Patent Application
20130039626
The present invention generally relates to fiber optic communication, and more particularly relates to small diameter multimode optical fiber that may be particularly useful for use in an optical backplane.
High performance computing and server installations typically require a large number of processor-to-processor interconnections and utilize optical backplanes. Optical interconnects advantageously require much less electrical power than conventional wired systems and offer high speed data communication at long distances. Conventional fiber optic communication systems employ single mode or multimode optical fibers to transfer the data between remote locations. Multimode fiber optic cable offers a plurality of modes, but conventional multimode fibers generally require a relatively larger diameter core and thus generally result in larger overall diameter fibers.
According to one embodiment, a multimode optical fiber is provided. The fiber includes a graded index glass core having a diameter in the range of 24 microns to 40 microns, a graded index having an alpha profile less than 2.12 and a maximum relative refractive index in the range between 0.6 percent and 1.9 percent. The fiber also includes a cladding surrounding and in contact with the core. The cladding includes a depressed-index annular portion. The fiber further has an overfilled bandwidth greater than 2.0 GHz-km at 1310 nm.
According to another embodiment, an optical backplane system is provided. The optical backplane system includes at least one transceiver, at least one optical connector, and a plurality of multimode optical fibers coupled to the at least one optical connector. Each multimode optical fiber includes a graded index glass core having a diameter in the range of 24 microns to 40 microns, a graded index having an alpha parameter less than 2.12 and a maximum relative refractive index in the range between 0.6 percent and 1.9 percent. The optical backplane further includes a cladding surrounding and in contact with the core. The cladding includes a depressed-index annular portion. Each multimode optical fiber also has an overfilled bandwidth greater than 2.0 GHz-km at 1310 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 that description or recognized by practicing the embodiments as 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 are merely exemplary, and are intended to provide an overview or framework to understanding 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 embodiments, and together with the description serve to explain principles and operation of the various embodiments.
Reference will now be made in detail to the present preferred embodiments, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.
The “refractive index profile” is the relationship between refractive index or relative refractive index and waveguide fiber radius.
The “relative refractive index” is defined as Δ=100×[n(r)2−ncl2)/2n(r)2, where n(r) is the refractive index at the radial distance r from the fiber's centerline, unless otherwise specified, and ncl is the refractive index of the cladding at a wavelength of 850 nm. In one aspect, the cladding comprises essentially pure silica. In other aspects, the cladding may comprise silica with one or more dopants (e.g., GeO2, Al2O3, P2O5, TiO2, ZrO2, Nb2O5 and/or Ta2O5) which increase the index of refraction, in which case the cladding is “up-doped” with respect to pure silica. The cladding may also comprise silica with one or more dopants (e.g., F and/or B) which decrease the index of refraction, in which case the cladding “down-doped” with respect to pure silica. As used herein, the relative refractive index is represented by delta or Δ and its values are typically given in units of “%,” unless otherwise specified. In cases where the refractive index of a region is less than that of the cladding, the relative index percent is negative and is referred to as having a depressed index, and 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 refractive index of silica, the relative index percent is positive and the region can be said to be raised or to have a positive index, and is calculated at the point at which the relative index is most positive, unless otherwise specified.
An “up-dopant” is herein considered to be a dopant which has a propensity to raise the refractive index relative to pure undoped SiO2. A “down-dopant” is herein considered to be a dopant which has a propensity to lower the refractive index relative to pure undoped SiO2. An up-dopant may be present in a region of an optical fiber having a negative relative refractive index when accompanied by one or more other dopants which are not up-dopants. Likewise, one or more other dopants which are not up-dopants may be present in a region of an optical fiber having a positive relative refractive index. A down-dopant may be present in a region of an optical fiber having a positive relative refractive index when accompanied by one or more other dopants which are not down-dopants. Likewise, one or more other dopants which are not down-dopants may be present in a region of an optical fiber having a negative relative refractive index.
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 Text Procedures-Numerical Aperture.”
The term graded index, “α-profile” or “alpha profile,” as used herein, refers to a relative refractive index profile, expressed in terms of Δ which is in units of “%”, where r is the radius and which follows the equation,
where Δ0 is the relative refractive index extrapolated to r=0, R1 is the radius of the core (i.e. the radius at which Δ(r) is zero), and α is an exponent which is a real number. For a step index profile, the alpha value is greater than or equal to 10. For a graded index profile, the alpha value is less than 10. The term “parabolic,” as used herein, includes substantially parabolically shaped refractive index profiles which may vary slightly from an α value of 2.0 at one or more points in the core, as well as profiles with minor variations and/or a centerline dip. The modeled refractive index profiles that exemplify the invention have graded index cores which are perfect alpha profiles. An actual fiber will typically have minor deviations from a perfect alpha profile, including features such as dips or spikes at the centerline and/or a diffusion tail at the outer interface of the core. However accurate values of alpha and Δ0 may still be obtained by numerically fitting the measured relative refractive index profile to an alpha profile over the radius range from 0.05 R1≦r≦0.95 R1. In ideal graded index fibers with no imperfections such as dips or spikes at the centerline, ΔO=Δ1MAX, where Δ1MAX is the maximum refractive index of the core. In other cases, the value from Δ0 obtained from the numerical fit from 0.05 R1≦r≦0.95 R1 may be greater or less than Δ1MAX.
