JP2005196012A - Dispersion compensated fiber and composite optical fiber transmission line - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a dispersion compensated fiber which is small in bending loss, large in RDS and small in residual dispersion after cumulative wavelength dispersion compensation over the entire range of the U-band, and to provide a composite optical fiber transmission line which uses the dispersion compensated fiber. <P>SOLUTION: The dispersion compensated fiber is provided with at least a center core 1, side cores 2, a ring core 3 and clads 4. In the fiber, radius r<SB>1</SB>is set to 1.2 to 2.0μm, radius r<SB>3</SB>is set to 6 to 8μm, r<SB>2</SB>/r<SB>1</SB>is set to 2.0 to 4.0, Δ1 is set to +1.3 to +2.0%, Δ2 is set to -1.3 to -0.6%, Δ3 is set to +0.1 to +0.5%, wavelength dispersion is set equal to or smaller than -20ps/nm/km at a wavelength selected from the U-band, RDS is set to 0.008 to 0.012nm<SP>-1</SP>, bending loss at a diameter of 20mm is made equal to or smaller than 40dB, and a cut-off wavelength is set shorter than 1.625μm under the operating condition. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a dispersion compensating fiber and a composite optical fiber transmission line using the same.

  At present, in long-distance optical communication, in order to suppress cumulative chromatic dispersion and four-wave mixing (hereinafter abbreviated as “FWM”), mainly, non-zero dispersion shifted fiber (Non-Zero Dispersion Shifted Fiber). Hereinafter, it may be abbreviated as “NZ-DSF”).

FIG. 10 is a graph showing an example of chromatic dispersion characteristics of a non-zero dispersion shifted fiber.
As shown in FIG. 10, in the non-zero dispersion shifted fiber, the zero dispersion wavelength is set at a wavelength slightly shifted from the wavelength 1.55 μm band. When this non-zero dispersion shifted fiber is used as an optical fiber transmission line, chromatic dispersion is accumulated. The chromatic dispersion accumulated by the non-zero dispersion shifted fiber is compensated by a dispersion compensating fiber module connected to the front stage or the rear stage of the non-zero dispersion shifted fiber, and is designed to reduce the dispersion in the entire optical fiber transmission line. Yes. As a dispersion compensating fiber used for such a purpose, for example, an optical fiber having characteristics as disclosed in Patent Document 1 can be cited.

 On the other hand, as shown in FIG. 11, a dispersion shifted fiber (hereinafter also referred to as “DSF”) in which a zero dispersion wavelength is shifted to a wavelength of 1.55 μm band has already been laid in a wide range. . Since this dispersion-shifted fiber has a small wavelength dispersion in the wavelength 1.55 μm band, long-distance high-speed communication is possible by combining it with an optical amplifier for the wavelength 1.55 μm band.

However, in wavelength division multiplexing (hereinafter abbreviated as “WDM”) transmission, the above-mentioned small chromatic dispersion is a problem, and transmission characteristics are deteriorated due to nonlinear phenomena such as FWM.
Therefore, in an optical fiber transmission line using a dispersion shifted fiber, it has already been proposed to perform WDM transmission in an L-band (1565 nm to 1625 nm) where the chromatic dispersion is not zero but relatively large. Since this optical fiber transmission line has the characteristics of a non-zero dispersion shifted fiber in L-band, WDM transmission is possible. Examples of the dispersion compensating fiber that compensates the dispersion shifted fiber with L-band include an optical fiber having characteristics as disclosed in Patent Document 2, for example.

 In addition, a standard single mode fiber (single-mode fiber, hereinafter abbreviated as “SMF”) having a wavelength dispersion of about +17 ps / nm / km in a 1.55 μm wavelength band, or a non-zero dispersion shifted fiber is used. Examples of the dispersion compensating fiber that compensates with U-band (1625 nm to 1675 nm) include an optical fiber having characteristics as disclosed in Patent Document 3, for example.

 As the wavelength band used for optical fibers, L-band (1565 nm to 1625 nm) that is gradually being used to expand the wavelength range, compared to C-band (1530 nm to 1565 nm), which is generally used at present. In the future, it is considered that S-band (1460 nm to 1530 nm) having a shorter wavelength and U-band (1625 to 1675 nm) having a longer wavelength will be used. In particular, a dispersion-shifted fiber having a relatively small wavelength dispersion in the U-band and a non-zero-dispersion shifted fiber having a zero-dispersion wavelength on the longer wavelength side than the wavelength of 1.55 μm are suitable for high-speed transmission.

However, the currently proposed dispersion compensating fiber as described above is not suitable for use in U-band in terms of dispersion slope compensation. In addition, since these dispersion compensating fibers are designed to be optimized in the used wavelength band, bending loss increases at wavelengths longer than the used wavelength band, making it difficult to use. .
JP 2003-255170 A JP 2002-277668 A JP 2003-207676 A

  The present invention has been made in view of the above circumstances. In the used wavelength band, the bending loss is small, the ratio of the dispersion slope to the chromatic dispersion is large, and the residual dispersion after the accumulated chromatic dispersion compensation is spread over the entire U-band. It is an object to provide a small dispersion compensating fiber and a composite optical fiber transmission line using the same.

