Band 5, Heft 1

We show the tradeoffs between the bandwidth-length product, drive voltage-length product and the optical insertion-loss of modulators butt-coupled to single-mode fibers. The numerical calculations are performed for a new design that permits fast Δβ-operation of directional coupler modulators.

We report on a very fast (≥ 6 Gbit/s) Δβ-reversal directional-coupler modulator with low insertion loss (2 dB) for 1.3 μm wavelength operation. The design of the modulator permits easy and reproducible fabrication.

Experimental and theoretical data are presented that relate to strip waveguides obtained by applying a voltage to electrodes deposited on a Ti:LiNbO 3 planar waveguide. The dependence of the modal picture on inducing voltage is registrated. Optical Field distributions in depth are calculated approximating the Gaussian index profile by a parabola and an exponent. The overlap between the optical intensity and the electric field is measured for different depth modes and is compared with calculated ones. A possibility of effective amplitude modulation is observed on the base of formations of successive lateral modes. A simple modulator is demonstrated.

Light propagation in X-cut LiNbO 3 waveguides fabricated by proton exchange is analyzed theoretically. Simple formulae are given for both (real) effective refractive indices and leaky mode loss of guided and leaky modes. An example of dispersion and loss curves for a typical proton-exchanged waveguide is presented. The similarities and differences with respect to titanium-indiffused and out-diffused LiNbO 3 waveguides are pointed out.

Time domain measurements of the bandwidth of concatenated cabled optical fibers at 1.3 μm has been carried out on two different cable constructions. Both time domain and frequency domain analysis are presented. For one cable type containing fibers exhibiting smooth frequency characteristics a concatenation exponent γ = 0.66 was found for a 9.5 km length of concatenated fibers. For the other cable type containing some fibers with irregular frequency characteristics such a fit was not possible for a 24 km length of concatenated fibers but an exponent γ = 0.65 could be fit to the last 14 km. Two splicing procedures were employed on the latter cable type, but no appreciable difference in the results was observed. When the frequency characteristic was regular the 3 dB optical bandwidth of the spliced fiber sections was found to be inversely proportional to therms pulse width broadening. The bandwidth-pulsewidth product agreed with results obtained for Gaussian pulse shapes for one cable type and hyperbolic secant pulse shapes for the other cable type.

The performance increase of optical transmission systems creates new problems like optical feedback and modal noise. It is very important to overcome these problems especially in systems composed by a lot of single optical elements. In this paper we describe a new method to determine the degree of mode selectivity produced by these devices. A simple model for modal noise description together with experimental results are presented.

A novel and simple method is described to characterize semiconductor lasers which may produce excessive modal noise in a transmission system. The method is based on an analysis of the speckle contrast at the end of a suitable length of multimode fiber. An optimal length of measuring fiber is calculated and verified experimentally. The qualitative dependence of speckle contrast on fiber length is shown. The measured speckle contrast allows to define an “effective” number of laser spectral lines accounting for the modal noise.