Development of long-wavelength avalanche photodiodes and vertical-cavity lasers for epitaxial integration as a vertical-cavity photon number amplifier

The goal of this research was to develop technology for building a vertical-geometry photon number amplifier (vertical PNA) that operates at a wavelength in the low-absorption window for optical fibers near 1.55 μm. An optical amplifier of this design would provide electrically-tappable low-noise polarization-independent optical amplification of laser pulses and serve as a stepping stone toward development of a tunable amplifying wavelength converter.; The vertical PNA design consists of a multiple active region (MAR) VCSEL integrated with an avalanche photodiode of the separate absorption, charge, and multiplication layer design (SACM APD): the VCSEL is intended to operate continuous wave (CW), modulated by the APD. Both components were selected for their high gain: in excess of 10 electrons out per photon in for the APD, and slightly more than 1 photon out per electron in for the MAR VCSEL under ideal circumstances. In working toward the vertical PNA, significant technical challenges were addressed: (1) Development of a long-wavelength MAR VCSEL capable of high-temperature CW operation. Although this goal was never achieved, efforts directed to this end resulted in an investigation of basic material science issues that are vital to future improvements of the device. Better DBR and active region designs were developed, the overall thermal impedance of the structure was reduced significantly, a rudimentary optical aperture compatible with InP-based materials was tested, and loss estimates for the device were put on solid ground. (2) Development of a low-noise SACM APD capable of modulating the MAR VCSEL at high speed. Here the vital relationships between growth conditions, material quality, and APD performance were established. Other achievements include demonstration of highly uniform arrays of these devices, extremely low-noise operation, and the largest area long-wavelength APDs ever reported. (3) Successful demonstration of the epitaxial integration of these two components as a functional vertical PNA. Without a MAR VCSEL capable of CW operation, a functional vertical PNA cannot be built. Nonetheless, functional APDs and VCSELs (under pulsed operation) were separately demonstrated on vertical PNA wafers, as was a basic integration scheme.