Compact polarization-independent Mach-Zehnder space switch combining carrier depletion and the quantum confined Stark effect

We have developed an n-doped InGaAs-InAsP quantum well between InP, which is suited for a polarization-independent Mach-Zender interferometric (MZI) space switch operating at 1.55 μm. The InAsP is compressively strained and the InGaAs is tensile strained for polarization independence and for strain balancing. The important boundary condition for the design of this structure is the waveguide loss, which we limit to 0.6 dB/cm, and the crosstalk due to imbalance in the MZI, which we limit to <-30 dB. To reduce the size of the phase shifting region, while imposing this boundary condition, we combine the quantum confined Stark effect (QCSE) effect and the carrier-depletion effect by using an n-doped quantum well. The QCSE was first optimized for an undoped InGaAs-InAsP quantum well. A polarization independent Δn of 7.8·10^-4 at 100 kV/cm was obtained at the expense of 0.2-dB/cm excess waveguide loss and 0.1-dB/mm electroabsorption loss. The carrier-depletion effect in a 2·10¹¹cm^-2-doped QW increases Δn with a factor 2.6 to 2·10^-3, at the expense of 0.4-dB/cm free-carrier absorption-induced waveguide loss. The combination of the QCSE and carrier depletion results in a phase-shifter length of 0.46 mm for an MZI in push-pull configuration.