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为了获得窄线宽的高频微波信号,在实验结构中加入级联光纤环结构和萨格纳克环结构,利用复合滤波的方式实现单纵模Stokes光信号的输出。级联光纤环结构中,环1是主环,环2是子环,根据维纳效应,级联环法布里-珀罗的有效自由光谱范围(FSR)为:
$$ FSR={n}_{1}FS{R}_{1}={n}_{2}FS{R}_{2} $$ (1) 式中:
$ {n}_{m} $ (m=1, 2)为整数;$ FS{R}_{m}=c/n{L}_{m} $ 为增益带宽;$ {L}_{m} $ 为长度。已知Brillouin增益带宽为20 MHz,对应的SMF长度为10 m,因此通过公式(1)可得环1和环2的Brillouin增益带宽分别为1 MHz和4 MHz。图2(a)和图2(b)分别为单个环1和级联光纤环的零差检测频谱,级联光纤环结构与单环结构相比,可增大纵模间隔,使其由1 MHz增大至4 MHz。但Brillouin增益带宽内依然有多个模式的存在,无法保证Stokes光信号的单纵模输出,因此需对其做进一步滤波处理。在级联光纤环结构之后加入萨格纳克环滤波结构,由于未泵浦的保偏掺铒光纤对Stokes光信号的饱和吸收效应,在萨格纳克环结构中顺时针和逆时针方向传输的相同阶次Stokes光信号会在其内部形成自写入式的窄带布拉格光栅滤波器,其滤波带宽为:$$ \Delta f=\frac{c}{{\lambda }_{B}}k\sqrt{\left(\frac{\Delta n}{2{n}_{0}}\right)^{2}+\left(\frac{{\lambda }_{B}}{2{n}_{0}{L}_{g}}\right)^{2}} $$ (2) 式中:真空中的光速
$c$ =3×108;窄带布拉格光栅的中心波长$ {\lambda }_{B} $ =1 550 nm;光纤介质的平均折射率$ {n}_{0} $ =1.45;未泵浦的保偏掺铒光纤长度$ {L}_{g} $ =2 m。自引入光栅的耦合系数为$ k=\pi \dfrac{\Delta n}{{\lambda }_{B}} $ ,折射系数与入射光功率有关,调节衰减器使折射系数$\Delta n$ =3×10−8[15],可计算出自写入式窄带布拉格光栅的半高全宽为3.145 MHz,小于两个纵模间隔8 MHz,因此在自写入式布拉格光栅滤波器的滤波带宽内只存在一个模式,实现了单纵模Stokes光信号的输出。图2(c)为级联光纤环结构和萨格纳克环结构复合滤波时的零差检测频谱,从图中可知输出的Stokes光信号为单纵模运行状态。图 2 (a) 环1零差检测频谱;(b) 级联光纤环零差检测频谱;(c) 级联光纤环和萨格纳克环零差检测频谱
Figure 2. (a) Homodyne detection spectrum of ring-1; (b) Homodyne detection spectrum of cascade optical fiber ring; (c) Homodyne detection spectrum of Cascade optical fiber ring and Sagnac ring
调节掺铒光纤放大器的输出功率,当功率增加至104 mW时,光谱分析仪上可观测到第一阶、第三阶和第五阶Stokes光信号的输出。调节泵浦光信号的输出波长使其产生的第三阶Stokes光信号波长与反射式光纤光栅的滤波中心波长对应,进而滤除第三阶Stokes光信号的输出,可得具有4倍Brillouin频移间隔的第一阶和第五阶Stokes光信号输出光谱如图3所示。
把图3中输出的4倍Brillouin频移间隔的单纵模双波长Stokes光信号输入光电探测器进行拍频检测,通过频谱分析仪观测到信噪比达19 dB的42.85 GHz高频微波信号产生,如图4所示。每间隔10 min记录一次输出频谱的变化情况,从图5中可知,产生的42.85 GHz高频微波信号的频率变化在0.83 MHz范围内,峰值功率变化在±0.8 dB范围内。
图 5 高频微波信号的频率漂移和功率变化
Figure 5. Frequency drift and power changes of high frequency microwave signals
通过Origin软件对产生的42.85 GHz高频微波信号进行非线性拟合,可得42.85 GHz高频微波信号的半高全宽为38 kHz,如图6所示。
Brillouin频移公式可表示为:
$$ {V}_{B}=\frac{2n{V}_{a}}{{\lambda }_{p}} $$ (3) 式中:n为光纤介质的折射率;
$ {V}_{a} $ 为光纤中的声波速度;$ {\lambda }_{p} $ 为泵浦激光器波长。通过公式(3)可知,由于n和$ {V}_{a} $ 是固定不变的,因此通过调谐泵浦激光器的波长即可实现高频微波信号的频率调谐。通过调谐泵浦激光器波长,可实现高频微波信号在42.25~43.51 GHz范围内的频率调谐,如图7所示。该实验结构可在较低泵浦功率下实现窄线宽的高频微波信号产生,具有很高的实用价值。此外,通过增加泵浦功率和反射式光纤光栅的滤波范围,可获得更高频率的微波信号产生,由于实验所用的频谱分析仪最高频率范围只能到44 GHz,因此最多观测到43.51 GHz的高频微波信号产生。
