Method of Fabricating an Optical Fibre Preform

Abstract

A method of fabricating an optical fibre preform is disclosed comprising using a subtractive process on an optical monolith to define therein at least a transverse section of the optical fibre preform, wherein the transverse section comprises at least two regions with different refractive indexes. An optical fibre preform fabricated in accordance with the method is also disclosed along with a method of assembling optical components using a subtractive process to define a first interconnecting feature in or for use with a first optical component; using a subtractive process to define a second interconnecting feature in or for use with a second optical component; and coupling the first and second components together using the first and second interconnecting features such that the coupling dictates a passive alignment of the first and second components.

Claims

1. A method of fabricating an optical fibre preform comprising: using a subtractive process on an optical monolith to define therein at least a transverse section of the optical fibre preform; and wherein the transverse section comprises at least two regions with different refractive indexes.

2. The method according to claim 1 wherein the subtractive process comprises laser assisted etching; optionally, comprising laser writing a structure in the optical monolith and chemically etching the structure to obtain the transverse section.

3. (canceled)

4. The method according to claim 1 wherein the transverse section comprises a structured core and/or a hollow core.

5. The method according to claim 1 comprising: using the subtractive process to define at least two transverse sections of the optical fibre preform; and stacking the at least two transverse sections to form a stacked optical fibre preform.

6. The method according to claim 5 further comprising coupling the at least two transverse sections to form the stacked optical fibre preform; optionally, wherein the coupling dictates a passive alignment of the at least two transverse sections.

7. (canceled)

8. The method according to claim 6 wherein the coupling is achieved using at least one interconnecting feature defined in at least one of the at least two transverse sections.

9. The method according to claim 8 wherein the at least one interconnecting feature in the at least one transverse section is configured for direct or indirect coupling to another transverse section.

10. The method according to claim 8 wherein the at least one interconnecting feature is configured for coupling using one or more of: a pin, a dowel, a sphere, a dove tail joint, a threaded portion, a ring, a plug and socket arrangement, a self-centring locator.

11. The method according to of claim 5 further comprising bonding the at least two transverse sections in the stacked optical fibre preform.

12. The method according to claim 11 wherein the bonding comprises one or more of: catalysis bonding; ultrafast laser bonding; optical contact bonding; or laser welding.

13. The method according to claim 1 wherein the optical fibre preform is fabricated with a longitudinally varying core structure.

14. The method according to claim 13 wherein the optical fibre preform is formed from two or more transverse sections having different core structures.

15. (canceled)

16. An optical fibre preform fabricated in accordance with claim 1.

17. (canceled)

18. A method of assembling optical components comprising: using a subtractive process to define a first interconnecting feature in or for use with a first optical component; using a subtractive process to define a second interconnecting feature in or for use with a second optical component; and coupling the first and second components together using the first and second interconnecting features such that the coupling dictates a passive alignment of the first and second components.

19. The method according to claim 18 wherein the first and second interconnecting features are configured for direct or indirect coupling.

20. The method according to claim 19 wherein the first and/or second interconnecting features are configured for coupling using one or more of: a pin, a dowel, a sphere, a dove tail joint, a threaded portion, a ring, a plug and socket arrangement, a self-centring locator.

21. The method according to claim 18 further comprising bonding the first and second optical components together once coupled.

22. The method according to claim 21 wherein the bonding comprises one or more of: catalysis bonding; ultrafast laser bonding; optical contact bonding; or laser welding.

23. The method according to claim 18 further comprising fabricating at least one of the first and second optical components using the subtractive process.

24. The method according to claim 18 wherein at least one of the first and second optical components comprises: an optical substrate; an image slicer; or at least a transverse section of an optical fibre preform.

25. (canceled)

Description

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] Some embodiments of the disclosure will now be described by way of example only and with reference to the accompanying drawings, in which:

[0051] FIG. 1 shows an optical fibre preform in accordance with the prior art;

[0052] FIG. 2 shows apparatus for drawing an optical fibre from an optical fibre preform in accordance with the prior art;

[0053] FIG. 3 shows apparatus for fabricating an optical fibre preform in accordance with the present disclosure;

[0054] FIGS. 4A, 4B and 4C show examples of optical components fabricated using the apparatus of FIG. 3;

[0055] FIG. 5A shows a transverse section for an optical fibre preform in accordance with the present disclosure;

