Patent Description:
More specifically, aspects relate to an optical limiter and a method for limiting the radiant flux of an optical source beam.

Optical networks are used to communicate data, encoded in optical signals, over fibre optic cables. To ensure optical signals are successfully transmitted from a source to a destination, without damage to any network components, it can be necessary to limit the radiant flux of optical beams carrying optical signals. This can be achieved using optical limiters.

Optical limiters are devices intended to limit the radiant flux of an optical beam to no more than a predetermined maximum. An idealised graph of input radiant flux versus output radiant flux for a flat maximum optical limiter is shown in <FIG>, wherein the output rises proportional to the input until a maximum output value M is reached, at which point the output is maintained at that level M no matter how much more the input rises.

Optical limiters can for example be formed using materials having negative thermal index coefficients, wherein heat generated by absorption of an optical beam decreases the index of refraction of the material, causing light rays to fan out in a defocused pattern such that only some of these light rays are received by a collimating lens. Other kinds of optical limiters make use of stabilised optical amplifiers whose outputs are kept constant by feedback loops, or which present saturation at their inputs.

Optical fuses are a particular kind of optical limiter intended to interrupt the passage of an optical beam if its radiant flux exceeds a predetermined maximum. A graph of input radiant flux versus output radiant flux for an idealised optical fuse is shown in <FIG>, wherein the output rises proportional to the input until a maximum output value M is reached, at which point the output falls to zero and remains zero for all higher input values.

Optical fuses can for example be constructed using light absorbing materials which are either destroyed by the heat generated when a powerful optical beam is incident on them, or whose transmittivity is changed by that heat (e.g. so that they become opaque). Optical fuses are therefore generally single use; they must be replaced to re-establish an optical connection along the path they reside in. What is needed is an alternative optical limiter which is reusable and does not require the use of exotic materials.

<CIT> discloses an optical limiter comprising a mirror which rotates in response to radiation pressure from an optical input beam, thereby reducing the optical output intensity.

Aspects of the present disclosure will now be described by way of example with reference to the accompanying figures. In the figures:.

The following description is presented to enable any person skilled in the art to make and use the system and is provided in the context of a particular application. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art.

The terms "top", "bottom", "side", "front", "back", "forward", "rear", "clockwise", "anticlockwise" and other terms describing the orientation of features are not intended to be limiting and, where used, are purely included in order to facilitate the description of the relative location of these features in the context of the accompanying drawings. In use, or during storage, the features may be disposed in other orientations.

It is proposed to use the light mill effect to route an optical signal between an input and an output of an optical limiter in such a way that the radiant flux of an output beam carrying the signal is limited to no more than a predetermined maximum.

A thermally driven light mill comprises at least one vane/blade whose two sides are thermally insulated from one another so that when one side is heated by light or other electromagnetic radiation it remains hotter than the other side. The vanes are located in fluid (typically low-pressure air) such that convection currents set up in the fluid by the asymmetry in temperature between the two sides of each vane are sufficient to cause the light mill to rotate.

In order to start a stationary light mill rotating the light mill effect (and therefore the radiant flux of the light source causing it) must be sufficient to overcome the light mill's inertia. In order to keep the light mill rotating, the light mill effect (and therefore the radiant flux of the light source causing it) must be sufficient to overcome frictional forces acting on the light mill during rotation, with respect to its mounting(s) and the surrounding fluid.

Optionally, the two sides of each vane can have different electromagnetic absorption characteristics so as to increase the rate at which the temperature differential is established and/or allow the temperature differential to become established in circumstances where both sides of the vane are exposed to the radiation.

<FIG> illustrates a Crookes radiometer <NUM>, the classical demonstration of the light mill effect. The Crookes radiometer <NUM> comprises four vanes <NUM> arranged to rotate around an axle <NUM> within a partially evacuated jar <NUM>. Each vane <NUM> is painted white on one side and black on the other, with the vanes arranged so that black and white sides alternate around the radiometer. (The black sides are indicated by cross-hatching. ) When light is shone on the radiometer <NUM> from the direction indicated by the arrow L1, the vanes rotate about the axle <NUM> in the direction indicated by the arrow R1, i.e. with the black sides trailing.

