Bipolar photodiode module operated at 4 K

An optoelectronic module for generating high frequency bipolar current pulses at 4 K was developed. Bipolar signals are generated by connecting a pair of photodiodes in series to coplanar waveguides on silicon. The transmission line pulse propagation was simulated in COMSOL. A planar optical fiber connector was fabricated, where the end fiber faces where angle polished to reflect the beam perpendicular to the fiber. An assembled prototype was fabricated by flip-chip bonding photodiodes to the silicon photodiode carrier. The optical connector was aligned and bonded using epoxy. The prototype was tested using a mode-locked laser producing pulses with sub-20 ps pulses. Bipolar output pulses were generated at 4 K and measured using an oscilloscope. The measured output pulses were affected by ringing and were broader than the reference signal. Comparison with simulations indicate that the ringing was caused by reflections at wirebonds.