JP7149529B2 - How to design a fiber fuse suppressing fiber - Google Patents

Description

Application of Article 30, Paragraph 2 of the Patent Act March 20-23, 2018 General Incorporated Association Institute of Electronics, Information and Communication Engineers 2018 General Conference held at Tokyo Denki University (publication date: March 6, 2018) Presented in

The present disclosure relates to a fiber fuse suppressing fiber, a fuse stopper, and a design method thereof for stopping or suppressing a fiber fuse phenomenon that can occur when high-power light on the order of several W (watts) is input to and transmitted through an optical fiber.

At present, the demand for expanding the transmission capacity of optical fiber communication is increasing day by day due to the development of the Internet, etc., and the total power of light input to optical fibers already installed in core optical networks is approaching 1 W. and is expected to increase further in the future. On the other hand, when light of the order of several watts is input to the optical fiber according to the related art, if some trigger such as bending, cutting, or heating is given, the glass portion of the core of the fiber is heated and melted. It is known that a phenomenon (fiber fuse phenomenon) occurs in which this becomes a high-temperature plasma state and propagates toward the light source side.

FIG. 1 is a conceptual diagram of the fiber fuse phenomenon. The optical fiber shown in FIG. 1 is composed of a clad and a core having different relative refractive index differences. The occurrence of this fiber fuse phenomenon itself occurs stochastically and does not necessarily occur even under specific conditions. However, unless the generation of the fiber fuse is prevented or the generated fiber fuse is stopped, not only the optical fiber transmission line from the generation point to the light source is destroyed, but also the light source (transmission device) is destroyed. Therefore, there is a strong demand for a method of preventing the fiber fuse phenomenon or stopping the fuse that has occurred. There are two ways to stop propagation of fiber fuses.
(Means 1) Make the power of the input light from the light source relatively smaller than a certain threshold (fuse propagation threshold P _th ) (Means 2) Cut the transmission line itself before the fuse propagates

The propagation threshold P _th of the fuse in means 1 has a certain range depending on the type of optical fiber in the optical fiber according to the related technology, but in SMF (1.3 μm band zero dispersion fiber, ITU-T, G.652) The propagation threshold is reported to be 1.5 W, and about 1.2 W for DSF (dispersion shifted fiber, ITU-T, G.653). In the optical fiber according to the related technology, this propagation threshold is known to have a strong correlation with the mode field diameter (MFD), and the two are in a substantially proportional relationship (for example, Non-Patent Document 1 , 4).

Therefore, various optical fiber type fuse stoppers have been proposed in the related art as a countermeasure in the transmission line. When using a fuse stopper, if the power of the input light is set below the propagation threshold of the stopper portion, even if a fuse occurs in a portion other than the stopper portion, the fuse will stop at the stopper portion and the remaining transmission path will remain. And the light source can be protected. As a fuse stopper, it has been proposed to insert a fiber having a particularly high propagation threshold into a part of the optical transmission line as a small optical component for stopping (fuse stopper).

For example, in Non-Patent Document 1, a multimode fiber (GIF: gretted index fiber) with a large MFD is fusion-spliced to an SMF, and the fiber fuse that has propagated is stopped under an input power condition of about 2 to 3 W. is realized. Non-Patent Document 2 proposes a method of heating a portion of a dispersion shift fiber (DSF) to expand the core diameter and MFD to form a Thermally-diffused Expanded Core (TEC) fiber. Non-Patent Document 3 proposes a method of improving the fuse propagation threshold by thinning the 125 μmφ cladding portion of the SMF to an outer diameter of about 10 to 30 μmφ by etching.

Fujita et al., "Breakdown of fiber fuse by GI fiber", 2004 Institute of Electronics, Information and Communication Engineers Communication Society Conference, B-10-5, 2004 Yanagi et al., "Development of Fiber Fuse Breaker Parts", IEICE Technical Report, OPE2004-178, 2004 E. M. Dianov et. al. , "Destruction of silica fiber cladding by the fuse effect", OPTICS LETTERS, 29, 16, 1852-1854, 2004 K. Takenaga et. al. , "Evaluation of high-power endurance of bend-insensitive fibers," in Proc. (OFC/NFOEC 2008), 2008, JWA. 11

However, the related art has problems as specifically described below. In the first related art, regarding the fiber fuse stopper, a GIF with a large MFD is fusion-spliced to an SMF, and the fiber fuse is stopped under an input power condition of about 2 to 3 W. However, the insertion loss is reduced. In order to reduce it, it is necessary to adjust the GIF to an optimum length on the order of several hundred μm, which is not easy to fabricate. Specifically, if the length is deviated from the optimum length by about 500 μm, a surplus loss of about 5 dB occurs. In addition, since SMF and GIF, which have significantly different core diameters, are connected, it becomes a factor in the generation of higher-order modes in the GIF part, and transmission errors must be avoided during high-speed signal transmission. There is also a problem that it remains at a relatively low level of about 3W.

