Communication network optical protection switching equipment and optical link

Abstract

Optical protection switching equipment (100) for switching optical signals between a working line and a protection line. The equipment comprises an electrical power module (104) operative to provide electrical power to control circuitry (106), the electrical power module comprises an optical-to-electrical converter configured to convert a part of a received optical signal into electrical power. The control circuitry (106) comprises an electrical hold-off element comprising a passive electrical device, wherein the passive electrical device is configured to store electrical power from the electrical power module for powering a switch (102) between the working line (W) and the protection line (P). Following a switch of optical signals between the working line and protection line, the electrical hold-off element takes a pre-determined recharge time period during which a further switch of optical signals between a working line and a protection line is not possible.

Description

[0001] COMMUNICATION NETWORK OPTICAL PROTECTION SWITCHING EQUIPMENT

[0002] AND OPTICAL LINK

[0003] Technical Field

[0004] The present disclosure relates to optical protection switching equipment and to an optical link comprising the optical protection switching equipment.

[0005] Background

[0006] Optical protection schemes have been widely used in different communication network architectures for many years. They typically include an Optical Protection module, OPM, based on an optical protection switch and an optical splitter. Figure 1 illustrates a known optical protection scheme for a bidirectional optical link. An OPM is used on one side of a link (the hub in a ring, the main site in a fronthaul link, etc.) and a splitter is provided at remote nodes / sites. The OPM and the splitter are connected by a working line, W, and a protection line, P. Dense wavelength division multiplexed, DWDM, optical signals are multiplexed / demultiplexed by the Mux / Dmux at either end of the optical link and are connected to the OPM and the splitter by common optical lines, COM.

[0007] The protection switch is an active device which requires an active host to power it. Alternative protection schemes use two optical protection switches at the ends of the optical link, an APS (Automated Protection Switching) protocol is generally required to coordinate switching actions. The APS protocol may be low loss, but it cannot be realized in a passive way because it requires an APS channel and processors at both ends. Splitters add significant loss to the optical budget reducing the possible number of concatenated remote nodes. Switches require active hosts to be powered preventing pure passive solutions preferred in cost sensitive environments like Radio Access Network.

[0008] In a hub and spokes optical fronthaul scenario, a Banded Mux / Dmux, BMD, or a splitter can be provided at the main site, at the first remote site or at a dedicated site between the main site and the remote sites. This dedicated site may require an indoor, outdoor solution or it may be required to place the splitter or banded filter in a manhole. In any case the BMD or splitter dedicated site is not powered so it is not possible to support active optical protection unit to protect the optical path between the splitter / BMD site and the remote site. In the optical fronthaul scenario in the remote site there are radio units with valuable 25G evolved Common Public Radio Interface, eCPRI, mobile traffic coming from macro and small cells to be protected.

[0009] The line protection introduces approximately 5.7dB insertion loss for the optical switch and splitter. In the fronthaul network segment where there are no amplifiers and where the optical modules do not have high performance because they must be low cost, it is difficult to protect more than one span. If more spans are to be protected it is advantageous to reduce the insertion loss for path protection by at least 2dB. In addition, commercially available line optical protection modules are not known to protect bidirectional or dual fibre grey signals on a passive network.

[0010] WO 2017 / 071827 A1 describes protection apparatus for a bidirectional optical link. The apparatus utilizes a protection switch at one end of the link and an optical interleaver, such as an optical splitter at the other end of the link. The detection of the downstream signals is over a further optical link (whilst traffic is transmitted over the bidirectional optical link), this detection is advantageously not affected by upstream signal reflections.

[0011] Summary

[0012] An aspect of the present disclosure provides optical protection switching equipment for switching optical signals between a working line and a protection line. The optical protection switching equipment comprises an electrical power module, control circuitry and a switch. The switch is between the working line and the protection line. The electrical power module is operative to provide electrical power to the control circuitry. The electrical power module comprises an optical-to-electrical converter configured to convert a part of a received optical signal into electrical power. The control circuitry comprises an electrical hold-off element comprising a passive electrical device. The passive electrical device is configured to store electrical power from the electrical power module for powering the switch. Following a switch of optical signals between the working line and protection line, the electrical hold-off element takes a pre-determined recharge time period during which a further switch of optical signals between a working line and a protection line is not possible.

[0013] The optical protection switching equipment may provide a highly reliable solution for protecting optical links. It is a low loss solution which enables an end-to-end 1 :1 Line Protection in any type of point-to-point optical link. The optical protection switching equipment is compatible with both Single Fiber Working, SFW, and Dual Fiber Working, DFW, applications.

[0014] The optical protection switching equipment is self-contained and operates without requiring management by external active units. The equipment enables a fully passive protection switching architecture, which may be particularly advantageous in Radio Access Network, RAN, applications.

[0015] The optical protection switching equipment may advantageously be used at each end of an optical link to be protected against accidental breaks or unwanted misconnections. It replaces the active switch and optical splitter of known optical protection schemes, which may enable lower insertion loss across the optical link; a greater than 2 dB insertion loss saving may be achieved. The optical protection switching equipment advantageously operates independent of the bit rate of traffic transmitted across an optical link being protected and may be used with both coloured and grey interfaces. In addition, the optical protection switching equipment imposes no limit on the number of optical channels which can be transmitted across the optical link being protected and it is entirely independent of the frequency band of the channels.

[0016] The optical protection switching equipment may obtain operational power directly from the optical signals carried within the optical link being protected. This capability allows it to function without any external power sources, which is particularly advantageous in remote or unpowered locations. The hold-off element may prevent spurious switching during handshake protocols.

[0017] The optical protection switching equipment advantageously eliminates the need to synchronize finite state machines, FSMs, at the endpoints of the optical link.

[0018] Corresponding advantages also apply to the optical link described below.

