Patent Publication Number: US-2020278536-A1

Title: Treating an optical waveguide

Description:
TECHNICAL FIELD OF THE INVENTION 
     The invention relates to an optical waveguide and, more particularly, to a method, device and system for treating an optical wave. 
     BACKGROUND TO THE INVENTION 
     A shaving device has been proposed in WO 2014/143670 that makes use of laser light. In particular, a laser light source is provided that is configured to generate laser light having a wavelength selected to target a predetermined chromophore to effectively cut a hair shaft. An optical fibre is located on a shaving portion of the device that is positioned to receive the laser light from the laser light source at a proximal end, conduct the laser light from the proximal end toward a distal end, and emit the light out of a cutting region of the optical fibre and toward hair when the cutting region is brought in contact with the hair. 
     Over time, use of such a shaving device may cause the optical fibre to become dirty. For example, debris, such as pieces of cut hair, dust or dirt may accumulate on the optical fibre. Such dirt may adversely affect the cutting ability of the cutting device, thereby reducing the effectiveness of the device. 
     Furthermore, debris built up on the optical fibre may be caused to heat up and eventually burn during use of the shaving device. Consequently, the optical fibre may be caused to heat up during use. If the temperature of the optical fibre increased too much, or too rapidly, then the optical fibre may be damaged. 
     Conventional optical fibre cleaning methods include wiping the dirt off the optical fibre, for example by using a cloth. However, dues to the fragility of the optical fibre, such cleaning methods may cause damage to the optical fibre. 
     It is to be noted that US 2005/0284185 A1 discloses a method for cleaning an optical fiber by bringing the optical fiber into a physical contact with the cleaning member. The cleaning member is disclosed to be formed of a porous or mesh member. 
     It is further to be noted that U.S. Pat. No. 6,227,942 B1 discloses a method for finishing a workpiece in which the workpiece is placed in a ferrofluid in which abrasive material is present. According to this method magnetic fields are alternately generated within the vessel comprising the workpiece and the ferrofluid which allegedly results in back and forth relative motion between the abrasive media and the workpiece. 
     SUMMARY OF THE INVENTION 
     Treating (e.g. cleaning) an optical fibre regularly may lengthen the life of the optical fibre and the shaving or cutting device in which it is installed. Furthermore, a cleaner optical fibre is likely to be more efficient at cutting hair than one on which dirt and debris has built up. Thus, an improved method for treating an optical fibre may improve the hair cutting experience for a user, and may improve the lifetime of the user&#39;s device. 
     Thus, there is a need for a method of treating an optical fibre which reduces the likelihood of damaging the optical fibre. 
     According to a first aspect, there is provided a method of treating an optical waveguide, the method comprising applying the volume of ferrofluid to the optical waveguide; and applying a magnetic field to the volume of ferrofluid to move the volume of ferrofluid at least partially along a length of the optical waveguide. By applying a volume of ferrofluid to the optical waveguide, it is possible to treat and clean the optical waveguide without applying direct pressure to the waveguide, which may result in the damage. The ferrofluid can be moved along the optical waveguide by controlling a magnetic field to manipulate the volume of ferrofluid in an intended manner. 
     According to some embodiments, applying a magnetic field may comprise applying a magnetic field generated by at least one of a permanent magnet and an electromagnet. 
     The ferrofluid may comprise a binder fluid. The binder fluid may, in some embodiments, comprise an oil-based binder fluid. 
     In some embodiments, the ferrofluid may comprise a chemical for providing a cleaning effect to the optical waveguide. In some embodiments, the chemical may dissolve pieces of hair that have accumulated on the optical waveguide. 
     The ferrofluid may comprise an additive for causing an abrasive effect when the additive is moved into contact with the optical waveguide. 
     The method may, in some embodiments, further comprise causing the magnetic field to move at least one of longitudinally relative to the optical waveguide and laterally relative to the optical waveguide. 
     The method may further comprise displacing at least a portion of the volume of ferrofluid from the optical waveguide. Such displacement may be used to remove some or all of the ferrofluid from the optical waveguide once the optical waveguide has been treated. 
