JP2005049796A - Optical fiber for ultraviolet ray transmission, method for manufacturing the same, and method for inspecting optical fiber preform - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber for UV transmission less liable to cause defects during use even when it is not treated in the atmosphere of hydrogen. <P>SOLUTION: The optical fiber for UV transmission is obtained by heating and drawing an optical fiber preform 1 comprising a core 11 and a clad 12. The core 11 comprises silica glass which contains at least OH groups and/or F in a total amount of 100-5,000 ppm and satisfies the following requirement (I) or (II) when irradiated with ArF excimer laser of 12 mJ/cm<SP>2</SP>at ordinary temperature and pressure for 10<SP>7</SP>shots: (I) neither E' center nor NBOHC is formed, (II) both E' center and NBOHC are formed. The optical fiber further has a coating layer, on the periphery, obtained by coating the periphery with molten Al or Al alloy and carrying out solidification. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to an optical fiber for transmitting ultraviolet light having a wavelength of 170 to 350 nm, in particular, N ₂ excimer laser (wavelength: 337 nm), XeCl excimer laser (wavelength: 308 nm), KrF excimer laser (wavelength: 248 nm), KrCl excimer. Laser (wavelength: 222 nm), ArF excimer laser (wavelength: 193 nm), Xe ₂ excimer laser (wavelength: 172 nm), Nd: YAG fourth harmonic laser (wavelength: 265 nm), Nd: YAG fifth harmonic laser (wavelength) The present invention relates to an optical fiber for ultraviolet transmission capable of suitably transmitting ultraviolet light from a laser beam having a high energy density such as 212 nm) and a deuterium lamp, a manufacturing method thereof, and an inspection method of an optical fiber preform used in the manufacturing.

  Conventionally, as an optical fiber for ultraviolet transmission, an optical fiber having a core / cladding structure mainly composed of silica glass and having a refractive index lower than that of the core has been used. However, when ultraviolet rays are transmitted to an optical fiber having silica glass doped with germanium oxide or the like as a core, defects are generated, resulting in large absorption and a significant decrease in transmittance. Therefore, it is inappropriate as an optical fiber for ultraviolet transmission. It is known that

  On the other hand, it has been shown that an optical fiber using silica glass containing OH groups and / or F (fluorine) as a core is excellent in radiation resistance (Patent Document 1). For this reason, a “pure silica core optical fiber” having pure silica as a core portion and fluorine-doped silica glass as a cladding portion has begun to be used as an optical fiber for ultraviolet transmission. However, even in the case of the pure silica core optical fiber, when irradiating and transmitting ultraviolet rays (radiation) having a high energy level, particularly KrF excimer laser or ArF excimer laser, light energy (hν) acts, It is known that radicals are generated according to the following formula (1) in the glass of an optical fiber, and the transmittance decreases.

Si-O-Si + hv → Si ^* + ^* O-Si (1)

  As a method for reducing the above-described decrease in transmittance, the optical fiber is exposed to a hydrogen-containing atmosphere, and hydrogen molecules are taken into the optical fiber to cause defects in the optical fiber to react with the hydrogen molecules, thereby causing the molecules of the optical fiber to react. It has been proposed that the defects in the structure can be filled and converted into a bond having no absorption in the ultraviolet region (eg, Si—OH or Si—H) (Patent Documents 2 to 5).

  However, since the hydrogen does not exist in a stable state as an OH group or the like, most of the hydrogen exists as hydrogen molecules or hydrogen ions. Recently, it has been found that the above method hardly contributes to suppressing the decrease in transmittance of the optical fiber.

  Furthermore, as a result of research and development, inventions in which ultraviolet rays are incident on an optical fiber while raising the temperature in a hydrogen atmosphere, or radiation is applied to an optical fiber that has been heated in a hydrogen atmosphere (Patent Documents 6 and 7). In addition, there is an invention (Patent Document 8) in which a metal is coated on the outer periphery of an optical fiber and a hydrogen treatment is performed under high temperature and pressure.

