CN103443673B - optical fiber and optical transmission system - Google Patents

Abstract

A low-cost and low-loss optical fiber (1) made of quartz glass is preferably used as an optical transmission path of an optical access network. The optical fiber is provided with: a core (11); an optical cladding (12) surrounding the core; and a sheath (13) surrounding the optical cladding. The core contains GeO ₂ . The relative refractive index difference (Δcore) of the _core with respect to the optical cladding is 0.35% to 0.50%, and the refractive index volume (v) of the core is 0.045 μm ² to 0.095 μm ² . The relative refractive index difference (ΔJ) of the sheath is 0.03% to 0.20%. The fictive temperature of the glass constituting the core is 1400°C to 1590°C. The residual stress in the core is compressive stress and the absolute value of the residual pressure is 5 MPa or more.

Description

Optical Fiber and Optical Transmission Systems

technical field

The present invention relates to optical fibers and optical transmission systems.

Background technique

G.Talli et al., J.Lightw.Technol., Vol.24, No.7, 2827-2834 (2006) described what is called "long-reach passive optical network (PON) (long-distance passive optical network )" optical access network. Long-distance PON is a transmission system used to transmit the signal light from the optical line terminal (OLT) of the telephone office to the optical network unit (ONU) in the user's home via a splitter, and allows the transmission of the optical transmission line between the OLT and the ONU The longer length reduces communication costs.

Such optical transmission systems preferably use low-attenuation optical fibers in order to extend the communication distance while maintaining the optical signal-to-noise ratio (OSNR). Regarding low-attenuation optical fibers, optical fibers comprising a pure silica core are disclosed in M. Kato et al., Electron. Lett., Vol. 35, No. 19, 1615-1617 (1999). Unfortunately, typical pure silica core fibers are expensive and for this economical reason no progress has been made in introducing pure silica core fibers into optical access networks.

S. Sakaguchi et al., Appl. Opt., Vol. 37, No. 33, 7708-7711 (1998) and JP2006-58494A disclose techniques for reducing attenuation in general-purpose optical fibers conforming to ITU-T G.652. According to this technique, when an optical fiber preform is drawn to form an optical fiber, the optical fiber is slowly cooled to lower the fictive temperature of the glass constituting the optical fiber, thereby reducing Rayleigh scattering in the optical fiber, thereby achieving low attenuation.

Contents of the invention

<technical problem>

The present invention provides an inexpensive low-attenuation optical fiber suitable for use as an optical transmission line in an optical access network and an optical transmission system using the optical fiber as a transmission line.

<Technical solution>

The present invention provides a silica-based glass optical fiber comprising: a core including a central axis; an optical cladding surrounding the core; and a sheath surrounding the optical cladding. The core contains GeO ₂ , the relative refractive index difference Δcore _{of the core is} greater than or equal to 0.35% and less than or equal to 0.50%, and the refractive index volume of the core

$\mathrm{<span\; class="notranslate">\; v</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; a</span>}}\mathrm{<span\; class="notranslate">\; \&Delta;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; dr</span>}<span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

greater than or equal to 0.045 μm ² and less than or equal to 0.095 μm ² , wherein Δ(r) represents the relative refractive index difference at the radial coordinate r, and a represents the radius of the core. The relative refractive index difference ΔJ of the sheath is greater than or equal to 0.03% and less than or equal to 0.20%. The fictive temperature of the glass constituting the core is higher than or equal to 1400°C and lower than or equal to 1590°C. The residual stress in the core is a compressive stress having an absolute value greater than or equal to 5 MPa.

In this specification, the term "relative refractive index difference" refers to the value of (nn _cladding )/n _cladding , which is based on the refractive index n of each part (core or sheath) relative to the refractive index of the optical cladding n _cladding . The term "refractive index of the core" refers to the equivalent step index (ESI). The term "outer diameter of the optical cladding" refers to the diameter at which the derivative of the refractive index with respect to the radial coordinate reaches a maximum value at the interface between the optical cladding and the sheath. The term "refractive index of the sheath" means an average value of the refractive indices from the portion of the optical cladding having the outer diameter of the optical cladding to the outermost peripheral portion of the glass.

In the optical fiber according to the present invention, the 2m fiber cutoff wavelength may be greater than or equal to 1260nm, the 22m cable cutoff wavelength may be less than or equal to 1260nm, and the mode field diameter at a wavelength of 1310nm may be greater than or equal to 8.2μm and less than or equal to 9μm, and the attenuation at a wavelength of 1550nm can be less than or equal to 0.18dB/km. In a cross-section of the optical fiber perpendicular to the axis, residual stress in a portion of 50% or more of a cross-sectional area of the sheath may be tensile stress. An absolute value of residual stress in the core may be less than or equal to 30 MPa. An absolute value of residual stress in the core may be less than or equal to 10 MPa. In the case of a wavelength of 1383 nm, the attenuation increment caused by the OH group may be less than or equal to 0.02 dB/km. The core may contain fluorine. The optical fiber may also include a primary coating and a secondary coating surrounding the jacket. The Young's modulus of the secondary coating may be greater than or equal to 800 MPa, and the Young's modulus of the primary coating may be greater than or equal to 0.2 MPa and less than or equal to 1 MPa.

