Simultaneous measurement of bending and temperature based on a single sampled chirped fiber Bragg grating embedded on a flexible cantilever beam

Intensity-interrogated hot-wire anemometer based on chirp effect of a fiber Bragg grating

An intensity-interrogated optical fiber hot-wire anemometer based on the chirp effect of fiber Bragg grating (FBG) is presented. The FBG is coated with a silver film and heated optically by a 1480 nm laser beam, which is coupled into the fiber cladding by a long-period grating (LPG) and absorbed by the silver film to convert to thermal heat. Due to the gradual decrease of laser power along the length of the FBG, a temperature gradient is formed that induces a chirp effect to the FBG. Bandwidth of the FBG's reflection spectrum is therefore broadened that increases its reflected light power. The chirp rate of the FBG reduces with airflow velocity since the temperature gradient is weakened under the cooling effect of the airflow, resulting in a certain relationship between the reflected power of the FBG and airflow velocity. In the experiment, by detecting the reflected power of the FBG, airflow velocity measurement is achieved successfully with a high sensitivity up to -28.60 µW/(m·s^-1) at airflow velocity of 0.1 m/s and a dynamic response time of under one second. The measurement range is up to 0 to 11 m/s. The intensity interrogation scheme of the FBG hot-wire anemometer reduces its cost greatly and makes it a promising solution for airflow velocity measurement in practical applications.

Femtosecond laser point-by-point inscription of helical-sampled fiber Bragg gratings

In this Letter, a helical-sampled fiber Bragg gratings (HSFBGs) fabrication using a femtosecond laser point-by-point (PBP) technique is proposed. The unique helical structure generates sampled gratings owing to its periodicity. A simple, single-step method for inscription of the sampled gratings is described. The effects of geometrical parameters, including length of grating, helical diameter, helical pitch, and off-axis distance on the resonances are studied, and a series of comb-like spectra are obtained.

Feasibility Study on Temperature Distribution Measurement Method of Thrust Sliding Bearing Bush Based on FBG Quasi-Distributed Sensing

According to the characteristics of the temperature distribution of the thrust sliding bearing bush, the principle and method of quasi-distributed fiber Bragg grating (FBG) sensing is used to measure it. The key problems such as calibration, arrangement and lying of optical FBG sensors are studied by using the simulated thrust sliding bearing bush, which was customized in the laboratory. Combined with the thrust sliding bearing bush, the measurement experiments were carried out, which were divided into two groups: Steady-state experiments and transient experiment. The steady-state experiments obtain the temperature data measured by the FBG temperature sensors at each setting temperature, and the transient experiment obtains the relationship between the measured temperature by each temperature sensor and time in the heating and cooling process. The experimental results showed that the FBG temperature sensors had good accuracy, stability and consistency when measuring the temperature distribution of bearing bush.

All fiber M-Z interferometer for high temperature sensing based on a hetero-structured cladding solid-core photonic bandgap fiber

We proposed and experimentally demonstrated a high temperature fiber sensor using a hetero-structured cladding solid-core photonic bandgap fiber (HCSC-PBGF) for the first time to our knowledge. A hetero-structured cladding solid-core photonic bandgap fiber is designed and fabricated that supports vibrant core mode and cladding mode transmission. Then, an all fiber M-Z interference sensor is constructed by splicing single mode fiber at both ends of HCSC-PBGF without any other micromachining. The transmission characteristics of HCSC-PBGF are analyzed with a full-vector beam propagation method and a full-vector finite element method, and the simulation results are consistent with experiment results. The sensitivity of this fiber sensor is as high as 0.09 nm/°C when operating from room temperature to 1000 °C, and the fringe contrast keeps stable and clear. It is obvious that this all fiber sensor will have great application prospects in fiber sensing with the advantages of a compact structure, high sensitivity, and cost-effectiveness.

Orientation-Dependent Displacement Sensor Using an Inner Cladding Fiber Bragg Grating

An orientation-dependent displacement sensor based on grating inscription over a fiber core and inner cladding has been demonstrated. The device comprises a short piece of multi-cladding fiber sandwiched between two standard single-mode fibers (SMFs). The grating structure is fabricated by a femtosecond laser side-illumination technique. Two well-defined resonances are achieved by the downstream both core and cladding fiber Bragg gratings (FBGs). The cladding resonance presents fiber bending dependence, together with a strong orientation dependence because of asymmetrical distribution of the “cladding” FBG along the fiber cross-section.

