GB2580607A

GB2580607A - Dual fibre optical apparatus

Info

Publication number: GB2580607A
Application number: GB1900561.0A
Authority: GB
Inventors: Cunningham Farries Mark; Howard Legg Thomas; Piotr Stabrawa Artur
Original assignee: Gooch & Housego Torquay Ltd
Current assignee: Gooch & Housego Torquay Ltd
Priority date: 2019-01-15
Filing date: 2019-01-15
Publication date: 2020-07-29
Anticipated expiration: 2039-01-15
Also published as: GB201900561D0; GB2580607B

Abstract

A device has first and second optical conduits that contain first 310 and second 320 optical fibres whose ends are arranged side-by-side and parallel to one another. For the same optical inputs their outputs differ in at least one optical characteristic due to the different configurations of the optical conduits. The different configuration may be due to a difference in configuration of the two optical fibres. The optical conduits may have at least one first optical component contiguous with their respective optical fibres. The output ends 315, 325 of the optical conduits may be offset from each other in a direction parallel to the optical axes of the end sections. The two end sections may be mounted in a single mount. The optical characteristic may comprise polarisation and the first optical output may be linearly polarised whilst the second optical output is circularly polarised. The first optical fibre may comprise first and second polarisation-maintaining sections along its length with a non-zero angular offset between their polarisation-maintaining axes. An atom interferometer, fluorescence microscope, laser machining apparatus or a Raman spectrometer may include the device.

Description

DUAL FIBRE OPTICAL APPARATUS

FIELD

[1] The present disclosure relates to optical apparatuses and optical systems. More particularly, the present disclosure relates to dual fibre optical apparatuses and systems comprising the same. The present disclosure also relates to methods of operation and uses of said apparatuses and systems.

BACKGROUND

[2] Optical fibres are flexible means for transmitting light. In its most basic form, an optical fibre comprises fibre core which keeps light propagating along the optical fibre inside it by the phenomenon of total internal reflection. In practicc, a cladding of lower refractive index to that of the fibre core is provided around the core in order to prevent damage to the core and while still allowing the total internal reflection to occur. Further layers, for example buffer and/or jacket layers, may be provided around the cladding to provide further protection. Light is transmitted from the location of the light source at an input end of the optical fibre to a location remote from the light source at an output end of the optical fibre. Optical fibres are used in many applications including telecommunications, illumination, imaging, optical probing, and laser materials processing.

[3] The end section of an optical fibre at the output end is often housed in a ferrule. The ferrule is a rigid housing which surrounds the outermost layer of the optical fibre and is used to facilitate mounting the of the output end of the fibre and for protecting the output end from damage. Typically, the ferrule is approximately an order of magnitude greater in diameter than the diameter of the fibre itself [4] Typically, light exiting the output end of the optical fibre diverges in a cone shape away from the output end. The light can be guided into a more useful form by optics placed in the path of the diverging light. For example, a lens or mirror can be used to transform the diverging cone into a collimated beam, which can propagate in useful form over much longer distances.

[5] Sonic applications, for example atom interfcrometry, laser machining and fluorescence microscopy, benefit from two or more overlapping beams propagating in the same direction, wherein one of the beams has at least one optical characteristic which is different from that of the other beam.

161 Existing optical apparatuses for producing two such overlapping beams include two separate standard optical fibres with different optical components placed in the paths of their optical outputs to achieve the difference in at least one optical characteristic between the two beams. Typically, lenses and other optical components, such as wave plates and/or polarisers, are many times larger than the diameter of the optical fibre. Therefore, to avoid interference between the optical components used with one optical fibre and the optical output of the other optical fibre, the end sections of the optical fibres are placed at right angles to each other and a knife-edge mirror is used after the optical components to redirect the path of one of the optical outputs to be parallel to the path of the other optical output. Examples of such arrangements are shown in Figures 1 and 2.

[7] Figure 1 is a cross-section of first example of an optical apparatus in which a knife edge mirror is used to combine the optical outputs of two optical fibres.

[8] Figure 1 shows an end section of a first optical fibre 110 housed in a first ferrule 181, a second optical fibre 120 housed in a second ferrule 182, a knife-edge minor 190, a focussing lens 161, a main collimating lens 162, and a polariser 185.

191 The first optical fibre 110 has a first optical axis 110a is collinear with that of the main collimating lens 162. A first optical output 151 is emitted from a first output end 115 of the first optical fibre 110 and has an angle of divergence al while propagating toward the main collimating lens 162. The first optical output 151 is collimated by the main collimating lens 162 to form a first collimated beam 171 after the main collimating lens 162.

[10] The second optical fibre 120 has a second output end 125 and a second optical axis I 20a that is perpendicular to, and in the same plane as, the first optical axis 110a. Once extrapolated beyond the respective output ends, the first optical axis 110a and second optical axis 120a intersect at an intersection point 100x, which is positioned a shorter distance from the end of the first output end 115 than from the second output end 125. A second optical output 152 is emitted from a second output end 125 of the second optical fibre 120 and diverges.

[11] The focussing lens 161 is placed in the path of the second optical output 152 between the second output end 125 and the intersection point I 00x. The focussing lens causes the second optical output 152 to converge to a focal point 152f beyond the intersection point 100x.

[12] The knife edge mirror 190 is positioned such that it avoids intersecting the first optical output 151 and such that its minor face 191 intercepts the second optical output 152 at the focal point 152f. The mirror face 191 is angled so that it redirects the second optical output 152 to be parallel to and overlapping with the first optical output 151. The second optical output 152 propagates after the focal point with a divergence angle a2 toward the main collimating lens 162. The second optical output 152 is collimated by the main collimating lens 162 to form a second collimated beam 172 after the main collimating lens 162. The divergence angle al of the first optical output 151 is greater than the divergence angle a2 of the second optical output 152 such that the diameter DI of the first collimated beam 171 is greater than the diameter D2 of the second collimated beam 172.

[13] The polariser 185, which may or may not be provided, polarises the first optical output 151 and second optical output 152.

[14] The result of the optical apparatus in Figure 1 is two overlapping cones of light which have substantially parallel optical axes, wherein one of the cones of light has an optical characteristic (in this case angle of divergence) which is different that of the other cone of light. Two overlapping collimated beams are created from the two light cones, wherein one of the collimated beams has an optical characteristic (in this case diameter) which is different from that of the other collimated beam.

[15] Figure 2 is a cross-section of second example of an optical apparatus in which a knife edge mirror is uscd to combinc the optical outputs of two optical fibres. The optical apparatus in Figure 2 is identical in many respects to that shown in Figure 1. For conciseness, the description of identical elements and arrangements is omitted and only the differences are described.

[16] As in Figure 1, in Figure 2, the first optical fibre 110 and second optical fibre 120 are standard optical fibres. Thus, light received at an input end of each optical fibre exits the fibre unpolarised. Therefore, the first optical output 151 and second optical output 152 are unpolarised.

1171 In Figure 2, a first set of optics 163 is positioned in front of the first output end 115 to focus, and change the polarisation of, the first optical output 151. The first set of optics 163 includes, in sequence from nearest to farthest from the first output end 115, a first converging lens 163a, a first polariser 163b, a quarter wave plate 163c, and second converging lens 163d. The first converging lens 163a receives and collimates the first optical output 151. The first polariser 163b linearly polarises the first optical output 15 I. The quarter wave plate I 63c converts the linear polarisation of the collimated first optical output 151 to circular polarisation. The second converging lens 163d focusses the circularly polarised first optical output 151 to a first focal point 151f between the point of intersection 100x and the second converging lens 163d.

[18] A second set of optics 164 is positioned in front of the second output end 125 to focus, and change the polarisation of, the second optical output 152. The second set of optics 164 includes, in sequence from nearest to farthest from the second output end 125, a third converging lens 164a, a second polariserl 64b, and fourth converging lens 164c. The third converging lens I 64a receives and collimates the second optical output 152. The second polariser 164b linearly polarises the collimated second optical output 152. The fourth converging lens 164c causes the linearly polarised second optical output 152 to converge toward the focal point 152f on the minor face 191.

[19] The optical power of the second converging lens 163d is greater than the optical power of the fourth converging lens 164c. As a result, the divergence angle at of the first optical output 151 after the first focal point 151f is greater than the divergence angle co of the second optical output 152 after the focal point I 52f such that the diameter of the first collimated beam 171 is greater than the diameter of the second collimated beam 172 after the main collimating lens 162.

[20] The result of the optical apparatus in Figure 2 is two overlapping cones of light which have substantially parallel optical axes, wherein one of the cones of light has two optical characteristics (in this case angle of divergence and polarisation) which is different those of the other cone of light. Two overlapping collimated beams are created from the light cones, wherein one of the collimated beams has two optical characteristics (in this case diameter and polarisation) which is different from those of the other collimated beam.

[21] The above described optical apparatuses are bulky and prone to optical aberrations.

[22] The claimed invention provides a solution to the above problems by providing an improved optical apparatus.

SUMMARY

[23] An invention is set out in the appended independent claim. Optional features are set out in the dependent claims.

[24] There is provided an optical apparatus comprising a first optical conduit, having a first input end and a first output end, and a second optical conduit having a second input end and a second output end. The first optical conduit comprises a first optical fibre and the second optical conduit comprises a second optical fibre. The end sections of the first and second optical conduits at the respective output ends are arranged side-by-side and substantially parallel to each other. The first optical conduit is configured to provide a first optical output at the first output end in response to an optical input at the first input end. The second optical conduit is configured differently from the first optical conduit such that, for the same optical input at the respective input ends, at least one optical characteristic of the second optical output differs from that of the first optical output.

