SiOH-CONTAINING OPTICAL FIBER PREFORM AND EVALUATION METHOD THEREFOR

Abstract

To provide a Si-OH-containing optical fiber preform which allows production of an optical fiber having low transmission loss caused by OH groups, desirably, an optical fiber having a transmission loss of 0.315 dB / km or less at 1385 nm, as well as an evaluation method therefor.SOLUTION: An optical fiber preform is manufactured through: a step of preparing a core rod comprising a core having a high refractive index and a cladding 1 mainly composed of SiO2 having a refractive index lower than that of the core; a step of producing a soot preform having a cladding 2 by depositing glass fine particles onto the core rod; a sintering step of producing the optical fiber preform by dehydrating and vitrifying the soot preform; and an evaluation step of assessing an estimated optical property value of the optical fiber preform. In the evaluation step, infrared light is transmitted through the cladding 2 of the optical fiber preform in the diameter direction from one end of its outer edge portion, and the infrared light transmitted from the other end is received, and an average OH concentration determined by processing the obtained infrared absorption spectrum is 0.5 ppm or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a SiOH-containing optical fiber preform and a method for evaluating the same.

Background Art

[0002] An optical fiber is composed of a core that confines and propagates light and a cladding having a refractive index lower than that of the core on its outer periphery. An optical fiber for communication is made of silica glass. In its manufacture, first, an optical fiber preform made of silica glass having a core and cladding structure similar to that of the optical fiber and having a larger diameter than the optical fiber is produced, and the optical fiber preform is heated and drawn to reduce its diameter and formed into an optical fiber. In communication using an optical fiber, light in a wavelength band around 1300 nm to 1600 nm is used. However, due to light absorption and light scattering in this wavelength band, the transmission loss characteristics deteriorate.

[0003] Conventionally, after drawing an optical fiber from an optical fiber preform, the transmission loss characteristics are measured by the OTDR method or the like, and only the optical fiber with small and good transmission loss characteristics is screened and used as an optical fiber for communication. In particular, a trace amount of OH groups contained in the optical fiber increases the transmission loss around a wavelength of 1385 nm and has an adverse effect on communication. It is considered that the content of OH groups in the optical fiber preform is distributed in the longitudinal direction of the preform. Therefore, screening has been performed by actually measuring the transmission loss at a wavelength of 1385 nm of the optical fiber obtained by drawing the optical fiber preform.

Summary of the Invention

Problems to be Solved by the Invention

[0004] An object of the present invention is to provide a Si-OH-containing optical fiber preform capable of producing an optical fiber with low transmission loss caused by OH groups, preferably an optical fiber having a transmission loss at 1385 nm of 0.315 dB / km or less, without performing pre-drawing screening, and to provide an evaluation method therefor.

Means for Solving the Problems

[0005] The Si-OH-containing optical fiber preform of the present invention comprises a core rod preparation step of preparing a core rod having a core with a high refractive index and a cladding 1 mainly composed of SiO having a lower refractive index than this, a deposition step of depositing glass fine particles on the core rod to produce a soot preform having a cladding 2, a sintering step of dehydrating and vitrifying the soot preform to produce an optical fiber preform, and an evaluation step of evaluating the estimated optical characteristics of the optical fiber preform. The optical fiber preform is produced through these steps. In the evaluation step, infrared light is transmitted in the diameter direction from one end of the outer edge of the cladding 2 of the optical fiber preform, the infrared light transmitted from the other end is received, the obtained infrared light absorption spectrum is processed to obtain the OH concentration, and the average OH concentration of the optical fiber preform is 0.5 (ppm) or less. 2 In addition, it is preferable that the transmission loss of the optical fiber obtained by drawing the optical fiber preform evaluated in claim 1 at a wavelength of 1385 nm is less than 0.315 [dB / km]. The optical fiber preform is obtained by dehydrating the soot preform after the deposition step in a dehydrating agent atmosphere and then vitrifying it.

