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Table 1

High-level requirements and performance of the GRAVITY integrated optics beam combiners (lines 2–5) and of the IO assembly (lines 6–11).

Quantity Requirement Performance Verification
Input waveguide spacings 1 mm–3 mm–1 mm 1 mm–3 mm–1 mm Design (Sect. 3.1)
Output waveguide spacing 180 μm 180 μm Design (Sect. 3.1)
Phase shift between ABCD outputs 90 ± 10 85 ± 3° Measurements (see Sect. 4.1)
Mode field radius 3.8 μmb 3.9 ± 0.1 μm Measurements (see Sect. 4.2.1)
Operation wavelength range [1.9 μm; 2.45 μm]c [1.9 μm; 2.45 μm] Measurements (see Sect. 4.3.4)
Operation pressure 10−6 mbar 10−6 mbar Design
Operation temperature 200 K 200 K Design
Transmission (with connections) ≥50%a,d ≥55% Measurements (see Sect. 4.2.1)
Instrumental contrast ≥95%a,e ≥95% Measurements (see Sect. 4.3.2)
Dispersion ≤5 μrad.cm2f ≤3 μrad.cm2 Measurementsg

Notes. (a) Over the K band. (b) At λ = 2.15 μm, this translates in a numerical aperture (NA) which encircles 95% of the emitted flux of the Gaussian intensity distribution of 0.218. (c) K band and metrology laser wavelength (1.908 μm). (d) Goal: ≥55%. (e) In polarized light. (f) For the entire fibre chain, including Fibre Control Unit. (g)This value includes the Fibre Control Unit that is the main contributor to this quantity due to the longer fibre length (see Paper II for details).

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