Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/02—Optical fibres with cladding with or without a coating
  • G02B6/028—Optical fibres with cladding with or without a coating with core or cladding having graded refractive index
  • G02B6/0288—Multimode fibre, e.g. graded index core for compensating modal dispersion
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/02—Optical fibres with cladding with or without a coating
  • G02B6/02004—Optical fibres with cladding with or without a coating characterised by the core effective area or mode field radius
  • G02B6/02009—Large effective area or mode field radius, e.g. to reduce nonlinear effects in single mode fibres
  • G02B6/02014—Effective area greater than 60 square microns in the C band, i.e. 1530-1565 nm
  • G02B6/02019—Effective area greater than 90 square microns in the C band, i.e. 1530-1565 nm
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/02—Optical fibres with cladding with or without a coating
  • G02B6/02047—Dual mode fibre
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/02—Optical fibres with cladding with or without a coating
  • G02B6/02214—Optical fibres with cladding with or without a coating tailored to obtain the desired dispersion, e.g. dispersion shifted, dispersion flattened
  • G02B6/02219—Characterised by the wavelength dispersion properties in the silica low loss window around 1550 nm, i.e. S, C, L and U bands from 1460-1675 nm
  • G02B6/02266—Positive dispersion fibres at 1550 nm

Definitions

• the present invention is directed generally to optical fiber for telecommunications and more specifically to an optical fiber capable of multimode operation at wavelengths below about 1 300 nm and single mode operation at wavelengths above about 1 300 nm, the optical fiber having reduced intermodal noise.
• the optical fiber typically used to wire homes and small businesses has undesirably low bandwidth.
• 850 nm multimode fiber is used for wiring homes and small businesses because the various system components (e.g. lasers, receivers) used in conjunction with this fiber are inexpensive.
• conventional 850 nm multimode fiber can support a relatively low bit rate.
• Intermodal noise is related to a variation of the optical intensity at a given optical fiber output location due to optical interference between modes of different phase. Many factors may act singly or in combination to produce phase changes that can cause intermodal noise. Example factors include, changes in temperature, mechanical distortions (including movement or vibration), as well as changes in optical source wavelength.
• Intermodal noise is a common problem in multimode fibers used with highly coherent light sources, e.g., lasers. This is because the relative coherence of the modes allows the modes to effect the intensity of the light by interfering with each other. Less coherent sources, such as LED's, have a short coherence length and therefore are only subject to intermodal noise in very short lengths of fiber. However, LED sources are polychromatic which causes significant pulse broadening in the fiber. This is a problem because pulse broadening reduces bandwidth. Therefore, it would be advantageous to have a fiber designed for operation with coherent light sources which does not suffer from intermodal noise.
• an optical fiber comprising a core having a refractive index profile that comprises an alpha profile with an alpha parameter in a range from approximately 2 to approximately 8, a maximum refractive index percent difference between the core and a cladding in a range from approximately 0.26% to approximately 0.5% and a core diameter in a range from approximately 6.0 to approximately 1 6.0 ⁇ m and a cladding, wherein the optical fiber has a bandwidth of at least approximately 0.6 GHz. km at 850 nm, and is configured for multimode operation at a wavelength less than 1 300 nm. Cut off wavelength of the optical fiber is in the range from about 1 050 nm to 1 300 nm so that single mode operation is exhibited at a wavelength of at least approximately 1 300 nm.
• the core diameter has a range of approximately 6.0 to 1 4.0 ⁇ m or a maximum refractive index percent difference in the range of approximately 0.3% to 0.4%.
• a preferred range of alpha parameter is from approximately 2 to approximately 4.
• the effective area of the optical fiber disclosed herein is greater than 70 ⁇ m 2 at 1 550 nm, and more preferably greater than 90 ⁇ m 2 .
• the pin array bend loss is preferably less than 4 dB at 1 550 nm and more preferably less than 2 dB.
• the mode field diameter is greater than or equal to 1 0 ⁇ m at 1 550 nm.
• the optical fiber comprises a core and a cladding, wherein the optical fiber is a multimode fiber at an operating wavelength and is configured in accord with a given operating wavelength, the desired bandwidth, and the length of the fiber.
• the peak modal bandwidth wavelength of the fiber is offset from the operating wavelength by an amount sufficient to reduce modal noise. The preferred amount of the offset depends upon fiber length, desired bandwidth, and operating wavelength.
• the alpha parameter is approximately 2
• the maximum refractive index percent difference between core and clad has a range from approximately 0.35% to 0.40%
• the core diameter is in the range from approximately 1 4.0 to 1 6.0 ⁇ m.
