Super Receivers for Visible Light Communications

Lead Research Organisation: University of Oxford
Department Name: Engineering Science


In the near future, light emitting diodes (LEDs) will replace all other sources of light - from the lamps that light homes and offices to the headlights of cars. As well as providing illumination, these LEDs can be used to transmit data, and so offer an opportunity to create a new wireless infrastructure for data transmission. The demand for wireless communications to smartphones, watches, tablets and other devices is growing at a rate of 50% per year, and new technologies are needed to augment the capacity of conventional WiFi. Using LEDs in visible light communications offers a huge potential capacity to support this growth and to provide new services that use localised wireless communications.

While LEDs can transmit the information, an optical receiver is needed to collect the transmitted light, convert it to an electrical signal and extract the transmitted data. The maximum amount of light that can be transmitted is limited by the illumination brightness and concerns for the eye safety and comfort of users. The sensitivity of the receiver therefore ultimately determines the range over which optical data can be transmitted and/or the maximum possible data rate. The sensitivity of existing receivers for visible light communications is limited by a combination of the methods used to collect light and the devices used to convert this light to an electrical signal.

In this project we aim to create new super receivers that are significantly more sensitive than existing optical receivers; that overcome conventional limits for combining speed, sensitivity and easy alignment; that are thin and flexible enough to be easily integrated onto any device. A dramatic change in performance will be achieved by combining two technologies- fluorescent concentrators and arrays of single-photon avalanche photodiodes- in a receiver for the first time. The first will use fluorescent materials to absorb the transmitted light signal and re-emit it at a different wavelength onto the detector. Using this method we will collect light over large areas using a thin, flexible layer which guides and concentrates the emitted light to its edges.

The second technology is a light detector capable of detecting individual photons. We will develop methods to count photons from the transmitter in the presence of ambient light. We will explore how to optimise the fluorescent materials and light collecting layer to efficiently concentrate light onto one or more light detectors, and develop methods to maximise the amount of data transmitted by optimising how the data is represented. These super receivers will be tested in free-space visible light communications links to quantify their performance. Our estimates suggest that this approach could lead to a 100 times improvement in performance over current receivers, enabling faster data transmission, longer transmission ranges and the ability to operate in difficult environments, such as in the presence of bright ambient light.

Planned Impact

This proposal aims to develop a new type of optical receiver that will achieve much higher sensitivity and thereby faster and more reliable visible light wireless communications. Optical wireless networks- known as LiFi- are expected to be a $75.5 Billion market by 2023, and the UK has recognized leaders in this emerging market (e.g. our project partner PureLiFi). Creating planar, efficient receiver structures that can be incorporated in almost any surface, with several orders of magnitude higher performance than currently available receivers has the potential to significantly increase the range of uses for LiFi. We therefore anticipate that this new kind of receiver could have a significant societal and economic impact in the UK in three main ways: through wider/faster adoption of LiFi in future wireless communications; new market applications of SPAD detectors; new internet services for the public:

Wider/faster adoption of LiFi in future wireless communications

A key part of the success of any communications technology is the data rate that can be achieved, and the ability to operate under a wide range of conditions. The orders of magnitude increase in receiver sensitivity that a successful project will achieve could be used to increase the data rate and/or reliability of LiFi communications systems. In addition the radical change in form-factor, from bulky 3D optics to a thin conformal layer, which will occur when these new receivers are employed, will mean that almost any surface could be used as a receiver. This will be an advantage in any LiFi system however this change in form factor will significantly increase the chances that LiFi will play a substantial role in the Internet of things, manufacturing 4.0, and future 'smart' environments. Overall the benefits of super receivers will significantly increase the chances that LiFi will become a widely adopted, every-day technology with an even bigger market than current predictions.

New market applications of SPAD detectors

The use of SPADs and fluorescent concentrators together is unknown, and therefore potential new applications and markets will be generated by the work. Our research will have an impact in the design of new SPAD detectors optimized for communications, or for other applications for which large area, sensitive optical detectors are required such as for sensing.

New internet services for the public

Communications and instant access to information are now a fourth utility, which is expected to be even more readily available than water, gas and electricity. Consequently, society increasingly relies on wireless access to information for entertainment and to function efficiently. Smart-cities and other information-centred organization structures are increasingly used to create efficient sustainable communities. LiFi is 'human centred' in that the light used to communicate can be seen by the user and sometimes controlled by them. This offers the potential to build trust in communications and interactions with infrastructure, with impact in areas such as smart-cities.
A successful outcome of this project would directly impact the users of wireless communications systems by allowing high-speed wireless access in new practical configurations with increased range and security. In turn, by providing improved underlying infrastructure, this will enable the continued growth of the digital economy. Areas in which there could be significant impact from improved wireless communications include healthcare, the armed forces and police, car-to-car communications on motorways, the airline industry and in sports stadia. In addition to existing uses the ability to provide site-specific information at different locations in a building creates new possibilities. For example, it will be possible to give museum visitors information about nearby exhibits or shoppers' information about nearby products.
Description Wireless communications such as WiFi are an important part of modern life. Unfortunately, in some situations, such as cafes, classes and offices, with many users these systems can't provide the expected quality of service. To enable the quality of services to be maintained, and indeed improved, in these situations visible light communications is being developed. In this new form of wireless communications data is transmitted using a light source such as a light emitting diode (LED) or a small eye-safe laser.

