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6 Fibre Optic Thermometers 6 .1 Properties of Optical Fibres Dielectric optical fibre, which is also referred to as a light guide, is composed of a rod core surrounded by a sheath as shown in Figure 6 .1 . The core, which conducts the electromagnetic wave, has a refractive index, n j , while the sheath, which contains the wave within the core, has a refractive index, n2 . To ensure that the wave is contained in the core it is necessary that the condition n j > n2 is satisfied . The electromagnetic waves incident on the front part of the optical fibre, within the angle cone, 2a, corresponding to the critical angle, 9 c , . , enter the fibre core . After multiple reflections from the core-sheath interface these waves leave the other end of the fibre . The waves from outside the 2a-cone leave the sheath to environment . Smolinski (1985) has shown that the value of critical angle, Bcr, , and of a are given by : cose cr = n2 / nj  (6 .1) sin a =  ni - n2  (6 .2) where n j is the refractive index of the core material and n 2 is the refractive index of the sheath material . In addition to the condition n 1 / n 2 > 0, optical fibre materials, whose core material has a low absorption coefficient at the transmitted wavelength, should be chosen . Optical fibres are used for wavelengths in the range from ultraviolet through visible up to infrared radiation . The most commonly used optical fibres have a core and sheath made of quartz i CORE  SHEATH  PROTECTIVE TUBE Figure 6 .1 Propagation of electromagnetic waves in optical fibre Temperature Measurement Second Edition L. Michalski, K. Eckersdorf, J. Kucharski, J. McGhee Copyright © 2001 John Wiley & Sons Ltd ISBNs: 0-471-86779-9 (Hardback); 0-470-84613-5 (Electronic) 126  FIBRE OPTIC THERMOMETERS glass with different chemical composition . Usually, sheath materials are made entirely of plastics . High temperature optical fibres have cores made of quartz and sapphire . Some optical fibres are covered by layers which reduce optical losses and provide good mechanical strength for protective purposes . In Table 6 .1, which gives parameters of some optical fibres, it can be seen that quartz optical fibres have the lowest absorption coefficient and thus the lowest transmission losses at the wavelengths 0 .85 gm, 1 .3 lm and 1 .55 l .tm . 6 .2  Classification of Fibre Optic Thermometers It is important to examine the grouping of fibre optic thermometers . McGhee and Henderson (1989) give a general classification of optical fibre sensors based upon previous groupings given by Culshaw (1982), Pitt et al . (1985) and Medlock (1986 ; 1987) . Further classification of optical fibre sensors is given byNing et al . (1991) . The general classification of optical fibre sensors provides the foundation for the grouping of optical fibre thermometers . From Grattan (1995) and Wickersheim (1992), it can be seen that there are two groups of fibre optic thermometers as shown in Figure 6 .2 . " Extrinsic sensing, also termed, indirect use thermometers, which are more popular, use the optical fibre for transmission of the electromagnetic waves between the sensor and the electronic temperature indicating system . In these sensing systems the light itself is modulated bysome process outside the optical transmission system . " Intrinsic sensing, also referred to as direct use thermometers, where the optical fibre itself is used as the temperature sensor, also exhibit the characteristics of modulating sensors described in Chapter 1 . In these sensors a light transmitting property of the optical fibre is modulated by the temperature . They indicate either the average temperature of the fibre or the temperature distribution along its length . Optical fibres are also applied in the construction of pyrometers as described in Chapter 10 . Table 6 .1 Types of optical fibre Fibre type  Properties  Core  Sheath Quartz  Material  Quartz glass Si0 2 doped  Quartz glass Refractive index  n j = 1 .48  n 2 = 1 .46 Diameter  50 to 300 pm  12 .5 to 400 pm Quartz  Material  Quartz glass  Polymers Refractive index  n, = 1 .46  n 2 = 1 .40 Diameter  200 to 1500 Itm  300 to 2000 pm Plastic  Material  Polymer e .g . polymethyl-methacrylate  Polymers Refractive index  n j = 1 .49  n 2 = 1 .40 Diameter  300 to 1500 Itm  500 to 2000 pm EXTRINSIC SENSING THERMOMETERS  127 FIBER OPTIC THERMOMETERS