[0006] The evaluation method of the Si-OH-containing optical fiber preform of the present invention is as follows: An optical fiber preform having a cladding 2 is further prepared outside a core rod having a core with a high refractive index and a cladding 1 mainly composed of SiO having a lower refractive index than this. Infrared light is irradiated in the diameter direction from one end of the outer edge of the cladding 2 of the optical fiber preform, the infrared light transmitted from the other end of the outer edge of the cladding 2 is received, the absorption loss height α is obtained from the obtained infrared light absorption spectrum, the OH concentration (ppm) is calculated from the following formula 1, and the optical fiber preform is evaluated with the calculated OH concentration. 2 [Formula 1] OH = Dα (ppm) D = 858.6 / d d; Infrared light transmission distance in the sample (mm) α; Height derived from the Si-OH absorption peak in the infrared light absorption spectrum In the formula 1, α is the value at the wave number of 3670 [cm -1 in the differential spectrum obtained by drawing a straight line passing through two points of 3400 [cm -1 and 3800 [cm -1 in the infrared absorption spectrum.

Advantages of the Invention

[0007] The present invention uses an evaluation method in which the infrared absorption spectrum caused by the OH group of the optical fiber preform is measured by infrared spectroscopy, and the OH concentration of the optical fiber preform is calculated from the measured OH absorption peak height and the optical path length, so that a preform with a low OH content over the longitudinal direction of the optical fiber preform can be non-destructively selected and provided. The optical fiber manufactured from this optical fiber preform has low transmission loss caused by the OH group and can be preferably used for optical communication applications.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0009] The optical fiber preform is silica (SiO 2It has a cylindrical shape formed of glass with as the main component, and a core is disposed at the center and a cladding having a refractive index lower than that of the core is disposed on its outer periphery. In order to impart a refractive index difference between the core and the cladding, the core is doped with an element that increases the refractive index such as germanium (Ge), or the cladding is doped with an element that decreases the refractive index such as fluorine (F). Furthermore, a complex refractive index distribution shape may be imparted, and the core or the cladding may be composed of multiple layers. An optical fiber preform is produced in a size such as a diameter of 100 mm and a length of 1500 mm, and this is heated and drawn and spun so as to have a diameter of, for example, 125 μm to obtain an optical fiber for communication.

[0010] The optical fiber preform is first produced by producing a core preform and externally attaching a cladding material to the outer periphery thereof. For example, in the case of the core preform, a porous core preform is produced by the VAD method, and then this is passed through the porous preform in a heating region where the heater temperature is heated to about 1200 °C to form a chlorine-containing atmosphere for dehydration, and then the heater temperature is heated to 1500 °C to form a helium gas-containing atmosphere, and the porous preform is passed through for sintering and vitrification into a transparent glass to obtain the core preform.

[0011] FIG. 1 is a cross-sectional view of a core preform having a core and a cladding 1. A porous glass layer corresponding to the cladding 2 is deposited on the outer periphery of this core preform by the OVD method, and this is further heated to 1500 °C in an atmosphere containing chlorine and helium gas using a sintering furnace shown in FIG. 2 for vitrification to obtain a transparent optical fiber preform. FIG. 3 is a cross-sectional view of an optical fiber preform having a core, a cladding 1, and a cladding 2.

[0012] The OH group content that affects the transmission loss of the optical fiber obtained by drawing the produced optical fiber preform is measured. This is measured using an OH group content evaluation apparatus shown in FIG. 4. The evaluation device consists of a mechanism for irradiating infrared light, a mechanism for receiving infrared light, and an arithmetic mechanism for performing arithmetic processing on the received infrared light and displaying the infrared spectrum. Each mechanism is controlled and operated by an arithmetic mechanism connected by a communication cable or the like indicated by a dotted line. As shown in Fig. 4(a), the infrared light emitted from the infrared light irradiation mechanism passes through the central part in the diameter direction of the glass base material cross-section, and its wavelength and intensity are measured by the light receiving mechanism. Further, as shown in Fig. 4(b), it is measured at an arbitrary position in the longitudinal direction of the glass base material, and the OH group content is evaluated by the arithmetic mechanism.

