Small diameter low attenuation optical fiber

This applicationis an application for reissue of U.S. Pat. No. 11,181,687, issued Nov. 23, 2021, filed as U.S. patent application Ser. No. 16/391,859 on Apr. 23, 2019, whichclaims the benefit of priority to U.S. Provisional Application Ser. No. 62/726,664 filed on Sep. 4, 2018,andwhich claims the benefit of priority to U.S. Provisional Application Ser. No. 62/664,359 filed on Apr. 30, 2018, the content of which is relied upon and incorporated herein by reference in its entirety.

The present disclosure relates generally to optical fibers. More particularly, this disclosure relates to small diameter low attenuation optical fibers having a refractive index profile with a depressed-index cladding region, a low modulus primary coating and a high modulus secondary coating. Most particularly, this disclosure relates to small-glass diameter optical fibers with a primary and secondary coatings thereon, such that the coated fibers exhibit low attenuation, large mode field diameters, low cutoff wavelengths and low bending losses.

Optical fibers with small cladding and coating diameters are attractive for reducing the size of cables, decreasing cable cost, and increasing the bandwidth density of optical interconnects. It is also desirable to use thinner layers as primary and/or secondary coatings in reduced-cladding diameter fibers. However, the smaller cladding diameter increases the microbending sensitivity, and thinner primary and coating diameter further compromise the microbend performance as well as the protective function of the coatings. As a result, commercially available reduced-cladding diameter fibers tend to have small mode field diameters, high numerical apertures and/or high cutoff wavelengths to reduce bend sensitivity at long wavelengths above 1530 nm.

Optical fiber designs with reduced coating diameters have been proposed, but the cladding diameter of such fibers is maintained at the conventional value of 125 microns. Decreasing the cladding diameter to 90 microns or smaller increases the microbending sensitivity by an order of magnitude compared to fibers with cladding diameters of 125 microns, and the coating solutions proposed in these references are not sufficient to achieve low attenuation and low bend losses.

It is therefore desirable to design a single mode optical fiber having reduced cladding and coating diameters, low attenuation, low bend losses, a G.657-compliant mode field diameter and a low cutoff wavelength.

The present disclosure discloses exemplary coated optical fiber embodiments having an outer cladding diameter of 90 microns or less (e.g., or 85 microns or less, or 70 microns or less) that possess large mode field diameters without experiencing significant bending-induced signal degradation. The coated fiber may comprise an internal glass region (glass cladding) having an outer radius not greater than 45 microns (for example not greater than 42 microns, or not greater than 35 microns, or not greater than or not greater than 34 microns, or not greater than 32.5 microns) which is surrounded by primary and secondary coatings. Representative fibers may include, in concentric order, a glass core, a glass cladding, a primary coating surrounding the glass cladding, and a secondary coating surrounding the primary coating. The glass cladding may include a first inner cladding region and a second inner cladding region. The first inner cladding region may have an outer radius not greater than 16 microns, or not greater than 14 microns, or not greater than 12 microns, or not greater than or not greater than 10 microns, or not greater than 8 microns. The fiber core has a higher refractive index than the maximum refractive index of the first inner cladding region. The fiber core also has a higher refractive index than the maximum refractive index of the outer cladding region. The second inner cladding regions may have a lower refractive index than the first inner cladding region. The second inner cladding regions may have a lower refractive index than the outer cladding region. The primary coating may be formed from a low modulus material and the secondary coating may be formed from a high modulus material.

According to some embodiments an optical fiber comprises:

According to some embodiments r=38 to 42 microns, r=60 microns to 65 microns, r=77.5 to 82.5 microns. According to some embodiments r=40.25 microns, r=62.5 microns, r=77.5 to 82.5 microns.

According to some embodiments the r=38 to 42 microns, r=45 to 55 microns (e.g., 48 to 52 microns), r=60 to 65 microns. According to some embodiments the r=40.25 microns, r=50 microns, r=62.5 microns.

According to some embodiments the fiber has the mode field diameter MFD≤9.5 microns at a wavelength of 1310 nm, for example MFD≤9.2 microns, or MFD≤9.0 microns. According to some embodiments the fiber has the mode field diameter MFD≥8.4 microns at a wavelength of 1310 nm, for example MFD≤9.2 microns, or MFD≤9.0 microns.