In the method of heating a part of the optical fiber to form a TEC (Thermally-diffused Expanded Core) fiber according to the second related technique, the stopper can be made into a small component, but the single mode condition is maintained. , there is an upper limit (approximately 20 to 30 μm) when enlarging the MFD, and therefore there is a problem that there is a limit to improving the fuse propagation threshold. In fact, the second related technique also shows the result of examination of a fuse stopper using a TEC fiber, but only confirms its effectiveness under the condition that the input power is about 2W. In addition, since the TEC is formed by heating and diffusing the dopant contained in the core part, it is necessary to control and set detailed conditions at the time of fabrication, making it difficult to fabricate the MFD characteristics, that is, the fuse propagation threshold, with good reproducibility. It is also disadvantageous in terms of cost.

Further, in the third related technique, in which the cladding portion of the fiber is thinned to an outer diameter of about 10 to 30 μmφ by etching, it is possible to reduce the insertion loss while maintaining the MFD at a constant value in the initial state. However, it takes a long time to sufficiently reduce the diameter of hard quartz glass by etching, which is disadvantageous in terms of cost. In addition, since the resin coating of the fiber is also removed at the portion where the diameter is reduced, in addition to the effects of the reduction in the diameter of the glass portion, the surface of the glass is more likely to be scratched, resulting in a decrease in mechanical strength. becomes. Also, in the third related art, the effectiveness has only been confirmed under the condition that the input power is 3W maximum.

As described above, with the method according to the related art, it is extremely difficult to realize a fuse stopper that is effective even when the input power is around 4W, or even at a high input power that greatly exceeds around 4W. Furthermore, it is extremely difficult to realize a fiber fuse stopper that satisfies all of the following conditions: insertion loss is sufficiently low, high-order mode generation and mechanical strength are less likely to occur, and production is possible at low cost and with high performance yield. It was difficult.

Furthermore, in the prior art, it is based on experimental verification assuming only general-purpose SMF (typical value of MFD at wavelength 1550 nm: 10.0 μm) as an optical fiber for transmission, and DSF (wavelength Structural requirements for optical fiber transmission lines with MFDs larger than the typical MFD at 1550 nm (8.0 μm) and general-purpose SMF are not specified. Moreover, the hole structure condition under the input power condition exceeding 10 W at a wavelength of 1550 nm, for example, has not been clarified.

In addition, a fiber fuse stopper using a hole-assisted optical fiber (HAF: Hole Assisted Fiber) is used by connecting it to an optical fiber for a transmission line such as SMF by a fusion splicing technique. It was also necessary to consider connectivity with However, when the HAF itself is used as an optical fiber for transmission of high optical power such as for communication or optical power supply, there is no need to consider the above connectivity problem, and the restriction on the value of MFD is eased. Such structural requirements are also different from fiber fuse stoppers. However, as far as I know, there is no example of studying the optimum hole structure condition of HAF as a fiber fuse resistant optical transmission line.

Therefore, in order to solve the above-mentioned problems, the present invention provides a fiber fuse suppressing apparatus having an HAF having a hole structure condition that can reliably prevent the occurrence of a fiber fuse, or a HAF having a hole structure condition that can reliably stop propagation. It is an object of the present invention to provide a fiber, a fuse stopper and a design method thereof.

In order to achieve the above object, the present invention uses D _melted , which is a parameter representing the spread of the heat distribution of the fiber fuse itself, as an important parameter in designing the hole structure.

Specifically, a fiber fuse-suppressed fiber according to the present invention includes a cladding region of uniform refractive index, a core centrally located in the cladding region and having a higher refractive index than the cladding region, and a core within the cladding region. A hole assisted optical fiber (HAF: Hole Assisted Fiber) having N holes with a diameter d arranged at equal intervals so as to circumscribe a circle with a diameter c on the outer periphery of the core of A fiber fuse inhibiting fiber for deactivating a fiber fuse,
For the diameter D _melted of the fiber fuse generation region where the fiber fuse is generated, calculated from the formula (A) based on the desired mode field diameter MFD and the input optical power P,
^In the cross section of the _HAF , the diameter c , the diameter d and the number N of the holes are set.
[Formula (A)]
If P is less than 2.0W:
_Dmelted = (62.76P/(11.44P+16.55)) ^0.5 x MFD
When P is 2.0 W or more:
D _melted = ((62.76P-52.72)/11.44P) ^0.5 x MFD
[Formula (B)]
When c/D _melted is 0.81 or less:
S≧0.12
When c/D _melted is 0.81 to 1.13:
S≧0.750(c/D _melted )−0.488
When c/D _melted is 1.13 to 1.22:
S≧2.778(c/D _melted )−2.779
When c/D _melted is 1.22 to 1.29:
S≧22.85(c/D _melted )−27.26
If c/D _melted is 1.29:
S≧2.21.

