Optical fiber termination structure, optical connection component, and method for manufacturing an optical fiber termination structure

Abstract

Optical fiber termination structure, optical connection component, and method for manufacturing the optical fiber termination structure to improve the transmission characteristics of hollow core optical fibers. [Solution] The optical fiber termination structure of the present invention comprises a hollow core optical fiber (1) having a hollow portion through which light is transmitted, a flat glass plate (2) covering the hollow portion, and an anti-reflection mechanism for preventing the reflection of light transmitted through the flat glass plate (2). An example of the anti-reflection mechanism is that an anti-reflection film is applied to both sides of the flat glass plate (2). The thickness of the flat glass plate (2) is preferably 100 μm or less. The flat glass plate (2) can be bonded to the end face of the hollow core optical fiber (1) using a jig. Furthermore, the optical connection component of the present invention consists of two flat glass plates (2) of optical fiber termination structures facing each other.

Description

Technical Field

[0001] The present invention relates to an optical fiber end structure, an optical connection component, and a method for manufacturing an optical fiber end structure.

Background Art

[0002] Optical connection components (optical connectors) for connecting optical fibers have been developed based on the technology of physically contacting the end faces of optical fibers, such as single-mode FC connectors, SC connectors, MU connectors, LC connectors, and multi-mode MT connectors, MPO connectors, etc. Details of these are described in Non-Patent Document 1.

[0003] In recent years, hollow-core optical fibers have attracted attention as optical fibers that can break through the limitations of conventional silica-based optical fibers (see Patent Document 1). The core of this optical fiber is air, which is a major difference from conventional optical fibers whose cores are made of solid glass. Hollow-core optical fibers have excellent characteristics: (1) the propagation speed (group velocity) is about 1.45 times faster, (2) the nonlinear coefficient is about three orders of magnitude smaller, and (3) the dispersion characteristics are small. The characteristic (1) is due to the fact that the refractive index of air is smaller than that of glass, and a reduction in delay time in online trading and online games is expected. The characteristics (2) and (3) are due to the fact that they can significantly reduce the limitation of the transmission capacity in conventional optical fibers using glass (solid) as the core.

[0004] In conventional optical fibers, the transmission capacity per fiber has been increased by means of multiplexing (wavelength multiplexing, multilevel modulation). However, no matter how multiplexing is performed, the total energy required for the total amount of transmitted data cannot be reduced. This means that as the capacity increases, the energy during transmission increases.

[0005] Conventional glass-core optical fibers suffer from signal degradation due to the nonlinear optical effects of the glass as energy increases, and a limitation in transmission capacity due to the fiber fuse limit (thermal breakdown limit) where the glass core melts and propagates towards the light source due to concentrated optical power. For single-mode fibers with a core diameter of about 10 μm, the limit is around 1 W, which limits the transmission capacity to around 100 Tbps. Consequently, it becomes impossible to keep up with the exponentially increasing network traffic. This rate-limiting factor is expected to be largely eliminated by changing the core from solid (glass) to hollow (air). Hollow-core optical fibers are expected to be the ultimate optical fiber that humanity can obtain.

[0006] However, with hollow-core optical fibers, it is not possible to apply detachable optical connection technology based on physical contact (Non-Patent Document 1), as is the case with conventional glass-core optical fibers. Hollow-core optical fibers come in various forms, such as photonic bandgap fibers, Kagome fibers, and anti-resonant fibers, as described in Patent Document 1, but all have a structure in which multiple thin (wall thickness of 1 μm or less) glass inner tubes are arranged around the hollow region that forms the core (see Patent Document 1). For this reason, their ends are more brittle than the ends of solid fibers, and if hollow-core optical fibers are brought into physical contact with each other, the ends may be damaged, and there is a concern that the fragments will enter the hollow core and cause deterioration of the transmission characteristics. In addition to this factor, means to prevent foreign matter from entering the hollow part from the outside are essential from the viewpoint of not degrading the transmission characteristics.

[0007] To solve this problem, means of protecting the hollow core portion have been considered. For example, Patent Documents 2 and 3 provide a means of sealing the hollow core portion at the fiber end by melting the cladding portion, thereby preventing the intrusion of foreign matter and achieving sufficient strength for physical contact. However, with such means, it is difficult to maintain the transmission mode of the hollow core optical fiber and it is also difficult to suppress reflections that occur at the boundary between the melted glass and air, leading to a deterioration of transmission characteristics.

[0008] As an alternative to melting, a termination structure has been disclosed in which the tip of a hollow core optical fiber is covered with a protective section having a cavity to prevent foreign matter from entering the hollow section (Patent Document 4). However, the presence of a space (cavity) at the fiber end face creates a gap of mm to cm between the fiber end and the window with an anti-reflective coating at the tip of the protective section. As a result, the light emitted from the fiber spreads out considerably, and when optical connections are made through the windows, there is a problem of increased insertion loss. [Prior art documents] [Non-patent literature]

[0009] NTT Technical Journal, vol.12, No.12, 2007, pp.74-78 [Patent Documents]

[0010] Special Publication No. 2019-504350 Japanese Patent Publication No. 2003-30765 Japanese Patent Publication No. 2002-323625 U.S. Patent No. 7373062 [Overview of the project] [Problems that the invention aims to solve]

[0011] In view of these circumstances, the present invention aims to improve the transmission characteristics of a hollow core optical fiber. [Means for solving the problem]

[0012] To achieve the above objective, the present invention is characterized by comprising: a hollow core optical fiber having a hollow portion through which light is transmitted; a light-transmitting member covering the hollow portion; and an anti-reflection mechanism for preventing the reflection of light transmitted through the light-transmitting member. Further details will be provided later. [Effects of the Invention]

[0013] According to the present invention, the transmission characteristics of the hollow-core optical fiber can be improved.