EXTRINSIC SENSORS  INTRINSIC SENSORS GaAs sensor absorption type  Raman scattering type thermochromic reflection type  change of refractive index fluorescent  optical coupling of two fibres L black body sensor thermal radiation type "Fabry-Perot" sensor optical interference type Figure 6 .2 Classification of fibre optic thermometers 6 .3  Extrinsic Sensing Thermometers 6 .3 .1  Thermometers with GaAs semiconductor sensors The operating principle of these thermometers, whose structure is given in Figure 6 .3, is based on the dependence of the absorption coefficient, A, of a semiconductor material upon its temperature and upon the frequency of the incident radiation . Samulski (1992) has shown that in the case of gallium arsenide (GaAs) this dependence is empirically given by : A = A o exp[a(h v - E g ) l kT ]  (6 .3) (b) 1,0 P 2s -__  ___ (a)  LED  GaAs SENSOR  z 0'8 ,  II r ~-~ TRANSMITTING OPTICAL FIBRE  L  i NwA  "' 0,6 P4u _ _- __ I  L~  N  40 C  I I  ~  a  25°C  I lED EMITTING I  Nyyh  a 0,4- I I LIGHT RECEIVING OPTICAL FIBRE  . .,  i L I PHOTODIODE DETECTOR  I 0 .2  ~ l w  1 l` 0 0 .88 0,90 0,92 WAVELENGTH 5,, Nm Figure 6 .3 A fibre optic thermometer with the basic operating structure of (a) uses a GaAs semiconductor sensor with the relative radiation transmitted through a 0 .25 mm thick prism in (b) 128  FIBRE OPTIC THERMOMETERS where A o is a constant, a is a material constant, h is the Planck constant, v is the frequency of incident radiation, E g is the energy gap, k is Boltzmann constant and T is the temperature . As the energy, h v , approaches E g , for a chosen wavelength, equation (6 .3) shows that the variation in absorption coefficient, A = f(T), really exhibits a very steep dependence . In this thermometer, a narrow-band LED light source, as shown in Figure 6 .3(a), emits the light along an input light guide to a GaAs prism sensor (Samulski, 1992) . After passing the prism the radiation returns by an output light guide to the diode photo-detector . The GaAs detecting prism exhibits the phenomenon of a shift of the radiation band border . Thus, the resulting output signal depends upon the thermally dependent optical transmission of the GaAs prism as shown in Figure 6 .3(b) . Fibre optic thermometers with GaAs semiconductor sensors are used in the temperature range below 50 °C usually in medicine and biology . 6 .3 .2  Thermochromic thermometers These thermometers are based on the temperature dependence of the reflection factor of liquid crystals . The reflection factor also depends upon the frequency of the incident visible radiation . Since they appear to change colour with temperature, liquid crystals are called thermochromic materials . In a thermometer described by Brenci et al (1984) and Grattan (1987), a thermochromic material consisting of a cobalt chloride solution in water/alcohol is used . This substance, which shows intensive colour change between 25 °C and 75 °C, has a spectral absorption with temperature given in Figure 6 .4(b) . The system design shown in Figure 6 .4(a) uses a halogen lamp whose light is chopped and transmitted by a silica optical fibre to a temperature sensor . The sensor is a small probe containing the thermochromic solution . Incoming light is reflected from the bottom of the mirrored probe to another optical fibre where it is split into two light beams at At = 0 .655 pm and Az = 0 .800 pm . As can be seen in Figure 6 .4(b) the light beam at Az = 0 .800 um, which is not absorbed by the cobalt chloride solution, serves as a reference value while the light absorption at At = 0 .655 pm is temperature dependent . Both signals are re-detected by photodiodes, (b! (a)  TRANSMITTING OPTICAL  CHOPPER  HALOGEN  75 °C 1_ FIBRE  LAMP FILTER 0,655pm OPTICAL  D  a  60 °C COUPLER ~,  U  o a SO°C RECEIVING  D  MICROPROCESSOR a FIBRE  /  PHOTODIODES  a  ~~ 40° 'SENSOR  FILTER O,BOOym  ~5° =5°C 0,4 0,5 0,6 0 .7 O,8 WAVELENGTH a, jim Figure 6 .4 A tbermochromic optical fibre thermometer with the basic block diagram of (a) uses the spectral absorption of cobalt chloride solution in water-alcohol at different temperatures in (b) EXTRINSIC SENSING THERMOMETERS  129 amplified and ac/dc converted . Subsequently their ratio, which is formed in a microprocessor, is a measure of temperature . The readings are independent of source intensity fluctuations and of transmission losses because both signals are transmitted along one common optical path . Thermochromic thermometers are applied in the temperature range from 25°C to 75 °C in medicine and biology, which is considered in Chapter 21 . 