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What is Intensity Modulation?
Intensity Modulation (IM) is a technique used in photonics to vary the optical power or brightness of a light source over time in accordance with an electrical input signal. Unlike phase modulation, which alters the timing of the wave's oscillations while keeping power constant, intensity modulation directly turns the light stream "on" and "off" or scales its amplitude smoothly between a maximum and minimum level.
Its primary purpose is to convert electrical data into distinct optical pulses, serving as the foundational format for digital optical networks, pulse shaping, and time-resolved optical sensing.
HOW Intensity Modulation Works
In high-speed photonic applications, intensity modulation is achieved externally by splitting and recombining light using an interferometric structure within an Electro-Optic Modulator (EOM), running through three main steps:
Wavefront Splitting
A continuous-wave (CW) laser beam enters the input waveguide of a Mach-Zehnder interferometer. The single channel splits evenly into two parallel waveguide arms (branches), dividing the optical power into two identical, in-phase traveling waves.
Differential Phase Shifting
An electrical drive voltage is applied to electrodes running alongside one or both of the waveguide branches. Via the electro-optic effect, this voltage alters the refractive index of the crystal substrate, accelerating the light wave in one arm while decelerating it in the other, creating a controlled phase difference between the two paths.
Interferometric Recombination
The two waves meet at the output Y-junction where they recombine. If the voltage creates a 0° phase difference, the waves combine constructively, resulting in maximum light intensity (On). If the voltage creates a 180°C (180°) phase shift, the waves combine destructively and cancel each other out, resulting in zero light output (Off).
