Peripheral-coupled-waveguide multiple quantum well electro-absorption modulator for high efficiency, high spurious free dynamic range and high frequency RF fiber-optic link

Currently the high-capacity long-span terrestrial and undersea transmission links are all fiber-optic links. For analog fiber-optic links, in order to have high RF link gain, large spurious free dynamic range (SFDR), small noise figure and large bandwidth, the requirements for the modulator are small insertion loss, large modulation efficiency, high optical power handling capacity and large bandwidth.; Electro-absorption modulators (EAM) play an important role in the analog fiber-optic link due to its large modulation efficiency, large bandwidth and small size. But in conventional EAMs, the electrical waveguide and the optical waveguide share the same waveguide structure. Many trade offs are made in order to meet both the electrical requirements and optical requirements. These trade-offs are: (1) The waveguide width is limited to 2--3mum. This narrow waveguide width reduces the coupling from/to the optical fibers. It also causes large optical propagation loss. Thus the insertion loss is big. The large propagation loss also limits the device length to 200--400mum and eventually limits the modulation efficiency. (2) The active intrinsic electro-absorption (EA) layer thickness is limited to 0.2--0.4mum. This prevents us from getting large modulation efficiency. (3) The device capacitance (in lumped-element EAM) or capacitance per unit length (in traveling-wave EAM) with these limited waveguide width, waveguide length, and EA layer thickness are still too large to achieve large bandwidth.; In this dissertation we propose a novel peripheral-coupled-waveguide (PCW) EAM, in which the electrical waveguide and the optical waveguide structure are decoupled, so that the optimizations for both structures are allowed. The low insertion loss, large modulation efficiency (i.e. low Vpi), high optical power handling and large bandwidth can be achieved simultaneously.; The proposed PCW-EAM has an optical waveguide width of 12mum, an electrical waveguide width of 1 mum and the active EA layer thickness of 0.1 mum. The 1.3mm long non-traveling wave PCW-EAM exhibits an insertion loss of 10.2dB, an equivalent Vpi of 1.6V and the optical saturation power larger than 80 mW. The optic link using this EAM has achieved RF link gain of -3dB, multi-octave SFDR of 118 dB-Hz2/3, sub-octave SFDR of 132 dB-Hz 4/5 and estimated noise figure of 14 dB.; With limited fabrication facility, the traveling wave PCW-EAM demonstrates a 3dB bandwidth of 10GHz with the electrical waveguide width of 3mum. This number agrees well with the theoretical calculation. A much larger 3dB bandwidth can be predicted using traveling-wave PCW-EAM with improvement in metal electrode thickness and electrical waveguide width.