JP4205125B2 - Optical fiber and optical transmission line using the same - Google Patents

Description

 The present invention relates to an optical fiber and an optical transmission line using the same, and more particularly to an optical fiber with reduced polarization mode dispersion (hereinafter abbreviated as “PMD”) and an optical transmission line using the same.

Data traffic represented by the Internet is increasing more and more, and an increase in communication transmission capacity is essential to support this data traffic.
WDM (wavelength multiplexing) transmission is a transmission method that meets the needs, and commercialization has already begun. Further, there are the following methods for increasing the capacity of the WDM transmission.
The first method is by increasing the number of multiplexed wavelengths (signals). This method can be realized by narrowing the signal wavelength interval or increasing the wavelength band to be used, but naturally, the number of transmission devices increases for each wavelength used, and there are problems such as cost and installation space.

The second method is by increasing the transmission speed per wavelength (signal). This has recently been attracting attention from the viewpoint of spectrum utilization efficiency. Currently, optical transmission systems with transmission speeds of 2.5 Gbit / s to 10 Gbit / s and even 40 Gbit / s have been studied and partially commercialized. Yes.
When such a transmission speed of 10 Gbit / s or higher is reached, wavelength dispersion and PMD of the optical fiber become problems. Regarding chromatic dispersion, we are trying to solve this problem by using a non-zero dispersion shifted fiber (hereinafter abbreviated as “NZ-DSF”), a dispersion slope compensation type dispersion compensating fiber (hereinafter abbreviated as “SC-DCF”), etc. Has achieved certain results.

On the other hand, various methods have been proposed for PMD. First, PMD will be described below.
PMD is caused by a difference in group delay time between the HE ^x ₁₁ mode and the HE ^y ₁₁ mode propagating through the optical fiber.
PMD is basically caused by birefringence induced in an optical fiber, although external factors such as lateral pressure also exist. Unless the birefringence is increased intentionally, such as in a polarization maintaining fiber, the birefringence inherent in the optical fiber is caused by the non-circularity of the core. Birefringence is classified into a non-circle of the core itself, that is, due to the non-circularity of the refractive index distribution, and a birefringence due to a deviation from the true circular distribution of stress caused by the non-circle.

Since GeO ₂ is usually added to the core of the optical fiber, the refractive index of the core increases from the refractive index of the cladding, and the thermal expansion coefficient also becomes larger than that of the cladding. This creates a stress distribution near the boundary between the core and the clad, and the core side has a large coefficient of thermal expansion, so it tends to shrink more strongly than the clad in the cooling process after drawing, but the clad side does not shrink, It is pulled by the tension.
On the other hand, the stress in the circumferential direction on the clad side becomes a compressive stress due to the contraction of the core, and the refractive index changes due to the photoelastic effect.
When the core is a perfect circle, the refractive index change due to the photoelastic effect is axially symmetric, so that they cancel each other and no difference in propagation constant occurs between the HE ^x ₁₁ mode and the HE ^y ₁₁ mode. If the core is non-circular, a difference in propagation constant occurs between both polarization modes. The difference in propagation constant caused by this stress asymmetry depends on the degree of non-circularity and the difference in thermal expansion coefficient between the core and the cladding.

When transmission is performed, anisotropy is caused in the way the signal travels due to birefringence, and the signal pulse shape deteriorates. Therefore, it is naturally preferable that the PMD is small. In particular, in high-speed transmission such as 40 Gbit / s, the influence of PMD is significant.
Several proposals have been made for PMD reduction. For example, there are a method of twisting the fiber in the drawing process (for example, see Non-Patent Document 1), a method of reducing the non-circle of the core by cutting off the cladding portion (for example, see Non-Patent Document 2), and the like.
Applied Optics, vol20, no. 17, p2962--1981 IEICE Electronics Society Conference, C-3-79, 1999

However, the method of twisting the fiber in the drawing process does not disclose a method for manufacturing an optical fiber having a PMD that can handle a transmission rate of 10 Gbit / s, and further 40 Gbit / s. For example, in an optical transmission line having a transmission rate of 40 Gbit / s and a transmission line length of 10,000 km, it is said that the allowable upper limit of PMD is about 0.025 ps / √km.
Moreover, regarding the method of reducing the non-circularity of the core by cutting off the cladding, it is unrealistic considering the cost. Furthermore, it is conceivable to select and use one having a small core non-circularity, but this is also difficult in terms of yield and cost.

 The present invention has been made in view of these circumstances, and provides an optical fiber having PMD suitable for high-speed transmission by reducing the birefringence by adjusting the thermal expansion coefficients of the core and the clad. An object of the present invention is to provide an optical transmission line using a fiber.

