Research On Highly Linearized Analog Fiber Optic Transmission Systems

Analog fiber-optic communication technique is the most popular method that has been used in telecom industry and will become the main development directions in the near future. In order to satisfy the tendency of becoming faster, larger capacity and lower loss, fiber optic communication technique possesses its own superiority and broad space for development in many areas, such as commercial, military industry and etc. Analog signal transmitted in the fiber-optic systems has made the best use of its lower loss and high transmission rate in the fiber-optic links, and become one of the key technologies. In these applications, the linearity and link gain are the most important figure of merit in such systems, and directly decide the link’s performance.However, the quality of analog signal which is transmitted in the fiber-optic systems is largely affected by the non-linear effects. Typically, the traditional system employs intensity modulation to transfer the electrical signals into optical domain, so that the signal can be transmitted through optical fibers. At the receiver end of the links, direct detection is more often used to demodulate the signal. However, such methods suffer from the system’s two main inherent drawbacks, which are strong nonlinear effects and low link gain. Both drawbacks have caused the decline in transmission quality and signal distortion. Meanwhile, the transmission link gain in such systems is proved to be less than -20dB.Typically, to overcome these drawbacks, two methods have been used. In one way, with the help of supplementary components, such as the high-power Erbium-doped fiber amplifiers, the non-linear effects can be reduced. However, such methods increase the complexity of the transmission systems, and to be best of our knowledge, none of them has derived the explicit expressions for the key optimization parameters of the low-biased analog optic links. Another one is to use the feedback mechanism to increase the system’s linearity. But, due to the phase modulation method, the systems have been largely affected by the modulated signal phase, and the transmission link therefore becomes not stable. Meanwhile, the transmission link gain in such systems is proved to be less than -20dB, which cannot be practically useful in these applications.Here, I studied the fiber-optic communication methods. Firstly, I provide the background, basic concepts and the characteristics of the transmission links. Based on the general concepts, I provide two methods to increase the link’s linearity below:Low biased externally modulated method employed in fiber optic analog link. I study the performance of an analog fiber-optic link that employs a low-biased Mach-Zehnder modulator. Explicit and exact formulas for the optimized bias angle, the maximized spurious free dynamic range (SFDR), and the minimized noise figure are all derived. Using these formulas, I discuss the SFDR and noise figure improvements obtained for a low-biased link over a quadrature-biased link. It is found that in order for the improvements to be practically useful, the power density of the relative intensity noise (RIN) of the transmitter laser must be at least 17 times larger than that of the thermal noise. We also provide a RIN-photocurrent phase diagram that allows the effectiveness of low biasing to be quickly determined.Feedback demodulation method based on intensity modulation. I propose a fiber-optic communication method based on the intensity modulation and feedback demodulation techniques. Detailed components of this transmission system are illustrated. With this system configuration, the explicit and simple expressions of both the transmission function and the third-order distortion have been derived. Compared with the traditional optic links, the improvements on the link linearity and link gain of this method have been demonstrated using theoretical analysis and been validate with numerical simulation results. Such method has been therefore proved to be practically useful to reduce the non-linear effects, and at the same time, increase the output signal gain in the analog fiber-optic links.Lastly, the above methods have been proved to be practically useful with the simulation results.