[0019] Another aspect of the present disclosure provides an optical link. The optical link comprises a first common optical line forming a first end of the optical link, a second common optical line forming a second end of the optical link, a working optical line, a protection optical line, a first communication network optical protection switching equipment and a second communication network optical protection switching equipment. The first communication network optical protection switching equipment is connected on one side to the first common optical line and connected on another side to the working line and the protection line. The second communication network optical protection switching equipment is connected on one side to the working line and the protection line and connected on another side to the second common optical line. Each optical protection switching equipment comprises an electrical power module, control circuitry and a switch. The switch is between the working line and the protection line. The electrical power module is operative to provide electrical power to the control circuitry. The electrical power module comprises an optical-to-electrical converter configured to convert a part of a received optical signal into electrical power. The control circuitry comprises an electrical hold-off element comprising a passive electrical device. The passive electrical device is configured to store electrical power from the electrical power module for powering the switch. Following a switch of optical signals between the working line and protection line, the electrical hold-off element takes a pre-determined recharge time period during which a further switch of optical signals between a working line and a protection line is not possible.

[0020] Another aspect of the present disclosure provides a method for switching optical signals between a working line and a protection line. The method comprises the following steps. A part of a received optical signal is converted into electrical power. The electrical power is stored for powering a switch between the working line and the protection line. Following a switch of optical signals between the working line and protection line, charging an electrical hold-off element (1506) for a pre-determined recharge time period during which a further switch of optical signals between a working line and a protection line is not possible.

[0021] Brief Description of the drawings

[0022] Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings.

[0023] Figs. 2, 11 and 13 illustrate embodiments of optical protection switching equipment';

[0024] Fig. 3 illustrates an electrical power module and an electrical hold-off element of an embodiment of optical protection switching equipment;

[0025] Fig. 4 shows a plot of recharge voltage as a function of time of the capacitor of Fig. 3;

[0026] Fig. 5 shows a plot of recharge voltage as a function of time of the capacitor of Fig. 3 when the variable resistor R1 is 250 ohms;

[0027] Fig. 6 illustrates part of the control circuitry of optical protection switching equipment of an embodiment;

[0028] Fig. 7 is a switching decision matrix implemented by the control circuitry of Fig. 6;

[0029] Fig. 8 is a transition state diagram for the decision matrix of Fig. 7;

[0030] Fig. 9 is a block diagram illustrating control circuitry of optical protection switching equipment of an embodiment;

[0031] Fig. 10 illustrates an optical link according to an embodiment, for single fibre working (SFW) applications;

[0032] Fig. 12 illustrates an optical link according to an embodiment, for dual fibre working (DFW) applications;

[0033] Fig. 14 illustrates an optical link according to an embodiment; and

[0034] Fig. 15 is a flow chart of steps of a method according to an embodiment.

[0035] Detailed description

[0036] The same reference numbers are used for corresponding features in different embodiments.

[0037] The present disclosure relates to optical communication systems, and more particularly to optical protection switching equipment (which may also be referred to as a passive Protection Switching Module, PSM) for optical path protection of an optical link and to an optical link including a PSM at each end. The present disclosure relates to a low loss Protection Switching Module suitable to implement an end-to-end 1 :1 Line Protection in any type of point-to-point optical link, which may be fully passive, and to an optical link having 1 :1 line protection provided by PSMs at each end.

[0038] The equipment may be powered by a portion of the optical power received from the optical link, thus eliminating the dependency on an external power source. The equipment can be positioned anywhere in the optical network, as it does not require power supply and management by active network elements.

[0039] The equipment may include a switch circuit, a bi-stable optical switch controlled by the switch circuit, an optical inter-leaver for monitoring the status of the optical link, and a self-contained commutation state machine. The bi-stable optical switch is driven by a switch circuit which makes decisions to change the position of the switch according to the commutation state machine triggered by external events, such as fibre break or cut, or cable disconnection. The switch circuit is powered by a small portion of the DWDM signal incoming power on the fibre. The state machine, in conjunction with a hold-off time realized through a charge capacitor, may automatically prevent unwanted switching during handshakes protocols across the optical link. The equipment avoids having to synchronize the state machines at either end of an optical link and provides high reliability.

[0040] The equipment may operate independent of the bit rate and may be used with coloured or grey optical interfaces. The equipment may be used in both single fibre working, SFW, and dual fibre working, DFW, applications. The same type of equipment may be used at both ends of an optical link to be protected against accidental breaks or unwanted disconnections. The solution may be self-contained and does not require management by external active units.

[0041] The equipment may offer improved insertion loss, potentially saving more than 2 dB compared to known optical protection switching schemes.

[0042] The equipment may be used in a fully passive architecture, making it suitable for environments like Radio Access Networks, RAN, where cost sensitivity is a concern. The equipment may also be used in datacentre scenarios to protect grey 200G / 400G signals. The disclosed equipment may provide a level 0 protection scheme requiring no management by an active unit in data centre applications.

[0043] Referring to Figure 2, an embodiment of an optical protection switching equipment 100 for switching optical signals between a working line and a protection line in single fibre working, SFW, applications is illustrated. The optical protection switching equipment 100 comprises an electrical power module 104, control circuitry 106, and optical switch apparatus 102. The electrical power module 104 is operative to provide electrical power to the control circuitry 106. The optical switch apparatus 102 is reconfigurable to have either a first condition, A, for connecting to the working line, W, or a second condition, B, for connecting to the protection line, P. The optical switch apparatus 102 of this embodiment is a bi-stable optical switch.

[0044] The electrical power module 104 comprises an optical-to-electrical, O-E, converter configured to convert a part of a received optical signal into electrical power. Part of a downstream, Tx, optical signal received from a common line, COM, is split off by an optical splitter 110 and sent along an optical path 124 to the O-E converter 104.

[0045] The control circuitry 106 comprises an electrical hold-off element comprising a passive electrical device configured to store electrical power from the electrical power module 104 for powering the optical switch 102. Following a switch of optical signals between the working line W and the protection line P, the electrical hold-off element takes a pre-determined recharge time period during which a further switch of optical signals between the working line and the protection line is not possible. In other words, during the pre-determined recharge time period, there is insufficient voltage or power to carry out the switch.

