Nonlinear fiber effects in ultra-high power 10 × 10 Gbit/s WDM-IM free-space systems for satellite links.

We investigate an unexplored type of nonlinear impairments that will take place in a very short fiber after the booster amplifier in a Free Space Optical (FSO) system for space communications. In Earth-satellite links, optical power levels up to 100 W could be required at the transmitter side to achieve the foreseen 100 Gbit/s capacity, because of the extremely high losses. These systems thus need an optical booster amplifier having very high optical power and it should be connected to the transmitting telescope by means of a short fiber (few meters). Here, we discuss and investigate the impact of the nonlinear fiber effects by means of numerical simulations, and estimate the impairments in a Wavelength Division Multiplexing (WDM) 10 × 10 Gbit/s system with intensity modulation. The obtained results clearly indicate that, in this system, the most relevant effect is Four Wave Mixing. We proved that this can be observed as soon as the total power exceeds 20 W. Due to the short fiber length, the system impairments are not affected by chromatic dispersion or channel spacing. We demonstrate that an effective means to reduce the impact is by adopting Polarization Interleaving, i.e., odd and even channels with orthogonal state of polarization. This solution could not work in long terrestrial links because of polarization mode dispersion, yet it can be effectively exploited in short fiber patch cords. These results can be used as a guideline to control this type of impairment in high-power FSO systems for satellite links.

Detecting WDM visible light signals by a single multi-color photodiode with MIMO processing.

For the first time, to the best of our knowledge, we experimentally demonstrate that multiple-input-multiple-output (MIMO) processing allows using a single photodiode to detect simultaneously a wavelength-division multiplexing (WDM) visible light communications (VLC) signal. The photodiode has a triple junction, and when it is illuminated by a WDM signal, the junctions produce inherently three photocurrents that are unusable for detecting any of the WDM signals. However, by means of linear MIMO processing, we are able to recover the transmitted signals exactly. Bit error rate measurements confirm the effectiveness of the proposed solution. This opens a new scenario for practical WDM-VLC systems.

Gigabit-class optical wireless communication system at indoor distances (1.5 ÷ 4 m).

In this paper we experimentally realized bidirectional optical wireless communication (OWC) link using four channel visible LED board exploiting wavelength division multiplexing (WDM) for the downlink and infrared LED for uplink. We achieved greater than 5 Gbit/s data rate at common indoor distance (1.5 to 4 m) for downlink and 1.5 Gbit/s for uplink using commercially available LEDs. We achieved these results after a careful choice of the LED emission wavelengths and the optical filter spectra. Moreover, we investigate the optimal LED working current and the optimal modulation depth. The bit error ratios of all the channels were maintained lower than the FEC limit (3.8·10(-3)).

Electrical filter-based and low-complexity DPSK coherent optical receiver.

Direct-detection of differential phase shifted keying (DPSK) optical signals is implemented either by ad hoc optical filtering or one-bit delayed optical interferometers. We show a coherent receiver where the filtering is performed in the electrical domain after down-converting the incoming signal to the baseband or to an intermediate frequency. This can be particularly advantageous whenever optical filters cannot be used or when extremely narrow filtering (sub-GHz) would be required [for example ultra-dense wavelength division multiplexing (WDM) passive optical networks]. Electrical filters can be realized more accurately than their optical counterpart. In addition, in a coherent receiver, this operation is colorless and avoids digital signal processing. We show experimentally this reduced complexity receiver and compare it with the classical one based on the delay and multiply block.

Effective homodyne optical phase locking to PSK signal by means of 8b10b line coding.

We demonstrate a novel technique that allows effective homodyne optical phase locking to a phase shift keying (PSK) signal with a residual carrier. We exploit 8b10b coding of the signal in order to reduce its low frequency spectral content, suppressing the data-to-phase crosstalk effect. In a transmission experiment on a 10 Gb/s binary PSK signal (8 Gb/s before coding), we achieved transmission over 215 km of dispersion-compensated, installed single-mode fibre, with no penalty compared to back-to-back at a bit error ratio of 10(-3). The proposed solution is applicable to other modulation formats, including multi-level amplitude and/or phase modulation formats.

System feasibility of using stimulated Brillouin scattering in self coherent detection schemes.

We demonstrate the first self-coherent detection of 10 Gbit/s BPSK signals based on narrow-band amplification of the optical carrier by means of Stimulated Brillouin effect in a common fiber. We found that this technique is very effective only if it is combined with proper line coding and high-pass electrical filtering at the receiver. In this case we obtain OSNR-performance close to the ideal coherent receiver.

A 80 km reach fully passive WDM-PON based on reflective ONUs.

We propose a novel line coding combination (Inverse RZ coding in downlink and RZ in uplink) that extends the reach of WDM Passive Optical Networks based on Reflective SOAs with no in-line amplification. We achieved full downstream remodulation even when feeding the reflective SOA with power levels as low as -35 dBm, thus increasing the system power budget. We experimentally assessed this scheme for a fully passive, full-duplex and symmetrical 1.25 Gb/s WDM-PON over a 80 km G.652 feeder.