High sensitivity optical fiber pressure sensor based on thin-walled oval cylinder

High sensitivity fiber optic pressure sensor is in great demand in in all aspects due to its advantages. However, the current high sensitivity optical fiber pressure sensor has the problems of large size and high cost. An optical fiber pressure sensor based on all-silica Fabry-Perot interference has been proposed in this paper. The sensor consists of single mode fiber, silica capillary and diaphragm made of coreless optical fiber. The diameter of the sensor is only 125 μm. With precise control of diaphragm thinning, the diaphragm thickness of F-P sensor is only 1.477 μm, the sensor manufactured in this way has sensitivity with 12.586 rad/MPa. The sensor is ease of manufacturing and low cost. Static calibration shown the advantages of the sensor are high sensitivity, good linearity, repeatability, reliability and low hysteresis.

In the field of fiber-optic sensing, effectively reducing the noise of sensing spectra and achieving a high signal-to-noise ratio (SNR) has consistently been a focal point of research. This study proposes a deep-learning-based denoising method for fiber-optic sensors, which involves pre-processing the sensor spectrum into a 2D image and training with a cycle-consistent generative adversarial network (Cycle-GAN) model. The pre-trained algorithm demonstrates the ability to effectively denoise various spectrum types and noise profiles. This study evaluates the denoising performance of simulated spectra obtained from four different types of fiber-optic sensors: fiber Fabry-Perot interferometer (FPI), regular fiber Bragg grating (FBG), chirped FBG, and FBG pair. Compared to traditional denoising algorithms such as wavelet transform (WT) and empirical mode decomposition (EMD), the proposed method achieves an SNR improvement of up to 13.71 dB, an RMSE that is up to three times smaller, and a minimum correlation coefficient (R2) of no less than 99.70% with the original high-SNR signals. Additionally, the proposed algorithm was tested for multimode noise reduction, demonstrating an excellent linearity in temperature response with a R2 of 99.95% for its linear fitting and 99.74% for the temperature response obtained from single-mode fiber sensors. The proposed denoising approach effectively reduces the impact of various noises from the sensing system, enhancing the practicality of fiber-optic sensing, especially for specialized fiber applications in research and industrial domains.

In this paper, a novel arc-shape FBG (Fiber Bragg Grating) load sensor is developed with a pair of FBGs embedded symmetrically in an elastic bending beam. The FBG pair is produced with peak wavelength separation of about 0.4 nm. The FBG with shorter wavelength is located above the neutral layer and another with longer wavelength below the neutral layer. As a result, the two peak wavelengths of the FBG pair move apart with lateral force applied but move together in the same direction when surrounding temperature varies, both in a linear manner. A very low temperature-dependency of only 0.04 pm/°C across the temperature range from 35°C to 82°C has been achieved, which makes it suitable for dynamic lateral force measurement even when the environmental temperature varies. Due to the fact that the sensor module consists of only the grating fiber with two FBGs and some elastic material, which are purely passive components, hence, the temperature effect of this sensor can be self-compensated without a need of any active temperature controller or other temperature-isolation mechanism, thus in principle, this sensor module is passively temperature-independent.

This thesis presents two designs for high power linearly polarized all-fiber linear cavity lasers, continuous wave (CW) and mode-locked. The cavity designs use Polarization Maintaining (PM) fibers for both gain medium and Fiber Bragg Gratings (FBGs). The FBG pairs select lasing wavelength and polarization. The fiber lasers incorporating specialty designed FBGs achieve an extinction ratio larger than 23 dB. Firstly, an all-fiber linear cavity design of a high power picoseconds mode-locked laser is introduced. The proposed configuration is based on Non-Linear Polarization (NPR) using PM Yb-doped active fiber and two matching FBGs to form the laser cavity. The combination of nonlinearity, gain and birefringence in cavity made the laser generate mode-locked pulses in picoseconds range and with high average output power. The output mode-locked pulses amplitude is modulated with an envelope whose mechanism is also investigated in this thesis project. Experimental data and numerical simulations of the self mode-locking fiber laser are presented. Main parameters affecting mode-locked pulses and its envelope are identified. In addition, a new theoretical model based on Nonlinear Schrödinger Equation (NLSE) is developed and implemented on the MATLAB platform. The model explains the self mode-locking mechanism and the source of the pulse envelope. In this model, it is proven that self phase modulation (SPM) plays an essential role in pulse formation and shaping. The theoretical model and experimental results are in a very good agreement at different pumping levels. A method of regulating the mode-locked pulses is presented. This is achieved by applying a pulsed current to pump diode. This method successfully stabilizes the mode-locked pulses underneath a Q-switched pulse envelope. Further scale-up of average power and pulse energy is realized by adding an amplifier stage. Secondly, a CW dual-wavelength all-fiber laser is presented. The laser consists of two pairs of FBGs and a PM Er/Yb co-doped fiber as a gain medium. The laser emits at both 1 μm and 1.5 μm wavelengths simultaneously with a stable output. This laser provides a compact fiber-based pumping source that is suitable for mid-IR generation.