Asymmetrical in-fiber Mach-Zehnder interferometer for curvature measurement

We demonstrated a compact and highly-sensitive curvature sensor based on a Mach-Zehnder interferometer created in a photonic crystal fiber. Such a Mach-Zehnder interferometer consisted of a peanut-like section and an abrupt taper achieved by use of an optimized electrical arc discharge technique, where only one dominating cladding mode was excited and interfered with the fundamental mode. The unique structure exhibited a high curvature sensitivity of 50.5 nm/m^-1 within a range from 0 to 2.8 m^-1, which made it suitable for high-sensitivity curvature sensing in harsh environments. Moreover, it also exhibited a temperature sensitivity of 11.7 pm/°C.

In-fiber directional coupler for high-sensitivity curvature measurement

A curvature fiber optic sensor using a two-core fiber (TCF) is proposed and demonstrated. The TCF is designed to operate as a directional coupler with one core located exactly at the center of the fiber and the other off-axis, but close to the center of the fiber. This design allows straightforward splicing of the TCF to single mode fibers (SMF), and alignment of the off-axis core is not strictly required for optimum operation. The sensor is fabricated by simply splicing a 5 cm long section of TCF between two SMF sections, which provides a sinusoidal spectral response. When the fiber is bent, the coupling parameters are modified due to stress-optic and effective length effects, effectively blue-shifting the sinusoidal spectral response of the sensor and allowing for the measurement of curvature. The sensor exhibits linear response and a sensitivity of -137.87 nm/m(-1) for curvature ranging from 0 to 0.27 m(-1), making it suitable to measure small curvatures with high sensitivity.

Direction-independent fiber inclinometer based on simplified hollow core photonic crystal fiber

A fiber optical inclinometer based on modal interferometer is demonstrated for bend angle sensing. The device consists of a piece of simplified hollow core photonic crystal fiber sandwiched between single mode fibers with lateral offset. The measurement of bend angles up to 45° is demonstrated, and the spectrum exhibits a blueshift of over 50 nm. The sensitivity is found to increase with the applied bend angles and reaches 2.4 nm/deg at 45°, and the response is independent of the direction of bending. A low temperature sensitivity of 0.5 pm/°C is observed between room temperature and 1000°C. Due to its capacity for withstanding high temperature, the device can work as a direction-independent inclinometer in high-temperature environments.

Sensitivity enhancement of strain sensing utilizing a differential pair of fiber Bragg gratings

In strain measurement applications, the matched fiber Bragg gratings (FBG) method is generally used to reduce temperature dependence effects. The FBG parameters have to be designed to meet the requirements by the particular application. The bandwidth and slope of the FBG has to be balanced well, according to the measurement range, accuracy and sensitivity. A sensitivity enhanced strain demodulation method without sacrificing the measurement range for FBG sensing systems is proposed and demonstrated utilizing a pair of reference FBGs. One of the reference FBGs and the sensing FBG have almost the same Bragg wavelength, while the other reference FBGs has a Bragg wavelength offset relative to the sensing FBG. Reflected optical signals from the sensing FBG pass through two reference FBGs, and subtract from each other after the detection. Doubled strain measurement sensitivity is obtained by static rail load experiments compared to the general matched grating approach, and further verified in dynamic load experiments. Experimental results indicate that such a method could be used for real-time rail strain monitoring applications.

Analysis for reflection peaks of multiple-phase-shift based sampled fiber Bragg gratings and application in high channel-count filter design

An analytical expression for calculating the reflection-peak wavelengths (RPWs) of a uniform sampled fiber Bragg grating (SFBG) with the multiple-phase-shift (MPS) technique is derived through Fourier transform of the index modulation. The new expression can accurately depict the RPWs incorporating various parameters such as the duty cycle and the DC index change. The effectiveness of the derived expression is further confirmed by comparing the RPWs estimated from the expression with the simulated reflective spectra using the piecewise uniform method. And the reflective spectrum has been well optimized by introducing the Gaussian apodization function to suppress the sidelobes without any wavelength shift on the RPWs. Then, a high-channel-count comb filter based on MPS is proposed by cascading two or more SFBGs with different Bragg periods but with the same RPWs. Noticeably, the RPWs of the new structured SFBG can also be accurately calculated through the expression. Furthermore, the number of spectral channels can be controlled by choosing gratings with specified difference Bragg periods.

Special optical fiber for temperature sensing based on cladding-mode resonance

A fiber-optic temperature sensor by using a multi-cladding special fiber is presented. It works on the basis of leaky mode resonance from fiber core to outer cladding. With the thin-thickness inner cladding, the cladding mode is strongly excited and the resonant spectrum is very sensitive to the refractive index variation of coating material. By coating the special fiber with temperature-sensitive silicone, the temperature response was investigated experimentally from -20 degrees C to 80 degrees C. The results show high temperature sensitivity (240 pm/degrees C at 20 degrees C) and good repeatability.