# Transmitters and Receivers for High Capacity Indoor Optical Wireless Communication
## 3 Receiver for Optical Wireless Communication
### State-of-the-Art  
There are several approaches to solving the problem of bandwidth-aperture and aperture-FoV optimization. Photodiode arrays have been proposed in [14,15]. Their advantage is that by arranging the photodiodes in a square matrix, the aperture and the generated photocurrent are increased but the bandwidth remains the same as of a single photodiode. A 8 \(\times\) 8 photodiode array presented in [14] achieved 3.3 Gbps with a field of view greater than 40\({}^{\circ}\) over the distance of 1 m, and an 8 \(\times\) 8 array presented in [15] showed transmission of 25 Gbps over 10 m. The performance of PD arrays can be improved by placing microlenses on top to expand the field of view and increase tolerance to misalignment [16]. In [17], 12 photodiodes are arranged in an array, however, each is processed in the electrical domain separately, which allows dynamic tracking and localization with which 40 Gbps was achieved. This concept is called an Angle Diversity Receiver (ADR), and in our previous work [18], we presented a quad photodiode array with which we achieved 20 Gbps and a field of view greater than 32\({}^{\circ}\). The field of view of the 2 \(\times\) 2 ADR PD array can also be improved to 60\({}^{\circ}\)with mechanical structures such as compound parabolic concentrators [19]. A nature-inspired angle diversity receiver, shown in [20], is based on an animal eye. The receiver consists of a photodiode array with microlenses placed on a hemisphere for a wide field of view. It is not a receiver for OWC but a camera; nonetheless, it is mentioned for its potential adaptation. Another type of OWC receiver that has been proposed is a large-area avalanche photodiode shown in [21]. In that work, 1 Gbps over 3.3 m with a field-of-view of 68\({}^{\circ}\) was achieved. The sensitivity of OWC receivers can be reduced to as low as one photon per bit as shown in [22], while still maintaining a high data rate of 10.5 Gbps. However, this solution must be cooled down to a few degrees Kelvin, deeming it impractical for widespread indoor OWC systems. Photoluminescent detectors have been presented in [23,24]. They can be omnidirectional receivers with very large aperture areas, which is promising for the indoor OWC, however the bandwidth of these devices is relatively low. Metasurfaces can be considered a likely candidate for future research, as shown in [25, 26, 27]. These materials absorb light coming from a wide range of angles and reemit collimated light at longer wavelengths, effectively reducing the etendue of the beam. However, etendue-reducing metasurfaces have low efficiency, at the time of writing, making them impractical for indoor OWC. An aperture of OWC receivers can be made larger with mechanical elements such as non-imaging optics [28,29]. A compound parabolic concentrator is a device which can make the aperture substantially larger at the cost of a lower field-of-view. However, the proposed existing receivers for optical wireless communication listed above hinder the scale up of transmission networks. The solution to overcome compromisesbetween the aperture, bandwidth, and field of view can be decoupling light collection from light detection. In this way, both can be independently optimized. Additionally, by decoupling light detection, the receiver can facilitate additional functionality such as WDM demultiplexing, or coherent detection. A device which allows that is a photonic integrated circuit. In [30], a surface grating coupler was connected to a waveguide photodiode with a bandwidth of more than 110 GHz, which could allow very high data rate transmission. The surface grating couplers can also be arranged in an array as shown in [31]. There, the array in combination with Mach-Zehnder Modulators is used to decouple SDM beams. Surface grating couplers can also be used for WDM signals. Refs. [32; 33] show grating couplers with an integrated wavelength demultiplexing function. However, these couplers can decouple only two and three wavelengths, respectively, which limits the scalability.