Patent Application
20110088433
1. Field
The present specification generally relates to optical fibers and, more specifically, to methods for drawing multimode bend resistant optical fiber from preforms.
2. Technical Background
Glass optical fibers with high bandwidth and improved bend-loss properties have recently been of significant interest in the telecommunications field. High bandwidth multimode fibers have been designed to meet the ever increasing demand for optical fibers capable of high volume data transmission. Such optical fiber designs may also be optimized to improve the bend-loss properties of the optical fiber. These fiber designs facilitate the use of such high bandwidth, multimode optical fiber for fiber to the home and fiber to the desk (FTTD) applications where bending loss has previously limited many designs from practical use.
While current fiber designs facilitate high bandwidth multimode bend-loss resistant optical fibers, a need exists for alternative methods for characterizing such optical fiber before the optical fiber is drawn from a preform.
According to one embodiment, a method for characterizing a multimode, bend resistant optical fiber may include determining a core refractive index profile for a core portion of a preform and determining a moat refractive index profile for a moat portion of the preform. Thereafter, a property of a multimode optical fiber is determined prior to drawing the multimode optical fiber from the preform. The property of the multimode fiber is determined based on the core refractive index profile of the preform, the moat refractive index profile of the preform, an inner radius rin and an outer radius rout of a depressed-index annular portion of the multimode optical fiber and fiber property coefficients.
In another embodiment, a method of drawing a multimode optical fiber from a preform may include determining a core refractive index profile of a core portion of a preform and determining a moat refractive index profile of a moat portion of the preform. Thereafter, a bend-loss of a multimode optical fiber is determined prior to drawing the multimode optical fiber from the preform. The bend-loss of the multimode optical fiber is determined based on the core refractive index profile of the preform, the moat refractive index profile of the preform, the inner radius rin and outer radius rout of a depressed-index annular portion of the multimode optical fiber and bend-loss coefficients. The bend-loss is compared to a target bend-loss range and, if the property is within the target bend-loss range, the multimode optical fiber is drawn from the preform.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description describe various embodiments and are intended to provide an overview or framework for understanding the nature and character of the claimed subject matter. The accompanying drawings are included to provide a further understanding of the various embodiments, and are incorporated into and constitute a part of this specification. The drawings illustrate the various embodiments described herein, and together with the description serve to explain the principles and operations of the claimed subject matter.
Various embodiments of the method for drawing an optical fiber will now be described with specific reference to the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.
As used herein, the phrase “refractive index profile” is the relationship between refractive index or relative refractive index and the radius of the optical fiber or preform.
As used herein, the phrase “relative refractive index percent” or “relative refractive index” of a region i is defined as
Δi%(r)=100−(ni(r)2−nREF2)/2ni(r)2,
where ni is the refractive index in region i at radius r, unless otherwise specified. The relative refractive index percent is measured at 850 nm unless otherwise specified. The reference index nREF is the refractive index of pure silica glass at 850 nm unless otherwise specified.
As used herein, the relative refractive index is represented by Δi% 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 refractive 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 refractive index percent is positive and the region can be said to be raised or to have a positive index. An “updopant” is herein considered to be a dopant which has a propensity to raise the refractive index relative to pure undoped SiO2. A “downdopant” is herein considered to be a dopant which has a propensity to lower the refractive index relative to pure undoped SiO2. An updopant may be present in a region of an optical fiber or preform having a negative relative refractive index when accompanied by one or more other dopants which are not updopants. Likewise, one or more other dopants which are not updopants may be present in a region of an optical fiber or preform having a positive relative refractive index. A downdopant may be present in a region of an optical fiber or preform having a positive relative refractive index when accompanied by one or more other dopants which are not downdopants. Likewise, one or more other dopants which are not downdopants may be present in a region of an optical fiber or preform having a negative relative refractive index.
When measured (as opposed to calculated) macrobend performance of a fiber is determined according to FOTP-62 (IEC-60793-1-47) by wrapping 1 turn around either a 15 mm or 30 mm diameter mandrel (the “1×15 mm diameter macrobend loss” or the “1×30 mm diameter macrobend loss”) and measuring the increase in attenuation due to the bending using an encircled flux launch (EFL) condition. The encircled flux was obtained by launching an overfilled pulse into an input end of a 2 m length of InfinicCor® 50 micron optical fiber which was deployed with a 1×25 mm diameter near the midpoint. The output end of the InfiniCor® 50 micron optical fiber was spliced to the fiber under test, and the measured bend loss was the ratio of the attenuation under the prescribed bend condition to the attenuation without the bend. For a fiber with low macrobend loss, the measurement is done by wrapping multiple turns on a mandrel to increase the accuracy. The macrobend loss was then normalized to 1 turn by dividing the total loss by the number of wraps around the mandrel.
