## 1 Introduction  
Over the last few years, the growing increase in traffic carried by emerging telecommunication network applications, which require higher data rates, higher security levels, along with lower power consumption and cost, has triggered remarkable scientific interest in the transition from the increasingly crowed radio frequency (RF) spectrum to the more bandwidth-effective optical spectrum, where OWC, commonly known as Free Space Optical (FSO) communications, have attracted particular attention lately for a variety of applications in both medical and telecommunication fields [1; 2]. In basic terms, this kind of technology uses an optical information-bearing carrier--i.e., visible, infrared, ultra-violet--which is propagated through the unguided transmission medium space, seawater, or skin, in order to transmit the information signal from one end to another. Indeed, FSO links use either Lasers or LEDs (light emitting diodes) at the transmitter's side and photo-detectors at the receiver, in order to detect the propagating optical beam. In fact, by modulating the intensity and/or the phase of light waves traversing atmosphere, space, seawater, or even biological tissues, the FSO links can reliably transmit huge amounts of data. FSO systems have concrete advantages in comparison with their RF counterparts, mainly due to their operation at optical wavelengths. In fact, by increasing the carrier frequency from RF to that of optical waves, the information capacity is increased by many orders of magnitude. Thus, FSO can establish ultra-high bit rate wireless links. Furthermore, FSO systems can offer higher security level because of the very narrow laser beam that carries the information, which makes undetected interception difficult. Additionally, considering that FSO terminals are more compact and less heavy, FSO systems can be installed rapidly with low installation and operational cost. Moreover, they operate with lower power consumption in the unlicensed optical spectrum, while they are practically immune to multi-path dispersion and electromagnetic interference (EMI) [3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13]. By focusing on medical applications with IMDs that have made also notable advances lately in commercial and research field, FSO, in terms of TOW, is a prime candidate for effective high bandwidth transdermal communication, such as recording signals from in-body devices and actuating these devices from extra-body signals [14; 15; 16; 17]. Nowadays, the common modalities used for transcutaneous communication are based on RF, acoustic wave, conductive, or electrical induction [9], and therefore, this kind of communication is realized by various conventional methods such as percutaneous wires, ultrasound or ultra-wide band (UWB) technology. Among them, percutaneous wires have a larger bandwidth and lower power consumption but physically traversing human skin increases the possibility of infection. Alternatively, an ultrasonic acoustic link has been used as a means of telemetry for cochlear implants, but utilizes piezoelectric materials that are not ideal for an IMD, due to high driving voltages [18]. On the other hand, wireless RF, which is a mature technology, is currently used in most implantable systems [19]. Indeed, RF telemetry has been used for diverse applications, including neuromuscular stimulators [17], commercial implantable cardioverter defibrillators and commercial cochlear implants for many years, while also neural microelectrode recording systems with integrated RF systems are being developed [20; 21; 22]. Current transdermal RF systems have been shown to achieve data rates up to several Mbps [20]. However, although a high-speed RF has been recently reported as being capable of transmitting data at 24 Mbps [23], it still requires a large amount of power consumption, ca. 30 mW. Additionally, transdermal RF systems cannot usually cover a sufficient bandwidth, since RF bandwidth is highly regulated.