In the optical fiber according to the present invention, when the wavelength is 1550nm, the bending loss when the bending radius is 15mm can be less than or equal to 0.002dB/turn, and the bending loss when the bending radius is 10mm can be less than or equal to 0.2dB/turn, The bending loss when the bending radius is 10mm can be less than or equal to 0.1dB/turn, and the bending loss when the bending radius is 7.5mm can be less than or equal to 0.5dB/turn. In addition, in the optical fiber according to the present invention, at a wavelength of 1625 nm, the bending loss when the bending radius is 15 mm can be less than or equal to 0.01 dB/turn, and the bending loss when the bending radius is 10 mm can be less than or equal to 0.4 dB/turn. When the bending radius is 10mm, the bending loss can be less than or equal to 0.2dB/turn, and when the bending radius is 7.5mm, the bending loss can be less than or equal to 1dB/turn.

In the fiber according to the invention, the MAC value (=MFD/λc), which is the ratio of the mode field diameter MFD at a wavelength of 1310 nm to the cut-off wavelength λc of a 2m fiber, may be less than or equal to 6.6. The attenuation difference (α_B-α_t) between attenuation α_B and attenuation α_t can be less than 0.01dB/km. Attenuation measured for longer fibers; said attenuation α_t is the attenuation measured for a fiber in a loosely wound loop at a wavelength of 1550 nm. In the optical fiber according to the present invention, the outer diameter of said optical fiber with the coating may be less than or equal to 210 μm. This can reduce the cross-sectional area, thereby improving space utilization after arrangement. In this case, in consideration of preventing breakage, the thickness of the secondary coating may be greater than or equal to 10 μm.

The present invention provides an optical transmission system, which is used to transmit the signal light from the optical line terminal of the telephone office to the optical network unit in the user's home via a splitter, wherein the optical line terminal of the telephone office The length of the optical transmission line between the splitter or the optical transmission line between the splitter and the optical network unit in the user's home is greater than or equal to 15 km, and the optical fiber according to the present invention is arranged in the More than 90% of the section of the optical transmission line. The present invention also provides an optical transmission system for optically transmitting a signal from a transmitter to a receiver, wherein the length of the optical transmission line between the transmitter and the receiver is greater than or equal to 40 km, and the optical fiber according to the present invention is arranged in more than 90% of the section of the optical transmission line.

The present invention also provides an optical transmission system, which is used to transmit the signal light from the optical line terminal of the telephone office to the optical network unit in the user's home via a splitter, wherein the optical fiber according to the present invention is arranged in In more than 50% of the section of the optical transmission line between the optical line terminal of the telephone office and the optical network unit in the user's home, and the signal light is not amplified in the optical transmission line. The present invention also provides an optical transmission system, which is used to transmit the signal light from the optical line terminal of the telephone office to the optical network unit in the user's home via a splitter, wherein the optical fiber according to the present invention is arranged in In more than 50% of the section of the optical transmission line between the optical line terminal of the telephone office and the optical network unit in the user's home, and amplify the signal light in the optical transmission line.

<Beneficial effect>

According to the present invention, an inexpensive low-attenuation optical fiber suitable for use as an optical transmission line in an optical access network is provided.

Description of drawings

Fig. 1 is a cross-sectional view of an optical fiber according to an embodiment of the present invention.

FIG. 2 is a graph showing the dependence of the fictive temperature attainable by an optical fiber including a GeO ₂ -containing core on L/V.

Fig. 3 is a graph showing the dependence of attenuation in an optical fiber on fictive temperature.

Fig. 4 is a graph showing the dependence of additional loss on residual stress in the core.

Fig. 5 is a graph showing the radial distribution of residual stress in an optical fiber.

FIG. 6 is a graph showing the relationship between Raman intensity and Raman shift.

detailed description

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings, which are for illustrative purposes only and are not intended to limit the scope of the present invention. In the drawings, the same reference numerals denote the same components, and repeated descriptions are omitted. The dimensions or proportions in the drawings are not necessarily exact.

The inventors of the present invention found that, in the case of _GeO2 in the core, if the Rayleigh scattering is reduced by lowering the fictive temperature of the glass by slow cooling, the loss component caused by factors other than the Rayleigh scattering (hereinafter Known as "excess loss") may increase, and low attenuation fiber may not necessarily be obtained. As far as the inventors of the present invention are aware, S. Sakaguchi et al. and JP2006-58494A disclose the reduction of attenuation in optical fibers by slowly cooling glass, M. Ohashi et al., IEEE Photon. Technol. Lett., Vol. 5, No. 7, 812-814 (1993) discloses the use of core-cladding viscosity matching to reduce attenuation in optical fibers, however none of these documents mention the additional loss in optical fibers including _GeO2 -doped cores and the residual stress in the core.

