JP2009216721A - Optical fiber, fusion bonding method for it and connector for optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber large in non-linearity, and small in connection loss when fusion bonded with a quartz-based glass fiber. <P>SOLUTION: The optical fiber is integrally composed of: a hollow glass fiber provided with a hollow portion; a light transmitting unit having a center axis portion on the center axis of the hollow portion and a peripheral portion concentric with it; and a platy support radially extending from a plurality of portions on the outer peripheral surface of the light transmitting unit to reach the inner peripheral surface of the hollow glass fiber. The optical fiber is also characterized in that the refractive index (n<SB>1</SB>) of the center axis portion at a wavelength of 1,550 nm of the light transmitting unit is larger than the refractive index (n<SB>2</SB>) at a wavelength of 1,550 nm of its peripheral portion. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical fiber and a fusion splicing method between the optical fiber and a silica-based optical fiber.

As an optical fiber having a large nonlinearity and having a hole, an optical fiber composed mainly of Bi ₂ O ₃ with a single glass has been proposed (see Patent Document 1).

JP 2004-294464 A

  However, when fusion-bonding the optical fiber and the silica-based optical fiber, if the fusion strength is given, the holes are crushed and easily deformed. In the optical fiber, since the air holes act as cladding, the waveguide structure disappears at the connection end face with the quartz optical fiber, leading to a significant increase in loss. In addition, there is a problem that if the fusion is performed such that the waveguide structure does not disappear, the fusion strength is low, and further, the fusion itself cannot be performed.

  An object of this invention is to provide the optical fiber which can solve such a problem, and its fusion | fusion method.

To achieve the above object, the present invention provides a hollow glass fiber having a hollow portion, an optical transmission portion having a central axis portion and a concentric peripheral portion on the central axis of the hollow portion, and an outer periphery of the optical transmission portion. A plate-like support portion extending radially from a plurality of locations on the surface and reaching the inner peripheral surface of the hollow glass fiber is integrally formed, and the refractive index at a wavelength of 1550 nm of the central axis portion of the optical transmission portion ( Provided is an optical fiber characterized in that n ₁ ) is larger than the refractive index (n ₂ ) at a wavelength of 1550 nm in the surrounding portion.

  Further, the end face of the optical fiber in which the hollow glass fiber, the optical transmission part and the support part are integrally formed of glass having a glass transition point of 500 ° C. or less and the end face of the silica type optical fiber are brought into contact with each other, and the silica type optical fiber The vicinity of the end face of the optical fiber is heated to fuse the two optical fibers together, and the hollow portion of the optical fiber is gradually narrowed as it approaches the end face to which the fusion splicing is performed, and the light is lost at the end face. A method for splicing fibers is provided.

  In addition, one or both end faces of the optical fiber in which the hollow glass fiber, the optical transmission part, and the support part are integrally formed of glass having a glass transition point of 500 ° C. or less and the end face of the silica-based optical fiber are fusion-bonded. An optical fiber connecting body, wherein the hollow portion of the optical fiber is gradually narrowed and disappears at the end face as it approaches the end face where the fusion splicing is performed, and the two optical fibers are fusion spliced. A difference in mode field diameter at a wavelength of 1550 nm on the end face is 2.0 μm or less in absolute value.

  According to the present invention, it is possible to obtain an optical fiber having a large non-linearity and a small connection loss when fusion-bonded to a silica glass fiber.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a view showing an example of the optical fiber of the present invention, and is a view schematically showing a cross section orthogonal to the central axis. The illustrated optical fiber 1 has a hollow glass fiber 3 having a hollow portion 2 extending in the axial direction (a direction perpendicular to the paper surface) at the center thereof, and extends along the central axis of the optical fiber 1 inside the hollow portion 2. The linear optical transmission unit 4 is arranged concentrically. The optical transmission part 4 and the hollow part 2 are support parts that extend radially from a plurality of locations (six in the example in the figure) on the outer peripheral surface of the optical transmission unit 4 and reach the inner peripheral surface of the hollow portion 2. 5 (plate glass). The support portion 5 is a plate-like member extending in the axial direction, and holds the optical transmission portion 4 at the center of the optical fiber 1.