• the embodiment provides an optical fiber having, at 1 550 nm, an effective area greater than 90 ⁇ m 2 and a mode field diameter greater than 1 1 ⁇ m at 1 550 nm. Pin array bend loss is less than 2 dB at 1 550 nm.
• the alpha parameter is approximately 3
• the maximum refractive index percent difference between the core and the cladding is in the range of approximately 0.35% to approximately 0.4% and the core diameter is in the range of approximately 1 2.0 to approximately 1 5.0 ⁇ m, to provide a waveguide fiber having effective area greater than 85 ⁇ m 2 at 1 550 nm, and mode field diameter greater than 1 0.5 ⁇ m at 1 550 nm.
• the pin array bend loss is less than 4 dB at 1 550 nm.
• the alpha parameter is approximately 4, the maximum refractive index percent difference between the core and the cladding is in the range from approximately 0.3% to approximately 0.4% and the core diameter is in the range from approximately 1 2.0 to approximately 1 6.0 /ym, to provide a waveguide fiber having, at 1 550 nm, an effective area greater than 85 ⁇ m 2 , and mode field diameter greater than 1 0.5 ⁇ m.
• the pin array bend loss at 1 550 nm is less than 3.5 dB.
• the offset between peak bandwidth wavelength and operating wavelength is selected to provide respective group time delays for modes which are either all positive or all negative. The sign of the delay is determined with reference to the arrival time of the lowest order mode (LP 01 mode). A positive group time delay pertains to a mode which arrives before the LP 01 mode and a negative arrival time is the converse.
• the absolute value of the sum of the respective group time delays is greater than 0.
• a method of designing an optical fiber having a bandwidth of at least 0.6 GHz. km at 850 nm in multimode operation and being in single mode operation at a wavelength of at least approximately 1 300 nm comprising, determining for a given length of optical fiber a minimum difference between the operating wavelength and a peak modal bandwidth wavelength such that the difference in the optical path lengths of the modes in multimode operation is greater than at least one coherence length of the fiber, the coherence length being associated with a source utilized to launch light within the optical fiber at the operating wavelength, and determining a refractive index profile associated with the optical fiber in accordance with the minimum difference.
• determining the minimum difference in offset between peak bandwidth wavelength and operating wavelength includes the step of calculating a speckle constant gamma, ⁇ .
• the speckle constant is calculated as a function of bandwidth, line width of the light source, intensity of the light source, and length of the optical fiber.
• the step of determining a minimum difference includes having the respective group delay times of the modes be all negative or all positive.
• the step of determining an index profile includes at least one of determining the operating wavelength, the desired bandwidth, or the length of the optical fiber during operation.
• an optical fiber system comprising an optical fiber w ith a core having a refractive index profile having an alpha profile with an alpha parameter from approximately 2 to approximately 8, a maximum refractive index percent difference between the core and a cladding from approximately 0.3% to approximately 0.5% and a core diameter of approximately 6.0 to approximately 16.0 ⁇ m and a light source optically coupled to the optical fiber.
• the alpha parameter, the maximum percent refractive index difference, and the core diameter are chosen to provide an offset between peak bandwidth wavelength and operating wavelength sufficient to reduce intermodal noise at the operating wavelength. >
• Also disclosed herein is a method of operating an optical fiber system comprising providing an optical fiber w herein the optical fiber is a multimode fiber at an operating wavelength and has a peak modal bandw idth wavelength offset from the operating wavelength and operating a light source at the operating wavelength.
• Figure 1 is a schematic diagram of an experimental apparatus.
• Figure 2A is a detail view of the fiber joint in Figure 1 illustrating aligned fibers.
• Figure 2B is a detail view of the fiber joint in Figure 1 illustrating offset fibers.
• Figure 3 is a plot comparing simulation data with actual data.
• Figure 4 is a plot illustrating the coherence damping for various source linewidths.
• Figure 5 is a simulated spectra for a fiber length of 1 0 meters.
• Figure 6 is a simulated spectra for a fiber length of 20 meters.
• Figure 7 is a simulated spectra for a fiber length of 50 meters.
• Figure 8 is a simulated spectra for a fiber length of 1 00 meters.
• Figure 9 is a simulated spectra for a fiber length of 500 meters.
• Figure 10 is a simulated spectra for a fiber length of 1 000 meters.
• Figure 1 1 is a simulated spectra for a fiber length of 2000 meters.
• Figure 1 2 is a simulated spectra for a fiber length of 5000 meters.
• Figure 1 3 is a chart of optimum values of the profile parameter versus wavelength.
• Figure 1 4 is a refractive index profile of an optical fiber, as disclosed herein.
