Patent Publication Number: US-2023140182-A1

Title: Optical limiter and method for limiting radiant flux

Description:
PRIORITY CLAIM 
     The present application is a National Phase entry of PCT Application No. PCT/EP2021/056074, filed Mar. 10, 2021, which claims priority from EP Patent Application No. 20166647.6, filed Mar. 30, 2020 and GB Patent Application No. 2004584.5, filed Mar. 30, 2020, each of which is hereby fully incorporated herein by reference. 
    
    
     FIELD 
     The present vapor disclosure relates to limiting the radiant flux of an optical beam; that is, limiting the radiant energy transmitted by the beam per unit time — its power. More specifically, aspects relate to an optical limiter and a method for limiting the radiant flux of an optical source beam. 
     BACKGROUND 
     Optical networks are used to communicate data, encoded in optical signals, over fiber 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 idealized graph of input radiant flux versus output radiant flux for a flat maximum optical limiter is shown in FIG.  1 A 1 , 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 stabilized 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 idealized optical fuse is shown in FIG.  1 A 2 , 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. 
     SUMMARY 
     According to a first aspect, there is provided an optical limiter for limiting the radiant flux of an optical source beam, the limiter comprising: an optical control port for illumination by an optical control beam originating from the source beam; an optical input port for illumination by an optical transmission beam originating from the source beam; an optical output port for illumination by the transmission beam; and a thermally driven light mill; wherein the light mill is arranged with respect to the input port, the control port and the output port 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; and rotation of the light mill causes an area of the output port illuminated by the transmission beam to change. 
     The light mill comprises: an axle; and at least one vane: arranged to rotate about the axle&#39;s axis in an environment comprising fluid, and having a first side and a second side which are thermally insulated from one another such that the light mill is driven by illumination of the first side by the control beam so that the first side absorbs more light energy than the second side, establishing a temperature gradient from the second side to the first side so that the vane rotates about the axle&#39;s axis with the first side trailing. 
     The predetermined radiant flux threshold depends on the light mill&#39;s inertia. 
     The limiter can be configured to permit rotation of the light mill by an angle large enough that the area of the output port illuminated by the transmission beam is zero, such that the limiter acts as a reusable optical fuse. 
     The source beam can originate from a laser. 
     A side of a vane of the light mill arranged to be illuminated by the control beam can have a higher optical absorptance than an opposing side of that vane. 
     Alternatively or additionally, a side of a vane of the light mill arranged to be illuminated by the control beam and an opposing side of that vane can be shaped such that, over a permitted range of rotation of the light mill, the side of the vane illuminated by the control beam receives a greater quantity of radiant energy from the control beam than the opposing side. 
     The limiter can further comprise an optical baffle apparatus arranged to prevent a portion of the transmission beam from illuminating the output port, that portion&#39;s size being dependent on an angle by which the light mill is rotated. 
     The baffle apparatus can comprise: one or more light directors; and/or one or more beam stoppers. 
     The input port and the output port can be coaxial with one another; the baffle apparatus can comprise one or more beam stoppers arranged to rotate with the light mill; and said one or more beam stoppers can be arranged to define an aperture that the transmission beam must pass through to reach the output port. 
     The aperture can be coaxial with the input and output ports when the angle is zero. 
     The input port can be the control port, the source beam itself serving as both the transmission beam and the control beam. 
     The limiter can further comprise an optical splitter arranged to split the source beam into the transmission beam and the control beam. 
     The limiter can further comprise a biasing element; wherein the light mill is coupled to the biasing element such that the light mill is biased towards a first rotational position in which the area of the output port illuminated by the transmission beam is maximized relative to any other rotational position of the light mill. 
     The biasing element can be configured to be adjustable so that a biasing force it exerts on the light mill can be modified. 
     The biasing element can be configured to: when the radiant flux of the source beam is below the predetermined threshold, prevent rotation of the light mill away from the first rotational position; and when the radiant flux of the source beam equals or exceeds the predetermined threshold, permit rotation of the light mill from the first rotational position by the angle large enough that the area of the output port illuminated by the transmission beam is zero. 
     The limiter can further comprise one or more stays, each arranged to prevent rotation of the light mill beyond a particular rotational position. 
     The limiter can further comprise a housing which encloses the light mill, the housing comprising an aperture configured for partial evacuation of fluid surrounding the light mill. 