  In addition, the thickness of the support part 5 becomes like this. Preferably it is 0.05-1.5 micrometers. If the thickness is less than 0.05 μm, the support 5 may be damaged when the optical fiber 1 is cut, and the optical transmission unit 4 may not be held. More preferably, it is 0.1 μm or more. On the other hand, if the thickness exceeds 1.5 μm, light leakage from the light transmission part 4 to the support part 5 becomes large, and the light confinement may be insufficient. More preferably, it is 0.5 μm or less.

  Moreover, although the hollow part 2 is divided equally by the support part 5, it is preferable that the number of divisions is three or more. In the case of two, the effect of preventing the hollow portion 2 from being crushed at the time of fusion splicing with a silica-based optical fiber is poor, and since the optical fiber 1 has a so-called air clad structure, light confinement becomes insufficient. There is a fear. Note that the upper limit of the number of sections is preferably 12 or less, more preferably 9 or less, in order to reduce light leakage from the plate-like support portion 5 or improve workability.

The above hollow glass fiber 3, the optical transmission unit 4, the support 5 is one member which is continuous, preferably as represented by mol% based on _{oxides, Bi 2 O 3: 40~75%} , B 2 O 3: _{12~45%, Ga 2 O 3:} 1~20%, In 2 O 3: 1~20%, ZnO: 0~20%, BaO: 0~15%, SiO 2 + Al 2 O 3 + GeO 2: 0~ 15%, MgO + CaO + SrO: 0-15%, SnO ₂ + TeO ₂ + TiO ₂ + ZrO ₂ + Ta ₂ O ₅ + Y ₂ O ₃ + WO ₃ : 0 to 10%, CeO ₂ : 0 to 5%, essentially Ga ₂ O ₃ + In ₂ O ₃ + ZnO is formed of glass with 5% or more. More _{preferably, Bi 2 O 3: 49~67%} , B 2 O 3: 13~30%, Ga 2 O 3: 5~13%, In 2 O 3: 0.5~8%, ZnO: 2~ _{8%, BaO: 0~6%,} CeO 2: 0.1 to 1%.

Furthermore, in the optical fiber 1, the optical transmission unit 4 has a central axis portion 6 at the center thereof. The diameter (d _c ) of the central shaft portion 6 is usually 0.2 to 10 μm, preferably 0.5 to 4 μm. The central axis portion 6 has a higher refractive index at a wavelength of 1550 nm than other portions (peripheral portions) of the light transmission section 4, and light propagates. Preferably, when the refractive index of the central axis portion 6 at a wavelength of 1550 nm is n ₁ and the refractive index of the surrounding portion at a wavelength of 1550 nm is n ₂ , the relationship of the following equation is preferably satisfied.
0.0005 ≦ (n ₁ −n ₂ ) / n ₁ ≦ 0.2

The central axis portion 6 is within the range of the glass composition, and the Bi ₂ O ₃ amount is 56% or more and 1% or more than the Bi ₂ O ₃ amount in the surrounding portion, and (B ₂ O ₃ + Ga ₂ O ₃ ) The amount is 1% or less less than the (B ₂ O ₃ + Ga ₂ O ₃ ) amount in the surrounding portion.

  Further, in the optical transmission unit 4, the peripheral part can be formed in a multilayer structure. At that time, in order to prevent light leakage, it is necessary that the refractive index at 1550 nm be maximized in the central axis portion 6.

In addition, as shown in an enlarged view in FIG. 2, the cross section of the hollow portion 2 is actually formed with a curved surface at the connection portion between the outer peripheral surface of the optical transmission portion 4 and the support portion 5. The outer peripheral surface also spreads outward with the middle point of the adjacent support portions 5 and 5 as the apex. In the present invention, the diameter (d) of the circle (C1) inscribed in the hollow portion 2 is preferably 0.2 to 10 μm, and more preferably 0.5 to 4 μm. The diameter (d ′) of the circle (C2) circumscribing the hollow portion 2 is preferably (1 + 2 ^1/2 ) d or more. If it is less than (1 + 2 ^1/2 ) d, the light is not sufficiently confined and the propagation loss may increase. More preferably, it is 3d or more, Most preferably, it is 4d or more.