When measured (as opposed to calculated), the numerical aperture of an optical fiber was measured using the method set forth in TIA SP3-2839-URV FOTP-177 IEC-60793-1-43 entitled “Measurement Methods and Test Procedures-Numerical Aperture.”
In the embodiments described herein, the refractive index profile of the preforms may be determined using the methodologies disclosed in U.S. patent application Ser. No. 12/414,188 filed Mar. 30, 2009 and entitled “METHODS OF MEASURING THE REFRACTIVE INDEX PROFILE OF A TRANSPARENT CYLINDRICAL OBJECT”, which is herein incorporated by reference.
The terms “α-profile” or “alpha profile” refer to a relative refractive index profile, expressed in terms of Δ %(r) which is in units of “%”, where r is the radius of the fiber and:
Δ %(r)=Δ(r0)%(1−[|r−r0|/(r1−r0)[α),
where r0 is the point at which Δ %(r) is a maximum, r1 is the point at which Δ (r)% is zero, and r is in the range ri≦r≦rf, where Δ % is defined above, ri is the initial point of the α-profile, rf is the final point of the α-profile, and α is an exponent which is a real number. For multimode optical fiber, r0=0 such that the above referenced equation becomes:
Δ %(r)=Δ(r0)%(1−[|r|/(r1)]α).
In the embodiments described herein, the multimode fibers comprise a depressed-index annular portion disposed around the core. The depressed-index annular portion has a profile volume V defined herein as:
where rin is the inner radius of the depressed-index annular portion and rout is the outer radius of the depressed-index annular portion where the radial thickness of the depressed-index annular portion is rth=rout−rin, as defined below. In the case where the refractive index of the moat has a constant value ΔM0, the moat volume is
V=Δ
M0(rout2rin2).
The units for moat volume are Δ %-μm2.
Methods of drawing a multimode, bend-loss resistant optical fiber will now be described in more detail with specific reference to the appended figures. The method generally includes characterizing an optical fiber preform prior to drawing optical fiber from the preform to verify that optical fiber drawn from the preform will have the desired bend-loss and/or numerical aperture properties. If it is determined that optical fiber drawn from the preform will have the desired bend-loss and/or numerical aperture properties, the optical fiber manufacturing process proceeds and optical fiber is drawn from the preform. However, if it is determined that optical fiber drawn from the preform will not have the desired bend-loss and/or numerical aperture properties, the preform is discarded and another preform is selected for characterization.
The method for drawing multimode, bend-loss resistant optical fiber begins with selecting an optical fiber preform which has the proper cross sectional area to facilitate a multimode, bend-loss resistant optical fiber. For example, referring to
The core portion 102 may have an index of refraction nc and a corresponding relative refractive index Δc% relative to pure silica glass and the moat portion 104 may have an index of refraction nM and a corresponding relative refractive index ΔM% relative to pure silica glass such that nC>nM and ΔC%>ΔM% as is graphically depicted in
In the embodiments shown and described herein, the core portion 102 of the preform has a graded index. For example, the refractive index profile of the core portion 102 has a parabolic or substantially parabolic shape. For example, in some embodiments, the refractive index profile of the core portion has an alpha-shape with a value of about 2, preferably between 1.8 and 2.3, as measured at 850 nm In some embodiments, the refractive index of the core portion may have a centerline dip wherein the maximum refractive index of the core portion, and the maximum refractive index of the entire preform, is located a small distance away from the center of the core portion of the preform. In other embodiments the refractive index of the core portion has no centerline dip, and the maximum refractive index of the core portion, and the maximum refractive index of the entire preform, is located at the center of the core portion of the preform. The parabolic shape extends to the radius R1 and preferably extends from the center of the core portion 102 to R1. As used herein, “parabolic” includes substantially parabolically shaped refractive index profiles which may vary slightly from an alpha value of 2.00 at one or more points in the core portion, as well as profiles with minor variations and/or a centerline dip.
The outer glass cladding portion 106 of the preform 100 may comprise an index of refraction nOC and a relative refractive index ΔOC% relative to pure silica glass which is generally greater than the relative refractive index ΔM% of the moat portion 104 (e.g., ΔOC%>ΔM%) such that the moat portion 104 is a depressed index trench disposed between the core portion 102 and the outer glass cladding portion 106.