BRIEF DESCRIPTION OF THE DRAWINGS

[25] Figure I illustrates a first example of an optical apparatus in which a knife edge mirror is used to combine the optical outputs of two optical fibres.

[26] Figure 2 illustrates a second example of an optical apparatus in which a knife edge minor s used to combine the optical outputs of two optical fibres.

[27] Specific embodiments are described below by way of example only and with reference to the accompanying drawings in which: [28] Figure 3 illustrates a dual fibre optical apparatus in which one fibre has a thermally expanded core.

[29] Figure 4a illustrates a dual fibre optical apparatus including two polarisation maintaining fibres.

[30] Figure 4b is a cross section through plane A:A' in Figures 4a, 5 and 6.

[31] Figure 5 illustrates a dual fibre optical apparatus including two polarisation maintaining fibres, wherein one of the fibres includes a thermally expanded core.

1321 Figure 6 illustrates a dual fibre optical apparatus including two polarisation maintaining fibres, wherein one of the fibres includes a thermally expanded core and both fibres are cleaved at an angle at the output ends.

[33] Figure 7 illustrates a dual fibre optical apparatus, wherein one fibre includes an optical component contiguous therewith.

[34] Figure 8a illustrates a polarisation maintaining fibre having two sections with different polarisation maintaining axes.

[35] Figure 8b is a cross-section through plane B:B' in Figure 8a [36] Figure 8c is a cross-section through plane C:C' in Figure 8a.

p71 Figure 9 illustrates a dual fibre optical apparatus in which the output end of one fibre is offset from that of the other fibre in the direction parallel to its optical axis.

[38] Figure 10 illustrates a system including a light source and a dual fibre optical apparatus.

p91 Figure 11 illustrates a simple prism magneto-optical trap cold atom interferometer schema c [40] Figure 12 is a flow chart illustrating a switch position for operating a dual fibre optical apparatus in an atom interferometer.

1411 The figures are not necessarily drawn to scale.

OVERVIEW

[42] This overview is provided to introduce a brief selection of disclosed concepts in a simplified form that are further described below in the section entitled 'detailed description of certain example embodiments' including the drawings provided. This overview is not intended to limit the scope of the claimed subject matter.

WI There is provided an optical apparatus having two optical conduits for carrying light from a light source to a location remote from the light source, each optical conduit being, or at least including, an optical fibre. End portions of the two optical conduits are provided side by side so that output light cones emerging from respective ends of each optical conduit overlap with each other as they diverge. The end portions of the optical conduits are arranged parallel to each other so that the central axes of the two emerging light cones are also parallel. The end portions may be arranged in such close proximity that the emerging light cones are almost entirely overlapping, and the central axes are almost collinear. One of the optical conduits is configured differently from the other so that if the same light is input into each optical conduit at the ends opposite said end portions, the two emerging light cones will differ in terms of at least one optical characteristic. The optical apparatus therefore creates two light cones having different optical properties, but which are overlapping and have substantially parallel and collinear optical axes.

WI The optical output of this optical apparatus may therefore be similar to that of the prior art arrangements in Figures 1 and 2. However, the difference in geometrical and configurational arrangement between this optical apparatus and those of the prior art leads to a more compact and robust optical apparatus with fewer (and/or less severe) aberrations in the optical output. The difference in configuration between the optical conduits themselves negates the need for bulky optical components in the path of one or both of the light cones in order to engender a difference in optical characteristics between the light cones. Faced with the task of producing two overlapping light cones having a difference in at least one optical characteristic between them, the person skilled in the art would be dissuaded from arranging the end portions of two optical conduits side by side and parallel to each other. This is because the prior art teaches that bulky optical components placed in the path of one of the light cones is the solution for producing the difference in the at least one optical characteristic, and that overlap must be achieved by redirecting one of the light cones using a mirror placed after the bulky optical components. If the skilled person were to place the optical conduits shown in Figures I or 2 side by side, the bulky optical components used to engender the difference in optical characteristics between the light cones would interfere with the two light cones identically and no such difference would be achieved. The inventors have recognised that creating a difference in configuration between the optical conduits themselves to engender the difference in optical characteristics between the two light cones, thus allowing placement of the optical conduits side by side and parallel to each other, alleviates the problems with the prior art arrangements.

[45] By optionally providing a lens spaced apart from the end portions of the optical conduits and in the path of the overlapping light cones, two collimated light beams can be formed which overlapping and substantially parallel. One of the light beams has at least one optical property which is different from the other light beam.

DETAILED DESCRIPTION OF CERTAIN EXAMPLE EMBODIMENTS

[46] Example embodiments are described with reference to the drawings, wherein like reference numerals are used to designate similar or equivalent elements.

[47] Figure 3 is a cross-section of a dual fibre optical apparatus according to an embodiment in which one fibre has a thermally expanded core. Figure 3 shows end sections of a first optical fibre 310 and a second optical fibre 320, a ferrule 380 and lens 360.

[48] The first optical fibre 310 includes a first core 311, and first cladding 312 surrounding the first core 31 I. The second optical fibre 320 includes a second core 321, and second cladding 322 surrounding the second core 321. The first optical fibre 310 has a first input end (not shown) for receiving a first optical input and a first output end 315 for outputting a first optical output 351 in response to the first optical input. The second optical fibre 320 has a second input end (not shown) for receiving a second optical input and a second output end 325 for outputting a second optical output 352 in response to the second optical input. The first core 311 has a first optical axis 310a which passes through the centre of the first core 311. The second core 321 has a second optical axis 320a which passes through the centre of the second core 321.

[49] Although two optical fibres are shown in Figure 3, in embodiments the first optical fibre 310 can be described more generally as being a first optical conduit, and the second optical fibre 320 may be described more generally as being a second optical conduit. An optical conduit may be understood to fulfil the function of carrying light from an input end thereof along its length to an output end thereof There may be low losses along the length of the optical conduit, for example less than 3dB per km.

[50] The term optical conduit may be understood to encompass an optical fibre alone, or an optical fibre having another optical component contiguous therewith or abutted thereto. The optical conduit may also comprise a further optical component contiguous with, or abutted to, said another optical component. That is, other light carrying or light guiding structures of similar cross-sectional dimensions to an optical fibre may be joined to the end of one or both optical fibres to create a composite structure(s) for carrying or guiding light from a light source to a location of use. An example of such a structure is shown in Figure 7.

1511 Thus, the tenn optical conduit may be understood to mean an optical fibre alone or an optical fibre having one or more optical components contiguous and in series therewith. The optical fibre and other/further light carrying structure(s) may be said to be abutted in a train with no gaps therebetween.

[52] It may therefore be understood that there is provided an optical apparatus comprising a first optical conduit having a first input end and a first output end, and a second optical conduit having a second input end and a second output end, wherein the first optical conduit comprises a first optical fibre and the second optical conduit comprises a second optical fibre.

[53] The terms 'first input end', 'first output end', 'second input end' and 'second output end' may each be considered to refer to a terminal face of the optical conduits, whereas the term 'end section' may be considered to refer to a length of the optical conduit proximate to the terminal face.

[54] In the end sections of the first optical fibre 310 and second optical fibre 320, the first optical axis 310a and the second optical axis 320a are side by side and parallel to each other to have a first separation yi. The first separation yi may be defined as the shortest distance between the first optical axis 310a and the second optical axis 320a, or as the distance between the first optical axis 310a and the second optical axis 320a in a direction which is perpendicular to the first optical axis 3I0a and second optical axis 320a.

[55] Although the first optical fibre 310 and second optical fibre 320 are shown as touching and exactly parallel to each other in Figure 3, it may be understood that there may be a small gap between them and a slight angular offset between their optical axes without deleteriously affecting the function of the dual fibre optical apparatus.

[56] It may therefore be understood that the end sections of the first and second optical conduits at the respective output ends are arranged side-by-side and substantially parallel to each other.

[57] The ferrule 380 is a cylindrical housing having a first through hole 381 and a second through hole 382 each parallel to the cylindrical axis of the ferrule 380. The first through hole 381 is dimensioned to receive and hold the end section of the first optical fibre 310 and the second through hole 332 is dimensioned to receive and hold the end section of the second optical fibre 320. The respective end sections are mounted within the ferrule 380 such the first output end 315 and second output end 325 are aligned in a plane perpendicular to the first optical axis 310a and second optical axis 320a. Although the output ends are shown as protruding from the end of the ferrule, the present disclosure is not limited to this configuration and the output ends may be flush with or recessed from the end of the ferrule.

[58] The ferrule may also have both holes 381 and 382 combined into an oval or rectangular hole that can receive and hold both optical fibres parallel and in contact with each other.

[59] Although a ferrule 380 is shown in Figure 3, any mount suitable for holding the end sections of the first optical fibre 310 and second optical fibre 320 in the desired position relative to each other may be substituted for the ferrule 380. Examples of alternatives to the ferrule are a resin block into which the end sections are set, or a silicon V-groove, which is a known optoelectronics packaging solution for accurately mounting and/or positioning optical fibres.