According to some embodiments the cladding includes a first inner cladding region having an outer radius r, and a second inner cladding region surrounding said first inner cladding region and having an outer radius r, said second inner cladding region having a relative refractive index Δwith a minimum value Δ, wherein Δ≤−0.2%.

According to some embodiments, t>8 microns. According to other embodiments t>10 microns. According to some embodiments, ris less than 45 microns. According to some embodiments, ris not greater than 42 microns, or not greater than 40 microns, or not greater than 35 microns, or not greater than 32.5 microns, or not greater than 30 microns. According to some embodiments, 25 microns≤r≤45 microns, or 30 microns≤r≤42 microns, or 35 microns≤r≤42 microns, or 38 microns≤r≤42 microns, or 25 microns≤r≤35 microns. According to some embodiments, r≤80 microns. According to some embodiments, 75 microns≤r≤85 microns. According to some embodiments, 39 microns≤r≤41 microns and 75 microns≤r≤85 microns. According to some embodiments, 30 microns≤r≤35 microns and 60 microns≤r≤70 microns.

According to some embodiments 0.5≤t/t≤1.5.

According to some embodiments, χ<1.3 MPa, or χ≤1 MPa, or χ≤0.9 MPa, or χ≤0.8 MPa. According to some embodiments, 0.5 MPa≤χ≤1.3 MPa. According to some embodiments, 0.5 MPa≤χ≤1.1 MPa. According to some embodiments, 0.5 MPa≤χ≤1 MPa. According to some embodiments, 0.5 MPa≤χ≤0.9 MPa.

According to some embodiments, the cladding includes a first inner cladding region having an inner radius r, an outer radius rand an outer cladding region surrounding the first inner cladding region and having the outer radius r, the inner cladding region having a relative refractive index Δwith a minimum value Δ, and a maximum value Δwherein Δ<0.1% and Δ>−0.1%. According to some embodiments, Δ<0.05% and Δ>−0.05%. According to some embodiments the refractive index Δis substantially constant between the inner radius rand the outer radius r. According to some embodiments, r>8 microns. According to other embodiments, r>9 microns, or r>10 microns. According to some embodiments, r<12 microns. According to other embodiments, r<11 microns, or r<10 microns, or 8 microns<r<12 microns, or 9 microns<r<11 microns.

According to some embodiments, the cladding includes a second inner cladding region surrounding the first inner cladding region and having an outer radius r, the second inner cladding region having a relative refractive index Δwith a minimum value Δ, and a maximum value Δwherein Δ<−0.2% and Δ>−0.7%. According to some embodiments, Δ<−0.25% and Δ>−0.6%. According to other embodiments, Δ<−0.3% and Δ>−0.5%. According to some embodiments the second inner cladding region is directly adjacent to the first inner cladding region and has an inner radius r. According to some embodiments, r<20 microns. According to other embodiments, r<18 microns, or r<16 microns or r<15 microns.

According to some embodiments the fiber exhibits a bend loss at a wavelength of 1550 nm, when turned about a mandrel having a diameter of 10 mm, of less than 1.0 dB/turn. According to other embodiments the fiber exhibits a bend loss at a wavelength of 1550 nm when turned about a mandrel having a diameter of 10 mm less than 0.5 dB/turn, or less than 0.3 dB/turn or even less than 0.2 dB/turn.

According to some embodiments the fiber exhibits a mode field diameter at a wavelength of 1310 nm greater than 8.2 microns. According to other embodiments the fiber exhibits a mode field diameter at a wavelength of 1310 nm greater than 8.4 microns, or greater than 8.6 microns. According to some embodiments the fiber exhibits a mode field diameter at a wavelength of 1310 nm less than 9.5 microns. According to other embodiments the fiber exhibits a mode field diameter at a wavelength of 1310 nm less than 9.2 microns, or less than 9.0 microns.

According to some embodiments the fiber exhibits a fiber cutoff wavelength less than 1310 nm. According to other embodiments the fiber exhibits a fiber cutoff wavelength less than 1300 nm, or less than 1280 nm. According to some embodiments the fiber exhibits a fiber cutoff wavelength greater than 1150 nm. According to other embodiments the fiber exhibits a fiber cutoff wavelength greater than 1200 nm, or greater than 1220 nm.

