A vapour containment structure

Abstract

This invention provides a vapour containment structure, a system comprising the vapour containment structure, and a method of operating the system, the vapour containment structure comprising: an elongate section defining an interior passageway, the elongate section having a major axis; a first optical access window permitting passage of an optical path into the elongate section and along the interior passageway of the elongate section; and a first sealable section further defining an interior passageway, the first sealable section having a major axis, wherein: the interior passageway of the first sealable section comprises a port for receiving vapour, the interior passageway of the first sealable section joins the interior passageway of the elongate section permitting fluidic communication between the interior passageway of the first sealable section and the interior passageway of the elongate section, the major axis of the first sealable section is offset from the major axis of the elongate section, and the first sealable section comprises a sealing point along its length that, when sealed, prevents fluidic communication into the interior passageway of the first sealable section.

Description

[0001] A VAPOUR CONTAINMENT STRUCTURE

[0002] Field of the Invention

[0003] The present invention relates to a vapour containment structure, a system comprising the vapour containment structure, and a method of operating the system.

[0004] Background

[0005] It is desirable to contain an atomic vapour, such as a vapour of alkali and alkali earth metals, for use in a variety of quantum technologies, such as an atomic receiver or atomic clock, and a variety of optical technologies, such as an optical reference or gas laser. Typically, a containment of atomic vapour is provided as a vessel. However, it is desirable to confine the containment of atomic vapour to a low-aspect ratio structure, such as a Hollow Core Fibre (HCF) or other waveguide, to enable direct integration with quantum and / or optical technologies. The ability to create an optical waveguide which ensures the optical path intercepts a volume of vapour is challenging for several reasons.

[0006] Firstly, the material that is typically contained for use in many quantum applications, such as Rubidium, tends to be highly reactive. For example, Rubidium exposed to air and / or water reacts violently and irreversibly, leading to compounds that no longer have the useful electronic configurations of the unreacted Rubidium vapour.

[0007] Secondly, the material is typically contained at a low pressure to ensure the material is in its gas phase. This presents a challenge when filling low aspect ratio structures, such as an HCF or other waveguide, as the narrow aperture of such structures limits the rate of filling.

[0008] Thirdly, when the material is contained within an HCF (or similar structure), there is a high surface area to volume ratio between the contained material and the surrounding structure. The resulting interactions between the surrounding structure and the material must be mitigated or managed.

[0009] It is therefore desirable to alleviate at least some of the above problems. Summary of the Invention

[0010] According to a first aspect of the invention, there is provided a vapour containment structure comprising: an elongate section defining an interior passageway, the elongate section having a major axis; a first optical access window permitting passage of an optical path into the elongate section and along the interior passageway of the elongate section; and a first sealable section further defining an interior passageway, the first sealable section having a major axis, wherein: the interior passageway of the first sealable section comprises a port for receiving vapour, the interior passageway of the first sealable section joins the interior passageway of the elongate section permitting fluidic communication between the interior passageway of the first sealable section and the interior passageway of the elongate section, the major axis of the first sealable section is offset from the major axis of the elongate section, and the first sealable section comprises a sealing point along its length that, when sealed, prevents fluidic communication into the interior passageway of the first sealable section.

[0011] The elongate section may comprise a first end and a second end and the interior passageway of the first sealable section joins the interior passageway of the elongate section at the first end thereof, and the vapour containment structure may further comprise: a second sealable section further defining an interior passageway, the second sealable section having a major axis, wherein: the interior passageway of the elongate section joins the interior passageway of the second sealable section permitting fluidic communication between the interior passageway of the second sealable section and the interior passageway of the elongate section, the major axis of the second sealable section is offset from the major axis of the elongate section, and the second sealable section comprises a sealing point along its length that, when sealed, prevents fluidic communication into the interior passageway of the second sealable section.

[0012] The vapour containment structure may further comprise a first elongate member having a first end and a second end and defining an interior passageway, the first elongate member forming a part of the elongate section such that the interior passageway of the first elongate member forms a part of the interior passageway of the elongate section.

[0013] The vapour containment structure may further comprise a first connector comprising the first sealable section and further comprising an interface, wherein the first connector is configured to receive the first end of the elongate member via the interface of the first connector, the interface and the received elongate member forming the elongate section.