     According to a second aspect, there is provided a method of limiting the radiant flux of an optical source beam, the method comprising: illuminating the input port of the optical limiter of any preceding claim with an optical transmission beam originating from the source beam; and illuminating the control port of the optical limiter of any preceding claim with an optical control beam originating from the source beam. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Aspects of the present disclosure will now be described by way of example with reference to the accompanying figures. In the figures: 
       FIG.  1 A 1  is an idealized graph of input radiant flux versus output radiant flux for a flat maximum optical limiter. 
       FIG.  1 A 2  is a graph of input radiant flux versus output radiant flux for an idealized optical fuse. 
       FIG.  1 B 1  illustrates a Crookes radiometer light mill. 
       FIG.  1 B 2  illustrates forces on each vane of the Crookes radiometer of FIG.  1 B 1 . 
         FIG.  1 C  illustrates another design of light mill. 
         FIG.  2 A  illustrates an example optical limiter in a first position. 
         FIG.  2 B  illustrates the limiter of  FIG.  2 A  in a second position. 
         FIG.  3 A  illustrates an example optical fuse in an “on” position. 
         FIG.  3 B  illustrates the fuse of  FIG.  3 A  in an “off” position. 
         FIG.  3 C  illustrates some internal components of the fuse of  FIG.  3 A . 
         FIG.  3 D  illustrates some external components of the fuse of  FIG.  3 A . 
         FIG.  4 A  illustrates another example optical limiter in a first position. 
         FIG.  4 B  illustrates the limiter of  FIG.  4 A  in a second position. 
         FIG.  5 A  illustrates another example optical fuse in an “on” position. 
         FIG.  5 B  illustrates the fuse of  FIG.  5 A  in an “off” position. 
         FIG.  6 A  illustrates another example optical limiter in a first position. 
         FIG.  6 B  illustrates the limiter of  FIG.  6 A  in a second position. 
         FIG.  7 A  illustrates another example optical fuse in an “on” position. 
         FIG.  7 B  illustrates the fuse of  FIG.  7 A  in an “off” position. 
         FIG.  8 A  illustrates another example optical limiter in a first position. 
         FIG.  8 B  illustrates the limiter of  FIG.  8 A  in a second position. 
         FIG.  9 A  illustrates another example optical fuse in an “on” position. 
         FIG.  9 B  illustrates the fuse of  FIG.  9 A  in an “off” position. 
     
    
    
     DETAILED DESCRIPTION OF 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&#39;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.  1 B 1  illustrates a Crookes radiometer  1100 , the classical demonstration of the light mill effect. The Crookes radiometer  1100  comprises four vanes  1110  arranged to rotate around an axle  1120  within a partially evacuated jar  1130 . Each vane  1110  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  1100  from the direction indicated by the arrow L 1 , the vanes rotate about the axle  1120  in the direction indicated by the arrow R 1 , i.e. with the black sides trailing. 
     FIG.  1 B 2  illustrates forces on each vane  1110  of the Crookes radiometer  1100  of FIG.  1 B 1 . In FIG.  1 B 2  a vane  1110  is shown side-on, with its white side  1111  to the left and its black side  1112  to the right. The white side  1111  and the black side  1112  are thermally insulated from one another. The black side  1112  absorbs more light energy than the white side  1111 . A temperature gradient is therefore established from the white side  1111  to the black side  1112  (left to right in FIG.  1 B 2 ). 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  1110  to rotate about the axle  1120  with the black side  1112  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.  1 C  is a plan view illustrating another design of light mill  1200  where a difference in light absorption between the two sides of each vane  1210  results from their geometry, not their color. The vanes  1210  are shaped so that they each have a concave side  1211  and a convex side  1212 , the convex and concave faces alternating around the radiometer. The shading in  FIG.  1 C  illustrates where shadow falls when light is shone on the light mill  1200  from the direction indicated by the arrow L 2 . It can be seen that, over the course of a full rotation, the convex sides  1212  of the vanes  1210  receive more light energy than the concave sides  1211 . This establishes a temperature gradient from the concave side  1211  to the convex side  1212  of each vane  1210 , so that the thermal creep force and the Einstein effect cause the vanes  1210  to rotate about the axle in the direction indicated by the arrow R 2 , i.e. with the convex sides  1212  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. 
       FIGS.  2 A and  2 B  illustrate schematic plan views of an example optical limiter  200  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  200  as shown in each of the two figures. 
