Xiujuan Yu

Frequency-Division Multiplexing Sensing System Based on Multilongitudinal Mode Fiber Lasers and Beat Frequency Demodulation

A five-channel frequency-division multiplexing (FDM) fiber laser sensor array based on multilongitudinal mode fiber lasers (MLMFLs) and beat frequency demodulation was proposed. Detailed theoretical analysis of the FDM principle was also described. The potential application of the MLMFL-based FDM for strain measurement was further experimentally studied. The strain sensitivity of -0.92 kHz/με at 1183.52 MHz is consistent with the theoretical calculation. The proposed FDM sensing system only needs one detector and one general frequency analyzer for monitoring all sensors. This all-electrical demodulation improves the stability of the fiber sensor and greatly decreases the cost of sensors. It offers a competitive technology for large-scale monitoring field.

Characteristics of in-fiber Mach-Zehnder interferometer formed by lateral offset splicing

We present an in-fiber Mach-Zehnder type interferometer (MZI) in single mode fiber based on lateral offset splicing technique. The MZI relies on the interference between the fundamental core mode and a co-propagating cladding mode. To build an in-fiber MZI, we need one element or device which excites two co-propagating modes and another one to recombine them. In our case, the excitation and recombination of modes is realized by two cascaded lateral offset splicing joints. The effects of the lateral offset amount, the interferometer cavity length, and the fusion splicing parameters on the property and performance of the in-fiber MZI were investigated experimentally. By choosing an appropriate lateral offset amount, the MZIs with good interference fringe contrast and low insertion loss can be fabricated. Several in-fiber MZIs with different interferometer lengths were built and it is found that the interference peak wavelength spacing is inversely proportional to the interferometer length L. The potential applications of the proposed in-fiber MZI were further investigated as strain sensors.

Hollow-core photonic bandgap fiber Mach-Zehnder interferometer based on a long period grating and an offset-splice joint

We report a compact in-fiber Mach-Zehnder interferometer (MZI) made along a hollow-core photonic bandgap fiber (HC-PBF). The MZI uses a long period grating (LPG) and an offset-splice joint (OSJ) which act as beam splitter and combiner respectively. The LPG was produced by use of a high frequency pulsed CO2 laser, and the OSJ was made in serial with the LPG by using a commercial fusion splicer. The interference is between the fundamental core mode (FCM) and a high-order core mode (HOCM). The proposed interferometer was experimentally tested for temperature and strain measurements, and the sensitivities of the interference fringe dip wavelength to temperature and strain are 107.5 pm/ (°C-m) and -1.24 pm/με, respectively.

Optical fiber relative humidity sensor based on a hydrogel coated long period grating

A hydrogel coated long period grating (LPG) used as relative humidity sensor is investigated theoretically and experimentally. It showed a high degree of sensitivity when relative humidity was above 50%RH. For humidity levels from 50%RH to 72%RH, a wavelength decrease of 15.4 nm is seen, with a sensitivity of 0.66 nm /%RH. For humidity levels from 72%RH to 98%RH, a wavelength increase of 9.5nm is observed, with a sensitivity of 0.36 nm /%RH. For humidity levels greater than 65%RH, the transmission loss at resonance increases rapidly, showing a high degree of sensitivity of 0.53 dB /%RH and offering a high resolution of 0.02%RH. The proposed relative humidity sensor offered good repeatability and low hysteresis errors, with a standard deviation of 0.005dB/%RH and 0.26% hysteresis error while RH levels greater than 65%RH during three cycles of increasing and decreasing humidity. The stability test was also carried out and a standard deviation of 0.14%RH was obtained. In addition, the durability test showed that the humidity sensor can work at least ten months.

Characteristics analysis of cascaded long-period gratings with nm-thick film coating

Cascaded long period grating with film coating is sensitive to the thickness and refractive index of the film material and the surrounding refractive index, so it can be used in the applications such as gas sensing, liquid concentration sensing and bio-sensing. In this paper, the characteristic of cascaded long period grating coating with higher refractive index nanostructured film overlay is investigated theoretically by using coupled-mode theory and transfer matrix method. The responses of interference fringe peak wavelength to the thickness and refractive index of the film and the surrounding refractive index are numerical calculated. The theoretical simulation results show that the minima wavelength of the interference fringes within the long period gratings attenuation bands are sensitive to the thickness and refractive index of the film coating and the refractive index of the surrounding environment. By choosing these parameters optimally, the sensitivity of the peak wavelength shift of the interference fringes can be enhanced. The theoretical analysis can provide guidance to the application of the nanostructured film coated cascaded long period grating sensor.

