic / control signals management is configured to manage the traffic and control signals within the communication network remote equipment 550. The DEMUX is configured to demultiplex downstream traffic and control signals from the high power cw downstream optical signal and to route downstream optical signals from the input / output port 202 to port 1 of the optical circulator 108.

[0092] Referring now to Figure 8, an embodiment of a communication network optical link 700 is illustrated. The communication network optical link 700 comprises a communication network local equipment 100, as described above, a communication network remote equipment 500, as described above, and a single fibre optical link 152 connecting the communication network local equipment and the communication network remote equipment.

[0093] It will be understood that a communication network local equipment 150, as described above, may alternatively be used. It will also be understood that any one of the communication network remote equipment 200, 250, 300, 400, 600 may alternatively be used.

[0094] The optical link 700 may advantageously provide a simple and effective way to ensure optical safety in systems powered by fibre with Power over Fibre (PoF) techniques. The optical link 700 advantageously provides a passive restart mechanism at the remote equipment side and a single source laser at the local equipment. The restart mechanism at the remote equipment is totally passive during the restart phase, not requiring power from the local equipment by PoF techniques, and the optical power of the restart pulses may therefore be extremely low, ensuring safety in Class 1 .

[0095] In certain embodiments, the communication network optical link 700 is a power over fibre optical link.

[0096] In certain embodiments, implementing a remote equipment 200, 300 as described above, a high power cw downstream optical signal is received at port #1 of the optical circulator and routed to port #2. At the optical splitter, the most part of the light is sent to the optical-to- electrical power conversion block, while a small part of the light is split off and sent to the optical attenuator, which has a low attenuation when not electrically powered.

[0097] When a high power cw downstream optical signal is received, there is power supply to the remote equipment 200, the optical attenuator can be modulated so that an optical tone is generated on the return optical signal, which is sent through the port #3 to #1 of the optical circulator to the input / output port, and then coupled into a single fibre optical link. The return optical signal power may be measured and attenuated by the optical attenuator so that it is below the threshold of saturation of the non-linear absorber 214. As soon as the optical path is open, i.e. there is a discontinuity in a single fibre optical link to which the remote equipment 200 is connected, there is no power supply to the remote equipment. The attenuation of the optical attenuator is then fixed at a low value and the return optical signal, with the optical tone, is no longer generated and transmitted. The local equipment 100, then, when it detects “tone absence” powers down the high-power laser and starts to send restart optical pulses at low power.

[0098] As soon as the optical path is restored, low power restart optical pulses are received at the remote equipment and return optical pulses are sent back to the local equipment. The shape of the return low power optical pulses is “marked” by the non-linear absorber 214. The presence of the non-linear absorber allows the local equipment receiving the return optical pulses to understand if they are from a reflection of an open connector (reflection is linear, equal to 15 dB) or a fibre scattering (that should be very low, in any case, because of the low power of the restart optical pulses) or from the remote equipment (indicating that the optical path has been restored).

[0099] The local equipment detects the return optical pulses from the remote equipment (when the optical path is restored) or the tone on the cw return optical signal from the remote equipment (when the remote equipment is fully powered). The absence of the tone can be used to cause power down of the high-power laser light source.

[0100] A further embodiment provides a communication network optical link 700 comprising a communication network local equipment 150, as described above, a communication network remote equipment 250, as described above, and a single fibre optical link 152 connecting the communication network local equipment and the communication network remote equipment.

[0101] The single fibre optical link 152 has a length of up to 1 km, meaning loss of up to 0.2 dB. If the length or loss is higher, there are no safety issues but the system does not start. The laser light source 104 has an optical power limited to the range 0.1 mW to 1 mW (-10 dBm to 0 dBm) in the restart mode. The mux 156, demux 252, optical attenuator 212 and optical connections have a combined maximum attenuation of 4 dB. The optical splitter 204 splits off 10% of a received downstream optical signal to form the return optical signal, with the remaining 90% transmitted to the O-E converter 216, meaning a reduction in power of 10 dB. The non-linear absorber 214 has an attenuation of 3 dB when it is operating in the linear zone, below its threshold.