  The outer diameter (D) of the optical fiber 1 is preferably 16d or less. If it exceeds 16d, the strength of the optical fiber 1 is reduced, foreign matter is likely to be mixed into the hollow portion 2, and the support portion 5 may be destroyed when attempting to cut the optical fiber 1. In addition, there is a concern that the support portion 5 cannot support the optical transmission portion 4 in the hole gradually decreasing portion 12 and the problem such as an increase in connection loss due to an axis shift occurs. More preferably, it is 12d or less.

  Next, the optical fiber connection method of the present invention will be described.

  FIG. 3 is a diagram schematically showing an example of the connection state along the central axis. The optical fiber 1 and the ITU-T Recommendation G. A state where a quartz optical fiber (SMF) 8 standardized by 652 is connected is shown.

  When connecting, first, end faces to be fused of the optical fiber 1 and the silica-based glass fiber 8 are arranged to face each other at a predetermined interval, and an axis between the central axis portion 6 of the optical fiber 1 and the core 10 of the silica-based optical fiber 8 is arranged. Align and align. Here, the end face is preferably flat, so that the optical fiber 1 and the silica-based glass fiber 8 can contact each other over the entire end face. Further, the angle (θ) between the end face and the axis of each of the fibers 1 and 8 may be 90 °, but may be preferably less than 90 °. For example, when [theta] is 90 [deg.], The light reflected and returned by the fused surfaces (connection surfaces) may cause problems such as unstable laser oscillation and increased spurs. . In such a case, θ is preferably 65 to 87 °.

  After alignment, both end faces are butted together and the silica glass fiber 8 is heated. Heating is preferably performed on a portion typically 100 μm or more away from the end face of the silica glass fiber 8. By such heating, heat from the quartz glass fiber 8 is gradually transmitted to the optical fiber 1 and both end faces are fused. At that time, in the optical fiber 1, the hollow portion 2 is gradually crushed by the surface tension, and gradually narrows as it approaches the fusion end face as shown, and further disappears at or near the fusion end face. In the following description, a region where the hollow portion 2 is narrowed and disappears in this way is referred to as a pore gradually decreasing portion 12. The length of the hole gradually decreasing portion 12 in the fiber axis direction is preferably 50 μm or more. If it is 50 μm or less, there is a possibility that light leaks at the hole gradually decreasing portion 12 due to a sudden change in the mode field diameter, and the connection loss increases. More preferably, it is 100 μm or more.

  When the heating is performed at a position closer to the end face of 100 μm or less, the pore gradually decreasing portion 12 is not formed, or the length of the pore gradually decreasing portion 12 is not sufficiently long, or a noticeable softening flow at the end surface of the optical fiber 1 or There is a risk that volatilization will occur and the end faces that are brought into contact with each other cannot be fused. More preferably, heating is performed at a position separated by 200 μm or more. However, if heating is performed at a position distant from 1000 μm or more, the fusion strength may be lowered, and heating is preferably performed at a position of 800 μm or less.

  The heating method is not particularly specified, and heating by discharge generated between electrodes arranged opposite to each other with the quartz optical fiber 8 interposed therebetween may be used, heating by laser, heating by a hydrogen burner, heating by heater resistance. Etc. may be used. Further, when it is desired to control the shape and length of the hole gradually decreasing portion 12, the heating portion may be moved.

The figure shows the mode field diameter at each position of the optical fiber 1 and the silica-based optical fiber 8 in the fused state. The mode field diameter of the optical fiber 1 is different from that of the silica-based optical fiber 8 in the hole gradually decreasing portion 12. It gradually expands toward the fusion end face. Further, in order to reduce the fusion loss by eliminating voids on the fusion surface, the mode field diameter (MFD _out ) at the wavelength 1550 nm at the connection end surface of the optical fiber 1 and the wavelength 1550 nm of the silica-based optical fiber 8 are used. Between the mode field diameter (MFD _SMF ) at
｜ MFD _out −MFD _SMF ｜ ≦ 2.0μm
It is preferable that this relationship is established. The difference between the two mode field diameters is more preferably 1.0 μm or less.

