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We present a numerical and experimental study of the impact of phase errors on the performance of large, high-resolution Arrayed Waveguide Gratings (AWG) for applications in astronomy. We use a scalar diffraction model to study the transmission spectrum of an AWG under random variations of the optical waveguide lengths. We simulate phase error correction by numerically trimming the lengths of the optical waveguides to the nearest integer multiple of the central wavelength. The optical length error distribution of a custom-fabricated silica AWG is measured using frequency-domain interferometry and Monte-Carlo fitting of interferogram intensities. In the end, we give an estimate for the phase-error limited size of a waveguide array manufactured using state-of-the-art technology. We show that post-processing eliminates phase errors as a performance limiting factor for astronomical spectroscopy in the H-band.

A comprehensive design of a folded-architecture arrayed-waveguide-grating (AWG)-device, targeted at applications as integrated photonic spectrographs (IPS) in near-infrared astronomy, is presented. The AWG structure is designed for the astronomical H-band (1500 nm-1800 nm) with a theoretical maximum resolving power R = 60,000 at 1630 nm. The geometry of the device is optimized for a compact structure with a footprint of 5.5 cm × 3.93 cm on SiO 2 platform. To evaluate the fabrication challenges of such high-resolution AWGs, effects of random perturbations of the effective refractive index (RI) distribution in the free propagation region (FPR), as well as small variations of the array waveguide optical lengths are numerically investigated. The results of the investigation show a dramatic degradation of the point spread function (PSF) for a random effective RI distribution with variance values above ∼10 −4 for both the FPR and the waveguide array. Based on the results, requirements on the fabrication technology for high-resolution AWG-based spectrographs are given in the end.

Caution must be exercised when evaluating data of supramolecular polymerization at high concentrations and various methods should be employed.

In the second part of our series on integrated photonic spectrographs for astronomy, we present theoretical and experimental results on the design, simulation and characterization of custom-manufactured silica-on-silicon arrayed waveguide gratings (AWGs) constructed using the three-stigmatic-point method. We derive several mid-to-high resolution field-flattened AWG designs, targeting resolving powers of 11,000 - 35,000 in the astronomical H-band, by iterative computation of differential coefficients of the optical path function. We use numerical simulations to study the imaging properties of the designs in a wide wavelength range between 1500 nm and 1680 nm. We theoretically discuss the design-specific degradation of spectral resolving power at far-off-centre wavelengths and suggest possible solutions. In the experimental section, we provide characterization results of seven manufactured AWG devices of varying free spectral range and resolution. We obtain estimates on spectral resolving powers of up to 27,600 for polarized input at 1550 nm from measurements of the channel transmission bandwidth. Furthermore, we numerically predict expected resolving powers of up to 36,000 in the polarized mode and up to 24,000 in the unpolarized mode for direct continuous imaging of the spectrum.

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Three-dimensional multi-mode interference devices are demonstrated using a single-mode fiber (SMF) center-spliced to a section of polygon-shaped core multimode fiber (MMF). This simple structure can effectively generate well-localized self-focusing spots that match to the layout of a chosen multi-core fiber (MCF) as a launcher device. An optimized hexagon-core MMF can provide efficient coupling from a SMF to a 7-core MCF with an insertion loss of 0.6 dB and a power imbalance of 0.5 dB, while a square-core MMF can form a self-imaging pattern with symmetrically distributed 2 × 2, 3 × 3 or 4 × 4 spots. These spots can be directly received by a two-dimensional detector array. The device can work as a vector curvature sensor by comparing the relative power among the spots with a resolution of ∼0.1° over a 1.8 mm-long MMF.