Terminated hollow fiber with suspended fiber end
By assembling the optical fiber end of the fiber optic fiber, and keeping a non -zero distance from the end cover, it solves the problem of being easily damaged by the traditional fiber end, and achieves higher protection effects and lower reflection losses.
Patent Information
- Application Number
- CN202380032419.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Priority Date
- 2022-04-07
- Filing Date
- 2023-04-05
- Publication Date
- 2025-01-28
AI Technical Summary
Traditional solid core optical fibers are susceptible to photodamage when transmitting high-power lasers, especially thermal damage caused by surface contaminants at the end of the optical fiber, and the hollow core optical fibers with structured cladding are susceptible to thermal melting and deformation during the termination process.
Using nested capillary structure, the fiber end of the optical fiber of the empty core fiber is suspended in the outer hair tube, so that the distance between the optical fiber end and the end cover is not zero. Fixed the optical fiber end through the inner capacity to avoid directly contacting other mechanical structures to protect the fiber end of the fiber end. And reduce Fresnel's reflection loss.
Effectively protect the optical fiber end of the core optical fiber, avoid heat damage and mechanical damage, and at the same time reduce Fresnel's reflection loss, and improve the performance and reliability of the fiber end.
Smart Images
Figure CN119384621A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the termination of optical fibers. In particular, the present invention relates to the termination of hollow core optical fibers used to transmit high optical powers in a small diameter core. Background Art
[0002] Optical fiber is made of an optically transmissive material (usually glass) that guides light from one end of the fiber to the other. Conventional optical fiber has a solid core with a higher refractive index than the surrounding cladding material. Total internal reflection at the interface between the core and cladding material causes light to be guided in the core.
[0003] Optical fibers confine the guided light to a small cross-sectional area. The diameter of the light-guiding core in many single-mode fibers is ten micrometers (μm) or less. Therefore, when using optical fibers to transmit high-power lasers, the laser intensity in the core can be very high. This can make the fiber susceptible to light-induced damage, especially at the fiber ends, where surface contaminants can act as light absorbers and cause thermal damage to the fiber ends. This damage is usually prevented by protective termination of the optical fiber, in which the fiber end is fused to a so-called "end cap." The end cap is a solid piece of glass with a diameter much larger than the core of the optical fiber.
[0004] The laser light emitted from an optical fiber is highly divergent due to diffraction and the small diameter of the fiber core. Unlike the fiber core, the end cap does not confine the laser beam to a small cross-sectional area. Therefore, the laser beam diameter at the distal surface of the end cap facing away from the fiber is typically at least an order of magnitude larger than the laser beam diameter at the end of the fiber itself. The problem of potential interactions between the laser and surface contaminants is shifted to the distal end cap surface, where absorption of light by surface contaminants at the distal surface of the end cap is less likely to damage the end cap because the laser intensity at the distal end cap surface is smaller. In addition, because the laser beam diameter at the distal surface of the end cap is larger, distortions of the laser beam mode characteristics by surface contaminants are typically reduced.
[0005] Conventional optical fibers with solid cores have certain limitations. Inevitably, the solid core material absorbs and / or scatters part of the light propagating in the fiber, resulting in losses that limit the transmission distance of the signal. In telecommunications, where optical signals need to be transmitted over long distances, such losses must be compensated for using optical amplifiers. The light-induced damage threshold of the solid core material also sets an upper limit on the optical power that can be transmitted by the fiber. Therefore, conventional optical fibers with solid cores are generally not well suited for transmitting lasers above a certain power. In applications with spectrally broad laser beams, dispersion in the core can temporally broaden the laser beam. In applications with high peak power laser pulses, nonlinear processes can cause optical losses or damage the fiber.
[0006] Hollow-core fibers, on the other hand, guide light in a hollow core, allowing the light to propagate in air, optically inert gases, or a vacuum. As a result, hollow-core fibers exhibit significantly reduced absorption / scattering losses and optical nonlinearities compared to conventional solid-core fibers. Hollow-core fibers are also capable of transmitting high-power lasers that exceed the photo-induced damage threshold of the solid core. The concept of hollow-core fibers was conceived more than fifty years ago as a low-loss alternative to conventional solid-core fibers. However, the first practical applications did not materialize until the 1990s. These first hollow-core fibers relied on the photonic bandgap effect to guide light in a hollow core surrounded by a microstructured photonic bandgap cladding with many smaller hollow channels arranged in a honeycomb pattern.
[0007] More recently, simpler designs have been developed that employ light guiding based on antiresonance, i.e., destructive optical interference of components of light that would otherwise propagate radially away from the optical axis defined by the center of the core. Examples of such simpler designs include Kagome fibers, twisted fibers, and nested antiresonant nodeless fibers. In each of these designs, a hollow core is surrounded by a structured cladding having multiple hollow channels. The radial components of the light are partially reflected at the walls of the structured cladding, which are configured so that the partial reflections interfere in a manner similar to a Fabry-Perot interferometer. There is constructive interference around the center of the core and destructive interference around the structured cladding. Thus, the structured cladding guides light of the design wavelength to propagate longitudinally through the hollow core, rather than radially outward through the cladding. SUMMARY OF THE INVENTION
[0009] The performance and ease of use of hollow core fibers will benefit from termination utilizing end caps. Contamination and damage to the ends of hollow core fibers present similar problems to those that exist with traditional solid core fibers. Additionally, if the ends of hollow core fibers are not protected, moisture from the surrounding environment tends to diffuse into the channels of the structured cladding, degrading the light guiding properties of the structure and even degrading the performance of the structure itself.