According to some embodiments the fiber exhibits an attenuation at 1550 nm less than 1.0 dB/km. According to other embodiments the fiber exhibits an attenuation at 1550 nm less than 0.7 dB/km, or less than 0.5 dB/km, or less than 0.4 dB/km, or less than 0.3 dB/km or even less than 0.25 dB/km. According to some embodiments the fiber exhibits an attenuation at 1600 nm less than 1.0 dB/km. According to other embodiments the fiber exhibits an attenuation at 1600 nm less than 0.7 dB/km, or less than 0.5 dB/km, or less than 0.4 dB/km, or less than 0.3 dB/km or even less than 0.25 dB/km.

According to some embodiments, the primary coating has an in situ modulus Eof 0.3 MPa or less. According to some embodiments, the secondary coating has an in situ modulus Eof 1800 MPa or greater.

The core may include silica glass or a silica-based glass. Silica-based glass may be silica glass modified with an alkali metal (e.g. Na, K), an alkaline earth metal (e.g. Mg, Ca), a column III element (e.g. B), or a column V element (e.g. P); or a dopant. The refractive index across the core may be constant or variable. The core refractive index may be at a maximum at or near the center of the core and continuously decreases in the direction of the outer core boundary. The core may include an updopant, for example, germania (GeO). The core refractive index profile may be or may approximate a Gaussian profile, may be an α profile, may be a step index profile, or may be a rounded step-index profile.

The cladding may include silica glass or a silica-based glass. The silica-based glass may be silica glass modified with an alkali metal (e.g. Na, K), an alkaline earth metal (e.g. Mg, Ca), a halogen (e.g. F, Cl), or other dopants (e.g. B, P, Al, Ti).

According to some embodiments, the cladding may include an inner cladding region and an outer cladding region, where the inner cladding region may have a lower refractive index than the outer cladding region. The inner cladding region may have a constant or continuously varying refractive index. The cladding may include a region that forms a reduced index trench in the index profile of the coated fiber. The outer cladding region may have a substantially constant refractive index.

The cladding may include a first inner cladding region adjacent the core and a second cladding region disposed between the first inner cladding region and the outer cladding region. The refractive index of the second inner cladding region may be lower than the refractive index of the first inner cladding region. The refractive index of the second inner cladding region may be lower than the refractive index of the outer cladding region. The refractive index of the second inner cladding region may be lower than the refractive indices of the first inner cladding region and the outer cladding region. The refractive index of the second inner cladding region may form a trench in the index profile of the coated fiber. The trench is a region of depressed refractive index relative to the outer cladding region.

The refractive index profiles of the core and cladding may be achieved through control of a spatial distribution of updopants and/or downdopants in silica or silica-based glass. The core may be updoped substantially with GeO, resulting in a refractive index delta (due to Ge) relative to pure silica given by the following equation Delta%=0.0601wt. % of GeO. The second inner cladding region may be downdoped substantially with Fluorine (F), resulting in a refractive index delta (due to F), relative to pure silica, given by the following equation: Delta%=−0.3053wt. % of F.

The primary coating may be formed from a curable composition that includes an oligomer and a monomer. The oligomer may be a urethane acrylate or a urethane acrylate with acrylate substitutions. The urethane acrylate with acrylate substitutions may be a urethane methacrylate. The oligomer may include urethane groups. The oligomer may be a urethane acrylate that includes one or more urethane groups. The oligomer may be a urethane acrylate with acrylate substitutions that includes one or more urethane groups. Urethane groups may be formed as a reaction product of an isocyanate group and an alcohol group.

The primary coating may have an in situ modulus of elasticity E(also referred to herein as elastic modulus) of 0.35 MPa or less, or 0.3 MPa or less, or 0.25 MPa or less, or 0.20 MPa or less, or 0.19 MPa or less, or 0.18 MPa or less, or 0.17 MPa or less, or 0.16 MPa or less, or 0.15 MPa or less. The glass transition temperature of the primary coating may be −15° C. or less, or −25° C. or less, or −30° C. or less, or −40° C. or less. The glass transition temperature of the primary coating may be greater than −60° C., or greater than −50° C., or greater than −40° C. The glass transition temperature of the primary coating may be or between −60° C. and −15° C., or between −60° C. and −30° C., or between −60° C. and −40° C., or between −50° C. and −15° C., or between −50° C. and −30° C., or between −50° C. and −40° C.

