Fiber-connectorized ultrafast-laser-inscribed K-band integrated optics beam combiner for the CHARA telescope array.

A fiber-connectorized K-band integrated-optics two-telescope beam combiner was developed for long-baseline interferometry at the CHARA telescope array utilizing the ultrafast laser inscription (ULI) technique. Single-mode waveguide insertion losses were measured to be ∼1.1d B over the 2-2.3 µm window. The development of asymmetric directional couplers enabled the construction of a beam combiner that includes a 50:50 coupler for interferometric combination and two ∼75:25 couplers for photometric calibration. The visibility of the bare beam combiner was measured at 87% and then at 82% after fiber-connectorization by optimizing the input polarization. These results indicate that ULI technique can fabricate efficient fiber-connectorized K-band beam combiners for astronomical purposes.

Six-telescope integrated optics beam combiner fabricated using ultrafast laser inscription for J- and H-band astronomy.

We have built and characterized, to our knowledge, the first six-telescope discrete beam combiner (DBC) for stellar interferometry in the astronomical J-band. It is the DBC with the largest number of beam combinations and was manufactured using ultrafast laser inscription in borosilicate glass, with a throughput of ≈56%. For calibration of the visibility-to-pixel matrix, we use a two-input Michelson interferometer and extract the complex visibility. A visibility amplitude of 1.05 and relative precision of 2.9% and 3.8% are extracted for 1328 nm and 1380 nm, respectively. Broadband (≤40n m) characterization is affected by dispersion but shows similar performance.

Design, simulation and characterization of integrated photonic spectrographs for astronomy II: low-aberration Generation-II AWG devices with three stigmatic points.

In the second part of our series on integrated photonic spectrographs for astronomy, we present theoretical and experimental results on the design, simulation and characterization of custom-manufactured silica-on-silicon arrayed waveguide gratings (AWGs) constructed using the three-stigmatic-point method. We derive several mid-to-high resolution field-flattened AWG designs, targeting resolving powers of 11,000 - 35,000 in the astronomical H-band, by iterative computation of differential coefficients of the optical path function. We use numerical simulations to study the imaging properties of the designs in a wide wavelength range between 1500 nm and 1680 nm. We theoretically discuss the design-specific degradation of spectral resolving power at far-off-centre wavelengths and suggest possible solutions. In the experimental section, we provide characterization results of seven manufactured AWG devices of varying free spectral range and resolution. We obtain estimates on spectral resolving powers of up to 27,600 for polarized input at 1550 nm from measurements of the channel transmission bandwidth. Furthermore, we numerically predict expected resolving powers of up to 36,000 in the polarized mode and up to 24,000 in the unpolarized mode for direct continuous imaging of the spectrum.

Design, simulation and characterization of integrated photonic spectrographs for astronomy: generation-I AWG devices based on canonical layouts.

We present an experimental study on our first generation of custom-developed arrayed waveguide gratings (AWG) on a silica platform for spectroscopic applications in near-infrared astronomy. We provide a comprehensive description of the design, numerical simulation and characterization of several AWG devices aimed at spectral resolving powers of 15,000-60,000 in the astronomical H-band. We evaluate the spectral characteristics of the fabricated devices in terms of insertion loss and estimated spectral resolving power and compare the results with numerical simulations. We estimate resolving powers of up to 18,900 from the output channel 3-dB transmission bandwidth. Based on the first characterization results, we select two candidate AWGs for further processing by removal of the output waveguide array and polishing the output facet to optical quality with the goal of integration as the primary diffractive element in a cross-dispersed spectrograph. We further study the imaging properties of the processed AWGs with regards to spectral resolution in direct imaging mode, geometry-related defocus aberration, and polarization sensitivity of the spectral image. We identify phase error control, birefringence control, and aberration suppression as the three key areas of future research and development in the field of high-resolution AWG-based spectroscopy in astronomy.

Simulations of mode-selective photonic lanterns for efficient coupling of starlight into the single-mode regime.

In ground-based astronomy, starlight distorted by the atmosphere couples poorly into single-mode waveguides, but a correction by adaptive optics, even if only partial, can boost coupling into the few-mode regime, allowing the use of photonic lanterns to convert into multiple single-mode beams. Corrected wavefronts result in focal patterns that couple mostly with circularly symmetric waveguide modes. A mode-selective photonic lantern is hence proposed to convert multimode light into a subset of single-mode waveguides of the standard photonic lantern, thereby reducing the required number of outputs. We ran simulations to show that only two out of the six waveguides of a 1×6 photonic lantern carry >95% of the coupled light to the outputs at D/r0<10 if the wavefront is partially corrected and the photonic lantern is made mode selective.

