Reduced diameter optical fiber and manufacturing method

Abstract

The invention relates to an optical fiber 1 comprising a core 2 and a cladding 3 surrounding the core 2 and having an outer diameter of 125 μm, the optical fiber 1 comprising a cured primary coating 4 directly surrounding the cladding 3 and a cured secondary coating 5 directly surrounding the cured primary coating 4, said cured primary coating 4 having a thickness t.sub.1 between 10 and 18 μm and an in-situ tensile modulus Emod.sub.1 between 0.10 and 0.18 MPa, said cured secondary coating 5 having a thickness t.sub.2 between 10 microns and 18 microns and an in-situ tensile modulus Emod.sub.2 between 700 and 1200 MPa, wherein said first and second thicknesses and said first and second in-situ tensile moduli satisfy the following equation:
4%<(t.sub.1×t.sub.2×E mod.sub.1×E mod.sub.2.sup.3)/(t.sub.1_norm×t.sub.2_norm×E mod.sub.1_norm×E mod.sub.2_norm.sup.3)<50%.

Claims

1. An optical fiber (1) having an optical-fiber diameter between 165 microns and 197 microns, the optical fiber comprising a core (2) and a cladding (3) surrounding the core (2) and having an outer diameter of 125 microns, the optical fiber (1) comprising a cured primary coating (4) directly surrounding the cladding (3) and a cured secondary coating (5) directly surrounding the cured primary coating (4), said cured primary coating (4) having a thickness t.sub.1 between 10 microns and 18 microns and an in-situ tensile modulus Emod.sub.1 between 0.10 MPa and 0.18 MPa, said cured secondary coating (5) having a thickness t.sub.2 between 10 microns and 18 microns and an in-situ tensile modulus Emod.sub.2 between 700 MPa and 1200 MPa, wherein said first and second thicknesses and said first and second in-situ tensile moduli satisfy the following equation:
4%<(t.sub.1×t.sub.2×E mod.sub.1×E mod.sub.2.sup.3)/(t.sub.1_norm×t.sub.2_norm×E mod.sub.1_norm×E mod.sub.2_norm.sup.3)<50% Where t.sub.1_norm is the thickness of the cured primary coating of a standard 245 μm-diameter optical fiber, which is equal to 33.5 microns, t.sub.2_norm is the thickness of the cured secondary coating of a standard 245 μm-diameter optical fiber, which is equal to 25 microns, Emod.sub.1_norm is the in-situ tensile modulus of the cured primary coating of a standard 245 μm-diameter optical fiber, which is equal to 0.4 MPa, and Emod.sub.2_norm is the in-situ tensile modulus of the cured secondary coating of a standard 245 μm-diameter optical fiber, which is equal to 800 MPa.

2. The optical fiber (1) according to claim 1, wherein both the core (2) and the cladding (3) are made of doped or un-doped silica.

3. The optical fiber (1) according to claim 1, wherein the cured primary coating (4) has a cure rate yield after UV curing between 80 and 90 percent one week after draw.

4. The optical fiber (1) according to claim 1, wherein the cured secondary coating (5) has a cure rate yield after UV curing between 94 and 98 percent.

5. The optical fiber (1) according to claim 1, wherein the primary coating (4) has a thickness t.sub.1 between 10 microns and 16 microns.

6. The optical fiber (1) according to claim 1, wherein the secondary coating (5) has a tensile modulus Emod.sub.2 higher than 1000 MPa.

7. The optical fiber (1) according to claim 1, wherein the optical fiber (1) complies with the macro-bend losses specified in the ITU-T G.657.A1 (October 2012) recommendations.

8. The optical fiber (1) according to claim 7, wherein the cladding (3) comprises a depressed area.

9. The optical fiber (1) according to claim 1, wherein, at a wavelength of 1550 nanometers and at a wavelength of 1625 nanometers, one kilometer of the optical fiber (1) in a free coil has temperature-induced attenuation losses of less than 0.05 dB/km as measured over a temperature range between −60° C. and +70° C.