In the embodiments shown and described herein, the core portion 102 may comprise silica glass (SiO2) comprising one or more index of refraction raising dopants such as, for example, GeO2, Al2O3, P2O5, TiO2, ZrO2, Nb2O5 and/or Ta2O5, such as when the core portion 102 is “up-doped.” The core portion 102 may be up-doped with one or more index of refraction increasing dopants such that ΔC% of the core portion 102 may be from about 0.2% to about 2%.
In some embodiments, the moat portion 104 comprises silica glass doped with fluorine. In some other embodiments, the moat portion 104 comprises silica comprising a plurality of non-periodically disposed voids. The voids may contain one or more gases, such as argon, nitrogen, krypton, CO2, SO2, or oxygen, or the voids may contain a vacuum with substantially no gas; regardless of the presence or absence of any gas, the refractive index in the moat portion 104 is lowered due to the presence of the voids. The voids can be randomly or non-periodically disposed in the moat portion 104, and in other embodiments, the voids are disposed periodically in the moat portion 104. Alternatively, or in addition, the moat portion 104 can also be provided by downdoping the moat portion 104 (such as with fluorine) or updoping one or more portions of the outer glass cladding portion and/or the core portion. Preferably, the minimum relative refractive index AM% of the moat portion 104, taking into account the presence of any voids, is less than −0.1%, more preferably less than about −0.2%, even more preferably less than about −0.3%, and most preferably less than about −0.4%.
As described above, the outer glass cladding portion 106 has a relative refractive index ΔOC%>ΔM%. Accordingly, it should be understood that the outer glass cladding portion 106 may comprise pure silica glass, silica glass up-doped with one or more dopants which increase the index of refraction of the outer glass cladding portion 106 with respect to pure silica glass, or silica glass down-doped with one or more dopants which decrease the index of refraction of the outer glass cladding portion 106 with respect to pure silica glass so long as ΔOC%>ΔM%.
It should be understood that the phrase “pure silica glass,” means that the silica glass does not contain material, such as dopants and/or other trace materials, in an amount which would significantly alter the refractive index of the silica glass. However, small amounts of dopants (e.g., chlorine and/or fluorine in an amount less than 1500 ppm of each) may be present in the silica glass which is otherwise pure silica.
Referring to
In this embodiment, the core portion 102 may have an index of refraction nC and a corresponding relative refractive index ΔC% relative to pure silica glass, the inner glass cladding portion 103 may have an index of refraction nIC and a corresponding relative refractive index ΔIC% relative to pure silica glass and the moat portion 104 may have an index of refraction nM and a corresponding relative refractive index ΔM% relative to pure silica glass such that nC>nIC>nM and ΔC%>ΔIC%>ΔM% which corresponds to the refractive index profile generally shown in
The core portion 102, moat portion 104 and outer glass cladding portion 106 may have relative refractive indices similar to those described above with respect to the embodiment of the preform 100 shown in
A multimode, bend-loss resistant optical fiber drawn from the preform 100 shown in
Similarly, a multimode, bend-loss resistant optical fiber drawn from the preform 101 shown in
After a preform with a structure appropriate for forming multimode, bend-loss resistant optical fiber has been selected, refractive index profiles for the core portion and the moat portion of the preform are determined using the measurement techniques referenced hereinabove. In one embodiment, the refractive index profiles are determined by direct measurement. For example, in one embodiment, a refractive index profile for the core portion of the preform may be determined after the core portion of the preform is formed and before the moat portion is formed around the core portion. A refractive index profile may thereafter be determined for the moat portion of the preform after the moat portion is formed around the core portion. Alternatively, refractive index profiles for both the core portion and the moat portion may be determined after the moat portion is formed around the core portion.
Further,
After refractive index profiles for the core portion 102 and the moat portion 104 of the preform are determined, the refractive index profiles may be used in conjunction with the desired dimensions of a multimode bend-loss resistant optical fiber to be drawn from the preform and fiber property coefficients to determine one or more properties of the multimode bend-loss resistant optical fiber prior to drawing the optical fiber from the preform.
For example, in one embodiment, the fiber property coefficients are bend-loss coefficients and the core refractive index profile of the preform, the moat refractive index profile of the preform, the inner radius rin and outer radius rout of the depressed-index annular portion of the multimode optical fiber are used in conjunction with the bend-loss coefficients to determine a bend-loss of a multimode optical fiber before the multimode optical fiber is drawn from the preform. In this embodiment, the bend-loss of the optical fiber is determined with the equation:
BL=A
BL
+B
BL·exp(CBL·V)−DBLΔC%−EBL·dC (1),
where BL is the bend-loss in dB/turn, ABL, BBL, CBL, DBL, and EBL are bend-loss coefficients, dc is the desired diameter of a core of the multimode optical fiber, V is the volume profile of the depressed-index annular portion of the multimode optical fiber, and ΔCMax% is the maximum relative refractive index of the core portion of the preform.
