Large-effective-area non-zero dispersion shifted optical fiber

Abstract

The invention discloses a large-effective-area non-zero dispersion shifted optical fiber which comprises a core layer, an inner cladding and an outer cladding, and the core layer comprises a first core layer, a second core layer, a third core layer and a fourth core layer. By adjusting the refractive index profile of the optical fiber, preventing the center of the first core layer from being sunken, and arranging the fluorine-doped second core layer and the outer cladding, the fixed effective refractive index area can be achieved under the condition of lower Ge doping amount, the related performance parameter indexes are met, the risk of attenuation caused by defects caused by chemical impurities can be reduced, and the optical fiber can be widely applied to the field of optical fiber communication. Meanwhile, the central recess of the first core layer is not arranged, the bending performance of the optical fiber is also helped to a certain extent, the leakage of optical signals in a bending state can be limited by the arrangement of the fluorine-doped second core layer, the bending loss is reduced, and the 1550nm attenuation can be controlled within 0.185 dB / km; by reasonably arranging the third core layer and the fourth core layer, light energy can be distributed on the two core layers, the diameter of an optical fiber mode field is increased, and the effective area of the optical fiber is increased.

Description

Technical Field

[0001] The invention belongs to the technical field of optical communication, and in particular relates to a large effective area non-zero dispersion shifted optical fiber. Background Art

[0002] The rapid development of data centers and cloud computing services has put forward higher and higher requirements on the transmission capacity and rate of optical transmission networks. The transmission network is facing an explosive growth in data traffic. At present, dense wavelength division multiplexing (DWDM) is the mainstream technology in the field of optical transmission. By increasing the single-channel transmission rate, the system transmission capacity can be increased. In the DWDM system, the dispersion tolerance decreases inversely proportional to the square of the transmission rate, and the optical signal-to-noise ratio increases in direct proportion to the square of the transmission rate. Therefore, dispersion, polarization mode dispersion, and nonlinear effects have become the main factors limiting long-distance transmission of optical fibers. As the single-channel transmission rate develops to a higher level, accurately controlling the accumulated dispersion of the system and reducing nonlinear effects are the key to improving the system transmission capacity and distance.

[0003] Non-zero dispersion-shifted fiber (G.655) is an optical fiber developed for DWDM transmission technology. It can overcome the problem of large dispersion of G.652 fiber (non-dispersion-shifted fiber) and serious four-wave mixing of G.653 fiber (dispersion-shifted fiber) at the low-loss wavelength of 1550nm. A small amount of dispersion-compensating fiber is required only when G.655 fiber is used to form a line for long-distance transmission of more than 10Gbit / s. Therefore, this fiber is the preferred fiber type for dense wavelength division multiplexing optical fiber communication systems that realize long-distance and large-capacity communications of more than 10Gbit / s.

[0004] The Chinese invention patent, publication number CN116643345A, discloses a structure of a non-zero dispersion-shifted optical fiber with a short cut-off wavelength and a large effective area. The optical fiber is prepared by an external vapor deposition process (OVD). The optical fiber includes a core layer, an inner cladding and an outer cladding. The core layer is composed of three core parts. The cross-section of the three core parts as a whole is a triangular core + ring type, and a central local depression and a base type structure are added. However, the central local depression structure will reduce the effective refractive index area. Although it is conducive to achieving a large effective area and reducing nonlinear effects, this structure will be detrimental to the bending performance of the optical fiber on the one hand, and it is difficult to achieve a balance between a large effective area and anti-bending. On the other hand, on the basis of ensuring that the effective refractive index area remains unchanged, the central local depression structure needs to increase the amount of the original positive dopant such as germanium Ge to improve the doping utilization rate. The increase in the amount of dopant used will lead to an increase in chemical impurities. These impurities form defects that will become obstacles in optical fiber transmission, causing scattering losses, which is not conducive to attenuation control. Summary of the invention

[0005] In order to solve the technical problems existing in the prior art, the object of the present invention is to provide a large effective area non-zero dispersion shifted optical fiber.

