Astrophotonics is the new frontier technology to make suitable diffraction-limited spectrographs for the next generation of large telescopes. Astrophotonic spectrographs are miniaturized, robust and cost-effective. For various astronomical studies, such as probing the early universe, observing in near infrared (NIR) is crucial. Therefore, our research group is developing moderate resolution (R ~ 1500) on-chip photonic spectrographs in the NIR bands (J Band: 1.1-1.4 μm; H band: 1.45-1.7 μm). To achieve this, we use the concept of arrayed waveguide gratings (AWGs). We fabricate the device using a silica-on-silicon substrate. The waveguides on this AWG are 2 μm wide and 0.1 μm high Si<sub>3</sub>N<sub>4</sub> core buried inside a 15 μm thick SiO<sub>2</sub> cladding.<p> </p> 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 three 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 device footprint is only 16 mm x 7 mm. 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.
We present a high resolution spectrometer consisting of dual solid Fabry-Perot Interferometers (FPI). Each FPI is made of a single piece of L-BBH2 glass which has a high index of refraction n~2.07. Each is then coated with partially reflective mirrors to achieve a spectral resolution of R~30,000. Running the FPIs in tandem reduces the overlapping orders and allows for a much wider free spectral range and higher contrast. Tuning of the FPIs is achieved by adjusting the temperature and thus changing the FPI gap and the refractive index of the material. The spectrometer then moves spatially in order to get spectral information at every point in the field of view. We select spectral lines for further analysis and create maps of the line depths across the field. Using this technique we are able to measure the fluorescence of chlorophyll in plants and observe zodiacal light. In the chlorophyll analysis we are able to detect chlorophyll fluorescence using the line depth in a plant using the sky as a reference solar spectrum. This instrument has possible applications in either a cubesat or aerial observations to measure bulk plant activity over large areas.
The Rapid infrared IMAger-Spectrometer (RIMAS) is a near-infrared (NIR) imager and spectrometer that will quickly follow up gamma-ray burst afterglows on the 4.3-meter Discovery Channel Telescope (DCT). RIMAS has two optical arms which allows simultaneous coverage over two bandpasses (YJ and HK) in either imaging or spectroscopy mode. RIMAS utilizes two Teledyne HgCdTe H2RG detectors controlled by Astronomical Research Cameras, Inc. (ARC/Leach) drivers. We report the laboratory characterization of RIMAS's detectors: conversion gain, read noise, linearity, saturation, dynamic range, and dark current. We also present RIMAS's instrument efficiency from atmospheric transmission models and optics data (both telescope and instrument) in all three observing modes.
Astrophotonics is the next-generation approach that provides the means to miniaturize near-infrared (NIR) spectrometers for upcoming large telescopes and make them more robust and inexpensive. The target requirements for our spectrograph are: a resolving power of 3000, wide spectral range (J and H bands), free spectral range of about 30 nm, high on-chip throughput of about 80% (-1dB) and low crosstalk (high contrast ratio) between adjacent on-chip wavelength channels of less than 1% (-20 dB). A promising photonic technology to achieve these requirements is Arrayed Waveguide Gratings (AWGs). We have developed our first generation of AWG devices using a silica-on-silicon substrate with a very thin layer of Si<sub>3</sub>N<sub>4</sub> in the core of our waveguides. The waveguide bending losses are minimized by optimizing the geometry of the waveguides. Our first generation of AWG devices are designed for H band have a resolving power of ~1500 and free spectral range of ~10 nm around a central wavelength of 1600 nm. The devices have a footprint of only 12 mm × 6 mm. They are broadband (1450-1650 nm), have a peak on-chip throughput of about 80% (~-1 dB) and contrast ratio of about 1.5% (-18 dB). These results confirm the robustness of our design, fabrication and simulation methods. Currently, the devices are designed for Transverse Electric (TE) polarization and all the results are for TE mode. We are developing separate J- and H-band AWGs with higher resolving power, higher throughput and lower crosstalk over a wider free spectral range to make them better suited for astronomical applications.
