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.