[0006] In order to achieve the above purpose and the above technical effect, the technical solution adopted by the present invention is:

[0007] A large effective area non-zero dispersion shifted optical fiber comprises a core layer, an inner cladding layer and an outer cladding layer arranged in sequence from the inside to the outside, wherein the core layer comprises a first core layer, a second core layer, a third core layer and a fourth core layer arranged in sequence from the inside to the outside.

[0008] Furthermore, the relative refractive index difference of the first core layer, the second core layer, the third core layer, the fourth core layer, the inner cladding layer and the outer cladding layer satisfies the following conditions:

[0009] △1>△3>△4>△5>△2>△6

[0010] Among them, △1 is the relative refractive index difference of the first core layer, △2 is the relative refractive index difference of the second core layer, △3 is the relative refractive index difference of the third core layer, △4 is the relative refractive index difference of the fourth core layer, △5 is the relative refractive index difference of the inner cladding, and △6 is the relative refractive index difference of the outer cladding.

[0011] Furthermore, the relative refractive index difference Δ1 between the first core layer and the pure silicon layer is 0.480% to 0.580%.

[0012] Furthermore, the relative refractive index difference Δ2 between the second core layer and the pure silicon layer is -0.050% to 0.

[0013] Furthermore, a relative refractive index difference Δ3 between the third core layer and the pure silicon layer is 0.035% to 0.065%.

[0014] Furthermore, a relative refractive index difference Δ4 between the fourth core layer and the pure silicon layer is 0.025% to 0.050%.

[0015] Furthermore, the inner cladding is a pure silicon layer with a relative refractive index difference of Δ5=0.

[0016] Furthermore, the relative refractive index difference Δ6 between the outer cladding layer and the pure silicon layer is -0.135% to -0.100%.

[0017] Furthermore, the radius R1 of the first core layer is 2.0~3.2μm, the radius of the second core layer is R2, R2: R1=2.0~2.5, the radius of the third core layer is R3, R3: R1=3.0~3.5, the radius of the fourth core layer is R4, R4: R1=3.5~4.0, the radius of the inner cladding is R5, R5: R1=6.0~7.0, and the radius of the outer cladding is R6=62.5μm.

[0018] Furthermore, the effective area of ​​the large effective area non-zero dispersion shifted optical fiber is greater than 70 μm2 , attenuation ≤0.400dB / km in the 1383nm band, ≤0.185dB / km in the 1550nm band, and ≤0.200dB / km in the 1625nm band; the mode field diameter of 1550nm is 9.2μm~9.8μm, the cabled cut-off wavelength is ≤1450nm, the optical fiber zero dispersion wavelength is ≤1520nm, the dispersion of 1530-1565nm is 2.0~6.0ps / nm·km, and the dispersion of 1565-1625nm is 4.5-11.2ps / nm·km; in terms of macrobending, the optical fiber is bent 100 times at a diameter of 60mm, and the bending loss at 1625nm wavelength is less than 0.05dB.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] (1) By adjusting the refractive index profile of the optical fiber, avoiding the depression in the center of the first core layer, and setting a fluorine-doped second core layer and an outer cladding layer, it is possible to ensure that a fixed effective refractive index area is achieved at a lower Ge doping dosage, meet the relevant performance parameter indicators, and reduce the risk of attenuation problems caused by defects caused by chemical impurities. At the same time, not setting the depression in the center of the first core layer is also helpful for the bending performance of the optical fiber. The setting of the fluorine-doped second core layer can limit the leakage of optical signals in the bending state, reduce bending losses, and the attenuation at 1550nm can be controlled within 0.185dB / km;

[0021] (2) The second core layer of the present invention is configured as a sunken structure, which can limit the leakage of optical signals in a bent state, reduce bending losses, increase the reliability of the optical fiber in complex situations, and also help reduce long wavelength attenuation;

[0022] (3) By properly setting the third and fourth core layers, the light energy can be distributed to the two core layers, increasing the mode field diameter of the optical fiber and the effective area of ​​the optical fiber. Since the mode field diameter and dispersion of G.655 optical fiber are both sensitive to the core diameter and refractive index, the third and fourth core layers are set, and a decreasing gradient refractive index is set, which can accurately control the core diameter and refractive index of the optical fiber and avoid a large impact on the performance indicators due to a large adjustment range. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a refractive index profile diagram of Example 1 of the present invention;

[0024] Figure 2 is the refractive index profile of Comparative Example 1;

[0025] Figure 3 is the refractive index profile of Comparative Example 2;

[0026] Figure 4is the refractive index profile of Comparative Example 3;

[0027] Figure 5 This is the refractive index profile of Comparative Example 4. DETAILED DESCRIPTION

[0028] The present invention is described in detail below so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more definite definition of the protection scope of the present invention.

