Nikos Raptis

Dynamic Properties of a WDM Switching Module Based on Active Microring Resonators

The switching properties of a resonator consisting of two serially cascaded active microrings (MRs) are reported. The switching is achieved through the variation of the refractive index and the losses of the MRs with current injection, which alters the overall output spectral response. The switch is suitable for optical burst switching systems and metro architectures as it provides fast optical switching in the nanosecond scale with contrast ratio higher than 30 dB for 10-Gb/s intensity modulated signals. The capacity of the switch can be remarkably increased if multiwavelength switching is employed.

An Optical Mode Filtering Technique for the Improvement of the Large Core SI-POF Link Performance

In this paper an investigation on the influence of the photodiodes active area and its position relative to the fiber end facet to the performance of a Large Core Step Index Polymer Optical Fiber link is presented. It is shown with extensive numerical simulations that an effective higher order mode filtering occurs, depending on the geometrical properties of the interface, which can significantly improve the performance of the link. These findings are confirmed experimentally. Depending on the photo-receivers axial offset relative to the fiber facet, an improvement in the BER performance of many orders of magnitude has been achieved for a fiber link of 37.5 m and a bit rate of 1.25 Gbps.

Space-time block code based MIMO encoding for large core step index plastic optical fiber transmission systems

The performance of Space-Time Block Codes combined with Discrete MultiTone modulation applied in a Large Core Step-Index POF link is examined theoretically. A comparative study is performed considering several schemes that employ multiple transmitters/receivers and a fiber span of 100 m. The performance enhancement of the higher diversity order configurations is revealed by application of a Margin Adaptive Bit Loading technique that employs Chows algorithm. Simulations results of the above schemes, in terms of Bit Error Rate as a function of the received Signal to Noise Ratio, are provided. An improvement of more than 6 dB for the required electrical SNR is observed for a 3 × 1 configuration, in order to achieve a 10(-3) BER value, as compared to a conventional Single Input Single output scheme.

Power losses in diffuse ultraviolet optical communications channels.

One of the most critical parameters in free-space optical communications systems operating in a non-line-of-sight regime are the optical losses. In this Letter, we numerically calculate these losses taking into account the scattering effects using the Monte Carlo simulation technique. The obtained results are compared with experimentally obtained data at 265 nm (solar-blind UV regime). A large set of measurements at distances up to 20 m, for different elevation angles of the transmitter (UV-LEDs) and receiver (photomultiplier tube) and for different atmospheric conditions has been taken for the characterization of the optical communications channel in terms of its loss properties.

Power-spectrum requirements in ultraviolet optical wireless networks

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.

Performance Evaluation of CDMA-DMT for 1-mm SI-POF Short-Range Transmission Links

In this letter, the code division multiple access discrete multitone (CDMA-DMT) modulation scheme is experimentally investigated for transmission over an intensity modulation direct-detection link of 100 m of 1-mm polymethyl methacrylate step-index plastic optical fibers. CDMA-DMT is a multicarrier modulation scheme combining the merits of both CDMA and DMT and in which CDMA is utilized to perform a spreading operation of the data symbols in the frequency domain using orthogonal spreading codes. It is experimentally shown that CDMA-DMT performs better than conventional DMT in terms of achieved bit-error rate, for transmission rates in the range of 1 Gbps and over the same experimental configuration.

Experimental Evaluation of Modulation Formats’ Performance in Diffuse UV Channels

The fundamental properties of free space optical channels in the ultraviolet (UV) spectral range between 200 and 280 nm are the enhanced scattering and the solar blind operation. Exploiting these properties, point-to-point links in a non-line-of-sight (NLOS) regime were experimentally tested and the performance of pulse position modulation of fourth order (4-PPM) and flip–orthogonal frequency division multiplexing (Flip-OFDM) modulation formats was evaluated. At the transmitting side, light-emitting diodes (LEDs) emitting at 265 nm were used. At the receiving side, a photo-multiplier tube was combined with an optical filter. Distances up to 20 m were covered and kilobits/s rates were applied. The performance was also evaluated after modifying artificially the atmospheric conditions by the operation of a fog machine. In short, 4-PPM outperformed Flip-OFDM, whereas a medium that offers enriched scattering due to fog may boost the performance of short-range NLOS links in the UV spectral range.

Performance evaluation of modulation and multiple access schemes in ultraviolet optical wireless connections for two atmosphere thickness cases.

The exploitation of optical wireless communication channels in a non-line-of-sight regime is studied for point-to-point and networking configurations considering the use of light-emitting diodes. Two environments with different scattering center densities are considered, assuming operation at 265 nm. The bit error rate performance of both pulsed and multicarrier modulation schemes is examined, using numerical approaches. In the networking scenario, a central node only receives data, one node transmits useful data, and the rest of them act as interferers. The performance of the desirable nodes transmissions is evaluated. The access to the medium is controlled by a code division multiple access scheme.

Performance evaluation of non-line-of-sight optical communication system operating in the solar-blind ultraviolet spectrum

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.

WDM Switching Module Based on Active Microring Resonators: Applications to Metro Networks

The switching properties of a resonator consisting of two serially cascaded active micro rings are reported. The switching is achieved through the variation of the refractive index and the losses of the micro rings with current injection, which alters the overall output spectral response. The switch is suitable for optical burst switching systems and metro architectures as it provides fast optical switching in the ns scale with contrast ratio higher than 30 dB for 10 Gbps intensity modulated signals. Its potential applications to a metro network node are discussed.