Today and development of the hottest optical fiber

Today and development of optical fiber sensors

this paper reports the current situation of the development of optical fiber sensors at home and abroad. This paper mainly introduces two aspects: the development status of the principle research of optical fiber sensor and the application and development status of optical fiber sensor products. The former reports the development of fiber grating, distributed fiber sensing technology and fiber sensing, which are the current research hotspots; The latter introduces optical tomography technology, intelligent materials, fiber optic gyroscope and inertial navigation system, industrial engineering sensors (including high-voltage and high current sensors for power industry, elastic-optic effect of optical fiber and stress sensors of FBG devices). Finally, new optical fiber materials and devices, fluoride glass fiber, carbon coated fiber, and honeycomb waveguide fiber and liquid crystal fiber under research are introduced

I. Introduction

with the development and maturity of Dense Wavelength Division Multiplexing DWDM technology, Erbium-doped Fiber Amplifier EDFA technology and optical time division multiplexing OTDR technology, optical fiber communication technology is developing in the direction of ultra-high speed and large capacity communication system, and gradually evolving to all-optical network. Driven by the rapid development of optical communication, optical fiber sensor, as a young member of the sensor family, has rapidly grown into an industry with an annual turnover of more than $1billion and is expected to have a market of more than $5billion in 2010 with its unique advantages in anti electromagnetic interference, lightness and sensitivity. The International Conference on optical fiber sensing (OFS), hosted by the American Society of optical engineers (OSA), reports the latest progress in the field of optical fiber sensing in a timely manner, and conducts useful discussions on optical fiber sensing and its corresponding technologies

at present, the development of optical fiber sensing field in the world can be divided into two directions: principle research and application development. With the growing maturity of optical fiber technology, the practical development of optical fiber sensors has become a hot spot and key in the development of the whole field. Because optical fiber sensing technology has not been industrialized as quickly as optical fiber communication technology, many key technologies are still in the laboratory prototype stage, and there is a certain distance from commercialization, so the theoretical research of optical fiber sensing technology is still in a very important position. Because many optical fiber sensors are developed to replace the traditional electromechanical sensing systems that are currently quite mature, whose reliability and cost have been recognized, and have been widely used, although these optical fiber sensors have many advantages such as electromagnetic insulation, high sensitivity, easy reuse and so on, the difficulties and challenges faced by their market penetration are conceivable. And those fiber-optic sensors with unprecedented new functions have obvious advantages in the competition. FBG and other grating sensors are the best example. Current theoretical research focuses on Fiber Bragg grating (FBG and LPG) sensors and distributed optical fiber sensing systems

the application research of optical fiber sensing technology mainly includes the following four categories: optical (fiber) tomography technology (OCT, OPT), smart materials, fiber optic gyroscope and inertial navigation system (IFOG, imiu) for various applications and conventional industrial engineering sensors. In addition, driven by the market demand of optical fiber communication and the special requirements of sensing technology, the research results of new devices and special optical fibers are also emerging in endlessly

at present, most of the research on optical fiber sensors in China is concentrated in Colleges and universities and scientific research institutions, and the transition from laboratory to product is still not completed. Among them, the more mature technologies include: the optical fiber oil tank liquid level and temperature measurement system jointly developed by the optical fiber sensing center of Tsinghua University and the General Logistics Department, which has been installed and operated for several years; The current index of the fiber optic gyroscope system developed by Beijing University of Aeronautics and Astronautics in cooperation with the general assembly is 0.2 °/hr; The distributed optical fiber sensing system developed by China Institute of metrology has been reported; High voltage and high current sensing system jointly developed by Huazhong University of technology and a company in Guangdong. In addition, many optical fiber passive component manufacturers have been established in Guangdong, Shenzhen and other places. Because optical fiber sensors have not crossed the threshold of productization and have not grown exponentially like the optical fiber communication industry, many sensor products (such as traffic management, alarm devices, etc.) and a large number of test instruments closely related to our daily life still rely on imports, and there is a very broad space for urgent development

II. Principle research of optical fiber sensor

1. Fiber Bragg grating

fiber Bragg grating FBG came out in 1978 [1]. This simple inherent sensing element can be written into the fiber core by using the ultraviolet photosensitivity of silicon fiber. Figure 1 describes the basic principle of fiber grating. Common FBG sensors detect the measured Bragg wavelength by measuring the drift of Bragg wavelength（ λ B) The condition can be expressed by formula (1):

where λ - grating period

n - refractive index

when a wide spectrum light source is incident into the fiber, the grating will reflect the narrow spectrum component with the Bragg wavelength LB as the central wavelength. In the transmission spectrum, this part of the component will disappear, and the drift of LB with stress and temperature is [2]: (2)

where, ε— Applied stress

pi, J - photoelastic tensor coefficient of optical fiber

ν— Poisson's ratio

α— Thermal expansion coefficient of optical fiber materials (such as quartz)