[0029] A brief summary of one or more aspects is given below to provide a basic understanding of these aspects. This summary is not an exhaustive overview of all conceived aspects, and is neither intended to identify the key or critical elements of all aspects nor to define the scope of any or all aspects. Its only purpose is to give some concepts of one or more aspects in a simplified form as a prelude to a more detailed description that will be given later.

[0030] like Figure 1 As shown, a large effective area non-zero dispersion-shifted optical fiber, whose waveguide structure includes a core layer, an inner cladding layer and an outer cladding layer arranged in sequence from the inside to the outside, wherein the core layer includes a first core layer, a second core layer, a third core layer and a fourth core layer arranged in sequence from the inside to the outside.

[0031] The first core layer is doped with germanium (Ge), and the refractive index satisfies the rule of decreasing with increasing radius. The radius R1 of the first core layer is 2.0-3.2 μm, and the relative refractive index difference △1 between the first core layer and the pure silicon layer is 0.480%-0.580%.

[0032] The second core layer is doped with fluorine (F), the radius of the second core layer is R2, R2:R1=2.0~2.5, and the relative refractive index difference △2 between the second core layer and the pure silicon layer is -0.050%~0.

[0033] The third core layer is doped with germanium (Ge), the radius of the third core layer is R3, R3:R1=3.0-3.5, and the relative refractive index difference △3 between the third core layer and the pure silicon layer is 0.035%-0.065%.

[0034] The fourth core layer is doped with germanium (Ge), the radius of the fourth core layer is R4, R4:R1=3.5-4.0, and the relative refractive index difference △4 between the fourth core layer and the pure silicon layer is 0.025%-0.050%.

[0035] The inner cladding is a pure silicon layer with a radius of R5, R5:R1=6.0~7.0, and a relative refractive index difference △5=0.

[0036] The outer cladding is doped with fluorine (F), and its radius R6=62.5μm. The relative refractive index difference △6 between the outer cladding and the pure silicon layer is -0.135% to -0.100%.

[0037] The relative refractive index difference of each layer satisfies: △1>△3>△4>△5>△2>△6.

[0038] The effective area of ​​the optical fiber disclosed in the present invention is greater than 70 μm 2 , attenuation ≤0.400dB / km in the 1383nm band, ≤0.185dB / km in the 1550nm band, and ≤0.200dB / km in the 1625nm band; the mode field diameter of 1550nm is 9.2μm~9.8μm, the cabled cut-off wavelength is ≤1450nm, the optical fiber zero dispersion wavelength is ≤1520nm, the dispersion of 1530-1565nm is 2.0~6.0ps / nm·km, and the dispersion of 1565-1625nm is 4.5-11.2ps / nm·km; in terms of macrobending, the optical fiber is bent 100 times at a diameter of 60mm, and the bending loss at 1625nm wavelength is less than 0.05dB.

[0039] The optical fiber disclosed in the present invention can be prepared by using the existing preparation process, and it only needs to control the drawing speed to 1000-1500m / min and match the drawing annealing process.

[0040] Example 1

[0041] like Figure 1 As shown, a large effective area non-zero dispersion shifted optical fiber, whose waveguide structure includes a core layer, an inner cladding layer and an outer cladding layer arranged in sequence from the inside to the outside, wherein the core layer includes a first core layer, a second core layer, a third core layer and a fourth core layer.

[0042] The first core layer is doped with germanium (Ge), the radius R1 of the first core layer is 2.8 μm, and the relative refractive index difference △1 between the first core layer and the pure silicon layer is 0.482%.

[0043] The second core layer is doped with fluorine (F), the radius R2 of the second core layer is 5.7 μm, and the relative refractive index difference △2 between the second core layer and the pure silicon layer is -0.050%.

[0044] The third core layer is doped with germanium (Ge), the radius R3 of the third core layer is 8.4 μm, and the relative refractive index difference △3 between the third core layer and the pure silicon layer is 0.035%.

