GB2550401A

GB2550401A - Limiting optical power in aircraft ignition risk zones

Info

Publication number: GB2550401A
Application number: GB1608833.8A
Authority: GB
Inventors: Cipullo Alessio
Original assignee: Airbus Operations Ltd
Current assignee: Airbus Operations Ltd
Priority date: 2016-05-19
Filing date: 2016-05-19
Publication date: 2017-11-22
Also published as: GB201608833D0

Abstract

An optical fibre assembly in an aircraft including a second portion 10B being in an ignition risk zone 20 of the aircraft, and an optical fuse assembly 100 optically coupled to the optical fibre. The assembly has an optical fuse (110, figure 2B) receiving an optical threat signal from a first portion 10A of the optical fibre and transmitting the signal, if it is lower than a threshold optical power to an optical attenuator (120, figure 2B) to attenuate the signal and transmit it to the second portion of the optical fibre 10B. The attenuator may have a variable attenuation constant and the optical fuse may have a non-varying threshold. The attenuator and fuse may be passive devices that are separately replaceable sub-assemblies. The fuse may have a transmitting configuration and may change irreversibly to a non-transmitting configuration. The optical fibre may carry optical data from a sensor coupled to the second portion to an optical interrogator coupled to the first portion. The ignition risk zone may be a fuel tank of an aircraft. The invention may also be an aircraft with one or more of the optical fibre assemblies.

Description

Limiting Optical Power in Aircraft Ignition Risk Zones

FIELD OF THE INVENTION

[001] The present invention relates to an assembly and method for limiting the optical power level of optical signals transmitted into optical fibre in an aircraft ignition risk zone such as a fuel tank

BACKGROUND OF THE INVENTION

[002] In modern aircraft data is increasingly transmitted via optical fibres rather than more traditional cabling. An example is fuel quantity measurement systems in which sensors within a wing fuel tank transmit data concerning fuel quantity to an optical interrogator within the aircraft fuselage via optical fibres.

[003] Areas of an aircraft that present an ignition risk, such as fuel tanks in the wings or fuselage, must comply with Intrinsic Safety requirements such as FAR 25.981 and FAA AC 25.981-1C to ensure that in all anticipated situations the energy transferred into such ignition risk zones is well below the energy required to initiate an explosion. For example, ignition risk zones must be protected against a potential ignition threat posed by lightning strikes to the aircraft.

SUMMARY OF THE INVENTION

[004] The present invention is concerned with ensuring that the optical power that can be transmitted via an optical fibre into an ignition risk zone of an aircraft is always below a safe optical power limit or irradiance limit.

[005] A first aspect of the invention provides an optical fibre assembly in an aircraft, the assembly including: an optical fibre having a first portion and a second portion, the second portion being in an ignition risk zone of the aircraft; and an optical fuse assembly optically coupled to the optical fibre, the optical fuse assembly comprising: an optical fuse arranged to receive an optical threat signal from the first portion of the optical fibre and transmit the optical threat signal only if it is lower than a threshold optical power; and an optical attenuator arranged to receive the optical threat signal transmitted by the optical fuse, attenuate the received optical threat signal, and transmit the attenuated optical threat signal to the second portion of the optical fibre.

[006] In this way, the maximum optical power that can enter the optical fibre within an ignition risk zone such as a fuel tank can be controlled. Moreover, the assembly according to the invention provides an intrinsically safe means of achieving such control.

[007] Another advantage is that it is possible to achieve a variety of different maximum optical power output levels for multiple such optical fibre assemblies across the aircraft by using identical optical fuses with the same optical power thresholds, and varying only the configuration of the optical attenuators to achieve different attenuation constants. This is particularly advantageous because it serves to reduce the overall bill of materials for the aircraft and reduces complexity in the aircraft assembly.

[008] At the time of writing there is some uncertainty about the level at which the maximum safe optical power limit for aircraft ignition risk zones will ultimately be set. It is envisaged that future changes may be made to the Intrinsic Safety requirements issued by aircraft certification authorities (e.g. the Civil Aviation Authority and Federal Aviation Administration) concerning the maximum safe optical power limit. It is therefore important that the present invention enables the maximum optical power output level of the optical fuse assembly to be adjustable. As discussed above, this adjustment is achieved by simply varying the attenuation constant of the optical attenuator, while making no changes to the optical fuse.