Brief Description of the Drawings

[0014] [Figure 1] It is a schematic diagram of the optical fiber end structure of the first embodiment. [Figure 2] (a) is an end face view of the hollow-core optical fiber, and (b) is a view with an adhesive applied. [Figure 3A] It is an explanatory diagram (1 / 2) of an example of the bonding process. [Figure 3B] It is an explanatory diagram (1 / 2) of an example of the bonding process. [Figure 4] It is an explanatory diagram of the mounting process of the optical fiber end structure of the second embodiment. [Figure 5] It is an explanatory diagram of the mounting process of the optical fiber end structure of the first modification of the second embodiment. [Figure 6] It is an explanatory diagram of the mounting process of the optical fiber end structure of the second modification of the second embodiment. [Figure 7] It is another example of the light transmission member. [Figure 8] (a) is a schematic diagram of the optical fiber end structure of other modification examples, and (b) is a view of the optical fiber end structure of (a) seen from the right. [Figure 9] It is a schematic diagram (Part 1) of an optical connection component using the optical fiber end structure of other modification examples. [Figure 10] It is a schematic diagram (Part 2) of an optical connection component using the optical fiber end structure of other modification examples.

Modes for Carrying Out the Invention

[0015] [First Embodiment] FIG. 1 shows the optical fiber termination structure of the first embodiment according to the present invention. In the optical fiber termination structure of this embodiment, a disk-shaped flat glass 2 with an antireflection film (not shown) applied on both sides as an optical transmission member is adhered to the end face of a hollow core optical fiber 1 (sometimes simply referred to as "optical fiber"). The hollow core optical fiber 1 has, for example, as shown in FIG. 2(a), six thin (wall thickness of 1 μm or less) glass inner tubes 4 arranged on the radially inner edge of a hollow portion H covered with a cylindrical glass 3, and the core through which light is transmitted is located in the region at the center in the radial direction of the hollow portion H (the region indicated by the dashed circle in FIG. 2(a)). The hollow region of the inner tube 4 constitutes the hollow portion H, and the inner tube 4 including this hollow region acts as a cladding. A jacket (not shown) may be applied to the outside of the glass 3 as necessary. The outer diameter (diameter) of the flat glass 2 is smaller than the through hole 61 of a ferrule 6 described later, and is approximately the same as the outer diameter of the hollow core optical fiber 1.

[0016] The flat glass 2 is adhered to the end face of the hollow core optical fiber 1. At the time of adhesion, by applying the adhesive 5 only to the portion of the glass 3 at the end of the hollow core optical fiber 1 as shown in FIG. 2(b), the adhesive 5 is prevented from entering the hollow portion H including the inner tube 4. The flat glass 2 can cover the hollow portion H.

[0017] FIGS. 3A and 3B show an example of the adhesion process. After inserting the hollow core optical fiber 1 into a ferrule 6 having a through hole 61 for accommodating the hollow core optical fiber 1 (FIG. 3A(a)), the tip of the hollow core optical fiber 1 is cleaved and once retracted into the ferrule 6 (FIG. 3A(b)). The material of the ferrule 6 is preferably zirconia but is not limited thereto.

[0018] Meanwhile, the adhesive 5 is applied to the flat glass 2. This application is carried out using an adhesive transfer jig 8 having an annular protrusion 81 slightly smaller than the glass region of the hollow core optical fiber 1, and a suction jig 7 for adsorbing and holding the flat glass 2. The adhesive 5 can be, for example, a thermosetting resin or an ultraviolet curing resin, but is not limited to these. The suction jig 7 is composed of, for example, a ferrule 6a and a hollow core optical fiber 1a (Figure 3A(c)). Here, the tip of the hollow core optical fiber 1a is fixed with a gap A exposed from the end face of the ferrule 6a, as shown in Figure 3A(c). The flat glass 2 can be adsorbed and held by equipping one end of the hollow core optical fiber 1a with a suction pump (not shown).

[0019] The transfer (application) of the adhesive 5 to the flat glass plate 2 is performed using an adhesive transfer jig 8 (Figure 3A(d)). The adhesive transfer jig 8 has an annular protrusion 81 that is slightly smaller than the glass region of the hollow core optical fiber 1. The adhesive transfer jig 8 is brought close to the adhesive 5 applied to the plate 5a, the adhesive 5 is pressed against the protrusion 81, and then the adhesive transfer jig 8 is released to transfer the adhesive 5 to the protrusion 81 (Figure 3A(d)).

[0020] Next, the transferred adhesive 5 is transferred to the flat glass 2 which is held by the suction jig 7 (Figure 3A(e)). The flat glass 2, to which the adhesive 5 has been transferred, is inserted into the ferrule 6 which houses the hollow core optical fiber 1, as shown in Figure 3B(f). The end faces of the ferrule 6 and the ferrule 6a of the suction jig 7 are brought into contact, and the hollow core optical fiber 1 is pushed in the direction of the arrow in the figure to bring it into contact with the flat glass 2 (Figure 3B(g)), and the adhesive 5 is cured. Note that the suction jig 7 and the adhesive transfer jig 8 can also apply or transfer the adhesive 5 to the hollow core optical fiber 1 instead of the flat glass 2 (see Figure 2(b)).

[0021] By following this procedure, the end face of the hollow core optical fiber 1 can be sealed by bonding the glass 3 at the end face of the hollow core optical fiber 1 to the flat glass 2 within the ferrule 6. This makes the distance from the side of the flat glass 2 facing the end face of the hollow core optical fiber 1 to the sealing surface of the hollow portion H (which is substantially the same as the end face of the hollow core optical fiber 1) almost zero. Therefore, compared to conventional examples where space (cavity) exists, the spread of emitted light from the hollow core optical fiber 1 can be minimized, thereby suppressing the increase in insertion loss of optical connection components using this and improving transmission characteristics.

[0022] In this embodiment, the optical connection component refers to a component that connects two optical fiber termination structures (a first optical fiber termination structure and a second optical fiber termination structure), with the flat glass plates 2,2 of each optical fiber termination structure facing each other. The facing of the flat glass plates 2,2 can be achieved by butting the ferrules 6,6 of the two optical fiber termination structures together. The optical connection component in this embodiment can be provided in a connector, and the optical connection component provided in a connector can achieve the butted state of the ferrules 6,6 together, thereby realizing the optical transmission characteristics according to the present invention. Preferably, in this state, the hollow core optical fiber 1 is bonded to the ferrule 6 by injecting adhesive 51 from the rear end of the ferrule 6 (Figure 3B(h)). Specifically, first, the hollow core optical fiber 1 is slightly inserted into the through hole 61 from the rear end of the ferrule 6. Insertion of the hollow core optical fiber 1 can be easily achieved by using the chamfered portion 62 formed around the through hole 61 at the rear end of the ferrule 6 as a guide. Next, adhesive 51 is applied to the chamfered portion 62. Next, the hollow core optical fiber 1 is further inserted to the specified position. In this embodiment, the specified position is the position that is close enough to the flat glass 2 to be bondable to it. As a result, as shown in Figure 3B(h)), adhesive 51 is applied to a portion of the side wall of the hollow core optical fiber 1 in the through hole 61 and hardened. At this time, the flat glass 2 will be fixed at a point set back inward from the tip of the ferrule by a distance A.

