W. A. Reed

Measurement of the nonlinear index of silica-core and dispersion-shifted fibers.

We have accurately measured the nonlinear refractive index, n(2), of the glass found in silica-core and dispersion-shifted optical fibers. We find that at 1.319 microm the n(2) of silica-core and dispersion-shifted fibers is 2.36 x 10(-16) and 2.52 x 10(-16) cm(2)/W (+/-5%), respectively. We also estimate that the n(2) of germania is three times the n(2) of pure silica. Because the wavelength dependence of n(2) between 1.3 and 1.5 microm is known to be small, our measured values should also be valid at 1.55 microm.

Birefringence reduction in side-written photoinduced fiber devices by a dual-exposure method.

An in situ birefringence measurement in conjunction with an atomic force microscope study shows that the geometric asymmetry of the side-writing process is a major cause of the induced birefringence in grating-based fiber devices. Measured refractive-index profiles of UV-exposed fibers clearly show the asymmetry in the induced index change. We demonstrate the use of a dual-exposure technique for producing low-birefringence devices.

Ultraviolet laser fabrication of ultrastrong optical fiber gratings and of germania-doped channel waveguides

We report the fabrication of low loss (<0.2 dB) fiber Bragg gratings of 10 nm reflection bandwidth, and of 19 nm bandwidth lossy gratings. With this technology we also demonstrate the direct writing of channel waveguides in commercial germania‐doped wafers without the use of photoresist and etching.

Phase matching in the minimum-chromatic-dispersion region of single-mode fibers for stimulated four-photon mixing.

We describe phase-matched stimulated four-photon mixing in single-mode fibers in the minimum-chromatic-dispersion region. Phase matching is achieved by using the refractive-index slope change and the guiding properties of the fundamental mode. Experimental results on large-shift (1600-cm(-1)) single-idler generation in single-mode fibers with 1.319-microm Nd:YAG laser pumping are presented.

Dispersion-compensating single-mode fibers: efficient designs for first- and second-order compensation.

Single-mode dispersion compensating fiber designs with absolute dispersion values of greater than 100 ps/(nm km) are described. A multiclad fiber with a triangular refractive-index profile in the core gives a dispersion of −250 ps/(nm km), resulting in a 15:1 compensation scheme. All fiber designs use only the fundamental LP01 mode for dispersion compensation and can account for first- and second-order dispersion effects.

Atomic force microscopy study of uv‐induced anisotropy in hydrogen‐loaded germanosilicate fibers

Thermally treated and uv‐exposed hydrogen‐loaded germanosilicate fibers are profiled with an atomic force microscope after cleaving the fiber in the exposed region and etching the cleaved endface in hydrofluoric acid. Thermally treated fibers etch symmetrically throughout the core region, but the uv‐exposed fibers etch differently. In the uv‐exposed fibers, both the etch depth and the refractive index are asymmetric. They are highest at the core‐cladding interface on the side exposed to the uv radiation. We propose that a photolytic process increases the refractive index across the entire core, but the asymmetry is the result of transient heating due to uv absorption. Furthermore, we demonstrate that uniform circumferential exposure symmetrizes the etch depth and refractive index change and reduces the induced birefringence.

Thermally enhanced ultraviolet photosensitivity in GeO2 and P2O5 doped optical fibers

The 248 nm UV photosensitivity of H2 loaded optical fibers is shown to be significantly enhanced by increasing the temperature during the UV exposure. Heating to 250–400 °C resulted in dramatic UV reaction rate increases in GeO2 doped fibers. Increasing the core temperature during 248 nm irradiation caused P2O5 doped fibers to become photosensitive.

Effect of refractive-index profiles on two-mode optical fiber dispersion compensators

We evaluate the effect of refractive-index profile variations on the performance of two-mode fibers used as dispersion compensators in long-distance transmission systems operated near 1550 nm. The comparison of various profiles is carried out for two cases: (i) For a constant operating wavelength away from the LP11-mode cutoff, the step-index fiber is shown to be the best dispersion compensator. (ii) For a constant power confinement in the core (Pcore/Ptotal), a matched-cladding, power-law profile with α = 6 presents a 20% performance improvement over step-index fibers.

Measurement of erbium confinement in optical fibers: a differential mode-launching technique

We present a selective mode-launching technique to determine the confinement of erbium in optical fibers. The loss per meter, alpha(lm), of the individual LP(lm) modes (LP(01) and LP(11) at 980 nm) is measured, and the ratio gamma = alpha(01)/alpha(11) is related to the concentration of erbium in the core. We assume a Gaussian diffusion profile for erbium and relate the dependence of gamma on erbium confinement by using exact mode profiles. This technique uses only two sets of measurements and is independent of erbium concentration.