[009] It is envisaged that an maximum safe continuous wave optical power limit for aircraft ignition risk zones such as a fuel tank may be considered to be in the range 15 to 150 mW, which equates to approximately 11.7 to 21.7 dBm (decebel-milliwatts). Thus, an appropriate optical power threshold for the optical fuse may be in the range 10 to 30 dBm. Similarly, an appropriate attenuation constant for the optical attenuator may be in the range 0 to 15 dBm.

[010] The arrangement of the optical fuse assembly whereby optical threat signals are transmitted first to the optical fuse and then to the optical attenuator before being transmitted to the second portion of the fibre within the ignition risk zone is especially important. The reverse arrangement, in which the optical attenuator receives optical threat signals before the optical fuse, would not be effective because the maximum output power that could enter the ignition risk zone would be controlled solely by the optical fuse threshold.

[011] In preferred embodiments the optical attenuator has a variable attenuation constant.

[012] Thus, the overall maximum power output of the optical fuse assembly may be adjusted easily by simply varying the attenuation constant of the attenuator. This arrangement serves to further reduce the bill of materials for the aircraft, and to further reduce aircraft assembly and maintenance complexity, since only one specification of optical attenuator should be required for all the optical fibre assemblies of the aircraft. Moreover, if the requirements for the overall maximum optical power output of the optical fuse assembly change during the life of the aircraft, for example because of a change in intrinsic safety requirements, this can be easily achieved simply by adjusting the attenuation constant of the attenuator.

[013] In further preferred embodiments the threshold optical power of the optical fuse is not variable. Thus, in embodiments in which the optical fuse is a consumable part (i.e. to be replaced after its optical power threshold has been exceeded) the maintenance task of replacing blown optical fuses is much simplified if there is only one specification of optical fuse used throughout the aircraft.

[014] The optical attenuator and optical fuse preferably each comprise a separately replaceable sub-assembly. That is, the optical attenuator and optical fuse are preferably not integrated into a single device with a single input port and single output port, but are instead separate devices each with their own input and output ports. This feature is particularly desirable in embodiments in which the optical fuse is a consumable part (i.e. to be replaced after its optical power threshold has been exceeded), since it would clearly be undesirable to discard an optical fuse assembly with a functioning optical attenuator part.

[015] The optical attenuator and optical fuse are preferably passive devices. That is, they are preferably devices which do not require additional power for their operation. This feature provides an additional degree of intrinsic safety, since power failure need not be considered as a risk.

[016] In preferred embodiments the optical fuse is initially set to a transmitting configuration in which it permits transmission of optical signals to the attenuator and is automatically switched to a non-transmitting configuration in which it prevents transmission of optical signals to the attenuator when the threshold optical power is exceeded. Thus, the optical fuse requires no further input beyond the optical threat signal itself to prompt immediate and automatic switching to the non-transmitting configuration.

[017] The optical fuse may be irreversibly switched to the non-transmitting configuration when the threshold optical power is exceeded. That is, the optical fuse may be a consumable, or replaceable, part which must be discarded and replaced once the threshold optical power has been exceeded.

[018] The optical attenuator may comprise an attenuating component that is arranged to reduce an optical power level of a received optical signal.

[019] The threshold optical power measured in decibel-milliwatts (dBm) is preferably higher than an attenuation constant of the optical attenuator measured in decibel-milliwatts (dBm). Thus, the maximum power output of the optical fuse assembly can potentially be varied between 0 mW and the threshold optical power.

[020] The optical fibre may be arranged to carry an optical data signal from a sensor optically coupled to the second portion to an optical interrogator optically coupled to the first portion. That is, the primary purpose of the optical fibre may be to carry data from the ignition risk zone to an optical interrogator in, e.g. the avionics bay of the aircraft.