[0056] FIG. 5B shows an alignment ring for use in an embodiment of the disclosure;

[0057] FIG. 5C shows a stacked optical preform with a socket for receiving a further transverse section in accordance with an embodiment of the disclosure;

[0058] FIG. 5D shows a stacked optical preform similar to that of FIG. 5C but with a further transverse section screwed into the socket;

[0059] FIG. 5E shows a transverse cross-sectional view of the further transverse section and uppermost alignment ring from FIG. 5D;

[0060] FIG. 6A shows a transverse section, including alignment holes, for an optical fibre preform in accordance with an embodiment;

[0061] FIG. 6B shows the transverse section of FIG. 6A with alignment dowels inserted in the alignment holes;

[0062] FIG. 7 shows possible chemical bonding between transverse sections in accordance with embodiments of the disclosure;

[0063] FIG. 8 shows two transverse sections with passive alignment achieved using self-centring conical pins in accordance with embodiments of the disclosure; and

[0064] FIGS. 9A and 9B show two transverse sections with passive alignment achieved using sliding dovetail joints in accordance with embodiments of the disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0065] Generally speaking, the disclosure provides a method of fabricating an optical fibre preform and a method of assembling optical components, such as, but not limited to said optical fibre preforms. The disclosure also provides for the manufacture of small scale glass preforms that can then be tapered on a fibre tapering rig.

[0066] In particular, FIG. 3 shows an apparatus 300 for fabricating an optical fibre preform 302 in accordance with the proposed method. Thus, the apparatus comprises an ultrafast pulsed laser 302 configured for use in a subtractive laser assisted etching process performed on an optical monolith 304, which is in the form of a cuboidal slab of fused silica. The laser 302 is configured to define at least a transverse section 306 of an optical fibre preform by laser writing a pre-defined three-dimensional pattern in the optical monolith 304. As shown, the laser 302 is oriented to direct a laser beam in a direction A which is parallel to a plane of the transverse section 306. In other embodiments, the laser 302 may be oriented to direct the laser beam in a direction B which is perpendicular to a plane of the transverse section 306. After the desired pattern is laser inscribed into the optical monolith 304, a wet chemical etching process is used to selectively remove the inscribed material and free the transverse section 306 from the optical monolith 304. This technique is known as ultrafast laser assisted etching (ULAE) and enables the removal of silica or other material from a substrate in three-dimensions with micrometre-scale precision. Further details of the proposed technique can be found in C. A. Ross, D. G. MacLachlan, D. Choudhury, and R. R. Thomson, “Optimisation of ultrafast laser assisted etching in fused silica,” Opt. Express 26(19), 24343 (2018), which is incorporated herein by reference.

[0067] The transverse section 306 comprises at least two regions with different refractive indexes. In particular, the transverse section 306 as shown has a hollow core 308 and six hollow segments 310 radially spaced from and surrounding the hollow core 308.

[0068] Although, the transverse section 306 is shown in FIG. 3 as elongated, in some embodiments much shorter transverse sections will be fabricated and subsequently stacked, aligned and bonded to form an optical fibre preform having a high degree of accuracy.

[0069] FIGS. 4A, 4B and 4C show examples of optical components fabricated using the apparatus of FIG. 3 and the method described above. In particular, FIG. 4A shows a star-shaped transverse section 400 with a hollow cylindrical core; FIG. 4B shows a transverse section having four diamond-shaped cut-outs 402; and FIG. 4C shows a preform comprising cylindrical portions of differing lengths. Notably, each of these optical components are of a scale suitable for embodiments of the invention.

[0070] FIGS. 5A through 5E illustrate a method of assembling optical components similar to those described above, to form an optical preform.

[0071] In particular, FIG. 5A shows a transverse section 500 for an optical fibre preform in accordance with the present disclosure. The transverse section 500 is in the form of a disc having a preform pattern 502 having a hollow core 504 etched out of a centre of the disc and an upper and a lower external screw thread 506 formed by laser assisted etching on the outer circumference of the disc. FIG. 5B shows an alignment ring 510, laser etched from the same material as the transverse section 500, and into which two transverse sections 500 may be partially screwed.

[0072] The alignment ring 510 comprises an upper internal channel 512 for receipt of an upper external screw thread 506 of a transverse section 500 and a lower internal channel 514 for receipt of a lower external screw thread 506 of a transverse section 500.