An optical fiber according to claim 1 of the present invention has a single-layer core and a two-layer clad, the refractive index of the outer clad is higher than the refractive index of the inner clad, and the main constituent material is quartz glass. a is a non-circularity of the core is 5% or less, wherein any or all of the portions of the cladding is made of silica glass doped with germanium and off Tsu containing the core germanium and made of silica glass doped with full Tsu element, wherein the concentration of germanium in the lowest part of the added germanium among the parts of the cladding and C2, the concentration of the most common sites of germanium germanium added to the core and C1 C2−C1 ≧ −0.5 wt%, the coefficient of thermal expansion of the glass constituting the core is α1, and the coefficient of thermal expansion is the highest among the respective portions of the cladding. The thermal expansion coefficient of the glass constituting the site is small when the [alpha] 2, was ^{-2.5 × 10 -7 /℃≦α1-α2≦1.0×10 -7 / ℃} , the polarization mode dispersion Is 0.03 ps / √km or less, and the relative refractive index difference Δ between the core and the outer cladding is 0.25% to 0.7%.

Optical fiber according to claim 2 of the present invention is to provide an optical fiber according to claim 1 of the present invention, the most thermal expansion coefficient of the prior thermal expansion coefficient of the glass α1 constituting the Kiko A, each part of the clad the thermal expansion coefficient of the glass constituting the site is small when the ^{α2, -1.5 × 10 -7 / ℃} ≦ α1-α2 ≦ 0 / ℃ der is, the polarization mode dispersion 0.015 ps / and wherein the der Rukoto following √km.

An optical fiber according to a third aspect of the present invention is characterized in that in the optical fiber according to the first or second aspect of the present invention, the transmission loss at a wavelength of 1550 nm is 0.22 dB / km or less .

An optical fiber according to a fourth aspect of the present invention has a three-layer core and a two-layer clad, the refractive index of the outer cladding is higher than or equal to the refractive index of the inner cladding, and the refractive index of the center core is a ring. Light that is higher than the refractive index of the groove, the refractive index of the ring groove is lower than the refractive index of the ring core, the refractive index of the ring core is higher than the refractive index of the cladding or the inner cladding and the outer cladding, and quartz glass is the main constituent material a fiber ratio circularity of the core is 5% or less, made of silica glass either or all of the sites were added germanium and off Tsu containing the cladding, of each part of the core made of silica glass either or all of the addition of germanium and off Tsu arsenide, fewest germanium added of each portion of the cladding When the concentration of germanium in a portion of the core is C2 and the concentration of germanium in the portion of germanium most added to the core is C1, C2-C1 ≧ −0.5 wt%, When the thermal expansion coefficient of the glass constituting the portion having the largest thermal expansion coefficient is α1, and the thermal expansion coefficient of the glass constituting the portion having the smallest thermal expansion coefficient among the portions of the clad is α2, −2. 5 × a 10 ^-7 /℃≦α1-α2≦1.0×10 ^-7 / ℃, the polarization mode dispersion is not more than 0.03 ps / √km, the ratio between the outer cladding and the center core The refractive index difference Δ is 0.25% to 0.7%.

Optical fiber according to claim 5 of the present invention is to provide an optical fiber according to claim 4 of the present invention, the thermal expansion coefficient of the glass constituting the most thermal expansion coefficient is large portion of the respective portions of the front SL core [alpha] 1, the thermal expansion coefficient of the glass constituting the most thermal expansion coefficient is small portions of each portion of the cladding is taken as [alpha] 2, was ^{-1.5 × 10 -7 / ℃ ≦ α1} -α2 ≦ 0 / ℃, The polarization mode dispersion is 0.015 ps / √km or less.

An optical fiber according to a sixth aspect of the present invention is the optical fiber according to the fourth or fifth aspect of the present invention, wherein a transmission loss at a wavelength of 1550 nm is 0.25 dB / km or less.

An optical fiber according to claim 7 of the present invention has a four-layer core and a two-layer clad, and the refractive index of the outer clad is higher than or equal to the refractive index of the inner clad. Lower than the refractive index of the inner ring core, the refractive index of the inner ring core is higher than the refractive index of the ring groove, the refractive index of the ring groove is lower than the refractive index of the outer ring core, and the refractive index of the outer ring core is the cladding or inner cladding and An optical fiber having a refractive index higher than that of the outer cladding and mainly composed of quartz glass, wherein the core has a specific circularity of 5% or less, and any or all of the portions of the cladding are made of germanium and made of silica glass doped with fine off Tsu containing any or all of the portions of the core was added germanium and off Tsu-containing quartz C2-C1 when the concentration of germanium in the portion of the clad with the least amount of germanium added is C2 and the concentration of germanium in the portion of the cladding with the most germanium added is C1. ≧ −0.5 wt%, α1 is the thermal expansion coefficient of the glass constituting the portion having the largest thermal expansion coefficient among the respective portions of the core, and the portion having the smallest thermal expansion coefficient among the respective portions of the cladding the thermal expansion coefficient of the glass when the α2 which is ^{-2.5 × 10 -7 /℃≦α1-α2≦1.0×10 -7 / ℃} , the polarization mode dispersion 0.03 ps / √km or less, and a relative refractive index difference between the inner ring core and the outer cladding is 0.25% to 0.7%.

An optical fiber according to an eighth aspect of the present invention is the optical fiber according to the seventh aspect of the present invention, wherein the thermal expansion coefficient of the glass constituting the portion having the largest thermal expansion coefficient among the respective portions of the core is α1, When the thermal expansion coefficient of the glass constituting the portion having the smallest thermal expansion coefficient among the respective portions of the clad is α2, −1.5 × 10 ⁻⁷ / ° C. ≦ α 1 −α 2 ≦ 0 / ° C. The polarization mode dispersion is 0.015 ps / √km or less .

