AN OPTICAL PRESSURE SENSING SYSTEM AND

A BIREFRINGENT SENSOR FOR USE THEREIN

Field of the Invention

This invention relates to birefringent pressure sensing means

Background of the Invention

It is desirable for optical sensors to detect temperature, pressure, torque, position, etc since optical sensors are immune to electrical interference Wesson descπbes in U S patent no 4,466,295 means for using the photoelastic effect to measure stress in plates that can be related to externally applied forces The sensor disclosed in Wesson is not particularly effective because the output signal is light intensity dependent and is adversely sensitive to environmental effects to the overall sensor system A further drawback is that the sensor output is linear over only a narrow pressure range Therefore, one must calibrate multiple sensors over different pressure ranges

Sensing devices utilizing birefringent crystals have been descπbed for temperature by Emo et al in U S patent no 5,255,068 entitled "Fringe Pattern Analysis of a Birefringent Modified Spectrum to Determine Environmental Temperature" which is incoφorated herein by reference Emo et al descπbe an optical high-temperature sensor based on a birefringent element made of a single crystal A broad band light spectrum 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 optical axis of the crystal The polaπzed wave can be represented by two equal linear polaπzed vectors which are

97/08653

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aligned along the optical axes. Propagation of these waves through the birefringent plate introduces a temperature dependent phase shift between the two waves Thereafter, a second linear polarizer combines the two waves creating a modulated spectrum. Information derived from this modulated spectrum, or fringe pattern, is then used to measure the temperature of the birefringent plate.

A pressure sensor can be created by combining a bias crystal arrangement with a sensor plate made of an isotropic material that is attached to a spacer πng which defines a cavity 25. The sensor plate deforms into the cavity 25 due to the application of external forces. In this arrangement, a light beam is directed through the top or bottom sections of the sensor plate so that the compression/tension terms do not cancel out the stress induced birefringence terms. The deficiency of this device is that methods for holding the sensor plate to the spacer ring produce extraneous stress terms that perturb the externally-induced stress levels inside the plate The magnitude of the externally-induced stress terms are due to a material constant (stress-optic coefficient) which is small in magnitude for appropriate materials and physical dimensions of the plate (thickness and path length) so that any perturbation can have a rather substantial effect on the reliability of the sensor This effect limits the sensitivity and resolution of the pressure sensor

Previous designs of this pressure sensor used o-rings or glass frit to seal the sensor plate to the spacer ring so that a pressure differential could be obtained across the plate. These fabrication methods subject the sensor plate to easily measurable levels of stress such that in the range of low external pressure the measured stress signal was overwhelmed by stresses attributable to the o-rings or glass frit thus rendering the sensor device ineffective. The sensors made with o- rings could be made to function if the sealing pressure was very low and the thermal conditions did not change. As thermal conditions changed, however, the o-ring produced more or less force on the sensor plate changing its sensitivity conditions.

Therefore there is a need to provide an optical pressure sensor configured and assembled in such a way so not to add extraneous stress terms to the stress terms indicative of the environmental conditions

Summary of the Invention:

Accordingly it is an object of the present invention to provide an optical sensor that is capable of accurately measuring the effects of external forces (pressure) applied to the sensor This is achieved by bonding a sensor plate or disk by means of diffusion or reactive bonding of identical or like materials onto a first face of a spacer πng in which the spacer ring defines an aperture The spacer πng also has an appropriate thickness and width so that other sealing requirements that induce stress into the spacer ring are not appreciably passed into the sensor plate A seal plate is attached to a second face of the spacer ring to form a sealed cavity which can then be evacuated or pressurized as required for the application An optical pressure sensing apparatus utilizing the invention comprises a broad band light source which is transmitted via a first fiber optic cable, a collimator and a first polarizer to the sensor plate and a bias crystal. The beam of light is transmitted and received so that the light only passes through the upper pan of the sensor plate thickness so that the light is restricted to the section of the sensor plate that is in compression due to the externally applied pressure and the zone created by the cantilever action of the supported section of the plate The sensor transmits a wavelength/polarization component of the light. Light exits the sensor and is captured by a second polarizer whose axis is parallel or peφendicular 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¬ electronic interface where an intensity vs. wavelength (fringe) pattern is extracted by a processing unit. The processing unit performs a Fourier transform on the fringe pattern, and the phase term of the selected frequency relates to the environmental pressure of the sensor.

