High-performance laser facet optical coatings fabricated by next generation microwave ion beam deposition

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BackgroundHigh power semiconductor lasers (laser diode bars) form and integral component within a vast array of systems for medical, military, commercial and industrial applications, in addition to advanced manufacturing techniques including surface treatment, cutting and welding. Often these semiconductor lasers are constructed by joining several emitters (width ~100um) in an array, with optical coatings applied at the ends to provide required optical feedback within the laser cavities of the laser diode bar. Many commercial and industrial devices can generate hundreds of Watts of output power, setting challenging requirements for the performance and robustness of the optical coatings.Ion beam deposition has been the method of choice for the highest performance optical coatings for the majority of laser applications. The energetic sputtered material results in a compact/dense film, with improved environmental stability compared to other processes, in addition to yielding ultra-smooth films with low optical scatter (rms surface roughness < 1nm).More broadly, many precision measurement and imaging systems, require extremely narrow linewidth lasers and the use of ultra-low optical loss coatings - typically classed as highly reflecting (HR), antireflection (AR), and optical filters (for spectral trimming). Applications often lie within the biomedical and regenerative medicine area. For example, Raman spectroscopy requires extremely narrowband optical filters, with the highest performance only being achieved using ion beam deposition. The University of Strathclyde is the only university in the UK that develops ion beam deposited optical coatings (in the Department of Biomedical Engineering, in collaboration with Physics, IoP and Fraunhoffer CAP). Narrowband optical coatings can also be used to enhance fluorescence imaging, a standard technique for imaging at the cellular level. Additionally, checking patient safety by monitoring the exhaled breath (capnography) can also be enhanced through the use of optical filters. Finally, many gas sensors designed for monitoring pollution will reply on optical cavities (two mirrors with highly-reflective coatings) for recycling the light and enhancing the signal associated with specular absorption.The industrial sponsorship included with this project demonstrates the commercial relevance.AimsThe University of Strathclyde has established novel ECR (electron cyclotron resonance) ion beam deposition, in addition to procuring the industry standard RF (radio frequency) ion deposition technology from Veeco (US). Scotland hosts a variety of world-leading laser engineering companies, and associated technologies, which could benefit from the capabilities at Strathclyde. Helia Photonics is a world-leader in advanced, high performance optical coating technologies, particularly for solid state laser coatings (laser diode bars). This project would be an ideal opportunity to explore a long-term, strategic partnership with Helia Photonics, in order to exploit the ECR ion beam deposition technology.Various optical materials will be investigated for optical coatings, including silica, silicon, tantalum pentoxide, zirconium dioxide, scandium oxide, hafnium dioxide and germanium. Alloys of these materials will also be investigated, as this can often help fabricated more stable amorphous structures, which in turn could improve the laser damage threshold performances.Working in partnership with Helia Photonics, we will aim to identify commercial applications for these coatings, with the aim to establish consultancy and contract work using the facilities at Strathclyde.