Fig. 1 is a cross-sectional view of an optical fiber 1 according to an embodiment of the present invention. The optical fiber 1 is an optical fiber composed of SiO ₂ glass and includes: a core 11 including a central axis; an optical cladding 12 surrounding the core 11 ; and a sheath 13 surrounding the optical cladding 12 . The core 11 contains GeO ₂ and may also contain fluorine. The refractive index of the optical cladding 12 is lower than that of the core 11 . The optical cladding 12 may consist of pure SiO ₂ glass or SiO ₂ glass doped with fluorine. The sheath 13 consists of pure _SiO2 glass and may contain chlorine.

Reducing Rayleigh scattering in the fiber 1 can reduce the attenuation in the fiber 1 . Lowering the fictive temperature of glass constituting the optical fiber 1 is effective for reducing Rayleigh scattering. Methods of lowering the fictive temperature of glass include the first method and the second method described below.

The first method is as follows (slow cooling method): During stretching of the optical fiber preform to form the optical fiber 1, the cooling rate of the formed optical fiber is slowed down to accelerate the relaxation of the network structure of the glass, thereby lowering the fictive temperature of the glass. The second method is as follows: add a very small amount of additives to the core 11, thereby lowering the fictive temperature of the glass, so that the attenuation due to light absorption is not increased while accelerating the structural relaxation of the core 11.

The Rayleigh scattering of the optical fiber 1 can be reduced by using the first method or the second method, or an appropriate combination of the above methods can be used to reduce the Rayleigh scattering of the optical fiber 1 . The slow cooling method is described below.

The method of manufacturing the optical fiber 1 is as follows. First, using vapor phase glass synthesis methods such as Vapor Axial Deposition (VAD), External Vapor Deposition (OVD), Modified Chemical Vapor Deposition (MCVD) or Plasma Chemical Vapor Deposition (PCVD) Forming the core for light propagation and forming a jacket surrounding the core using VAD, OVD, Advanced Plasma Vapor Deposition (APVD), rod collapse, or other similar methods to form an optical fiber preform . The intermediate optical cladding between the core and the jacket can be formed using VAD, OVD, MCVD, rod collapse, or other similar methods. The optical fiber preform thus formed is held by a wire drawing machine, the lower end of the preform is heated to a working temperature or higher, and the droplet-shaped end of the molten glass is appropriately drawn to form an optical fiber, thereby producing a glass optical fiber. The drawing speed is controlled so that the glass optical fiber has a predetermined outer diameter. The glass optical fiber is coated with a resin, thereby forming a coated optical fiber. Coated fiber is wound onto a take-up bobbin.

The coating of the resin has a double-layer structure and includes: a primary coating that prevents the glass optical fiber from being directly applied with an external force; and a secondary coating that protects the glass optical fiber from external damage. During the optical fiber formation stage, the mold for applying the coating can be continuously provided. Alternatively, the resin coating can be applied using a mold that injects both coatings simultaneously. In this case, the height of the wire drawing machine can be lowered. Therefore, the construction cost of a building for accommodating the wire drawing machine can be reduced.

In addition, a device for controlling the cooling rate of the formed glass optical fiber can be installed between the drawing furnace and the mold, so that the surface temperature of the glass optical fiber entering the mold can be controlled at an appropriate temperature. Preferably, the Reynolds number of the gas flowing through the means for controlling the cooling rate is low, since this reduces the vibrations caused by the turbulent flow occurring on the formed fiber. In addition, by controlling the cooling rate of the glass fiber, Rayleigh scattering can be reduced, thereby obtaining a fiber with low attenuation.

In the UV oven for curing the resin, the curing speed of the resin can be appropriately controlled by feedback control of the intensity of UV light and the temperature inside the oven. Magnetrons or UV LEDs are suitable for use in UV ovens. In the case of ultraviolet LEDs, since this light source does not generate heat, a mechanism for supplying hot air is additionally provided in order to control the furnace at an appropriate temperature. Components separated from the resin have the potential to adhere to the inner surface of the furnace tube of the UV furnace, thereby altering the power of the UV light reaching the coating during stretching. Therefore, the reduction degree of the UV light power during stretching can be monitored in advance, and the UV light power can be controlled according to the stretching time, thereby controlling the UV light power applied to the coating to be a constant power. Alternatively, the UV light leakage from the furnace tube can be monitored and the UV light power can be controlled such that the UV light power applied to the coating is controlled to a constant power. This gives the fiber a uniform breaking strength throughout its length.

