*4.3. Receiving Unit*

The receiving unit is illustrated in Figure 6. An aperture (Thorlabs, SM05D5D) and a CaF2 wedged window (Thorlabs, WW50530) are fixed inside of a 12.7 mm diameter adjustable lens tube to allow the laser beam to couple directly into the N2-purged environment after passing the burner. Thus, the absorption path is confined within the flame gases and minimizes interference of ambient water vapor in the beam path outside of the flame. Optics and detectors of the receiving unit are contained inside an aluminum box

(10-mm wall thickness) with a 25.4 mm diameter optical access port originally designed for shielding the probe from excessive acoustic noise and thermal load when placed close to the gas turbine test site. Lens tubes and the whole receiver box are purged with dry nitrogen supplied through a Vortex tube (Vortec, 208-25H, Cincinnati, OH, USA) that simultaneously keeps the internal atmosphere cooled to approx. 293 K even at elevated environmental temperature.

**Figure 6.** Schematics of the receiving unit.

The four incoming collinearly propagating transmitted laser beams are split by a 45:55 pellicle beam splitter (Thorlabs, BP145B4), originally designed for the 3–5 μm wavelength range, which also has a transmittance for the NIR-lasers of 85% and 63% near 1392 and 1469 nm, respectively. In the MIR detection beam path, a second 3–5 μm 45:55 pellicle beam splitter delivers a reflected and transmitted beam towards two TE-cooled IR photovoltaic detectors (Vigo Systems, 2 mm diameter active area, 1 MHz bandwidth), where they are focused with parabolic mirrors (Thorlabs, MPD019-M01). Two narrow bandpass filters (Spectrogon, NB4560-135 nm as M1 and Laser Components, SNB-4860-001793 as M2 in Figure 6) transmit the respective beams and block unwanted background emission from hitting the detectors. The fraction of the NIR beams transmitted through the first pellicle beam splitter to the NIR detection side is again divided into two separate beams with a third 45:55 pellicle beam splitter for 1–2 μm (Thorlabs, BP145B3) and directed to two InGaAs free-space amplified photodetectors (Thorlabs, PDA20CS2, 3.14 mm<sup>2</sup> active area, max. 11 MHz bandwidth). Narrow-bandpass spectral filters (Thorlabs, FB1480-12 as N1 and FB1400-12 as N2 in Figure 6) are mounted in front of the respective detectors. All the optics are fixed on a threaded aluminum breadboard (300 × 300 mm2) inside the closed box.
