In-line Interferometer Research Articles

Quasi-Distributed 3-cm Vibration and Strain Monitoring Using OFDR and In-Line Interferometers

Quasi-Distributed 3-cm Vibration and Strain Monitoring Using OFDR and In-Line Interferometers

Temperature-insensitive fiber-optic refractive index sensor based on cascaded in-line interferometer and microwave photonics interrogation system

Temperature-insensitive fiber-optic refractive index sensor based on cascaded in-line interferometer and microwave photonics interrogation system

Vibration measurement technique by using OFDR with in-line interferometers

Due to their compact size, electromagnetic interference immunity, and compatibility with embedding in composite materials, optical fiber sensors have emerged as a versatile and integral solution for structural health monitoring (SHM) in various structures. Among the key contributors to the evolution of this field is the significant progress in optical frequency domain reflectometry (OFDR), a technology that offers unparalleled high resolution and sensitivity in distributed strain sensing, thereby establishing itself as an indispensable tool in the realm of SHM. Through the implementation of OFDR, we devised a distributed fiber optic sensor tailored for the precise measurement of vibration or strain within structures. In previous works, vibration monitoring has been achieved using an OFDR with an array of 20 in-line interferometers, serving as weak reflectors that not only facilitated vibration monitoring but also furnished valuable strain information. In this study takes a step further by introducing an innovative technique wherein in-line interferometers are strategically arranged in a non-uniform fashion along the optical fiber. To validate the practical utility of this technology, extensive simulations and experiments were conducted, utilizing a cantilever beam as a representative structure. The results obtained from these endeavors showcase the tangible potential and reliability of optical fiber sensors in the practical implementation of SHM across various applications. By enhancing the precision and scope of structural monitoring, optical fiber sensors equipped with non-uniform interferometer arrangements pave the way for more effective and comprehensive structural health assessment in diverse settings.

High-precision magnetic field sensor via optoelectronic oscillator having taper-based in-line modal interferometer

High-precision magnetic field sensor via optoelectronic oscillator having taper-based in-line modal interferometer

Fs-Laser Fabricated Miniature Fabry-Perot Interferometer in a No-Core Fiber for High-Temperature Applications.

This paper reports a fiber in-line Fabry-Perot interferometer (FPI) fabricated in a no-core fiber using the direct femtosecond laser writing technique for high-temperature sensing applications. Two in-line reflectors are directly inscribed in a no-core fiber to construct a low-finesse FPI. Fringe visibility greater than 10 dB is obtained from the reflection spectra of the fabricated no-core fiber FPIs. Temperature responses of a prototype no-core fiber FPI are characterized up to 1000 °C. The proposed configuration is compact and easy to fabricate, making it attractive for sensing applications in high-temperature harsh environments.

Wire grid polarizer based in-line laser interferometer for macroscopic and microscopic phase estimation of transmissive and reflective phase samples

This work describes a full-field and near-common-path in-line laser interferometer and interferometric microscope utilizing a wire-grid polarizer (WGP) placed normally on the laser beam illuminating the sample. The WGP serves the dual purpose of a beam splitter and a polarization separator where the reference and sample beams reflected and transmitted from it respectively are orthogonally polarized so that, unlike other conventional interferometers, polarization phase shifting is inherent in its architecture. This arrangement presents experimental results showing quantitative phase analysis of transparent and reflecting phase samples.

Signal identification based on modified filter bank feature and generalized regression neural network for optical fiber perimeter sensing

• Signal recognition method based on the generalized regression neural network (GRNN). • Feature extraction algorithm based on the modified filter bank (FB) feature. • Endpoint detection algorithm based on the short-time energy. • Little parameter adjustment and human influence in the training process. • High identification rate, short recognition time and broad application prospects. A method based on the modified filter bank (FB) feature and a generalized regression neural network (GRNN) is proposed to raise the accuracy and real-time performance of signal recognition in optical fiber perimeter sensing systems. The FB feature is modified by adding root mean square information of the signal power under the Mel filter bank. Four kinds of sensing signals under three types of weather conditions are obtained experimentally by a fenced perimeter sensing system based on an in-line Sagnac interferometer. The endpoint detection algorithm based on short-term energy is performed on the sensing signals to obtain the effective signal segments. Then the modified FB features of signal segments are extracted and randomly divided into training and testing samples. The optimal GRNN classifier model is generated by performing a 10-fold cross-validation on the training samples and used to recognize the testing samples. The average accuracy can reach 98.22 % and average recognition time is only about 0.07 s. This method is expected to meet the requirements for real-time and accuracy in practical application of optical fiber perimeter sensing signal recognition.