However, according to such a manufacturing method, processing in a hydrogen atmosphere is essential, and the manufacturing process becomes complicated. For this reason, new equipment is required, and there is a problem that the cost of the optical fiber to be obtained, and further a product (such as a bundle light guide) using the same increases.
JP-A-5-147966 JP 60-90853 A JP-A-6-34830 JP-A-6-56457 JP-A-12-226223 JP-A-12-86272 JP-A-12-86273 JP-A-9-309742

  SUMMARY OF THE INVENTION An object of the present invention is to provide an optical fiber for ultraviolet transmission that is less prone to defects during use without being treated in a hydrogen atmosphere. In addition, it is an object of the present invention to provide a method for manufacturing an optical fiber for ultraviolet transmission as described above and a method for inspecting an optical fiber preform used for the manufacturing.

  The optical fiber for ultraviolet transmission of the present invention is as follows. Moreover, the manufacturing method of the optical fiber for ultraviolet transmission of this invention and the inspection method of an optical fiber preform are as follows.

(1) An optical fiber for ultraviolet transmission formed by heating and drawing an optical fiber preform having a core and a cladding,
The core of the optical fiber preform contains at least 100 to 5000 ppm of OH groups and / or F, and when irradiated with 10 ⁷ shots of ArF excimer laser at 12 mJ / cm ² at normal temperature and normal pressure (I ) Or (II), the silica glass satisfying the requirements,
An optical fiber for ultraviolet transmission, further comprising a coating layer formed by coating and solidifying molten aluminum or an alloy thereof on the outer periphery of the optical fiber.
(I) No E ′ center is generated, and NBOHC is not generated.
(II) E ′ center is generated and NBOHC is generated.

(2) An inspection method of an optical fiber preform having a core and a cladding,
A step of irradiating the core with 12 mJ / cm ² of ArF excimer laser at normal temperature and normal pressure for 10 ⁷ shots, and a step of measuring the presence or absence of E ′ center and NBOHC in the core after the step An optical fiber preform inspection method for determining that an optical fiber preform satisfying the following requirement (I) or (II) in the measurement is acceptable.
(I) No E ′ center is generated, and NBOHC is not generated.
(II) E ′ center is generated and NBOHC is generated.

(3) An optical fiber preform determined to be acceptable by the inspection method described in (2) above, and the core of the optical fiber preform has at least OH groups and / or F in a total amount of 100 to 5000 ppm. A process of forming an optical fiber by heating and drawing an optical fiber preform made of silica glass containing;
Forming a coating layer on the outer periphery of the optical fiber by coating and solidifying molten aluminum or an alloy thereof;
A method for producing an optical fiber for ultraviolet transmission, comprising:

In the optical fiber of the present invention (particularly its core), defects newly generated by ultraviolet irradiation can be effectively eliminated by hydrogen. In the optical fiber of the present invention, it becomes possible to hold hydrogen released when molten aluminum (or an alloy thereof) is coated on the outer periphery of the optical fiber and cooled and solidified in the optical fiber (particularly the core). Therefore, the optical fiber of the present invention has an ultraviolet ray having a wavelength of 170 to 350 nm, particularly an N ₂ excimer laser (wavelength: 337 nm), an XeCl excimer laser (wavelength: 308 nm), a KrF excimer laser (wavelength: 248 nm), a KrCl excimer laser ( Wavelength: 222 nm), ArF excimer laser (wavelength: 193 nm), Xe ₂ excimer laser (wavelength: 172 nm), Nd: YAG fourth harmonic laser (wavelength: 265 nm), Nd: YAG fifth harmonic laser (wavelength: 212 nm) ) And the like can be suitably transmitted, and are less likely to deteriorate (the ultraviolet transmittance is less likely to decrease over the long term). Since the optical fiber of the present invention is excellent in ultraviolet resistance, it can be applied to a bundle light guide for measuring light in the ultraviolet region.

