Astrophotonic Spectrographs

Abstract

Astrophotonics is the application of photonic technologies to channel, manipulate, and disperse light from one or more telescopes to achieve scientific objectives in astronomy in an efficient and cost-effective way. Utilizing photonic advantage for astronomical spectroscopy is a promising approach to miniaturizing the next generation of spectrometers for large telescopes. It can be primarily attained by leveraging the two-dimensional nature of photonic structures on a chip or a set of fibers, thus reducing the size of spectroscopic instrumentation to a few centimeters and the weight to a few hundred grams. A wide variety of astrophotonic spectrometers is currently being developed, including arrayed waveguide gratings (AWGs), photonic echelle gratings (PEGs), and Fourier-transform spectrometer (FTS). These astrophotonic devices are flexible, cheaper to mass produce, easier to control, and much less susceptible to vibrations and flexure than conventional astronomical spectrographs. The applications of these spectrographs range from astronomy to biomedical analysis. This paper provides a brief review of this new class of astronomical spectrographs.Appl. Sci. 2019, xx, 5 2 of 18 of the telescope), creating new challenges for instrumentation [7]. At the same time, there is an explosion in the number of large astronomical surveys discovering a multitude of new transients, exoplanets, and galaxies. This necessitates development of new instruments for large telescopes that are compact and cost-effective while providing the flexibility to address the challenge of high-throughput characterization for these large surveys.Photonic technologies provide a promising platform for building next-generation instruments that are flexible (in terms of light manipulation), compact (volumes of a few tens of cubic centimetres) and lightweight (a few hundreds of grams), thanks to manipulation of guided light [8,9]. In addition, they are cost-effective, due to the advantages of mass-fabrication. Therefore, the astronomical community has pursued this direction very positively and built a wide variety of instruments from the near-ultraviolet (NUV) [10] to mid-infrared (MIR) wavebands [11]. As an example of the growth of astrophotonics, Figure 1 shows the number of citations of papers related to astrophotonic spectrographs. However, the current technical know-how is still far from complete or ideal, leaving many challenges that will likely be addressed in the near future.In this paper, we focus on the recent developments in the field of astrophotonic spectrographs. To paint a complete picture, in Sections 2 and 3, respectively, we briefly discuss the steps to channel the light from the telescope to the spectrograph and condition it. Photonic dispersion concepts, and their implementation and challenges are described in Section 4. In Section 5, we discuss new developments in calibration and detection that are particularly of interest to astrophotonic spectrographs. Lastly, in Section 6, we summarize and discuss the impact of photonic spectrogr...

Potential of commercial SiN MPW platforms for developing mid/high-resolution integrated photonic spectrographs for astronomy

“…When operating at the diffraction-limit, it is possible to consider replacing the bulk optics with photonic circuits. In terms of spectrographs, there are numerous technologies that could be used including planar concave gratings, arrayed waveguide gratings (AWGs), and Fourier transform spectrometers, for example [15]. The additional benefit of using these devices is an even more compact instrument than can be achieved with bulk optics, reducing volume and mass, and increasing thermal and mechanical stability.…”

Development of an integrated near-IR astrophotonic spectrograph

“…These techniques are described in more detail from an astronomical perspective in a recent review paper. 4 Of these techniques, AWG is the most promising for astronomical spectroscopy, thanks to its ease of fabrication, flexibility, and relatively higher throughput that can be achieved.…”

“…The photonic platform of guided light in fibers and waveguides has opened the doors to next-generation instrumentation for both ground-and space-based telescopes in optical and near/mid-IR bands, particularly for the upcoming extremely large telescopes (ELTs). [1][2][3][4][5] The key idea here is to leverage the ability of guiding the light in waveguides (using total internal reflection) to collapse the conventional optical setups into 2D "optical circuits". These optical circuits make use of single-mode waveguides or fibers, which are equivalent to diffractionlimited operation.…”

Status and future developments of integrated photonic spectrographs for astronomy and Earth and planetary sciences

“…Several IPS variants exist including Planar Concave Gratings and Arrayed Waveguide Gratings (AWGs) to name a few (please see 12 for a review on the topic). Given that AWGs are the most mature versions of IPSs, development and testing for astronomical applications has almost exclusively been reserved to them and hence they form the focus of the discussion in this paper.…”