[0045] The fourth core layer is doped with germanium (Ge), the radius R4 of the fourth core layer is 9.8 μm, and the relative refractive index difference Δ4 between the fourth core layer and the pure silicon layer is 0.028%.

[0046] The inner cladding is a pure silicon layer with a radius R5 of 18.2 μm and a relative refractive index difference △5=0.

[0047] The outer cladding is doped with fluorine (F), and its radius R6=62.5μm. The relative refractive index difference △6 between the outer cladding and the pure silicon layer is -0.110%.

[0048] The relative refractive index difference of each layer satisfies: △1>△3>△4>△5>△2>△6.

[0049] Example 2

[0050] like Figure 1 As shown, a large effective area non-zero dispersion shifted optical fiber, whose waveguide structure includes a core layer, an inner cladding layer and an outer cladding layer arranged in sequence from the inside to the outside, wherein the core layer includes a first core layer, a second core layer, a third core layer and a fourth core layer.

[0051] The first core layer is doped with germanium (Ge), the radius R1 of the first core layer is 3.0 μm, and the relative refractive index difference △1 between the first core layer and the pure silicon layer is 0.490%.

[0052] The second core layer is doped with fluorine (F), the radius R2 of the second core layer is 6.1 μm, and the relative refractive index difference △2 between the second core layer and the pure silicon layer is -0.050%.

[0053] The third core layer is doped with germanium (Ge), the radius R3 of the third core layer is 9.7 μm, and the relative refractive index difference △3 between the third core layer and the pure silicon layer is 0.038%.

[0054] The fourth core layer is doped with germanium (Ge), the radius R4 of the fourth core layer is 11.1 μm, and the relative refractive index difference △4 between the fourth core layer and the pure silicon layer is 0.030%.

[0055] The inner cladding is a pure silicon layer with a radius R5 of 19.8 μm and a relative refractive index difference △5=0.

[0056] The outer cladding is doped with fluorine (F), and its radius R6=62.5μm. The relative refractive index difference △6 between the outer cladding and the pure silicon layer is -0.100%.

[0057] The relative refractive index difference of each layer satisfies: △1>△3>△4>△5>△2>△6.

[0058] The rest is the same as in Example 1.

[0059] Example 3

[0060] like Figure 1 As shown, a large effective area non-zero dispersion shifted optical fiber, whose waveguide structure includes a core layer, an inner cladding layer and an outer cladding layer arranged in sequence from the inside to the outside, wherein the core layer includes a first core layer, a second core layer, a third core layer and a fourth core layer.

[0061] The first core layer is doped with germanium (Ge), the radius R1 of the first core layer is 2.9 μm, and the relative refractive index difference △1 between the first core layer and the pure silicon layer is 0.482%.

[0062] The second core layer is doped with fluorine (F), the radius R2 of the second core layer is 5.9 μm, and the relative refractive index difference △2 between the second core layer and the pure silicon layer is -0.050%.

[0063] The third core layer is doped with germanium (Ge), the radius R3 of the third core layer is 8.8 μm, and the relative refractive index difference △3 between the third core layer and the pure silicon layer is 0.035%.

[0064] The fourth core layer is doped with germanium (Ge), the radius R4 of the fourth core layer is 10.7 μm, and the relative refractive index difference Δ4 between the fourth core layer and the pure silicon layer is 0.028%.

[0065] The inner cladding is a pure silicon layer with a radius R5 of 18.9 μm and a relative refractive index difference △5=0.

[0066] The outer cladding is doped with fluorine (F), and its radius R6=62.5μm. The relative refractive index difference △6 between the outer cladding and the pure silicon layer is -0.110%.

[0067] The relative refractive index difference of each layer satisfies: △1>△3>△4>△5>△2>△6.

[0068] The rest is the same as in Example 1.

[0069] Example 4

[0070] like Figure 1 As shown, a large effective area non-zero dispersion shifted optical fiber, whose waveguide structure includes a core layer, an inner cladding layer and an outer cladding layer arranged in sequence from the inside to the outside, wherein the core layer includes a first core layer, a second core layer, a third core layer and a fourth core layer.

