JP2006133277A - Quality assurance method for optical fiber, optical fiber, and optical cable - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new quality assurance method for preventing deterioration in the transmission quality in a single optical fiber caused by dispersion of a polarization mode, and to provide an optical fiber assured by the above assurance method. <P>SOLUTION: The quality assurance method for an optical fiber is characterized in that: the birefringence of an optical fiber before processed into a cable is measured; and only an optical fiber showing a measurement value lower than a preliminarily determined reference value is selected to suppress the probability of inducing deterioration in the transmission quality caused by dispersion of a polarization mode in a transmission line after the fiber is processed into a cable. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical fiber used for a transmission line and the like, and relates to an optical fiber quality assurance method for preventing deterioration of transmission quality caused by polarization mode dispersion of the transmission line, an optical fiber quality-guaranteed by the method, and the optical fiber. The present invention relates to an optical cable manufactured using

In an optical fiber used for a transmission line or the like, dispersion is an important characteristic in terms of transmission quality because it induces distortion of a signal pulse waveform.
One type of dispersion is polarization mode dispersion. As the transmission speed of optical communication increases, this polarization mode dispersion has been highlighted as a factor for determining the transmission speed limit of the transmission path (see, for example, Non-Patent Document 1).
F. Kapron, et. al. , NFOEC '96, pp. 757-768, 1996 Okamoto, “Fundamentals of Optical Fiber Communication, Part 3: Factors of Optical Fiber Dispersion”, Oplus E, pp. 706-714, June 1999 Tanikawa et al., 2004 Shingaku Sodai, B-10-37, 2004 M.M. Wilpart et. al. , ECOC2002, PMD / PDL 9.3.3, 2002

  However, polarization mode dispersion varies depending on the external environment (temperature, side pressure, bending, etc.), and fiber manufacturers measure the polarization mode dispersion at the time of shipment and deliver it as satisfying certain standards. However, when a cable manufacturer uses the fiber as a cable, the value of the polarization mode coupling changes due to the change in the external environment, resulting in an increase in polarization mode dispersion. There is a possibility that it cannot be used as a transmission path of the route that has been used.

  Also, the polarization mode dispersion changes depending on the cable structure and manufacturing skill, so it is impossible to determine whether the change is due to cabling or the influence of the external environment described above. There was also a problem that it was not possible to divide the responsibility at the laying manufacturer.

  As described above, various problems have arisen in setting the polarization mode dispersion to the quality assurance value of the optical fiber, and a new quality for preventing transmission quality deterioration due to the polarization mode dispersion in the optical fiber alone. A guarantee method is required.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a new quality assurance method for preventing transmission quality deterioration due to polarization mode dispersion in an optical fiber alone and an optical fiber guaranteed by the assurance method. .

  In order to achieve the above-mentioned object, the present invention measures the birefringence of an optical fiber before cable formation, and selects only the optical fibers whose measured values are lower than a preset reference value. Provided is an optical fiber quality assurance method characterized by suppressing the probability of transmission quality degradation caused by polarization mode dispersion in a transmission line.

  In the optical fiber quality assurance method of the present invention, it is preferable to measure the birefringence distribution along the longitudinal direction of the optical fiber.

  In the optical fiber quality assurance method of the present invention, it is preferable to measure the birefringence of the optical fiber using the polarization OTDR.

In the optical fiber quality assurance method of the present invention, the birefringence reference value is preferably 2 × 10 ⁻⁷ or less.

In the optical fiber quality assurance method of the present invention, the birefringence reference value is preferably 8 × 10 ⁻⁸ or less.

In the optical fiber quality assurance method of the present invention, the birefringence reference value is preferably 3 × 10 ⁻⁸ or less.

  The present invention also provides an optical fiber that is selected by the above-described optical fiber quality assurance method according to the present invention and displays the measured birefringence value or polarization mode dispersion value of the transmission line after cable formation. To do.

The present invention also provides an optical fiber that is cabled and used as a transmission line, and has a birefringence value of 2 × 10 ⁻⁷ or less.

