U.S. patent application number 11/567600 was filed with the patent office on 2008-12-04 for optical sensor for extreme environments.
Invention is credited to Frank Perez, Nabeel Agha Riza.
Application Number | 20080297808 11/567600 |
Document ID | / |
Family ID | 40140047 |
Filed Date | 2008-12-04 |
United States Patent
Application |
20080297808 |
Kind Code |
A1 |
Riza; Nabeel Agha ; et
al. |
December 4, 2008 |
Optical Sensor For Extreme Environments
Abstract
An optical sensing probe includes a tube having a tip portion
configured for placement in an environment in which conditions are
to be sensed and an etalon having a known characteristic disposed
proximate the tip portion. The tube also includes a head portion
remote from the tip portion containing a light directing element
for directing light beams at the etalon and receiving reflected
light beams from the etalon wherein the received reflected light
beams are used for determining an environmental condition proximate
the tip portion. A method for measuring a thickness of the etalon
may include directing a light beams at different frequencies at the
etalon and receiving the light beams from the etalon. The method
may also include identifying conditions of the respective light
beams condition received from the etalon and then calculating a
first thickness of the etalon responsive to the respective
conditions and the known characteristic.
Inventors: |
Riza; Nabeel Agha; (Oviedo,
FL) ; Perez; Frank; (Tujunga, CA) |
Correspondence
Address: |
BEUSSE WOLTER SANKS MORA & MAIRE, P. A.
390 NORTH ORANGE AVENUE, SUITE 2500
ORLANDO
FL
32801
US
|
Family ID: |
40140047 |
Appl. No.: |
11/567600 |
Filed: |
December 6, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60742813 |
Dec 6, 2005 |
|
|
|
Current U.S.
Class: |
356/503 ;
356/630 |
Current CPC
Class: |
G01B 2290/25 20130101;
G01B 9/02072 20130401; G01B 9/02028 20130101; G01B 9/02004
20130101; G01B 9/02023 20130101; G01B 9/02056 20130101; G01B 9/0209
20130101 |
Class at
Publication: |
356/503 ;
356/630 |
International
Class: |
G01B 11/28 20060101
G01B011/28; G01B 11/02 20060101 G01B011/02 |
Claims
1. A method for measuring a thickness of an etalon comprising:
directing a first light beam at a first frequency and a second
light beam at a second frequency at a first portion of an etalon
having a known characteristic; receiving the first light beam and
the second light beam from the etalon; and identifying a first
condition of the first light beam and a second condition of the
second light beam received from the etalon; and calculating a first
thickness of the etalon responsive to the first condition, the
second condition, and the known characteristic.
2. The method of claim 1, wherein the first condition comprises at
least one of an interference maximum and an interference minimum of
the first light beam.
3. The method of claim 2, wherein the second condition comprises at
least one of an interference maximum and an interference minimum of
the second light beam.
4. The method of claim 3, further comprising determining respective
refractive indices of the etalon for the at least one of the
interference maximum and the interference minimum of the first
light beam and for the at least one of the interference maximum and
the interference minimum of the second light beam.
5. The method of claim 4, wherein the respective refractive indices
are determined according to a Sellmeier equation.
6. The method of claim 4, wherein the first thickness is calculated
according to the formula:
t=(.lamda.1*.lamda.2)/(2*(.lamda.2*n1-.lamda.1*n2)); where t is the
first thickness; .lamda.1 is a wavelength of the first frequency,
.lamda.2 is a wavelength of the second frequency, n1 is the
refractive index at the first frequency, and n2 is the refractive
index at the second frequency.
7. The method of claim 3, wherein the first condition and second
condition correspond to adjacent at least one of the interference
maximum and minimum of the first light beam and at least one of the
interference maximum and minimum of the second light beam.
8. The method of claim 7, further comprising: calculating at least
a second thickness for a different first frequency and a different
second frequency; and calculating an average thickness
corresponding to at least the first thickness and the second
thicknesses.
9. The method of claim 1, further comprising: moving the etalon
relative to the first light beam and the second light beam to align
the beams with a second portion of the etalon; performing the steps
of claim 1 to calculate the thickness of a second portion of the
etalon.
10. The method of claim 1, further comprising: disposing the etalon
in an environment in which conditions are to be sensed; and using
the calculated thickness to determine an environmental condition
proximate the etalon.
11. The method of claim 10, wherein the environmental conditions
comprise at least one of a temperature and a pressure.
12. The method of claim 1, wherein the light beams are directed to
impinge upon a surface of the etalon at an angle normal to the
surface.
13. The method of claim 1, wherein the known characteristic
comprises a relationship between a refractive index of the etalon
and wavelength of light incident on the etalon.
14. The method of claim 13, wherein the relationship comprises a
Sellmeier equation for the etalon.
15. The method of claim 1, wherein the etalon comprises silicon
carbide.
16. The method of claim 1, wherein the etalon comprises a single
crystal silicon carbide.
17. A system for measuring a thickness of an etalon comprising: a
first light source for directing a first light beam having a first
wavelength at a first portion of an etalon having a known
characteristic; a second light source for directing a second light
beam having a second wavelength different from the first wavelength
at the etalon; an optical receiver for receiving the first light
beam and the second light beam from the etalon and for providing a
first power signal corresponding to the first light beam received
from the etalon and a second power signal corresponding to the
second light beam received from the etalon; and a processor for
identifying a first condition of the first power signal and a
second condition of the second power signal received from optical
receiver and calculating a thickness of the etalon responsive to
the first condition, the second condition, and the known
characteristic.
18. The system of claim 17, wherein the first light beam and the
second light beam are directed to impinge upon a surface of the
etalon at an angle normal to the surface.
19. The system of claim 17, wherein the first light source and the
second light source comprise a single laser capable of selectively
generating light at the first wavelength and the second
wavelength.
20. The system of claim 17, wherein the first light source and the
second light source comprise a single broadband light source.
21. The system of claim 17, wherein the light beams are directed at
the etalon along a light path comprising at least a free space
portion.
22. The system of claim 21, wherein the light path comprises a
single mode optical fiber and a collimator.
23. The system of claim 22, wherein the collimator comprises at
least one of a fiber collimating self imaging lens and a fiber
imaging lens
24. The system of claim 21, wherein the etalon is disposed at a
minimum light beam waist location in the free space portion.
25. The system of claim 21, further comprising a polarizer disposed
in the light path in the free space portion.
26. The system of claim 17, further comprising a circulator for
separating the light beams directed at the etalon from the light
beams received from the etalon.
27. The system of claim 22, wherein the single mode fiber is
configured to function as a pin hole for allowing optimization of a
light beam incidence angle on the etalon.
28. The system of claim 17, wherein the optical receiver comprises
at least one of an optical detector and an optical spectrum
analyzer.
29. An optical sensing probe comprising a tube having a tip portion
configured for placement in an environment in which conditions are
to be sensed, an etalon having a known characteristic that changes
responsive to an environmental condition disposed proximate the tip
portion; and a head portion remote from the tip portion containing
a light directing element for directing light beams at the etalon
and receiving reflected light beams from the etalon wherein the
received reflected light beams are used for determining an
environmental condition proximate the tip portion.
30. The probe of claim 29, further comprising a fiber bundle for
conducting respective portions of the light beams between the head
portion and the etalon.
31. The probe of claim 29, further comprising a light source for
providing a first light beam at a first frequency and a second
light beam at a second frequency to the light directing
element.
32. The probe of claim 29, further comprising a processor for
identifying a first condition of the first light beam reflected
from the etalon and a second condition of the first light beam
reflected from the etalon and calculating a thickness of the etalon
responsive to the first condition, the second condition, and the
known characteristic.
33. The probe of claim 29, wherein the etalon is configured to seal
an interior of the tube proximate the tip.
34. The probe of claim 33, further comprising a window configured
to seal an interior the tube proximate the head.
35. The probe of claim 34, wherein the interior of the tube
contains at least a partial vacuum.
36. The probe of claim 29, further comprising a tip cage disposed
around the tip portion proximate the etalon for providing
protection of the etalon.
38. The probe of claim 29, wherein the etalon comprises single
crystal silicon carbide.
40. The probe of claim 29, wherein the tip portion comprises a
material having a coefficient of thermal expansion about the same
as the etalon effective to limit heat induced stresses on the
etalon.
42. The probe of claim 29, wherein at least a portion of the tube
comprises a material having a lower coefficient of thermal
conductivity than the tip portion.
43. The probe of claim 29, further comprising at least one
telescoping portion between the head portion and the tip
portion.
44. The probe of claim 29, wherein the light directing element
further comprises a mechanism for aiming the light beams at the
etalon to achieve a desired light incidence angle with respect to a
surface of the etalon.
45. The probe of claim 29, wherein the light directing element
further comprises a polarizer disposed in a light beam path of the
light beams.
46. The probe of claim 29, wherein the tip portion and the head
portion comprise two separate elements configured for allowing
passage of the light beams therethrough.
47. The probe of claim 29, wherein a plurality of tubes are
disposed within a tip housing around a least the respective tip
portions of the tubes.
48. The probe of claim 47, wherein the respective head portions
corresponding to the plurality of tubes comprise separate elements
disposed within a head housing around the respective head portions
of the tubes.
49. The probe of claim 29, wherein the etalon is attached to a
rotating element and the tube is disposed relative to the rotating
element for directing the light beams at the etalon and receiving
the reflected light beams from the etalon when the etalon is
positioned within a light path of the light beams as the rotating
element moves the etalon into the light path.
50. The probe of claim 49, wherein the rotating element comprises
at least one of a wheel and a turbine blade.
51. The probe of claim 50, wherein the tube is disposed proximate a
support structure of the wheel.
52. The probe of claim 49, further comprising a support element for
attaching the etalon to the rotating element.