  The optical fiber for ultraviolet transmission of the present invention (hereinafter also simply referred to as “optical fiber”) is similar to a conventional optical fiber in that an optical fiber preform having a core / cladding structure is heated and drawn. It is characterized in that a specific silica glass is used for the core of the optical fiber preform, and that the optical fiber after drawing further has a coating layer formed by solidifying molten aluminum or an alloy thereof. The core / cladding structure exists in both the optical fiber preform before drawing and the optical fiber after drawing, but the present invention focuses on the core in the optical fiber preform before drawing.

  The effect of the optical fiber of the present invention to achieve the above-mentioned problem is not known in detail. However, the present inventors consider as follows. An optical fiber formed by drawing a base material having a core having the characteristics described later is in a state in which hydrogen is more easily retained than a conventional optical fiber, so that it does not have to be treated in a hydrogen atmosphere under high temperature and high pressure. It is also possible to fill in the defects of the optical fiber. Therefore, hydrogen released when the molten aluminum coated on the outer periphery of the optical fiber is cooled and solidified can be held in the optical fiber. Further, the retained hydrogen is difficult to escape to the outside due to the solidified aluminum or its alloy (coating layer), so that the ultraviolet ray permeability is unlikely to deteriorate for a long time. Furthermore, since the conventional optical fiber is subjected to rapid cooling at the time of drawing, there are many drawing defects, and these defects have become precursors of defects that prevent the transmission of ultraviolet rays, but by coating molten aluminum, Since the cooling rate of the optical fiber is reduced, the occurrence of drawing defects is suppressed.

  The optical fiber preform used for producing the optical fiber for ultraviolet transmission of the present invention is silica glass having a core-clad structure. FIG. 1 is a diagram schematically showing a cross section of an optical fiber preform used in the present invention. The optical fiber preform 1 has a core 11 and a cladding 12 on the outer periphery thereof. One of the features of the present invention resides in the core 11 of the optical fiber preform 1. The clad 12 is formed on the outer periphery of the core 11 and is made of silica glass having a refractive index lower than that of the core 11. As the optical fiber preform 1 (particularly the material of the core 11) used in the present invention, pure quartz glass that is not intentionally doped with germanium, phosphorus, boron, metal, or the like is preferable. Specifically, silica glass in which impurities other than OH groups and / or F (fluorine) described below are preferably 100 ppb or less, more preferably 10 ppb or less is used.

  The core 11 of the optical fiber preform 1 used in the present invention contains at least OH groups and / or F (fluorine) in a total amount of 100 to 5000 ppm. When the content is less than 100 ppm, there is an inconvenience that the transmittance of ultraviolet rays tends to be lowered in the obtained optical fiber. When the content is more than 5000 ppm, it becomes difficult to produce the optical fiber preform 1 and more than the core. However, it is difficult to reduce the refractive index of the cladding. When both OH groups and F are contained, the total amount of both is 100 to 5000 ppm, preferably 300 to 3000 ppm. The reason is that if it exceeds 5000 ppm, the production becomes difficult, and if it is less than 100 ppm, it is difficult to exhibit ultraviolet resistance. When F is not contained, the content of OH groups is preferably from 100 to 2000 ppm, more preferably from 300 to 1500 ppm from the viewpoint that the production of the optical fiber preform 1 becomes easy. In the case where no OH group is contained, the content of F is preferably 100 to 4000 ppm, more preferably 300 to 3000 ppm from the viewpoint of making the refractive index of the clad 12 easier than the core 11. Here, the content is a weight ratio in the core 11 of the optical fiber preform 1. In the above, the meaning of “when it does not contain” F or OH group will be explained in the quantitative method described later.

  The OH group content in the core 11 of the optical fiber preform 1 can be determined by the FT-IR method (Fourier transform infrared spectroscopy). Specifically, the core 11 is measured by an infrared spectrophotometer, and absorption at a wavelength of 2.7 μm is performed according to the method described in JP Wiliams et. Al., Ceramic Bulletin, 55 (5), pp. 524, 1976. The OH group content can be determined from the peak. When no OH group can be detected by this method, the core 11 is assumed to contain no OH group.