  In the optical fiber, an optical fiber that is cabled and used as a transmission line, and in order to prevent transmission quality deterioration due to polarization mode dispersion of the transmission line, birefringence in the state of the optical fiber before cable formation Preferably, the rate is measured and sorted by the measured birefringence value corresponding to the required transmission quality.

  In the optical fiber, it is preferable that the birefringence is measured in the longitudinal direction to ensure that the entire length is not more than a desired birefringence value.

  In the optical fiber, it is preferable that the birefringence measurement method is polarization OTDR.

In the optical fiber, the birefringence value is preferably 8 × 10 ⁻⁸ or less.

In the optical fiber, the birefringence value is preferably 3 × 10 ⁻⁸ or less.

  In the optical fiber quality assurance method according to the present invention described above, when measuring the birefringence, the bending radius R of the optical fiber is expressed by the following equation.

(Where n is the refractive index of the optical fiber, p ₁₁ and p ₁₂ are Pockels coefficients, ν is the Poisson's ratio, and r is the optical fiber radius.)
It is preferable to measure at a bending radius that satisfies the following relationship.

  The present invention also provides an optical fiber characterized in that the birefringence value is guaranteed by the quality assurance method.

  The present invention also provides an optical fiber characterized in that the birefringence value is priced by the quality assurance method.

  The present invention also provides an optical cable manufactured using the above-described optical fiber according to the present invention.

  In the optical cable, it is preferable that two or more optical fibers are bundled.

According to the present invention, it is possible to provide a new quality assurance method for preventing transmission quality deterioration due to polarization mode dispersion in an optical fiber alone and an optical fiber guaranteed by the assurance method.
Further, by producing an optical cable using an optical fiber guaranteed by the quality assurance method of the present invention, a transmission line capable of high-quality and high-speed communication can be realized.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 3 is an end view showing an example of a non-polarization-maintaining optical fiber that is normally used for optical transmission, for example, a single mode fiber. This optical fiber includes a central core 1 and a clad 2 surrounding the core 1.
Polarization mode dispersion is caused by the non-circularity of the core shape of the optical fiber, the non-circularity of the stress generated in the core, etc., and the propagation speed differs between the two intrinsic polarizations propagating in the optical fiber ( This is a dispersion caused by a group velocity difference).

  In a non-polarization-maintaining optical fiber used for optical transmission, there are three parameters that determine polarization mode dispersion, and the local birefringence magnitude and birefringence axis of the optical fiber, respectively. The polarization mode coupling constant, which is an indicator of the fluctuation (rotation) in the direction, and the length of the optical fiber (transmission path).

  Approximately in a practical range, in a non-polarization maintaining optical fiber used for transmission, polarization mode dispersion (PMD) is expressed by the following equation (1) using these three parameters. Can be represented.

Here, B represents the birefringence, C represents the speed of light, L represents the strip length, and h represents the mode coupling constant.
More precisely, the polarization mode dispersion is expressed by the following equation (2).

However, since the wavelength-dependent term (λ represents the light wavelength) has a smaller influence (about several percent) than B, there is no practical problem even if this term is ignored. In the following description, for ease of understanding, the description will be made using the formula (1).

The birefringence can be roughly divided into two factors: the birefringence of the fiber itself and the birefringence due to stress applied from the outside due to bending, lateral pressure, or the like.
Mode coupling is a characteristic mainly originating from the outside caused by a shift in the direction of the birefringence due to the birefringence of the fiber itself and a stress applied from the outside, twisting of the fiber, and the like. This is quantified by the mode coupling constant (see Non-Patent Document 2).

  Since the last transmission path length varies depending on actual installation conditions, it cannot be an index to be adjusted at the optical fiber design stage, and is an index performance that aims to be used as long as possible when designing an optical fiber. Therefore, this parameter is not particularly described.

  Here, when considering the polarization mode dispersion before and after the optical fiber cable, the birefringence of the fiber itself does not change between the strand before cable and the strand after cable. The birefringence due to external causes is different from the polarization mode coupling constant.