Further, a method for designing a fiber fuse-suppressed fiber according to the present invention includes a cladding region having a uniform refractive index, a core having a higher refractive index than the cladding region disposed in the center of the cladding region, and A hole assisted optical fiber (HAF: Hole Assisted Fiber) having N holes with a diameter d arranged at equal intervals so as to circumscribe a circle with a diameter c on the outer periphery of the core of 1. A method of designing a fiber fuse suppressing fiber that terminates a fiber fuse, comprising:
a calculation step of calculating the diameter D _melted of the fiber fuse generation region where the fiber fuse is generated from the equation (A) based on the desired mode field diameter MFD and the input optical power P;
Diameter c and diameter d where the ratio S (=N(d/D _melted ) ² ) of the area of the fiber fuse generation region to the area occupied by all the holes in the cross section of the HAF satisfies formula (B) and a detection step of detecting the number N of the holes;
characterized by performing

First, an approximation function evaluation formula for input power P and MFD of the diameter D _melted of the molten/vaporized region around the core generated after fiber fuse propagation is derived. Next, the structure of the HAF capable of stopping the fiber fuse is determined by the ratio (parameter X) to the approximate value of D _melted and the diameter of the circle inscribed in the hole (twice the distance from the center of the fiber to the hole), and the melting area It is expressed by the area ratio (parameter Y) of vacancies to the area (π(D _melted /2) ² ). Then, by setting the diameter c, the diameter d, and the number N of holes so that the fiber fuse does not propagate on the plane of the parameter X and the parameter Y, the propagation of the fiber fuse can be stopped at the HAF. .

Accordingly, the present invention provides a fiber fuse suppression fiber having an HAF having a hole structure condition that can reliably prevent the occurrence of fiber fuses, or a fiber fuse suppression fiber that has a HAF having a hole structure condition that can reliably stop propagation, and a design method thereof. be able to.

In the fiber fuse suppressing fiber according to the present invention, the mode field diameter MFD is 10 μm or more, more preferably 15 μm or more, and the maximum input power assumed to be used is P _{max ,} P _th =0.135 (W/μm)× It is characterized by satisfying the hole structure conditions of the above formulas (A) and (B) at an input power P in the range of _Pth or more and _Pmax or less when MFD is used.

The mode field diameter MFD is characterized by being approximately equal to the mode field diameter of a single mode optical fiber connected to the HAF. It is possible to reduce the connection loss when connecting the HAF and the single mode optical fiber.

A fuse stopper according to the present invention comprises the fiber fuse suppressing fiber.

The present invention provides a fiber fuse suppressing fiber, a fuse stopper, and a method for designing the HAF having a hole structure condition that can reliably prevent the occurrence of a fiber fuse, or a HAF that has a hole structure condition that can reliably stop propagation. can do.

It is a figure which shows the conceptual diagram of a fiber fuse phenomenon. FIG. 4 is a diagram showing an example of a measurement evaluation system for a melted region diameter D _melted after propagation through a fiber fuse; 3 is a diagram showing an example of the relationship between v(D _melted ) ² (the product of the propagation velocity v and the square of D _melted ) and the input power P; FIG. FIG. 4 is a diagram showing an example of the relationship between v(MFD) ² (the product of v and the square of MFD) and input power P; FIG. 10 is a diagram showing an example of the relationship between the coefficient K and (D _melted /MFD) ² under the condition of P=2.0 W or less; FIG. 4 is a diagram showing an example of the relationship between D _melted /MFD and P; It is a figure which shows an example of sectional drawing of HAF with which the fiber fuse suppression fiber which concerns on this invention, and a fuse stopper are provided. FIG. 5 is a diagram showing an example of an experimental system used for evaluation of fiber fuse propagation or termination and measurement evaluation of propagation threshold power for the fiber fuse suppressing fiber and the fuse stopper according to the present invention. 4 is a table showing specific examples of HAF structures in which the presence or absence of fiber fuse propagation has been confirmed. FIG. 5 is a diagram showing c/D _melted capable of stopping the propagation of the fiber fuse and the area ratio S of the holes and the melted/vaporized regions with respect to the fiber fuse suppressing fiber and the HAF provided in the fuse stopper according to the present invention; FIG. 5 is a diagram showing c/D _melted capable of stopping the propagation of the fiber fuse and the area ratio S of the holes and the melted/vaporized regions with respect to the fiber fuse suppressing fiber and the HAF provided in the fuse stopper according to the present invention;

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, the present invention is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present invention can be implemented in various modified and improved forms based on the knowledge of those skilled in the art. In addition, in this specification and the drawings, constituent elements having the same reference numerals are the same as each other.

