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如图4所示,通过对模拟AOMⅠ使用对称三角波形调制,最终输出一个脉冲宽度为309 ns的高斯脉冲。通过改变声光调制器的驱动信号亦可输出平顶脉冲。由图4中可以看出,高斯脉冲顶部平缓光滑,无明显的受激布里渊散射(SBS)出现。布里渊散射是光和传播光的介质之间的相互作用,当入射光功率达到一定值时,大部分的功率会被散射回来。通过在光纤上施加不均匀的梯度应力,影响布里渊的增益谱线,产生不同的布里渊频移,从而提高受激布里渊散射阈值,抑制放大器中的SBS[17]。如图5所示,将设计的应变梯度应用于Er/Yb共掺商用PM 25/300光纤中,光纤的起始段和末端段设计为无应变,以便于和其它光纤熔接,使用一个直径为100 mm的带有凹槽的金属圆盘来缠绕PM 25/300增益光纤,以获得更好的光束质量和热耗散。
激光功率指标主要受SBS效应的影响,SBS阈值的简化公式如下:
式中:Aeff为光学有效模面积;gB为SBS增益系数;Leff为有效光纤长度;α为吸收系数。在确定光纤种类的条件下,为了提高SBS阈值,应尽可能地缩短各器件的光纤长度以减小Leff的值。如图6所示,输出功率随泵浦电流的增加近似线性变化,在重频10 kHz条件下,最大平均输出功率高达1.6 W。整个系统电功耗小于60 W。
利用日本横河AQ6370D光谱分析仪对放大系统产生的激光光谱特性进行分析。输出脉冲光谱如图7所示,信噪比大于25 dB。如果加入窄带滤波器,信噪比可大于40 dB。
L波段可切换双波长高能量窄线宽脉冲光纤激光器基本参数如表 1所示。整机效果如图8所示,激光器运行过程中底部应加入相同规格尺寸的散热尺以避免因热效应引起的系统性能的变化。
Parameter Test Center wavelength/nm 1572.018 &1572.48 Wavelength accuracy/pm ±2 Repetition frequency/kHz 10 Pulse width/ns 309 SNR/dB >25 Pulse energy/uJ >150 Peak power/W >485 Energy stability <5% AOM extinction ratio/dB >80 AOM frequency shift/MHz >80 Polarization extinction ratio/dB >20 Beam quality <1.5 Power consumption/W <60 Case size/mm3 387×340×81 Cooling method Air Table 1. Specification of pulsed fiber laser
L-band switchable dual-wavelength, high-energy pulsed fiber laser
doi: 10.3788/IRLA20200370
- Received Date: 2020-09-20
- Rev Recd Date: 2020-12-01
- Available Online: 2021-07-23
- Publish Date: 2021-07-25
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Key words:
- fiber laser /
- pulsed fiber amplifier /
- L-band /
- single frequency
Abstract: An L-band switchable dual-wavelength, frequency stabilized, high energy, single frequency, single mode linearly polarized, pulsed laser based on a master osocillator power amplifier (MOPA) configuration was reported. It could be used as the emission source of detecting atmospheric CO2 LIDAR system. This pulsed fiber laser system was mainly composed of two single frequency narrow linewidth external cavity semiconductor lasers, pulse modulation system, and multi-stage fiber amplifiers. The wavelengths of 1572.018 nm and 1572.480 nm could be switched freely by controlling a magneto-optical switch. Using the closed loop temperature control technology, the locked central frequency and output optical power were implemented. The on-off extinction ratio of 80 dB was achieved by using digital and analog acousto-optic modulators in series. The Stimulated Brillouin Scattering (SBS) threshold was increased by applying non-uniform stress to the fiber. With a common commercial gain fiber and commercial polarization maintaining components, an average output power of 1.5 W, pulse width of 309 ns, the peak power of 485 W, pulse energy of 150 μJ and signal to noise ratio of 25 dB were generated at a repetition rate of 10 kHz and the wavelength of 1572 nm. The power consumption of the whole laser system which used air cooling was less than 60 W.