Alexander Arriola

Ultrafast laser inscription of mid-IR directional couplers for stellar interferometry

We report the ultrafast laser fabrication and mid-IR characterization (3.39 μm) of four-port evanescent field directional couplers. The couplers were fabricated in a commercial gallium lanthanum sulfide glass substrate using sub-picosecond laser pulses of 1030 nm light. Straight waveguides inscribed using optimal fabrication parameters were found to exhibit propagation losses of ∼0.8 dB·cm(-1). A series of couplers were inscribed with different interaction lengths, and we demonstrate power-splitting ratios of between 8% and 99% for mid-IR light with a wavelength of 3.39 μm. These results clearly demonstrate that ultrafast laser inscription can be used to fabricate high-quality evanescent field couplers for future applications in astronomical interferometry.

Fabrication of high quality sub-micron Au gratings over large areas with pulsed laser interference lithography for SPR sensors

Metallic gratings were fabricated using high energy laser interference lithography with a frequency tripled Nd:YAG nanosecond laser. The grating structures were first recorded in a photosensitive layer and afterwards transferred to an Au film. High quality Au gratings with a period of 770 nm and peak-to-valley heights of 20-60 nm exhibiting plasmonic resonance response were successfully designed, fabricated and characterized.

Fabrication of fully integrated antiresonant reflecting optical waveguides using the femtosecond laser direct-write technique

Utilizing a recently developed two-step fabrication process, we have experimentally demonstrated what is to the best of our knowledge the first ever integrated antiresonant reflecting optical waveguide (ARROW), directly inscribed into bulk optical glass. The waveguide consists of an array of high-index contrast rods that surround an unmodified core. The transmission spectrum reveals the formation of discrete bandgaps that can be tuned by varying the diameter of the individual rods.

Mid-infrared astrophotonics: study of ultrafast laser induced index change in compatible materials

The mid-infrared wavelength regime 3.5 – 4.1μm, known as the astronomical L’ band is of special interest for exoplanet hunting. Mid-IR compatible photonic technologies are an enabling platform for a range of critical observational science using compact instruments on the next generation of Extremely Large Telescopes. Pupil remapping interferometry is a technique in which subapertures of the telescope pupil (2D) are reformatted into a 1D linear array. This can be done efficiently using 3D photonics. One of the most important techniques to fabricate 3D photonic devices in glass is ultrafast laser inscription. However, common silicate glasses are opaque above 2–2.2 μm and therefore not useful for the fabrication of waveguides at mid-infrared wavelengths. Here we present a study of mid-infrared transparent materials that are compatible with the ultrafast laser inscription technique. This study will inform the development of mid-infrared photonic devices for future exoplanetary discovery.

Efficient photonic reformatting of celestial light for diffraction-limited spectroscopy

The spectral resolution of a dispersive astronomical spectrograph is limited by the trade-off between throughput and the width of the entrance slit. Photonic guided wave transitions have been proposed as a route to bypass this trade-off, by enabling the efficient reformatting of incoherent seeing-limited light collected by the telescope into a linear array of single modes: a pseudo-slit which is highly multimode in one axis but diffraction-limited in the dispersion axis of the spectrograph. It is anticipated that the size of a single-object spectrograph fed with light in this manner would be essentially independent of the telescope aperture size. A further anticipated benefit is that such spectrographs would be free of ‘modal noise’, a phenomenon that occurs in high-resolution multimode fibre-fed spectrographs due to the coherent nature of the telescope point spread function (PSF). We seek to address these aspects by integrating a multicore fibre photonic lantern with an ultrafast laser inscribed three-dimensional waveguide interconnect to spatially reformat the modes within the PSF into a diffraction-limited pseudo-slit. Using the CANARY adaptive optics (AO) demonstrator on the William Herschel Telescope, and 1530 ± 80 nm stellar light, the device exhibits a transmission of 47–53 per cent depending upon the mode of AO correction applied. We also show the advantage of using AO to couple light into such a device by sampling only the core of the CANARY PSF. This result underscores the possibility that a fully optimized guided-wave device can be used with AO to provide efficient spectroscopy at high spectral resolution.

