Routing Light on a Chip
Photonic optical switches route light between waveguide ports without converting to electrical signals, enabling transparent optical networking. The directional coupler — two parallel waveguides in close proximity — is the simplest and most widely used switching element. Evanescent field overlap between the guides creates periodic power exchange, and by perturbing one guide's refractive index, light can be steered to either output port on demand.
Coupled-Mode Theory
The physics of the directional coupler is elegantly described by coupled-mode theory. Two identical waveguides exchange power sinusoidally with propagation distance, with full transfer occurring at the coupling length Lc = π/(2κ). Detuning — making the guides non-identical by changing the refractive index — reduces the maximum power transfer and shortens the beat length. The coupling coefficient κ decreases exponentially with gap spacing, making precise fabrication essential.
Switching Mechanisms
Three main effects drive optical switches in integrated photonics. The thermo-optic effect (dn/dT ≈ 10⁻⁵/K in silicon) is simple and reliable but slow (microseconds) and power-hungry. The electro-optic Pockels effect in lithium niobate is fast (picoseconds) but requires centimeter-scale devices. Free-carrier dispersion in silicon offers a middle ground — nanosecond switching with compact footprints, though it introduces optical loss from carrier absorption.
Large-Scale Switch Fabrics
Optical data centers and telecom networks demand switches with hundreds of ports. These are built by cascading 2×2 directional coupler elements in Benes or Clos network topologies. Silicon photonic switches with 32×32 ports have been demonstrated on a single chip. Key metrics include insertion loss (cumulative through the cascade), crosstalk (determined by individual element extinction), and reconfiguration speed (set by the switching mechanism).