[0014] The vapour containment structure may further comprise a second connector comprising the second sealable section and further comprising an interface, wherein the second connector is configured to receive the second end of the elongate member via the interface of the second connector, the interface of the second connector and the received elongate member forming the elongate section.

[0015] The vapour containment structure may further comprise a third sealable section having an interior passageway and extending between the first connector and second connector, wherein: the interior passageway of the third sealable section permits fluidic communication between the first connector and second connector, the interior passageway of the third sealable section has a larger diameter than the interior passageway of the elongate member, and the third sealable section comprises a sealing point along its length that, when sealed, prevents fluidic communication into the interior passageway of the third sealable section.

[0016] The first elongate member may be one of a plurality of elongate members, and the vapour containment structure may further comprise: at least one elongate member connector for connecting a pair of elongate members of the plurality of elongate members.

[0017] According to a second aspect of the invention, there is provided a system comprising: the vapour containment structure of the first aspect of the invention; a vapour source; a connector comprising a first connector portion configured to connect the vapour source to the first sealable section of the vapour containment structure, the connector defining an interior passageway for fluidic communication from the vapour source to the interior passageway of the first sealable section via the port of the first sealable section, wherein the vapour containment structure is detachable from the first connector portion when sealed at the sealing point of the first sealable section; and an optical monitoring device configured to generate a first optical signal and transmit the first optical signal along the optical path, wherein a property of the first vapour containment structure is determinable from the first optical signal. The system may further comprise a vacuum pump connected to the first sealable section of the first vapour containment structure via the first connector portion, the vacuum pump configured to evacuate the respective interior passageways of the first sealable section and elongate section.

[0018] The connector may comprise a second connector portion configured to connect to the second sealable section of the first vapour containment structure, the vacuum pump may be configured to evacuate the interior passageway of the second sealable section.

[0019] The vacuum pump may be configured to evacuate the interior passageway of the third sealable section.

[0020] The optical monitoring device may be configured to transmit the first optical signal along the optical path during passage of the vapour from the interior passageway of the first sealable section to the interior passageway of the elongate section.

[0021] The optical monitoring device may be configured to transmit a second optical signal along the optical path, the first and second optical signals causing an Electromagnetically Induced Transparency, EIT, effect when interacting with the vapour in the elongate section of the first vapour containment structure.

[0022] According to a third aspect of the invention, there is provided a method of operating the system of the second aspect of the invention, the method comprising the steps of: connecting, by the first connector portion, the vapour source to the first sealable section of the vapour containment structure; filling the elongate section of the vapour containment structure with vapour from the vapour source; transmitting the first optical signal along the optical path; determining, by the optical monitoring device, the property of the vapour containment structure from the first optical signal; and sealing the first sealable section of the vapour containment structure such that the vapour containment structure is detachable from the first connector.

[0023] The method may further comprise the step of: transmitting a second optical signal along the optical path so as to generate an Electromagnetically Induced Transparency, EIT, effect in the vapour contained in the vapour containment structure. The method may further comprise the step of: evacuating the interior passageway of the elongate section.

[0024] Brief Description of the Figures

[0025] In order that the present invention may be better understood, embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings in which:

[0026] Figure 1 is a schematic diagram of a filling apparatus;

[0027] Figure 2 is a schematic diagram of an assembly of the filling apparatus of Figure 1 ;

[0028] Figure 3 is a side cross-sectional view of a first filling structure of the assembly of Figure 2;

[0029] Figure 4 is a top cross-sectional view of the first filling structure of the assembly of Figure 2;

[0030] Figure 5 is a flow diagram illustrating a method of filling a Hollow Core Fibre (HCF) of the assembly of Figure 2;

[0031] Figure 6A is a schematic diagram of the filling apparatus in a first state;

[0032] Figure 6B is a schematic diagram of the filling apparatus in a second state;

[0033] Figure 6C is a schematic diagram of the filling apparatus in a third state; and Figure 6D is a schematic diagram of the filling apparatus in a fourth state.

