Chunyang Han

Vernier effect has been captivated as a promising approach to achieve high-sensitivity optical sensors. Nevertheless, their practical applications are hindered by the ultra-fine manufacturing process and spectrum-based demodulation method. Since the optical path differences (OPDs) of the sensing and reference interferometer need to be extremely close to each other, very accurate fabrication technique is required that is quite difficult to satisfy in many laboratories. Additionally, the spectrum-based demodulation method requires a broadband light source and an optical spectrum analyzer to acquire its spectrum in a broad wavelength range, which is not readily applicable. In this work, we demonstrate an easy-to-fabricate and spectrometer-free Vernier-effect-based optical fiber sensor. A short section of polarization maintaining fiber (PMF) is inserted into one arm of Mach-Zehnder interferometer to form sensing area and an all-fiber variable optical delay line (VODL) is utilized to drive OPD scan. Thanks to the birefringence effect of PMF, two slightly detuned interferometric signals can be generated and overlap at the output of sensor during OPD scanning process. In our design, the instantaneous interference intensity and OPD are obtained in an accurate way via three photodetectors so that the detuned interferograms caused by Vernier effect can be worked out. Our device is independent of the precise and complicated fabrication procedures, the sensing signal can be demodulated without broadband light source and OSA. We envision that the proposed structure will inspire a new concept for constructing simple and cheap Vernier-effect based sensors that are well suited for practical applications.

In this Letter, we demonstrate an easy-to-fabricate time-domain Vernier-effect-based sensor. An all-fiber variable optical delay line (VODL) is utilized to drive an OPD scan of two interferometers simultaneously, and fiber Bragg gratings are used to filter out two slightly detuned time-domain interferometric signals. Then two normalized interferograms with different spatial frequencies can be achieved and utilized to generate an envelope modulation, viz., a Vernier envelope, with enhanced sensitivity in comparison to the native state of the interferometers used. The sensitivity magnification factor of our structure can be regulated simply via altering the resonant wavelength difference of FBGs rather than optimizing the OPDs of the interferometers. The proposed sensor is independent of the precise and complicated fabrication procedures. The Vernier signal can be demodulated without a broadband light source and spectrometer. We argue that the proposed structure may inspire a new concept for constructing simple and cheap Vernier effect-based sensors that are well suited for practical applications.

A novel dual parallel structured fiber-optic Fabry-Perot interferometers (FPIs) for ultrasensitive hydrogen ( ${{\text{H}}_{2}}$ ) sensing using the Vernier effect is proposed and demonstrated. Leveraging the high mechanical strength and chemical stability of hexagonal boron nitride (hBN), a nanoscale hydrogen-sensitive thin film of palladium (Pd)-decorated hBN is prepared and integrated with optical fiber to form a probe-type FPI hydrogen sensor. To further enhance the sensitivity, dual FPIs with similar but different cavity lengths are connected in parallel to exploit the Vernier effect, resulting in a significant amplification factor. The sensitivity of dual parallel structured FPIs based on the Vernier effect is $-$ 14.19 nm/%, which is 9.27 times higher than that of the individual FPI within the ${{\text{H}}_{2}}$ concentration range of 0 $ \sim$ 1%. Moreover, as the dual FPIs possess similar temperature coefficients, the Vernier effect reduces the impact of ambient temperature on the sensor, yielding a temperature coefficient as low as $ \sim$ 46 pm/ ${}^\circ \text{C}$ . Additionally, the sensor offers several advantages, including good repeatability, fast response time ( $ \sim$ 31.3 s), compact structure, easy preparation, and low cost, making it a practical solution for detecting trace amounts of hydrogen.

Vibration measurement is a frequent measurement requirement in a number of areas. Optical vibration sensors have many advantages over electrical counterparts. A common approach is to optically detect the vibration induced mechanical movement of a cantilever. Nevertheless, their practical applications are hindered by the cross-sensitivity of temperature and dynamic instability of the mechanical structure, which lead to unreliable vibration measurements. Here, we demonstrate a temperature insensitive vibration sensor that involves an enclosed suspended cantilever integrated with a readout fiber, providing in-line measurement of vibration. The cantilever is fabricated from a highly birefringent photonic crystal fiber by chemical etching and fused to a single-polarization fiber. Mechanical vibration induced periodic bending of the cantilever can significantly modify the state of polarization of the light that propagates along the photonic crystal fiber. The single-polarization fiber finally converts the state of polarization fluctuation into the change of output optical power. Therefore, the vibration could be demodulated by monitoring the output power of the proposed structure. Due to the special design of the structure, the polarization fluctuation induced by a variation of the ambient temperature can be significantly suppressed. The sensor has a linear response over the frequency range of 5 Hz to 5 kHz with a maximum signal-to-noise ratio of 60 dB and is nearly temperature independent.