[0046] The optical protection switching equipment further comprises optical monitoring apparatus 108 comprising a first monitoring channel 130 configured to detect upstream light received from the working line W and a second monitoring channel 132 configured to detect upstream light received from the protection line P. Each monitoring channel comprises an optical tap 112, 114 configured to tap off a portion of an optical signal received from the respective line to form a monitoring signal, and optical detection apparatus configured to output detection signals responsive to detecting monitoring signals at defined upstream wavelengths.

[0047] Each optical detection apparatus comprises an optical filter 116, 118 and a photodetector 120, 122. The optical filters 116, 118 are configured to receive the monitoring signal from the respective optical tap and are configured to pass light at defined upstream wavelengths. The photodetectors 120, 122 are configured to output detection signals responsive to detecting light passed by their respective optical filter.

[0048] In this SFW application, the working line W comprises single optical fibre for transmission of both downstream optical signals and upstream optical signals and the protection line P comprises a further single optical fibre for transmission of both downstream optical signals and upstream optical signals.

[0049] In certain embodiments, the optical switch which is a bi-stable switch and can assume only two positions, A or B. The optical switch is driven by a logic circuit which bases a switch commutation decision on the status of the switch itself and on the presence / absence of light on the working line, W, and the protection line, P, monitored through monitoring optical splitters (also referred to as optical taps) 112, 114. The split ratio of the optical taps can vary in general according to the application, but a 90:10 splitting ratio is suitable, in many cases, to guarantee enough optical powerto monitoring optical channels, while maintaining a low loss on the main optical path.

[0050] In the SFW scenario, the light tapped is sent to an optical filter 116, 118 that passes only light coming from the opposite direction of the optical link, i.e. upstream signals, so to avoid switch commutation decisions being distorted by light reflected from a fiber break point in case of a fault. The design of the optical filter depend on the SFW scenario, however it is generally an interleaver filter on the upstream and downstream wavelength channel grid of the SFW application. In case of a bidirectional grey application, the interleaver filter can be designed on the two frequencies used by the specific bidirectional pair of transceivers. Of course, in a SFW scenario, the filters used at the two endpoints of the optical link must be designed in a complementary way; the frequencies filtered at one end of the link must be complementary to the frequencies filtered the other end. After the filter, the light is detected at the photodetectors 120, 122 sent to the decision circuit; the photodetectors can reveal the presence or absence of light in the relevant path at the monitoring points Mw and Mp.

[0051] So that the optical protection switching equipment 100 is passive, the logic circuit is powered by the optical light coming from the COM port, through the optical tap 110. In passive scenarios the logic circuit shall operate only when the voltage supply is enough to drive the photodetectors 120, 122 and (if necessary) to commutate the optical switch 102.

[0052] One of the advantages of the present disclosure is that the optical protection switching equipment 100 introduces a low insertion loss on the express channel. Assuming that the optical tap 110 and the optical taps 112, 114 have a 10% of splitting ratio, the overall insertion loss on the express path can be maintained below 1.8 dB (including 2 x LC connectors, 4 x Splices, 1 x Optical Switch and 2 x Taps). Since an optical link would have an equipment 100 at each end, the overall express insertion loss in less than 3.6 dB instead of 5.7 dB of the conventional solution that foresees an Optical Protection Switching on one side and a Splitter unit on the opposite side of the protected line. A loss saving of around 2dB allows to use such an optical link in many more network topologies and also in subsequent spans in a hop-by-hop scenario.

[0053] Figure 3 illustrates an electrical power module 204 and an electrical hold-off element of control circuitry of an optical protection switching equipment according to another embodiment.

[0054] The electrical power module comprises an O-E converter, in this example a photodiode 204. The photodiode is configured to convert a portion of a received optical signal into electrical power. The electrical hold-off element comprises a passive electrical device and a threshold circuit 210.

[0055] The passive electrical device comprises a capacitor 208 and a recharge circuit 212 for the capacitor. The capacitor 208 is configured to store electrical power from the photodiode 204. The recharge circuit 212 is configured to recharge the capacitor 208 to the threshold voltage in the pre-determined recharge time period. The threshold voltage is sufficient to power the control circuitry.

[0056] The recharge circuit 212 comprises a variable resistor Ri. The resistance of the variable resistor Ri may be configured to cause the capacitor 208 to charge to the threshold voltage in the predetermined recharge time period. This may allow for control over the recharge time period, providing flexibility in the operation of the optical protection switching equipment.

[0057] The threshold circuit 210 is configured to prevent electrical power output to the control circuitry for a pre-determined recharge time period. The threshold circuit 210 is operative to prevent electrical power output to the control circuitry until the capacitor 208 has reached the threshold voltage. The threshold circuit 210 comprises a second resistor R2 and an electrical switch S2 in parallel with the capacitor 208, and a further electrical switch Si between the variable resistor and the voltage supply, Vcc.

[0058] In an embodiment, the optical splitter 110 is a 10% optical tap that taps off 10% of a received downstream optical signal, which may as an example be 34.7 mW.

[0059] The photodiode 204, used in photovoltaic mode, is equivalent to a current source proportional to the incident light, with a limited voltage at open circuit, for example the voltage limit may typically be 2.2V.

[0060] The photodiode has a responsivity, Re, of about 0.1 A / W, giving an output current from the photodiode of:

[0061] A

[0062] I = p . Re = 0.0347 W ■ 0.1 — = 3.4 mA W

[0063] A voltage of 2V is required for the control circuitry. Using a capacitor of 0.1 Farad, this gives an energy stored in the capacitor of:

[0064] 1 1

[0065] E = - CV2= - 0.1 ■ 2.042= 0.208 J 2 2J

[0066] The threshold circuit 210 is an electronic circuit which monitors the voltage across the capacitor and closes the switch Si to feed the control circuitry when the voltage reaches 2V. The threshold circuit 210 works as follows: once a voltage of 2V is reached on the capacitor switch Si is automatically closed, and the power is enough to continuously feed the control circuitry 106. When protection switching is triggered, and the optical switch 102 is to be commutated, switch S2 is also closed, so that the capacitor is fully discharged. At the end, that is with no voltage, both Si and S2 are normally open and the whole cycle starts again according to the designed recharge time period (also referred to as the hold-off time).

