Strip vs rib waveguide
The two standard cross-sections of integrated waveguides: fully etched strip for tight confinement and small bends; partially etched rib for lower loss and the slab access that electrical devices need.
Most PIC platforms offer the same silicon (or nitride) layer in two etch depths, and the choice threads through every layout decision.
Strip (channel) waveguide: full etch to the buried oxide; the core is a rectangle clad in oxide (SOI standard: ~450–500 nm wide × 220 nm tall). Maximum index contrast in both axes gives the tightest mode (~0.1 µm² in silicon) and the smallest usable bends (a few microns of radius), which is what dense routing, small rings, and compact couplers are made of. The cost is proportionality between confinement and sidewall interaction: the mode presses against two full-height etched walls, and sidewall roughness scattering dominates loss (order 1–2 dB/cm in typical foundry silicon strip).
Rib (ridge) waveguide: partial etch (SOI standard: 220 nm layer with a ~90–110 nm slab left). The mode sits mostly under the ridge but spreads into the slab, touching less etched surface: loss drops (sub-dB/cm figures are routine where strip loses more), power handling improves, and (the decisive feature) the continuous slab provides a lateral electrical path. Carrier-depletion modulators, PIN phase shifters, and detectors are built almost exclusively in rib, with doped regions in the slab flanking the junction under the ridge. The price: weaker lateral confinement demands bend radii of tens to hundreds of microns, and the slab can leak light between adjacent structures if layout ignores it.
Design practice uses both, converting between them with short strip↔rib tapers: strip for routing, bends, and passive density; rib for active devices, low-loss straights, and higher power. Single-mode conditions differ (the rib's effective lateral contrast depends on etch ratio; classic large-cross-section rib designs stay single-mode at dimensions where a strip would be massively multimode), and polarization behavior differs too, so the conversion tapers and the etch-depth choice are proper design variables, not afterthoughts.