Large numerical aperture bend resistant multimode optical fiber

Bend resistant optical fibers which are multi-moded at 1300 nm include a core, an inner cladding, a low index ring and an outer cladding. The core has a graded index of refraction with a core alpha profile where 1.9≦αC≦2.1, a maximum relative refractive index percent Δ1Max%, and a numerical aperture NA of greater than 0.23. The inner cladding surrounds the core and has a maximum relative refractive index percent Δ2Max%, a minimum relative refractive index percent Δ2Min%, and a radial thickness ≧0.5 microns, wherein Δ1Max%>Δ2Max%. The low index ring surrounds the inner cladding and has a relative refractive index percent Δ3%, a radial thickness of at least 0.5 microns, a profile volume with an absolute magnitude of greater than 50%-μm2, wherein Δ2Min%≧Δ3%. The outer cladding surrounds the low index ring and has a relative refractive index percent Δ4%, such that Δ1Max%>Δ4%≧Δ2Max%.

BACKGROUND

The present specification generally relates to optical fibers and, more specifically, to multimode optical fibers having large numerical apertures and improved bend performance.

2. Technical Background

Corning Incorporated manufactures and sells InfiniCor® 62.5 μm optical fiber, which is multimode optical fiber having a core with a maximum relative refractive index delta of about 2% and 62.5 μm core diameter, as well as InfiniCor® 50 μm optical fiber, which is multimode optical fiber having a core with a maximum relative refractive index delta of about 1% and 50 μm core diameter. It would be desirable to develop alternative multimode fiber designs, particularly optical fiber designs with high numerical apertures that would enable improved bend performance and higher bandwidth.

SUMMARY

According to one embodiment, a bend resistant optical fiber which is multi-moded at 1300 nm includes a core, an inner cladding, a low index ring and an outer cladding. The core may be formed from silica-based glass and has a graded index of refraction with a core alpha profile where 1.9≦αC≦2.1, a maximum relative refractive index percent Δ1Max% relative to the outer cladding, and a numerical aperture NA of greater than 0.23. The inner cladding surrounds and is in direct contact with the core, the inner cladding having a maximum relative refractive index percent Δ2Max% relative to the outer cladding, a minimum relative refractive index percent Δ2min% relative to the outer cladding, and a radial thickness of at least 0.5 microns. Δ1Max% of the core may be greater than Δ2Max% of the inner cladding. The low index ring may surround and be in direct contact with the inner cladding such that the low index ring is spaced apart from the core. The low index ring has a relative refractive index percent Δ3% relative to the outer cladding, a radial thickness of at least 0.5 microns and a profile volume with an absolute magnitude of greater than 50%-μm2. The minimum relative refractive index percent Δ2Min% of the inner cladding is greater than or equal to Δ3% of the low index ring. The outer cladding surrounds and is in direct contact with the low index ring and may have a relative refractive index percent Δ4% relative to pure silica glass such that Δ1Max%>Δ4%≧Δ2Max%.

In another embodiment, a bend-resistant optical fiber which is multi-moded at 1300 nm includes a core, an inner cladding, a low index ring, and an outer cladding. The core may be formed from silica-based glass and comprises a graded index of refraction with a core alpha profile where 1.9≦αC≦2.1, a maximum relative refractive index percent Δ1Max% relative to the outer cladding, and a numerical aperture NA of greater than 0.23. The inner cladding may surround and be in direct contact with the core and have a graded index of refraction with an inner cladding alpha profile αIC, a maximum relative refractive index percent Δ2Max% relative to the outer cladding, and a minimum relative refractive index percent Δ2Min% relative to the outer cladding, wherein Δ1Max%>Δ2Max%. The low index ring may surround and be in direct contact with the graded index inner cladding such that the low index ring is spaced apart from the core, the low index ring having a relative refractive index percent Δ3% relative to the outer cladding, a radial thickness of at least 1 micron and a profile volume with an absolute magnitude of greater than 50%-μm2, wherein Δ2Min%≧Δ3%. The outer cladding may surround and be in direct contact with the low index ring, the outer cladding comprising a relative refractive index percent Δ4% relative to pure silica glass, wherein Δ1Max%>Δ4%≧Δ2Max%.

