Silicon Photonics: Fundamentals and Devices

7.5 Modulators: Some Useful Characteristics

Before discussing the construction and performance of the actual Si-based modulators, it may be useful to have some introductory discussion on the basic figures of merit of a modulator or switch 13.

In many cases the same device can function as either a modulator or a switch, depending on the strength of interaction between the optical wave and the controlling electrical signal. The device is a modulator if the controlling signal changes one of the properties of the medium to effect a change in the light signal emerging out of the device. A switch, on the other hand, changes the spatial position of the light, or else turns it off and on. Both the devices possess the same figures of merit, some of which are discussed throughout this section.

7.5.1 Modulation Depth

The presence of a controlling signal s(t) causes the optical output of the device, I₀, to vary between maximum and minimum values. The modulation depth is defined as

(7.16)

For a switch, this modulation depth should approach unity. For analog modulation, a modulation depth much less than unity may sometimes prove satisfactory.

7.5.2 Modulation Bandwidth

For analog modulation, the bandwidth, that is, the range of frequencies over which the device can be operated, is an important parameter. By convention, the bandwidth of a modulator is the difference between upper- and lower-half ...