GB2572621A

GB2572621A - Adhesive curing for affixing optical elements to substrates

Info

Publication number: GB2572621A
Application number: GB1805680.4A
Authority: GB
Inventors: Robertson Andrew; Richard Rider Andrew
Original assignee: Gooch & Housego Torquay Ltd
Current assignee: Gooch & Housego Torquay Ltd
Priority date: 2018-04-05
Filing date: 2018-04-05
Publication date: 2019-10-09
Anticipated expiration: 2038-04-05
Also published as: GB201904886D0; GB2572696B; GB2572696A; GB201805680D0; GB2572621B

Abstract

Assembling optical devices includes providing a glass-based optical element and a substrate with an ultraviolet (UV) light curable adhesive positioned in two locations between the optical element and the substrate. The substrate is within 5 mm of a UV mirror positioned below the substrate. The adhesive is cured using at least one UV light source which emits primarily at a UV wavelength or in a range of UV wavelengths that is positioned above the optical device. At the UV wavelength or range of UV wavelengths the substrate comprises a UV transmissive material and the UV mirror provides high UV reflectance. A portion of the UV light that is transmitted through the substrate is reflected by the UV mirror back through the substrate to provide additional substrate curing. The adhesive may be an acrylic resin such as epoxy acrylate which is cured without the need for any heating. The light source may be a single source and may include LEDs. The substrate may lie directly on the UV mirror. After curing, the substrate may be removed from the UV mirror. Assembling a fused-fiber device also follows the steps above with a silica substrate in direct contact with a UV mirror.

Description

ADHESIVE CURING FOR AFFIXING OPTICAL

ELEMENTS TO SUBSTRATES

FIELD [0001] Disclosed embodiments relate to the affixing of glass-based optical elements such as fused couplers or multimode combiners to substrates, and optical devices therefrom.

BACKGROUND [0002] Glass-based optical elements include fiber-based components and devices such as couplers, attenuators, wavelength division multiplexers/demultiplexers, connectors, filters, switches, fiber-pigtailed semiconductor lasers, and isolators, including for use in fiber-optic communication systems, sensors and instrumentation. In nearly all of these applications employing fiber-based components or devices, design-specific mounting fixtures (or ‘substrates’) are utilized to precisely align, position or secure optical fibers or optical elements within such optical fiber components or devices. In most of these applications, it is common for such mounting fixtures or substrates to be formed of a fused silica because its low coefficient of thermal expansion closely matches that of the optical fibers and other optical components or optical devices thereon.

[0003] Glass-based optical elements such as fused-fiber device (e.g., fused couplers or multimode combiners) are typically secured to a substrate by the curing of an acrylate resin.. The two most common types of such adhesives are cured from their resin state by exposure to either ultraviolet (UV) light and/or by exposure to heat. UV light is generally regarded as having a wavelength from 10 nm to 400 nm, and UV curing as known in the art is a process in which UV light is used to initiate a photochemical reaction that generates a crosslinked network of polymers.

SUMMARY [0004] This Summary is provided to introduce a brief selection of disclosed concepts in a simplified form that are further described below in the Detailed Description including the drawings provided. This Summary is not intended to limit the claimed subject matter's scope.

[0005] Disclosed embodiments recognize for forming glass-based optical elements adhesively affixed to a substrate, in the case of UV curing of UV curable adhesives, when a conventional single overhead UV light source is used, weak adhesive affixments can result between the substrate and the optical element. Weak adhesive affixments may result because the adhesive is only being illuminated from the top side, so that less UV light intensity reaches adhesive locations near the substrate as compared to adhesive locations on the top side. Weak adhesive affixments can result in post-assembly movement of the optical element resulting in beam misalignment or even detachment of the optical element from the substrate, including while in field use of the optical device.

[0006] Disclosed embodiments include a method of assembling optical devices. A glassbased optical element and a substrate with a UV light curable adhesive positioned in a first and a second location between the optical element and the substrate is provided. The substrate is within 5 mm of a UV mirror positioned below the substrate. The adhesive is cured using at least one UV light source emitting primarily at a single UV wavelength or in a range of UV wavelengths that is positioned above the optical device. At the UV wavelength or the range of UV wavelengths the substrate comprises a UV transmissive material and the UV mirror provides high UV reflectance. A portion of the UV light transmitted through the substrate is reflected by the UV mirror back through the substrate to provide additional curing of the adhesive.