(Embodiment 1)
This embodiment relates to an approximation formula and method for estimating and evaluating the diameter D _melted of the melting/vaporization region that occurs in the cross section when a fiber fuse occurs in an optical fiber, which is a reference for hole design of the HAF according to the present invention. It is. As described above, D _melted is a parameter representing the spread of the heat distribution of the fiber fuse itself, which varies depending on the input power P and the MFD of the fiber.

FIG. 2A shows a propagation experimental system of a fiber fuse. A commercial single-mode optical fiber (SMF) 41 and a test single-mode optical fiber 42 are fusion spliced, a fiber fuse is generated at the end of the test single-mode optical fiber 42, and propagation evaluation of the fiber fuse is performed. I did an experiment. In the evaluation, the wavelength of the light source 44 was around 1550 nm, which is a typical wavelength used in optical communication. The propagation velocity v was obtained from a moving image of the state of fiber fuse propagation, and D _melted was estimated from a micrograph of the cross section of the fiber after propagation. FIG. 2B shows an example of a photograph of a fiber cross section after propagation. FIG. 2(B1) is a cross section taken along a plane perpendicular to the longitudinal direction. FIG. 2(B2) is a cross section along a plane parallel to the longitudinal direction. In the cross-section of the fiber after propagation, the melted/vaporized region was cooled and vitrified again, resulting in a ring-shaped trace, and the diameter of this ring was determined as D _melted .

A fiber fuse propagation experiment was conducted using three types of single-mode optical fibers as the test single-mode optical fiber 42 . The breakdown is a general-purpose commercially available SMF, a commercially available single-mode fiber for short wavelengths (SSMF), and a commercially available dispersion-shifted fiber DSF. The MFD values at a wavelength of 1550 nm were 10.1 μm for SMF, 9.2 μm for SSMF, and 7.7 μm for DSF. The input power P ranged from 1.4 to 6.6W during the experiment.

FIG. 3 shows the relationship between v(D _melted ) ² (the product of the propagation velocity v and D _melted squared) and the input power P. As shown in FIG. FIG. 4 shows the relationship between v(MFD) ² (the product of v and the square of MFD) and input power P. In FIG. As can be seen from the figure, the values of v(D _melted ) ² and v(MFD) ² are in good agreement for the three types of optical fibers although the MFDs are sufficiently different. A straight line in each figure is obtained by using the method of least squares for data (n=13) that satisfies the condition (i) that P is 2.0 W or more. In particular, under condition (i), the following approximations (1-1), (1-2), and (2) hold with very good accuracy. Here, K is a proportionality coefficient, and since K is a constant of 62.76 under condition (i) as shown in equation (1-2), the approximation of D _melted is given by equation (3).

The coefficient K (= v(D _melted ) ² /P) is considered to represent the plasmatization efficiency due to the light source power P. That is, in the fiber fuse phenomenon, the work P×t(J) performed by the light source per unit time t is
(a) temperature rise and plasmatization in the D _melted region around the core;
(b) This process is considered to be consumed by the temperature rise outside the D _melted region.

Since in condition (i) D _melted (plasma size) is large, the light from the light source is converted to heat with high efficiency, so the main part of the light source work P×t(J) is given above in (a) is consumed inside the D _melted region of , and the contribution of the above (b) (increase in temperature of the clad and coating due to heat transfer and radiation and dissipation of light) is presumed to be relatively small. At this time, the contribution of the above (a) is proportional to the volume of the cylindrical region (=π(D _melted /2) ² vt) turned into plasma during the time t (s), so (1-1) It is thought that the relationship of the formula arose.

