Widely-wavelength-tunable brillouin fiber laser with improved optical signal-to-noise ratio based on parity-time symmetric and saturable absorption effect
doi: 10.37188/CO.EN-2024-0016
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摘要:
本文提出了一种基于宇称时间对称与饱和吸收效应的宽可调谐高光信噪比布里渊光纤激光器。这种新型布里渊光纤激光器是通过使用保偏掺铒光纤Sagnac环实现宇称时间对称和饱和吸收效应的。保偏掺铒光纤Sagnac环是由一个保偏掺铒光纤、一个耦合器和两个偏振控制器构成的。利用保偏掺铒光纤固有的双折射特性，在注入Stokes信号时形成两个处于正交偏振态的反馈环。其中一个环路在Sagnac环内提供顺时针方向的增益，而另一个环路在逆时针方向产生损耗。当饱和吸收效应参与的受激布里渊散射增益和损耗相平衡，并且增益值大于耦合系数时，由于宇称时间对称性被破坏，通过调整偏振控制器改变保偏掺铒光纤的偏振态，可获得单纵模布里渊光纤激光器。与以往的布里渊光纤激光器相比，本文提出的激光器具有更精简的结构和更宽的波长可调范围，且不受掺铒光纤放大器带宽的限制，同时仍保持窄线宽单纵模输出。此外，由于保偏掺铒光纤的饱和吸收效应，提高了宇称时间对称受激布里渊散射增益对比度，因此获得了更高的光信噪比。实验结果表明，该激光器具有
1526.088 ～1565.498 nm宽可调范围、77 dB光信噪比和140.5 Hz线宽。Abstract:A widely-wavelength-tunable Brillouin fiber laser (BFL) with improved optical signal-to-noise ratio (OSNR) based on parity-time (PT) symmetric and saturable absorption (SA) effect is present. This novel BFL realizes PT symmetry and SA effect through polarization-maintaining erbium-doped fiber (PM-EDF) Sagnac loop, which is composed of a PM-EDF, a coupler and two polarization controllers (PCs). By using the inherent birefringence characteristic of PM-EDF, two feedback loops in orthogonal polarization state are formed when the Strokes signal in injected. One of these loops provides gain in the clockwise direction with in the Sagnac loop, while the other loop generates loss in the counterclockwise direction. By adjusting the PCs to control the polarization state of the PM-EDF, a single-longitudinal-mode (SLM) BFL can be achieved, as the PT symmetry is broken when the SA participating stimulated Brillouin scattering (SBS) gain and loss are well-matched and the gain surpasses the coupling coefficient. Compared to previous BFLs, the proposed BFL has a more streamlined structure and a wider wavelength tunable range, at the same time, it is not being limited by the bandwidth of the erbium-doped fiber amplifier while still maintaining narrow linewidth SLM output. Additionally, thanks to SA effect of the PM-EDF, the PT symmetric SBS gain contract is enhanced, resulting in a higher optical signal-to-noise (OSNR). The experimental results show that the laser has a wide tunable range of
1526.088 nm to1565.498 nm, an improved OSNR of 77 dB, and a fine linewidth as small as 140.5 Hz.-
Key words:
- Brillouin fiber laser /
- widely-wavelength-tunable /
- parity-time symmetric /
- high OSNR /
- narrow linewidth
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Figure 1. Experimental setup. TLS: tunable laser source; Cir: circulator; PC: polarization controllers; OC: optical coupler; SMF: single-mode fiber; ESA: electrical spectrum analyzer; PM-EDF: polarization maintaining erbium-doped fiber; PD: photodetector; OSA: optical spectrum analyzer.
Figure 2. The foundational concept behind the BFL based on PT symmetric and SA effect. (a) The occurrence of Stimulated Brillouin Scattering in the optical spectrum. (b) The mode selection operational principle when the PT symmetry breaking occurs within the PT-symmetric BFL
Figure 3. Measured optical spectrum. (a) The optical spectrum of the BFL and
1550 nm pump wavelength and (b) the threshold curve between the pump intensity and BFL output intensity
Figure 4. The wavelength tunability measurement results of the BFL based on SA effect and PT symmetric
Figure 5. The spectral analysis results of the frequency beat signals emitted from the PD. (a) Unbroken PT-symmetry and (b) broken PT-symmetry with their magnified view under 0−4 MHz frequency range; (c) and (d) are enlarged views at 100−300 kHz respectively; (e) and (f) present a comparative diagram of the measured spectra, demonstrating the effects of PT-symmetry under 300−500 kHz and 500−700 kHz frequency ranges respectively
Figure 6. The linewidth measurement setup of the BFL based on PT symmetric and SA effect
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