WO1996031763A1

WO1996031763A1 - Birefringent optical temperature sensor with adjustable temperature sensitivity

Info

Publication number: WO1996031763A1
Application number: PCT/US1996/004555
Authority: WO
Inventors: William R. Rapoport; Devlin M. Gualtieri; Janpu Hou; Herman Van De Vaart
Original assignee: Alliedsignal Inc.
Priority date: 1995-04-03
Filing date: 1996-04-03
Publication date: 1996-10-10
Also published as: EP0819242A1

Abstract

An optical temperature sensor for use in a temperature detector system having at least two birefringent crystal elements arranged in tandem. A collimated broad band light source is transmitted via a fiber optic cable, a polarizer to a first birefringent crystal element. The first crystal element decomposes the light wave into first and second orthogonally polarized waves and transmits the wave components to a second birefringent crystal element. The linearly polarized waves propagate through the birefringent crystals, and the environmental temperature introduces a temperature dependent phase shift between the two polarized waves. The light waves exit the second crystal to a second polarizer producing a modulated light spectrum. A focusing element collects the light and transmits it down another fiber optic cable. The cable transmits the light to an opto-electronic interface where the fringe pattern is extracted and a computer compatible signal is generated for a CPU. The CPU performs a Fourier transform on the fringe pattern, where the phase term for a selected frequency is the measure of the environmental temperature experienced by the birefringent crystals.

Description

BIREFRf CT T OPTICAL TEMPERATTTRE SENSOR WITH ADJUSTABLE TEMPERATURE SENSlTTVrrv

Field of the Invention;

The invention relates to birefringent optical temperature sensors.

Background of the Invention;

It is desirable to have optical sensors to detect temperature, pressure, torque, position etc. that would be immune to electrical interference. Temperature sensing devices utilizing birefringent crystals are described by Emo et al. in U S patent no. 5,255,068 entitled "Fringe Pattern Analysis ofa Birefringent Modified Spectrum to Determine Environmental Temperature" which is incorporated herein by reference. Emo et al. describe an optical high temperature sensor based on a birefringent element made ofa single crystal. A broad band light speαrum 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 μlaie introduces a temperature dependent phase shift between the two waves Thereafter, a second linear polarizer combines the two waves creating a modulated speαrum. Information derived from this modulated speαrum or fringe pattern is then used to measure the temperature of the bireπ-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.

Summary of the Invention; Accordingly it is an objeα of the present invention to increase the sensitivity of an optical temperature sensor. The sensor consists of two or more single birefringent crystal elements in tandem and the total birefringence length produα remains within the accepted tolerances of current devices. Each crystal element has a birefringence (B), a dB/dT and a coefficient of thermal expansion (α) 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. Light exits the crystals and is captured by a second polarizer whose axis is parallel or perpendicular to the first polarizer producing a wavelength modulated light spectrum A focusing element collects the light and transmits it via a second fiber optic cable to an opto- eleαronic 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 seleαed frequency relates to the environmental temperature of the crystals.

Brief Description of the Drawings FIGURE 1 is a schematic ofa 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 speαrum generated by an opto-electronic interface;

FIGURE 5 is a Fourier transform of the waveform of Fig. 4 at a seleαed frequency; and

I FIGURE 6 is a graphical representation illustrating the increased sensitivity of a tandem birefringent optical temperature sensor.

Detailed Description of the Preferred Embodiments of the Invention The preferred embodiments of this invention will be better understood by those skilled in the art by reference to the above Figures. The preferred embodiments of this invention illustrated in the Figures are neither intended to be exhaustive nor to limit the invention to the precise form disclosed. The Figures are chosen to describe or to best explain the principles of the invention, and its application and praαical use to thereby enable others skilled in the an to best utilize the invention.