[0071] The first core layer is doped with germanium (Ge), the radius R1 of the first core layer is 2.8 μm, and the relative refractive index difference △1 between the first core layer and the pure silicon layer is 0.495%.

[0072] The second core layer is doped with fluorine (F), the radius R2 of the second core layer is 5.7 μm, and the relative refractive index difference △2 between the second core layer and the pure silicon layer is -0.050%.

[0073] The third core layer is doped with germanium (Ge), the radius R3 of the third core layer is 8.4 μm, and the relative refractive index difference △3 between the third core layer and the pure silicon layer is 0.035%.

[0074] The fourth core layer is doped with germanium (Ge), the radius R4 of the fourth core layer is 9.8 μm, and the relative refractive index difference Δ4 between the fourth core layer and the pure silicon layer is 0.028%.

[0075] The inner cladding is a pure silicon layer with a radius R5 of 18.2 μm and a relative refractive index difference △5=0.

[0076] The outer cladding is doped with fluorine (F), and its radius R6=62.5μm. The relative refractive index difference △6 between the outer cladding and the pure silicon layer is -0.110%.

[0077] The relative refractive index difference of each layer satisfies: △1>△3>△4>△5>△2>△6.

[0078] The rest is the same as in Example 1.

[0079] Figure 1 is the refractive index profile of Example 1, Figure 2 is the refractive index profile of Comparative Example 1, Figure 3 is the refractive index profile of Comparative Example 2, Figure 4 is the refractive index profile of Comparative Example 3, Figure 5 This is the refractive index profile of Comparative Example 4.

[0080] The waveguide structural parameters of Examples 1-4 and Comparative Examples 1-4 are shown in Table 1.

[0081] Table 1

[0082]

[0083]

[0084] The optical fiber preform designed according to the above waveguide structure was drawn, the drawing speed was set to 1200m / min, and the drawing annealing process was adopted. The relevant parameters of the optical fiber after drawing are shown in Table 2.

[0085] Table 2

[0086]

[0087] It can be seen from Table 1-2 that the effective areas of the optical fibers of Examples 1-4 are all greater than 70 μm 2 , attenuation ≤0.185dB / km at 1550nm, attenuation ≤0.200dB / km at 1625nm; mode field diameter of 1550nm is 9.2μm~9.8μm, cable cut-off wavelength ≤1450nm, fiber zero dispersion wavelength ≤1520nm, dispersion of 1530-1565nm is 2.0~6.0ps / nm·km, dispersion of 1565-1625nm is 4.5-11.2ps / nm·km; in terms of macrobending, the fiber is bent 100 times at a diameter of 60mm, and the bending loss of 1625nm wavelength is less than 0.05dB;

[0088] Compared with Example 1, Example 3 has a slightly larger core diameter. Within a certain range, the diameter of the first core layer increases, which will cause the optical fiber mode field diameter to decrease, the cutoff wavelength to increase, the zero dispersion wavelength to decrease, the zero dispersion slope to decrease, and the dispersion coefficient to increase. Compared with Example 1, Example 4 has a slightly larger relative refractive index difference △1 of the first core layer, which will cause the mode field diameter to decrease, the cutoff wavelength to increase, the zero dispersion wavelength to increase, the zero dispersion slope to decrease, and the dispersion coefficient to decrease. In the adjustment process, it is necessary to balance the indicators such as mode field diameter and attenuation.

[0089] Comparing Comparative Example 1 with Example 1, Comparative Example 1 increases the relative refractive index difference between the first and second core layers and the relative refractive index difference of the outer cladding. It is clearly seen that the attenuation at 1550nm and 1625nm bands increases, and the macrobending performance deteriorates.

[0090] Comparing Comparative Example 2 with Example 1, in Comparative Example 2, the third core layer and the fourth core layer are not subjected to gradient refractive index control. The two core layers have a significant effect on the change of the overall mode field diameter, which can distribute the transmitted light to this position and increase the mode field diameter. Although the effective area is increased, the non-gradient refractive index control increases the difficulty of controlling the mode field diameter and dispersion.

[0091] Comparing Comparative Example 3 with Example 1, in Comparative Example 3, since the cladding is not provided with a depression, the effective refractive index is reduced and the G.655 related parameters are not met. Although the effective area is increased, the drawing results in a large mode field diameter and a large dispersion.