<FIG> illustrates forces on each vane <NUM> of the Crookes radiometer <NUM> of <FIG>. In <FIG> a vane <NUM> is shown side-on, with its white side <NUM> to the left and its black side <NUM> to the right. The white side <NUM> and the black side <NUM> are thermally insulated from one another. The black side <NUM> absorbs more light energy than the white side <NUM>. A temperature gradient is therefore established from the white side <NUM> to the black side <NUM> (left to right in <FIG>). This sets up convection currents in the thin air surrounding the vane, resulting in unbalanced forces on the vane. A "thermal creep" force TC acts in a direction opposite to the temperature gradient. An additional "Einstein effect" force E acts on the edges of the surfaces which lie perpendicular to the temperature gradient, again in a direction opposite to the temperature gradient. The forces TC and E therefore cause the vane <NUM> to rotate about the axle <NUM> with the black side <NUM> trailing.

Hettner radiometers are similar to Crookes radiometers but with horizontal (as opposed to vertical) vanes, the exposed face of each vane being painted half black and half white, the black and white sides alternating around the radiometer. The Einstein effect is not present in a Hettner radiometer, but it still rotates with the black sides of the vanes trailing due to the thermal creep force.

<FIG> is a plan view illustrating another design of light mill <NUM> where a difference in light absorption between the two sides of each vane <NUM> results from their geometry, not their colour. The vanes <NUM> are shaped so that they each have a concave side <NUM> and a convex side <NUM>, the convex and concave faces alternating around the radiometer. The shading in <FIG> illustrates where shadow falls when light is shone on the light mill <NUM> from the direction indicated by the arrow L2. It can be seen that, over the course of a full rotation, the convex sides <NUM> of the vanes <NUM> receive more light energy than the concave sides <NUM>. This establishes a temperature gradient from the concave side <NUM> to the convex side <NUM> of each vane <NUM>, so that the thermal creep force and the Einstein effect cause the vanes <NUM> to rotate about the axle in the direction indicated by the arrow R2, i.e. with the convex sides <NUM> trailing.

The example light mill designs described above all incorporate vanes in which there is asymmetry between the light absorption characteristics of two sides of the vane. However, motion will result from the thermal creep force and (depending on the geometry of the design) the Einstein effect even without this asymmetry, provided a temperature gradient can be established between two sides of a vane so that it is thermally driven.

Thermally driven light mills typically operate in low pressure gases or gas mixtures (such as air) but can generally function in any fluid capable of carrying convection currents.

While the example light mill designs described above each comprise four vanes, any number of vanes can be subject to the light mill effect.

It will also be appreciated that light mills can be driven by other forms of electromagnetic radiation than visible light, for example infrared or ultraviolet radiation could also be used.

Several example designs of optical limiters for limiting the radiant flux of an optical source beam will now be described. Each example limiter comprises an optical input port, an optical output port and an optical control port. The control port is arranged to be illuminated by an optical control beam originating from the source beam. The input port is arranged to be illuminated by an optical transmission beam, also originating from the source beam. The output port is arranged to be illuminated by the transmission beam. In addition, each example limiter comprises a thermally driven light mill arranged such that illumination of the control port by the control beam drives the light mill to rotate only when the control beam has a radiant flux equal to or in excess of a predetermined radiant flux threshold. Rotation of the light mill in turn causes an area of the output port illuminated by the transmission beam to change. In this way, the radiant flux of an output beam emitted through the output port can be limited.

<FIG> illustrate schematic plan views of an example optical limiter <NUM> comprising a light mill respectively in first and second positions. To avoid cluttering these two figures, reference numerals not relevant to the specific description of each figure are omitted in that figure, though all of the components referred to are present in the limiter <NUM> as shown in each of the two figures.

The light mill comprises a rigid assembly configured to rotate about an axle <NUM> on which it is centred. The rigid assembly comprises a first vane <NUM> rigidly connected to a second vane <NUM> which acts as a counterweight to the first vane <NUM>. The light mill is located in a sealed housing <NUM> which keeps the air around the light mill at low pressure, but not entirely evacuated.