• the inventors have discovered that it is possible to design a fiber capable of multimode operation at 850 nm having high modal bandwidth and single mode operation at wavelengths of at least approximately 1 300 nm. This can be accomplished by producing a fiber with an alpha profile having an alpha parameter of approximately 2-8, a maximum index percent difference between the core and a cladding of approximately 0.26%-0.5% and a core diameter of approximately 6.0- 1 6.0 ⁇ m. In addition to being capable of both multimode and single mode operation, these fibers have higher effective areas and lower pin array bend loss than conventional single mode optical waveguide fiber such as SMF-28TM, available from Corning, Inc.
• refractive index difference or, simply, index difference
• relative refractive index difference or, simply, relative index difference
• ⁇ is also referenced in the art as relative refractive index.
• the maximum index difference ⁇ may be approximated
• the fiber preferably has an alpha parameter in the range of approximately
• the alpha parameter is in the range of approximately
• the alpha parameter is in the range of approximately 2.5 to approximately 3.3.
• An alpha parameter that is too large or small can result in an unacceptable decrease in modal equalization (modal equalization occurs when all of the modes travel with the same velocity) . Because bandwidth is a maximum at modal equalization, an increase in the difference in the modal velocities results in a decrease in bandwidth.
• the maximum index difference ⁇ between the core and the cladding is preferably in the range of approximately 0.26% to approximately 0.5%. More preferably, the maximum index difference ⁇ is in the range of 0.3%-0.5%, and more preferably still in the range of 0.3% to 0.4%. A maximum index difference ⁇ that is too low results in an unacceptably high bend loss. A maximum index difference ⁇ that is too high can require an unacceptably small core diameter.
• the diameter of the fiber core is in the range of approximately 6.0 to approximately 1 6.0 ⁇ m. More preferably, the core diameter is in the range of approximately 8.0 to approximately 1 4.0 ⁇ m. Even more preferably, the core diameter is approximately 1 2.0 ⁇ m.
• the diameter of the core is too small, then it may become difficult to connect fiber segments because even small absolute misalignments of the fiber segments at the joint results in a large percentage misalignment of the fiber cores. If the core diameter is too large, then too many modes may propagate through the fiber, which may result in an increase in intermodal noise.
• properties of exemplary fibers designed in accordance with the present invention are listed in Table I along with properties of a standard SMF-28TM optical fiber, manufactured by Corning Incorporated. In this table, pin array bend loss and effective area (A eff ) were determined at a wavelength of 1 550 nm.
• the dispersion was measured at 1 525 nm and 1 575 nm and the dispersion slope calculated from these points. Using the dispersion slope, the dispersion curve was extrapolated to determine the zero dispersion wavelength ⁇ 0 (the wavelength at which the dispersion goes to zero) . Cut off wavelength of the fiber was measured. As can be seen from Table I, fibers made according to the invention have cut off wavelengths ⁇ c between 1 220 nm and 1 250 nm. At the fiber cutoff wavelength ⁇ c , the fiber transitions from multimode to single mode. In actual applications, fibers generally are packaged in fiber optic cables. Cabling changes the cutoff wavelength.
• cabling lowers the cutoff wavelength between 50 nm to 400 nm, depending on the type of fiber. In the type of fibers disclosed in Table I, cabling lowers the cutoff wavelength between 50 nm and 1 00 nm. As shown in Table I, the cabled cutoff wavelengths of the fibers produced in accordance with the present invention are between 1 1 25 nm and 1 1 50 nm. Thus, all of those fibers are capable of multimode operation at 850 nm and single mode operation at 1 300 nm in either uncabled or cabled configuration.
• Attenuation induced in a fiber by bending can be measured as pin array bend loss.
• pin array bend loss Attenuation is measured for a fiber configured to have essentially no induced bending loss. The fiber is then woven in a serpentine path through a pin array.
• the pin array is a set of ten cylindrical pins arranged in a single row and held in a fixed vertical position on a flat surface.
• the pin spacing is 5 mm, center to center, and the pin diameter is 0.67 mm.
• sufficient tension is applied to make the serpentine woven fiber conform to the portions of the pin surface at which there is contact between fiber and pin. Attenuation of the fiber in the pin array is then measured.
• the pin array bend loss is the difference between the two measured attenuation values expressed in dB. All of the fibers shown in
• Table I that were designed in accordance with the present invention have lower pin array bend loss than the standard SMF-28TM optical fiber manufactured by Corning Incorporated.
• the pin array bend losses are as low as 1 /20 the pin array bend loss of the standard SMF-28TM optical fiber.