The optical fiber according to claim 9 of the present invention is the optical fiber according to claim 7 or 8 of the present invention , characterized in that the transmission loss at a wavelength of 1550 nm is 0.30 dB / km or less.

An optical fiber according to a tenth aspect of the present invention is the optical fiber according to any one of the first to ninth aspects of the present invention, wherein the concentration of germanium added to the core is 1.5 wt% or less even at the highest concentration. The fluorine concentration is also 1.5 wt% or less .

An optical fiber according to an eleventh aspect of the present invention is the optical fiber according to any one of the first to tenth aspects of the present invention, wherein the optical fiber preform is spun and twisted, and polarization mode dispersion is performed. Is 0.01 ps / √km or less .

An optical transmission line according to a twelfth aspect of the present invention is a combination of the optical fiber according to any one of the first to eleventh aspects of the present invention and an optical fiber that compensates for the chromatic dispersion and dispersion slope of the optical fiber. It is formed .

According to the present invention, when the thermal expansion coefficient of the cladding and the thermal expansion coefficient of the core are adjusted, the thermal expansion coefficient of the glass constituting the core is α1, and the thermal expansion coefficient of the glass constituting the cladding is α2. by 2.5 as a × ^{10 -7} /℃≦α1-α2≦1.0×10 ^-7 / ℃ to produce an optical fiber, the polarization mode dispersion less 0.03 ps / √km Therefore, it is possible to realize an optical fiber capable of preventing high-speed and high-quality transmission by preventing deterioration of the signal pulse shape due to birefringence.
Further, by producing an optical fiber so that −1.5 × 10 ⁻⁷ / ° C. ≦ α 1 −α 2 ≦ 0 / ° C., the polarization mode dispersion can be made 0.015 ps / √km or less. Furthermore, an optical fiber capable of high-speed and high-quality transmission can be realized.

In addition, by manufacturing an optical fiber so that the core is made of pure silica glass and the clad is made of silica glass to which fluorine is added, the thermal expansion coefficient of the clad can be relatively increased, and a suitable PMD can be obtained. Can be realized.
In addition, by fabricating an optical fiber such that the core is made of quartz glass to which germanium and / or fluorine is added and the cladding is made of quartz glass to which fluorine is added, the case where the core has a refractive index distribution, Even in the case where a dopant is necessary for the core from the viewpoint of hydrogen characteristics, an optical fiber capable of satisfying PMD and other characteristics at the same time while sufficiently satisfying these optical characteristics and hydrogen characteristics is realized. be able to.

Further, by adding germanium to the cladding in addition to fluorine, the hydrogen resistance can be improved, so that an optical fiber with improved long-term reliability can be realized.
Further, when the concentration of germanium added to the clad is C2 and the concentration of germanium added to the core is C1, C2−C1 ≧ −0.5 wt%, so that refractive index control and thermal expansion are achieved. An optical fiber with reduced PMD while satisfying both conditions of coefficient control can be realized.
Further, by setting the concentration of germanium added to the core to 1.5 wt% or less and the concentration of fluorine to 0.5 wt% or less, it is possible to reduce the Rayleigh scattering loss and realize a low-loss optical fiber.
Further, by applying twist to the optical fiber preform and spinning it, the polarization mode dispersion can be reduced to 0.01 ps / √km or less.

 The above effects are not limited to an optical fiber having a single-layer core and a single-layer clad, but an optical fiber having a single-layer core and a double-layer clad, and a three-layer core and a single-layer or double-layer clad. Also, an optical fiber having a four-layer core and a one-layer or two-layer clad can be obtained by setting the above parameters in the same manner as described above.

 In addition, by forming an optical transmission line by combining the optical fiber of the present invention and an optical fiber that compensates for the chromatic dispersion and dispersion slope of this optical fiber, the PMD conditions allowed in the dispersion compensating optical fiber are greatly relaxed. In addition to providing a margin for the design of the dispersion compensating optical fiber, it is possible to realize an optical transmission line capable of high-speed and high-quality transmission by lowering the PMD of the entire optical transmission line.

Hereinafter, the present invention will be described in detail.
FIG. 1 shows an example of an optical fiber according to the present invention. FIG. 1A shows the simplest example of a cross section with respect to the longitudinal direction of the optical fiber. In FIG. Reference numeral 2 denotes a clad formed around the core 1. FIG. 1B shows the refractive index distribution of this optical fiber, and the core 1 is formed so that the refractive index is larger than that of the clad 2. The cross-sectional structure and refractive index distribution of another example of the optical fiber of the present invention are also shown in FIGS.
The birefringence of the optical fiber can be attributed to the non-circularity of the refractive index distribution due to the non-circular core 1, and the birefringence caused by the deviation of the stress generated from the non-circular core 1 from the perfect circular distribution. As described above. Of these, when the refractive index difference between the core 1 and the cladding 2 at a level that is commonly used in transmission optical fibers, specifically, the relative refractive index difference Δ is about 0.25% to 0.7%. The contribution of deviation from the perfect circle distribution of stress is large. The birefringence due to the deviation from the true circular distribution of stress increases as the difference between the thermal expansion coefficients of the core and the clad increases as compared with the case of having the same non-circle.