BRIEF DESCRIPTION OF THE DRAWINGS [0007] Reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, wherein:

[0008] FIG. 1 shows a depiction of a glass-based optical element being adhesively affixed to a UV transmissive substrate that is within 5 mm of a UV reflective mirror using UV light curable adhesive blobs positioned between the optical element and the substrate. A portion of the UV light from an overhead UV light source that is transmitted through the substrate is reflected by the UV reflective mirror back through the substrate to provide additional adhesive curing, according to an example aspect.

[0009] FIG. 2A is a depiction of the optical element shown comprising a fused-fiber device being adhesively affixed to a UV transmissive substrate that is within 5 mm of a UV reflective mirror using UV light curable adhesive blobs positioned between the fused-fiber device and the substrate, where a portion of the UV light from an overhead UV light source that is transmitted through the substrate is reflected by the UV reflective mirror back through the substrate to provide additional adhesive curing, according to an example aspect.

[0010] FIG. 2B shows a depiction of a fused-fiber device being adhesively affixed to a UV transmissive substrate that is in direct contact with a UV reflective mirror using UV light curable adhesive blobs positioned between the fused-fiber device and the substrate, where a portion of the UV light from an overhead UV light source that is transmitted through the substrate is reflected by the UV reflective mirror back through the substrate to provide additional adhesive curing, according to an example aspect.

[0011] FIG. 3 is a flow chart that shows steps in an example method of adhesive curing for affixing optical elements to substrates using a UV reflective mirror below the substrate so that UV light from an overhead UV light source that is transmitted through the substrate is reflected by the UV reflective mirror to provide additional adhesive curing, according to an example aspect.

DETAILED DESCRIPTION [0012] Example embodiments are described with reference to the drawings, wherein like reference numerals are used to designate similar or equivalent elements. Illustrated ordering of acts or events should not be considered as limiting, as some acts or events may occur in different order and/or concurrently with other acts or events. Furthermore, some illustrated acts or events may not be required to implement a methodology in accordance with this disclosure.

[0013] This Disclosure recognizes when affixing glass-based optical elements such as fused-fiber devices (e.g., fiber couplers or multimode combiners) to a substrate, a UV cure adhesive is conventionally utilized. Although generally described herein for affixing fused-fiber device to a substrates, disclosed embodiments are not limited to the affixment of fused-fiber devices to substrates, since the optical device can be other than a fused-fiber device.

[0014] After fiber fusion to form the fused-fiber device, the suspended region of the fused-fiber device is typically secured in a static position, a substrate is then raised from below to the close vicinity of the suspended taper region, typically within 1 mm, and then a blob of uncured adhesive (which can be considered to be a ‘drop’ or a ‘bead’) is generally applied to locations at both ends of the tapered region. The suspended region comprises a region of the untapered fiber(s) at each end, a biconical down-taper, uniform center waist region, and biconical up-taper. The applied blobs are generally applied in a measured amount sufficient to generally both cover the fiber and extend to touch the substrate.

[0015]

Disclosed aspects include having the active portion of a fused-fiber device affixed to a substrate at two (or more) locations that are outside the fused area. A fused-fiber device has at least two fibers fused together in a controlled way. Before fusing the respective fiber sections, each fiber section to be fused needs to have its jacket (also called a buffer or cladding) removed. This exposes the outside surface of the cladding layer. During the fusion operation, the cladding layers of two or more fibers are brought together, heated up by using a flame or a CO2 laser, and the section is pulled to a longer length. The cladding layers of participating fibers fuse together. [0016] Due to the elongation of the fused section, each fiber diameter, and also their core diameters, will become smaller to the point where launched light will leak into the cladding area of the fiber. This light can then couple into the core of one or several of the other fibers in the fused section. To keep the light loss minimal, it is generally important that the exposed cladding surface is protected from making contact with any other surfaces such as substrate, and housing. Such accidental contact can induce phase shifts or attenuation of the cladding modes that propagate close to that surface. Therefore, this Disclosure recognizes it to be helpful to package the fused section so it is a suspended region over the substrate, not touching the substrate, between two blobs of adhesives. The blobs of adhesive are applied to encase the up and down taper regions opposite the uniform thickness waist region, and where the fibers separate. The blobs typically also extend to encase all individual fiber sections to where the cladding is present on the fibers.