10. An optical cable (6) comprising at least one optical fiber (1) according to claim 1.

11. An optical fiber (1) having an optical-fiber diameter between 165 microns and 197 microns, the optical fiber comprising a core (2) and a cladding (3) surrounding the core (2) and having an outer diameter of 125 microns, the optical fiber (1) comprising a cured primary coating (4) directly surrounding the cladding (3) and a cured secondary coating (5) directly surrounding the cured primary coating (4), said cured primary coating (4) having a thickness t.sub.1 between 10 microns and 18 microns, an in-situ tensile modulus Emod.sub.1 between 0.10 MPa and 0.18 MPa, and a cure rate yield after UV curing between 80 and 90 percent one week after draw, said cured secondary coating (5) having a thickness t.sub.2 between 10 microns and 18 microns, an in-situ tensile modulus Emod.sub.2 between 700 MPa and 1200 MPa, and a cure rate yield after UV curing between 94 and 98 percent, wherein said first and second thicknesses and said first and second in-situ tensile moduli satisfy the following equation:
4%<(t.sub.1×t.sub.2×E mod.sub.1×E mod.sub.2.sup.3)/(t.sub.1_norm×t.sub.2_norm×E mod.sub.1_norm×E mod.sub.2_norm.sup.3)<50% where t.sub.1_norm is the thickness of the cured primary coating of a standard 245 μm-diameter optical fiber, which is equal to 33.5 microns, t.sub.2_norm is the thickness of the cured secondary coating of a standard 245 μm-diameter optical fiber, which is equal to 25 microns, Emod.sub.1_norm is the in-situ tensile modulus of the cured primary coating of a standard 245 μm-diameter optical fiber, which is equal to 0.4 MPa, and Emod.sub.2_norm is the in-situ tensile modulus of the cured secondary coating of a standard 245 μm-diameter optical fiber, which is equal to 800 MPa.

12. The optical fiber (1) according to claim 11, wherein the primary coating (4) has a thickness t.sub.1 between 10 microns and 16 microns.

13. The optical fiber (1) according to claim 11, wherein the secondary coating (5) has a tensile modulus Emod.sub.2 higher than 1000 MPa.

14. The optical fiber (1) according to claim 11, wherein the cladding (3) comprises a trench.

15. The optical fiber (1) according to claim 11, wherein the optical fiber has a cable cut-off wavelength less than or equal to 1260 nanometers.

16. The optical fiber (1) according to claim 11, wherein, at a wavelength 1310 nanometers, the optical fiber has a Mode Field Diameter (MFD) between 8.6 and 9.5 microns.

17. The optical fiber (1) according to claim 11, wherein the optical fiber has a zero-dispersion wavelength between 1300 and 1324 nanometers.

18. The optical fiber (1) according to claim 11, wherein, at a wavelength of 1550 nanometers and at a wavelength of 1625 nanometers, one kilometer of the optical fiber (1) in a free coil has temperature-induced attenuation losses of less than 0.05 dB/km as measured over a temperature range between −60° C. and +70° C.

19. An optical cable (6) comprising at least one optical fiber (1) according to claim 11.

Description

5. BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention can be better understood with reference to the following description and drawings, given by way of example and not limiting the scope of protection, and in which:

(2) FIG. 1 is a schematic view of the cross section of an optical fiber according to an embodiment of the invention;

(3) FIG. 2 is a schematic view of the cross section of an optical cable according to an embodiment of the invention;

(4) FIGS. 3a, 3b, 3c and 3d are four illustrations of different steps of a sample preparation when performing a primary In situ modulus Emod.sub.1 test on fiber;

(5) FIGS. 4a, 4b are two curves obtained after performing DMA;

(6) FIG. 5 is a flowchart illustrating steps according to one embodiment of the invention.

(7) The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

6. DESCRIPTION OF AN EMBODIMENT

(8) The present invention relates to optical fibers and targets reaching micro bending losses and other fiber performances similar to what is obtained with 245 μm fibers, but with a reduced fiber size up to 180 μm, thanks to a specific combination of primary and secondary coating monomer-polymer ratios, thicknesses and tensile moduli.

(9) Many specific details of the invention are set forth in the following description and in FIGS. 1 to 5. One skilled in the art, however, will understand that the present invention may have additional embodiments, or that the present invention may be practiced without several of the details described in the following description.

