Quad photoreceivers, namely a 2 × 2 array of p-i-n photodiodes followed by a transimpedance amplifier (TIA) per diode,
are required as the front-end photonic sensors in several applications relying on free-space propagation with position and
direction sensing capability, such as long baseline interferometry, free-space optical communication, missile guidance,
and biomedical imaging. It is desirable to increase the active area of quad photoreceivers (and photodiodes) to enhance
the link gain, and therefore sensitivity, of the system. However, the resulting increase in the photodiode capacitance
reduces the photoreceiver's bandwidth and adds to the excess system noise. As a result, the noise performance of the
front-end quad photoreceiver has a direct impact on the sensitivity of the overall system. One such particularly
challenging application is the Laser Interferometry Space Antenna (LISA), which proposes to detect gravity waves in
space by measuring distance at 1064 nm wavelength with ~10 pm/√Hz accuracy over a baseline of 5,000,000 kilometers.
Currently, LISA's sensitivity is restricted by the noise arising from ~20 pF capacitance per quadrant demonstrated by
typical 1 mm diameter InGaAs quad photodiodes.
We present a 1 mm diameter quad photoreceiver having an equivalent input current noise density of <3.2 pA/√Hz per
quadrant up to a 3 dB bandwidth of ~20 MHz. This performance is primarily enabled by a rad-hard-by-design dualdepletion
region InGaAs quad photodiode having 2.5 pF capacitance per quadrant, which allows ~17dB improvement in
sensitivity over the state-of-the-art. Moreover, the quad photoreceiver demonstrates a crosstalk of <-52 dB between the
neighboring quadrants, which ensures a direction sensing resolution of <30 nrad in LISA.