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
  • G02B6/02214—Optical fibres with cladding with or without a coating tailored to obtain the desired dispersion, e.g. dispersion shifted, dispersion flattened

Definitions

• Degradation of an optical pulse propagating through an optical fiber is the result of attenuation and dispersion.
• Dispersion is the broadening of discrete data bits as they propagate through the media. Pulse broadening results in an overlap between sequential data bits causing an increase in the uncertainty whether a bit is interpreted as logic 0 or 1. This uncertainty in logic state is quantified in terms of bit error rate (BER), where the BER is defined as the number of error bits divided by the total number of bits transmitted in a given period of time. For high-speed Ethernet, the BER cannot exceed 1 error bit for every 1 trillion bits transmitted (BER ⁇ 10 "12 ).
• chromatic dispersion or material dispersion
• modal dispersion There are two contributions to the total dispersion in multimode fiber: chromatic dispersion, or material dispersion, and modal dispersion.
• Chromatic or material dispersion occurs because the refractive index of a material changes with the wavelength of light. This is due to the characteristic resonance frequencies at which the material responds to light (light is a propagating electromagnetic field). Shorter wavelengths encounter a higher refractive index (i.e., greater optical density) and consequently travel slower than longer wavelengths. Since a pulse of light typical comprises several wavelengths, the spectral components of the optical signal spread in time, or disperse, as they propagate, causing the pulse width to broaden.
• Optical fiber is nearly pure silica (SiO 2 ), so the chromatic or material dispersion of fiber is essentially the same as pure fused silica.
• FIG. 1 we plot the material dispersion of fused silica and the refractive index as a function of wavelength. Since the refractive index of a material is wavelength dependent, n( ⁇ ), the velocity of light in a material is also wavelength dependent related by,
• c is the speed of light in vacuum (299,792,458 meters/second).
• the refractive index for a short wavelength (referred to as “blue” light) is larger than that for a longer wavelength (referred to as “red” light) so that light of longer wavelengths ("red”) travels faster than shorter wavelengths (“blue”).
• optical signals traversing optical waveguides such as a multimode fiber optic cable (MMF) also undergo modal dispersion, which is generally a much larger effect in MMF.
• MMF multimode fiber optic cable
• the optical power of the pulse is carried by the sum of the discrete modes.
• MMF is optimized so that all modes arrive at the output of the fiber at the same time. This is achieved by adjusting or "grading" the refractive index profile of the fiber core. Modes traveling with larger angles (and consequently traverse longer distances) must travel faster. These are called high-order modes. Modes traveling with small angles (low-order modes) travel slower in graded-index fiber.
• the difference in propagation delays between the fastest and slowest modes in the fiber is used to determine the inter-modal dispersion or simply modal dispersion.
• GI-MMF Graded Index Multimode Fiber
• a is a number close to 2 (and specific to each fiber manufacturer)
• R is the radius of the fiber core
• ⁇ is given by
• the metric used to characterize modal dispersion in MMF is Differential Mode Delay (DMD), specified in Telecommunications Industry Association Document No. TIA-455-220-A and expressed in units of picoseconds per meter (ps/m) so that the total delay is normalized by fiber length.
• DMD Differential Mode Delay
• Low modal dispersion as measured by DMD generally results in higher-bandwidth MMF.
• Better control in the manufacturing process produces a profile closer to the standard parabolic refractive index profile which minimizes modal dispersion.
• a method for manufacturing an improved multimode fiber optic cable which compensates for both material dispersion and modal dispersion effects.
• the method includes, but is not limited to, coupling a laser with a reference multimode fiber optic cable and generating and launching a plurality of pulses of light radiation by the laser into the reference multimode fiber optic cable. Each pulse of light radiation is launched at different radial offset.
• the method also includes, but is not limited to, determining a DMD waveform profile along with a pulse delay for each pulse of light at each radial offset and determining if there are differences in pulse delays for each DMD waveform profile.
• the method also includes, but is not limited to, designing the improved multimode fiber optic cable with an improved refractive index profile which compensates for any differences in pulse delay present in each DMD waveform profile, and which compensates for at least a portion of the material dispersion present in the reference multimode fiber optic cable.
• a method for designing an improved multimode fiber optic cable which compensates for both material dispersion and modal dispersion effects includes, but is not limited to, determining an amount of material and modal dispersion within a reference multimode fiber optic cable resulting from a pulse of light radiation launched into the multimode fiber optic cable using a laser and designing an improved refractive index profile for the improved multimode fiber optic cable which compensates for at least a portion of the material dispersion present in the reference multimode fiber optic cable.
• a method for designing an improved multimode fiber optic cable which compensates for both material dispersion and modal dispersion effects includes, but is not limited to, generating and launching a plurality of pulses of light radiation into a reference multimode fiber optic cable. Each pulse of light radiation is launched at different radial offset.