A key part of any communications system is the receiver, which receives the transmitted data. The aim of this project was to dramatically improve the performance of VLC receivers by reducing the minimum intensity of light needed to support the transmission of data. To achieve this the detector used in existing receivers has been replaced by a silicon photomultiplier (SiPM). These devices have been used because, unlike other detectors, they are capable of detecting individual photons. Consequently, they are more sensitive than other detectors. Using this sensitivity the project team were able to demonstrate the world's most sensitive VLC receiver. This is particularly important for VLC system because too much visible light can cause harm and there are therefore limits on the amount of light that can be used to transmit data.

The performance of a VLC receiver depends upon a combination of their sensitivity and their speed of response, characterised by their bandwidth. A well-known theorem suggests that the most important characteristic of a receiver is its bandwidth. An advantage of SiPMs is that they consist of many individual elements, known as microcells, acting in parallel. Consequently, unlike other detectors, they can have a wide bandwidth and a large area. However, larger SiPMs have a lower bandwidth. This means that previously all research groups have investigated systems using 3 mm by 3 mm SiPMs. However, we realised that the limited power from transmitters gave larger area SiPMs an advantage and we then demonstrated that 6 mm by 6 mm SiPMs are a better choice for data rates up to approximately 2 Gbps. This insight enabled the team to show for the first time that using a SiPM receiver, it is possible to transmit 1 Gbps to each 2 m by 2 m section of a room. This data rate is more than the data rate recommended for running an office with 5 or more users. The data rate is therefore high enough to provide the quality of service in those situations where WiFi can give a disappointing quality of service.
A disadvantage of the method used to detect each photon in a SiPM is that after a photon has been detected by a microcell the microcell's ability to detect photons is reduced for a time known as the recovery time. This creates a non-linear SiPM response. Depending upon the details of its operation microcells, and hence SiPMs, can be either paralysable or non-paralysable. This distinction is important because as the intensity of light reaching the SiPM increases a paralysable SiPM has a peak response, whilst the response of a non-paralysable SiPM becomes a constant. Previously, other researchers assumed that the currently available SiPMs are paralysable and therefore in some situations reducing the voltage applied to the SiPM will improve the receiver's performance. The team demonstrated that in fact these SiPMs are non-paralysable and so reducing their bias voltage won't increase their performance.

The performance of a SiPM receiver can be improved significantly by protecting the SiPM from ambient light. Conventionally, photodetectors are protected from unwanted light using optical filters that reflect or absorb the unwanted wavelengths. The project team showed that a better way to protect the SiPM from ambient light is to use fluorescence. In this process the wavelengths from the transmitter are selectively absorbed by a fluorophore in an optical element such as an optical fibre. The fluorophore can emit light at a different wavelength which is retained within the optical element until it reaches the photodetector. The team demonstrated that one advantage of using this new approach was that it allowed the receiver to have a very wide field of view. In addition, any light that isn't absorbed by the fluorophore passes through the optical element. Optical elements containing different fluorophores can then be stacked to create a system that can receive data transmitted on different colours (wavelengths) of light. This ability can be used to increase the received data rate and/or to avoid interference from transmitters covering a neighbouring area. The disadvantage of using fluorophores in optical elements is that they can limit the systems bandwidth. This can also be limited by the time it takes for light to travel within the optical element. A relationship for the optimum length of a fluorophore doped optical fibre and its bandwidth has been described by the team. In addition, a new method of processing the data before it has been transmitted to increase the effective bandwidth of the VLC system has been developed.

The best existing SiPM for a particular application depends upon some factors related to the application, for example the required data rate and the ambient light level, and factors from the SiPM, including its area, bandwidth and non-linear response. These multiple factors make selecting an existing SiPM for a particular application difficult. The team has therefore constructed and validated a simulation of a SiPM which can be used to compare the performance of existing SiPMs in different situations. In addition, it can be used to predict the performance of future SiPMs. Using this simulation the team has demonstrated that, when the safety of transmitters is taken into account, existing SiPMs will be limited to data rates less than 10 Gbps. However, they also showed that using the recently developed ability to stack two integrated circuits it will be possible to create SiPMs that can support much higher data rates, possibly several hundred Gbps. In addition, these SiPMs would be much better than the receivers used in existing fibre optic communications systems. One of the project partners could potential manufacture these receivers.
Exploitation Route We have used a validated numerical model to predict that, by using the recently developed ability to stack two integrated circuits, it will be possible to create SiPMs that can support data rates of several hundred Gbps. In addition, these SiPMs would be much better than the receivers used in existing fibre optic communications systems. One of the project partners is currently considering their response to these predictions.
Sectors Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics