chapter  5
22 Pages

Electroabsorption Modulators

ByHaruhisa Soda, Ken Morito

As the ˜rst generation of electroabsorption modulators, very simple bulk-type electroabsorption modulators were used. ™is modulator was based on the Franz-Keldysh eŸect. It comes from an electric ˜eld-assisted tunneling of electrons into the band-gap of the semiconductor material. When the electric ˜eld is applied to the semiconductor, the eŸective absorption edge moves to lower energies. Figure 5.1a shows the theoretical absorption characteristics of indium-gallium-arsenide-phosphide (InGaAsP) having 1.50-μm band-gap wavelength calculated by Aspnes’ theory [1]. It was found that the light with lower energies than the band-gap of the semiconductor can be modulated by the applied electrical signal. Actually, high speed and low voltage operation were achieved successfully in semiinsulating buried heterostructure (BH) structure InGaAsP electroabsorption modulator [2]. Such bulk-type InGaAsP/ InP electroabsorption modulators were put to practical use in long-distance 2.5-Gb/s transmission systems due to the lower chirp characteristics [3] compared to a directly modulated distributed feedback (DFB) lasers. However, it was very dišcult to obtain a negative chirp parameter, which is indispensable

References ......................................................................................................149

for long-distance 10-Gb/s transmission applications. Figure 5.1b shows the calculated chirp parameters of such bulk-type InGaAsP/InP electroabsorption modulators. ™e chirp parameters were ˜xed to be almost positive values for low insertion loss regions. A multiple quantum well electroabsorption modulator resolved this engineering problem and realized current long-haul 10 Gb/s transmission system.