Optical fiber thermometry technology for high-temperature measurement is briefly reviewed in this paper. The principles, characteristics, recent progresses and advantages of the technology are described. Examples of using the technology are introduced. Many blackbody, infrared, and fluorescence optical thermometers are developed for practical applications.
Temperature measurement is very important in fire studies and combustion research. Thermocouples are commonly used as their operation principle is simple and easy to use. However, there are some limitations [
Of all the developed OFTs, blackbody and fluoroscopic sensors are widely used. Blackbody sensors consist of a high-temperature optical fiber with an opaque cavity attached to the sensing tip. The spectral radiative flux detected at the end of the fiber is related to the temperature of the cavity via Planck’s law. Temperature is obtained by measuring the spectral intensity or the intensity distribution.
Fluoroscopic sensors have a photo-luminescent material attached to the active end of an optical fiber. The sensing tip is activated by an excitation pulse from a pulsed laser or flash lamp. Temperature can be deduced from the intensity and decay-time (which depends on temperature) of the photo-luminescent signal. Fluoroscopic sensors are very sensitive, but their temperature ranges are limited by the properties of the material. In general, the fluorescence intensity is weak at high temperatures due to the quenching effect. Meanwhile, the background of blackbody radiation becomes stronger at high temperature. Therefore, the signal-to-noise ratio (SNR) is poor, limiting application to high-temperature measurement.
Blackbody sensors can operate over a wide range of temperatures in principle. However, the signal intensity is weak in the lower-temperature region as the radiance intensity is nearly exponentially related to temperature. The thermometer has a strong radiance signal and higher resolution at the higher temperature region. Therefore, blackbody sensors are generally used in high-temperature applications. The optimum measuring temperature of blackbody OFT sensor varies normally from 500 to
To widen the measuring range, a complex OFT scheme [
In this article, blackbody, fluoroscopic and non-contact infrared OFTs will be reviewed. It is suggested that OFTs have potential advantages to be used in studying fire and combustion.
Blackbody OFT is based on Planck’s radiation law that describes the spectral distribution of radiance for an ideal blackbody. The radiant power (
The above equation applies to ideal blackbody radiation only. In most practical cases, the emissivity of energy radiated is corrected by a factor
The sight path is mainly formed by an optical fiber, which is immunised to the impact of the surrounding atmosphere observed in the open path pyrometer, that is, refractive changes in air and other disturbing effects. The fiber bending and connecting points would give transmission loss and hence errors in
In addition, the total emission is measured by some radiation thermometers to get the temperature based on the Stefan-Boltzman law,
However, most of the photodetectors have limited response wavelength region. Correction to the above equation is required in calibrating the instrument.
By plotting
A typical inexpensive silicon photodiode, with its long wavelength response cutting off at ~1.1
A typical blackbody OFT sensor system consists of three basic elements: an optical signal generator (the blackbody cavity), an optical-signal-transmitting system including one or more transmitting optical fibers, and an optical detecting system, as illustrated in Figure
A schematic diagram for high-temperature optical fiber sensor system (from [
The optical signal generator is the temperature sensor which transfers thermal signals to optical ones. This is a quasi blackbody cavity at the tip of the high-temperature fiber. It is constructed with a thin (3 to 5
The principal components of the detector system are a light gathering lens, a narrow band filter, and a photodiode or photomultiplier that converts the radiation signals to electric ones. These three elements construct a basic optical fiber thermometer. For a practical temperature measurement system, there is an electronic signal and data processing system including signal amplification, A/D conversion and PC data processing. Such an electronic system is required to be able to measure the weak signal of 7
The optical fiber thermometer mentioned above has many advantages, such as high accuracy, intrinsic immunity to electromagnetic inference, and long lifetime, which make it very successful in various applications. Some of these excellent performances cannot be achieved by other pyrometers [
An improved method was proposed by Zhang et al. [
This equation is deduced from the ratio expression (see (
Another improved measurement method was proposed by Tong et al. [
The configuration of a curved sapphire optic fiber thermometer (from [
Thermal characteristics of Cr3+ fluorescence in LiSAF. The solid squares and open triangles represent the data on fluorescence lifetime and intensity, respectively (from [
Blackbody OFTs have been proved to be very successful in measuring high temperatures [
High-temperature processing operations in cement, refractory, and chemical industries often use fiber optic temperature sensing. At somewhat lower temperatures, plastics processing, papermaking and food processing operations are making more use of the technology. Fiber optic temperature sensors are also used in fusion, sputtering, and crystal growth processes in the semiconductor industry.
