This paper presents a comprehensive review of recent advancements in surface engineering of metals, encompassing techniques, characterization methods and applications. The study emphasizes the significance of surface engineering in enhancing the performance and functionality of metallic materials in various industries. The paper discusses the different techniques employed in surface engineering, including physical techniques such as thermal spray coatings and chemical techniques such as electroplating. It also explores characterization methods used to assess the microstructural, topographical, and mechanical properties of engineered surfaces. Furthermore, the paper highlights recent advancements in the field, focusing on nanostructured coatings, surface modification for corrosion protection, biomedical applications, and energy-related surface functionalization. It discusses the improved mechanical and tribological properties of nanostructured coatings, as well as the development of corrosion-resistant coatings and bioactive surface treatments for medical implants. The applications of surface engineering in industries such as aerospace, automotive, electronics, and healthcare are presented, showcasing the use of surface engineering techniques to enhance components, provide wear resistance, and improve corrosion protection. The paper concludes by discussing the challenges and future directions in surface engineering, highlighting the need for further research and development to address limitations and exploit emerging trends. The findings of this review contribute to advancing the understanding of surface engineering and its applications in various sectors, paving the way for future innovations and advancements.

Surface Engineering of Metals: Techniques, Characterizations and Applications

Nowadays, there have been extensive applications of biosensors for industrial production, environment monitoring, disease diagnosis, and food safety. Attributed to periodically refractive index modulation, long period fiber grating (LPFG) can couple the core mode to forward propagating cladding modes at specific wavelengths, so it is sensitive to the variations of environmental refractive index. In addition, LPFG sensors have been ubiquitous in the biological detecting field with advantages of high sensitivity, compact structure, resistance to electromagnetic interference, and long-distance monitoring. In this paper, we focus on the review of sensing mechanisms, signal demodulation, fabrication and origin and recent development of biosensors based on LPFG. Besides, we discuss the methods for improving detection limits such as reducing cladding diameter, working near dispersion turning point (DTP), and within mode transition (MT) region. Applications on the detection of glucose, bacteria, immunoglobulins, biotin, DNA, hemoglobin, virus, drugs and so on are also classified and summarized in detail. Finally, several problems that hinder the practical use of LPFG biosensors are presented and some possible solutions are proposed, and the future development of biosensors is conceived. Over the past few years, biosensors based on LPFG have made remarkable achievements in the diversification of detected objects and improvement of extreme sensitivity. However, there are still some problems in practical applications for LPFG biosensors, such as temperature cross-sensitivity and stable wavelength demodulation, which need to be solved in the future. 

Biological sensors based on long period fiber grating

Resonance optical fiber sensors are widely studied for their high sensitivity, small size, and anti‐electromagnetic interference. However, traditional resonance optical fiber sensors have encountered bottlenecks in the trace detection due to limited localized field intensity and low molecules affinity. With the development of materials technology, low‐dimensional materials such as quantum dots, graphene, and transition metal sulfides have excellent properties such as high carrier mobility, large specific surface area, and flexible structure controllable artificial technologies that can also be used to synthesize new materials with desired characteristics, which can enhance the performance of sensors fundamentally. The introduction of low‐dimensional materials can not only achieve the performance optimization of sensors, but also make sensors functional to realize the diversification of detection objects. This paper reviews the research progress of resonance optical fiber sensors based on low‐dimensional materials. The detection principle and performance indexes related to resonance optical fiber sensing are elaborated, and resonance optical fiber sensors modified by low‐dimensional materials are introduced. Furthermore, the role of low‐dimensional materials in resonance fiber sensing is summarized, the direction of performance optimization in the production process of resonance fiber sensors is analyzed, and the future prospect of new resonance optical fiber sensors is given.

Research Progress of Resonance Optical Fiber Sensors Modified by Low‐Dimensional Materials

There are review articles focusing on the condition monitoring (CM) of rotating machines but there is no review article on the CM of Oil and Gas pumps that are critically important and expensive equipment in the Oil and Gas industry. The objective of this article is to fill this void and review studies related to CM of oil pumps, focusing on sensing technologies. In this review, we touch upon different aspects of CM namely the maintenance fundamentals, harsh environment definition and applications, and the two approaches used for dealing with the harsh environment. We found that though CM is a widely explored area for rotating machines, it is comparatively less explored for pumps in the Oil and Gas industry. Encapsulation of sensors to protect them from the harsh environment is widely investigated compared to the novel sensor development. The encapsulation material properties, its reactivity to harsh environments, and its effects on the intended function of the sensor are important aspects to study in the investigation of the degradation of the encapsulation material in the harsh environment. Different novel sensing technologies deliver different temperature ranges, frequency ranges, and sensitivity, for high-temperature sensing applications. There is scope for the development of novel sensors for high-temperature applications as well as other harsh environmental conditions. One of the important directions for future research is the investigation of the effects of the harsh environment on the sensors and sensing technologies in CM of pumps in the Oil and Gas industry. 