[0010] We have recognized that conventional processes for splicing an end cap to an optical fiber end are generally not suitable for hollow core optical fibers with structured cladding. Splicing is typically accomplished by heat fusing the interface between the optical fiber and the end cap. The heat required to fuse a hollow core optical fiber to an end cap tends to melt and deform the thin walls of the structured cladding. Furthermore, conventional fusion splicing is incompatible with having an anti-reflection coating on the surface of the end cap that contacts the optical fiber end. Thus, conventional fusion splicing of an end cap to a hollow core optical fiber will be at the expense of Fresnel reflection losses from that surface. We have further recognized that the ends of hollow core optical fibers with structured cladding are so fragile, at least after stripping and cleaving, that even simple contact between the optical fiber end and an end cap or other mechanical structure presents a significant risk of damaging the optical fiber end.
[0011] This article discloses a hollow core fiber termination solution, which utilizes an end cap to protect the hollow core fiber and is applicable to a hollow core fiber with a structured cladding. The solution suspends the fiber end of the hollow core fiber in an outer capillary so that the distance between the fiber end and the end cap located on the end face of the outer capillary is not zero. The fiber end is fixed in the outer capillary by an inner capillary used as a radial spacer. This nested capillary structure places the fiber end in a protected environment close to the end cap while avoiding direct contact between the fiber end and other mechanical structures. The nested capillary structure allows the fiber end to be precisely positioned relative to the end cap, but can also tolerate defects in the relative positioning of the fiber end and pointing errors of the light beam. The fiber termination solution can be achieved without fusing anything to the light-transmitting surface of the end cap, so that an anti-reflection coating can be directly applied on the two light-transmitting surfaces of the end cap to eliminate or at least reduce Fresnel reflection losses.
[0012] In one aspect, a terminated hollow core optical fiber includes: an outer capillary having an end face; a hollow core optical fiber having an optical fiber end located inside the outer capillary, the optical fiber end being at a first non-zero distance from the end face of the outer capillary; an optical fiber jacket disposed on an outer surface of the hollow core optical fiber; and an inner capillary disposed between an outer surface of the optical fiber jacket and an inner surface of the outer capillary. The inner capillary secures the hollow core optical fiber through the optical fiber jacket so that the optical fiber end extends from the inner capillary and is suspended within the outer capillary. The terminated hollow core optical fiber also includes an end cap disposed near the end face of the outer capillary for transmitting light toward or away from the optical fiber end.
[0013] In another aspect, a method of terminating a hollow core optical fiber includes: (a) sliding a housing, an inner capillary, and an outer capillary onto a hollow core optical fiber protected by an optical fiber jacket, (b) stripping the optical fiber jacket from an optical fiber end of the hollow core optical fiber, (c) cleaving the stripped optical fiber end, (d) positioning the inner capillary and the outer capillary such that (i) the cleaved optical fiber end is located within the outer capillary at a non-zero distance from an end face of the outer capillary, and (ii) the inner capillary serves as a radial spacer between the optical fiber jacket and the outer capillary with the optical fiber end suspended from the inner capillary, (e) positioning the outer capillary in a channel of the housing, and (f) after the positioning step, securing an end cap in an opening of the housing channel so that the end cap is adjacent to the outer capillary and seals the channel. BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate schematically preferred embodiments of the invention and, together with the general description above and the detailed description of the preferred embodiments below, serve to explain the principles of the invention.
[0015] Figure 1A and Figure 1BA terminated hollow core optical fiber is shown with a nested capillary configuration that suspends the fiber end of the hollow core optical fiber at a non-zero distance from an end cap, according to an embodiment.
[0016] Figure 2 express Figure 1A and Figure 1B Some exemplary dimensions for terminated hollow core optical fibers.
[0017] Figure 3 It shows that according to Figure 1A and Figure 1B Embodiments of hollow core optical fibers terminated with a scheme.
[0018] Figure 4 Shown in Figure 1A and Figure 1B In an embodiment of a terminated hollow core optical fiber, light propagates toward or from the hollow core optical fiber, and the embodiment is characterized in that the distance between the optical fiber end face and the end cap is small and non-zero.
[0019] Figure 5 Shows Figure 1A and Figure 1B In the embodiment of the mid-terminated hollow core fiber, light propagates toward or from the hollow core fiber, and the embodiment is characterized in that the distance between the fiber end face and the end cap is large and non-zero.
[0020] Figure 6 A terminated hollow core fiber with a nested capillary configuration that suspends the fiber end of the hollow core fiber at a non-zero distance from the end cap is shown according to one embodiment. The terminated hollow core fiber utilizes a housing for mechanical support and to isolate the fiber end face from the surrounding environment.
