Wireless and Fiber Optics

RESEARCH TOPICS

Speciality Optical Fibers
 +  anti-resonant fibres, hollow-core photonic bandgap fibres

With the invention of photonic crystal fibres (PCFs, or also denoted as microstructured optical fibres, MOFs) in 1996, a new field of research opened for optical fibres as for telecommunications (wavelengths 1260 – 1620 nm) the conventional silica single-mode fibre was already a mastered technology. PCFs allow vast opportunities of parameter design by modification of their inner air-hole structure, such as the chromatic dispersion, attenuation, single-mode regime, nonlinearity and many more. Furthermore, with the advances in glass processing, non-silica glasses (fluoride, lead-silicate, chalcogenide, etc.) can be exploited to move deeper into the near and mid-infrared region. In the past few years, hollow-core fibres, as the masterpiece of PCFs, have been prepared with reasonably low attenuation enabling cutting-edge applications.

At the moment the research of our team is mainly focused on hollow-core photonic bandgap fibres (HC-PBGFs) and anti-resonant fibres (ARFs), where both fibre types guide light in a hollow air-core. Hollow-core fibres offer many advantages over conventional solid-core fibres due to the strongly reduced interaction of the guided light with the glass material. Examples of such benefits are the extremely low fibre non-linearity, low and stable signal latency, the possibility to construct long-length gas cells.

We concentrate on the efficient light coupling from standard solid-core fibres into hollow-core fibres. We have designed and developed the interconnection using a world-unique approach. We propose anti-reflective or highly reflective layers with respect to state-of-the-art research applications. All this is made on an international level in cooperation with one of the world-leading institutes in PCFs, the University of Southampton.

Our background includes experience with PCF characterization, fibre tapering for sensor purposes, rare-earth doped amplifiers and lasers. Furthermore, we have a brand new glass fusion station, the top-shelf Fujikura/AFL LZM-100 giving us countless possibilities of research extensions.

Fiber-Optic Sensors
 +  detection of liquids, displacement measurement, fiber-optic gyroscope

Fibre-optic sensors (FOS) can be found almost anywhere, preventing disasters, improving healthcare, analyzing food quality, enhancing performance in industry and helping to reveal fundamental natural phenomena. By using optical fibres, physical quantities such as temperature, pressure, force, flow, rotation, position and radiation can be measured. Moreover, electromagnetic field can be analyzed in terms of magnetic fields having effects on polarization in the optical fibre.

The most successful representatives are fibre Bragg grating (FBG) sensors, which monitor buildings, bridges and other constructions for possible cracks. Also, also fibre-optic gyroscopes (FOG), which are the most precise gyroscopes used especially for aerial and space inertial navigation.

In our team, we focus on three kinds of FOS: i) We investigate refractometric sensors for detection of liquid quality, either by refractive index or by spectroscopic analysis. We developed fibre tapers with enhanced evanescent-wave overlap, designed and prepared photonic crystal fibres for sensing and also experimented with chalcogenide multimode sensors for MIR spectroscopy; ii) We are involved in the research of precise displacement measurement using Fabry-Perot interferometers (FPI). Furthermore, we now focus on FPI in combination with hollow-core fibres (see the section above); iii) Last but not least we have been involved in FOG development, both in hardware and software.

Our background includes experience with fibre-optic sensor design, spectroscopy, fibre-optic gyroscopes, fibre tapering and extremely precise interferometers. Furthermore, we have a brand new glass fusion station, the top-shelf Fujikura/AFL LZM-100 giving us countless possibilities of research extensions and sensor preparation.

Microwave Photonics
 +  5G systems, radio-over-fibre

Microwave photonics combines multidisciplinary fields of optical, microwave and electrical engineering. Therefore, it must cover the spectrum area from very low frequencies (below 1 kHz) up to hundreds of THz associated with the optical domain. In our team, we mainly focus on characterisation, testing and optimisation of optical components for utilisation in new wireless networks operating in high-frequency bands.  Also, overall systems as radio over fibre (RoF) and radio over free-space optics (RoFSO) are tested to maximise transmission capacity and flexibility under different conditions. We have several running projects and international collaboration on microwave photonics with universities in Southampton and Valencia.