[0034] Detailed Description

[0035] Figure 1 illustrates a filling apparatus 100 comprising a first vacuum flange 110, a second vacuum flange 120, a vapour reservoir 130, a turbo pump 140, a roughing pump 150, a set of valves (collectively 160, and comprising a first valve 160a to a fifth valve 160e) and an optical monitoring device 170. The first and second vacuum flanges may be ultra-high vacuum flanges. Figure 1 also illustrates an assembly 200 attached between the first vacuum flange 110 and the second vacuum flange 120, which may be partially or completely removed from the filling apparatus 100 following a filling operation by the filling apparatus 100 as described in more detail below.

[0036] Figure 2 illustrates the assembly 200 in more detail. The assembly 200 comprises a first filling structure 210 and a second filling structure 220. A vapour containment structure, in this example a Hollow Core Fibre (HCF) 300, to be filled by the filling apparatus 100 extends between the first and second filling structures 210, 220 of the assembly 200. The first filling structure 210 is attached to the first vacuum flange 110 of the filling apparatus 100 via a first burn off point 230. Similarly, the second filling structure 220 is attached to the second vacuum flange 120 of the filling apparatus 100 via a second burn off point 240. A cross tube 250 extends between the first and second filling structures 210, 220, and comprises a third burn off point 170. The first and second filling structures 210, 220 and the cross tube 250 are hollow to enable fluidic communication therethrough.

[0037] As shown in Figure 2, the optical monitoring device 170 comprises a first optical source 171 (hereinafter referred to as a probe laser), a first mirror 172, a second optical source 173 (hereinafter referred to as a control laser), a dichroic mirror 174, a first lens 175, a second lens 176, a prism 177 and a photodiode 178. The optical monitoring device 170 is described in more detail below.

[0038] Figure 3 is a side cross-sectional view of the first filling structure 210 and its attachment to a first end of the HCF 300. Figure 4 is a top view of the first filling structure 210 and its attachment to the first end of the HCF 300. The first filling structure 210 comprises a glass structure 211 and a capillary 213 (which may be constructed from silica). An outer diameter of the first end of the HCF 300 matches an inner diameter of the capillary 213 enabling the first end of the HCF 300 to extend at least partially into and within the capillary 213. An overlapping section of the first end of the HCF 300 and the capillary 213 is hermetically sealed, such as by fusing the glass of the first end of the HCF 300 and the glass of the capillary 213 and / or by applying an adhesive that is resistant to corrosion by the vapour of the vapour reservoir 130 that will fill the HCF 300 (as described in more detail below). The overlap between the first end of the HCF 300 and the capillary 213 provides mechanical stability to the first end of the HCF 300.

[0039] An outer diameter of the capillary 213 matches the inner diameter of the glass structure 211 enabling the capillary 213 and glass structure 211 to overlap and be hermetically sealed (again, by fusing the glass and / or by applying adhesive). The glass structure 211 and capillary 213 are both hollow and therefore define a passageway extending from the junction between the glass structure 211 and the first vacuum flange 110 of the filling apparatus 100 (to enable fluidic communication between the first vacuum flange 110 and the glass structure 211 ) to the junction between the capillary 213 and the first end of the HCF 300 (to enable fluidic communication between the capillary 213 and a hollow core 301 of the HCF 300).

[0040] A vapour path therefore extends from the first vacuum flange 110 of the assembly to the hollow core 301 of the HCF 300, via the glass structure 211 and capillary 213. This vapour path enables the HCF 300 to be evacuated and filled by a filling operation as described in more detail below. Furthermore, an optical path extends from an optical access window of the glass structure 211 to the hollow core 301 of the HCF 300, via the glass structure 211 and capillary 213.

[0041] The optical path and vapour path include a section that is at least partially offset. As shown in Figure 3, a section of the vapour path is perpendicular to the optical path. This offset section is beneficial as the remainder of the assembly 200 and the filling apparatus 100 is positioned outside the optical path, thus enabling the optical monitoring device 170 to monitor the optical path extending through the hollow core 301 of the HCF 300 before, during and after the filling operation (as described in more detail below).

[0042] Figure 4 provides example diameters of the hollow core 301 of the HCF 300, the HCF 300, the capillary 213, and the glass structure 211 . The diameter of the hollow core 301 of the HCF 300 may be in the range of 0.005mm (inclusive) to 0.05mm (inclusive), the diameter of the HCF may be in the range of 0.08mm (inclusive) to 0.250mm (inclusive), the diameter of the capillary 213 may be in the range of 1 mm (inclusive) to 3mm (inclusive), and the diameter of the glass structure may be in the range of 5mm (inclusive) to 10mm (inclusive). However, the size of each component, including the diameter of the HCF 300 and its hollow core 301 , may vary depending on the optical requirements, coupling requirements and vapour requirements of the HCF 300. These requirements may be based on a future application of the HCF 300 once filled. Figure 4 also illustrates 6 inner circular structures surrounding the hollow core 301 of the HCF 300, thus providing an anti-resonant HCF.