[0021] Figure 7 Shown in more detail Figure 6 A housing with a hollow core optical fiber terminated in the middle.
[0022] Figure 8 Another terminated hollow core optical fiber having a housing with an end cap secured to the housing and the outer capillary is shown according to an embodiment.
[0023] Fig. 9 Another terminated hollow core optical fiber with a housing according to an embodiment is shown. The housing holds the end cap at a non-zero distance from the outer capillary end face.
[0024] Fig.10 is a flow chart of a method for terminating a hollow core optical fiber. DETAILED DESCRIPTION OF THE INVENTION
[0026] Referring now to the drawings, wherein like parts are indicated by like numerals, Figure 1A and Figure 1BA terminated hollow core fiber 100 is shown having a nested capillary configuration that suspends the fiber end 110E of the hollow core fiber 110 at a non-zero distance from the end cap 150 . Figure 1A and Figure 1B is an orthogonal cross-sectional view of the terminated hollow core optical fiber 100. Figure 1A , the cross section is taken along the longitudinal axis of the hollow core optical fiber 110. Figure 1B The cross section is orthogonal to the longitudinal axis of the hollow core optical fiber 110 and is Figure 1A The hollow core optical fiber 110 has a structured cladding. In addition to the hollow core optical fiber 110 and the end cap 150 , the terminated hollow core optical fiber 100 further includes an optical fiber jacket 120 , an inner capillary 130 , and an outer capillary 140 .
[0027] The optical fiber jacket 120 is disposed on the outer surface 112S of the hollow core optical fiber 110. The hollow core optical fiber 110 passes through the inner capillary 130, and the optical fiber end 110E protrudes a non-zero distance 182 from the inner capillary 130. The inner capillary 130 is disposed within the outer capillary 140, between the outer surface 122S of the optical fiber jacket 120 and the inner surface 142S of the outer capillary 140. The inner capillary 130 thereby secures the hollow core optical fiber 110 through the optical fiber jacket 120. The hollow core optical fiber 110 is longitudinally positioned so that the optical fiber end 110E protrudes a non-zero distance 180 from the inner capillary 130 away from the end face 140F of the outer capillary 140. Thus, the optical fiber end 110E is suspended on the inner capillary 130 within the outer capillary 140. The inner capillary 130 essentially serves as a radial spacer between the optical fiber jacket 120 and the outer capillary 140. The end cap 150 is located near the end surface 140F of the outer capillary 140 , and the end surface 110F of the optical fiber end 110E is at least 180 meters away from the light-transmitting surface 150F of the end cap 150 facing the optical fiber end 110E.
[0028] The non-zero distance 180 ensures a gap between the end cap 150 and the fiber end face 110F. The inner capillary 130 ensures a significant radial gap between the fiber end 110E and the outer capillary 140. By suspending the fiber end 110E in this manner, mechanical contact between the fiber end 110E and other mechanical structures is avoided, thereby protecting the potentially fragile fiber end 110E from structural damage.
[0029] The longitudinal positions of the hollow core fiber 110, the inner capillary 130, and the outer capillary 140 relative to each other can be locked by an adhesive 170 applied at the distal ends of the capillaries 130 and 140 (furthest from the end cap 150). Although not shown, the distal ends of the capillaries 130 and 140 can be longitudinally offset from each other. The inner capillary 130 can extend out of the outer capillary 140 in a direction away from the end cap 150. In this configuration, the portion of the inner capillary 130 extending out of the outer capillary 140 can provide additional strain relief for the hollow core fiber 110. Alternatively, the inner capillary 130 can be completely contained by the outer capillary 140, wherein the ends of the inner capillary 130 are longitudinally offset from the corresponding ends of the outer capillary 140. When the distal ends of the capillaries 130 and 140 are longitudinally offset from each other, the application of the adhesive 170 (if included) can be adjusted accordingly, requiring the adhesive 170 to be deposited at two separate locations in some cases.
[0030] The light-transmitting surface 150F and the opposite light-transmitting surface 152F of the end cap 150 may be planar. In one implementation, the end cap 150 is a cylindrical block, such as Figure 1A As shown. In another implementation, the end cap 150 has a different shape, such as a solid shape that radially expands from the direction of the surface 150F to the surface 152F, such as a conical block. Each of the end cap 150, the inner capillary 130 and the outer capillary 140 can be made of glass. Each of the capillaries 130 and 140 can be a hollow cylinder. The optical fiber jacket 120 can include or consist of a polymer.
