Automatic active alignment of substrate-free thin-film filters on a photonic platform using single photon detectors

We present an automated active alignment procedure for assembling miniaturized photonic quantum circuits specifically designed to handle single-photon-level signals (i.e. low photon count). This process is exemplarily used for producing a polarization-based encode-and-measure quantum key distribution emitter. For this, we use the integration of miniaturized substrate-free thin-film filter elements into laser-induced deep-etched pockets on a photonic platform. The filter elements function as splitters to combine/divide four beams with different linear polarization states, as required for the BB84 quantum communication protocol. Bare-die laser diodes bonded to the same platform are used to create the single-photon level signals. The single filter chips are placed under an angle of 45 degrees to the propagation direction of the photon radiation vertically into the deep-etched pockets and fixed on the platform surface with UV-curable adhesive. The resulting signal is subsequently coupled into a single-mode fiber. For active alignment, a single photon avalanche detector is used in a feedback loop with a precision-optics assembly system, exploiting six degrees of freedom for the alignment of the assembly with stacked translation and rotation stages. The single photon detector is connected to an oscilloscope where a single voltage peak signals the detection of a photon. Aligning the filter elements changes the number of photons detected. The average voltage is a measure for the number of photons detected per integration time and used for the active alignment loop. This technique enables the active alignment of optical components for single photon-level signals, otherwise not detectable with conventional power meters.