Therefore, in the optical fiber of this example, in reducing the PMD, in the conventional single mode fiber having a core doped with germanium, the core 1 and the clad 2 when the relative refractive index difference Δ is about 0.3% The difference in thermal expansion coefficient is about 3 × 10 ⁻⁷ / ° C., whereas the difference in thermal expansion coefficient is made sufficiently small.
Further, by reversing the signs of birefringence caused by non-circularity of the refractive index distribution generated from the non-circle of the core 1 and birefringence caused by deviation of the stress generated from the non-circle from the true circular distribution, PMD Can be reduced. That is, PMD can be reduced by making the thermal expansion coefficient of the clad 2 larger than the thermal expansion coefficient of the core 1.

Based on these policies, studies were conducted with the aim of reducing PDM. If the upper limit of the non-circularity is set to about 5%, which is the same as that of the conventional optical fiber, in order not to deteriorate the yield of the current non-circularity, the thermal expansion coefficient of the glass constituting the core 1 is α1. In order to make the PDM 0.03 ps / √km or less when the thermal expansion coefficient of the glass constituting the cladding 2 is α2, −2.5 × 10 ⁻⁷ / ° C. ≦ α1−α2 ≦ 1. In order to control within the range of 0 × 10 ⁻⁷ / ° C. and make PDM 0.015 ps / √km or less, control within the range of −1.5 × 10 ⁻⁷ / ° C. ≦ α1-α2 ≦ 0 / ° C. I knew that I should do it.
Here, the thermal expansion coefficient is a numerical value from near the softening temperature to room temperature. This softening temperature is used as an index indicating the state of glass. The softening temperature varies depending on the material, the type and concentration of the additive, and the production method. In quartz glass, the softening temperature decreases as the additive increases. As an example, in the case of quartz glass prepared by the inventors and containing about 1% by weight of fluorine, the softening temperature is about 800 ° C.

Next, a specific method for setting the thermal expansion coefficients of the core 1 and the clad 2 in the above range will be described.
The coefficient of thermal expansion is greatly influenced by the composition of the constituent materials. For example, it is known that quartz glass doped with 1% by weight of germanium has a thermal expansion coefficient that is about 1 × 10 ⁻⁷ / ° C. higher than that of quartz glass. Therefore, in order to adjust the thermal expansion coefficient, it is necessary to carefully examine the materials constituting the core 1 and the clad 2 and the composition ratio thereof. Furthermore, attention is paid to satisfying basic conditions such as optical characteristics, transmission loss, environmental characteristics, mechanical characteristics, manufacturing costs, etc. as a general transmission fiber at that time. Therefore, it is necessary to examine the material and its composition ratio.
As a result of various examinations, the combinations of the materials of the core 1 and the clad 2 that satisfy the basic requirements for the general transmission fiber and the difference in thermal expansion coefficient between the core 1 and the clad 2 and the composition ratio thereof are as follows. It became.

 First, the first combination uses a substantially pure quartz glass for the core 1, that is, a quartz glass that is intentionally free of impurities and uses a quartz glass in which fluorine is added to the cladding 2. Fluorine has the effect of lowering the refractive index, and when fluorine is added, the thermal expansion coefficient increases near the fictive temperature. Therefore, since the refractive index of the core 1 can be made relatively high, there is no problem as a waveguide structure, and an optical fiber having a suitable PMD is produced by relatively increasing the thermal expansion coefficient of the cladding. Can do.

The second combination uses quartz glass in which one or both of germanium and fluorine are added to the core 1 and fluorine is added to the cladding 2 in consideration of manufacturability, hydrogen resistance, optical characteristics, and the like. Even if the core 1 has a refractive index profile or a dopant is required for the core 1 in terms of hydrogen resistance, the optical characteristics and hydrogen characteristics are sufficiently satisfied depending on the requirements from the optical characteristics. PMD and other characteristics can be satisfied at the same time.
With the above combination, the transmission loss at a wavelength of 1550 nm can be reduced to 0.20 dB / km or less.
In the case of adding a dopant for improving the hydrogen resistance, there is no problem even if a dopant is uniformly added to the core 1, but when the refractive index distribution is given inside the core 1 according to the requirement from the optical characteristics, the additive is thermally expanded. Since it affects not only the coefficient but also the refractive index, it is necessary to create a refractive index distribution that satisfies the requirements of optical characteristics. Therefore, it is necessary to realize a dopant distribution that satisfies both the thermal expansion coefficient and the refractive index distribution. However, since the refractive index distribution varies depending on the required optical characteristics, the thermal expansion coefficient also changes each time. It is necessary to design the dopant distribution inclusive.