[60] It may therefore be understood that the end section of the first optical conduit and the end section of the second optical conduit may be mounted in a single mount, for example a ferrule, silicon V-groove or resin block. Advantageously, a single mount allows the alignment to be set during manufacturing and significantly reduces the possibility of misalignment between the two end sections after manufacture. Furthermore, a single mount simplifies alignment of the end sections during manufacture, thereby providing a more robust and easy to manufacture dual fibre optical apparatus.

[61] However, embodiments are not limited to a single mount having both end sections therein. It is envisaged that a separate mount may be provided for each end section. The two mounts may have engaging portions which may mutually interlock to provide any of the desired alignments of the end sections relative to each other as herein described (e.g. proximity and/or relative orientation). For example, the separate mounts may be different from standard ferrules in that they permit the close alignment (e.g. touching) of the end sections as herein described. Such separate mounts may mate at a plane positioned between the end sections to bring the end sections into close proximity to each other or cause them to touch. Engaging portions may be provided on the mating faces to facilitate alignment of the end sections relative to each other. The engaging portions may include complimentary pairings such as grooves and ridges or locating holes and protrusions. Alternatively, or in addition, the two separate mounts may be aligned and/or held relative to each other by a third member, or by an adhesive or a frame or housing.

[62] In Figure 3, the first core 311 has a constant diameter equal to a first diameter along the full length of the first optical fibre 310. The first optical fibre 310 may therefore be unmodified and the first optical output 351 has a divergence determined by the properties of the fibre.

[63] In the most widely used optical fibre for wavelengths of 1310 mm or 15501am (Corning SMF28), the core diameter is around 8 pm, the cladding around 125 pm and the MFD is around 10 Km. If the first optical fibre 310 is of this type, the first optical output 352 would typically have a near-Gaussian mode with a waist in the range of 4-6 pm located at, or at least very near to the first output end 315. The first optical fibre 310 and/or second optical fibre 320 may include the Corning SMF28 fibre or any other optical fibre having a core and a cladding. For example, for wavelengths of 780 nm Nufern 780-HP fibre may be used, with core diameter around 4.4 pm and MFD of around 5 [64] The second core 321 has a constant diameter equal to the first diameter along the full length of the second optical fibre 320 except along an end portion of the end section of the second optical fibre 320 adjacent to the second output end 325. In the end portion, the diameter of the second core 321 gradually increases in a direction toward to the second output end 325. That is, the second core 321 can be said to have a fimnel-shaped core profile in the end portion so that the second core 321 has a diameter at the second output end 325 which is greater than the core diameter of the first core 311 at the first output end 315. The second optical fibre 320 may therefore be said to have an expanded core at the second output end 325. A subset of such optical fibres types is a thermally expended core (TEC) fibre, the fabrication of which will be now be described.

[65] The method for forming Thermally Expanded Core (TEC) fibres and is commonly used to produce high power handling patch chords or to make low loss connections between fibres with different mode field diameters (MFD's) such as between single mode fibre (SMF) and large mode area (LMA) fibre in fibre amplifiers and lasers, or for connecting fibre to other waveguides.

[66] As described earlier, optical fibres consist of a core and cladding of glass with different refractive indices. The core is where light is guided. In the most widely used optical fibre (Corning SMF28) the core diameter is around 10pm and the cladding around 125ipn. To increase the refractive index of the glass core, it is doped, for example with Germanium. To produce a TEC fibre the output end of the fibre is heated in a controlled manner to above 1200°C. At this temperature the Ge dopant diffuses into the cladding this changes the refractive index profile of the optical fibre. Alternatively, the optical fibre is heated at a point along its length other than at the output end, and the fibre is cleaved at the midpoint of the heat affected zone (i.e. the midpoint of the thermally expanded core region).

[67] Thus, the process involves heating the fibre in a controlled manner to allow the core dopants to diffuse into the cladding of the fibre in the transverse direction (perpendicular to the optical axis of the core). in the longitudinal direction (i.e. parallel to the optical axis of the core) the heat profile or sweep of the fibre/heat source may be controlled to ensure that the change in core profile is adiabatic. This is preferred to prevent light loss from the fibre. The resulting fibre has a larger MFD, typically between 2-3x the original MFD. The divergence in the optical output from the output end is accordingly reduced by approximately the same factor compared with an untreated fibre.

[68] The funnel-shaped core profile at the end of the TEC fibre causes the propagating mode to expand, resulting in a larger MFD. The resulting free space beam (e.g. the second optical output 352) will have a lower divergence than from the untreated fibre (e.g. the first optical fibre 310).

[69] A more detailed explanation of how to manufacture a thermally expanded core optical fibre can be found in the paper Xuanfeng Zhou, Zilun Chen, Hang Zhou, and Jing Hou, "Mode-field adaptor between large-mode-area fiber and single-mode fiber based on fiber tapering and thermally expanded core technique," Appl. Opt. 53, 5053-5057 (2014). US5301252A describes a similar process using a fusion splicer. Another method of fusing dissimilar diameter fibres which achieves a similar result is described in patent US6244757B1.

1701 The effect of the difference in core profile between the first core 311 and second core 321 is that, for the same optical input at the first input end and second input end, a divergence angle al of the first optical output 351 is greater than a divergence angle az of the second optical output 352.

[71] it may therefore be understood that the first optical fibre comprises a first core profile and the second optical fibre comprises a second core profile which is different from the first core profile.

[72] it may also be understood that the difference in the core profile may be in the end section of either optical conduit, optionally at or proximate to the output end thereof For example, one of the first optical fibre and second optical fibre may comprise a thermally expanded core at the output end of the respective optical conduit. Advantageously, this allows a difference in configuration between the first and second optical fibre to be provided without providing a difference in the manner in which light may be coupled into the respective input ends. That is, no difference in light coupling means between those provided for the first and second input ends is required to launch light from the same light source into the fibre.

[73] In Figure 3 the difference between the first optical output 351 and second optical output 352 is the angle of divergence and this is caused by the difference in configuration of the first optical fibre 310 relative to that of the second optical fibre 320.

[74] However, alternatively, or in addition, differences in other optical characteristics between the first optical output 351 and second optical output 352 may be caused by configuring the first optical fibre 310 differently from the second optical fibre 320 in other ways. Further examples of differences in configurations between the optical fibres are provided later in this description. However, these are provided for illustrative purposes and the appended claims are not limited to the examples described.

[75] It may therefore be understood that the first optical conduit is configured to provide a first optical output at the first output end in response to an optical input at the first input end. The second optical conduit is configured differently from the first optical conduit such that, for the same optical input at the respective input ends, at least one optical characteristic of the second optical output differs from that of the first optical output.

[76] The difference in configuration between the first and second optical conduits may comprise a difference in configuration of the first optical fibre relative to the configuration of the second optical fibre. The at least one optical characteristic may comprise divergence.

1771 The optical characteristic may not be wavelength. That is, it is envisaged that in any of the embodiments described herein, the first optical output and second optical output may comprise identical wavelengths or wavelengths that are very close (<1nm apart). That is, the wavelength spectra of the first optical output and second optical output may be substantially identical. Alternatively, the first optical output and second optical output might have different spectral characteristic (sidebands or multiple wavelengths depending on the laser source used) but have substantially the same peak wavelength to within several nm. The peak wavelengths being too close to be selected/split/combined with a colour/bandpass filter [781 The lens 360 is a converging lens arranged to receive and reduce the divergence of the first optical output 351 and second optical output 352. More particularly, the lens 360 shown is a biconvex lens having positive optical power. The optical power of the lens 360 and distance zi from the first output end 315 and second output end 325 are selected so that the lens 360 collimates the first optical output 351 and second optical output 352 to form a first beam 371 having a first beam diameter di and second beam 372 having a second beam diameter cb. The lens 360 has a third optical axis (not shown), which may pass between the first optical axis 310a at the first output end 315 and second optical axis 320a at the second output end 325.

[791 In operation, light is coupled (or launched) into the first core 311 at the first input end of the first optical fibre 310 and into the second core 321 at the second input end of the second optical fibre 320. The light propagates along the first optical fibre 310 and second optical fibre 320 to the first output end 315 and second output end 325 respectively. The light is emitted from the first output end 315 and second output end 325 to form the first optical output 351 and second optical output 352, respectively. The first optical output 351 and second optical output 352 diverge with an angle al and az, respectively, while propagating toward the lens 360. Due to the divergence, the first optical output 351 and second optical output 352 begin to overlap at a distance.z2 from the first output end 315 and second output end 325.

1801 At the lens 360, second optical output 352 in its entirety overlaps a portion of the first optical output 351. After the lens, the second collimated beam 372 in its entirety overlaps a portion of the first collimated beam 371. The first collimated beam 371 has a third optical axis 371a and second collimated beam 372 has a fourth optical axis 372a The third optical axis 37Ia and fourth optical axis 372a have a second separation y2. The second separation y2 may be proportional to the first separation yi. The second separation y2 may be defined as the shortest distance between third optical axis 37Ia and the fourth optical axis 372a at the lens 360, or as the distance between the third optical axis 371a and the fourth optical axis 372a in a direction which is perpendicular to the third optical axis 371a and fourth optical axis 372a.

1811 The person skilled in the art of optical design knows how to select the optical power of the lens and its position relative to the output ends of the first optical fibre 310 and second optical fibre 320 to achieve collimation of the first optical output 35 I and second optical output 352.

[82] Although Figure 3 shows a lens 360, the dual fibre optical apparatus may not include the lens 360 and no, or other, optical components may be placed in the path of the first optical output 351 and second optical output 352.