[0043] The second filling structure 220 is substantially the same as the first filling structure 210 but may differ in orientation (as shown in Figure 2). Furthermore, the second filling structure 220 is shaped and dimensioned so as to attach to a second end of the HCF 300. Turning back to Figure 2, it is shown that the probe laser 171 of the optical monitoring device 170 is configured to transmit a probe signal (illustrated by a solid line) which is reflected by the mirror 172 to be coaxial with the optical path (passing through the glass structure 211 , capillary 213 and hollow core 301 of the HCF 300). The dichroic mirror 174 is configured to allow passage of the probe signal without reflection. The probe signal therefore passes through the dichroic mirror 174 and continues to be coaxial with the optical path. The control laser 173 of the optical monitoring device 170 is configured to transmit a control signal (illustrated by a dashed line) which is reflected by the dichroic mirror 174 to be coaxial with the probe signal and the optical path. The first lens 175 focuses the probe and control signals onto the optical access window of the glass structure 211 of the first filling structure 210. The probe and control signals therefore enter the glass structure 211 and are guided along the optical path, through the glass structure 211 and capillary 213 of the first filling structure 210 and through the hollow core 301 of the HCF 300.

[0044] The optical access window of the glass structure 211 is designed so as to maximise optical coupling of the probe and control signals from outside the first filling structure 210 to inside the first filling structure 210. The optical access window is therefore transparent to the wavelengths of the probe and control signals, and may further: be flat or of minimal curvature, have an applied antireflective coating, angled with respect to the optical path to prevent back reflections to the probe and / or control lasers, and / or shaped to create a lens to further focus the probe and control signals to the optical path within the glass structure 211 .

[0045] The probe and control signals are guided from the first end of the HCF 300 to the second end of the HCF 300. The probe and control signals are thereafter guided along an optical path defined by a glass structure and capillary of the second filling structure 220 and exit the glass structure at an optical access window. The second lens 176 focuses the probe and control signals onto the prism 177, which separates the probe and control signals such that only the probe signal enters the photodetector 178. The photodetector 178 may then analyse the probe signal.

[0046] A method of filling the HCF 300 will now be described with reference to the flow diagram of Figure 5 and the schematic diagrams of Figures 6A to 6D. In a first step, S101 , as shown in Figure 6A, the set of valves 160 are configured such that the first valve 160a, third valve 160c and fourth valve 160d are closed and the second valve 160b and fifth valve 160e are open. In step S103, the turbo pump 140 and roughing pump 150 are then operated so as to generate a vacuum throughout the filling apparatus 100 (but not including the vapour reservoir). The roughing pump exhausts to ambient pressure.

[0047] In step S105, as shown in Figure 6B, the assembly 200 is attached to the filling apparatus 100 by attaching the first filing structure 210 of the assembly 200 to the first vacuum flange 110 of the filling apparatus 100 and by attaching the second filling structure 220 of the assembly 200 to the second vacuum flange 120 of the filling apparatus 100. In step S107, the set of valves 160 are reconfigured so as to open the third valve 160c and the fourth valve 160d (the first, second and fifth valves 160a, 160b, 160e remain open). The turbo pump 140 and roughing pump 150 may continue to operate during steps S105 and S107 such that the assembly 200 is brought under vacuum.

[0048] A hollow interior of the cross tube 250 has a larger diameter than the hollow core 301 of the HCF 300. The cross tube 250 therefore reduces the amount of time to evacuate the assembly 200 and HCF 300 as the air does not have to flow through the relatively narrow aperture hollow core 301 of the HCF 300. Once the assembly 200 and HCF have been brought under vacuum, then the cross tube 250 is no longer required. Therefore, in step S109, the cross tube 250 is burnt off at the third burn off point 170. This may be achieved by application of heat to melt the glass at the third burn off point 170, such as by a blowtorch flame, an electrically heated tungsten wire, or any other source of significant heat. The burn off process seals the cross tube 250 so as to create a first and second burr 250a, 250b (as shown in Figure 6C) and prevents any material from entering the assembly 200.