In yet another embodiment, a bend resistant optical fiber which is multi-moded at 1300 nm includes a core, an inner cladding, a low index ring and an outer cladding. The core may be formed from silica-based glass and comprises a graded index of refraction with a core alpha profile where 1.9≦αC≦2.1, a maximum relative refractive index percent Δ1Max% relative to the outer cladding, and a numerical aperture NA of greater than 0.23. The inner cladding may surround and be in direct contact with the core, the inner cladding having a maximum relative refractive index percent Δ2Max% relative to the outer cladding, a minimum relative refractive index percent Δ2Min% relative to the outer cladding and a radial thickness of at least 0.5 microns, wherein Δ2Max%−Δ2Min%≦0.1% and Δ1Max%>Δ2Max%. The low index ring may surround and be in direct contact with the inner cladding such that the low index ring is spaced apart from the core, the low index ring having a relative refractive index percent Δ3% relative to the outer cladding, a radial thickness of at least 0.5 microns and a profile volume with an absolute magnitude of greater than 50%-μm2, wherein the minimum relative refractive index percent Δ2Min% of the inner cladding is greater than or equal to Δ3%. The outer cladding may surround and be in direct contact with the low index ring, the outer cladding comprising a relative refractive index percent Δ4% relative to pure silica glass, wherein Δ1Max%>Δ4%≧Δ2Max%.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of bend resistant multimode optical fibers, examples of which are illustrated in the accompanying drawings.FIG. 1schematically depicts a cross section of an optical fiber according to one or more embodiments shown and described herein. The optical fiber generally comprises a core, an inner cladding, a low index ring and an outer cladding. The structure of the optical fibers as well as the properties of the optical fibers will be described in more detail herein. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.

The phrase “refractive index profile,” as used herein, refers to the relationship between refractive index or relative refractive index and optical fiber radius.

The phrase “relative refractive index percent,” as used herein, is defined as Δ%=100×(ni2−nREF2)/2ni2, where niis the maximum refractive index in region i, unless otherwise specified. The relative refractive index percent is measured at 1300 nm unless otherwise specified. Unless otherwise specified herein, nREFis the average refractive index of the outer cladding140, which can be calculated, for example, by taking “N” index measurements (nC1, nC2, . . . nCN) in the outer annular region of the cladding (which in some preferred embodiments may be undoped silica), and calculating the average refractive index by:

As used herein, the relative refractive index is represented by Δ and its values are given in units of “%,” unless otherwise specified. In cases where the refractive index of a region is less than the reference index nREF, the relative index percent is negative and is referred to as having a depressed region or depressed-index, and the minimum relative refractive index is calculated at the point at which the relative index is most negative unless otherwise specified. In cases where the refractive index of a region is greater than the reference index nREF, the relative index percent is positive and the region can be said to be raised or to have a positive index.

Macrobend performance is measured according to FOTP-62 (IEC-60793-1-47) by wrapping 1 turn around either a 6 mm, 10 mm, 15 mm, 20 mm, 30 mm or other diameter mandrel as stated (e.g. “1×10 mm diameter macrobend loss” or the “1×15 mm diameter macrobend loss”) and measuring the increase in attenuation due to the bending using an overfilled launch (OFL) condition. The minimum calculated effective modal bandwidths (Min EMBc) may be measured differential mode delay spectra as specified by TIA/EIA-455-220.

Bandwidth may be measured at 1300 nm (unless another wavelength is specified) according to FOTP-204 with overfilled launch.

As used herein, numerical aperture of the fiber means numerical aperture as measured using the method set forth in TIA SP3-2839-URV2 FOTP-177 IEC-60793-1-43 titled “Measurement Methods and Test Procedures-Numerical Aperture.”

The term “α-profile” or “alpha profile” refers to a relative refractive index profile, expressed in terms of Δ(r) which is in units or “%”, where r is the radius, which follows the equation,
Δ(r)=Δ(r0)(1[|r−30|/(r1−r0)]α),
where r0is the point at which Δ(r) is maximum, r1is the point at which Δ(r)% is zero with respect to pure silica glass, and r is in the range ri≦r≦rf, where Δ is defined above, riis the initial point of the α-profile, rfis the final point of the α-profile, and α is an exponent which is a real number. For a profile segment beginning at the centerline (r=0), the α-profile has the simpler form
Δ(r)=Δ(0)(1−[|r|/(r1)]α),
where Δ(0) is the refractive index delta at the centerline.