[Formula column]
v(D _melted ) ² = K×P (1-1)
K _(i) = 62.76 in condition (i) (1-2)
v _(i) (MFD) ² =11.44P _(i) +16.55 (2)
D _{melted (i)}
=(62.76P _(i) /(11.44P _(i) +16.55)) ^0.5 MFD
... (3)
K _(ii) = 19.70 (D _{melted (ii)} /MFD) ² (1-3) in condition (ii)
P _(ii) =K _(i) P _(i) /K _(ii) (4)
D _{melted (ii)}
= ((62.76P _(ii) −52.72 )/11.44P _(ii) ) ^0.5 MFD
... (5)

On the other hand, FIG. 5 shows the relationship between K and (D _melted /MFD) ² under condition (ii) of P=2.0 W or less. From the figure, it is considered that there is a proportional relationship between the two. This is probably because the area of the high-temperature portion of the plasma (considered to be proportional to D _melted ² ) decreases as P decreases, and the efficiency K decreases. This is a straight line determined by assuming that K _(i) = K _(ii) = 62.76 when the boundary P = 2.0 W, taking into account the property, and is given by equation (1-3) .

FIG. 6 shows the relationship between D _melted /MFD and P. In FIG. Under the condition (i), each point of the measured value and the approximate predicted value (dotted line) by the formula (3) agreed well. However, under condition (ii), the divergence between the two increases particularly at SMF P=1.4 and 1.5W. The main reason is that K _(i) = 62.76 was assumed in formula (1-2), but the measured value is 62.76 or less according to K _(ii) in formula (1-3). (Fig. 5). Therefore, equation (4) is assumed to compensate for the reduction in K _(ii ), and this is used as the predicted value P _(ii) under condition (ii). That is, by substituting equations (1-3) and (4) into equation (3), approximate equation (5) representing the relationship between P _(ii) and D _melted /MFD was determined. In the figure, the solid line indicates the approximation value obtained by the equation (5). The suffixes (i) and (ii) used in each formula represent the condition (i) and the condition (ii), respectively.

The experimental values (each plotted point) and the predicted values (condition (i): dotted line, condition (ii): solid line) tend to agree well, especially in the range of condition ( _ii ). (5) reproduces well. The relative error between the true value (measured value) and the estimated value is -8.7% to +8.6% (RMS = 3.4 %), confirming good estimation accuracy. By using the approximate expressions (3) and (5) above, it is possible to estimate the value of D _melted with high accuracy for any input power P and MFD value of the optical fiber.

(Embodiment 2)
The present embodiment relates to an example of the cross-sectional structure of the HAF provided in the fuse stopper and an example of use in an optical transmission line.
FIG. 7 is a diagram showing an example of a cross-sectional view of the HAF. The optical fiber (HAF) shown in the figure has a clad 11, a core 12, and holes 13, and the clad 11 is composed of the core 12 and a plurality of holes 13 arranged in an annular shape. In FIG. 7, the hole diameter is shown as d, and the inscribed circle diameter of the hole 13 is shown as c.

As described above, D _melted is the diameter of the circular region in the cross section melted and vaporized by the fiber fuse, and the vicinity of the outside is exposed to extremely high temperatures and pressures even if the glass state is maintained during propagation through the fiber fuse. It is easily deformed and destroyed. Therefore, the smaller the value of c/D _melted , the ratio of c to D _melted , the more holes exist in the vicinity of the hottest region in the center of the fiber fuse.

At that time, high-temperature gas and plasma generated by the fiber fuse flow into the void region, expand rapidly, and lose their energy, so that the temperature tends to be low. In other words, the c/D _melted value is an index of the HAF structure used for the fiber fuse stopper because the hole structure with a smaller c/D _melted is easier to terminate the fiber fuse.

Another index is the ratio S between the pore area (=Nπ(d/2) ² ) and the area of the melted/vaporized region (=π(D _melted /2) ² ). The larger S is, the easier it is for the fiber fuse to stop. This is because high-temperature gas and plasma are likely to expand in volume and lose their energy, so they tend to be low in temperature. Note that S=N(d/D _melted ) ² where N is the total number of vacancies.

FIG. 8 is a diagram showing an example of a system for measuring and evaluating the HAF propagation threshold. HAF 45 and a commercial single mode optical fiber (SMF) 46 are fusion spliced, a fiber fuse is generated at the end of the single mode optical fiber 46 (SMF), the fiber fuse passes through the connection point 43, and the HAF 45 is spliced. It was confirmed and evaluated whether or not it was propagated. In the evaluation, the wavelength of the light source 44 was around 1550 nm, which is a typical wavelength used in optical communication. In the case of HAF, it has been reported that when the input power is low, the fiber fuse penetrates the HAF longer, that is, it propagates more easily through the HAF. Evaluation was performed at P=2.0 W, which is close to the propagation threshold. For the HAF in which the fiber fuse stopped under the condition of 2.0 W, evaluation was also performed at 15.0 W, and it was confirmed that the fiber fuse stopped even under the condition of 15.0 W.