[831 For example, when no lens or other alternative optical component is used, the first optical output 351 and second optical output 352 may be used in their raw form. Alternatively, another optical component(s) of positive optical power, for example a concave mirror or a diffractive lens, may be provided in place of the lens 360 to provide the collimation or merely to reduce the divergence.

1841 it may therefore be understood that the optical apparatus as herein described may comprise a second optical component arranged to receive the first optical output and second optical output. The second optical component may be configured to reduce the divergence of the first optical output and second optical output. The second optical component may be configured to collimate the first optical output to produce a first collimated output and to collimate the second optical output to produce a second collimated output. Alternatively, another optical component may be provided in place of the lens 360 to provide another optical effect other than collimation depending on the application.

1851 The end sections of the first optical fibre 310 and second optical fibre 320 may be held in position by the ferrule 380 substantially parallel to each other and side by side. The end sections may be held such to achieve the shortest possible distance yi between the first optical axis 310a and the second optical axis 320a.

1861 This has the advantage of maximising the degree of overlap and/or collinearity of propagation between the first optical output 351 and second optical output 352, and hence also the first collimated beam 371 and second collimated beam 372.

1871 It may therefore be understood that the optical axes of the first collimated output and second collimated output may be substantially parallel and have a spatial offset of the same order of magnitude as the separation distance between the optical axes of the end sections. The first output end and the second output end may be spaced close enough to allow the first optical output and the second optical output overlap at the second optical component. The first output end and the second output end may be spaced close enough to allow one of the first optical output and second optical output overlap to entirely overlap the other at the second optical component.

1881 One way in which this can be achieved is to mount the end sections of the first optical fibre 310 and the second optical fibre 320 such that they are touching. If (i) the first optical fibre 310 is identical in diameter to the second optical fibre 320, (ii) the respective cores are centrally positioned with the respective claddings, and (iii) the respective claddings are circular in cross-section (these three criteria being met by many, if not most, standard optical fibres), then the distance yi between the first optical axis 310a and second optical axis 320a may be said to be equal to half the sum of the diameters of the first optical fibre 310 and second optical fibre 320 when the end sections touch each other.

1891 The distance yi can be advantageously reduced below this value by etching the cladding along the length of the end section of either or both of the first optical fibre 310 and second optical fibre 320. Another way to reduce the distance yi is to manufacture the first optical fibre 310 and/or second optical fibre 320 such that the optical axis of the respective core is offset from the central axis of the respective cladding. The fibre may then be rotated around the optical axis of the core such that the point on the outer edge of the cladding closest to the optical axis of the core is also closest to the other optical fibre. The first optical fibre 310 and/or second optical fibre 320 may be D-shaped optical fibres (i.e. optical fibres having a D-shaped cross-section) with the flat side of the respective optical fibre being oriented to face the other fibre. D-shaped optical fibres are most commonly used for sensing applications and are standard optical fibres which have been polished to have a flat surface on one side thereof The optical axis of the core is therefore closest to the edge of the cladding at the midpoint of the flat side. When one or both of the optical fibres are D-shaped along the end sections and are oriented such that the flat surface(s) face the other fibre, this minimises the separation between the optical axes of the two fibre cores.

[90] Although touching end sections is preferred, a small gap between the end sections may be provided while still enjoying a degree of the benefit of having touching end sections in that overlap between the parallel optical outputs may occur to a sufficient level for some applications.

pil it may therefore be understood that a separation between the optical axes of the end sections is less than five times the sum of maximum diameter of the first optical conduit and second optical conduit, optionally less than or equal to half the sum of maximum diameter of the first optical conduit and second optical conduit.

[92] It may therefore be understood that there is provided an optical apparatus comprising a first optical conduit having a first input end and a first output end, and a second optical conduit having a second input end and a second output end. The first optical conduit comprises a first optical fibre and the second optical conduit comprises a second optical fibre. The end sections of the first and second optical conduits at the respective output ends are arranged side-by-side and substantially parallel to each other. The first optical conduit is configured to provide a first optical output at the first output end in response to an optical input at the first input end. The second optical conduit is configured differently from the first optical conduit such that, for the same optical input at the respective input ends, at least one optical characteristic of the second optical output differs from that of the first optical output.

[931 Figure 4a is a cross-section of a dual fibre optical apparatus including two polarisation maintaining fibres. The optical apparatus of Figure 4a is identical in many respects to that shown in Figure 3. For conciseness, the description of identical elements and arrangements is omitted and only the differences are described.

NI The configurations of the first optical fibre 310 and second optical fibre 320 in Figure 4a differ from those in Figure 3 in that the first optical fibre 310 and second optical fibre 320 are polarisation maintaining optical fibres (PM fibres) each having identical core profiles.

[951 PM fibres arc known in the art and are widely commercially available.

[961 Standard PM fibre guides two modes with orthogonal linear polarisations. However, embodiments are not limited to standard PM fibres and the use of any PM fibre is envisaged. The term PM fibre may be taken to mean any fibre which is configured to guide at least one first mode having a first axis of polarisation preferentially over at least one second mode having a second axis of polarisation rotated at a non-zero angle with respect to the first axis. A PM fibre may therefore be considered to be an optical fibre having a substantially greater propagation constant in at least one first axis compared with that of at least one second axis rotated at a non-zero angle with respect to the first axis.

[971 Typically, PM fibres include one or more of an elliptical fibre core, elliptical fibre cladding, or rods in the fibre cladding which induce a stress across the fibre core. These structures cause there to be a significantly different propagation constant in two orthogonal axes, compared to other axes. This is caused by the birefringence inherent in the fibre, leading to Eigen-modes which are linearly polarized and perpendicular to each other (fast and slow propagating modes). That is, with these stress-birefringent fibers, orthogonally polarized fast and slow Eigen-modes may propagate in the fibre. To ensure linearly polarized light at the output, it is sufficient to launch light at the input end of the fibre in just one of those modes.

[981 In the dual fibre optical apparatus shown in Figure 4a, the first optical fibre 310 and second optical fibre 320 each have a pair of rods 313, 323 in the fibre cladding. Each pair of rods runs through the fibre cladding, parallel to the fibre core.

1991 More generally, it may be understood that the first optical fibre 310 is a polarisation maintaining fibre of any type and/or the second optical fibre 320 is a polarisation maintaining fibre of the same or any other type as the first optical fibre 310.

[1001 Figure 4b shows a cross section of the first optical fibre 310 and second optical fibre 320 in Figure 4a through plane A:A'. In Figure 4b, the cross section of the first optical fibre 310 shows the first core 311 in the centre of the first cladding 312 and a pair of first stress rods 313, placed either side of and equidistant from the centre of the first core 311. The cross section of the second optical fibre 320 shows the second core 321 in the centre of the second cladding 322 and a pair of second stress rods 323, placed either side of and equidistant from the centre of the second core 321. A first polarisation maintaining axis 313a is perpendicular to and passes through the optical axis of the first core 311 and the central axis of each of the first stress rods 313. A second polarisation maintaining axis 323a is perpendicular to and passes through the optical axis of the second core 321 and the central axis of each of the second stress rods 323. A third polarisation maintaining axis 313b is perpendicular to the first polarisation maintaining axis 313a and the optical axis of the first core 311. A fourth polarisation maintaining axis 323b is perpendicular to the second polarisation maintaining axis 323a and the second optical axis 310a of the second core 321. The first polarisation maintaining axis 313a is rotated at a non-zero angle 01 relative to the second polarisation maintaining axis 323a. That is, the first polarisation maintaining axis 313a is offset by an angle 01 relative to the second polarisation maintaining axis 323a.

[101] More generally, it may be said that the first optical fibre may have a first polarisation maintaining axis offset at a non-zero angle with respect to a second polarisation maintaining axis of the second optical fibre.

[102] Referring again to Figure 4a, in operation, the first optical output 351 and second optical output 352 diverge with the same angle, while propagating toward the lens 360. Therefore, the first beam 371 and second beam 372 exiting the lens 360 have the same diameter. However, the first optical output 351 differs from the second optical output 352 in that first optical output 351 has an axis of polarisation which is offset by the angle 01 relative to an axis of polarisation of the second optical output 352. That is, the angle between the axis of polarisation of the first optical output 351 and that of the second optical output 352 is the same as the offset angle 01 between the first polarisation maintaining axis 313a and the second polarisation maintaining axis 323a.

[103] The offset angle 0, may be 45 degrees, which advantageously makes the dual fibre optical apparatus particularly suitable for use in certain applications, such as in an atom interferometer as described later with reference to Figure 11.

[104] it may be understood that two polarisation maintaining fibres may secured side-by-side in a ceramic or glass ferrule with a suitable bore diameter. The fibres may be rotated so that their stress rods are orientated at 45° with respect to each other. Linearly polarised light is guided in each fibre with the polarisation direction aligned with either the slow or fast axis (no preference).

[105] It may therefore be understood that the second optical conduit may be configured differently from the first optical conduit such that, for the same optical input at the respective input ends, the polarisation of the second optical output differs from that of the first optical output. The first optical fibre may be arranged to maintain linear polarisation in a first axis, and the second optical fibre may be arranged to maintain linear polarisation in a second axis having an angular offset relative to the first axis. Advantageously, two parallel and overlapping fibre outputs are provided having differing polarisation without the need for the bulky and aberration-prone arrangement shown in Figure 2.