Photothermal multi-species detection in a hollow-core fiber with frequency-division multiplexing

A photothermal gas sensor has been developed for simultaneous multi-species detection in a hollow-core fiber based on the frequency-division multiplexing technique. By passing multiple pump lasers and a probe laser through a gas-filled hollow-core anti-resonant fiber (HC-ARF), the absorption-generated modulation of the refractive index at various frequencies is sensitively detected by an in-line interferometer. Three distributed-feedback diode lasers at C-band, L-band and U-band are employed as pump lasers for C2H2, CO2 and CH4 detection. Assisted by optical power amplifiers, the sensor achieves noise equivalent concentrations of 2.5 ppb, 21 ppm and 200 ppb for C2H2, CO2 and CH4, respectively. The sensor demonstrates excellent linear response to gas concentrations with R-square values between 0.996 and 0.999. Measurement of the three gas species with time-varying concentrations, as well as the aforementioned experiments, further validates the simultaneous multi-species detection with high sensitivity, compact size, and extremely low gas consumption.

Observation on the Droplet Ranging from 2 to 16 μm in Cloud Droplet Size Distribution Based on Digital Holography

Cloud droplets size distribution (DSD) is one of the significant characteristics for liquid clouds. It plays an important role for the aerosol–droplet–cloud mechanism and variation in cloud microphysics. However, the minuscule sampling space is insufficient for the observation of whole DSD when using high-magnification optical systems. In this paper, we propose an observation method for cloud droplets ranging from 2 to 16 μm, by which the balance relationship between sampling space and optical magnification is realized. The method combines an in-line digital holographic interferometer (DHI) with the optical magnification of 5.89× and spatial stitching technique. The minimum size in DSD is extended to 2 μm, which improves the integrity of size distribution. Simultaneously, the stability of DSD is enhanced by increasing the tenfold sampling volume of cloud droplets. The comparative experiment between the in-line DHI and fog monitor demonstrates that the DSD obtained by this method is reliable, which can be used for the analysis of microphysical parameters. In the Beijing Aerosol and Cloud Interaction Chamber (BACIC), the observation results show that the size of cloud droplets follows the Gamma distribution, which is consistent with the theoretical DSD. The results of cloud microphysical parameters indicate that each pair of parameters has a positive correlation, and then the validity of observation method is confirmed. Additionally, the high-concentration aerosol condition significantly mitigates the effect of random turbulence and enhances the robustness of the microphysical parameter data.

Common-path off-axis single-pixel holographic imaging.

Common-path off-axis single-pixel holographic imaging (COSHI) is proposed to obtain complex amplitude information using an in-line interferometer and a single-pixel (point-like) detector. COSHI is more robust to disturbances such as vibration than the conventional single-pixel digital holography technique because of its common-path configuration. In addition, the number of measurements can be reduced due to COSHI's reconstruction process based on the Fourier fringe analysis. In COSHI, an off-axis digital hologram can be obtained using the structured patterns composed of Hadamard basis patterns and stationary tilted phase distribution. Interestingly, COSHI's space bandwidth is larger than of the conventional off-axis digital holography because COSHI does not reconstruct the self-correlation term of an object. The proposed method is theoretically confirmed and numerical and experimental results show its feasibility.

Dual-comb photothermal spectroscopy

Dual-comb spectroscopy (DCS) has revolutionized optical spectroscopy by providing broadband spectral measurements with unprecedented resolution and fast response. Photothermal spectroscopy (PTS) with a pump-probe configuration offers a highly sensitive gas sensing method, which is normally performed using a single-wavelength pump laser. The merging of PTS with DCS may enable a spectroscopic method by taking advantage of both technologies, which has never been studied yet. Here, we report dual-comb photothermal spectroscopy (DC-PTS) by passing dual combs and a probe laser through a gas-filled anti-resonant hollow-core fiber, where the generated multi-heterodyne modulation of the refractive index is sensitively detected by an in-line interferometer. As an example, we have measured photothermal spectra of acetylene over 1 THz, showing a good agreement with the spectral database. Our proposed DC-PTS provides opportunities for broadband gas sensing with super-fine resolution and high sensitivity, as well as with a small sample volume and compact configuration.