The table in FIG. 9 shows the results of an experiment in which 21 kinds of HAFs were checked for the presence or absence of fiber fuse propagation in the experimental system of FIG. In the table, the hole diameter d, the hole inscribed circle diameter c, the number of holes N, the MFD at a wavelength of 1550 nm for HAF45, P = 2.0 W, obtained from the approximate expression (3) above, The assumed D _melted (calculated value), c/D _melted , and the area ratio of the vacancies to the area of the melted/vaporized region for each HAF were described as S (=N(d/D _melted ) ² ). 8 is a single mode fiber (SMF) 46 assumed as a transmission line fiber, and its MFD at a wavelength of 1550 nm is about 10 μm. The fuse propagation threshold was also evaluated for the eight types of HAFs for which fuse propagation was confirmed, and all of them were about 1.5W.

FIG. 10 is a graph showing the relationship between fiber fuse propagation or termination in the HAF 45 and the area ratio S and c/D _melted of voids to the melted/vaporized area. As can be seen from the figure, structures suitable as fiber fuse stoppers or fiber fuse suppressing fibers of the present invention are included in structural regions where the value of c/D _melted is 1.29 or less and the value of S is 0.12 (12%) or more.

However, the above region also includes a region where the fiber fuse propagates in the experiment. Therefore, when the structure of HAF45 is represented by coordinates (c/D _melted , S), A1 (0.81, 0.12), A2 (1.13, 0.36), A3 (1.22, 0. 61), A4(1.24, 1.07), A5(1.29, 2.21), the straight line c/D _melted = 1.29, the straight line S = 0.12, and the coordinates A1 and A2 , a straight line including coordinates A2 and A3, and a straight line including coordinates A3, A4, and A5, HAF 45 can stop the fiber fuse, and the fiber fuse stopper of the present invention can Alternatively, it is suitable as a fiber fuse suppression fiber. Since the coordinates A3, A4 and A5 are approximately collinear, this straight line is determined by the method of least squares, and the above region is defined using equation (A) and inequality (B). . In FIG. 10, the boundary of the structural area of the above "fiber fuse stop" is indicated by a dotted line.

[Formula column]
For condition (ii) D _melted = (62.76P/(11.44P+16.55)) ^0.5 MFD
In case of condition (i) D _melted = ((62.76P-52.72)/11.44P) ^0.5 MFD (A)

When c/D _melted is 0.81 or less S≧0.12
When c/D _melted is 0.81 to 1.13 S≧0.750 (c/D _melted )−0.488
When c/D _melted is 1.13 to 1.22 S≧2.778(c/D _melted )−2.779
When c/D _melted is 1.22 to 1.29 S≧22.85(c/D _melted )−27.26
When c/D _melted is 1.29 S≧2.21 (B)

Regarding the value of S, a larger value is more preferable for stopping the fiber fuse, but there is a geometrical upper limit value that depends on the value of c. On the other hand, if the value of S is excessively large, deterioration of manufacturing accuracy and reduction of mechanical strength are likely to occur. Therefore, taking these factors into consideration, the value of S may be appropriately set in the structural region within the conditional range of "stopping the fiber fuse". Also, as shown in FIG. 6, D _melted decreases as the input power P decreases. Therefore, when the value of P is relatively small, the value of c should be decreased to bring the position of the hole closer to the center of the fiber. must be placed Therefore, the value of c can be determined based on the value of D _melted obtained by substituting the lower limit value P _min of the input power or the propagation threshold value P _th of the fiber fuse into the equation (3) or (5). Conversely, for example, when P _min is 4 W or more, D _melted is always 2 times or more of MFD, and when P is 6 W or more, D _melted is always 2.1 times or more of MFD. Therefore, when the lower limit _Pmin of the assumed input power is relatively large, around 5 W, the value of c may be relatively large. In general, if the value of c is small, it tends to be difficult to design and manufacture the HAF. The above regions are suitable from the viewpoint of HAF design and manufacturability.

In the HAF having the structure shown in FIG. 10, when verification was performed using the experimental system shown in FIG. I didn't. Therefore, when the HAF having the structure described above is used as a fuse stopper, it is preferable to set the length of the HAF to about 0.5 to 5 mm. Moreover, it is also possible to adopt a configuration in which a single-mode optical fiber having an appropriate length is fusion-spliced to the HAF having the above length. However, in this case, depending on the conditions, the holes in the vicinity of the connection may be crushed by electric discharge during fusion splicing. It is necessary to confirm whether 5 mm or more is secured. Also, in order to suppress the fusion splicing loss, it is preferable that the MFD of the HAF and the MFD of the single mode optical fiber are substantially equal. For example, the error between the MFD of the HAF and the MFD of the single mode optical fiber is preferably ±5-10%.