Another type of OFTs is non-contact infrared (IR) thermometer. Temperature measurements can be classified into two types: invasive and noninvasive, or contact and non-contact. The first type requires direct contact between the measurement device and the specimen, as thermocouple temperature measurement system. The blackbody OFT mentioned above is one kind of contact thermometers. No direct contact is required for the second type. The measured specimen can be observed at distance away from the instrument.
The contact instrumentation must be able to stand the high temperature concerned. In high-temperature or chemically reactive applications such as flames or plasmas, invasive instrumentation can degrade with time. If operating above the material limits, it would be disintegrated completely. On the other hand, thermal probes are needed in most contact thermometers. A thermal equilibrium state between the probes and the measured specimen should be reached while measuring the temperature, and therefore this confines the response of the OFTs. Noninvasive methods are not bound by these constraints. In addition, noninvasive instrumentation can be useful in determining the temperature of moving components without any additional telemetry systems. Both temperature measurements at a point and the variation over a region, by scanning, can be made. Most noninvasive techniques measure temperature from the electromagnetic spectrum emitted by the measured target, so are in fact radiometers. Infrared devices are sensitive to that part of the spectrum, and infrared thermometers are the most popular non-contact temperature measurement instruments.
Infrared thermometry is based on the same theoretical background as the blackbody OFT, that is, the blackbody emission law. But the blackbody cavity is not used in the infrared thermometry. The detected emission comes directly from the surfaces of those measured targets. There is not a blackbody cavity at the end of the IR OFT, and the end is placed away from the measured target. So, no high-temperature fibers are needed. However, in comparing with the blackbody OFT, the effects of emission and path factors and environment on the measurements are greater, and the accuracy is lower. With the rapid development of the electronic data processing technology and the measurement methods, the performance of the infrared thermometers is greatly improved.
Fluorescence-based OFT is based on the temperature-dependent fluorescence decay time or fluorescence intensity of the appropriate materials. Figure
Most of the earliest fluorescence OFT systems are based on the fluorescence intensity of materials. An early commercial system utilized europium-activated lanthanum and gadolinium oxysulphide as alternate sensor materials [
Although a number of intensity-based fluorescence thermometry systems are available, it can be seen that the technique has certain limitations in performance and cost [
Two main methods are used in the measurement of fluorescence decay time, namely the “pulse method” and the “phase detection method.” In the first method, the sample is excited by short pulse of light and the resulting emission in the longer wavelength is an exponentially decaying function whose rate of decay can be measured. In the second method, the sample is excited with sinusoidally modulated light, which results in a sinusoidal fluorescent emission that lags in phase with the original excitation sinusoidal light. This phase shift gives an indication of the decay time. The major advantage of such decay-time measurement techniques is that they obviate the need for accurate measurements of the input light intensity for reference purposes, a very important consideration in fiber-optical sensors.
The common feature of the schemes in this method is that the excitation light applied to the fluorescence material is a high intensity
This is a very straightforward method which was used by several groups in the early stage of the development of fluorescence OFT systems [
The principle of two-point time constant measurement technique.
This type of method is simple and inexpensive in relation to the electronic components used. Since the fluorescence signal is measured after the excitation pulse is over, the detector optics do not have to be designed to discriminate strongly against stray signal from the excitation source. However, a significant disadvantage of this method is that the signal is only measured at two special times, and as a result, the precision is greatly limited.
