Demonstration of an efficient, photonic-based astronomical spectrograph on an 8-m telescope.

We demonstrate for the first time an efficient, photonic-based astronomical spectrograph on the 8-m Subaru Telescope. An extreme adaptive optics system is combined with pupil apodiziation optics to efficiently inject light directly into a single-mode fiber, which feeds a compact cross-dispersed spectrograph based on array waveguide grating technology. The instrument currently offers a throughput of 5% from sky-to-detector which we outline could easily be upgraded to ∼ 13% (assuming a coupling efficiency of 50%). The isolated spectrograph throughput from the single-mode fiber to detector was 42% at 1550 nm. The coupling efficiency into the single-mode fiber was limited by the achievable Strehl ratio on a given night. A coupling efficiency of 47% has been achieved with ∼ 60% Strehl ratio on-sky to date. Improvements to the adaptive optics system will enable 90% Strehl ratio and a coupling of up to 67% eventually. This work demonstrates that the unique combination of advanced technologies enables the realization of a compact and highly efficient spectrograph, setting a precedent for future instrument design on very-large and extremely-large telescopes.

The surface composition of asteroid 162173 Ryugu from Hayabusa2 near-infrared spectroscopy.

The near-Earth asteroid 162173 Ryugu, the target of the Hayabusa2 sample-return mission, is thought to be a primitive carbonaceous object. We report reflectance spectra of Ryugu's surface acquired with the Near-Infrared Spectrometer (NIRS3) on Hayabusa2, to provide direct measurements of the surface composition and geological context for the returned samples. A weak, narrow absorption feature centered at 2.72 micrometers was detected across the entire observed surface, indicating that hydroxyl (OH)-bearing minerals are ubiquitous there. The intensity of the OH feature and low albedo are similar to thermally and/or shock-metamorphosed carbonaceous chondrite meteorites. There are few variations in the OH-band position, which is consistent with Ryugu being a compositionally homogeneous rubble-pile object generated from impact fragments of an undifferentiated aqueously altered parent body.

Subaru telescope observations of Deep Impact.

The impact cratering process on a comet is controversial but holds the key for interpreting observations of the Deep Impact collision with comet 9P/Tempel 1. Mid-infrared data from the Cooled Mid-Infrared Camera and Spectrometer (COMICS) of the Subaru Telescope indicate that the large-scale dust plume ejected by the impact contained a large mass (approximately 10(6) kilograms) of dust and formed two wings approximately +/-45 degrees from the symmetric center, both consistent with gravity as the primary control on the impact and its immediate aftermath. The dust distribution in the inner part of the plume, however, is inconsistent with a pure gravity control and implies that evaporation and expansion of volatiles accelerated dust.

Polymer waveguide star coupler.

A polymer waveguide star coupler formed by the selective photopolymerization method is described. This polymer waveguide star coupler has low insertion loss independent of the core and cladding dimensions of the optical fiber. The six-port star coupler obtained, with input and output fiber arrays, had an insertion loss of 2.6 dB and a loss variation across the six ports of 1.2 dB at 0.84 microm.

Polymer optical circuits for multimode optical fiber systems.

Polymer optical waveguides for multimode optical fiber systems have been formed by the selective photopolymerization method, and their waveguiding properties have been investigated. It is shown that the polymer optical waveguides match multimode optical fiber systems, with respect to guide dimensions and index differences, and have a transmission loss sufficiently low for circuit designs in the infrared region (0.19 dB/cm at 0.83 microm). In addition, effective applications of polymer optical circuits to compact optical dividers and lower loss couplers are demonstrated.

Tapered fiber-optic sheet formation and its image-guiding properties.

Polymer fiber-optic sheets with tapered guides have been made by selective photopolymerization for use as image transformers. A new fabrication technique has been proposed, and good image-guiding properties have been achieved. It has been confirmed theoretically and experimentally that scatterers in cladding layers permit high resolution and surface cladding layers with large refractive-index differences by polymer coating give high lighting efficiency. These tapered fiber-optic sheets used as an image reducer have resolutions of up to 6 lines/mm and lighting efficiency of ~4%.

Frequency response of thermo-optic tuning mechanism of planar silica optical multiplexer.

The thermo-optic mechanism provides a simple means of tuning the wavelength of interferometric optical multiplexers constructed from planar silica waveguides upon a silicon substrate. Frequencydomain measurements of the tuning response permit an understanding of the tuning mechanism that is relevant to the design of components for applications in which wavelength-tracking error and cross-talk penalty are important. Measurements on a prototype device have shown a primary low-pass breakpoint at 360 Hz but have also highlighted the possibility of secondary mechanisms affecting the low-frequency response. A continually increasing phase lag at higher frequencies has also been detected and must be allowed for in control-circuit design.

Fiber optic sheet formation by selective photopolymerization.

Fiber optic sheets (8 lines/mm), applicable as image convertors in a facsimile input system, have been formed by a selective monomer photopolymerization in polymer films. In this method, the monomer is doped in a plane film with a higher refractive index and then photopolymerized selectively through a mask pattern by uv light irradiation, so that all light paths are formed simultaneously, instead of arranging individual optical fibers. The attenuation was 0.15-0.20 dB/cm at lambda = 633 nm, and the output light intensity distribution formed by a test chart through light paths showed a resolution of up to ~5 lines/mm.

Characterization of silica-based waveguides with an interferometric optical time-domain reflectometry system using a 1.3-microm-wavelength superluminescent diode.

Backscattering of silica-based glass waveguides is characterized for the first time to our knowledge by using an interferometric optical time-domain reflectometry system. High spatial resolution, as short as 15 microm, is obtained by using a newly developed 1.3-microm-wavelength superluminescent diode. Scattering centers produced by waveguide irregularities are clearly observed in glass optical waveguides. Waveguide loss and bend loss in the curved regions are estimated from the backscattered light intensity distribution.