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Four-port frustrated total internal reflection couplers in InP-based GalnAsP quantum-well substrates are realized and characterized. Each coupler forms an "X" at the perpendicular intersection of two ridge waveguides and is aligned 45° to the optical path. The 180-nm-wide couplers are fabricated by dry etching deep trenches through the quantum wells and backfilling with alumina (n = 1.71) by atomic layer deposition. Coupling coefficients for the fabricated coupler are in good agreement with a three-dimensional finite-difference time-domain theory, and an 82% coupler efficiency is estimated.