A relatively low-power optical beam (such as a laser beam) is indicated in <FIG> by relatively broad horizontal hatching. (Both uses of "relatively" in the preceding sentence indicate a comparison to <FIG>, which will be described below. ) The beam enters the housing <NUM> through an input port <NUM> to be incident on the first vane <NUM>. The surface of the first vane <NUM> on which the beam is incident is partially reflective, so a portion of the beam is reflected off the first vane <NUM> to be incident on an output port <NUM>. The various components of the limiter <NUM> are arranged such that all of the reflected portion of the beam is incident on the output port <NUM> when the light mill is in the position shown in <FIG>.

The surface of the first vane <NUM> on which the beam is incident is configured to absorb some of the electromagnetic radiation carried by the beam (the portion not reflected). That surface is thermally insulated from the surface on the opposite side of the first vane <NUM> so that a temperature gradient arises from the shaded side to the illuminated side. The light mill effect thus tends to cause the light mill to rotate clockwise so that the illuminated first vane <NUM> retreats from the beam incident on it. The beam shown in <FIG> is however of sufficiently low power that the light mill effect is not sufficient to overcome the light mill's inertia. The light mill therefore remains in its initial position abutting a first stay <NUM>.

The first stay <NUM> is a post which prevents anticlockwise motion of the light mill beyond the initial position shown in <FIG> so that the reflected portion of the beam cannot be misdirected partly or fully to the right of the output port <NUM>, for example in response to external vibrations. It can for example be formed of a material capable of buffering impact forces to reduce wear on both the stay <NUM> and the portion of the light mill which comes into contact with it.

<FIG> shows a relatively high-power optical source beam entering the housing <NUM> via the input port <NUM> as indicated by relatively narrow horizontal hatching. (Both uses of "relatively" in the preceding sentence indicate a comparison to <FIG>. ) In this case, the radiant flux of the beam striking the first vane <NUM> is high enough to cause a sufficient temperature gradient between the illuminated and shaded sides of the first vane <NUM> that the light mill effect causes the light mill to rotate clockwise about the axle <NUM>, away from the first stay <NUM> and towards a second stay <NUM>. Thus the input port <NUM> acts as a control port for rotation of the light mill, the light beam depicted entering and travelling within the housing <NUM> being all of a source beam, a transmission beam and a control beam as referred to above.

A biasing element (not shown) in the form of an elastic member attaching the light mill to the housing <NUM> is provided to slightly bias the light mill towards the position shown in <FIG>. (The elastic member could optionally be the axle <NUM>. ) This reduces the risk of the light mill rotating clockwise under any influences except for the light mill effect, for example in response to external vibrations. (The biasing element also increases the threshold radiant flux required to start the light mill rotating relative to a light mill whose acceleration is only limited by its own inertia. ) Such a biasing element, correctly calibrated, can also be used to control the limiter <NUM>'s response. This is because rotation of the light mill will halt at the point that the light mill effect forces are balanced by the biasing force. For example, if the light mill is attached to the housing <NUM> via an elastic member then an angle by which the light mill is rotated from the position shown in <FIG> will be approximately proportional to the input beam power. Such a biasing element can be adjustable; for example the tension of an elastic member could be adjusted by winding or unwinding it from a reel. A suitable biasing element could take other forms than an elastic member, for example a spring or a magnetic apparatus.

With the light mill in the position shown in <FIG>, a portion of the beam is still reflected generally towards the output port <NUM>, but at an angle such that only some of the reflected portion of the beam is incident on the output port <NUM>, the remainder being blocked by a beam stopper <NUM>. Therefore, in <FIG>, the radiant flux of the output beam exiting the housing <NUM> via the output port <NUM> is lower than the radiant flux of the source(/control/transmission) beam entering the housing <NUM> via the input(/control) port <NUM>. It can be seen that the higher the radiant flux of the beam entering via the input port <NUM> the more the light mill will turn and thus the lower the area of the output port <NUM> that will be illuminated. In this way, the mirror provided by the reflective surface of the first vane <NUM> and the beam stopper <NUM> together form an optical baffle apparatus arranged to prevent a portion of the beam from illuminating the output port <NUM>, that portion's size being dependent on the angle by which the light mill is rotated.

