一种耐静压分布反馈式光纤激光水听器探头设计
1 引言
分布反馈式光纤激光水听器具有灵敏度高,抗电磁干扰能力强,传播损耗低和便于大规模复用的优点,在水声探测领域具有广阔的应用前景[1-5]。目前,分布反馈式光纤激光水听器已经由单个阵元研究走向多阵元研究,复用技术的成熟使分布反馈式光纤激光水听器能够大规模空间布置,得到更好的水声探测信号[6-12]。由于光纤激光水听器阵列工作时处于不同水深,因此设计出适应静水压环境下频率响应一致的光纤激光水听器阵元,是分布反馈式光纤激光水听器工程化急需解决的问题。在抗加速度方面,分布反馈式光纤激光水听器劣于普通干涉型光纤水听器。
静水压对光纤光栅水听器主要产生2个方面的影响:1) 影响水听器阵列大规模复用,静水压作用下,光纤光栅水听器的出射激光中心波长会漂出解调系统波分复用窗口,导致光纤水听器无法解调出目标信号;2) 影响水听器阵元一致性,在不同静压作用下,水听器阵元的灵敏度也会发生变化。目前国内外对光纤光栅水听器耐静水压结构已进行了相应研究,2008年Steven Goodman设计了一种压力补偿结构,通过柔性气囊与水听器相连的方式,使得水听器的增敏薄板两侧压力平衡,经试验证明该水听器能在50 m水深下正常工作[13]。2014年新加坡Kuttan Chandrika Unnikrishnan将压力补偿结构以滑块室的形式集成到水听器内部,通过滑块调节滑块室的压力,从而达到静压平衡,该结构能满足40 m水深要求[14]。2012年李东明设计了一种弹性膜片增敏探头,该结构的耐压性能取决于弹性膜片的刚度,通过优化结构该探头的耐压性能达2 MPa[15]。2013年Zhang提出了一种静压补偿结构,通过筒壁上的圆孔来实现静压平衡,该结构的声压灵敏度频响曲线在2 kHz以下的波动范围不大于±2 dB[16]。
本文主要研究能在不同水深下保持正常工作的分布反馈式光纤激光水听器探头。通过优化结构的尺寸参数,获得在工作频段内理想的频响曲线,拓宽分布反馈式光纤激光水听器的应用范围。
1 理论分析
耐静水压的DFB光纤激光水听器的结构如
图 1. Structural diagram of DFB fiber laser hydrophone with resistant static pressure
Fig. 1. Structural diagram of DFB fiber laser hydrophone with resistant static pressure
图 2. Acoustic equivalent circuit of DFB fiber laser hydrophone with resistant static pressure
Fig. 2. Acoustic equivalent circuit of DFB fiber laser hydrophone with resistant static pressure
图 3. Simplified acoustic equivalent circuit of DFB fiber laser hydrophone with resistant static pressure
Fig. 3. Simplified acoustic equivalent circuit of DFB fiber laser hydrophone with resistant static pressure
端盖通孔的声阻抗为
套筒通孔的声阻抗为
近似为圆柱壳的端盖总等效声阻抗为
圆膜片可视为固定边界的圆板,圆膜片的总等效声阻抗为
短腔声阻
对于
由于该光纤激光水听器的尺寸结构很小,声压频率较低时,声波的波长比水听器大得多,端盖处的激励声压幅值和套筒开孔处的激励声压幅值近似相同,即
2 仿真分析
直接由声压传递函数表达式难以确定各尺寸参数对声压传递函数幅值的影响。为了能确定分布反馈式光纤激光水听器端盖孔半径a1、套筒孔半径a2、膜片厚度t、短腔长度L1、长腔长度L2和腔体半径R2,通过计算机软件对结构各参数与声压传递函数幅值的关系进行分析。端盖和套筒使用的材料为殷钢,弹性模量E1=1.42×1011 Pa,泊松比ν1=0.28,密度ρ1=8 100 kg/m3;膜片使用的材料为不锈钢,弹性模量E2=1.93×1011 Pa,泊松比ν2=0.31,密度ρ1=7 750 kg/m3;切边粘滞系数μ=0.001 14,水中声速c0=1 500 m/s;光纤激光水听器壳体厚度l1=2 mm,端盖孔数量n1=2,套筒孔数量n2=40。
图 4. Influence of end cap through-hole radius on sound pressure transfer function of hydrophone
Fig. 4. Influence of end cap through-hole radius on sound pressure transfer function of hydrophone
图 5. Influence of sleeve through-hole radius on sound pressure transfer function of hydrophone
Fig. 5. Influence of sleeve through-hole radius on sound pressure transfer function of hydrophone
图 6. Influence of diaphragm thickness on sound pressure transfer function of hydrophone
Fig. 6. Influence of diaphragm thickness on sound pressure transfer function of hydrophone
