High speed, low driving voltage vertical cavity germanium-silicon modulators for optical

1.3 Research on Optical Modulators
Typical modulators can be categorized as amplitude modulated or phase modulated. In 
practical applications, amplitude modulation is normally used, because it is hard for 
photo detectors to distinguish phase change. The studies of different modulation 
mechanisms and recent advances on silicon based modulators are introduced below. 
1.3.1 Modulation Mechanism Study 
1.3.1.1 Thermo-Optic Effect 
The thermo-optic effect is the thermal modulation of the refractive index of a material. 
The modulation of the refractive index of a material depends on its thermo-optic 

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coefficient α and can be expressed as n(T)=n
0
+αT. In a modulator, thermal optical 
modulation is typically applied using a Mach-Zehnder (MZ) structure. When two light 
beams pass through the two arms of the MZ structure, one of the arms can be heated, 
which changes the index, leading to different optical path lengths. One of the beams 
will change accordingly. Therefore the superposition of the two beams at the other end 
will be modulated. Si MZ modulators based on SOI have been studied extensively 
[22-24]. The disadvantage of this structure is the high power consumption and slow 
transition time. (~10ns) 
1.3.1.2 Electro-Optic Effect 
The change of a material’s refractive by an electric field is called the electro-optic 
effect. When the refractive index depends linearly on electric field, it is called the Kerr 
effect; if the relationship is quadratic, it is called the Pockels effect. The first mature 
technology using this effect was in lithium-niobate modulators [25]. The electro-optic 
effect is relatively weak, so another stronger effect was studied for high-speed 
operation [26-28]. The MOS capacitor on SOI was used. The refractive index is tuned 
using carrier injection in MOS transistors in a MZ structure. Ring structures have been 
thoroughly studied as well [29].