If it is difficult to set | MFD _out −MFD _SMF | ≦ 2.0 μm, the distance between the optical fiber 1 and the silica-based optical fiber 8 is closer to MFD _out than MFD _SMF , as shown in FIG. A second silica-based fiber 13 having a mode feel diameter (MDF _Si ) at a wavelength of 1550 nm is selected and interposed between the optical fiber 1 and the silica-based optical fiber 8, and the optical fiber 1 and the second silica-based optical fiber. 13 is fused and connected in the same manner as described above, and the second silica-based optical fiber 13 and the silica-based optical fiber 8 are further connected by so-called TEC (Terminal Expansion Core) fusion. Note that reference numeral 14 in the figure denotes a core of the second silica-based optical fiber 13. In this case as well, in order to reduce the connection loss,
｜ MFD _out −MFD _Si | ≦ 2.0 μm
It is preferable that The difference between the two mode field diameters is more preferably 1.0 μm or less.

MFD _out is determined by the difference in refractive index between n ₁ and n ₂ and the diameter (d _c ) of the central axis portion 6 of the optical fiber 1 if the hole gradually decreasing portion 12 is sufficiently long. Accordingly, the diameter of the central axis portion of the optical fiber 1 (d _c) is within the range defined above, and, as MFD _out which is magnified by the holes tapering portion 12 meets the condition is suitably designed It is preferable. Moreover, it is preferable that n _{1 is} also appropriately designed so that the MFD _out expanded by the hole gradually decreasing portion 12 satisfies the above-described conditions.

  The present invention also includes a connection body of the optical fiber 1 and the silica-based optical fiber 8 that are fusion-spliced in this way, or a connection body that further has a second silica-based optical fiber 13 interposed therebetween.

(Raw material preparation)
The raw materials were prepared and mixed so as to have a composition represented by mol% in the columns from Bi ₂ O ₃ to CeO _{2 in} Table 1 to prepare 250 g of prepared raw materials 1 to 4. This prepared raw material is put into a platinum crucible and melted by holding at 1000 ° C. for 2 hours in an air atmosphere. The obtained molten glass is poured into a plate shape, and then kept at 370 ° C. for 4 hours and then cooled to room temperature. went. Then, a glass plate having a thickness of 1 mm and a size of 20 mm × 20 mm is prepared from the obtained glass, and a refractive index n with respect to light having a wavelength of 1550 nm is set to a model 2010 manufactured by Metricon Co., Ltd. Measurement was performed using a prism coupler. The measurement results are shown in Table 1.

Example 1
Using the mixed raw material 2, the molten glass obtained in the same manner as described above is poured into a SUS310S tea tube mold (cylindrical mold having a bottom surface) having an inner diameter of 10 mm and a height of 180 mm, followed by slow cooling. Thus, a glass rod A was obtained. This glass rod A was redrawn at 418 ° C., that is, heated and stretched to obtain a glass rod A having a diameter of 4.5 mm.

  Also, using the blended raw material 1, the molten glass obtained in the same manner was poured into a SUS310S tea cylinder mold (cylindrical mold having a bottom surface) having an inner diameter of 28 mm and a height of 120 mm, and slowly cooled. A glass rod B was obtained. A glass tube A was obtained by forming a through-hole having an inner diameter of 8 mm at the center of the glass rod B using an ultrasonic machine USM-3CNC manufactured by Prosonic Corporation.

  Next, one end of the glass tube A is sealed, the sealing part is turned down, the glass rod A is inserted, and the space between the glass rod A and the glass tube A is heated to 444 ° C. while reducing the pressure at −60 kPa. The glass rod A and the glass tube A were simultaneously redrawn to obtain a two-layer glass rod A having a diameter of 14.5 mm. At this time, the diameter of the glass rod A was 2.5 mm.