[0073] FIG. 5C shows a stacked optical preform 520 with a socket for receiving a further transverse section 500 in accordance with an embodiment of the disclosure. The stacked optical preform 520 comprises multiple transverse sections 500 screwed into multiple alignment rings 510 in a stacked configuration to form a solid preform without air gaps. FIG. 5D shows the stacked optical preform 520 similar to that of FIG. 5C but with a further transverse section 500 screwed into the socket of the top alignment ring 512 such that the upper external screw thread 506 of the further transverse section 500 projects above the alignment ring 512 for screwing into a yet further alignment ring 512 and so on. FIG. 5E shows a transverse cross-sectional view of a transverse section 500 screwed into the upper internal channel 512 of an alignment ring 510, with the lower internal channel 514 of the alignment ring 510 awaiting insertion of a further transverse section 500.

[0074] Notably, each transverse section 500 has two separate 360 degree threads 506, allowing the transverse section 500 to be stacked indefinitely and coupled or locked together using complementary alignment rings 510. The pitch of the threads 506 is such that the individual transverse sections 500 contact one another within the alignment rings 510 upon a full 360 degree rotation and a thread terminator (not shown) prevents over-tightening—this ensures precise lateral, longitudinal and rotational alignment. The threads 506 also provide compressive pressure between transverse sections 500, which hold the preform 520 together ready for drawing.

[0075] The exposed upper and lower flat surfaces of the transverse sections 500 are optically polished and are not substantially altered from the original optical monolith of pristine silica which laser is written to form the individual transverse sections 500. As a result, optical contacting between the surfaces means that no further bonding is required. During drawing, any discontinuity of the structure at an interface between transverse sections 500 (or between the transverse sections 500 and the alignment rings 510) is annealed and drawn out over a large distance such that the individual transverse sections 500 (and alignment rings 510) are no longer distinguishable in the final fibre.

[0076] FIG. 6A shows another transverse section 600 in accordance with an embodiment, which is in the form of a disc having a preform pattern 602 and a hollow core 604 etched out of a centre of the disc. However, in this case, instead of external screw threads, three alignment holes 606 are laser etched (equally spaced apart) into a rim of the transverse section 600 and extend from the upper to lower flat surfaces of the transverse section 600. As shown in FIG. 6B, alignment dowels 610 (e.g. pins), etched from the same material as the transverse section 600, are inserted halfway into each of the alignment holes 606 such that the projecting halves serve to locate and align a further transverse section 600 placed on top and so on. In some embodiments, the alignment dowels 610 may be longer than the thickness of the transverse sections 600 such that a single alignment dowel may extend through multiple alignment holes 606 in multiple transverse sections 600 for ease of assembly.

[0077] Although the alignment holes 606 are shown as extended completely through each transverse section 600, in some embodiments, blind alignment holes may be provided to locate the alignment dowels 610 partway within each transverse section 600. However, in this case, separate alignment holes would need to be etched in both the upper and lower surfaces of each transverse section 600.

[0078] The alignment holes 606 and alignment dowels 610 serve to align the transverse section 600 both laterally and rotationally. An advantage of this method is that there is no need to fabricate a complex screw or alignment ring and so fabrication time is reduced. However, the alignment dowels 610 must be placed in the alignment holes 606 by hand or by a machine which increases the time for assembly and there may be an additional need to bond the surfaces together.

[0079] The transverse sections of silica may be bonded together using any of several “direct bonding” techniques that do not rely on any foreign substance, such as glue, which would remain as an impurity in the fibre after drawing. Here, direct bonding means any technique in which a chemical fusion of the silicate components takes place but also extends to optical contacting in which two extremely flat surfaces are held together purely by intermolecular forces. Although optical contacting may not result in a permanent bond immediately, the process of drawing the preform in a furnace results in a blending of the interface which does result in a permanent and indistinguishable bond. During optical contacting, a small quantity of isopropanol or similar can be dropped onto the surface of one of the parts to allow for some alignment time before evaporation makes the parts immovable.

