1:61 photonic lanterns for astrophotometry: A performance study

In this paper, we study how the spectral information is transmitted through a 1:61 photonic lantern (PL) when illuminating the multimode (MM) end with a point source. We found that the spectrum undergoes two major effects between the MM input and the independent single-mode (SM) outputs of the PL. First, at resolution R = 775 (at 1550 nm), the overall output spectrum envelope strongly differs from SM to SM outputs. The spectra measured at the 61 SM outputs also vary when the small point object (10 μm in diameter, compared with the 100 μm diameter MM core) is moved across the MM fibre end-face of the PL. However, the sum of all the spectra measured at the SM outputs is very close to the spectrum of the point source, and is insensitive to the position of the point source. Secondly, at resolution R = 77 500 a fine 150 pm periodic structure was observed at the SM outputs. We found that a PL in the MM–SM configuration (i.e. MM input/SM outputs) is a very stable system regarding variations of the transmitted spectra when it is left mechanically non-perturbated and only submitted to environment-related changes, compared to a conventional step-index MM fibre of same core diameter. The latter clearly shows characteristic modal noise. Furthermore, we showed that at resolution R = 77 500 the sum of the spectra measured at 61 the SM outputs is very close to the spectrum of the point source, and does not present the 150 pm periodic oscillations. We also studied two PLs spliced together in the MM–SM–MM configuration and the results showed that the spliced PLs perform very well as mode scramblers, especially under conditions where conventional step-index MM fibres usually fail at performing effective scrambling.