[0067] The hold-off time can be regulated by changing the value of the variable resistor Ri. In fact, the recharge relation of the capacitor can be represented for this circuit as a diagram, shown in Figure 4, that is linear in the first part 302, until the voltage across the equivalent current generator, represented by the photodiode 204, has reached its maximum value of 2.2 V and becomes equivalent to a voltage generator, and exponential in the second part 304 with time constant T = RC. In this second part the recharge time period To can be regulated (lengthened or shortened) as desired, by adjusting the variable resistor, i.e. by acting on the charge time constant T = RC.

[0068] So, for example, if the variable resistor is configured with a resistance R=250 ohm, the capacitor is charged at 2 V in a time To of around 60 seconds, as illustrated in Figure 5.

[0069] Figure 6 illustrates control circuitry 700 of an optical protection switching equipment 100, as described above, according to an embodiment.

[0070] The control circuitry comprises a logic circuit 700 for controlling switching of a optical switch apparatus 710. The logic circuit 700 is configured to make one of a Yes decision to switch the optical switch apparatus or a No decision not to switch the optical switch apparatus, based on a current condition of the optical switch apparatus, whether or not a detection signal is received from the optical detection apparatus for the working line, and whether or not a detection signal is received from the optical detection apparatus for the protection line.

[0071] The logic circuit 700 comprises a NOT gate 702, an AND gate 704, an OR gate 706, a driver circuit 708, an oscillator 712 and a flip-flop 714.

[0072] The driver circuit 708 is configured to output a control signal to the optical switch apparatus 710 responsive to receiving a Yes decision and electrical power being provided from the electrical power module. The control signal is configured to cause the optical switch apparatus to switch from the current condition to the other of the first condition or the second condition.

[0073] The optical switch apparatus 710 is a bi-stable optical switch configured to switch between two stable states, corresponding to the first condition and the second condition. The driver circuit is configured to control the state of the bi-stable optical switch based on the output of the logic circuit 700.

[0074] It is assumed that the optical switch apparatus 710 has a simple auxiliary contact with two states (open, closed) referred to as the "Switch Status," which indicates the physical state in which the switch itself is. Thus, even in the total absence of power supply this "memory" is retained and can be read the next time the logic circuit is reactivated. The flip flop thus always has the correct input signals to possibly switch its outputs even after a complete shutdown of the circuit.

[0075] The oscillator is needed so that the flip flop can operate. There are commercially available oscillators with power consumption on the order of a few pW’s. The logic gates in the circuit also have the same low power consumption.

[0076] The consumption of the driver circuit depends on the optical switch apparatus, which may require less than 200 mW for 5 ms to switch (i.e., 1 mJ). So, also considering some power leakage, this circuit needs less than 300mWfor 10ms (i.e., 3mJ) to turn on, process inputs, and eventually switch the optical switch apparatus.

[0077] Based on the above considerations, the switching time of optical protection switching equipment is of the order of magnitude of the discharge time of the energy accumulated in the capacitor, that is around 10ms.

[0078] The logic circuit 700 is configured to implement the switching decision matrix shown in Figure 7. The switching decision matrix represents the possible states of the optical switch apparatus 710 and the corresponding actions taken by the logic circuit 700. For example, if the current condition of the optical switch apparatus is the first condition A, and a detection signal is received from the optical detection apparatus for the working line but not for the protection line, the logic circuit 700 may make a No decision not to switch the optical switch apparatus. On the other hand, if the current condition of the optical switch apparatus is the first condition A, and no detection signal is received from the optical detection apparatus for either the working line or the protection line, the logic circuit 700 may make a Yes decision to switch the optical switch apparatus to the second condition B.

[0079] The logic circuit 700 can make a commutation decision based only on information that is locally available: the status of the optical switch apparatus (or switches in case of DFW described below); the presence of light on Mw (or MWRX in case of DFW) and on Mp (or MPRX in case of DFW).

[0080] The switching decision matrix can also be described through a transition state diagram 800, as shown in Figure 8. The transition state diagram 800 includes a working state 802, a protection state 804, a Transition to Protection state 806, and a Transition to Working state 808.

[0081] Referring now to Figure 9, an alternative control circuitry 900 is depicted. The control circuitry 900 comprises interface circuitry 902, at least one processor 904, and memory 906 comprising instructions 908 executable by the processor 904. The control circuitry 900 is operative to make one of a Yes decision to switch the optical switch apparatus or a No decision not to switch the optical switch apparatus based on a current condition of the optical switch apparatus, whether or not light is received from the working line, and whether or not light is received from the protection line.

[0082] The control circuitry 900 is further operative to output a control signal to the optical switch apparatus responsive to a Yes decision and electrical power being provided from the electrical power module. The control signal is configured to cause the optical switch apparatus to switch from the current condition to the other of the first condition or the second condition.

[0083] Referring to Figure 10, a single fibre working (SFW) optical link 1000 according to an embodiment is depicted. The optical link 1000 comprises a first common optical line 1002 forming a first end of the optical link, a second common optical line 1004 forming a second end of the optical link, a working optical line 1006, a protection optical line 1008, a first communication network optical protection switching equipment 100(1) and a second communication network optical protection switching equipment 100(2).

[0084] The first optical protection switching equipment 100(1) is connected on one side to the first common optical line 1002 and connected on the other side to the working optical line 1006 and the protection optical line 1008. The second optical protection switching equipment 100(2) is connected on one side to the working optical line 1006 and the protection optical line 1008 and connected on the other side to the second common optical line 1004.

[0085] The optical link 1000 further comprises a first multiplexer / demultiplexer, Mux / Demux, 1010 for multiplexing / demultiplexing optical signals to and from the first common optical line 1002 and a second Mux / Demux 1012 for multiplexing / demultiplexing optical signals to and from the second common optical line 1004.

[0086] The optical protection switching equipment 100(1), 100(2) may be independent of the bit rate, and it may be used with colored or grey optical interfaces. This may provide flexibility in the operation of the optical link 1000, as it can accommodate various types of optical signals.