The secondary coating may be formed from a curable secondary composition that includes one or more monomers. The one or more monomers may include bisphenol-A diacrylate, or a substituted bisphenol-A diacrylate, or an alkoxylated bisphenol-A diacrylate. The alkoxylated bisphenol-A diacrylate may be an ethoxylated bisphenol-A diacrylate. The curable secondary composition may further include an oligomer. The oligomer may be a urethane acrylate or a urethane acrylate with acrylate substitutions. The secondary composition may be free of urethane groups, urethane acrylate compounds, urethane oligomers or urethane acrylate oligomers.

The secondary coating may be a material with a higher modulus of elasticity and higher glass transition temperature than the primary coating. The in situ modulus of elasticity Eof the secondary coating may be 1200 MPa or greater, and preferably 1300 MPa or greater, or 1500 MPa or greater, or 1800 MPa or greater, or 2100 MPa or greater, or 2400 MPa or greater, or 2700 MPa or greater. The secondary coating may have an in situ modulus Ebetween about 1500 MPa and 10,000 MPa, or between 1500 MPa and 5000 MPa. The in situ glass transition temperature of the secondary coating may be at least 50° C., or at least 55° C., or at least 60° C. or between 55° C. and 65° C.

The radius of the coated fiber may coincide with the outer diameter of the secondary coating. The radius of the coated fiber may be 85 microns or less, or 80 microns or less, or 75 microns or less. In some embodiments the radius of the coated fiber may be between 75 and 85 microns. In some embodiments the radius of the coated fiber may be between 55 and 75 microns. In other embodiments the radius of the coated fiber may be between 60 and 70 microns.

The radius of the coated fiber may coincide with the outer diameter of a tertiary coating that may comprise a UV-curable ink. The radius of the coated fiber may be 85 microns or less, or 80 microns or less, or 75 microns or less. In some embodiments the radius of the coated fiber may be between 75 and 85 microns. In other embodiments the radius of the coated fiber may be between 60 and 75 microns. In other embodiments the radius of the coated fiber may be between 60 and 65 microns.

Within the coated fiber, the glass radius (coinciding with the outer diameter of the cladding) may be less than 45 microns, or less than 42 microns, or less than 40 microns, or not greater than 38 microns. In some embodiments the glass radius (or the outer radius of the outer cladding) of the fiber may be at least 24 microns, and in some embodiments at least 30 microns. In some embodiments the glass radius (or the outer radius of the outer cladding) of the fiber may be between 24 microns and 35 microns, or between 27 microns and 35 microns, or between 30 microns and 35 microns or between 24 and 26 microns. The glass may be surrounded by the primary coating. According to some embodiments the primary coating has a thickness tgreater than 8 microns, for example 8 microns<t≤20 microns, 8 microns<t≤16 microns, or 8 microns<t≤12 microns. According to some embodiments the primary coating has a thickness tgreater than 12 microns, or greater than 15 microns, or greater than 20 microns, for example 15 microns≤t≤35 microns, 15 microns≤t≤30 microns, or 20 microns≤t≤30 microns, 20 microns≤t≤35 microns, or 25 microns≤t≤35 microns. The balance of the coated fiber diameter may be provided by the secondary coating or by the combination of the secondary and the (optional) tertiary coating. According to some exemplary embodiments the secondary coating has thickness tgreater than 10 microns, or greater than 12 microns, or greater than 15 microns, for example 10 microns to 15 microns, 10 microns to 20 microns, 12 microns to 20 microns, or 15 microns to 25 microns. In some embodiments an optional tertiary coating is situated over the secondary coating, and in such embodiments the outer radius of the coated fiber is the outer radius of the tertiary coating. In some embodiments the tertiary coating has a thickness tof 5 microns or less (e.g., 2 to 5 microns or 3 to 4 microns). According to at least some embodiments, the ratio t/(t+t) is preferably 0.5<t/(t+t)<1.5.