The optical core diameter 2*Roptis measured using the technique set forth in IEC 60793-1-20, titled “Measurement Methods and Test Procedures—Fiber Geometry,” in particular using the reference test method outlined in Annex C thereof titled “Method C: Near-field Light Distribution.” To calculate the optical core radius Roptfrom the results using this method, a 10-80 fit is applied per section C.4.2.2 to obtain the optical core diameter, which is then divided by 2 to obtain the optical core radius.

The low index ring has a profile volume, V3, defined herein as:

where Riis the innermost radius where Δ2(r)% is negative with respect to the outer cladding and Rois the outermost radius of the depressed-index annular region where Δ3(r)% is negative with respect to an outer cladding after passing through a minimum. For the fibers disclosed herein, the absolute magnitude of V3is preferably greater than 50%-μm2, more preferably greater than 140%-μm2. In some cases, V3is greater than 180%-μm2or even greater than 200%-μm2.

Referring toFIG. 1, a cross section of an optical fiber100multi-moded at 1300 nm is schematically illustrated. The optical fiber generally comprises a core110an inner cladding120, a low index ring130, and an outer cladding140each of which is formed from silica-based glass. The cross section of the optical fiber100may be generally circular-symmetric with respect to the center of the core110.

In the embodiments described herein the core110generally comprises silica glass doped with one or more dopants which increase the index of refraction of the glass. In some embodiments, the core comprises silica doped with germanium (i.e., germania (GeO2). However, it should be understood that dopants other than germanium such as Al2O3or P2O5, individually or in combination, may be employed within the core. In some embodiments, the refractive index profile of the optical fiber disclosed herein is non-negative from the centerline to the outer radius of the core. In some embodiments, the optical fiber contains no index-decreasing dopants in the core110. When dopants are present in the core, the dopants may be distributed throughout the core to obtain the desired refractive index profile. The core110has a relative refractive index percent Δ1% relative to the outer cladding and a maximum relative refractive index percent Δ1Max% of greater than 1.6% and less than 2.2%, more preferably greater than 1.6% and less than 2.0%, and, most preferably, greater than 1.6% and less than 1.9%. The numerical aperture of the core is greater than 0.23, more preferably from about 0.26 to 0.31, and even more preferably from about 0.27 to about 0.29.

The core110has a graded index in a radial direction from the center of the core such that the refractive index profile of the core has a parabolic or substantially parabolic shape. In some embodiments the refractive index profile of the core has core alpha profile with an α value (αC) between 1.9 and 2.1 as measured at 1300 nm. In some embodiments the refractive index profile of the core may have a centerline dip such that the maximum relative refractive index percent Δ1Max% of the core110(and the maximum relative refractive index percent of the entire optical fiber) is located a small distance away from the centerline of the optical fiber. However, in other embodiments, the refractive index profile of the core has not centerline dip such that the maximum relative refractive index percent Δ1Max% of the core110(and the maximum relative refractive index percent of the entire optical fiber) is located at the center of the optical fiber.

The core110generally has a physical core radius R1and an optical core radius Ropt. The physical core radius, as used herein, is the radius at which the relative refractive index percent Δ1% of the core first reaches zero in a radial direction from the center of the core110. The optical core radius Ropt, as used herein, is half of the optical core diameter. For refractive index profiles of the type shown inFIG. 2, Roptis approximately equal to R2. For refractive index profiles of the type shown inFIG. 3, R1≦Ropt≦R2and is modeled by determining the radius at which the refractive index equals the effective refractive index of the highest mode group with leaky losses of less than 1 dB/m. In the embodiments depicted inFIGS. 2 and 3, R2is the innermost radius at which the relative refractive index of the optical fiber first reaches a minimum in a radial direction from the center of the core110. In the embodiments shown and described herein, the core110has a physical core radius R1from 26 microns to 33 microns, more preferably less than 31 microns, even more preferably less than 30.5 microns and, most preferably, less than 30 microns. Further, in the embodiments described herein the core110has an optical core radius Roptfrom 28 microns to 34 microns, more preferably from 29 to 33 microns and, most preferably, from 30 to 32.5 microns.

