The present paper reports the detailed study of various metal oxide nanomaterials as opto-electronic humidity sensor The sensor elements were fabricated and characterized as thin or thick film based on the semi-conducting metal oxide As semi-conducting metal oxides are known for their n-type conduction because of the presence of oxygen vacancies and change in refractive indices with the exposure of moisture therefore they were proven to be very good sensors for humidity Depending upon the need, the sensing materials were deposited either on the base of the prism or on the U-shaped borosilicate glass rods or on some other transparent substrates for opto-electronic sensing Light from LED or He–Ne laser was launched into the sensing element from one side and collected into the other side by optical power meter The mode of propagation of light was transmission, reflection or partially refraction Modulations in the intensity of light with changes in humidity were recorded Being optical in nature, such types of sensors are very useful for detection of moisture level at remote places or unmanned stations The primary purpose of this paper is to focus on the techniques used in these sensors

Opto-electronic humidity sensor: A review

This review covers photonic crystal cavities (PCCs) and their applications in optical sensors, with a particular focus on the structures of different PCCs. For each kind of optical sensor, the specific measurement principle, structure of PCC, and the corresponding sensing properties are all presented in detail. The summary of the reported works and the corresponding results demonstrate that it is possible to realize miniature and high-sensitive optical sensors due to the ultra-compact size, excellent resonant properties, and flexibility in structural design of PCCs. Finally, the key problems and new directions of PCCs for sensing applications are discussed.

/pdf/a-review-for-optical-sensors-based-on-photonic-crystal-wcqhfbx0s5.pdf

A review for optical sensors based on photonic crystal cavities

A novel parallel structured fiber-optic Fabry-Perot interferometer (FPI) based on Vernier-effect is theoretically proposed and experimentally demonstrated for ultrasensitive strain measurement. This proposed sensor consists of open-cavity and closed-cavity fiber-optic FPI, both of which are connected in parallel via a 3 dB coupler. The open-cavity is implemented for sensing, while the closed-cavity for reference. Experimental results show that the proposed parallel structured fiber-optic FPI can provide an ultra-high strain sensitivity of −43.2 pm/μe, which is 4.6 times higher than that of a single open-cavity FPI. Furthermore, the sensor is simple in fabrication, robust in structure, and stable in measurement. Finally, the parallel structured fiber-optic FPI scheme proposed in this paper can also be applied to other sensing field, and provide a new perspective idea for high sensitivity sensing.

Ultrasensitive strain sensor based on Vernier- effect improved parallel structured fiber-optic Fabry-Perot interferometer

In-fiber interferometric-based sensors are a rapidly growing field, as these sensors exhibit many desirable characteristics compared to their regular fiber-optic counterparts and are being implemented in many promising devices. These sensors have the capability to make extremely accurate measurements on a variety of physical or chemical quantities such as refractive index, temperature, pressure, curvature, concentration, etc. This article is a comprehensive overview of the different types of in-fiber interferometric sensors that presents and discusses recent developments in the field. Basic configurations, a brief approach of the operating principle and recent applications are introduced for each interferometric architecture, making it easy to compare them and select the most appropriate one for the application at hand.

https://www.mdpi.com/2304-6732/8/7/265/pdf

In-Fiber Interferometric-Based Sensors: Overview and Recent Advances

In this paper, the thermo-optic coefficient of silica fiber is analyzed and measured within a wide temperature range of 0–1200 °C for the first time. To obtain the thermo-optic coefficient, experiments were carried out by using fiber optic extrinsic Fabry–Perot interferometers (EFPIs) embedded in silica fiber. The optical cavity lengths of the EFPIs were measured with white-light interferometry. The experimental results show that the first and second orders of the thermo-optic coefficient are 1.090 × 10−5/°C and 1.611 × 10−9/°C2, respectively. The quadratic model for the temperature dependence of the basic optical parameter, the refractive index, is rigorous within a wide temperature range. Furthermore, this paper also provides a temperature calibration method for extrinsic Fabry–Perot interferometric temperature sensors. The temperatures measured by the sensors are in excellent agreement with those measured by the reference thermocouple. Hence, this calibration method is promising for temperature sensors with a wide working temperature range.