  Next, six through holes having an inner diameter of 2.5 mm were formed in the double-layer glass rod A using the ultrasonic machine. The center axis of these six through holes was 3.5 mm away from the center axis of the double-layer glass rod A, and the distance between adjacent holes was 1 mm. And the double-layer glass rod A in which the through-hole was formed was redrawn at 425 degreeC, and the double-layer glass rod B with a diameter of 3.5 mm was obtained.

  In addition, two glass rods B having an outer diameter of 15 mm and a height of 130 mm are prepared using the blended raw material 1, and a through-hole having a diameter of 4 mm is formed in the center of the glass rod B using the ultrasonic processing machine. Glass tube B and glass tube C with a through hole having a diameter of 6 mm were produced.

  Next, one end of the two-layer glass rod B was sealed, put in the glass tube B with the sealing portion down, and then the lower end of the glass tube B was sealed. Then, the space between the two-layer glass rod B and the glass tube B is depressurized at −60 kPa, and the through hole of the two-layer glass rod B is heated to 425 ° C. while being pressurized and expanded at 30 to 40 kPa. The two-layer glass rod B and the glass tube B were simultaneously redrawn to obtain a primary preform having a diameter of 4.7 mm.

  Here, pressurizing and expanding the through-holes means the following. That is, although the trace of the outer periphery of the heat-stretched double-layer glass rod B is observed in the cross section of the primary preform, the ratio of the diameter of the peripheral trace and the diameter of the double-layer glass rod B before being heat-stretched is squared. The value β obtained by dividing the scale ratio of the cross-sectional area perpendicular to the central axis of the through hole of the primary preform by α is larger than 1, whereas the area scale ratio α is less than 1. Say. In the above primary preform, β was 3.9.

  One end of this primary preform was sealed, put into the glass tube C with the sealing portion down, and then the lower end of the glass tube C was sealed. Then, the space between the primary preform and the glass tube C is depressurized at −60 kPa, and the primary preform is heated to 425 ° C. while being expanded by pressurizing the through holes of the primary preform at 30 to 40 kPa. The reforming and the glass tube C were simultaneously redrawn to obtain a preform having a diameter of 5 mm. In this case, β was 11.7.

Then, the preform was drawn under the conditions of a drawing temperature of 425 ° C. and a drawing speed of 6 mm / min while pressurizing the through hole at 5 kPa, and the diameter d _c of the central axis portion was 1.6 μm and the diameter d of the inscribed circle C 1 was An optical fiber having a diameter of 2.8 μm, a circumscribed circle diameter d ′ of 17.3 μm, an MFD of 1.9 μm, a fiber diameter of 125 μm, and a support portion thickness of 0.25 μm was obtained.

The group velocity dispersion GVD of the present optical fiber with respect to light having a wavelength of 1550 nm was measured by homodyne interferometry using an Agilent 81910A and found to be −10 ± 20 ps / nm / km. Further, γ was measured by four-wave mixing as follows. That is, an optical fiber having a length of 1 m is prepared, light having a wavelength of 1550 nm is used as pump light, and signals having wavelengths away from the pump light wavelength in increments of 0.5 nm such as 1549.5 nm, 1549 nm, and 1548.5 nm. The light was simultaneously incident on the optical fiber 2 through the coupler, the output was observed with an optical spectrum analyzer, and the ratio r between the idler light and the signal light at this time was measured. Γ was calculated from r thus obtained and the formula (1), which was 700 ± 90 W ⁻¹ km ⁻¹ . In the equation (1), P is an average pump power passing through the optical fiber, and z is the length of the optical fiber.
r = (γ × P × z) ² (1)

  The above optical fiber was cut into a length of 1000 mm, and an optical fiber having an end face θ of 90 ° was prepared. A quartz glass fiber having an MFD of 10.5 μm, a θ of 90 °, and a length of 1000 mm was prepared. Then, the optical fiber and the silica-based optical fiber are once subjected to power alignment with an interval of 1 μm, and after alignment, the distance between both ends is separated by 15 μm, and then the discharge current = 12 mA, the discharge time = 0.01 seconds, The discharge was performed under the conditions of the number of times = 220 times and the discharge pause time = 0.05 seconds, and both were fusion-spliced. As the discharge electrode, a tungsten electrode having a bottom cone diameter of 1 mm and a height of 1.2 mm is used as the discharge electrode, the distance between the opposite electrode tips is 0.6 mm, and the straight line connecting the discharge electrode tips is quartz glass. The distance from the intersecting point perpendicular to the fiber axis to the end face to be fused was 285 μm.