     The light mill comprises a rigid assembly configured to rotate about an axle  214  on which it is centered . The rigid assembly comprises a first vane  211  rigidly connected to a second vane  212  which acts as a counterweight to the first vane  211 . The light mill is located in a sealed housing  240  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.  2 A  by relatively broad horizontal hatching. (Both uses of “relatively” in the preceding sentence indicate a comparison to  FIG.  2 B , which will be described below.) The beam enters the housing  240  through an input port  221  to be incident on the first vane  211 . The surface of the first vane  211  on which the beam is incident is partially reflective, so a portion of the beam is reflected off the first vane  211  to be incident on an output port  225 . The various components of the limiter  200  are arranged such that all of the reflected portion of the beam is incident on the output port  225  when the light mill is in the position shown in  FIG.  2 A . 
     The surface of the first vane  211  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  211  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  211  retreats from the beam incident on it. The beam shown in  FIG.  2 A  is however of sufficiently low power that the light mill effect is not sufficient to overcome the light mill&#39;s inertia. The light mill therefore remains in its initial position abutting a first stay  281 . 
     The first stay  281  is a post which prevents anticlockwise motion of the light mill beyond the initial position shown in  FIG.  2 A  so that the reflected portion of the beam cannot be misdirected partly or fully to the right of the output port  225 , 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  281  and the portion of the light mill which comes into contact with it. 
       FIG.  2 B  shows a relatively high-power optical source beam entering the housing  240  via the input port  221  as indicated by relatively narrow horizontal hatching. (Both uses of “relatively” in the preceding sentence indicate a comparison to  FIG.  2 A .) In this case, the radiant flux of the beam striking the first vane  211  is high enough to cause a sufficient temperature gradient between the illuminated and shaded sides of the first vane  211  that the light mill effect causes the light mill to rotate clockwise about the axle  214 , away from the first stay  281  and towards a second stay  282 . Thus the input port  221  acts as a control port for rotation of the light mill, the light beam depicted entering and travelling within the housing  240  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  240  is provided to slightly bias the light mill towards the position shown in  FIG.  2 A . (The elastic member could optionally be the axle  214 .) 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  200 &#39;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  240  via an elastic member then an angle by which the light mill is rotated from the position shown in  FIG.  2 A  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.  2 B , a portion of the beam is still reflected generally towards the output port  225 , but at an angle such that only some of the reflected portion of the beam is incident on the output port  225 , the remainder being blocked by a beam stopper  290 . Therefore, in  FIG.  2 B , the radiant flux of the output beam exiting the housing  240  via the output port  225  is lower than the radiant flux of the source(/control/transmission) beam entering the housing  240  via the input(/control) port  221 . It can be seen that the higher the radiant flux of the beam entering via the input port  221  the more the light mill will turn and thus the lower the area of the output port  225  that will be illuminated. In this way, the mirror provided by the reflective surface of the first vane  211  and the beam stopper  290  together form an optical baffle apparatus arranged to prevent a portion of the beam from illuminating the output port  225 , that portion&#39;s size being dependent on the angle by which the light mill is rotated. 
     A second stay  282 , similar to the first stay  281 , is provided by another post to set a maximum rotation angle for the light mill from the initial position shown in  FIG.  2 A . This prevents the second vane  212  from rotating so far that it hits the housing  240 . The second stay  282 &#39;s positioning can be chosen to have one of two effects on the response of the limiter  200  to high-power inputs. If the second stay  282  is positioned far enough around the light mill&#39;s rotational path that the light mill can rotate far enough that the beam misses the output port  225  altogether then the limiter  200  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  282  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  282  can be positioned such that the light mill&#39;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  290  could be omitted from the limiter  200  and, with the light mill in the position shown in  FIG.  2 B , a portion of the beam would still miss the output port  225 , striking the housing  240  adjacent the output port  225  instead. However, the limiter  200 &#39;s response can be controlled to be flatter by using a dedicated beam stopper  290  than if the housing  240  surrounding the output port  225  is relied upon as part of the baffle apparatus. 
       FIGS.  3 A and  3 B  illustrate schematic plan views of an example optical fuse  300  of a similar design to the optical limiter  200  of  FIGS.  2 A and  2 B . 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  300  as shown in each of the two figures. The optical fuse  300  comprises a light mill having a first vane  311  and second vane  312  configured to rotate about an axle  314 . The light mill is enclosed in a housing  340  having an input port  321  and an output port  325 . The light mill&#39;s rotation is constrained by first and second stays  381  and  382 . All of these components function in the same way as the corresponding components of the limiter  200  of  FIGS.  2 A and  2 B . 