Investigation on the Thermo-Optic Coefficient of Silica Fiber Within a Wide Temperature Range

A photonic crystal fiber (PCF) based high-temperature fiber-optic sensor is proposed and experimentally demonstrated. The sensor head is a Fabry-Perot cavity manufactured with a short section of endless single-mode photonic crystal fiber (ESM PCF). The interferometric spectrum of the Fabry-Perot interferometer is collected by a charge coupled device linear array based micro spectrometer. A high-resolution demodulation algorithm is used to interrogate the peak wavelengths. Experimental results show that the temperature range of 1200 °C and the temperature resolution of 1 °C are achieved.

High-temperature fiber-optic Fabry-Perot interferometric sensors.

A filmed extrinsic Fabry–Perot interferometric (FEFPI) sensor for the measurement of arbitrary refractive index (RI) is experimentally demonstrated. The FEFPI sensor was constructed by aligning two fiber endfaces, on which the multilayer dielectric mirrors are deposited. There is a fixed reflection at the filmed fiber endface no matter how much the RI is, and we can obtain a clear interferogram with a high visibility. A resolution of up to 1.64 × 10 −5 refractive index unit is achieved in our experiment.

Filmed extrinsic Fabry–Perot interferometric sensors for the measurement of arbitrary refractive index of liquid

An Agarose gel filled photonic crystal fiber in-line interferometric sensor for the measurement of relative humidity is proposed and experimentally demonstrated. The sensor is constructed by filling the Agarose gel between the aligned single mode fiber (SMF) and the photonic crystal fiber (PCF). A fiber in-line interferometer is fabricated by splicing the other end of the PCF and another SMF. Due to the tunable refractive index property of the Agarose gel, the mode field diameter of the propagation light is changed with the external relative humidity, which induces the change of coupling ratio between the PCF and the SMF. The relative humidity is measured by interrogating the fringe visibility of the white-light interferogram. The experimental results show that the sensitivity of up to 2.2 dB/RH is achieved. The proposed method possesses high resolution and low temperature cross-sensitivity.

Agarose gel filled temperature-insensitive photonic crystal fibers humidity sensor based on the tunable coupling ratio

A fiber optic white-light interferometry based on cross-correlation calculation is presented. The detected white-light spectrum signal of fiber optic extrinsic Fabry-Perot interferometric (EFPI) sensor is firstly decomposed by discrete wavelet transform for denoising before interrogating the cavity length of the EFPI sensor. In measurement experiment, the cross-correlation algorithm with multiple-level calculations is performed both for achieving the high measurement resolution and for improving the efficiency of the measurement. The experimental results show that the variation range of the measurement results was 1.265 nm, and the standard deviation of the measurement results can reach 0.375 nm when an EFPI sensor with cavity length of 1500 μm was interrogated.

A cross-correlation based fiber optic white-light interferometry with wavelet transform denoising

A white-light interferometry based on the Fourier transform method is proposed to measure the optical path difference (OPD) of the high-finesse fiber optic extrinsic Fabry–Perot interferometric (EFPI) sensor. The Fourier spectrum of the transmission spectrum signal consists of multiple frequency components because of the multiple-beam interference occurring in the high-finesse EFPI sensor. The high-order frequency component of the Fourier spectrum can be extracted by the Fourier transform method to recover the OPD in order to overcome the spectrum overlapping, which happens when the OPD of the EFPI sensor is short. In the experiment, a high-finesse fiber optic EFPI sensor with the cavity length of 120 \(\mu \) 
m is measured, and the second-order frequency component was extracted to recover the cavity length. The standard deviation of the measurement results was 9.132 nm. The measurement range of the Fourier transform method was effectively extended to the short OPD.

A White-Light Interferometry for the Measurement of High-Finesse Fiber Optic EFPI Sensors

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