Complex phase masks for OH suppression filters in astronomy: part I: design

Abstract

The design of a complex phase mask (CPM) for inscribing multi-notch fiber Bragg grating filters in optical fibers for OH suppression in astronomy is presented. We demonstrate the steps involved in the design of a complex mask with discrete phase steps, following a detailed analysis of fabrication constraints. The phase and amplitude of the complex grating is derived through inverse modelling from the desired aperiodic filter spectrum, following which the phase alone is encoded into the surface relief of a CPM. Compared to a complicated “running-light” Talbot interferometer based inscription setup where the phase of the inscribing beam is controlled by electro- or acousto-optic modulators and synchronized to a moving fiber translation stage, CPM offers the well-known convenience and reproducibility of the standard phase mask inscription technique. We have fabricated a CPM that can suppress 37 sky emission lines between 1508 nm to 1593 nm, with a potential of increasing to 99 channels for suppressing near-infrared (NIR) OH-emission lines generated in the upper atmosphere and improving the performance of ground-based astronomical telescopes.

PAWS and POCO: NIR Astrophotonic Instruments for Astronomy

“…Long-distance transmission and distribution of POCO are possible for several tens or even hundreds of kilometers, provided single-mode fibers are used. The next step is to develop BluePOCO, a visible wavelength frequency comb for current observatories like STELLA and CAHA, and for future instruments like BlueMUSE (Roth et al 2022;Richard et al 2021), and ELT/MOSAIC (Floriot et al 2022).…”

“…Sky emission filters based on fiber Bragg gratings (FBGs) have already shown the effectiveness of OH suppression in reducing the sky background (Ellis et al 2020(Ellis et al , 2012. These aperiodic complex filters can be fabricated using a "running light" interferometer (Trinh et al 2013), complex phase masks (Rahman et al 2020), or by ultrafast laser inscription (ULI) method (Goebel et al 2018), in single-mode optical fibers of a photonic lantern (Leon-Saval et al 2005) that distributes the multimode point spread function (PSF) of a seeing-limited telescope into multiple single modes or can be incorporated as waveguide gratings in PICs.…”

Astrophotonics: photonic integrated circuits for astronomical instrumentation

“…While FBG have been reported to be feasible with ULI inscription in optical fibers [59], it would be interesting to demonstrate the same in photonic chips. A competing technology is currently under development using phase mask inscription with ultraviolet laser light [60].…”

Starlight coupling through atmospheric turbulence into few-mode fibers and photonic lanterns in the presence of partial adaptive optics correction

“…Photonic spectrographs (Blind et al 2017), e.g. arrayed waveguide gratings (AWGs) (Bland-Hawthorn & Horton 2006), fiber Bragg gratings for OH suppression (Bland-Hawthorn et al 2011;Rahman et al 2020), and photonic beam combiners, e.g. GRAVITY (Eisenhauer et al 2008) and discrete beam combiners (DBCs) (Minardi 2015(Minardi , 2012, need to operate in the single mode regime in order to deliver their promised spectral resolution, filter characteristics and phase retrieval capabilities, respectively, while avoiding modal noise and focal ratio degradation.…”