A second stay <NUM>, similar to the first stay <NUM>, is provided by another post to set a maximum rotation angle for the light mill from the initial position shown in <FIG>. This prevents the second vane <NUM> from rotating so far that it hits the housing <NUM>. The second stay <NUM>'s positioning can be chosen to have one of two effects on the response of the limiter <NUM> to high-power inputs. If the second stay <NUM> is positioned far enough around the light mill's rotational path that the light mill can rotate far enough that the beam misses the output port <NUM> altogether then the limiter <NUM> will act to limit the output power up to a certain input power, then effectively acts as a fuse, causing the output power to drop to zero for all higher input powers. Alternatively, the second stay <NUM> can be positioned to limit rotation of the light mill more, such that the output radiant flux can be prevented from ever falling to zero if an input signal is present. That is, the second stay <NUM> can be positioned such that the light mill's clockwise rotation is halted just short of the point where the reflected portion of the beam would entirely miss the output port. (This would however allow the output power to increase beyond the level the limiter is intended to restrict it to, so use of a backup optical fuse in conjunction with a limiter of this type may be advisable if there is a risk of damage to network apparatus from power surges in excess of the limiter level.

The beam stopper <NUM> could be omitted from the limiter <NUM> and, with the light mill in the position shown in <FIG>, a portion of the beam would still miss the output port <NUM>, striking the housing <NUM> adjacent the output port <NUM> instead. However, the limiter <NUM>'s response can be controlled to be flatter by using a dedicated beam stopper <NUM> than if the housing <NUM> surrounding the output port <NUM> is relied upon as part of the baffle apparatus.

<FIG> illustrate schematic plan views of an example optical fuse <NUM> of a similar design to the optical limiter <NUM> of <FIG>. To avoid cluttering these two figures, reference numerals not relevant to the specific description of each figure are omitted in that figure, though all of the components referred to are present in the limiter <NUM> as shown in each of the two figures. The optical fuse <NUM> comprises a light mill having a first vane <NUM> and second vane <NUM> configured to rotate about an axle <NUM>. The light mill is enclosed in a housing <NUM> having an input port <NUM> and an output port <NUM>. The light mill's rotation is constrained by first and second stays <NUM> and <NUM>. All of these components function in the same way as the corresponding components of the limiter <NUM> of <FIG>.

In contrast to the limiter <NUM> however, the fuse <NUM> does not comprise the elastic member present in the limiter <NUM>. In addition, the second vane <NUM> of the fuse <NUM> is magnetic (for example due to being made of or coated in a layer of iron) and the fuse <NUM> further comprises a biasing element in the form of a magnet <NUM>. (In the design shown, the magnet <NUM> is external to the housing <NUM> for ease of adjustment as will be described below, but it could be within the housing <NUM> instead. ) Magnetic attraction between the magnet <NUM> and the second vane <NUM> keeps the light mill abutting the first stay <NUM> in the position shown in <FIG> provided the radiant flux of a beam input through the input port <NUM> remains below a threshold value.

If the radiant flux of the input beam reaches or exceeds that threshold value then the light mill effect overcomes the magnetic attraction and the light mill (unconstrained by any elastic member) swings suddenly to the position shown in <FIG>, abutting the second stay <NUM>, where the reflected portion of the beam misses the output port <NUM> entirely so that the output power falls sharply to zero.

The light path in <FIG> is respectively indicated by thin dashed and dot-dashed lines (rather than hatched regions as in <FIG>) since the sharp response of the fuse at the threshold input power means that the width of the beam is irrelevant. The beam's full width is either transmitted through the output port <NUM> as shown in <FIG> or misses the output port <NUM> entirely as shown in <FIG>. (There is of course an interval as the light mill swings between the positions shown in <FIG> when the output port <NUM> is illumination by only a part of the beam, but this interval is extremely brief.

If and when the source beam is switched off, or its radiant flux falls below the threshold value, the magnet <NUM> causes the light mill to swiftly rotate back anticlockwise to the position shown in <FIG>. The fuse <NUM> is therefore reusable and self-resetting.

