Abstract: A new annular-core optical fiber is useful, inter alia, for stabilizing the behavior of an optical attenuator based on a Long Period Grating (LPG) against fluctuations in the polarization of the fundamental optical signal. The fiber dimensions and ?n are selected such that when a LPG of some period is used to couple fundamental-mode radiation in the fiber into higher-order modes, the wavelength of peak coupling into an HE mode is removed by at least 30 nm from the wavelength of peak coupling into the nearer of a TE mode or a TM mode.

Abstract: A technique for measuring the modal power distribution of an optical source (for example, a laser) launching pulses into a multimode fiber involves a characterization of the multimode fiber itself in terms of its differential modal delay. A reverse differential mode delay measurement is then performed to characterize the interaction of the optical source with the multimode fiber. By knowing these characteristics, the modal power distribution of the source into the fiber can then be determined by using a reconstruction algorithm.

Abstract: Embodiments of the invention include an optical communications system including a multimode optical fiber having improved overfill-launch bandwidth performance without disturbing existing laser-launch bandwidth performance. The multimode optical fiber has a characteristic differential mode delay with a first portion associated with lower order modes that behaves conventionally and a second portion associated with higher order modes that deviates from conventional behavior in a way that improves overfill-launch bandwidth performance at one operating window without adversely impacting the laser-launch bandwidth performance at the same and other operating windows. Multimode optical fibers conventionally optimized for operation at 850 nm are configured in such a way that their characteristic differential mode delay, at 1300 nm, initially increases in a conventional manner and then flattens out to approximately zero at the higher order modes.

Abstract: Our method of making high bandwidth silica-based multimode optical fiber comprises provision of a non-circular preform, and drawing fiber of chiral structure from the preform. The non-circular preform can be made by maintaining the inside of the tubular preform under reduced pressure during at least part of the collapse, resulting in a non-circular core and cladding. It can also be made by removal (e.g., by grinding or plasma etching) of appropriate portions of the preform, resulting in a circular core and non-circular cladding. In the latter case, fiber is drawn at a relatively high temperature such that, due to surface tension, the cladding assumes substantially circular shape and the core assumes a non-circular shape. The chiral structure is imposed on the fiber in any appropriate way, e.g., by twisting during fiber drawing the fiber alternately in clockwise and couterclockwise sense relative to the preform.

Abstract: A method and apparatus for overcoming modal noise penalties associated with multi-mode fiber optic links operating at high data rates on the order of 10 Gb/s. The method involves restricting the center launch condition of optical signals into the multi-mode fiber while permitting loose tolerances between source and fiber.

Abstract: Disclosed is a method of launching a light beam from a semiconductor laser into a multimode optical fiber, as well as apparatus for launching the light beam. The light beam is directed at nominally the center of the input face of the multimode fiber such that the (unguided) light beam makes an angle &thgr;b (typically in the range 1-20 degrees) with the direction perpendicular to the input face of the multimode fiber. The angle &thgr;b is selected such that at least one lower order mode of the guided radiation in the multimode fiber is substantially not excited in the fiber, with at least some higher order modes being substantially excited. Several specific embodiments are disclosed.

Abstract: Our method of making high bandwidth silica-based multimode optical fiber comprises provision of a non-circular preform, and drawing fiber of chiral structure from the preform. The non-circular preform can be made by maintaining the inside of the tubular preform under reduced pressure during at least part of the collapse, resulting in a non-circular core and cladding. It can also be made by removal (e.g., by grinding or plasma etching) of appropriate portions of the preform, resulting in a circular core and non-circular cladding. In the latter case, fiber is drawn at a relatively high temperature such that, due to surface tension, the cladding assumes substantially circular shape and the core assumes a non-circular shape. The chiral structure is imposed on the fiber in any appropriate way, e.g., by twisting during fiber drawing the fiber alternately in clockwise and couterclockwise sense relative to the preform.

Abstract: A method is disclosed for qualifying a multimode optical fiber 150 for bandwidth performance when used with a particular laser source. The method combines the modal power distribution (MPD) excited by a particular laser source with the differential mode delay (DMD) characteristic of the fiber. The DMD of the fiber is measured by injecting test pulses into one end of the fiber and detecting the resulting output pulse(s) at the other end. The test pulses are adapted to excite only a small number of the modes supported by the fiber. The test pulses are scanned across the core of the fiber at close intervals with the output pulse(s) stored at each radial position. A weighted sum of the output pulses is formed to determine a time-domain impulse response, where the weighting used corresponds to the MPD excited by the laser source. Bandwidth is then determined by standard methods for transforming the impulse response into the frequency domain.

Abstract: A multi-mode optical fiber having an improved refractive index profile is disclosed. The refractive index profile is modified from a conventional by at least one of (i) a step formed in the index profile at the boundary of the core and cladding regions, in combination with a linear correction; (ii) a ripple near the core-cladding boundary, in combination with a linear correction, with or without an index step; and (iii) an annular ridge formed in the index profile of an &agr;-profile with a center dip defect. The invention further relates to a communications system comprising the inventive fiber.

Abstract: Our method of making high bandwidth silica-based multimode optical fiber comprises provision of a non-circular preform, and drawing fiber of chiral structure from the preform. The non-circular preform can be made by maintaining the inside of the tubular preform under reduced pressure during at least part of the collapse, resulting in a non-circular core and cladding. It can also be made by removal (e.g., by grinding or plasma etching) of appropriate portions of the preform, resulting in a circular core and non-circular cladding. In the latter case, fiber is drawn at a relatively high temperature such that, due to surface tension, the cladding assumes substantially circular shape and the core assumes a non-circular shape. The chiral structure is imposed on the fiber in any appropriate way, e.g., by twisting during fiber drawing the fiber alternately in clockwise and couterclockwise sense relative to the preform.

Abstract: A method for estimating a real function that describes a phenomenon occurring in a space of any dimensionality is disclosed. The function is estimated by taking a series of measurements of the phenomenon being described and using those measurements to construct an expansion that has a manageable number of terms. A reduction in the number of terms is achieved by using an approximation that is defined as an expansion on kernel functions, the kernel functions forming an inner product in Hilbert space. By finding the support vectors for the measurements one specifies the expansion functions. The number of terms in an estimation according to the present invention is generally much less than the number of observations of the real world phenomenon that is being estimated. In one embodiment, the function estimation method may be used to reconstruct a radiation density image using Positron Emission Tomography (PET) scan measurements.