     In contrast to the limiter  200  however, the fuse  300  does not comprise the elastic member present in the limiter  200 . In addition, the second vane  312  of the fuse  300  is magnetic (for example due to being made of or coated in a layer of iron) and the fuse  300  further comprises a biasing element in the form of a magnet  385 . (In the design shown, the magnet  385  is external to the housing  340  for ease of adjustment as will be described below, but it could be within the housing  340  instead.) Magnetic attraction between the magnet  385  and the second vane  312  keeps the light mill abutting the first stay  381  in the position shown in  FIG.  3 A  provided the radiant flux of a beam input through the input port  321  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.  3 B , abutting the second stay  382 , where the reflected portion of the beam misses the output port  325  entirely so that the output power falls sharply to zero. 
     The light path in  FIGS.  3 A and  3 B  is respectively indicated by thin dashed and dot-dashed lines (rather than hatched regions as in  FIGS.  2 A and  2 B ) since the sharp response of the fuse at the threshold input power means that the width of the beam is irrelevant. The beam&#39;s full width is either transmitted through the output port  325  as shown in  FIG.  3 A  or misses the output port  325  entirely as shown in FIG.  3 B. (There is of course an interval as the light mill swings between the positions shown in  FIGS.  3 A and  3 B  when the output port  325  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  385  causes the light mill to swiftly rotate back anticlockwise to the position shown in  FIG.  3 A . The fuse  300  is therefore reusable and self-resetting. 
     The magnet  385  has an external member so that its position can be adjusted by screwing it closer to or further from the second vane  312  within an internally threaded nut  386 . In this way, the threshold power for tripping the fuse can be adjusted. If the nut is sufficiently long the magnet  385  could even be retracted far enough from the second vane  312  that the fuse  300  responds in much the same way as the limiter  200 . (In that case, a beam stopper similar to the beam stopper  290  of the limiter  200  could be added to flatten the response as discussed above in relation to  FIG.  2 B .) Thus the apparatus  300  could in fact be multi-functional; providing an adjustable, reusable, self-resetting optical fuse/limiter. 
       FIG.  3 C  illustrates the relative positions of the light mill (comprising first and second vanes  311  and  312  configured to rotate about an axle  314 ), input and output ports  321  and  325 , first and second stays  381  and  382  and magnet  385  of the fuse  300  three-dimensionally. 
       FIG.  3 D  is a plan view of the housing  340  and nut  386  of the fuse  300 . The thickness of these components is indicated in phantom using dashed lines for their internal walls. Also shown in phantom are two apertures  341  and  345  in the housing  340 , configured to respectively assist in coupling an input optical fiber (not shown) to the input port  321  and an output optical fiber (not shown) to the output port  325 . The apertures  341  and  345  have a stepped profile with a relatively narrow internal portion (for example 3 mm in diameter) and a relatively wide external portion (for example 4.6 mm in diameter). The input and output ports  321  and  325  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 fibers (not shown) in a snug interference fit. 
     The housing  240  of the limiter  200  can be identical to the housing  340  of the fuse  300 . The housings  240 ,  340  of the limiter  200  and fuse  300  can for example be approximately 11 mm tall, 7 mm wide and 27 mm long with walls approximately 2 mm thick. They can for example be made of plastic, metal, or another impermeable solid. 
     In both the limiter  200  and the fuse  300  a further port can be provided in the housing  240 ,  340  (not shown in any of  FIGS.  2 A to  3 D ) 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 300 and 600 mTor. 
     The angle between the input and output ports  221 ,  321  and  225 ,  325  in the limiter  200  and the fuse  300  can for example be an obtuse angle, e.g. approximately 120°. 
     The light mills of the limiter  200  and fuse  300  can for example have vanes approximately 1 mm thick and 10 mm long from axle to tip. 
     The entire limiter/fuse assembly  200 ,  300  can for example have a mass of approximately 15 g. 
     The beam stopper  290  of the limiter  200  can for example be approximately 3 mm wide. 
     Further example limiters and fuses will now be described with reference to  FIGS.  4 A to  9 B . In these figures similar conventions are used to those employed in  FIGS.  2 A to  3 B . 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. 
