Vertical Cavity X-Modulators

Recently, we have experimentally developed two types of ideal optoelectronic switches using Fabry-Perot vertical, III-V semiconductor structures. Zero chirp reflectivity modulators operate as pure optical amplitude switches;1 vertical cavity phase flip modulators (VCPPM) operate as pure optical phase switches.2 The later display large intensity output in both of the device’s states and switching occurrs by altering phase between 0° and 180°. In this work we describe a photon conserving, reversible intensity switch. In this device, known as an X-modulator, the incident power is either transmitted through or reflected from the device. Reversibility indicates that the device is designed so that the outputs for light incident from both the top and bottom of the structure are nearly identical. The device ideally does not modify total amplitude or phase, but merely routes incident photons. Thus the X-modulator provides the third of the three desirable optical modulators: amplitude modulator, phase modulator, direction modulator. By being both conservative and reversible, the device is uniquely capable of performing complex optical switching, routing, and logic.3 The device is essentially a vertical cavity, electro-optical realization of an X-gate (sometimes referred to as a Fredkin Gate) which is a primitive structure into which all logic functions can be decomposed. The first experimental device, when switched with an applied bias of 40V, modulates from (T ~ 60%, R~ 6%) to (T ~ 6%, R~ 60%). We will discuss individual device design including the optimization of the device parameter space: lower voltage, optical bandwidth, modulation ratio, insertion loss, and how to match these devices to electronics. In addition, theoretical and experimental results of stacked devices and the use of graded buffers to reduce electric field requirements will be explained as will issues such as device crosstalk in cross bar switches.