  In general, when optical fibers are shipped recently, they are wound around a shipping bobbin with as low tension as possible in order to reduce the change in polarization mode dispersion in consideration of polarization mode dispersion. Therefore, when the polarization mode dispersion is measured in such a state, since the side pressure is relatively limited, the birefringence due to the internal property of the fiber itself is dominant for the birefringence.

  On the other hand, the twist of the fiber, which is one of the characteristics affecting polarization mode coupling, is difficult to control at the time of winding in the bobbin, and it is considered that a relatively large amount of twist exists at the time of shipment. That is, the polarization mode dispersion at the time of shipment is measured in a state where the polarization mode coupling is relatively large.

  Next, the state of the optical fiber after being cabled varies greatly depending on the state of the cable structure and the cable manufacturing process, and is very different. That is, some cables have a large birefringence due to the external environment, and some cables do not change with respect to the state at the time of shipment. In some cases, some fibers may have a low birefringence. In addition, the polarization mode coupling has a large cable and a small cable with respect to the state at the time of shipment of the strands. In addition, the degree of change in the longitudinal direction may vary depending on the level of manufacturing technology.

  Considering these combinations, from equation (1), the polarization mode dispersion increases as the birefringence increases, and the polarization mode dispersion decreases as the mode coupling constant increases. Table 1 shows the possible states of polarization mode dispersion after cable formation with respect to wave mode dispersion.

  Here, when looking at the relationship between these characteristics and cable manufacturing, the fact that the birefringence increases after cable formation corresponds to the fact that lateral pressure and strain are applied to the optical fiber. An optical cable is designed and manufactured so as not to be subjected to side pressure or distortion as much as possible. Therefore, it is a problem on the optical cable side that the birefringence increases after the formation of the cable.

  On the contrary, the polarization mode coupling tends to increase as the side pressure and twist of the fiber increase, which is well known to those skilled in the art. Accordingly, it can be said that an optical cable having a small polarization mode coupling is manufactured with an excellent cable structure or an excellent cable manufacturing technique. In other words, for optical fibers whose polarization mode coupling has decreased due to cable formation and polarization mode dispersion has become larger than that at the time of shipment, there was a problem with the quality of the optical fiber, that is, due to the internal factors of the optical fiber. It should be considered that the polarization mode dispersion is increasing.

Here, it is considered to guarantee using polarization mode dispersion at the time of shipment.
When cabled using an optical fiber guaranteed by polarization mode dispersion at the time of shipment, No. in Table 1 above. In the case of 1, 4, 7, 8 and 9, the polarization mode dispersion after cable formation may be larger than the polarization mode dispersion at the time of shipment. Furthermore, since the polarization mode coupling is a parameter due to only the external environment for the optical fiber as described above, there was no way to know the degree of polarization mode coupling after cable formation when shipping the optical fiber. There is no known method for measurement without breaking the polarization mode coupling in the cable, that is, while maintaining a state where it can be used as it is as a transmission line. In the case of such an optical fiber, even if quality assurance is performed by polarization mode dispersion at the time of shipment, polarization mode dispersion after cable formation cannot be guaranteed.

Therefore, the present inventors thought that if the birefringence of the optical fiber is directly measured, transmission quality deterioration due to polarization mode dispersion after cable formation due to the optical fiber can be predicted, The concrete method has been studied earnestly. As a result, for example, it has been found that the birefringence of an optical fiber can be directly measured by using an apparatus such as a polarization OTDR in a state where the optical fiber is wound with a relatively gentle diameter and tension. Can now be established.
By establishing a method for measuring this birefringence, the present inventors examined quality assurance against transmission quality degradation due to polarization mode dispersion after cable formation, using the birefringence of the optical fiber. I can do it now.
Therefore, in addition to the study of the birefringence measuring method, the quality assurance method using the birefringence of the optical fiber was invented for the first time by further studying the quality assurance using the birefringence.

