In this thesis microwave photonic (MWP) technology is central. This technology is extremely flexible. Its possible applications are very diverse: ranging from broadband wireless networks, to radar and satellite communications, sensing, and autonomous traffic.

Nowadays, several advantages are inherent to optical systems, such as high speed, low and frequency-independent propagation loss, and reduced electromagnetic interference. MWP is applicable across the entire microwave (tens of GHz) and millimetre range (hundreds of GHz), with potentially smaller size and lower weight than the well-established electronic systems, Caterina Taddei explains in her thesis work. 

'Because microwave photonics had to rely on discrete components thus far, the systems were often bulky, unstable and fragile', she says. This is why microwave photonics systems are not yet been applied in the a real context.

'Integration in fully hybrid optical systems was the main challenge in my PhD work', Caterina says. 'Apart from a theoretical and descriptive approach, I worked on fabricating and testing optical chip configurations. As an engineer, I enjoyed very much to show reliable and robust technical proof of principles. We really could show practical applications of novel optical functionalities'. 

Together with the testing of the different functionalities such as filters, optical beamforming networks, splitters and combiners - which were all successfully demonstrated - the other main goal of research was to explore the viability of the concept of hybrid integration, in particular, integration of indium phosphide (InP) and silicon nitride platforms.

With the goal to enable the processing of microwave and millimetre waves via photonic chips, Caterina investigated how photonic technologies can be exploited, to realize integrated microwave photonic systems. In particular, the interest was on the design, fabrication, integration and testing of state-of-the-art integrated microwave photonic optical beamforming networks (for Ku-, and Ka-band satellite communications).

'We focussed on hybrid integration of III-V semiconductor (indium phosphide, InP) components with low-loss and high index contrast dielectric waveguide circuits (silicon nitride),' Caterina says. 'The focus on InP is somewhat new for Mesa+. Together with spin-off Lionix, here in Twente, we work on this promising key point technology. I am a believer of the flexible and practical approach we demonstrate, in order to come up with better and novel active platforms.'

Building blocks

In the thesis work, first the basic working principles and properties of analogue photonic links were presented, as well as a description of the hybrid integrated microwave photonic demonstrators to be studied. Also the most essential photonic building blocks were quantitively described such as: interferometers, micro-ring resonators, and other combinations.

Then, a complex reconfigurable photonic filter was realized, based on silicon nitride. The filter consists of a combination of eight mutually coupled ring resonators in order to have a highly selective on-chip passband filter with a bandwidth of 72 MHz and an out-of-band rejection of at least 51 dB.

'This configuration is highly relevant for future applications,' Caterina says. 'Such a filter can be potentially used in a satellite payload, to increase the capacity and enhance the flexibility of high throughput satellites.'

Demonstrators

Further, two hybrid integrated microwave photonic systems were tested, forming again the state of the art.

The processing cores of these demonstrators were formed by true-time delay optical beam forming networks. The demonstrators were fully integrated, meaning that the inputs and outputs are signals in the microwave domain, while also the processing is done in the optical domain. Both systems were equipped with an internal hybrid laser, consisting of indium phosphide semiconductor optical amplifiers (SOAs), coupled to an external cavity arm fabricated on the silicon nitride platform, to provide an on-chip optical carrier.

'The practical relevance is taken into account fully,' Caterina says. Both, continuously tuneable delay lines and switched delay times, were tested. The two systems comprise hybrid integrated photodetectors for optical-to-electrical conversion.

Crucial step

In the last chapter a fully successful hybrid integration was reached. 'This demonstration I consider as a crucial step for the next generation of hybrid integrated photonic systems,' Caterina says. 

'After my PhD Defense I will join Lionix, working further on the integration of photonic chip systems. I am glad that I have put my industrial career on a hold, in taking up this fundamental as well as application oriented PhD project. I learnt to carefully plan my research steps, and take a critical as well as a constructive attitude towards literature and experiments already performed.' 

'I have developed my research skills enormously. Especially collaborating and sparring with the cleanroom experts within Mesa+ opened my eyes. Their support was a great advantage. Making use of their expertise, I was able to build feasible, robust and reliable, almost 'bug-free', chip configurations. Also collaborating with company Neways was instructional and provided me with up-to-date technological knowledge of this important field of research.'