△ F - temperature change

in the above formula: the typical value of the factor is 0.22. Therefore, it can be deduced that the FBG stress and temperature response conditions under normal temperature and constant stress conditions are as follows:

the wavelength resolution of 1pm roughly corresponds to the temperature and stress measurement accuracy of 0.1 ℃ or 1me at 1.3mm

in addition to all the advantages of fiber sensors, fiber Bragg gratings also have the characteristics of self calibration and easy integration of multiple sensors in the same fiber. Figure 2 is an example of fiber Bragg grating sensor realizing multi-point measurement in one fiber [3]

there are many applications that grating sensors can expand, such as embedding distributed fiber grating sensors into materials to form intelligent materials, which can carry out real-time safety monitoring of load, stress, temperature, vibration and other parameters of large components; Grating can also replace other types of optical fiber sensors for chemical, pressure and acceleration sensing

Figure 3 shows the comparison of test results between traditional impedance meter and FBG sensor. The FBG application system si425[9] (see Figure 4) developed by micron-options in the United States can measure up to 512 FBG sensors in 4 channels at the same time, with a scanning range of 50nm, a resolution of 1pm and a measurement frequency of 244hz

long period grating refers to the grating with a period greater than 100mm, which is also another important branch of fiber grating sensor after FBG. Because the measurement uses the principle of cladding film coupling, it has the advantages of excellent sensitivity and simple fabrication. Fig. 5 is the transmission spectrum of long-period grating. Other branches of fiber Bragg grating include chirped grating, oblique grating, etc. [2], which have also been applied and studied [6]

2. Distributed optical fiber sensing system

worldwide, due to the continuous improvement of safety and benefit requirements for industrial and civil construction and industrial facilities, the demand for integrated safety detection system is gradually rising. The distributed optical fiber sensing system with continuous, uninterrupted, long-distance measurement and strong affinity with the measured medium seems to be tailored for this purpose. There are usually three types of distributed optical fiber sensing systems: Raman, Brillouin and FBG

Raman distributed optical fiber sensing system is a continuous sensor based on Fiber Raman scattering effect. Its working principle is shown in Figure 6. Applications of three types of sensing systems have been reported. Among them, Raman distributed sensing system is the most mature one, which has been successfully loaded on A340 transport aircraft (Figure 7)

fbg distributed sensing system has unique advantages in the multi-point distributed measurement of stress, and can complete the two parameter measurement of temperature and stress at the same time [5], which opens up a broader prospect for the application of FBG. Figure 8 introduces the topology of FBG array using wdm/tdm demodulation [4]

III. application and development of optical fiber sensor products

Application and development of optical fiber sensor can be roughly divided into four major directions according to the current hot application fields and technology types: optical (fiber) tomography analysis technology OCT, optical fiber intelligent material (smart material), optical fiber gyroscope and inertial navigation system, and conventional industrial engineering sensors. The year 2002 is the 25th anniversary of the birth of fiber optic gyroscope (I-FOG). At the 15th ofs annual conference, a special venue was opened for fiber optic gyroscope

1. Optical tomography technology

optical fiber tomography analysis technology has only 20 or 30 years from its rise to application. According to different principles and applications, optical fiber tomography technology can be divided into optical coherence tomography analysis (OCT) and optical process tomography analysis technology (OPT)

optical tomography technology is derived from X-ray tomography analysis (CT), and its basic principle is shown in Figure 9. When X-ray or light transmission passes through the tested sample, different sample materials have different absorption characteristics of rays. Therefore, the required sample parameters can be obtained by measuring and analyzing the rays or light passing through the sample and solving according to the predetermined topology and design

optical fiber coherence tomography (OCT) is mainly used in biology, medicine, chemical analysis and other fields, such as optic film scanning, gastrointestinal internal vision and color Doppler (CDOCT) blood flow imaging, which is used to realize that color users should not only choose the right manufacturer when purchasing tensile testing machines. Its working principle is based on the coherent detection principle of light, and the basic system structure is shown in Figure 10

oct provides an effective way to detect the activity of biological cells and bodies, and many countries in the world have developed corresponding products. Figure 11 shows the CT scanning image of the optic membrane. German scientists recently launched an OCT device that can be used to diagnose skin cancer. In addition, OCT can realize the advantage of depth measurement (~1mm), and examples have been applied to the observation and monitoring of growing cells

opt is oriented to industrial engineering - oil wells, pipelines and other places, and solves the problem of fluid process measurement with high accuracy. Since opt is concerned with the integration process on the light path, the related system integration design, unit segmentation and signal processing in measurement theoretical analysis are the key. Figure 12 briefly depicts the measurement principle of traditional opt. Because opt has many advantages, such as being suitable for narrow or irregular space, high security, free of electromagnetic interference in the measurement area, and can form a measurement network, it provides an excellent means for the safety measurement of industrial processes

2. Intelligent materials

intelligent materials have been proposed and studied for quite a long time, and are familiar to insiders. Smart material refers to embedding sensitive components into the body and materials of the tested components, so that the components or materials often