(10) 6.1 Particular Embodiment of the Reduced Diameter Optical Fiber

(11) FIG. 1 illustrates schematically an optical fiber 1 according to one embodiment, which is defined about an axis of revolution X that is orthogonal to the plan of FIG. 1. The fiber 1 comprises a core 2 and a cladding 3 surrounding the core 2, both made of un-doped or doped silica. The cladding 3 has an outer diameter of about 125 μm. A cured primary coating 4 having a cure rate yield between 80 and 90%, preferably between 82 and 87%, is directly surrounding the cladding 3, with a thickness t.sub.1 between 10 and 18 μm and an in-situ tensile modulus Emod.sub.1 between 0.10 and 0.18 MPa. A cured secondary coating 5 having a cure rate yield between 94 and 98%, preferably between 95 and 97%, directly surrounds the cured primary coating 4, with a thickness t.sub.2 lower or equal to 18 μm and an in-situ tensile modulus Emod.sub.2 between 700 and 1200 MPa, with a ratio (t.sub.1×t.sub.2×Emod.sub.1×Emod.sub.2.sup.3)/(t.sub.1_norm×t.sub.2_norm×Emod.sub.1_norm×Emod.sub.2_norm.sup.3) between 4 and 50%.

(12) Where (t.sub.1_norm; t.sub.2_norm; Emod.sub.1_norm; Emod.sub.2_norm) are the featuring values of a standard 245 μm-diameter optical fiber and are equal to (33.5 μm; 25 μm; 0.4 MPa; 800 MPa).

(13) If those characteristics are not verified, the reduced diameter fiber cannot present acceptable attenuation losses under stress (notably micro bending losses would be higher than those of a standard 245 μm-diameter fibers and attenuation variation at 1550 nm could not be kept within 0.05 bB/km under thermo cycling between −60° C. and +85° C.).

(14) In one embodiment, a plurality of these optical fibers 1 is regrouped within the sheath 7 that defines the outline of an optical cable 6, as illustrated by FIG. 2.

(15) 6.2 Method for Manufacturing a Reduced Diameter Optical Fiber

(16) The core and cladding of the present optical fibers may be produced by a variety of chemical vapor deposition methods that are well known in the art for producing a core rod, such as Outside Vapor Deposition (OVD), Axial Vapor Deposition (VAD), Modified Chemical Vapor Deposition (MCVD), or Plasma enhanced Chemical Vapor Deposition (PCVD, PECVD). In one embodiment, the core rods produced with the above described processes may be provided with an additional layer of silica on the outside using prefabricated tubes, such as in Rod-in-Tube or Rod-in-Cylinder processes, or by outside deposition processes such as Outside Vapor Deposition (OVD) or Advanced Plasma Vapor Deposition (APVD). The preforms thus obtained are drawn into optical fiber in a fiber draw tower in which the preform is heated to a temperature sufficient to soften the glass, e.g. a temperature of about 2000° C. or higher. The preform is heated by feeding it through a furnace and drawing a glass fiber from the molten material at the bottom of the furnace. In subsequent stages the fiber while being drawn is cooled down to a temperature below 100° C. and provided with the reduced diameter coating.

(17) The coating is provided on the outer surface of the glass part of the optical fiber, by passing the fiber through a coating applicator. In the applicator liquid unreacted coating is fed to the fiber and the fiber with coating is guided through a sizing die of appropriate dimensions. Some processes use applicators in which both coatings, primary and secondary are applied the fiber (so called wet-on-wet). The fiber with two layers of coating subsequently passes through a curing system for curing both coatings. Other processes use a first applicator for applying the primary coating on the fiber which is subsequently cured. After (partial) curing of the primary coating the secondary coating is applied in a second applicator, after which a second curing occurs. The UV source can be provided notably from microwave powered lamps or LED lamps.

(18) After curing of the coatings the fiber is guided over a capstan, which pulls the molten fiber out of the drawing furnace. After the capstan the fiber is guided to a take up spool.

(19) 6.3 Tests Procedures to be Performed on Optical Fibers to Determine the Primary and Secondary In-Situ Tensile Modulus Emod.sub.1 and Emod.sub.2

(20) The primary modulus Emod.sub.1 can be either directly measured on fiber or with the help of a Dynamic Mechanical Analyzer (DMA) using film or bulk coating sample.

(21) In contrast, it is not possible to measure the secondary modulus Emod.sub.2 directly on the fiber 1.