Free-space Optics Systems
 +  diversity techniques, relay links, thermal and atmospheric turbulence

Free-space optics (FSO) introduces a line-of-sight optical wireless communication (OWC) technology which offers many advantages for modern communication infrastructures including large frequency bandwidth and substantially high available data rates, immunity to electromagnetic interference, license-free spectrum, higher security of transmission due to narrow optical beams etc. Nevertheless, many factors may affect an optical beam causing fluctuations of the received optical signal like a scattering of small particles, beam wandering and scintillation caused by thermal turbulence or atmospheric turbulence, which is the major factor of bit error rate performance degradation.

In our research, we exploit several wireless optical links in the CTU campus connected into a simple network and as well our dedicated turbulent chamber and OWC laboratory. We characterize atmospheric parameters and from that develop behavioural models of atmospheric turbulence. The error performance of the FSO links under turbulences can be significantly improved using diversity techniques, e.g. by employing multiple transmit or receive apertures or by utilization of several relay FSO links (here, we experimentally test both amplify and forward and regeneration and forward techniques).

Our international collaborators include academic research groups from Northumbria University, KU Leuven, Indian Institute of Technology Delhi and other teams.

Visible Light Communications
 +  digital signal processing, optical camera communications, organic LEDs

In recent years, visible light communications (VLC) has rapidly gained interest among research communities worldwide. It is an emerging technology for future high capacity communication links utilizing the visible range of the electromagnetic spectrum (~370-780 nm), which is not only licensed free but free from spectral overcrowding unlike radio frequencies (RF). VLC utilizes light-emitting diodes (LEDs), modulating them at high speeds that are much faster than the human eye can detect, to simultaneously provide data transmission and room illumination. A major challenge in VLC is the LED modulation bandwidths, which are limited to a few MHz.

Recently, we have been focused mainly on the signal processing techniques enabling effective utilization of the transmission bandwidth in VLC. We have adopted multi-band carrier-less amplitude and phase (m-CAP) modulation in VLC domain, originally developed for optical fibre networks, and proposed and verified a number of techniques improving m-CAP performance in terms of bit rate, spectral efficiencies, and computational complexity. Among inorganic LEDs, organic-based LEDs (OLEDs) represent a possible solution for solid-state lighting applications due to their advantage such as ultra-low costs, mechanical flexibility, and large photoactive areas. However, their modulation bandwidth is limited to a few hundreds of kHz introducing significant bottle-neck in VLC networks. Thus, we are also focused on equalization schemes that enhance VLC systems performance and are necessary for designing OLED based networks.

We have many international academic collaborations with world-leading research groups in the field of VLC, including Northumbria University and Newcastle University.

Optical camera communication (OCC) can be considered a convenient and versatile short-range communication technology within the framework of optical wireless communications. OCC is a pragmatic version of VLC based on a smart device camera that allows easier implementation of various services in smart devices. OCC can be a more favourable solution, especially in indoor environments, due to one compelling fact that the OCC is based on a camera as the receiver and nearly six billion smartphones fitted with cameras are available worldwide.

We have addressed the issue of longer processing time in OCC using neural network-based processing for applications such as motion detection over the existing OCC links. The motion detection is considered as an add-on functionality in OCC to control various smart devices in smart home environments. Recently we have focused on OCC System for Internet of Things based on OLEDs. Performance of outdoor OCC links using focusing and defocusing techniques for applications such as vehicle-to-vehicle communications is also performed in an outdoor environment. Currently, we are working on OCC link analysis to develop multiuser environment as well as to provide mobility in indoor OCC scenarios. Collaborative work with international academic institutes working in the same field gives a way out to interesting results for the ongoing research. Our international collaborators include academic research groups from the University of Las Palmas de Gran Canaria and Northumbria University.