The magnet <NUM> has an external member so that its position can be adjusted by screwing it closer to or further from the second vane <NUM> within an internally threaded nut <NUM>. In this way, the threshold power for tripping the fuse can be adjusted. If the nut is sufficiently long the magnet <NUM> could even be retracted far enough from the second vane <NUM> that the fuse <NUM> responds in much the same way as the limiter <NUM>. (In that case, a beam stopper similar to the beam stopper <NUM> of the limiter <NUM> could be added to flatten the response as discussed above in relation to <FIG>. ) Thus the apparatus <NUM> could in fact be multi-functional; providing an adjustable, reusable, self-resetting optical fuse/limiter.

<FIG> illustrates the relative positions of the light mill (comprising first and second vanes <NUM> and <NUM> configured to rotate about an axle <NUM>), input and output ports <NUM> and <NUM>, first and second stays <NUM> and <NUM> and magnet <NUM> of the fuse <NUM> three-dimensionally.

<FIG> is a plan view of the housing <NUM> and nut <NUM> of the fuse <NUM>. The thickness of these components is indicated in phantom using dashed lines for their internal walls. Also shown in phantom are two apertures <NUM> and <NUM> in the housing <NUM>, configured to respectively assist in coupling an input optical fibre (not shown) to the input port <NUM> and an output optical fibre (not shown) to the output port <NUM>. The apertures <NUM> and <NUM> have a stepped profile with a relatively narrow internal portion (for example <NUM> in diameter) and a relatively wide external portion (for example <NUM> in diameter). The input and output ports <NUM> and <NUM> are sealed within the internal portions so that the housing is airtight, allowing the air inside the housing to be kept at an optimal low pressure for operation of the light mill. The external portions are configured to receive optical fibres (not shown) in a snug interference fit.

The housing <NUM> of the limiter <NUM> can be identical to the housing <NUM> of the fuse <NUM>. The housings <NUM>, <NUM> of the limiter <NUM> and fuse <NUM> can for example be approximately <NUM> tall, <NUM> wide and <NUM> long with walls approximately <NUM> thick. They can for example be made of plastic, metal, or another impermeable solid.

In both the limiter <NUM> and the fuse <NUM> a further port can be provided in the housing <NUM>, <NUM> (not shown in any of <FIG>) for a vacuum apparatus to be attached so that the air pressure inside the housing can be kept at an optimal level for operation of the light mill, for example between <NUM> and <NUM> mTor.

The angle between the input and output ports <NUM>, <NUM> and <NUM>, <NUM> in the limiter <NUM> and the fuse <NUM> can for example be an obtuse angle, e.g. approximately <NUM>°.

The light mills of the limiter <NUM> and fuse <NUM> can for example have vanes approximately <NUM> thick and <NUM> long from axle to tip.

The entire limiter/fuse assembly <NUM>, <NUM> can for example have a mass of approximately <NUM>.

The beam stopper <NUM> of the limiter <NUM> can for example be approximately <NUM> wide.

Further example limiters and fuses will now be described with reference to <FIG>. In these figures similar conventions are used to those employed in <FIG>. That is: (i) to avoid cluttering the figures, not all reference numerals are repeated between multiple views of a particular apparatus; (ii) relatively broad and narrow horizontal hatching is used to indicate relatively low-power and high-power beams in depictions of limiters; and (iii) thin dashed lines are used to indicate relatively low-power beams in depictions of fuses, in contrast to thin dot-dash lines to indicate relatively high-power beams.

<FIG> illustrate schematic plan views of an example optical limiter <NUM> comprising a light mill respectively in first and second positions. The limiter <NUM> functions in a very similar way to the limiter <NUM> of <FIG>. It comprises a light mill having a first vane <NUM> and second vane <NUM> configured to rotate about an axle <NUM>. The light mill is biased towards the position shown in <FIG> by virtue of being attached to the housing <NUM> by an elastic member (not shown). The light mill is enclosed in a housing <NUM> having an input port <NUM> and an output port <NUM>. The light mill's rotation is constrained by first and second stays <NUM> and <NUM>. A beam stopper <NUM> is also provided. All of these components function in the same way as the corresponding components of the limiter <NUM> of <FIG>, the only difference being the geometry of their arrangement. Specifically, the input port <NUM> is perpendicular to the output port <NUM> in this limiter <NUM>, as opposed to the input port <NUM> being at an obtuse angle to the output port <NUM> in the limiter <NUM>.