图 7. Influence of length of short cavity on sound pressure transfer function of hydrophone
Fig. 7. Influence of length of short cavity on sound pressure transfer function of hydrophone
图 8. Influence of long cavity length on sound pressure transfer function of hydrophone
Fig. 8. Influence of long cavity length on sound pressure transfer function of hydrophone
图 9. Influence of cavity radius on sound pressure transfer function of hydrophone
Fig. 9. Influence of cavity radius on sound pressure transfer function of hydrophone
声压灵敏度高,灵敏度频率响应平坦是分布反馈式光纤激光水听器结构设计的目标。综合以上分析,膜片厚度、短腔长度和腔体半径主要影响分布反馈式光纤激光水听器声压传递函数的幅值,即水听器结构对声压的放大倍数,膜片越薄,腔体半径越大,短腔越长,则光纤激光水听器灵敏度越高。但在实际应用中,考虑到分布反馈式光纤激光水听器的优点是体积小,因此主要通过减小膜片厚度来提高水听器灵敏度,而不是增大腔体半径或短腔长度。端盖通孔、套筒通孔和长腔长度主要影响分布反馈式光纤激光水听器声压传递函数的平坦度。端盖通孔半径越小,套筒通孔半径越大,长腔越短,则光纤激光水听器的响应越平坦。考虑到水听器结构稳定性,由于套筒通孔数量较多,因此套筒通孔半径的增大应保证光纤水听器的结构稳定;同时长腔长度不能无限减小,应以分布反馈式光纤激光器的长度为下限,因此主要通过减小端盖通孔半径来提高传递函数的平坦度。
3 实验分析
根据理论分析,设计加工了耐静压分布反馈式光纤激光水听器样品N002。该光纤激光水听器出射激光的中心波长为1 530.798 nm,膜片厚度t=0.1 mm,端盖通孔的半径a1=1 mm,套筒通孔的半径a2=1 mm,腔体半径R2=2 mm。在10 Hz~10 kHz频率范围内,对耐静压光纤激光水听器的声压灵敏度进行测量,声压灵敏度按照参考文献[19]中定义计算.通过解调系统得到分布反馈式光纤激光水听器相位变化信息,再通过示波器读取电压值得到声压信息,最后经计算得到水听器声压灵敏度。低频段用振动液柱法进行光纤激光水听器校准实验,实验装置如
图 10. Experimental device diagram of vibration liquid column method
Fig. 10. Experimental device diagram of vibration liquid column method
图 11. Schematic diagram of sound-absorbing pool experimental system
Fig. 11. Schematic diagram of sound-absorbing pool experimental system
2次测量得到的实验数据如
图 12. Response curve of measured frequency for resistant static pressure fiber laser hydrophone
Fig. 12. Response curve of measured frequency for resistant static pressure fiber laser hydrophone
为验证耐静压光纤激光水听器的耐压性能,搭建耐静压实验平台,实验系统如
图 13. Schematic diagram of resistant static pressure experimental system
Fig. 13. Schematic diagram of resistant static pressure experimental system
实验数据如
图 14. Static pressure test of resistant static pressure fiber laser hydrophone
Fig. 14. Static pressure test of resistant static pressure fiber laser hydrophone
4 总结
本文针对光纤激光水听器阵列在实际应用中面临的耐静水压问题,提出一种静压补偿结构。该结构能解决分布反馈式光纤激光水听器在静压环境下的大规模复用问题及水听器成阵的阵元灵敏度一致性问题。基于电-力-声类比理论,建立了耐静压分布反馈式光纤激光水听器探头模型。并分析了结构参数对传递函数的影响,为耐静压分布反馈式光纤激光水听器探头响应平坦化设计提供理论依据。根据分析结果,制作了耐静压光纤激光水听器样品,实验结果表明:该光纤激光水听器平均声压灵敏度达到−146.4 dB,在0 Hz~10 kHz工作频率范围内起伏不大于±2.4 dB,在0 MPa~2.3 MPa静压环境下出射激光的中心波长漂移量不大于0.06 nm,满足光纤激光水听器阵列波分复用的要求,对深水分布反馈式光纤激光水听器工程化具有重要的指导意义。
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