Signal recognition method based on Mel frequency cepstral coefficients and fast dynamic time warping for optical fiber perimeter defense systems.

To improve the accuracy of signal recognition in optical fiber perimeter defense systems, a method based on Mel frequency cepstral coefficients (MFCCs) and a fast dynamic time warping (FastDTW) algorithm is proposed. Four kinds of sensing signals are acquired by employing an in-line Sagnac interferometer system. MFCC features of the signals are extracted. The FastDTW algorithm is utilized to calculate distances of the optimal warping paths between a test sample and all reference templates. According to the nearest neighbor criterion, signal recognition results are obtained. The average accuracy is over 96%. This proposed identification method is highly efficient and easy to implement.

Super-resolved second harmonic generation imaging by coherent image scanning microscopy

We extend image scanning microscopy to second harmonic generation (SHG) by extracting the complex field amplitude of the second-harmonic beam. While the theory behind coherent image scanning microscopy (ISM) is known, an experimental demonstration was not yet established. The main reason is that the naive intensity-reassignment procedure cannot be used for coherent scattering as the point spread function is now defined for the field amplitude rather than for the intensity. We use an inline interferometer to demonstrate super-resolved phase-sensitive SHG microscopy by applying the ISM reassignment machinery on the resolved field. This scheme can be easily extended to third harmonic generation and stimulated Raman microscopy schemes.

Temperature compensated fiber-optic liquid level sensor by micro-sphere based in-line Michelson interferometer

In this paper, a novel micro-sphere based in-line Michelson interferometer is proposed and completed experimentally through arc-discharge technique. The light field distributions are theoretically investigated to gain optimal core offset and diameter of micro-sphere. Multiple fabricated samples are compared and the comprehensive tests are then performed in terms of liquid level and temperature. The experimental results show that obvious blue shift of fringes is demonstrated with the rise of liquid level and the average sensitivity reaches 62.6 pm/mm with high linearity in the range of 0–40 mm. Moreover, owing to highly-consistent temperature response (∼99.5%), the crosstalk caused by ambient temperature is eliminated by differential method and the measurement error is compressed within 1.33%. Due to the merits of compactness, low cost, ease of fabrication and operation, our sensor is very potential in the liquid-level related engineering applications.

Compact strain fiber sensor based on Fabry-Perot microstructural air cavity

A Fabry-Perot air chamber was prepared at the splicing point of two single-mode fibers by the heating-expansion of liquid glycerin. The morphology of the Fabry-Perot air chamber was optimized by the splicing parameters (drawing process, discharging location, time and intensity) and the fibers’ end-face (plane or arc). The in-line Fabry-Perot interferometer was employed to determine the tensile strain from 0 to 1.2 N. A sensitivity of 3.628 nm/N and the resolution of ∼0.005 N were experimentally demonstrated. The reported Fabry-Perot strain sensor is easy to fabricate, low cost, and flexible to connect with other fibers.

Tapered multicore fiber interferometer for ultra-sensitive temperature sensing with thermo-optical materials.

An in-line interferometer based on tapered multicore embedded into a flexible thermo-optical material is proposed and investigated, theoretically and experimentally. The device consists of a tapered multicore fiber spliced between two single-mode fibers covered with PDMS, with high thermo-optic coefficient. The temperature sensitivity improvement obtained from PDMS applied on a tapered multicore fiber (TMCF) interferometer has been fundamentally and experimentally verified. The experimental results show the temperature sensitivity can be improved by reducing the tapered waist diameter of TMCF. The sensor exhibits the high sensitivity of 5-25 nm/°C within the decreasing temperature range from 50 °C down to 10 °C. A sequence of simulations and corresponding experiments are performed to clarify the evolution of the interference fading and consequently build the criteria for sensor design and reachable lower limit of temperature sensing. The proposed sensor can be employed as photonic thermometer with ultra-high sensitivity for biological and deep-sea applications, particularly based on the claimed quantitative criteria.