     According to a second aspect, there is provided a personal care device comprising an optical waveguide having a sidewall and an optical axis, wherein a portion of the sidewall forms a cutting face for contacting hair. The personal care device further comprises a magnetic field source configured to generate a magnetic field for engaging a volume of ferrofluid present on the optical waveguide. The personal care device includes means for manipulating the ferrofluid. A magnetic field movement mechanism causes the magnetic field to move relative to the optical waveguide, along an optical axis of the optical waveguide. 
     In some embodiments, the personal care device may further comprise an applicator for applying a volume of ferrofluid to the optical waveguide. 
     The personal care device may further comprise a fluid displacement device for displacing at least a portion of the volume of the ferrofluid from the optical waveguide. 
     According to a third aspect, there is provided a system comprising a personal care device, the personal care device comprising an optical waveguide having a sidewall and an optical axis, wherein a portion of the sidewall forms a cutting face for contacting hair. The system further comprises a docking unit for receiving at least a portion of the personal care device. The docking unit comprises a magnetic field generator for generating a magnetic field to engage a volume of ferrofluid disposed on the optical waveguide of the personal care device. With this arrangement, the components used to manipulate the ferrofluid are included in the docking unit rather than in the personal care device. Therefore, the personal care device (or components thereof) may be disposable or replaceable. The docking unit comprises a magnetic field movement mechanism for causing the magnetic field to move relative to the optical waveguide, along an optical axis of the optical waveguide. 
     The docking unit may further comprise an applicator for applying a volume of ferrofluid to the optical waveguide. 
     The docking unit may in some embodiments, further comprise a fluid displacement device for displacing at least a portion of the volume of the ferrofluid from the optical waveguide. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which: 
         FIG. 1  is a block diagram of a hair cutting device according to embodiments of the invention; 
         FIG. 2  is a pair of schematic drawings showing different views of an exemplary hair cutting device; 
         FIG. 3  is a schematic illustration of an example of an arrangement for manipulating a ferrofluid on an optical waveguide; 
         FIG. 4  is a schematic illustration of a further example of an arrangement for manipulating a ferrofluid on an optical waveguide; 
         FIG. 5  is a flowchart of an example of a method of treating an optical waveguide; 
         FIG. 6  is a flowchart of a further example of a method of treating an optical waveguide; 
         FIG. 7  is a flowchart of a further example of a method of treating an optical waveguide; 
         FIG. 8  is a schematic illustration of an example of a personal care device; 
         FIG. 9  is a schematic illustration of an example of a portion of the personal care device of  FIG. 8 ; 
         FIG. 10  is a schematic illustration of an example of a personal care system; and 
         FIG. 11  is a schematic illustration of an example of a portion of the personal care system of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is a block diagram of a hair cutting device  100 .  FIG. 2  shows a hair cutting device  100  in the form of a handheld razor according to an exemplary embodiment of the invention. The hair cutting device  100  is for cutting (e.g. shaving) hair on a body of a subject. The subject may be a person or an animal. The hair may be facial hair (i.e. hair on the subject&#39;s face), or hair on the subject&#39;s head or other part of their body (legs, chest, etc.). 
     The hair cutting device  100  comprises a cutting element  102  that enables hair to be cut as the hair cutting device  100  is moved over the skin of a subject. The cutting element  102  is an optical waveguide  102  that is arranged on the hair cutting device  100  so that the optical axis of the optical waveguide  102  (i.e. the line along which light typically propagates through the optical waveguide  102 ) is generally perpendicular to the direction in which the hair cutting device  100  is moved so that hairs contact the sidewall of the optical waveguide  102  (the sidewall corresponding to the long edge of the optical waveguide  102 ) as the hair cutting device  100  is moved across the skin of the subject. In some embodiments, the optical waveguide  102  is an optical fibre, although those skilled in the art will be aware of other types of optical waveguide that can be used according to the invention, such as a slab waveguide, a strip waveguide or a photonic crystal waveguide. An optical fibre comprises a core, and in some embodiments also comprises a cladding, which may or may not fully encompass the core (e.g. part of the core may be exposed). The optical waveguide  102  may form part of a cutting assembly of the hair cutting device  100 . The cutting assembly may, in some embodiments, be a detachable and/or replaceable component, and may be designed to be replaced as the optical waveguide  102 , or other components of the cutting assembly, become worn or damaged. 