       FIGS.  4 A and  4 B  illustrate schematic plan views of an example optical limiter  400  comprising a light mill respectively in first and second positions. The limiter  400  functions in a very similar way to the limiter  200  of  FIGS.  2 A and  2 B . It comprises a light mill having a first vane  411  and second vane  412  configured to rotate about an axle  414 . The light mill is biased towards the position shown in  FIG.  4 A  by virtue of being attached to the housing  440  by an elastic member (not shown). The light mill is enclosed in a housing  440  having an input port  421  and an output port  425 . The light mill&#39;s rotation is constrained by first and second stays  481  and  482 . A beam stopper  490  is also provided. All of these components function in the same way as the corresponding components of the limiter  200  of  FIGS.  2 A and  2 B , the only difference being the geometry of their arrangement. Specifically, the input port  421  is perpendicular to the output port  425  in this limiter  400 , as opposed to the input port  421  being at an obtuse angle to the output port  425  in the limiter  200 . 
       FIGS.  5 A and  5 B  illustrate schematic plan views of an example optical fuse  500  of a similar design to the optical limiter  400  of  FIGS.  4 A and  4 B . The optical fuse  500  comprises a light mill having a first vane  511  and second vane  512  configured to rotate about an axle  514 . The light mill is enclosed in a housing  540  having an input port  521  and an output port  525 . The light mill&#39;s rotation is constrained by first and second stays  581  and  582 . All of these components are identical to the corresponding components of the limiter  400  of  FIGS.  4 A and  4 B . 
     In contrast to the limiter  400  however, in the fuse  500  there is no elastic member, the second vane  512  is magnetic and the fuse  500  further comprises a biasing element in the form of a magnet  585 . The magnet  585  functions in a similar way to the magnet  385  of  FIGS.  3 A to  3 C , only it is located within the housing  540  and is not adjustable. The fuse  500  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  300  however, the fuse  500  is reusable and self-resetting. 
       FIGS.  6 A and  6 B  illustrate schematic plan views of another example optical limiter  600  comprising a light mill respectively in first and second positions. The limiter  600  functions in a similar way to the limiter  400  of  FIGS.  4 A and  4 B . It comprises a light mill enclosed in a housing  640  having an input port  621  and an output port  625 . The light mill is attached to the housing  640  via an elastic member (not shown) which biases it towards the position shown in  FIG.  6 A . A beam stopper  690  is provided, which functions in the same way to the beam stopper  490  of the limiter  400 . 
     The limiter  600  however differs from the limiter  400  of  FIGS.  4 A and  4 B  in that control of the light mill&#39;s rotation and direction of light from the input port  621  towards the output port  625  are provided separately, rather than both being provided by a first vane of the light mill as in the limiter  400 . As shown in  FIGS.  6 A and  6 B , a source beam arriving from the left-hand side encounters an optical splitter  661  which splits the source beam into a control beam and a transmission beam. 
     The control beam is directed to a mirror  671  which routes it on through a control port  631  to be incident on a first vane  611  of the light mill. The first vane  611  absorbs some or all of the energy of the control beam. Where the source beam is relatively powerful, as shown in  FIG.  6 B , 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  661  and the input port  621  to be incident centrally on a light director  613  such as a mirror. The light director  613  is part of the rigid assembly of the light mill, centered on the axle  614  and configured to turn with the first and second vanes  611  and  612  about the axle  614 . 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.  6 A , the transmission beam is incident centrally on the output port  625  so that the power of the output beam is maximized . With the light mill in the position shown in  FIG.  6 B , part of the transmission beam is blocked by the beam stopper  690  so the power of the output beam is reduced. In this way, the light director  613  and the beam stopper  690  together form an optical baffle apparatus arranged to prevent a portion of the transmission beam from illuminating the output port  625 , that portion&#39;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  681 . Clockwise rotation of the light mill is constrained by a pair of second stays  682 . One of the pair of second stays  682  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.  2 B , the positioning of the second stays  682  determines the response of the limiter  600  to high-power source beams. 
       FIGS.  7 A and  7 B  illustrate schematic plan views of an example optical fuse  700  of a similar design to the optical limiter  600  of  FIGS.  6 A and  6 B . The optical fuse  700  comprises a light mill having a first vane  711 , a second vane  712  and a light director  713  rigidly connected between them. All of the first and second vanes  711 ,  712  and the light director  713  are configured to rotate together about an axle  714 . The light mill is enclosed in a housing  740  having an input port  721 , an output port  725  and a control port  731 . The light mill is attached to the housing  740  via an elastic member (not shown) which biases it towards the position shown in  FIG.  7 A . A splitter  761  is configured to split a source beam into a transmission beam incident on the input port  721  and a control beam incident on the control port  731 , via reflection by a mirror  771 . The light mill&#39;s rotation is constrained by a first stay  781  and a pair of second stays  782 . All of these components function in the same way as the corresponding components of the limiter  600  of  FIGS.  6 A and  6 B . 