[0023] The thickness of the flat glass plate 2 is preferably 100 μm or less. The reason for this is explained below. When an optical fiber is cleved with a commercially available cleaver, the cutting angle varies from 90 degrees, and this variation is distributed within a range of up to about 1 degree. If the flat glass plate 2 is bonded to the end face of the fiber with a cutting angle deviating from 90 degrees, the deviation in the cutting angle directly leads to optical axis misalignment (because both ends of the flat glass plate 2 are filled with air, the optical axis is misaligned in parallel). This optical axis misalignment is proportional to the thickness of the flat glass plate 2. As the core diameter of the hollow core optical fiber 1 is about 20 μm to 50 μm, as described in Patent Document 1, in order to construct a low-loss optical connection component, this optical axis misalignment needs to be approximately 1 μm or less. When a flat glass plate 2 with a refractive index of 1.45 is mounted on the end face of a hollow core optical fiber 1 with a cleavage angle deviation of 1 degree, which is the worst case, and its thickness is 100 μm, the optical axis misalignment remains at about 0.5 μm. Therefore, by applying a flat glass plate 2 with a thickness of 100 μm or less, when the ferrule end faces of the optical fiber termination structure of this embodiment are brought together to form an optical connection component, low-loss transmission becomes possible even when considering the manufacturing tolerances of the mechanical components.

[0024] The spacing A is preferably 5 μm to 50 μm. This spacing A can be easily defined by the suction jig 7. In this case, when the end faces of these ferrules 6, 6 are brought together to form an optical connection component, the spacing between the flat glass plates 2, 2 (the distance between one side of one flat glass plate 2 and one side of the other flat glass plate 2 facing that side) can be 10 μm to 100 μm. Since there is no contact between the flat glass plates 2, 2, stable optical connection (attachment and detachment) can be performed repeatedly without concern for damage to the flat glass plates 2.

[0025] The reason why the spacing between the two flat glass plates 2,2 should be 100 μm or less is explained below. When two single-mode fibers with an MFD (mode field diameter) of 10 μm and an NA of 0.11 are connected via a gap, the insertion loss is approximately 0.5 dB when the spacing is 100 μm. Compared to single-mode fibers, hollow core optical fiber 1 has a larger MFD and a smaller NA due to its structural characteristics. Therefore, when two hollow core optical fibers 1,1 are connected via a gap, the loss is less compared to the case of single-mode fibers. This means that if the spacing is set to 100 μm or less, optical transmission can be achieved with an insertion loss of 0.5 dB or less.

[0026] In this embodiment, the distance between the fiber end faces is increased by the thickness of the two flat glass plates 2,2 (~200 μm). However, in the case of an air-glass-air path, the refraction of light in the glass portion suppresses the spreading of light. Therefore, if the spacing between the flat glass plates 2, which form the glass portion, is set to 100 μm or less when configuring the optical connection component, low insertion loss transmission can be achieved. Note that if the spacing between the flat glass plates 2,2 is on the order of wavelength (a few μm or less), the transmittance may fluctuate greatly with even slight changes in the gap, but this problem can be avoided by setting the spacing to 10 μm or more.

[0027] The suction jig 7 used during adhesive curing can be configured in two ways: either by suctioning the flat glass plate 2 or by pressurizing the flat glass plate 2 with air (gas). When the adhesive 5 is cured while suction is applied, the surface of the flat glass plate 2 can be aligned perpendicular to the axial direction of the ferrule 6 (the optical axis direction of the hollow core optical fiber 1), allowing light from the hollow core optical fiber 1 to be emitted from the end face of the ferrule 6 without any optical axis misalignment. When the adhesive 5 is cured under pressure, the end face of the hollow core optical fiber 1 and the flat glass plate 2 can be closely bonded together in line with the cleave surface, resulting in a more reliable seal.

[0028] The spacing A can be determined by considering the thermal expansion coefficients of the ferrule 6 and the hollow core optical fiber 1, so that the flat glass 2 is not exposed from the end face of the ferrule 6 within the operating temperature range. If a glass fiber is bonded only to the rear end of a 10 mm long zirconia ferrule, a 50-degree drop in temperature will cause the fiber to move in a direction that exposes 8 μm due to the difference in thermal expansion coefficients. In such an expected operating environment, setting the spacing A to approximately 20 μm will prevent the flat glass 2 from being exposed from the end face of the ferrule 6 even when the ambient temperature fluctuates significantly, and will also prevent a gap on the order of the wavelength. This makes it possible to provide a stable optical connection component under various temperature conditions.

[0029] Since both sides of the flat glass 2 are coated with an anti-reflective coating (not shown), no reflection occurs at the interface between the hollow core optical fiber 1 (air) and the flat glass 2, and at the interface between the flat glass 2 and the space A at the tip of the ferrule 6, thus enabling the construction of an optical connection component with good transmission characteristics. As a result of these effects, it becomes possible to provide an optical connection component that does not cause the deterioration of transmission characteristics that occurs with conventional solidification by melting.

[0030] Furthermore, the optical fiber termination structure is not limited to the configuration housed in the ferrule 6 as illustrated in Figure 3B(i); for example, a configuration without the ferrule 6 (Figure 1) is also acceptable.

