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本文提出并验证了一种基于直调激光器与全光锁模产生微波频率梳的方案. 理论分析表明, 通过调节光纤环形腔的参数, 可对直调激光器不同动力学态下的模式实现谐波锁模或有理数谐波锁模, 从而获得梳间距可调节的频率梳. 在此基础上进行实验验证, 直调激光器在不同频率与幅度的正弦信号的调制下, 可以激发出多种典型动力学态, 这些动力学态可在环形激光器腔内实现全光锁模, 产生频率梳. 在平坦度为±5 dB的标准下, 不同动力学态作为种子信号, 可获得带宽为13, 15 GHz, 19.8, 19.5和22 GHz的频率梳; 通过直调激光器与全光锁模的有效结合, 梳间距的连续可调谐范围可达200 MHz—3 GHz; 生成的所有微波频率梳一阶梳线的单边带相位噪声测量值均低于–100 dBc/Hz@10 kHz. 理论分析和实验结果表明, 该方案调制信号的参数调节灵活, 且所生成的微波频率梳在平坦性、带宽及调谐性方面均表现出显著优势.In this paper, a novel scheme is proposed and experimentally demonstrated. It is based on a directly modulated laser (DML) and all-optical mode-locking for generating tunable microwave frequency combs (MFCs). Theoretical analysis reveals that harmonic or rational harmonic mode-locking can be achieved by adjusting the parameters of the fiber ring cavity, which enables the generation of MFCs with adjustable comb spacing. Based on this, experimental verification shows that the DML can be driven to exhibit various typical dynamical states under sinusoidal modulation with different frequencies and amplitudes. These states serve as seeding signals that subsequently undergo all-optical mode-locking within the ring laser cavity, resulting in the generation of MFCs. The bandwidths of the MFCs are 13, 15, 19.5, 19.8, and 22 GHz, respectively, all of which satisfy the ±5 dB flatness criterion. A continuously tunable comb-spacing range of 200 MHz to 3 GHz is attained through the effective combination of the DML and all-optical mode-locking. The single-sideband (SSB) phase noise of the first comb line remains below –100 dBc/Hz at a 10 kHz offset. Theoretical analysis and experimental results demonstrate that the modulated signals of the proposed scheme support flexible parameter tuning over a wide range. Furthermore, the generated MFCs have remarkable advantages in flatness, bandwidth, and tunability.
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Keywords:
- directly modulated laser /
- dynamic states /
- all-optical mode-locking /
- microwave frequency comb
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图 1 基于DML与全光锁模产生可调谐MFC的结构图(RF, 射频信号源; DML, 直调激光器; VOA, 可变光衰减器; Cir, 光环行器; PC, 偏振控制器; SOA, 半导体光放大器; ODL, 光延迟线; TOF, 可调光滤波器; EDFA, 掺铒光纤放大器; OC, 光耦合器; ISO, 光隔离器)
Fig. 1. Schematic of tunable MFCs generated by DML and all-optical mode-locking. RF, radio frequency source; DML, directly modulated laser; VOA, variable optical attenuator; Cir, optical circulator; PC, polarization controller; SOA, semiconductor optical amplifier; ODL, optical delay line; TOF, tunable optical filter; EDFA, erbium-doped fiber amplifier; OC, optical coupler; ISO, optical isolator.
图 2 全光锁模的实验验证 (a) 测量FRL的自由光谱范围(FSR); (b) 谐波锁模; (c) 二阶有理数谐波锁模; (d) 三阶有理数谐波锁模
Fig. 2. Experimental verification of all-optical mode-locking: (a) The measured free spectral range (FSR) of the FRL; (b) Harmonic mode-locking; (c) second-order rational harmonic mode-locking; (d) third-order rational harmonic mode-locking.
图 3 不同动力学态作为种子信号产生MFC (a) SS; (b) CPSSH; (c) CPS2SH; (d) PP2SH; (e) PPSH(第1列为DML输出的时间序列, 第2列为DML输出的频谱, 第3列为系统输出的时间序列, 第4列为系统输出的频谱)
Fig. 3. The MFCs generated by using different dynamical states as the seed signals: (a) SS; (b) CPSSH; (c) CPS2SH; (d) PP2SH; (e) PPSH (Column 1 represents the time series at the DML output; Column 2 represents corresponding frequency spectrum at the DML output; Column 3 represents the time series at the system output; Column 4 represents corresponding frequency spectrum at the system output).
图 4 不同RF信号产生的MFC的频谱 (a) 200 MHz, 插图为0—5 GHz放大图; (b) 300 MHz, 插图为0—7 GHz放大图; (c) 400 MHz, 插图为0—10 GHz放大图; (d) 500 MHz; (e) 1 GHz; (f) 1.5 GHz; (g) 2 GHz; (h) 2.5 GHz; (i) 3 GHz
Fig. 4. Spectra of microwave frequency combs generated by different RF signals: (a) 200 MHz, the illustration is an enlarged view from 0 to 5 GHz; (b) 300 MHz, the illustration is an enlarged view from 0 to 7 GHz; (c) 400 MHz, the illustration is an enlarged view from 0 to 10 GHz; (d) 500 MHz; (e) 1 GHz; (f) 1.5 GHz; (g) 2 GHz; (h) 2.5 GHz; (i) 3 GHz.
图 5 MFC的相位噪声与稳定性 (a) 不同动力学态产生的MFC的相位噪声; (b)动力学态为SS与PPSH时, 产生的MFC的一阶梳线的稳定性; (c)种子信号为PPSH时, 不同梳间距的MFC在10 kHz偏移处的一阶梳线相位噪声
Fig. 5. The phase noise diagram of MFCs: (a) Phase noise of MFCs generated under different dynamical states; (b) the stability of the first comb line of the MFCs when the dynamical states are SS and PPSH; (c) phase noise of the first comb line at a 10 kHz offset for MFCs with various comb spacings when the seed signal is PPSH.
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