**5. Conclusion**

In this Chapter, a detailed comparative experimental study was carry out to pursue advanced performances corresponding to the key parameters of two VCSELbased photodetectors operating in free-running or optically injection locked mode as well as an inherent pin-photodetector. During the preliminary study, the key static and dynamic parameters were quantitatively determined and the optimal operating modes were derived for both versions of VCSEL-based photodetector as a separate microwave-photonics circuit element. Based on them, a final study was carried out to evaluate the processing quality when the both versions of VCSELbased photodetectors or an inherent pin-photodetector work as an optical-toelectrical converter of a typical microwave circuit, which can become the core when developing a cost- and power-efficient base station for a mobile cellular telecommunication system of fifth generation [27], as well as for an advanced Wi-Fi system [28] of fiber-wireless architecture, which will significantly increase its service area. As a result, it was confirmed that better processing quality can be obtained by using the free-running VCSEL-based photodetector version.

Further research will focus at a detailed comparative studying the features of the VCSEL-based photodetectors, at expanding their bandwidth to the millimeter wavelength range, as well as at the development of optimal microwave-photonics circuits using one of these devices as an optical-to-electrical converter.