[021] In preferred embodiments the ignition risk zone comprises a fuel tank of the aircraft.

[022] A second aspect of the invention provides an aircraft comprising one or more optical fibre assemblies according to the first aspect.

[023] A third aspect of the invention provides a method of controlling transmission of an optical threat signal to a second portion of an optical fibre in an ignition risk zone of the aircraft, the method including the steps of: providing an optical fuse assembly comprising an optical fuse and an optical attenuator optically coupled to the optical fibre; receiving an optical threat signal at the optical fuse; transmitting the optical threat signal from the optical fuse to the optical attenuator only if the optical threat signal has an optical power below a threshold optical power; attenuating the transmitted optical threat signal by the optical attenuator; and transmitting the attenuated optical threat signal to the second portion of the optical fibre.

[024] The method of the third aspect may be carried out with the optical fuse assembly of the first aspect, and/or in the aircraft of the second aspect.

[025] Like the first aspect, the method according to the third aspect enables the maximum optical power that can enter the optical fibre within an ignition risk zone such as a fuel tank to be controlled. Moreover, the method provides an intrinsically safe means of achieving such control.

[026] Another advantage is that it is possible to achieve a variety of different maximum optical power output levels for multiple such optical fibre assemblies across the aircraft by using identical optical fuses with the same optical power thresholds, and varying only the configuration of the optical attenuators to achieve different attenuation constants. This is particularly advantageous because it serves to reduce the overall bill of materials for the aircraft and reduces complexity in the aircraft assembly.

[027] The method may include varying an attenuation constant of the optical attenuator to achieve a desired maximum optical power of the attenuated optical threat signal.

[028] Thus, the overall maximum power output of the optical fuse assembly may be adjusted easily by simply varying the attenuation constant of the attenuator. This arrangement serves to further reduce the bill of materials for the aircraft, and to further reduce aircraft assembly and maintenance complexity, since only one specification of optical attenuator should be required for all the optical fibre assemblies of the aircraft. Moreover, if the requirements for the overall maximum optical power output of the optical fuse assembly change during the life of the aircraft, for example because of a change in intrinsic safety requirements, this can be easily achieved simply by adjusting the attenuation constant of the attenuator.

[029] In further preferred embodiments the method comprises maintaining the threshold optical power of the optical fuse at a constant level. Thus, in embodiments in which the optical fuse is a consumable part (i.e. to be replaced after its optical power threshold has been exceeded) the maintenance task of replacing blown optical fuses is much simplified if there is only one specification of optical fuse used throughout the aircraft.

[030] The optical fuse is preferably initially set to a transmitting configuration in which it permits transmission of optical signals to the attenuator and automatically switched to a non-transmitting configuration in which it prevents transmission of optical signals to the attenuator only if the optical threat signal has an optical power exceeding the threshold optical power. Thus, the optical fuse requires no further input beyond the optical threat signal itself to prompt immediate and automatic switching to the non-transmitting configuration.

[031] In some embodiments the optical threat signal has an optical power exceeding the threshold optical power, and the method includes the step of receiving the optical threat signal at the optical fuse and irreversibly switching the optical fuse to the nontransmitting configuration. That is, the optical fuse may be a consumable, or replaceable, part which must be discarded and replaced once the threshold optical power has been exceeded.

[032] In such embodiments the method may include replacing the optical fuse with an identical optical fuse set to the transmitting configuration, without replacing the optical attenuator. That is, the optical attenuator and optical fuse are preferably not integrated into a single device with a single input port and single output port, but are instead separate devices each with their own input and output ports. This feature is particularly desirable in embodiments in which the optical fuse is a consumable part (i.e. to be replaced after its optical power threshold has been exceeded), since it would clearly be undesirable to discard an optical fuse assembly with a functioning optical attenuator part.

[033] The method may include transmitting an optical data signal from a sensor optically coupled to the second portion to an optical interrogator optically coupled to the first portion. That is, the primary purpose of the optical fibre may be to carry data from the ignition risk zone to an optical interrogator in, e.g. the avionics bay of the aircraft.