[0017] The substrate can have a D-shaped cross-section and have all surfaces ground. Alternate substrates can have a rectangular cross-section. The substrate is generally always longer than the fused section, and substrates are typically 1 nm to 5 mm wide.

[0018]

The optical device with the fused section, is held between clamps that hold the jacketed fibers at each end under slight tension. The mounting clamp for the substrate can be on a stage that is then moved upward on an adjustable stage to approach, but not touch, the fused section including the waist. The distance between the substrate and the cladding is generally less than 2 mm, but large enough to avoid the substrate from accidental touching even under large acceleration forces. This distance can typically be 0.125 mm to 0.25 mm.

[0019] An example assembly jig is now described. In general, a silica substrate sits on a custom jig with a flat metal base plus a fixed and moving jaw that is adjusted to suit the substrate size and allow adequate clamping force. The clamping jaw is generally 10 mm wide, the base about 20 mm wide but a few versions exist. For a known assembly process the section of the substrate with the adhesive applied is in free-space, or as disclosed herein may instead sit directly on a reflective mounting platform. The mounting platform is at least as long as the substrate and the area towards the end need to be reflective. This can be done for example by gluing two rectangular mirrors to the left and right of the jaw, but extending far enough. The fused-fiber device is then illuminated from above with a UV source to cure the adhesive to provide the needed affixment between the fused-fiber device and the substrate.

[0020] In this conventional adhesive affixment process, the UV adhesive is illuminated from the top only, and as a result the UV curable adhesive does not harden (i.e., crosslink) in a uniform manner which can lead to weakness of the adhesive bond at locations in the adhesive closer to the substrate. The UV light exposure time may be lengthened to try to ensure better adhesive bonding in the adhesive closer to the substrate. However, this results in a lengthening of manufacturing cycle time thus increasing cost assembly, and may generally still result the adhesive bond being weak enough to allow in movement of the fused-fiber device or other optical element causing beam misalignment or even detachment of the optical element from the substrate, including while in field use.

[0021] FIG. 1 shows a depiction of a glass-based optical element 110 being assembled to a UV transmissive substrate 120 shown being within (<) 5 mm from a UV reflective mirror 125 using a UV light curable adhesive 115. The adhesive is shown as blobs 115a and 115b (referred to herein collectively as adhesive 115) positioned between respective ends of the suspended region 110c of the optical element 110 and the substrate 120. The UV reflective mirror 125 can also be in direct physical contact with the substrate 120. (See FIG. 2B described below).

[0022] The adhesive 115 is cured using UV light from at least one overhead UV light source 105, where a portion of the UV light that is transmitted through the substrate 120 is reflected by the UV reflective mirror 125 back through the substrate 120 shown as reflected UV 129. The path of the reflected UV 129 through the adhesive 115 from below thus provides additional adhesive curing. The curing of the adhesive 115 thus occurs faster and in a more uniform manner as compared to the conventional adhesive curing process described above.

[0023] The UV light source 105 is for emitting UV illumination 109 primarily at a single UV wavelength or in a range of UV wavelengths that is directed through a collimating lens 108 to the adhesive 125. The lens 108 can be complex to try to achieve good uniformity and controlled angular spread across the aperture. If a discharge lamp is used, a cold light mirror or heat shield can be used to avoid heat from reaching the work area.

