Research progress of the hottest optical fiber hyd

Research progress of optical fiber hydrogen sensor

I. Introduction

whether in order to prevent the accumulation of hydrogen or to use hydrogen, it is necessary to measure and monitor the content of hydrogen, and safe, reliable and cheap hydrogen measurement or sensing technology is the necessary guarantee for the wider and safe use of hydrogen. Up to now, a variety of non optical fiber sensors have been made and put into use. These sensors can provide fast and effective response at room temperature and standard atmospheric pressure, but mainly electrochemical sensors. The connection between the sensor and the system is completed through metal copper wire, which increases the weight of the load and power consumption - especially for multi-point detection, and more importantly, Potential electromagnetic interference and electric sparks may cause disastrous consequences. However, the optical fiber hydrogen sensor based on optical fiber technology can meet the requirements of safety, real-time detection and non-interference with the measured environment at the same time. Moreover, it also has the characteristics of small volume, light weight and good flexibility, which is especially suitable for applications requiring light load and small installation space; In addition, optical fiber can resist chemical corrosion, harsh environmental temperature, no electromagnetic interference, etc., which is suitable for long-distance large-scale multi-point multiplexing detection. The working principle of optical fiber hydrogen sensor can be referred to the relevant literature

II. Performance analysis of several typical optical fiber hydrogen sensors

(I) interference optical fiber hydrogen sensor

it is well known that metal palladium (PD) absorbs hydrogen in a low partial pressure hydrogen environment to produce elongation effect. If the metal palladium is evaporated on a certain section of optical fiber, the elongation effect will cause the radial and axial strain of the optical fiber. Taking the palladium plated optical fiber as the signal arm of the M-Z interferometer, the hydrogen concentration can be obtained indirectly by detecting the phase change of the light. The detectable hydrogen concentration is 20 PA ~ 200Pa, and its detection principle is shown in Figure 1

Figure 1 Detection schematic diagram of interferometric optical fiber hydrogen sensor

interferometric optical fiber hydrogen sensor has a development history of decades. Theoretically, this kind of sensor has high sensitivity. Moreover, this sensor shows good reusability, small accumulated error in continuous use, short rise and fall time of response, and its sensitivity can be controlled by controlling the length of the signal arm. However, devices based on this principle need electric drive to produce interference conditions, so they no longer have the advantages of all-optical system, because they are not widely used. In addition, this kind of sensor has complex structure and is greatly affected by temperature, which significantly reduces the accuracy of the sensor

(II) evanescent field optical fiber hydrogen sensor

its probe structure is shown in Figure 2

Figure 2 probe structure of evanescent field optical fiber hydrogen sensor

evanescent field optical fiber hydrogen sensor is a new type of optical fiber hydrogen sensor. The sensor uses the change of evanescent field in the metal coating to detect the concentration of hydrogen. The cladding of the optical fiber is cut off and coated with a hydrogen sensitive film. When light passes through the optical fiber, an evanescent field is generated around it. Evanescent wave is a traveling wave along the propagation direction, and its amplitude is transmitted. Due to the close connection between China's extruder products and strategic new industries, the propagation direction attenuates exponentially. Its penetration depth is:

Where is the wave vector in the electromagnetic wave, N2 is the refractive index of the hydrogen sensitive film, and its value changes with concentration. Palladium or oxides of palladium and tungsten are generally used as the sensing medium. Among them, palladium acts as a catalyst. Pure WO3 reacts with hydrogen only when the temperature is higher than 400 ℃, but it can react even at room temperature with metal catalysts (such as palladium)

its characteristics are as follows:

1. the regeneration ability of the sensor

the regeneration ability in inert gas is very weak, and when re exposed to air, the light energy increases rapidly, indicating that the existence of oxygen helps to enhance the regeneration ability of the sensor. The good regeneration of this kind of sensor can be observed by repeatedly exposing the sensor to N2 and air