53. The probe of claim 52, wherein the support element comprises a
material having a coefficient of thermal expansion about the same
as the etalon for limiting heat induced stresses on the etalon.
54. The probe of claim 52, wherein the support element comprises a
material having a lower coefficient of thermal conductivity than
the etalon.
55. The probe of claim 29, wherein the etalon is configured to
deform responsive to a pressure differential on opposite surfaces
of the etalon sufficiently for being sensed by the probe as a
received light beam cross section difference from a transmitted
light beam cross section.
56. The probe of claim 55, further comprising at least one beam
expansion lens disposed in a light path of the light beams for
increasing the transmitted light beam cross section for impinging
on a relatively larger surface of the etalon.
57. The probe of claim 29, further comprising a two dimensional
optical detector for generating an image responsive to the received
beams.
58. The probe of claim 57, further comprising an image processor in
communication with the two dimensional optical detector for
analyzing at least a portion of the image to determine a
temperature of the etalon.
59. The probe of claim 57, further comprising an image processor in
communication with the two dimensional optical detector for
analyzing the image to determine at least one of a pressure and a
pressure distribution on the etalon.
60. The probe of claim 57, further comprising a beam splitter
disposed in a light path of the light beams for directing the
received light beams to the two dimensional optical detector.
61. The probe of claim 29, further comprising a point detector for
receiving at least a portion of the received light beams for
indicating an alignment condition of the light beams with respect
to the etalon.
62. An optical sensor comprising: a chamber having an inlet for
receiving a fluid into the chamber, an aperture formed in a wall of
the chamber, and an etalon having a known characteristic that
changes responsive to an environmental condition sealing the
aperture; and a light directing element for directing light beams
at the etalon and receiving reflected light beams from the etalon,
wherein the reflected light beams are used for determining an
environmental condition in the chamber.
63. The sensor of claim 62, further comprising a light source for
providing a first light beam at a first frequency and a second
light beam at a second frequency to the light directing
element.
64. The sensor of claim 62, further comprising a processor for
identifying a first condition of the first light beam reflected
from the etalon and a second condition of the first light beam
reflected from the etalon and calculating a thickness of the etalon
responsive to the first condition, the second condition, and the
known characteristic.
65. The sensor of claim 62, further comprising a two dimensional
optical detector for generating an image responsive to the received
light beams.
66. The sensor of claim 65, further comprising an image processor
in communication with the two dimensional optical detector for
analyzing the image to determine a pressure on the etalon.
67. The sensor of claim 62, wherein the light directing element
comprises a single mode fiber for emitting the light beams along a
free space path to the etalon and receiving the reflected light
beams from the free space path.
68. The sensor of claim 67, further comprising a lens for focusing
the emitted light beams and received light beams on a focused spot
of the etalon.
69. The sensor of claim 67, further comprising a lens for
collimating the emitted light beams and received light beams on
majority portion of the surface of the etalon.
70. The sensor of claim 62, wherein the light directing element
further comprises a mechanism for aiming the light beams at the
etalon to achieve a desired light incidence angle with respect to a
surface of the etalon.
71. The sensor of claim 70, wherein the mechanism comprises a tilt
element.
72. The sensor of claim 70, wherein the mechanism comprises a
translation element.
Description
SPECIFIC DATA RELATED TO INVENTION
[0001] This application claims the benefit of U.S. provisional
application No. 60/742,813 filed on Dec. 6, 2005.
BACKGROUND OF INVENTION
[0002] The present invention relates to optical sensors and, more
particularly, to optical sensors using etalons for remote sensing
in extreme environments.
[0003] There are numerous vital sensing scenarios in commercial and
defense sectors where the environment is extremely hazardous.
Specifically, the hazards can be for instance due to extreme
temperatures, extreme pressures, highly corrosive chemical content
(liquids, gases, particulates), nuclear radiation, biological
agents, and high Gravitational (G) forces. Realizing a sensor for
such hazardous environments remains to be a tremendous engineering
challenge. One specific application is fossil fuel fired power
plants where temperatures in combustors and turbines typically have
temperatures and pressures exceeding 1000.degree. C. and 50
Atmospheres (atm). Future clean design zero emission power systems
are expected to operate at even high temperatures and pressures,
e.g., >2000.degree. C. and >400 atm [J. H. Ausubel, "Big
Green Energy Machines," The Industrial Physicist, AIP, pp. 20-24,
October/November, 2004.] In addition, coal and gas fired power
systems produce chemically hazardous environments with chemical
constituents and mixtures containing for example carbon monoxide,
carbon dioxide, nitrogen, oxygen, sulphur, sodium, and sulphuric
acid. Over the years, engineers have worked very hard in developing
electrical high temperature sensors (e.g., thermo-couples using
platinum and rodium), but these have shown limited life-times due
to the wear and tear and corrosion suffered in power plants [R. E.
Bentley, "Thermocouple materials and their properties," Chap. 2 in
Theory and Practice of Thermoelectric Thermometry: Handbook of
Temperature Measurement, Vol. 3, pp. 25-81, Springer-Verlag
Singapore, 1998].
[0004] Researchers have turned to optics for providing a robust
high temperature sensing solution in these hazardous environments.
The focus of these researchers have been mainly directed in two
themes. The first theme involves using the optical fiber as the
light delivery and reception mechanism and the temperature sensing
mechanism. Specifically, a Fiber Bragg Grating (FBG) present within
the core of the single mode fiber (SMF) acts as a temperature
sensor. Here, a broadband light source is fed to the sensor and the
spectral shift of the FBG reflected light is used to determine the
temperature value. Today, commercial FBG sensors are written using
Ultra-Violet (UV) exposure in silica fibers. Such FBG sensors are
typically limited to under 600.degree. C. because of the
instability of the FBG structure at higher temperatures [B. Lee,
"Review of the present status of optical fiber sensors," Optical
Fiber Technology, Vol. 9, pp. 57-79, 2003]. Recent studies using
FBGs in silica fibers has shown promise up-to 1000.degree. C. [M.
Winz, K. Stump, T. K. Plant, "High temperature stable fiber Bragg
gratings, "Optical Fiber Sensors (OFS) Conf. Digest, pp. 195 198,
2002; D. Grobnic, C. W. Smelser, S. J. Mihailov, R. B. Walker,"
Isothermal behavior of fiber Bragg gratings made with ultrafast
radiation at temperatures above 1000 C," European Conf. Optical
Communications (ECOC), Proc. Vol. 2, pp. 130-131, Stockholm, Sep.
7, 2004]. To practically reach the higher temperatures (e.g.,
1600.degree. C.) for fossil fuel applications, single crystal
Sapphire fiber has been used for Fabry-Perot cavity and FBG
formation [H. Xiao, W. Zhao, R. Lockhart, J. Wang, A. Wang,
"Absolute Sapphire optical fiber sensor for high temperature
applications," SPIE Proc. Vol. 3201, pp. 36-42, 1998; D. Grobnic,
S. J. Mihailov, C. W. Smelser, H. Ding, "Ultra high temperature FBG
sensor made in Sapphire fiber using Isothermal using femtosecond
laser radiation," European Conf. Optical Communications (ECOC),
Proc. Vol. 2, pp. 128-129, Stockholm, Sep. 7, 2004]. The single
crystal Sapphire fiber FBG has a very large diameter (e.g., 150
microns) that introduces multi-mode light propagation noise that
limits sensor performance. An alternate approach [see Y. Zhang, G.
R. Pickrell, B. Qi, A. S. -Jazi, A. Wang, "Single-crystal
sapphire-based optical high temperature sensor for harsh
environments," Opt. Eng., 43, 157-164, 2004] described replaced the
Sapphire fiber frontend sensing element with a complex assembly of
individual components that include a Sapphire bulk crystal that
forms a temperature dependent birefringent Fabry-Perot cavity, a
single crystal cubic zirconia light reflecting prism, a
Glan-Thompson polarizer, a single crystal Sapphire assembly tube, a
fiber collimation lens, a ceramic extension tube, and seven 200
micron diameter multimode optical fibers. Hence this described
sensor frontend sensing element not only has low optical efficiency
and high noise generation issues due to its multi-mode versus SMF
design, the sensor frontend is limited by the lowest high
temperature performance of a given component in the assembly and
not just by the Sapphire crystal and zirconia high temperature
ability. Add to these issues, the polarization and component
alignment sensitivity of the entire frontend sensor assembly and
the Fabry-Perot cavity spectral notch/peak shape spoiling due to
varying cavity material parameters. In particular, the Sapphire
Crystal is highly birefringent and hence polarization direction and
optical alignment issues become critical.
[0005] An improved packaged design of this probe using many
alignment tubes (e.g., tubes made of Sapphire, alumina, stainless
steel) was shown in Z. Huang. G. R. Pickrell, J. Xu, Y. Wang, Y.
Zhang,, A. Wang, "Sapphire temperature sensor coal gasifier field
test," SPIE. Proc. Vol. 5590, p. 27-36, 2004. Here the fiber
collimator lens for light collimation and the bulk polarizer (used
in Y. Zhang, G. R. Pickrell, B. Qi, A. S. -Jazi, A. Wang,
"Single-crystal sapphire-based optical high temperature sensor for
harsh environments," Opt. Eng., 43, 157-164, 2004) are interfaced
with a commercial Conax, Buffalo multi-fiber cable with seven
fibers; one central fiber for light delivery and six fibers
surrounding the central fiber for light detection. All fibers have
200 micron diameters and hence are multi-mode fibers (MMF). Hence
this temperature sensor design is again limited by the spectral
spoiling plus other key effects when using very broadband light
with MMFs. Specifically, light exiting a MMF with the collimation
lens has poor collimation as it travels a free-space path to strike
the sensing crystal. In effect, a wide angular spread optical beam
strikes the Sapphire crystal acting as a Fabry-Perot etalon. The
fact that broadband light is used further multiplies the spatial
beam spoiling effect at the sensing crystal site. This all leads to
additional coupling problems for the receive light to be picked up
by the six MMFs engaged with the single fixed collimation lens.
