Ultracompact, VCSEL-based photon source for prepare-and-measure QKD

We present the design and first performance evaluation of an ultra-compact, hybrid faint pulse source (FPS) at 850 nm, making use of a linear eight VCSELs array at spectral (<1 pm wavelength difference, and >90% overlap at FWHM) and temporal (<1 ps) indistinguishability, as well as polarization quality in the four H/V/D/A BB84 channels >15..20 dB. A VCSEL array on a single substrate is used, at a pitch of 250 μm and with integrated polarizers, having a spectral indistinguishability, if the substrate is temperature levelled < 0.5 K. Each VCSEL represents either faint or full amplitude signal for the H/V/D/A channels of the BB84 protocol. The temperature levelling heatsink is made of Molybdenium, integrated on LTCC board to host the emitter substrate and its respective DAC driving circuit at speeds up to 6 GHz. VCSEL integrated micro-lenses and two additional micro-lens arrays fully collimate the beams and refocus them into a waveguide combiner chip which realizes the polarization independent coupling of all eight VCSEL free-space beams with a pulse delay variation <1 ps.