*4.4. Auxiliary Measurements*

A split-off fraction of the incoming beams enters the auxiliary unit (details Figure 7) through a separate side window in the lens tube of the sending unit, where they are again split by a 45:55 pellicle beam splitter (coated for 3–5 μm; Thorlabs, BP145B4). Depending on the wavelength, they are then focused with parabolic mirrors (Thorlabs, MPD00M9- M01, *f* = 15 mm) on an InGaAs free-space amplified photodetector (Thorlabs, PDA20CS2, 0.8 mm2 active area, max. 11 MHz bandwidth) or a TE-cooled IR photovoltaic detector (Vigo Systems, 1 mm active diameter, 1 MHz bandwidth), respectively. A flip mirror in front of the beam splitter can in addition switch the optical path towards a MIR wavemeter (Bristol 621B) for monitoring the absolute wavelength.

**Figure 7.** Schematics of the auxiliary unit.