As described hereinabove, the desired diameter dC of the core of the multimode optical fiber may be from about 45 microns to about 56 microns, preferably from about 47 microns to about 53 microns.
The volume profile V of the depressed-index annular portion of the multimode optical fiber may be calculated according to the equation:
where ΔM% is the relative refractive index of the moat portion of the preform, rin is the inner radius of the depressed-index annular portion of the multimode optical fiber and rout is the outer radius of the depressed-index annular portion of the multimode optical fiber such that the radial thickness of the depressed-index annular portion is rth=rout−rin. Where the multimode optical fiber is drawn from a preform having a cross section as shown in
Alternatively, where the multimode optical fiber is drawn from a preform having a cross section as shown in
The relative refractive index ΔM%(r) of the moat portion of the preform may be calculated according to the equation:
ΔM%(r)=100·(nM(r)2−nREF2)/2nM(r)2 (3),
where nM(r) is the index of refraction in the moat portion of the optical fiber preform at a radius r as determined from the moat refractive index profile of the preform.
The maximum relative refractive index ΔCMax% of the core portion of the preform may be calculated according to the equation:
ΔCMax%=100·(nCMax2−nREF2)/2nCMax2 (4),
where nCMax is the maximum index of refraction in the core portion of the optical fiber preform as determined from the core refractive index profile of the preform.
Referring to
After the bend losses are plotted as a function of the volume profile, a line may be fit through the resulting curve yielding a relationship between the volume profile and the bend-loss. The bend-loss coefficients ABL, BBL, CBL, DBL, and EBL were determined through this curve fitting process. For a 15 mm bend diameter at a wavelength the 850 nm the bend-loss coefficient ABL may generally be greater than 0 and less than 10; the bend-loss coefficient BBL may be greater than 10 and less than 1000; the bend-loss coefficient CBL may be greater than 0 and less than 1; the bend-loss coefficient DBL may be greater than 0 and less than 10; and the bend-loss coefficient EBL may be greater than 0 and less than 1. For a 30 mm bend diameter at a wavelength the 850 nm the bend-loss coefficient ABL may generally be greater than 0 and less than 10; the bend loss coefficient BBL may be greater than 10 and less than 1000; the bend-loss coefficient CBL may be greater than 0 and less than 2; the bend loss coefficient DBL may be greater than 0 and less than 10; and the bend-loss coefficient EBL may be greater than 0 and less than 1. Table 1, shown below, contains exemplary values for the bend-loss coefficients for 15 mm and 30 mm diameter bend conditions. Both the 15 mm and 30 mm of bend-loss coefficients were determined at 850 nm. Further, while Equation (1) indicates an exponential relationship between the depressed-index annular portion volume profile and the bend-loss, it should be understood that a linear relationship between the depressed-index annular portion volume profile and the bend-loss may also be established through appropriate curve fitting techniques with corresponding linear bend-loss coefficients. Accordingly, is should be understood that other equations relating the volume profile of the depressed-index annular portion to bend-loss and the corresponding bend-loss coefficients may be used in conjunction with the methods described herein.
Accordingly, it should now be understood that Equation (1) may be used to predict the bend-losses for a multimode optical fiber of specified dimensions before the fiber is drawn from a preform having a specific core refractive index profile and a specific moat refractive index profile. In this manner, the suitability of the preform for forming multimode optical fiber with the desired bend-loss properties may be evaluated prior to drawing the multimode fiber from the preform.
For example, the multimode optical fibers may have a 15 mm target bend-loss range at 850 nm of less than 0.2 dB/turn, more preferably less than 0.1 dB/turn and, even more preferably, less than 0.05 dB/turn. Equation (1) and the coefficients of Table 1 may be used in conjunction with the desired dimensions of the optical fiber, the core refractive index profile of the preform and the moat refractive index profile of the preform to determine if the preform is suitable for drawing multimode optical fiber having bend-loss properties within the target bend-loss range. If the bend-loss BL for the desired optical fiber, as determined with Equation (1), is within the target bend-loss range, the preform is suitable for forming multimode optical fiber with the desired bend-loss properties and, thereafter, multimode fiber may be drawn from the optical fiber preform. However, if the bend-loss BL, as determined from Equation (1), is not within the target bend-loss range, the preform is not suitable for forming multimode optical fiber with the desired bend-loss properties and the multimode optical fiber is not drawn from the preform.
