KEYWORDS: Crystals, Modulation, Monte Carlo methods, Laser crystals, Modulators, Laser communications, Electro optics, Signal attenuation, Ocean optics, Refractive index
Indirect modulation with electro-optical crystal is a useful way to generate optical signal for underwater blue-green laser communication. However, as crystal surface is not strictly flat in practical application, light intensity distribution in the cross section is non-uniform, which would affect extinction ratio of modulated signal and system performance. In this letter, we study this issue with Monte Carlo method. The result shows that with the increase of crystal flatness, extinction ratio is decreasing dramatically, and it should be smaller than 0.78μm in order to make the extinction ratio greater than 10dB while 0.25μm for 20dB, 0.08μm for 30dB, and 0.025μm for 40dB.
Lots of researches have simulated performance of optical system based on plane wave or spherical wave model, but
optical field is Gauss distribution in real optical communication system. So we derive the relationships between the
signal-to-noise ratio (SNR), bit error rate (BER), channel capacity (C) and the transmission distance of the space laser
communication system, which is obtained by the Gaussian beam propagation model for on-off keying (OOK) modulated
signal under horizontal link. We numerically study the influence of atmospheric turbulence on system performance. we
get the conclusions: Under weak turbulence intensity, as the turbulence intensity increases, the C and SNR for OOK
modulated space laser communication system decrease while the BER increase; In the case of strong turbulence intensity,
the intensity scintillation and the BER dramatically increase with the increase of transmission distance, and then become
saturated; In the condition of turbulence intensity and transmission distance are both constant, the longer the wavelength
results in greater C, higher SNR and lower BER. Selecting longer communication wavelength can mitigate the impact of
the atmosphere turbulence on the communication system in some extent.
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