BRIEF DESCRIPTION OF THE DRAWINGS

[034] Embodiments of the invention will now be described with reference to the accompanying drawings, in which: [035] Figure 1 is a schematic illustration of an optical threat to an ignition risk zone of an aircraft; [036] Figure 2A is a schematic illustration of an attenuated optical fuse assembly according to an embodiment of the invention, and Figure 2B is a detail view of that optical fuse assembly; [037] Figure 3 is a plan view of an aircraft showing in schematic form an optical fibre assembly according to an embodiment of the invention; and [038] Figures 4A and 4B are graphs showing the relationship between output power and input power for an optical fuse (Figure 4A) and an attenuated optical fuse assembly according to embodiments of the invention (Figure 4B). DETAILED DESCRIPTION OF EMBODIMENT(S) [039] Figure 1 schematically illustrates a potential problem that is addressed by the present invention. An optical fibre 10 extends between an optical sensor 12 in a wing fuel tank 20 of an aircraft, and an optical interrogator 14 with an optical source 16 in the fuselage 30 of the aircraft. Under normal operation, optical signals are used to send data from the sensor 12 to the interrogator 14. However, in some circumstances it is conceivable that light travelling in the optical fibre 10 may leak into the fuel tank 20 via the sensor 12 (as indicated by reference 18) or via a damaged portion of the optical fibre 10 in the fuel tank 20.

[040] The optical source 16 typically has a limited optical power output to attempt to control the power level of light leaking into the fuel tank, but nonetheless there may be modes by which such leaking light may be uncontrolled. For example, a lightning strike 40 to the aircraft may cause light with a high optical power to enter the optical fibre 10 via an unprotected portion of the fibre such as a portion in which the protective coating has been damaged and which passes through an unprotected part of the wing leading or trailing edge. Alternatively, light with a high optical power may derive from a faulty optical source in the optical fibre assembly, for example in the avionics bay of the fuselage.

[041] There is therefore a desire to provide an intrinsically safe means by which to prevent such leaking light from having a sufficiently high optical power to present an ignition risk in ignition risk zones of aircraft such as fuel tanks. Packages of optical power which may cause leaking light in an ignition risk zone will be referred to herein as optical threat signals. It should be understood that such optical threat signals may comprise modulated or unmodulated light, and may derive from any source of light including, but not limited to, lightning strikes to the aircraft and faulty optical sources or other faulty optical equipment.

[042] A solution proposed by the present inventor is illustrated in Figs. 2A and 2B. In this embodiment an attenuated optical fuse assembly 100 is optically coupled between a first portion 10A of the optical fibre 10 which extends between the location at which the fibre crosses the boundary of the fuel tank 20 and the interrogator 14, and a second portion 10B of the optical fibre 10 which extends from the first portion 10A into the fuel tank 20. In this embodiment the attenuated optical fuse assembly 100 is located outside the fuel tank 20, preferably on the boundary of the fuel tank 20, i.e. just outside the ignition risk zone. For example, the attenuated optical fuse assembly 100 may be fixed to the dry side of a fuel tank boundary such as a wing spar or rib. In other embodiments the attenuated optical fuse assembly 100 may be located inside the fuel tank 20.

[043] The attenuated optical fuse assembly 100 comprises an optical fuse 110 optically coupled to a variable optical attenuator 120, the optical fuse being optically coupled to the first portion 10A of the fibre and the variable optical attenuator 120 being optically coupled to the second portion 10B of the fibre. The optical fuse 110 and the optical attenuator 120 are thus arranged so that the attenuator 120 is optically coupled between the fuse 110 and the second portion 10B of the fibre in the fuel tank 20. This arrangement ensures that an optical threat signal 200 transmitted via the first portion 10A of the fibre first encounters the optical fuse 110 and then the optical attenuator 120, before being transmitted to the second portion 10B of the fibre as an attenuated optical threat signal 210.