[0092] Comparing Comparative Example 4 with Example 1, the relative refractive index difference between the third core layer and the fourth core layer in Comparative Example 4 is set to increase in gradient, resulting in a mode field diameter that is too large and close to being unqualified, and the dispersion value is also in a close range. The refractive index gradients of the third and fourth core layers are set in the opposite direction as expected, which makes it difficult to accurately control the light transmission area, and easily causes the light transmission position to extend outward, increasing the mode field diameter, and having a greater impact on parameters such as dispersion.

[0093] Parts or structures not specifically described in the present invention may adopt existing technologies or existing products and will not be described in detail here.

[0094] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention specification, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A large effective area non-zero dispersion-shifted optical fiber, characterized in that: It comprises a core layer, an inner cladding layer and an outer cladding layer which are arranged in sequence from the inside to the outside, wherein the core layer comprises a first core layer, a second core layer, a third core layer and a fourth core layer which are arranged in sequence from the inside to the outside.

2. A large effective area non-zero dispersion-shifted optical fiber according to claim 1, characterized in that: The relative refractive index differences of the first core layer, the second core layer, the third core layer, the fourth core layer, the inner cladding layer and the outer cladding layer satisfy the following conditions: △1>△3>△4>△5>△2>△6 Among them, △1 is the relative refractive index difference of the first core layer, △2 is the relative refractive index difference of the second core layer, △3 is the relative refractive index difference of the third core layer, △4 is the relative refractive index difference of the fourth core layer, △5 is the relative refractive index difference of the inner cladding, and △6 is the relative refractive index difference of the outer cladding.

3. A large effective area non-zero dispersion-shifted optical fiber according to claim 2, characterized in that: The relative refractive index difference Δ1 between the first core layer and the pure silicon layer is 0.480% to 0.580%.

4. The large effective area non-zero dispersion-shifted optical fiber according to claim 2, characterized in that: The relative refractive index difference Δ2 between the second core layer and the pure silicon layer is -0.050% to 0.

5. The large effective area non-zero dispersion-shifted optical fiber according to claim 2, characterized in that: The relative refractive index difference △3 between the third core layer and the pure silicon layer is 0.035% to 0.065%.

6. The large effective area non-zero dispersion-shifted optical fiber according to claim 2, characterized in that: The relative refractive index difference Δ4 between the fourth core layer and the pure silicon layer is 0.025% to 0.050%.

7. The large effective area non-zero dispersion-shifted optical fiber according to claim 2, characterized in that: The inner cladding is a pure silicon layer with a relative refractive index difference of △5=0.

8. The large effective area non-zero dispersion-shifted optical fiber according to claim 2, characterized in that: The relative refractive index difference Δ6 between the outer cladding layer and the pure silicon layer is -0.135% to -0.100%.

9. The large effective area non-zero dispersion-shifted optical fiber according to claim 1, characterized in that: The radius R1 of the first core layer is 2.0-3.2 μm, the radius of the second core layer is R2, R2: R1 = 2.0-2.5, the radius of the third core layer is R3, R3: R1 = 3.0-3.5, the radius of the fourth core layer is R4, R4: R1 = 3.5-4.0, the radius of the inner cladding is R5, R5: R1 = 6.0-7.0, and the radius of the outer cladding is R6 = 62.5 μm.

10. The large effective area non-zero dispersion-shifted optical fiber according to claim 1, characterized in that: The effective area of ​​the large effective area non-zero dispersion shifted optical fiber is greater than 70 μm 2 , attenuation ≤0.400dB / km in the 1383nm band, ≤0.185dB / km in the 1550nm band, and ≤0.200dB / km in the 1625nm band; the mode field diameter of 1550nm is 9.2μm~9.8μm, the cabled cut-off wavelength is ≤1450nm, the optical fiber zero dispersion wavelength is ≤1520nm, the dispersion of 1530-1565nm is 2.0~6.0ps / nm·km, and the dispersion of 1565-1625nm is 4.5-11.2ps / nm·km; in terms of macrobending, the optical fiber is bent 100 times at a diameter of 60mm, and the bending loss at 1625nm wavelength is less than 0.05dB.

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