<FIG> illustrate schematic plan views of an example optical fuse <NUM> of a similar design to the optical limiter <NUM> of <FIG>. The optical fuse <NUM> comprises a light mill having a first vane <NUM> and second vane <NUM> configured to rotate about an axle <NUM>. The light mill is enclosed in a housing <NUM> having an input port <NUM> and an output port <NUM>. The light mill's rotation is constrained by first and second stays <NUM> and <NUM>. All of these components are identical to the corresponding components of the limiter <NUM> of <FIG>.

In contrast to the limiter <NUM> however, in the fuse <NUM> there is no elastic member, the second vane <NUM> is magnetic and the fuse <NUM> further comprises a biasing element in the form of a magnet <NUM>. The magnet <NUM> functions in a similar way to the magnet <NUM> of <FIG>, only it is located within the housing <NUM> and is not adjustable. The fuse <NUM> thus has a fixed threshold input power value which will cause it to trip and cannot be made to function as a limiter. Like the fuse <NUM> however, the fuse <NUM> is reusable and self-resetting.

<FIG> illustrate schematic plan views of another example optical limiter <NUM> comprising a light mill respectively in first and second positions. The limiter <NUM> functions in a similar way to the limiter <NUM> of <FIG>. It comprises a light mill enclosed in a housing <NUM> having an input port <NUM> and an output port <NUM>. The light mill is attached to the housing <NUM> via an elastic member (not shown) which biases it towards the position shown in <FIG>. A beam stopper <NUM> is provided, which functions in the same way to the beam stopper <NUM> of the limiter <NUM>.

The limiter <NUM> however differs from the limiter <NUM> of <FIG> in that control of the light mill's rotation and direction of light from the input port <NUM> towards the output port <NUM> are provided separately, rather than both being provided by a first vane of the light mill as in the limiter <NUM>. As shown in <FIG>, a source beam arriving from the left-hand side encounters an optical splitter <NUM> which splits the source beam into a control beam and a transmission beam.

The control beam is directed to a mirror <NUM> which routes it on through a control port <NUM> to be incident on a first vane <NUM> of the light mill. The first vane <NUM> absorbs some or all of the energy of the control beam. Where the source beam is relatively powerful, as shown in <FIG>, the control beam is sufficiently powerful to cause the light mill to rotate clockwise due to the light mill effect induced by this absorption.

The transmission beam continues on through the splitter <NUM> and the input port <NUM> to be incident centrally on a light director <NUM> such as a mirror. The light director <NUM> is part of the rigid assembly of the light mill, centred on the axle <NUM> and configured to turn with the first and second vanes <NUM> and <NUM> about the axle <NUM>. The transmission beam is reflected by the light director at an angle dependent on the rotational position of the light mill. With the light mill in the position shown in <FIG>, the transmission beam is incident centrally on the output port <NUM> so that the power of the output beam is maximised. With the light mill in the position shown in <FIG>, part of the transmission beam is blocked by the beam stopper <NUM> so the power of the output beam is reduced. In this way, the light director <NUM> and the beam stopper <NUM> together form an optical baffle apparatus arranged to prevent a portion of the transmission beam from illuminating the output port <NUM>, that portion's size being dependent on the angle by which the light mill is rotated.

Any anticlockwise rotation of the light mill which may be caused by influences other than the light mill effect, such as external vibrations, is constrained by a first stay <NUM>. Clockwise rotation of the light mill is constrained by a pair of second stays <NUM>. One of the pair of second stays <NUM> could be omitted, though including both balances the forces on the two sides of the light mill when it is abutting them, reducing the risk of it bending or snapping. As explained above in relation to <FIG>, the positioning of the second stays <NUM> determines the response of the limiter <NUM> to high-power source beams.