The case where the quality of the optical fiber is guaranteed by using the birefringence at the time of shipment will be specifically described below.
The birefringence is different from the polarization mode dispersion when care is taken to suppress the occurrence of externally generated birefringence, that is, when the influence of bending and side pressure is small. It can be measured as a characteristic of the optical fiber itself without being affected by polarization mode coupling caused by twisting or lateral pressure of the optical fiber.

First, the case where the birefringence at the time of a strand is larger than the birefringence at the time of cable formation is assumed (refer No. 1-3 of Table 1).
As described above, when the birefringence measurement is performed while paying attention to suppress the occurrence of externally-induced birefringence, only the internal-induced birefringence is measured. Because it is measured, it cannot be realistic. Even if it exists, the influence of the externally generated birefringence is negligible, so the difference in birefringence before and after cable formation is very small, and it is not necessary to take this into consideration.

Next, the case where the birefringence at the time of a strand is substantially the same as the birefringence at the time of cable formation is assumed (refer No. 4-6 of Table 1).
At this time, if the birefringence at the time of shipment of the wire is limited so that the polarization mode coupling after cable formation is within a certain limit value, the minimum polarization mode coupling constant that can be assumed by any method is obtained. From there, it is necessary to set the upper limit of the birefringence. In other words, the polarization mode dispersion value after cable formation is the largest, in Table 1. With the case of 4 in mind, the upper limit value of the birefringence is set. The method of obtaining the minimum possible polarization mode coupling constant is, for example, a method of obtaining an optical fiber that is not cabled so that it is not twisted, or the smallest possible polarization mode coupling constant. There is a method for experimentally obtaining the polarization mode coupling of an optical cable having a cable structure. When the upper limit value is set in such a form, No. For 4 to 6, the assumed polarization mode dispersion value after cable formation is smaller than the limit value, so that the polarization mode dispersion value after cable formation can be ensured in the optical fiber before cable formation by this method.

Finally, it is assumed that the birefringence at the time of the wire is smaller than the birefringence at the time of cable formation (see Nos. 7 to 9 in Table 1).
When measuring the birefringence with care to suppress the occurrence of externally generated birefringence during strand measurement, this state is caused by the birefringence induced by external force being induced after cable formation. Means that A large birefringence due to external force means that large lateral pressure and distortion are applied to the optical fiber as described above, and this increase in polarization mode dispersion is due to the cable structure or cable manufacturing technology. This is not something that optical fiber manufacturers should follow.

  In summary, the optical fiber whose birefringence is limited while paying attention to suppress the occurrence of externally generated birefringence at the time of shipment is related to the degree of polarization mode coupling. In other words, the optical fiber can avoid transmission quality deterioration due to polarization mode coupling caused by the optical fiber.

The present inventors have intensively studied based on the above-mentioned technical idea, a method of selecting an optical fiber based on the measured value (quality assurance method), and further using this method, a small birefringence We have developed a new optical fiber with a high rate (if there is no problem with the cable, the polarization mode dispersion after the cable becomes small as a result).
This has the advantage that the quality of the optical fiber can be assured regardless of the skill of the cable, that is, the quality of the optical fiber can be selected before it is cabled. .

Needless to say, a cable having a small polarization mode dispersion can be manufactured by using the optical fiber having a small birefringence of the present invention. In particular, when an optical cable composed of a plurality of optical fibers is manufactured, the quality of polarization mode dispersion (after cable formation) of the optical fiber of the constituent element is guaranteed. As a result, it is possible to greatly suppress the occurrence of defects in the entire cable due to the optical fiber, and it is possible to manufacture an optical cable having a small average polarization mode dispersion and a small link design value.
Since the transmission path using this optical fiber or optical cable has a small polarization mode dispersion, the signal degradation due to the polarization mode dispersion is small, and the transmission path is a high performance transmission line.

  Furthermore, as one aspect of the present invention, by measuring the birefringence in the longitudinal direction of the optical fiber, ensuring the birefringence over the entire length of the optical fiber ensures transmission quality at any position of the optical fiber. This is more desirable.

