Phase Shifters

What is a Phase Shifter?

Light signals (photonic signals) can be described using wave characteristics. Just like AC electric signals, photonic signals have frequencies, amplitudes, and phases (in addition to other properties such as polarization). Information can be encoded in photonic signals in several ways, including amplitude modulation, frequency modulation, and phase modulation.

Phase shifters are active photonic devices that enable phase modulation of photonic signals. Phase shifters can also be used to “tune” light signals, or in other words, adjust the phase of the signal as it travels through a waveguide. Generally phase shifters can be categorized as modulators or tuners. Modulators encode information into photonic signals, and tuners maintain or adjust the phase of photonic signals during transmission.

Phase Shifting in a Photonic Circuit: A Simple Application

Let’s say we have two equally polarized light signals travelling through a photonic circuit that are going to be combined using a y-bridge. Depending on the phase difference between the two signals, the signals may interfere with one another constructively or destructively. In this case, the phase of one light signal may be tuned using a phase shifter, so that the two light signals combine in some desired way.

TODO: Insert illustration of example.

How does a Phase Shifter Work?

Phase shifters are regions with indices of refraction that can be changed. When a light signals passes through a materials with a different index of refraction than the material it’s propagating through, the frequency of the light signal “expands” or “compresses” according to the difference between the two refractive indices. The frequency of a light signal travelling from a first material to a second material with a higher index of refraction will compress (i.e. the signal will oscillate more times per unit length within the new material). The frequency of a light signal travelling from a first material to a second material with a lower index of refraction will expand (i.e. the signal will oscillate fewer times per unit length within the new material). For illustrative purposes, see the figure below.

TODO: Insert figure

Notice how the relative phase of the signal changes as it passes through the material.