The miniaturization of spectrometers have attracted much attention owning to the demand for portable or in situ spectral analysis in a wide variety of fields, but it is a great challenge to push them into practical applications due to high cost, complicated configuration, and sensitivity to external disturbance. We report on a miniature Fourier transform (FT) spectrometer based on fiber-tip Fizeau interferometer. Hand pulling or any other types of force can be used to drive optical path difference (OPD) scan. Interferences are monitored as a function of time by two photodetectors, one is used to detect the whole interferogram while the other to measure single-wavelength interferogram. In this design, the instantaneous interference intensity as well as OPD can be obtained in an accurate way so that the exact spatial interferogram of the incident spectrum can be worked out. Consequently, the incident spectrum can be retrieved by FT method. A resolution of 7.69 cm⁻¹ in the wavelength range of 1400 nm ∼ 1700 nm is achieved. Experimental results show that the performance of our device is comparable to the commercial benchtop spectrometer. Our device is independent of the complicated fabrication procedures, easy of usage, and cost effective. We envision that the proposed design will inspire a new concept for constructing simple and cheap spectrometers that is well suited for practical applications.

We report a self-assembly composite photonic crystal with humidity-controlled structure color, towarding to easy humidity detection by a smartphone. As the key issue, the composite photonic sensor is developed via quasi-coating the silica colloidal photonic crystal (SPC) with hydrophilic metal-organic-framework (MOF). Thus the sensor combines the complementary adsorption properties between SPC and MOF, so that realizes a sensitive humidity sensor with wide dynamic detection range. The performance of the sensor was validated via water vapor testing, that its reflection spectrum shifts ~ 9 nm towards to longer wavelength with the concentration from 500 ppm to 20000 ppm under room temperature. The humidity-controlled structure color of the sensor paves a way for smartphone detection, so that a simple yet powerful smartphone spectrometer is established to analyze the reflection spectrum of the sensor. Thus, a lab-on-a-phone humidity measurement platform is developed. Thanks to the portability, low-cost and user-friendliness, this platform is promising to be a powerful tool for humidity IoT application, from personal health monitoring to environment monitoring.

Precise measurement of temperature is vital in a variety of industries and basic sciences. Numerous techniques have been proposed for temperature measurement, such as fiber Bragg gratings and Fabry-Perot interferometers. Despite several important accomplishments, such optical thermometers are still limited in their performances. Two major limitations are the low operational temperature, and the paradox between a large dynamic range and a high resolution. Here, we propose a fiberoptic thermometer based on fiber-tip micro-spherical Fizeau interferometer made from high temperature resistant material. The sensor is fabricated by inserting a section of single-mode fiber into a spherical microcavity to form a Fizeau interferometer. Thanks to the thermo-optic effect and thermal expansion effect, the variation of ambient temperature modifies the refractive index and the interferometric length of the microcavity, and then a corresponding change in the interferometric spectrum can be observed. The demodulation method is based on the combination measurement of interferometric fringe shift and free spectral range variation, which is capable of providing high resolution and wide dynamic range. Experimental results show that by using a simple wavelength tracking demodulation method, a sensitivity of 22 pm/°C with a high resolution and a dynamic range over 1000 °C can be achieved. Since the sensor has small size, it exhibits fast response and recovery time which makes it can monitor ambient temperature variation in real time. Additionally, the structure is robust and compact which makes the technology an interesting alternative to conventional thermometers for temperature measurement

Resonators are promising platforms for sensing applications. Nevertheless, their practical applications are hindered by the complicated waveguide-resonator coupling configuration, which requires very precise position control of the separation distance between resonator and waveguide to match the coupling condition. Here, we demonstrate a single-sided microsphere resonator based compact fiber-optic sensor for real-time detection of humidity. Instead of coupling light via evanescent waves, the light is injected into the cavity by connecting a single-mode-fiber and a agarose-coated microsphere directly. The light on resonance will be absorbed by microsphere and the rest will be partially reflected back. Since the agarose is hydrophilic, the variation of ambient humidity can change the resonance condition, which shifts the fringe of reflection spectrum. Therefore, the humidity could be demodulated by monitoring the reflection spectrum of the proposed structure at the end of the fiber. The sensor was successfully used to detect ambient humidity over the relative humidity range of 30%- 90% under room temperature (25°C). Thanks to the small size, the sensor exhibits fast dynamic response and recovery time which makes it can monitor human respiration and water evaporation in real time. Due to the compact structure and fast response/recovery time, the sensor is promising for applications in personal health monitoring as well as environmental monitoring.