The pump laser excites a small spot within the nonlinar crystal, where the conversion takes place and the generated photons are subsequently coupled into single-mode fibres. Therefore the focus of the pump laser and the focusing optics of the fibres need to be matched. The optical path between the pump laser and the corresponding fibre is more than 1.5 meter. To align the overall system, different options are available:

[ManC] Manual coupling:

• With standard 100 TPI drive screws an accurate fibre and lens alignment it is possible to manually adjust the optical element in order to select corresponding pair photons. Due to a low beam height and a design with improved passive stability long-term operation over hours in optical laboratories should be possible only with minor further optimizations.

[AutoC] Automated coupling:

• This portable source of entangled photon pairs can also be used in an environment not set up especially for optical systems as in typical office workspace or even containers used in outdoor-experiments, such source was originally designed for. Therefore higher temperature changes may influence the source and the count rates as well as the quality of entanglement would decrease. For long term operation over days, the self-adjusting mode is preferable by using corresponding count-rates as a feedback signal.
• Depending on the pump laser and the used environment, temperature fluctuations within the laser diode manifest themselves as a beam pointing scintillation over time. This effect is reduced by appropriate cooling, but can not be avoided completely. Only this automated coupling option overcomes this disturbance completely within short time of the control cycle.
• Manual control with screws used in the option “Manual coupling” is still possible, if the automated feedback loop is switched off. Moreover this option “Automated coupling” additionally allows an accurate adjustment with piezo-driven actuators over a switching board.

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