Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
  • G01K11/00—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00
  • G01K11/32—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in transmittance, scattering or luminescence in optical fibres
  • G01K11/3206—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in transmittance, scattering or luminescence in optical fibres at discrete locations in the fibre, e.g. using Bragg scattering
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
  • G01K11/00—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00
  • G01K11/12—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in colour, translucency or reflectance
  • G01K11/18—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in colour, translucency or reflectance of materials which change translucency

Definitions

• the invention relates to birefringent optical temperature sensors.
• Emo et al. describe an optical high temperature sensor based on a birefringent element made of a single crystal. A broad band light spe ⁇ rum is transmitted through a first linear polarizer creating a linearly polarized wave The linearly polarized wave passes through a single crystal birefringent plate at 45° to the opticai axis of the crystal.
• the polarized wave can be represented by two equal linear polarized vectors which are aligned along the optical axes. Propagation of these waves through the birefringc.u ⁇ laie introduces a temperature dependent phase shift between the two waves Thereafter, a second linear polarizer combines the two waves creating a modulated spe ⁇ rum. Information derived from this modulated spe ⁇ rum or fringe pattern is then used to measure the temperature of the bire ⁇ -ngent plate.
• the deficienc y of this device is that the temperature sensitivity of the birefringent material is fixed by the constraints of the physical constants involving refractive index and the expansion ofa single crystal birefringent element. Furthermore, the resolution is limited by the parameters of the detection system. Accordingly, it would be desirable that an optical temperature sensor has the capability of accurately measuring environmental temperatures with sensitivities greater than currently available sensor systems.
• the sensor consists of two or more single birefringent crystal elements in tandem and the total birefringence length produ ⁇ remains within the accepted tolerances of current devices.
• Each crystal element has a birefringence (B), a dB/dT and a coefficient of thermal expansion ( ⁇ ) term such that when the crystal are arranged in tandem the combined birefringence terms equal the required birefringence and the dB/dT terms equal the required temperature sensitivity.
• a broad band light source is transmitted via a first fiber optic cable, a collimator and a first polarizer to the birefringent crystals.
• the birefringent crystals transmit a wavelength polarization component of the light.
• a focusing element collects the light and transmits it via a second fiber optic cable to an opto- ele ⁇ ronic interface where an intensity vs. wavelength (fringe) pattern is extracted by a CPU.
• the CPU performs a Fourier transform on the fringe pattern, and the phase term of the sele ⁇ ed frequency relates to the environmental temperature of the crystals.
• FIGURE 1 is a schematic of a preferred embodiment of the invention
• FIGURE la is a schematic of an alternate embodiment of the invention.
• FIGURE 2 is a schematic exemplifying the concept of birefringence of a linearly polarized wave
• FIGURE 3 is the amplitude frequency waveform ofa broad band light source useful in the practice of the invention
• FIGURE 4 is an intensity vs. wavelength waveform of a modulated light spe ⁇ rum generated by an opto-electronic interface
• FIGURE 5 is a Fourier transform of the waveform of Fig. 4 at a sele ⁇ ed frequency
• FIGURE 6 is a graphical representation illustrating the increased sensitivity of a tandem birefringent optical temperature sensor.
• Fig. 1 illustrates a temperature sensing system 18 utilizing a temperature sensor 20 that comprises at least two birefringent crystal elements arranged in tandem.
• sensor 20 comprises of two birefringent elements 30 and 32, however, any pra ⁇ ical number of birefringent elements may be employed
• System 18 utilizes a broad band light source 40 as may be generated by a plurality of LEDs having an exemplary waveform illustrated in Fig. 3.
• the broad band light source 40 is randomly polarized and is focused by lens 42 into a multi-mode optical fiber 44.
• the light output of fiber 44 is collimated by lens 46. such as a -
• Polarizer 48 is aligned so that it transmits the linearly polarized light at 45° to the optical axis of the birefringent crystal elements 30 and 32.
• crystals are anisotropic with respect to their physical properties, that is, their property values vary with the direction in the crystal.