     A light source  104  is provided in the hair cutting device  100  that generates laser light at one or more specific wavelengths. The light source  104  is optically coupled to the optical waveguide  102  so that the laser light generated by the light source  104  is coupled into the optical waveguide  102  (and specifically coupled into at least one end of the optical waveguide  102  so that the laser light propagates through the optical waveguide  102 ). 
     The light source  104  is configured to generate laser light at one or more specific wavelengths that can be used to cut or burn through hair. In particular, each wavelength corresponds to the wavelength of light absorbed by a chromophore that is found in hair. As is known, a chromophore is the part of a molecule that provides the molecule with its colour. Thus, the laser light will be absorbed by the chromophore and converted into heat which will melt or burn the hair or otherwise destroy the bonds in the molecules of the hair, and it is this melting or burning that provides the cutting action of the hair cutting device  100 . 
     Suitable chromophores that can be targeted by the laser light generated by the light source  104  include, but are not limited to, melanin, keratin and water. Suitable wavelengths of laser light that can be used include, but are not limited to, wavelengths selected from the range 380 nm (nanometres) to 500 nm and 2500 nm to 3500 nm. Those skilled in the art will be aware of the wavelengths of light that are absorbed by these chromophores, and thus also the specific wavelengths of light that the light source  104  should generate for this purpose, and further details are not provided herein. 
     In some embodiments the light source  104  can be configured to generate laser light at a plurality of wavelengths (either simultaneously or sequentially), with each wavelength being selected to target a different type of chromophore. This can improve the cutting action of the optical waveguide  102  since multiple types of molecules in the hair may be burnt using the laser light. Alternatively multiple light sources  104  can be provided that each generate laser light at a respective wavelength. The multiple light sources may be coupled to a single optical waveguide, or each light source  104  can be coupled to a respective optical waveguide  102  to provide multiple cutting elements  102  in the device  100 . 
     The hair cutting device  100  may also comprise a control unit  106  that controls the operation of the hair cutting device  100 , and in particular may be connected to the light source  104  to control the activation and deactivation of the light source  104  (and in some embodiments control the wavelength and/or intensity of the light generated by the light source  104 ). The control unit  106  may activate and deactivate the light source  104  in response to an input from a user of the hair cutting device  100 . The control unit  106  can comprise one or more processors, processing units, multi-core processors or modules that are configured or programmed to control the hair cutting device  100 . 
     As noted above,  FIG. 2  shows a hair cutting device  100  that is in the form of a handheld wet razor.  FIG. 2  shows a side view and a bottom view of the razor  100 . The razor  100  comprises a handle  108  for the subject (or other user of the device  100 ) to hold, and a head portion  110  that includes the cutting element  102  (optical waveguide/fibre). As shown, the optical waveguide  102  is arranged along an edge of the head portion, and a part of the optical waveguide  102  forms (or corresponds to) a cutting face  112 . The cutting face  112  is the part of the optical waveguide  102  that is intended to come into contact with hair as the hair cutting device  100  is moved across the skin of the subject. A light source  104  and control unit  106  are shown as being incorporated into the head portion  110  and handle  108  respectively, but it will be appreciated that the positions of these components in the hair cutting device  100  as shown in  FIG. 2  is not limiting. Likewise it will be appreciated that the embodiment shown in  FIG. 2  is merely an example, and the invention can be incorporated or used in any type of hair cutting device  100  that comprises an optical waveguide cutting element  102  as described herein. 
     As is known, the optical waveguide  102  acts as a waveguide for the light coupled from the light source  104  through the occurrence of total internal reflection, since the refractive index of air is lower than that of the optical waveguide  102 . However, if an object that has a refractive index higher than the optical waveguide  102  is put into contact with the optical waveguide  102 , then the total internal reflection is ‘frustrated’ and light can couple from the optical waveguide  102  into that object. Thus, in order for light to be coupled into a hair from the optical waveguide  102  (to provide the cutting action according to the invention), the optical waveguide  102  should preferably have the same or a lower refractive index than hair at the point at which the hair contacts the optical waveguide  102 . Thus, the optical waveguide  102  should preferably have the same or a lower refractive index than hair at least at the cutting face  112  portion of the optical waveguide  102 . Preferably the refractive index of the optical waveguide  102  at the cutting face  112  is the same as that of hair since that provides the best coupling of light from the optical waveguide  102  to the hair. Light may still be able to couple from the optical waveguide  102  into an object (e.g. a hair) brought into contact with the cutting face  112  of the optical waveguide even if the refractive index of the optical waveguide is higher than that of the object, due to a high numerical aperture in the cutting face. 