Recall that the best Fabry-Perot effect is obtained when incident
light is highly collimated; meaning it has high spatial coherence.
Another problem plaguing this design is that any unwanted
mechanical motion of any of the mechanics and optics along the
relatively long (e.g., 1 m) freespace optical processing path from
seven fiber-port to Sapphire crystal cannot be countered as all
optics are fixed during operations. Hence, this probe can suffer
catastrophic light targeting and receive coupling failure causing
in-operation of the sensor. Although this design used two sets of
manual adjustment mechanical screws each for 6-dimension motion
control of the polarizer and collimator lens, this manual alignment
is only temporary during the packaging stage and not during sensing
operations. Another point to note is that the tube paths contain
air undergoing extreme temperature gradients and pressure changes;
in effect, air turbulence that can further spatially spoil the
light beam that strikes the crystal and also for receive light
processing. Thus, this mentioned design is not a robust sensor
probe design when using freespace optics and fiber-optics.
[0006] Others such as Conax Buffalo Corp. U.S. Pat. No. 4,794,619,
Dec. 27, 1988 have eliminated the freespace light path and replaced
it with a MMF made of Sapphire that is later connected to a silica
MMF. The large Numerical Aperture (NA) Sapphire fiber captures the
Broadband optical energy from an emissive radiative hot source in
close proximity to the Sapphire fiber tip. Here the detected
optical energy is measured over two broad optical bands centered at
two different wavelengths, e.g., 0.5 to 1 microns and 1 to 1.5
microns. Then the ratio of optical power over these two bands is
used to calculate the temperature based on prior 2-band power ratio
vs. temperature calibration data. This two wavelength band power
ratio method was described earlier in M. Gottlieb, et. al., U.S.
Pat. No. 4,362,057, Dec. 7, 1982. The main point is that this
2-wavelength power ratio is unique over a given temperature range.
Using freespace optical infrared energy capture via a lens, a
commercial product from Omega Model iR2 is available as a
temperature sensor that uses this dual-band optical power ratio
method to deduce the temperature. Others (e.g., Luna Innovations, V
A and Y. Zhu, Z. Huang, M. Han, F. Shen, G. Pickrell, A. Wang,
"Fiber-optic high temperature thermometer using sapphire fiber,"
SPIE Proc. Vol. 5590, pp. 19-26, 2004.) have used the Sapphire MMF
in contact with a high temperature handling optical crystal (e.g.,
Sapphire) to realize a temperature sensor, but again the
limitations due to the use of the MMF are inherent to the
design.
[0007] It has long been recognized that SiC is an excellent high
temperature material for fabricating electronics, optics, and
optoelectronics. For example, engineers have used SiC substrates to
construct gas sensors [A. Arbab, A. Spetz and I. Lundstrom, "Gas
sensors for high temperature operation based on metal oxide silicon
carbide (MOSiC) devices," Sensors and Actuators B, Vol. 15-16, pp.
19-23, 1993]. Prior works include using thin films of SiC grown on
substrates such as Sapphire and Silicon to act as Fabry Perot
Etalons to form high temperature fiber-optic sensors [G. Beheim,
"Fibre-optic thermometer using semiconductor-etalon sensor,"
Electronics Letters, vol. 22, p. 238, 239, Feb. 27, 1986; L. Cheng,
A. J. Steckl, J. Scofield, "SiC thin film Fabry-Perot
interferometer for fiber-optic temperature sensor," IEEE Tran.
Electron Devices, Vol. 50, No. 10, pp. 2159-2164, October 2003; L.
Cheng, A. J. Steckl, J. Scofield, "Effect of trimethylsilane flow
rate on the growth of SiC thin-films for fiber-optic temperature
sensors," Journal of Microelectromechanical Systems, Volume: 12,
Issue: 6, Pages: 797-803, December 2003]. Although SiC thin films
on high temperature substrates such as Sapphire can operate at high
temperatures, the SiC and Sapphire interface have different
material properties such as thermal coefficient of expansion and
refractive indexes. In particular, high temperature gradients and
fast temperature/pressure temporal effects can cause stress fields
at the SiC thin film-Sapphire interface causing deterioration of
optical properties (e.g., interface reflectivity) required to form
a quality Fabry-Perot etalon needed for sensing based on SiC film
refractive index change. Note that these previous works also had a
limitation on the measured unambiguous sensing (e.g., temperature)
range dictated only by the SiC thin film etalon design, i.e., film
thickness and reflective interface refractive
indices/reflectivities. Thus maker a thinner SiC film would provide
smaller optical path length changes due to temperature and hence
increase the unambiguous temperature range. But making a thinner
SiC film makes the sensor less sensitive and more fragile to
pressure. Hence, a dilemma exists. In addition, temperature change
is preferably estimated based on tracking optical spectrum minima
shifts using precision optical spectrum analysis optics, making
precise temperature estimation a challenge dependent on the
precision (wavelength resolution) of the optical spectrum analysis
hardware. In addition, better temperature detection sensitivity is
achieved using thicker films, but thicker etalon gives narrower
spacing between adjacent spectral minima. Thicker films are harder
to grow with uniform thicknesses and then one requires higher
resolution for the optical spectrum analysis optics. Hence there
exists a dilemma where a thick film is desired for better sensing
resolution but it requires a better precision optical spectrum
analyzer (OSA) and of course thicker thin film SiC etalons are
harder to make optically flat. Finally, all to these issues the
Fabry-Perot cavity spectral notch/peak shape spoiling due to
varying cavity material parameters that in-turn leads to
deterioration in sensing resolution.
[0008] Material scientists have also described non-contact laser
assisted ways to sense the temperature of optical chips under
fabrication. Here, both the chip refractive index change due to
temperature and thermal expansion effect have been used to create
the optical interference that has been monitored by the traditional
Fabry-Perot etalon fringe counting method to deduce temperature.
These methods are not effective to form a real-time temperature
sensor as these prior-art methods require the knowledge of the
initial temperature when fringe counting begins. For industrial
power plant applications, such a prior knowledge is not possible,
while for laboratory material growth and characterization, this a
prior knowledge is possible. As shown later in this application,
our described sensor designs solve this problem and no longer need
the initial temperature data as real-time fringe counting is not
used. Prior works in this general laser-based materials
characterization field include: F. C. Nix & D. MacNair, "An
interferometric dilatometer with photographic recording," AIP Rev.
of Scientific Instruments (RSI) Journal, Vol. 12, February 1941; V.
D. Hacman, "Optische Messung der substrat-temperatur in der
Vakuumaufdampftechnik," Optik, Vol. 28, p 115, 1968; R. Bond, S.
Dzioba, H. Naguib, J. Vacuum Science & Tech., 18(2), March
1981; K. L. Saenger, J. Applied Physics, 63(8), Apr. 15, 1988; V.
Donnelly & J. McCaulley, J. Vacuum Science & Tech., A 8(1),
January/February 1990; K. L. Saenger & J. Gupta, Applied
Optics, 30(10), Apr. 1, 1991; K. L. Saenger, F. Tong, J. Logan, W.
Holber, Rev. of Scientific Instruments (RSI) Journal, Vol. 63, No.
8, August 1992; V. Donnelly, J. Vacuum Science & Tech., A
11(5), September/October 1993; J. McCaulley, V. Donnelly, M.
Vernon, I. Taha, AIP Physics Rev. B, Vol. 49, No. 11, 15 Mar. 1994;
M. Lang, G. Donohoe, S. Zaidi, S. Brueck, Optical Engg., Vol. 33,
No. 10, October 1994; F. Xue, X. Yangang, C. Yuanjie, M. Xiufang,
S. Yuanhua, SPIE Proc. Vol. 3558, p. 87, 1998.
SUMMARY DESCRIPTION OF THE INVENTION
[0009] Ideally, one would like a robust optical sensor that can be
remoted, is minimally invasive, works at high temperatures (e.g.,
2000.degree. C.) and pressures including chemically corrosive
environments, requires low cost low loss optics, has high sensing
resolution over any extended wide unambiguous range, and provides
easy access to many sensing points. In commonly assigned U.S.
patent application Ser. No. 11/185,540 incorporated herein by
reference, modules for the needed extreme environment minimally
invasive optical free-space laser beam targeted optical sensor
using preferably single crystal Silicon Carbide (SiC) optical
sensor chip(s) acting as a natural Fabry-Perot Etalon(s) are
described. A key point in these designs is that laser light is
launched into freespace using a Single-Mode Fiber (SMF); hence the
light beam striking the temperature sensing SiC crystal has
excellent spatial coherence (or collimation properties), making the
optically sensed reading highly accurate and sensitive. The present
application describes robust optical probe designs for these
earlier described SiC single crystal optical sensors, including
novel techniques for measurement of refractive index, temperature,
pressure, and thickness.
[0010] This application describes self-calibrating and aligning
aspects of the probe design via the use of SMF optics.
Specifically, the SMF acts as a confocal optical system that
insures that the sensor is properly aligned and hence providing the
correct sensing data. It is shown that for a given fixed
temperature, the described sensor set-up can provide accurate
optical sample thickness measurements using the known Sellmeier
equation (that gives the wavelength dependent refractive index) for
the given sample. Similarly, knowing the sample thickness, the
sample Sellmeier equation can be found.
[0011] Mechanical motion of alignment mirrors have been described
to keep the freespace laser beam targeting on the SiC chip and
receive optical detector. In the present application, described is
a simpler non-mirror electronically actuated mechanical alignment
assembly technique for the SMF-Lens combination so the sensor probe
can implement fast real-time alignment operations for the
free-space beam striking the sensing SiC single crystal that is
acting as a temperature sensor. In other words, using for example
fine tilt control piezo-electric motion control mechanics on the
entire SMF-Fiber lens (FL) assembly, one can accurate point the
laser beam (LB) to the correct retroreflective position on the SiC
chip, thus enabling robust temperature probe design in harsh
environments.