[0049] Step S111 represents the start of a monitoring process performed by the optical monitoring device 170. The monitoring process is performed before, during and after the steps of filling the HCF 300 with vapour (described below). Before the steps of filling the HCF 300 with vapour, a probe signal is generated by the probe laser 171 and transmitted through the hollow core 301 of the HCF 300. The received power of the probe signal, as measured at the photodetector 178, is measured. This measurement of the received power may be used when calibrating further measurements. The probe signal is transmitted (continuously or periodically) in the following steps. In step S113, the set of valves 160 are reconfigured so as to 1) close the second and fifth valves 160b, 160e, and then 2) open the first valve 160a. This reconfiguration sequence ensures that the turbo and roughing pumps 140, 150 are not damaged by the vapour as it exits the vapour reservoir 130 when the first valve 160a is opened. In step S115, the section of the filling assembly between the first valve 160a and third valve 160c is heated to increase the local vapour pressure. The vapour then diffuses through the HCF 300, driven by the pressure differential between the first and second ends of the HCF 300.

[0050] Whilst the HCF 300 is being filled with vapour, the optical monitoring device 170 contemporaneously monitors the received power of the probe signal being transmitted through the hollow core 301 of the HCF 300. A change in received power at the photodetector 178 is used to determine a density of vapour inside the hollow core 301 of the HCF 300. A correlation between received power at the photodetector 178 and vapour density may be determined through a calibration phase. This correlation may be based on the change in received power at the photodetector 178, such that the received power as measured before the steps of filling the HCF 300 is utilised in the determination.

[0051] In step S117, it is determined whether the filling process is complete. This determination may be based on a comparison of the vapour density (as determined in step S115) to a desired vapour density. If the vapour density is less than the desired vapour density, then the filling and monitoring processes continue. If the vapour density equals (or is greater than) a desired vapour density, then, in step S119, the filling process is complete and the first, third and fourth valves 160a, 160c, 160d are closed.

[0052] Following completion of the filling process, the monitoring process continues by generating and transmitting probe and control signals along the optical path so as to interact with the vapour in the hollow core 301 of the HCF 300. The probe and control signals generate an Electromagnetically Induced Transparency (EIT) effect due to their interactions with the vapour. The photodetector 178 may then analyse the probe signal to determine whether the desired optical properties of the HCF 300 (and vapour contained in the HCF 300) have been achieved post-filling. The analysis may be based on, for example, optical absorption spectrum, optical phase shift spectroscopy, and / or photoluminescence. In step S121 , a burn off process seals the first filling structure 210 so as to create a third burr 210a and further seals the second filling structure 220 so as to create a fourth burr 220a (as shown in Figure 6D). The assembly 200 is thereafter detachable from the filling apparatus 100 by virtue of the burn off process, such that the assembly 200 may be thereafter used in any quantum or optical technology application. This may involve a connector being installed to one or both of the glass structures of the first and second structure 210, 220 of the assembly 200, such that the assembly 200 may be integrated with other hollow core or single mode fibres.

[0053] The above method of filling the HCF 300 utilising the filling apparatus 100 and assembly 200 provides many advantages over conventional methods. Conventional methods of filling an HCF, such as pouring, local saturations or extrusions, are constrained by the narrow aperture of the HCF, leading to long filling times. Further conventional methods of filling an HCF involve surrounding the HCF in a vapour chamber, but this increases the risk of contaminants entering the HCF when removing the HCF from the vapour chamber (or the HCF remains in the vapour chamber and the system is undesirably large). The method and apparatus described above enables the HCF 300 to be evacuated, filled and sealed without any interaction with external material (e.g. atmospheric gases) which may otherwise react with the vapour and inhibit the desired active properties of the final assembly 200. This is not possible with the conventional vapour chamber filling method, in which the HCF is connected to the filling system inside the vapour chamber and therefore cannot be readily removed without risk of contamination.

[0054] Furthermore, the offset optical and vapour paths allow the hollow core 301 of the HCF 300 to be optically monitored before, during and after the filling process. The filling process may therefore achieve target properties for the vapour and / or HCF 300 (e.g. a target density) and any changes in chemical and / or optical properties during the filling process can be tracked in real time, including the equilibration of the vapour through the HCF 300.

