Towards a multi-input astrophotonic AWG spectrograph

Abstract: Astrophotonics is the new frontier technology to develop diffraction-limited, miniaturized, and cost-effective instruments for the next generation of large telescopes. For various astronomical studies such as probing the early universe, observing in near infrared (NIR) is crucial. To address this, we are developing moderate resolution (R = 1500) on-chip astrophotonic spectrographs in the NIR bands (J Band: 1.1-1.4 $\mu m$; H band: 1.45-1.7 $\mu m$) using the concept of arrayed waveguide gratings (AWGs). We fabricate the AWGs using a silica-on-silicon substrate. The waveguides on these AWGs are 2 $\mu m$ wide and 0.1 $\mu m$ high Si3N4 core buried inside a 15 $\mu m$ thick SiO2 cladding. To make the maximal use of astrophotonic integration such as coupling the AWGs with multiple single-mode fibers coming from photonic lanterns or fiber Bragg gratings (FBGs), we require a multi-input AWG design. In a multi-input AWG, the output spectrum due to each individual input channel overlaps to produce a combined spectrum from all inputs. This on-chip combination of light effectively improves the signal-to-noise ratio as compared to spreading the photons to several AWGs with single inputs. In this paper, we present the design and simulation results of an AWG in the H band with 3 input waveguides (channels). The resolving power of individual input channels is 1500, while the overall resolving power with three inputs together is 500, 600, 750 in three different configurations simulated here. The free spectral range of the device is 9.5 nm around a central wavelength of 1600 nm. For the standard multi-input AWG, the relative shift between the output spectra due to adjacent input channels is about 1.6 nm, which roughly equals one spectral channel spacing. In this paper, we discuss ways to increase the resolving power and the number of inputs without compromising the free spectral range or throughput.

• The paper demonstrates a multi-input AWG design that integrates multiple single-mode fibers to produce a unified on-chip spectrograph.
• It employs simulations varying input waveguide spacing to optimize spectral resolution and improve signal-to-noise ratios in the NIR bands.
• The study highlights the potential for compact, robust, and cost-effective solutions in high-precision astronomical instrumentation.

Towards a Multi-Input Astrophotonic AWG Spectrograph

Introduction

Astrophotonics represents a significant technological advancement in the development of diffraction-limited spectrographs for future large telescopes. Its miniaturized, robust, and cost-effective nature makes it essential for astronomical applications, particularly in the near-infrared (NIR) spectrum. The focus on moderate resolution ($R \sim 1500$) on-chip spectrographs in NIR bands such as J (1.1–1.4 $\mu m$) and H (1.45–1.7 $\mu m$) bands is of high importance. Utilizing Arrayed Waveguide Grating (AWG) technology fabricated on a silica-on-silicon substrate, this paper explores a multi-input AWG design aiming to integrate with multiple single-mode fibers.

Figure 1: Analogy between conventional grating spectrograph (left) and arrayed waveguide gratings (right), from Gatkine et al. 2016.

Scientific Motivation

The investigation of galaxy evolution and the early universe demands high-resolution observations in the NIR spectrum due to the redshift from ultraviolet wavelengths. Conventional spectrographs scale adversely with telescope aperture, resulting in mechanical and thermal stability challenges. The integration of AWG technology offers a way to mitigate these issues by reducing the size, weight, and cost of instrumentation. The adoption of integrated photonic spectrographs is especially relevant for large telescopes where maintaining instrument stability is a critical challenge.

AWG Design and Implementation

The AWG design offers a solution similar to a traditional diffraction grating spectrograph, where the waveguide array induces a path difference correlating with the spectral order, leading to constructive interference peaks.

Figure 2: A schematic of the full setup of an integrated photonic spectrograph with OH-suppression.

The paper presents a multi-input AWG in the H band, accommodating three input waveguides designed to maximize the use of astrophotonic integration by combining outputs into a singular spectrum, enhancing the signal-to-noise ratio effectively. The resolving power of individual channels achieves $\sim$1500; however, combining the inputs alters resolving powers to 500, 600, and 750 in varied simulated configurations.

Figure 3: The CAD of the designed multi-input AWG. The input waveguides are on the left, and the output is sampled by five waveguides.

Simulation and Performance Evaluation

The multi-input AWG simulations reveal the output spectra responses at varied input configurations. As the central output channel is examined, it becomes evident that the wavelength shift correlates directly with input waveguide spacing, affecting overall resolution.

Figure 4: Simulation results: normalized throughput variation when input channels are progressively modified.

Simulations demonstrate that decreasing input waveguide separation minimizes wavelength shifts, reducing resolution degradation effectively — a crucial finding for future high-resolution design efforts. For example, reducing input waveguide spacing from 6 $\mu m$ to 3 $\mu m$ improved the degradation factor significantly.

Conclusion

The advancements in multi-input AWG design presented demonstrate significant potential for improving astrophotonic spectrograph performance, particularly regarding maintaining high resolution and signal-to-noise ratios. This paper sets the stage for further development in integrated multi-input photonic spectrographs, particularly relevant for high-precision astronomical investigations. Continued evolution of this technology underscores a promising avenue for efficient and compact spectroscopic solutions tailored to modern astrophysical challenges. Future efforts will likely aim at enhancing spectral order and resolution capabilities for ongoing astronomical exploration initiatives.