[0031] [Second Embodiment] Figure 4 shows an optical fiber termination structure of a second embodiment related to the present invention. The main difference between this embodiment and the first embodiment is that a recess 63 for accommodating a flat glass 2 is provided at the tip of the ferrule 6, and the tip of the ferrule 6 (where the through hole 61 leads to the recess 63) is chamfered. Here, the outer diameter (diameter) of the flat glass 2 is set to be greater than the chamfering range L1 and less than or equal to the diameter L2 of the recess 63 (see Figure 4(d)). The flat portion 63a of the recess 63 is approximately perpendicular to the axial direction of the ferrule 6 (the optical axis direction of the hollow core optical fiber 1). Furthermore, the depth of the recess 63 is greater than the thickness of the flat glass 2 so that when bonded, the flat glass 2 is positioned recessed from the tip surface of the ferrule 6. This difference (depth of recess 63 - thickness of flat glass 2) is preferably set to 5 μm or more and 50 μm or less.

[0032] The implementation process of this embodiment is described below. After inserting the hollow core optical fiber 1 into the ferrule 6, the tip is cleaved (Figure 4(a)), and before retracting the hollow core optical fiber 1, adhesive 5 is applied to the recess 63, which includes the chamfered portion 64 formed by chamfering the through hole 61 of the ferrule 6 (Figure 4(b)). Next, the flat glass plate 2 is pressed against the end face of the hollow core optical fiber 1 and against the flat portion 63a of the recess 63 (Figure 4(c)). This process can be carried out, for example, using the suction jig 7 illustrated in Figure 3A(c). In this case, as shown in Figure 3A(e), the adhesive 5 may be transferred to the flat glass plate 2 so that adhesion can be made between the glass 3 at the end of the hollow core optical fiber 1 and the flat glass plate 2. Alternatively, the adhesive 5 may be applied directly to the flat portion 63a of the recess 63 (existing technology).

[0033] The outer diameter of the flat glass 2 is set to be larger than the chamfering range L1 and less than or equal to the diameter of the recess 63, so that when the flat glass 2 is housed in the ferrule 6, it is locked in the flat portion 63a of the recess 63. In this process, as the hollow core optical fiber 1 retracts through the through hole 61, the adhesive 5 applied to the chamfered portion 64 flows into the through hole 61 through the side of the hollow core optical fiber 1 (Figure 4(c)). By using this process, the adhesion of the flat glass 2 to the recess 63 of the ferrule 6 and the adhesion of the vicinity of the tip of the hollow core optical fiber 1 to the ferrule 6 can be performed simultaneously while the end face of the hollow core optical fiber 1 and the flat glass 2 are in contact, thereby simplifying the mounting process and reducing mounting costs.

[0034] Here, since the flat glass 2 is bonded to the flat portion 63a of the recess 63 of the ferrule 6, the surface of the flat glass 2 is perpendicular to the axial direction of the ferrule 6 (the optical axis direction of the hollow core optical fiber 1). Therefore, even if the cleave angle of the hollow core optical fiber 1 is not 90 degrees, the optical axis will not be misaligned. When the cleave angle is not 90 degrees, a small gap will be created in the optical path between the two, but since this gap is air (the amount of adhesive 5 is appropriately designed so that the adhesive 5 flows reliably into the through hole 61), it will have the same refractive index as the core of the hollow core optical fiber 1, and the optical axis perpendicular to the surface of the flat glass 2 will remain unchanged.

[0035] Furthermore, since the hollow core optical fiber 1 is bonded to the vicinity of the tip of the ferrule 6, the relative position of the two hardly changes even under temperature fluctuations. Therefore, there is no concern that excessive pressure will be applied to the flat glass plate 2 to which the end face of the hollow core optical fiber 1 is in contact, or that a separation of the order of micrometers will occur due to fiber pulling.

[0036] However, due to the difference in thermal expansion coefficients between glass and zirconia, a pistoning phenomenon occurs, causing positional shifts on the order of tens of nanometers in the axial direction. This positional shift is directly transmitted to the flat glass 2 in contact with the end face of the hollow core optical fiber 1, raising concerns that it could, in the worst case, lead to breakage.

[0037] This concern can be resolved by the chamfered portion 64 provided on the ferrule 6. This is because, in the optical fiber termination structure of this embodiment, there is an adhesive 5 or space in the chamfered portion 64 that is less hard than zirconia. Even when pressure due to pistoning is applied to the flat glass 2 and the position of the flat glass 2 fluctuates in the axial direction of the fiber (up and down direction in the figure), the presence of this adhesive 5 or space can alleviate this stress, thereby preventing damage to the flat glass 2. This stress relief is more effective the larger the area of ​​the chamfered portion 64. For example, if the radius of the hollow core optical fiber 1 is a, sufficient stress relief can be achieved by chamfering with a diameter of Ca (diagonal chamfering at position a from the tip of the corner) or Ra (circular chamfering at radius a) or greater, or by making the apex angle θ of the chamfer 90 degrees or greater (see Figure 4(d)).

[0038] Now, in the chamfered portion 64, the hollow core optical fiber 1 will be exposed from the through hole 61, and from the viewpoint of suppressing optical axis misalignment, it is desirable for this exposed length to be as short as possible. On the other hand, from the viewpoint of stress relief, it is desirable to increase the bonding area between the flat glass 2 and the adhesive 5. By setting the apex angle θ of the chamfer to 90 degrees or more, it is possible to achieve both securing a bonding area between the flat glass 2 and the adhesive 5 and shortening the length of the hollow core optical fiber 1 exposed from the through hole 61.

[0039] As a result, compared to conventional examples, the distance between the hollow core optical fiber 1 and the flat glass plate 2 can be minimized, thereby reducing the spread of light emitted from the hollow core optical fiber 1 and realizing an optical fiber termination structure without optical axis misalignment.

[0040] Furthermore, since the flat glass 2 is positioned 5 μm to 50 μm behind the end face of the ferrule 6, when the end faces of the ferrules 6, 6 are brought together to form an optical connection component, contact between the flat glass 2, 2 can be avoided over a wide temperature range, and the gap between them can be kept between 10 μm and 100 μm, thereby providing a low-loss optical connection component. In addition, since both sides of the flat glass 2 are coated with an anti-reflective film, no reflection occurs at the interface between the hollow core optical fiber 1 (air) and the flat glass 2, and at the interface between the flat glass 2 and the space at the tip of the ferrule 6, thus enabling the construction of an optical connection component with good transmission characteristics. As a result of these effects, it is possible to provide an optical connection component that does not have the concern of causing degradation of transmission characteristics, as is the case with conventional solidification by melting.