[0013] In evaluating the OH group content of the optical fiber base material, as a blank in advance, infrared light is irradiated and transmitted through a reference sample glass with an extremely low OH content, preferably 5 ppb or less, more preferably 1 ppb or less, and still more preferably 0.5 ppb or less to obtain an infrared spectrum. Next, the optical fiber base material to be measured is installed in the evaluation device, and the transmitted infrared light is measured to obtain an infrared spectrum. The difference between the infrared spectra of the obtained reference sample glass and the optical fiber base material to be measured is calculated, and a baseline is drawn at two points of wave numbers 3400 [cm -1 and 3800 [cm -1 to obtain the difference spectrum shown in Fig. 5.

[0014] In the difference spectrum shown in Fig. 5, the height at the position farthest from the baseline in the range of wave number 3670 ± 50 [cm -1 is defined as α, and this is taken as the absorbance peak height of the infrared light by the optical fiber base material. When determining the OH concentration of the optical fiber base material, the value of the OH molar absorption coefficient is 90 L / mol·cm, and the density of the glass base material is 2.2 g / cm 3 and it is calculated based on the Lambert-Beer law. TIFF2025084270000001.tif53170 That is, the OH concentration (ppm) is obtained by the following formula 1. [Formula 1] [OH] = Dα D = 858.6 / d d; Infrared light transmission distance within the sample in mm, i.e., the diameter of the optical fiber base material α; Height derived from the Si-OH absorption peak in the infrared light absorption spectrum

[0015] By setting the OH concentration in the diameter direction of the optical fiber base material thus obtained to 0.5 ppm or less, an optical fiber base material capable of producing an optical fiber with a transmission loss of 0.315 dB / km or less at 1385 nm can be obtained. Measurement is performed at a plurality of positions in the longitudinal direction of the base material, and the longitudinal range where the OH concentration exceeds 0.5 ppm may be cut and removed. In addition, when there are many longitudinal regions where the OH concentration exceeds 0.5 ppm, the temperature conditions and atmosphere gas conditions for sintering the porous glass base material may be adjusted so that the longitudinal regions where the OH concentration becomes 0.5 ppm or less increase.

[0016] Note that the OH concentration of the core base material before depositing and adding the cladding 2 on the outer periphery can also be measured in the same manner as above. For the core base material, it is preferable that the OH concentration is 0.5 ppb or less or less than the detection limit by the above measurement method. The region of the core base material corresponds to the region through which the optical signal of the optical fiber passes and is easily affected by the OH concentration. Therefore, it is preferable to suppress the OH concentration to about one-thousandth of the level. In addition, the ratio of the diameter of the cladding 1 to the diameter of the cladding 2 of the optical fiber base material is preferably produced to be 0.2 or more and 0.4 or less. When the diameter ratio is less than 0.2, the optical signal in the produced optical fiber also passes through the region corresponding to the cladding 2 and is easily affected by the OH concentration of the cladding 2.

Example

[0017] First, a porous core base material having a core and a cladding 1 was produced by the VAD method. After dehydrating this by passing it through a heating region heated to 1200 °C in a sintering furnace and made into a chlorine-containing atmosphere, the heater temperature was heated to 1500 °C, and the porous core base material was passed through as a helium gas-containing atmosphere and sintered and vitrified to produce a core base material. A porous glass layer corresponding to the cladding 2 was deposited on the outer periphery of this core base material by the OVD method, and this was further heated to 1500 °C in an atmosphere containing chlorine and helium gas using a sintering furnace and vitrified to produce an optical fiber preform. In this way, an optical fiber preform with a diameter of 100 mm and a length of 1500 mm was produced, and the OH concentration was measured at a plurality of locations in the longitudinal direction to obtain Examples 1 to 6 and Comparative Examples. Then, the portions where the OH concentration was evaluated were marked to produce an optical fiber with a diameter of 125 μm, and the transmission loss at 1385 nm was actually measured.

[0018]

Table 1

[0019] From Table 1, the OH concentration of the optical fiber preforms of Examples 1 to 6 was all 0.5 ppm or less, and the transmission loss at 1385 nm of the optical fibers marked from these was all 0.315 dB / km or less. The OH concentration at the measurement location used as the comparative example was 0.505 ppm, and the transmission loss at 1385 nm of the optical fiber marked from this was 0.316 dB / km. Therefore, the portion used as the comparative example of the optical fiber preform was removed. In this way, it is possible to pre-screen the portions of the optical fiber preform with an OH concentration of 0.5 ppm or less, and all the marked optical fibers have a transmission loss at 1385 nm of 0.315 dB / km or less.