• the method also includes, but is not limited to determining a DMD waveform profile along with a pulse delay for each pulse of light at each radial offset and designing an improved refractive index profile for the improved multimode fiber optic cable which compensates for at least a portion of the material dispersion present in the reference multimode fiber optic cable by correcting for any differences in pulse delay present in each DMD waveform profile.
• FIG. 1 depicts a graph of material dispersion and a refractive index of pure silica as a function of wavelength, in accordance with one embodiment of the present invention.
• FIG. 2A depicts a first cross-sectional perspective view of a graded index
• FIG. 2B depicts a second cross-sectional perspective view of a graded index MMF having a standard parabolic refractive index profile that equalizes the velocities of the various modes traversing the fiber, in accordance with one embodiment of the present invention. Different colors are used for clarification only. All modes are supposed to have the same wavelength in prior models.
• FIG. 3 depicts a graph of a DMD waveform to measure the difference in arrival time of all modes in a GI-MMF, in accordance with one embodiment of the present invention. From that time difference, normalized by the length of the fiber, the fiber is graded and classified to comply as OM3 or OM4 type fiber. Different colors are used for clarification purposes only. All modes have the same wavelength.
• FIGS. 4A and 4B depict graphs of DMD waveform profiles of two fibers with similar DMD and EMB values, in accordance with one embodiment of the present invention. Both fibers are from the same fiber cable. Both have an EMB of4540 MHz-km.
• FIGS. 5A and 5B depict eye diagrams for Blue and Brown fibers, in accordance with one embodiment of the present invention.
• Blue is on the left side and exhibits a wider eye opening, which indicates a larger signal-to-noise ratio and therefore transmits information with fewer errors (better BER performance).
• FIG. 6 depicts a graph of BER traces of Blue and Brown fibers as a function of received power, in accordance with one embodiment of the present invention.
• a received optical power of -9.9dBm minimum optical power for 1 OGBASE-SR
• the difference in BER performance is more than two orders of magnitude.
• FIG. 7 depicts graphs illustrating that minimizing modal dispersion requires that all the modes arrive at the same time at the output end of the fiber (illustrated for two low order modes), in accordance with one emboidment of the present invention. In previous implementations, modes have been assumed to have same wavelength.
• FIG. 8 depicts a graph of a wavelength dependence of a BERT Vertical Cavity Surface Emitting Lasers (VCSEL) as a function of offset from the center of a device, in accordance with one embodiment of the present invention.
• VCSEL Vertical Cavity Surface Emitting Lasers
• FIG. 9 depicts graphs illustrating that when a wavelength variation of modes is taken into consideration, material dispersion effects, D(D), will spread the components of a pulse even for a standard parabolic refractive index profile, in accordance with one embodiment of the present invention.
• the "blue” mode will arrive at the output of the fiber at a later time than the "red” mode.
• FIG. 10 depicts graphs illustrating that a fiber with lower than standard parabolic index of refraction (dotted line) in the outer region of the core will speed up higher order modes ("blue"), while not affecting the lower order modes (“red”) cancelling the material dispersion effects, in accordance with one embodiment of the present invention.
• FIG. 1 1 depicts graphs ilustrating that a fiber with higher than the standard index of refraction in the outer region of the core will slow down higher order modes ("blue"), while not affecting the lower order modes (“red”), exacerbating the material dispersion effects, in accordance with one embodiment of the present invention.
• the present invention makes use of the discovery that multimode fiber optic cable having a refractive index profile may be designed to compensate for both material dispersion as well as modal dispersion when used with light sources that emit modes having different optical wavelengths in different emission patterns.
• the proposed multimode fiber optic cable compensates for the spatial spectral distribution of laser launch modes when coupled into fiber modes to reduce overall modal dispersion.
• the disclosed multimode fiber optic cable exhibits improved Bit Error Rate (BER) system performance by balancing the wavelength dependency of the VCSEL modes and the refractive index profile of the multimode fiber optic cable to reduce modal dispersion.
• the disclosed multimode fiber optic cable also increases the maximum reach over which a signal can be transmitted with acceptable error rates.
• the refractive index profile of the multimode fiber optic cable exhibits a Differential Mode Delay (DMD) waveform profile that shifts to the left (in ps/m as shown in standard graphical depictions of DMD) at larger radial offsets to compensate for a spatial distribution of emitted optical wavelengths in an optical source.
• DMD Differential Mode Delay
• a method for manufacturing a multimode fiber which compensates for both material dispersion and modal dispersion effects.
• the method first includes determining an amount of material and modal dispersion within a reference multimode fiber resulting from a pulse of light radiation launched into the multimode fiber using a laser.
