Carrier-injection-type high-speed semiconductor optical switches have been of interest in recent years due to
their nanosecond switching times, their immunity to variations in temperature, wavelength, polarization, etc, and the
ease with which they can be monolithically integrated with other optoelectronic components and electronic circuitry.
Their drawbacks, however, have been high insertion loss and excessive power dissipation. To overcome these limits,
a novel, large cross-section, single-mode AlGaAs/GaAs optical switch has been designed and fabricated. The
switch's strip-loaded waveguide uses a five-layer W-shaped heterostructure and a 1.7&mgr;m-thick core layer, which
provides high fiber-coupling efficiency. Since the constituent heterojunction band discontinuities can impede the
current across the junction, the addition of 20nm-40nm thick, compositionally graded interfaces significantly
reduces the switching voltage. In addition, using a lightly doped core layer can reduce the series resistance of the
switch, which is important in heat reduction. The core doping needs to be low otherwise it will cause increased free-carrier
absorption, which contributes to high insertion loss. We have fabricated switches with different core doping
levels using both abrupt and graded heterojunctions. The measured on-chip optical propagation losses are 0.3dB/cm
for unintentionally doped core, 1.5dB/cm for n = 1x10<sup>16</sup>cm<sup>-3</sup> doped core, and 2.7dB/cm for n = 5x10<sup>16</sup>cm<sup>-3</sup> doped
core. The measured I-V curves show that the switching voltage can be reduced by changing abrupt heterojunctions
to graded ones. The calculated theoretical band structure for switches with abrupt/graded heterojunctions based on
thermionic emission clearly demonstrated the advantages of applying grading in semiconductor optical switches.
Optical switches based on the electro-optic effect in III-Vs such as GaAs have fast optical switching times, typically shorter than 1 ns, and thus are promising candidates for a wide variety of optical network applications ranging from fault restoration and network configuration to optical packet switching. A Mach-Zehnder interferometer (MZI) is often used to implement a 2x2 electo-optic switch in which two identical multi-mode interference (MMI) couplers connected by two identical parallel single mode waveguides (two MZI arms) with electrodes allowing to vary the phase difference between the two MZI arms based on the electro-optic effect. In this paper, we report the design, fabrication, and test of MMI couplers and 2x2 MZI-MMI optical switches based on these couplers. The waveguide structure has 5 undoped GaAs-GaAlAs layers with a 1.7um-thick core layer. In both simulation and fabrication, various values of MMI width, MMI length, and waveguide width have been considered. Both simulation and experimental results have indicated that the device performance is most sensitive to the MMI width and is less sensitive to the MMI length. From simulation and experimental results, optimized structures have been obtained for 2x2 MZI-MMI optical switches. Devices based on the optimized structure have been fabricated without electrodes. The fabricated MZI-MMI optical switches have shown very promising switching properties such as low insertion loss and low cross-talk. The propagation loss of straight waveguides is typically around 0.3 dB/cm. There is almost no measurable additional loss due to the MMI couplers. A cross-talk of - 22 dB has been achieved.