In order to solve the above problems, the present invention provides at least a center core having a higher refractive index than the cladding, a side core having a lower refractive index than the cladding provided on the outer periphery of the center core, and a cladding provided on the outer periphery of the side core. The center core has a radius of 1.2 μm to 2.0 μm, the distance from the fiber center to the outer periphery of the ring core is 6 μm to 8 μm, and the center core radius. The ratio of the distance from the fiber center to the outer periphery in the side core to 2.0 is 4.0 to 4.0, the relative refractive index difference of the center core to the clad is + 1.3% to + 2.0%, and the relative refractive index difference of the side core to the clad Is -1.3% to -0.6%, and the relative refractive index difference of the ring core with respect to the cladding is + 0.1% to + A .5%, at a wavelength selected from U-band 'is a wavelength range of 1.625Myuemu～1.675Myuemu, chromatic dispersion is -20 ps / nm / miles or less, the ratio for the wavelength dispersion of the dispersion slope 0.008nm Dispersion compensating fiber having a cutoff loss of 40 dB / m or less at a diameter of ^{−1 to} 0.012 nm ⁻¹ and a diameter of 20 mm and a cutoff wavelength shorter than 1.625 μm in use.

By having such a refractive index distribution, the dispersion compensating fiber of the present invention has excellent chromatic dispersion and dispersion slope compensation performance while maintaining low bending loss that can be modularized in U-band. Also, the dispersion compensating fiber of the present invention, by a 0.008nm ^-1 ~0.012nm ^-1 the ratio chromatic dispersion of the dispersion slope at U-band ', it is possible to decrease the residual dispersion in the used wavelength band Therefore, the transmission speed can be increased. Further, the dispersion compensating fiber of the present invention suppresses the bending loss while increasing the ratio of the dispersion slope to the chromatic dispersion, so that there is almost no loss degradation even when modularized, and the U-band has high performance with low loss. It is possible to provide a dispersion compensating fiber module for use.

  In the dispersion compensating fiber having the above-described configuration, it is preferable that a difference between a curve representing the chromatic dispersion in the U-band and a straight line obtained by linear approximation of the curve is 10 ps / nm / km or less.

  Even if the dispersion slope at a certain wavelength is completely compensated, the fact that the unevenness of the curve representing the chromatic dispersion is large in the used wavelength band indicates that the deviation of the residual dispersion in the used wavelength band is large. When the deviation of the residual dispersion is large, dispersion compensation becomes complicated. In the dispersion compensating fiber of the present invention, the difference between the curve representing the chromatic dispersion in the U-band and the straight line obtained by linearly approximating the curve is set to 10 ps / nm / km or less, whereby the residual in the U-band. The dispersion can be reduced. In addition, even when dispersion compensation is required for each wavelength, dispersion compensation can be performed by a simple method, and can be applied to high-speed transmission.

  In the dispersion compensating fiber having the above configuration, it is preferable that an absolute value of a value obtained by dividing chromatic dispersion by transmission loss is 100 ps / nm / dB or more.

  In the dispersion compensating fiber of the present invention, if the absolute value of the value obtained by dividing the chromatic dispersion by the transmission loss is 100 ps / nm / dB or more, a dispersion compensating fiber module using this dispersion compensating fiber has a low loss.

In the dispersion compensating fiber having the above configuration, at a wavelength selected from L-band in the wavelength range of 1.565 μm to 1.625 μm, the chromatic dispersion is −20 ps / nm / km or less and the ratio of the dispersion slope to the chromatic dispersion is 0. It is preferably 0.01 nm ⁻¹ or more.

The dispersion compensating fiber of the present invention has a wavelength dispersion in the L-band of −20 ps / nm / km or less and a ratio of the dispersion slope to the wavelength dispersion of 0.01 nm ⁻¹ or more. The slope, the chromatic dispersion in the L-band and the dispersion slope can be compensated simultaneously. By using a dispersion compensating fiber module using this dispersion compensating fiber, for example, for the time being, an optical transmission line that uses only L-band will use U-band by expanding the future wavelength range. Can be prepared for the case. That is, it is possible to carry out optical transmission with an expanded usable wavelength range without replacing the dispersion compensating fiber module.

  In the dispersion compensating fiber having the above configuration, the center core preferably contains fluorine, and the fluorine concentration in the center core is preferably 0.02 wt% or more and 0.3 wt% or less.

  In the dispersion compensating fiber of the present invention, germanium is added to the center core in order to make the refractive index of the center core higher than that of the cladding. In the dispersion compensating fiber of the present invention, a small amount of fluorine is added to the center core together with germanium. As a result, the dispersion compensating fiber of the present invention has a reduced absorption loss due to the OH group that causes an increase in transmission loss in the wavelength 1.38 μm band. As a result, the dispersion compensating fiber of the present invention is one in which an increase in transmission loss is suppressed.