The two-point measurement technique was used in the system reported by Wickersheim and Sun [
To achieve higher precision from the pulse measurement approach, several techniques have been developed based on the integration of the decaying fluorescence signal over different periods of time. One example of this method is the signal-processing scheme used by Sholes and Small [
Another example is the balanced integration method described by Sun [
The first integration is carried out over a fixed time interval between the predetermined time
Another modular system, WTS-11, introduced by Luxtron company, was designed to monitor winding temperature in power transformer [
In this system, the excitation spectrum of the fluorescence material allows the use of a convenient excitation source, for example, red LED or laser diode. When the sensor is excited by sequential light pulses from an LED or laser diode, a periodic decaying luminescent signal is resulted. A selected portion of each decay curve is digitalized, after the detected signal has passed through a low noise and wide bandwidth amplifier. The digital samples, after correction for any offset, are then processed by the DSP to provide the best exponential decay curve by means of the least squares curve fitting technique. The exponential is first converted to a straight line by taking the natural logarithm of the digitised signal. The slope of the best fit straight line is proportional to the lifetime of the luminescence. The results of a number of curve fits are averaged further to reduce the effect of noise. The average lifetime is then compared with the values stored in a digital look-up table to determine the temperature of the sensor.
As mentioned earlier, the fluorescence sample is excited with sinusoidally modulated light, which results in a sinusoidal fluorescent emission that lags in phase with the original excitation sinusoidal light. It can be shown that the phase difference between the input light and the excited fluorescence optical signals is
During the early stage of development, the lack of convenient and economic excitation modulation schemes limited the use of the phase shift technique in fluorescence thermometry. Now, with the wide availability of cheap and easily modulated high-power LEDs or laser diode, this technique has found its wide application in the thermometry area.
A signal-processing scheme, phase-locked detection of fluorescence lifetime [
Phase-lock detection system for fluorescence lifetime using single reference signal. (FID: the fluorescence inducing and detecting devices,
A fluorescence lifetime measurement system recently developed by Grattan et al. [
The temperature probe of this system is a single-crystalline sapphire fiber with Cr3+-doped (ruby) tip, as shown in Figure
A schematic diagram of the fluorescence lifetime measurement system, with an enlarged view of the fluorescent fibre optic probe (from [
As mentioned before, the fluorescence intensity is weak at high temperatures owing to the quenching effect, and meanwhile the background of blackbody radiation becomes stronger. Then the SNR is poor, which limits the high-temperature application of this scheme. The blackbody sensors are generally used in high-temperature applications. To enlarge the measuring range, a complex OFT scheme [
Here, an example of the cross-referencing OFT, which was developed by Shen et al. [
A systematic diagram of a cross-referencing OFT (from [
The fundamental differences between an optical fiber and a metal wire for signal transmission give OFTs the following advantages [
The materials in the OFT probes are typically good electrical insulators. Since they do not conduct electricity, the probes cannot (in principle) introduce electrical shorting path or cause electrical safety problems. Likewise, they do not absorb significant amount of electromagnetic radiation or become heated by such fields. In addition, stray fields cannot induce electrical noise in fibers, so the probes exhibit a very high level of electromagnetic immunity.
Since the typical sensor is as small as in diameter with the fiber itself, the sensor, in principle, can be extremely small. This allows its use in applications such as in medicine or in microelectronics where size is critical, and allows a more accurate temperature distribution. Furthermore, since small size means small thermal mass, the fiber optic sensors typically exhibit a very rapid thermal response. Blackbody OFTs of kHz frequency response have been achieved [
Safety may be the main reason for the use of OFTs in some particular areas of application. Most OFTs require no electric power at the sensor end of the system. They generate their own optical signal or they are “powered” remotely by radiation from a light source located within the OFT instrument, therefore introducing no danger of electrical sparks in hazardous environments. There is a reason to believe that at normal levels of optical power coupled into fiber optics, that is, levels of up to several hundred million-watts of optical power, there is almost no hazard with any accidental fracture of cable and the possible focusing of the optical radiation by the lensing effects of the broken end. Particularly in the chemical industry, where highly explosive gases or gas mixtures may be used, this is an important consideration, but on the whole in normal use, optical sensing systems can be considered intrinsically safe.
The small size of fiber and its electrical, chemical and thermal inertness allow for long-term location of the sensor deep inside complex equipment and thereby provide access to locations which are difficulty to address, where monitoring the temperature may be of interest. Beyond this, some OFTs allow non-contact or remote sensing of the temperature, and this is easily accomplished with infrared thermometers.
Besides these generic advantages, some of the optical techniques exhibit an unusually wide range of operation with precision good enough to meet reasonable requirements. At the same time, these techniques provide simplicity of calibration, or as in the fluorescence lifetime-based OFT, the absence of the need to calibrate individual probes.
