Silicon photonics
Which correctly describes how a Mach-Zehnder modulator converts a change in optical phase into a change in optical intensity?
- AIt splits light into two arms, creates a phase difference between the arms, then recombines them so they interfere
- BIt circulates light many times in a ring-shaped resonator so that only specific wavelengths are absorbed
- CShining phase-shifted light directly onto a photodetector turns the phase difference into a difference in current
Show answer and explanation
Answer: A. It splits light into two arms, creates a phase difference between the arms, then recombines them so they interfere
In a Mach-Zehnder modulator, incoming light is split equally into two arms, the arms' refractive indices are changed by an electrical signal to create a phase difference, and the light is recombined into one path. If the phase difference between the two beams is 0 they interfere constructively (bright); if it is π (half a wavelength) they cancel (dark); in between, the intensity depends on the phase difference. This places the signal on intensity changes that a photodetector can detect. A photodetector only outputs current according to light intensity and cannot directly detect phase differences. Using a resonator is the approach of the ring modulator.