• the method then includes designing an improved refractive index profile for the improved multimode fiber which compensates for at least a portion of the material dispersion present in the reference multimode fiber optic cable.
• determining an amount of material and modal dispersion within a reference multimode fiber requires first implementing a revised DMD measurement test method, wherein a temporally short and spectrally pure optical pulse of light radiation is generated and transmitted by a laser and launched into the core of a reference MMF under test.
• the optical pulse of light radiation is first launched along the center axis of the fiber and an output pulse waveform is measured with a photo-detector and sampling oscilloscope.
• the output pulse waveform is stored for subsequent analysis.
• the launched optical pulse of light radiation is then displaced a small radial distance from a central core of the reference MMF, typically 1 or 2 microns, and an output waveform is again measured and recorded.
• X is approximately 23 microns ( ⁇ 5 microns) for a 50 micron core diameter.
• a small diameter single-mode fiber is preferably used to launch the optical pulse of light radiation into the core of the MMF.
• the DMD or modal dispersion of a MMF is calculated by subtracting the launch pulse temporal width from the difference in arrival times between the leading edge of the fastest pulse and the falling edge of the slowest pulse.
• MMF fiber can be classified as laser optimized (i.e., OM3), capable of supporting 10Gb/s Ethernet communications up to 300m (in theory).
• the fiber must meet 1 of 6 DMD mask templates, which specify the maximum modal dispersion (i.e., DMD) within radial regions of the core. If the fiber meets more stringent DMD requirements (to be specified by TIA), the fiber is characterized as OM4, capable of supporting a greater reach. Low modal dispersion as measured by DMD, is believed to translate to higher MMF performance.
• EMB Effective Modal bandwidth
• VCSELs Vertical Cavity Surface Emitting Lasers
• VCSELs emit a single longitudinal mode coupled with multiple transverse modes resulting in a distribution of light with slightly different wavelengths over the area of emission.
• Each VCSEL mode has a defined polar emission pattern. This physical effect is referred to herein as a polar pattern having a radius-dependent wavelength.
• Equation 2 The standard parabolic refractive index profile (based on an ⁇ value, as described by Equation 2) is currently designed to minimize the spread of all modes travelling through the fiber, with the assumption that all the modes have substantially the same wavelength (color). With reference to FIG. 7, laser source emission patterns and wavelength distribution effects have been completely neglected.
• VCSELs used in high-speed optical transceivers emit light with different wavelengths across the aperture of the device (modes). Longer wavelengths are emitted into smaller angles whereas short wavelengths are emitted into larger angles normal to the surface (polar emission pattern). With reference to FIG. 8, this VCSEL spatial spectral distribution is preserved when coupled into fiber modes.
• the standard parabolic DMD waveform profile used today is valid only for the same wavelength of light across the aperture of the VCSEL, where all coupled fiber modes are believed to have the same center wavelength.
• the spatial distribution of optical wavelengths launched into the MMF requires a new preferred DMD waveform profile since the modes, with their radius-dependent wavelengths, are influenced by material dispersion effects. The effect of the radius-dependent emission pattern coupled with modal dispersion is illustrated in FIG. 9. Lower order modes have longer wavelengths ("red”) and therefore travel faster than high order modes having shorter wavelengths ("blue").
• fibers that exhibit a "left" shift in the radial pulse waveforms in the DMD profile correspond to a lower-than-standard parabolic index of refraction in the outer region of the MMF core. This is observed at large radial offsets in the DMD waveform profile for the Blue fiber in FIG. 4A.
• the lower-order modes will not be affected by this shift in refractive index, as they do not travel through the outer region.
• higher-order modes are slowed down as a result of the wavelength-dependent index of refraction, as shown in FIG. 9, since they travel in a region of lower than the so-called standard parabolic refractive index profile they in fact catch up with the "red" light (See FIG. 10).
• the index of refraction is higher than the standard parabolic refractive index in the outer region of the core. This is seen in the DMD waveforms of the Brown fiber at high radial offsets, as shown in FIG. 4B.
• the lower-order modes will not be affected by this shift in refractive index, as they do not travel through that region.
• Higher order modes that were slowed down by the wavelength-dependent index of refraction, as shown in FIG. 9, will travel through a region with higher than the standard parabolic index profile, and therefore slow down the "blue” light even further (See FIG. 1 1).
• Minimizing modal dispersion can transform modal wavelength
• an improved refractive index profile can be designed for an improved MMF that compensates for at least a portion of the wavelength dependency of the modes, allowing for modal dispersion to be further reduced from previous compensation methods.

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• Optics & Photonics (AREA)
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• Optical Couplings Of Light Guides (AREA)

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• 2010
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