High-power handling, ultra-fast, GRIN lens-coupled photodetectors

Abstract

The performance of microwave photonic systems can be improved by utilizing high power handling photodetectors. Operation at higher photocurrents enables larger output RF signals to be produced directly by the photodetector. This reduces the requirement of signal amplification by RF amplifiers, thereby simultaneously improving the dynamic range and the noise figure. In optical coherent systems, high power handling photodetectors enable operation at high local oscillator power levels to boost the coherent gain and the detection sensitivity. Thus, techniques to enhance the power handling capability of photodetectors are of interest for both free space and fiber based applications. Photodetector current saturates at high optical power levels due to space-charge screening effect. The saturation effect is maximized where the illumination intensity, and the resulting photocurrent density, is largest. In this work, we focus on optimizing the optical field profile incident on top-illuminated InGaAs photodiodes to minimize the peak photocurrent density. This was achieved by employing graded-index (GRIN) lens coupling to uniformly distribute the optical power across the diode cross-section. We demonstrate 5dB improvement in photodiode's power handling capability and linearity by employing GRIN lens coupling as compared to single mode fiber (SMF) coupling. Our GRIN lens-coupled photodetectors have achieved small-signal 1dB compression current of >50mA and 12.5dBm amplifier-free RF output. These devices also exhibit linear behavior for a peak-to-peak RF pulse output of >2.5V, at ~30ps pulse width. This constitutes a 100% improvement over SMF coupled devices. Further, the GRIN photodiodes demonstrate pulse broadening =0.65ps/mW, as compared to 2ps/mW for SMF devices.