[0012] Described are packaged probe designs using laser-bonded
single crystal SiC chip on-to a SiC (e.g., re-crystallized SiC)
tube assembly that provides an excellent match of the SiC tube and
SiC single crystal material Coefficients of Thermal Expansions
(CTEs), hence preventing breaking of the SiC chip due to extreme
temperature swings. Also provided is a vacuum sealed tube design
using a glass window made for example from low CTE Borosilicate
Glass (called Pyrex by Coming) or low CTE (0.55 microns/meter-deg.
C.) synthetic fused silica or BK7 Glass (Schott Glass). Key points
to note is that the SiC tube materials (e.g., polycrystalline SiC)
has a lower Coefficient of Thermal Conductivity (CTC: units W/m-K
of 33 to 270 W/K-m)) than high optical quality single crystal SiC
(CTE: 330 W/K-m), yet both have similar CTEs around 4.2 to 4.68
ppm/K. This helps in preventing heat transfer from the hot chip end
of the tube to the glass window end of the tube. Further low CTC
tubing such as made from ceramics such as Alumina Al.sub.2O.sub.3
or Silicon (CTC: 150 W/K-m) is also used from the glass window edge
to the cooler insertion point zone of the probe where the alignment
and launch-receive optics sits in an ambient (e.g., <65.degree.
C.) temperature chamber. The described probe can also use
convention cooling on the tubing near the transmit/receive fiber
and bulk optics assembly to keep temperatures down to
<65.degree. C. in the optical enclosure, as typically needed for
SMF optics. Such a probe is ideal for power plant systems where
extreme temperatures range from 1400.degree. C. to 1600.degree. C.
in the hot zones of combustion chambers that are near a meter
inside the chamber with chamber wall temperatures around
300.degree. C. to 500.degree. C. Using a variety of SiC tubes and
other low CTC ceramic tubes, various probe designs (see FIG. 3-4)
are described for extreme temperature, pressure, and corrosion
conditions.
[0013] Described in FIG. 5 is a multi-tip probe design using
multiple independent SiC crystal chips in separate SiC tubes all
encased in one larger SiC tube to enable fault-tolerant and
self-calibrating sensing.
[0014] An example application of the described hybrid SMF-freespace
temperature probe shown in FIG. 6 is a temperature sensing probe
for moving parts such as a wheel of an aircraft landing gear or a
turbine blade in a aircraft engine or power generation system.
Described is an assembly to meet these temperature sensing
requirements when temperature of a moving part is to be determined.
Here, if needed, one can use a higher power laser to burn-off any
debris and the infrared (IR) red laser to read the moving part
temperature.
[0015] FIG. 7 shows an alternate mounting condition for the SiC
chip that is less effected by pressure differentials between the
two faces of the chip.
[0016] Described also is a probe design in FIG. 8 that can
simultaneously measure temperature and pressure using localized
optical beam targeting for temperature sensing and spatial or
global optical targeting of SiC crystal chip for pressure sensing
via optical interference pattern detection and processing.
[0017] General applications for the described sensor include use in
fossil fuel-based power systems, aerospace/aircraft systems,
satellite systems, deep space exploration systems, and drilling and
oil mining industries. In other words, both extremely hot or
extremely cold conditions can use the described temperature probes.
In fact, cold conditions tend to be more inert and stable and hence
one can expect exceptional performance from the described SiC chip
and tube-based probes.
[0018] An optical pressure sensor can be designed when using a High
Pressure Capsule (HPC) design where the HPC uses the single crystal
SiC chip as an optical window accessed by a laser beam. The HPC can
be made with the sintered SiC material so CTE's of chip/window and
HPC material are matched to prevent thermal deformation effects on
the chip. The sensors can also be designed into small fiber-optic
packages.
DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 shows an example embodiment of etalon measurement
system using hybrid freespace-fiber optics and wavelength selective
interferometry and processing.
[0020] FIG. 2 shows another example embodiment of a high spatial
resolution etalon measurement system design using an imaging type
GRIN fiber lens.
[0021] FIG. 3 shows another example embodiment for a probe package
design using SiC and low thermal conductivity tubes.
[0022] FIG. 4. shows another example embodiment for a probe design
using SiC tube-on-tube protection.
[0023] FIG. 5. shows an example embodiment for a Fault-tolerant
self-Calibrating probe design.
[0024] FIG. 6. shows an example embodiment for a Wireless
fault-tolerant probe design for moving parts temperature
sensing.
[0025] FIGS. 7a-7b show an example embodiments of a mounting
mechanism for the SiC crystal chip on (a) a SiC tube and (b) a SiC
substrate mount using laser bonding.
[0026] FIG. 8 shows an example embodiment for a design of
simultaneous temperature and pressure sensing probe.
[0027] FIG. 9 shows an example embodiment of a SiC-chip based
remote wireless optical pressure sensor.
[0028] FIG. 10 shows a Weak Lens (WL) optical ray-trace model that
describes how the SiC chip acts as a pressure dependant concave
lens that diverges and magnifies the input laser beam.
[0029] FIGS. 11a-11b show an example embodiments of a compact
fiber-connected SiC frontend sensor designs for (a) temperature and
(b) pressure sensing.
[0030] FIG. 12 shows an example embodiment of a packaged
pressure-only sensor design for long range access of hot zone.
[0031] FIG. 13 shows an example embodiment of a Wireless Hybrid
Optical Sensor for Simultaneous Temperature and Pressure
Measurements for short access points.
DETAILED DESCRIPTION OF THE INVENTION
[0032] Knowing the exact thickness of optical components is a
critical need when designing high quality large, miniature, and
super small optics for numerous platforms such as
integrated-waveguide optics, bulk-optics, and fiber-optics. Over
the years, many methods have been described to measure thickness
from the thin-film level (i.e., smaller than the optical
wavelength) to thick plates (i.e., hundreds of wavelengths).
Perhaps the most tried method is based on the classic Michelson
optical interferometry using a broadband optical source such as
white light [see P. A. Flourney, R. W. McClure, G. Wyntjes, "White
light interferometric thickness gauge," Appl. Opt., 11, 1907
(1972); L. M. Smith and C. C. Dobson, "Absolute displacement
measurements using modulation of the spectrum of white light in a
Michelson interferometer," Appl. Opt., 28, 3339 (1989)]. Here, the
short coherence length of the broadband source is used to produce
interferometer arms path-length difference-based output detected
white light fringes that are processed to obtain the sample
thickness measurement. These white light fringes can be captured in
the time domain by scanning a reference mirror over a known scan
range and recording the fringe power, with the maximum optically
detected power position giving data to calculate the sample
thickness. This method has also been called low coherence
reflectometry or time domain optical coherence tomography (OCT)
[see R. C. Youngquist, S. Carr, and D. E. N. Davies, "Optical
coherence-domain reflectometry: a new optical evaluation
technique," Opt. Lett. 12, 158 (1987); K. Takada, I. Yokohama, K.
Chida, and J. Noda, "New measurement system for fault location in
optical waveguide devices based on an interferometric technique,"
Appl. Opt., 26, 1603 (1987); M. Haruna, M. Ohmi, T. Mitsuyama, H.
Tajiri, H. Maruyama, and M. Hashimoto, "Simultaneous measurement of
the phase and group indices and the thickness of transparent plates
by low-coherence interferometry," Opt. Lett. 23, 966 (1998)].
[0033] An alternate twist to white light interferometry is
wavelength scanning interferometry where the broadband source is
produced in time by tuning a laser and the interferometer output is
observed on a per wavelength basis [see Y. Ishii, J. Chen, and K.
Murata, "Digital phase-measuring interferometry with a tunable
laser diode," Opt. Lett., 12, 233 (1987); M. Suematsu and M.
Takeda, "Wavelength-shift interferometry for distance measurements
using the Fourier transform technique for fringe analysis," Appl.
Opt., 30, 4046 (1991); F. Lexer, C. K. Hitzenberger, A. F. Fercher,
and M. Kulhavy, "Wavelength-tuning interferometry of intraocular
distances," Appl. Opt., 36, 6548 (1997); K. Hibino, B. F. Oreb, P.
S. Fairman, and J. Burke, "Simultaneous measurement of surface
shape and variation in optical thickness of a transparent parallel
plate in wavelength scanning Fizeau interferometer," Appl. Opt.,
43, 1241 (2004)].
[0034] For instance, the interferometer output can be dispersed
into spatially independent bins representing different wavelengths
along a linear detector array. In this case, by Fourier
transforming the spatially observed optical spectrum, the sample
thickness is determined without using any motion of the reference
mirror or sample along the optical axis. In a similar fashion by
temporally sweeping the laser wavelength and temporally Fourier
transforming the fringe data, one can determine the sample optical
thickness. This Fourier domain approaches are now be referred to as
spectral domain OCT. Even ultra-fast light in the TeraHertz (THz)
bandwidth range has been described for measuring optical thickness
[see F. Huang, J. F. Federici, and D. Gary, "Determining thickness
independently from optical constants by use of ultrafast light,"
Opt. Lett. 29, 2435-2437 (2004)].
[0035] As is clear with the previously mentioned techniques that
are considered fore-runners in thickness measurement instruments,
one must use broadband light of the widest optical spectrum, as the
shorter the coherence length, the finer the resolution of the
thickness measurement. This in-turn puts an extreme performance
requirement on all components used to assemble the interferometer
(e.g., Michelson, Fizeau); in particular, material dispersion
effects in the optical components can become significant, not to
mention increased non-linear effects arising from non-perfect
linear tuning of the laser. In addition, interference fringe power
data is acquired across the entire optical spectrum to enable the
best approximation of the sample thickness. Thus, the broader the
spectrum to get a better measurement resolution, more data points
are needed to obtain this resolution. Finally and most importantly,
a fundamental limit with these broadband interferometric
measurement methods is that they ignore the test sample material
dispersion effects and hence indeed are only approximations to the
real sample thickness. In fact, the instrument designer faces a
dilemma where a broader spectrum is expected to give the better
measurement resolution but at the same time will add more component
and sample material dispersion effects to the measurement, hence
reducing the accuracy of the measurement.