[106] A polariscr or wave plate 385 may be placed in the optical path of the first optical output 351 and second optical output 352 before the lens 360 (i.e. between the end sections and the lens 360) to adjust the polarisation of both the first optical output 351 and second optical output 352. Alternatively, or in addition, another polariser or wave plate 386 may be placed in the optical path of the first beam 371 and second beam 372 after the lens 360 to adjust the polarisation of both the first beam 371 and second beam 372. Alternatively, only one or no such polarisers or wave plates 385, 386 may be present.

[107] Figure 5 is a cross-section of a dual fibre optical apparatus including two polarisation maintaining fibres, wherein one of the fibres includes a thermally expanded core. The optical apparatus of Figure 5 is identical in many respects to that shown in Figure 3. For conciseness, the description of identical elements and arrangements is omitted and only the differences are described. I5

[108] in Figure 5, the first optical fibre 310 and second optical fibre 320 not only have different core profiles as described with reference to Figure 3 but are both also polarisation maintaining fibres which have the same cross-sections through section A:A' as those shown in Figures 4b. That is, the dual fibre optical apparatus shown in Figure 5 may be a hybrid of those described with reference to Figures 3 and 4a/4b.

[109] In operation, the first optical output 351 and second optical output 352 differ in both divergence angle as described with reference to Figure 3 and their relative axes of polarisation as described with reference to Figures 4a and 4b.

[110] Wave plate(s) and/or polariser(s) 385, 386 may be disposed before and/or after the lens 360 in the same way as described with reference to Figure 4a.

[111] Figure 6 is a cross-section of a dual fibre optical apparatus including two polarisation maintaining fibres, wherein one of the fibres includes a thermally expanded core and both fibres are cleaved at an angle at the output ends.

[112] The optical apparatus of Figure 6 is identical in many respects to that shown in Figure 5. For conciseness, the description of identical elements and arrangements is omitted and only the differences are described.

[113] The first output end 315 of the first optical fibre 310 is cleaved at an angle pi relative to a plane lying perpendicular to the first optical axis 310a. The angle pi is swept out in the plane in which the first optical axis 310a and second optical axis 320a lie.

[114] The second output end 325 of the second optical fibre 320 is cleaved at an angle 92 relative to a plane lying perpendicular to the second optical axis 320a. The angle 92 is swept out in the plane in which the first optical axis 310a and second optical axis 320a lie.

[115] In operation, the cleaved first output end 315 causes the optical axis of the first optical output 351 to have a first angular offset ipi relative to the first optical axis 310a of the first optical fibre 310. Likewise, the cleaved second output end 325 causes the optical axis of the second optical output 352 to have a second angular offset itrii relative to the second optical axis 320a of the second optical fibre 320.

[116] The first optical axis 310a and second optical axis 320a have a third angular offset V3 relative to a third optical axis 360a of the lens 360. That is the first optical fibre 310 and second optical fibre 320 are tilted with respect to the third optical axis of the lens 360. The first angular offset pi, second angular offset kitz, and third angular offset tp3 are swept out in the plane in which the first optical axis 310a and second optical axis 320a lie.

[117] It may therefore be understood that at least one of the first and second output ends may be cleaved at a non-zero angle with respect to a first plane perpendicular to the optical axis of the respective end section, wherein the angle is in a second plane in which the optical axes of the end sections he.

[118] Advantageously, the arrangements described above allow the first optical output 351 and second optical output 352 to propagate along the optical axis of the lens 360 while any back reflections of the first optical output 351, second optical output 352, first beam 371 or second beam 372 back into the first core 3I I or second core 312 are significantly reduced. Thus, back reflections into the optical fibres are reduced without any significant reduction in quality of the dual beam output.

[119] Figure 7 is a cross-section of a dual fibre optical apparatus, wherein one fibre includes a GRIN lens or multimode grin-core optical fibre at the output end.

11201 The optical apparatus of Figure 7 is identical in many respects to that shown in Figure 5 For conciseness, the description of identical elements and arrangements is omitted and only the differences are described.

[121] Thus far, we have described the first and second optical conduits as a first optical fibre 310 and second optical fibre 320, respectively. In the following description of Figure 7, the second optical fibre 320 is re-named second optical conduit 1320 because, in this case, the second optical conduit 1320 may include an optical component in addition to a single optical fibre as per the definition of optical conduit' as herein described.

[122] In Figure 7, the second optical conduit 1320 comprises a first section 1320A and a second section 1320B. The first section 1320A makes up the majority of the length of the second optical conduit 1320 and the second section 1320B makes up a remaining length of the second optical conduit 1320 proximate to the second output end 325.

[123] The first section 1320A and a second section 1320B are arranged end-to-end, contiguous with each other, and such that the longitudinal axes of the first section 1320A and a second section 1320B are substantially collinear.

[124] The second section 1320B may be a section of grin-core multimode optical fibre or a grin lens which acts in an analogous manner to the thermally expanded core in Figure 3, i.e. by reducing the divergence angle of the second optical output 352 compared with that of the first optical output 351. That is, the first optical output 351 and second optical output 352 are substantially the same as those described in relation to Figure 3.

[125] It may therefore be understood that one of the first optical conduit and second optical conduit further comprises at least one first optical component contiguous with the respective optical fibre. For example, the one of the first optical conduit and second optical conduit further comprises a grin lens or multimode-grin-core optical fibre at the respective output end. Advantageously, this provides a manufacturing alternative to modifying the first or second optical fibre to achieve the difference in configuration between the first optical conduit and second optical conduit suitable for producing the difference in the at least one optical characteristic between the first and second optical output.

[126] The first optical component may be provided in the respective end section such that an end of the first optical component opposite the end contiguous with the optical fibre is the output end of the optical conduit. Advantageously, this allows a difference in configuration between the first and second optical conduit to be provided without providing a difference in the manner in which light is coupled into the respective input ends. That is, no difference in light coupling means between the those provided for the first and second input ends is required to launch light from the same light source into the fibre.

[127] The first optical component may be mechanically held to the end of the optical fibre, or may be spliced to the optical fibre, for example with the fusion splicing method described with reference to Figure 8a. Other means for attaching the first optical component to the respective optical fibre are also envisaged.

[128] Figure 8a illustrates a spliced polarisation maintaining (PM) fibre 3200 having a first section 3320 with a first polarisation maintaining axis in a first direction and a second section 4320 with polarisation maintaining axis in a second direction having an angular offset from the first direction.

[129] In Figure 8a, the first section 3320 includes a first section fibre core 3321, a first section fibre cladding 3322 surrounding the first section fibre core 3321, and a pair of first section stress rods 3323 embedded in the fibre cladding. The first section fibre core 3321 and the first section stress rods 3323 nn parallel to one another through the full length of the first section fibre cladding 3322. The first section fibre core 3321, first section fibre cladding 3322 and first section stress rods 3323 each have a circular cross-section as shown in Figure 8b.

[130] Figure 8b is a cross-section in the plane B:131 shown in Figure 8a. Central axes of the first section stress rods 3323 are respectively offset either side of the central axis shared by the first section fibre core 332I and first section fibre cladding 3322. The first polarisation axis 3329 is in a first direction perpendicular to and intersecting each of the central axes of the first section fibre core 3321, first section fibre cladding 3322, and first section stress rods 3323.

[131] Turning back to Figure 8a, the second section 4320 includes a second section fibre core 4321, a second section fibre cladding 4322 surrounding the second section fibre core 4321, and a pair of second section stress rods 4323 embedded in the second section fibre cladding 4322. The second section fibre core 4321 and the second section stress rods 4323 run parallel to one another through the length of the second section fibre cladding 4322.

[132] The second section fibre core 4321, second section fibre cladding 4322 and second section stress rods 4323 each have a circular cross-section as shown in Figure 8c. Figure 8c is a cross-section in the plane C:C' shown in Figure 8a.

[133] Central axes of the second section stress rods 4323 are respectively offset either side of the central axis shared by the second section fibre core 4321 and second section fibre cladding 4322. The second polarisation axis 4329 is in a second direction perpendicular to and intersecting each of the central axes of the second section fibre core 4321, second section fibre cladding 4322, and second section stress rods 4323.

[134] The first section 3320 and second section 4320 are arranged end-to-end, contiguous with each other, and such that the central axis shared by the first section fibre core 3321 and first section fibre I8 cladding 3322 and the central axis shared by the second section fibre core 4321 and second section fibre cladding 4322 are collinear. The second section 4320 has a length zc, which is equal to a whole number of boat lengths (where the minimum number is zero) plus or minus a quarter of a beat length at the wavelength for which the spliced PM fibre 3200 is designed. The beat length is the length over which the phase difference between the slow and fast axes advances by 221 radians. There is an angular offset of 45 degrees between the first polarisation axis 3329 and second polarisation axis 4329. Other angles of offset are also envisaged.

11351 In operation, linearly polarised light having a wavelength Xi and a polarisation axis aligned with that of the first polarisation direction can be input (i.e. launched) into one end of the first section 3320. The linearly polarised light propagates along the first section 3320 and is coupled into the second section 4320 at the interface 3300 between the first section 3320 and the second section 4320. The second section 4320 acts as a quarter wave 0,/4) retarder with the result that light output from the end of the second section 4320 is circularly polarised.