For the past forty years, optical fibres have found widespread use in ground-based and space-based instruments. In most applications, these fibres are used in conjunction with conventional optics to transport light. But photonics offers a huge range of optical manipulations beyond light transport that were rarely exploited before 2001. The fundamental obstacle to the broader use of photonics is the difficulty of achieving photonic action in a multimode fibre. The first step towards a general solution was the invention of the photonic lantern<sup>1</sup> in 2004 and the delivery of high-efficiency devices (< 1 dB loss) five years on<sup>2</sup>. Multicore fibres (MCF), used in conjunction with lanterns, are now enabling an even bigger leap towards multimode photonics. Until recently, the single-moded cores in MCFs were not sufficiently uniform to achieve telecom (SMF-28) performance. Now that high-quality MCFs have been realized, we turn our attention to printing complex functions (e.g. Bragg gratings for OH suppression) into their <i>N</i> cores. Our first work in this direction used a Mach-Zehnder interferometer (near-field phase mask) but this approach was only adequate for <i>N</i>=7 MCFs as measured by the grating uniformity<sup>3</sup>. We have now built a Sagnac interferometer that gives a three-fold increase in the depth of field sufficient to print across <i>N</i> ≥ 127 cores. We achieved first light this year with our 500mW Sabre FRED laser. These are sophisticated and complex interferometers. We report on our progress to date and summarize our first-year goals which include multimode OH suppression fibres for the Anglo-Australian Telescope/PRAXIS instrument and the Discovery Channel Telescope/MOHSIS instrument under development at the University of Maryland.
The Rapid infrared IMAger-Spectrometer (RIMAS) is a rapid gamma-ray burst afterglow instrument that will provide photometric and spectroscopic coverage of the Y, J, H, and K bands. RIMAS separates light into two optical arms, YJ and HK, which allows for simultaneous coverage in two photometric bands. RIMAS utilizes two 2048 x 2048 pixel Teledyne HgCdTe (HAWAII-2RG) detectors along with a Spitzer Legacy Indium- Antimonide (InSb) guiding detector in spectroscopic mode to position and keep the source on the slit. We describe the software and hardware development for the detector driver and acquisition systems. The HAWAII- 2RG detectors simultaneously acquire images using Astronomical Research Cameras, Inc. driver, timing, and processing boards with two C++ wrappers running assembly code. The InSb detector clocking and acquisition system runs on a National Instruments cRIO-9074 with a Labview user interface and clocks written in an easily alterable ASCII file. We report the read noise, linearity, and dynamic range of our guide detector. Finally, we present RIMAS’s estimated instrument efficiency in photometric imaging mode (for all three detectors) and expected limiting magnitudes. Our efficiency calculations include atmospheric transmission models, filter models, telescope components, and optics components for each optical arm.
The Rapid Infrared Imager/Spectrometer (RIMAS) is designed to perform follow-up observations of transient
astronomical sources at near infrared (NIR) wavelengths (0.9 - 2.4 microns). In particular, RIMAS will be used to
perform photometric and spectroscopic observations of gamma-ray burst (GRB) afterglows to compliment the <i>Swift</i>
satellite’s science goals. Upon completion, RIMAS will be installed on Lowell Observatory’s 4.3 meter Discovery
Channel Telescope (DCT) located in Happy Jack, Arizona. The instrument’s optical design includes a collimator lens
assembly, a dichroic to divide the wavelength coverage into two optical arms (0.9 - 1.4 microns and 1.4 - 2.4 microns
respectively), and a camera lens assembly for each optical arm. Because the wavelength coverage extends out to 2.4
microns, all optical elements are cooled to ~70 K. Filters and transmission gratings are located on wheels prior to each
camera allowing the instrument to be quickly configured for photometry or spectroscopy. An athermal optomechanical
design is being implemented to prevent lenses from loosing their room temperature alignment as the system is cooled.
The thermal expansion of materials used in this design have been measured in the lab. Additionally, RIMAS has a guide
camera consisting of four lenses to aid observers in passing light from target sources through spectroscopic slits. Efforts
to align these optics are ongoing.
The Observational Cosmology Laboratory at NASA’s Goddard Space Flight Center (GSFC), in collaboration with the
University of Maryland, is building the Rapid Infrared Imager/Spectrometer (RIMAS) for the new 4.3 meter Discovery
Channel Telescope (DCT). The instrument is designed to observe gamma-ray burst (GRB) afterglows following their
initial detection by the Swift satellite. RIMAS will operate in the near infrared (0.9 – 2.4 microns) with all of its optics
cooled to ~60 K. The primary optical design includes a collimator lens assembly, a dichroic dividing the wavelength
coverage into the “YJ band” and “HK band” optical arms, and camera lens assemblies for each arm. Additionally, filters
and dispersive elements are attached to wheels positioned prior to each arm’s camera, allowing the instrument to quickly
change from its imaging modes to spectroscopic modes. Optics have also been designed to image the sky surrounding
spectroscopic slits to help observers pass light from target sources through these slits. Because the optical systems are
entirely cryogenic, it was necessary to account for changing refractive indices and model the effects of thermal
contraction. One result of this work is a lens mount design that keeps lenses centered on the optical axis as the system is
cooled. Efforts to design, tolerance and assemble these cryogenic optical systems are presented.