Design and analysis of In0.53Ga0.47As/InP symmetric gain optoelectronic mixers

Optoelectronic mixing devices mix a modulated optical signal with a reference electrical signal to obtain an electrical low frequency difference signal. These devices are particularly attractive for chirped-FM laser detection and ranging (LADAR) systems. The Army Research Laboratory (ARL) has been developing chirped-FM LADAR systems for applications such as reconnaissance, terrain mapping, force protection, facial recognition, robotic navigation and weapons fuzing [1]. Signal processing of a chirped-FM LADAR system is simplified if the photodetector in the receiver is used as an optoelectronic mixer (OEM). While a DC biased phototransistor can be used as an optoelectronic mixer [2], a symmetric I–V characteristic photodetector allows driving the OEM directly with the local oscillator (LO) signal, without a DC bias [3]. Sensitivity to background light is reduced, as the response from background light averages to zero. An additional 3 dB signal processing gain is also obtained. As the OEM output is the low frequency difference signal, the gain of the following transimpedance amplifier (TZA) can be increased, improving LADAR performance. ARL has previously demonstrated chirped FM LADAR systems with GaAs and InGaAs metal-semiconductor-metal (MSM) Schottky photodetector OEMs for operation at the 800 nm and 1550 nm wavelengths [3,4]. A symmetric photodetector with gain would improve overall system performance, while preserving the advantages offered by MSM OEM devices. We previously reported on symmetric gain optoelectronic mixers based on a symmetric heterojunction phototransistor using In 0.52 Al 0.48 As/ In 0.53 Ga 0.47 As heterostructures [5]. In this work, the In 0.52 Al 0.48 As wide bandgap layers are replaced with InP to improve thin film growth quality and device performance.