Narrow linewidth high frequency microwave signal generator based on composite filter structure
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摘要: 为了在较低泵浦功率下实现单纵模双波长激光信号的输出,进而获得窄线宽的高频微波信号,设计并实验了一种基于复合滤波结构的窄线宽高频微波信号产生装置。通过8字腔结构布里渊增益腔和反射式光纤光栅构成的波长选择滤波器实现了4倍布里渊频移间隔的双波长斯托克斯光信号输出,采用200 m长单模光纤作为增益介质,同时与50 m长单模光纤构成级联光纤环结构,采用三端口耦合器与2 m长未泵浦的保偏掺铒光纤构成萨格纳克环结构,利用级联光纤环结构和萨格纳克环结构的复合滤波作用实现了斯托克斯光信号模式的选择,使输出的斯托克斯光信号由多纵模运行状态变为单纵模运行状态。实验证明:通过对输出的单纵模双波长斯托克斯光信号进行拍频检测可得42.85 GHz的高频微波信号产生,线宽为38 kHz;通过改变可调谐泵浦激光器的输出波长,可实现42.25~43.51 GHz范围内的频率调谐;通过稳定性测试,产生的42.85 GHz高频微波信号的频率变化在0.83 MHz内,峰值功率变化在±0.8 dB内,稳定性良好,满足实际应用需求。Abstract: In order to realize the output of single longitudinal mode dual-wavelength laser signal at lower pump power and obtain the high frequency microwave signal with narrow line width, the narrow linewidth high frequency microwave signal generator based on multiple filter compound structure was proposed and demonstrated. The dual-wavelength Stokes optical signal with four times Brillouin frequency shift interval was realized through the eight shaped Brillouin cavity structure and the wavelength selective filter composed by reflective fiber grating. The 200 m length single mode fiber was used as gain medium, and it forms a cascaded fiber ring structure with 50 m long single-mode fiber. A three-port coupler and 2 m long unpumped polarization-maintaining erbium-doped fiber was used to form a Sagnac ring structure. The cascaded fiber ring configuration and Sagnac ring configuration were designed to select mode for single longitudinal mode Stokes optical signal. The experiment proves that the high-frequency microwave signal of 42.85 GHz can be generated by the beat frequency detection of the output single-longitudinal-mode dual-wavelength Stokes optical signal, and the line width is 38 kHz; Changing the output wavelength of the tunable pump laser, frequency tuning in the range of 42.25-43.51 GHz can be achieved; Through the stability test, the frequency change of the 42.85 GHz high-frequency microwave signal is in the range of 0.83 MHz, and the peak power change is in the range of ±0.8 dB. It has good stability and meets the actual application requirements.
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Key words:
- fiber optics /
- high frequency microwave signal /
- cascaded fiber ring /
- Sagnac ring
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