## 6 Summary  
Borrowing heavily from the telecommunication industry, the field of astrophotonic spectrographs has seen tremendous growth since its inception about two decades ago. Every year, new devices and improvements are being reported in every aspect of spectroscopic instrumentation discussed in this paper, i.e., guiding, filtering, combining, dispersing, and the detection of the light. In this paper, we have focused on three photonic spectroscopic techniques which show the best potential for astronomical spectroscopy--arayed waveguide gratings, photonic echelle gratings, Fourier transform spectrographs. For each technique, we briefly discussed their relevance to astronomy, their underlying concept, fabrication methods, challenges, and their upcoming solutions in the context of astronomical spectroscopy. We have summarized various key aspects of these techniques in Table 1. To match the requirements of the photon-starved field of astronomy, efforts are being taken to achieve a throughput and performance comparable or even better than the existing conventional spectrographs, while miniaturizing the devices by orders of magnitude. In many aspects, the photonic approach is opening doors for new ways of processing the light, such as OH-emission suppression and active control/manipulation of optical path. These developments are well timed for the emerging era of thirty-meter scale terrestrial telescopes (e.g., TMT, ELT, and GMT). The next generation of space-based telescopes are also going to benefit greatly from the photonic dividends, thanks to the stability, compactness, and cost-effectiveness of photonic instrumentation.  
For building astrophotonic spectrographs, it is important to address the challenges at hand, mainly from a fabrication perspective. In a broad context, the key improvements-in-waiting are: a) Exploring new material platforms and stable processing technologies (over the full device size) for achieving high throughput over a broad band, b) developing polarization insensitive devices to cater to the unpolarized astronomical sources, c) integrating most of the devices in a spectrograph pipeline on a single chip to minimize the number of interfaces and subsequent loss of photons, and d) detectors optimized for direct coupling with photonic chips. The developments described in this paper are not just limited to astronomy, but also applicable to a diverse set of fields, such as remote sensing, biophotonics, biomedical analysis, microfluidics, and the telecommunication industry. It is clear that the field of astrophotonics is gearing up towards building an ecosystem of instruments and spectrographs providing a diffraction-limited performance for the next generation of telescopes.  
All authors contributed to the writing of the paper. [PERSON], [PERSON], and M.D. conceptualized the paper. [PERSON] prepared the draft and visualizations. Review and editing were performed by [PERSON] and M.D. Funding acquisition and administration was done by [PERSON] and M.D.  
This research was funded by a W.M. Keck Foundation grant and is currently funded by NSF/ATI grant 1711377, NASA/APRA grant 80 NS18K0242, and NASA Earth and Space Science Fellowship ASTRO18F-0085.  
The authors acknowledge [PERSON], [PERSON], [PERSON], and [PERSON] at Univ of Sydney for insights on various photonic instrumentation at Univ of Sydney. We also acknowledge [PERSON] at AAO and [PERSON], [PERSON], and [PERSON] at Macquarie University for extensive discussion on current astrophotonics instrumentation at Australian National Fabrication Facility. The authors are very grateful to members of The Maryland Astrophotonics Laboratory [PERSON] (former), [PERSON] (former), [PERSON], [PERSON], and [PERSON] for lending their expertise in various photonic devices.  
The authors declare no conflict of interest.