In this paper, we propose a comparative performance study to characterize the transmission of multiple subcarrier modulation signals with different M-ary phase shift keying (M-ary PSK) schemes over a radio-on-free-space optical system using the aperture averaging (AA) technique. First, we derive modified expressions for carrier-to-noise and distortion ratio (CNDR), outage probability, and bit-error rate (BER) based on the AA technique and take into consideration the laser diode nonlinearity effect. We also show that there is a design trade-off between the receiver lens aperture and the required average CNDR to achieve a given average BER, ensuring substantial scintillation fade reduction. Further, we perform a comparison between different M-ary PSK modulation schemes, i.e., BPSK, QPSK, and 16-PSK, to characterize the achieved performance improvement by using the AA technique. We found out that when higher order PSK modulation techniques are used such as 16-PSK, the fading reduction is far less impressive than that for schemes employing smaller constellation sizes such as BPSK and QPSK. We thus conclude that for more complex modulation techniques no matter how large the receiver aperture size, the accepted BER level cannot be attained only with the use of the AA technique.
Proc. SPIE. 7814, Free-Space Laser Communications X
KEYWORDS: Modulation, Receivers, Phase shift keying, Turbulence, Free space optics, Picosecond phenomena, Intermodulation, Systems modeling, Free space optical communications, Current controlled current source
In this paper, we propose a Free-Space Optical (FSO) system model based on multiple subcarrier modulation
(MSM) using M-ary Phase Shift Keying (M-ary PSK) scheme over Intensity Modulation Direct Detection optical
link. We investigate the impact of aperture averaging on our model across weak-to-strong turbulence regimes
taking into consideration the intermodulation distortion term due to the laser diode non-linearity. By using
a modified model for aperture averaging technique, we also show that there is a design tradeo¤ between the
receiver lens aperture and the required average Carrier-to-Noise and Distortion Ratio to achieve a given average
bit error rate, ensuring substantial scintillation fade reduction.
We analyse the coupling characteristics of dual-core photonic crystal fibre couplers by a 3D finite difference vector beam propagation method. Beam propagation analysis of photonic crystal fibre couplers is performed in terms of coupling length and coupling efficiency. The determination of the guiding properties such as the propagation constants is evaluated using a mode solver based on plane wave method. We study the influence of the photonic crystal fibre coupler geometrical parameters on the coupling length at different wavelengths. Variable size of the central hole is considered to improve the coupling between the two cores. It is shown that it is possible to design shorter photonic crystal fibre couplers with coupling lengths of hundred micrometers compared to conventional optical fibre couplers. We demonstrate that the designed coupler can operate as a polarization preserving directional coupler. This study confirms that this device can act as an efficient ultra small wavelength selective coupler.