Brief Description of the Drawings

FIGURE 1 is a schematic exemplifying the concept of birefringence of a linearly polarized wave, FIGURE 2a is an elevation view of the invention,

FIGURE 2b is an elevation view of the invention with external forces applied showing an exaggerated view of the deformation of sensing plate 20; FIGURE 3 a is one embodiment of the sensor plate 20; FIGURE 3b is an alternate embodiment of the sensor plate 20; FIGURE 4a-d are graphs illustrating changing sensitivities due to alternate embodiments of the invention;

FIGURE 5 is a block diagram of a pressure sensing system utilizing the invention;

FIGURE 6 is the amplitude/frequency waveform of a broad band light source useful in the practice of the invention;

FIGURE 7 is an intensity vs. wavelength waveform of a modulated light spectrum generated by an opto-electronic interface; and

FIGURE 8 is a Fourier transform of the waveform of Fig. 7. FIGURE 9 is a graphical representation of phase vs. pressure for an optical pressure system in accordance with the invention.

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 practical use to thereby enable others skilled in the art to best utilize the invention.

The present invention provides for a pressure sensor plate that is capable of accurately measuring the effects of external forces The invention is applicable for use in a pressure detector system as disclosed in the following copending applications serial no 08/406,33 1 , filed on March 17, 1995 entitled "System to Determine Environmental Pressure and Birefringent-Biased Cladded Optical Sensor for use Therein ^" , serial no 08/41 1, 186. filed on March 27, 1995, entitled ^" Integrated Birefringent-Biased Pressure and Temperature Sensor System ^" , and serial no 08/545,455, filed on October 19, 1995, entitled "A Birefringent-Biased Sensor", all assigned to the same assignee as the present application and incoφorated herein by reference. The present invention is also useful in other birefringent sensing methods and other sensor applications that require force measurements, such as accelerometers, as is commonly known to those skilled in the art

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 _e -n ₀ where n _e is the extra ordinary index of refraction and n ₀ is the ordinary index of refraction Uniaxial crystals can be categorized as positive or negative depending on whether the n _e term is larger or smaller than n ₀ . Exemplary uniaxial crystals are sapphire, magnesium fluoride and crystalline quartz. The terms n _e and n ₀ 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 a, - C2/c and are exemplified by Lanthanum Beryllate or Berylluim Lanthanate (La ₂ Be ₂ Oj 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") For biaxial crystals, terms such as n _a , n^, and n _c can be used, or any two such terms and their respective temperature dependent birefringent terms can be substituted giving a total of 3 separate cases for this class of crystals

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Fig 1 illustrates the principles of birefringence Two orthogonally polaπzed waves 144 and 146 enter and propagate through a birefringent element 150 The electric polarization vectors of these two waves are oπented in the X and Z directions, and the waves propagate in the Y direction On entering face 152, the linearly polaπzed 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 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

Fig 2a illustrates a pressure sensor 10 that utilizes the principles of birefringence to accurately measure externally applied forces without having to take into consideration any extraneous forces that may result from the manufacture of pressure sensor 10 A sensor plate or disk 20 is bonded to a first face 22 of a spacer ring 24 by means of diffusion or reactive bonding of identical or like materials Spacer ring 24 defines a cavity 25 to allow sensor plate 20 to deform upon the application of an external force F as shown in Fig. 2b Sensor plate 20 will deform into the cavity 25 anywhere from 0- 10 microns The depth of deformation is limited by the maximum stress in a given sensor plate 20 A pre¬ determined maximum stress level will determine the maximum depth of deformation for a given sensor 10 geometry and material Spacer ring 24 also has a thickness and width so that other sealing requirements that induce stress into the ring are not appreciably passed into sensor plate 20. A seal plate 28 is bonded to a second face 23 of spacer ring 24 preferably using a glass frit The sealed cavity 25 created by sensor plate 20, spacer ring 24 and seal plate 28 is evacuated or pressurized as required for the application