• the effective area (A eff ) is generally defined as, where ⁇ is the electric field associated with the propagated light and r is the radial distance from the center axis of the fiber.
• a larger effective area is beneficial because it results in a lower power density across the fiber, improving fiber performance.
• the improved performance includes reduced nonlinear effects and lower splicing and connecting loss.
• All of the fibers shown in Table I that were designed as disclosed herein have larger effective areas than the standard commercially available single mode fiber designed for use at a wavelength near 1 300 nm, for example Coming's SMF-28TM fiber.
• the first listed fiber has an effective area (Aeff) that is approximately 33% higher than the standard SMF-28TM fiber ( 1 08.5 ⁇ m 2 versus 81 .5 ⁇ m 2 ).
• the bandwidths of these fibers were calculated via computer simulation. All of the fibers were found to have bandwidths of at least approximately 0.6 GHz. km at 850 nm in multimode operation, some having 850 nm bandwidth greater than 1 .0 GHz. km.
• the operating wavelength ⁇ op is the wavelength at which the fiber is intended to be used.
• the peak bandwidth wavelength ⁇ p is the wavelength at which equalization of the group time delays occur, i.e., the sum of the group time delay differences of all the modes propagating in a multimode fiber are nearly equal to zero.
• the group time delay difference of the modes is zero at ⁇ p which corresponds to a bandwidth which is large without bound.
• the bandwidth versus wavelength curve exhibits a large but finite value of bandwidth at wavelength ⁇ p .
• the fiber has its highest bandwidth.
• there is a significant amount of intermodal noise when the operating wavelength ⁇ op is at the peak bandwidth wavelength ⁇ p .
• operating at the peak wavelength results in a noisy signal while operating at any other wavelength results in a decrease in bandwidth.
• the average difference between group time delays of the modes in multimode operation is sufficiently greater than zero, to provide an optimum configuration in which bandwidth is as high as possible because intermodal noise has been limited.
• the bandwidth is effectively optimized by selecting refractive index profile parameters that move ⁇ p away from ⁇ op .
• the reduction in bandwidth due to operating away from ⁇ p is more than made up by the increase in bandwidth due to the reduction in intermodal noise.
• Design parameters to overcome intermodal noise that is the parameters to offset ⁇ p from ⁇ op by a desired amount, can be determined by calculation and then implemented in a fiber. The case for a 2-mode fiber is discussed below. Similar derivations have been developed for the general N-mode fiber as is discussed below.
• the optical phase difference between the lowest order modes, LP and LP 01 can result in optical interference.
• external perturbations such as changes in temperature and mechanical distortions, can produce such phase differences which can result in optical interference and thus changes in the output optical intensity at a given transverse position on the output end of the fiber as a function of time.
• This modal interference condition which varies in time, creates an intensity variation in the modal pattern, i.e., modal noise.
• the time it takes for both modes to travel the length of the fiber is the same.
• a preferred embodiment of an optical fiber has an alpha parameter selected to nearly equalize mode group delay time. Sufficient difference in mode group time delay is maintained to limit intermodal noise.
• a number of factors, including index perturbations in the fiber can modify the optimum alpha parameter for equalization.
• optimum alpha parameter is defined by a range.
• Fibers designed in accordance with these embodiments have an alpha parameter in the range of approximately 2 to approximately 8. More
• optimum parameter is meant the value of of core refractive index profile that provides maximum bandwidth.
• an examination of curve 1 02 in Figure 1 3 shows that for the values of ⁇ % and diameter of the core selected (see below), an of 2.5 provides a ⁇ p of 0.75 ⁇ m, a wavelength that can be desirable in a system having an operating wavelength ⁇ op of 0.85 ⁇ m, for example.
• curve 1 02 shows corresponding optimum values in the range from about 2.5 to 4.
• Curve 1 02 is representative of the core configuration in which ⁇ % is about 0.3% and core diameter is about 1 2 ⁇ m.
• curve 1 04 is included in Figure 1 3.
• Curve 1 04 is representative of the same core configuration as that of curve 1 02 except that the refractive index profile exhibits a depression on the centerline of the fiber.
• the root mean square width of the depression is about 0.1 ⁇ m and has a minimum relative refractive index percent of about zero. The presence of the centerline depression lowers the optimum alpha parameter, particularly at longer wavelengths.
• the fiber disclosed is designed so that A ⁇ is not 0 at the operating wavelength ⁇ op .
• the peak bandwidth wavelength ⁇ p for time delay equalization is purposely offset in wavelength from the operating wavelength ⁇ op .
• the subsequent reduction in bandwidth is not severe, and allows a significant reduction in the intermodal noise. This reduction is due to coherence damping which depends on the source spectral width and the fiber propagation characteristics.