PDMS-based liquid-core optical fiber fluorescence pressure sensor

In this paper, we propose a simple fiber-optic sensing system to measure the concentration of the methane based on a special catalyst and fiber Bragg gratings (FBGs). The temperature around the catalyst rises rapidly as the concentration of methane increases, and hence can be detected by an FBG-type sensor through monitoring the wavelength shift of the reflection spectrum. By employing a well-matched FBG as a reference, the wavelength shift of the sensing FBG can be transformed effectively into the variation in the measured optical intensity. An experiment is demonstrated to show the good performances of our system, such as the high resolution and good stability to the environmental temperature.

The wavelength shift of single Bragg grating integrated into a waveguide in optical MEMS pressure sensors gives inaccurate result due to the cross sensitivity caused by multiple external factors including temperature. Novel dual waveguide Bragg gratings (WBGs)-based microelectromechanical system pressure sensors with silicon micromachined diaphragm are presented here. The sensor consists of a curved waveguide with two identical WBGs located on the diaphragm. When pressure is applied on the diaphragm, the pitch of the gratings changes, and hence, there is a shift in the corresponding Bragg wavelength. The effect of temperature is assumed to be the same for both the gratings. As temperature shifts the Bragg wavelength of the two gratings equally, the error due to the temperature change on pressure sensitivity is eliminated. This method of pressure measurement effectively cancels out the temperature sensitivity. Three different configurations of the sensor with circular, square, and rectangular diaphragms are designed and simulated using COMSOL Multiphysics. Pressure sensitivity for the circular diaphragm configuration is found to be 2.1 and 0.744 pm/Pa for the two gratings, which are higher than the square and rectangular diaphragm configuration. Hence, the circular diaphragm configuration is better suited for multiplexing. The temperature independent properties of the sensors as well as the adjustability of sensitivity and measurement range by means of altering the dimensions of the sensors are studied. The temperature sensitivity is the same for both the gratings and is found to be 11.6 ppm/°C. Using modulation techniques and suitable designs, these sensors find applications in remote distributed sensing.

This paper presents an optical fibre pressure and temperature sensor (OFPTS) system, which is adapted for use as a urodynamic pressure measurement system (UPS) for differential pressure measurement with temperature compensation. The OFTPS is based on a Fabry Perot interferometer (FPI), which acts as a pressure sensor and includes an embedded fibre Bragg grating (FBG) for temperature measurement. The sensor system is evaluated in a lower urinary tract (LUT) simulator, which simulates the bladder, rectum and detrusor muscle. The system was benchmarked against a commercially available urodynamic system, at the University Hospital Limerick (UHL) Urology Clinic. Both systems demonstrate a high correlation with a relative pressure variation of less than ±2.8cmH₂O for abdominal and ±4cmH₂O for vesical pressure. The repetitive measurement of the OFPTS system in the LUT simulator against the commercial system demonstrated the high repeatability. Furthermore, the low fabrication cost makes the OFPTS a potentially interesting instrument for urodynamic and other medical applications.

In this paper, we fabricate a pair of fiber Bragg gratings (FBGs) in large mode area double cladding fiber (LMA-DCF, core/inner cladding diameter: 20/400 μm) by visible (515 nm) femtosecond laser and phase mask for the first time to the best of our knowledge. Then this pair of FBGs are utilized to construct a high power fiber oscillator operated near 1080 nm. The maximum output laser power is more than 3.2 kW with a slope efficiency of ∼77.9%, and the beam quality factor M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is about 1.28. The FBGs are simply fixed on water-cooling plate without any special package, and the temperature of the FBGs is about 40 °C at the maximum output power. Our research work confirms the reliability of visible femtosecond laser in fabrication of FBGs intended to be applied in high power fiber oscillators.