Alternatively, the multimode optical fibers may have a 30 mm target bend-loss range at 850 nm of less than 0.2 dB/turn, more preferably less than 0.1 dB/turn and, even more preferably, less than 0.05 dB/turn. Accordingly, if the bend-loss BL for the desired optical fiber, as determined with Equation (1) and the appropriate coefficients from Table 1, is within the target bend-loss range the preform is suitable for forming multimode optical fiber with the desired bend-loss properties and, thereafter, multimode fiber may be drawn from the optical fiber preform.
In another embodiment, the fiber property coefficients are numerical aperture coefficients and the core refractive index profile of the preform, the moat refractive index profile of the preform, the inner radius rin and outer radius rout of the depressed-index annular portion of the multimode optical fiber are used in conjunction with the numerical aperture coefficients to determine a numerical aperture of a multimode optical fiber before the multimode optical fiber is drawn from the preform. In this embodiment, the numerical aperture of the multimode optical fiber is determined with the equation:
NA=NA
C
−A
NA
V−B
NA (5),
where NAC is the numerical aperture of the core, V is the volume profile of the depressed-index annular portion of the optical fiber as defined in Equation (2) above, and ANA and BNA are the numerical aperture coefficients.
The numerical aperture of the core may be expressed mathematically as:
where nREF is the index of refraction of pure silica glass. In the embodiments described herein, nREF is the index of refraction of pure silica glass at 850 nm which is 1.4524982. ΔC% is the relative refractive index of the core portion of the preform as defined in Equation (4) and described above.
Referring to
After the measured numerical aperture values are plotted as a function of the volume profile, a line may be fit through the resulting curve yielding a relationship between volume profile and numerical aperture. At a wavelength of 850 nm the numerical aperture coefficient ANA may be greater than 0 and less than 1 while the numerical aperture coefficient BNA may be greater than 0 and less than 1. For example, in one embodiment, the numerical aperture coefficients ANA and BNA were determined through this curve fitting process to be 0.000296474 and 0.022110436, respectively.
Equation (5) may be utilized to predict the numerical aperture of a multimode optical fiber of specified dimensions before the multimode optical fiber is drawn from a preform having a specific core refractive index profile and a specific moat refractive index profile. In this manner, the suitability of the preform for forming multimode optical fiber with the desired numerical aperture may be evaluated prior to drawing the multimode fiber from the preform.
For example, the multimode optical fibers described herein may have a target numerical aperture range from 0.17 to about 0.23, more preferably greater than 0.18 and, most preferably from about 0.185 to about 0.215. Equation (5) may be used in conjunction with the desired dimensions of the multimode optical fiber, the core refractive index profile of the preform and the moat refractive index profile of the preform to determine if the preform is suitable for drawing multimode optical fiber having a numerical aperture within the target range. If the numerical aperture NA for the desired optical fiber, as determined with Equation (5), is within the target range, the preform is suitable for forming multimode optical fiber with the desired numerical aperture and, thereafter, multimode fiber may be drawn from the optical fiber preform. However, if the numerical aperture NA, as determined from Equation (5), is not within the target range, the preform is not suitable for forming multimode optical fiber with the desired numerical aperture and the multimode optical fiber is not drawn from the preform.
In the embodiments described herein either the bend-loss or the numerical aperture of multimode optical fiber are determined from the desired dimensions of the optical fiber, the core refractive index profile of the preform and the moat refractive index profile of the preform prior to the fiber being drawn from the preform. However, it should be understood that, in other embodiments, both the bend-loss and the numerical aperture may be determined from the desired dimensions of the optical fiber, the core refractive index profile of the preform and the moat refractive index profile of the preform and the multimode optical fiber is drawn from the optical fiber preform when the determined values for the bend-loss and numerical aperture are within the respective target ranges.
It should also be understood that the methods described herein may be used to characterize the properties of a multimode optical fiber before the fiber is drawn from a preform. This methodology may be used to verify that the preform is suitable for producing an optical fiber having the desired properties before the optical fiber is actually drawn from the preform. The methodologies described herein may be used to improve fiber quality and reduce manufacturing costs and waste. For example, suitable preforms may be selected based on their physical characteristics without having to draw and measure fiber to determine the suitability of the preform for forming optical fiber with the desired characteristics thereby avoiding product waste and lost manufacturing time. Further, the methods described herein may be used to improve the quality of the optical fiber produced as non-conforming preforms can be identified early in the manufacturing process and discarded or otherwise disposed of thereby insuring that only optical fiber with the desired characteristics are produced.
It will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments described herein without departing from the spirit and scope of the claimed subject matter. Thus it is intended that the specification cover the modifications and variations of the various embodiments described herein provided such modification and variations come within the scope of the appended claims and their equivalents.
