Reliable Packaging and Development of Photodiode Module for Operation at 4 K

Abstract

AC voltage standards based on pulse-driven Josephson junction arrays, operated at 4 K, enable quantum-accurate generation of arbitrary voltage waveforms. These devices are operated by biasing the arrays with high-frequency current pulses. Utilizing an optically controlled current source, consisting of a modulated laser source and optical fibercoupled photodiodes, operated at 4 K, rather than an electrical link, is expected to have several advantages in terms of performance.

The PhD-project has focused on the development of an optoelectronic module for realizing this concept. Packaging techniques and characterization of the module was investigated. In particular, designs and materials for reducing thermal stress at 4 K were considered.

A packaging technique for assembling multiple optical fiber-coupled high-speed photodiodes on silicon substrates was demonstrated. The assembly was shown to have sufficient precision and to be robust against thermal stress. Finite element simulations of the thermomechanical stresses were performed in order to validate the robustness claim.

The frequency response of commercial InGaAs/InP photodiodes up to 14 GHz, as well as DC-response, was measured at room temperature and at 4 K. It was shown that the effect of low temperature did not negatively affect the frequency response.

The thesis also includes a review of devices, packaging techniques and materials for cryogenic optoelectronics. In addition, initial work on semiconductor simulations of photodiodes at low temperature is discussed.