BER Analysis of Coherent Free Space Optical Communication Systems with Holographic Modal Wavefront Sensor

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  • ABSTRACT

    Degradation of bit-error-rate (BER), caused by atmospheric turbulence, seriously hinders the performance of coherent Free Space Optical (FSO) communication systems. An adaptive optics system proves to be effective in suppressing the atmospheric turbulence. The holographic modal wavefront sensor (HMWFS) proposed in our previous work, noted for its fast detecting rates and insensitivity to beam scintillation, is applied to the coherent FSO communication systems. In this paper, based on our previous work, we first introduce the principle of the HMWFS in brief and give the BER of the coherent FSO with homodyne detection in theory, and then analyze the improvement of BER for a coherent FSO system based on our previous simulation works. The results show that the wavefront sensor we propose is better for weak atmospheric turbulence. The most obvious advantages of HMWFS are fast detecting rates and insensitivity to beam scintillation.


  • KEYWORD

    BER , Coherent FSO communication , Holographic modal wavefront sensor , Aberrations correction

  • I. INTRODUCTION

    For the free space optical communication (FSO) system, the performance of the coherent detection scheme is better than that of the intensity modulation direct detection, with higher spectral efficiencies and data rates, a greater ability to decrease both background and thermal noise, and more sensitive receivers [1-4]. However, the atmospheric turbulence greatly degrades the performance of the coherent FSO systems [5]. The wavefront distortion, scintillation, beam wandering and spreading caused by a turbulent atmosphere will not only degrade the entrance efficiency of the receiving antenna but also cause mismatch of the field of the signal beam and the local oscillator (LO) [6].

    An adaptive optics (AO) system is identified as an effective way to compensate atmospheric turbulence in a coherent FSO system [7, 8]. And as the main component of an AO system, the wavefront sensor is attracting extensive attention [9]. The Shack-Hartmann wavefront sensor (SHWFS) is widely used for turbulence compensation for a coherent FSO system [10]. Belmonte put emphasis on elucidating how the addition of AO to the transmitter or receiver can reduce the effects of atmospheric propagation and on quantifying the improvement on the performance of FSO systems regarding coherent detection [11, 12]. Zuo investigated the bit-error-rate (BER) performance of the FSO links in weak non-Kolmogorov turbulence and showed that BER decreased sharply with AO corrections. Considering the influence of both the amplitude fluctuation and spatial phase aberrations, when the ratio of receiving aperture diameter D to the coherent length r0 (D/r0) was large enough, the Zernike mode was accurate [13, 14]. Li evaluated the performance of the coherent FSO system employing quadrature array phase-shift keying modulation over the maritime atmosphere with AO system based on SHWFS for atmospheric turbulence compensation [15]. C. Liu and J. Huang analyzed the improvement of BER for the coherent FSO system with AO system based on SHWFS through numerical simulation and experimental data of the 1.8 m telescope with a 127-subaperture SHWFS, under different D/r0 [16-19]. However, the SHWFS was so sensitive to laser scintillation that the performance was limited seriously for a coherent FSO system. The focal plane wavefront sensor solved this problem. It had higher power usage ratio, but it was much more costly in time [20]. In 2000, Ghebremichael et al. applied computer-generated holographic elements to wavefront sensing and corrected the response curve [21-25]. Changhai et al. used the plane wave in combination with the Fourier lens for the replacement of spherical waves to improve the efficiency of energy utilization [26-28].

    In this paper, based-on our previous work, HMWFS is applied to the coherent FSO systems to evaluate the BER performance improvement. First, the principle of HMWFS and the BER of the coherent FSO communication are given in brief in Section 2. Then, the numerical simulation is used to verify the feasibility of the HMWFS for the coherent FSO system, and the BER improvement for the coherent FSO system with HMWFS based on our previous works are presented in Section 3. Finally, we summarize and present our conclusions in Section 4.

    II. THEORETICAL ANALYSIS