[136] The spliced PM fibre 3200 shown in Figure 8a can be implemented in place of the second optical fibre 320 in the arrangement shown in Figure 4a. In this implementation, the first optical output 351 remains linearly polarised whereas the second optical output 352 is circularly polarised. If the second section fibre core 4321 has the same diameter as the first core 311, the beam divergence of the first optical output 351 is substantially the same as that of the second optical output 352.

[137] The spliced PM fibre 3200 can alternatively be implemented in place of the first optical fibre 310 in the arrangement shown in Figure 5. in this implementation, the second optical output 352 remains linearly polarised whereas the first optical output 351 is circularly polarised. Provided that the second section fibre core 4321 has a smaller diameter than the diameter of the expanded core at the second output end 325, the divergence of the first optical output 351 is greater than the divergence of the second optical output 352 in this implementation.

[138] A summary of a method of splicing two sections of PM fibre to obtain a fibre as shown in Figure 8a now follows.

[139] The spliced PM fibre shown in Figure Ra may be produced splicing a first section of standard PM fibre onto a second section of standard PM fibre to create a single spliced PM fibre having the first and second sections. Fibre splicing methods are known in the art. For example, in fusion splicing, the first and second sections may be prepared by stripping any protective coating from the optical fibres from which they originate, cleaning the stripped fibres, and cleaving sections of appropriate length using, for example, the score-and-break method. Before splicing, the second section has its polarisation maintaining axis rotated with respect to that of the first section so that the polarisation maintaining axes have an angular offset. The first and second sections are then aligned by aligning the core and/or cladding and spliced together using a fusion splicer, which is known art. The result is that the spliced PM fibre having an optical input of linear polarisation into one end will result in an optical output of elliptical or circular polarisation from the other end.

[140] For example, it is possible to manufacture a spliced PM fibre to produce a circularly polarised beam at the output. If a first section of PM fibre has a structure for guiding a linearly polarised mode along one of its polarisation maintaining axes (slow or fast) then circular polarisation can be obtained by splicing a second section of PM fibre onto the output with one of its polarisation maintaining axes rotated at 45° with respect to that of the first section and the length of the second section cleaved or polished to a quarter of a beat length at the wavelength of interest. The beat length of the fibre is the length over which the phase difference between the slow and fast axis advances by 2a radians. 11411 That is, by butt-coupling a birefringent fiber (second section) rotated relative to the first section, this launches a fraction of the power, cos(angle)^2, into one of the orthogonal modes, and sin(angle)A2 into the other. After propagating through the second section, the phases of the modes will not be aligned and the light exiting will generally be elliptically polarised.

[142] It may therefore be understood that the first optical fibre may comprise a first polarisation-maintaining section along a first length of the first optical fibre and a second polarisation-maintaining section along a second length of the first optical fibre. A polarisation-maintaining axis of the first polarisation-maintaining section may have a non-zero angular offset relative to a polarisation-maintaining axis of the second polarisation-maintaining section. Advantageously, the first optical output is elliptically polarised while the second optical output is linearly polarised without the need for a bulky and aberration-prone arrangement such as that shown in Figure 2.

[143] The second length may be a whole number of beat lengths plus 'A or'/ of a beat length at an operating wavelength of the first optical fibre, and the non-zero angular offset may be about 45 degrees. Advantageously, the first optical output is circularly polarised while the second optical output is linearly polarised without the need for a bulky and aberration-prone arrangement such as that shown in Figure 2.

[144] it may therefore be understood that there is more than one way in which the second optical conduit may be configured differently from the first optical conduit such that, for the same optical input at the respective input ends, the polarisation of the second optical output differs from that of the first optical output. The first optical output may be linearly polarised, and the second optical output may be elliptically or circularly polarised or vice versa due to the difference in configuration between the first optical conduit and second optical conduit.

[145] Patent US 2003/0138224A1 describes a process to create such a spliced PM fibre. Fibres of this type are commonly used in fibre-optic current sensors.

[146] in one possible generalisation of the examples provided in Figures 3, 5, 6, 7 and 8, it may be understood that the first optical conduit may comprise a first optical fibre of uniform cross-section from the first input end to the first output end. In contrast, the second optical conduit may comprise (i) a first portion comprising a second optical fibre identical in cross-section to the first optical fibre between the second input end and an interim position short of the second output end; and (ii) a second portion having a different cross section to that of the first portion, the second portion extending from the interim position to the second output end.

[147] The interim position may lie within a distance less than or equal to 100 times the diameter of the second core.

[148] It may therefore be understood that the difference in configuration of the second optical conduit relative to the first optical conduit is provided proximate to the second output end.

[149] Advantageously, the above described arrangements comprise standard optical fibre at the input ends of the optical conduits and therefore allow the dual fibre optical apparatus to be connected to standard equipment at the input ends. Thus, a more compatible dual fibre optical apparatus is provided.

[150] Figure 9 is a cross-section of a dual fibre optical apparatus in which the output end of one fibre is offset from that of the other fibre in the direction parallel to its optical axis. Such arrangements allow the relative focal positions of the first optical output 351 and second optical output 352 to be adjusted.

[151] Figure 9 shows the first optical fibre 310 and second optical fibre 320 of any of Figures 3 to 8, but wherein the first output end 315 is offset by a distance Az (= zB -ZA) forward of the second output end 325 in a direction parallel to the second optical axis 320a and in the propagation direction of the second optical output 352. If the distance between the second optical axis 320a and the side of the first optical fibre 310 closest to the second optical axis is y3, the closest the waist w(zB) of the second optical output can be to the first optical fibre 310 at the first output end 315 is a half of y3 if beam clipping is to be prevented.

[152] The outputs of the two optical fibres may be offset in the Z direction as shown in Fig 9 up to a limit where the beam from the rear fibre intersects with the front fibre. At this point some beam distortion will occur. An example derivation of a maximum offset follows, but this is not intended to be limiting and other maximum offsets may be derived depending on the application.

[153] Assuming a minimum beam waist radius con reaching the Rayleigh limit at the second output end 325 (i.e. at zA), a beam waist propagation w(z) given by equation I and the Rayleigh range zR defined by equation 2 below, the maximum output end 325 is given by equation 3. offset Az between the first output end 315 and the second w(z) = 2 1//2 AN" 2 (1) (2) (3) (Z-Zo 1]1/2 [1+ )

ZR ZR =

"R too Az = ZR

-2 coo

In equation 2, M2 is the beam quality factor. The beam vaist w(z) is defined as the distance from the second optical axis 320a at which the intensity of the second optical output 352 is lies of the maximum intensity of the second optical output 352, it is assumed that the second output end 325 is flat and perpendicular to the second optical axis 320a. It may therefore be understood that the first output end 315 and second output end 325 are offset in the direction parallel to the first optical axis 310a (or second optical axis 320a) by less than or equal to the offset defined in equation 3. A larger offset is possible, but artefacts from clipping the second optical output on the first optical fibre may occur. Az/zR derived from a simple rearrangement of equation 3 is a dimensionless parameter relating the maximum offset to the Rayleigh range of the beam from the second output end 325.

11541 It may therefore be understood that the first output end 315 of the first optical conduit and second output end 325 of the second optical conduit may be offset in the direction parallel to the first optical axis 310a (or second optical axis 320a), optionally by less than or equal to the offset defined in equation 3. Said offset may be zero so that the output ends arc aligned or non-zero.

[155] Figure 10 illustrates a system including the dual fibre optical apparatus of Figure 5. Although the dual fibre optical apparatus of Figure 5 is illustrated, it may be understood that the system may instead incorporate any of the dual fibre optical apparatuses previously described with reference to Figures 3-9.

[156] The system 500 shown in Figure 10 includes a light source 510, a feed optical fibre 520, an optical switch 530, a light source driver 515, a switch driver 535, and a controller 550. The dual fibre optical apparatus 300 of Figure 5 is also shown in Figure 10. However, it may be understood that any dual fibre optical apparatus as herein described may be substituted for the dual fibre optical apparatus shown.

[157] The light source 510 is optically coupled via the feed optical fibre 520 to the optical switch 530, which is, in turn, coupled to the first input end of the first optical fibre 310 and the second input end of the second optical fibre 320.

[158] it may therefore be understood that there is provided a system including any one of the dual fibre optical apparatuses herein described including any of the variants herein described and a light source, wherein the first and second input ends are optically connected to the light source.

[159] The controller 550 is connected to the light source driver 515 and the switch driver 535, which are, in turn, respectively connected to the light source 510 and optical switch 530.

[160] In operation, the controller 550 commands the light source driver 515 to drive the light source 510 to provide light having a first set of optical parameters. The light is fed via the feed optical fibre 520 to the optical switch 530. The controller 550 commands the switch driver 535 to drive the optical switch 530 to control the coupling of light into the first input end of the first optical fibre 310 or the coupling of light into the second input end of the second optical fibre 320.

[161] The optical switch 530 may perform the selective switching operation previously described by selectively switching between coupling the light from the light source 510 into the first input end of the first optical fibre 310, or the second input end of the second optical fibre 320, or a beam dump (not shown). In addition, the optical switch 530 may operate in a further mode in which light from the light source 510 is split, for example with a beam splitter in the optical switch 530, so as to be simultaneously coupled into both the first input end of the first optical fibre 310 and the second input end of the second optical fibre 320.

[162] As a result of the arrangement in Figure 10, light from a single light source may be selectively coupled into the first input end of the first optical conduit and, independently of this, selectively coupled into the second input end of the second optical conduit.