[0041] Alternatively, the hollow core optical fiber 1 may be bonded to the rear end of the ferrule 6 by injecting adhesive 51 from the rear end of the ferrule 6 (see Figure 3B(h)). This method can further strengthen the bond between the hollow core optical fiber 1 and the ferrule 6.

[0042] [First modified example of the second embodiment] Figure 5 shows an optical fiber termination structure of a first modification of a second embodiment related to the present invention. The main difference between this embodiment and the embodiment shown in Figure 4 is the absence of the chamfered portion 64 of the ferrule 6. The absence of the chamfered portion 64 has the advantage of allowing the use of an inexpensive ferrule. The outer diameter of the flat glass 2 is set to be larger than the through hole 61 of the ferrule 6 and less than or equal to the diameter of the recess 63. Also, the depth of the recess 63 is deeper than the thickness of the flat glass 2 so that when bonded, the flat glass 2 is positioned recessed from the tip surface of the ferrule 6. This difference (depth of recess 63 - thickness of flat glass 2) is preferably set to 5 μm or more and 50 μm or less.

[0043] The implementation process of this embodiment is described below. The cleave hollow core optical fiber 1 is retracted into the through hole 61 of the ferrule 6 (Figure 5(a)). In this state, the flat glass 2 is pressed against the flat portion 63a of the recess 63 of the ferrule 6 and bonded (Figure 5(b)). This bonding can be carried out by transferring the adhesive to the portion of the flat glass 2 that will come into contact with the flat portion 63a of the recess 63, as shown in Figure 3A, or by directly applying the adhesive 5 to the flat portion 63a of the recess 63 (existing technology).

[0044] After bonding, the hollow core optical fiber 1 inside the ferrule 6 is raised to a predetermined position (Figure 5(c)). This predetermined position is one in which the ferrule does not come into contact with the flat glass 2, maintaining a predetermined distance. For example, by placing a camera (not shown) above the figure (on the opposite side of the flat glass 2 from the ferrule 6) and monitoring the through-hole 61 of the ferrule 6 through the flat glass 2, the tip of the hollow core optical fiber 1 can be kept in the predetermined position. In this state, bonding is performed by injecting adhesive 51 from the rear end of the ferrule 6 (Figure 5(d)) (see Figure 3B(h)).

[0045] This predetermined spacing is preferably set to about 10 μm. This takes into account that if a glass fiber is bonded only to the rear end of a 10 mm long zirconia ferrule, a 50-degree drop in temperature will cause the fiber to move in a direction that exposes 8 μm due to the difference in thermal expansion coefficients. Therefore, by setting the spacing to about 10 μm, the tip of the hollow core optical fiber 1 will not come into contact with the flat glass plate 2 even when the ambient temperature fluctuates greatly.

[0046] This allows the gap between the hollow core optical fiber 1 and the flat glass 2 to be minimized without worrying about damaging the flat glass 2, thus realizing an optical fiber termination structure that suppresses light diffusion. Furthermore, since the flat glass 2 is bonded to the recess 63 of the ferrule 6, the surface of the flat glass 2 is perpendicular to the axial direction of the ferrule 6 (the optical axis direction of the hollow core optical fiber 1). Therefore, even if the cleave angle of the hollow core optical fiber 1 is not 90 degrees, the optical axis will not be misaligned. Although there is a gap of about 10 μm in the optical path between the two, this gap is air (the amount of adhesive 5 is appropriately designed so that the adhesive 5 flows reliably into the through hole 61), so it has the same refractive index as the core of the hollow core optical fiber 1, and the optical axis perpendicular to the surface of the flat glass 2 remains unchanged.

[0047] As a result, compared to conventional examples, by minimizing the distance between the hollow core optical fiber 1 and the flat glass plate 2, the light diffusion from the hollow core optical fiber 1 can be minimized, and an optical fiber termination structure without optical axis misalignment can be realized.

[0048] Since the tip of the flat glass 2 is positioned 5 μm to 50 μm behind the end face of the ferrule 6, when the end faces of two ferrules 6 are brought together to form an optical connection component, contact between the flat glass 2s can be avoided, and the gap between them can be 10 μm to 100 μm. Therefore, the spread of light can be suppressed without worrying about damage to the flat glass 2, and repeated, stable, low-loss optical connections (attachments and detachments) can be performed. Furthermore, since both sides of the flat glass 2 are coated with an anti-reflective film, no reflection occurs at the interface between the hollow core optical fiber 1 (air) and the flat glass 2, or at the interface between the flat glass 2 and the space at the tip of the ferrule 6. As a result, it is possible to provide an optical connection component that does not cause the degradation of transmission characteristics that occurs with conventional solidification by melting.

[0049] [Second variation of the second embodiment] Figure 6 shows a second modified optical fiber termination structure of a second embodiment related to the present invention. The main difference between this embodiment and the embodiment shown in Figure 4 is that the ferrule 6 does not have a recess 63. Therefore, a chamfered portion 64 is formed on the end face of the ferrule 6 by chamfering the through hole 61 of the ferrule 6. The absence of the recess 63 has the advantage of allowing the use of an inexpensive ferrule. The outer diameter of the flat glass 2 is set to be larger than the chamfering range L1 (see Figure 4(d)) and smaller than the outer diameter of the ferrule 6.

[0050] The implementation process of this embodiment is described below. After inserting the hollow core optical fiber 1 into the ferrule 6, the tip is cleaved (Figure 6(a)), and adhesive 5 is applied to the chamfered portion 64 before retracting the hollow core optical fiber 1 (Figure 6(b)). Next, the flat glass 2 is pressed against the end face of the hollow core optical fiber 1 and brought into contact with the end face of the ferrule 6 (Figure 6(c)). Here, the application of adhesive 5 to the chamfered portion 64 can be done by transferring the adhesive 5 to the portion of the flat glass 2 that comes into contact with the ferrule 6, as shown in Figure 3A, or by directly applying the adhesive to the chamfered portion 64 on the end face of the ferrule 6 (existing technology).

