OPTICAL FIBER WITH LOW STIMULATED BRILLOUIN

SCATTERING, AND OPTICAL TRANSMISSION LINE AND

OPTICAL TRANSMISSION SYSTEM USING THE SAME

Technical Field

[1] The present invention relates to a single-mode optical fiber used in a signal transmission system, and more particularly to a single-mode optical fiber having an improved SBS (Stimulated Brillouin Scattering) threshold so as to transmit an analog signal having a particularly high power satisfactorily even under an environment having severe bending. Background Art

[2] If a photon is input to a material, a phonon is generated, and this phonon is classified into an optical phonon and an acoustic phonon. The optical phonon is to generate oscillations of molecular stretching due to photon, and the acoustic phonon is to generate collective oscillations of matter lattices or the like. Brillouin scattering is caused by interaction between the photon and the acoustic phonon.

[3] If an optical signal of a high power is input to a general single-mode optical fiber, back scattering is generated due to SBS (Stimulated Brillouin Scattering). This back scattering weakens a transmission signal, thereby causing noise, and this phenomenon is particularly serious in PON (Passive Optical Network).

[4] In an analog signal transmission, using a PON element requires input of an optical signal with a higher power in order to diverge the optical signal into more branches. However, if the analog signal has an increased power as mentioned above, SBS in the optical fiber is increased, thereby seriously deteriorating the signal quality. Thus, in order to diverge an analog signal with higher power to more users into a single optical fiber, it is very important to reduce SBS in the optical fiber.

[5] An analog optical signal is transmitted through an optical fiber in a triple play system that transmits voice, video and data at the same time through an optical fiber under FTTx circumstance or when a CATV signal is transmitted. In this case, if a transmission signal power is increased, the signal is distorted due to SBS. In order to minimize this distortion, an optical fiber with a reduced SBS, namely an optical fiber with a high SBS threshold, is required.

[6] In an optical fiber, the SBS threshold SBS is expressed using the threshold power following equation 1. [7] [8] Equation 1

[9] SBS = (21A V(L -g ) threshold power eff eff B

[10] (here, A is an effective sectional area of the optical fiber, L is an effective length eff eff of the optical fiber, and g is a Brillouin gain coefficient.)

B

[H]

[12] As understood from the equation 1, in order to increase a SBS threshold (or, in order to reduce a magnitude of SBS), it is required to increase the effective sectional area A , shorten the effective length L , or reduce the Brillouin gain coefficient g by eff eff B changing a refractive index profile of the optical fiber.

[13] However, in order to reduce SBS with respect to the same length with keeping an effective area and a loss level regularized in ITU-T G.652 for an optical fiber to be made, it is preferred to reduce a Brillouin gain coefficient, namely to change a refractive index profile of the optical fiber. Disclosure of Invention Technical Problem

[14] An object of the invention is to provide a single-mode optical fiber suitable for transmission of an analog signal with high power and also having a small bending loss.

[15] In addition, another object of the present invention is to provide a single-mode optical fiber having an improved threshold for SBS (Stimulated Brillouin Scattering) as well as satisfying standards required for a common single-mode optical fiber (e.g., an effective sectional area and a bending loss).

[16] In addition, still another object of the present invention is to provide an optical transmission line using the above optical fiber, and an optical communication system adopting this optical transmission line.

[17] These and other objects and advantages of the present invention will be described below and acknowledged using embodiments of the present invention. In addition, the objects and advantages of the present invention may be realized using means and combinations mentioned in the appended claims. Technical Solution

[18] In order to reduce SBS (Stimulated Brillouin Scattering), a refractive index structure should be changed in a radial or longitudinal direction so that the stimulated acoustic phonons induced by an input signal advance at the same speed at each portion of an optical fiber and thus they are not resonant with each other.

