Arrayed waveguide gratings (AWG) have gained attention as promising integrated spectrographs for ground-based telescopes, airborne applications, and spaceborne instrumentation due to their low mass, diffraction limit characteristics, thermal stability, and robustness against vibrations and misalignment. The Potsdam Arrayed Waveguide Spectrograph (PAWS) is a cross-dispersive instrument based on an integrated photonic spectrograph (IPS) that is optimized for the astronomical H-Band and was designed and developed by innoFSPEC at the Leibniz Institute for Astrophysics Potsdam (AIP). The main element is a second-generation AWG that is fibre coupled and works as a first dispersive element. To work as an IPS, the dispersed light of the AWG is sampled at the output facet and magnified by a microscope objective. The light is then fed into a free-space optical system housed in a cryostat working at 140 K. An afocal relay re-images the exit pupil of the microscope into the plane of a cross-dispersive element containing a diffractive grating. Subsequently, an objective focuses the resulting echellogram on a Teledyne 2k × 2k H2RG near-infrared array working at 80 K. To validate the functionality of the system, different light sources have been used. A tunable laser source generated an echellogram through frame stacking. Furthermore, the cross-dispersed output of a supercontinuum source and of an in-house developed frequency comb system were recorded under varying input conditions of the AWG, successfully achieving first light laboratory measurements. Throughout multiple cycles and measurements of the instrument, several parameters and characteristics were identified, providing opportunities for optimization to enhance the instrument’s performance and facilitate the miniaturization of future iterations. In this manuscript, we will provide a concise overview of the PAWS instrument, the preliminary results of laboratory measurements, and lessons learned to improve the future iterations of the next generation near-infrared cross-dispersed integrated photonic spectrograph. PAWS represents a pioneering demonstration of an astronomy optimized AWG chip, showcasing the advantageous capabilities of integrated photonic spectrograph.
Photonic Integrated Circuits (PIC) are best known for their important role in the telecommunication sector, e.g. high speed communication devices in data centers. However, PIC also hold the promise for innovation in sectors like life science, medicine, sensing, automotive etc. The past two decades have seen efforts of utilizing PIC to enhance the performance of instrumentation for astronomical telescopes, perhaps the most spectacular example being the integrated optics beam combiner for the interferometer GRAVITY at the ESO Very Large Telescope. This instrument has enabled observations of the supermassive black hole in the center of the Milky Way at unprecedented angular resolution, eventually leading to the Nobel Price for Physics in 2020. Several groups worldwide are actively engaged in the emerging field of astrophotonics research, amongst them the innoFSPEC Center in Potsdam, Germany. We present results for a number of applications developed at innoFSPEC, notably PIC for integrated photonic spectrographs on the basis of arrayed waveguide gratings and the PAWS demonstrator (Potsdam Arrayed Waveguide Spectrograph), PIC-based ring resonators in astronomical frequency combs for precision wavelength calibration, discrete beam combiners (DBC) for large astronomical interferometers, as well as aperiodic fiber Bragg gratings for complex astronomical filters and their possible derivatives in PIC.
The Potsdam Arrayed Waveguide Spectrograph (PAWS) is built upon an integrated photonic spectrograph designed for astronomy. Similar to integrated optic beam combiners for interferometry, PAWS is intended to demonstrate on sky how a traditional bulk optics spectrograph with R = 15.000 in the H band can be miniaturized to fit on a chip. The integrated photonic spectrograph is based on second-generation Arrayed Waveguide Gratings (AWG) with unprecedented performance in terms of spectral resolution and throughput. The fibre-coupled AWG serves as a first dispersive element. The pre-dispersed light is fed into a free-space optical system located in a cryostat. Here the overlapping spectral orders are separated by cross-dispersion. The resulting echellogram is recorded by a Teledyne 2kx2k H2RG near-infrared array. Locally controlled constant cryogenic temperatures are required for the operation of the cryostat. This was achieved by fine-tuning and optimizing the original cryostat design using experimental data from multiple cryogenic cycles. These steps included the optimization of thermal interfaces, gold coating of the radiation shield, and an appropriate cooling sequence using the constraints of the allowed cooling rate for H2RG focal plane arrays. Using the readout electronics and GEIRS software provided by the Max Planck Institute for Astronomy (MPIA), frames of the H2RG were obtained, allowing performance calculations and dark pre-characterization of the system. For the optimum alignment of the optical system, the coefficient of thermal expansion (CTE) was measured with an interferometric set-up that recorded the spatial displacements of two reflecting optical elements within the cryostat during a cryogenic cycle. An appropriate strategy was developed to adjust the cryogenic cross dispersion optics inside the vacuum chamber to the AWG coupling optics outside the chamber.