[0031] The inner diameter 130D of the inner capillary tube 130 may be equal to the outer diameter 120D of the optical fiber jacket 120, and the outer diameter 132D of the inner capillary tube 130 may be equal to the inner diameter 140D of the outer capillary tube 140, as shown in FIG. Figure 1BAs shown. However, in practice, there may be a gap of, for example, tens or hundreds of microns between the inner capillary 130 and each of the optical fiber jacket 120 and the outer capillary 140 to simplify the process of sliding the inner capillary 130 onto the optical fiber jacket 120 and sliding the outer capillary 140 onto the inner capillary 130. Therefore, at least for practical reasons, the inner diameter 130D of the inner capillary 130 may exceed the outer diameter 120D of the optical fiber jacket 120, and the outer diameter 132D of the inner capillary 130 may be smaller than the inner diameter 140D of the outer capillary 140. In one such embodiment, the capillaries 130 and 140 are made of glass, while the optical fiber jacket 120 is made of a polymer. When the inner capillary 130 is slid onto the optical fiber jacket 120, the polymer material of the optical fiber jacket 120 causes considerable friction between the optical fiber jacket 120 and the inner capillary 130. In addition, the polymer optical fiber jacket 120 may have significant thickness variations. Therefore, in this embodiment, the inner diameter 130D of the inner capillary 130 is at least 100 μm larger than the outer diameter 120D of the optical fiber jacket 120. On the other hand, the friction between the glass capillaries 130 and 140 may be relatively small, and their sizes may be controlled to a higher degree of accuracy. In this embodiment, the inner diameter 140D of the outer capillary 140 is therefore less than 100 μm larger than the outer diameter 132D of the inner capillary, for example, only tens of microns. In the case where the thickness of the polymer optical fiber jacket 120 is relatively well controlled, the difference between the inner diameter 130D of the inner capillary 130 and the outer diameter 120D of the optical fiber jacket 120 can be reduced to less than 100 μm, for example, tens of microns. This situation is conducive to improving the centering of the hollow core optical fiber 110 relative to the outer capillary 140.
[0032] In certain embodiments, the surface 150F of the end cap 150 is positioned against the end surface 140F of the outer capillary 140, such as Figure 1A This configuration can help control the distance from the fiber end face 110F to the end cap 150. The end cap 150 can be fused to the end face 140F, bonded to the outer capillary 140 by an adhesive, or by Figure 1A A mechanical structure not shown in the figure is positioned against the end face 140F. In an embodiment where the end cap 150 is bonded to the outer capillary 140 by an adhesive, the adhesive may be applied directly at the interface between the end face 140F of the outer capillary 140 and the surface 150F of the end cap 150. However, due to the risk of adhesive outgassing, which may contaminate the optical fiber end face 110F, it is preferred that the adhesive be applied at a position radially outward of the interface between the outer capillary 140 and the end cap 150.
[0033] In some cases, it is desirable or necessary to minimize Fresnel reflection losses at the light-transmissive surface of the end cap 150. Advantageously, for this purpose, the terminated hollow core optical fiber 100 may include an antireflection coating on each of surfaces 150F and 152F. In embodiments where the end cap 150 is fused to the outer capillary 140, or even in embodiments where the end cap 150 is adhered to the outer capillary 140, maintaining the integrity of the antireflection coating on surface 150F may be challenging. However, in embodiments where the end cap 150 and the outer capillary 140 are merely positioned in contact or proximity to one another, depositing and retaining the antireflection coating on surface 150F is simple. Therefore, when surface 150F has an antireflection coating, a mechanical structure that maintains the end cap 150 in this position relative to the outer capillary 140 may be advantageous. Reference is made below to Figure 6 , Figure 7 and Fig. 9 Several such implementations are discussed.
[0034] The local environment at the fiber end face 110F can be hermetically sealed from the surrounding environment to prevent external moisture and contaminants from reaching the fiber end face 110F and degrading the performance of the hollow core fiber 110. In particular, in embodiments where the surface 150F has an anti-reflection coating, it may be advantageous to incorporate a housing to provide such a seal. Figure 6-9 The exemplary housing is discussed in further detail. However, the optical fiber end face 110F may be sealed from the surrounding environment without using an outer housing. The end cap 150 is directly bonded to the outer capillary 140 by adhesive or fusion, and can be used in conjunction with the adhesive 170 to seal both ends of the outer capillary 140. If the fit of the inner capillary 130 between the optical fiber jacket 120 and the outer capillary 140 is tight enough, the adhesive 170 may not be needed to isolate the optical fiber end face 110F from the surrounding environment.
[0035] Figure 2 100. The inner capillary 130 and the outer capillary 140 have respective lengths 230L and 240L. In embodiments including the adhesive 170, it may be advantageous for the length 230L to be relatively large in order to prevent or at least reduce contamination of the optical fiber end face 110F by outgassing of the adhesive 170. In such embodiments, the length 230L may be at least 5 millimeters (mm), such as between 5 and 20 mm. When the distal ends of the capillaries 130 and 140 that are furthest from the end cap 150 are at the same longitudinal position relative to the hollow core optical fiber 110, such as Figure 1A and Figure 2As shown, length 240L is greater than length 230L by the sum of non-zero distances 180 and 182. In one embodiment, distance 180 is at least 50 μm (or at least 100 μm to further reduce the risk of accidental contact between fiber end 110E and end cap 150), and distance 182 is at least 500 μm, for example, in the range between 500 μm and 5 mm.
