Dian Fan

Continuous-Wave Frequency-Shifted Interferometry Cavity Ring-Down Gas Sensing With Differential Optical Absorption

This paper proposes a novel frequency-shifted interferometry (FSI) fiber cavity ring-down (CRD) gas sensing with dual-wavelength differential optical absorption that requires no modulation to a continuous-wave source or without fast detection and switching electronics. The fiber cavity is constructed from standard fiber optical components that include a micro-optical gas cell. FSI-CRD experiments are carried out with two wavelengths at 1531.770 and 1532.000 nm. The technique is successfully carried out by measuring acetylene-nitrogen mixtures with acetylene concentrations varying from 0% to 1.0%. A resolution of 7.8125%/dB is obtained. A minimum detectable acetylene concentration of 105.25 ppm was achieved with a 48-mm gas cell. The results show a good linear relationship between acetylene concentration and absorption loss and are in good agreement with existing theories. Dual-wavelength differential optical absorption can enhance measurement precision efficiently and eliminate the influence of various external factors. The relative deviation of measured concentration is less than ±0.29%, measured at 1.0% acetylene concentration over 70 min.

Large-capacity multiplexing of near-identical weak fiber Bragg gratings using frequency-shifted interferometry

We demonstrate interrogation of a large-capacity sensor array with nearly identical weak fiber Bragg gratings (FBGs) based on frequency-shifted interferometry (FSI). In contrast to time-division multiplexing, FSI uses continuous-wave light and therefore requires no pulse modulation or high-speed detection/acquisition. FSI utilizes a frequency shifter in the Sagnac interferometer to encode sensor location information into the relative phase between the clock-wise and counter-clockwise propagating lightwaves. Sixty-five weak FBGs with reflectivities in the range of -31 ~-34 dB and with near identical peak reflection wavelengths around 1555 nm at room temperature were interrogated simultaneously. Temperature sensing was conducted and the average measurement accuracy of the peak wavelengths was ± 3.9 pm, corresponding to a temperature resolution of ± 0.4 °C. Our theoretical analysis taking into account of detector noise, fiber loss, and sensor cross-talk noise shows that there exists an optimal reflectivity that maximizes multiplexing capacity. The multiplexing capacity can reach 3000 with the corresponding sensing range of 30 km, when the peak reflectivity of each grating is -40 dB, the sensor separation 10 m and the source power 14 mW. Experimental results and theoretical analysis reveal that FSI has distinct cost and speed advantages in multiplexing large-scale FBG networks.

Large WDM FBG Sensor Network Based on Frequency-Shifted Interferometry

We combine wavelength-division multiplexing (WDM) and frequency shifted interferometry (FSI) to interrogate a large-scale ultra-weak fiber Bragg grating (FBG) array. Based on Sagnac interference, FSI can location resolve sensors without using pulses or fast detection, therefore considerably lowers the system cost. By combining FSI with WDM, higher spatial resolution can be achieved. We demonstrated simultaneous interrogation of 363 FBG sensors, grouped into 121 identical units of 3 FBGs of different central wavelengths and spaced two meters apart. Based on the performance of the 363 grating system, we show the potential for interrogating 3207 sensors with good signal-to-noise ratio. Stability test and temperature sensing were carried out, and the obtained temperature resolution was ± 0.4 ° C. The results indicate the proposed scheme can greatly enhance the multiplexing capacity and meet the requirements of large-scale optical fiber networks.

Doppler Effect-Based Optical Fiber Vibration Sensor Using Frequency-Shifted Interferometry Demodulation

A novel optic-fiber vibration sensing system based on frequency-shifted interferometry (FSI) is proposed and demonstrated in this paper. The sensor is based on the phenomenon that the light frequency transmitted through a segment of bent fiber can be Doppler shifted by vibration. The Doppler frequency shift is measured using FSI demodulation. Compared to traditional fiber Bragg grating vibration sensor, the proposed sensor is easy to fabricate, of low cost and without directional dependency. The application of FSI demodulation allows for custom tailoring of measurement sensitivity, and has superior signal stability due to its same-path interference nature. The measured Doppler frequency shift using the FSI system agrees well with the calculated one, derived from strain measurements. The results show that the system has a high sensitivity and wide frequency range, limited only by the excitation apparatus used in the experiment.

Fiber optic magnetic field sensor based on magnetic fluid and etched Hi-Bi fiber loop mirror

A novel magnetic field sensor consisting of magnetic fluid (MF) and etched highly birefringent fiber loop mirror (Hi-Bi FLM) is proposed in the paper. The sensor is based on the etched FLM interferometer by using the property of the controllable refractive index of MF under external magnetic field. The refractive index of MF is changed by a tunable magnetic field and the resonant dip wavelength produced by the FLM shifts correspondingly. The magnetic field intensity can be measured by detecting wavelength shift. High sensitivity of 11.31pm/Oe and a resolution of 0.1Oe are obtained for the proposed magnetic field sensor.

