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What is PHASE MODULATION?
Phase Modulation (PM) is a technique used in photonics to alter the instantaneous phase of an optical carrier wave in accordance with an electrical data or drive signal. Unlike amplitude modulation, which changes the brightness or intensity of the light, phase modulation shifts the timing of the wave's oscillations while keeping its optical power and frequency constant.
Its primary purpose is to encode high-speed digital data onto light waves for telecommunications, or to introduce precise, dynamic optical delays required for advanced sensing and quantum frequency manipulation.
HOW Phase Modulation works
In modern photonic systems, optical phase modulation is typically achieved using the linear electro-optic effect (Pockels effect) inside a specialized crystal waveguide, operating in three distinct steps:
Carrier Wave Injection
A continuous-wave (CW) laser beam with a stable, unmodulated phase enters the optical waveguide of an Electro-Optic Modulator (EOM). The waveguide material is typically made of an anisotropic crystal, such as Lithium Niobate (LiNbO3).
Voltage Application
An electrical drive signal (RF signal) is applied via electrodes positioned adjacent to the crystal waveguide. This applied voltage generates an electric field that dynamically alters the refractive index of the crystal substrate.
Phase Shifting
As the light travels through the modified refractive index zone, its velocity changes. A higher refractive index slows the wave down, delaying its wavefront, while a lower index speeds it up. This controlled acceleration or deceleration creates a precise shift in the exiting wave's phase, directly proportional to the applied voltage.