As mentioned above, optical fiber thermometers have many advantages over conventional thermometers. However, in selecting a measurement method and the associated instrument among the various types of OFTs to suit a particular application, it is still necessary to make some considerations such as measurement range, sensitivity, accuracy, response, service-life, stability, contact method, and cost. A comparison among the different OFTs is presented in the following.
Among the different OFTs, the blackbody OFT is most suitable for high-temperature measurement (up to
For non-contact IR thermometers, they are based on the same measurement principle as the blackbody OFTs, and so are better used in high-temperature measurement. Although the blackbody-radiation-based thermometer can measure lower temperatures up to room temperature, the accuracy and sensitivity are generally low. To raise the accuracy and sensitivity for low-temperature measurement, some special photodetectors, complicated signal, and data processing systems are required. This would make the systems expensive and slow in response. IR thermometers are suitable for qualitative measurement due to its low accuracy. In addition, the accuracy of IR thermometers has to be ensured by detailed knowledge of the detected targets. This makes the calibration difficult in measuring the temperatures of different detected targets.
Since there are no thermal probes in non-contact IR thermometers, thermal-equilibrium processing between the probes and the measured specimen is not needed. The response speed is not confined by the properties of the thermal probe. Therefore, non-contact IR thermometers have faster response and are suitable for quick and qualitative measurements. However, the contact OFTs also exhibit quite rapid thermal response due to the small size (in the order of micrometer in diameter) of the fiber optic probes. Except for the size of the probe (or the blackbody cavity), the heat conduction between the external and inner sapphire parts has also a considerable effect on the response time of OFTs. Therefore, in the construction of the probe, the size and heat conduction of the probe should be considered seriously.
For blackbody-radiation-based OFT, the impact of the variance in the ambient temperature, emissivity and sight path factors would induce some instability in measurement, and a real-time calibration is required. For high temperature, this unstable impact is very small, but for lower temperature, it becomes considerable. Unlike blackbody radiation thermometer, the measurement for lifetime-based fluorescence OFT is free of any impact of the variance in ambient temperature, emissivity and sight path factors. Thus, generally, the lifetime-based fluorescence OFT provides a very stable measurement.
Both blackbody and fluorescence OFTs have a long service-life because their probes are coated with some protective films. Generally, the radiation and thermal properties of the blackbody cavity can be kept unchanged for a long time at high temperature. However, the fluorescent properties of OFT sensor would have some changes when it stays in high temperature for a long time, although this can be improved through some heat treatments. The non-contact IR OFT should have a longer service-life than the above two, because there is no thermal probe that contacts with the measured objects. But the measurement by non-contact IR OFT is of lower accuracy and unstable due to its strong impact of ambient temperature, emissivity and sight path factors.
In summary, in comparison with traditional thermometers, OFTs have the advantages of electromagnetic immunity, high sensitivity, small probe size and quick response, long-term stability, safety, and they can sustain under harsh environmental conditions. Among the different OFTs described above, blackbody OFTs are most suitable for high-temperature applications, fluorescence OFTs for lower-temperature measurements, and non-contact IR thermometers for the application in quick and qualitative measurements.
Phase lag of the period
Wavelength (m)
Narrow wave-band (m)
Peak-emission wavelength given by Wien’s displacement law (
Sight path factor
Emissivity
Stefan-Boltzman constant = 5.6687
Fluorescence life-time (s)
Phase difference between the input light and the excited fluorescence
Frequency of reference signal from VCO in Figure
Integration of fluorescence signal over fixed periods in (
Integration of fluorescence signal over fixed periods in (
First radiation constant = 1.4388
Second radiation constant = 1.43879
Base of the natural logarithms = 2.7321
Frequency of fluorescence
Emission intensity of fluorescence
First value of decaying signal defined in Figure
Area of the observed surface (m2)
Time (s)
Selected time in Figure
Fixed periods
A preset time at which the fluorescence decays to zero
Temperature (K)
Reference temperature
Period
Signal to modulate the output intensity of the excitation light source
Fluorescence signal
The reference signal
Radiation power (W
Total emission power (W
Gain of the lifetime-to-modulation signal.
The work is supported by The Hong Kong Polytechnic University under the postdoctoral fellowship scheme (G-YW68).