(22) 6.3.1 Primary In Situ Modulus Emod.sub.1 Test Procedure on Fiber

(23) A. Sample Choice

(24) Representative fiber samples are chosen two weeks after drawing, coming from the middle part of a preform.

(25) B. Sample Preparation

(26) Three fiber samples are cut of about 50 to 60 cm each. 2 mm of coating is then stripped at a distance of about 10 cm from the end, as illustrated by FIG. 3a.

(27) Each sample of fiber is then glued in glass slides.

(28) In this matter, a glass slide 9 is placed on an Aluminum support 20, which has been prepared to fit this glass slide. A landmark at 1 cm from the bottom limit of the glass is then made before fixing an adhesive tape 10 at this landmark, as illustrated by FIG. 3b.

(29) The fiber sample 1 is then positioned on the glass slide so that the 2 mm stripped position 11 is laying just out the glass slide. The fiber is subsequently glued to the glass slide, preferably with a two component Epoxy resin. A 1 cm-diameter resin dot 8 is used to fix the fiber to the glass slide, as illustrated by FIG. 3c

(30) C. In-Situ Modulus Emod Test

(31) When the glue is hardened, the fiber is cut on top of the glass slide and the prepared sample is placed on an aluminum support plate 20, as illustrated by FIG. 3d. and placed under a video microscope. The support plate 20 has a groove 23 for guiding the fiber and a small pulley 24 to allow the fiber to move during the test. The glass slide is fixed in a slot 21 by hold dawn clamps 22a, 22b. The 2 mm stripped fiber 11 is above the inspection slot 25.

(32) A curve of displacement versus weight is obtained by measuring the displacement of the fiber in the stripped 2 mm zone under influence of several (typically four) different weights. Care is taken that for each displacement measurement the fiber stops moving after 4 to 5 seconds and that after releasing all weights from the fiber, the fiber returns to its original position.

(33) This measurement is repeated for each fiber sample.

(34) A suitable apparatus for performing such measurement is a microscope with top and bottom illumination, equipped with a color video camera connected to a video color monitor and a displacement measurement system.

(35) The diameter of resin dot 8 is measured with a caliper. The cross sectional dimensions of the fiber are measured on a geometrical bench in order to check the exact value of the primary coating diameter and the bare fiber 11 diameter.

(36) D. Results

(37) Following the displacements measurements, the shear modulus and the tensile modulus is calculated. Firstly the shear modulus is calculated, in dynes/cm.sup.2. The usual formula is:

(38) $G_{eq}=\left(\frac{980.7}{m}\right)\frac{\ln \left(2R_2/2R_1\right)}{2\pi L}$
With:

(39) G.sub.eq: shear modulus (dynes/cm.sup.2)

(40) m: slope of the linear function of displacement vs. weight (cm/g)

(41) R1: diameter of the bare fiber (μm)

(42) R2: diameter of the primary coating (μm)

(43) L: length of the isolated section of coated fiber on the glass slide (cm), under the resin dot.

(44) The shear modulus in units of dynes/cm.sup.2 can be converted to tensile modulus E.sub.eq in MPa by using the usual formula below.
E.sub.eq=2G.sub.eq(1+ν)×10.sup.−7

(45) E.sub.eq: tensile modulus (MPa)

(46) G.sub.eq: shear modulus (dynes/cm.sup.2)

(47) ν: Poisson's ratio

(48) In this relation, the Poisson's ratio () is approximated to 0.5, considering the primary coating material type is an ideal rubber within the extension experienced during the measurement.

(49) 6.3.2 Secondary In Situ Modulus Emod.sub.2 Test Procedure on Film

(50) The secondary in situ modulus Emod.sub.2 is measured using fiber tube-off samples.

(51) To obtain a fiber tube-off sample, a 0.14 mm Miller stripper is first clamped down approximately 2.5 cm from the end of the coated fiber. The 2.5 cm region of fiber extending from the stripper is plunged into a stream of liquid nitrogen and held for 3 seconds. The fiber is then removed from the stream of liquid nitrogen and quickly stripped. The stripped end of the fiber is inspected to insure that the coating is removed. If coating remains on the glass, the sample is prepared again. The result is a hollow tube of primary and secondary coatings. The diameters of the glass, primary coating and secondary coating are measured from the end-face of the unstripped fiber. To measure secondary in situ modulus, fiber tube-off samples can be run with an instrument such as a Rheometries DMT A IV instrument at a sample gauge length 11 mm. The width, thickness, and length of the sample are determined and provided as input to the operating software of the instrument. The sample is mounted and run using a time sweep program at ambient temperature (21° C.) using the following parameters:

(52) Frequency: 1 Rad/sec

(53) Strain: 0.3%

(54) Total Time=120 sec.