  The present invention is a composite light obtained by combining the dispersion compensating fiber and an optical fiber having a zero dispersion wavelength in the wavelength range of 1.525 μm to 1.575 μm (hereinafter sometimes abbreviated as “dispersion shifted fiber”). Provide a fiber transmission line.

  By combining the dispersion-compensating fiber of the present invention and this dispersion-shifted fiber, a length capable of dispersion compensation can be increased, and a composite optical fiber transmission line with little characteristic deterioration due to high-speed transmission can be constructed. . Furthermore, by using this composite optical fiber transmission line, high-speed transmission such as 10 Gbit / s can be performed in U-band.

  The composite optical fiber transmission line having the above configuration has a residual dispersion of 3 when converted to the length of the optical fiber having a zero dispersion wavelength in the wavelength range of 1.525 μm to 1.575 μm at all wavelengths in the U-band. It is preferably 25 ps / nm / km or less.

  The composite optical fiber transmission line of the present invention has a residual dispersion of 3.25 ps / nm / km or less in all wavelengths in the U-band, in terms of the length of the optical fiber having a zero dispersion wavelength in the C-band. Then, further high-speed transmission of 10 Gbit / s or 10 Gbit / s or more can be performed in the U-band. In addition, the relay interval of the composite optical fiber transmission line can be increased.

  The present invention provides a composite optical fiber transmission line formed by combining the dispersion compensating fiber and an optical fiber having a zero dispersion wavelength in L-band.

  By combining the dispersion compensating fiber of the present invention and this non-zero dispersion shifted fiber, a length capable of dispersion compensation can be increased, and a composite optical fiber transmission line with less characteristic deterioration due to high-speed transmission is constructed. Can do. Furthermore, by using this composite optical fiber transmission line, high-speed transmission such as 10 Gbit / s or 10 Gbit / s or more can be performed in U-band.

  The dispersion compensating fiber of the present invention is a fiber having a small bending loss, a large ratio of the dispersion slope to the chromatic dispersion, and a small residual dispersion after the accumulated chromatic dispersion compensation over the entire U-band in the used wavelength band. Therefore, according to the dispersion compensating fiber of the present invention, chromatic dispersion and dispersion slope can be compensated simultaneously without causing loss deterioration due to bending loss in the U-band. In particular, the dispersion compensating fiber of the present invention can compensate for chromatic dispersion accumulated when a dispersion shifted fiber having a zero dispersion wavelength in the 1.55 μm wavelength band is used in U-band.

Hereinafter, a dispersion compensating fiber embodying the present invention will be described with reference to the drawings.
FIG. 1 is a schematic diagram showing an example of the refractive index distribution of the dispersion compensating fiber of the present invention.
1A and 1B, reference numeral 1 denotes a center core, reference numeral 2 denotes a side core provided on the outer periphery of the center core 1, reference numeral 3 denotes a ring core provided on the outer periphery of the side core 2, reference numeral Reference numeral 4 denotes a clad provided on the outer periphery of the ring core 3.

In FIG. 1, the radius of the center core 1 is r ₁ , the distance from the fiber center to the outer periphery of the side core 2 is r ₂ , and the distance from the fiber center to the outer periphery of the ring core 3 is r _3. The difference is Δ1, the relative refractive index difference of the side core 2 with respect to the cladding 4 is Δ2, and the relative refractive index difference of the ring core 3 with respect to the cladding 4 is Δ3. In the present invention, the fiber center is the center of the center core 1 (the center of the dispersion compensating fiber).
The center core 1 has a higher refractive index than the cladding 4, the side core 2 has a lower refractive index than the cladding 4, and the ring core 3 has a higher refractive index than the cladding 4. The center core 1 has a higher refractive index than the ring core 3.

In the dispersion compensating fiber of the present invention, the radius r _{1 of the} center core 1 is 1.2 μm to 2.0 μm, the distance r ₃ from the fiber center to the outer periphery of the ring core ₃ is 6 μm to 8 μm, and the side core 2 with respect to the radius r ₁ of the center core 1 The ratio (r ₂ / r ₁ ) of the distance r ₂ from the fiber center to the outer periphery is 2.0 to 4.0, Δ1 is + 1.3% to + 2.0%, Δ2 is −1.3% to − 0.6%, Δ3 is 0.1% to 0.5%, and chromatic dispersion is −20 ps / nm / km or less at a wavelength selected from U-band (1.625 μm to 1.675 μm). the ratio for the wavelength dispersion of the dispersion slope (relative dispersion slope, hereinafter referred to as "RDS".) is 0.008nm ^-1 ~0.012nm ^-1, loss bend of diameter 20mm There follows 40 dB / m, and has a short cutoff wavelength than 1.625μm in use.

The center core 1 is made of quartz glass mainly containing quartz glass and containing germanium (Ge) as an additive in the form of germanium oxide (GeO ₂ ). Here, germanium is added to make the refractive index of the center core 1 higher than that of the clad 4.

  The side core 2 is made of quartz glass mainly containing quartz glass and containing fluorine (F) as an additive. Here, fluorine (F) is added to make the refractive index of the side core 2 lower than the refractive index of the clad 4.