  The content of F in the core 11 of the optical fiber preform 1 can be obtained by an ion selective electrode analysis method. Specifically, the method is described in Journal of the Chemical Society of Japan, 1972 (2), pp.350. That is, an aqueous solution in which concentrated hydrochloric acid having a concentration of about 35% and water are mixed at a volume ratio of 1: 1 is added to a melt obtained by heating and melting a measurement sample together with anhydrous sodium carbonate to obtain a sample solution. The electromotive force of the sample solution is measured with a radiometer using a fluoride ion selective electrode and a reference electrode (No. 945-220 and No. 945-468, manufactured by Radiometer Trading Co., Ltd.). The content of F in the core 11 is obtained from the electromotive force using a calibration curve obtained in advance using a fluoride ion standard solution. When F cannot be detected by this method, the core 11 is assumed to be “contains no F”.

  Control of the content of OH groups and F contained in the core 11 of the optical fiber preform 1 is known. For example, in the direct method or the VAD method known as a manufacturing method of an optical fiber preform, a silicon compound such as silicon tetrachloride, oxygen and hydrogen are used as raw materials, and a fluorine compound such as silicon tetrafluoride is used as the raw material. Furthermore, the content of OH groups and F in the obtained optical fiber preform can be controlled by adding the amount in a prescribed manner.

The core 11 of the optical fiber preform 1 used in the present invention has the following two defects that occur when an ArF excimer laser of 12 mJ / cm ² is irradiated at room temperature and normal pressure for 10 ⁷ shots: E ′ center and NBOHC (non- It is also characterized by the presence or absence of non-bridging oxygen hole centers. In the silica glass that can be used as the core 11 of the optical fiber preform 1 of the present invention, there are roughly two types of defect generation modes. The first mode is a mode in which no E ′ center is generated and NBOHC is not generated. The second mode is a mode in which the E ′ center is generated and NBOHC is generated. The optical fiber preform 1 having the core 11 made of silica glass according to the first aspect has a strong glass structure, and the optical fiber preform having the core 11 made of silica glass according to the second aspect. The present inventors presume that the material 1 can be an excellent optical fiber preform because defects caused by ultraviolet irradiation have a glass structure that easily disappears due to hydrogen.

  Thus, paying attention to the presence / absence of generation of defects in the core 11 of the optical fiber preform 1 is a technical idea unique to the present invention that is not present in the prior art. Conventionally, an optical fiber or product after drawing has been inspected to determine whether it can be used for ultraviolet transmission. However, it takes a lot of time and money to make a pass / fail decision. On the other hand, in the present invention, it can be determined whether or not the final product is suitable for ultraviolet transmission by inspection after manufacturing the optical fiber preform 1 before drawing, so that the yield and production efficiency in the subsequent steps are significantly improved.

As used herein, "an ArF excimer laser room temperature, when 10 ⁷ shots under normal pressure", an electromagnetic wave with a wavelength of 193 nm, about 25 ° C., about 1 atm, at 50 Hz, which means that irradiation ¹⁰⁷ shots . The technical significance of irradiating the ArF excimer laser in this way is to make it easy to generate defects in order to detect the presence / absence of the generation of the two defects with high sensitivity. Therefore, it is within the scope of implementation of the present invention to correct the actual use conditions to the extent that the inspection result for the presence / absence of defects is not changed.

  The above defects will be described. The E 'center is a kind of defect having an unpaired electron and has a chemical structure as shown in FIG. In general, it is considered that the E ′ center is generated by breaking the bond between silicon and oxygen in the optical fiber by high energy ultraviolet light having a peak at a wavelength of 215 nm. On the other hand, NBOHC is a kind of defect having holes, and has a chemical structure as shown in FIG.