Strain measurement of steel structure using a novel FBG sensor packaged by titanium alloy slice

In recent years, Fiber Bragg grating (FBG) sensors become a focus in civil structure health monitoring because they have many advantages and show superior potential for health monitoring to ensure their structural integrity, durability and reliability. In this paper, fiber Bragg grating (FBG) sensors are used to monitoring the strain of steel structure. A designed novel titanium alloy slice is introduced to package the used FBG. A shallow rectangular groove is notched on the titanium alloy slice. Coated with a thin layer of epoxy, the FBG is fixed and protected on the groove. In order to eliminate to the temperature vibration effect on the wavelength shift of FBG sensors, a FBG temperature sensor is used for compensation. The traditional resistance strain gauges are also used to measure the strain for the comparison with the FBG sensors to validate the effectiveness. The strain of the flat steel at the elastic and yield stage of the steel are measured by monitoring the shift of center wavelength of FBG sensors and the change of the resistance strain gauges. The experimental results show that FBG sensors packaged by titanium alloy slice possess good stain sensing property and the strain sensitivity coefficients are 1.25pm/με. The shifts of center wavelengths of FBG are in good linear relationship with the tensile force and the strain measured by the resistance strain gauge. The linear correlation coefficients both reach 0.999.

A 16-channel multi-longitudinal mode fiber laser sensor array system based on wavelength/frequency division multiplexing

The 16-channel multi-longitudinal mode fiber laser sensor array is investigated experimentally by the wavelength/frequency division multiplexing technique. In the proposed sensing system, a 4×4 sensor array is established by a few different coupling-ratio couplers, and four different fiber Bragg gratings (FBG) with different center wavelengths are used as four different sensing units. In each sensor unit, four parallel fiber laser sensors have the same operating wavelengths FBGs, but their effective laser cavity lengths slightly different from each other. Every cavity is formed by a fiber Bragg grating (FBG) serving as one reflection mirror, a piece of erbium-doped fiber (EDF) acting as the gain medium, and a Faraday rotator mirror (FRM) serving as other reflection mirror. When the pump power is higher than threshold value, the 16-channel fiber laser sensor array is stimulated stably. The frequency of the beat signal of the fiber laser sensor with different cavity lengths is used to realize frequency division multiplexing, and the wavelength division multiplexing is realized according to the operation wavelength of the fiber laser. The beat frequency signals are generated on a photodetector(PD), and monitored by a frequency spectrum analyzer(FSA). By tracking the shift of the beat frequency, all of the 16 laser sensors can be demodulated and real-time discriminated. The result of the experiment shows that different channels can be demodulated independently. The applications of the sensor array for strain and temperature measurements are also investigated. The strain or temperature information can be extracted from the change of the beat frequency signals according to the wavelength-frequency division multiplexing and the beat signal demodulation. The proposed hybrid multiplexing system can greatly reduce the weight, volume, and cost of the fiber laser sensors system while increasing the amount of the sensors multiplexable, which making it very competitive in some applications fields requiring large scale arrays such as space vehicles, marine infrastructure systems and constructional engineering.

Pd-Ag film coated cascaded long period gratings for hydrogen gas sensing

A Mach-Zehnder-like interferometer based on cascaded long period gratings with palladium silver (Pd-Ag) film coating is developed for monitoring the hydrogen concentration. The visibility of fringes is measured using Fourier analysis. The basic theory was given and preliminary experiment had been proved that this sensor can used to monitor the hydrogen concentration. The sensor showed a good response.

Research on the characteristics of hydrogel coated long period gratings

In this paper, the characteristics of the LPG with thin film coating are analyzed theoretically by using the optical couple-mode theory and cylindrical four-layer waveguide model. The influences of the refractive index and thickness of film overlay on the sensitivity of LPG are analyzed in detail. By choosing properly the thickness and refractive index of the thin-film overlay, the sensitivity of the LPG can be enhanced significantly. In order to verify the theoretical analysis and enhance the sensitivity of LPG to humidity, hydrogel is chosen as coating material. The characteristics on the hydrogel coated long period grating are investigated experimentally. The transmission spectral response of hydrogel coated LPG to relative humidity is measured. It shows that the resonant wavelength and the amplitude of attenuation bands are high sensitive to relative humidity when RH is above 50%RH.

Kerr effect in a passive optical ring-resonator gyroscope using a hollow-core photonic-band fiber

Optical Kerr effect in the passive ring-resonator optical fiber gyroscope (PORG) made from hollow-core photonic band-gap (PBG) fiber is analyzed theoretically using a simple optical field overlap method, comparing it with the passive ring-resonator optical fiber gyroscope made from conventional single mode fiber (SMF). The theoretical evidence show that optical Kerr effect induced rotation rate shift in PORG made from hollow-core PBG fiber should be reduced by 1~2 orders at different ring resonator length when the optical source spectral width is fixed.