In a typical optical link, data is first transferred from the electrical to the optical domain using an or a directly modulated laser. An electro-optic modulator can vary the intensity and/or the phase of the optical carrier. In silicon photonics, a common technique to achieve modulation is to vary the density of free charge carriers. Variations of electron and hole densities change the real and the imaginary part of the refractive index of silicon as described by the empirical equations of Soref and B.
[pdf] Attenuators are usually made from simple networks. between different resistances forms adjustable stepped attenuators and continuously adjustable ones using. For higher frequencies precisely matched low networks are used. Fixed attenuators in circuits are used to lower voltage, power, and to improve.
[pdf] 
