The Laser Interferometry Space Antenna’s (LISAs) scientific mission to detect gravity waves demands stringent noise limits on the system components, especially the large-area quad photoreceiver front end, for ultra-high-precision (≈10 pm/Hz1/2) distance measurements. The optical LO (local oscillator) power on LISA is limited to 100 μW to keep power dissipation and thermal fluctuation low on the optical bench. Consequently, a large-area quad photoreceiver having an equivalent input current noise of the order of 1 pm/Hz1/2 is needed. Additionally, the quad photoreceiver must demonstrate low crosstalk between individual quadrants to allow accurate direction sensing of the incoming optical beam. This performance must be achieved over a bandwidth of 2 to 20 MHz to meet LISA’s requirements. Commercially available quad photodetectors do not have the combination of large area, low capacitance (and therefore low noise), and low crosstalk, which is critical for LISA’s scientific objectives.

Large-area quad photoreceivers, namely a 2×2 array of p-i-n photodiodes followed by a transimpedance amplifier per diode, are required in several applications relying on freespace optical propagation with position and/or direction sensing capability. The performance of such systems may be limited by the noise arising from the capacitance of the large-area quad photodiodes. This innovation builds on the prior invention of the dual-depletion region p-i-n photodiodes in order to simultaneously attain large area and low capacitance in a p-i-n photodiode. For example, a 1-mm-diameter quad InGaAs photodiode having 2.5-pF capacitance per quadrant was developed using this innovation. This device was used to develop a 1-mm-diameter quad photoreceiver having 17 dB better sensitivity than that of the state of the art.

This work was done by Abhay Joshi of Discovery Semiconductors, Inc. for Goddard Space Flight Center. GSC-15997-1

Photonics Tech Briefs Magazine

This article first appeared in the March, 2012 issue of Photonics Tech Briefs Magazine.

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