[0024] Systems designed for UV curing typically are generally simpler. One such system has flood lamps and does not even need a lens. At the UV wavelength or range of UV wavelengths the substrate 120 comprises a UV transmissive material and the UV mirror 125 provides high UV reflectance. As used herein, a UV transmissive material for the substrate 120 transmits > 80% of the power at the UV wavelength or range of UV wavelengths provided by the UV light source 105 for a substrate thickness of 2 mm, generally providing > 80% power transmission between 250 nm and 400 nm. For example, pure silica has over 90% power transmission for a 2 mm thickness. Most of the light loss is reflection loss from the surfaces in uncoated silica substrates, about 8% from the two surfaces. It is noted UV light curing the adhesive from below (reflected UV 129) passes through the substrate 120 twice and gets reflected by the UV mirror 125 as well.

[0025] A high UV reflectance for the UV mirror 125 as used herein provides a reflectance of > 80% for the UV wavelength or range of UV wavelengths provided by the UV light source 105, generally providing > 90% reflectance for UV light between 250 nm and 400 nm. One UV mirror 125 choice is protected aluminum. Protected aluminum has good UV reflectivity, and the overcoating makes it harder to be scratched if the substrate 120 gets placed onto the surface directly.

[0026] In a typical embodiment the material for the substrate 120 comprises fused silica (also known as fused quartz) due to its reliability, rigidity and thermal expansion characteristics that match silica that may be used for the optical element 110. Fused quartz is known to provide 90 % power transmission at a thickness of 10 mm from 230 nm to 790 nm. However, other materials matching properties of the optical element 110 can generally be used for the substrate 120. The thickness of the substrate 120 is generally within the range from 0.5 mm to 2 mm. This thickness range is generally sufficiently thin to allow efficient thermal conduction, while generally being thick enough to be mechanically robust. The substrate 120 can be in various shapes, such as D-shaped as described above (for circular housings), or a slab-shaped geometry.

[0027] The UV light source 105 can comprise mercury vapor lamp(s) which are known for curing products with UV light. The mercury vapor lamp works by high voltage passing therethrough, which vaporizes the mercury. An arc is created within the mercury vapor which emits a spectral output in the UV with light intensity occurring in the 240 nm to 270 nm range and 350 nm to 380 nm range. Xenon lamp(s) can also be used. Besides a single light source, the UV light source comprises can also comprise an array of light sources, such as an LED array which emit at essentially a single wavelength. Fluorescent lamps made specifically for UV curing are also commercially available.

[0028] The adhesive 115 can comprise a UV acrylate resin employing free radical curing. The adhesive can also comprise an epoxy resin, which as known in the polymer arts are also known as polyepoxides. The adhesive 115 can be cured exclusive of applying any externally applied heating. The adhesive 115 can be selected to provide a coefficient of thermal expansion (CTE) value that reasonably matches the substrate 120, such as a substrate having a TCE of about 0.5xl0'⁶/°C and an adhesive having a CTE of about 50 to 60 x 10'⁶/°C. CTE matching can be an important consideration for applications where exposure to wide temperature ranges exist. Another selection consideration can be is that the adhesive should have low shrinkage upon curing. If there is significant shrinkage, it can put strain on the fibers which can induce stress birefringence which can degrade performance of certain optical devices. The adhesive 115 selected can providing < 1 % shrinkage upon curing.

[0029] The UV reflective mirror 125 can comprise a variety of mirrors and is generally a planar (flat) mirror. One example UV reflective mirror as describe above is an aluminum mirror with a protective coating (e.g., silicon oxide) thereon to resist scratching, such as sold by

Thorlabs Inc, Newton, New Jersey, USA. This UV reflective mirror provides > 90% reflectance from 250 nm to 450 nm (thus extending into the visible spectrum).

[0030] FIG. 2A shows a depiction of a fused-fiber device 110’ being adhesively affixed to a UV transmissive substrate 120 that is within (<) 5 mm of a UV reflective mirror 125 using a UV light curable adhesive blobs 115a and 115b positioned between a suspended region 110c comprising a region of the un-tapered fiber(s) at each end, a biconical down-taper region 110ci, uniform thickness center waist region 110c2, and a biconical up-taper 11 Oct. The length of the suspended regions 110c may be in the range 7.5 mm to 90 mm, and typically 20 to 24 mm. A fused multimode combiner may comprise a fused and tapered fiber bundle spliced to a single delivery fiber. In this case the UV adhesive may be applied to the glass fiber (stripped of buffer coating) or even directly on to the acrylate buffer coating in some cases. The fibers 110a, 110b are typically 0.125 mm to 0.25 mm above the substrate 120 for 2x2 splitters using a 0.125 mm fiber. The fused-fiber device 110’ can comprise a fused-fiber coupler (single or multimode) or a multimode combiner.