[0055] Furthermore, the amount of additional material (glass and / or silica) attached to the HCF 300 following the filling process is minimised, allowing a relatively small final assembly (relative to, for example, an HCF surrounded by a vapour chamber). The first and second structures attached to the HCF 300 define an interior passageway having a first passageway section (covering the optical path and extending from the end of the HCF 300 to the optical access window) and a second passageway section covering the vapour path and offset from the first passageway section. A length of the first passageway section is only sufficiently long to provide a junction with the second passageway section. The second passageway section may be burnt off at the junction with the first passageway section. The amount of additional material of the first and second structures defining these first and second passageways may therefore be minimised.

[0056] Furthermore, the ability to maintain the system at high vacuum for a significant period of time enables the narrow aperture HCF to be filled without any risk of contamination.

[0057] It is desirable to reuse the filling apparatus 100 so as to fill a plurality of assemblies, each assembly of the plurality of assemblies being the same or similar to the assembly 200 described above. Therefore, in step S123, heat may be applied in the section between the third burr 210a and the first vacuum flange 110 so as to chase any vapour from this section into the filling apparatus 100 (that is, to the section between the first and third valves 160a, 160c). The third valve 160c may then be closed. This step is repeated for the section between the fourth burr 220a and the second vacuum flange 120 so as to chase any vapour from this section into the filling apparatus 100. The fourth valve 160d may then be closed. This step of “chasing” the vapour prevents any contamination entering the filling apparatus 100.

[0058] In step S125, the sections between the first vacuum flange 110 and the third burr 210a and between the second vacuum flange 120 and the fourth burr 220a are removed. The process may then be repeated with a new assembly of the plurality of assemblies being attached in step S105.

[0059] The filling apparatus 100 may also be modified so as to fill a plurality of assemblies in a single filling operation (that is, each assembly of the plurality of assemblies may be filled in series in the single filling operation). This may be achieved by connecting the assemblies as a chain of assemblies such that each intermediate assembly of the chain of assemblies is connected to its adjacent intermediate assemblies via their respective first and second filling structures such that the vapour path extends through a passageway between the respective first and second filling structures. Therefore, each assembly (and its corresponding HCF) of the chain of assemblies may be evacuated and filled by the same filling operation as described above (and illustrated in Figure 5).

[0060] An array of optical monitoring devices, each being the same or similar to the optical monitoring device 170 described above, may be provided to monitor each assembly of the chain of assemblies before, during and after the filling operation. Alternatively, a single optical monitoring device may be provided, with additional components to direct and / or guide the probe and control signals along the optical path of each HCF of each assembly of the chain of assemblies.

[0061] As noted above, ends of the HCF 300 are inserted into the capillary of each first / second filling structure and fused thereto. This provides mechanical stability to the ends of the HCF 300. However, this is optional and the ends of the HCF may be inserted directly into the glass structure 211 of the filling structure and fused thereto.

[0062] The method of filling the HCF 300 may be enhanced by flushing the filling apparatus 100 and assembly 200 with inert gas to displace unwanted chemical species in the filling apparatus 100 and assembly 200 and further enhanced by application of heat to the filling apparatus 100 and assembly 200 to aid in the removal of impurities. The method of filling the HCF 200 described above may therefore be modified such that the assembly is attached to the filling apparatus 100 prior to the step of generating a vacuum, the system is flushed and heated, and then the filling apparatus 100 and assembly 200 are evacuated as described above.

[0063] In the above description, the turbo and roughing pumps are used to bring the filling apparatus 100 and assembly 200 to a hard vacuum (e.g. less than 106pascals). Ultra high vacuum techniques may therefore be used to generate the hard vacuum, such as system bakeout, outgassing and the elimination of parasitic and virtual leaks. However, this is non-essential and the filling apparatus 100 and assembly 200 may be evacuated to a particular threshold as required to achieve decontamination of the hollow core 301 of the HCF 300 during evacuation and to achieve vaporisation of the vapour in the HCF 300 during filling.

