What is a fiber optic temperature sensor?

This paper analyzes the advantages of fiber optic temperature sensors in temperature detection, and introduces the working principles of distributed fiber optic temperature sensors, fiber grating temperature sensors, interferometric fiber optic temperature sensors and fiber optic fluorescence temperature sensors. Application in modern industry and life.

Working principle of fiber optic temperature sensor

The optical fiber temperature sensor uses the principle that the spectrum absorbed by some substances changes with temperature, and analyzes the spectrum transmitted by the optical fiber to understand the real-time temperature.

The optical fiber sensor uses a polymer temperature-sensitive material that matches the refractive index of the optical fiber to coat the outside of two fused optical fibers, so that light can be input from one optical fiber to the reflective surface and output from the other optical fiber. Due to this The new temperature-sensitive material is affected by temperature, and the refractive index changes, so the output optical power is a function of temperature.

Its physical essence is to use the characteristic parameters of light waves transmitted in the optical fiber, such as amplitude, phase, polarization state, wavelength and mode, etc., which are sensitive to external environmental factors such as temperature, pressure, and radiation. It belongs to non-contact temperature measurement.

Various fiber optic temperature sensors

1. Semiconductor absorption temperature sensor

Semiconductor absorption temperature sensor consists of semiconductor absorber, light source, transmission fiber, photodetector, preamplifier and signal processing system. It is realized by utilizing the characteristic that the absorption spectrum of semiconductor material changes with temperature. When the temperature rises, because the spectral line absorbed by the semiconductor material moves to the long-wave direction, the transmitted light intensity decreases, and the output voltage of the photodetector will also decrease with the increase of temperature. Therefore, by measuring the output of the photodetector The voltage can be used to obtain the corresponding temperature, so as to obtain the measured temperature.

The advantages of semiconductor absorption temperature sensors are small size, high sensitivity, fast time response, reliable operation, and can overcome the shortcomings of thermocouple sensors that are susceptible to interference in electromagnetic fields or test points that cannot be excited by electric fields.

2. Optical fiber fluorescent temperature sensor

Optical fiber fluorescence temperature sensor consists of light source, conductive fiber bundle, coupling, sensing probe, photodetector and signal processing display. It uses the characteristics of the fluorescent brightness of the substance to change with temperature to measure the temperature. When an object is irradiated by light or radiation, its atoms are in an excited state, and when the atoms return to their original state, they emit fluorescence. The intensity of some spectral lines emitted by fluorescence will change with the change of the temperature of the phosphor, so as long as the change of the intensity of a spectral line of the emission spectrum is measured, the change of the temperature of the phosphor can be determined accordingly.

The advantages of optical fiber fluorescent temperature sensor are small heat capacity, fast response, anti-electromagnetic interference, and high measurement accuracy. The measured temperature is a direct function of the temperature of the phosphor, and the sensor is not affected by changes in the light source or by bending the fiber optic lead.

3. Optical fiber liquid temperature sensor

The optical fiber liquid temperature sensor is composed of light source, optical fiber, liquid core waveguide probe, optical detector and signal processing device. A section of unclad optical fiber is immersed in some kind of liquid, and the liquid becomes the cladding of the optical fiber. The refractive index of the liquid changes with the temperature, so the numerical aperture of the waveguide also changes accordingly. Therefore, only a part of the light is transmitted, and this part of the light is sent from the output optical fiber to the photodetector, and the energy flow of the output light also changes. The temperature of the liquid to be tested can be obtained by measuring the change of the transmitted light intensity.

The advantages of this sensor are simple signal processing method, small size, light weight and high sensitivity.

4. Optical fiber thermochromic temperature sensor

Optical fiber thermochromic temperature sensor consists of light source, chopper, optical fiber, probe, optical fiber coupler, filter, photodetector and signal processing device. It is based on the thermochromic effect. The color of many inorganic solutions changes with temperature, so the optical absorption line of the solution also changes with the temperature. Using the thermochromic characteristics of inorganic solutions, an optical fiber thermochromic temperature sensor can be made. Since the color changes with temperature, the transmission spectrum will also vary with temperature. The light with the wavelength of and respectively is used as the temperature signal beam and the reference signal beam, then the ratio of the light intensity of the two beams will only decrease monotonously with the increase of temperature, so the measured light intensity ratio of the two beams can be calculated as The temperature to be measured can be obtained.

The advantage of the optical fiber thermochromic temperature sensor is that it uses ratio signal processing, thereby eliminating the influence of power fluctuations and temperature-independent factors in the optical fiber on the measurement, ensuring the accuracy of the system measurement. This temperature sensor has a measuring range of 5-75 degrees Celsius with an accuracy of +/- degrees Celsius and is immune to interference from microwave radiation fields.

5. Optical Fiber Polarization Temperature Sensor

The optical fiber polarization temperature sensor is composed of light source, optical fiber, probe, photodetector and signal processing device. It measures the temperature characteristic by detecting the change of the polarization state of light. The detection of light wave polarization state includes two parts: one part is to convert the angle information of the rotation of the light wave polarization plane into the light intensity information that the photodetector can receive, and the other part is to identify the temperature information from the light intensity information.

The advantages of optical fiber polarization temperature sensor are: high sensitivity, small interaction with electromagnetic field. The temperature measurement range is 18-180 degrees Celsius, and the resolution is 2 degrees Celsius.

