Polarization mode dispersion (PMD) in fibers for high-power lasers can induce significant frequency modulation to amplitude modulation (FM-to-AM) conversion. However, existing techniques are not sufficiently flexible to achieve efficient compensation for such FM-to-AM conversion. By analyzing the nonuniform transmission spectrum caused by PMD, we found that the large-scale envelope of the transmission spectrum has more serious impacts on the amount of AM. In order to suppress the PMD-induced FM-to-AM conversion, we propose a novel tunable spectral filter with multiple degrees of freedom based on a half-wave plate, a nematic liquid crystal, and an axis-rotated polarization-maintaining fiber. Peak wavelength, free spectral range (FSR), and modulation depth of the filter are decoupled and can be controlled independently, which is verified through both simulations and experiments. The filter is utilized to compensate for the PMD-induced FM-to-AM conversion in the front end of a high-power laser facility. The results indicate that, for a pulse with phase-modulation frequency of 22.82 GHz, the FM-to-AM conversion could be reduced from 18% to 3.2% within a short time and maintained below 6.5% for 3 h. The proposed filter is also promising for other applications that require flexible spectral control such as high-speed channel selection in optical communication networks.

中文翻译：

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使用多自由度光纤滤波器抑制偏振模色散引起的幅度调制

用于高功率激光器的光纤中的偏振模色散 (PMD) 可以引起显着的频率调制到幅度调制 (FM-to-AM) 的转换。然而，现有技术不足以灵活地实现对这种 FM 到 AM 转换的有效补偿。通过对PMD引起的透射谱不均匀的分析，我们发现透射谱的大尺度包络对AM量的影响更为严重。为了抑制 PMD 引起的 FM 到 AM 转换，我们提出了一种基于半波片、向列液晶和轴旋转保偏光纤的多自由度可调谐光谱滤波器。滤波器的峰值波长、自由光谱范围 (FSR) 和调制深度是解耦的，可以独立控制，这是通过模拟和实验验证的。该滤波器用于补偿高功率激光设备前端由 PMD 引起的 FM 到 AM 转换。结果表明，对于相位调制频率为 22.82 GHz 的脉冲，FM 到 AM 的转换可以在短时间内从 18% 降低到 3.2%，并在 3 h 内保持在 6.5% 以下。所提出的滤波器也适用于需要灵活光谱控制的其他应用，例如光通信网络中的高速通道选择。短时间内达到2%，并保持在6.5%以下3小时。所提出的滤波器也适用于需要灵活光谱控制的其他应用，例如光通信网络中的高速通道选择。短时间内达到2%，并保持在6.5%以下3小时。所提出的滤波器也适用于需要灵活光谱控制的其他应用，例如光通信网络中的高速通道选择。