[0064] In the above steps, a probe and control laser 171 , 173 are used to determine properties of the hollow core 301 of the HCF 300. Additionally, or alternatively, a broadband source may be used instead of the probe laser 171 so as to transmit a broadband signal through the hollow core 301 of the HCF 300. The broadband transmission properties, as measured by the photodetector 178, may therefore be characterised by the broadband signal. The photodetector 178 may detect, form the broadband signal, changes in the HCF structure over time as vapour interacts with the HCF 300. This allows for the detection of changes in the guidance properties of the HCF 300, or changes in absorption of particular wavelengths that would depend on the chemical species that lie in the optical path e.g. impurities.

[0065] The skilled person will understand that the method and apparatus described above is not limited to filling HCF. That is, the method and apparatus may be used to fill any form of hollow elongate containment structure, such as a capillary or an optical waveguide. These structures are characterised as being relatively narrow in two dimensions relative to a third dimension, and further characterised as having a narrow aperture to their hollow interior (in which the aperture is narrow in two dimensions relative to a third dimension).

[0066] Furthermore, the skilled person will understand that the method and apparatus described above may be used to fill the HCF (or other containment structure) with alternative materials, such as any atomic, molecular, gas or suspended powder vapour.

[0067] In the above description, the optical monitoring device 170 monitored the received power of the probe signal being transmitted through the hollow core 301 of the HCF 300 during the filling operation and a change in received power at the photodetector 178 was used to determine a density of vapour inside the hollow core 301 of the HCF 300. However, the skilled person will understand that the optical monitoring device 170 may monitor other properties of the probe signal to measure or estimate the vapour density. For example, the vapour density may be correlated with an aggregate loss of optical power in the probe signal, and further correlated with a temperature sweep. In further examples, the characteristic absorptions, de-excitations or phase shifts may be analysed and correlated with the vapour density. The properties of the optical signal may be correlated with other properties of the HCF 300 (i.e. other than the vapour density).

[0068] Furthermore, in the above description, only the probe signal is transmitted along the optical path during the filling process. However, this is non-essential and both the probe and control signals may be transmitted along the optical path during the filling process. This enables the optical monitoring device 170 to detect the presence of an EIT peak in the probe signal so as to ensure that optically active vapour is present in the HCF 300. The EIT peak may be monitored periodically so as to not interfere with the filling process.

[0069] The photodetector 178 may be, for example, a photodiode or an Optical Spectrum Analyser (OSA).

[0070] The skilled person will understand that the section of the first / second filling structure defining only the vapour path that is perpendicular to the section of the first / second filling structure defining both the vapour and optical path does not need to be perpendicular. That is, a major axis of the section of the first / second filling structure defining only the vapour path may be offset from the major axis of the section of the first / second filling structure defining both the vapour and optical path so as to enable the remainder of the filling apparatus 100 to be external to the optical path.

[0071] The skilled person will also understand that the vacuum pump and evacuation steps are non-essential as the HCF (or other containment structure) may already be evacuated prior to attachment to the filling apparatus.

[0072] The skilled person will also understand that it is non-essential that the HCF (or other containment structure) extends between both a first and second filling structure. That is, the HCF (or other containment structure) may be attached at one end to a first filling structure, closed at the other end, and evacuated and filled according to the method described above.

[0073] The skilled person will understand that any combination of features is possible within the scope of the invention, as claimed.

Claims

CLAIMS1 . A vapour containment structure comprising : an elongate section defining an interior passageway, the elongate section having a major axis; a first optical access window permitting passage of an optical path into the elongate section and along the interior passageway of the elongate section; and a first sealable section further defining an interior passageway, the first sealable section having a major axis, wherein: the interior passageway of the first sealable section comprises a port for receiving vapour, the interior passageway of the first sealable section joins the interior passageway of the elongate section permitting fluidic communication between the interior passageway of the first sealable section and the interior passageway of the elongate section, the major axis of the first sealable section is offset from the major axis of the elongate section, and the first sealable section comprises a sealing point along its length that, when sealed, prevents fluidic communication into the interior passageway of the first sealable section.