[19] However, since a productivity problem is caused in the manufacturing process if a refractive index is changed in a length direction of the optical fiber, the inventors propose a technique of changing a refractive index structure only in a radial direction with keeping it constantly in a length direction. In order to enhance a SBS threshold by effectively dispersing a propagation rate of acoustic phonon in an optical fiber, it is

desirable to give many changes to the acoustic propagation rate by means of effective changes of refractive index structure according to a radius. Rather than a stepped refractive index profile shown in FIG. 1, an optical fiber having a parabolic refractive index profile (see FIG. 2) or a triangular refractive index profile (see FIGs. 7 and 8) is more suitable for enhancing a SBS threshold.

[20] In addition, with respect to a bending loss, if an optical fiber is bent, a refractive index distribution experienced by an advancing optical signal is deformed and inclined, and the power departed out is partially not advancing along the optical fiber but escaping out of it, thereby causing a loss. In case an optical fiber is wound several times at a certain bending radius, power continuously comes out through the wound portion. In addition, in case a basic mode passing through a bent optical fiber then advances to a straight optical fiber, the power is partially lost due to mismatch of both field shapes.

[21] A field shape of the basic mode passing through an optical fiber is differently changed depending on a refractive index structure of the optical fiber. A general single-mode optical fiber has a refractive index profile with a stepped shape, and its field is nearly a Gaussian shape. Meanwhile, a field of an optical fiber whose refractive index is higher as closer to a core center has a shape in which power is localized to the core center, rather than a Gaussian shape. Thus, in case of a refractive index profile in which a refractive index is increased as a core center is closer, a relatively small power gets out when an optical fiber is bent, thereby capable of reducing a bending loss.

[22] In addition, as a scheme of increasing an effective sectional area with keeping a bending loss constantly, there is a method of lowering a refractive index at a core center and increasing a refractive index around it. If the refractive index at the core center is set lower than its surroundings as mentioned above, the field is dispersed out from the core center, thereby increasing an effective sectional area. That is to say, a bending loss is decreased with respect to the same effective sectional area. In addition, if an effective sectional area of an optical fiber is increased, an intensity of an advancing signal per a unit area is reduced, and thus all non-linear phenomenon including SBS is reduced.

[23] In order to accomplish the above object, the present invention provides a single- mode optical fiber including a core region with a radius (r ) from an optical center core axis and a clad region surrounding the core region, wherein the core region has a refractive index profile in which a specific refractive index difference (Δ ) at the optical center axis is not equal to a specific refractive index (Δ ) at the radius (r ) (Δ ≠Δ ) (here, Δ (%)=[(n ² -n ² )/2n ² ]xl00, Δ (%)= [(n ² -n ² )/2ri ₂ ² ]χl00, n : a refractive index at the optical center axis, n 2 : a refractive index at the radius (r core ), n c : a refractive index of the clad region). For example, the single-mode optical fiber

according to the present invention allows both cases that the specific refractive index difference (Δ ) at the optical center axis is grater than the specific refractive index (Δ ) at the radius (r ) (Δ >Δ ) and that the specific refractive index difference (Δ ) at core 1 2 1 the optical center axis is smaller than the specific refractive index (Δ ) at the radius (r core ) (A 1 <Δ 2 ).

[24] In particular, in case the refractive index profile of the core region has a triangular structure, a ratio of the specific refractive index differences (Δ /Δ ) is preferably 0.5 or less (Δ /Δ <0.5), and more preferably the ratio of the specific refractive index differences (Δ /Δ ) is not less than 0.25 and not more than 0.45 (0.25<Δ /Δ <0.45).

2 1 2 1

Thus, if the ratio of the specific refractive index differences (Δ /Δ ) is 0.5 or above, features of the optical fiber are changed into a multi-node optical features, and also the SBS threshold is improved to a satisfactory level.

[25] In addition, in case the refractive index profile of the core region has an inverted- triangular structure, the ratio of the specific refractive index differences (Δ /Δ ) is preferably 2 or above (Δ /Δ >2), and more particularly the ratio of the specific refractive index differences (Δ 2 /Δ 1 ) is not less than 3 and not more than 4 (3<Δ 2 /Δ 1

<4). Thus, if the ratio of the specific refractive index differences (Δ /Δ ) is less than 0.2, a satisfactory SBS threshold or bending loss cannot be realized.

