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NExt generation of Tuneable LASers for optical coherence tomography

Periodic Reporting for period 1 - NETLAS (NExt generation of Tuneable LASers for optical coherence tomography)

Reporting period: 2020-02-01 to 2022-01-31

NETLAS is an interdisciplinary/intersectoral European Training Network providing state-of-the-art research training in the design and build of the next generation of tunable optical sources for optical coherence tomography (OCT) applied to medical imaging and non-destructive testing. NETLAS will foster training and education of 15 Early Stage Research (ESRs) at 8 world-leading academic institutions, and 6 non-academic institutions.
The training and research programme is born out of a strong and clear need to respond to the challenges of providing faster, deeper, higher resolution imaging and with a smaller footprint (portable) at a low cost.
Societal benefits: Stimulate the development of cutting edge photonic technologies with direct applications in biomedicine and elsewhere, improving medical care and outcomes for patients, creating new markets for European engaged in building components for photonics research.
NetLaS training concept is to build a scientific and technology development program comprising multidisciplinary and intersectoral crossovers. Training objectives are defined to:
• enhance the appeal for an academic and industrial research career by delivering a structured training program taught by leading scientists
•provide academic and private-sector employers with researchers skilled in a wide range of techniques and methods
•produce researchers with excellent transferable skills able to transform abstract and challenging ideas into influential and practical outcomes
•create an active, long-term network of young researchers whose personal contacts and expertise will help Europe shaping the future of laser technologies and applications, delivering researchers with the skills/ability to become leaders in fields of Photonics.
NETLAS Training Programme for young scientists offers a combination of scientific, technical, entrepreneurship, innovation and management skills in 3 categories:
1.Network-wide Training (individual research projects; secondments; joint summer training schools; international conference).
2.Local Training (host institutions provides scientific/technical transferable skills courses and training activities).
3.External Training (courses; visits; conferences outside the network).
A student coming in contact with different partners in the consortium have the unique experience of visiting and learning from world leading groups and world leadership in industry: Superlum Photonics (Ireland) - maker of the best 800 nm semiconductor devices, Innolume (Germany )- maker of the best 1050 nm devices, NKT Photonics (Denmark) - maker of the best photonic crystal fibres and best supercontinuum sources, Optores (Germany) - maker of the fastest commercial swept laser systems, Centervue (Italy) – offering eye diagnostics/ophthalmic products worldwide and OCTLIGHT (Germany) – a high-tech company developing and selling OCT Swept Lasers.
ESRs have the chance for training/internships and acquire knowledge in OCT-based non-destructive testing within the Research Center for Materials Characterization and Non-Destructive Testing (Germany), internationally well-recognized institution covering two main areas on optical and acoustic non-destructive testing.
ESRs will attend lecturers and training within Northwick Park Hospital, Biomedical Research Centre at UCL Institute of Ophthalmology and Moorfields Eye Hospital (all in London).
This is the first time to our knowledge that a strategic approach is taken to optical source development for coherence imaging, drawing on world leading expertise in semiconductors, supercontinuum and swept sources.
Preliminary research work was conducted on three of the four strands:

1st: Three technologies of emitters (WP2).
* An experimental MOPA system was assembled to determine the possible spectral and power characteristics at 1060 nm with the SOAs available at Superlum. The experiment showed the possibility of achieving an output power of 50 mW at a spectrum width of 28 nm with a center of 1080 nm. In future, it is planned to grow the optimized wafer structure by combining the experience of Superlum and Tampere University (Finland).
* The MEMS fabrication was optimised in terms of DBR composition and cleanroom processing, and a new mask design.
* A first iteration on the development of red-emitting gain membranes for high-brightness wavelength tunable membrane external cavity surface emitting lasers (MECSELs) was completed. These novel lasers should enable operation at red-visible wavelengths with tunability > 10 mW emission. Although in this 1st iteration the lifetime was not sufficiently high for full-scale MECSEL development, the membrane fabrication processes were established. A first MECSEL with more than 60 nm tuning range was demonstrated at 1 µm region and used to experiment the automated wavelengths tuning function based on birefringent elements.
* Training of students on cleanroom processing is in progress. Design of 1060nm and 1300nm wide bandwidth devices with good thermal dissipation.

2nd: Four Technologies of novel tuning modalities (WP3)
* Bidirectional tuning of MEMS VCSELs using 3 contacts to the movable mirror has been analysed and demonstrated. Using fixed voltages on the outer contact the voltage requirement to the center tuning contact is reduced and accurate MHz tuning is possible. Custom electronics were made inserting step-up transformers in the signal path. The fastest sweep speed from MEMS VCSELs were accomplished.
* All components for both FDML lasers have been received or are being manufactured. We demonstrate an ultra-high-accuracy chromatic dispersion measurement with a timing measurements accuracy down to ~200 fs. The 1st order dispersion coefficient for a wavelength range of 850 ± 30 nm is -100.446 ps/nm/km. The high repeatability proves the reliability of our setup which can be easily adapted to other wavelengths like 1200 nm.
* in record of 5 months, a time stretch laser was assembled, characterised and shipped to Kent Univ, operating at 80 MHz,1060 nm;
* the concept of dual resonance in dispersive cavities using an intensity modulator has been proven, as a stable operation that reported so far employing SOA modulation;
* a time stretch laser based on Erbium doped fibre was designed, assembled and being optimised.

3rd: Three Technologies of novel signal processing
* First evaluation of high-speed GHz optical signal detection systems relevant for later optical clocking and synchronisation; worked on GHz arbitrary signal generation transimpedance generation of photoreceivers.
* Postaquisition Master Slave method advanced using 20 GHz oscilloscope.
12 technologies will be developed: 3 technologies of semiconductor lasers that allow optical tuning (Emitters), 4 technologies of novel solutions for swept source (tunable) laser systems (Tuning modalities), 3 Technologies of novel signal processing for OCT imaging and testing applications of SS based systems (Signal processing) and evaluation of the suitability of the 10 technologies above to two specific application technologies: high resolution medical imaging and non-destructive testing.

NETLAS will provide the ESRs with an excellent foundation for their future careers through integrated training far beyond what would be possible at any single institution.

The ESRs will gain a strong advantage compared to their peers who often lack the cross-sectoral and interdisciplinary understanding of their research field.
Schematic diagram of interactions within NETLAS
Characteristics of devices produced by Superlum part of NETLAS
Spectral dispersion measured
ESR S1 at Superlum
ESRs at UzL