[163] Alternatively, or in addition, a first variable beam splitter or first variable attenuator (for example, an acousto-optic modulator with variable RF input power) may be provided in the switch to vary the amount of light coupled into the first input end. Alternatively, or in addition, a second variable beam splitter or second variable attenuator may be provided in the switch to vary the amount of light coupled into the second input end. The variable beam splitter(s) or variable attenuator(s) may be controlled by commands sent from the controller via the switch driver.

[164] it may therefore be understood that light from the light source may be variably coupled into the first input end of the first optical conduit and, independently of this, variably coupled into the second input end of the second optical conduit by means provided in the optical switch 530.

[165] The controller may issue first commands to the switch driver 535 to vary the coupling of light into the first input end in a first time dependent cycle and may issue second commands to the switch driver 535 to vary the coupling of light into the second input end in a second time dependent cycle. The first time dependent cycle may be different from the second time dependent cycle.

[166] it may therefore be understood that the system may further comprise a controller configured to operate the at least one switch such that the power of first optical output varies according to a first time-dependent cycle and the power of the second optical output varies according to a second time-dependent cycle different from the first time-dependent cycle.

[167] Advantageously, a robust and compact means for providing two beams of light having differing time dependent and time independent optical properties to or near the same location is provided.

[168] Alternatively, the input end of each optical fibre may be connected to a separate light source, each light source being operated independently of the other.

[169] Figure 11 is a schematic of a simple prism magneto-optical trap cold atom interferometer 1100 including a dual fibre optical apparatus 300 similar to that shown in Figure 5. The dual fibre optical apparatus 300 may be a part of the system shown in Figure 10.

[170] Preferably, the dual fibre optical apparatus in Figure I I is substantially the same as that shown in Figure 5. Preferably, a quarter wave plate 385 is placed in the path of the first optical output 351 and second optical output 352 with its optical axis aligned with the polarisation axis of the second optical output 352.

[171] As the polarisation maintaining axis of the first optical fibre 310 is aligned relative to that of the second optical fibre as shown in Figure 4b, the quarter wave plate 385 acts to circularly polarise the first optical output 351, which has its polarisation axis angled at 45° to the second optical output and quarter wave plate 385 optical axis. The second optical output 352, having an axis of polarisation aligned with the optical axis of the quarter wave plate 385, will experience no change in polarisation, but a delay in phase will occur.

[172] Alternatively, the quarter wave plate 385 shown in Figure 5 is not present and a quarter wave plate 386 may instead be placed in the path of the first collimated beam 371 and second collimated beam 372. However, as the first collimated beam 371 and second collimated beam 372 are larger than the first optical output 351 and second optical output 352, respectively, it may be preferable from a manufacturing point of view to instead use the quarter wave plate 385 in the path of the first optical output 351 and second optical output 352 before the lens as described above. The disadvantage being that this will reduce the quality of polarisation in the periphery of the first optical output 351 and second optical output 352.

[173] The first collimated beam 371 has a larger diameter than the second collimated beam 372. The first collimated beam 371 may be used as the cooling beam and the second collimated beam 372 may be used as the Raman beam in the atom interferometer.

[174] The position of the output ends of each optical fibre relative to the lens may be optimised to fine tune the divergence of each of the first collimated beam 371 and second collimated beam 372. As a result, there may be a slight difference between divergence angle of first and second beam after collimating lens. The Raman beam may be collimated, whereas cooling beam may be slightly convergent due to the difference in focal position of the respective optical fibres. However, in the arrangement shown in Figure 11 there is a relatively short working range, so this effect is negligible. By providing a small angular offset between the end sections, while keeping them substantially parallel to each other, an apparatus can be conceived whereby this small angular offset is instead replaced by a small lateral offset between the first collimated beam 371 and second collimated beam 372. However, manufacture of such an optical apparatus would be complicated. The angular divergence could be reduced by etching or polishing the fibre cladding to decrease the distance between the fibre cores as described earlier.

[175] Figure 11 shows a first magnetic coil 1111 and a second magnetic coil 1112, a set of four prisms 1121, 1122, 1123, 1124, a quarter wave plate 1130, a plane mirror 1140, and the location of an atom cloud 1150, all of which are contained in an ultra-high vacuum chamber 1160.

[176] The first magnetic coil 1111 and second magnetic coil 1112 have collinear coil axes and are spaced apart from one another in the direction of the coil axes. The set of four prisms 1 121, 1122, 1123, 1124, quarter wave plate 1130, plane mirror 1140, and atom cloud 1150 are positioned in the space between the first magnetic coil 1111 and second magnetic coil 1112. The atom cloud lies on a point along the coil axes and is held in place by the magnetic fields generated by the first magnetic coil 1 111 and second magnetic coil 1 112 and the optical field generated by the cooling beam.

[177] The four prisms 1121, 1122, 1123, 1124, are each a triangular prism having a cross-section in the shape of a 45-degree isosceles triangle. The four prisms 1121, 1122, 1123, 1124, arc arranged around the atom cloud 1150 so as to define a square aperture lying in a plane perpendicular to the coil axes and having a centre aligned with the coil axes. The hypotenuse face of each prism faces the atom cloud 1150 and is angled at 45 degrees to the plane of the square aperture. The atom cloud 1150 is located between the first magnetic coil 1111 and the square aperture.

[178] The quarter wave plate 1130 lies perpendicular to the coil axes, has a centre aligned with the coil axes, and is positioned between the square aperture and the second magnetic coil 1112.

[179] The plane minor 1140 lies perpendicular to the coil axes, has a centre aligned with the coil axes, and is positioned between the quarter wave plate 1130 and the second magnetic coil 1112.

[180] The dual fibre optical apparatus 300 is arranged such that the first collimated beam 371 and second collimated beam 372 enter the ultra-high vacuum chamber 1160 through a window therein (not shown) to propagate through the first magnetic coil 1 11 1 and second magnetic coil 1112 along the coil axes.

[181] The diameter of the first collimated beam 371 is greater than the length of one side of the square aperture such that outer portions of the first collimated beam 371 impinge on the hypotenuse face of each prism 1121, 1122, 1123, 1124, while a central portion of the first collimated beam 371 passes through the square aperture. Owing to the angle of each hypotenuse face relative the direction of propagation of the first collimated beam 371, the outer portions of the first collimated beam 371 which are impingent on each hypotenuse face are reflected by the hypotenuse face to propagate in a direction which is perpendicular to the coil axes to impinge on the atom cloud. That is, the atom cloud 1150 is irradiated by the outer portions of the first collimated beam 371 from four directions lying in a plane perpendicular to the coil axes.

[182] The central portion of the first collimated beam 371 passes sequentially through the atom cloud 1150, the square aperture, and then the quarter wave plate 1130 before being reflected by the plane minor 1140 to pass sequentially back through the quarter wave plate 1130, the square aperture and the atom cloud 1150. Therefore, the atom cloud 1150 is additionally irradiated by the first collimated beam 371 from a further two (opposing) directions parallel to the coil axes. That is, the first collimated beam 371 irradiates the atom cloud from six sides due to the arrangement of the dual fibre optical apparatus 300, the four prisms 1121, 1122, 1123, 1124, and the mirror relative to each other and the atom cloud 1150. The first collimated beam 371 is circularly polarised due the configuration of the first optical fibre 310.

[183] The diameter of the second collimated beam 372 is less than the length of one side of the square aperture such that the second collimated beam 372 passes through the square aperture without impinging the prisms 1121, 1122, 1123, 1124. The second collimated beam 371 passes sequentially through the atom cloud 1 ISO, the square aperture, and then the quarter wave plate 1130 before being reflected by the plane mirror 1140 to pass sequentially back through the quarter wave plate 1130, the square aperture and the atom cloud 1150. The purpose of the quarter wave plate 1130 is to ensure that the reflected light has appropriate polarisation for atom interaction.

[184] The first collimated beam 371 having circular polarisation acts as the cooling beam in the atom interferometer. The second collimated beam 372 having the linear polarisation acts as the Raman beam.

[185] Figure 12 is a flow chart illustrating a switch sequence for operating the dual fibre optical apparatus for use with the atom interferometer described with reference to Figure 11. initially, in step S1201, the optical switch 530 is operated to couple the light from the light source 510 into the first optical fibre 310 only, so that the first optical output 351 (and hence the first collimated beam 371, i.e. cooling beam) is on and the second optical output 352 (and hence the second collimated beam 372, i.e. Raman beam) is off. Next, in step S1202, the optical switch 530 is operated to couple the light from the light source 510 into the second optical fibre 320 for a period sufficient to create a TE/2 Raman pulse in the second optical output 352. Next, in step S 1203, the optical switch 530 is operated to couple the light from the light source 510 into the beam dump so that no light from the light source 510 is coupled into either the first optical fibre 310 or second optical fibre 320, hence both the first optical output 351 and second optical output 352 are both off Next, in step S1204, the optical switch 530 is operated to couple the light from the light source 510 into the second optical fibre 320 for a period sufficient to create a It Raman pulse in the second optical output 352. The next two successive steps, step S1205 and step 51206, are identical to steps S1203 and 51202, respectively. Finally, in step 51207 the optical switch 530 is operated to switch the cooling beam on to couple the light from the light source 510 into the first optical fibre 320 for a period sufficient to create a detection pulse in the first optical output 351. The person skilled in the art of atom interfcrometry can determine an appropriate length of time for the detection pulse based on other variables in the atom interferometer without undue skill or burden.

