Influence of amplitude modulator on time-frequency characteristics of broadband low coherence light
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摘要: 通过改变马赫-曾德干涉型幅度调制器的射频系数和偏置电压，调制光脉冲的强度。研究了幅度调制器对宽带低相干光时频特性影响的规律，分析调制后的光脉冲时域波形分布、光谱和复相干度模值曲线，结果表明，射频系数对光脉冲的光谱成分和时间相干性无明显的调制，射频系数存在最佳工作区间使得输出光脉冲的波形保真度最佳。当偏置电压处于半波电压时，光脉冲的时域波形保真度最好，时间相干性最低，但光谱成分会缺失。理论仿真了调制器的臂长差、偏置电压对宽带低相干光频域特性的影响，与由实测光谱计算出调制器的臂长差，实验结果进行了对比，结果基本符合。由于实际的电光重叠积分因子随加载电压值变化，因而模拟与实测结果存在误差，但研究得出的规律将为低相干脉冲精密整形系统提供更为明确的方向。Abstract: Low-coherent light pulses with precise time-shaping capability have the potential to suppress the instability of laser-plasma interactions in laser inertial confinement fusion, but related research is currently lacking. In this paper, the law of the influence of the amplitude modulator on the time-frequency characteristics of broadband low-coherent light was studied, and the intensity of the light pulse was modulated by changing the RF coefficient and bias voltage of the Mach-Zehnder interferometric amplitude modulator. The time domain waveform distribution, spectrum and complex coherence modulus curve of the modulated light pulse were analyzed. The research shows that the RF coefficient has no obvious modulation on the spectral composition and temporal coherence of the light pulse, and the RF coefficient has an optimal working range, which makes the waveform fidelity of the output light pulse the best. When the bias voltage is at half-wave voltage, the time domain waveform fidelity of the optical pulse is the best, and the temporal coherence is the lowest, but the spectral components are missing. The arm length difference of the modulator is calculated from the measured spectrum, and the influence of the arm length difference and bias voltage on the frequency domain characteristics of the broadband low-coherent light is simulated theoretically, the results are in good agreement with the experimental results. As the actual electro-optic overlap integration factor changes with the voltage, there is an error between the simulation and the actual measurement results. However, the laws obtained from the research will provide a more clear direction for low coherent pulse precision shaping system development.
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图 1 M-Z型铌酸锂幅度调制器示意图
Figure 1. Schematic diagram of M-Z lithium niobate amplitude modulator
图 4 加载在幅度调制器上的电脉冲波形和幅度调制器输出光脉冲波形对比
Figure 4. The electrical pulse loaded on the amplitude modulator is compared with the output light pulse of the amplitude modulator
图 7 调制器偏置电压和输出电脉冲波形的关系
Figure 7. Relationship between modulator bias voltage and output electrical pulse
图 9 不同偏置电压时的调制器输出光谱
Figure 9. Spectra of output light with different modulator bias voltages
图 10 调制器射频系数为0.46时，不同偏置电压下输出光脉冲的复相干度曲线
Figure 10. Complex coherence curves of output optical pulses under different bias voltages when the RF coefficient of the modulator is 0.46
图 11 调制器射频系数为0.46时，AM偏置电压对光脉冲时间相干性的影响
Figure 11. Influence of AM bias voltage on the temporal coherence when the RF coefficient of the modulator is 0.46
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