Coated fibers in accordance with the present disclosure may be reduced diameter fibers that exhibit low attenuation and low bending losses while providing a mode field diameter that minimizes losses associated with splicing and connecting to standard single-mode fibers. The mode field diameter (MFD) at 1310 nm may be greater than 8.2 microns, greater than 8.4 microns, or greater than 8.6 microns. For example, in some embodiments, 8.2 microns≤MFD≤9.5 microns, or 8.2 microns≤MFD≤9.2 microns, or 8.2 microns≤MFD≤9.0 microns, at 1310 nm.

The coated fibers may exhibit a macrobend loss at 1550 nm of less than 1.0 dB/turn when wrapped around a mandrel with a diameter of 10 mm, or less than 0.5 dB/turn when wrapped around a mandrel with a diameter of 10 mm. In some embodiments the coated fibers may exhibit a macrobend loss at 1550 nm of less than less than 0.25 dB/turn when wrapped around a mandrel with a diameter of 15 mm. In some embodiments the coated fibers may exhibit a macrobend loss at 1550 nm less than 0.15 dB/turn when wrapped around a mandrel with a diameter of 20 mm.

The optical and mechanical characteristics of the fibers of the present disclosure may be compliant with the G.657 standard. The coated fibers may have a cabled cutoff wavelength of 1260 nm or less. The fibers may have a zero dispersion wavelength λin the range 1300 nm≤λ≤1324 nm.

Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims hereof, as well as the appended drawings.

It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework to understand the nature and character of the claims.

The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the description serve to explain principles and operation of the various embodiments.

The present disclosure concerns coated optical fibers that may combine small cladding and coating diameters, a large mode field diameter, low fiber cutoff wavelength, low attenuation and low macrobend loss. A brief explanation of selected terminology used herein is now presented:

The “refractive index profile” is the relationship between refractive index or relative refractive index and fiber radius.

The “relative refractive index delta” is defined as

where n(r) is the refractive index of the fiber at the radial distance r from the fiber's centerline, unless otherwise specified, and n=1.444 is the refractive index of pure silica at a wavelength of 1550 nm. As used herein, the relative refractive index percent (also referred herein as the relative refractive index) is represented by Δ (or “delta”), Δ % (or “delta %”), or %, all of which are used interchangeably herein, and its values are given in units of percent or %, unless otherwise specified. Relative refractive index may also be expressed as Δ(r) or Δ(r) %.

“Chromatic dispersion”, which may also be referred to as “dispersion”, of a waveguide fiber is the sum of the material dispersion, the waveguide dispersion, and the intermodal dispersion at a wavelength λ. In the case of single-mode waveguide fibers, the inter-modal dispersion is zero. Dispersion values in a two-mode regime assume intermodal dispersion is zero. The zero dispersion wavelength (λ) is the wavelength at which the dispersion has a value of zero. Dispersion slope is the rate of change of dispersion with respect to wavelength.

The term “α-profile” refers to a relative refractive index profile Δ(r) that has the following functional form:

where ris the point at which Δ(r) is maximum, ris the point at which Δ(r) is zero, and r is in the range r≤r≤r, where ris the initial point of the α-profile, ris the final point of the α-profile, and α is a real number. In some embodiments r=0 and r=r.

The mode field diameter (MFD) is measured using the Petermann II method and is determined from:

where f(r) is the transverse electric field distribution of the LP01 mode and r is the radial position in the fiber.

The microbend resistance of a waveguide fiber may be gauged by induced attenuation under prescribed test conditions. Various tests are used to assess microbending losses including the lateral load microbend test, wire mesh covered drum microbend test, and mandrel wrap test.

In the lateral load test, a prescribed length of waveguide fiber is placed between two flat plates. A #70 wire mesh is attached to one of the plates. A known length of waveguide fiber is sandwiched between the plates, and a reference attenuation at a selected wavelength (typically within the range of 1200-1700 nm, e.g., 1310 nm or 1550 nm or 1625 nm) is measured while the plates are pressed together with a force of 30 Newtons. A 70 Newton force is then applied to the plates, and the increase in attenuation at the selected wavelength in dB/m is measured. The measured increase in attenuation is the lateral load wire mesh (LLWM) attenuation of the waveguide.