[0051] In this process, as the hollow core optical fiber 1 retracts through the through hole 61, the adhesive 5 applied to the chamfered portion 64 flows into the through hole 61 through the side of the hollow core optical fiber 1 (Figure 6(c)). By using this process, the end face of the hollow core optical fiber 1 and the flat glass 2 can be in contact with each other, and the adhesion of the vicinity of the tip of the hollow core optical fiber 1 to the ferrule 6 can be performed simultaneously, thereby simplifying the mounting process and reducing mounting costs.

[0052] Here, since the flat glass 2 is bonded to the end face of the ferrule 6, the surface of the flat glass 2 is perpendicular to the axial direction of the ferrule 6 (the optical axis direction of the hollow core optical fiber 1). Therefore, even if the cleave angle of the hollow core optical fiber 1 is not 90 degrees, the optical axis will not be misaligned. When the cleave angle is not 90 degrees, a small gap will be created in the optical path between the two, but since this gap is air (the amount of adhesive 5 is appropriately designed so that the adhesive 5 flows reliably into the through hole 61), it will have the same refractive index as the core of the hollow core optical fiber 1, and the optical axis perpendicular to the surface of the flat glass 2 will remain unchanged.

[0053] Furthermore, since the hollow core optical fiber 1 is bonded to the tip of the ferrule 6, the relative position of the two hardly changes even under temperature fluctuations. Therefore, there is no concern that excessive pressure will be applied to the flat glass plate 2 to which the end face of the hollow core optical fiber 1 is in contact, or that a separation of the order of micrometers will occur due to fiber pulling.

[0054] However, due to the difference in thermal expansion coefficients between glass and zirconia, a pistoning phenomenon occurs, causing positional shifts on the order of tens of nanometers in the axial direction. This positional shift is directly transmitted to the flat glass 2 in contact with the end face of the hollow core optical fiber 1, raising concerns that it could, in the worst case, lead to breakage.

[0055] This concern can be resolved by the presence of the chamfered portion 64 provided on the ferrule 6. This is because, in the optical fiber termination structure of this embodiment, there is an adhesive 5 or space in the chamfered portion 64 that is less hard than zirconia. Even when pressure due to pistoning is applied to the flat glass 2 and the position of the flat glass 2 fluctuates in the axial direction of the fiber (up and down direction in the figure), the presence of this adhesive 5 or space can alleviate this stress, thereby preventing damage to the flat glass 2. This stress relief is more effective the larger the area of ​​the chamfered portion.

[0056] For example, if the radius of the hollow core optical fiber 1 is a, sufficient stress relief can be achieved by chamfering with a material of Ca or Ra or higher, or by setting the chamfer apex angle θ to 90 degrees or more (see Figure 4(d)). Setting the chamfer apex angle θ to 90 degrees or more makes it possible to secure the bonding area between the flat glass plate 2 and the adhesive 5, and to shorten the length of the hollow core optical fiber 1 exposed from the through hole 61.

[0057] As a result, compared to conventional examples, by minimizing the distance between the hollow core optical fiber 1 and the flat glass plate 2, the light diffusion from the hollow core optical fiber 1 can be minimized, and an optical fiber termination structure without optical axis misalignment can be realized.

[0058] The optical connector that connects these optical fiber termination structures is configured with a spacer 9, as shown in Figure 6(d). Preferably, the spacer 9 is ring-shaped, with its inner diameter larger than the outer diameter of the flat glass 2 and its outer diameter less than or equal to the outer diameter of the ferrule 6. Furthermore, the thickness of the spacer 9 is greater than the thickness of the flat glass 2. Preferably, the thickness of the spacer 9 is 5 μm to 50 μm thicker than the thickness of the flat glass 2. Note that this configuration with the spacer 9 can also be used as an optical fiber termination structure.

[0059] In this embodiment, the optical connection component refers to a component that connects two optical fiber termination structures equipped with a spacer 9, so that the flat glass plates 2, 2 of each optical fiber termination structure face each other. The spacer 9 can surround the flat glass plates 2 that are bonded to the end face of the ferrule 6. The spacer 9 can also be bonded to the end face of the ferrule 6 as appropriate (not shown in Figure 6(d)).

[0060] By providing such a spacer 9 between the end faces of the optical fiber termination structure shown in Figure 6, when the end faces of the ferrules 6, 6 are facing each other via the spacer 9 to form an optical connection component, contact between the flat glass plates 2, 2 can be avoided, and the distance between them can be between 10 μm and 100 μm. Therefore, when connecting optical fiber termination structures without optical axis misalignment, the spread of light can be suppressed without worrying about damage to the flat glass plates 2, and repeated, stable, low-loss optical connections (attachments and detachments) can be performed.

[0061] Furthermore, since both sides of the flat glass 2 are coated with an anti-reflective film, no reflection occurs at the interface between the hollow core optical fiber 1 (air) and the flat glass 2, nor at the interface between the flat glass 2 and the space at the tip of the ferrule 6, thus enabling the construction of an optical connection component with good transmission characteristics. Therefore, it becomes possible to provide an optical connection component that does not have the concern of causing degradation of transmission characteristics, as is the case with conventional solidification by melting.

[0062] Furthermore, the optical fiber termination structure is not limited to the embodiment illustrated in Figure 6; a shape without a chamfered portion 64 on the ferrule 6 may also be used. In this case, the advantage of being able to use an even less expensive ferrule arises. In this case, as illustrated in the embodiment in Figure 5, preferably, the hollow core optical fiber 1 and the flat glass plate 2 are not in contact, and the bonding is performed with a predetermined distance between them so that they do not come into contact even under temperature fluctuations. Furthermore, the spacer 9 does not need to be provided for each optical fiber termination structure; one spacer 9 may be provided as an optical connection component. In this case, the thickness of the spacer 9 is preferably 10 μm to 100 μm thicker than twice the thickness of the flat glass 2 (the sum of the thicknesses of the two flat glass 2). The optical connection component in this embodiment is a component that connects two optical fiber termination structures (a first optical fiber termination structure with a spacer 9 and a second optical fiber termination structure without a spacer 9), with the flat glass 2s of each optical fiber termination structure facing each other. The spacer 9 can surround the flat glass 2 that is bonded to the end face of the ferrule 6. The spacer 9 can also be appropriately bonded to the end face of the ferrule 6.