We present the optomechanical design of the Potsdam Arrayed Waveguide Spectrograph (PAWS), which is the first on-sky demonstrator of an integrated photonic spectrograph specifically designed and optimized for astronomy. The instrument is based on an arrayed waveguide grating (AWG) that was designed by and custom fabricated for the innovation center innoFSPEC Potsdam. The commissioning of the instrument is planned at the Calar Alto 2:2m Telescope in southern Spain. The core of the instrument is the AWG-chip as the primary dispersive element. The AWG device is coupled to the telescope module via a single-mode fibre (SMF). The spectral image on the output facet of the AWG is a superposition of multiple spectral orders due to the cyclic dispersive behavior of the waveguide array. The output of the AWG is fed into a free-space optical system housed inside a cryostat via an infinity-corrected microscope objective. The overlapping spectral orders are separated by a second dispersion stage using a ruled grating as a cross-dispersive element, and the resulting echellogram is projected onto a Teledyne 2k x 2k H2RG near-infrared array. The requirement of sub-micron accuracy of the fibre-chip alignment has led to an advanced photonic packaging method. In order to avoid on-site alignment procedures during the on-sky testing, the AWG mount, fibre-support, and microscope objective were integrated into a single monolithic module. Optical and thermal simulations and the design of the cryostat were realized by Andes Scientific. The read-out electronics and the compatible operating software for the detector was provided by the Max Planck Institute for Astronomy (MPIA). Data analysis is performed using the open-source data reduction software P3D, which provides functionality for the removal of the instrument signature, extraction of the spectra, correction for the blaze function, wavelength calibration, and processed data file export.
In pursuit of miniaturization of spectrograph systems, various wavelength-dispersive technologies such as arrayed waveguide gratings (AWGs) [1] and stationary-wave integrated Fourier transform spectrographs (SWIFTS) [2] have been studied as possible candidates for practical implementations of compact, lightweight integrated spectrographs. Integrated echélle-grating (EG) based wavelength demultiplexers have been proposed as an alternative to AWGs for use as the main diffractive element in such a compact spectrograph [3]. Apart from the simple Rowland mount type, more sophisticated geometries, such as the perfect chirped grating (PCG) [4] and two-stigmatic-point gratings (SEG) [5] exist. In this work, we present the first planar integrated echélle grating based on SEG geometry and specifically designed for astronomical spectroscopy in the NIR range ~1500 nm to 1600 nm.
Compact yet highly functional optical components are desired in modern astronomical instruments targeted at low system cost and reduced maintenance complexity. Integrated photonic spectrometers based on planar lightwave circuits are attractive as the planar miniature device can provide high spectral resolution but also great robustness and flexibility in the design of spectrograph systems. Arrayed waveguide gratings (AWGs) have the potential to be adapted and optimized to function as compact spectrometers in astronomical spectrographs. In this work high-resolution AWGs based on low-loss silica waveguides have been designed, fabricated and characterized. The measured spectral resolution exceeds 104 with Δλ = 150 pm at 1548 nm. The insertion loss (including two times fiber-chip coupling) is merely 2.07 dB, amounting to a peak throughput of 62%. Adiabatic fiber taper is developed to bring down the mode field diameter of a standard single mode fiber to match the mode size of the designed waveguide, resulting in almost lossless coupling from the fiber to the waveguide. The free-spectrum range is 48 nm and the side-band suppression is 22 dB. The AWG is also polarization-insensitive. Rotating the linearly polarized input light by 180° results in a slight shift of the central wavelength ~ 30 pm. The excellent overall performance makes this AWG an ideal candidate as the key building block for the development of an integrated astronomical spectrograph module.