Specific methods for measuring the birefringence include a method of measuring a group delay time difference between polarized waves in a short length (JME method and interferometry are considered), a method obtained from a long period grating, and a method using polarized wave OTDR. Since it is difficult to use other than the polarization OTDR for the optical fiber to be shipped (because it is necessary to cut the optical fiber), the polarization OTDR method is most preferable. Furthermore, since the longitudinal direction distribution of the birefringence can be measured in the polarization OTDR, it is preferable from this viewpoint.
A method for measuring the birefringence in the polarization OTDR is shown in, for example, Non-Patent Document 2 and the like, but this is merely an example, and the present invention is not limited to this.

Here, a specific method for realizing a state in which attention is paid so as to suppress the occurrence of externally generated birefringence is illustrated, but the present invention is not limited to this.
For example, after winding the optical fiber with a low tension so that the side pressure is not applied as much as possible or removing the side pressure by a mechanism that can remove the side pressure, the bending radius R of the optical fiber is expressed by the following equation (3).

It is very effective and preferable to measure the birefringence as follows. Here, R is the bending radius of the optical fiber, n is the refractive index of the optical fiber, p ₁₁ and p ₁₂ are Pockels coefficients, ν is the Poisson's ratio, and r is the optical fiber radius.

  Since the optical fiber selected by such a guarantee method can suppress deterioration in transmission quality due to polarization mode dispersion, it can be suitably used as a transmission line for high-speed communication as long as there is no problem with cable formation.

  Furthermore, by measuring the birefringence in the longitudinal direction of the optical fiber, ensuring the birefringence over the entire length of the optical fiber guarantees the transmission quality at any position of the optical fiber. It is more desirable that the optical fiber has a birefringence measured in the direction. In addition, since the polarization OTDR method is the simplest and can measure the birefringence distribution in the longitudinal direction, it is more preferable that the optical fiber has a birefringence measured by the polarization OTDR method.

Furthermore, the present inventors diligently studied and experimentally obtained a specific limit range of the birefringence index of the optical fiber that satisfies the required polarization mode dispersion after cable formation.
The present invention is the first to clarify the upper limit to be satisfied by the birefringence. As described above, this is the first time that a birefringence measurement method has been established.

As described above, if the birefringence at the time of shipment is limited so that the polarization mode coupling after cable formation is within a certain limit value, it is necessary to estimate the polarization mode coupling constant after cable formation by some method. There is. The polarization mode coupling constant in the case of a loose tube cable that is considered to have the smallest polarization mode coupling constant is known in the literature, and the smallest one is 0.005 / m or more. This is shown in Non-Patent Document 3 and Non-Patent Document 4 (in Non-Patent Document 4, the reciprocal of Lc corresponds to h in this specification). Moreover, the same numerical value is obtained also in the experiment using the optical fiber extended by the present inventors.
It has also been reported that the polarization mode coupling tends to increase as the birefringence decreases.

FIG. 1 shows the results of measuring the birefringence and the polarization mode dispersion for several optical fibers based on these. FIG. 1 shows the experimental results (diamond points) and the relationship between the polarization mode dispersion and the birefringence obtained by the calculation (solid line) when the polarization mode coupling is 0.005 m ⁻¹ . The calculation reflects experimental values of the birefringence dependence of polarization mode coupling. The fiber type is a normal single mode fiber. If the birefringence is below the standard value (vertical dotted line in the figure), it can be seen that the polarization mode dispersion after cable formation satisfies the standard (horizontal dotted line in the figure).

As a result of examination based on this result, when a specific limit range of the birefringence of the optical fiber is set, for example, the birefringence is set to about 2 × 10 ⁻⁷ or less, so that the design / production is not particularly problematic. It is possible to set the polarization mode dispersion value in the optical cable to 0.20 ps / √km or less required for 40G × 80 km transmission. Similarly, in the case of a polarization mode dispersion value of 0.10 ps / √km required for 40 G × 400 km transmission, the birefringence can be surely satisfied if it is about 8 × 10 ⁻⁸ or less. Similarly, in the case of a polarization mode dispersion value of 0.03 ps / √km required for 40 G × 2000 km transmission, the birefringence can be reliably satisfied if it is about 3 × 10 ⁻⁸ or less.