[0036] In most implementations, the optical fiber jacket 120 terminates at a non-zero distance 284 from the optical fiber end face 110F, such that the optical fiber end 110E protrudes from the optical fiber jacket 120, as shown in FIG. Figure 1A and Figure 2 As shown. Distance 284 is, for example, in a range between 0.5 and 4 mm. In one embodiment, the optical fiber jacket 120 initially extends to the entire length of the hollow core fiber 110, but is stripped from the optical fiber end 110E to facilitate cutting the hollow core fiber 110 to form the optical fiber end face 110F. In embodiments where the optical fiber jacket 120 terminates at a non-zero distance 284 from the optical fiber end face 110F, the optical fiber jacket 120 may terminate outside the inner capillary 130 (e.g., Figure 1A and Figure 2 130 ) or terminated inside the inner capillary 130. Terminating the fiber jacket 120 outside the inner capillary 130 provides maximum mechanical support for the fiber end 110E. Terminating the fiber jacket 120 inside the inner capillary 130 provides maximum protection for the fiber jacket 120 from potentially damaging laser exposure. Stripped fiber is much more brittle than jacketed fiber. Fibers are best jacketed when in contact with other glass, and therefore, the hollow core fiber 110 can be jacketed along most of the length 230L of the inner capillary 130.
[0037] In one embodiment, the inner diameter 140D of the outer capillary 140 is at least 0.5 mm, for example, in the range of 0.5 to 3 mm. Such a value of the inner diameter 140D ensures that the laser beam emitted from or coupled into the core of the hollow core fiber 110 at the fiber end face 110F is not clamped by the outer capillary 140 for a wide range of distances 180 and mode field diameters as low as, for example, 5 micrometers (μm).
[0038] Figure 3 Embodiments of hollow core fiber 110 that may be terminated according to the scheme for terminating hollow core fiber 100 are shown: a spun fiber 310, a photonic crystal fiber 320, a Kagome fiber 330, and a nested antiresonant node-free fiber (NANF) 340. Figure 3 A cross-sectional view of each of the optical fibers 310, 320, 330, and 340 is shown. The applicability of the termination scheme for terminating the hollow core optical fiber 100 is not limited to Figure 3 Fiber types shown in .
[0039] The spun fiber 310 includes a plurality of tubes 314 arranged on the inner surface of a solid outer wall 312. The tubes 314 form an anti-resonant structured cladding around a hollow core 318. The photonic crystal fiber 320 has a microstructured cladding 324 in which there are a large number of hexagonal channels arranged in a honeycomb pattern. The microstructured cladding 324 surrounds a hollow core 328. The Kagome fiber 330 includes a cladding 334 composed of thin walls arranged in a three-hexagonal tile pattern, which is known from the Japanese Kagome basket. The cladding 334 is supported by a solid outer wall 332 and surrounds a hollow core 338. The NANF 340 is an extension of the spun fiber 310, which further includes a smaller tube 316 within each tube 314. The presence of the tube 316 reduces the optical loss of the NANF 340 relative to the spun fiber 310.
[0040] Each of the spun fiber 310, photonic crystal fiber 320, kagome fiber 330, and NANF 340 has a hollow core surrounded by a structured cladding. Many other types of hollow core fibers have the same basic design, including hexagonal hollow core fibers, antiresonant fibers with parachute-shaped cells, and tube-connected antiresonant fibers. In addition, many of these types of fibers are available in different versions on the market. For example, a spun fiber can be configured with adjacent tubes 314 that are in contact with each other, such as Figure 3 As shown, or leaving a space between each pair of adjacent tubes.
[0041] Hollow core optical fibers, such as reference Figure 3 Those discussed above may be characterized by a relatively small mode field diameter. Thus, in one embodiment, the hollow core fiber 110 is a single mode fiber characterized by a mode field diameter less than 12 μm, such as in the range of 5 μm to 12 μm. The laser beam emitted from the core of the hollow core fiber 110 of this embodiment at the fiber end face 110F diverges relatively quickly. Similarly, effectively coupling the laser beam into the core of the hollow core fiber 110 of this embodiment requires that the laser beam be tightly focused on the fiber end face 110F.
[0042] Figure 4 FIG. 1 shows light propagating to (or from) a hollow core fiber 110 in one implementation of a terminated hollow core fiber 100, wherein there is a relatively small non-zero distance 480 between the fiber end face 110F and the end cap 150. The distance 480 can be in the range of 50 μm to 500 μm. However, the distance 480 can also be less than 50 μm. A laser beam 490 emitted from the core of the hollow core fiber 110 at the fiber end face 110F diverges and expands to a lateral 1 / e distance at the surface 150F of the end cap 150. 2The diameter 492. The laser beam 490 can be infrared, visible light or ultraviolet light. In one embodiment, the wavelength of the laser beam 490 is in the range of 650 to 1100 nanometers (nm).
[0043] exist Figure 4 In one embodiment of the implementation, the hollow core fiber 110 is a single mode fiber characterized by a mode field diameter of 8 μm, a wavelength of the laser beam 490 of 1 μm, and a distance 480 of 500 μm. Therefore, 1 / e 2 The diameter 492 is about 80 μm. For most practical diameters of the hollow core fiber 110, the fiber jacket 120, and the inner capillary 130, the final value of the inner diameter 140D of the outer capillary 140 is at least 500 μm, thereby avoiding the laser beam 490 from being cut off by the outer capillary 140 with a considerable safety margin. When evaluating the risk of laser beam cutoff, the 1 / e 2 It is reasonable to operate with a safety margin of three times the diameter 492. In this embodiment, 3x1 / e 2 The diameter 492 is about 250 μm.