[0102] The attenuation of the optical path (one way) is therefore: - 0.2 - 4 - 10 - 3 = -17.2 dB

[0103] When the optical path is closed, the received optical power from the local equipment when transmitting 0 dBm (1 mW) is: 0 dBm - 17.2dB -17.2dB = -34.4 dBm

[0104] When the optical path is closed, the received optical power from the local equipment when transmitting -10 dBm (0.1 mW) is: -10 dBm - 17.2dB -17.2dB = -44.4 dBm The non-linear optical absorber threshold is therefore specified so that the received optical power will show saturation in the range -34.4 to -24.4 dBm (trace 906 in Figure 9). So, the threshold of the non-linear absorber is specified for an optical power of -5 dBm (i.e. the middle of the range -10 dBm to 0 dBm) as -5 -17.2 = -22.2 dBm

[0105] Referring now to FIG. 10, a flowchart illustrating steps of a method 800 for controlling a laser light source at a communication network local equipment is shown. The method 800 comprises steps that are performed by the communication network local equipment, such as the communication network local equipment 100 or 150 described in previous embodiments.

[0106] The method 800 comprises a step of, in an operational mode, generating a high power continuous wave, cw, downstream optical signal 802, for transmission on a single fibre optical link. This step may be performed by a laser light source, such as the laser light source 104 described above.

[0107] The method 800 also comprises a step of receiving an upstream optical signal 804 from the single fibre optical link. The upstream optical signal has an amplitude characteristic. This step may be performed by a monitoring apparatus, such as the monitoring apparatus 106 described above.

[0108] The method 800 comprises a step of detecting the upstream optical signal amplitude characteristic 806. This step may be performed by a monitoring apparatus, such as the monitoring apparatus 106 described above.

[0109] The method 800 comprises a step of determining 808, based on the amplitude characteristic, whether the upstream optical signal is from a remote node connected to the single fibre optical link or the upstream optical signal is a reflection from within the single fibre optical link. This step may be performed by a monitoring apparatus, such as the monitoring apparatus 106 described above.

[0110] The method 800 comprises a step of generating a control signal 810. The control signal may comprise a first indication indicative that the upstream optical signal is a signal from a remote node. Alternatively, the control signal may comprise a second indication indicative that the upstream optical signal is a reflection from within the single fibre optical link. This step may be performed by a monitoring apparatus, such as the monitoring apparatus 106 described above.

[0111] The method 800 comprises a step of receiving a said control signal and controlling 812 the generation of the downstream optical signal dependent on whether the control signal comprises a first indication or a second indication. The first indication is indicative that the upstream optical signal is a signal from a remote node and the second indication is indicative that the upstream optical signal is a reflection from within the single fibre optical link. This step may be performed a monitoring apparatus, such as the monitoring apparatus 106 described above, to control a laser light source, such as the laser light source 104 described above. In certain embodiments, the method 800 comprises further steps that are performed by the communication network local equipment.

[0112] The method comprises a further step of, in a restart mode, generating a pulsed downstream optical signal of low power restart optical pulses for transmission on the single fibre optical link. The method comprises a further step of continuing to operate in the restart mode when the control signal comprises a second indication indicative that the upstream optical signal is a reflection from within the single fibre optical link. The method comprises a further step of switching to operating in the operational mode when the control signal comprises a first indication indicative that the upstream optical signal is a signal from a remote node. In certain embodiments, the method 800 comprises further steps that are performed by the communication network local equipment.

[0113] The method comprises a further step of determining that the upstream optical signal is from a remote node responsive to detecting an amplitude modulation and determining that the upstream optical signal is a reflection from within the single fibre optical link responsive to detecting a constant amplitude. The method comprises a further step of automatically shutting down the laser light source when the control signal comprises a second indication indicative that the upstream optical signal is a reflection from within the single fibre optical link.