• Anisotropy of the refractive index is called birefringence and is defined as n ⁇ -n- where n € is the extra-ordinary index of refraction and n 0 is the ordinary index of refra ⁇ ion.
• Uniaxial crystals can be categorized as positive or negative depending on whether the ⁇ i ⁇ term is larger or smaller than n 0 .
• Explemplary uniaxial crystals are sapphire, magnesium fluoride and crystalline quartz.
• the terms n e and n ⁇ , are not used for biaxial crystals that have 3 separate refractive indices.
• Examples of biaxial crystals are c-centered monoclinic crystals defined as space group C 6 2h - C2/c and exemplified by Lanthanum Beryllate or Berylluim Lanthanate (La 2 Be 2 O 5 or "BeL ”) as referenced by H. Harris and H.L.
• biaxial crystals include alexandrite and yittrium aluminum pervoskite ("YAP").
• YAP yittrium aluminum pervoskite
• terms such as n ⁇ nj,, and ti ⁇ can be used and any 2 such terms and their respe ⁇ ive temperature dependent birefringent terms can be substituted giving a total of 3 separate cases for this class of crystals.
• Fig. 3 illustrates the principles of birefringence.
• Two orthogonally polarized waves 144 and 146 enter and propagate through a birefringent element 150.
• the electric polarization vectors of these two waves are oriented in the X and Z dire ⁇ ions, and the waves propagate in the Y dire ⁇ ion.
• the linearly polarized wave On entering face 152, the linearly polarized wave, propagates through element 150 at different velocities due to different refractive indices in the x and z planes. Therefore, waves 144 and 146, which exhibited a zero phase difference before entering element 150, now exhibit a certain phase difference ⁇ on exiting face 154
• -5- phase difference depends on the difference in the indices of refraction, the path length, L, through the birefringent element 150, the temperature of crystal 150 and the wavelength of the broad band light source.
• an optical temperature sensing system 18 includes a sensor 20 that comprises a first birefringent crystal 30 in tandem with a second birefringent crystal 32.
• the total birefringence is such that
• ABS'fL Bi ⁇ L2'B 2 ] X' ⁇ [ 1]
• Coefficient X represents the approximate number of orders of the effe ⁇ ive waveplate. It is the canonical value used in available temperature sensing devices that describes the number of full cycle polarization rotations that the linearly polarized broad band light undergoes while traversing the crystal. Coefficient X is a fun ⁇ ion of the overall system design, including the wavelength and band of light source 40 and the opto-electronic interface 58 that has its own chara ⁇ eristic wavelength range and resolution.
• Desired system accuracy determines the amount of birefringent and the value of X
• Exemplary values of X may be in the range from about 20 to 60. It is possible to increase or decrease the birefringent in relation to changes of other system parameters and still maintain overall system accuracy.
• the crystal elements 30 and 32 are also sele ⁇ ed so that the respective L(dB/dT) terms add or subtra ⁇ to yield the desired temperature sensitivity Accordingly,
• Li and L 2 are the respective crystal lengths
• dB ⁇ /dT t and dB 2 /dT 2 are the respective birefringence change as a function of temperature and ⁇ .
• ⁇ 2 are the respective thermal expansion coefficients ofthe crystals.
• dB./dTi and dB 2 /dT 2 are defined as di-e/dT - div/dT for consistency ofthe respective materials. Equations [1] and [2] can be solved for Li and L 2 for two given crystal materials and knowing their respe ⁇ ive orientations.
• the linearly polarized light passes through crystal elements 30 a-id 32 whose axes are aligned 45° to that of polarizer 48.
• the polarized light is decomposed into two orthogonal polarization states by the tandem birefringent crystal elements 30 and 32.
• the two orthogonal polarized light waves experience a temperature dependent phase shift propagating through crystals 30 and 32
• the output ofthe crystals is colle ⁇ ed by a second polarizer 52, commonly known as an analyzer, having the same or a 90° orientation to polarizer 48.
• Polarizer 52 combines the two orthogonal phases to form a modulated light spectrum.
• the light spe ⁇ rum is focused down a second fiber optic cable 56 by a second coUimating means 54.