     Pieces of hair that have been cut by the optical waveguide  102  may rest on the optical waveguide, or may become stuck to the optical waveguide, for example by a shaving liquid, such water or shaving gel applied to the user&#39;s face. Particles of dust, dirt and other debris may also accumulate on the optical waveguide  102  and, as noted above, the high temperatures experienced by the cutting face of the optical waveguide may cause such debris to burn. In order to assist a user in removing such debris from the optical waveguide  102 , the present invention involves the use of a ferrofluid. A ferrofluid is a substance that becomes magnetized in the presence of a magnetic field. A ferrofluid is a colloidal suspension which includes ferromagnetic, or ferrimagnetic, particles suspended in a carrier fluid. The particles may, in some examples, be nanoparticles. The carrier fluid may be water, or some other liquid, such as an organic solvent. 
     In general, the invention relates to the use of a ferrofluid to treat an optical waveguide. Due to the inherent properties of a ferrofluid (i.e. its magnetization in a magnetic field), a volume of ferrofluid, such as a droplet of ferrofluid, can be manipulated by controlling a magnetic field acting on the ferrofluid. 
       FIG. 3  is a schematic illustration of an arrangement  300  showing an example of how a ferrofluid may be used to treat an optical waveguide in accordance with embodiments of the invention. In the arrangement  300  of  FIG. 3 , a portion of the optical waveguide  102  is shown. It will be appreciated that the optical waveguide  102  may form a part of the cutting device  100  discussed above. The cutting device  100  is omitted from  FIG. 3  for clarity. A volume of ferrofluid  302  is present on the optical waveguide  102 . Any volume of ferrofluid may be applied to the optical waveguide  102 . However, it will be apparent that a volume sufficient to extend around the circumference of the optical waveguide  102  may result in a more efficient treatment process. If the volume of ferrofluid applied to the optical waveguide  102  is too large, then it may be more difficult to manipulate the ferrofluid using an applied magnetic field. Therefore, in some embodiments, a droplet of ferrofluid may be applied to the optical waveguide  102 . 
     A magnetic field source  304  is provided proximal to the optical waveguide  102 , and is configured to generate a magnetic field B. The magnetic field source  304  is positioned such that the generated magnetic field B is able to interact with the volume of ferrofluid  302 . In the example shown in  FIG. 3 , the magnetic field source  304  comprises an electromagnetic coil capable of generating a magnetic field when a current I is passed through the coil. In other examples, alternative magnetic field sources may be used, such as one or more permanent magnets. While, in  FIG. 3 , the magnetic field source  304  is shown to one side of the optical waveguide  102 , it will be appreciated that the magnetic field source may extend partially around the optical waveguide, for example around half of the circumference of the optical waveguide. 
     The magnetic field B shown in the arrangement  300  of  FIG. 3  can be manipulated in such a way that it moves along an optical axis of the optical waveguide  102 , for example in the direction x indicated by the arrows. Manipulation of the magnetic field B may be achieved in a number of ways. In some examples, the magnetic field B may be generated by a magnetic field source  304  (e.g. a coil) extending along only a portion of a length of the optical waveguide  102 . The magnetic field source  304  may be moveable along the length of the optical waveguide  102 , so that the magnetic field B can be moved in the direction x. In this way, the magnetic field source  304  and the magnetic field B it generates can be moved in either direction along the length of the optical waveguide  102 . The magnetic field B interacts with the volume of ferrofluid  302  and may, for example, cause the ferrofluid to be attracted towards (or repelled away from) the magnetic field source  304 . By moving the magnetic field source  304  in an appropriate manner (e.g. along the length of the optical waveguide), the volume of ferrofluid  302  can be caused to move along the optical waveguide  102 . In other examples, a magnetic field source  304  may extend along the length of the optical waveguide  102 , for example as a series of electromagnetic coils. With such an arrangement, portions of the magnetic field source  304  may be independently controlled and actuated (e.g. by switching on a first coil, then switching off the first coil and switching on a second, adjacent coil, then switching off the second coil and switching on a third coil, and so on), to cause the volume of ferrofluid to be moved along the optical waveguide  102 . In this arrangement, the magnetic field source  304  does not move relative to the optical waveguide  102 , so a mechanism to move the magnetic field source is not included. 