Preferably, the thickness of the secondary coating of the two coatings is appropriately set to maintain resistance to external damage. Typically, the thickness of the secondary coating is preferably greater than or equal to 10 μm, more preferably greater than or equal to 20 μm. The optical fiber 1 thus produced and wound on the winding bobbin is colored as necessary and used as a final product such as an optical cable or an optical cord.

According to an embodiment of the present invention, the optical fiber is formed in a drawing furnace, passes through a slow cooling unit and a heating furnace after leaving the drawing furnace, and then enters a mold. The slow cooling unit continuously cools the following area at a cooling rate greater than or equal to 1000°C/s and less than or equal to 20000°C/s: from the tapered part (with a diameter range of 90% to 5%) to the part where the temperature of the formed optical fiber is 1400°C. The heating furnace is disposed below the plane (exit of the drawing furnace) which is the lower part of the drawing furnace and at which the formed optical fiber is generally discharged from the drawing furnace. The distance between the exit of the drawing furnace and the entrance of the heating furnace is 1 m or less. Preferably, the slow cooling unit provided between the exit of the drawing furnace and the entrance of the heating furnace has an insulating structure for preventing the temperature of the formed optical fiber from decreasing. When the fiber enters the furnace, the temperature of the fiber is preferably higher than or equal to 1000°C, more preferably higher than or equal to 1400°C.

This reduces the length of the furnace where the fiber is reheated to a temperature that substantially allows the structure to relax (typically, this temperature is at or above the glass transition point). Therefore, the time required for the structure to relax can be increased. If V represents the stretching speed, the length L of the heating furnace is set such that L/V is greater than or equal to 0.05 s. Preferably, the furnace comprises a plurality of furnaces. In this way, the cooling rate of the fiber can be more precisely controlled. Preferably, the cooling rate is greater than or equal to 5000°C/s until the optical fiber in the heating furnace is cooled to below 1100°C. An optical fiber with reduced Rayleigh scattering can be obtained by using the above-mentioned heating furnace in the manufacture of an optical fiber.

Increasing L/V can lower the fictive temperature of the glass. In consideration of economy, the stretching speed V is preferably greater than or equal to 20 m/s. For example, in order to realize L/V=0.2s, the length L of heating furnace must reach 4m. As described above, there is a certain limit to the coordination between the construction cost of equipment or buildings affected by the length of the heating furnace and the stretching speed. FIG. 2 is a graph showing the dependence of the fictive temperature attainable by an optical fiber including a GeO ₂ -containing core on L/V. Figure 2 is formed on the basis of the data in Table 1 in K.Saito et al., J.Am.Ceram.Soc., Vol.89[1], 65-69 (2006). In the case of allowing L/V<0.5s in consideration of economic requirements, the achievable fictive temperature is 1400°C.

FIG. 3 is a graph showing the dependence of attenuation in an optical fiber on fictive temperature, and FIG. 3 is formed on the basis of equation (2) of K. Saito et al. The term "additional loss" in FIG. 3 refers to losses (including macrobending loss and microbending loss) other than losses caused by Rayleigh scattering, Brillouin scattering, and Raman scattering. When the additional loss is greater than or equal to 0.030dB/km, if the fictive temperature is 1400°C, it is difficult to achieve an attenuation of 0.18dB/km at a wavelength of 1550nm.

As mentioned above, although the Rayleigh scattering is reduced by lowering the fictive temperature of the glass by slow cooling, the additional loss caused by factors other than the Rayleigh scattering component increases, and in this case, it is difficult to make the wavelength 1550nm The attenuation under the circumstances is stably less than or equal to 0.18dB/km. The inventors of the present invention identified factors that lead to an increase in parasitic loss, and found a good correlation between parasitic loss and residual stress in the core. Fig. 4 is a graph showing the dependence of additional loss on residual stress in the core. The graph shows that when the residual stress in the core is a compressive stress greater than or equal to 5MPa in absolute value (in Figure 4, at or below -5MPa), the additional loss can be less than or equal to 0.02dB/km, which is more reliable is less than or equal to 0.025dB/km.

When the residual stress in the core is a compressive stress greater than or equal to 5 MPa in absolute value, the additional loss is approximately less than or equal to 0.02 dB/km. Therefore, at a wavelength of 1550nm, attenuations of 0.180dB/km, 0.183dB/km and 0.185dB/km can be achieved at fictive temperatures of 1530°C, 1560°C, and 1590°C, respectively.