We demonstrate a dual-wavelength linearly polarized all-fiber laser emitting 1- μm and 1.5- μm wavelengths with a total output power of 1.3 W. The laser consists of two pairs of fiber Bragg gratings (FBGs) fabricated on PM fibers to form a cavity and an Er/Yb co-doped fiber as a gain medium. A high extinction ratio of 23 dB is achieved using specially designed PM FBGs. The laser output has a narrow bandwidth of 26 pm at both 1057 and 1554 nm. When the dual-wavelength all-fiber laser is used to pump a PPLN crystal, the generation of mid-infrared emission at 3.3 μm is successfully demonstrated.

We present in this paper an optical fiber pressure and temperature sensor (OFPTS) with multi Fibre Bragg Grating (FBG) array. The sensor based on an extrinsic Fabry Perot interferometer and is fabricated from silica glass. A femtosecond laser (FSL) was used to inscribe multiple FBGs proximately close to the diaphragm, parallel to each other. This concepts allows a chain of FBGs with miniature active length which can be a significant important tool for medical application, like radio frequency ablation (RFA) cancer treatment.

A high sensitive pressure sensor based on Fiber Bragg grating (FBG) integrated with a thin metal diaphragm was designed and demonstrated. To enhance the pressure sensitivity FBG is firmly glued across the diameter of the diaphragm. Under pressure, the diaphragm deforms and produces an induced strain along the length of the fiber causes shift in Bragg wavelength of the FBG. Pressure measurement is made by measuring the Bragg wavelength shift against change in pressure. The sensor was tested up to the maximum pressure of 140 psi and the corresponding pressure sensitivity was found to be 0.0204 nm/psi, which is approximately 970 times higher than that can be achieved with a bare FBG. The experimental results show good agreement with the theoretical results and possess good linearity and repeatability. This sensor can be used for the measurement of medium pressure, liquid level and depth of underwater.

Due to the advantages of small size, high sensitivity and strong anti-interference ability, optical fiber sensors are widely used in gas detection. Meanwhile, concentration and humidity detection based on optical fiber sensing are paramount technologies in the field of gas sensing. However, how to solve the issue of cross sensitivity of concentration and humidity of optical fiber gas sensors and realize synchronous detection of two parameters are current common challenges. In this study, taking carbon dioxide as an example, a detection method based on double fiber Bragg grating (FBG) string is proposed to measure carbon dioxide concentration and air humidity simultaneously. Here, a pair of FBGs are welded in series on an optical fiber after being filled with special materials. Using the control variable method, the performance parameters of the sensor can be determined through calculation. The experimental results demonstrate that the gas concentration sensitivity is 0.5952 pm/ppm in the range of 0 ppm~2250 ppm. In the range of 25%~65%, the air humidity sensitivity is 28.62 pm/RH. Moreover, it is less affected by temperature and has satisfactory repeatability and reproducibility. The sensor realizes high sensitivity and low crosstalk measurement of concentration and humidity and can be applied to the simultaneous detection of carbon dioxide concentration and air humidity in specific confined space.

We propose and experimentally demonstrate the feasibility of an integrated hybrid optical fiber sensing interrogation technique that efficiently combines distributed Raman-based temperature sensing with fiber Bragg grating (FBG)-based dynamic strain measurements. The proposed sensing system is highly integrated, making use of a common optical source/receiver block and exploiting the advantages of both (distributed and point) sensing technologies simultaneously. A multimode fiber is used for distributed temperature sensing, and a pair of FBGs in each discrete sensing point, partially overlapped in the spectral domain, allows for temperature-independent discrete strain measurements. Experimental results report a dynamic strain resolution of 7.8 nε/√Hz within a full range of 1700 με and a distributed temperature resolution of 1°C at 20 km distance with 2.7 m spatial resolution.

We propose and demonstrate a relative humidity (RH) sensor based on a polarimetric fiber laser and beat frequency (BF) demodulation. The laser cavity of the proposed fiber laser sensor consists of a pair of fiber Bragg gratings (FBGs) and a high-birefringence erbium-doped fiber (Hi-Bi EDF) coated by a thin polyvinyl alcohol (PVA) film layer. This thin-film layer responds to humidity changes in the environment by swelling or contracting, inducing stress effects on the EDF. The stress effect, in turn, results in changes to the polarization mode beat frequency (PMBF) detected by the photodetector (PD) in the sensing system. The inherent high-birefringence property of the Hi-Bi EDF significantly enhances the sensitivity of the PMBF to humidity-induced stress. Humidity sensing experiments are conducted utilizing the proposed sensor. Experiment results show that the sensor has a high average humidity sensitivity of 13.9 MHz/%RH within the humidity range of 20.4%RH–36.0%RH. A stability test is also conducted, demonstrating that the proposed sensor achieves an accuracy of 0.65%RH.