The ring core 3 is made of quartz glass mainly containing quartz glass and containing germanium (Ge) as an additive in the form of germanium oxide (GeO ₂ ). Here, germanium is added to make the refractive index of the ring core 3 higher than that of the cladding 4.

  Here, in order to make the refractive index of the center core 1 higher than the refractive index of the ring core 3, the concentration of germanium contained in both is adjusted, and the germanium (Ge) concentration contained in the source gas is increased.

The clad 4 is made of pure quartz alone or made of quartz glass co-doped with germanium oxide (GeO ₂ ), fluorine (F), and phosphorus oxide (P ₂ O ₅ ).

  For example, when the clad 4 is made of pure quartz, the germanium concentration contained in the center core 1 is 13 wt% to 20 wt% and the fluorine contained in the side core 2 ( The concentration of (dopant) is 2.4 wt% to 5.2 wt%, and the concentration of germanium contained in the ring core 3 is 1.0 wt% to 5.0 wt%.

Further, in addition to setting the concentration of the additive in each part of the dispersion compensating fiber within the above range, the radius r _{1 of the} center core ₁ is 1.2 μm to 2.0 μm, and the distance r ₃ from the center of the fiber to the outer periphery in the ring core 3. Is 6 μm to 8 μm, and the ratio (r ₂ / r ₁ ) of the distance r ₂ from the fiber center to the outer periphery of the side core 2 to the radius r ₁ of the center core 1 is 2.0 to 4.0.
By doing so, optical characteristics desirable as a dispersion compensating fiber can be obtained.

The dispersion compensating fiber of the present invention has such a refractive index distribution, maintains a low bending loss that can be modularized in U-band, has a large chromatic dispersion coefficient, and has excellent dispersion slope compensation performance. Become.
In addition, if the chromatic dispersion coefficient of the dispersion compensating fiber is large, the length of the fiber to be used is shortened. A dispersion compensation fiber having excellent dispersion slope compensation performance means a fiber that has a small residual dispersion and is suitable for high-speed and long-distance transmission.

Also, the dispersion compensating fiber of the present invention, in the wavelength selected from U-band, (1) the chromatic dispersion is -20 ps / nm / miles or less, (2) RDS is 0.008nm ^-1 ~0.012nm ^-1 (3) The bending loss at a diameter of 20 mm is 40 dB / m or less, and (4) has a cutoff wavelength shorter than 1.625 μm in use.

  In the dispersion compensating fiber of the present invention, (1) since the chromatic dispersion is −20 ps / nm / km or less, the dispersion compensating fiber to be used can be short, which is advantageous for low loss, downsizing, and cost reduction.

The dispersion compensating fiber according to the present invention has (2) an R ⁻ S of 0.008 nm ^{−1 to} 0.012 nm ⁻¹ , so that the transmission optical fiber U− having a zero dispersion wavelength in the wavelength range of 1.525 μm to 1.575 μm is used. It is possible to reduce the residual dispersion in the band. As a result, high-speed and long-distance transmission is possible.

  The dispersion compensating fiber of the present invention (3) has a bending loss of 40 dB / m or less at a diameter of 20 mm. For example, even when it is wound on a small reel having a body diameter of about 90 mm, it is caused by the bending loss that is noticeable on the long wavelength side. It is possible to provide a dispersion compensating fiber module that can suppress loss deterioration, has low loss and stable loss wavelength characteristics, and has low loss temperature characteristics.

  Since the dispersion compensating fiber of the present invention has a cutoff wavelength shorter than 1.625 μm in the use state, it is possible to suppress deterioration of transmission characteristics due to the remaining higher-order mode.

Transmission speed of this manner, by the RDS in U-band 'of the dispersion compensating fiber and 0.008nm ^-1 ~0.012nm ^-1, it is possible to reduce the residual dispersion in the used wavelength band, dispersion compensating fiber Can be made faster. In addition, since the dispersion compensation fiber of the present invention suppresses the bending loss while increasing the RDS, there is almost no loss deterioration even when modularized, and a low-loss and high-performance U-band dispersion compensation fiber module is provided. It becomes possible to provide.

  The dispersion compensating fiber of the present invention is a U-band curve representing chromatic dispersion (hereinafter referred to as “wavelength dispersion curve”) and a straight line obtained by linear approximation of this chromatic dispersion curve (hereinafter referred to as “ The difference from “a straight line obtained by linear approximation of a chromatic dispersion curve” is sometimes referred to as a “linear approximation straight line”) is preferably 10 ps / nm / km or less.

  The difference between the chromatic dispersion curve and the linear approximation line is said to be dispersion deviation from linearity (for example, “deviation from the linear approximation line to the chromatic dispersion curve”).

The deviation of the linear approximation line from the wavelength dispersion curve is defined as follows.
As shown in FIG. 2, the deviation of the linear approximation line B from the chromatic dispersion curve A is expressed by the difference between the chromatic dispersion curve A at an arbitrary wavelength and the linear approximation line B obtained by linear approximation of the chromatic dispersion curve A. Is done.