Claims

CLAIMS1 . A communication network local equipment comprising: an input / output port for connection to a single fibre optical link (152); a laser light source operative in an operational mode to generate a high power continuous wave, cw, downstream optical signal, for transmission on the single fibre optical link; and monitoring apparatus configured to: receive an upstream optical signal from the single fibre optical link, the upstream optical signal having an amplitude characteristic; detect the upstream optical signal amplitude characteristic; determine, based on the amplitude characteristic, whether the upstream optical signal is from a remote node connected to the single fibre optical link or the upstream optical signal is a reflection from within the single fibre optical link; and generate a control signal comprising a first indication indicative that the upstream optical signal is a signal from a remote node or comprising a second indication indicative that the upstream optical signal is a reflection from within the single fibre optical link, and wherein the laser light source is operative to receive a said control signal and to control generation of the downstream optical signal dependent on which of the first indication or the second indication the control signal comprises.

2. A communication network local equipment as claimed in claim 1 , wherein the laser light source is additionally operative in a restart mode to generate a pulsed downstream optical signal of low power restart optical pulses for transmission on the single fibre optical link, and wherein the laser light source is configured to continue to operate in the restart mode when the control signal comprises a said second indication and configured to switch to operating in the operational mode when the control signal comprises a said first indication.

3. Communication network local equipment as claimed in claim 2, wherein the low power restart optical pulses have a peak pulse power not exceeding an accessible emission limit, AEL, of the laser light source and less than a nonlinear threshold of the single fibre optical link, such as up to 5 mW4. Communication network local equipment as claimed in any one of claim 2 or claim 3, wherein restart optical pulses have a first pulse shape and the upstream optical signal comprises upstream optical pulses having a second pulse shape, and wherein the monitoring apparatus is configured in the restart mode to: detect upstream optical pulses and determine the second pulse shape; compare the second pulse shape to the first pulse shape;determine that upstream optical pulses are reflections from within the single fibre optical link responsive to determining that the second pulse shape is substantially the same as the first pulse shape; and determine that upstream optical pulses are received from a remote node responsive to determining that the second pulse shape is different to the first pulse shape.

5. Communication network local equipment as claimed in claim 4, wherein the restart optical pulses have a pulse duration and wherein the monitoring apparatus is configured in the restart mode to: compare a total received optical power of an upstream optical pulse with a total transmitted optical power of a restart optical pulse; determine that upstream optical pulses are reflections from within the single fibre optical link responsive to determining a constant difference across said pulse duration between the total transmitted optical power of a restart optical pulse and the total received optical power of an upstream optical pulse; and determine that upstream optical pulses are received from a remote node responsive to determining an increasing difference across said pulse duration between the total transmitted optical power of a restart optical pulse and the total received optical power of an upstream optical pulse.

6. Communication network local equipment as claimed in claim 5, wherein the increasing difference increases nonlinearly across at least part of said pulse duration.

7. A communication network local equipment as claimed in any one of the preceding claims, wherein: the amplitude characteristic is one of an amplitude modulation or a constant amplitude; the monitoring apparatus is configured in the operational mode to determine that the upstream optical signal is from a remote node responsive to detecting an amplitude modulation and to determine that the upstream optical signal is a reflection from within the single fibre optical link responsive to detecting a constant amplitude; and the laser light source is configured in the operational mode to automatically shut down when the control signal comprises a said second indication.

8. A communication network remote equipment comprising: an input / output port for connection to a single fibre optical link for receiving a downstream optical signal, having a received optical power, from the single fibre optical link; an optical splitter in communication with the input / output port and configured to split off a portion of the downstream optical signal to form a return optical signal having a return optical power;an optical to electrical converter configured to receive the downstream optical signal from the optical splitter, convert the downstream optical signal into electrical power and to provide electrical power to optical signal conditioning apparatus; optical signal conditioning apparatus operative to apply an amplitude characteristic to the return optical signal to form a modified return optical signal, wherein the applied amplitude characteristic is dependent on the received optical power; and an optical routing element configured to direct the modified return optical signal back to the input / output port for upstream transmission on the single fibre optical link.