• the output ofthe fiber optic cable is directed to an opto- ele ⁇ ronic interface 58, such as a spectrometer having a fiber optic input and a CCD array output.
• the light spectrum is focused onto an array of photodetectors or a charge coupled device (CCD) dete ⁇ or associated with conditioning ele ⁇ ronics which yields the intensity vs. time (intensity vs. wavelength) fringe pattern signal as shown in Fig 4
• a CPU 60 digitizes the signal and performs a Fourier transform on the signal, which resultant is shown in Fig. 5.
• the measured phase shift ofthe transformed signal is a direct representation ofthe environmental temperature of crystals 30 and 32 17US96/04555
• Fig. la is an alternate embodimet ofthe invention where an optical temperature sensing system 18a includes a sensor 20a that comprises a first birefringent crystal 30a in tandem with a second birefringent crystal 32a.
• the total birefringence is such that
• Coefficient X represents the approximate number of orders ofthe effe ⁇ ive waveplate. It is the canonical value used in available temperature sensing devices that describes the number of full cycle polarization rotations that the linearly polarized broad band light undergoes while double traversing the crystals. Coefficient X is a fiin ⁇ ion ofthe overall system design, including the wavelength and band of light source 40a and the opto ⁇ electronic interface 58a that has its own chara ⁇ eristic wavelength range and resolution.
• Desired system accuracy determines the amount of birefringent and the value of X.
• Exemplary values of X may be in the range from about 20 to 60 It is possible to increase or decrease the birefringent in relation to changes of other system parameters and still maintain overall system accuracy.
• the crystal elements 30a and 32a are also selected so that the respective L(dB/dT) terms add or subtract to yield the desired temperature sensitivity Accordingly,
• Li and L 2 are the respe ⁇ ive crystal lengths
• dB ⁇ /dT ⁇ and dB 2 /dT 2 are the birefringence change as a function of temperature and c-i and ⁇ are the respecti .
• «- thermal expansion coefficients of the crystals dB ⁇ /dT ⁇ and dB 2 /dT 2 are defined a . dnjdl - dno dT for consistency ofthe respective materials.
• Equations [la] and [2a] can be solved for Li and L 2 for two given crystal materials and knowing their respective orientations.
• the ⁇ terms are due to the natrue ofthe crystals involved and their respe ⁇ ive signs of birefringence and dB/dT terms.
• the orientation ofthe crystals with respe ⁇ to each other also determines the sign. Once the crystals and their respe ⁇ ive orientations have been sele ⁇ ed on the basis of equation [la], the same sign is used in equation [2a].
• the linearly polarized light passes through crystal elements 30a and 32a whose axes are aligned 45° to that of polarizer 48a.
• the polarized light is decomposed into two orthogonal polarization states by the tandem birefringent crystal elements 30a and 32a.
• the two orthogonal polarized light waves experience a temperature dependent phase shift propagating through crystals 30a and 32a.
• Crystal 32a has a high refle ⁇ ive coating 34a, or other common reflective means, such as a mirror, applied to the back of crystal 32a.
• Coating 34a causes the orthogonally polarized light waves to double-pass through crystals 30a and 32a which causes further temperature dependent phase shifting ofthe waves.
• Polarizer 48a combines the two orthogonal phases to form a modulated light spe ⁇ rum
• the light spe ⁇ rum is focused into fiber optic cable 44a by coUimating means 46a
• the Ught wave is spUt by a Y-splitter 59a which dire ⁇ s some pre-determined fraction ofthe Ught wave to an opto-ele ⁇ ronic interface 58a, such as a spe ⁇ rometer having a fiber optic input and a CCD array output.
• the light spe ⁇ rum is focused onto an array of photodete ⁇ ors or a charge coupled device (CCD) detector associated with conditioning ele ⁇ ronics which yields the intensity vs. time (intensity vs wavelength) fringe pattern signal as shown in Fig. 4.
• CCD charge coupled device
• a CPU 60a digitizes the signal and performs a Fourier transform on the signal, which resultant is shown in Fig. 5.