[0044] In order to achieve a relatively small non-zero distance 480, special care may be required when assembling the terminated hollow core fiber 100. An optical inspection tool may be used to ensure that the distance 480 meets the requirements. Using such a tool, the distance 480 can be set to within an accuracy of about 50 μm. However, the design of the terminated hollow core fiber 100 is also feasible in implementations that do not benefit from a high degree of control of the non-zero distance 180. When the distance 180 is not tightly controlled, it is best to set the distance 180 to a conservatively high value to positively eliminate any risk that the fiber end 110E protrudes from the outer capillary 140 and may be damaged by the end cap 150 during the assembly process.
[0045] Figure 5 In another implementation of terminating hollow core fiber 100, light propagates to (or from) hollow core fiber 110, wherein a distance 580 between fiber end face 110F and end cap 150 is larger. Distance 580 may be greater than 500 μm, such as several millimeters. Figure 5 An implementation of may involve a situation where the assembly process is configured to ensure that the distance 180 is at least 50 μm, and the process sets the distance 180 to a tolerance of up to ±1 mm. Figure 5 In the relevant embodiment of the implementation scheme, the hollow core fiber 110 is a single mode fiber, characterized by a mode field diameter of 8 μm, a wavelength of the laser beam 490 of 1 μm, and a distance 580 of 2.05 mm. Therefore, 1 / e 2 The diameter of 492 is about 330μm, and 3x1 / e 2The diameter 492 is slightly less than 1.0 mm. The inner diameter 140D of the outer capillary 140 is 1.0 mm or more, thereby ensuring that the laser beam 490 is not intercepted by the outer capillary 140.
[0046] The above discussion regarding the truncation of the laser beam 490 assumes that the propagation of the laser beam 490 between the fiber end face 110F and the end cap 150 is centered on the central axis 548 of the capillary 140 (see Figure 5 ). If the core of the hollow core fiber 110 is not centered and parallel to the central axis 548 and / or the fiber end face 110F is not orthogonal to the central axis 548, the laser beam 490 may propagate in a manner that is offset from the central axis 548 and / or at a certain angle to the central axis 548. Figure 5 In the illustrated embodiment, the fiber end face 110F is cut at an angle 510A relative to the central axis 548 so that the laser beam 490 is directed at Figure 5 In one embodiment, angle 510A deviates from 90 degrees by at most 1 or 2 degrees. The inner diameter 140D of outer capillary 140 may be selected to accommodate such imperfections, optionally in combination with a relatively large nominal value of distance 180.
[0047] Figure 6 A terminated hollow core optical fiber 600 is shown, which utilizes a housing 660 for mechanical support and sealing the optical fiber end face 110F from the surrounding environment. The terminated hollow core optical fiber 600 is an embodiment of the terminated hollow core optical fiber 100, which further includes a housing 660. Figure 7 The housing 660 is shown in more detail. The housing 660 has a channel 710 with an opening 720. The outer capillary 140 is located in the channel 710. The housing 660 can be made of metal. The end cap 150 is located in the opening 720 and is placed against the end face 140F of the outer capillary 140. The end cap 150 is fixed to the housing 660. The hollow core optical fiber 110 and the optical fiber jacket 120 extend out of the housing 660 through the port 730 in a direction away from the end cap 150.
[0048] Since the end cap 150 is fixed to the housing 660, it is not necessary to fix the end cap 150 to the outer capillary 140. Therefore, in terminating the hollow core fiber 600, it is simple to include an anti-reflection coating on the surface 150F of the end cap 150 (as well as on the surface 152F).
[0049] Optionally, as Figure 6 and Figure 7 As shown, the housing 660 forms a side port 740. The side port 740 can receive an adhesive 672 that secures the hollow core fiber 110 (and the optical fiber jacket 120) in the port 730, thereby locking the position of the hollow core fiber 110 in the housing 660.
[0050] The diameter 720D of the opening 720 can match the outer diameter 750D of the end cap 150, thereby helping to support the end cap 150. Alternatively, the diameter 720D is greater than the diameter 750D, in which case the end cap 150 can be supported by the housing 660 indirectly through adhesive and / or mechanical hardware (e.g., set screws or washers). (It is assumed here that the end cap 150 is cylindrical, but the relationship between the diameters 720D and 750D can be easily extended to end caps 150 of other shapes.)
[0051] In one embodiment, if Figure 6 As shown, the end cap 150 is bonded to the housing 660 by an adhesive bead 670 extending around the entire circumference of the outer surface 650S of the end cap 150. The adhesive 670 secures the end cap 150 to the housing 660 and seals the opening 720 of the passage 710. In order to completely seal and isolate the optical fiber end 110E from the surrounding environment, the port 730 is also sealed. In one embodiment, the size of the port 730 is adjusted to form a close fit with the optical fiber jacket 120, and the optical fiber jacket 120 is flexible enough to seal the port 730. Alternatively, or in combination therewith, the port 730 can be sealed by an adhesive 672 applied through a side port 740 of the housing 660.