Temperature Sensor Based on Er-Doped Cascaded-Peanut Taper Structure In-Line Interferometer in Fiber Ring Laser

A temperature sensor based on erbium-doped fiber cascaded peanut taper (EDFCPT) structure in fiber ring laser (FRL) cavity is proposed and studied. The cascaded peanut structure was fabricated by erbium-doped fiber. The cladding mode and core mode interference and the strong thermo-optical effect of erbium-doped fiber were used to realize the high sensitivity measurement of temperature. The mode interference and thermal sensitivity effects of EDFCPT in a broadband super-continuous light source and a fiber ring laser cavity were studied and compared experimentally. Besides, Erbium-doped fiber cascaded peanut (EDFCP) structure was designed to compare performance with EDFCPT. The experimental results show that EDFCPT has a higher signal to noise ratio (SNR) (~50dB) and a narrower 3-dB bandwidth (~0.12nm) than the broadband light source. Among them, the temperature sensitivity of EDFCPT is 571pm/°C which is two times higher than the EDFCP (~249nm/°C) structure at range from 5°C-55°C. The proposed temperature sensor has the advantages of high sensitivity, good repeatability, simple fabrication, compact structure, etc. It has a good application prospect in the field of aerospace and life health monitoring.

Highly Birefringent Microfiber Hybrid Interferometer Sensor

In this work, an optical microfiber-based hybrid interferometer is presented. A combination of two types of in-line interferometers is achieved by inserting a section of nonadiabatic tapered polarization maintaining microfiber into a Lyot filter. The hybrid interference spectrum shown as large spectrum envelope and fine fringes, corresponding to the polarized and modal interferences is obtained. We experimentally investigate the response characteristics of the two-interference system using both spectral interrogation and spatial frequency domain analysis. The refractive index sensitivities are 1150.0nm/RIU and 1779.8nm/RIU, and the temperature sensitivities are 190pm/°C and -12.7 pm/°C for two interferences, respectively. The proposed hybrid interferometer has the advantages of simple device and compact structure, and has the potential of simultaneous measurement of multiple parameters.

Lab on optical fiber: surface nano-functionalization for real-time monitoring of VOC adsorption/desorption in metal-organic frameworks

Abstract Metal-organic framework (MOF) nanomaterials are emerging porous coordinative polymers with large surface area and high porosity. Their application scenarios highly depend on adsorption/desorption dynamics of guest molecules in the framework. For representative ZIF-8 with framework flexibility, the study of molecule transportation in the pore channels of ZIF-8 will address the ambiguity of unclear application scenarios. In this study, the integration of lab-on-fiber technology and nanotechnology are demonstrated for real-time monitoring of adsorption/desorption dynamics of heterocyclic volatile compounds (VOCs) with kinetic diameters larger than the window aperture of ZIF-8. The in-line fiber interferometer with cascaded long-period gratings is used to monitor the real-time refractive index change of VOC adsorption/desorption. The structure-effect relationship between guest VOCs and framework flexibility is analyzed. It shows that the adsorption dynamics is highly related to the molecular geometry and kinetic diameter. The framework flexibility results in the trapping of guest VOCs toluene, pyridine, and tetrahydrofuran in the frameworks. The methanol adsorption/desorption is an effective strategy for the fast desorption of trapped residual VOCs in the framework. Finally, we conceptually demonstrated the real-time monitoring of trace toluene enrichment using ZIF-8 for indoor air purification. This study paves the way for the in-depth understanding of framework flexibility for MOF’s application.

Dual-core fiber based in-line Michelson interferometer for humidity sensing

A high-sensitive optical fiber humidity sensor based on in-line Michelson interferometer (MI) is proposed and demonstrated. The MI is formed by an asymmetric dual-core fiber (DCF) spliced with a short section of multimode fiber (MMF). The MMF serves as fiber coupler, while the DCF creates a strong intermodal interference to strengthen the interaction between light and ambient moisture. The sensor presents a humidity sensitivity of −0.199 dB/%RH in a wide relative humidity (RH) range of 20–90%RH without any humidity-sensitive material. An experiment based on human breathing shows that the response time and recovery time are 562 ms and 308 ms, respectively. In addition, the performance of stability and temperature response are tested, the standard measurement error is 0.21 %RH and the temperature sensitivity is 0.0398 nm/°C. Experimental results indicate the sensor has a good practical prospect.