• the measured phase shift ofthe transformed signal is a dire ⁇ representation ofthe environmental temperature of crystals 30a and 32a.
• Advantages of this embodiment are that a single optical fiber is connected to sensor 20a and the length of crystal 30a and 32a are decreased by two as compared to sensor 20 (crystals 30 and 32) for the same amount of birefringence. This embodiment also provides for faster thermalization time and fewer components.
• a typical optical temperature sensor uses BeL as the sensing media. It has a birefringence of 0.0714 (tic - n,)and a dB/dT of -9.5x10 " V*C (di /dt - dru/dt) and a ⁇ of 8.0x10 "6 cm cm/°C.
• a second crystal element made from Yittrium Vanadate (YVO ) may be combined with the BeL crystal to increase the temperature sensitivity.
• YVO 4 has a birefringence of 0.2152 (n. - ru), a dB/dT of -6.68xlO "6 /°C and an ⁇ of 7.3x10 " * cm/cm/°C.
• the crystals are arranged with the resulting absolute value for the total birefringent length produ ⁇ dereasing compared to the largest birefringent length produ ⁇ ofa single BeL crystal. This indicates taht the signs of equations [1] and [2] are negative.
• Fig. 6 graphically illustrates the concept ofthe invention.
• the light source 40 used in a demonstration unit consists of a single LED package that contains three LEDs. This generates a wavelength spectrum as shown in Fig. 3
• the 10% end points are at 760 and 900 nm respectively; the pixel numbers associated with Fig. 3 are the CCD array element numbers.
• the opto-electronic interface 58 has a 256 element CCD array as the dete ⁇ ion system. Dispersion elements inside the unit have pixel number 1 at 748 nm and pixel number 256 at 960 nm. The entire LED spe ⁇ rum is therefore observed on the CCD array yielding intensity vs wavelength information.
• Fig. 4 shows a number of fringes are within the optimum conditions ofthe dete ⁇ ion system.
• Fig. 5 shows the Fourier Transform of Fig. 4. The largest amplitude signal peaked at 0 frequency (arb. units) is due to dc terms. The small amplitude feature peaked at frequency 22 (arb.
• This Ught source, birefringent, and dete ⁇ ion system form a self-consistent arrangement that is capable ofthe required accuracy.
• the LED light source can be located at another wavelength and have a width that is considerably narrower than that used in the previous example. This would require a detection system that operates at a different wavelength and has a higher resolution requirement so as to spread out the vyavelengths over the same number of pixels
• the birefringent will have to be increased which can be accomplished by changing the birefringent crystal and/or changing its propagation length.
• line 70 represents the temperature/phase relationship of a single y-axis BeL crystal having a length of 0.505 mm and a sensitivity of -4.56 nm/°C.
• Line 72 represents the temperature/phase relationship of a tandem of birefringent elements, BeL and YVO 4 .
• the BeL has a length of 1.04 mm (sensitivity of -9.38 nm/°C), and the YVO 4 has a length of 0.20 mm (sensitivity of -1.02 nm/°C).
• the combined sensitivity of line 72 is 1 8 times greater than that of line 70.
• equations [I] and [2] It is possible using the relationship of equations [I] and [2] to find any desired sensitivity at a given value for X from equation [1].
• a birefringent element of any type, positive or negative, uniaxial or biaxial can be used. The arrangements ofthe elements must be such that they satisfy the equations.
• the crystal types that yield the highest multipUcative value for the minimum thickness should be oriented such that the total birefringence length produ ⁇ s meet the conditions of equation [1] and the temperature dependent terms of equation [2] such that:
• ABS «[L, «(dB . /dT, + ⁇ ,'B,)] + ABS « [L 2 » (dB 2 /dT 2 + ⁇ 2 'B 2 )] ABS «[L, «(dB,/dT, + ⁇ ,*B.) + L 2 « (dB 2 /dT 2 + ⁇ 2 *B 2 )].

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• 1996
  • 1996-04-03 WO PCT/US1996/004555 patent/WO1996031763A1/en not_active Application Discontinuation
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