In addition, it is more preferable to co-add germanium to the clad 2 in consideration of hydrogen resistance characteristics in the case of a submarine fiber or the like in which long-term reliability is highly important.
Furthermore, regarding germanium, when it is added to quartz glass, the change (increase) in the thermal expansion coefficient is large, so special care must be taken when using it as a dopant. That is, when germanium is used for controlling the refractive index, the thermal expansion coefficient of the core 1 becomes large if the amount of germanium added to the core 1 is much larger than the amount of germanium added to the cladding 2. , It becomes a problem.
In order to realize low PMD, the upper limit of the difference between the amount of germanium added to the core 1 and the amount of germanium added to the cladding 2 was estimated. The concentration of germanium added to the cladding 2 was C2, and the germanium added to the core 1 was Assuming that the concentration of C1 is C1, if C2−C1 ≧ −0.5 wt%, PMD is 0.03 ps / √km or less, and it has been found that a good PMD value is obtained. When the germanium concentration of the core 1 is 0.5 wt% or less, it is not necessary to add germanium to the clad 2.

Furthermore, since germanium and fluorine increase Rayleigh scattering loss, which is the main cause of transmission loss, in order to reduce transmission loss, the concentration of germanium should be 1.5 wt% or less even at the highest location, and the fluorine concentration should also be Similarly, it is preferably 1.5 wt% or less.
Furthermore, when these optical fibers are drawn, if the optical fiber is twisted and drawn, PMD can be further reduced. Specifically, PMD can be reduced to 0.01 ps / √km or less. It becomes possible.

According to the optical fiber of this example, the thermal expansion coefficient of the clad 2 and the thermal expansion coefficient of the core 1 are adjusted, the thermal expansion coefficient of the glass constituting the core 1 is α1, and the thermal expansion coefficient of the glass constituting the clad 2 is when the [alpha] 2, by producing the optical fiber as the ^{-2.5 × 10 -7 /℃≦α1-α2≦1.0×10 -7 / ℃} , the polarization mode dispersion 0. It can be set to 03 ps / √km or less, and the deterioration of the signal pulse shape due to birefringence can be prevented, and an optical fiber capable of high-speed and high-quality transmission can be realized.
Further, by producing an optical fiber so that −1.5 × 10 ⁻⁷ / ° C. ≦ α 1 −α 2 ≦ 0 / ° C., the polarization mode dispersion can be made 0.015 ps / √km or less. Furthermore, an optical fiber capable of high-speed and high-quality transmission can be realized.

Further, by manufacturing the optical fiber so that the core 1 is made of substantially pure quartz glass and the cladding 2 is made of quartz glass to which fluorine is added, the thermal expansion coefficient of the cladding 2 can be relatively increased. An optical fiber having a suitable PMD can be realized.
In addition, when the optical fiber is manufactured such that the core 1 is made of quartz glass to which germanium and / or fluorine is added and the clad 2 is made of quartz glass to which fluorine is added, the core 1 has a refractive index distribution. Even in the case where a dopant is required for the core 1 in terms of hydrogen resistance, light that can satisfy PMD and other characteristics at the same time while sufficiently satisfying these optical characteristics and hydrogen characteristics. A fiber can be realized.

Moreover, since hydrogen resistance can be improved by adding germanium to the cladding 2 in addition to fluorine, an optical fiber with improved long-term reliability can be realized.
Further, when the concentration of germanium added to the clad 2 is C2 and the concentration of germanium added to the core 1 is C1, the refractive index control is performed by setting C2−C1 ≧ −0.5 wt%. An optical fiber with reduced PMD while satisfying both conditions of thermal expansion coefficient control can be realized.
Further, even if the concentration of germanium added to the core 1 is the highest, 1.5 wt% or less, and the fluorine concentration is also 1.5 wt% or less, so that the Rayleigh scattering loss can be reduced and a low loss optical fiber can be obtained. Can be realized.
Further, by applying twist to the optical fiber preform and spinning it, the polarization mode dispersion can be reduced to 0.01 ps / √km or less.

The refractive index distribution of the optical fiber of the present invention is not limited to the refractive index distribution by the one-layer core and the one-layer clad shown in FIG.
FIG. 2 shows an optical fiber mainly composed of quartz glass having a single-layer core and a two-layer clad, and the refractive index of the outer clad 2b being higher than that of the inner clad 2a. Is such that α1 and α2 satisfy the above relationship, where α1 is the thermal expansion coefficient of the glass constituting the core and α2 is the thermal expansion coefficient of the glass constituting the portion of the clad where the thermal expansion coefficient is the smallest. To.
In this optical fiber, the cladding is made of at least fluorine-added quartz glass, and the core is made of pure silica glass, or the core is made of germanium and / or fluorine-added quartz glass. It can be 0.22 dB / km or less.

3 and 4 each have a three-layer core and a one-layer or two-layer clad, and the refractive index of the outer clad 2b is higher than or equal to the refractive index of the inner clad 2a. Quartz having a higher refractive index than the ring groove 1b, a lower refractive index than the ring core 1c, and a higher refractive index than the cladding 2 or the inner cladding 2a and the outer cladding 2b. An optical fiber mainly composed of glass, and in this case, the thermal expansion coefficient of glass constituting the portion having the highest thermal expansion coefficient among the respective portions of the core is α1, and the most thermal among the respective portions of the cladding. Α2 is set so that α1 and α2 satisfy the above relationship, where α2 is the thermal expansion coefficient of the glass constituting the portion having a small expansion coefficient.
In this optical fiber, the cladding is made of at least fluorine-added quartz glass, and the core is made of pure silica glass, or the core is made of germanium and / or fluorine-added quartz glass. It can be 0.25 dB / km or less.