Explanation of symbols

[0020] 1. Core, 2. Cladding.

Claims

1. A core rod having a core with a high refractive index and a cladding 1 mainly composed of SiO with a lower refractive index than this is prepared in a core rod preparation step. In a deposition step, glass fine particles are deposited on the core rod to produce a soot base material having a cladding 2. In a sintering step, the soot base material is dehydrated and vitrified to produce an optical fiber preform. An optical fiber preform is produced through an evaluation step of evaluating the estimated optical characteristics of the optical fiber preform. In the evaluation step, infrared light is transmitted in the diameter direction from one end of the outer edge portion of the cladding 2 of the optical fiber preform, and the infrared light transmitted from the other end is received. The OH concentration is obtained by processing the obtained infrared light absorption spectrum. The Si-OH-containing optical fiber preform is characterized in that the average OH concentration of the optical fiber preform is 0.5 (ppm) or less. ２ A core rod having a core with a high refractive index and a cladding 1 mainly composed of SiO with a lower refractive index than this is prepared in a core rod preparation step. In a deposition step, glass fine particles are deposited on the core rod to produce a soot base material having a cladding 2. In a sintering step, the soot base material is dehydrated and vitrified to produce an optical fiber preform. An optical fiber preform is produced through an evaluation step of evaluating the estimated optical characteristics of the optical fiber preform. In the evaluation step, infrared light is transmitted in the diameter direction from one end of the outer edge portion of the cladding 2 of the optical fiber preform, and the infrared light transmitted from the other end is received. The OH concentration is obtained by processing the obtained infrared light absorption spectrum. The Si-OH-containing optical fiber preform is characterized in that the average OH concentration of the optical fiber preform is 0.5 (ppm) or less.

2. The Si-OH-containing optical fiber preform according to claim 1, wherein the optical fiber obtained by drawing the optical fiber preform evaluated in claim 1 has a transmission loss at a wavelength of 1385 nm of less than 0.315 [dB / km].

3. The Si-OH-containing optical fiber preform according to claim 1, wherein the optical fiber preform evaluated in claim 1 is an optical fiber preform obtained by dehydrating the soot preform after the deposition process in an atmosphere of a dehydrating agent and then vitrifying it.

4. A core having a high refractive index and a cladding 1 mainly composed of SiO having a lower refractive index than this, an optical fiber preform having a cladding 2 is further prepared outside the core rod, infrared light is irradiated in the diameter direction from one end of the outer edge portion of the cladding 2 of the optical fiber preform, and the infrared light transmitted from the other end of the outer edge portion of the cladding 2 is received. The absorption loss height α is obtained from the obtained infrared light absorption spectrum, the OH concentration is calculated from the following formula 1, and the optical fiber preform is evaluated with the calculated OH concentration. A method for evaluating a Si-OH-containing optical fiber preform, characterized in that: ２ On the outside of a core rod having a core having a high refractive index and a cladding 1 mainly composed of SiO having a lower refractive index than this, an optical fiber preform having a cladding 2 is further prepared. Infrared light is irradiated in the diameter direction from one end of the outer edge portion of the cladding 2 of the optical fiber preform, and the infrared light transmitted from the other end of the outer edge portion of the cladding 2 is received. The absorption loss height α is obtained from the obtained infrared light absorption spectrum, the OH concentration is calculated from the following formula 1, and the optical fiber preform is evaluated with the calculated OH concentration. A method for evaluating a Si-OH-containing optical fiber preform, characterized in that: [Formula 1] OH = Dα (ppm) D = 858.6 / d d; infrared light transmission distance in the sample (mm) α; height derived from the Si-OH absorption peak in the infrared light absorption spectrum

5. α in the formula (1) is the value at a wave number of 3670 [cm -1 in the difference spectrum obtained by drawing a straight line passing through two points of 3400 [cm -1 and 3800 [cm -1 in the infrared absorption spectrum. The method for evaluating a Si-OH-containing optical fiber preform according to claim 4.

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