Ultrafast EAMs (Electro-Absorption Modulators) which can function in the femtosecond-timescale are increasingly required for attaining ultrahigh-speed data transmission of ∼400Gb/s per channel for the next generation of telecommunication networks. The achievable bandwidth in conventional InP-based EAMs functioning based on IBTs (Interband Transitions) is ∼80Gb/s. This restriction of speed is due to the comparatively slow interband recombination lifetimes caused by the limited absorption saturation recovery time restricted by the sweep-out time of the photo-generated carriers. Alternatively, InP-based EAMs functioning based on ISBTs (Intersubband Transitions) in QW (Quantum Well) owing to the ultrafast intersubband recombination lifetimes could offer enormous bandwidth of ≥400Gb/s per channel. This thesis firstly describes the design and modelling of an ultrafast waveguided EAM based on ISBT in InGaAs/AlAs/AlAsSb asymmetric coupled double quantum well lattice-matched to InP at telecommunication wavelength (λ=1.55 μm). The investigated EAM is expected to have a RC-limited speed (f_(3dB )) of ∼300GHz, 5.1 dB insertion loss, 10 dB extinction ratio, and 5.18 dB/V modulation efficiency at a peak-to-peak voltage of 2.0 V which can support a data rate of ≥600Gb/s. Furthermore, electro-absorption modulation characteristics of various InAs-GaAs QD (Quantum Dot) waveguides on silicon substrate at wavelength ranges of 1290-1340 nm are measured. The 400 µm AR (Anti-Reflection coated) InAs-GaAs QD waveguide at 1302 nm has 2.2 dB extinction ratio, 0.7 dB/V modulation efficiency, and 19 dB insertion loss at a peak-to-peak voltage of 3.0 V. The anticipated modulation bandwidth is ∼2GHz which can support a data rate of ≥4Gb/s. The modulation bandwidth is limited due to the large device capacitance and relatively large active resistance; therefore, it could be significantly improved by decreasing the waveguide length and waveguide width, and by increasing the top and bottom contacts widths. ISBTs-based EAMs due to the ultrafast intersubband recombination lifetimes offer significantly larger modulation bandwidth compared to IBTs-based EAMs.