[0031] Although the fused-fiber device 110’ is shown having a single input fiber 110a and a single output fiber 110b, the fused-fiber device 110’ can have multiple input fibers and multiple output fibers. For example, a 2x2 port fused-fiber coupler that comprises first and second input fibers and first and second output fibers with the tapered regions HOcl and 11 Oct between the input fibers and the output fibers.

[0032] The substrate 120 for the fused-fiber device 110’ can be in various shapes, such as D-shaped (for circular housings), slab-shaped geometry, and may have a slotted region in which to locate the tapered region 110c of the device. In a typical embodiment the material for the substrate 120 for a fused-fiber device 110’ can comprise fused silica (also known as fused quartz) due to its reliability, rigidity and thermal expansion characteristics that match conventional silica optical fiber in typical applications where the fused-fiber device 110’ comprises silica. For silica optical fiber the CTE is around 5xlO^_7/°C.

[0033] FIG. 2B shows a depiction of a fused-fiber device 110’ being adhesively affixed to a UV transmissive substrate 120 that is in direct contact with a UV reflective mirror 125 using an adhesive 115 positioned between the fused-fiber device 110’ and the substrate 120, where a portion of the UV light from an overhead UV light source that is transmitted through the substrate 120 is reflected by the UV reflective mirror 125 back through the substrate 120, according to an example aspect. The substrate 120 can be simply laid on the surface of the UV reflective mirror 125, or affixed thereto.

[0034] FIG. 3 is a flow chart that shows steps in an example method 300 of adhesive curing for bonding glass-based optical elements to substrates. Step 301 comprises providing a glass-based optical element and a substrate with a blob of an UV light curable adhesive positioned in a first and a second location between the optical element and the substrate. The substrate is within 5 mm of a UV mirror positioned below the substrate. As described above, UV mirror can comprise a planar metal coating that provides > 90% reflectance from 250 nm to 400 nm, and the substrate can comprise fused quartz or silica. Example optical elements can comprise a fused-fiber device comprising a fused-fiber coupler (single or multimode) or a multimode combiner.

[0035] Step 302 comprises curing the blobs using at least one UV light source emitting primarily at a UV wavelength or in a range of UV wavelengths that is positioned above the optical device. Although most UV curable adhesives can be accelerated by applying heat, no external heat is generally applied in disclosed curing.

[0036]

At the UV wavelength or range of UV wavelengths provided by the UV light source the substrate comprises a UV transmissive material and the UV mirror provides high UV reflectance. A portion of the UV light that is transmitted through the substrate is reflected by the UV mirror back through the substrate to provide adding substrate curing. Step 303 comprises removing the substrate from the UV mirror. The optical element affixed to the substrate is then generally positioned with housing. The device can be entered into a housing tube, generally made of stainless steel. The inner diameter of the tube can be close to twice the radius of the Dshape of the substrate and the length can be at least as long as the substrate. The substrate can be affixed to the tube by glue as long as the fused section is not touched by any applied glue. The pigtails that come out at each end generally need to be secured and the tube inside sealed. This can be done by applying a blob of elastomer that will close up the tube end and also provide some strain relief to the fibers. The sealing can be important to prevent particulates from entering the tube, potentially interfering with the optical device.

[0037] Advantages of disclosed methods of adhesive curing for affixing glass-based optical elements to substrates include a shortened cure time and more uniform adhesive curing, including better adhesive curing proximate to the substrate to provide better optical device stability to resist later misalignment from occurring due to more even adhesive curing. Moreover, for fused-fiber devices, strain on the fibers in the suspended region 110c including the tapers and the waist 110c2 which is known to be vulnerable can lead to changes in coupling, making the device performance less consistent. Since most UV curing adhesives shrink upon curing, a directional cure that generally happens with a conventional top side positioned UV light source 105 being a one-sided illumination will tend to bend the ends of the fiber in the affixment regions upward, which can lead to strain-induced effects after cure. Such strain-induced effects are essentially eliminated by disclosed adhesive curing which adds additional curing to the bottom side of the adhesive using reflected UV.

