Open Access
Table 1
Main characteristics of the fibers and Fiber Control Unit elements of GRAVITY.
| Item | Performance | Comment |
|---|---|---|
| Fibers and FCU specifics | ||
| Operating temperature | 240 K | Tested in cryogenic environment |
| Operating pressure | 10−6 mbar | Tested in cryogenic environment |
| Core diameter | 6.55 µm | By design |
| Numerical aperture | 0.22 | at 1.9 µm |
| Cut-off wavelength | 1.88–1.9 µm | First and second batch values (Sect. 3.1) |
| Mode field radius (w0) | 3.78–3.82 µm | First and second batch values (Sect. 3.1) |
| Fiber attenuation | 6.5 dB km−1 @ 2 µm – 3.4 dB km−1 @ 2.5 µm | Measured with the cut-back method (Sect. 3.1) |
| Fiber connector losses | 0.05–0.1 dB | With accurate alignment of cores (Sect. 3.4) |
| Minimum curvature radius | 2 cm | By design |
| Total throughput | ⩾81.5% | All fibers and connectors of Fig. 1 (Sect. 3.4) |
| Birefringence beating length | 297–1110 m | See Sect. 3.2 |
| Contrast loss due to birefringence | ⩽6.9% more likely ⩽5% | See Sect. 3.2 |
| Differential dispersion | ⩽2.04 µrad cm2 | With FDDLs at 0 stroke (Sect. 3.3) |
| Contrast loss due to diff. disp. | ⩽0.6% | With FDDLs at 0 stroke at R = 22 (Sect. 3.3) |
| FDDL specifics | ||
| Stroke of fibered delay lines | ⩾5.7 mm | See Sect. 4 |
| OPD difference at zero stroke | ⩽2.29 mm | See Sect. 4 |
| Differential dispersion variation | 1 µrad cm2 per mm of OPD | See Sect. 3.3 |
| Max contrast loss due to diff. disp. | ⩽5.8% | At 6 mm stroke with R = 22 (Sect. 3.3) |
| Command relative OPD accuracy | 40 nm | See Sect. 4 |
| Relative OPD accuracy | 0.6 nm in 100 s | Controlled with the GRAVITY metrology (Sect. 4) |
| FPR specifics | ||
| Range of polarization rotators | ⩾180° | Measured with the Malus law (Sect. 5) |
| Command accuracy | ⩽1° | See Sect. 5 |
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