     In contrast to the limiter  600  however, in the fuse  700  there is no elastic member, the second vane  712  is magnetic and the fuse  700  further comprises a biasing element in the form of a magnet  785 . The magnet  785  functions in the same way as the magnet  585  of the fuse  500 . 
       FIGS.  8 A and  8 B  illustrate schematic plan views of another example optical limiter  800  comprising a light mill respectively in first and second positions. The limiter  800  functions in a similar way to the limiter  600  of  FIGS.  6 A and  6 B . It comprises a light mill enclosed in a housing  840  having an input port  821 , an output port  825  and a control port  831 . The light mill is attached to the housing  840  via an elastic member (not shown) which biases it towards the position shown in  FIG.  8 A . A splitter  861  is configured to split a source beam into a transmission beam incident on the input port  821  and a control beam incident on the control port  831 , via reflection by a mirror  871 , just like in the limiter  600  of  FIGS.  6 A and  6 B . The light mill&#39;s clockwise rotation is constrained by a pair of stays  882  corresponding to the pair of second stays  682  of the limiter  600 . 
     The limiter  800  however differs from the limiter  600  of  FIGS.  6 A and  6 B  in that the light mill of the limiter  800  does not comprise a light director. The limiter  800  has an optical baffle apparatus comprising two beam stoppers  891  and  892  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  891  and  892  are rigidly connected between the first and second vanes  811  and  812  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  825  is parallel to and coaxial with the input port  821 . When the light mill is in the position shown in  FIG.  8 A , the aperture between the beam stoppers  891  and  892  is intermediate and coaxial with the input port  821  and output port  825  so that the power of the output beam is maximized. As shown in  FIG.  8 B , as the light mill rotates from the position shown in  FIG.  8 A  to the position shown in  FIG.  8 B  an increasing area of the transmission beam is blocked by the beam stoppers  891  and  892  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.  8 A  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  811  and  812  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  840 . Alternatively, the elastic member could be sufficient to perform this function. 
       FIGS.  9 A and  9 B  illustrate schematic plan views of an example optical fuse  900  of a similar design to the optical limiter  800  of  FIGS.  8 A and  8 B . The optical fuse  900  comprises a light mill having a first vane  911 , a second vane  912  and two beam stoppers  991  and  992 , defining an aperture, rigidly connected between the first and second vanes  911  and  912 . All of the first and second vanes  911 ,  912  and the first and second beam stoppers  991 ,  992  are configured to rotate together about an axle (not shown). The light mill is enclosed in a housing  940  having an input port  921 , an output port  925  and a control port  931 . A splitter  961  is configured to split a source beam into a transmission beam incident on the input port  921  and a control beam incident on the control port  931 , via reflection by a mirror  971 . All of these components are identical to the corresponding components of the limiter  800  of  FIGS.  8 A and  8 B . 
     In contrast to the limiter  800  however, in the fuse  900  there is no elastic member, the second vane  912  is magnetic and the fuse  900  further comprises a biasing element in the form of a magnet  985 . The magnet  985  functions in the same way as the magnet  785  of the fuse  700 . 
     The fuse  900  also comprises first and second stays  981  and  982  to respectively constrain anticlockwise and clockwise rotation of the light mill in a similar manner to the first and second stays  581  and  582  of the fuse  500  of  FIGS.  5 A and  5 B . 
     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 aluminum foil, anodized aluminum 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.  1 C  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  200 ,  300 ,  400  and  500  described above in relation to  FIGS.  2 A to  5 B , 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) counter-rotation 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 fibers . They can optionally comprise lenses to focus or defocus that light as appropriate. 
     Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the embodiments disclosed herein. It is intended that the specification and examples be considered as exemplary only. In addition, where this application has listed the steps of a method or procedure in a specific order, it could be possible, or even expedient in certain circumstances, to change the order in which some steps are performed, and it is intended that the particular steps of the method or procedure claims set forth herein not be construed as being order-specific unless such order specificity is expressly stated in the claim. That is, the operations/steps may be performed in any order, unless otherwise specified, and embodiments may include additional or fewer operations/steps than those disclosed herein. It is further contemplated that executing or performing a particular operation/step before, contemporaneously with, or after another operation is in accordance with the described embodiments.