Electro-optics is a branch or topic in photonics that deals with the modulation, switching and redirection of optical signals. These functions are produced through the application of an electric field, which alters the optical properties of a material, such as the refractive index. The refractive index refers to the speed of light propagation in a medium relative to the speed of light in a vacuum.
Modulators vs. Switches
In a number of situations, the same device may function as both a modulator and a switch. One dependent factor on whether the device would be suitable or not for a switch as opposed to a modulator would be the strength of the effect that an electric field may have on the device. If the device’s primary role is to impress information onto a light wave signal through temporary varying of the signal, then it is referred to as a modulator. A switch on the other hand either changes the direction or spatial position of light or turns it off completely.
Theory of Operation
The electro-optic effect presumes the dependence of the refractive index on the the applied electric field. The change in refractive index, although small allows for various applications. For instance, a lens may be applied an electric field and depending on the material and the applied field, the focal length of the lens can change. Other optical instruments that utilize this effect may also see use, such as a prism. A very small adjustment to the refractive index may still produce a delay in the signal, still large enough to detect and, if information was implied by the delay that was produced on the signal, the delay can be phase demodulated at the receiving end.
Electroabsorption is also another effect that is used to modify the optical properties of a material by the application of an electric field. An applied electrical field may increase the bandgap of the optical semiconductor material, turning the material from optically transparent to optically opaque. This process is useful for making modulators and switches.
Kerr Effect and Pockels Effect
The Pockels Effect and the Kerr Effect both account for the change in refractive index through the application of an electric field. The Kerr Effect states that this effect is nonlinear, while the Pockels Effect states that the effect is linear. Although the Pockels Effect is more pronounced in Electro-optical modulator design, both are applied in many situations. The linear electro-optic effect exists only in crystals without inversion symmetry. The design of electro-optic modulators or switches requires special attention to the waveguide material and how the electric field reacts with the material. Common materials (also maintaining large Pockels coefficients) are GaAs, GaP, LiNbO3, LiTaO3 and quartz. The Kerr Effect is relatively weak in commonly used waveguide materials.
Properties of the Electro-Optic Modulator
Important for both modulators and switches is the modulation depth, also known as the modulation index. Modulation depth has applications for the several types of optical modulators, such as intensity modulators, phase modulators and interference modulators. The modulation depth may be conceptually understood as the ratio of effect that is applied to the signal. In other words, is the modulation very noticeable? Is it a strong modulation or is it a weak modulation?
The bandwidth of the modulator is critically important as it determines what range of signal frequencies may be modulated onto the optical signal. Switching time or switching speed may be equally applied to an optical switch.
Insertion loss of optical modulators and switches is a form of optical power loss and is expressed in dB. However, the result of insertion loss often results in the system requiring more electrical power and would not explicitly reduce performance of the modulation or switching function of the device.
In distinction from the electric field, a modulator or switch also needs a power supply for itself. The amount of power required increases with modulation frequency. A common figure of merit is the drive power per unit bandwidth, typically expressed in milliwatts per megahertz.
References: , ,