The core 11 of the optical fiber 1 contains GeO ₂ , the relative refractive index difference Δ _{core of} the core 11 is greater than or equal to 0.35% and less than or equal to 0.50%, and the refractive index volume ν of the core 11 is greater than or equal to 0.045 μm and less ^than or equal to Equal to 0.095 μm ² , the refractive index volume is expressed as equation (2):

$\mathrm{<span\; class="notranslate">\; v</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; a</span>}}\mathrm{<span\; class="notranslate">\; \&Delta;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; dr</span>}<span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

Among them, Δ(r) represents the relative refractive index difference at the radial coordinate r, and a represents the radius of the core. More preferably, the refractive index volume ν is greater than or equal to 0.06 μm ² and less than or equal to 0.085 μm ² . The relative refractive index difference ΔJ of the sheath 13 is greater than or equal to 0.03% and less than or equal to 0.20%. The fictive temperature of the glass constituting the core 11 is higher than or equal to 1400°C and lower than or equal to 1590°C, preferably lower than or equal to 1560°C, more preferably lower than or equal to 1530°C. In addition, the residual stress in the core 11 is a compressive stress whose absolute value is greater than or equal to 5 MPa.

In the optical fiber 1, preferably, the 2m fiber cutoff wavelength is greater than or equal to 1260nm, the 22m cable cutoff wavelength is less than or equal to 1260nm, and the mode field diameter at the wavelength of 1310nm is greater than or equal to 8.2μm and less than or equal to 9μm, in The attenuation at the wavelength of 1550nm is less than or equal to 0.18dB/km. More preferably, the attenuation at a wavelength of 1550nm is less than or equal to 0.178dB/km, and the cutoff wavelength of the 2m optical fiber is greater than or equal to 1290nm.

Birefringence in the fiber is used to measure the residual stress in the fiber as described in JP2009-168813A. Alternatively, the residual stress in the fiber can be measured based on the amount of change in the refractive index using area analysis of the refractive index in the cross-section of the fiber and the intrinsic photoelastic coefficient of the material. get. Fig. 5 is a graph showing the radial distribution of residual stress in an optical fiber. In an optical fiber comprising a _GeO2 -containing core and a sheath substantially composed of pure silica, since the viscosity of the core is lower than that of the sheath at the same temperature, the core of the drawn fiber remains Compressive stress (the curve of L/V=0s in Figure 5). This compressive stress changes based on the tensile force. It is known that basically the greater the tensile force, the greater the residual compressive stress.

Furthermore, in the slowly cooled optical fiber, the compressive stress is relaxed in the slow cooling unit, so that the absolute value of the compressive stress decreases. FIG. 5 shows changes in residual stress obtained by changing L/V to 0 s, 0.12 s, and 0.40 s. It is known that increasing the residence time of the fiber in the slow cooling unit gradually reduces the absolute value of the compressive stress in the core. In order to make the absolute value of the compressive stress in the core greater than or equal to 5 MPa, it is preferable that L/V is less than 0.4 s. In addition, during cooling of the optical fiber, the temperature of the optical fiber is kept higher than the inner surface temperature of the slow cooling unit until the optical fiber reaches the slow cooling unit, thereby preventing excessive reduction of compressive stress in the core. Basically, the longer the slow cooling unit, the more effective the slow cooling will be. Therefore, the amount of change in compressive stress is increased.

For example, in the case where the length of the slow cooling unit is greater than or equal to 2 m, it is important to keep L/V equal to or lower than 0.2 s. In addition, the pulling force applied to the drawn fiber optic glass is preferably greater than or equal to 50 g, more preferably greater than or equal to 100 g.

Regarding another method of controlling the stress in the core, an additive for reducing the viscosity of the core is added to the core so that the absolute value of the compressive stress of the core can be controlled to be equal to or greater than 5 MPa. Alkali metal elements are preferably used as additives, since very small amounts of alkali metal elements can significantly reduce the viscosity of quartz glass. Excessive addition of alkali metal elements will unfortunately lead to increased structural defects in the glass, resulting in degradation of hydrogen and radiation properties. Therefore, it is preferable to adjust the amount of the additive to an appropriate value. Preferably, after stretching, the concentration of the alkali metal element in the core is greater than or equal to 1 wtppb and less than or equal to 10 wtppm.

FIG. 6 is a graph showing the relationship between Raman intensity and Raman shift. The fictive temperature of the optical fiber can be estimated based on the relationship between the ratio of the D1 (490 cm ⁻¹ ) peak area to the 800 cm ⁻¹ peak area in the microscopic Raman scattering spectrum in each portion constituting the optical fiber. A baseline is drawn in the wavenumber range between 525 cm ^-1 and 475 cm ^-1 and the D1 peak area between this baseline and the spectrum is calculated. In addition, a baseline is drawn in the wavenumber range between 880cm ^-1 and 740cm ^-1 , and the 800cm- ¹ peak area between this baseline and the spectrum is calculated. The fictive temperature can use the ratio of the peak area of 800cm- ¹ and the peak area of D1 to compare with that of the bulk glass (bulk glass) by means of the IR method (D.-L.Kim and M.Tomozawa, J.Non-Cryst.Solids, Vol. 286, 132-138 (2001)) to obtain the relationship between the measured fictive temperatures.

