单端面长周期光栅透射模式测量技术

摘要

In order to realize remote monitoring or measuring in the way of transmission mode for long period fiber grating (LPFG), the measurement device system based on a single end face coated with reflective film was designed.The sensing principle of LPFG with a single end face coated with reflective film was explained and analysed.The responses of the single end face system and the direct transmission mode system to environmental media with different refractive index were investigated by experiments.Firstly, 2×2 single-mode fiber coupler was used to connect spectrometer, light source and LPFG, respectively.Then, the reflective silver film was fabricated in the other end face of the optical fiber including LPFG.Finally, by measuring a series of environmental media with different refractive indices, the response spectra of LPFG were compared in the two cases of the direct transmission mode and the silver film coated single end face mode.When the wavelength demodulation is adopted, the resonance wavelength corresponding to the same medium is nearly same in the above two cases.When the power/peak demodulation is adopted, the optical loss gradually changed from-6.05 dB to-9.22 dB for the direct transmission mode, but from-8.03 dB to-11.33 dB for the silver film coated single end face mode with the increase of glycerin concentration in solutions.Compared with the direct transmission mode, the relative optical loss of LPFG based on the single end face mode increases significantly.The LPFG in the single end face mode has more sharp resonance peaks than that in the direct transmission mode, which makes the resonance wavelengths and the optical loss value being recognized more easily.The measurement device system based on the single end face coated with silver film not only retains the transmission sensing mode of LPFG, but also makes the operation of measuring environment medium more flexible and convenient.In particular, it has a unique advantage for the measurement of refractive index in long distance, deep liquid, and severe environment.%为了实现长周期光栅透射谱测量模式的远距离监测,设计了单端面镀反射膜的测量装置系统,对单端面镀银膜长周期光栅的传感原理做了分析,并从实验的角度分别对单端面镀银膜模式系统和直接透射模式系统的长周期光栅在不同折射率的环境介质中的响应进行了研究,比较了它们的异同.首先,采用2×2单模光纤耦合器分别连接光谱分析仪、光源、长周期光栅.然后,在包含长周期光栅的光纤的另一个端面制备反射银膜.最后,通过测量一系列不同折射率的环境介质,比较了直接透射模式与单端面镀银膜模式下的长周期光栅的响应光谱.实验结果表明:采用波长解调表达时,对于同一种环境介质,两种模式下长周期光栅的响应光谱的谐振波长基本相同;采用功率/峰值解调表达时,随着甘油浓度从水变为80%的甘油溶液,直接透射模式下的光损耗从-6.05 dB变为-9.22 dB, 单端面镀银膜模式下的光损耗从-8.03 dB变为-11.33 dB.与直接透射模式相比,单端面镀银膜的长周期光栅光谱中的相对光损耗明显增加,谐振峰更尖锐,更有利于谐振波长和谐振峰光损耗值的识别.本研究设计的单端面镀银膜的长周期光栅测量系统不仅保留了长周期光栅透射谱的感应模式,而且使长周期光栅在对环境介质的测量中操作更加灵活方便,尤其是在远距离、恶劣环境或深层液体的折射率测量中具有独特的优势.