2. A vapour containment structure as claimed in Claim 1 , wherein the elongate section comprises a first end and a second end and the interior passageway of the first sealable section joins the interior passageway of the elongate section at the first end thereof, the vapour containment structure further comprising: a second sealable section further defining an interior passageway, the second sealable section having a major axis, wherein: the interior passageway of the elongate section joins the interior passageway of the second sealable section permitting fluidic communication between the interior passageway of the second sealable section and the interior passageway of the elongate section, the major axis of the second sealable section is offset from the major axis of the elongate section, andthe second sealable section comprises a sealing point along its length that, when sealed, prevents fluidic communication into the interior passageway of the second sealable section.

3. A vapour containment structure as claimed in any one of the preceding claims, further comprising: a first elongate member having a first end and a second end and defining an interior passageway, the first elongate member forming a part of the elongate section such that the interior passageway of the first elongate member forms a part of the interior passageway of the elongate section.

4. A vapour containment structure as claimed in Claim 3, further comprising: a first connector comprising the first sealable section and further comprising an interface, wherein the first connector is configured to receive the first end of the elongate member via the interface of the first connector, the interface and the received elongate member forming the elongate section.

5. A vapour containment structure as claimed in Claim 3 or Claim 4 as dependent on Claim 2, further comprising: a second connector comprising the second sealable section and further comprising an interface, wherein the second connector is configured to receive the second end of the elongate member via the interface of the second connector, the interface of the second connector and the received elongate member forming the elongate section.

6. A vapour containment structure as claimed in Claim 5, further comprising a third sealable section having an interior passageway and extending between the first connector and second connector, wherein: the interior passageway of the third sealable section permits fluidic communication between the first connector and second connector, the interior passageway of the third sealable section has a larger diameter than the interior passageway of the elongate member, and the third sealable section comprises a sealing point along its length that, when sealed, prevents fluidic communication into the interior passageway of the third sealable section.

7. A vapour containment structure as claimed in Claim 3, wherein the first elongate member is one of a plurality of elongate members, the vapour containment structure further comprising: at least one elongate member connector for connecting a pair of elongate members of the plurality of elongate members.

8. A system comprising: the vapour containment structure of any one of Claims 1 to 7; a vapour source; a connector comprising a first connector portion configured to connect the vapour source to the first sealable section of the vapour containment structure, the connector defining an interior passageway for fluidic communication from the vapour source to the interior passageway of the first sealable section via the port of the first sealable section, wherein the vapour containment structure is detachable from the first connector portion when sealed at the sealing point of the first sealable section; and an optical monitoring device configured to generate a first optical signal and transmit the first optical signal along the optical path, wherein a property of the first vapour containment structure is determinable from the first optical signal.

9. A system as claimed in Claim 8, further comprising: a vacuum pump connected to the first sealable section of the first vapour containment structure via the first connector portion, the vacuum pump configured to evacuate the respective interior passageways of the first sealable section and elongate section.

10. A system as claimed in Claim 9 as dependent on Claim 2, wherein the connector comprises a second connector portion configured to connect to the second sealable section of the first vapour containment structure, the vacuum pump configured to evacuate the interior passageway of the second sealable section.

11. A system as claimed in Claim 10 as dependent on Claim 6, wherein the vacuum pump is configured to evacuate the interior passageway of the third sealable section.

12. A system as claimed in any one of Claims 8 to 11 , wherein the optical monitoring device is configured to transmit the first optical signal along the optical path during passage of the vapour from the interior passageway of the first sealable section to the interior passageway of the elongate section.

13. A system as claimed in any one of Claims 8 to 11 , wherein the optical monitoring device is configured to transmit a second optical signal along the optical path, the first and second optical signals causing an Electromagnetically Induced Transparency, EIT, effect when interacting with the vapour in the elongate section of the first vapour containment structure.

14. A method of operating the system of any one of Claims 8 to 13, comprising the steps of: connecting, by the first connector portion, the vapour source to the first sealable section of the vapour containment structure; filling the elongate section of the vapour containment structure with vapour from the vapour source; transmitting the first optical signal along the optical path; determining, by the optical monitoring device, the property of the vapour containment structure from the first optical signal; and sealing the first sealable section of the vapour containment structure such that the vapour containment structure is detachable from the first connector.

15. A method as claimed in Claim 14, further comprising the step of: transmitting a second optical signal along the optical path so as to generate an Electromagnetically Induced Transparency, EIT, effect in the vapour contained in the vapour containment structure.

16. A method as claimed in Claim 14 or Claim 15, further comprising the step of: evacuating the interior passageway of the elongate section.

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