[0063] [Other variations] (a) In the embodiments described above, a flat glass plate 2 is used as the light-transmitting member, but it is not limited to this, and any material that transmits light may be used, such as Si or resin. The shape of the light-transmitting member does not necessarily have to be a disc, and may be a square or other shape. Furthermore, as shown in Figure 7, it may be a shape that has a function such as a plano-convex lens 10 or a prism 11. When a plano-convex lens 10 or a prism 11 is applied, it is possible to provide flexibility in connection distance and connection direction, thereby diversifying the configuration of optical connection components.

[0064] (b) The hollow core optical fiber is not limited to those exemplified in Figure 2(a), but can be any form such as a photonic bandgap fiber, kagome fiber, anti-resonant fiber, NANF, etc., as long as the core is hollow.

[0065] (c) The material of ferrule 6 is not limited to zirconia, but may be resin, glass, metal or other materials.

[0066] (d) Although this embodiment illustrates an optical fiber termination structure using ferrules 6, it can also be applied to other forms such as V-groove arrays (optical fiber termination structures without ferrules).

[0067] (e) The end face and recess 63 of the ferrule 6, which serve as the bonding surface to the flat glass 2, are shown in an example where they are perpendicular to the axial direction of the ferrule 6 (the optical axis direction of the hollow core optical fiber 1) (a configuration in which the optical axis direction of the hollow core optical fiber 1 coincides with the normal direction of the flat surface of the end face and recess 63 of the ferrule 6). However, for example, they may be inclined at any (predetermined) angle (preferably 8 degrees or less) with respect to the axial direction of the ferrule 6, rather than perpendicular. That is, the normal direction of the flat surface of the end face and recess 63 of the ferrule 6 may be inclined with respect to the axial direction of the ferrule 6. Figure 8(a) illustrates an optical fiber termination structure in which the flat portion 63a of the recess 63 formed on the end face of the ferrule 6 is inclined. In this case, the reflection angle of the light returned from the flat glass 2 increases according to the inclination angle of the flat portion 63a of the recess 63, and a predetermined amount of return loss (e.g., 40 dB) can be reproducibly achieved without applying an extremely low-reflection anti-reflective coating to the flat glass 2. Since a glass with an extremely low-reflection anti-reflective coating is not required, there is an advantage in being able to use less expensive components. Furthermore, in a configuration in which the flat glass 2 is equipped with an extremely low-reflection anti-reflective coating, the wavelength band in which extremely low reflection can be achieved is limited due to the material selection of the anti-reflective coating, etc. In contrast, in a configuration in which the flat portion 63a of the recess 63 is tilted and the flat glass 2 is not equipped with an extremely low-reflection anti-reflective coating, there is an advantage in being able to obtain good extremely low-reflection characteristics over a wide wavelength band. However, even if it is not extremely low-reflection, by applying an anti-reflective coating to the flat glass 2 and then tilting it, connection loss due to Fresnel reflection can be reduced. The anti-reflective coating applied to both sides of the flat glass 2 and the tilt of the recess 63 (flat portion 63a) described in this embodiment, together with the tilt of the end face of the ferrule 6 shown in Figure 10, are specific examples of an anti-reflective mechanism that prevents the reflection of light transmitted through the flat glass 2. A preferred example of an optical connection component with two optical fiber termination structures facing each other, as shown in Figures 8(a) and 8(b), is shown and explained in Figure 9. The optical fiber termination structure comprises a ferrule 6 that houses a hollow core optical fiber 1, a flange 20 into which the ferrule 6 is press-fitted, and a housing 21 that houses the flange 20. This optical connection component connects two optical fiber termination structures facing each other via an adapter 30. When housing the flange 20 in the housing 21, the keyway 22 of the housing 21 fits into the projection 23 of the flange 20, thus uniquely determining the relative rotation angle of the flange 20 and the housing 21. Here, when press-fitting the ferrule 6 into the flange 20, for example, the shallowest part of the flat portion 63a of the inclined recess 63 is aligned with the keyway 22 of the housing 21. In other words, the relative rotation angle of the ferrule 6 and the housing 21 is determined. Furthermore, when forming optical connection components by facing the optical fiber termination structures, the keys 24, 24 of the housings 21, 21 are fitted into the keyways 31, 31 of the adapter 30, so that the projections 23, 23 of the flanges 20, 20 face each other. As a result, as shown in Figure 9, an optical connection component can be constructed in which the shallowest parts of the flat portion 63a of the recess 63 face each other, and the inclined apex portions 2a of the flat glass 2 (the parts of the flat glass 2 that are located in the shallowest part of the flat portion 63a of the recess 63) face each other (including the meaning of nearly facing each other). If the inclination of the flat portion 63a of the recess 63 is 8 degrees and the thickness of the flat glass 2 is 100 μm, the optical axis offset in the flat glass 2 will be large, about 4 μm. However, as shown in Figure 9, if the inclined apex portions 2a of the flat portions 63a, 63a of the flat glass 2 are facing each other, then no optical axis misalignment will occur in the optical connection component. When optical connection components are constructed by facing optical fiber termination structures where the inclination angles of the end faces of the ferrule 6 and the flat portion 63a of the recess 63 are the same, no optical axis misalignment will occur even if the inclination angle is any angle (even if it is greater than 8 degrees). Therefore, by giving the flat glass 2 a predetermined inclination, a predetermined amount of return loss can be achieved with good reproducibility, and an optical connection component with low insertion loss can be constructed. The above explanation also applies to the configuration shown in Figure 10, where the end faces of the ferrules 6,6 are inclined with respect to the axial direction of the ferrules 6,6. As shown in Figure 10, most of the end faces of the ferrules 6,6 are inclined, but a portion of the region 65,65 located radially outward by a predetermined amount from the central axis of the ferrules 6,6 is not inclined and serves as the abutting surface when the optical fiber termination structures face each other to form an optical connection component. The thickness and diameter of the flat glass plates 2,2, and the inclined region of the end faces of the ferrules 6,6 are determined so that the flat glass plates 2,2 on the end faces of the ferrules 6,6 do not come into contact with each other when the optical connection component is formed with a predetermined inclination angle. Furthermore, the above explanation also applies to the configuration in which a flat glass plate 2 is bonded to a hollow core optical fiber 1 as shown in Figure 1, specifically to the configuration in which the hollow core optical fiber 1 is cleved at an angle.

