KEYWORDS: Optical amplifiers, Fiber amplifiers, Raman spectroscopy, Optical fibers, L band, Signal attenuation, Control systems, S band, Amplifiers, Telecommunications
To meet the ever-increasing traffic demands and to reduce the equipment as well as operation costs, recent developments in fiber-optic communication systems are focused on four areas: longer reach, wider bandwidth, higher bit rate and the intelligence of the system. In this presentation, recent advances in optical fiber amplifiers will be reviewed with the emphasis on the attributes supporting system requirements in these four areas. Topics to be addressed include ultra wideband amplifiers and amplifiers with new bands, a system perspective of Raman amplification, management of gain ripple, and intelligent amplifier control.
A subpicosecond electro-optic sampling system was used to measure the picosecond characteristics of silicon-based, metal- semiconductor-metal photodiodes made on both bulk-silicon and silicon-on-sapphire (SOS) substrates with submicrometer finger spacings and widths. The temporal response of bulk-silicon diodes was strongly dependent on the wavelength of excitation light because of the effect of different penetration depths. On the other hand, for the SOS diodes, the device speed is nearly independent of wavelength since the thickness of the silicon layer limits the depth of photogenerated carriers. The external quantum efficiency of SOS diodes was measured at several selected wavelengths and shown to be dominated by the photon absorption coefficient.
The characteristics of a family of coplanar transmission lines have been studied at frequencies extending to the terahertz range. Traditional wide-ground coplanar waveguides and coplanar strip lines were investigated together with a coplanar waveguide with narrow ground planes. The technique of nonuniform gap illumination was used to excite subpicosecond electrical pulses as a testing tool of transmission lines for the first time. It is shown that this method is versatile and convenient for testing ultrafast devices and circuits. The experimental results, extracted by both time- and frequency-domain analyses, indicate several interesting features. In the subterahertz frequency range, the 50-micrometers transmission lines are dominated by dispersion, while the narrower 10-micrometers lines are dominated by loss. The characteristics of traditional (wide-ground) coplanar waveguides and coplanar strips are in agreement with theory and comparable to each other up to very high frequencies. The implementation of narrow ground planes can considerably reduce attenuation and dispersion in coplanar waveguides. In some geometries, radiation loss can be eliminated completely. The reduction in radiation is attributed to the change of field patterns at the dielectric interface, which leads to reduced coupling between the coplanar waveguide mode and radiative substrate modes.
The ultrafast characteristics of crystalline-silicon metal-semiconductor-metal (MSM) photodiodes with finger widths and spacings down to 200 nm, subjected to femtosecond optical pulse excitations, was measured with a subpicosecond electro-optic sampling system. Electrical responses with fullwidth at half-maximum (FWHM) as short as 3.7 ps, at a corresponding 3 dB bandwidth of 110 GHz, were generated by violet-light excitation. These diodes are the fastest silicon photodetectors reported to date. Detailed bias and light-intensity dependence of the diode response has been measured. These results are used to obtain the velocity-field relation of electrons in silicon and to demonstrate the ideal transit-time-limited response of the diodes.
Conference Committee Involvement (2)
Optical Transmission, Switching, and Subsystems II
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