Simulation and optimisation of an astrophotonic reformatter

Image slicing is a powerful technique in astronomy. It allows the instrument designer to reduce the slit width of the spectrograph, increasing spectral resolving power whilst retaining throughput. Conventionally this is done using bulk optics, such as mirrors and prisms, however, more recently astrophotonic components known as photonic lanterns and photonic reformatters have also been used. These devices reformat the multimode input light from a telescope into single-mode outputs, which can then be re-arranged to suit the spectrograph. The photonic dicer (PD) is one such device, designed to reduce the dependence of spectrograph size on telescope aperture and eliminate modal noise. We simulate the PD, by optimizing the throughput and geometrical design using SOAPY and BEAMPROP. The simulated device shows a transmission between 8 and 20 per cent, depending upon the type of adaptive optics correction applied, matching the experimental results well. We also investigate our idealized model of the PD and show that the barycentre of the slit varies only slightly with time, meaning that the modal noise contribution is very low when compared to conventional fibre systems. We further optimize our model device for both higher throughput and reduced modal noise. This device improves throughput by 6.4 per cent and reduces the movement of the slit output by 50 per cent, further improving stability. This shows the importance of properly simulating such devices, including atmospheric effects. Our work complements recent work in the field and is essential for optimizing future photonic reformatters.