  For example, as shown in FIG. 2, when the chromatic dispersion curve A is convex upward, chromatic dispersion at both end wavelengths (1.625 μm and 1.675 μm in U-band) in an arbitrary wavelength band (for example, U-band). The absolute values of the differences in chromatic dispersion between the curve A and the linear approximation line B are D1 (ps / nm / km) and D3 (ps / nm / km), respectively. Further, in the vicinity of the center of the wavelength band, the absolute value of the difference between the chromatic dispersion of the chromatic dispersion curve A and the linear approximation line B at the wavelength where the divergence between the chromatic dispersion curve A and the linear approximation line B is the largest is D2 (ps / nm / km). By adding the larger absolute value of D1 and D3 to the absolute value of D2, the deviation of the linear approximation line B from the chromatic dispersion curve A can be expressed.

  Even if the dispersion slope at a certain wavelength is completely compensated, the fact that the unevenness of the wavelength dispersion curve is large in the used wavelength band indicates that the deviation of the residual dispersion in the used wavelength band is large. When the deviation of the residual dispersion is large, dispersion compensation becomes complicated. Therefore, in the dispersion compensating fiber of the present invention, the residual dispersion in the U-band can be reduced by setting the deviation of the linear approximation line from the chromatic dispersion curve to 10 ps / nm / km or less. In addition, even when dispersion compensation is required for each wavelength, dispersion compensation can be performed by a simple method, and can be applied to high-speed transmission.

  If the deviation of the linear approximate straight line from the chromatic dispersion curve exceeds 10 ps / nm / km, the residual dispersion in the U-band of the dispersion compensating fiber becomes large and cannot be applied to high-speed transmission.

In the dispersion compensating fiber of the present invention, the absolute value of the value obtained by dividing the chromatic dispersion by the transmission loss is preferably 100 ps / nm / dB or more.
The absolute value of the value obtained by dividing the chromatic dispersion by the transmission loss is a value called a figure of merit, and is an index indicating the amount of dispersion compensation per loss. When the figure of merit of the dispersion compensating fiber is less than 100 ps / nm / dB, the dispersion compensating fiber module using this dispersion compensating fiber has an increased transmission loss.

In the dispersion compensating fiber of the present invention, the wavelength dispersion is −20 ps / nm / km or less and the RDS is 0.01 nm ⁻¹ at a wavelength selected from a wavelength range (L-band) of 1.565 μm to 1.625 μm. The above is preferable.

  Such a dispersion compensating fiber can simultaneously compensate for the chromatic dispersion and dispersion slope in the U-band and the chromatic dispersion and dispersion slope in the L-band. By using a dispersion compensating fiber module using this dispersion compensating fiber, for example, for the time being, an optical transmission line that uses only L-band will use U-band by expanding the future wavelength range. Can be prepared for the case. That is, it is possible to perform optical transmission with an expanded usable wavelength range without replacing the dispersion compensating fiber module.

Dispersion compensating fiber when wavelength dispersion exceeds −20 ps / nm / km or RDS is less than 0.01 nm ⁻¹ at a wavelength selected from a wavelength range (L-band) of 1.565 μm to 1.625 μm Cannot simultaneously compensate for the chromatic dispersion and dispersion slope in the U-band and the chromatic dispersion and dispersion slope in the L-band.

Furthermore, in the dispersion compensating fiber of the present invention, it is preferable that the center core 1 contains fluorine, and the concentration of fluorine in the center core 1 is 0.02 wt% or more and 0.3 wt% or less.
In the dispersion compensating fiber of the present invention, for example, the relationship between the fluorine concentration and the refractive index distribution is as shown in FIG.

  In the dispersion compensating fiber of the present invention, the center core 1 contains a small amount of fluorine (F) together with germanium. As a result, the dispersion compensating fiber of the present invention has reduced absorption loss due to OH groups that causes an increase in transmission loss in the wavelength 1.38 μm band. As a result, the dispersion compensating fiber of the present invention suppresses an increase in transmission loss.

  When the concentration of fluorine contained in the center core 1 is less than 0.02 wt%, the effect of fluorine addition is small, and the maximum value of the transmission loss in the wavelength 1.38 μm band of the dispersion compensating fiber is large. If the absorption loss (transmission loss) in the wavelength 1.38 μm band is large, the curve representing the absorption loss of the dispersion compensating fiber spreads to the short wavelength side and the long wavelength side where the tail is far from the peak, resulting in loss degradation. Therefore, also in the U-band, the transmission loss of the dispersion compensating fiber increases due to the influence of the bottom of the absorption loss. On the other hand, when the concentration of fluorine contained in the center core 1 exceeds 0.3 wt%, the dispersion compensating fiber has a transmission loss in a wide wavelength band from C-band to U-band due to degradation of Rayleigh scattering and structural irregularity loss. To increase.

Here, the absorption loss due to OH groups in the optical fiber will be described.
The absorption loss due to the OH group causes an increase in transmission loss in the wavelength band of 1.38 μm. The absorption loss due to the OH group is that the moisture remaining in the optical fiber in the manufacturing process becomes a bonded state called OH group, and vibration absorption of the OH group exists in a wavelength band centered on 1.38 μm. This is a loss due to the phenomenon of light absorption at the center.