Fig. 1 illustrates a temperature sensing system 18 utilizing a temperature sensor 20 that comprises at least two birefringent crystal elements arranged in tandem. In this embodiment, sensor 20 comprises of two birefringent elements 30 and 32, however, any praαical 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 -

-4- gradient index lens, and is directed through a polarizer 48 that passes only linear polarized light preferably with a > 100: 1 extinction ratio to provide an acceptable signal-to-noise ratio. However, an extinction ration of 2: 1 would still provide an acceptable signal for this invention. 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.

Generally, 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 n₀ is the ordinary index of refraαion.

Uniaxial crystals can be categorized as positive or negative depending on whether the τi_ς term is larger or smaller than n₀. 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 C⁶2h - C2/c and exemplified by Lanthanum Beryllate or Berylluim Lanthanate (La₂Be₂O₅ or "BeL ") as referenced by H. Harris and H.L. Yakel, Acta Cryst., B24, 672-682 ( 1968) Other biaxial crystals include alexandrite and yittrium aluminum pervoskite ("YAP"). In that case, terms such as n^ nj,, and tiς can be used and any 2 such terms and their respeαive 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 direαions, and the waves propagate in the Y direαion. 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 Δθ on exiting face 154 The -

-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.

Referring again to Fig. 1, 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'B₂] = X'λ [ 1]

where Li and L₂ are the respeαive crystal lengths, λ is the central wavelength of the light source 40, Bj and B₂ are the respeαive birefringences and ABS refers to the absolute value of the enclosed terms. Coefficient X represents the approximate number of orders of the effeαive 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 funαion of the overall system design, including the wavelength and band of light source 40 and the opto-electronic interface 58 that has its own charaαeristic 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 seleαed so that the respective L(dB/dT) terms add or subtraα to yield the desired temperature sensitivity Accordingly,

Lι'(dBι/dT, + cti'B.) ± L_:'(dB_:/dT₂ +α₂'B₂) = Sensitivity [2] Li and L₂ are the respective crystal lengths, dBι/dT_t and dB₂/dT₂ are the respective birefringence change as a function of temperature and α. and α₂ are the respective thermal expansion coefficients ofthe crystals. dB./dTi and dB₂/dT₂ are defined as di-e/dT - div/dT for consistency ofthe respective materials. Equations [1] and [2] can be solved for Li and L₂ for two given crystal materials and knowing their respeαive orientations. The ± terms are due to the natrue ofthe crystals involved and their respeαive signs of birefringence and dB/dT terms. The orientation of the crystals with respeα to each other also determines the sign. Once the crystals and their respeαive orientations have been selected on the basis of equation [1], the same sign is used in equation [2].

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 colleαed 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 speαrum 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- eleαronic 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) deteαor associated with conditioning eleαronics 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

-7-

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

2*ABS^»[Lι^»Bι ± L₂-B₂] = X^»λ [ la]

where L_t and L₂ are the respective crystal lengths, λ is the central wavelength of the light source 40a, B\ and B₂ are the respeαive birefringences and ABS refers to the absolute value ofthe enclosed terms. Coefficient X represents the approximate number of orders ofthe effeαive 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 fiinαion ofthe overall system design, including the wavelength and band of light source 40a and the opto¬ electronic interface 58a that has its own charaαeristic 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,

2"[Lι«(dBι/dTι + α,«B,) ± L₂ ^«(dB₂/dT₂-t-α₂ ^«B₂)] = Sensitivity [2a]