The optical fiber according to this embodiment preferably complies with ITU-T G.657.A1, and preferably also has a bending loss conforming to G.657.A2. Since the fiber according to this embodiment complies with G.657.A2, this fiber can be connected with a general-purpose single-mode fiber with low attenuation that complies with G.652.D, and can be used in the same way as the G.652.D fiber Transmission system.

In the optical fiber according to the present embodiment, preferably, in the cross section of the optical fiber perpendicular to the axis, the residual stress in a portion of 50% or more of the cross-sectional area of the sheath is tensile stress. In order for the residual stress in the core to be compressive, a tensile stress must be applied to the sheath such that the value of the tensile stress is equal to the value of the compressive stress applied to the core. Tensile force, thermal history and composition of the fiber are controlled such that the residual stress in more than 50% of the cross-sectional area of the jacket is tensile, thereby easily making the residual stress in the core compressive.

In the optical fiber according to the present embodiment, preferably, the absolute value of the residual stress in the core is less than or equal to 30 MPa. More preferably, the absolute value of the residual stress in the core is less than or equal to 10 MPa. Make the stress in the core part compressive stress and make the absolute value of the stress less than or equal to 30MPa, so that the additional loss can be less than or equal to 0.02dB/km, and at the same time fully obtain the effect of reducing Rayleigh scattering by slow cooling stretching Effect.

In the optical fiber according to this embodiment, preferably, the attenuation increment caused by the OH group is less than or equal to 0.02 dB/km at a wavelength of 1383 nm. The presence of OH absorption leads to increased attenuation at a wavelength of 1550 nm. When the attenuation increase caused by the OH group is less than or equal to 0.02 dB/km at the wavelength of 1383 nm, the attenuation increase at the wavelength of 1550 nm may be less than or equal to 0.004 dB/km.

In an embodiment according to the invention, preferably, the core is doped with fluorine. Since the core contains fluorine, the viscosity of the core is lowered, so that the residual stress in the core can be easily made compressive. Therefore, wavelength-independent attenuation can be reduced. It should be noted that increasing the fluorine content increases Rayleigh scattering caused by concentration fluctuations. Therefore, it is preferable to add fluorine at a concentration such that the decrease in the relative refractive index due to the addition of fluorine is greater than or equal to −0.1% and less than or equal to 0%.

An optical fiber according to this embodiment may also include a primary coating and a secondary coating surrounding the jacket. Preferably, the Young's modulus of the secondary coating is greater than or equal to 800 MPa, and the Young's modulus of the primary coating is greater than or equal to 0.2 MPa and less than or equal to 1 MPa. Therefore, microbend loss can be reduced, thereby preventing an increase in attenuation after the cable is formed.

According to the optical fiber of this embodiment, when the wavelength is 1550nm, the bending loss when the bending radius is 15mm is preferably less than or equal to 0.002dB/turn, and the bending loss when the bending radius is 10mm is preferably less than or equal to 0.2dB/turn , the bending loss at a bending radius of 10 mm is preferably less than or equal to 0.1 dB/turn, and the bending loss at a bending radius of 7.5 mm is preferably less than or equal to 0.5 dB/turn. Furthermore, in the optical fiber according to the present embodiment, at a wavelength of 1625 nm, the bending loss when the bending radius is 15 mm is preferably less than or equal to 0.01 dB/turn, and the bending loss when the bending radius is 10 mm is preferably less than or equal to 0.4dB/turn, the bending loss at a bending radius of 10mm is preferably less than or equal to 0.2dB/turn, and the bending loss at a bending radius of 7.5mm is preferably less than or equal to 1dB/turn.

In the optical fiber according to this embodiment, preferably, the MAC value is less than or equal to 6.6, wherein the MAC value is the ratio (MFD/λc) of the mode field diameter MFD to the cut-off wavelength λc of a 2m fiber at a wavelength of 1310 nm. Such control makes it possible to obtain an optical fiber having the above-mentioned microbend loss with a high yield.

In the optical fiber according to this embodiment, preferably, the attenuation difference (α_B-α_t) between the attenuation α_B and the attenuation α_t is less than 0.01dB/km, wherein the attenuation α_B is at a wavelength of 1550nm, and is wound at 140φ The attenuation measured on a fiber with a length of 10 km or more on a winding bobbin; the attenuation α_t is the attenuation measured on the fiber in a loosely wound loop at a wavelength of 1550 nm. Assuming that the fiber exhibits low attenuation in the coated state, if the attenuation in actual use cannot be reduced, the desired result may not be obtained. Reducing the attenuation difference (α_B-α_t) to less than 0.01dB/km can reduce the attenuation in actual use.

In the optical fiber according to this embodiment, preferably, the outer diameter of the coated optical fiber is less than or equal to 210 μm, and the thickness of the secondary coating is greater than or equal to 10 μm. A need may arise to pass many optical fibers through restricted ducts, such as existing ducts. This need can be met.

A preferred embodiment of an optical transmission system including the optical fiber according to the present invention as an optical transmission line is as follows.