• the apparatus 2 includes a white light source 1 0, a 20 m long SMF-28TM optical fiber 20 (single mode at wavelength greater than about 1 300 nm) having a joint region 25, a 2-mode fiber 40 fabricated as disclosed herein having a joint region 45 and an optical analyzer 50. Holding the joint regions 25 of the single mode fiber 20 and the joint region 45 of the 2-mode fiber 40 is an XYZ micro-positioner 35.
• Figures 2A and 2B illustrate alternative fiber alignments used to test the 2-mode fiber fabricated as disclosed herein.
• the joint regions 25 of the single mode fiber 20 and the joint region 45 of the 2-mode fiber 40 can be aligned as in Figure 2A or offset as in Figure 2B. If the fibers are aligned as in Figure 2A, only the LP 01 mode is present in the 2-mode fiber 40. However, if the fibers are offset as in Figure 2B, both the LP 01 and LP modes are present in the 2- mode fiber 40.
• Fiber designs and simulations were generated based on the following general equation for the speckle constant, ⁇ (the standard deviation of the optical power fluctuations caused by optical interference among the modes of a multimode fiber),
• C p (v) is the source spectral intensity and h(v) is the impulse response function.
• a satisfactory evaluation of the speckle constant can be made by making a Gaussian approximation for C p (v) an ⁇ expanding h(v) a
• the second term of the two mode equation dominates.
• This term includes an oscillatory (cosine) term and, more importantly, an exponential decay term.
• the inventors have discovered that intermodal noise can be reduced as the magnitude of the argument of the exponential term increases.
• the argument is at least approximately - 1 . More preferably, the argument is between approximately - 1 and approximately - 3.
• the intermodal noise can be significantly reduced. That is, the noise can be reduced to a level satisfactory for commercial use.
• the intermodal noise is made to be less than 0.5 dB.
• the intermodal noise is made to be less than 0.25 dB.
• ⁇ r is summed over all N modes. This calculation has been done to give an expression including the optimum alpha parameter in an equation including the group index N v fiber length L, speed of light c, relative index ⁇ , and the number of modes corresponding to a particular alpha profile N( ).
• the decreasing intensity portion of the curve, 53 is indicative of the cut off wavelength for the third mode, that is, the wavelength above which the fiber supports two modes.
• the 2-mode fiber length was approximately 5 meters and had a measured bandwidth of 1 .3 GHz-km at 850 nm.
• the simulated data has been offset from the actual data for clarity. The simulation was done using a wavelength data point spacing of 0.5 nm, a ⁇ p of 770 nm, and a full width half maximum (FWHM) spectral width of the source of 2 nm. As can be seen by comparing curves 52 and 54, the simulation is a very good match for the actual fiber data.
• the damping curve depends upon the FWHM source spectral width as can be seen by comparing curve 55 illustrative of damping for a 0.02 nm source spectral width, to curves 56 and 57, illustrative of damping for 0.2 nm and 2.0 nm spectral width, respectively.
• curve 55 illustrative of damping for a 0.02 nm source spectral width
• curves 56 and 57 illustrative of damping for 0.2 nm and 2.0 nm spectral width, respectively.
• Figures 5-1 2 illustrate simulated spectra for fibers of lengths varying from 1 0 meters to 5 kilometers.
• Curve 60 in figure 6, calculated for a 20 m length of fiber shows modal noise near zero at operating wavelength 61 .
• Bandwidth is greater than 0.6 GHz. km but less than 1 .3 GHz. km.
• curve 62 in figure 7, calculated for fiber length of 50 m shows modal noise of zero at operating wavelength 61 .
• curve 63 in figure 8 calculated for 1 00 m fiber length
• curve 64 in figure 9 calculated for 500 m fiber length
• curve 66 in figure 1 1 calculated for a 2000 m length
• curve 67 in figure 1 2 calculated for a 5000 m length.
• Curve 67 in figure 1 2 shows essentially no intermodal noise.
• the inventors have determined that longer fibers fabricated as disclosed herein allow the use of operating wavelengths ⁇ op closer to the peak wavelength ⁇ p .
• Combinations of fiber length and peak separation which the inventors have found to be particularly advantageous include, 1 0-20 m with the absolute value of the difference between ⁇ op and ⁇ p of approximately 80 to approximately 1 50 nm (bandwidth 0.6-1 .2 GHz. km); 20-1 00 m with the absolute value of the difference between ⁇ op and ⁇ p of approximately 1 2 to approximately 80 nm (bandwidth 1 .2-7 GHz.

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