Li and L₂ are the respeαive crystal lengths, dBι/dTι and dB₂/dT₂ are the

birefringence change as a function of temperature and c-i and α are the respecti . «- thermal expansion coefficients of the crystals dBι/dTι and dB₂/dT₂ are defined a . dnjdl - dno dT for consistency ofthe respective materials. Equations [la] and [2a] can be solved for Li and L₂ for two given crystal materials and knowing their respective orientations. The ± terms are due to the natrue ofthe crystals involved and their respeαive signs of birefringence and dB/dT terms. The orientation ofthe crystals with respeα to each other also determines the sign. Once the crystals and their respeαive orientations have been seleαed 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 refleαive 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 speαrum The light speαrum is focused into fiber optic cable 44a by coUimating means 46a The Ught wave is spUt by a Y-splitter 59a which direαs some pre-determined fraction ofthe Ught wave to an opto-eleαronic interface 58a, such as a speαrometer having a fiber optic input and a CCD array output. The light speαrum is focused onto an array of photodeteαors or a charge coupled device (CCD) detector associated with conditioning eleαronics which yields the intensity vs. time (intensity vs wavelength) fringe pattern signal as shown in Fig. 4. 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 direα 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.

Examples The foUowing examples describe the invention consisting of two birefringent crystal elements by way of example only and is not intended to limit the scope of this invention. Any number of crystals aligned in tandem are possible to accomplish the objeαives ofthe invention.

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 α 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. YVO₄ has a birefringence of 0.2152 (n. - ru), a dB/dT of -6.68xlO^"6/°C and an α of 7.3x10^"* cm/cm/°C. The crystals are arranged with the resulting absolute value for the total birefringent length produα dereasing compared to the largest birefringent length produα ofa single BeL crystal. This indicates taht the signs of equations [1] and [2] are negative.

By varying the length of each crystal, virtually any value of sensitivity can be obtained with these two crystals. The sensitivity is much greater with the crystals arranged in tandem than either crystal alone since the birefringences are opposite in sign. A 1.04 mm thick sample of BeL has a sensitivity of -9 38 nm °C and a 0.20 thick sample of YVO₄ has a sensitivity of -1 02 nm °C This combination increases the sensitivity of the sensor by 1.85 times over a sensor consisting of only 0,5 mm of BeL while maintaining the same effective birefringence [-9.38 - (-1 02)]/-4 51 Appropriate thickness changes can increase the sensitivity by 10 times or more The sensitivity is only limited by the desired physical size ofthe crystals and thermal lag terms due to the finite thermal conduαivity ofthe materials. Fig. 6 graphically illustrates the concept ofthe invention. The light source 40 used in a demonstration unit consists ofa 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 deteαion system. Dispersion elements inside the unit have pixel number 1 at 748 nm and pixel number 256 at 960 nm. The entire LED speαrum is therefore observed on the CCD array yielding intensity vs wavelength information. In the preferred case there wiU be six to ten fringes produced on this CCD array due to the action ofthe two polarizers and the tandem crystals located between them. Six to ten fringes have been determined to give the required system accuracy and low produαion costs ofthe hardware involved. This number of fringes determines the amount of total birefringence length product that the crystals must provide. Fig. 4 shows a number of fringes are within the optimum conditions ofthe deteαion 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. units) is due to the wavelength spacing ofthe three LEDs that comprise the Ught source 40 The larger ampUtude signal peaked at frequency 33 (arb. units ) is due to the birefringent length produα. The phase information at this frequency will be related to temperature-induced birefringence generated in crystals 30 and 32 which is a direα fiinαion ofthe environmental temperature.

This Ught source, birefringent, and deteαion system form a self-consistent arrangement that is capable ofthe required accuracy. There are many other combinations that can achieve the same results. For example; 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 To achieve the same number of fringes, the birefringent will have to be increased which can be accomplished by changing the birefringent crystal and/or changing its propagation length.

Referring again to Fig. 6, line 70 represents the temperature/phase relationship ofa 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 YVO₄. The BeL has a length of 1.04 mm (sensitivity of -9.38 nm/°C), and the YVO₄ 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.

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 produαs meet the conditions of equation [1] and the temperature dependent terms of equation [2] such that:

ABS«[L,«(dB_./dT, + α,'B,)] + ABS^«[L₂ ^»(dB₂/dT₂+α₂'B₂)] = ABS«[L,«(dB,/dT, + α,*B.) + L₂ ^«(dB₂/dT₂ + α₂*B₂)].