The inner cladding120surrounds and is in direct contact with the core110and extends from the physical core radius R1to the radius R2. Accordingly, it should be understood that the inner cladding has a radial thickness T2=R2−R1. In the embodiments described herein, the radial thickness T2of the inner cladding120is generally from about 0.5 microns to about 5.0 microns.

The inner cladding120has a relative refractive index percent Δ2% relative to the outer cladding140and a minimum relative refractive index percent Δ2Min% and a maximum relative refractive index Δ2Max%. The inner cladding120may comprise silica glass which is substantially free from dopants (i.e., the inner cladding120is formed from pure silica glass). Alternatively, the inner cladding120may comprise one or more dopants which increase or decrease the index of refraction of the inner cladding120. However, the maximum relative refractive index percent Δ2Max% of the inner cladding120will generally be less than or equal to the relative refractive index percent Δ1% of the core and, more specifically, the maximum relative refractive index percent Δ2Max% of the inner cladding120is less than the maximum relative refractive index percent Δ1Max% of the core110.

Referring now toFIG. 2, the relative refractive index percent as a function of the radius of an optical fiber is graphically depicted for a bend resistant optical fiber according to one or more embodiments shown and described herein. In the embodiment of the bend resistant optical fiber depicted inFIG. 2, the relative refractive index percent Δ2% is substantially uniform through the radial thickness of the inner cladding120. For example, in one embodiment shown inFIG. 2, the maximum relative refractive index percent Δ2Max% and the minimum relative refractive index percent Δ2Min% are the same (i.e., Δ2Max%=Δ2Min%). In another embodiment, the difference between Δ2Max% and Δ2Min% is less than or equal to 0.1% (i.e., Δ2Max%−Δ2Min%≦0.1%) such that the relative refractive index percent Δ2% is substantially uniform through the radial thickness of the inner cladding. For example, in one embodiment, Δ2Max%≦0.05% while Δ2Min%≧−0.05% such that the difference between Δ2Max% and Δ2Min% is less than or equal to 0.1%. In embodiments where the relative refractive index percent Δ2% is substantially uniform through the radial thickness of the inner cladding120, the radial thickness of the inner cladding120is from about 0.5 microns to about 4.0 microns, more preferably from about 0.75 microns to about 2 microns and, most preferably, from about 0.1 micron to about 1.5 microns. Further, in these embodiments, the physical core radius R1is from about 27 microns to about 33 microns, more preferably from about 28 microns to about 32 microns and, most preferably, from about 29 microns to about 31 microns.

WhileFIG. 2depicts the relative refractive index Δ2% of the inner cladding120as being substantially uniform through the radial thickness T2of the inner cladding120, it should be understood that in other embodiments the relative refractive index Δ2% may vary through the radial thickness of the inner cladding120.

For example, referring toFIG. 3, a refractive index profile of one embodiment of a bend resistant multimode optical fiber is graphically illustrated where the relative refractive index percent Δ2% varies through the radial thickness of the inner cladding120. In one embodiment, the index of refraction of the inner cladding decreases between R1and R2such that the relative refractive index percent Δ2% is graded in a radial direction, as depicted inFIG. 3. For example, the refractive index profile of the inner cladding may have an inner cladding alpha profile with an α value (αIC). In some embodiments, the refractive index profile of the inner cladding120may be an extension of the refractive index profile of the core110. For example, the inner cladding may have an α value αICfrom about 1.9 to about 2.1 such that the inner cladding is a continuation of the graded index profile of the core. In this embodiment the graded index of the refraction of the core continues past R1and into the inner cladding120where the relative refractive index percent Δ2% is negative between R1and R2.

In another embodiment, the α-shape of the inner cladding120is a function of the α-shape of the core110. For example, in this embodiment, the inner cladding may have an α value αICfrom 0.8*αCto 1.2*αC. In this embodiment the graded index of refraction of the core also continues past R1and into the inner cladding120where the relative refractive index percent Δ2% is negative between R1and R2. However, in this embodiment the α-shape of the inner cladding may be slightly different than the α-shape of the core. In either embodiment the inner cladding120has a maximum relative refractive index percent Δ2Max% at R1, which decreases over the radial thickness of the inner cladding to a minimum relative refractive index percent Δ2Min% at R2. Accordingly, it should be understood that Δ1%≧Δ2Max% and Δ1Max%>Δ2Max%. In these embodiments Δ2Max%<−0.05%.