[0068] (f) Furthermore, it is also possible to realize technologies that appropriately combine the various technologies described in this embodiment.

[0069] (g) In addition, the shape, material, function, etc. of the components of the present invention may be modified as appropriate without departing from the spirit of the present invention. [Explanation of Symbols]

[0070] 1,1a hollow core optical fiber 2. Flat glass (light-transmitting material) 3 Glass 4 Inner Tubes 5.51 Adhesive 5a plate 6,6a Ferrule 61 Through hole 62 Chamfered section 63. Indentation 63a Flat area 64 Chamfered section 65 (Partial area of ​​the inclined end face of the ferrule) 7. Suction jig (jig) 8. Adhesive transfer jig (jig) 81 Convex part 9 Spacers 10 Plano-convex lens 11 Prisms 20 flanges 21 Housing 22 keyways 23 Protrusion 24 keys 30 adapters H Hollow part

Claims

1. A hollow core optical fiber having a hollow portion through which light is transmitted, A light-transmitting member which is a flat glass plate covering the hollow portion, The end face of the ferrule through which the hollow core optical fiber is inserted is provided with a recess for accommodating the light-transmitting member, The light-transmitting member is bonded to the flat portion of the recess. The aforementioned light-transmitting member is an optical fiber termination structure made of a flat glass plate with anti-reflective coatings on both sides.

2. The light-transmitting member is made of a disc-shaped flat glass plate. The optical fiber termination structure according to claim 1, wherein the normal direction of the flat portion of the recess is inclined at a predetermined angle with respect to the axial direction of the ferrule.

3. The optical fiber termination structure according to claim 1, wherein the light-transmitting member is positioned set back from the end face of the ferrule, or the light-transmitting member is positioned set back within a range of 5 μm to 50 μm from the end face of the ferrule.

4. A hollow core optical fiber having a hollow portion through which light is transmitted, A light-transmitting member which is a flat glass plate covering the hollow portion, An adhesive is provided between the chamfered portion of the ferrule through which the hollow core optical fiber is inserted and the light-transmitting member. The adhesive has a hardness lower than that of the ferrule (zirconia) and a hardness higher than that of the light-transmitting member. The hollow core optical fiber is exposed through the through hole in the chamfered portion, An optical fiber termination structure in which the apex angle θ of the chamfered portion is 90 degrees or more.

5. The flange into which the ferrule is press-fitted, A housing for accommodating the flange, The optical fiber termination structure according to claim 1 or 4, wherein when the flange is housed in the housing, at least one of the irregularities of the housing is fitted to at least the other of the irregularities of the flange.

6. The housing is equipped with a keyway, The flange is provided with a projection, The keyway of the housing and the projection of the flange are fitted together. The optical fiber termination structure according to claim 5, wherein the shallowest part of the flat portion of the inclined recess of the ferrule is aligned with the keyway of the housing.

7. Includes an adapter, The optical fiber termination structure according to claim 6, wherein the keyway of the housing is fitted into the keyway of the adapter.

8. The optical fiber termination structure according to claim 1, wherein the thickness of the light-transmitting member is 100 μm or less.

9. By adhesive applied to or transferred to the light-transmitting member or the hollow core optical fiber The optical fiber termination structure according to claim 1, wherein the light-transmitting member is bonded to the end face of the hollow core optical fiber.

10. The optical fiber termination structure according to claim 9, wherein the outer diameter of the light-transmitting member is smaller than the diameter of the through-hole of the ferrule through which the hollow core optical fiber is inserted.

11. The light-transmitting member is bonded to the end face of the ferrule through which the hollow core optical fiber is inserted. The optical fiber termination structure according to claim 1, further comprising a spacer surrounding the light-transmitting member.

12. The ferrule is provided with a chamfered portion formed by chamfering the through hole, The optical fiber termination structure according to claim 1, wherein the outer diameter of the light-transmitting member is larger than the chamfered range of the chamfered portion.

13. The optical fiber termination structure according to claim 11, wherein the thickness of the spacer is greater than the thickness of the light-transmitting member, or has a thickness of 10 μm or more and 100 μm or less compared to twice the thickness of the light-transmitting member.

14. The optical fiber termination structure according to claim 1, wherein the normal direction of the end face of the hollow core optical fiber is inclined at a predetermined angle with respect to the axial direction of the hollow core optical fiber.

15. The optical fiber termination structure according to claim 11, wherein the end face of the ferrule is inclined at a predetermined angle with respect to the axial direction of the ferrule.

16. An optical connection component comprising optical fiber termination structures according to claim 1 or claim 4, arranged facing each other.

17. The optical connection component according to claim 16, wherein the spacing between the light-transmitting members in the optical connection component is within the range of 10 μm or more and 100 μm or less.

18. An insertion process in which a hollow core optical fiber is inserted into a ferrule with a through hole, After the insertion process, a cleave-retraction process is performed in which the tip of the hollow core optical fiber is cleave and retracted into the interior of the ferrule, The transfer process involves bringing an adhesive transfer jig close to the adhesive applied to the plate, pressing the adhesive onto the convex part of the adhesive transfer jig which is an annular shape slightly smaller than the glass area of ​​the hollow core optical fiber, and then moving the adhesive transfer jig away to transfer the adhesive to the convex part. A re-transfer step in which the adhesive transferred in the transfer step is re-transferred to the flat glass held by the adsorption jig, After the retransfer step, the flat glass is inserted into the ferrule containing the hollow core optical fiber after the cleave retraction step, and the hollow core optical fiber is brought into contact with the flat glass in an insertion step. A method for manufacturing an optical fiber termination structure, comprising a curing step of curing an adhesive after the insertion step.

19. The adhesive is a thermosetting resin or an ultraviolet curing resin. The method for manufacturing an optical fiber termination structure according to claim 18, wherein the curing step includes a UV curing step or a thermocuring step.

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