[186] More generally, it may be understood that there is provided an atom interferometer including any of the optical apparatuses or systems described herein, wherein the first optical output is used to form a cooling beam and the second optical output is used to form a Raman beam.

[187] Although the example of an atom interferometer is provided, many other applications may benefit from the use of the dual fibre optical apparatuses and systems described herein. Examples of such applications include, but are not limited to, laser materials processing, fluorescence microscopy and Raman spectroscopy. In general, the dual fibre optical apparatuses provide two overlapping optical outputs which can be combined or alternately used in rapid succession at the same target.

[188] In laser materials processing, for example laser machining, the dual fibre optical apparatuses offer a compact and low aberration means for allowing beam switching on the fly during processing. It is known that polarisation can affect the coupling of light into material surfaces differently depending on the direction of travel of the beam. The dual fibre optical apparatus shown in Figure 4a can be used, for example, in conjunction with a focussing lens arranged to focus the first collimated beam 371 and second collimated beam 372 onto a material surface. The first optical output could be turned on when machining in a first direction, and the second output could be turned on in place of the first optical output when machining in a second direction that is angled at 45 degrees to the first direction, thereby adjusting the angle of polarisation of the beam to suit the direction of the travel of the focussed beam relative to the material surface.

[189] In another example of use, the dual fibre optical apparatus may be used in dual focus laser machining. Dual focus laser machining is usually carried out with a dual focus lens and a single beam, for example from a single optical fibre. The dual focus lens has a central portion arranged to focus light incident thereon to a first focal position, and an annular portion arranged to focus light incident thereon to a second focal position closer to the lens than the first focal position. Thus, the lens creates two focal positions, which enhances focus position tolerance and increases cutting speed in laser machining. The dual focus lens can be a bifocal refractive lens or mirror. Comparable results may be achieved using any of the dual fibre optical apparatuses described herein with output ends set at different distances from a standard converging lens (therefore without the need for a dual focus lens). The added ability to change polarisation between beams using any of the arrangements described with reference to Figures 4a/4b and 8a/8b/8c, may improve the machining quality yet further.

[190] More generally it may be understood that there is provided a laser machining apparatus comprising any of the optical apparatuses or systems described herein.

[191] Dual spot laser welding is yet another laser material processing application which may benefit from the use of the dual fibre optical apparatuses herein described, owing to the creation of two beams having differing optical characteristics without the need for a bulk optic for splitting a single beam.

[192] The dual fibre optical apparatuses as herein described may also be useful in other applications.

[193] In fluorescence microscopy, for example, the first optical output may be used to excite the material and the second optical output may be used to de-excite the material.

[194] it may be understood that there is provided a fluorescence microscope comprising any of the optical apparatuses or systems described herein, wherein the first optical output is used to form a beam for exciting a material and second optical output is used to form a beam for de-exciting the material.

[195] In Raman spectroscopy, for example, the first optical output may be used to excite the material and the second optical output may be used to probe the material.

[196] It may therefore be understood that there is provided a Raman spectrometer comprising any of the optical apparatuses or systems described herein, wherein the first optical output is used to form a beam for exciting a material and the second optical output is used to form a beam for probing the material properties.

[197] Those skilled in the art to which this disclosure relates will appreciate that many other embodiments and variations of embodiments are possible within the scope of the appended claims, and further additions, deletions, substitutions and modifications may be made to the described embodiments without departing from the scope of the appended claims.

[198] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of this Disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Moreover, all ranges disclosed herein are to be understood to encompass any and all sub-ranges subsumed therein. For example, a range of "less than 10" can include any and all sub-ranges between (and including) the minimum value of zero and the maximum value of 10, that is, any and all sub-ranges having a minimum value of equal to or greater than zero and a maximum value of equal to or less than 10, e.g., I to 5.

[199] While various disclosed embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Numerous changes to the subject matter disclosed herein can be made without departing from the scope of the appended claims. In addition, while a particular feature may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application.

Claims

CLAIMS1. An optical apparatus comprising: a first optical conduit having a first input end and a first output end, and a second optical conduit having a second input end and a second output end; wherein the first optical conduit comprises a first optical fibre and the second optical conduit comprises a second optical fibre, end sections of the first and second optical conduits at the respective output ends are arranged side-by-side and substantially parallel to each other, and the first optical conduit is configured to provide a first optical output at the first output end in response to an optical input at the first input end, and the second optical conduit is configured differently from the first optical conduit such that, for the same optical input at the respective input ends, at least one optical characteristic of the second optical output differs from that of the first optical output.
2. The optical apparatus according to claim 1, wherein the difference in configuration between the first and second optical conduits comprises a difference in configuration of the first optical fibre relative to the configuration of the second optical fibre.
3. The optical apparatus according to claim 1 or 2, wherein the first optical conduit or second optical conduit further comprises at least one first optical component contiguous with the respective optical fibre.
4. The optical apparatus according to claim 1, 2 or 3, wherein a separation between the optical axes of the end sections is less than five times the sum of maximum diameter of the first optical conduit and second optical conduit, optionally less than or equal to half the sum of maximum diameter of the first optical conduit and second optical conduit.
The optical apparatus according to any preceding claim, wherein the second output end is offset from the first output end in a direction parallel to the optical axes of the end sections.
6. The optical apparatus according to any preceding claim, wherein the two end sections arc mounted in a single mount.
7. The optical apparatus according to any preceding claim, wherein the at least one optical characteristic comprises polarisation.
8 The optical apparatus according to claim 7, wherein the first optical output is linearly polarised, and the second optical output is circularly polarised.
9. The optical apparatus according to claim 7 or 8, wherein the first optical fibre comprises a first polarisation-maintaining section along a first length of the first optical fibre and a second polarisation-maintaining section along a second length of the first optical fibre, and a polarisation-maintaining axis of the first polarisation-maintaining section has a non-zero angular offset relative to a polarisation-maintaining axis of the second polarisation-maintaining section.
10. The optical apparatus according to claim 9, wherein the second length is a whole number of beat lengths plus 'A or of a beat length at an operating wavelength of the first optical fibre, and the non-zero angular offset is about 45 degrees.
11. The optical apparatus according to claim 7, wherein the first optical fibre is arranged to maintain linear polarisation in a first axis, and the second optical fibre is arranged to maintain linear polarisation in a second axis having an angular offset relative to the first axis.
12. The optical apparatus according to any preceding claim, wherein the at least one optical characteristic comprises divergence.
13. The optical apparatus according to any preceding claim, wherein the first optical fibre comprises a first core profile and the second optical fibre comprises a second core profile which is different from the first core profile.
14. The optical apparatus according to any preceding claim, wherein one of the first optical fibre and second optical fibre comprises a thermally expanded core at the output end of the respective optical conduit.
15. The optical apparatus according to any preceding claim, wherein one of the first optical conduit and second optical conduit further comprises a grin lens or multimode-grin-core optical fibre at the respective output end.
16. The optical apparatus according to any preceding claim, further comprising a second optical component arranged to receive the first optical output and second optical output.
17. The optical apparatus according to claim 16, wherein the second optical component is configured to reduce the divergence of the first optical output and second optical output.
I 8. The optical apparatus according to claim 16, wherein the second optical component is configured to collimate the first optical output to produce a first collimated output and to collimate the second optical output to produce a second collimated output.
19. The optical apparatus according to claim 18, wherein the optical axes of the first collimated output and second collimated output are substantially parallel and have a spatial offset of the same order of magnitude as the separation distance between the optical axes of the end sections.
20. The optical apparatus according to any of claims 16-19, wherein the first output end and the second output end are configured such that the first optical output and the second optical output overlap at the second optical component.
21. The optical apparatus according to any preceding claim wherein at least one of the first and second output ends is cleaved at a non-zero angle with respect to a first plane perpendicular to the optical axis of the respective end section, wherein the angle is in a second plane in which the optical axes of the end sections lie.
22. A system comprising the optical apparatus of any preceding claim and a light source, wherein the first and second input ends are optically connected to the light source.
23. The system according to claim 22, wherein the system further comprises at least one optical switch configured to selectively couple light from the light source into the first optical conduit and second optical conduit independently of each other.
24. The system according to claim 23, wherein the system further comprises a controller configured to operate the at least one optical switch such that the power of first optical output varies according to a first time-dependent cycle and the power of the second optical output varies according to a second time-dependent cycle different from the first time-dependent cycle.
25. The system according to claim 24, wherein the first time-dependent cycle comprises: a first period in which the power of the first optical output is lower than the power of the second optical output, and a second period in which the power of the first optical output is higher that the power of the second optical output.
26. An atom interferometer comprising the optical apparatus of any of claims 1-21 or the system of any of claims 22-25, which optical apparatus or system is arranged to form a cooling beam from the first optical output and a Raman beam from the second optical output.
27 A fluorescence microscope comprising the optical apparatus of any of claims 1-21 or the system of any of claims 22-25, which optical apparatus or system is arranged to form a beam for exciting a material from the first optical output and a beam for de-exciting the material from the second optical output.
28. A laser machining apparatus comprising the optical apparatus of any of claims 1-21 or the system of any of claims 22-25.
29. A Raman spectrometer comprising the optical apparatus of any of claims 1-21 or the system of any of claims 22-25, which optical apparatus or system is arranged to form a beam for exciting a material from the first optical output and a beam for probing the material properties from the second optical output.