[0038] 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 claimed invention, and further additions, deletions, substitutions and modifications may be made to the described embodiments without departing from the scope of this disclosure.

[0039] 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., 1 to 5.

[0040] 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 in accordance with this Disclosure without departing from the spirit or scope of this Disclosure. 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

1. A method of assembling optical devices, comprising:

providing a glass-based optical element and a substrate with a blob of an ultraviolet (UV) light curable adhesive positioned in a first and a second location between said optical element and said substrate, and wherein said substrate is within 5 mm of a UV mirror positioned below said substrate, and curing said blobs using at least one UV light source emitting primarily at a UV wavelength or in a range of UV wavelengths positioned above said optical device, wherein at said UV wavelength or said range of UV wavelengths said substrate comprises a UV transmissive material and said UV mirror provides high UV reflectance, wherein a portion of said UV light that is transmitted through said substrate is reflected by said UV mirror back through said substrate to also cure said blobs.

2. The method of claim 1, wherein said UV light curable adhesive comprises an acrylic resin, and wherein said curing is exclusive of applying any external heating.

3. The method of claim 1 or 2, wherein said UV light source comprises a single light source.

4. The method of claim 1 or 2, wherein said UV light source comprises an array of light emitting diodes (UEDs).

5. The method of any preceding claim, wherein said substrate comprises fused quartz.

6. The method of claim any of claims 1-4, wherein said substrate comprises silica.

7. The method of any preceding claim, wherein said substrate transmits > 80% at said UV wavelength or said range of UV wavelengths for a thickness of 2 mm, and wherein said UV mirror comprises a planar metal coating that provides > 90% reflectance in a wavelength range from 250 nm to 400 nm.

8. The method of any preceding claim, wherein said optical element comprises a fused-fiber device that includes a first fiber and at least a second fiber that are fused together, and wherein there is a suspended region including a uniform thickness fused waist region between said blobs, optionally wherein said fused-fiber device comprises a fused-fiber coupler or a multimode combiner.

9. The method of any of claims 1 -7, wherein said optical element comprises a fusedfiber device comprising a fused-fiber coupler or a multimode combiner.

10. The method of claim 2, wherein said curing comprises free radical curing, and wherein said acrylic resin comprises a UV acrylic resin having a shrinkage upon said curing of < 1%.

11. The method of claim 2, wherein said acrylic resin comprises an epoxy acrylate.

12. The method of any preceding claim, wherein said substrate is directly on said UV mirror.

13. The method of any preceding claim, further comprising after said curing removing said substrate from said UV mirror.

14. A method of assembling a fused-fiber device, comprising:

providing said fused-fiber device and a silica substrate with a blob of ultraviolet (UV) light curable acrylate resin positioned on respective sides between a tapered region of said fused-fiber device and said silica substrate, wherein said silica substrate is in direct physical contact with a UV mirror positioned below said silica substrate, and curing said acrylic resin using at least one UV light source emitting primarily at a UV wavelength or in a range of UV wavelengths positioned above fused-fiber device, wherein at said UV wavelength or said range of UV wavelengths said silica substrate comprises a UV transmissive material and said UV mirror provides a high UV reflectance;

wherein a portion of said UV light that is transmitted through said silica substrate is reflected by said UV mirror back through said silica substrate to also cure said acrylic resin.

15. The method of claim 14, wherein said UV mirror comprises a planar metal coating that provides > 90% reflectance in a wavelength range from 250 nm to 400 nm.

16. The method of claim 14 or 15, wherein said curing comprises free radical curing, and wherein said acrylic resin comprises a UV acrylic resin have a shrinkage upon said curing of < 1%.

17. The method of claim 14 or 15, wherein said acrylic resin comprises an epoxy acrylate.