  Conventionally, in a method for manufacturing an optical fiber preform, particularly a VAD (Vapor Phase Axial Deposition) method, a dehydration operation using a chlorine-based gas has been performed in order to suppress absorption loss due to OH groups. However, in particular, in the MCVD method in which vitrification proceeds almost simultaneously with the reaction, unlike the VAD method in which OH groups are generated during the reaction, no dehydration work is performed. However, since this method cannot sufficiently suppress absorption loss due to OH groups, in the present invention, a small amount of fluorine is added to the center core 1 together with germanium. By adding fluorine to the center core 1, -H can be separated from the OH group, and absorption loss due to the OH group is suppressed.

Next, a method for manufacturing the dispersion compensating fiber of the present invention will be described.
In order to manufacture the dispersion compensating fiber of the present invention, for example, MCVD (Modified Chemical Vapor Deposition) method, VAD method, or the like is used.
In the MCVD method, first, SiCl _{4 as} a main raw material in a liquid state and GeCl ₄ and POCl _{3 in a} liquid state as additives for increasing the refractive index of glass are vaporized with oxygen gas and introduced into a quartz tube. . At this time, fluorine (F) may be added to lower the refractive index of the glass.

The quartz tube rotates about its axis at a constant speed and is heated by an oxyhydrogen burner. By heating the quartz tube, an oxidation reaction of SiCl ₄ represented by the following formula (1) occurs in the quartz tube, and oxide fine particles are generated.
SiCl ₄ + O ₂ → SiO ₂ (oxide fine particles) + 2Cl ₂ (gas) (1)

Then, SiO ₂ particles containing GeO ₂ or the like are deposited in the quartz tube on the downstream side of the oxygen flow from the position where the burner is arranged. Thereafter, the quartz tube is heated with a high-temperature burner, whereby the SiO ₂ particles are vitrified to obtain a fiber preform.

  The quartz glass deposition layer in this manufacturing method is around several tens of μm, and the refractive index distribution of the optical fiber can be formed by changing the composition of the flow rate of each layer and adjusting the refractive index and composition of the glass. it can.

In the VAD method, vaporized chloride raw materials (SiCl ₄ , GeCl ₄ , POCl _3, etc.) are supplied into an oxyhydrogen flame through a burner, and oxide fine particles are generated by a hydrolysis reaction that occurs in the flame. A rod-shaped porous body is produced by pulling up the glass rod at a constant speed while depositing the oxide fine particles on the tip of a rotating glass rod called a target rod.

Since this porous body contains OH groups and impurities generated in the manufacturing process, dehydration and removal of impurities are performed with chlorine or a chlorine-based gas.
Thereafter, the porous body is sintered to obtain a fiber preform.

  The fiber preform thus obtained is melt-drawn to obtain a dispersion compensating fiber having a predetermined refractive index distribution.

Next, the composite optical fiber transmission line of the present invention will be described.
The first embodiment of the composite optical fiber transmission line of the present invention is a combination of the dispersion compensating fiber of the present invention and an optical fiber having a zero dispersion wavelength in a wavelength range of 1.525 μm to 1.575 μm. .
As such a composite optical fiber transmission line, for example, the above dispersion compensating fiber is wound around a reel having a diameter of 100 mm or less, and a 1.3 μm band zero dispersion single mode fiber or a non-zero dispersion is formed at both ends of the dispersion compensating fiber. For example, a shift fiber is connected.

  Since the center wavelength of the U-band is 1650 nm, the wavelength dispersion of the single mode fiber is 20 ps / nm / km or more because it is on the long wavelength side of about 100 nm with respect to 1550 nm, which is almost the center wavelength of the C-band. And become very big. Therefore, the cumulative dispersion in the optical transmission line using the single mode fiber also increases, and the transmission characteristics are likely to deteriorate.

  A non-zero dispersion shifted fiber having a zero dispersion wavelength shorter than 1550 nm has a chromatic dispersion of several ps / nm / km in C-band. An optical fiber having a large dispersion slope has a chromatic dispersion of about 10 ps / nm / km in the L-band, and exceeds 10 ps / nm / km in the U-band.

  In contrast, the dispersion-shifted fiber has a chromatic dispersion at 1650 nm of around 7 ps / nm / km, and has a desirable chromatic dispersion value in terms of nonlinear characteristics. Therefore, by combining the dispersion shifted fiber and the dispersion compensating fiber of the present invention, the length capable of dispersion compensation can be increased, and a composite optical fiber transmission line with less characteristic deterioration due to high speed transmission can be constructed. .

  Further, by using the composite optical fiber transmission line of this embodiment, it becomes possible to carry out further high-speed transmission of 10 Gbit / s or 10 Gbit / s or more in U-band.