For the polarization mode coupling constant, here, a value in a setting with a considerable margin is used. Therefore, even with an optical fiber having a birefringence greater than the birefringence shown here, the polarization mode dispersion may fall within the standard depending on the degree of polarization mode coupling after cable formation. It is not based on the ability of optical fiber.
Of course, it is preferable to use the above-described method for ensuring the birefringence.

FIG. 4 is a graph illustrating the refractive index distribution in the radial direction of each optical fiber, illustrating the structure of the optical fiber to which the present invention can be applied.
The optical fiber shown in FIG. 4A includes a central high refractive index core 1 and a clad 2 having a low refractive index and a flat refractive index distribution around the core.
The optical fiber of FIG. 4B is composed of the core 1 and the clad 2 as in FIG. 4A, but the core 1 has a refractive index distribution in which the refractive index gradually increases toward the center, and at the center of the core 1. The low refractive index region 3 is provided.
In the optical fiber of FIG. 4C, an inner cladding 4 having an intermediate refractive index between the core 1 and the cladding 2 is provided outside the core 1, and the cladding 2 having a low refractive index is disposed outside the inner cladding 4. It has a stepped refractive index distribution.
The optical fiber of FIG. 4D includes a core 1 having a trapezoidal refractive index profile that spreads from the bottom, a cladding 2 around the core 1, and an outer core 5 having a higher refractive index than the cladding 2 outside the core 1. It is configured.
The optical fiber of FIG. 4 (e) is provided with a core 1 having an inverted V-shaped refractive index profile, and a trench 6 having the lowest refractive index provided on the outer side thereof via an inner cladding 4 and an outer core 5, and the outer side thereof. The clad 2 is provided.
The optical fiber of FIG. 4 (f) has a configuration including a core 1 having an inverted U-shaped refractive index profile, a trench 6 having the lowest refractive index provided on the outer side thereof, and a cladding 2 on the outer side thereof. Yes.
The optical fiber of FIG. 4 (g) includes a core 1 having an inverted V-shaped refractive index profile with a flat center, a trench 6 having the lowest refractive index on the outer side, an outer core 5 on the outer side, and an outer core on the outer side. The second trench 7 and the outer cladding 2 are provided.
The optical fiber shown in FIG. 4 (h) includes a core 1 having a low-refractive region 3 in the center, a trench 6 having the lowest refractive index on the outer side, an outer core 5 on the outer side, and a second trench 7 on the outer side. And the outer cladding 2 thereof.
Note that the refractive index distributions of the optical fibers shown in FIGS. 4A to 4H are merely examples, and the present invention is not limited to these examples, and various optical fibers having other refractive index distributions are used. The present invention can also be applied.

  The optical fiber of the present invention is selected by the optical fiber quality assurance method according to the present invention described above, and the measured birefringence value or polarization mode dispersion value of the transmission line after cable formation is displayed. It is characterized by. For example, the display may be a form printed on a label, tag, instruction manual attached to or bundled with the optical fiber, and can be appropriately selected according to the packaging form of the optical fiber.

  Finally, the novelty of the quality assurance method of the present invention based on the measurement of the birefringence of the optical fiber will be described. Until now, when measuring the birefringence of an optical fiber, it was necessary to measure the optical fiber in a state of being cut short. With this method, as shown in some known examples, it is possible to obtain a birefringence value. However, since the optical fiber measured by such a method is cut into a short length, it can no longer be cabled. According to the present invention, the birefringence can be measured without cutting the optical fiber. Therefore, it is not necessary to measure the birefringence of the optical fiber itself that is actually cabled, and to confirm that there is no deterioration in transmission quality due to the birefringence of the product and the polarization mode dispersion after cabling. It means that.

