Packaging of fiber-coupled optoelectronic module for cryogenic voltage standards

The Josephson Arbitrary Waveform Synthesizer (JAWS) is currently being developed for generating precise voltage waveforms, for use in measurement standards. These system is based on superconducting Josephson junction arrays, and are operated by controlling the JAWS with high-speed (>10 GHz) current pulse trains in a cryogenic environment at 4K. Using an optical input rather than an electrical input, has several advantages, including shielding from electromagnetic noise and easier parallel operation of Josephson arrays Therefore, a customized optical input module, where high speed photodiodes converts short optical pulses to current pulses, is being developed. The module consists of multiple lensed InGaAs/InP photodiodes are flip chip bonded to a silicon carrier with gold stud bumps and a customized fiber attachment. Early prototypes showed that chip cracking of the silicon carrier occurred due to thermal stresses caused by the fiber attachment. A technique for assembling multiple optical fibers was developed, using silicon fixtures for mechanically aligning and fixing borosilicate alignment sleeves. In order to simplify assembly, the fixture is aligned by the flip chip technique and adhesively bonded to the carrier with an epoxy. Sufficient alignment accuracy has been achieved, with an average misalignment of 13 μm between the fixture and the photodiode position. Testing of an assembled module showed promising behavior, achieving a stable optical coupling when immersed in liquid Nitrogen and having sufficient strength for operation down to 4 K.