[Paper Review] Photonics-based mid-infrared interferometry: 4-year results of the ALSI project and future prospects

Lucas Labadie, Stefano Minardi+8 more|arXiv (Cornell University)|Mar 5, 2020

Laser Material Processing Techniques28 references3 citations

TL;DR

This paper presents the 4-year results of the ALSI project, demonstrating ultrafast laser-written mid-infrared integrated optics with low propagation losses in GaLaS and ZBLAN glasses, achieving a 60% total throughput in a 4-telescope beam combiner. The study confirms feasibility for high-contrast, broadband interferometry in the L, L′, and M bands, with chromatic dispersion below 0.35 μm⁻¹ and instrumental contrasts up to 90%.

ABSTRACT

In this contribution, we review the results of the ALSI project (Advanced Laser-writing for Stellar Interferometry), aimed at assessing the potential of ultrafast laser writing to fabricate mid-infared integrated optics (IO) devices with performance compatible with an implementation in real interferometric instruments like Hi5 or PFI. Waveguides for the L, L' and M bands with moderate propagation losses were manufactured in Gallium Lanthanum Sulfide and ZBLAN glasses and used to develop photonic building blocks as well as a full mid-IR 4-telescope beam combiner. We discuss the advantages and disadvantages of the tested combiners and discuss a possible roadmap for the continuation of this work.

Motivation & Objective

• To assess the feasibility of ultrafast laser writing (ULI) for fabricating mid-infrared integrated optics compatible with real astronomical interferometers.
• To develop low-loss waveguides in GaLaS and ZBLAN glasses for L, L′, and M bands to support long-baseline interferometry.
• To demonstrate a 4-telescope beam combiner with high throughput and low chromatic dispersion suitable for on-sky use.
• To evaluate performance metrics such as instrumental contrast, propagation losses, and spectral stability for future interferometric instruments.
• To establish a roadmap for advancing mid-infrared integrated optics toward operational use in instruments like Hi5 and PFI.

Proposed method

• Ultrafast laser writing (ULI) was used to fabricate channel waveguides in GaLaS and ZBLAN glasses with core/cladding index contrasts enabling single-mode operation.
• Waveguides were characterized for propagation losses, with measured values indicating moderate loss levels suitable for astronomical applications.
• A 4-telescope beam combiner (DBC) was implemented using directional couplers and waveguide routing to combine light from four telescopes into 23 outputs.
• Polychromatic interferometric testing was performed using a 300 nm bandwidth centered at 3.8 μm to assess white-light interferogram contrast and spectral stability.
• Fourier-transform spectroscopy (FTS) was used to measure the splitting ratio of the DBC across 100 nm bandwidth, quantifying chromatic dispersion.
• Throughput was measured by injecting light into each of the four inputs individually and summing the total output power to determine overall efficiency.

Experimental results

Research questions

• RQ1 Can ultrafast laser writing produce mid-infrared integrated optics with propagation losses low enough for on-sky interferometric use?
• RQ2 To what extent does the DBC architecture maintain high instrumental contrast and low chromatic dispersion in broadband polychromatic light?
• RQ3 What is the total throughput of a 4-telescope beam combiner fabricated via ULI in GaLaS and ZBLAN glasses?
• RQ4 How does the chromatic behavior of the beam combiner compare to existing near-infrared solutions like Gravity’s fibered combiner?
• RQ5 Can the DBC architecture be scaled to six telescopes while maintaining high efficiency and low loss?

Key findings

• Propagation losses in GaLaS and ZBLAN waveguides were measured to be moderate, enabling feasibility for astronomical interferometry.
• The 4-telescope beam combiner (DBC) achieved a total throughput of up to 60% when injecting light into each of the four inputs, indicating strong potential for on-sky use.
• Instrumental contrasts of 40–90% were measured in white-light interferograms, with the highest contrasts observed on the central baseline (waveguides #10 and #14).
• Chromatic dispersion of the DBC splitting ratio was found to be less than 0.35 μm⁻¹ over a 100 nm bandwidth, approaching the performance of state-of-the-art near-infrared devices.
• The DBC design avoids bending losses by using only channel waveguides, offering a potential advantage over architectures with complex routing.
• Despite low signal-to-noise ratios limiting full visibility and phase retrieval on some baselines, the results suggest the DBC is a viable path toward high-throughput, broadband mid-infrared interferometry.