  As a result of the above examination, the upper limit value of the birefringence index of the optical fiber to be targeted has been clarified. The relationship with the rate was investigated.

  As a result, the eccentricity and non-circularity of the optical fiber preform, the non-circularity of the portion corresponding to the core of the optical fiber, the circumferential nonuniformity of the heating temperature when spinning the optical fiber preform into the optical fiber, etc. Strict management in the manufacturing process has increased the probability of obtaining an optical fiber having a desired birefringence or lower. Table 2 shows the birefringence value of the optical fiber and the polarization mode dispersion value after cable formation obtained by such efforts.

The probability of a strand having a birefringence value of 3 × 10 ⁻⁸ or less corresponding to a polarization mode dispersion value of 0.03 ps / √km, which is said to be necessary for 40G × 2000 km transmission, which is the most severe, is approximately The probability of a strand having a birefringence value of 2 × 10 ⁻⁷ or less corresponding to a polarization mode dispersion value of 0.20 ps / √km, which is said to be required for 22%, 40 G × 80 km transmission, As a result, the polarization mode dispersion can be suppressed in an optical cable actually manufactured using these fibers. That is, it became clear that the optical fiber of the present invention can suppress signal degradation caused by polarization mode dispersion.

  It should be noted that here, as described above, the polarization mode coupling constant is set to a value with a slight margin. As can be seen from FIG. 2, the PMD value of the optical fiber actually cabled is considerably small. Actually, the polarization mode dispersion value of 0.20 ps / √km, which is said to be required for transmission of 40G × 80 km, with a probability of a strand having a polarization mode dispersion value of 0.03 ps / √km or less is approximately 43%. The probability of the following wire is 100% satisfied. This is because the polarization mode coupling constant in the actual cable is larger than expected. The reason for this difference is that, with the same idea as taking a measurement error margin in quality assurance, a margin is given to the expected polarization mode dispersion, and the optical fiber before cable is judged strictly. It is.

  The present optical fiber could never be realized when the polarization mode dispersion was treated as the characteristic value of the optical fiber as in the prior art, and was realized for the first time by the method of the present invention.

  Specifically, when using polarization mode dispersion as an index, the correlation with characteristics such as non-circularity in various manufacturing processes is erased by polarization mode dispersion variations caused by polarization mode coupling. This is because the effect could not be grasped.

  In addition, it is known that the control value (such as non-circle) in each specific process varies greatly depending on the manufacturing apparatus and the product type, and it cannot be uniformly described here. When a normal single mode fiber is manufactured with one of the devices owned by the present applicant, the core non-circle needs to be 0.3% or less.

Further, FIG. 2 shows the results of measuring the birefringence of the optical fibers in Table 2 at the time of shipment of the optical fibers, and then measuring the polarization mode dispersion of each fiber after forming into a loose tube cable. The lines shown in the figure correspond to lines having birefringence of 2 × 10 ⁻⁷ , 8 × 10 ⁻⁸ and 3 × 10 ⁻⁸ , respectively, and the PMDs after cable formation are 0.20 ps / √km and 0.10 ps / respectively. It can be seen that √km and 0.03 ps / √km or less can be realized.

  In some cases, even if the birefringence is large, the polarization mode dispersion value is below the target value, but this was larger than the lowest value assumed for the polarization mode coupling of the cable at the time of the experiment. It means that.

Furthermore, an optical cable using this optical fiber was prototyped and the effect was verified.
Optical fibers were selected with each birefringence being 2 × 10 ⁻⁷ , 8 × 10 ⁻⁸ and 3 × 10 ⁻⁸ as the upper limit, and five optical cables were produced for each group.