[0087] The optical link 1000 may provide a passive solution for optical path protection in optical communication systems. The same type of optical protection switching equipment 100(1), 100(2) may be used at both ends of the optical link 1000 to be protected against accidental breaks or unwanted disconnections. The solution may be self-contained and may not require management by external active units. This may provide a reliable and efficient protection mechanism for optical communication systems.

[0088] The overall switching time, that is the time needed for a complete switch from the working path to the protection path, is the time required by the pair of optical protection switching equipments 100 (at each end of the optical link) to switch: provided they are both “ready to switch” (that is both charged, which is the normal operating condition) in the worst case we can assume a switching time of 20ms (i.e. 10ms+10ms).

[0089] Protection switching operation of the optical link 100 may be described referring to the switching decision matrix of Figure 7 and the transition state machine of Figure 8, as follows.

[0090] From the perspective of the optical protection switching equipment, PSE, 100(1). Supposing to start with the optical switch apparatus in state A, any further transition of the state evolves based on the light presence on the two lines Mi and on M2 The two transition states, TP (Transition to Protection) 806 and TW (Transition to Working) 808 account for the possibility that the PSE 100(2) at the other end of the optical link is not yet switched to the non-broken path and avoid spurious switching during handshake procedures. The principle of operation is that in case a loss of signal is detected on one line, the selector tries to switch to the other line and waits for the PSE 100(2) to do the same; this will happen because the PSE 100(2) follows exactly the same logic. The permanence time on a Transition state depends on different factors. Normally (in case of a fiber break of a line and with both PSEs 100 are full charged) it is just a hop. But if PSE 100(2) is not yet ready to switch (i.e. not yet charged) PSE 100(1) must wait until PSE 100(2) is recharged and so the permanence time on the TP or TW state depends on the recharge time period. There is also the possibility of a momentary loss of signal, LOS, on a line (not caused by a real fiber break but only due to a loss of light in the fiber, because of the optical transmitters at the other end of the link are briefly switched off for any reason) and in this case a PSE 100 needs to induce the opposite PSE to switch and wait until it has completed the switch).

[0091] In any case the above mechanism guarantees that both PSEs 100 will align autonomously in a time that is few tens of milliseconds (normally) or few minutes (due to the recharge time period) if one of the two PSEs 100 is not immediately ready to switch.

[0092] In addition to this, another important aspect of the PSE 100 is the Hold-Off time that is strictly linked to the recharge time period and, in the end, to the capacitor’s recharge circuit dimensioning, as described above. The Hold-off time comes into play in the steady states “W” 802 and “P” 804, to avoid spurious transitions simply due to handshake protocols between end terminals (e.g. Radio units and Baseband units) connected to the optical link 1000. The duration of the protocol depends on the type of endpoints and a reasonable value could be one minute.

[0093] By dimensioning the capacitor’s recharge circuit and / or, the capacitor, a minimum hold-off time can be set. For example, if we want to have a minimum hold-off of 1 min we need to limit the capacitor’s maximum recharge current so that in the best case of optical power conditions of our specific link, the time required for a complete capacitor recharge is just a minute. In all other conditions (i.e. with lower optical power available and so lower current in the recharge circuit) the recharge time period will be greater, but this is not a problem.

[0094] This minimum recharge time period (Hold-Off) time can be set by the operator during installation and according to the setting the PSE shall set a value for a current limiter so to obtain the desired recharge time period.

[0095] An advantage of the optical protection switching equipment 100 is that it can be realized in a passive way, without connection to an external electrical supply. Thus, the protection mechanism can be implemented in any type of infrastructure and topology. All the energy required by the decision circuit and by the optical switch can be obtained through the optical to electrical conversion of the optical signal split via the optical tap from the incoming optical signal and stored in the capacitor. The optical tap is positioned on the optical signal coming from the side near to the optical sources so that the optical power available is attenuated only by the multiplexer and so great enough to guarantee a reasonable recharge time period after any cycle.

[0096] As an example, to obtain an energy of 10mJ (rounding up the 3mJ required discussed above) from the O-E conversion circuit, starting from a WDM signal of 6 channels at 1 dBm, considering a loss of the multiplexer (or Add / Drop) filter of 5.6 dB, a tap at 10%, and also accounting for a conversion inefficiency, the recharge time period is approximately 100 seconds. This time can varies depending on the number of channels, the loss of the filter the launch power of the transceivers and so on, but the order of magnitude does not change much in any case. So normally the switching circuit is ready to operate a decision and the overall switching time is within a few milliseconds apart from the worst case in which the opposite PSM is waiting (for any reason) to be recharged and in such case the first PSE 100(1) moves to the transitory state waiting for the other PSE 100(2) to switch.

[0097] Moreover, after each switch event the PSE 100 will be again available to do another evaluation cycle (decision and switch if requested) every 1 or 2 minutes. This makes the protection mechanism quite reliable because normally after a break the time requires to repair the fiber is much longer time and in any case the circuit can remain on the protection path w / o any necessity to revert to the former path.

[0098] In the end another important aspect is the dimensioning of the capacitor’s recharge circuit, in fact in this way we have the possibility to fix a minimum hold-off time that avoids spurious switch transitions and makes the mechanism robust. For example, if we suppose that a minimum hold-off time of 1 minute is necessary, we need to limit the capacitor’s maximum recharge current so that, in the maximum optical power conditions of our link, the time required for a complete capacitor recharge is just 1 minute. In all other conditions (i.e. with lower optical power available and so lower current in the recharge circuit) the recharge time period will be greater, but this is not a problem because the important is to fix a minimum hold-off time.

[0099] Referring to Figure 11 , an embodiment of an optical protection switching equipment 1100 for switching optical signals between a working line and a protection line in dual fibre working, DFW, applications is illustrated.

[0100] The optical protection switching equipment 1100 of this embodiment is broadly the same as the equipment 100, described above, with certain modifications to adapt it for DFW. Elements with the same reference numbers as use above operate in the same way as described above.

[0101] The working line comprises a first downstream optical fibre, WTX, for transmission of downstream optical signals and a first upstream optical fibre, WRX, for transmission of upstream optical signals. The protection line comprises a second downstream optical fibre, PTX, for transmission of downstream optical signals and a second upstream optical fibre, PRX, for transmission of upstream optical signals.