[0036] In this application, described is a solution to eliminate
the effects of using very broadband light, whether in the
interferometer optics or the sample. First, described is the use of
a direct free-space material dispersion-free laser beam to
interrogate the test sample in air that is expected to naturally
act as the interferometer via the Fabry-Perot effect. Thus removed
is the need for many material dispersion-free optical components
required to assemble for example a Michelson interferometer.
Second, described is the use of only a few (e.g., five) closely
spaced accurately measured discrete wavelengths, thus removing the
need to acquire optical power data over a very broad continuous
optical spectrum. Third, sample refractive index data at only the
chosen few (e.g., five) adjacent wavelengths is needed for
processing. Note that today extensive and accurate material
dispersion data in the form of the Sellmeier equation is available
for key optical substrate materials such as Silicon (Si) [see D. F.
Edwards, "Silicon (Si)", in E. D. Palik (Ed.), Handbook of Optical
Constants of Solids, Inc., 547 (Academic Press 1985)] and Silicon
Carbide (SiC) [see W. Martienssen and H. Warlimont, Eds., Handbook
of Condensed Matter and Materials Data, XVII (Springer 2005)].
Fourth, the described thickness calculation uses a closed form
expression based on Fabry-Perot interferometry; hence providing an
exact measured value for the sample thickness. Finally, the use of
a Single Mode Fiber (SMF) coupled to fiber lens optics insures a
confocal self-aligning set-up and hence provides the launching and
reception of a high collimation on-axis wireless optical beam
needed for proper sample access. In effect, the described system is
self-calibrating and insures a true thickness measurement. The rest
of the application describes the described thickness (or
temperature or refractive index) measurement hybrid fiber-freespace
system design. Such a system is ideal for measuring the large
thickness of sample wafers such as Si and 6H--SiC Crystals such as
recently described to form wireless optical sensors for temperature
sensing [see U.S. patent application Ser. No. 11/185,540 and N. A.
Riza, M. A. Arain, and F. Perez, "Harsh Environments Minimally
Invasive Optical Sensing Technique for Extreme Temperatures:
1000.degree. C. and Approaching 2500.degree. C.," in Proceedings of
the 17.sup.th Optical Fiber Sensors Conference, (SPIE 2005) Vol.
5855, p. 687; N. A. Riza, M. A. Arain, and F. Perez, "Harsh
Environments Minimally Invasive Optical Sensor using Freespace
Targeted Single Crystal Silicon Carbide," IEEE Sensors J., accepted
(2005); N. A. Riza, M. A. Arain, and F. Perez, "6-H Single Crystal
Silicon Carbide Thermo-optic Coefficient Measurements for Ultra
High Temperatures up to 1273 K in the Telecommunications IR band,"
J. Appl. Phys., 98, (2005)]. FIG. 1 shows the described hybrid
fiber-freespace thickness sensor system that is based on this
earlier high temperature freespace targeted temperature sensor
design. Because this system relies on the natural Fabry-Perot
interferometry produced by the sample placed in air (and if needed
vacuum), the optical sample must be a non-scattering partially
reflecting (or semi-transparent) parallel-plate structure over the
optical observation spot. Earlier, a multi-beam interference
approach for thickness measurements was described that was focused
on using broadband light for thin-film (<.lamda.) thickness
measurements for films on thick substrates [see R. Swanepoel,
"Determination of the thickness and optical constants of amorphous
silicon," J. Phys. E: Sci. Instrum. 16, 1214 (1983)]. This study
concluded that direct use of all the wideband spectra optical power
maxima (or minima) and related wavelength values is not a very
accurate method for measuring the thin film thickness. In fact, it
was correctly shown that although multi-beam interferometry
provides an exact closed form expression for the sample thickness,
the technique is extremely sensitive to the sample material
dispersion data and requires further optical data and processing to
produce better accuracy results [see R. Swanepoel, "Determining
refractive index and thickness of thin films from wavelength
measurements only," J. Opt. Soc. Am. A, 2, 1339 (1985)]. A key
reason for this limitation was the thin-film nature of the sample
that caused a large (e.g., 45 nm) separation between adjacent
spectra maxima (or minima). This large wavelength separation
translated to a large (e.g., 0.05) refractive index change for the
adjacent wavelength positions for the used amorphous silicon
material.
[0037] In addition, these earlier multi-beam interferometry methods
provided no instrument self-calibration (or self-alignment) method
so the placement of the sample guaranteed a true thickness reading.
In an aspect of the invention, an example method for measuring a
thickness of an etalon may include directing a first light beam at
a first frequency and a second light beam at a second frequency at
a first portion of an etalon having a known characteristic and
receiving the first light beam and the second light beam from the
etalon. The method may also include identifying a first condition
of the first light beam and a second condition of the second light
beam received from the etalon; and calculating a first thickness of
the etalon responsive to the first condition, the second condition,
and the known characteristic. The first condition may include at
least one of an interference maximum and an interference minimum of
the first light beam and the second condition may include at least
one of an interference maximum and an interference minimum of the
second light beam. The method may further include determining
respective refractive indices of the etalon for the at least one of
the interference maximum and the interference minimum of the first
light beam and for the at least one of the interference maximum and
the interference minimum of the second light beam, such as by using
the known Sellmeier equation and then calculating the thickness
according to equation (2) above.
[0038] The described FIG. 1 system solves these problems by using a
hybrid design that uses both fiber and free-space optics and
etalons such as Si and 6H--SiC whose material dispersion data via
the Sellmeier equations are accurately available in the literature
[see D. F. Edwards, "Silicon (Si)", in E. D. Palik (Ed.), Handbook
of Optical Constants of Solids, Inc., 547 (Academic Press 1985); W.
Martienssen and H. Warlimont, Eds., Handbook of Condensed Matter
and Materials Data, XVII (Springer 2005)]. Furthermore, the typical
mentioned Si and 6H--SiC substrates are thick (d>>probe
wavelength .lamda., e.g., d=300 .mu.m) leading to small
inter-wavelength gaps (e.g., <2 nm) that reduces thickness
measurement errors due to inaccuracies in prior reported wavelength
dependent refractive index data. These sample conditions are indeed
met for the concerned temperature sensor applications using Si and
SiC substrates.
[0039] The FIG. 1 system 10 uses two optical processing trains.
First, a moderate bandwidth (e.g., 10 nm) broadband source 12
directs light beams at the sample 26, such as an etalon, via
control of a 2.times.1 fiber-optical switch 14. Light beams from
the broadband source 12 pass via the switch 14 to enter a
fiber-optic circulator 16 that directs the light via an SMF 18 to a
fiber lens 20. This fiber lens 20 is a special Gradient Index
(GRIN) lens that produces an output Gaussian beam 30 with its
minimum beam waist 24 located at a distance d from the output GRIN
lens surface 22. The location of the freespace beam waist 24 also
marks the location of the sample plane. Note that this minimum beam
waist 24 location is also where the laser beam has near perfect
collimation, a condition required for high quality Fabry-Perot
interferometry. Hence, plane waves are launched into the parallel
faces of the sample 26 at the localized minimum beam waist 24 spot.
To profile the entire sample 26, the sample 26 is physically
translated in the plane of the beam cross-section by translation
element 40 or stage. For high accuracy thickness measurements, the
beam 30 must strike at normal incidence on the sample 26. In the
described system, this is naturally achieved by aligning the sample
26 to maximize the received optical power coupled back into the SMF
18. In other-words, only when the sample acts like a
retro-reflecting surface in the minimum beam waist 28 plane of the
incident beam 30 does one get the lowest freespace-to-SMF coupling
loss [see M. van Buren and N. A. Riza, "Foundations for low loss
fiber gradient-index lens pair coupling with the self-imaging
mechanism," Applied Optics, Appl. Opt., 42, 550 (2003)]. Thus by
monitoring the received optical power, one can conclude that the
sample 26 is correctly aligned for a true thickness measurement.
This same principle is also true for a true temperature (or
refractive index) measurement via the optical chip (e.g., SiC
crystal) used in the FIG. 1 system that acts as a freespace laser
beam targeted temperature sensor.
[0040] Because the sample refractive index and optical loss due to
all system components is known, one can use Fresnel reflection
coefficient theory to estimate the maximum optical power reflected
from a given substrate. Hence, knowing the total expected losses
from all components in the system including a test sample, one can
approximate the expected optical power detected when the sample 26
is correctly aligned. In short, the described system provides the
self-calibration feature needed for true measurements via classic
Fabry-Perot interferometry. This maximized light re-coupled into
the SMF 18 passes via the circulator 16 and another fiber-optic
1.times.2 switch set 32 such that the sample reflected broadband
light enters an optical receiver, such as fiber-coupled optical
spectrum analyzer 34. The nature of the used broadband source is to
provide a few observable interference fringes for the output
spectrum. Hence, perhaps two to 5 fringes are needed to add a 2 to
5 redundancy into the described thickness measurement. Hence,
unlike previous approaches that rely on extensive broadband data
over continuous and very wide spectra, the described sensor only
needs spectra data over a narrow (e.g., 10 nm) range. Specifically,
the OSA 34 is used to read the wavelength location of say 5 fringe
minima positions. The accuracy of the wavelength reading depends on
the resolution of the OSA 34, both in terms of wavelength and
optical power reading. Thus, using the broadband source and OSA 34,
a first reading of wavelength positions is noted. Next, the two
optical switches 14, 32 in the FIG. 1 system are flipped and a
tunable laser 36 and optical power meter 38 are switched into the
measurement system 10 to take a second reading for wavelength
positions. Here the tuning resolution of the laser 36 combined with
the accuracy of the power meter 38 will determine the accuracy of
the taken wavelength readings. Hence, two sets of wavelength
readings can be taken to add fault-tolerance to the measurement
system. Pairs of these adjacent spectral minima readings in
conjunction with the pre-known sample refractive index data via the
known Sellmeier equations is then used to compute the sample
thickness value at the given probed sample location set by the
mechanical translation stage.