Various embodiments of a multimode optical fiber exhibiting a small diameter with enhanced performance characteristics are provided. Multimode optical fiber is disclosed having a graded index glass core and a cladding surrounding and in contact with the core. The core has a small diameter in the range of 24 microns to 40 microns or a radius in the range of 12 microns to 20 microns. The core also includes a graded index having an alpha (α) value less than 2.12, preferably less than 2.04 and more preferably between 1.95 and 2.04. The core further has a maximum refractive index in the range between 0.6 percent and 1.9 percent. The cladding includes a depressed-index annular portion. The fiber further has an overfilled bandwidth greater than 2.0 GHz-km at 1310 nm.
Referring to
Referring to both
The inner cladding portion 30 of cladding 60 has an outer radius R2, a width W2, relative refractive index Δ2, and a maximum relative refractive index Δ2MAX. R2 is defined as the radius at which the derivative of the normalized refractive index profile with respect to the normalized radius, d(Δ/Δ1MAX)/d(r/R1), has a local minimum, as shown in
The depressed-index annular portion 40 of cladding 60 has a minimum relative refractive index Δ3MIN and extends from R2 to R3, wherein R3 is the radius at which Δ3(r) first reaches a value of greater than −0.05%, going radially outwardly from the radius at which Δ3(r)=Δ3MIN. The depressed-index annular portion 40 has a radial width W3=R3−R2. In one embodiment, the depressed-index annular portion 40 has a width W3 of at least 1 micron. W3 is preferably between 2 μm and 10 μm, more preferably between 2 μm and 8 μm and even more preferably between 2 μm and 6 μm. The depressed-index annular portion 40 may have an outer radius R3 in the range of 13 to 23 microns. The depressed-index annular portion 40 has a minimum relative refractive index Δ3MIN of less than about −0.2 percent, and more preferably refractive index Δ3MIN may be in the range of −0.2 to −0.54. The low index ring has a minimum relative refractive Δ3MIN which is less than or equal to Δ2MAX and less than Δ1MAX.
The outer annular portion 50 of cladding 60 has an outer radius R4 and has relative refractive index Δ4 which is greater than Δ2MAX and greater than Δ3MIN and less than ΔMAX. Accordingly, Δ1MAX>Δ4>Δ2MAX>Δ3MIN in this embodiment. However, it should be understood that other embodiments are possible. For example, Δ4 may be equal to Δ2MAX. Alternatively, Δ2MAX may be greater than Δ4. According to one embodiment, the outer radius R4 is less than 60 microns, thereby resulting in a diameter of less than 120 microns. According to another embodiment, the outer radius R4 of outer annular portion 50 has an outer radius of less than 50 microns, or a diameter of less than 100 microns. It should be appreciated that by using a small diameter core 20 in the range of 24-40 microns, an overall reduced diameter of the fiber 10 as indicated by the outer radius R4 is likewise reduced thereby providing a smaller cross section which allows for efficient coupling to devices, such as an array planar waveguide.
Fiber 10 preferably has an overfilled bandwidth greater than 2.0 GHz-km at 1310 nm, and a numerical aperture greater than about 0.185. The cutoff wavelengths of modes in the eleventh (11th) mode group are less than 1310 nm, and more preferably the fiber guides fewer than ten mode groups at a wavelength of 1310 nm or greater.
The refractive index profile of a radially symmetric optical fiber depends on the radial coordinate r and is independent of the azimuthal coordinate φ. In most optical fibers, including the examples disclosed below, the refractive index profile exhibits only a small index contrast, and the fiber can be assumed to be only weakly guiding. If both of these conditions are satisfied, Maxwell's equations can be reduced to the scalar wave equation, the solutions of which are linearly polarized (LP) modes.