In the present invention, whether or not the E ′ center and NBOHC are generated is determined by ESR (electron spin resonance method) measurement one day after the electromagnetic wave (ArF excimer laser) irradiation described above. Detailed conditions for ESR measurement can be appropriately set by those skilled in the art. Since the peak indicating the presence of both defects appears in the magnetic field region of 3410 to 3510 gauss, measurement is performed so as to sweep the magnetic field region including that region. Specific means for obtaining the peak due to each of the E ′ center and NBOHC from the ESR spectrum obtained by the ESR measurement is also known. If the value of the peak intensity of the peak due to the E ′ center (obtained by integrating twice) is 1 × 10 ¹³ spins / g or more, it is determined that the E ′ center is generated, otherwise It is determined that the E ′ center has not been generated. If the peak intensity value of the peak due to NBOHC (obtained by integrating twice) is 1 × 10 ¹⁴ spins / g or more, it is determined that NBOHC is generated, otherwise NBOHC is generated. Judge that it is not. As an apparatus which can perform such a measurement, ESP-300E (made by BRUKER) etc. are mentioned, for example.

“A silica glass in which E ′ center does not form and NBOHC does not form when irradiated with an ArF excimer laser of 12 mJ / cm ² at normal temperature and normal pressure” (the first aspect) is, for example, in the direct method It is obtained by making the silica glass oxygen-excess than SiO ₂ ( _x in SiO _{2 + x} is 0 <x <1).

The above-mentioned “silica glass produced by the E ′ center and produced by NBOHC when irradiated with a 12 mJ / cm ² ArF excimer laser at room temperature and normal pressure” (the second aspect) is, for example, in the direct method It is obtained by reducing the reducing defects by causing the silica glass to have oxygen excess as compared with SiO ₂ and further performing an acid heat treatment at 600 to 1500 ° C.

  In the optical fiber preform 1 used in the present invention, a clad 12 is provided on the outer periphery of the core 11 described above. The clad 12 may be silica glass having a refractive index lower than that of the core 11. As a means for lowering the refractive index than that of the core 11, a known method, a method of doping an element for reducing the refractive index (eg, fluorine or boron) into a portion of the silica glass that becomes the clad 12 using a known manufacturing method, an apparatus, or the like. It is done.

  The optical fiber preform 1 used in the present invention may be the same as a known optical fiber preform except that the core 11 and the cladding 12 described above are provided. The optical fiber preform 1 may be manufactured from, for example, chemically synthesized synthetic silica glass or fused silica glass obtained by melting natural quartz powder. Among them, it is preferable to use a synthetic silica glass having a small impurity content. Synthetic silica glass can be produced by a known method, and specific examples of the production method include VAD method, MCVD method, plasma CVD method, OVD method, and direct method.

  A method of obtaining an optical fiber by drawing the optical fiber preform 1 may be a conventionally known method, an example of which will be described later. However, the optical fiber of the present invention is characterized in that it further has a coating layer formed by solidifying molten aluminum or an alloy thereof on its outer periphery. The means for melting aluminum or an alloy thereof can be a known means, and examples thereof include high frequency induction heating and resistance heating. In general, melting takes place in air or preferably in a hydrogen-containing atmosphere. The reason is that hydrogen can be obtained from moisture adhering to aluminum or its alloy or moisture from the atmosphere, and hydrogen is supplied to the optical fiber when solidified from a molten state.

  The coating of aluminum or an alloy thereof may be performed by means such as vapor deposition or sputtering, in addition to the means for solidifying the molten aluminum after coating. However, when the film thickness of the coating layer obtained by solidifying after coating molten aluminum or its alloy is measured by cross-sectional observation or the like, it is usually 5 μm or more (usually 5 to 50 μm), and other means (evaporation, It is much thicker than a film obtained by sputtering or the like (normally the film thickness is 1 μm or less).