Sensing technologies for condition monitoring of oil pump in harsh environment

A novel π-phase-shifted (PS) long-period fiber-grating (LPFG) sensor operating near the phase-matching turning point (PMTP) was proposed. The PS-LPFG sensor can simultaneously detect temperature, strain, refractive index (RI), and pH. Due to the introduction of π phase shift, the double resonance peak of the selected cladding mode was divided into four loss peaks, which is convenient for multiparameter measurement. By monitoring the resonant-wavelength shifts of four dips of PS-LPFG, simultaneous detection of temperature, strain, RI, and pH can be achieved. Moreover, using a simple LPFG design on one fiber and a multiparameter matrix algorithm, the crosstalk between multiple signals was eliminated. Graphene oxide/polyvinyl alcohol (GO/PVA) hydrogel was coated onto the grating sensing area to measure the change in pH, and the sensitivity was higher than that of the same type of fiber-grating sensor. Results showed that the maximum sensitivity of temperature, strain, RI, and pH of the fabricated phase-shifted grating sensing structure were 0.2128 nm/°C, 0.1225 nm/μϵ, 718 nm/RIU, and -1.0166 nm/pH, respectively. Overall, the proposed sensor had great potential applications in monitoring marine environments and biochemistry sensing. 

Simultaneous detection of temperature, strain, refractive index, and pH based on a phase-shifted long-period fiber grating

• 1. The optimal optical sensor is selected and systematically studied to determine the detection object. • 2. The real-time synchronous measurement of multi parameters is realized with multi parameter matrix. • 3. The optimized cascade mode is combined with WGM technology to realize the integration of sensors. Multi parameter measurement based on optical fiber sensor has been a research hotspot in recent years. In this work, four kinds of optical fiber sensors, LPFG, FBG, PMF and SMS are used. The transmission characteristics and sensing characteristics of each sensor are analyzed in detail. Based on the modulation of resonance wavelength by environmental variables, the accurate and synchronous real-time detection of temperature, stress, refractive index and humidity is realized. We use all-optical coupling cascade and wavelength division multiplexing (WDM) technology to prepare a highly integrated sensing device. The problem of signal crosstalk of each sensor is solved, and the cost of signal transmission and reception is greatly reduced. It is of great significance for monitoring a variety of environmental variables under extreme conditions. 

Temperature, stress, refractive index and humidity multi parameter highly integrated optical fiber sensor

Increasing sensitivity, measuring points, and stability have always been the pursuit of sensors. ZnSe9:CO1 and Ag composite nano films were coated on polarization maintaining fiber (PMF). Then, the coated PMF was nested in capillary and hose which was encapsulated with polydimethylsiloxane (PDMS) and epoxy resin. The integrated capillary sensor and thermoplastic hose sensor were prepared. The gradient sensitization of various measurement parameters such as temperature, stress, and micro bending is realized. The temperature sensitivity is 1.49 nm/°C, the micro bending sensitivity is 1.72 nm/102 g, and the stress sensitivity is 6.27 nm/mε. The sensors maintain good linearity and instantaneous response while having high sensitivity. By adjusting the length of PMF, the number of troughs is increased in the same band range, and different troughs have different sensitivities, which solves the inherent problem of cross sensitivity and realizes multiparameter measurement. Capillary sensors are used for remote safe real-time monitoring of mechanical overheating, and hose sensors are used for real-time monitoring of bridge load and human joint bending. This work is of great significance to the extension of the application range of optical fiber sensor.

Polarization Maintaining Fiber Temperature and Stress Gradient Sensitization Sensor Based on Semiconductor-Metal-Polymer Three-Layer Film Coating.

A novel graphene tilt fiber grating sensor prepared by PLD and its photosensitive characteristics

The silicon Mach–Zehnder modulator is a significant type of optical modulator, which is widely utilized in optical communication and microwave photonics. The performance of the second harmonic microwave generation by a silicon Mach–Zehnder modulator is simulated and analyzed. The power and the purity of the second harmonic microwave mainly depend on two significant parameters: 1) the electrical nonlinearity of the diodes on arms of the modulator, 2) the bias voltage on the diode. Simulations show following results:1) when the built-in voltage Vb decreases from 8V to 3.5V, the incremental amounts of second harmonics and the fourth harmonics are 7.9dB and 15.6dB under the condition that the RF power is 15dBm and the bias voltage is 0V; 2) when the bias voltage increases from -0.5V to 4V, the ratio between second harmonics and the fourth harmonics varies from 20.0dB to 11.3dB under the condition that the RF power is 15dBm and the built-in voltage Vb is 3.5V. 3) when the RF power increases from 5dBm to 20dBm, the ratio between second harmonics and the fourth harmonics varies from 27.6dB to 11.6dB under the condition that is the bias voltage is 2V and the built-in voltage Vb is 3.5V. To conclude, the powers of second harmonics will increase with the reduction of the ratios between the second harmonics and the fourth harmonics.

Second harmonic generation by the silicon Mach-Zehnder modulator with symmetric arm

When we use FBG to filter the optical signal, we pay more attention to its optical power spectrum and less attention to its dispersion fluctuation. However, if the dispersion fluctuation of FBG is too large, it may lead to signal distortion, so it is very important to detect the dispersion fluctuation of FBG. In this paper, we choose the FBG with wavelength of 1550nm and reflectivity of 95%, and use the method of phase to amplitude conversion to detect the second-order dispersion fluctuation of FBG. In the temperature range of 23 ° - 33.6 °, we have measured the optical power spectrum and the second-order dispersion fluctuation curve of the fiber grating, and analyzed the experimental results. Then we find that when the FBG with flat optical power spectrum but large dispersion fluctuation is used for filtering, the output signal will be distorted.

Measurement of second-order dispersion fluctuation of fiber grating based on optical phase modulator

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