FIG. 5 includes a four-layer core and a one-layer or two-layer clad, and the refractive index of the outer clad 2b is higher than or equal to the refractive index of the inner clad 2a, and the refractive index of the center groove 1a is the inner ring core. The refractive index of the inner ring core 1b is lower than the refractive index of the ring groove 1c, the refractive index of the ring groove 1c is lower than the refractive index of the outer ring core 1d, and the refractive index of the outer ring core 1d is clad. 2 or higher than the refractive index of the inner cladding 2a and the outer cladding 2b, and is an optical fiber mainly composed of quartz glass. In this case, the portion having the largest thermal expansion coefficient is formed among the respective portions of the core. Α1 is the thermal expansion coefficient of the glass to be processed, and α2 is the thermal expansion coefficient of the glass constituting the part having the smallest thermal expansion coefficient among the parts of the cladding. Thus, α1 and α2 are set to satisfy the above relationship.
In this optical fiber, the cladding is made of at least fluorine-added quartz glass, and the core is made of pure silica glass, or the core is made of germanium and / or fluorine-added quartz glass. It can be 0.30 dB / km or less.

Next, an example of the optical transmission line of the present invention will be described.
The optical transmission line of the present invention is formed by combining the above-described optical fiber and an optical fiber that compensates for the chromatic dispersion and dispersion slope of the optical fiber.
For long-distance transmission, a composite optical transmission line with reduced signal distortion can be realized by using a dispersion-compensating optical fiber for compensating the chromatic dispersion and dispersion slope of the transmission optical fiber. When the optical fiber of the present invention is used for a simple optical transmission line, the PMD condition allowed in the dispersion compensating optical fiber is greatly relaxed because the PMD of the optical fiber of the present invention is small, and there is a margin in the design of the dispersion compensating optical fiber. Can be given. Also, the PMD of the dispersion compensating optical fiber can be made the same size as before, and the PMD of the entire optical transmission line can be lowered.

Specific examples are shown below.
Example 1
1. An optical fiber having a refractive index profile as shown in FIG. 1 and an optical fiber in which germanium is added to the core (prototype 1), and an optical fiber in which the core is almost pure quartz and fluorine is added to the cladding. (Prototype 2) was prepared and its PMDs were compared.
Prototype 1 contains about 2.7 wt% germanium in the core, and the clad is silica containing no intentional impurities other than a very small amount of chlorine for dehydration. Prototype 2 is silica containing about 1.1 wt% fluorine in the clad and containing no intentional impurities in the core other than a very small amount of chlorine for dehydration. In both optical fibers, the relative refractive index difference between the core and the clad is about 0.33%.
In Prototype 1, PMD was about 0.065 ps / √km, but in Prototype 2, PMD was about 0.012 ps / √km, and when Prototype 2 was spun to produce an optical fiber, PMD was about 0 Reduced to 0.004 ps / √km. At this time, the difference α1-α2 in the thermal expansion coefficient was estimated to be −0.8 × 10 ⁻⁷ / ° C. (3.3 × 10 ⁻⁷ / ° C. in the trial production 1). Moreover, the optical characteristics other than PMD were the same as those of prototype 1.

(Example 2)
1 is an optical fiber having a refractive index profile as shown in FIG. 1. The clad contains about 2.2 wt% fluorine, and the core contains intentional impurities other than a very small amount of chlorine for dehydration. An optical fiber with no silica was made (Trial Production 3).
The relative refractive index difference between the core and the clad is about 0.69%. PMD was about 0.026 ps / √km, and when an optical fiber was produced by span spinning, PMD was about 0.010 ps / √km.
At this time, the difference α1-α2 between the thermal expansion coefficients was estimated to be −1.7 × 10 ⁻⁷ / ° C.

(Example 3)
An optical fiber having a refractive index profile as shown in FIG. 1 was fabricated, with about 0.9 wt% fluorine added to the cladding and about 0.4 wt% germanium added to the core ( Prototype 4).
The relative refractive index difference between the core and the clad is about 0.33%. PMD was about 0.009 ps / √km. When an optical fiber was produced by span spinning, PMD was about 0.006 ps / √km.
Moreover, there was no problem in optical characteristics and hydrogen characteristics.

(Example 4)
1. An optical fiber having a refractive index profile as shown in FIG. 1, wherein about 1.1 wt% fluorine is added to the clad, and about 0.4 wt% germanium and about 0.14 wt% fluorine are added to the core. The added optical fiber was prototyped (prototype 5).
The relative refractive index difference between the core and the clad is about 0.34%. PMD was about 0.015 ps / √km, and when an optical fiber was produced by span spinning, PMD was about 0.006 ps / √km.

(Example 5)
1. An optical fiber having a refractive index profile as shown in FIG. 1, wherein about 1.3 wt% fluorine and about 0.4 wt% germanium are added to the cladding, and about 0.4 wt% germanium is added to the core. An optical fiber to which about 0.2 wt% of fluorine was added was prototyped (prototype 6).
The relative refractive index difference between the core and the clad is about 0.33%. PMD was about 0.021 ps / √km, and when an optical fiber was produced by span spinning, PMD was about 0.008 ps / √km.