[26] The single-mode optical fiber according to the present invention is characterized in that a mode field diameter (MFD) at a wavelength of 1310nm is 8.6 ~ 9.5 D, more preferably 8.8 ~ 9.4 D; a zero dispersion wavelength exists between 1300 nm and 1324 nm, more preferably between 1302 nm and 1322 nm; a dispersion value at a wavelength of 1550 nm is 18 ps/nm-km or less; and a bending loss at 1550 nm is 0.1 dB or less when the optical fiber is wound one time with a bending radius of 16 mm.

[27] In another aspect of the present invention, there are provided an optical transmission line adopting the above single-mode optical fiber, and an optical communication system adopting this optical transmission line as at least a part of an optical transmission path. Brief Description of the Drawings

[28] These and other features, aspects, and advantages of preferred embodiments of the present invention will be more fully described in the following detailed description, taken accompanying drawings. In the drawings:

[29] FIG. 1 shows a refractive index profile of a conventional single-mode optical fiber having a stepped refractive index structure;

[30] FIG. 2 shows a refractive index profile of a single-mode optical fiber having a parabolic refractive index structure according to the present invention;

[31] FIG. 3 shows a refractive index profile of a single-mode optical fiber having a

triangular refractive index structure according to the present invention; [32] FIG. 4 shows a refractive index profile of a single-mode optical fiber having a inverted-triangular refractive index structure according to the present invention; [33] FIG. 5 is a graph showing a SBS (Stimulated Brillouin Scattering) threshold for an optical fiber according to a first embodiment; [34] FIG. 6 is a graph showing a SBS threshold for an optical fiber according to a second embodiment; [35] FIG. 7 is a graph showing a SBS threshold for an optical fiber according to a first comparative example; and [36] FIG. 8 is a graph showing a SBS threshold for an optical fiber according to a second comparative example.

Best Mode for Carrying Out the Invention [37] Embodiment 1

[38] An optical fiber of this embodiment has a parabolic refractive index profile as shown in FIG. 2. Assuming that a refractive index at an optical center axis is n , a refractive index at a radius r is n , and a refractive index of a clad region (having the core 2 same refractive index as a silica tube used as a preform) is n , specific refractive index differences Δ (%) and Δ (%) satisfy the following equations, and its refractive index structure and optical features are as follows.

[41] (1) Radius: r = 4.8 D core

[42] (2) Specific refractive index difference: Δ (%) = 0.346, Δ (%) = 0.132

[43] (3) Zero dispersion wavelength: 1319 nm

[44] (4) Dispersion (1550 nm): 16.2 ps/nm-km

[45] (5) Effective sectional area (1550 nm) : 89.2 ² J

[46] (6) Cut-off wavelength: 1286 nm

[47] (7) Mode field diameter

[48] 1310 nm : 9.00 D, 1550 nm : 9.2 D

[49] (8) Dispersion slope: 0.088 ps/nm ² /km

[50]

[51] Embodiment 2

[52] An optical fiber of this embodiment has a parabolic refractive index profile as shown in FIG. 2. Assuming that a refractive index at an optical center axis is n , a refractive index at a radius r is n , and a refractive index of a clad region (having the core 2 same refractive index as a silica tube used as a preform) is n , specific refractive index differences Δ (%) and Δ (%) satisfy the following equations, and its refractive index

structure and optical features are as follows.

[55] (1) Radius: r = 4.9 D core

[56] (2) Specific refractive index difference: Δ (%) = 0.356, Δ (%) = 0.132

[57] (3) Zero dispersion wavelength: 1318 nm

[58] (4) Dispersion (1550 nm): 16.2 ps/nm-km

[59] (5) Effective sectional area (1550 nm) : 91.2 ² J

[60] (6) Cut-off wavelength: 1273 nm

[61] (7) Mode field diameter

[62] 1310 nm : 8.90 D, 1550 nm : 9.5 D

[63] (8) Dispersion slope: 0.088 ps/nm /km

[64]

[65] Embodiment 3

[66] An optical fiber of this embodiment has a triangular refractive index profile as shown in FIG. 3. Assuming that a refractive index at an optical center axis is n , a refractive index at a radius r core is n 2 , and a refractive index of a clad region (having the same refractive index as a silica tube used as a preform) is n , specific refractive index differences Δ (%) and Δ (%) satisfy the following equations, and its refractive index structure and optical features are as follows.