     Parameters of the magnetic field B and, therefore, parameters of the magnetic field source may be selected or controlled such that the volume of ferrofluid may be moved along the optical waveguide, but is not removed from the waveguide. In other words, the magnetic field may be controlled such that the force exerted on the ferrofluid in a direction away from the optical waveguide is not stronger than the surface tension causing the ferrofluid to remain in contact with the optical waveguide. 
     As the ferrofluid moves along the optical waveguide  102 , particles of cut hair, dust and other debris may be removed from the optical waveguide and may accumulate in the volume of ferrofluid  302 . Causing the volume of ferrofluid  302  to move along the length of the optical waveguide  102  repeatedly may result in the optical waveguide becoming cleaner, with fewer particles of debris remaining on the waveguide. As explained below, once the ferrofluid has been moved along the optical waveguide  102 , at least a portion of the volume of ferrofluid  302  may be removed from the optical waveguide, along with debris that has been removed from the waveguide. 
       FIG. 4  is a schematic illustration of an arrangement  400  showing an example of how a ferrofluid may be used to treat an optical waveguide in accordance with embodiments of the invention. In the arrangement  400  of  FIG. 4 , a portion of the optical waveguide  102  is shown with a volume of ferrofluid  302  disposed on the optical waveguide. In the arrangement  400 , a first magnetic field source  304  is provided on one side of the optical waveguide  102 , and a second magnetic field source  306  is provided on an opposing side of the optical waveguide. In some embodiments, the first and second magnetic field sources  304 ,  306  may form part of the same magnetic field source, but may be individually operable and controllable. Using the arrangement  400  of  FIG. 4 , the magnetic field sources  304 ,  306  may be actuated (i.e. caused to generate a magnetic field B alternately, or simultaneously but at different strengths. In other words, the first magnetic field source  304  may generate a magnetic field B during a first time period t 1  while the second magnetic field source  306  does not generate a magnetic field. The first magnetic field source  304  may then be de-activated (e.g. switched off) so that it does not generate a magnetic field for a timer period t 2 , during which period the second magnetic field source  306  may generate a magnetic field B. Applying an alternating magnetic field in this way may cause the volume of ferrofluid  302  to move in an oscillatory fashion between the first and second magnetic field sources, in a direction y indicated by the arrow in  FIG. 4 . By causing the ferrofluid to move in this way, dirt and debris stuck to the optical waveguide  102  may be released and suspended in the ferrofluid. 
     In a further example, a magnetic field source  304 ,  306  may be disposed at one or more positions along the optical waveguide  102 . For example, a magnetic field source may be positioned at a first end of the optical waveguide. In this way, a volume of ferrofluid deposited at a second end (i.e. opposite to the first end) may be attracted to, by the magnetic field, to the magnetic field source at the first end. In some examples, a magnetic field source may be positioned at each end of the optical waveguide  102 . In this way, alternately causing each of the magnetic field sources to generate a magnetic field will cause a volume of ferrofluid to travel along the length of the optical waveguide in a first direction, then back in the opposite direction, and so on. 
     In a further example, the arrangements  300  and  400  may be combined such that the volume of ferrofluid  302  may be moved in an oscillatory manner (e.g. vibrated) in the y direction while it is moved along the length of the optical waveguide  102  in the x direction. In this way, the effectiveness of the ferrofluid in removing debris from the optical waveguide may be further improved. 
     According to a first aspect, a method is provided.  FIG. 5  is a flowchart of an example of a method  500  of treating an optical waveguide  102 . The method comprises, at step  502 , applying the volume of ferrofluid  302  to the optical waveguide  102 . At step  504 , the method  500  comprises applying a magnetic field to the volume of ferrofluid  302  to move the volume of ferrofluid at least partially along a length of the optical waveguide  102 . 
