The angle of linearly polarized laser light emitted from the laser diode is adjusted with the half-
wave plate. This have-wave plate can be inserted rotated by 180° to manipulate the down-
converted state. In the default position the pump laser enters through the circular aperture on
the back. The nonlinear down-conversion crystals are mounted at the focus of the laser beam.
Additionally, a pinhole with two adjacent crosses is mounted in the pump beam path. This
enables the realignment from scratch in case the couplers completely lost signal (see Figure 8).
All the components of the down-conversion block are positioned along the two down-
conversion emission directions and form two arms. Either arm comprises a mirror, a rotatable
polarizing filter optionally motorized, a long-pass filter, an adjustable optical coupler and a
polarization maintaining (PM) single-mode fiber (12). Long-pass filters are inserted into the
paths of down-conversion photons to block the residual laser-diode light and stray light.
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