[0102] The optical protection switching equipment 1100 comprises an electrical power module 104, as described above, control circuitry 1106, optical switch apparatus 1102, 1104 and optical monitoring apparatus 1108. The optical switch apparatus comprises a first bi-stable optical switch 1102 and a second bistable optical switch 1104, control circuitry 1106. The first bi-stable optical switch 1102 and the second bi-stable optical switch 1104 are each reconfigurable to have either a first condition, A, for connecting to the working line, W, or a second condition, B, for connecting to the protection line, P. The first bi-stable optical switch is for switching between the first downstream optical fibre, WTX, and the second downstream optical fibre, PTX. The second bi-stable optical switch is for switching between the first upstream optical fibre, WRX, and the second upstream optical fibre, PRX.

[0103] The electrical power module 104 comprises O-E converter, as described above. Part of a downstream, Tx, optical signal received from a first common line, COMTX, is split off by the optical splitter 110 and sent along the optical path 124 to the O-E converter 104.

[0104] The control circuitry 1106 comprises an electrical hold-off element comprising a passive electrical device configured to store electrical power from the electrical power module 104 for powering the bi-stable optical switches 1102, 1104, as described above. Following a switch of optical signals between the working line W and the protection line P, the electrical hold-off element takes a pre-determined recharge time period during which a further switch of optical signals between the working line and the protection line is not possible.

[0105] The optical monitoring apparatus 1108 comprising a first monitoring channel 130 configured to detect upstream light received from the working line W and a second monitoring channel 132 configured to detect upstream light received from the protection line P. Each monitoring channel comprises an optical tap 112, 114 configured to tap off a portion of an optical signal received from the respective upstream optical fibre to form a monitoring signal, and photodetectors 120, 122 configured to output detection signals responsive to detecting upstream light received from the respective line W, P.

[0106] In certain embodiments, the optical switches are driven by a logic circuit 700, as described above.

[0107] In certain embodiments, the control circuitry 1106 is operative to control the switching of the first bi-stable optical switch 1 102 and the second bi-stable optical switch 1104 based on a current condition of the optical switch apparatus, whether or not light is received from the working line, and whether or not light is received from the protection line. The control circuitry 1106 is operative to output a control signal to the first bi-stable optical switch 1102 and the second bi-stable optical switch 1104 responsive to a Yes decision and electrical power being provided from an electrical power module. The control signal is configured to cause the first bistable optical switch 1102 and the second bi-stable optical switch 1104 to switch from the current condition to the other of the first condition or the second condition.

[0108] Advantageously, the optical protection switching equipment 1100 may be positioned anywhere in the optical network because it does not require power supply and management by active network elements. This flexibility allows for a wide range of deployment scenarios and network configurations.

[0109] The optical protection switching equipment 1 100 may advantageously be independent of the bit rate, and it may be used with colored or grey optical interfaces. This flexibility can provide advantages in various applications, such as single fiber working (SFW) and dual fiber working (DFW) applications. The optical protection switching equipment 1100 may also offer improved insertion loss, potentially saving more than 2 dB compared to traditional solutions.

[0110] The optical protection switching equipment 1100 may be used in a fully passive architecture, making it suitable for environments like Radio Access Networks (RAN) where cost sensitivity is a concern. The optical protection switching equipment 1100 may also be used in datacenter scenarios to protect valuable gray 200G / 400G signals. The disclosed optical protection switching equipment 1100 and associated protection scheme may provide a level 0 protection scheme with no management by the generic active unit, potentially simplifying the design of the switch / router in the data centre scenario.

[0111] The two optical bi-stable switches can assume only two positions synchronously, both on A or both on B. The switches are driven by a “Switch Circuit” which bases the commutation decision on the status of the switches itself and on the presence / absence of light on the receiver side of the two working and protection lines, monitored through the two monitoring taps, one per each receiver line, which monitor the signal coming from the opposite sides of the optical links (worker and protection).

[0112] Also in this case, the monitor taps split ratio can vary in general according to the application, and a 90:10 splitting ratio is generally enough.

[0113] In this DFWscenario, the light tapped is sent directly to the photodetector to reveal the presence or absence of upstream light from the relevant path; no optical filter is required since there is no potential for reflected downstream optical signals.

[0114] Similar considerations on the attenuation of the express channel can be repeated also in this case, but here the advantage is also greater in fact, per each direction (Tx and Rx) only one Tap is necessary, and the express insertion loss of the optical protection switching equipment 1100 is therefore less than 1.6 dB, that means a total of 3.2 dB instead of 5.7 dB of the conventional solution constituted by an Optical Protection Switching on one side and a Splitter unit on the opposite side of the protected line.

[0115] Referring now to Figure 12, a dual fibre working, DFW, optical link 1200 according to an embodiment is depicted. In some aspects, the optical link 1200 comprises a first downstream common optical line 1202 and a first upstream common optical line 1204 forming a first end of the optical link, a second downstream common optical line 1206 and a second upstream common optical line 1208 forming a second end of the optical link, a downstream working optical line 1210 and an upstream working optical line 1212, and a downstream protection optical line 1214 and an upstream protection optical line 1216.

[0116] The optical link 1200 further comprises a first communication network optical protection switching equipment 1100(1) and a second communication network optical protection switching equipment 1100(2). The first optical protection switching equipment 1100(1) is connected on one side to the first downstream common optical line 1202 and the first upstream common optical line 1204 and is connected on the other side to the downstream working optical line 1210, the upstream working optical line 1212, the downstream protection optical line 1214, and the upstream protection optical line 1216. The second optical protection switching equipment 1100(2) is connected on one side to the downstream working optical line 1210, the upstream working optical line 1212, the downstream protection optical line 1214, and the upstream protection optical line 1216 and is connected on the other side to the second downstream common optical line 1206 and the second upstream common optical line 1208.