9. Communication network remote equipment as claimed in claim 8, wherein the downstream optical signal is one of a high power continuous wave, cw, optical signal or low power optical pulses and wherein the optical signal conditioning apparatus comprises: an optical attenuator configured to apply an optical attenuation to the return optical signal when electrically powered and configured to transmit the return optical signal without applying optical attenuation when not electrically powered; and an optical limiter configured to prevent transmission of light above a pre-configured optical power threshold, wherein the optical splitter and the optical attenuator are configured to produce an attenuated return optical signal by attenuating the return optical signal to have an optical power lower than the optical power threshold and to produce an unattenuated return optical signal having a total optical power higher than the optical power threshold, such that amplitude shaping is applied to an unattenuated return optical signal.

10. Communication network remote equipment as claimed in claim 9, wherein the optical power threshold is configured to apply amplitude shaping to low power optical pulses.11 . Communication network remote equipment as claimed in any one of claim 9 or claim10, wherein the single fibre optical link has a nonlinear threshold above which fibre nonlinearities occur and the optical power threshold is configured to limit a peak pulse optical power of a return optical signal of optical pulses to less than the nonlinear threshold.

12. Communication network remote equipment as claimed in any one of claims 9 to 11 , wherein the optical limiter is nonlinear absorber having an absorption threshold in the range -23 dBm to -16 dBm.

13. Communication network remote equipment as claimed in any one of claims 9 to 12, wherein the optical attenuator is additionally operative to apply an amplitude modulation when electrical power is received.

14. Communication network remote equipment as claimed in claim 8, wherein the downstream optical signal is a high power continuous wave, cw, optical signal and wherein the optical signal conditioning apparatus comprises an optical modulator operative when electrical power is received to apply an optical modulation to the return optical signal to form an amplitude modulated return optical signal.

15. A communication network optical link comprising: a communication network local equipment as claimed in any one of claims 1 to 7; a communication network remote equipment as claimed in any one of claims 8 to 14; and an single fibre optical link connecting the communication network local equipment and the communication network remote equipment.

16. The communication network optical link of claim 15, wherein the optical link is a power over fibre optical link.

17. A method of controlling a laser light source at a communication network local equipment, the method comprising steps of:- in an operational mode, generating a high power continuous wave, cw, downstream optical signal, for transmission on an single fibre optical link;- receiving an upstream optical signal from the single fibre optical link, the upstream optical signal having an amplitude characteristic;- detecting the upstream optical signal amplitude characteristic;- determining, based on the amplitude characteristic, whether the upstream optical signal is from a remote node connected to the single fibre optical link or the upstream optical signal is a reflection from within the single fibre optical link;- generating a control signal comprising a first indication indicative that the upstream optical signal is a signal from a remote node or comprising a second indication indicative that the upstream optical signal is a reflection from within the single fibre optical link; and- receiving a said control signal and controlling generation of the downstream optical signal dependent on which of the first indication or the second indication the control signal comprises.

18. The method of claim 17, further comprising steps of:- in a restart mode, generating a pulsed downstream optical signal of low power restart optical pulses for transmission on the single fibre optical link;- continuing to operate in the restart mode when the control signal comprises a said second indication; and- switching to operating in the operational mode when the control signal comprises a said first indication.

19. The method of any one of claim 17 or claim 18, further comprising steps in the operational mode of:- determining that the upstream optical signal is from a remote node responsive to detecting an amplitude modulation and determining that the upstream optical signal is a reflection from within the single fibre optical link responsive to detecting a constant amplitude; and- automatically shutting down the laser light source when the control signal comprises a said second indication.

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