| With the growing demand for Relative Humidity（RH）regulation,quick and precise humidity measurement has become critical.When compared with traditional electrical humidity sensors,optical fiber humidity sensors have better stability and are less susceptible to electromagnetic interference.However,the wavelength detection-based optical fiber humidity sensor is difficult to build,huge in size,and expensive,making it unsuitable for large-scale applications.The optical power detection-based evanescent wave type fiber optic humidity sensor has a simple construction,is easy to miniaturize and integrate,but is currently limited by the regulation of moisture-sensitive material physical characteristics,influencing sensing performance improvement.As a result,this paper proposes a composite film-based evanescent wave fiber optic humidity sensor,which primarily includes the theoretical analysis and simulation of evanescent wave transmission in media with varying optical properties,the controllable preparation method of composite moisture-sensitive film,the sensor’s humidity sensing performance,and the sensor’s moisture-sensitive mechanism.The following might serve as a summary of this article’s key works:（1）The transmission characteristics of evanescent waves in media with different optical properties are theoretically analyzed and simulated using the evanescent wave circular waveguide electromagnetic theory.When the cladding refractive index is close to the core refractive index,the deeper the penetration depth,the greater the optical power attenuation;and when the cladding material’s absorption coefficient increases,the sensor’s transmission loss increases as well.This paper provides theoretical and technical support for the sensing mechanism of a evanescent wave type optical fiber humidity sensor.（2）This paper designs and fabricates a PVA/PMMA composite film-based evanescent wave type fiber optic humidity sensor.The sensor’s luminescence intensity decreases with increasing relative humidity and improves sensing performance at high humidity,which is consistent with the analysis of the evanescent wave transmission theory.The composite thin-film sensor has a better average sensitivity than the PVA-coated sensor in the range of 10%to85%RH,81.3 lux/%RH,and response and recovery durations of 49 and 30 s respectively.In addition,the sensor is stable for long-term humidity monitoring.In conclusion,the PVA/PMMA composite film sensor may be used to assess high humidity levels since it has a better sensitivity under higher humidity.（3）This paper designs and fabricates a SiO2/PMMA composite film-based evanescent wave type fiber optic humidity sensor,and its transmission mechanism was theoretically analyzed and simulated.The sensor absorbs water molecules as humidity rises,and as ambient humidity changes,so do the film’s refractive index and absorption coefficient,modulating the amount of light transmitted through the fiber.The composite moisture-sensitive film can be prepared by sol-gel method and improved lifting method,and the SiO2-2/PMMA-3 composite film shows the best performance,and the luminescence intensity of the sensor shows a decreasing trend with the increase of humidity,and it shows excellent sensing performance at low humidity,which is in accordance with the analytical results of the evanescent wave transmission theory,and the sensor establishes a strong linear relationship between the logarithm of the output light intensity and the relative humidity with a slope of-0.03.The best average sensitivity of the composite thin-film fiber sensor was 188.3 lux/%RH in the range of5-95%RH,an improvement in average sensitivity of approximately 12 times.The response recovery performance simultaneously demonstrates clear benefits due to the reaction time of 8s and the recovery duration of 23 s.In addition,the sensor exhibits low hysteresis,as well as excellent stability and repeatability.In conclusion,the SiO2/PMMA composite thin film-based optical fiber humidity sensor offers a broad range of application possibilities due to its high sensitivity to low humidity,wide dynamic response range,quick response recovery speed,and outstanding repeated stability performance.（4）This paper designs and fabricates a SiO2/PEG composite film-based evanescent wave type fiber optic humidity sensor.The sol-gel method could be used to create the composite humidity sensitive film,and the SiO2-2/PEG-3 composite film performed best.The luminescence intensity of this sensor showed a linear increase trend with the increase in humidity,with a slope of 0.28 and linearity up to 0.996,and this sensing law is favorable for the sensor’s widespread application.In the range of 30%-80%RH,the average sensitivity of SiO2/PEG composite films reaches 277.6 lux/%RH,which is about 18 times that of single SiO2films.Its response time is 22 s and recovery time is 38 s,showing good response-recovery performance.In addition,the sensor has less hysteresis and can perform humidity sensing steadily for a long time.In conclusion,the SiO2/PEG composite thin film-based optical fiber humidity sensor,which has a wide variety of potential applications,has the benefits of excellent linearity,high sensitivity,and quick reaction times. |
PDF Full Text Request