  When the connected surfaces were visually observed, neither noticeable softening flow nor volatilization was observed, and the end surfaces that were abutted were well fused. In addition, the connection strength was sufficiently high, and the two fibers could not be separated by pulling them by hand. The connection loss in this connection state was estimated to be 4.6 dB.

Next, when the connected optical fiber and the silica-based optical fiber were peeled off and the end face of the optical fiber was observed, it was confirmed that the hollow portion had disappeared cleanly. FIG. 5 shows a micrograph of a cross section along the central axis of the optical fiber at this time. Further, the MFD _{OUT of the} pore gradually decreasing portion was measured and found to be 8.9 μm.

  In addition, the above optical fiber and silica optical fiber are cut again so that θ = 90 °, and after power alignment with an interval of 1 μm, the interval is as close as possible to 0 μm, and connection loss is caused by so-called butt joint connection. Was estimated to be 10.2 dB.

  Therefore, the splice loss of the present invention has decreased the connection loss by 5.6 dB.

(Example 2)
An optical fiber is produced in the same manner using the blended raw material 3 shown in Table 1 instead of the blended raw material 2 in Example 1. The diameter _{d c} of the center axis portion 1.0 .mu.m, the diameter d of the inscribed circle C1 is the same as the optical fiber of Example 1 except that a 1.9 .mu.m.

Next, “H1980” (MFD 6.0 μm) manufactured by Corning is used as the silica-based optical fiber, and the optical fiber is fused and connected in the same manner as in Example 1. The MFD _OUT of the optical fiber is 6.7 μm. Then, when the connection loss is estimated in the same manner as in the first embodiment, the decrease in connection loss is 7.1 dB.

(Example 3)
An optical fiber is prepared in the same manner using the preparation raw material 4 shown in Table 1 instead of the preparation raw material 2 of Example 1 and the preparation raw material 3 instead of the preparation raw material 1. The diameter _{d c} of the center axis portion 0.7 [mu] m, the diameter d of the inscribed circle C1 is is the same as the optical fiber of Example 1 except that the 1.3 .mu.m.

Next, “UHNA4” (MFD 4.0 μm) manufactured by Nufern is used as the silica-based optical fiber, and the optical fiber is fusion-spliced in the same manner as in Example 1. The MFD _OUT of the optical fiber is 4.1 μm. Then, when the connection loss is estimated in the same manner as in the first embodiment, the reduction in the connection loss is 5.8 dB.

  By providing an optical fiber having a large γ and a small D in a form connected to a silica-based optical fiber, the optical fiber of the present invention may be used for 2R and 3R in ultrahigh-speed optical processing, for example.

It is a figure which shows an example of the optical fiber of this invention, and is a figure which shows typically the cross section orthogonal to a central axis. It is an enlarged view inside the hollow part of the optical fiber 1 of FIG. It is a figure which shows an example of the connection body of the optical fiber of this invention, and is a schematic diagram which shows the cross section along a central axis. It is a figure which shows the other example of the connection body of the optical fiber of this invention, and is a schematic diagram which shows the cross section along a central axis. It is the microscope picture which image | photographed the side surface along the central axis of the optical fiber connection body produced in the Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical fiber 2 Hollow part 3 Hollow glass fiber 4 Optical transmission part 5 Support part 6 Center axis part 8 Silica-type optical fiber 12 Hole decreasing part 13 2nd silica-type optical fiber

Claims (11)