[0052] The channel 710 may be cylindrical, with a diameter 710D that generally matches the outer diameter 142D of the outer capillary 140 (see Figure 1B ). However, the channel 710 does not have to be cylindrical. For example, the channel 710 may have a rectangular cross section, and the outer surface of the outer capillary 140 may also be rectangular.
[0053] Figure 8 A terminated hollow core fiber 800 is shown that secures the end cap 150 to the housing and the outer capillary 140. The terminated hollow core fiber 800 is an embodiment of the terminated hollow core fiber 600 that implements the housing 860. In the housing 860 (which is an embodiment of the housing 660), the lateral extension (e.g., diameter 720D) of the opening 720 is slightly larger than the lateral extension (e.g., outer diameter 750D) of the end cap 150. The adhesive 870 forms a bead around the entire circumference of the surface 650S of the end cap 150. In addition, the adhesive 870 extends along the surfaces 650S and 150F of the end cap 150 to the outer capillary 140. The adhesive 870 thereby bonds the end cap 150 to the housing 860 and directly to the outer capillary 140.
[0054] The direct bonding between the end cap 150 and the outer capillary 140 can help prevent or reduce any relative movement between the end cap 150 and the outer capillary 140. Such movement may be caused by vibration, shock, and thermal expansion / contraction, and may generate debris if not prevented. On the other hand, when the risk of generating such debris is small, it is preferred to maintain a relatively long path length between the fiber end face 110F and any adhesive that may outgas, such as in the terminated hollow core fiber 600, where the adhesive 670 is farther away from the fiber end face 110F.
[0055] Fig. 9 A terminated hollow core fiber 900 is shown which holds the end cap 150 at a non-zero distance 988 from the end face 140F of the outer capillary 140. The terminated hollow core fiber 900 is a variation of the terminated hollow core fiber 600. In the terminated hollow core fiber 600, the outer capillary 140 protrudes from the channel 710 into the opening 720 so that the end cap 150 can rest on the end face 140F of the outer capillary 140. On the other hand, in the terminated hollow core fiber 900, the housing 960 is configured so that the outer capillary 140 is completely contained within the channel 710, and the surface 150F of the end cap 150 is located on a shelf 968 that is a distance 988 from the end face 140F of the outer capillary 140. This prevents direct contact between the end face 140F 150 of the outer capillary 140 and the surface 150F of the end cap 150, allowing movement therebetween without generating any debris and facilitating the implementation of an anti-reflection coating on the surface 150F. In one embodiment, the distance 988 is less than 250 μm and therefore does not result in a detrimental increase in the overall distance between the fiber end face 110F and the end cap 150. The housing 960 is otherwise similar to the housing 660.
[0056] Fig.10 1 is a flow chart of a method 1000 for terminating a hollow core optical fiber. The method 1000 can be used to assemble each of the terminated hollow core optical fibers 600, 800, and 900. The method 1000 includes steps 1010, 1020, 1030, 1040, 1050, and 1060.
[0057] Step 1010 slides a housing (e.g., housing 660, 860, or 960), an inner capillary 130, and an outer capillary 140 onto the hollow core optical fiber 110. At this stage of the method 1000, the hollow core optical fiber 110 is protected by the optical fiber jacket 120. Step 1010 positions the housing and capillaries 130 and 140 sufficiently far from the optical fiber end 110E to allow stripping and cleaving of the optical fiber end 110E. Then, step 1020 strips the optical fiber jacket 120 from the optical fiber end 110E, and step 1030 cleaves the now stripped optical fiber end 110E.
[0058] After stripping and cleaving the fiber end 110E, step 1040 positions the capillaries 130 and 140 such that (a) the fiber end 110E terminates within the outer capillary 140, and (b) the inner capillary 130 serves as a radial spacer between the fiber jacket 120 and the outer capillary 140. Step 1040 results in the fiber end face 110F being longitudinally offset from the end face 140F of the outer capillary 140 by a non-zero distance 180, and the fiber end 110E is suspended from the inner capillary 130. Step 1040 may include applying an adhesive 170 to lock the longitudinal positions of the hollow core fiber 110, the inner capillary 130, and the outer capillary 140 relative to each other.
[0059] After the hollow core fiber 110 (and the fiber jacket 120) are assembled with the capillaries 130 and 140 in step 1040, step 1050 positions the outer capillary 140 in the channel 710 of the housing. Step 1050 may slide the housing onto the assembly formed in step 1040. Step 1050 may include bonding the housing to the fiber jacket 120 to lock the longitudinal position of the housing and the hollow core fiber 110 relative to each other. In one embodiment, step 1050 applies adhesive 672 in the side port 740 of the housing. Adhesive 672 may also be used to seal the port 730 of the housing. When used to assemble the terminated hollow core fiber 600 or 800, step 1050 positions the housing so that the outer capillary 140 protrudes from the channel 710 into the opening 720. When used to assemble the terminated hollow core fiber 900, step 1050 positions the housing so that the outer capillary 140 is a non-zero distance 988 from the shelf 968.
