Page 234 - Introduction to Information Optics

4.3. Fast Electro-optic Switches: Modulators

Data as control

Regenerated
data
Clock as data

Fig. 4.15. Optical digital regenerator.

4.3. FAST ELECTRO-OPTIC SWITCHES: MODULATORS

Modulators are devices that alter one of the detectable characteristics
(amplitude, phase, or frequency) of a coherent optical wave according to an
external electrical signal. They are essential components for converting electri-
cal data into optical ones in optical communication systems. Generally
speaking, we can make a fast optical modulator by using an electrical pulse to
induce a dielectric change in the medium through which the (carrier) optical
signal is to pass. In electro-optic (EO) modulators, an external electrical signal
modulates the gain, absorption, or refractive index of the medium. The output
optical signal, therefore, is altered according to the external electrical signal.
Research on electro-optic modulators started with bulk devices based on the
electro-optic effect that changes the optical properties of the medium. Modu-
lation bandwidths on the order of gigahertz were achieved. However, there are
limitations related to diffraction effects and drive power in bulk devices with
small transverse dimensions. Thin film and waveguide modulators were inves-
tigated both for their compatibility with fibers and for their potentials for
optical integration. Optical waveguide modulators normally need much
smaller drive power than bulk ones. In this type of modulator, the optical beam
is coupled into a waveguide the refractive index of which is higher than that of
the substrate and of any covering layer. Materials suitable for waveguide
modulators are those that possess desired optical properties (for example, a
good electro-optic merit figure), and are also capable of being configured in a
waveguide. There are two main types of waveguide structures, planer wave-
guides and stripe waveguides. Waveguides have been produced by sputtering,
epitaxial layers, ion implantation, ion exchange, and diffusion; the main