Multi-aperture direct-detection receiver based on electrical combining for free-space optical communication systems

Abstract

Free-space optical communications (FSOCs) have recently re-emerged as an alternative to wireless radio frequency (RF) communications. By taking advantage of modern fiber-optic transmission technologies, FSOC systems can deliver high-speed data over a long distance with ultra-broad unlicensed spectrum, low crosstalk, high security, and low power consumption. FSOCs face a couple of technical challenges including the mitigation of channel effects (induced by atmospheric turbulence) and the precise alignment between the transmitter and receiver. Atmospheric turbulence gives rise to scintillation, beam wandering, angle-of-arrival fluctuation, and beam spreading. These effects eventually reduce the optical power collected by the receiver and degrade the performance of FSOCs even in a clear day. There are several solutions to mitigate the deleterious effects of atmospheric turbulence. On the transmitter side, the scintillation effects can be reduced by using a partially coherent beam. The scintillation can be interpreted as multi-path interference and this effect can be mitigated by using incoherent optical beam realized in space and/or time. On the other hand, one way to reduce the effect of atmospheric turbulence on the receiver side is to utilize the adaptive optics, where the wavefront distortions induced by atmospheric turbulence are first detected by using a wavefront sensor, and then this information is used to control a deformable mirror to correct the distortions. However, this approach is typically too expensive to be used for most FSOC systems. Also, the implementation complexity becomes very high when the atmospheric turbulence is strong. Another popular receiver-side approach is the aperture averaging. By using a large aperture receiver, the spatial variations of optical intensity caused by scintillation can be averaged out when the aperture size is larger than the spatial scale of irradiance correlation. Instead of using a single large-aperture receiver, array receivers or multiple small apertures can be used to collect the optical beam at different positions on the receiver side, and then the received signals are combined either optically or electrically. In this thesis, I study the multiple-aperture receiver to mitigate the adverse effects of atmospheric turbulence. Typical diversity techniques such as selective combining, equal gain combining, and maximal ratio combining are employed to combine the signal collected by multiple apertures. The optical signal is detected directly by using a photo-detector and the multiple signals are combined in the electrical domain for simplicity and cost-effectiveness. The number of receiver apertures is also examined to optimize both the system performance and cost. I experimentally demonstrate the multiple-aperture direct-detection receiver for FSOC system. The angle-of-arrival fluctuation is known to degrade the performance of FSOC systems. This is because the deviation of propagation direction of wavefront from the optical axis makes the received light fall outside of the fiber tip or detector when the received optical beam is collimated by using a lens. I attempt to mitigate the adverse effects of angle-or-arrival fluctuation by tilting the angle of some apertures in multiple-aperture receiver. I show through computer simulation that the system performance is improved considerately in comparison with multiple apertures all placed on a single plane. The pitch between apertures is also investigated to optimize both the “dead” zone and beam collected area. To investigate the proposed method, I developed a computer simulation program for the FSOC system. I model the atmospheric channel using multiple phase screens and solve the stochastic wave equation by using the split-step Fourier method. Numerical simulation is used to represent a series of independent apertures. To evaluate the performance of our proposed system, bit-error ratio (BER) is calculated in relationship with the transmission power, the angle of tilting aperture, the distance between adjacent apertures, and the number of receiver apertures. Comparing BER performance between diversity techniques is also examined to select the most effective combining method for our proposed system. Moreover, the scintillation index is computed to present the channel atmospheric turbulence condition. The system losses including geometric loss and coupling loss are also measured. The simulation results indicate that the multiple-aperture direct-detection receiver using the maximal ratio combining technique outperforms the single-aperture receiver or other diversity schemes. The performance of our proposed system is improved significantly when I tilt the angle of aperture to the direction of the transmitter. The experiment is also carried out to validate the proposed scheme over a 100-m free-space link. Atmospheric turbulence is emulated by using a few fans and heaters. The receiver having seven apertures is designed and realized for this experiment. The received signals are digitized by using a real-time oscilloscope and I combine the signal with the selective combining, equal gain combining, or maximal gourain combining. The results show that the multiple-aperture receiver based on the maximal ratio combining provides us the sensitivity improvement of > 2 dB. The sensitivity improvement is increased as the strength of turbulence increases. These findings agree well with my simulation study.