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图1给出了文中所提出的主动锁模OEO系统的结构及工作原理。主动锁模OEO的系统结构与传统OEO类似,其工作原理简述如下:半导体激光二极管输出窄线宽激光,由可调谐光衰减器控制进入OEO腔内的光功率,从而实现环腔内增益的精细调节,保证OEO能够进入稳定的锁模状态。直流光进入电光强度调制器(IM),IM输出的调制光信号通过一段非零色散位移光纤(NZ-DSF),在光电探测器内转换成微波信号,之后依次经过带通滤波器、低噪声放大器后,被电耦合器分成两路,一路用于输出OEO的振荡信号,一路连接至IM的射频输入端口,实现OEO闭环。与传统OEO不同的是,IM的偏置端口不再由直流信号驱动,而是由函数发生器(FG)输出的正弦信号进行控制,该正弦信号实现OEO环腔内损耗的周期性调控,当调控周期为环腔自由光谱范围的整数倍
$N$ 时,实现主动锁模。图 1 主动锁模OEO系统结构(a)及工作原理(b)示意图
Figure 1. Schematic diagram of system structure (a) and work principle (b) of the proposed actively mode-locked OEO
为了实现稳定的锁模,需要通过可调光衰减器控制输入OEO环腔的光功率,并调节FG输出的驱动信号电平值,获得如图1所示的周期性损耗变化。当IM偏置端口加载的正弦信号频率
${f_{bias}}$ 与OEO环腔的自由光谱范围$\Delta {f_{FSR}}$ 满足$$ {f_{bias}} = N \times \Delta {f_{FSR}}{\rm{ = }}N \times \frac{c}{{nL}}(N \geqslant {\rm{1}}{\text{且为整数}}) $$ (1) 时,腔内的净增益在调制损耗最小值附近大于0,其余时刻均小于0,进而产生重复频率
${f_{bias}}$ 、周期${{\rm{1}} / {{f_{bias}}}}$ 的微波脉冲信号。微波脉冲信号的载频由带通滤波器的中心频率决定,脉冲宽度取决于环腔内的净增益谱(主要由带通滤波器的通带宽度决定)。当$N = 1$ 时,实现基频锁模;当$N \geqslant 2$ 时,实现$N$ 阶谐波锁模。由于在所提方案中采用了控制IM的偏置电压来对OEO环腔进行损耗调制,相比于使用电学幅度调制器而言[5],系统结构更加简单,并且可以实现更高阶的谐波锁模,产生重复频率高达数MHz的超短微波脉冲信号。
Harmonically mode-locked optoelectronic oscillator (Invited)
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摘要:
提出了一种主动锁模光电振荡器(OEO)方案,可以实现高阶谐波锁模,从而产生具有高重复频率的微波脉冲信号。在所提方案中,通过在OEO腔内的电光强度调制器直流偏置端口引入一个正弦驱动信号,当该正弦信号的频率
为OEO环腔自由光谱范围的整数倍
时,实现基频(
)或谐波(
)锁模,输出重复频率为
的微波脉冲信号。实验中分别实现了10阶、50阶和100阶谐波锁模,输出微波脉冲信号的重复频率分别为360 kHz、1.8 MHz和3.6 MHz。该方案为脉冲多普勒雷达等系统应用提供了一种全新的、具备低相噪潜力的微波脉冲信号产生的技术途径。
Abstract:An actively mode-locked optoelectronic oscillator (OEO) scheme was proposed to generate high-repetition-rate microwave pulse signals based on high-order harmonic mode locking. In the proposed scheme, the bias port of the electro-optic intensity modulator in the cavity was driven by a sinusoidal signal. The frequency of the sinusoidal signal (
) was set to be
times of the free spectral range of the OEO cavity to realize fundamental (
) and harmonic (
) mode locking, where the repetition rate of the generated microwave pulse signal was equal to
. In the proof-of-concept experiment, 10th-, 50th- and 100th-order harmonic mode locking were realized, where the repetition rate of the generated microwave pulse signal was 360 kHz, 1.8 MHz and 3.6 MHz, respectively. The proposed actively mode-locked OEO scheme provides a new way to generate low-phase-noise microwave pulse signals for pulse Doppler radar application.
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