Multiple-Octave-Spanning Vibration Sensing Based on Simultaneous Vector Demodulation of 499 Fizeau Interference Signals from Identical Ultra-Weak Fiber Bragg Gratings Over 2.5 km

Multi-point vibration sensing at the low frequency range of 0.5–100 Hz is of vital importance for applications such as seismic monitoring and underwater acoustic imaging. Location-resolved multi-point sensing using a single fiber and a single demodulation system can greatly reduce system deployment and maintenance costs. We propose and demonstrate the demodulation of a fiber-optic system consisting of 500 identical ultra-weak Fiber Bragg gratings (uwFBGs), capable of measuring the amplitude, frequency and phase of acoustic signals from 499 sensing fibers covering a total range of 2.5 km. For demonstration purposes, we arbitrarily chose six consecutive sensors and studied their performance in detail. Using a passive demodulation method, we interrogated the six sensors simultaneously, and achieved a high signal-to-noise ratio of 22.1 dB, excellent linearity, phase sensitivity of around 0.024 rad/Pa, and a dynamic range of about 38 dB. We demonstrated a frequency response flatness of <1.2 dB in the range of 0.5–100 Hz. Compared to the prior state-of-the-art demonstration using a similar method, we have increased the sensing range from 1 km to 2.5 km, and increased the frequency range from 0.4 octaves to 7.6 octaves, in addition to achieving sensing in the very challenging low-frequency range of 0.5–100 Hz.

Hollow-glass-microsphere-structured Fabry-Perot interferometric sensor for highly sensitive temperature measurement

We propose and demonstrate a miniature Fabry-Perot (F-P) interferometric sensor based on a hollow glass microsphere (HGM) for highly sensitive temperature measurement. The sensor head is fabricated by sticking a HGM on the end face of a single-mode fiber, and it consists of a short air F-P cavity between the front and the rear surfaces of the HGM. A sensor with 135.7280-μm cavity length was tested for temperature measurement from −5 °C to 50 °C. The obtained sensitivity reached up to 24.5 pm/°C and the variation rate of the HGM-F-Ps cavity length was2.1 nm/°C. The advantages of compact size, easy fabrication and low cost make the sensor suitable for highly sensitive temperature sensing.

High spatial resolution multiplexing of fiber Bragg gratings using single-arm frequency-shifted interferometry

We demonstrate a high spatial resolution multiplexing scheme for fiber Bragg grating (FBG) sensors based on single-arm frequency-shifted interferometry (SA-FSI). The SA-FSI system uses an incoherent broadband source, a slow detector, and an electro-optic modulator (EOM). By sweeping the frequency of EOM and taking the fast Fourier transform (FFT) of the interference signal, we resolved the locations of FBGs distributed both in parallel and in series along fiber links despite their reflection spectral overlap. Eighteen weak FBGs (∼5% reflectivity) separated by ∼0.1 m were clearly resolved experimentally, sweeping EOM modulation frequency in the range of 2–11 GHz.

Novel glucose fiber sensor combining ThFBG with GOD

We propose a novel glucose fiber optic sensor combining a thinned cladding fiber Bragg grating (ThFBG) with glucose oxidase (GOD). By immobilizing GOD on the surface of a ThFBG, the fabricated sensor can obtain a high specificity to glucose. Because of the evanescent field, the sensor is very sensitive to the ambient refractive index change arising from the catalytic reaction between glucose and GOD. A four-level fiber model was simulated and verified the precision of the sensing principle. Two methods, glutaraldehyde crosslinking method (GCM) and 3-aminopropyl triethoxysilane covalent coupling method (ATCCM), were experimentally utilized to immobilize GOD. And sensor fabricated with the method ATCCM shows a measurement range of 0-0.82 mg/mL which is better than the sensor fabricated with the method GCM with measurement range of 0-0.67 mg/mL under the same condition. By using ATCCM to immobilize GOD with different concentrations, three sensors were fabricated and used for glucose measurement by monitoring the Bragg wavelength (λb) shifts, the results indicate a good linear relationship between wavelength shift and glucose concentration within a specific range, and the measurement range increases as GOD concentration increases. The highest sensitivity of sensor reaches up to 0.0549 nm/(mg.mL-1). The proposed sensor has distinct advantages in sensing structure, cost and specificity.

Novel gas sensor combined fiber cavity ring-down and frequency-shifted interferometry

A novel gas senor combined fiber cavity ring-down (CRD) with frequency-shifted interferometry (FSI) is proposed and demonstrated. Compared to the conventional fiber CRD techniques, the CRD decay transient was originated from continuous-wave light as a function of the distance transmitted in the cavity, therefore this method needs neither pulsed light nor fast detection. As a proof-of-concept experiment, we employed a micro-optical gas cell as the sensing element and measured different concentrations of acetylene samples. The experimental results show that the FSI-CRD system has a resolution of 0.15798%/dB, and it provides a simple and cost-effective scheme for gas sensing.