[0117] The optical link 1200 further comprises a first multiplexer / demultiplexer, Mux / Demux, 1218 and a second Mux / Demux 1220. The first Mux / Demux 1218 is for multiplexing / demultiplexing optical signals to and from the first downstream common optical line 1202 and the first upstream common optical line 1204. The second Mux / Demux 1220 is for multiplexing / demultiplexing optical signals to and from the second downstream common optical line 1206 and the second upstream common optical line 1208.

[0118] The optical protection switching equipment 1100(1), 1100(2) may be independent ofthe bit rate, and it may be used with coloured or grey optical interfaces. This may provide flexibility in the operation of the optical link 1200, as it can accommodate various types of optical signals.

[0119] The optical link 1200 may provide a passive solution for optical path protection in optical communication systems. The optical protection switching equipment 1100(1), 1100(2) may be powered by a portion of the optical power of all channels multiplexed in the fiber of the optical link 1200, thus eliminating the dependency on an external power source. This may provide advantages in terms of cost and deployment flexibility.

[0120] The same type of optical protection switching equipment 1100(1), 1100(2) may be used at both ends of the optical link 1200 to be protected against accidental breaks or unwanted disconnections. The solution may be self-contained and may not require management by external active units. This may provide a reliable and efficient protection mechanism for optical communication systems.

[0121] The overall switching time, that is the time needed for a complete switch from the working path to the protection path, is the time required by the pair of optical protection switching equipments 1100 (at each end of the optical link) to switch: provided they are both “ready to switch” (that is both charged, which is the normal operating condition) in the worst case we can assume a switching time of 20ms (i.e. 10ms+10ms). Protection switching operation of the optical link 1200 may be described referring to the switching decision matrix of Figure 7 and the transition state machine of Figure 8, as described above.

[0122] Referring to Figure 13, an embodiment provides optical protection switching equipment 1300 for switching optical signals between a working line, W, and a protection line, P. The equipment 1300 comprises an electrical power module 1304, control circuitry 1306 and a switch 1302. The switch 1302 is between the working line and the protection line. The electrical power module 1304 is operative to provide electrical power to the control circuitry 1306. The electrical power module comprises an O-E converter configured to convert a part of a received optical signal into electrical power.

[0123] The control circuitry comprises an electrical hold-off element comprising a passive electrical device. The passive electrical device is configured to store electrical power from the electrical power module for powering the switch 1302. Following a switch of optical signals between the working line and protection line, the electrical hold-off element takes a pre-determined recharge time period during which a further switch of optical signals between the working line and the protection line is not possible.

[0124] An embodiment provides an optical link 1400, as illustrated in Figure 14. The optical link comprises a first common optical line 1402 forming a first end of the optical link, a second common optical line 1404 forming a second end of the optical link, a working optical line 1406, and a protection optical line 1408. The optical link further comprises a first communication network optical protection switching equipment 100(1) and a second communication network optical protection switching equipment 100(2). The first communication network optical protection switching equipment 100(1) is connected on one side to the first common optical line and connected on the other side to the working optical line and the protection optical line. The second communication network optical protection switching equipment 100(2) is connected on one side to the working optical line and the protection optical line and connected on the other side to the second common optical line.

[0125] Referring to Figure 15, an embodiment provides a method 1500 for switching optical signals between a working line and a protection line. The method comprises converting 1502 a part of a received optical signal into electrical power. The electrical power is stored 1504 for powering a switch between the working line and the protection line. The method further comprises, following a switch of optical signals between the working line and protection line, charging 1506 an electrical hold-off element (e.g. charging the passive electrical device, for example a capacitor) for a pre-determined recharge time period. During the recharge time period a further switch of optical signals between a working line and a protection line is not possible.

[0126] In certain embodiments, the method further comprises detecting upstream light received from the working line and detecting upstream light received from the protection line. The method further comprises making one of a Yes decision to switch the switch or a No decision not to switch the switch based on a current condition of the switch, whether or not there is detection of upstream light received from the working line, and whether or not there is detection of upstream light received from the protection line. A control signal is output to the switch responsive to receiving a Yes decision and electrical power being provided to the switch. The control signal is configured to cause a switch of optical signals between the working line and protection line.

[0127] In certain embodiments, the step of detecting upstream light received from the working line comprises tapping off a portion of an optical signal received from the working line to form a working line monitoring signal, Mw, and outputting a first detection signal responsive to detecting a working line monitoring signal, Mw, at defined upstream wavelengths.

[0128] The step of detecting upstream light received from the protection line comprises tapping off a portion of an optical signal received from the protection line to form a protection line monitoring signal, Mp, and outputting a second detection signal responsive to detecting a protection line monitoring signal, Mp, at defined upstream wavelengths.

[0129] The step of making one of a Yes decision to switch the switch or a No decision not to switch the switch is based on a current condition of the switch, whether or not a first detection signal is received for the working line, and whether or not a second detection signal is received for the protection line.

Claims

CLAIMS1. Optical protection switching equipment (100; 1100; 1300) for switching optical signals between a working line and a protection line, the equipment comprising: an electrical power module (104; 204; 1304) operative to provide electrical power to control circuitry (106; 206; 1306), the electrical power module comprising an optical-to- electrical converter (204) configured to convert a part of a received optical signal into electrical power; the control circuitry (106; 1106; 1306) comprising an electrical hold-off element comprising a passive electrical device (208), wherein the passive electrical device is configured to store electrical power from the electrical power module for powering a switch (102; 710; 1102; 1104; 1302) between the working line (W) and the protection line (P); wherein, following a switch of optical signals between the working line and protection line, the electrical hold-off element takes a pre-determined recharge time period during which a further switch of optical signals between a working line and a protection line is not possible.

2. Equipment as claimed in claim 1 , wherein the electrical hold-off element further comprises a threshold circuit (210) configured to prevent electrical power output to the control circuitry for the pre-determined recharge time period.

3. Equipment as claimed in claim 2, wherein the passive electrical device (208) comprises a capacitor and the threshold circuit (210) is operative to prevent electrical power output to the control circuitry until the capacitor has reached a threshold voltage.

4. Equipment as claimed in claim 3, wherein the passive electrical device further comprises a recharge circuit (212) for the capacitor and the recharge circuit is configured to recharge the capacitor to the threshold voltage in the pre-determined recharge time period, wherein the threshold voltage is sufficient to power the control circuitry.