[69] (1) Radius: r core = 5.65 D

[70] (2) Specific refractive index difference: Δ (%) = 0.456, Δ (%) = 0.132

[71] (3) Zero dispersion wavelength: 1315 nm

[72] (4) Dispersion (1550 nm): 17.3 ps/nm-km

[73] (5) Effective sectional area (1550 nm) : 86.5 θ

[74] (6) Cut-off wavelength: 1220 nm

[75] (7) Mode field diameter

[76] 1310 nm : 9.45 D, 1550 nm : 10.7 D

[77] (8) Dispersion slope: 0.090 ps/nm /km

[78]

[79] Embodiment 4

[80] An optical fiber of this embodiment has a triangular refractive index profile as shown in FIG. 3. Assuming that a refractive index at an optical center axis is n , a refractive index at a radius r core is n 2 , and a refractive index of a clad region (having the same refractive index as a silica tube used as a preform) is n , specific refractive index differences Δ (%) and Δ (%) satisfy the following equations, and its refractive index

structure and optical features are as follows.

[83] (1) Radius: r core = 5.06 D

[84] (2) Specific refractive index difference: Δ (%) = 0.467, Δ (%) = 0.198

[85] (3) Zero dispersion wavelength: 1312 nm

[86] (4) Dispersion (1550 nm): 17.2 ps/nm-km

[87] (5) Effective sectional area (1550 nm) : 80.1 &

[88] (6) Cut-off wavelength: 1230 nm

[89] (7) Mode field diameter

[90] 1310 nm : 9.10 D, 1550 nm : 10.3 D

[91] (8) Dispersion slope: 0.078 ps/nm /km

[92]

[93] Embodiment 5

[94] An optical fiber of this embodiment has an inverted-triangular refractive index profile as shown in FIG. 4. Assuming that a refractive index at an optical center axis is n 1 , a refractive index at a radius r core is n 2 , and a refractive index of a clad region

(having the same refractive index as a silica tube used as a preform) is n , specific refractive index differences Δ (%) and Δ (%) satisfy the following equations, and its refractive index structure and optical features are as follows.

[97] (1) Radius: r = 4.03 D core

[98] (2) Specific refractive index difference: Δ (%) = 0.152, Δ (%) = 0.533

[99] (3) Zero dispersion wavelength: 1309 nm

[100] (4) Dispersion (1550 nm): 17.2 ps/nm-km

[101] (5) Effective sectional area (1550 nm) : 84.7 ² J

[102] (6) Cut-off wavelength: 1270 nm

[103] (7) Mode field diameter

[104] 1310 nm : 9.01 D, 1550 nm : 10.2 D

[105] (8) Dispersion slope: 0.090 ps/nm ² /km

[106]

[107] Embodiment 6

[108] An optical fiber of this embodiment has an inverted-triangular refractive index profile as shown in FIG. 4. Assuming that a refractive index at an optical center axis is n 1 , a refractive index at a radius r core is n 2 , and a refractive index of a clad region

(having the same refractive index as a silica tube used as a preform) is n , specific refractive index differences Δ (%) and Δ (%) satisfy the following equations, and its

refractive index structure and optical features are as follows.