[0060] After step 1050, step 1060 secures the end cap 150 in the opening 720 of the channel 710 so that the end cap 150 is located adjacent the end face 140F of the outer capillary 140. When used to assemble the terminated hollow core fiber 600 or 800, step 1060 places the end cap 150 against the end face 140F of the outer capillary 140. When used to assemble the terminated hollow core fiber 900, step 1060 places the end cap 150 on the shelf 968. Step 1060 may include bonding the end cap 150 to the housing, and optionally, to the outer capillary 140, using adhesive 670 or 870, as described above with reference to FIG. Figure 6 and Figure 8 As discussed, adhesive 870 may also be used to seal opening 720 .
[0061] The present invention has been described above in terms of preferred embodiments and other embodiments. However, the present invention is not limited to the embodiments described and depicted herein. Instead, the present invention is limited only by the claims appended hereto.
Claims
1. Terminate hollow core optical fiber, including: an outer capillary tube having an end surface; a hollow core optical fiber having an optical fiber end, wherein the optical fiber end is located inside the outer capillary tube and at a first non-zero distance from the end face of the outer capillary tube; An optical fiber sheath, the optical fiber sheath being arranged on the outer surface of the hollow core optical fiber; An inner capillary tube, the inner capillary tube being disposed between the outer surface of the optical fiber sheath and the inner surface of the outer capillary tube, and being used to hold the hollow-core optical fiber through the optical fiber sheath, so that the optical fiber end extends out of the inner capillary tube and is suspended inside the outer capillary tube; as well as An end cap is disposed adjacent to the end face of the outer capillary for transmitting light toward or away from the optical fiber end.
2. The terminated hollow core optical fiber according to claim 1, wherein the hollow core optical fiber is a single mode optical fiber, characterized in that The mode field diameter does not exceed 12 microns.
3. The terminated hollow core optical fiber according to claim 1 or claim 2, wherein the optical fiber end is between 50 and 500 microns from the end cap.
4. A terminated hollow core optical fiber according to any one of the preceding claims, wherein the inner diameter of the outer capillary is at least 500 microns.
5. A terminated hollow core optical fiber according to any one of the preceding claims, wherein the end cap has two light-transmitting surfaces facing towards and away from the optical fiber end, respectively, each of the two light-transmitting surfaces comprising an anti-reflection coating.
6. A terminated hollow core optical fiber according to any one of the preceding claims, wherein the end cap abuts against the end face of the outer capillary.
7. A terminated hollow core optical fiber according to any one of the preceding claims, wherein the outer capillary and the end cap are made of glass.
8. The terminated hollow core optical fiber according to any of the preceding claims, further comprising a housing, the outer capillary being at least partially accommodated by a channel of the housing, the end cap being arranged in an opening of the channel and fixed to the housing.
9. The terminated hollow core optical fiber of claim 8, wherein the end cap is disposed on the open shelf and a gap exists between the end cap and the outer capillary.
10. The terminated hollow core optical fiber of claim 9, wherein the gap is less than 250 microns.
11. The terminated hollow core optical fiber of claim 8, wherein the end cap is secured to the housing by a first adhesive that seals the opening around the end cap.
12. The terminated hollow core optical fiber according to any one of claims 8 to 11, wherein the hollow core optical fiber and the optical fiber jacket extend out of the housing via a port, the port and the opening being located at opposite ends of the passage, the port being sealed.
13. The terminated hollow core optical fiber according to any one of claims 8 to 12, wherein the housing is made of metal.
14. A terminated hollow core optical fiber according to any one of the preceding claims, wherein respective ends of the inner and outer capillaries which are furthest from the end cap are adhered to each other.
15. A terminated hollow core optical fiber according to any one of the preceding claims, wherein the inner capillary has a length of at least 5 mm.
16. A terminated hollow core optical fiber according to any one of the preceding claims, wherein the end cap abuts against an end face of the outer capillary and is bonded to the outer capillary.
17. A terminated hollow core optical fiber according to any one of the preceding claims, wherein the optical fiber end protrudes at least 500 microns from the inner capillary.
18. A method for terminating a hollow core optical fiber, comprising the steps of: Slide the housing, inner capillary, and outer capillary onto the hollow-core optical fiber protected by the optical fiber jacket; stripping the optical fiber sheath from the optical fiber end of the hollow core optical fiber; Cutting the stripped fiber end; Positioning the inner capillary and the outer capillary such that (a) the cleaved optical fiber end is located inside the outer capillary at a non-zero distance from an end face of the outer capillary, and (b) the inner capillary acts as a radial spacer between the optical fiber jacket and the outer capillary, with the optical fiber end depending from the inner capillary; positioning the outer capillary tube in the channel of the housing; as well as After the positioning step, an end cap is secured in the opening of the passageway of the housing such that the end cap is positioned adjacent the outer capillary and seals the passageway.
19. The method of claim 18, wherein the positioning and securing steps result in the optical fiber end being located between 50 and 500 microns from the end cap.
20. A method according to claim 18 or claim 19, wherein the step of securing comprises applying an adhesive between the end cap and the housing, the adhesive spanning the entire circumference of the end cap.