William Alfred Reed

Gradient-index fiber-optic microprobes for minimally invasive in vivo low-coherence interferometry

We describe the design, construction, and application of what are believed to be the smallest fiber-optic probes used to date during imaging or diagnosis involving low-coherence interferometry (LCI). The probes use novel fiber-optic gradient-index (GRIN) lenses fabricated by a recently developed modified chemical-vapor-deposition (MCVD) process that avoids on-axis aberrations commonly marring MCVD-fabricated GRIN substrate. Fusing GRIN fiber lenses onto single-mode fiber yields automatically aligned all-fiber probes that insert into tissue through hypodermic needles as small as 31-gauge (inner diameter, 127 mum). We demonstrate the use of such probes with LCI by measuring microscopic brain motions in vivo.

All-fiber grating-based higher order mode dispersion compensator for broad-band compensation and 1000-km transmission at 40 Gb/s

We use a novel fiber-grating device to demonstrate the first polarization-insensitive all-fiber higher order mode dispersion compensator for broad-band dispersion compensation. Its low loss and high effective area have enabled transmission through 1000 km (10/spl times/100 km) of nonzero dispersion-shifted fiber (NZDSF) at 40 Gb/s.

Quantitative estimates of mode coupling and differential modal attenuation in perfluorinated graded-index plastic optical fiber

The performance of plastic optical fiber is greatly influenced by the related but distinct effects of mode coupling and differential modal attenuation (DMA). We establish a method for estimating the matrix that governs both of these effects and allows us to distinguish the two. We obtain partial quantitative estimates of this matrix for a particular graded-index plastic optical fiber (GI-POF). The sample we studied exhibited strong but incomplete mode coupling over 100-m lengths, while DMA was largely limited to a centerline defect. We show that much of the loss of the fiber can be attributed to mode coupling between mode groups with similar effective indexes.

Tunable dispersion compensators utilizing higher order mode fibers

We demonstrate a novel tunable dispersion compensator that utilizes higher order mode fibers and switchable fiber gratings. The device is broad band and wavelength continuous, yielding a bit rate, bit format, and signal bandwidth as well as channel-spacing transparent adjustable dispersion compensator. The novel device design is free from tradeoffs between tuning range and bandwidth. The tuning range is 435 ps/nm, with a bandwidth of 30 nm. Its all-fiber configuration yields the lowest loss (average /spl sim/3.7 dB) tunable dispersion compensator reported to date. 40-Gb/s transmission tests reveal penalty-free operation.

Splice loss of single-mode fiber as related to fusion time, temperature, and index profile alteration

The lower bound in loss for fusion splices is ∼ 0.01 dB due principally to lateral offset of the cores and alteration of the index profile. This paper investigates the effects on loss of viscous flow and diffusion of the glass constituents during fusion, defining their time-temperature dependencies, and changes in index profile.

Intermodal dispersion and mode coupling in perfluorinated graded-index plastic optical fiber

Graded-index multimode perfluorinated plastic optical fibers typically exhibit bandwidths much greater than would be expected from their index profiles. To resolve this discrepancy, we have conducted the first measurements of differential mode delay in such fibers. These measurements show intermodal dispersion that increases as the square root of fiber length, implying strong mode coupling in these fibers. Significant power transfer between modes occurs at lengths less than 20 m, so that mode coupling results in improved bandwidth on length scales relevant for local area networks. The observed coupling arises from extrinsic nonuniformities of the waveguide.

Highly efficient single-mode fiber for broadband dispersion compensation

The advent of erbium-doped optical fiber amplifiers which operate at 1.55 µm has triggered considerable interest in schemes that will permit the upgrade of installed systems operating at 1.31 µm to operate in the 1.55 µm window. Since installed systems use standard single mode fibers with a dispersion-zero at 1.31 µm, any upgrade scheme must incorporate a method of compensating for the positive dispersion of the standard (non-dispersion-shifted) fiber at 1.55 µm if high transmission rates are to be achieved. An equalization technique that uses two-mode fibers was suggested by Poole et al..1 While high compensation ratios (defined as the ratio of the length of the communications fiber to the length of the compensating fiber) of 30:1 have been achieved, the higher-order mode technique requires intermodal convertors and polarization rotators, thus adding to the system cost and complexity. Dugan et al. 2 and Izadpanah et al.3 demonstrated the use of single-mode fiber compensators (using the fundamental LP01 mode). However, the single-mode fiber-based compensating schemes could not compensate for the second-order dispersion and absolute values of the dispersion were typically less than 100 ps/nm-km. In this paper, we present a multiclad dispersion compensating fiber (DCF) operated in the single-mode regime that has a dispersion of -212 ps/nm-km and is capable of compensating dispersion over a broad wavelength range.

A new modal power distribution measurement for high-speed short-reach optical systems

We propose a pair of related new techniques for measuring the modal power distribution (MPD) of light launched into a multimode fiber (MMF). For modern applications involving laser launches, our method has several potential advantages over ray optics-based methods such as and recent extensions of them . Instead of relying on a time-averaged measurement of near-field intensity with a CCD camera to spatially resolve the launched power, our method uses a specially designed probe fiber to temporally resolve the launched power. If the source can be modulated fast enough (as is necessary for 10 Gbps serial transmission, for instance), the measurement is straightforward and the calculation of the MPD is trivial. For slower sources, a more involved measurement and inversion algorithm are required.

Ultraviolet photosensitive response in an antimony-doped optical fiber.

A silica optical fiber doped with Sb is fabricated with a refractive-index profile that is comparable with standard single-mode fiber. In D(2)-loaded samples, we observe UV photosensitivity with an initial refractive-index-modulation growth rate six times higher than that of the equivalent Ge-doped standard fibers. Enhanced temperature stability of the Bragg grating strength up to 200 degrees C is also observed. Grating growth kinetics in the Sb-doped fiber is compared with those of other Ge-doped photosensitive fibers.

The fabrication and performance of long lengths of silica core fiber

Three hundred kilometers of single-mode fiber exhibiting median optical losses of 0.19 dB/km at 1.57 μm have been fabricated from preforms made by a high-rate Modified Chemical Vapor Deposition (MCVD) process. A new fiber design [1] was utilized which minimizes Rayleigh scattering loss by reducing the amount of dopants in the core. Milestone systems experiments incorporating this fiber have already demonstrated 420-Mbit transmission through 203 km [2], 2-Gbit transmission through 130 km [3], 1.37 Tbit km/s using 10 wavelength division multiplexed lasers [4], 4-Gbit through 102 km using a novel electronic multiplexer/demultiplexer [5], and 4 Gbit through 117 km using a Ti:LiNbO 3 external modulator [6]. Additionally, very low induced losses from hydrogen and radiation are reported.