(Example 6)
2. An optical fiber having a refractive index profile as shown in FIG. 2, wherein about 1.0 wt% fluorine and about 0.4 wt% germanium are added to the outer cladding 2b, and about 1.2 wt. An optical fiber in which about 0.4 wt% of germanium and about 0.4 wt% of germanium are added and about 0.4 wt% of germanium is added to the core and about 0.2 wt% of fluorine is manufactured (prototype 7).
The relative refractive index difference between the core and the clad is about 0.25%. PMD was about 0.011 ps / √km. When an optical fiber was produced by span spinning, PMD was about 0.005 ps / √km.

(Example 7)
4. An optical fiber having a refractive index profile as shown in FIG. 4, wherein about 1.4 wt% fluorine and about 0.8 wt% germanium are added to the outer cladding 2b, and about 2.0 wt. % Of fluorine and about 0.8 wt% of germanium are added, about 0.8 wt% of germanium and about 0.7 wt% of fluorine are added to the ring core 1c, and about 1.4 wt% of fluorine is added to the ring groove 1b. Then, an optical fiber in which about 1.2 wt% germanium was added to the center core 1a was prototyped (prototype 8).
The relative refractive index difference between the central core and the outer cladding is about 0.50%. PMD was about 0.027 ps / √km, and when an optical fiber was produced by span spinning, PMD was about 0.009 ps / √km.

(Example 8)
5. An optical fiber having a refractive index profile as shown in FIG. 5, wherein about 1.6 wt% fluorine and about 0.8 wt% germanium are added to the outer cladding 2b, and about 1.8 wt. % Fluorine and about 0.8 wt% germanium are added, about 0.8 wt% germanium and about 0.7 wt% fluorine are added to the outer ring core 1d, and about 1.4 wt% fluorine is added to the ring groove 1c. In addition, about 1.3 wt% germanium was added to the inner ring core 1b, and about 1.2 wt% fluorine was added to the center groove 1a, and an optical fiber was prototyped (prototype 9).
The relative refractive index difference between the central core and the outer cladding is about 0.53%. PMD was about 0.026 ps / √km. When an optical fiber was produced by span spinning, PMD was about 0.009 ps / √km.

(Comparative Example 1)
4. An optical fiber having a refractive index profile as shown in FIG. 4, wherein about 1.2 wt% fluorine and about 0.7 wt% germanium are added to the outer cladding 2b, and about 1.7 wt. % Fluorine and about 0.7 wt% germanium are added, about 0.7 wt% germanium and about 0.6 wt% fluorine are added to the ring core 1c, and about 1.4 wt% fluorine is added to the ring groove 1b. Then, an optical fiber in which about 1.8 wt% germanium was added to the center core 1a was prototyped (prototype 10).
The relative refractive index difference between the central core and the outer cladding is about 0.50%. PMD was about 0.054 ps / √km.

(Comparative Example 2)
3. An optical fiber having a refractive index profile as shown in FIG. 3, wherein about 1.1 wt% fluorine is added to the clad 2, and about 0.4 wt% germanium and about 0.3 wt% are added to the ring core 1c. An optical fiber in which fluorine is added, about 0.4 wt% germanium and about 1.5 wt% fluorine are added to the ring groove 1b, and about 2.4 wt% germanium and about 0.2 wt% fluorine are added to the center core 1a. (Prototype 11).
The relative refractive index difference between the central core and the clad is about 0.60%. PMD was about 0.090 ps / √km, and when an optical fiber was produced by span spinning, PMD was about 0.060 ps / √km.

It is a figure which shows an example of the cross section with respect to the longitudinal direction of an optical fiber, and refractive index distribution. It is a figure which shows an example of the cross section with respect to the longitudinal direction of an optical fiber, and refractive index distribution. It is a figure which shows an example of the cross section with respect to the longitudinal direction of an optical fiber, and refractive index distribution. It is a figure which shows an example of the cross section with respect to the longitudinal direction of an optical fiber, and refractive index distribution. It is a figure which shows an example of the cross section with respect to the longitudinal direction of an optical fiber, and refractive index distribution.

Explanation of symbols

1 ... core, 2 ... cladding.

Claims (12)