(55) Time Per Measurement=1 sec

(56) Initial Static Force=15.0 [g]

(57) Static>Dynamic Force by=10.0 [%]

(58) Once completed, the last five E′ (storage modulus) data points are averaged. Each sample is run three times (fresh sample for each run) for a total of fifteen data points. The averaged value of the three runs is reported as the secondary in situ modulus.

(59) 6.4 Test Procedure to Measure Coating Cure Yield by FTIR

(60) A—As Per the Primary Coating Cure Yield:

(61) a) Measure of Acrylate Area Ratio in the Resin State

(62) A background spectrum is firstly realized on the FTIR apparatus.

(63) Then a droplet of primary resin is positioned on the top of the FTIR cell. The FTIR spectrum is then realized. The FTIR subtracts the background spectrum to obtain the primary FTIR spectrum.

(64) On the spectrum, the area of the residual acrylate peak is measured between 813 and 798 cm.sup.−1.

(65) The area of a reference peak is then measured between 1567 and 1488 cm.sup.−1.

(66) The resin acrylate ratio in then obtained by dividing the acrylate peak area by the reference peak area.

(67) b) Measure of Acrylate Area Ratio in the Coating State

(68) A background spectrum is firstly realized on the FTIR apparatus.

(69) Then a 5 mm piece of coating is removed from the coated fiber one week after draw using a razor blade and the convex side is positioned on the top of the FTIR cell. The FTIR spectrum is then realized. The FTIR subtracts the background spectrum to obtain the primary FTIR spectrum.

(70) On the spectrum, the area of the residual acrylate peak is measured between 813 and 798 cm.sup.−1.

(71) The area of a reference peak is then measured between 1567 and 1488 cm.sup.−1.

(72) The coating acrylate ratio in then obtained by dividing the acrylate peak area by the reference peak area.

(73) c) Measure of the Primary Coating Cure Yield

(74) The primary coating cure yield is obtained according to the formula below:
Primary cure (in %)=(1−coating acrylate ratio/resin acrylate ratio)*100
B—As Per the Secondary Coating Cure Yield:

(75) a) Measure of Acrylate Area Ratio in the Resin State

(76) The same procedure is applied as for the primary resin to obtain the secondary resin ratio.

(77) b) Measure of Acrylate Area Ratio in the Coating State

(78) A background spectrum is firstly realized on the FTIR apparatus.

(79) Then a 30 cm-coated fiber is cut one week after draw into 2 to 3 cm-lengths that are assembled to form a bundle, which is positioned on the top of the FTIR cell. The FTIR spectrum is then realized. The FTIR subtracts the background spectrum to obtain the primary FTIR spectrum.

(80) On the spectrum the area of the residual acrylate peak is measured between 813 and 798 cm.sup.−1.

(81) The area of a reference peak is then measured between 1567 and 1488 cm.sup.−1.

(82) The coating ratio in then obtained by dividing the acrylate peak area by the reference peak area.

(83) c) Measure of the Secondary Coating Cure Yield

(84) The secondary coating cure yield is obtained according to the formula below:
Secondary cure (in %)=(1−coating acrylate ratio/resin acrylate ratio)*100
6.5 Tests Performed to Determine the Thermal Stability of the Optical Fibers

(85) Tests have been performed in order to challenge the thermal stability of an optical fiber according to the invention. In this matter, 1 km of such a fiber in a free coil has been operated under temperatures ranging between −60° C. to +70° C. As a result, the change in attenuation of a light signal with a wavelength of 1550 nm and 1625 nm have been measured under 0.05 dB/km for a fiber of the known G657A2-type (BendBright.sup.XS© FTTH optical fiber, produced by Prysmian Group). Such a minimization of the light attenuation in an optical fiber is undoubtedly a major performance that distinguishes the invention from the prior art.