[111] (1) Radius: r core = 4.10 D

[112] (2) Specific refractive index difference: Δ (%) = 0.141, Δ (%) = 0.501

[113] (3) Zero dispersion wavelength: 1309 nm

[114] (4) Dispersion (1550 nm): 17.2 ps/nm-km

[115] (5) Effective sectional area (1550 nm) : 89.4 ² J

[116] (6) Cut-off wavelength: 1240 nm

[117] (7) Mode field diameter

[118] 1310 nm : 9.26 D, 1550 nm : 10.5 D

[119] (8) Dispersion slope: 0.090 ps/nm ² /km

[120]

[121] Comparative Examples

[122] An optical fiber of this comparative example has a stepped refractive index profile as shown in FIG. 1. Assuming that a refractive index at an optical center axis is n and a refractive index of a clad region (having the same refractive index as a silica tube used as a preform) is n c , a specific refractive index difference Δ 1 (%) satisfies the following equation, and its refractive index structure and optical features are as follows. [123] Δ 1 (%) = [(n 1 ² -n c ² )/2n 1 ² ] x 100

[124]

[125] Comparative Example 1

[126] (1) Radius: r = 4.83 D

[127] (2) Specific refractive index difference: Δ (%) = 0.311

[128] (3) Zero dispersion wavelength: 1308 nm

[129] (4) Dispersion (1550 nm): 16.8 ps/nm-km

[130] (5) Effective sectional area (1550 nm) : 85.9 S

[131] (6) Cut-off wavelength: 1230 nm

[ 132] (7) Mode field diameter

[133] 1310 nm : 9.31 D, 1550 nm : 9.1 D

[134] (8) Dispersion slope: 0.088 ps/nm ² /km

[135]

[136] Comparative Example 2

[137] (1) Radius: r = 4.45 D

[138] (2) Specific refractive index difference: Δ (%) = 0.355

[139] (3) Zero dispersion wavelength: 1311 nm

[140] (4) Dispersion (1550 nm): 16.5 ps/nm-km

[141] (5) Effective sectional area (1550 nm) : 86.4 θ [142] (6) Cut-off wavelength: 1220 nm [143] (7) Mode field diameter [144] 1310 nm : 9.34 D, 1550 nm : 9.9 D [145] (8) Dispersion slope: 0.087 ps/nm /km [146] [147] Comparative Example 3 [148] (1) Radius: r = 4.40 D core [149] (2) Specific refractive index difference: Δ (%) = 0.331 [150] (3) Zero dispersion wavelength: 1304 nm [151] (4) Dispersion (1550 nm): 17.3 ps/nm-km [152] (5) Effective sectional area (1550 nm) : 84.8 S [153] (6) Cut-off wavelength: 1190 nm [154] (7) Mode field diameter [155] 1310 nm : 9.29 D, 1550 nm : 10.5 D [156] (8) Dispersion slope: 0.091 ps/nm /km [157] [158] Experiments [159] For the optical fibers of the embodiments 1 to 6 and the comparative examples 1 to 3, ® a one time bending loss (dB) at a radius of 16mm and ® SBS threshold (mW) were measured, and their results are shown in the following table 1.

[160] [161] Table 1

[162]

[163] As understood from the table 1, the optical fibers of the embodiments have smaller mode field diameters (or, effective sectional areas) than those of the comparative examples, but they have higher SBS thresholds. It is derived from differences of Brillouin gain coefficients (g ) caused by difference of the refractive index profiles of

B the optical fibers.

[164] FIGs. 5 to 8 are graphs showing SBS thresholds measured at a wavelength of 1550 nm for the optical fibers of the embodiments 1 and 2 and the comparative examples 1 and 2.

[165] In particular, the optical fibers of the embodiments 1 to 4 show lower bending losses than the optical fibers of the comparative examples, and also show higher SBS thresholds. In addition, the optical fibers of the embodiments 5 and 6 show much higher SBS thresholds than the optical fibers of the comparative examples.

[166] As mentioned above, it would be understood that a SBS threshold may be increased by dispersing a propagation rate of an acoustic wave in a way of changing the refractive index structure of an optical fiber in a radial direction, not in a stepped shape.

[167] It should be understood that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present invention on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation.

[168] Therefore, the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the invention, so it should be understood that other equivalents and modifications could be made thereto without departing from the spirit and scope of the invention. Industrial Applicability

[169] The optical fiber of the present invention increases a SBS threshold with keeping a bending loss to a low level by changing a refractive index of a core region in a radial direction. Thus, it allows transmission of an optical signal with a higher power without a sacrifice of other transmission characteristics under an installation circumstance having severe bending, when an analog signal is transmitted.