6 .3 .3  Fluorescent thermometers In this type of thermometer a fluorescent material, which is placed at the end of the light guide, is excited by radiation of a given wavelength . The external excitation, which is essentially an interrogative stimulation, causes the sensor to fluoresce at a different wavelength from the exciting light source . The fluorescent radiation and the exciting radiation are subjected to monochromatic filtering, which allows easy separation of the radiation from the two sources . First generation fluorescent thermometers were based on temperature dependence of monochromatic repartition of emitted radiation : E e,~ - .f(T)  (6 .4) e, where E e, A  and Ee,, ;,2 are radiation intensities respectively at wavelength ~ l and X z and T is temperature . The operating principle of a fluorescent thermometer offered by ABB (Sweden) is shown in Figure 6 .5 (Grattan, 1987) . The GaAs sensor, which is placed at the end of the light guide, is excited by a LED modulated light when it emits fluorescent radiation . The combined light from the visible fluorescent light emitted by the sensor and the exciting light is transmitted by the same light guide to an optical bifurcator which splits the light into two beams . Each beam then passes through a filter . The output signal of each filter, which is proportional to the respective intensities, E e, A  and E, , ~ 2 , are applied to detecting CHOPPER LED I I U I I  I I I  D I D I I L -  _  _ J OPTICAL PHOTODIODES DIVIDER FILTERS  SENSOR GaAs OPTICAL FIBRE Figure 6 .5 Fluorescent optical fibre thermometer 130  FIBRE OPTIC THERMOMETERS photodiodes before being amplified . Taking the ratio gives a signal which is a function of the measured temperature . The readings are independent of the exciting radiation intensity, nevertheless they may be influenced by bending of the light guide (Grattan, 1987) . Second generation fluorescent thermometers are based on the decay-time concept of a periodically excited probe . Mei Sun (1992) and Samulski (1992) state that the emitted radiation intensity as a function of time E e /t) is given by : E e (t) = E e (0)exp(-t /'Z)  (6 .5) where E e (0) is the radiation intensity at the instant the exciting impulse stops, z is a so called decay time and t is time . The decay time is usually defined, as shown in Figure 6 .6, as the time difference t 2 - t l such that Ee(t2) = E e (t t ) / e , where e is the base of the natural logarithms . Grattan and Zhang (1995) provide a survey of other measurement methods of fluorescent lifetime . Figure 6 .7(a) shows decay time, z, as a function of temperature, S, of magnesium fluoro- germanate activated with tetravalent manganese . The wavelength bands of exciting and j I  EXCITING IMPULSE 1 E Q (O) - I z Eelt'1  E e (t 1=E, (0) exp (-t 1e) I 1 z  II  I a E . (t,) -i -_ t ._ _ 0 1  I 0  t,  tz TIME t Figure 6 .6 Impulse excited radiation intensity of a fluorescent material as a function of time (6) (a)  EXCITATION - _ EMISSION _ REGION  ~ REGION tr S  z_ 0,9 t W z 4  20,6- 3  t a >0 1 v  ~0"4  \ : 1 0 2   , >  i 1 0 .2 w 0 0 -200 -100 0  100 200 300 400  200 300 400 S00 600 700 TEMPERATURE 4 . °C  WAVELENGTH 7, , mm Figure 6 .7 Fluorescent properties of magnesium fluoro-germanate activated with tetravalent Mn . (a) decay time, r as a function of temperature, S, . (b) relative intensity of exciting and emitted radiation as a function of wavelength, A EXTRINSIC SENSING THERMOMETERS  131 emitted radiation shown in Figure 6 .7(b) are quoted by Mei Sun (1992) and Ballico (1997) . Properties of different materials used for fluorescent temperature sensors in the temperature range from -190 °C to 1300 °C are given by Fernicola and Galleano (1997) and Grattan and Zhang (1995) . Fernicola and Galleano (1997) and Zhang et al (1997) state that red and blue laser light are used as exciting radiation . An example of a second generation fluorescent thermometer is shown in Figure 6 .8 . In this Model 3000 thermometer by Luxtron, the sensor is excited periodically by the microsecond long pulses of a xenon flash lamp (Samulski, 1992) . The radiation emitted by the sensor is conveyed by a light guide and optical system to the photodiode, whose output