First stellar photons for an integrated optics discrete beam combiner at the William Herschel Telescope.

We present the first on-sky results of a four-telescope integrated optics discrete beam combiner (DBC) tested at the 4.2 m William Herschel Telescope. The device consists of a four-input pupil remapper followed by a DBC and a 23-output reformatter. The whole device was written monolithically in a single alumino-borosilicate substrate using ultrafast laser inscription. The device was operated at astronomical H-band (1.6 µm), and a deformable mirror along with a microlens array was used to inject stellar photons into the device. We report the measured visibility amplitudes and closure phases obtained on Vega and Altair that are retrieved using the calibrated transfer matrix of the device. While the coherence function can be reconstructed, the on-sky results show significant dispersion from the expected values. Based on the analysis of comparable simulations, we find that such dispersion is largely caused by the limited signal-to-noise ratio of our observations. This constitutes a first step toward an improved validation of the DBC as a possible beam combination scheme for long-baseline interferometry.

Performance limits of astronomical arrayed waveguide gratings on a silica platform.

We present a numerical and experimental study of the impact of phase errors on the performance of large, high-resolution arrayed waveguide gratings (AWG) for applications in astronomy. We use a scalar diffraction model to study the transmission spectrum of an AWG under random variations of the optical waveguide lengths. We simulate phase error correction by numerically trimming the lengths of the optical waveguides to the nearest integer multiple of the central wavelength. The optical length error distribution of a custom-fabricated silica AWG is measured using frequency-domain interferometry and Monte-Carlo fitting of interferogram intensities. In the end, we give an estimate for the phase-error limited size of a waveguide array manufactured using state-of-the-art technology. We show that post-processing eliminates phase errors as a performance limiting factor for astronomical spectroscopy in the H-band.

Chromatic response of a four-telescope integrated-optics discrete beam combiner at the astronomical L band.

We show the results of simulation and experimental study of a 4-telescope zig-zag discrete beam combiner (DBC) for long-baseline stellar interferometry working at the astronomical L band (3 - 4 µm) under the influence of a narrow bandwidth light source. Following Saviauk et al. (2013), we used a quasi-monochromatic visibility-to-pixel matrix (V2PM) for retrieving the complex coherence functions from simulated and experimentally measured power at the output of the device. Simulation and coefficient of determination (R2) measurements show that we are able to retrieve the visibility amplitudes with >95 % accuracy of our chromatic model source up to a bandwidth of 100 nm centred at 3.5 µm. We characterized a DBC manufactured by 3D ultra-fast laser inscription (ULI) written on gallium lanthanum sulphate (GLS). Experimental results showed retrieval of visibility amplitude with an accuracy of 80-90 % at 69 nm bandwidth, validating our simulation. The standard deviation of experimental phase residuals are between 0.1-0.4 rad, which shows that the retrieval procedure is sufficient to get good quality images, where phase perturbations of less than 1 rad are expected under good seeing conditions for astronomical applications.

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

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.

Fiber Vector Bend Sensor Based on Multimode Interference and Image Tapping.

A grating-less fiber vector bend sensor is demonstrated using a standard single mode fiber spliced to a multimode fiber as a multimode interference device. The ring-shaped light intensity distribution at the end of the multimode fiber is subject to a vector transition in response to the fiber bend. Instead of comprehensive imaging processing for the analysis, the image can be tapped out by a seven-core fiber spliced to the other end of the multimode fiber. The seven-core fiber is further guided to seven single mode fibers via a commercial fan-out device. By comparing the relative light intensities received at the seven outputs, both the bend radius and its direction can be determined. Experiment has shown that a slight bend displacement of 10 µm over a 1.2-cm-long multimode fiber in the X direction (bend angle of 0.382°) causes a distinctive power imbalance of 4.6 dB between two chosen outputs (numbered C4 and C7). For the same displacement in the Y direction, the power ratio between the previous two outputs C4 and C7 remains constant, while the imbalance between another pair (C3 and C4) rises significantly to 7.0 dB.