Diffuse optical tomography is an emerging biomedical imaging technique, due to its numerous advantages, such as low cost and non-ionizing radiation. In this work, we have developed a very simple setup, which included a single source – photodiode (SP) pair for scanning a sample of water with diluted Intralipid, simulating a biological tissue. LEDs emitting at 470, 525 and 624 nm, as well as a 650 nm Fabry Perot laser, were used as light sources. Scattered light from the sample was detected by the photodiode placed next to the LED. The SP distance could vary and the phantom could be scanned by moving the SP pair in precise, small and automated steps without any intervention during the measurements. Therefore, we obtained measurements from multiple locations on the sample, with just one SP pair. The presented experimental system verified the feasibility of deploying extremely low cost devices for detection and imaging absorbing objects of 1 cm height, placed inside a scattering medium. Maximum depth detection was 2.5 cm. As expected, the quality of the obtained images was degrading, as the object’s depth or the scanning step was increasing. Additionally, we developed Monte Carlo simulations of the setup, which achieved good agreement with the experimental results. We also conducted another set of simulations, studying the depth sensitivity of a single static SP pair considering a scattering medium similar to the experimental phantom with and without object. We observed that the depth sensitivity increases as the source wavelength increases from 450 nm to 650 nm.
It is known that the tempting features of free space Non-Line-Of-Sight (NLOS) communications systems operating in the Ultraviolet C-band between 200 and 280 nm are the significantly reduced solar irradiance on ground level, the intense scattering and its combination with strong absorption which ensures the covertness against distant eavesdroppers or jammers. In the majority of the experimental surveys that have been published so far, the performance of point-to-point links has been evaluated under clear atmosphere without taking into account the weather conditions. In this work, it is shown that harsh atmospheric conditions due to fog appearance can be advantageous to short distance NLOS transmissions at 265 nm. Initially, the impact of fog on the losses of the diffuse wireless channels was investigated theoretically. Afterwards, an experimental survey of both the losses and the performance of low rate amplitude signals’ transmissions for two atmosphere cases followed. Initially, the satisfactory relation between scattering and absorption at 265 nm was verified by deploying outdoor NLOS point-to-point links under clear atmosphere. The transmitter consisted of 4 Light Emitting Diodes and the optical part of the receiver included a filter and a Photo-Multiplier tube. Then, the beneficial impact of artificially generated fog on scattering was exploited not only to enhance the system performance but also to identify the modification of the conditions. The experimental results showed a clear decrease of both the losses and the Bit Error Rate under fog conditions making such a system a perfect candidate for low rate communications under dense atmosphere.
For several years, it has been examined if the attributes of the wavelengths in C band of the Ultraviolet (UV) spectrum that lie between 200 and 280 nm can be exploited in order to set up short range covert links of low rate in a Non-Line-of-Sight (NLOS) regime. In the present work, it is experimentally investigated and verified that using this band, short range and low rate NLOS links using the same transmitter/receiver pair under different atmospheric conditions without applying extreme power levels can be implemented rather effectively. The transmitter was composed of four Light Emitting Diodes. At the receiving side, an optical filter was followed by a Photo-Multiplier Tube. Initially, we measured the power losses of the channels with and without fog (artificially generated) for ranges up to 20 meters and several transmitters/receiver configurations. Secondly, the performance of Fourth-order Pulse Position Modulation (4-PPM) and Flip Orthogonal Frequency Division Multiplexing (Flip-OFDM) was evaluated for such channels and 10 Kbit/s rate. Applying emissions at 265 nm, NLOS links can operate efficiently especially in harsh environments, as the power losses were lowered when fog appeared. In terms of the modulation formats, 4-PPM performed better in most cases. Better results were obtained for both schemes when the medium became thicker due to the presence of fog. Finally, some initial measurements were realized with a Silicon Carbide PiN photodiode for the same rate but low elevation angles. The performance was exactly the opposite compared to a receiver with inherent gain when the atmosphere thickened.
Short range communications using an optical wireless channel in a non-line-of-sight regime can be attained by exploiting the solar-blind UVC band. Firstly, the power loss and the bandwidth of the diffused wireless channel are examined for several particle and molecular densities of the medium when a receiver with a wide Field-Of-View is considered. Proper transmissions were simulated for two channel cases in order to investigate how the channel affects the signals. The investigated modulation schemes were On-Off-Keying (OOK) and 4-Pulse-Position-Modulation (4-PPM). At the receiving side, estimations with photoelectrons were considered. Compared to OOK, 4-PPM is favored by the detection without threshold and the higher peak optical power. Concerning the influence of the transmission medium, it appears that a sparse medium may limit the performance of both PPM and OOK due to the increased losses and the slight Inter- Symbol-Interference that appears. On the contrary, the estimated channel with broader bandwidth and lower losses for a thicker atmosphere ameliorated the performance of both schemes. Using the previous indication, Code Division Multiple Access (CDMA) transmissions were investigated utilizing the already defined receiver configuration. In CDMA, if the coded signals are transmitted with the same mean and peak power, 4-PPM seems to need more power in order to achieve similar performance to OOK. This confinement of 4-PPM is mitigated in a dense channel. Finally, the linearity of the Power-Current curve of the LEDs at the transmitting side was also taken into account as a factor that may increase the consumption of the sources.