Claims

--12-CLAIMS

We claim:

1 An optical temperature sensor for use in a temperature detector system to provide an optical signal indicative ofthe environmental temperature experienced by said sensor, said sensor comprising: a) a first linear polarizing element for orienting an initial randomly polarized broad band light source into a linearly polarized light wave; b) a first birefringent crystal for receiving said linearly polarized light wave from said first polarizing element, said linearly polarized wave decomposing into first and second orthogonally polarized waves, said first and second orthogonally polarized waves experiencing a phase difference on propagating through said birefringent crystal; c) a second birefringent crystal for receiving said phase-shifted first and second orthogonaUy polarized waves, said phase-shifted experiencing a funher phase difference on propagating through said second birefringent crystal, and d) a second polarizing element for receiving said phase-shifted first and second orthogonaUy polarized waves and combining said first and second orthogonaUy polarized waves to create a modulated light spectrum having a fringe pattern, said fringe pattern being a function ofthe environmental temperature experienced by said first and second birefringent elements.

2. The optical temperature sensor of claim 1 wherein the sensitivity of said sensor is defined as: Lι*(dB|/^'dTι -<- α_.-Bij ± L₂ ^«(dB₂. d i ₂ +α₂*B₂)

3 The optical temperature sensor of claim 1 wherein said sensor further comprises a plurality of birefringent crystals in tandem

4. The optical temperature sensor of claim 3 wherein the sensitivity of said sensor is defined as:

L,^»(dB,/dT, + α,^»B₁) ± L₂'(dB₂/dT₂ +α₂ ^»B₂).±. . . U'CdB_n/dT_n+ c_tn-B-,)_.

5. The optical temperature sensor of claim I wherein said crystals are uniaxial or biaxial crystals.

6. A deteαor system for providing a frequency signal indicative of the environmental temperature comprising: a) a first linear polarizing element for orienting an initial randomly polarized broad band Ught source into a Unearly polarized Ught wave; b) a first birefringent crystal for receiving said Unearly polarized light wave from said first polarizing element, said Unearly polarized wave decomposing into first and second orthogonaUy polarized waves, said first and second orthogonaUy polarized waves experiencing a phase difference on propagating through said birefringent crystal; c) a second birefringent crystal for receiving said phase-shifted first and second orthogonaUy polarized waves, said phase-shifted experiencing a further phase difference on propagating through said second birefringent crystal; d) a second polarizing element for receiving said phase-shifted first and second orthogonaUy polarized waves and combining said first and second orthogonally polarized waves to create a modulated light speαrum having a fringe pattern, said fringe pattern being a tuπction of the environmental temperature experienced by said first and second birefringent elements; e) an opto-eleαronic interface for accepting said modulated light speαrum output from said second polarizer and producing a corresponding electrical signal; and -14- e) signal conditioning electronics for analyzing said electrical signal and to extraα a waveform at a pre-selected frequency indicative of said environmental temperature.

7. The pressure sensor system of claim 6 further comprising means for coUimating said initial randomly polarized broad band light spectrum.

8. The optical temperature sensor of claim 6 wherein the sensitivity of said first and second birefringent crystals is defined as: L_.^dB_./dT, + α,'B,) ± L₂ ^»(dB₂/dT₂ +α₂ ^«B₂).

9. The optical temperature sensor of claim 6 wherein said sensor further comprises a pluraUty of birefringent crystals in tandem.

10. The optical temperature sensor of claim 9 wherein the sensitivity of said plurality of birefringent crystals is defined as:

L,'(dB_./dT_. + α.^«B,) ± L₂ ^«(dB₂/dT₂ +α₂ ^«B₂).±. . Iv(dB_n/dT_n + α^B,,)

11. The optical temperature sensor of claim 6 wherein said crystal elements are uniaxial or biaxial crystals.