  Furthermore, the composite optical fiber transmission line of this embodiment has a residual dispersion of 3.25 ps / nm / in terms of the length of an optical fiber having a zero dispersion wavelength in C-band at all wavelengths in U-band. It is preferable that it is km or less. In order to enable high-speed and long-distance transmission, it is desirable that the residual dispersion be as small as possible. Here, for example, from the graph showing the relationship between the residual dispersion and the transmission speed shown in FIG. 4, the distance that can be transmitted at 10 Gbit / s is 1040 / 3.25 = 320, so the residual dispersion is 3.25 ps / nm. That it is less than / km means that transmission of 10 Gbit / s and 320 km is possible.

  The second embodiment of the composite optical fiber transmission line of the present invention is a combination of the dispersion compensating fiber of the present invention and an optical fiber having a zero dispersion wavelength in the L-band (non-zero dispersion shifted fiber). It is.

  By using the composite optical fiber transmission line of this embodiment, it becomes possible to carry out further high-speed transmission of 10 Gbit / s or 10 Gbit / s or more in U-band.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further more concretely, this invention is not limited to a following example.

(Example 1)
Using the MCVD method, it has the refractive index distribution shown in FIG. 1A, and Δ1, Δ2, Δ3, r ₂ / r ₁ , r ₃ / r ₁ , and r ₃ are the values shown in Table 1. Thus, four types of dispersion compensating fibers a, b, c, and d were produced.
Table 2 shows the optical characteristics of these dispersion compensating fibers. In Table 2, 2mλc represents a cutoff wavelength measured with a length of 2 m, and Aeff represents an effective cross-sectional area of the dispersion compensating fiber.
Further, the chromatic dispersion characteristics of these dispersion compensating fibers are shown in FIG.
Furthermore, the difference between the chromatic dispersion curve shown in FIG. 5 and a straight line obtained by linear approximation of this chromatic dispersion curve was measured. The results are shown in Table 3.

In Table 2, the polarization mode dispersion shows the measurement result in the L-band because of the measurement device. However, since the polarization mode dispersion has a small wavelength dependency of the measurement value, it is almost the same in the U-band. Is considered to indicate the value of.
Further, the ratio (RDS) of the dispersion slope to the chromatic dispersion is 0.0082 nm ^{−1 to} 0.011 nm ⁻¹ at the wavelength of 1650 nm, and almost completely compensates for the chromatic dispersion and the dispersion slope of the dispersion compensating fiber in the U-band. I found out that I could do it.
Further, in the dispersion compensating fibers a, b, c, and d, the difference between the chromatic dispersion curve and a straight line obtained by linearly approximating the chromatic dispersion curve is 7.5 ps / nm / km or less. confirmed.

(Example 2)
Dispersion-compensating fibers a, b, c, and d are respectively wound on reels having a diameter of 90 mm on reels of 10.25 km, 12.06 km, 16.19 km, and 13.77 km, respectively. Dispersion compensating fiber modules A, B, C, and D were manufactured by connecting (MFD at a wavelength of 1550 nm = 7.2 to 8.8 μm, chromatic dispersion at wavelengths of 1525 nm to 1575 nm ± 3.5 ps / nm / km). The dispersion compensation fiber module A uses a dispersion compensation fiber a, the dispersion compensation fiber module B uses a dispersion compensation fiber b, the dispersion compensation fiber module C uses a dispersion compensation fiber c, and the dispersion compensation fiber module D uses a dispersion compensation fiber. It was produced using d.
Table 4 shows the optical characteristics of these dispersion compensating fiber modules.
FIG. 6 shows residual dispersion characteristics when dispersion compensating fiber modules A, B, C, and D are connected to a dispersion compensating fiber having a length of 100 km.

  From FIG. 6, in U-band, the residual dispersion when using module A is ± 19 ps / nm or less, the residual dispersion when using module B is ± 32.5 ps / nm or less, and module C is used. It was found that the dispersion can be compensated so that the residual dispersion is ± 22 ps / nm or less and the residual dispersion when the module D is used is ± 25 ps / nm or less. From these results, it was found that the dispersion compensating fiber modules A, B, C, and D can sufficiently withstand high-speed transmission of 10 Gbit / s or 10 Gbit / s or more depending on the transmission distance.

(Comparative Example 1)
Using the MCVD method, the refractive index distribution shown in FIG. 1A is obtained, and Δ1, Δ2, Δ3, r ₂ / r ₁ , r ₃ / r ₁ , and r ₃ are the values shown in Table 5. In this way, a dispersion compensating fiber e was produced.
Table 6 shows the optical characteristics of the dispersion compensating fiber e.
Further, FIG. 7 shows the chromatic dispersion characteristics of the dispersion compensating fiber e.

(Comparative Example 2)
A dispersion compensating fiber module E was manufactured by winding the dispersion compensating fiber e around a reel of 90 mm in diameter of 16.75 km and connecting dispersion shift fibers with connectors to both ends of the dispersion compensating fiber.
Table 7 shows the optical characteristics of the dispersion compensating fiber module E.
FIG. 8 shows residual dispersion characteristics when the dispersion compensating fiber module E is connected to a dispersion compensating fiber having a length of 100 km.
Furthermore, the wavelength characteristic of the insertion loss of the dispersion compensating fiber module E is shown in FIG.