5. Equipment as claimed in claim 4, wherein the recharge circuit comprises a variable resistor (Ri) and wherein a resistance of the variable resistor is configured to cause the capacitor to charge to the threshold voltage in the pre-determined recharge time period.

6. Equipment as claimed in claim 1 , further comprising optical switch apparatus reconfigurable to have one of a first condition (A) for connecting to the working line (W) or a second condition (B) for connecting to the protection line (P).

7. Equipment as claimed in any one of claims 1 to 6, further comprising optical monitoring apparatus comprising: a first monitoring channel (130) configured to detect upstream light received from the working line; and a second monitoring channel (132) configured to detect upstream light received from the protection line.

8. Equipment as claimed in claim 7, wherein each monitoring channel comprises: an optical tap (112; 114) configured to tap off a portion of an optical signal received from the respective line to form a monitoring signal; and optical detection apparatus (116; 118; 120; 122) configured to output detection signals responsive to detecting monitoring signals at defined upstream wavelengths.

9. Equipment as claimed in claim 8, wherein the working line (W) comprises a first optical fibre for transmission of both downstream optical signals and upstream optical signals and the protection line (P) comprises a second optical fibre for transmission of both downstream optical signals and upstream optical signals.

10. Equipment as claimed in claim 9, wherein the optical switch apparatus comprises a bistable optical switch (102) for switching optical signals between the working line (W) and protection line (P).11 . Equipment as claimed in claim 8, wherein the working line comprises a first downstream optical fibre (WTX) for transmission of downstream optical signals and a first upstream optical fibre (WRX) for transmission of upstream optical signals, and the protection line comprises a second downstream optical fibre (PTX) for transmission of downstream optical signals and a second upstream optical fibre (PRX) for transmission of upstream optical signals, and wherein each monitoring channel comprises: an optical tap (112; 114) configured to tap off a portion of an optical signal received from the respective upstream optical fibre to form a monitoring signal; and optical detection apparatus (120; 122) configured to output a detection signal responsive to detecting a monitoring signal.

12. Equipment as claimed in claim 10, wherein the optical switch apparatus comprises a first bi-stable optical switch (1102) and a second bi-stable optical switch (1104), wherein the first bi-stable optical switch is for switching between the first downstream optical fibre (WTX) and the second downstream optical fibre (WRX), and the second bistable optical switch is for switching between the first upstream optical fibre (PRX) and the second upstream optical fibre (PTX).

13. Equipment as claimed in any one of claims 8 to 12, and wherein an optical detection apparatus comprises an optical filter (116; 118) configured to receive the monitoring signal and configured to pass light at defined upstream wavelengths and a photodetector (120; 122) configured to output a detection signal responsive to detecting light passed by the optical filter.

14. Equipment as claimed in claim 8 and claim 6, wherein the control circuitry comprises: a logic circuit (700) configured to make one of a Yes decision to switch the optical switch apparatus or a No decision not to switch the optical switch apparatus based on a current condition of the optical switch apparatus, whether or not a detection signal is received from the optical detection apparatus for the working line, and whether or not a detection signal is received from the optical detection apparatus for the protection line; and a driver circuit (708) configured to output a control signal to the optical switch apparatus responsive to receiving a Yes decision and electrical power being provided from the electrical power module, the control signal configured to cause the optical switch apparatus to switch from the current condition to the other of the first condition or the second condition.

15. Equipment as claimed in claim 8 and claim 6, wherein the control circuitry (900) comprises interface circuitry (902), at least one processor (904) and memory (906) comprising instructions (908) executable by said processor whereby the control circuitry is operative to: make one of a Yes decision to switch the optical switch apparatus or a No decision not to switch the optical switch apparatus based on a current condition of the optical switch apparatus, whether or not light is received from the working line, and whether or not light is received from the protection line; and output a control signal to the optical switch apparatus responsive to a Yes decision and electrical power being provided from the electrical power module, the control signal configured to cause the optical switch apparatus to switch from the current condition to the other of the first condition or the second condition.

16. An optical link (1000; 1200; 1400) comprising: a first common optical line (1002; 1202; 1204; 1402) forming a first end of the optical link; a second common optical line (1004; 1206; 1208; 1404) forming a second end of the optical link; a working optical line (1006; 1210; 1212; 1406); a protection optical line (1008; 1214; 1216; 1408);a first communication network optical protection switching equipment (100(1); 1100(1)) as claimed in any one of claims 1 to 14 connected on one side to the first common optical line and connected on another side to the working optical line and the protection optical line; and a second communication network optical protection switching equipment (100(2);1100(2)) as claimed in any one of claims 1 to 14 connected on one side to the working optical line and the protection optical line and connected on another side to the second common optical line.

17. A method (1500) for switching optical signals between a working line and a protection line, the method comprising steps of:- converting (1502) a part of a received optical signal into electrical power;- storing (1504) the electrical power for powering a switch between the working line and the protection line; and- following a switch of optical signals between the working line and protection line, charging an electrical hold-off element (1506) for a pre-determined recharge time period during which a further switch of optical signals between a working line and a protection line is not possible.

18. A method as claimed in claim 17, further comprising steps of:- detecting upstream light received from the working line;- detecting upstream light received from the protection line;- making one of a Yes decision to switch the switch or a No decision not to switch the switch based on a current condition of the switch, whether or not there is detection of upstream light received from the working line, and whether or not there is detection of upstream light received from the protection line; and- outputting a control signal to the switch responsive to receiving a Yes decision and electrical power being provided to the switch, the control signal configured to cause a switch of optical signals between the working line and protection line.

19. A method as claimed in claim 18, wherein the steps of detecting upstream light comprise:- tapping off a portion of an optical signal received from the respective line to form a monitoring signal; and- outputting detection signals responsive to detecting monitoring signals at defined upstream wavelengths; and wherein making one of a Yes decision to switch the switch or a No decision not to switch the switch is based on a current condition of the switch, whether or not adetection signal is received for the working line, and whether or not a detection signal is received for the protection line.

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