The optical transmission system according to the first embodiment is an optical transmission system for transmitting signal light from an optical line terminal of a telephone office to an optical network unit in a user's home via a splitter. The length of the optical transmission line between the optical line terminal of the telephone office and the splitter is greater than or equal to 15km. The optical fiber according to the invention is arranged in more than 90% of the sections of the optical transmission line.

The optical transmission system according to the second embodiment is an optical transmission system for transmitting signal light from an optical line terminal of a telephone office to an optical network unit in a user's home via a splitter. The length of the optical transmission line between the splitter and the optical network unit in the user's home is greater than or equal to 15 km. The optical fiber according to the invention is arranged in more than 90% of the sections of the optical transmission line.

The optical transmission system according to the third embodiment is an optical transmission system for transmitting signal light from a transmitter to a receiver. The length of the optical transmission line between the transmitter and the receiver is greater than or equal to 40km. The optical fiber according to the invention is arranged in more than 90% of the sections of the optical transmission line.

The optical transmission system according to the fourth embodiment is an optical transmission system for transmitting signal light from an optical line terminal of a telephone office to an optical network unit in a user's home via a splitter. The optical fiber according to the present invention is arranged in more than 50% of the section of the optical transmission line between the optical line terminal of the telephone office and the optical network unit in the user's home. Signal light is not amplified in the optical transmission line.

The optical transmission system according to the fifth embodiment is an optical transmission system for transmitting signal light from an optical line terminal of a telephone office to an optical network unit in a user's home via a splitter. The optical fiber according to the present invention is arranged in more than 50% of the section of the optical transmission line between the optical line terminal of the telephone office and the optical network unit in the user's home. Signal light is amplified in an optical transmission line.

Using the optical fiber according to the present invention as an optical transmission line in an optical transmission system can improve OSNR=10log(Aeff×α)-αL compared to a system using an optical fiber in the prior art. Here, Aeff represents the effective area at the wavelength of the signal light, α represents the attenuation at the wavelength of the signal light, and L represents the transmission distance or length.

As the transmission length L increases, the amount of improvement in OSNR of an optical transmission system comprising an optical fiber according to the invention also increases. Compared with a system including a general-purpose single-mode fiber conforming to ITU-T G.652, if the transmission length L is 15 km or more, the OSNR of the optical transmission system including the fiber according to the present invention is improved by more than 0.1 dB. The typical connection loss of optical fiber is equal to or lower than 0.1dB. Thus, the OSNR can be increased by an amount corresponding to more than one connection using an optical fiber according to the invention. Furthermore, an OSNR margin against connection failure etc. can be provided. In addition, since the transmission length can be increased, the population coverage of a telephone exchange can be increased. Therefore, the construction cost of the transmission system in the metro access network can be reduced.

A system in which the distance between the OLT and the ONU is in the range of about 10 km to about 25 km is a typical access system without optical amplification. Using the optical fiber according to the present invention in at least 50% of the section of the optical transmission line between the OLT and the ONU can extend the transmission distance without optical amplification.

As an access system based on optical amplification, a system in which the distance between the OLT and the ONU is in the range of about 20 km to about 100 km is under study. Using the optical fiber according to the present invention in more than 50% of the section of the optical transmission line between the OLT and the ONU can further extend the transmission distance with optical amplification. In addition, since an increase in noise during amplification can be suppressed and a high OSNR can be obtained over such a distance, the OSNR required by other devices can be reduced. Therefore, a system with high economic efficiency can be constructed.

<Industrial applicability>

Optical fibers according to the invention can be used as optical transmission lines in optical access networks.

Claims (22)