[044] The optical fuse 110 is a passive component (i.e. non-powered) that has a region, layer, or thin film of material between an input port and output port that is substantially transparent when the input power is below a given optical power threshold and substantially opaque above that threshold. Thus, the output power is zero, or close to zero, when the input power exceeds the optical power threshold. This is illustrated in Fig. 4A, which shows the power drop to zero, or near zero, at the optical power threshold, identified as T. The maximum output power, Pmax, is thus equal to the optical power threshold, T, which equates to the maximum input power.

[045] Known optical fuses are single use products, since once the optical power threshold is exceeded it is not possible to return the material from opaque to transparent. Thus, once the threshold is exceeded the optical fuse 110 must be discarded and replaced.

[046] Examples of an appropriate optical fuse are described in US8463090. A suitable optical fuse for embodiments of the invention is the Molex™ 86560 product. That product is based on a non-linear, power-induced light scattering phenomenon created by a nanostructure thin film placed in the light path. When the optical power threshold is overcome the thin film becomes opaque and it does not either transmit or reflect significant optical power. Hence this is a fully passive optical device.

[047] The variable optical attenuator 120 is also a passive device. The attenuator 120 acts to attenuate (i.e. reduce the optical power of) a signal received at its input port and output the attenuated signal to the second portion 10B of the optical fibre. The attenuator 120 includes a means by which the attenuation constant, C, (i.e. the degree to which the optical power is reduced) can be varied. For example, the attenuator may include an adjustment screw that can be manually rotated to vary the attenuation constant. The variable optical attenuator 120 may be step-wise variable (i.e. variable by given increments, or steps) or continuously variable.

[048] Variable optical attenuators typically include a lens to collimate light from an input fibre, and a blocking device or window can be manually adjusted by a screw to vary the attenuation constant. In narrowband attenuators a blocking device is movable across the direction of light travel to block a desired proportion of the light transmitted to a second lens that couples light into the output fibre. In broadband attenuators a window is rotated relative to the direction of light travel to cause a beam path deviation that changes the coupling efficiency into the output fibre via the second lens. Suitable variable optical attenuators are produced by Thorlabs™.

[049] Fig. 4B illustrates the relationship between input power and output power for the whole attenuated optical fuse assembly 100, i.e. the combined effect provided by the optical fuse 110 and optical attenuator 120. The line Ci shows the relationship between input power and output power when the attenuation constant, Ci, of the attenuator is set to 0 dBm (decebel-milliwatts), line C2 when the attenuation constant, C2, is 5 dBm, line C3 when the attenuation constant, C3, is 10 dBm, and line C4 when the attenuation constant, C4, is 15 dBm.

[050] It can be seen that, although the optical power threshold, T, of the optical fuse 110 remains the same, the maximum output power of the attenuated optical fuse assembly 100, Pmax, is reduced by an amount proportional to the attenuation constant.

[051] The maximum output power, Pmax, of the attenuated optical fuse assembly 100, measured in dBm, can be calculated as follows:

[052] Pmax — T - C

[053] Where T is the optical power threshold of the optical fuse 110 measured in dBm, and C is the attenuation constant of the optical attenuator 120 measured in dBm.

[054] Thus, for attenuation constant Ci, which is 0 dBm (i.e. no attenuation), the maximum output power Pmaxi is equal to the optical power threshold T. For attenuation constant C2, which is 5 dBm, the maximum output power Pmax2 is equal to T - 5 dBm. Similarly, Pmax3 is equal to T - 10 dBm, and Pmax4 is equal to T - 15 dBm.

[055] In this way, the maximum output power of the attenuated optical fuse assembly 100 may be controlled by simply varying the attenuation constant of the optical attenuator 120, without it being necessary to make any changes to the optical fuse 110.

[056] An appropriate optical fuse 110 for use in embodiments of the invention has an optical power threshold of approximately 25 dBm (decebel-milliwatts), which equates to approximately 316 mW. Thus, in this example T is equal to 25 dBm.