<FIG> illustrate schematic plan views of an example optical fuse <NUM> of a similar design to the optical limiter <NUM> of <FIG>. The optical fuse <NUM> comprises a light mill having a first vane <NUM>, a second vane <NUM> and a light director <NUM> rigidly connected between them. All of the first and second vanes <NUM>, <NUM> and the light director <NUM> are configured to rotate together about an axle <NUM>. The light mill is enclosed in a housing <NUM> having an input port <NUM>, an output port <NUM> and a control port <NUM>. The light mill is attached to the housing <NUM> via an elastic member (not shown) which biases it towards the position shown in <FIG>. A splitter <NUM> is configured to split a source beam into a transmission beam incident on the input port <NUM> and a control beam incident on the control port <NUM>, via reflection by a mirror <NUM>. The light mill's rotation is constrained by a first stay <NUM> and a pair of second stays <NUM>. All of these components function in the same way as the corresponding components of the limiter <NUM> of <FIG>.

In contrast to the limiter <NUM> however, in the fuse <NUM> there is no elastic member, the second vane <NUM> is magnetic and the fuse <NUM> further comprises a biasing element in the form of a magnet <NUM>. The magnet <NUM> functions in the same way as the magnet <NUM> of the fuse <NUM>.

<FIG> illustrate schematic plan views of another example optical limiter <NUM> comprising a light mill respectively in first and second positions. The limiter <NUM> functions in a similar way to the limiter <NUM> of <FIG>. It comprises a light mill enclosed in a housing <NUM> having an input port <NUM>, an output port <NUM> and a control port <NUM>. The light mill is attached to the housing <NUM> via an elastic member (not shown) which biases it towards the position shown in <FIG>. A splitter <NUM> is configured to split a source beam into a transmission beam incident on the input port <NUM> and a control beam incident on the control port <NUM>, via reflection by a mirror <NUM>, just like in the limiter <NUM> of <FIG>. The light mill's clockwise rotation is constrained by a pair of stays <NUM> corresponding to the pair of second stays <NUM> of the limiter <NUM>.

The limiter <NUM> however differs from the limiter <NUM> of <FIG> in that the light mill of the limiter <NUM> does not comprise a light director. The limiter <NUM> has an optical baffle apparatus comprising two beam stoppers <NUM> and <NUM> defining the limits of an aperture. (The optical baffle apparatus could alternatively be provided by a single ring-shaped beam stopper which would look identical when cross-sectioned through the level of the aperture. ) The beam stoppers <NUM> and <NUM> are rigidly connected between the first and second vanes <NUM> and <NUM> and are configured to turn along with them about an axle which is not shown (since it is attached to the depicted components of the light mill above and/or below the level of the aperture). The output port <NUM> is parallel to and coaxial with the input port <NUM>. When the light mill is in the position shown in <FIG>, the aperture between the beam stoppers <NUM> and <NUM> is intermediate and coaxial with the input port <NUM> and output port <NUM> so that the power of the output beam is maximised. As shown in <FIG>, as the light mill rotates from the position shown in <FIG> to the position shown in <FIG> an increasing area of the transmission beam is blocked by the beam stoppers <NUM> and <NUM> so that the power of the output beam is reduced.

No stay is depicted to hold the light mill in the initial position shown in <FIG> against influences other than the light mill effect, such as external vibrations, though one could be provided. For example, one or both of the first and second vanes <NUM> and <NUM> and/or the spokes on which they are mounted could be configured to be weakly attracted to one or more magnets in corresponding locations on the base and/or roof of the housing <NUM>. Alternatively, the elastic member could be sufficient to perform this function.

<FIG> illustrate schematic plan views of an example optical fuse <NUM> of a similar design to the optical limiter <NUM> of <FIG>. The optical fuse <NUM> comprises a light mill having a first vane <NUM>, a second vane <NUM> and two beam stoppers <NUM> and <NUM>, defining an aperture, rigidly connected between the first and second vanes <NUM> and <NUM>. All of the first and second vanes <NUM>, <NUM> and the first and second beam stoppers <NUM>, <NUM> are configured to rotate together about an axle (not shown). The light mill is enclosed in a housing <NUM> having an input port <NUM>, an output port <NUM> and a control port <NUM>. A splitter <NUM> is configured to split a source beam into a transmission beam incident on the input port <NUM> and a control beam incident on the control port <NUM>, via reflection by a mirror <NUM>. All of these components are identical to the corresponding components of the limiter <NUM> of <FIG>.