[0041] It is well known that the optical power reflectance from a
Fabry-Perot cavity is given by:
R FP = R 1 + R 2 + 2 R 1 R 2 cos .phi. 1 + R 1 R 2 + 2 R 1 R 2 cos
.phi. , ( 1 ) ##EQU00001##
where R.sub.1 and R.sub.2 are the Fabry-Perot etalon front and back
mirror reflectances, respectively. .phi. is the round-trip
propagation phase accumulated by an optical beam while passing
through the sample etalon of thickness t and refractive index
n(.lamda.) at a wavelength of .lamda., and is given by
.phi. = 4 .pi. n ( .lamda. ) t .lamda. ##EQU00002##
at normal incidence. For the test substrate sample in air,
R.sub.1=R.sub.2=R=r.sup.2, where r=[n(.lamda.)-1]/[n(.lamda.)+1] is
the Fresnel amplitude reflection coefficient of an air-sample
interface. Eq. 1 indicates that the maximum reading of the
described FIG. 1 system are taken when cos(.phi.)=1 or
.phi.=2.pi.m, where m=0, 1, 2, 3, . . . , while the minimum reading
of the received optical power are taken when cos(.phi.)=-1 or
.phi.=(2m-1).pi.. Hence, as the wavelength .lamda. of the system
optical source engaging the sample changes from one spectrum
minimum (or maximum) position to the adjacent minimum position, the
optical path length in the substrate has changed causing .phi. to
change by 2.pi. radians. Given that the first chosen power minimum
occurs at a measured .lamda..sub.1, the sample round-trip
propagation phase accumulated is given by:
.phi..sub.1={4.pi.n(.lamda..sub.1)t/.lamda..sub.1}. Similarly, for
the adjacent power minimum occurring at a measured .lamda..sub.2
value, the sample round-trip propagation phase accumulated is given
by: .phi..sub.2={4.pi.n(.lamda..sub.2)t/.lamda..sub.2}. Given that
for any two chosen adjacent spectra power minima with
.lamda..sub.2>.lamda..sub.1 the roundtrip optical phase changes
by 2.pi., .phi..sub.1-.phi..sub.2 can be written to give the sample
closed-form exact optical thickness value t of:
t = .lamda. 1 .lamda. 2 2 ( .lamda. 2 n 1 - .lamda. 1 n 2 ) , ( 2 )
##EQU00003##
where n(.lamda..sub.1)=n.sub.1 and n(.lamda..sub.2)=n.sub.2.
[0042] The sample substrate cuts can be chosen such that the
material has one refractive index in the plane containing the
linear polarization and these refractive indices are given by the
following Sellmeier equations:
[0043] For 6H:SiC (ordinary index);
n 2 ( .lamda. ) = A + B .lamda. 2 .lamda. 2 - C , ( 3 )
##EQU00004##
where A=1, B=5.5515, C=0.026406, and .lamda. is in .mu.m. In
particular, the crystal or c-axis for the given 6H--SiC chip is
along the optical beam propagation direction and the crystal
ordinary index is given to be normal to the crystal c-axis. Hence
the incident linear polarization sees the given ordinary index in
Eqn. 3 for the 6H--SiC chip.
[0044] For Si,
n 2 ( .lamda. ) = + A .lamda. 2 + B .lamda. 1 2 .lamda. 2 - .lamda.
1 2 , ( 4 ) ##EQU00005##
where .lamda..sub.1=1.1071 .mu.m, .epsilon.=11.6858, A=0.939816,
B=0.00810461, and .lamda. is in .mu.m.
[0045] Do note from FIG. 2 that improved localized thickness
measurements are possible using a focused probe beam such as from a
0.29 pitch imaging type fiber GRIN lens 21. In conclusion, this
application shows that high measurement accuracy from the described
thickness measurement sensor is possible given the high performance
of today's state-of-the-art wavelength tunable lasers, optical
spectrum analyzers, optical power meters, fiber-optics, optical
chip fabrication methods, and well documented optical material
dispersion data. The described method provides a simple,
self-calibrating, non-contact mechanism for accurate optical chip
thickness measurements such as needed for sensors based on a
variety of optical crystal chips.
[0046] According to the description above, in an example
embodiment, a system for measuring a thickness of an etalon may
include a first light source, e.g., tunable laser 36 or broadband
light source 12, for directing a first light beam having a first
wavelength at a first portion of an etalon having a known
characteristic and a second light source e.g., tunable laser 36 or
broadband light source 12, for directing a second light beam having
a second wavelength different from the first wavelength at the
etalon, e.g. sample 26. The system may also include an optical
receiver, such as optical spectrum analyzer 34 or power meter 38,
for receiving the first light beam and the second light beam from
the etalon and for providing a first power signal corresponding to
the first light beam received from the etalon and a second power
signal corresponding to the second light beam received from the
etalon. The system may also include a processor 42 for identifying
a first condition of the first power signal and a second condition
of the second power signal received from optical receiver and
calculating a thickness of the etalon responsive to the first
condition, the second condition, and the known characteristic.
[0047] For extreme temperature, pressure, and corrosive fluid/gas
species environments, the FIG. 1 and FIG. 2 sensing/measurement
systems require appropriate probe designs and packaging. For these
extreme environments, an insertion-type probe design is described
in FIG. 3 where the sensor probe 44/stick is inserted into the
harsh sensing zone 46 via an inlet with a sealed gasket G1 48 and
fitting FT 50 on the probe engaging another high pressure gasket
and fitting in the chamber wall inlet. The sensing chamber gasket
& fitting couples with the sealed gasket-fitting G1 48-FT 50 on
the probe 44, making a temperature, pressure and gas isolating
interface between the extreme sensing environment 46 (e.g.,
combustion chamber, turbine engine, etc) and external ambient
environment 52 where instrument controls and technicians operate
and service the industrial systems. A typical extreme or hot zone
temperature T1 a certain distance equivalent to the L1+L2+L3/2
length of probe (e.g., 100 cm) inside the chamber away from the
internal wall 54 might be T1=1600.degree. C., while the temperature
at the chamber internal wall 54 might be T2=600.degree. C. At the
exterior wall 56 location, the temperature T3 will be lower (e.g.,
400.degree. C.) compared to the interior wall 54. Further, the
probe temperature T4 a certain distance L3 away from the exterior
wall 56 and in the ambient conditions (e.g., 60.degree. C.
temperature) environment will be a much lower value, e.g.,
T4=200.degree. C., due to thermal cooling due to ambient air
convection and thus heat transfer over the probe surface. This T4
to T3 temperature range probe length can have a typical, e.g.,
L4+L3/2=30 cm length in the ambient conditions zone 52 so it is
away from the chamber wall hot zone 46 and hence safer for
handling. An additional thermally insulating probe section 60 Of
length L5 (e.g., 10 cm) is added to take the probe temperature down
from T4 to T5 where T5 is the ambient temperature, e.g.,
T5=60.degree. C.). This T5 temperature section of the probe 44 is
the probe terminal head section "H" 62 that contains a light
directing element, or probe transmit and receive beam conditioning
and control optics 63. The total probe length from hot zone tip
portion 64 to head portion 62 can be estimated to be
L1+L2+L3+L4+L5, e.g., 140 cm. Hence, the freespace light beam 66
must travel a distance L1+L2+L3+L4+L5 (e.g., about 140 cm) to
strike an etalon, for example, 6H--SiC sensor crystal/chip 68. For
the extreme hot temperature conditions, the hotter probe sections
from temperature T1 to T4 (or lengths L1+L2+L3+L4) are vacuum
sealed to prevent air turbulence inside the probe 44 that can
spatially spoil the laser beam 66 to and from the SiC chip 68. The
use of a vacuum in the sealed SiC is a design option for reducing
possible beam spoiling/wander effects on the laser beam 66 in case
the tube is containing air. The L5 section made for a low CTC
material can contain air under favorable ambient conditions with
minimal turbulence. Alternately, the entire length of the probe 44,
i.e., L1+L2+L3+L4+L5 can be vacuum sealed to insure that no air
turbulence affects the propagating a light beam, such as freespace
laser beam 66.
[0048] SiC tubes (labeled as ST's in the FIGS. 3-7) may be made of
a variety of SiC material forms (see Morgan Advanced
Ceramics/HaldenWanger, Germany) such as Halsic-R (recrystallized
SiC), Halsic-I (silicon infilterated SiC), Halsic-S (sintered SiC),
Halsic-RX (recrystallized and doped SiC). Some of these SiC tubes
are porous (e.g., Halsic-R) while others (e.g., Halsic-S) are
impermeable to gases. Hence, also described is possible use of the
porous-type SiC tubes for gas species optical sensing using the
described freespace laser beam sensor probe designs. For vacuum
seal tube designs, the impermeable SiC tubes are required for probe
assembly. If sealing of SiC tubes is not essential for the
described probe design under given applications, then one can
deploy the porous SiC tubes for probe assembly. Typically,
commercial SiC tube are available with lengths up-to 3 m and
outer-diameters ranging from 15 mm to 80 mm and inner diameters
ranging from 5 mm to 66 mm. Hence, as shown in FIG. 3, a SiC tube
e.g. 70a can be inserted into another SiC tube e.g. 70b to enable
longer probe lengths and probes with large surface area for heat
dissipation. For the lower CTC high insulation tubes labeled as "I"
tubes 60 to house the optics 63, other ceramic materials can be
used such as alumina (see Morgan Technical Ceramics, Catalog,
Fairfield, N.J., USA) or sheath materials such as Tantalum,
Molybdenum, Platinum/Rhodium, Inconel 600, Nickel-Chrome based
Super OMEGACLAD XL and insulators such as Hafnium Oxide, Magnesia,
and Alumina (see Omega Exotic Thermocouple Probes Catalog,
Stamford, Conn., USA). Depending on the porosity of the SiC tube
used for the described probe design, the quality of the vacuum
sealing will vary. To make sure excellent vacuum sealing is
possible if needed, one can encase the entire SiC tube 70a-70e and
related optics 63 in one larger non-porous tube such as made of
stainless steel that will realize a high quality vacuum seal.
