In this work, a novel Michelson interferometer (MI) based on the core-offset splicing between the single-mode fiber (SMF) and thin-core fiber (TCF) is proposed and experimentally demonstrated. By using the layer-by-layer self-assembly method, a novel metal-organic framework UiO-66 film is coated on the TCF surface. Then, the MI for fluoride-ion detection is obtained. The adsorption of fluoride ions by the sensing film induces the change in the effective refractive index (RI) of the coated cladding, which results in the regular redshifts of the interference dip. Different fluoride-ion concentrations can be obtained in real-time by monitoring the dip wavelength shifts. The proposed sensor exhibits an excellent linear response in the fluoride-ion concentration range up to 0.1 ppb, a high sensitivity of 612 pm/ppb and a fast response time of 10 s.
A carbon monoxide (CO) sensor based on a Michelson interferometer combined with α-Fe2O3/reduced graphene oxide quantum dots (rGOQDs) composite film is proposed and fabricated. First, a waist-enlarged taper is formed between the single-mode fiber (SMF) and the no-core fiber (NCF), then the other end of the NCF is spliced with a section of thin-core fiber (TCF). The end of the TCF is coated with a layer of silver film to enhance the reflection. Thus, a Michelson interferometer comprising SMF–NCF–TCF is formed. The α-Fe2O3/rGOQDs composite film is deposited on the outside surface of the TCF. The specific adsorption of CO by the composite film leads to a change in the sensor's effective refractive index, realizing the detection of CO. The results show that the interference intensity of the monitoring valley decreases with increase in the concentration of CO. The sensitivity of the sensor is 0.057 dBm ppm−1, the detection limit of the sensor is 105 ppb and the response time and recovery time are 70 s and 100 s, respectively. The sensor has the advantages of high sensitivity, high selectivity and simple structure, and it is expected to be used for the detection of low concentrations of CO gas.
A novel optic-fiber intensity-modulated carbon monoxide (CO) gas sensor based on cerium dioxide (CeO 2 ) sensing film which coated thin-core fiber (TCF) was proposed and fabricated. Single-mode fiber (SMF), no-core fiber (NCF) and TCF are symmetrically spliced to form a SMF-NCF-TCF-NCF-SMF Mach-Zehnder interference structure. The crystal structure, surface morphology and composition of CeO 2 sensing membrane have been characterized by using X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscope, high resolution transmission electron microscopy, high-angle annular dark-field and energy dispersive spectrum. The sensing film with the porous appearance can be successfully achieved by the hydrothermal reaction and dip-coating method. While the CO concentration is ranging from 0 to 70 ppm, the sensitivity and detection limit of the optic-fiber CO gas sensor are, respectively, 0.03224 dB/ppm and 0.07630 ppm, and it has a good linearity (0.99127) and almost insensitive to temperature. The response time and recovery time are about 40 s and 47 s, respectively. The surface adsorption and desorption sensing mechanism has been discussed in detail. In addition, CO can be detected by the sensor with unexceptionable selectivity, which has also been verified by the surface adsorption energy results between CeO 2 and seven gases, i.e., oxygen (O 2 ), carbon dioxide (CO 2 ), argon (Ar), ammonia (NH 3 ), nitrogen (N 2 ), hydrogen sulfide (H 2 S) and carbon monoxide (CO). This sensor is simple in structure, and easy to manufacture. It has a good application prospect in the toxic CO monitoring.
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