  The aluminum of the coating layer or its alloy may be a known one. Examples of components other than aluminum in the case of an alloy include silicon, magnesium, copper, or combinations thereof, but are not limited thereto. From the viewpoint of wettability with silica glass, melting point, and the like, an eutectic alloy having an aluminum content of 80% by weight or more is preferable, and aluminum having a purity of 99.9% or more is more preferable. Examples of such aluminum or an alloy thereof include, for example, 1035, 1040, 1045, 1050, 1050A, 1060, 1065, 1070, 1070A, 1080, 1080A, 1085, 1090, 1098, 1100, 1110, indicated by international alloy symbols. 1200, 1200A, 1120, 1230, 1135, 1235, 1435, 1145, 1345, 1445, 1150, 1350 (18), 1350A, 1450, 1260, 1170, 1370, 1175, 1275, 1180, 1185, 1285, 1385, 1188 1190, 1193, 1198, 1199 and the like.

  The aluminum coating on the outer periphery of the optical fiber may be of a thickness that can hold hydrogen of the optical fiber, and is preferably 10 to 80 μm, more preferably 20 to 40 μm. If the coating is thinner than 10 μm, the contribution of hydrogen doping is small, and if it is thicker than 80 μm, the flexibility of the manufactured optical fiber tends to be low.

  Next, an example of a method for producing the optical fiber of the present invention from the optical fiber preform will be described with reference to the drawings. FIG. 2 is a diagram schematically showing a method of manufacturing an optical fiber from an optical fiber preform. This manufacturing method will be described by taking, as an example, a process A in which the optical fiber preform 1 is heated and drawn and molten aluminum 5 (hereinafter, aluminum having a purity of 99.9% is used), but is not limited to this example. ) Is roughly divided into step B. The temperature at which the optical fiber preform 1 is heated in the step A is usually about 2000 to about 2400 ° C. For example, when the optical fiber 2 having an outer diameter of 70 to 2500 μm is manufactured, the drawing temperature is set to 0. Drawing can be performed at 1 to 1000 m / min. Heating can be performed using a heater 91 or the like. Next, in step B, the molten aluminum 5 is held, and the molten aluminum 5 is covered with a dice tank 6 that also serves as a die to adjust the shape. The temperature of the molten aluminum 5 in the die tank 6 may be any temperature that can be melted, and is preferably 670 to 800 ° C. When the temperature is lower than 670 ° C., the film thickness of the molten aluminum 5 is not stable, and characteristics such as the strength of the coated optical fiber 3 tend to vary in the longitudinal direction. If the temperature of the molten aluminum 5 in the die tank 6 is higher than 800 ° C., the optical fiber 2 tends to deteriorate due to the reaction with the molten aluminum 5. The temperature control of the molten aluminum 5 in the die tank 6 is performed by, for example, immersing a thermocouple 7 and monitoring it with a temperature measuring device 8 or measuring the temperature with a non-contact type temperature sensor or the like (not shown). However, there is a method of adjusting the temperature using a heater 92 or the like.

  The molten aluminum 5 on the outer periphery of the optical fiber 2 may be cooled and solidified before being wound on a bobbin (not shown). At this time, the cooling means and the cooling rate are not particularly limited, but it is preferably a slow cooling rate that does not decrease the production efficiency. FIG. 3 is a sectional view perpendicular to the longitudinal direction of the optical fiber of the present invention thus obtained. The optical fiber 3 has a core 31 and a clad 32, and further has the coating layer 4 described above on its outer periphery. The thickness of the coating layer 4 is as described above.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention more concretely, this invention is not limited at all by these description. As described above, OH content, F content, and paramagnetic defect content were measured for various optical fiber preforms obtained by the direct method (Tables 1 and 2). The refractive index of the optical fiber preform was measured with a preform analyzer. In the table, “ΔN” in the column of “cladding” is a difference in refractive index between the core and the cladding of the optical fiber preform (for example, “0.013” in Example 1 indicates the refraction of the cladding). This means that the refractive index is 0.013 smaller than the refractive index of the core.)