[057] The maximum output power, Pmax, of the attenuated optical fuse assembly 100 can thus be varied between 25 dBm and 10 dBm by varying the attenuation constant C between Ci and C4. That is, attenuation constant Ci provides zero attenuation and therefore provides a maximum output power Pmaxi of 25 dBm. Attenuation constant C2 provides a 5 dBm attenuation and therefore provides a maximum output power Pmax2 of 20 dBm. Similarly, attenuation constant C3 provides a 10 dBm attenuation and therefore provides a maximum output power Pmax3 of 15 dBm, and attenuation constant C4 provides a 15 dBm attenuation and therefore provides a maximum output power P,nax4 of 10 dBm.

[058] In use, an optical threat signal enters the first portion 10A of the optical fibre (by means of a lightning strike, faulty optical component, or other optical threat as discussed above) and travels along the optical fibre to the attenuated optical fuse assembly 100. If the optical power of the optical threat signal exceeds the optical power threshold, T, then the optical fuse 110 will change state (i.e. become opaque) such that the optical threat signal is not transmitted to the optical attenuator 120. Alternatively, if the optical power of the optical threat signal is below the optical power threshold, T, then the optical threat signal will be transmitted by the optical fuse 110 to the optical attenuator 120.

[059] An optical threat signal received by the optical attenuator 120 therefore always has an optical power that is equal to or less than the optical power threshold T. The attenuator 120 attenuates the optical threat signal to the degree set by the attenuation constant C. Thus, the optical threat signal that is transmitted to the second portion 10B of the optical fibre, and thus into the fuel tank ignition risk zone, always has a maximum optical power, Pmax, of T minus C.

[060] In this way, the maximum optical power output to an ignition risk zone such as a fuel tank can be controlled by simply varying the attenuation constant of the optical attenuator 120 in the attenuated optical fuse assembly 100.

[061] In other embodiments the variable optical attenuator 120 may be substituted with a non-variable optical attenuator, i.e. an optical attenuator with a non-variable attenuating constant, also referred to as a fixed annuator. In such embodiments the attenuating constant of the attenuator is selected so that the overall optical power threshold of the attenuated optical fuse assembly 100 is at a desired level. Thus, if there are different requirements for overall optical power threshold for different applications across the aircraft the same optical fuse can be used, but an optical attenuator selected from a plurality of fixed optical attenuators having different attenuation constants according to the particular requirements of the application.

[062] Fig. 3 shows how the optical fibre assembly of Figs. 2A and 2B may be applied to an aircraft 60. In an outer fuel tank 20' an optical sensor 12' is optically connected to an attenuated optical fuse assembly 100' via a second portion 10B' of an optical fibre, and a first portion 10A' of the optical fibre is optically connected between the attenuated optical fuse assembly 100' and the interrogator 14 in the fuselage 30. Similarly, in an inner fuel tank 20" another optical sensor 12" is optically connected to another attenuated optical fuse assembly 100" via a second portion 10B" of another optical fibre, and a first portion 10A" of the fibre is optically connected between the attenuated optical fuse assembly 100" and the interrogator. The optical sensors 12', 12" may be sensors for detecting fuel levels or fuel temperature, for example. The interrogator 14 includes an optical time domain reflectometry (OTDR) system which is able to detect when the optical fuse 110 becomes opaque.

[063] The attenuated optical fuse assemblies 100', 100" are each located on the dry side of the fuel tank boundary 22', 22" of their respective fuel tanks 20', 20". In this embodiment the fuel tank boundaries 22', 22" are provided by a rear spar of the aircraft wing 50 which separates the wing box 56 from the wing trailing edge 52. The first portions 10A', 10A" of the optical fibres pass through the trailing edge 52 and enter the fuselage 30 at the wing root, and are ultimately optically coupled to the interrogator 14 in the avionics bay. In other embodiments the attenuated optical fuse assemblies 100', 100" may instead be located on the front spar which separates the wing box 56 from the wing leading edge 54, or at any other appropriate fuel tank boundary location.

[064] The first portion 10A', 10A" of the fibre may be particularly vulnerable in the trailing edge 52 or leading edge 54 because movement of the control surfaces of the aircraft such as flaps, slats, ailerons and spoilers (not shown) means that the fibre may be exposed to damage by fretting or by debris thrown up by the aircraft's wheels during take-off or landing, for example. Additionally, when the control surfaces are deployed any damaged portions of the fibre (i.e. portions in which the optical fibre core becomes exposed) or any unprotected optical connectors may be exposed to light ingress from lightning strikes.

