Subnanosecond pulses in the 1120-1550-nm region are generated by multiple-order stimulated Raman scattering in a small core single-mode silica fiber pumped by a Q-switched and mode-locked Nd:YAG laser at 1064 nm. These near ir pulses are injected into various km long test fibers, and relative time delay changes between different wavelengths are used to determine dispersion in a region where fiber material dispersion is small. Zero material dispersion has been observed in germanium and boron-doped single-mode and multimode est fibers.

Pulse delay measurements in the zero material dispersion wavelength region for optical fibers.

A sinusoidal technique is reported, which allows simple and accurate measurements of chromatic dispersion in optical fibers. It is based on the phase shift which a sinusoidally modulated fight beam undergoes while traveling along a fiber when its wavelength is changed. The choice of a multiple LED's source permits the continuous spectral covering from 750 to 1600 nm;easily available instrumentation and devices are needed for the measurement setup. The technique is reported in detail by showing results obtained in multimode fibers; statistical evaluation of its accuracy and a comparison with conventional methods are carried out. An accuracy of a few picoseconds in relative delay and of ∓1 ps/nm . km in chromatic dispersion are demonstrated, that compare very favorably with the existing techniques.

Phase shift technique for the measurement of chromatic dispersion in optical fibers using LED's

The round dielectric waveguide exhibits a surprising variety of characteristics that are not accurately inferable from the slab model. The forceful effort of recent years has grately extended the knowledge of these structures and added new and exciting modifications. An attempt to unify these results in a simplified picture is made. Specific phenomena relevant to optical fiber design and fabrication are then brought into focus. Some of the problems discussed are cross sectional loss variations, various core index profiles and the tolerances required in their preparation, the necessary cladding thickness, directional changes, and sources of mode coupling affecting signal distortion and loss.

Propagation Effects in Optical Fibers

The variation of the core-cladding refractive index difference Deltan(lambda) is determined as a function of wavelength for GeO(2)-SiO(2), B(2)O(3)-SiO(2), TiO(2)-SiO(2), P(2)O(5)-SiO(2), Al(2)O(3)-SiO(2), and Cs(2)O-SiO(2) optical fibers. The measurements are obtained by electronically processing the output of an interference microscope illuminated by a monochromatic light source variable over the 0.5-1.1-microm range. The Deltan(lambda) results are utilized to calculate the precise grading of the refractive index profile characterized by an exponent alpha(lambda) required to achieve a minimum modal dispersion. The material dispersion for these compositions is also calculated with the aid of available n(lambda) data for SiO(2).

Binary silica optical fibers: refractive index and profile dispersion measurements.

This paper describes a high-sensitivity, high-dynamic range experimental method for measuring the frequency response of the underwater optical channel in the forward direction for the purpose of wireless optical communications. Historically, there have been few experimental measurements of the frequency response of the underwater channel, particularly with regard to wireless communication systems. In this work, the frequency response is measured out to 1 GHz over a wide range of water clarities (approximately 1-20 attenuation lengths). Both spatial and temporal dispersions are measured as a function of pointing angle between the transmitter and the receiver. We also investigate the impact of scattering function and receiver field of view. The impact of these results to the link designer is also presented.

Temporal Response of the Underwater Optical Channel for High-Bandwidth Wireless Laser Communications

Two synchronously self-pulsing GaAlAs lasers, emitting at slightly different wavelengths, are combined in a setup which evaluates mode and material dispersion in multimode fibers. The results show that material dispersion which causes a pulse broadening of 3.6 ns/km, which may represent a serious limit to the bit rates achievable in a multimode fiber system using an LED source.

GaAs Twin-Laser Setup to Measure Mode and Material Dispersion in Optical Fibers.

Incoherent light, transmitted via multimode fibers, represents a potential carrier for information that is directly modulated on the light envelope. The amplitude and phase characteristics of the envelope (baseband) signal are affected by the fiber waveguide because of group delay differences among the modes. We report on a technique that determines the baseband frequency response of the fiber system by comparing the beat spectra of light from a free-running laser before and after transmission through the fiber. As an example, we describe the measurement of 30 m of cladded multimode fiber. The 3-dB bandwidth was 700 MHz, somewhat larger than predicted on the basis of the computed group velocity differences.

E. L. Chinnock

Papers

GaAs Twin-Laser Setup to Measure Mode and Material Dispersion in Optical Fibers.

Direct measurement of the (baseband) frequency response of multimode fibers.