Figures 3 a and 3 b illustrate two sensor geometries that work equally well Fig 3a illustrates a sensor plate 20 having two opposing flats 30a and 30b An

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optical beam is directed through flats 30a and 30b Fig 3b illustrates a generally rectangular sensor plate 20 having two opposing flats 30c and 30d In both embodiments the flats are ground and polished Preferably, sensor plate 20 is symmetπcally placed over cavity 25 and has an overlap 21 of about 0 060 inches beyond the cavity 25 to allow for sufficient bonding area as shown in Fig 2a

.Alternatively, overlap 21 may be expanded allow for a greater bonding length thus greater strength The longer bonding length implies that sensor plate 20 is held down firmer Referring to Fig. 2b, when force F is applied to sensor plate 20, there is less deformation due to the greater firmness of the bond on the part of sensor plate 20 over spacer ring 24 This results in a stress field change near the edge of aperture 25 and at the top of sensor plate 20 which can give rise to greater sensitivity as shown in Table 1 below The drawback to the longer bond 21 is that the collimated beam must propagate that much further through sensor plate 20 which diminishes the available signal. Accordingly, the sensitivity of sensor 10 is determined by the stress birefringence constant and geometric dimensions of sensor plate 20 and the bonding areas of the diffusion or reactive bond in relation to the light input and light output areas of sensor plate 20 The last consideration is due to the desired compression terms of the force F applied and other stress terms due to the cantilever action of plate 20 being bonded to the spacer ring 24 in the support role as shown in Fig. 2b Figs 4a and 4b indicate the respective sensitivities (deg phase/psi) for sensor plate 20 thicknesses of 0 040" and 0 080" respectively Work on the design guide has indicated that the sensitivity of the sensor scales with the square of the aperture of cavity 25, that is, the larger the aperture, the more sensitive the sensor as shown in Table 1 The range is however, limited by the stress/strain in the sensor plate under the loading condition Due to the large number of deflection cycling expected during the sensor lifetime, a large safety factor must be used. This in effect limits the standard 0 040" sensor plate with a 0 60" aperture to about 120 psi.

Table No. 1

Sensitivity (degrees phase/psi) as a Function of

Sensor Plate Thickness and Aperature Diameter

In addition, the thickness of sensor plate 20 was varied for a constant apeture of 0 40" The results depicted in Figure 4c show the relationship This relationship has no simple mathamatical formula Work showed that keeping a constant sensor plate 20 diameter at 0 12" larger than the aperture yielded a nice relationship, shown in Fig. 4d Extending that overlap changed the results considerably and so far unpredictably This appears to be due to strong surface tensile terms that are created when sensor plate 20 is more firmly bound to the spacer ring 24

Preferred materials for the sensor plate 20, spacer ring 24 and seal plate 28 are clear fused silica (CFQ) or Corning 1737F aluminosilicate glass Both these materials exhibit high stress optic coefficients and excellent thermal properties

The puφose of spacer ring 24 is to create a cavity 25 that can be evacuated so that pressure sensor 10 can act as an absolute pressure sensor Alternatively, cavity 25 can act as a pressure reference Spacer ring 24 is required to be thicker for CFQ material since the frit used to bond the spacer ring 24 and seal plate 28 has a coefficient of thermal expansion (CTE) mis-match to the CFQ The CTE mis-match is due to the very low expansion coefficeint of fused silica and the desire

to frit the parts together at lower temperatures This creates stress in spacer πng 24 which can propagate up through sensor plate 20 and add unwanted localized birefringent bias terms to the light propagating through sensor plate 20 The thickness of spacer ring 24 range from about 0 12 to 0 16 inches for CFQ and from about 0 086 to 0 16 inches for 1737F glass.

