摘要
光学调制器是光通信系统中的关键器件。在Radio-over-Fiber(ROF)系统中,传统的调制方案是双边带调制,由于光纤的色散效应,调制光的两个边带分别与中心载波拍频而产生的两个拍频项的相位不同,干涉相消,导致射频功率代价。为了解决这个问题,很多单边带调制的方案被提出,但是均存在一定的缺陷。
本文针对上述问题,提出了实现单边带调制的铌酸锂行波电极结构的调制器方案。周期反相电极能够实现行波电极的速度匹配,在此基础上,如果改变电极反相点的位置,就能够改变总的相移函数的相位。当M-Z型调制器的两个臂的电极周期反相点横向移动达到四分之一的周期,那么就能够实现90°相移,从而实现单边带调制。该调制器能够工作在60GHz以上的频率,只用一路驱动信号就能够简单的实现单边带调制。
论文中在共面波导和非对称带状传输线结构的基础上,提出了两种实现单边带调制器的周期反相电极结构。并利用保角变换法得到了两种电极结构的静场解析解。分析了电极结构参数对微波有效折射率、特征阻抗、半波电压等调制器参数的影响,并得到了一组优化的结构参数和性能参数列表。
利用系统传递函数的分析方法,得到了周期反相电极的频域和时域响应,分析了工作频率为100GHz的单边带调制器的频域响应,当分段数越多,电极响应的带通特性越强,频带宽度越窄。在分段数为4时,模拟了微波调制和加载10Gb/s非归零码(NRZ)的副载波调制的时频响应,得到了较理想的结果。
建立了60GHz的单边带调制器的模型,在光通信系统仿真软件OptiSystem中,模拟了60GHz单边带调制的ROF传输系统。实现了边带抑制比大于40dB的单边带调制,并在长距离的光纤传输中,将色散引起的射频功率代价减小到2dB以内。
Optical external modulators are essential components for the transmission of analog microwave/millimeter-wave and high-speed digital signals over optical fibers. In the radio-over-fiber (ROF) systems, the fiber chromatic dispersion will cause severe RF power penalty due to the optical double-sideband (DSB) modulation scheme. In order to overcome the power penalty, some optical single-sideband (SSB) modulation techniques have been implemented. However, all above schemes are complicated to implement and require a phase shift device.
We designed a LiNbO3 traveling-wave modulator with the period phase reversal electrode to implement SSB modulation. The optical structure of the modulator is a Mach-Zehnder interferometer and the electrode which consists of a feeder and two modulating period phase reversal electrodes that are used to overcome the velocity mismatch between microwaves and optical signals. Due to the velocity mismatch, we can control the optical phase shift of the each arm in the modulator by changing the structure of the period phase reversal electrode. To shift the reversal position along the traveling-wave of the electrode on one arm, the phase shift difference of the two arms is 90-degree. Thus, the SSB optical signal is reached after interference at output of the modulator.
We describe the fundamental theory and types of the electro-optical modulators in detail, and make an introduction to the means used to analyze the modulators, especially, to the Schwartz-Christoffel (SC) conformal mapping (CM) technique. Then a quasi-static analysis using the conventional conformal mapping approaches based on elliptic integrals has been used in evaluation of the microwave characteristics of the coplanar waveguide modulator and asymmetrical strip line modulator, the effective index, the characteristic impedance and the half wave voltage.
We theoretically analyze the principle of the SSB modulator and the time-domain and frequency-domain response of the period phase reversal electrode. The simulation results show that the SSB modulation can be implemented well and an optical sideband suppression ratio is over 40dB in 60GHz ROF system that could successfully reduce the power penalty caused by the chromatic dispersive. Comparing to conventional EO SSB modulators, this scheme does not require a 90-degree RF hybrid circuit or tuning of a phase of RF modulation signals and it can be operated under a high microwave frequency as long as setting an appropriate period.
引文
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