As described above, the HAF used in the present invention may be very short, from 0.5 mm to several mm, so the connection point with the single mode fiber may be included inside the connector to protect it. In the case of such an optically coded structure, it is easy to insert in the vicinity of a transmission device or in the middle of a transmission line. Even if a fiber fuse occurs in the optical fiber transmission line, the fuse stopper can stop the fiber fuse, so that the transmission device and the optical fiber transmission line can be protected from destruction by the fiber fuse.

(Embodiment 3)
This embodiment relates to an example of a cross-sectional structure of the HAF according to the present invention.
In the present invention, the HAF used for the fiber fuse stopper assumes a wide optical power range. Therefore, even when the input power P is changed within a specific range of use, it is confirmed that the combination of the parameters (c/D _melted , S) described in the above embodiment is included in the "fiber fuse stop" condition range. It is necessary to confirm. For this purpose, the following verification experiment was conducted.

No. in the table of FIG. 2 HAF and the experimental system of FIG. 8, a fiber fuse termination experiment was conducted. The input power conditions at that time were P=2W, 5W, 10W, 15W, and 22W. In an actual termination experiment, the fiber fuse terminated under all conditions, and the penetration depth of the fiber fuse into the HAF was 0.3 mm or less (maximum value of 0.3 mm when P was 22 W). Regarding these conditions, no. The coordinate points of (c/D _melted , S) are shown in FIG. In the drawing, as in FIG. 10, the boundary indicating the range of "fiber fuse stop" is indicated by a dotted line. From the figure, No. Each coordinate of (c/D _melted , S) was all within the range of "fiber fuse stop" even when the value of P was greatly changed for the HAF of 2. This result supports the validity of the HAF and the method for defining the pore structure of the HAF of the present invention.

(Embodiment 4)
This embodiment relates to an example of a cross-sectional structure of the HAF according to the present invention.
When the HAF itself is used alone as a fiber fuse suppression fiber for communication or high optical power transmission such as optical feeding, it is not necessary to consider the problem of connectivity with transmission optical fibers such as SMF. In this case, there are two main problems: nonlinear optical phenomena (mainly stimulated Raman scattering), which are factors limiting optical power, and occurrence of fiber fuses. For example, the optical power that can be transmitted using light with a wavelength near 1550 nm by stimulated Raman scattering is about 15 W at maximum for SMF when a transmission distance of 1 km is assumed, and 1.5 W at maximum when a transmission distance of 10 km is assumed. limited to an extent.

In order to avoid the nonlinear optical phenomenon, a method of enlarging the effective cross-sectional area A _eff (approximately proportional to the square of the MFD), that is, the MFD can be considered. In addition, since the assumed use power range of 1.5 W to 15 W or more may greatly exceed the SMF fiber fuse propagation threshold (P _th ) of about 1.5 W, it is necessary to take measures against the occurrence of fiber fuses as described above. .

From the above point of view, the HAF, in which the MFD is expanded to the value of the SMF (approximately 10 μm) or more, preferably 15 μm or more, is promising as a transmission line fiber in that case. However, as shown in FIG. 6, D _melted sharply decreases near the propagation threshold, so it is necessary to make the diameter c of the inscribed circle of the holes relatively small. This means that the holes are arranged near the center of the core, which makes it difficult to expand the MFD and the effective cross-sectional area _Aeff . In other words, it is necessary to increase the effective cross-sectional area A _eff to avoid the nonlinear optical phenomenon and to reduce the diameter c of the inscribed circle of the air hole to stop the fiber fuse (the effective cross-sectional area A _eff cannot be increased). is in a trade-off state.

In this embodiment, we describe an HAF structure that greatly alleviates this trade-off. As shown in Non-Patent Document 4, it is known that the fiber fuse propagation threshold P _th is proportional to the MFD in various ordinary (not hole-type) single-mode optical fibers, and the relationship is Roughly, it is given by the following equation (6).
[Formula column]
Pth= _0.135MFD (6)

Therefore, up to around 2.0 W, which is a singular point where the change rate of D _melted in FIG. Trade-offs can be mitigated. That is, from the result of substituting P _th =2.0 W into Equation (6), the MFD at the working wavelength should be increased to about 15 μm or more. At this time, assuming that the assumed maximum input power (for example, the maximum output power of the light source) is P _max , the value of D _melted is calculated from Equation (3) in the condition range of P = P _th ~ P _max , and (c/ Structural parameters such as c, d, number of holes N are chosen such that the coordinates of D _melted , S) fall within the range of "fiber fuse stop" in FIG.