     The magnetic field may be applied to the volume of ferrofluid by using a magnetic field source such as the source  304  and/or the source  306 , as discussed above. The volume of ferrofluid may, in some embodiments, be a single droplet. The ferrofluid may be applied to the optical waveguide  102  using any suitable dispensing means. For example, the ferrofluid may be applied manually using a device, such as a dropper or pipette. Alternatively, the ferrofluid may be applied using an automatic device configured, for example, to deposit a defined amount (e.g. volume) of ferrofluid from a ferrofluid reservoir onto the optical waveguide  102 . 
     As noted above, the magnetic field may be generated using one or more various sources. In some embodiments, the step of applying ( 504 ) a magnetic field may comprise applying a magnetic field generated by at least one of a permanent magnet and an electromagnet. In the case of an electromagnet being used as the magnetic field source, one or more electromagnetic coils may be used. In some embodiments, a single coil may be movable along the length of the optical waveguide while, in other embodiment, one or more coils which are stationary with respect to the optical waveguide may be positioned along the length of the optical waveguide. In embodiments in which one or more stationary coils are implemented, each coil may be selectively controlled to generate a magnetic field of a particular strength at a particular time and for a particular duration. 
     In addition to ferromagnetic particles, the ferrofluid may include one or more additives for providing additional benefits. In some embodiments, the ferrofluid may comprise a binder fluid. A binder fluid may be included to improve the binding effect of the ferrofluid to the optical waveguide. A volume of ferrofluid which includes such a binder fluid may deposit a portion of the binder fluid onto the optical waveguide as the volume of ferrofluid moves along the length of the optical waveguide. The binder fluid may be selected such that the coating, or film, deposited on the optical waveguide improves the cutting effectiveness of the optical waveguide. For example, a binder fluid may be chosen which, when deposited on the optical waveguide, helps to initiate the hair cutting process. Such an effect may, for example, be achieved using an oil-based binder fluid. 
     In some embodiments, the ferrofluid may comprise a chemical, for example in addition to, or instead of, the binder fluid. The chemical may, in some examples, provide a cleaning effect to the optical waveguide. In one embodiment, the ferrofluid may comprises a chemical for targeting a chromophore present in hair, such as keratin. For example, the chemical may break down or dissolve keratin. In this way, the chemical may serve to dissolve pieces of hair disposed on (e.g. stuck to) the optical waveguide, thereby further improving the cleaning effect of the ferrofluid. 
     The ferrofluid may, in some embodiments, comprise an additive for causing an abrasive effect when the additive is moved into contact with the optical waveguide. For example, the ferrofluid may comprise micro-particles. An abrasive additive such as micro-particles may serve to knock or rub against the optical waveguide as the ferrofluid moves over the optical waveguide. In some examples, particles (e.g. micro-particles) of an abrasive additive may be caused to move around within the volume of ferrofluid as the volume of ferrofluid oscillates (e.g. vibrates) on the optical waveguide due to the magnetic field, as exemplified in the arrangement  400  of  FIG. 4 . 
     The ferrofluid may comprise one or more of the fluids, chemicals and additives discussed above, and/or one or more other additives. 
       FIG. 6  is a flowchart of an example of a method  600  of treating an optical waveguide, such as the optical waveguide  102 . The method  600  may comprise one or more of the steps of the method  500  above. The method  600  may comprise, at step  602 , causing the magnetic field to move at least one of longitudinally relative to the optical waveguide and laterally relative to the optical waveguide. In other words, the magnetic field may be caused to move in the x direction (see  FIG. 3 ) and/or in they direction (see  FIG. 4 ). In order to move the magnetic field, the magnetic field source may be caused to move at least one of longitudinally relative to the optical waveguide and laterally relative to the optical waveguide. 
       FIG. 7  is a flowchart of an example of a method  700  of treating an optical waveguide, such as the optical waveguide  102 . The method  700  may comprise one or more of the steps of the methods  500 ,  600  above. The method  700  may comprise, at step  702 , displacing at least a portion of the volume of ferrofluid from the optical waveguide. Some or all of the volume of ferrofluid may be displaced from the optical waveguide  102  once it is has been used to treat (e.g. clean) the optical waveguide. For example, after a defined time of moving the ferrofluid along the optical waveguide, the ferrofluid (or a portion thereof) may be removed. Debris removed from the optical waveguide by the ferrofluid may also be removed by displacing the volume of ferrofluid. 