signal corresponds to decaying radiation intensity . After the transformation, this signal is a measure of decay time, z and thus also of measured temperature . Neither variations of radiation intensity nor bending of the light guide influence the readings . Figure 6 .9 illustrates an interesting construction of a fluorescent thermometer for use in aviation as described by Phillips and Tilstra (1992) . When the sensor is excited by a LED radiation of wavelength 0 .65 to 0.69 lun it subsequently emits radiation in the wavelength range 0 .7 to 0.98 pm transmitted to the photodiode . The temperature measure is decay time, r, of the photodiode signal . The thermometer is used in the temperature range from -75 °C to 300°C and is intended for temperature measurement of rapidly flowing gas with the sensor structure shown in Figure 6 .9(b) . The sensor cavity, which slows down the gas flow, is shaped so that the sensor temperature is as close as possible to that of the gas . Further details of temperature measurement of rapidly flowing gas are discussed in Chapter 17 . An interesting application of the phenomenon of fluorescence is an arrangement =A TO OTHER CHANNELS = 3PUMP FIBER FLAS TRANSFORMATION PHOTODIODE BEAM SYSTEM  SPUTTER SENSOR  OPTICAL FIBRE Figure 6 .8 Optoelectronic arrangement of a fluorescent thermometer by Luxtron, USA . (Samulski, 1992) (bl la! LED  FLUORESCENT CAVITY MATERIAL  E- . GAS SENSOR SHEATH d=1, 6 mm OPTICAL FIBRE  OPTICAL OPTICAL COUPLER  FIBRE SIGNAL PHOTODIODE PROCESSOR Figure 6 .9 Optoelectronic arrangement of a fluorescent thermometer TRD (Phillips and Tilstra, 1992) in (a) has the sensor arrangement in (b) 132  FIBRE OPTIC THERMOMETERS to measure the surface temperature of a turbo-generator rotor in a Canadian 540 MW hydro- electric power station (Mannik and Brown, 1992) described in Chapter 19 . Second generation fluorescent thermometers, which are the most popular of all fibre optic thermometers, give high measurement precision and do not need any periodical calibration . The fundamental principles of these thermometers with all the important aspects of the subject are given by Grattan and Zhang (1995) . Their typical applications are concerned with temperature measurement : "  in medicine and biology, which is considered in more depth in Chapter 21, "  in high voltage appliances (Wickersheim, 1991), "  of rotating bodies, which is discussed in Chapter 9, "  of microwave and dielectric heated bodies, "  in chemical and physical research . 6 .3 .4  Thermometers with black body sensors The operating principle of these thermometers is based on the temperature dependence of spectral radiant intensity emitted by a black body, as given by the Planck's law in equation (8 .7) : Cl ~ -s Wok - ec 2 /,,T _ 1 where cl =3 .7415xl0-16 Win 2, C2 =14 388 pm K, A is the wavelength in pm and T is the temperature in K . The basic diagram of a fibre optic thermometer with black body sensor is shown in Figure 6 .10 (Adams, 1992) . At the end of the high-temperature optical fibre, the cavity, which is covered by a noble metal layer, makes a sensor exhibiting the properties of a black body . Its radiation is sent by a high-temperature fibre optic light guide, with a low- temperature extension to a photodiode . The measured temperature is obtained from the photodiode output signal . The high-temperature light guide, which is made of mono- crystalline sapphire operating up to 2000°C (Grattan and Zhang, 1995), is resistant to the majority of oxidising media . The low-temperature light guide is made of quartz with a HIGH TEMPERATUREE LOW TEMPERATURE  TO SIGNAL PROCESSOR OPTICAL FIBRE  OPTICAL FIBRE "BLACK BODY" SENSOR OPTICAL COUPLER CAVITY  CERAMIC PROTECTIVE FILM LENS / PHOTODIODE NARROW BAND FILTER THIN FILM METAL COATING Figure 6 .10 Fibre optic thermometer with a `black body' sensor EXTRINSIC SENSING THERMOMETERS  133 polymer sheath . Depending on the application range of the thermometer, the noble metals platinum, rhodium or iridium are used for the black body sensor cavity . This thermometer which is used in the range from 300 to 1900 °C, has an indication error below ±0 .2 % at a level of 1000 °C and a resolution up to 0 .01 °C . The thermometer sensor follows temperature variations up to the frequency of 10 kHz . Adams (1992) describes the applications of this small diameter thermometer sensor, which is immune to the presence of electromagnetic fields, has a large temperature range, high precision and resolution . Application examples are quoted as : " semiconductor production, "  ceramic products sintering,  - "  hard soldering, "  plasma etching, "  deposition of diamond film . 