With the selected structural parameters, fiber fuses do not occur in the condition range of P = 1.5 W to P _max , and the expansion of the MFD also increases the threshold of nonlinear optical phenomena by about 2.3 times that of the SMF (= MFD ratio 1.5 squared) or more, it is greatly advantageous and suitable compared to the case of using SMF, which is about 1.5 W or more and has a concern of fiber fuse occurrence. That is, in this structure, fiber fuses do not occur due to the expansion effect of the MFD under the condition of P=1.5 W to P _th and due to the effect of air holes under the condition of P=P _th to P _max .

In this case, when predicting D _melted , the MFD of HAF should be set to 15 μm or more and P should be set to _2.0 W or more. The above HAF may be single mode or several modes at the wavelength used. In the case of a few-mode structure, the LP ₀₁ mode, which is the fundamental propagation mode, should be used for transmission.

[Appendix]
The following is a description of the HAF used in the fiber fuse suppressing fiber and fuse stopper of this embodiment.
(1):
The HAF includes a cladding having a uniform refractive index, a core having a higher refractive index than the cladding and located in the center of the cladding region, and a circumference having a diameter of c an optical fiber having N holes of diameter d equally spaced circumscribing thereon,
MFD is the mode field diameter defined by the cladding and core and the wavelength used, P is the input optical power, D _melted is the diameter of the region where the optical fiber melts and vaporizes when the fiber fuse occurs, and the circle defined by D _melted When the ratio of the area to the area occupied by the vacancies is S (=N(d/D _melted ) ² ), the c, MFD, D _melted , P, and S are the above formulas (A) and (B). ) at the same time.

(2):
When the MFD of the fundamental propagation mode (LP ₀₁ mode) at the wavelength used is 10 μm or more, preferably approximately 15 μm or more, and the maximum input power assumed to be used is P _max , P _th = 0.135 MFD. The pore structure condition described in (1) above is satisfied at an input power in the range of _th or more and P _max or less.

(effect)
INDUSTRIAL APPLICABILITY The present invention can provide a HAF that reliably stops propagation of a fiber fuse, a fiber fuse stopper using the HAF, an optical connector, an optical transmission system, an optical fiber, and a fiber fuse stopping method. In addition, the insertion loss is sufficiently low, there is little concern about the occurrence of higher-order modes and the decrease in mechanical strength, etc., and it can be manufactured at a low cost with a high performance yield. On the other hand, it is possible to provide a fiber fuse stopper that stops the generated fiber fuse, an optical connector, and an optical fiber transmission line that suppresses the generation of the fiber fuse.

11: Cladding 12: Core 13: Hole 41: Single-mode optical fiber 42: Single-mode optical fiber (for testing)
43: Connection point 44: Light source 45: HAF

Claims (3)

a cladding region with a uniform refractive index; a core having a higher refractive index than the cladding region and disposed in the center of the cladding region; 1. A method for designing a fiber fuse suppressing fiber comprising a hole assisted optical fiber (HAF) having N holes of diameter d equally spaced at , wherein the fiber fuse is stopped, comprising: ,
a calculation step of calculating the diameter D _melted of the fiber fuse generation region where the fiber fuse is generated from the equation (A) based on the desired mode field diameter MFD and the input optical power P;
Diameter c and diameter d where the ratio S (=N(d/D _melted ) ² ) of the area of the fiber fuse generation region to the area occupied by all the holes in the cross section of the HAF satisfies formula (B) and a detection step of detecting the number N of the holes;
A design method characterized by performing
[Formula (A)]
When P is 2.0 W or more :
_Dmelted = (62.76P/(11.44P+16.55)) ^0.5 x MFD
If P is less than 2.0W:
D _melted = ((62.76P-52.72)/11.44P) ^0.5 x MFD
[Formula (B)]
If c/D _melted is 0.81 :
S≧0.12
When c/D _melted is 0.81 to 1.13:
S≧0.750(c/D _melted )−0.488
When c/D _melted is 1.13 to 1.22:
S≧2.778(c/D _melted )−2.779
When c/D _melted is 1.22 to 1.29:
S≧22.85(c/D _melted )−27.26
If c/D _melted is 1.29:
S≧2.21 When the mode field diameter MFD is 10 μm or more and the maximum input power assumed to be used is P _{max and} P _th =0.135 (W/μm)×MFD, an input in the range of P _th or more and P _max or less 2. The designing method according to claim 1 , wherein the power P satisfies the hole structure conditions of the formulas (A) and (B). 2. The design method according to claim 1 , wherein the mode field diameter MFD has an error within 10% of the mode field diameter of a single mode optical fiber connected to the HAF.

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