[0080] In the case that optical contacting is not sufficient, chemically activated direct bonding may be used. In the case of silica, hydroxide-catalysis bonding may be employed such as those described in detail in E. J. Elliffe, J. Bogenstahl, A. Deshpande, J. Hough, C. Killow, S. Reid, D. Robertson, S. Rowan, H. Ward, and G. Cagnoli, “Hydroxide-catalysis bonding for stable optical systems for space,” in Classical and Quantum Gravity (2005), 22(10) and Anna-Maria A. van Veggel and Christian J Killow, “Hydroxide catalysis bonding for astronomical instruments”, Adv. Opt. Techn. 2014; 3(3): 293-307. These documents describe the process of chemically activating one surface by producing dangling silicon bonds which preferentially attach to hydroxide molecules of the counterpart surface. The process works as follows: an alkaline solution such as potassium hydroxide is placed on one surface and the other surface is placed in contact. The OH— ions etch the silica surface liberating silicate ions. The released ions decrease the pH of the solution until dissociation occurs and siloxane polymer chains form. The chains create a strong bond between the surfaces and become ridged as a water by-product dehydrates.

[0081] FIG. 7 illustrates such possible chemical bonding 702 between two bulk silica components 700, which may be transverse sections and/or alignment features, in accordance with embodiments of the disclosure.

[0082] FIG. 8 shows two transverse sections 800 having structured cores and with passive alignment achieved using two self-centring conical pins 802, which are inserted into two corresponding conical recesses 804 in a rim of each surface of the transverse section 800, in accordance with embodiments of the disclosure. The conical pins 802 are similar to the dowels described above but have short cylindrical bodies with conical ends which are easier to insert and locate within each transverse section 800 due to their conical nature helping to guide the components into alignment. This may therefore speed up the process of assembling the optical preform and allow for automated assembly. Alternatively, micromachined fused silica spheres could be used instead of pins, further simplifying the assembly process since no pin orientation is required.

[0083] As illustrated in FIG. 8, the two conical recesses 804 in the lower surface of each transverse section 800 are offset by 90 degrees with respect to the two conical recesses 804 in the upper surface of each transverse section 800. This helps to prevent any interference or overlap between the conical recesses 804 in the upper surface and lower surface and allows the conical recesses 804 to each have a depth which is 50% or greater than the thickness of the transverse section 800 for secure coupling and interlocking. Notably, the offsetting of the alignment features in this embodiment is possible due to the symmetrical nature of the structured cores. This may not always be the case.

[0084] FIGS. 9A and 9B show two transverse sections 900 with passive alignment achieved using sliding dovetail joints in accordance with embodiments of the disclosure. The transverse sections 900 each comprise a structured core and two dovetail channels 902 in an upper surface with two corresponding dovetail projections 904 in a lower surface. The dovetail projections 904 on an upper transverse section 900 are configured to slide laterally into the dovetail channels 902 in a lower transverse section 900 to couple the two transverse sections 900 together in a predetermined alignment during assembly of the preform. In this case, direct bonding of the two transverse sections 900 may be by optical contacting or chemical bonding as described above. The dovetail channels may additionally be tapered such that the upper transverse section can be wedged into the lower transverse section.

[0085] In general, an aspect of the invention relates to a method of assembling optical components comprising: using a subtractive process (e.g. laser assisted etching) to define a first interconnecting feature in or for use with a first optical component; using a subtractive process (e.g. laser assisted etching) to define a second interconnecting feature in or for use with a second optical component; and coupling the first and second components together using the first and second interconnecting features such that the coupling dictates a passive alignment of the first and second components.

[0086] Although most of the drawings relate to the fabrication and alignment of transverse sections for optical preforms, it will be understood that aspects of the invention, such as that described above, are applicable more widely to any optical components. For example, macro-scale glass components (e.g. an image slicer or other components) may be manufactured by combining two or more individual components, which may themselves be fabricated using a subtractive process such as laser assisted etching. Furthermore, aspects of the invention extend to the alignment of optical microcomponents on a glass “breadboard” or optical substrate which comprises laser written alignment and joining features as described above.

[0087] The various aspects of the invention have multiple advantages over the prior art, for example, in creating more accurate and pure optical fibre preforms and ensuring ease of alignment and coupling of optical components in general.

[0088] The skilled person will understand that in the preceding description and appended claims, positional terms such as ‘upper’, ‘lower’, ‘top’, ‘side’, etc. are made with reference to conceptual illustrations, such as those shown in the appended drawings. These terms are used for ease of reference but are not intended to be of limiting nature. These terms are therefore to be understood as referring to an object when in an orientation as shown in the accompanying drawings.

[0089] Although the disclosure has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in any embodiments, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.