The fuse <NUM> also comprises first and second stays <NUM> and <NUM> to respectively constrain anticlockwise and clockwise rotation of the light mill in a similar manner to the first and second stays <NUM> and <NUM> of the fuse <NUM> of <FIG>. In all of the example limiters and fuses described above, at least one region of at least one face of at least one vane of each light mill is configured to have light incident upon it, and to absorb energy from that light so that it heats up relative to an opposing region of an opposing face of that vane. The absorbing region can for example be coated with graphite, black aluminium foil, anodised aluminium or Litho-Black™. Provided the opposing regions are thermally insulated from one another and the absorbing region is illuminated to a greater extent than the opposing region, there is no need for there to be any asymmetry in their optical absorptance. (Optical absorptance is defined as the ratio of absorbed to incident radiant power. ) The illuminating light being a laser beam sufficiently narrow with respect to the size of the vane that only one side of the vane is targeted enhances the effect. However, the light mill effect can be enhanced by providing the absorbing region with a higher optical absorptance than the opposing region. For example, the opposing region can be covered with a reflective metal, such as silver, or a dielectric material. Alternatively or additionally, the absorbing region can be shaped such that it receives a greater quantity of radiant flux than the opposing region over a permitted range of rotation of the light mill, making use of the principle described in relation to <FIG> above.

The light mills of all of the example fuses and limiters described above comprise two vanes; a first vane configured to be struck by a control beam and a second vane which acts as a counterweight. The second vane could be omitted, and the light mills would still rotate in response to the control beams. Alternatively, the light mills could be provided with more than two vanes.

Some of the example limiters and fuses described above comprise one or more light directors such as mirrors. Such a mirror can be provided by a surface which is at least partially reflective. If a mirror needs to be capable of absorbing some light, for example in the example switches and limiters <NUM>, <NUM>, <NUM> and <NUM> described above in relation to <FIG>, its surface can for example be made partially reflective by layering a thin dielectric over a light absorbent surface (e.g. a surface coated with a light-absorbent material as described above). Other optical components, such as prisms, could alternatively be employed as light directors.

In all of the example limiters and fuses described above, at least one component of an optical baffle apparatus is arranged to rotate with the light mill. However, other arrangements could be envisaged wherein motion of the light mill causes redirection or blocking of the transmission beam in some other way. For example, a cam arrangement could be used to translate the rotational motion of a light mill into linear motion of a light director.

In all of the example limiters and fuses described above, motion of the light mill is constrained by stays in the form of buffer/bumper/rest elements provided for one or more of the light mill vanes (and/or spokes on which they are carried) to but up against. Alternatively, a single stay could be provided for multiple vanes, e.g. so that in a two-vane example the light mill rotates almost a full circle between its two positions.

Other forms of stay could also be used; any element that prevents or impedes rotation beyond a certain position in one direction, while allowing (some) counterrotation away from that position, would be suitable. For example other kinds of mechanical stays, such as catches, could be envisaged, in addition to magnetic stays such as those described above.

The optical transmission (input/output) and control ports used in limiters and fuses according to the present disclosure can be used to couple light from/to optical fibres. They can optionally comprise lenses to focus or defocus that light as appropriate.

Claim 1:
An optical limiter (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) for limiting radiant flux of an optical source beam, the limiter comprising:
an optical control port (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) for illumination by an optical control beam originating from the optical source beam;
an optical input port (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) for illumination by an optical transmission beam originating from the optical source beam; and
an optical output port (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) for illumination by the optical transmission beam; the optical limiter being characterised in that it further comprises a thermally driven light mill;
wherein the light mill is arranged with respect to the optical input port (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>), the optical control port (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) and the optical output port (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) such that:
illumination of the optical control port (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) by the optical control beam drives the light mill to rotate only when the optical control beam has a radiant flux equal to or in excess of a predetermined radiant flux threshold; and
rotation of the light mill causes an area of the optical output port (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) illuminated by the optical transmission beam to change.