Further, where the relative refractive index percent Δ2% varies through the radial thickness of the inner cladding120, the inner cladding120has radial thickness T2from about 1 micron to about 5 microns, more preferably greater than about 1.5 microns and, most preferably, greater than about 2.0 microns. Further, in these embodiments, the physical core radius is from about 26 microns to about 31 microns, more preferably from about 27 microns to about 30 microns.

Referring now toFIGS. 1-3, the low index ring130surrounds and is in direct contact with the inner cladding120such that the low index ring130is spaced apart from the core110. The low index ring extends from the optical radius R2to a radius R3such that the low index ring has a radial thickness T3=R3−R2. The radius R3, as used herein, refers to the radius of the optical fiber100at which the relative refractive index of the optical fiber100reaches a value of 0.05% after passing through a minimum in the radial direction from the centerline of the optical fiber. In the embodiments described herein the radial thickness T3of the low index ring130may be from about 2.0 microns to about 8.0 microns, more preferably from about 4 microns to about 6 microns. The low index ring130may be formed from silica glass which includes one or more dopants which decrease the index of refraction of the silica glass. For example, the low index ring130may include silica glass doped with fluorine, boron or various combinations thereof. However, it should be understood that other dopants may be used to decrease the index of refraction of the low index ring130.

The low index ring130generally has a relative refractive index percent Δ3% with respect to the outer cladding with a minimum relative refractive index percent Δ3Min% and a maximum relative refractive index percent Δ3Max%. The relative refractive index Δ3% of the low index ring130is less than zero through the radial thickness of the low index ring. In one embodiment, the relative refractive index percent Δ3% is substantially uniform through the radial thickness of the low index ring130(i.e., from R2to R3) such that Δ3Min%=Δ3Max%. However, it should be understood that, in other embodiments, Δ3% may vary between R2and R3. In general, the relative refractive index Δ3% of the low index ring130is less than or equal to Δ2Min%. As described hereinabove, the low index ring130may have a profile volume V3with an absolute magnitude preferably greater than 50%-μm2, more preferably greater than 100%-μm2and even more preferably greater than 140%-μm2.

An outer cladding140is disposed around the low index ring130such that the outer cladding140surrounds and is in direct contact with the low index ring130. The outer cladding140extends from R3to R4. In the embodiments described herein, R4may be from about 40 microns to about 62.5 microns. The outer cladding140may generally have a radial thickness T4=R4−R3. In the embodiments described herein, T4may be from about 10 microns to about 30 microns, more preferably less than about 25 microns. In some embodiments the outer cladding140is formed from pure silica glass. The term pure silica glass, as used herein, means that the silica glass does not contain dopants in concentrations which would significantly modify (i.e., increase or decrease) the index of refraction of pure silica glass. In these embodiments, the relative index of refraction Δ4% of the outer cladding140is zero relative to pure silica glass. In other embodiments, the outer cladding140has a maximum relative index of refraction percent Δ4Max% which is less than 0.05% and a minimum relative index of refraction percent Δ4Min% which is greater than −0.05%. In this embodiment, the low index ring130ends where Δ3% reaches a value of greater than −0.05% going radially outward after passing through Δ3Min%. In general, the outer cladding140has a relative refractive index Δ4% such that Δ1Max%>Δ4%≧Δ2Max%.

Accordingly, the glass portion of the optical fiber100(e.g., the core102, the inner cladding104, the low index ring106, and the outer cladding108) may have a diameter of 2R4. In the embodiments described herein, the diameter of the glass portion of the optical fiber is between 120 and 130 μm, preferably about 125 μm.

The optical fiber100shown inFIG. 1may be formed by conventional fiber manufacturing techniques. For example, the various layers (e.g., the inner cladding120, the low index ring130, and the outer cladding140) may be formed on a core cane member to create a fiber preform using various vapor phase deposition techniques such as chemical vapor deposition (CVD), modified chemical vapor deposition (MCVD), or any other vapor phase deposition technique used in the manufacture of optical fiber preforms. Alternatively, the fiber preform may be formed using rod-in-tube techniques where a core cane member is “sleeved” with a glass tube or tubes having the desired characteristics. The resulting fiber preform formed from the aforementioned processes may thereafter be drawn into optical fiber.