For a given wavelength, the radial equation of the scalar wave equation for a given refractive index profile has solutions which tend to zero for r going to infinity only for certain discrete values of the propagation constant β. These Eigen vectors (transverse electric field) of the scalar wave equation are guided modes of the fiber, and the Eigen values are the propagation constants βlm, where l is the azimuthal index and m is the radial index. In a graded index fiber, the LP modes can be divided into groups, designated by common values of the principle mode number, p=1+2m−1. The modes in these groups have nearly degenerate propagation constants and cutoff wavelengths and tend to propagate through the fiber with the same group velocity.
The cutoff wavelength of a particular mode group is the minimum wavelength beyond which all modes from that mode group cease to propagate in the optical fiber. The cutoff wavelength of a single mode fiber is the minimum wavelength at which an optical fiber will support only one propagating mode. The cutoff wavelength of a single mode fiber corresponds to the highest cutoff wavelength among the higher order modes. Typically the highest cutoff wavelength corresponds to the cutoff wavelength of the LP11 mode. If the operative wavelength is below the cutoff wavelength, multimode operation may take place and the introduction of additional sources of dispersion may limit a fiber's information carrying capacity. A mathematical definition can be found in Single Mode Fiber Optics, Jeunhomme, pp. 39 44, Marcel Dekker, New York, 1990 wherein the theoretical fiber cutoff is described as the wavelength at which the mode propagation constant becomes equal to the plane wave propagation constant in the outer cladding. This theoretical wavelength is appropriate for an infinitely long, perfectly straight fiber that has no diameter variations.
For example, the LP modes in a fiber that propagates 7 mode groups are: LP01, LP11, (LP02 and LP21), (LP12 and LP31), (LP03, LP22 and LP41), (LP13, LP32 and LP51) and (LP04, LP23, LP42, LP61). The requirements for a fiber propagating fewer than 7 mode groups is that the cutoff wavelengths of all of the modes in the 7th mode group are less than the operating wavelength λ. The mode with the lowest/value generally has the highest cutoff wavelength in a graded index multimode fiber. If the operating wavelength is 1310 nm, it is sufficient to require that the LP04 cutoff wavelength is less than 1310 nm. Similarly, the requirements for a fiber propagating fewer than 8, 9, 10 or 11 mode groups at 1310 nm are that the LP14, LP05, LP15 and LP06 cutoff wavelengths, respectively, are less than 1310 nm.
The numerical aperture (NA) is defined as the sine of the maximum angle (relative to the axis of the fiber) of the incident light that becomes completely confined in the fiber by total internal reflection. It can be shown that this condition yields the relationship NA=√{square root over (n12−n22)}. Using the definition of delta (A), this expression can be transformed into the following equation:
The overfilled bandwidth at a given wavelength is measured according to measurement standard FOTP-204 using an overfilled launch. The modeled bandwidth may be calculated according to the procedure outlined in T. A. Lenahan, “Calculation of Modes in an Optical Fiber Using the Finite Element Method and EISPACK,” Bell Sys. Tech. J., vol. 62, pp. 2663-2695 (1983), the entire disclosure of which is hereby incorporated herein by reference. Equation 47 of this reference is used to calculate the modal delays; however note that the term dkclad/dω2 must be replaced with dkclad2/dω2, where kclad=2π*nclad/λ and ω=2π/λ. The modal delays are typically normalized per unit length and given in units of ns/km. The calculated bandwidths also assume that the refractive index profile is ideal, with no perturbations such as a centerline dip, and as a result, represent the maximum bandwidth for a given design.
The tables 1-5 presented below summarize various examples generally arranged in five sets of embodiments of multimode fibers that were modeled having various characteristics in accordance with the embodiments disclosed herein and compared to a comparative fiber shown in
In the examples presented below, the multimode fiber generally exhibit a core diameter between 24 and 40 microns and a core maximum relative refractive index Δ1MAX between 0.6 and 1.9%, wherein the core diameter and core delta provide for (in terms of narrowness): LP06 cutoff wavelength less than 1310 nm (fewer than 11 mode groups); LP15 cutoff wavelength less than 1310 nm (fewer than 10 mode groups—preferred); LP05 cutoff wavelength less than 1310 nm (fewer than 9 mode groups—more preferred); and LP14 cutoff wavelength less than 1310 nm (fewer than 8 mode groups—most preferred).
The first set of embodiments identified as Fibers 1-3 in Table 1 below has approximately the same core maximum relative refractive index Δ1MAX as the comparative fiber example, which is a commercially available multimode fiber sold by Corning Inc. under the name ClearCurve® MMF. The comparative fiber has a large core radius of about 23.7 microns and generally exhibits a large number of mode groups, shown as 11 mode groups at 1310 nm. In example fibers 1-3, only 7 mode groups propagate at 1310 nm due to the smaller core diameter, and this enables bandwidths as large as 14 GHz-km at either 1310 or 1550 nm. The numerical apertures of these first three embodiments are between 0.185 and 0.215.