     Displacement of at least a portion of the volume of ferrofluid may be achieved in various ways. For example, some or all of the ferrofluid may be displayed (e.g. removed) from the optical waveguide using a fluid-flow device arranged to direct a fluid (e.g. a stream, jet or curtain of air, water or another gas or liquid) towards the volume of ferrofluid so as to remove an amount of the ferrofluid from the optical waveguide. It will be appreciated that, in some scenarios, it may be desirable for an amount of the volume of ferrofluid to remain on the optical waveguide, for example to coat a surface of the optical waveguide to improve the effectiveness of the coupling of light from the waveguide into hair to be cut. As such, while the device to displace the ferrofluid may, in some embodiments, be directed or aimed at the entire length of the optical waveguide, in other embodiments, the device may be directed towards a particular portion of the optical waveguide. 
     According to another aspect, the invention relates to a personal care device, such as the hair cutting device  100  discussed above.  FIG. 8  is a schematic illustration of an example of a personal care device  800 . The personal care device  800  comprises an optical waveguide  102  having a sidewall, wherein a portion of the sidewall forms a cutting face  112  for contacting hair. The personal care device  800  further comprises a magnetic field source  802  configured to generate a magnetic field for engaging a volume of ferrofluid  302  present on the optical waveguide  102 . In some embodiments, the personal care device  800  may include a handle portion  108  and a head portion  110 . In the embodiment shown in  FIG. 8 , the magnetic field source  802  is located in the head portion  110 . However, it will be appreciated that, in other embodiments, the magnetic field source  802  may be located elsewhere in the personal care device  800 , such as in the handle portion  108 . 
     The magnetic field source  802  is shown in  FIG. 8  to extend substantially along the length of the optical waveguide  102 . However, it will be appreciated that the magnetic field source  802  may be sized, positioned and/or configured to focus a magnetic field on a particular portion of the optical waveguide, and may be moveable (e.g. in the direction of the optical axis of the optical waveguide), as discussed above. 
       FIG. 9  is a schematic illustration of a further example of the personal care device  800 . According to various embodiments, the personal care device  800  may be provided with one or more additional or alternative components. In some embodiments, the personal care device  800  may comprise a magnetic field movement mechanism  804  for causing the magnetic field to move relative to the optical waveguide  102 . The magnetic field movement mechanism  804  may move the magnetic field source  802 , or cause the magnetic field source to move, for example across or along the optical waveguide. Alternatively, the magnetic field movement mechanism  804  may manipulate the magnetic field itself, for example by directing the magnetic field and/or controlling the strength of the magnetic field at particular places along the optical waveguide. 
     The personal care device  800  may, in some embodiments, comprise an applicator  806  for applying a volume of ferrofluid  302  to the optical waveguide. The applicator, or ferrofluid applicator,  806  may comprise any mechanism suitable for applying a volume, or multiple volumes, of ferrofluid (e.g. in the form of a droplet) to a particular location of the optical waveguide. The applicator  806  may, for example, comprise a spray mechanism for spraying the ferrofluid onto the optical waveguide  102 , a nebuliser device for depositing the ferrofluid on the optical waveguide, a mechanism similar to, or based on, a mechanism used in a printing apparatus for depositing ink onto a printable substrate, or a droplet depositing mechanism. Those skilled in the art will appreciate that other means for depositing or applying ferrofluid onto the optical waveguide  102  may be implemented for use as the applicator  806 . 
     In some embodiments, the personal care device  800  may comprise a fluid displacement device  808  for displacing at least a portion of the volume of the ferrofluid  302  from the optical waveguide  102 . The fluid displacement device  808  may be configured to remove some or all of the ferrofluid from the optical waveguide  102 , for example after the ferrofluid has been used to treat the optical waveguide. In some embodiments, the fluid displacement device  808  may comprise a fluid flow device, such as a device configured to direct fluid (e.g. air or water) towards the optical waveguide  102  and/or towards the volume of ferrofluid to remove excess ferrofluid from the optical waveguide. 