[065] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

1. An optical fibre assembly in an aircraft, the assembly including: an optical fibre having a first portion and a second portion, the second portion being in an ignition risk zone of the aircraft; and an optical fuse assembly optically coupled to the optical fibre, the optical fuse assembly comprising: an optical fuse arranged to receive an optical threat signal from the first portion of the optical fibre and transmit the optical threat signal only if it is lower than a threshold optical power; and an optical attenuator arranged to receive the optical threat signal transmitted by the optical fuse, attenuate the received optical threat signal, and transmit the attenuated optical threat signal to the second portion of the optical fibre.

2. An assembly according to claim 1, wherein the optical attenuator has a variable attenuation constant.

3. An assembly according to claim 1 or claim 2, wherein the threshold optical power of the optical fuse is not variable.

4. An assembly according to any of claims 1 to 3, wherein the optical attenuator and optical fuse each comprise a separately replaceable sub-assembly.

5. An assembly according to any of claims 1 to 4, wherein the optical attenuator and optical fuse are passive devices.

6. An assembly according to any of claims 1 to 5, wherein the optical fuse is initially set to a transmitting configuration in which it permits transmission of optical signals to the attenuator and is automatically switched to a non-transmitting configuration in which it prevents transmission of optical signals to the attenuator when the threshold optical power is exceeded.

7. An assembly according to claim 6, wherein the optical fuse is irreversibly switched to the non-transmitting configuration when the threshold optical power is exceeded.

8. An assembly according to any of claims 1 to 7, wherein the optical attenuator comprises an attenuating component that is arranged to reduce an optical power level of a received optical signal.

9. An assembly according to any of claims 1 to 8, wherein the threshold optical power measured in decibel-milliwatts (dBm) is higher than an attenuation constant of the optical attenuator measured in decibel-milliwatts (dBm).

10. An assembly according to any of claims 1 to 9, wherein the optical fibre is arranged to carry an optical data signal from a sensor optically coupled to the second portion to an optical interrogator optically coupled to the first portion.

11. An assembly according to any of claims 1 to 10, wherein the ignition risk zone comprises a fuel tank of the aircraft.

12. An aircraft comprising one or more optical fibre assemblies according to any of claims 1 to 11.

13. A method of controlling transmission of an optical threat signal to a second portion of an optical fibre in an ignition risk zone of the aircraft, the method including the steps of: providing an optical fuse assembly comprising an optical fuse and an optical attenuator optically coupled to the optical fibre; receiving an optical threat signal at the optical fuse; transmitting the optical threat signal from the optical fuse to the optical attenuator only if the optical threat signal has an optical power below a threshold optical power; attenuating the transmitted optical threat signal by the optical attenuator; and transmitting the attenuated optical threat signal to the second portion of the optical fibre.

14. A method according to claim 13, comprising varying an attenuation constant of the optical attenuator to achieve a desired maximum optical power of the attenuated optical threat signal.

15. A method according to claim 13 or claim 14, comprising maintaining the threshold optical power of the optical fuse at a constant level.

16. A method according to any of claims 13 to 15, wherein the optical fuse is initially set to a transmitting configuration in which it permits transmission of optical signals to the attenuator and is automatically switched to a non-transmitting configuration in which it prevents transmission of optical signals to the attenuator only if the optical threat signal has an optical power exceeding the threshold optical power.

17. A method according to claim 16, wherein the optical threat signal has an optical power exceeding the threshold optical power, and the method includes the step of receiving the optical threat signal at the optical fuse and irreversibly switching the optical fuse to the non-transmitting configuration.

18. A method according to claim 17, comprising replacing the optical fuse with an identical optical fuse set to the transmitting configuration, without replacing the optical attenuator.

19. The method according to any of claims 13 to 18, comprising transmitting an optical data signal from a sensor optically coupled to the second portion to an optical interrogator optically coupled to the first portion.