Silicon Photonics: Fundamentals and Devices

7.2 Physical Effects

In an intensity modulator, the light intensity is controlled or modulated in accordance with the change of a controlling or modulating signal, usually in the form of an electrical wave. In optical communication, the intensity of the carrier wave is changed by the modulating signal derived from speech, music, a video signal, or computer data. The phase modulator changes the phase of the carrier wave, in conformity with the amplitude of the modulating signal impressed onto the material. The modulating signal basically changes either the absorption coefficient or the refractive index (RI) of the material. When an electrical signal causes such changes, the change in absorption is termed as electroabsorption, while the change in RI is called electrorefraction.

Other methods exist for introducing a change in the RI of a semiconductor. An acoustic wave impressed upon certain materials may change their RI through the acousto-optic effect. In order that the effect produces perceptible change in the RI, the acousto-optic coefficient of the material must be very high. Another viable method of producing a RI change, particularly suitable for Si and its alloys, is to heat the material. The RI change occurs due to a thermo-optic effect. An optical field of high intensity may induce a change in RI of a material by nonlinear effects; sometimes the effect is called an opto-optic effect.

In the present section, we shall discuss the methods of changing the absorption and RI in ...