[0049] In FIG. 3 and related described designs, optics may be
mounted on a thermally stable substrate Base Plate B 72 with
appropriate low CTE and CTC materials. FIG. 3 and later related
designs use a Single Mode Fiber (SMF) 74 coupled to a collimating
Fiber Lens (FL) 76, with alignment mechanism, such as a tip/tilt
element and/or a translation element, controlled precisely by a
piezoelectric motion stage 78 coupled to the FL 76. This
piezoelectrically controlled FL 76 is critical for exactly
targeting the center of the SiC crystal chip C 68 a distance
L1+L2+L3+L4+L5 from the lens. More importantly, the
piezoelectrically controlled FL tip/tilt position makes sure the
laser beam 66 produces a retroreflective or collinear beam that can
be coupled back into the SMF 74. Hence, as in designs in FIGS. 1
& 2, the FIG. 3 probe 44 is self-aligning and self-calibrating
as only for the correct tip/tilt/translation setting of the FL 74
will produce the correct power of the receive coupled light into
the SMF, this insuring that the sensor is correctly calibrated. In
addition, if there are unwanted mechanical vibrations in the probe,
the long length of the freespace laser beam path 67 will not be a
limitation as active laser beam alignment is present in the
described probe 44. Also, hitting the correct spot on a target
etalon, such as the SiC chip 68, for all temperature readings is
important because the chip 68 has a given thickness and refractive
index change behavior with temperature given its specific packaging
with the SiC tube ST1 70a and ST5 70e. As the probe 44 is
calibrated for a given laser beam hit location on the SiC chip 68,
the same location must be struck during all operations of the probe
44. There are a number of companies that make precision motion
controls using piezoelectric ceramics for fiber-optical alignment.
These include ultrafast piezo tip/tilt platform and Z (on axis)
positioner Model S-325 from Physik Instruments (PI), Auburn, Mass.,
USA (and Germany), 2-axis tilter stages from Piezo Systems, Inc.,
Cambridge, Mass., and Nano-MTA series tip/tilt actuators from Mad
City Labs, Madison, Wis. In short, the described probe 44 may use a
state-of-the-art tip/tilt motion stage to make sure the laser beam
66 is correctly aligned in the insulation I 60 and SiC ST tubes
70a-70e and also strikes the SiC crystal chip C 68 for perfect
retro-reflective beam operations. SMF-FL freespace alignment
constraints and quantified limits has been earlier reported in
Martin van Buren and N. A. Riza, "Foundations for low loss fiber
gradient-index lens pair coupling with the self-imaging mechanism,"
Applied Optics, LP, Vo. 42, No. 3, Jan. 20, 2003. and Shifu Yuan
and N. A. Riza, "General formula for coupling loss characterization
of single mode fiber collimators using gradient-index rod lenses,"
Applied Optics, Vol. 38, No. 15, pp. 3214-3222, May 20, 1999.
Erratum, Applied Optics, Vol. 38, No. 30, p. 6292, October 1999.
Light launched from the SMF-FL can preferably use the self-imaging
condition in Martin van Buren and N. A. Riza, "Foundations for low
loss fiber gradient-index lens pair coupling with the self-imaging
mechanism," Applied Optics, LP, Vo. 42, No. 3, Jan. 20, 2003, this
making sure of high coupling efficiency for receive light back into
the SMF. Note FIG. 3 only shows the SMF 74, although as in FIG. 1
and FIG. 2, the SMF 74 is connected to other optics such as a
fiber-optic circulator, tunable and broadband laser, switches, OSA,
and optical power meter and data processor. For alignment purposes,
one can tune the laser to get maximum light power back into the SMF
74 to simulate constructive interference in the Fabry-Perot effect
in the SiC chip 68 under probe calibration conditions. Along with
the SMF 74, an electrical cable E 80 is connected to the
piezo-motion stage M 78 for controls of the stage 78. One can
envision simultaneously using a visible light laser for alignment
purposes while the infrared laser for the temperature sensing
operations. Appropriate optical filters may be used at the receive
optics to prevent any unwanted optical bands from saturating or
adding noise to the photodetection process such as via Black-body
thermal radiation. The SMF 74 and E 80 are in a protective cable PC
82 that has a gasket and fitting inside the I-tube to keep the
optical chamber isolated and clean. The optical bench has an
optional high extinction ratio polarizer optic 84 such as a calcite
crystal polarizer that can improve interference fringe visibility
off the SiC chip 68. One can also use a polarization maintaining
SMF and hence remove the need for the polarizer P 84 in the probe
Head H 62. The SiC tube ST4 70d is closed with an optional glass
window W1 86 that makes the SiC tube assembly e.g. 70a-70e vacuum
tight or in the least air-isolated from the air in the I tube 60
that contains the optical bench. The need for these isolating
windows 86 will depend on the temperature and pressure levels in
the extreme environment, the chosen probe design and dimensions,
and the chamber testing and insertion zone conditions.
[0050] To prevent any possible damage/breakage to the sensing SiC
single crystal chip (labeled as C 68 in the FIGS. 3-8) due to
accidental probe drop or fast hard moving object striking the tip,
an optional protective tip cage 88 can be designed around a portion
of the probe tip 64. Possible laser drilled holes in the SiC tube
cover (ST2 90 in FIG. 4) can allow the hot gases to easily access
the chip 68. FIG. 3 and FIG. 4 shows various designs using the SiC
tube to protect the SiC chip 68. In FIG. 3, ST5 70e SiC tube is
used in two arrangements to protect the SiC chip C 68. Laser
bonding can be used to attach the SiC tubes 70a, 70e to each other
or to the SiC chip 68. In either design in FIG. 3, the hot gases
can access the SiC chip 68 directly producing a direct and fast
thermal contact for fast temperature assessment in the hot
placement zone of the probe 44. In FIG. 4, a closed SiC tube ST2 90
completely protects the SiC chip C 68. In this case, a physical
contact is preferably made between the SiC chip surface and the
inside of the ST2 90 closed tube surface to make fast heat transfer
into the chip. One can also laser drill tiny holes into the ST2
cover 69 to provide access for the hot gases to the SiC chip
surface. Note the inside surface of the ST2 tube cover 69 is
optically rough so light passing through the SiC chip 68 is not
specularly reflected back to the SMF 74.
[0051] The FIG. 4 probe embodiment uses one long SiC tube SL1 92 to
form the sealed chamber. In addition, it uses one long SiC tube ST2
90 to act as a protection tube around ST1 92. Also, an adaptor
insulator tube I1 94 is used to connect to the optical assembly
sitting in another isolated insulating tube I2 96. Here, two
optional glass windows W1 86 and W2 87 are used in the tubes to
realize seals and isolate thermal transfer to optics.
[0052] FIG. 5 shows an alternate design that uses four independent
SiC chips C1, C2, C3, C4, 68a-68d each one mounted in its own SiC
tube assembly 92a-92d as described earlier. In addition, the
optical assembly now has four SMF-FL-M assemblies, each matched to
its own SiC chip target. One large polarizer P 84 is used for all
four optical beams. Four SMFs 74a-74d and related four E 80a-80d
cables exit the optical assembly encased in the insulating tube I2
96. The four FL-M's 76a-76d are mounted in a holder H 98. Using a
1.times.4 optical switch, any one of the SiC chip probes can be
activated to provide the temperature reading. On the other hand,
all four SMFs 74a-74d can be activated simultaneously for multiple
readings of temperature. Hence, the FIG. 5 probe has built-in
fault-tolerance and a self-calibration feature. In other words, all
four SiC crystals 68a-68d and their related probe sub-assemblies
should provide the same temperature reading, given the temperature
is considered the same and localized due to the small size of the
probe tip region. If the readings from any one or more of the four
crystals is different, then the given probe chip is no-longer
calibrated. One can design the probe using different chip
conditions. The baseline design could use four SiC chips 68a-68d of
same thicknesses (and refractive index), and same read and
processing wavelengths. Hence one should expect same temperature
readings for the same normalized optical power data from all four
probe sub-assemblies. This design ensures that the overall
4-channel probe is calibrated and providing correct temperature
readings. One can also choose SiC chips 68a-68d with different
thicknesses and possibly different refractive indices via different
dopant levels in the SiC chips during fabrication. Also, one can
use different wavelengths for processing for the four different
probe sub-assemblies to compute the actual temperature reading. In
short, the FIG. 5 probe design provides four independent yet
simultaneous channels of optical power data that can be used for
multi-dimensional signal processing to produce a robust and highly
accurate temperature measurement. In effect, note that the chip
thicknesses (and refractive indices) and wavelengths used for
optical power data generation can be chosen such that unambiguous
temperature measurements can be made over a designed temperature
range using a particular signal processing formula consisting of a
function of the measured normalized minimal four optical power
values. The nature of the formula is unique to the probe design and
measurement ranges. A simple example formula might be (P1+P3+
P2P4)/(P1+P2+P3+P4+ P1P2). The idea is that the function value is
unique over the design temperature range and hence the temperature
measurement is unambiguous. Of course, the earlier described two
wavelength phase-based signal processing (see N. A. Riza and F.