Comparing FIG. 8 and FIG. 6, it was found that the residual dispersion of the dispersion compensating fiber module E is large.
Further, FIG. 9 confirms that the insertion loss of the dispersion compensating fiber module E increases on the long wavelength side due to the influence of bending loss. Therefore, it was found that the wavelength range in which the dispersion compensating fiber module E can be used is limited.

  From the above results, it was found that the dispersion compensating fiber module E requires a further dispersion compensation in order to perform a high-speed transmission in addition to a narrow usable wavelength range when performing optical transmission with U-band. .

The dispersion compensating fiber of the present invention can be applied not only to a module but also to an optical fiber cable using the dispersion compensating fiber itself as a transmission line. It can also be used for Raman amplification using the dispersion compensating fiber itself as an amplification medium.
The composite optical fiber transmission line of the present invention is applicable to a wavelength band wider than U-band.

It is a schematic diagram which shows an example of the refractive index distribution in the dispersion compensation fiber of this invention. It is a figure explaining the difference of a wavelength dispersion curve and the linear approximation straight line obtained by carrying out the linear approximation of this wavelength dispersion curve. It is a figure which shows the relationship between the density | concentration of fluorine, and refractive index distribution. 6 is a graph showing the relationship between residual dispersion and transmission speed in a dispersion compensating fiber. 3 is a graph showing chromatic dispersion characteristics of the dispersion compensating fiber of Example 1. It is a graph which shows the residual dispersion characteristic when the dispersion compensation fiber module of Example 2 and the dispersion compensation fiber of length 100km are connected. 6 is a graph showing the chromatic dispersion characteristics of the dispersion compensating fiber of Comparative Example 1. It is a graph which shows the residual dispersion characteristic when the dispersion compensation fiber module of the comparative example 2 and the dispersion compensation fiber of length 100km are connected. 10 is a graph showing the wavelength characteristics of insertion loss in the dispersion compensating fiber module of Comparative Example 2. It is a graph which shows an example of the wavelength dispersion characteristic of NZ-DSF. It is a graph which shows an example of the wavelength dispersion characteristic of a dispersion shift fiber.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Center core, 2 ... Side core, 3 ... Ring core, 4 ... Cladding.

Claims (8)

At least a center core having a refractive index higher than that of the cladding, a side core having a refractive index lower than that of the cladding provided on the outer periphery of the center core, a ring core having a refractive index higher than that of the cladding provided on the outer periphery of the side core, and an outer periphery of the ring core And a clad provided in the
The radius of the center core is 1.2 μm to 2.0 μm, the distance from the fiber center to the outer periphery in the ring core is 6 μm to 8 μm, and the ratio of the distance from the fiber center to the outer periphery in the side core to the radius of the center core is 2.0 to 4.0. Yes,
The relative refractive index difference of the center core relative to the cladding is + 1.3% to + 2.0%, the relative refractive index difference of the side core to the cladding is −1.3% to −0.6%, and the relative refractive index difference of the ring core to the cladding is +0. 1% to + 0.5%,
At a wavelength selected from U-band in the wavelength range of 1.625 μm to 1.675 μm, the chromatic dispersion is −20 ps / nm / km or less, and the ratio of the dispersion slope to the chromatic dispersion is 0.008 nm ^{−1 to} 0.012 nm. ⁻¹ , Dispersion compensating fiber characterized by having a cutoff loss of 40 dB / m or less at a diameter of 20 mm and a cutoff wavelength shorter than 1.625 μm in use.   2. The dispersion compensating fiber according to claim 1, wherein, in the U-band, a difference between a curve representing the chromatic dispersion and a straight line obtained by linearly approximating the curve is 10 ps / nm / km or less. .   The dispersion compensating fiber according to claim 1 or 2, wherein an absolute value of a value obtained by dividing the chromatic dispersion by a transmission loss is 100 ps / nm / dB or more. In a wavelength selected from L-band which is a wavelength range of 1.565 μm to 1.625 μm, the chromatic dispersion is −20 ps / nm / km or less and the ratio of the dispersion slope to the chromatic dispersion is 0.01 nm ⁻¹ or more. The dispersion compensating fiber according to any one of claims 1 to 3, wherein:   The dispersion compensating fiber according to any one of claims 1 to 4, wherein the center core contains fluorine, and the concentration of fluorine in the center core is 0.02 wt% or more and 0.3 wt% or less.   A composite optical fiber transmission comprising the dispersion compensating fiber according to any one of claims 1 to 5 and an optical fiber having a zero dispersion wavelength in a wavelength range of 1.525 to 1.575 μm. Road.   Residual dispersion is 3.25 ps / nm / km or less when converted to the length of the optical fiber having a zero dispersion wavelength in the wavelength range of 1.525 μm to 1.575 μm at all wavelengths in the U-band. The composite optical fiber transmission line according to claim 6. 6. A composite optical fiber transmission line comprising a combination of the dispersion compensating fiber according to claim 1 and an optical fiber having a zero dispersion wavelength in L-band.

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