Sensor plate 20 and spacer ring 24 are joined together by means of optical bonding, such as that process developed by Onyx Optics of Dulin, California Spacer ring 24 is preferably polished to optical bonding tolerances in the area where plate 20 and ring 24 overlap Preferably, the optical bond is a low temperature molecular bond that increases in strength with increasing temperature .Alternatively, plate 20 and ring 24 are joined by a reaction bonding process, such as that described in United States Patents 5,407,506 and 5,427,638 to Goetz et al , both incoφorated herein by reference.

Preferably, spacer ring 24 and seal plate 28 are joined by means of a glass frit Preferably, glass is used instead of fused silica to allow for a lower temperature process. An exemplary frit is Dow Corning 7070, 325 mesh As would be obvious to those skilled in the art, in an accelerometer application using the present invention, a seal plate 28 is not required since an accelerometer does not need a differential pressure. Fig 5 illustrates a pressure sensing system 18 utilizing pressure sensor 20 having sensor plate 24 in combination with two bias birefringent crystal elements arranged in tandem as disclosed in 08/545,455 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. 6. 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 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 polaπzed light at 45° to the optical axis of the birefringent crystal elements 30 and 32 The crystals are further arranged so that the αBo and dB/dT terms cancel so that no significant temperature effects contπbute to the signal modification The polaπzed light is decomposed into two orthogonal polaπzation states by the tandem birefringent crystal elements 30 and 32 The two orthogonal polaπzed light waves expeπence a temperature compensated phase shift propagating through crystals 30 and 32 The birefringently-biased light then passes through pressure sensing plate

24, which is an isotropic media such as fused silica, YAG or glass Sensor 24 is typically a disk with two opposing flats cut on the disk and polished to pass light and two major faces, which on one, a force F is exerted Sensor 24 is supported by spacer πng 24 which in combination with seal plate 28 defines a sealed cavity 25 The areas of maximum stress occur near the surfaces plate 24 so the light output of elements 30 and 32 is configured to pass near light at the surface of plate 24

The output of plate 24 is collected by a second polaπzer 52, commonly known as an analyzer, having the same or a 90° oπentation to polaπzer 48 Polaπzer 52 combines the two orthogonal phases to form a modulated light spectrum The light spectrum is focused down a second fiber optic cable 56 by a second collimattng means 54 The output of the fiber optic cable is directed to an opto-electronic interface 58, such as a spectrometer having a fiber optic input and a charge coupled device (CCD) array output The light spectrum is focused onto an array of photodetectors or a CCD detector associated with conditioning electronics which yields the intensity vs time (intensity vs wavelength) fringe pattern signal as shown in Fig 7 The opto-electroruc interface 58 has a 256 element CCD array as the detection system Dispersion elements inside the unit have pixel number 1 at 748 nm and pixel number 256 at 960 nm The entire LED spectrum is therefore observed on the CCD array yielding intensity versus wavelength information In

the preferred case there will be six to ten fringes produced on this CCD array due to the action of the two polarizers and the tandem crystals located between them Six to ten fringes have been determined to give the required system accuracy and low production costs of the hardware involved This number of fringes determines the amount of total birefringence-length product that the crystals must provide A CPU 60 digitizes the signal and performs a Fourier transform on the signal, which resultant is shown in Fig. 8 The measured phase shift of the transformed signal is a direct representation of the pressure exerted on crystal 34 The largest amplitude signal peaked at frequency zero (arb units) is due to dc terms The small amplitude feature peaked at frequency 22 (arb uruts) is due to the wavelength spacing of the three LEDs that comprise the light source 40 The larger amplitude signal peaked at frequency 33 (arb units ) is due to the birefringent length product The phase information at this frequency is related to environmental pressure sensed by sensor 20 Fig 9 graphically illustrates the linear relationship between the phase information and sensed environmental pressure

Pressure sensing system 18 is bidirectional, such that the light input may alternatively travel in cable 56 and output via cable 44 Imperfections due to scattering can cause small offset differences in the results between the two directions This light source, birefringent, and detection system form a self-consistent arrangement that is capable of the 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 wavelengths 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.