2. sensitivity characteristics

in a dry or wet environment, the light energy hardly changes, indicating that the sensitivity is not affected by humidity. But the response rate and recovery rate become slower. Moreover, the sensitivity of exposure to air containing 1%h2 is 40% lower than that of exposure to H2 containing 1%n2

3. Humidity characteristics

humidity has a great influence on the recovery rate, and the recovery rate in humid air is about 10 times that in dry air. At room temperature and in dry air, it takes several hours for the sensor to recover its original performance. Therefore, the control of the amount of water in the production of membrane is very critical

4. temperature characteristic

sensitivity and response speed decrease with the decrease of ambient temperature, but light energy changes little. In addition, calcining the sensor at 500 ℃ revealed that this kind of sensor has excellent temperature characteristics

this sensor has high sensitivity and response speed, and has good temperature performance. It can be used in a large temperature range near room temperature. In addition, the traditional sensor can only measure the hydrogen concentration at a certain point or several points, while the optical fiber hydrogen sensor using evanescent wave absorption can be wound on a large container or pipe to measure the three-dimensional space

(III) micro lens optical fiber hydrogen sensor

evaporate a layer of palladium film on the end face of single-mode or multimode optical fiber, and the thickness of palladium film is 10 mm ~ 50 mm. The structural principle of the sensor probe is shown in Figure 3

Fig. 3 structural principle diagram of micro lens optical fiber hydrogen sensor

the light injected into the optical fiber is reflected on the output end face. When the sensor is exposed to hydrogen, the reaction between palladium and hydrogen causes the refractive index of palladium film to change, thus changing the light intensity. The change of refractive index is related to the hydrogen concentration. The hydrogen concentration can be determined by detecting the change of light intensity. Its characteristics are as follows:

1. Sensitivity

this sensor can detect 1% hydrogen in the air, and the response time is not more than 10s. When the incident angle is changed within the range of not more than 20O, the response remains unchanged. In low concentration of hydrogen, the response and response time have obvious changes with the change of hydrogen concentration, which can accurately measure hydrogen. When the concentration is higher than 2%, the response changes little with the increase of concentration. The response time decreases with the increase of hydrogen concentration, and it is longer when the concentration is close to the phase transition. Alaintrouillet et al. Designed a pure Pd film microlens type micro hydrogen testing instrument, put the hydrogen sensitive optical fiber probe into a protective sleeve and fix it with glue. This optical fiber hydrogen sensing test device has been installed on Ariane V rocket engine. The experimental data show that when the hydrogen concentration is 15% and the temperature is 20 ℃ ~ 100 ℃, the response time is within 10s. If the concentration is less than 15% and the temperature is less than 70O, the response time is less than 10s

2. Regeneration ability

put the sensor alternately in pure N2 and medium N2 containing 4% H2, and it can be observed that the response and response time of the sensor have good regeneration ability in a wide temperature range (-196 ~ 23 ℃). However, with the extension of service time, the response tends to decrease, while the response time tends to increase

3. Temperature

in N2 containing 4% H2, the phase transition occurs near 36 ℃. At 75 ℃, the sensor operates at α Phase, the response time is very short but the sensitivity is small; At -45 ℃, the sensor operates at β Relatively, the response time is large, but the current Prime Minister Erdogan is sensitive, and the election degree reaches the maximum with a majority of 52%. The experimental results show that the phase transition depends on two factors: temperature and concentration. In addition, in order to obtain a fixture sensor based on such three standards, it is necessary to α In order to obtain high response speed, the temperature of palladium membrane should be increased at a certain concentration. This conclusion is inconsistent with the operation of the device at low temperature, so local heating of the palladium membrane should be carried out so that a small response time can be obtained at any ambient temperature under the guidance of China's recent comprehensive, harmonious and sustainable scientific concept of development

4. Thickness

palladium membrane is the most critical part of the sensor, and the thickness of the membrane has an impact on the response time and sensitivity. For films of various thicknesses（