|
An InGaAs/InP coupled quantum well layer sequence specially designed for Mach-Zelmder interferometric space switching is presented. Each coupled quantum well consists of three 27A InGaAs strained quantum wells separated by 1 iA InP barriers. The structure shows a red shift of the absorption edge as high as 8Onm with 1OV applied bias. Using these coupled quantum wells, we realized a Mach-Zehnder interferometric space switch with low attenuation and a switching voltage of 3 .1V for 4mm long phase shifting sections. Furthermore, we realized full polarization independent switching using 0.85% tensile strained and coupled quantum wells. However, we found that the electrorefraction in this structure was not optimal since the red shift of the lowest confined level and the blue shift of higher confined levels yield opposite contributions to the electrorefraction. This compensation can be circumvented in asymmetrical coupled quantum wells resulting in a 1 0 times larger electrorefraction. Coupled quantum wells (CQW) increase the degree of freedom in material design. For instance, the value ofthe bandgap, the total CQW well width and the bias induced change in overlap between electron and hole envelope wavefunctions can be tuned. These tools can be used to optimize a material design for specific applications. In this paper we first present a MZI space switch using a CQW design with optimized Quantum Confined Stark Effect (QCSE) bandgap shift. Secondly we will introduce an improved layer design using asymmetric coupled quantum wells. These wells are optimized for large bias induced changes in the electron-hole envelope wavefunction overlap. The most important design constraints for a material for a Mach-Zehnder interferometric (MZI) space switch at 1 .55tm are waveguide transparency, physical length and polarization independence. When using QCSE tuning sections with InGaAs/InP quantum wells, the QW-thickness is limited to a maximum of 40A for preserving waveguide transparency. On the other hand, such a small QWthickness is far from ideal for an appreciable QCSE resulting in a small index ofrefraction change and thus in long switches. The bandgap shift due to the Quantum Confined Stark Effect increases rapidly with increasing quantum well width [1]. The QCSE in a CQW structure with three 27A InGaAs quantum wells and very thin 1 1A InP barriers is similar to the QCSE in a single 103A quantum well (3x27A InGaAs + 2x1 iA InP). This is due to the excellent coupling of the carrier wavefunctions between the neighboring 27A quantum wells. The inset in figure 1 shows such CQW structure with the envelope wavefunctions of electron and hole ground level. These CQW's combine a room temperature bandgap at 1 390nm, necessary for waveguide transparency at 1 550nm, with a total CQW well width of 103A required for an optimized QCSE red shift
|