The second set of embodiments identified as example fibers 4-8 in Table 2 below support the propagation of 10 mode groups at 1310 nm. The maximum achievable bandwidth is significantly lower than the value that is possible with fewer than 10 mode groups. The numerical apertures of embodiments 4-8 are greater than 0.22 and the overfilled bandwidths at 1310 nm are greater than 3700 MHz-km. Embodiments 6-8 illustrate than an overfilled bandwidth greater than 5500 MHz-km at 1310 nm is possible when the core maximum relative refractive index Δ1MAX), is less than 1.7% and the numerical aperture is less than 0.27.
The third set of embodiments identified as example fibers 11-15 in Table 3 below supports 7 mode groups at 1310 nm, as do the examples in the first set above, but have different combinations of the core relative refractive index Δ1 and radius R1. Embodiments 11-15 have numerical apertures greater than 0.185 and overfilled bandwidths at 1310 nm greater than 5000 MHz-km. Embodiments 14 and 15 illustrate that bandwidths as large as 14 GHz-km are possible when the core maximum relative refractive index Δ1MAX is less than 1.0% and the numerical aperture is between 0.185 and 0.215.
The fourth set of embodiments identified as example fibers 16-18 in Table 4 below has fiber designs which support 9 mode groups at 1310 nm while example fibers 19-21 support 6 mode groups at 1310 nm. In comparison with the second set of embodiments, which support 10 mode groups at 1310 nm, the designs with 9 mode groups offer a slight improvement, but still do not enable 10 GHz-km. Examples 16-18 have numerical apertures greater than 0.22 and overfilled bandwidths at 1310 nm greater than 5000 MHz-km. The designs designated as examples 19-21 propagate 6 mode groups and enable higher bandwidth than embodiments that support 7 mode groups, but the smaller core radius may result in higher sensitivity to misalignments. Examples 19-21 have numerical apertures between 0.185 and 0.22 and overfilled bandwidths at 1310 nm greater than 15000 MHz-km.
The fifth set of embodiments identified as example fibers 22-26 in table 5 below has fiber designs in which the inner cladding region is an extension of the graded index core, as shown in
Table 6 presented below provides a further example, labeled example 27, of parameters measured for a multimode fiber made having characteristics similar to those shown in example 1 of the multimode fiber. Further parameters of the multimode fiber 27 are expected to have similar properties to those in the modeled version of example 1.
The fiber examples in Tables 1-6 illustrate that a reduced diameter graded index core in the range of 24 microns to 40 microns employed in a multimode optical fiber with a core maximum relative refractive index Δ1MAX in the range of 0.6 to 1.9 percent with a cladding surrounding the core and comprising a depressed-index annular portion, and the fiber having an overfilled bandwidth greater than 2.0 GHz-km at 1310 nm.
Referring to
The electrical/mechanical backplane 120 includes electrical circuitry 122 which engages electrical connectors 132 on each of the system cards 130. Additionally, the electrical/mechanical backplane 120 includes a plurality of optical connectors 124 which matingly engage optical connectors 134 on each of the system cards 130. The optical connectors 134 and 124 may perform an optical connection at an angle, such as 90°. This may be achieved by using a mirror to direct the optical signals at 90° or by bending a fiber optic cable at an angle of 90°. It should be appreciated that each of the optical connectors 124 may engage an optical connector 134 from a corresponding system card 130.
The system card 130 is shown further employing a plurality of transceivers 136 for receiving and sending optical signals. Transceiver 136 include both a transmit fiber and a receive fiber, according to one embodiment. Transceiver 136 may further be associated with electrical circuitry that converts the optical signal to electrical signals. Transceivers 136 are coupled to optical cross connect devices 138 which direct the optical signals from the optical connector 134 to the appropriate receiver 136. It should be appreciated that the optical backplane system 100 may be configured in any of a number of arrangements to provide optical signals between optical circuits, each employing a multimode fiber as described herein.
Accordingly, the multimode optical fiber and optical backplane system 100 utilizing the multimode optical fiber advantageously provides for a reduced footprint and good performance optical communication. The multimode optical fiber employs a reduced diameter core resulting in overall reduced diameter fiber thereby reducing the footprint of the fiber. The multimode fiber advantageously provides optimum core maximum relative refractive index values and alpha profiles. The cladding provides a depressed-index angular portion with an optimal overfill bandwidth value to achieve enhanced performance in a small diameter multimode fiber.
It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the claims.
This application claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 61/522,278, filed on Aug. 11, 2011, the content of which is relied upon and incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61522278 | Aug 2011 | US |