The SCExAO high contrast imager: transitioning from commissioning to science

SCExAO is the premier high-contrast imaging platform for the Subaru Telescope. It offers high Strehl ratios at near-IR wavelengths (y-K band) with stable pointing and coronagraphs with extremely small inner working angles, optimized for imaging faint companions very close to the host. In the visible, it has several interferometric imagers which offer polarimetric and spectroscopic capabilities. A recent addition is the RHEA spectrograph enabling spatially resolved high resolution spectroscopy of the surfaces of giant stars, for example. New capabilities on the horizon include post-coronagraphic spectroscopy, spectral differential imaging, nulling interferometry as well as an integral field spectrograph and an MKID array. Here we present the new modules of SCExAO, give an overview of the current commissioning status of each of the modules and present preliminary results.

Increasing the spectral coverage of interferometric integrated optics: K/L and N-laser-written beam combiners

Integrated optics (IO) has proven to be a competitive solution for beam combination in the context of astronomical interferometry (e.g. GRAVITY at the VLTI). However, conventional silica-based lithographic IO is limited to wavelengths shorter than 2.2μm. We report in this paper the progress on our attempt to extend the operation of IO to longer wavelengths. Previous work has demonstrated the suitability of chalcogenide devices in the MID-IR in the N band and monochromatically at 3.39 μm. Here, we continue this effort with the manufacturing of new laser written GLS IO as beam combiners designed for the astronomical L band and characterized interferometrically at 3.39 μm. In the era of multi-telescope interferometers, we present a promising solution to strengthen the potential of IO for new wavelength ranges.

Creating large second-order optical nonlinearity in optical waveguides written by femtosecond laser pulses in boro-aluminosilicate glass

The thermal poling technique was applied to optical waveguides embedded in a commercial boro-aluminosilicate glass, resulting in high levels of induced second-order optical nonlinearity. The waveguides were fabricated using the femtosecond laser direct-write technique, and thermally poled samples were characterized with second harmonic optical microscopy to reveal the distribution profile of the induced nonlinearity. It was found that, in contrast to fused silica, the presence of waveguides in boro-aluminosilicate glass led to an enhancement of the creation of the second-order nonlinearity, which is larger in the laser written waveguiding regions when compared to the un-modified substrate. The magnitude of the nonlinear coefficient d33 achieved in the core of the laser-written waveguides, up to 0.2 pm/V, was comparable to that in thermally poled fused silica, enabling the realization of compact integrated electro-optic devices in boro-aluminosilicate glasses.

Modal noise characterisation of a hybrid reformatter

This paper reports on the modal noise characterisation of a hybrid reformatter. The device consists of a multicore-fibre photonic lantern and an ultrafast laser-inscribed slit reformatter. It operates around 1550 nm and supports 92 modes. Photonic lanterns transform a multimode signal into an array of single-mode signals, and thus combine the high coupling efficiency of multimode fibres with the diffraction-limited performance of single-mode fibres. This paper presents experimental measurements of the device point spread function properties under different coupling conditions, and its throughput behaviour at high spectral resolution. The device demonstrates excellent scrambling but its point spread function is not completely stable. Mode field diameter and mode bary-centre position at the device output vary as the multicore fibre is agitated due to the fabrication imperfections.

Precision single mode fibre integral field spectroscopy with the RHEA spectrograph

The RHEA Spectrograph is a single-mode echelle spectrograph designed to be a replicable and cost effective method of undertaking precision radial velocity measurements. Two versions of RHEA currently exist, one located at the Australian National University in Canberra, Australia (450 - 600nm wavelength range), and another located at the Subaru Telescope in Hawaii, USA (600 - 800 nm wavelength range). Both instruments have a novel fibre feed consisting of an integral field unit injecting light into a 2D grid of single mode fibres. This grid of fibres is then reformatted into a 1D array at the input of the spectrograph (consisting of the science fibres and a reference fibre capable of receiving a white-light or xenon reference source for simultaneous calibration). The use of single mode fibres frees RHEA from the issue of modal noise and significantly reduces the size of the optics used. In addition to increasing the overall light throughput of the system, the integral field unit allows for cutting edge science goals to be achieved when operating behind the 8.2m Subaru Telescope and the SCExAO adaptive optics system. These include, but are not limited to: resolved stellar photospheres; resolved protoplanetary disk structures; resolved Mira shocks, dust and winds; and sub-arcsecond companions. We present details and results of early tests of RHEA@Subaru and progress towards the stated science goals.