6 .3 .5  Thermometers with Fabry-Perot sensors The temperature dependent spectral reflection coefficient of a thin mono-crystalline Si film provides the operating principle of this thermometer . The arrangement of the thermometer using a LED radiation source is shown in Figure 6 .11 (Saaski and Hartl,1992) . At the end of the optical fibre, a thin Si monocrystal layer is tightly enclosed between two glass layers . The radiation of a LED is transmitted to thin film Si sensor, where it is reflected with a reflection coefficient depending on the wavelength . The reflected radiation is transmitted by the same optical fibre to a semi-transparent mirror splitting the incoming radiation into two radiation beams of wavelengths A1 and AZ . The temperature of the thin film sensor is related to the ratio of the radiation intensity at these two wavelengths, which is directly proportional to the resulting ratio of the two electrical signals from the respective photodetectors . These thermometers, which are immune to electromagnetic fields, exhibit high resolution and operate in the temperature range from 0 to 400°C . Saaski and Hard (1992) report the applications of, Fabry-Perot thermometers as : " cross-linking of polymer composites, " microwave drying and food processing, "  research on thermal effects of induced electromagnetic fields in aircraft . }  OPTICAL COUPLER SPECTRAL  LED  OPTICAL FIBRE SIGNAL MIRROR PROCESSOR  LJ a  'FABRY-PEROT° SENSOR ' a2 LENS  c  LIGHT SHIELD GLASS Si MONOCRYSTAL PHOTODETECTORS PEDESTAL Figure 6 .11 Fibre optic thermometer with "Fabry-Perot" sensor 134  FIBRE OPTIC THERMOMETERS 6 .3 .6  Optical coupling thermometers A thermometer, based on extrinsic optical coupling of two light guides, is shown in Figure 6 .12 . Two sheathed optical fibres, which are aligned in parallel, have their sheaths partially removed at the measuring point . The measuring part is immersed in the liquid, which makes one common sheath around the two bared cores . If the liquid refractive index, n 2 , is smaller than that of the core, n 1 , no coupling between the two optical fibres occurs . As the temperature changes, the refractive index, n 2 , becomes larger than n l , so initiating the optical coupling, which allows transmission of the signal from the light source to the detector . It can be seen that this thermometer is especially suited to temperature limit detection, thus approaching the idea of temperature indicators . 6 .4  Intrinsic Sensing Thermometers 6 .4 .1  Raman scattering thermometers Optical fibres may be applied for the direct measurement of the temperature distribution or average temperature along its length . When a light pulse is propagatedthrough an optical fibre, temperature dependent Raman scattering of the light intensity components occurs . As a result the Raman scattering of two components of a back-scattering light pulse, of wavelengths different from that of the incident light, are observed . These are called, Stokes light and Anti-Stokes light . Also a third component called Rayleigh scattering light is observed as given in Figure 6 .13 . Iida et al (1992) describe how back-scattering Stokes and Anti-Stokes lights are used in the measuringarrangement on their return to the incident light point of the optical fibre . The ratio of their intensities is a function of temperature, T, of that part of the optical fibre at which the scattering occurs . The path length covered by the input pulses in the optical fibre up to the point of scattering, is measured by a reflectometcr . The block diagram of a fibre optic thermometerbased on Raman scattering, for temperature distribution measurement, is shown in Figure 6 .14 (Iida et al, 1992) . The thermometers based on Raman scattering are usually designed for a certain well specified problem . Their main application range is to measure the temperature distribution and the average temperatures on large surfaces and along long objects like pipe-lines (Sandberg and Haile, 1987) . FROM LIGHT  TO DETECTOR SOURCE FIBRE SHEATH SENSING  FIBRE CORE PART _  LIQUID Figure 6 .12 Optical fibres with variable optical coupling