1. INTRODUCTION

Sydney, with a population of over 5 million people, is the largest city of the state of New South Wales in Southeastern Australia. It is the home of five major universities, several major research institutes and private industry. Sydney has a long and distinguished history in optics with seminal research in interferometry carried out at the Commonwealth Scientific and Industrial Research Organization (CSIRO) in the 2nd half of the 20th century. The center of activity then shifted to universities, for example through the Optical Fibre Technology Centre and its successor the Australian Photonics CRC, the Centre for Lasers and Application, and the Centre for Ultrafast Devices for Optical Systems, all headquartered in Sydney. Though all of these organizations have ceased to operate, many of their researchers and students have stayed in Sydney, or have returned after stints elsewhere. With their students, and with other researchers attracted by the optics-related activities in Sydney, they have spawned a flourishing optics research and development scene in government labs, universities and industry. Over the last few decades it has been driven particularly by leadership in and membership of major national research collaborations, expertise in astronomy and space science driving the need for astronomical instrumentation, through close links with defense establishments, and more generally, through strong international links. This regional focus issue showcases work in applied optics and photonics from four Sydney universities–the University of Sydney, the University of New South Wales, Macquarie University, and the University of Technology Sydney, and their collaborators in Australia and elsewhere. The papers cover a remarkable breadth ranging from astronomical and space instrumentation, fiber preform fabrication, sensors, microwave photonics, fiber lasers, neural interfaces to integrated optical device design.

2. DESCRIPTION OF THE PAPERS

A. Advancing Sensing Technology through Optical Innovations

This special issue highlights significant advancements in sensor technology driven by optical innovations, illustrating the depth and breadth of research being conducted. It encompasses a range from advanced microwave photonic sensing technology to innovative neural interfaces and magnetic field sensing, demonstrating Sydney’s leadership in applied optics and photonics research. Such advancements not only enhance our understanding of optical phenomena but also pave the way for practical applications across various scientific and industrial fields.

Chen et al. [1] delve into the realm of microwave photonic sensors, employing a reflective microring resonator, enhanced by self-attention-assisted convolutional neural networks. This study exemplifies the integration of photonic devices with machine learning to achieve high precision and sensitivity in temperature sensing, demonstrating a significant improvement over traditional sensing models.

Ladouceur et al. [2] explore the capabilities of all-optical neural interfaces using liquid-crystal-based transducer technology. This research highlights the potential of optical approaches to brain-computer interfaces, offering a platform to revolutionise how neural activity is monitored and interfaced with electronic systems, thus opening new avenues for medical diagnostics and rehabilitative technologies.

Authored by Guo et al. [3], this paper presents a novel approach to magnetic field sensing utilizing a compact, remote optical waveguide with double-pass Faraday rotation-induced optical attenuation. This technique showcases the potential of optical components in detecting and measuring magnetic fields with high sensitivity and spatial resolution, which could be crucial for applications ranging from industrial monitoring to environmental sensing.

B. Astrophotonics

In this focus issue of Applied Optics, we are privileged to showcase cutting-edge optics research and innovations from Sydney, a hub of expertise in the field of astrophotonics. The collaborative nature of this community is demonstrated by the shared authorship between the featured articles.

Among the standout contributions is an invited review article by Norris et al. [4], which delves into the current advancements and future prospects of astrophotonics, particularly its application in observational astronomy. This article focuses on developments at Sydney-based institutions and underscores the transformative impact of photonic technologies on astronomical instrumentation. The authors elaborate on enhancements in sensitivity, resolution, and versatility brought about by advances in photonic wavefront sensing, imaging, interferometry, and spectroscopy. Additionally, the review discusses the development of innovative photonic devices, such as photonic lanterns for wavefront error correction and arrayed waveguide gratings for high-resolution spectroscopy, which are poised to revolutionise astronomical observations alongside the next generation of Extremely Large Telescopes.

Betters et al. [5] discuss a low-cost alternative to complex laser metrology systems in their article “Photonic comb: a stabilised single-mode fibre etalon for wavelength calibration”. This system utilizes a single-mode fiber Fabry-Perot etalon for spectrograph wavelength calibration, offering high precision and competitiveness by referencing hyperfine transition lines of rubidium and finely controlling the etalon’s cavity dimensions through temperature adjustment.

Yu et al. [6] explore the inverse design and optimisation of aperiodic multi-notch fiber Bragg gratings using artificial neural networks and genetic algorithms. This innovative method enhances the optimisation process, preserving key spectral characteristics and improving computational efficiency, which could significantly alter photonic design approaches, especially in fields like astronomical OH suppression and gas detection.

Taras et al. [7] outline the design of next-generation instruments for the Very Large Telescope Interferometer (VLTI) in their article focussing on three core modules: Heimdallr, Baldr, and Solarstein. These modules are designed to enhance high-angular-resolution imaging for studying exoplanetary systems, tackling atmospheric phase errors, enhancing wavefront sensing, and ensuring precise alignment and calibration to improve sensitivity, spectral resolution, and nulling interferometry capabilities at VLTI.

C. Breadth and Depth

The final set of papers illustrates the breadth and depth of optics research carried out in Sydney, covering preform fabrication, laser science and integrated optical device design.

Optical preforms, the precursors of optical fibers, are traditionally fabricated using chemical vapor deposition or rod and tube stacking. More recently, preforms have been fabricated by 3D printing which has advantages of rapid prototyping, lower cost and flexibility. Kong et al. [8], consider the effect of the inclusion of a UV-sensitive resin in the preform that allows it to be sculpted by UV irradiation. Optimizing the conditions they find that the resolution of this process can be significantly improved.

Fiber lasers that emit visible radiation have a number of applications in material processing and photodynamic therapy. In particular yellow light is useful in dermatology treatments. Lee et al. [9] describe the results of numerical simulations of ${{\rm Dy}^{3 +}}$-doped aluminosilicate fiber lasers, which emit visible light in the yellow. Through their simulations the authors show that by careful design it will be possible to improve the performance of these lasers considerably.

Directional couplers tend to be strongly wavelength dependent. Reducing this dependence reduces the complexity and cost of the circuit of which they are part and makes the couplers more robust against fabrication imperfections. Passarelli et al. [10] describe a systematic procedure combining an approximate analytic method and sophisticated numerical simulations that leads to directional couplers with very wide bandwidths.

We thank the Editor of Applied Optics for the opportunity to provide an overview of some of the optics and photonics research in Sydney. We also thank the contributors, the editorial staff, associate editors and the reviewers for helping to make this focus issue a reality.