1. A quartz-based glass optical fiber, comprising: a core having a central axis; an optical cladding surrounding the core; and a jacket, which surrounds the optical cladding, wherein, The core contains GeO ₂ , the relative refractive index difference Δcore _{of the core is} greater than or equal to 0.35% and less than or equal to 0.50%, and the refractive index volume of the core $\mathrm{<span\; class="notranslate">\; \&nu;</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; a</span>}}\mathrm{<span\; class="notranslate">\; \&Delta;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; ...</span><span\; class="notranslate">\; ...</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$ greater than or equal to 0.045 μm ² and less than or equal to 0.095 μm ² , wherein Δ(r) represents the relative refractive index difference at the radial coordinate r, a represents the radius of the core, the sheath has a relative refractive index difference ΔJ greater than or equal to 0.03% and less than or equal to 0.20%, the fictive temperature of the glass constituting the core is higher than or equal to 1400°C and lower than or equal to 1590°C, and The residual stress in the core is a compressive stress having an absolute value greater than or equal to 5 MPa. 2. The optical fiber of claim 1, wherein, The fictive temperature is lower than or equal to 1530°C. 3. The optical fiber according to claim 1 or 2, wherein, 2m fiber cut-off wavelength is greater than or equal to 1260nm, 22m cable cut-off wavelength is less than or equal to 1260nm, a mode field diameter at a wavelength of 1310 nm of greater than or equal to 8.2 μm and less than or equal to 9 μm, and The attenuation is less than or equal to 0.18dB/km at a wavelength of 1550nm. 4. The optical fiber according to claim 1 or 2, wherein, In the cross-section of the optical fiber perpendicular to the axis, the residual stress in a portion of 50% or more of the cross-sectional area of the sheath is tensile stress. 5. The optical fiber according to claim 1 or 2, wherein, The absolute value of the residual stress in the core is less than or equal to 30 MPa. 6. The optical fiber according to claim 1 or 2, wherein, The attenuation increase caused by the OH group in the case of a wavelength of 1383 nm is less than or equal to 0.02 dB/km. 7. The optical fiber according to claim 1 or 2, wherein, The core is doped with fluorine. 8. The optical fiber according to claim 1 or 2, further comprising: a primary coating and a secondary coating surrounding the sheath, wherein, The Young's modulus of the secondary coating is greater than or equal to 800 MPa, and the Young's modulus of the primary coating is greater than or equal to 0.2 MPa and less than or equal to 1 MPa. 9. The optical fiber according to claim 1 or 2, wherein, The bending loss in the case of a bending radius of 15mm and a wavelength of 1550nm is less than or equal to 0.002dB/turn. 10. The optical fiber according to claim 1 or 2, wherein, The bending loss in the case of a bending radius of 10 mm and a wavelength of 1550 nm is less than or equal to 0.2 dB/turn. 11. The optical fiber according to claim 1 or 2, wherein, The bending loss in the case of a bending radius of 7.5 mm and a wavelength of 1550 nm is less than or equal to 0.5 dB/turn. 12. The optical fiber according to claim 1 or 2, wherein, The bending loss in the case of a bending radius of 15 mm and a wavelength of 1625 nm is less than or equal to 0.01 dB/turn. 13. The optical fiber according to claim 1 or 2, wherein, The bending loss in the case of a bending radius of 10 mm and a wavelength of 1625 nm is less than or equal to 0.4 dB/turn. 14. The optical fiber according to claim 1 or 2, wherein, The bending loss in the case of a bending radius of 7.5 mm and a wavelength of 1625 nm is less than or equal to 1 dB/turn. 15. The optical fiber according to claim 1 or 2, wherein, The MAC value is less than or equal to 6.6, and the MAC value is the ratio of the mode field diameter MFD to the cut-off wavelength λc of the 2m optical fiber at a wavelength of 1310 nm. 16. The optical fiber according to claim 1 or 2, wherein, The attenuation difference between attenuation α_B and attenuation α_t measured at a wavelength of 1550 nm on an optical fiber with a length of 10 km or more wound on a 140φ bobbin is less than 0.01 dB/km The attenuation; the attenuation α_t is: at a wavelength of 1550 nm, the attenuation measured for an optical fiber in a loosely wound loop. 17. The optical fiber of claim 8, wherein, The outer diameter of the optical fiber with the coating is less than or equal to 210 μm, and the thickness of the secondary coating is greater than or equal to 10 μm. 18. An optical transmission system, which is used to transmit the signal light from the optical line terminal of the telephone office to the optical network unit in the user's home via a splitter, wherein, The length of the optical transmission line between the optical line terminal of the telephone exchange and the splitter is greater than or equal to 15 km, and The optical fiber according to any one of claims 1 to 17 is arranged in more than 90% of the section of the optical transmission line. 19. An optical transmission system, which is used to transmit the signal light from the optical line terminal of the telephone office to the optical network unit of the user's home via a splitter, wherein, The length of the optical transmission line between the splitter and the optical network unit in the user's home is greater than or equal to 15 km, and The optical fiber according to any one of claims 1 to 17 is arranged in more than 90% of the section of the optical transmission line. 20. An optical transmission system for optically transmitting a signal from a transmitter to a receiver, wherein, the length of the optical transmission line between the transmitter and the receiver is greater than or equal to 40 km, and The optical fiber according to any one of claims 1 to 17 is arranged in more than 90% of the section of the optical transmission line. 21. An optical transmission system, which is used to transmit the signal light from the optical line terminal of the telephone office to the optical network unit in the user's home via a splitter, wherein, The optical fiber according to any one of claims 1 to 17 is arranged in more than 50% of the section of the optical transmission line between the optical line terminal of the telephone exchange and the optical network unit in the user's home, and The signal light is not amplified in the optical transmission line. 22. An optical transmission system, which is used to transmit the signal light from the optical line terminal of the telephone office to the optical network unit of the user's home via a splitter, wherein, The optical fiber according to any one of claims 1 to 17 is arranged in more than 50% of the section of the optical transmission line between the optical line terminal of the telephone exchange and the optical network unit in the user's home, and The signal light is amplified in the optical transmission line.