| Liquid level detection has important application value in petroleum storage,chemical production,agricultural planting,etc.Compared to traditional electricity sensors,optical fiber sensors have the characteristics of immunity from electromagnetic interference,high sensitivity,and low cost.In recent years,the research on optical fiber liquid level sensors has attracted the attention of many scholars.Cascading different types or the same type of fiber optic sensors can help improve and expand the performance of fiber optic sensors.In order to optimize the performance of optical fiber liquid level sensors,three cascaded systems have been designed to achieve simultaneous detections of liquid level and temperature,liquid level measurement with refractive index compensated compensation,and simutaneous detections of liquid level and strain.Firstly,in order to achieve simultaneous detection of liquid level and temperature,a sensing system for liquid level and temperature detection with two mode interference sensors cascaded was designed.Using a section of 2 cm no core fiber(NCF),a fiber structure of single mode fiber-no core fiber-single mode fiber(SMF-NCF-SMF)was fabricated,which could excited multi-mode interference(MMI)to detect the variation of liquid level.Using a section of 1 cm NCF to fabricated SMF-NCF-SMF,and coating polydimethylsiloxane(PDMS)on the NCF to excite MMI.The strong photothermal and thermal expansion effects of PDMS are used to detect changes in external temperature.Experiments data show that in a cascade system,two sensors could work normally,and the variation of temperature have little influence on the liquid level sensor.The sensitivities of liquid level and temperature sensors were 580 pm/mm and 1 nm/℃,respectively.A matrix method was used to understand the simultaneous detection of liquid level and temperature of this sensing system.Secondly,in order to achieve refractive index compensated liquid level sensing,a refractive index compensated liquid level sensing system with two MMI sensors cascaded was designed.Using a section of 2 cm NCF to fabricated SMF-NCF-SMF optical fiber structure to excite MMI and detect variation of liquid level.Using a section of 1 cm of NCF and 1 cm of photonic crystal fiber(PCF)to fabricated the SMF-NCF-PCF-SMF fiber structure,which could excited MMI and detect hte variation of refractive index.The sensing characteristics of liquid level sensors under different refractive index liquids were tested experimentally,and the performance of liquid level sensors and refractive index sensors in the cascade system was also tested.Experimental data show that the sensitivity of the liquid level sensor increased with the increase of liquid refractive index,and the maximum sensitivity is approximately 875 pm/mm.In this cascade system,refractive index sensors could compensate for liquid level sensing in the range of 1.3335-1.3717.The liquid level sensing process with refractive index compensation in the sensing system was explained using contour map.Finally,in order to achieve simultaneous detections of liquid level and strain,an experimental study was conducted on a cascaded sensing system of optical fiber MMI liquid level sensor and optical fiber Sagnac interference(SI)strain sensor,achieving the goal of dual parameter detections of liquid level and strain.The SMF-NCF-SMF optical fiber structure was prepared using a section of 2 cm NCF to excite MMI and measure the variation of liquid level.Using a section of 10 cm polarization-maintaining fiber(PMF)and a 3 d B coupler to build an SI loop.The sensing system was built through cascading the SI loop and the liquid level sensor.Experimental data showed that both sensors could work normally in the cascade system and performed small hysteresis effect.The variation of strain had little influence on the detection of liquid level,but the variation of liquid level had obvious influence on the detection of strain.The sensitivities of liquid level and strain sensors were 525 pm/mm and33 pm/με,respectively. |