A hollow glass fiber having a hollow part, a light transmission part having a central axis part and a concentric peripheral part on a central axis of the hollow part, and extending radially from a plurality of locations on the outer peripheral surface of the light transmission part A plate-like support part reaching the inner peripheral surface of the hollow glass fiber is integrally formed, and the refractive index (n ₁ ) at a wavelength of 1550 nm of the central axis part of the optical transmission part is the wavelength of the surrounding part. An optical fiber having a refractive index (n ₂ ) greater than 1550 nm. The (n ₁ ) and (n ₂ ) in the optical transmission unit are 0.0005 or more and 0.2 or less in [(n ₁ −n ₂ ) / n ₁ ] value. Optical fiber.   3. The optical fiber according to claim 1, wherein a peripheral portion of the optical transmission portion is formed of a plurality of concentric layers, and a refractive index at 1550 nm is maximum at a central axis portion.   The optical fiber according to any one of claims 1 to 3, wherein a diameter (d) of a circle inscribed in the hollow portion is 0.2 to 10 µm in a cross section perpendicular to the central axis. 5. The diameter according to claim 1, wherein a diameter of a circle circumscribing the hollow portion is not less than (1 + 2 ^1/2 ) times the diameter (d) in a cross section orthogonal to the central axis. Optical fiber.   The optical fiber according to any one of claims 1 to 5, wherein a thickness of the support portion is 1.5 µm or less in a cross section orthogonal to the central axis. The glass material forming the hollow glass fiber, the optical transmission part and the support part is expressed in terms of mol% based on oxide, Bi ₂ O ₃ : 40 to 75%, B ₂ O ₃ : 12 to 45%, Ga ₂ O _{3 : 1~20%, In 2 O 3} : 1~20%, ZnO: 0~20%, BaO: 0~15%, SiO 2 + Al 2 O 3 + GeO 2: 0~15%, MgO + CaO + SrO: 0~15% SnO ₂ + TeO ₂ + TiO ₂ + ZrO ₂ + Ta ₂ O ₅ + Y ₂ O ₃ + WO ₃ : 0 to 10%, CeO ₂ : 0 to 5%, and Ga ₂ O ₃ + In ₂ O ₃ + ZnO is 5 % Or more, The optical fiber according to any one of claims 1 to 6. The glass material forming the hollow glass fiber, the optical transmission part and the support part is expressed in terms of mol% based on oxide, Bi ₂ O ₃ : 49 to 67%, B ₂ O ₃ : 13 to 30%, Ga ₂ O ₃ 5 to 13%, In ₂ O ₃ : 0.5 to 8%, ZnO: 2 to 8%, BaO: 0 to 6%, CeO ₂ : 0.1 to 1%. 7. The optical fiber according to 7. The amount of Bi ₂ O _{3 in} the central axis portion of the optical transmission portion is 56% or more and 1% or more higher than the amount of Bi ₂ O _{3 in the} surrounding portion in terms of oxide-based mol%. _{B 2 O 3 + Ga 2 O} 3) amount of the peripheral portion _{(B 2 O 3 + Ga 2} O 3) amount the optical fiber according to claim 8, wherein the less than 1% than.   The end face of the optical fiber according to any one of claims 1 to 9, wherein the hollow glass fiber, the optical transmission part, and the support part are integrally formed of glass having a glass transition point of 500 ° C or lower, and the silica-based optical fiber. The end face of the optical fiber is brought into contact with each other, the vicinity of the end face of the silica-based optical fiber is heated to fuse the two optical fibers together, and the hollow portion of the optical fiber is gradually narrowed as it approaches the end face to which the fusion is connected. An optical fiber fusion splicing method characterized in that it is eliminated at the end face.   A hollow glass fiber, an optical transmission part, and a support part are integrally formed with glass having a glass transition point of 500 ° C or lower, and one or both end faces of the optical fiber according to any one of claims 1 to 9, and quartz An optical fiber connector in which the end surface of the system optical fiber is fusion-connected, and gradually disappears at the end surface as the hollow portion of the optical fiber approaches the end surface that has been fusion-connected, The optical fiber connector is characterized in that the difference in mode field diameter at a wavelength of 1550 nm at the end face where both optical fibers are fusion-spliced is 2.0 μm or less in absolute value.

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