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基于光注入半导体激光器技术的光电振荡器系统结构图如图1所示,该系统主要包括可调谐激光器(Tunable Laser Source, TLS),相位调制器(Phase Modulator, PM),马赫曾德尔强度调制器(Mach-Zehnder Modulator, MZM),偏振控制器(Polarization Controller, PC),光环行器(Optical Circulator, OC),分布反馈式半导体激光器(Distributed-Feedback Semiconductor Laser Diode, DFB-LD),单模光纤(Single-Mode Fiber, SMF),光电探测器(Photodetector, PD),电滤波器(Electrical Band-Pass Filter, EBPF)和电放大器(Electrical Amplifier, EA)。通过电频谱分析仪(Electrical Spectrum Analyzer, ESA)和信号源分析仪(Signal Source Analyzer, SSA)对OEO产生的微波信号进行测量分析。
图 1 基于光注入半导体激光器技术的光电振荡器结构图
Figure 1. Schematic diagram of the optoelectronic oscillator of optically-injected-based semiconductor laser
如图1所示,将可调谐激光器作为主激光器(Master Laser, ML),不含隔离器的半导体激光器作为从激光器(Slave Laser, SL)。当链路中无次谐波信号调制时,主激光器输出的光信号直接经过光环行器注入到从激光器中。由于从半导体激光器只对注入光信号的TE模式起作用,因此通过调节链路中的偏振控制器调整注入光信号的偏振态,以获得合适的注入效率。在适当的光注入条件下,从半导体激光器会处于单周期振荡工作状态,此时从激光器的谐振腔模式会发生红移并产生光学增益区。此后,从激光器输出的光信号再次经光环行器输入到强度调制器中,此时强度调制器工作在正交偏置点下。强度调制器输出的光信号经过一定长度的单模光纤后输入到光电探测器中,光电探测器输出的电信号经电放大器的放大与电滤波器的滤波后获取笔者所需要的纯净微波信号。最后,使用电分路器将系统生成的微波信号分成两路,其中一路信号输入到频谱仪与信号源分析仪中进行测量,另一路信号输入到强度调制器的射频输入端口中实现OEO的闭环振荡。
光注入半导体激光器技术的原理图如图2所示。主激光器输出的光信号频率与从激光器自由运行时的频率之间的差值被定义为主激光器与从激光器间的失谐频率,因此,主、从激光间的失谐频率可以表示为:
$$\Delta f = {f_m} - {f_s}$$ (1) 在经过光环行器前,通过使用偏振控制器调节注入光的偏振态,获取合适的注入效率,从而保证从激光器将处于单周期振荡工作态。
在单周期振荡态下,从半导体激光器的谐振腔模式将受载流子浓度变化的影响红移至频率fcav处[15-16]。因此,新的信号频率fcav会随着半导体激光器自由运行频率fs的消失而产生,且红移后的频率fcav可以表示为:
$${f_{cav}} = {f_s} + \frac{1}{2}\alpha g(N - {N_{th}})$$ (2) 式中:α为线宽增强因子;g为线性增益系数;N为腔内载流子数量;Nth表示阈值载流子数量。主激光器与谐振腔模式红移后的从激光器间的频率差值为:
$${f_c} = {f_m} - {f_{cav}}$$ (3) 结合公式(1)~(3)可以得到:
$${f_c} = \Delta f - \frac{1}{2}\alpha g\left( {N - {N_{th}}} \right)$$ (4) 因此,当从激光器输出的光信号经光电探测拍频后会产生如图2中所示频率为fc的微波信号。通过单独或同时调节主、从激光器间的失谐频率和注入系数,fc可以由几GHz调节至数十GHz。其中,注入系数被定义为注入光功率与从激光器自由运行时功率之比的平方根。然而,由于单周期振荡态是一种非稳定的工作状态,处于单周期振荡工作状态下的半导体激光器的光频率会随着注入系数或失谐频率的变化而发生变化,因此外部实验环境的轻微变化都会对实验结果产生影响,从而导致OEO振荡环路生成微波信号的稳定性与频谱纯度的劣化。
而通过在基于光注入半导体激光器技术的OEO振荡环路中引入次谐波信号调制,能够有效提升系统生成微波信号的稳定性与频谱纯度,主激光器输出的光信号经相位调制器被频率为f0/N(N为整数,且频率f0与频率fc相近)的次谐波微波信号调制后,经光环行器注入到从半导体激光器中。由光注入半导体激光器的原理可知,在适当的光注入条件下,从半导体激光器的谐振腔模式在发生红移的同时会产生光学增益区,因此,调制信号的N阶边带,即频率为fm − f0的光信号将位于激光器的锁定区内并将其锁定,同时只有最接近N阶调制边带的振荡模式将被锁定,如图3所示。因此,通过在基于光注入半导体激光器的光电振荡器中引入次谐波信号调制可以实现OEO的稳定振荡,生成具有低相位噪声的稳定高质量单模微波信号。
Tunable optoelectronic oscillator based on optically injected distributed-feedback semiconductor laser diode under subharmonic microwave modulation
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摘要: 为实现具有高频谱纯度、低相位噪声的宽带可调谐微波信号生成,提出并通过实验验证了一种次谐波信号调制下光注入半导体激光器结构的光电振荡器,其原理为通过利用光注入半导体激光器的单周期(P1)振荡工作状态和波长选择放大特性实现可调微波信号生成,并进一步通过在光电振荡环路中引入次谐波信号调制对系统生成微波信号的频率稳定性、边模抑制比与频谱纯度进行优化。实验结果表明,文中方案提出的光电振荡器可以生成输出功率大于5 dBm,频率调谐范围为12~18 GHz的微波信号。同时,系统生成的微波信号的3 dB带宽为100 kHz,边模抑制比可达 51 dB,且信号在频偏量为100 Hz和10 kHz处的相位噪声分别为−78 dBc/Hz和−109 dBc/Hz。此外,光电振荡器生成微波信号的频率调谐范围只受系统中使用的各类光电器件工作带宽的限制,通过采用具有更大带宽的光电器件可以实现更高频率的微波信号生成。Abstract: In order to obtain microwave signal with high spectral purity, low phase noise and flexible tunability, a novel approach to achieving a tunable optoelectronic oscillator (OEO) which was based on optically injected semiconductor laser and subharmonic microwave modulation for microwave signal generation was proposed and experimentally demonstrated. The fundamental concepts for realizing the OEO were based on the wavelength-selective amplification effect and the period-one(P1) oscillation state of optically injected semiconductor laser. The frequency stability, side-mode-suppression ratio and spectral purity of the generated microwave signal could be optimized by introducing subharmonic microwave modulation via a phase modulator in the OEO loop. The experimental results show that the central frequency of the microwave signal generated by the proposed OEO could be tuned from 12 GHz to 18 GHz, and output power of the generated signal was more than 5 dBm. At the same time, the generated signal had a side-mode-suppression ratio of 51 dB and a 3 dB bandwidth of 100 kHz. Finally, the phase noise of the measured microwave signal could be optimized to −78 dBc/Hz and −109 dBc/Hz at 100 Hz and 10 kHz frequency offset by introducing subharmonic microwave modulation in the system, respectively. Furthermore, the tunable frequency range of the generated signal was restricted by the operating bandwidths of the optic-electronic devices which were utilized in the system. A higher frequency of the generated microwave signal could be achieved by using the devices with larger bandwidths in the OEO loop.
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