The total number of fibers included in each group of cables is 206, 140, and 108, and the average values of polarization mode dispersion thereof are 0.028 ps / √km, 0.014 ps / √km, 0. Link Design Value used for performance index of polarization mode dispersion of optical cable is PMD _Q described in ITU-T Recommendation G.652, and is 0.042 ps / √km and 0.026 ps, respectively. / √km, 0.016 ps / √km. In an optical cable using a conventional optical fiber, the average value of polarization mode dispersion is 0.057 ps / √km and PMD _Q is 0.125 ps / √km in the sample measured by sampling. Polarization mode dispersion is sufficiently reduced. Thus, it can be said that the effect of the present invention has been demonstrated.

It is a graph which shows the experimental result (diamond point) of the Example of this invention, and the relationship between the polarization mode dispersion | distribution and birefringence obtained by the calculation (solid line) when polarization mode coupling | bonding is 0.005m- ¹ . It is a graph which shows the result of having measured birefringence at the time of an optical fiber, and measuring polarization mode dispersion of each fiber after making into a loose tube cable after that. It is an end view which shows an example of the structure of a normal optical fiber. It is a graph which illustrates the structure of the optical fiber which can apply this invention, and shows the refractive index distribution of radial direction.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Core, 2 ... Cladding, 3 ... Low refractive index area | region, 4 ... Inner clad, 5 ... Outer core, 6 ... Trench, 7 ... 2nd trench.

Claims (18)

  By measuring the birefringence of the optical fiber before cable formation and selecting only the optical fiber whose measured value is below the preset reference value, transmission due to polarization mode dispersion of the transmission line after cable formation An optical fiber quality assurance method characterized by suppressing the occurrence probability of quality degradation.   2. The optical fiber quality assurance method according to claim 1, wherein the distribution of birefringence is measured along the longitudinal direction of the optical fiber.   3. The optical fiber quality assurance method according to claim 1, wherein the birefringence of the optical fiber is measured using a polarization OTDR. 4. The optical fiber quality assurance method according to claim 1, wherein the reference value of the birefringence is 2 × 10 ⁻⁷ or less. 4. The optical fiber quality assurance method according to claim 1, wherein the reference value of the birefringence is 8 × 10 ⁻⁸ or less. 4. The optical fiber quality assurance method according to claim 1, wherein a reference value of the birefringence is 3 × 10 ⁻⁸ or less.   An optical fiber selected by the quality assurance method for an optical fiber according to any one of claims 1 to 6 and displaying a measured birefringence value or a polarization mode dispersion value of a transmission line after cable formation. An optical fiber that is cabled and used as a transmission line, and has a birefringence value of 2 × 10 ⁻⁷ or less.   An optical fiber that is cabled and used as a transmission line, in order to prevent transmission quality deterioration due to polarization mode dispersion of the transmission line, the birefringence is measured in the state of the optical fiber before cable formation, 9. The optical fiber according to claim 8, wherein the optical fiber is selected based on the measured birefringence value corresponding to the required transmission quality.   10. The optical fiber according to claim 8, wherein the birefringence is measured in the longitudinal direction, and it is guaranteed that the birefringence is not more than a desired birefringence value in the entire length.   The optical fiber according to any one of claims 8 to 10, wherein the birefringence measuring method is polarization OTDR. The optical fiber according to claim 8, wherein the birefringence value is 8 × 10 ⁻⁸ or less. The optical fiber according to claim 12, wherein the birefringence value is 3 × 10 ⁻⁸ or less. When measuring the birefringence, the bending radius R of the optical fiber is given by

(Where n is the refractive index of the optical fiber, p ₁₁ and p ₁₂ are Pockels coefficients, ν is the Poisson's ratio, and r is the optical fiber radius.)
4. The optical fiber quality assurance method according to claim 1, wherein the optical fiber quality assurance method is measured at a bending radius that satisfies the following relationship.   An optical fiber characterized in that the birefringence value is guaranteed by the method according to claim 1.   An optical fiber characterized in that the birefringence value is priced by the method according to any one of claims 1 to 3, 14, and 15.   The optical cable produced using the optical fiber in any one of Claims 7-13, 15, 16. The optical cable according to claim 17, wherein two or more optical fibers are bundled.

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