These optical fiber preforms are heated to 2150 ° C., drawn at a drawing speed of 60 m / min, and molten aluminum having a purity of 99.99% (referred to as “molten aluminum” in Tables 1 and 2). Covered. Then, it cooled by standing_to_cool and obtained the optical fiber. However, in Comparative Example 3, an ultraviolet curable resin (indicated as “UV resin” in Table 2) was coated instead of the aluminum.
[Evaluation of the obtained optical fiber]
The content of hydrogen molecules in the core of the obtained optical fiber was measured according to the method described in VS Khotimchenko et al., J. Appl. Spectrospec., 46 (1987) 632-635. The ultraviolet resistance of the obtained optical fiber was evaluated by guiding ultraviolet light (215 nm) from a deuterium lamp, measuring the emitted light with an instantaneous multi-photometry system, and calculating the transmittance. The case where the transmittance after light guiding for 10 hours was 70% or more of the initial transmittance was evaluated as having UV resistance (◯), and the case where it was less than 70% was evaluated as having no UV resistance (×). Here, the deuterium lamp is a deuterium lamp (D2 lamp) manufactured by Hamamatsu Photonics, the power source for the light source is C3150, the housing is MC-962A, the lamp is L1314 (window material: fused silica), and 150 W (irradiation light power). : 0.21 mJ / cm ² ) irradiation at 215 nm. For the instantaneous multi-photometry system, an instantaneous spectrophotometer MCPD-1100 manufactured by Otsuka Electronics Co., Ltd. was used.

  Tables 1 and 2 summarize the parameters of each optical fiber (or its base material) manufactured in the examples and comparative examples.

It is a figure which shows typically the cross section of the optical fiber preform | base_material used by this invention. It is a figure which shows typically the method of manufacturing an optical fiber from an optical fiber preform. It is sectional drawing perpendicular | vertical to the longitudinal direction of the optical fiber of this invention. It is a figure which shows typically E 'center of a paramagnetic defect. It is a figure which shows typically NBOHC of a paramagnetic defect.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical fiber base material 11 Core 12 Cladding 16 Unpaired electron 17 Silicon atom 18 Chemical bond 19 Oxygen atom 2 Optical fiber 3 Optical fiber 31 Core 32 Cladding 4 Coating layer 5 Molten aluminum (alloy)
6 tanks 7 thermocouple 8 temperature measuring instrument 91, 92 heater

Claims (3)

An optical fiber for ultraviolet transmission formed by heating and drawing an optical fiber preform having a core and a cladding,
The core of the optical fiber preform contains at least 100 to 5000 ppm of OH groups and / or F, and when irradiated with 10 ⁷ shots of ArF excimer laser at 12 mJ / cm ² at normal temperature and normal pressure (I ) Or (II), the silica glass satisfying the requirements,
An optical fiber for ultraviolet transmission, further comprising a coating layer formed by coating and solidifying molten aluminum or an alloy thereof on the outer periphery of the optical fiber.
(I) No E ′ center is generated, and NBOHC is not generated.
(II) E ′ center is generated and NBOHC is generated. An inspection method for an optical fiber preform having a core and a cladding,
A step of irradiating the core with 12 mJ / cm ² of ArF excimer laser at normal temperature and normal pressure for 10 ⁷ shots, and a step of measuring the presence or absence of E ′ center and NBOHC in the core after the step An optical fiber preform inspection method for determining that an optical fiber preform satisfying the following requirement (I) or (II) in the measurement is acceptable.
(I) No E ′ center is generated, and NBOHC is not generated.
(II) E ′ center is generated and NBOHC is generated. A silica glass that is an optical fiber preform that is determined to be acceptable by the inspection method according to claim 2 and that the core of the optical fiber preform contains at least OH groups and / or F in a total amount of 100 to 5000 ppm. Forming an optical fiber by heating and drawing an optical fiber preform comprising:
Forming a coating layer on the outer periphery of the optical fiber by coating and solidifying molten aluminum or an alloy thereof;
A method for producing an optical fiber for ultraviolet transmission, comprising:

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