After the optical fiber100is drawn from the fiber preform, the optical fiber100may be coated with one or more coatings (not shown). For example, in one embodiment, optical fiber100may be coated with a low modulus primary coating and a high modulus secondary coating.

Optical fibers according to the embodiments described herein have large numerical apertures (e.g., NA≧0.23) and overfilled launch (OFL) bandwidths at 1300 nm of greater than 1000 MHz-km, more preferably greater than 1500 MHz-km and, more preferably, greater than 2000 MHz-km. In some embodiments, the OFL bandwidth at 1300 nm of the optical fibers described herein is greater than 3000 MHz-km, more preferably greater than 4000 MHz-km and, most preferably, greater than 5000 MHz-km. In addition, optical fibers according to the embodiments described herein have improved bending performance. For example, the optical fibers exhibit a 1×10 mm diameter macrobend loss of less than 0.2 dB at a wavelength of 1300 nm, more preferably less than 0.1 dB and, most preferably, less than 0.05 dB.

EXAMPLES

The various embodiments of optical fibers will be further clarified by the following modeled examples of various embodiments of the high numerical aperture, bend resistant optical fibers set forth in Tables 1-4 below. Specifically, Tables 1-4 list various modeled values for Δ1Max%, R1, αC, Δ2Max%, R2, Δ3Max%, R3, V3, the OFL bandwidth of the optical fiber at 1300 nm, and the 1×10 mm diameter macrobend loss for 20 modeled optical fibers.

The optical fiber examples contained in Table 1 have physical core radii fixed at 31.25 microns while the maximum relative refractive index Δ1Max% of the core ranges from about 1.9% to about 2.0%. The embodiments of the optical fibers shown in Table 1 have numerical apertures of 0.29 and optical core diameters of greater than 62.5 microns.

Referring toFIG. 4, the modeled overfill launch (OFL) bandwidth at 1300 nm for an optical fiber according to one or more embodiments shown and described herein, is graphically depicted as a function of the radius R2of the inner cladding. The various data points comprising the curve represent optical fibers with a fixed optical core radius R1of 31.25 microns (i.e., optical fibers such as those described in Table 1). As shown inFIG. 4, high OFL bandwidths may be achieved when the radial thickness T2of the inner cladding is greater than about 0.5 microns and less than about 4 microns, more preferably greater than about 1 micron and less than 2 microns, and most preferably, greater than about 1 micron and less than about 1.5 microns.FIG. 4also graphically demonstrates that a peak OFL bandwidth of greater than 3500 MHz-km may be achieved utilizing the fiber designs described herein.

The optical fiber examples contained in Table 2 have a physical core radius fixed at 31.25 microns, as with the optical fiber examples in Table 1. However, the maximum relative refractive index Δ1Max% of the core in optical fiber Examples 7-10 is less than 1.9%. Maintaining the maximum relative refractive index Δ1Max% below 1.9% reduces the numerical aperture NA of the optical fiber to approximately 0.28 which improves the compatibility of the optical fiber with existing 62.5 micron optical core diameter fiber. However, the fiber designs shown in Table 2 have improved OFL bandwidth at 1300 nm compared to existing 62.5 micron fibers in addition to low bend losses.

The optical fiber examples contained in Table 3 have optical core diameters of 62.5 microns and numerical apertures NA of 0.28 which are compatible with existing 62.5 micron fiber designs. However, the fiber designs shown in Table 3 have improved OFL bandwidth at 1300 nm compared to existing 62.5 micron fibers in addition to low bend losses. Examples 15 and 16 are optical fiber designs with outer cladding diameters of 90 microns and 80 microns respectively.

The optical fiber examples contained in Table 4 have an inner cladding which has a graded index of refraction extending between the physical core radius R1and an outer core radius R2. The optical fibers in these examples have R1≦Ropt≦R2, an optical core diameter of 62.5 microns with numerical apertures of at least 0.23, high OFL bandwidths at 1300 nm and low bend losses.