6. Heterodyne interference temperature sensor

The heterodyne interference temperature sensor is composed of a laser, a coupler, a polarization maintaining fiber, a polarization analyzer, a photodetector and a signal processing circuit. When the temperature is different, the refractive index of the optical fiber in the y direction is very different, resulting in a different phase in the x, y direction. The change of the phase difference of light in the r, y direction is linearly related to the change of temperature , so as long as the phase difference is measured, the measured temperature can be obtained. In order to measure the phase difference, the sensor adopts the detection method of transverse Zeeman laser and double optical path, and obtains the phase difference through the phase comparator.

7. Published fiber optic temperature sensor

Distributed optical fiber temperature sensor is composed of laser light source, optical fiber, coupler, photoelectric detector and signal processing system. It realizes distributed measurement by means of optical time domain backscattering technology. When the optical pulse is transmitted in the optical fiber, it will produce Rayleigh scattering, Brillouin scattering and Raman scattering, so there are three distributed temperature sensors based on Rayleigh scattering, Brillouin scattering and Raman scattering in the distributed temperature sensor. Temperature sensor. Among them, the research technology based on Raman scattering is the most in-depth. For Raman scattering, when light is injected into the optical fiber, it continuously generates backscattered light waves while propagating in the optical fiber, emitting a light longer than the wavelength of the light source called Stokes light and a light shorter than the wavelength of the light source called reflection Stokes Light. Anti-Stokes light scattering is particularly sensitive to temperature, the fiber is modulated by the external temperature to change the anti-Stokes light intensity in the fiber, but Stokes light scattering has nothing to do with temperature. In order to eliminate the influence of light source instability and fiber bending loss on the scattering intensity, a dual-channel dual-wavelength comparison method is adopted in the system design. The anti-Stokes light is used as the signal channel and the Stokes light is used as the reference channel to detect both ratio to achieve absolute temperature measurement.

Distributed optical fiber temperature sensor has the advantages of anti-electromagnetic field interference, large signal transmission bandwidth, high spatial resolution and high precision. The temperature measurement range is between 77K-800K.

Application of fiber optic temperature sensor

Since the advent of optical fiber temperature sensing, it has been mainly used in power systems, construction, chemical industry, aerospace, medical and marine development and other fields, and has achieved a large number of reliable application results.

(1) Optical fiber temperature sensors have important applications in power systems. Surface temperature of power cables and temperature monitoring of cable-intensive areas; monitoring of heat-prone parts in high-voltage power distribution devices; ambient temperature detection and fire alarm systems of power plants and substations; various large and medium-sized generators, transformers, motors Temperature distribution measurement, thermal protection and fault diagnosis; thermal power plant heating system, steam pipeline, oil pipeline temperature and fault point detection; geothermal power station and indoor closed substation equipment temperature monitoring, etc.

(2) Optical fiber temperature sensor, especially fiber grating temperature sensor, is easy to embed in materials to measure the internal temperature with high resolution and wide range, so it is widely used in buildings and bridges. Some developed countries such as the United States, Britain, Japan, Canada and Germany have already carried out research on bridge safety monitoring. Bridge safety monitoring and early warning systems have been installed on major bridges. It is used to monitor key safety indicators such as strain, temperature, acceleration and displacement of bridges. In the summer of 1999, 120 optical fiber grating temperature sensors were installed on a steel structure bridge of Interstate Highway 10 in Las Cmces, New Mexico, USA. A record for the most sensors of this type used on a single bridge.

(3) The aerospace industry is a sensor-intensive place. An aircraft needs more than 100 sensors to monitor pressure, temperature, vibration, fuel level, landing gear status, wing and rudder positions, etc., so the size and weight of the sensors become very important. Few other sensors can compare with fiber optic sensors in terms of small size and light weight.

(4) The small size of the sensor is very meaningful in medical applications. Fiber Bragg grating sensors are the smallest sensors that can be achieved today. Fiber Bragg grating sensors can perform internal measurements of human tissue functions in a minimally invasive way. Provides precise local information on temperature, pressure and acoustic wave fields. Fiber Bragg Grating sensors cause very little damage to human tissue, enough to avoid interference with normal medical procedures.

(5) Fiber Bragg grating sensors are very suitable for downhole sensing because of their anti-electromagnetic interference, high temperature resistance, long-term stability and high radiation resistance. Norway's Optoplan is developing fiber optic grating temperature and pressure sensors for permanent downhole measurement.

Advantages of fiber optic temperature sensor

Fiber optic temperature sensors offer several advantages, including:

• High precision and accuracy: Optical fiber enables precise temperature measurement with minimal error.

• Electromagnetic Interference (EMI): Unlike electrical temperature sensors, fiber optic sensors are immune to EMI and are suitable for harsh electromagnetic environments.

• Wide temperature range: Fiber optic sensors can measure temperatures from very low to very high.

• Fast Response Time: They offer fast temperature sensing for real-time monitoring.

Summarize

Optical fiber temperature sensor is a new type of temperature sensor, which has the advantages of anti-electromagnetic interference, high pressure resistance, corrosion resistance, explosion-proof and flame-proof, small size, light weight, etc. Among them, several main optical fiber temperature sensors: distributed optical fiber temperature sensor, Fiber Bragg grating temperature sensors, interferometric fiber optic temperature sensors, and fiber optic fluorescence temperature sensors have their own unique advantages. Broad application prospects.