We present results of comprehensive re-design of an arrayed waveguide grating (AWG)-based integrated photonic spectrograph (IPS), using Silica-on-Silicon (SOS) technology, to tailor specific performance parameters of interest to high-resolution (resolving power, R = λ/Δλ= 60,000) exoplanet astronomy and stellar seismology. The compactness, modularity, stability, replicability and small-lightweight-payload of the IPS are a few promising and innovative features in the design of high-resolution spectrographs for astronomy or other areas of sciences. The IPS is designed to resolve up to 646 spectral lines per spectral order, with a wavelength spacing of 25 pm, at a central wavelength of 1630 nm (Hband). The fabricated test waveguides have been stress engineered in order to compensate the inherent birefringence of SOS waveguides. The birefringence values of fabricated test structures were quantified, to be on the order 10-6 (theoretical value required to avoid the formation of ghost-images), through inscription of Bragg-gratings on straight waveguides and subsequent measurement of Bragg-reflection spectra. An interferometer system has been integrated with the SOS-IPS (in the same chip) for the characterization of phase errors of the waveguide array. Moreover, promising results of first fabricated key photonics components to form other complex integrated photonic circuits (IPCs), such as astro-interferometers, using silicon nitride-on-insulator (SNOI) technology are also presented. The fabricated IPCs include multimode interference based devices (power splitter/combiners, optical cross/bar-switches), directional-couplers with varying power ratios, Mach-Zehnder interferometers and an AWG. The first results of annealed, low-hydrogen SNOI based devices are promising and comparable to SOI and commercial devices, with device excess-loss less than 2 dB and under 1 dB/cm waveguide-loss in the IR-wavelength.
A conventional Arrayed Waveguide Grating (AWG) has been modified, without output receiver waveguides, for nonconventional
applications such as Astrophotonics and spectroscopy sensing where the input signal can have information
over the entire band and a continuum of light/spectrum. The material system chosen for the AWG design is siliconnitride/
SiO2/Si (Si3N4-SiO2-Si) for its relatively high refractive index, which for a given channel spacing allowing a
more compact device than Silicon-on-Silica. Further, CMOS compatibility and the presence of high non-liner optical
coefficient would be an added advantage to design and fabricate densely integrated photonic sub-systems, such as
calibration source and AWG, for astrophotonics and spectroscopy. The proposed AWG utilizes a flat image plane
optimized for minimal aberration. An analytical calculation, based on Gaussian beam approximation, was used to
determine the optimal flat plane position where the non-uniformity in 1/e electric field widths is minimal. This plane can
be used as the dicing plane to re-image the entire output of the AWG onto a detector array to sample the entire spectrum.
Tailored AWG, with flat image-plane, designed to resolve 48 spectral channels with 0.4nm (50GHz) resolution and
adjacent channel cross-talk level within a 0.2nm window (ITU-grid) ~ -28dB. Calculated insertion loss non-uniformity is
close to 3dB. The foot-print of high index contrast (Δn=23%) IPS is ~ 12x8.5 mm2. The modelled mean spectral
resolving power, R, at the flat image-plane is ~ 7,600. The design principle could be utilised for devices using other
material systems with different parameters.
A conventional Arrayed Waveguide Grating (AWG) has been tailored for non-conventional applications such as Astro-Photonics, Life-science and spectroscopy where the input signal can have information over the full continuum of
light/spectrum, compared to discrete optical channels in optical communication systems. The material system chosen for
the AWG design is silicon-nitride/SiO2/Si (Si3N4-SiO2-Si) for it's relatively high refractive index, which for a given
channel spacing allowing a more compact device than Silicon-on-Silica. While existing conventional AWGs cannot be
utilized in spectroscopy when the input is a continuum, due to the fixed output waveguides where the centre wavelength
λc and therefore rest of the wavelength channels have been assigned to predetermined output waveguides, the device
under development has no output waveguides permitting to utilize the entire-image plane of the output star-coupler. The
output of the AWG can then be re-imaged onto a detector array to sample the entire output spectrum, such as the 2-D
infrared arrays used in astronomy. The designed AWG can resolve up to 40 spectral channels with wavelength spacing
0.4nm (50GHz), adjacent channel cross-talk level < -25dB at the ITU grid (25GHz) and non-uniformity ~ 2.5dB. The
modeled mean spectral resolving power, R, at the flat image-plane is ~ 12,200.
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