5:00 PM - EN04.06.28
Luminescent Solar Concentrators as Detectors in Free-Space Optical Communication Systems and Their Bandwidth Limits
Ioannis Papakonstantinou1,Mark Portnoi1
University College London1
Luminescent solar concentrators (LSCs) were originally proposed for the efficient collection of solar energy and have since found a prominent place in building integrated photovoltaics. In its simplest form, an LSC device is composed of a transparent host matrix doped with fluorescent materials. The fluorophores absorb the incident light and typically re-emit it isotropically at a longer wavelength, due to the Stokes shift effect. The majority of the re-emitted light is trapped within the host matrix by means of total internal reflection, where it is waveguided towards the edges of the device. A thin solar cell module integrated on the sides of the LSC is then used for the photon-to-electron conversion. The ability of LSCs to concentrate light efficiently makes them a powerful photonic platform compatible with a diverse range of applications. Apart from photovoltaics, LSCs have been used in dark-field imaging, as chemical microreactors, in greenhouse coatings, and even in dynamic systems (where the time of photon arrival is of essence), such as in image recording and movement detection technologies and in free-space optical communications. In the latter case, LSCs were introduced as an efficient means to collect the diffuse light generated by fastly modulated light-emitting diodes (LEDs) in visible light communication (VLC) systems.
The first demonstrations of LSCs for VLC predominantly focused their attention on how to increase their optical gain (optical power out divided by optical power in) and field-of-view (range of solid angles for which the detector accepts light). However, little attention has been devoted to the performance of such devices in the time and frequency domains. Typically, fluorophore lifetimes are between 1 and 100 ns, which is orders of magnitude slower than the response time of currently employed high-quality communications semiconductor components, indicating that LSCs likely form the weakest link in optical communication channels. Worse still, the overlap between the absorption and emission spectra of fluorophores causes reabsorption, inducing further temporal delays and adding to the bandwidth (BW) limitations. In spite of LSCs potentially acting as barriers to high data-rate transmission, there is no existing study into the profound question of bandwidth limits in optical systems employing them.
This study makes a threefold contribution. i) We present a general analytical model for the prediction of the impulse response in arbitrary LSC geometries and show that for a large class of LSCs, for which reabsorption can be ignored, LSCs behave as simple low-pass, RC circuits. Crucially, we subsequently show that in the case of optically dense LSCs, for which multiple reabsorption events occur, the above equation becomes a strict upper bound and hence sets the fundamental bandwidth limit for any LSC integrated optical communication channel. ii) The next contribution of this work is the development of a powerful time-domain, Monte Carlo (MC) modelling framework for LSCs. With the MC model the received power and the system’s field-of-view can accurately be predicted, inferring fundamental quantities, such as the signal-to-noise ratio (SNR) and the bit-error ratio of the link. iii) Finally, we highlight a subtle trade-off between maximising the collection efficiency of LSCs and increasing their bandwidth, accentuating the importance of evaluating LSC dynamic performance when employed for use in optical communication systems.
1. Portnoi, M., Haigh, P. A., Macdonald, T. J., Ambroz, F., Parkin, I. P., Darwazeh, I., & Papakonstantinou, I., "Bandwidth Limits of Luminescent Solar Concentrators as Detectors in Free-Space Optical Communication Systems". Light: Science & Applications, 10 (3), 2021