An optical fiber having a core of one layer and a clad of two layers, the refractive index of the outer cladding being higher than the refractive index of the inner cladding, and comprising quartz glass as a main constituent material,
The specific circle ratio of the core is 5% or less,
Wherein any or all of the portions of the cladding is made of silica glass doped with germanium and off Tsu element made of silica glass in which the core was added germanium and off Tsu element,
C2-C1 ≧ −0..., Where C2 is the concentration of germanium with the least amount of germanium added to each portion of the clad and C1 is the concentration of germanium with the most germanium added to the core. 5 wt%,
When the thermal expansion coefficient of the glass constituting the core is α1, and the thermal expansion coefficient of the glass constituting the portion having the smallest thermal expansion coefficient among the portions of the clad is α2, −2.5 × 10 ^−7. / ° C. ≦ α1-α2 ≦ 1.0 × 10 ⁻⁷ / ° C., and its polarization mode dispersion is 0.03 ps / √km or less,
An optical fiber having a relative refractive index difference Δ between the core and the outer cladding of 0.25% to 0.7%. When the thermal expansion coefficient of the glass constituting the core is α1, and the thermal expansion coefficient of the glass constituting the portion having the smallest thermal expansion coefficient among the portions of the clad is α2, −1.5 × 10 ⁻⁷ The optical fiber according to claim 1, wherein / ° C. ≦ α 1 −α 2 ≦ 0 / ° C. and the polarization mode dispersion is 0.015 ps / √km or less.  The optical fiber according to claim 1 or 2, wherein a transmission loss at a wavelength of 1550 nm is 0.22 dB / km or less. It has a three-layer core and a two-layer clad, and the refractive index of the outer cladding is higher than or equal to the refractive index of the inner cladding, and the refractive index of the center core is higher than the refractive index of the ring groove. An optical fiber whose refractive index is lower than the refractive index of the ring core, the refractive index of the ring core is higher than the refractive index of the cladding or the inner cladding and the outer cladding, and whose main constituent material is quartz glass,
The specific circle ratio of the core is 5% or less,
From said any or all of the portions of the cladding is made of silica glass doped with germanium and off Tsu arsenide, quartz glass any or all of the portions of the core was added germanium and off Tsu containing Become
C2-C1 ≧ −0..., Where C2 is the concentration of germanium with the least amount of germanium added to each portion of the clad and C1 is the concentration of germanium with the most germanium added to the core. 5 wt%,
Α1 represents the thermal expansion coefficient of the glass constituting the portion having the largest thermal expansion coefficient among the respective portions of the core, and α2 represents the thermal expansion coefficient of the glass constituting the portion having the smallest thermal expansion coefficient among the respective portions of the cladding. when a ^{-2.5 × 10 -7 /℃≦α1-α2≦1.0×10 -7 / ℃} , the polarization mode dispersion is not more than 0.03 ps / √km,
Optical fiber wherein the relative refractive index difference of the center core and said outer cladding Δ is characterized in that 0.25% to 0.7%. Α1 represents the thermal expansion coefficient of the glass constituting the portion having the largest thermal expansion coefficient among the respective portions of the core, and α2 represents the thermal expansion coefficient of the glass constituting the portion having the smallest thermal expansion coefficient among the respective portions of the cladding. 5, wherein −1.5 × 10 ⁻⁷ / ° C. ≦ α 1 −α 2 ≦ 0 / ° C. and the polarization mode dispersion is 0.015 ps / √km or less. Optical fiber.  6. The optical fiber according to claim 4, wherein a transmission loss at a wavelength of 1550 nm is 0.25 dB / km or less. It has a four-layer core and a two-layer clad, and the refractive index of the outer cladding is higher than or equal to the refractive index of the inner cladding, and the refractive index of the center groove is lower than the refractive index of the inner ring core. The refractive index of the ring groove is higher than the refractive index of the ring groove, the refractive index of the ring groove is lower than the refractive index of the outer ring core, the refractive index of the outer ring core is higher than the refractive index of the cladding or inner cladding and outer cladding, and quartz glass is used. An optical fiber as a main constituent material,
The specific circle ratio of the core is 5% or less,
From said any or all of the portions of the cladding is made of silica glass doped with germanium and off Tsu arsenide, quartz glass any or all of the portions of the core was added germanium and off Tsu containing Become
C2-C1 ≧ −0..., Where C2 is the concentration of germanium with the least amount of germanium added to each portion of the clad and C1 is the concentration of germanium with the most germanium added to the core. 5 wt%,
Α1 represents the thermal expansion coefficient of the glass constituting the portion having the largest thermal expansion coefficient among the respective portions of the core, and α2 represents the thermal expansion coefficient of the glass constituting the portion having the smallest thermal expansion coefficient among the respective portions of the cladding. when a ^{-2.5 × 10 -7 /℃≦α1-α2≦1.0×10 -7 / ℃} , the polarization mode dispersion is not more than 0.03 ps / √km,
An optical fiber having a relative refractive index difference Δ between the inner ring core and the outer cladding of 0.25% to 0.7%. Α1 represents the thermal expansion coefficient of the glass constituting the portion having the largest thermal expansion coefficient among the respective portions of the core, and α2 represents the thermal expansion coefficient of the glass constituting the portion having the smallest thermal expansion coefficient among the respective portions of the cladding. 8, wherein −1.5 × 10 ⁻⁷ / ° C. ≦ α 1 −α 2 ≦ 0 / ° C., and its polarization mode dispersion is 0.015 ps / √km or less. Optical fiber.  The optical fiber according to claim 7 or 8, wherein a transmission loss at a wavelength of 1550 nm is 0.30 dB / km or less.  The concentration of germanium added to the core is 1.5 wt% or less even at the highest concentration, and the fluorine concentration is also 1.5 wt% or less. Optical fiber.  11. The optical fiber preform is spun by twisting so that the polarization mode dispersion is 0.01 ps / √km or less. 11. Optical fiber.   An optical transmission line formed by combining the optical fiber according to any one of claims 1 to 11 and an optical fiber that compensates for chromatic dispersion and dispersion slope of the optical fiber.

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