RF Pulse Distribution Network for a Compact Photon Pulse Source for Quantum Key Distribution

In the context of the design of a compact faint pulse source, laser-optic and RF driver circuitry for the control of the vertical-cavity surface-emitting laser diodes must be designed. Our system design comprises an array of eight VCSEL diodes. Key parameters are a spectral wavelength difference of less than 1 pm and a temporal indistinguishability of less than 1 ps, as well as a polarization purity of more than 20 dB. Especially the temporal indistinguishability requires a length-matched design of the RF transmission lines from the electronic pulse source up to the optical laser array. In a first step, experimental investigations were performed to determine the feasibility to meet this specification. For the concept verification, a set of sixteen transmission lines was designed on a LTCC substrate with a RF connector at the input and wafer probe tips at the output. A back-to-back configuration was used to measure the performance of the setup. The results revealed the necessity of matching the lengths of the lines in order to meet the synchronization requirements between the laser control pulses. A redesign was done which eventually met the specification.