     It will be appreciated that, while the magnetic field movement mechanism  804 , the applicator  806  and the fluid displacement device  808  are shown in  FIG. 9  to be components of the head portion  110  of the personal care device  800 , those features may, in other embodiments, be components of another part of the personal care device, and/or positioned elsewhere in the personal care device, such as in the handle portion  108 . 
     In the embodiment shown in  FIG. 8 , the magnetic field source  802  is located in the personal care device  800 . However, in other embodiments, the magnetic field source  802  may be located remote from the personal care device.  FIG. 10  is a schematic illustration of an example of a system  1000 , such as a personal care system. The system  1000  comprises a personal care device  1002  and a docking unit  1004 . The personal care device  1002  comprises an optical waveguide  102  having a sidewall, wherein a portion of the sidewall forms a cutting face  112  for contacting hair. The personal care device  1002  may include a head portion  110  and a handle portion  108 . The docking unit  1004  is for receiving at least a portion of the personal care device. For example, the docking unit  1004  may be configured to receive the head portion  110  of the personal care device  1002  in a suitably sized and shaped receiving portion (not shown). The docking unit  1004  comprises a magnetic field generator  1006  for generating a magnetic field to engage a volume of ferrofluid  302  disposed on the optical waveguide  102  of the personal care device  1002 . The magnetic field generator  1006  may be the same, or similar to the magnetic field source  802  discussed above. 
     A user of the system  1000  may use the personal care device  1002  to perform a personal care activity, such as trimming facial hair. Once the personal care activity has been completed, the user may place the personal care device  1002  in or on the docking unit  1004  to treat (e.g. clean) the optical waveguide  102 . In some embodiments, a user may initiate treatment of the optical waveguide  102  manually, for example by pressing a button. In other examples, treatment of the optical waveguide  102  may commence automatically upon detection that personal care device  1002  has been docked. 
     A volume of ferrofluid  302  may be applied to the optical waveguide  102  using any of the method described herein. The magnetic field generator  1006  may be configured of controlled to generate a magnetic field which engages and interacts with the volume of ferrofluid, thereby causing the ferrofluid to move along the optical waveguide  102 . The ferrofluid may treat the optical waveguide  102  in the various ways discussed herein. For example, the ferrofluid may cause debris (e.g. pieces of cut hair) to be removed from the optical waveguide  102 , and a film or coating may be formed on the optical waveguide, which may improve the effectiveness of hair cutting. 
       FIG. 11  is a schematic illustration of a further example of the system  1000 . In  FIG. 11 , only the docking unit  1004  is shown. The docking unit  1004  includes the magnetic field generator  1006 . According to various embodiments, as shown in  FIG. 11 , the docking unit  1004  may also include additional functionality, as discussed herein. 
     In some embodiments, the docking unit  1004  may comprise a magnetic field movement mechanism  1008  for causing the magnetic field to move relative to the optical waveguide  102 . The magnetic field movement mechanism  1008  may be the same as, or similar to, the magnetic field movement mechanism  804  discussed above. In this embodiment, however, the mechanism  1008  is located in the docking unit  1004  rather than in the personal care device. The magnetic field movement mechanism  1008  may, in some embodiments, move the magnetic field generator  1006 . 
     The docking unit  1004  may, in some embodiments, comprise an applicator  1010  for applying a volume of ferrofluid to the optical waveguide  102 . The applicator  1010  may be the same as, or similar to, the applicator  806  discussed above. In this embodiment, however, the mechanism  1008  is located in the docking unit  1004  rather than in the personal care device. 
     In some embodiments, the docking unit  1004  may comprise a fluid displacement device  1012  for displacing at least a portion of the volume of the ferrofluid from the optical waveguide  102 . The fluid displacement device  1012  may be the same as, or similar to, the fluid displacement device  808  discussed above. In this embodiment, however, the fluid displacement device  1012  is located in the docking unit  1004  rather than in the personal care device. 
     It will be appreciated by those skilled in the art that the personal care devices and/or the systems disclosed herein may comprise one or more additional components not discussed herein. For example, the devices and/or systems may include a power source for supplying power to one or more other components. 
     The methods, devices and systems described herein provide an effective means for treating an optical waveguide of a personal care device in manner which reduces the likelihood of damaging the optical waveguide, and which can improve the hair cutting effectiveness of the device. 
     Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.