Perez, "High Temperature Minimally Invasive Optical Sensing
Modules," for which a provisional application was filed on Jul. 23,
2004, Application No. 60/590,672; a second provisional was filed on
Dec. 7, 2004, Application No. 60/633,900; and for which a
non-provisional application for United States patent was filed on
Jul. 20, 2005, application Ser. No. 11/185,540; N. A. Riza, M. A.
Arain, and F. Perez, "Harsh Environments Minimally Invasive Optical
Sensing Technique for Extreme Temperatures: 1000.degree. C. and
Approaching 2500.degree. C.," in Proceedings of the 17.sup.th
Optical Fiber Sensors Conference, (SPIE 2005) Vol. 5855, p. 687; N.
A. Riza, M. A. Arain, and F. Perez, "Harsh Environments Minimally
Invasive Optical Sensor using Freespace Targeted Single Crystal
Silicon Carbide," to appear in IEEE Sensors J., accepted (2005) can
also be used with the described probe designs to produce the
unambiguous temperature data.
[0053] FIG. 6 shows an another example probe 44 that has a wireless
implementation allowing temperature sensing of a moving part such
as a rotating element, such as a wheel 114 or a turbine blade. The
SiC chip C1 101a is mounted on a short SiC tube ST1 110 using laser
bonding. The SiC tube 110 is mounted using a fitting FT1 112 to the
wheel base 104 of the wheel 114. The probe 44 is disposed proximate
to a support structure, such as by being inserted into a wheel axle
rod fitting 108. The probe 44 is based on a SiC tube 100 with an
optional exit glass window W1 86. The optical assembly 63 in the
probe 44 is similar to the earlier described (e.g., FIG. 3) design
with an optional glass window W2 87. The laser beam 66 from the
probe 44 hits the chip C1 101a every one revolution of the wheel
114. The probe 44 can use a pulsed high power laser synchronized
with the wheel rotation to enable high efficiency optical data
processing. Note that multiple SiC chips (e.g., C2 101 b and C3
101c) can be added to the moving part to add
redundancy/fault-tolerance to the measurement system. Also, using
multiple SiC chips at different locations produces a distributed
wireless temperature measurement system. Note that a collinear
additional visible laser beam can be used for alignment as well as
keeping the SiC chip clean off debris, etc, by laser burning any
deposits on the chips 101a-101c.
[0054] FIG. 6 shows a SiC single crystal chip 101a mounting where
the laser bonding zone 116 forms at outer ring on the chip 101a.
Hence, the chip 101a is in a way clamped to the SiC tube 110 with
the internal chip region 118 free to deform under high temperature
high pressure differential conditions. Given a large diameter
(e.g., 5 mm) of the SiC chip 101a, thermal and pressure effects
have minimal local effects in the inner ring (e.g., <3 mm
diameter) of the chip. This inner localized and flat region of the
chip is targeted by the laser beam for proper temperature
readings.
[0055] FIG. 7 shows an alternate mounting of the SiC chip 101a for
moving-parts or stationary but targeted temperature sensing
applications. FIG. 7(a) shows one design using the previously
mentioned SiC chip 101a laser-bonded mounting onto a SiC tube 110.
The key point to note here is that the tube 110 outer diameter is
smaller than the SiC chip 101a diameter, thus allowing the chip
boundary to expand as needed due to temperature effects without
deforming the outer part of the chip 101a. More importantly, under
high pressure conditions, the pressure is the same on both sides of
the chip 101a in the region that is the outer boundary 118 of the
chip 101a. Hence, pressure will essentially not effect the chip
flatness at these boundary 118 location. Hence, the targeted
temperature sensing beam must strike the chip at this outer
free-moving boundary 118 zone of the SiC chip 101a. FIG. 7(b) shows
an alternate mounting design where the SiC chip 101a is laser
bonded to a SiC form (e.g., recrystalized or polycrystalline SiC)
substrate 120 where the entire central region 116 of the SiC chip
101a is laser-bonded to the substrate 120. This design again leaves
the boundary 118 of the chip 101a free for expansion due to thermal
effects. Again, pressure or changing pressure does not effect the
SiC chip 101a as the pressure P is the same on both sides of the
SiC chip 101a.
[0056] Previously, numerous works have been conducted to measure
pressure. Pressure sensors have been built by utilizing the
variation in the resistance or capacitance of a device under
pressures. Prototype silicon carbide (SiC) high temperature
piezoresistive pressure sensors were batch-fabricated at the NASA
John Glenn Research Center by producing the diaphragms using a
chemical micromachining process, and the sensors showed promise and
were demonstrated to operate up to 500.degree. C. [A. A. Ned, A. D.
Kurtz and R. S. Okojie, High temperature pressure sensors made from
silicon carbide, NASA Tech Briefs, LEW-16772, Glenn Research
Center, Cleveland, Ohio, January 2000]. Okojie et al. [R. S.
Okojie, A. A. Ned and A. D. Kurtz, Operation of .alpha.(6H)--SiC
Pressure Sensor at 500.degree. C., 1997 International Conference on
Solid-State Sensors and Actuators, (Institute of Electrical and
Electronics Engineers, Inc., New Jersey, 1997), pp. 1407-1409]
fabricated and tested piezoresistive pressure sensors with full
scale output 40.66 and 20.03 mV at 23.degree. C. and 500.degree.
C., respectively, at 1000 psi. Ziermann et al. [R. Ziermann, J. von
Berg, W. Reichert, E. Obermeier, M. Eickhoff and G. Krotz, A High
Temperature Pressure Sensor with .beta.-SiC Piezoresistors on SOI
Substrates, 1997 International Conference on Solid-State Sensors
and Actuators, (Institute of Electrical and Electronics Engineers,
Inc., New Jersey, 1997), pp. 1411-1414] used Silicon Carbide on
Insulator (SiCOI) to create a piezoresistive pressure sensors and
tested its operation between room temperature and 500.degree. C.
They reported the sensitivity of the device to be 20.2 .mu.V/VkPa
at room temperature. Since these SiC sensors are based on the
principle of piezoresistance, micropipe defects in SiC negatively
impact performance. Further research is necessary to harness the
full potentials of SiC as efficient high temperature pressure
sensors surpassing the capability of silicon-based sensors.
Moreover, these SiC MEMS pressure sensors are not wireless passive
devices as described for our optical sensor. In other words,
electronic power and processing is done on chip that is also being
simultaneously exposed to the changing high pressure and
temperature environment. In effect, all the processing in the chip
must withstand the environmental effects.
[0057] Works on producing a wireless pressure sensor includes: A
Dehennis, K. D. Wise, "A double-sided single-chip wireless pressure
sensor," 15th IEEE International Conference on MEMS, 2002; O Akar,
T Akin, K Najafi, "A wireless batch sealed absolute capacitive
pressure sensor," Sensors and Actuators A: Physical, 2001; G
Schimetta, F Dollinger, R Weigel, "A wireless pressure-measurement
system using a SAW hybrid sensor," IEEE Transactions on Microwave
Theory and Techniques, 2000. This highlighted sensors require
on-chip power plus electronics and contacts that are non-robust to
high temperatures. Another design described is passive, that by M A
Fonseca, J M English, M von Arx, M G Allen, "Wireless micromachined
ceramic pressure sensor for high-temperature applications," Journal
of Microelectromechanical Systems, 2002. Nevertheless, this design
presently has limitations in temperature (<400 C) and pressure
(<7 bars) ranges of operations.
[0058] In silicon technology, p-n junction-isolated piezoresistors
are used as pressure sensors for temperatures less than 175.degree.
C., and silicon-on-insulator (SOI) sensors for temperatures up to
500.degree. C. Other techniques have also been investigated to
measure pressure. Leading fiber-optic sensors such as using fiber
Fabry-Perot interference or in-fiber Bragg Gratings with
wavelength-based processing by use of the fiber wire for light
delivery and light return do not form the needed wireless pressure
sensor (see C. E. Lee and H. F. Taylor, "Sensors for smart
structures based on the Fabry-Perot interferometer," Chapter 9, pp.
249-270, Fiber Optic Smart Structures, Ed. Eric Udd, Wiley, 1995;
R. Duncan, D. Gifford, V. Rajendran, "OFDR tracks temperatures on
power generators," Laser Focus World Magazine, p. 89, October 2003;
A. D. Kersey, et.al., "Fiber Grating Sensors," IEEE/OSA J.
Lightwave Tech., Vol. 15, No. 8, pp. 1442-1463, August 1997; Brian
Culshaw, "Optical Fiber Sensor Technologies: Opportunities and
Perhaps Pitfalls," IEEE/OSA Journal of Lightwave Technology, Vol.
22, No. 1, pp 39-50, January 2004). Optically reflective [L.
Tenerz, L. Smith and B. Hok, A Fiber Optic Silicon Pressure
Microsensor for measurements in Coronary Arteries, in Proc. Sixth
Int. Conf. Solid State Sensors and Actuators, Transducers '91, San
Francisco, 1991, pp. 1021-1023] and interferometric [T. Katsumata,
Y. Haga, K. Minami and E. Esashi, Micromachined 125 .mu.m Diameter
Ultra-Miniature Fiber-Optic Pressure Sensor for Catheter, Trans.
Inst. Electr. Eng. Jpn. Part E, Vol. 120E, 2000, pp. 58-63; J.
Zhou, S. Dasgupta, H. Kobayashi, J. M. Wolff, H. E. Jackson and J.
T. Boyd, Optically interrogated MEMS pressure sensors for
propulsion applications, Opt. Eng., Vol. 40, 2001, pp. 598-604; D.
C. Abeysinghe, S. Dasgupta, J. T. Boyd and H. E. Jackson, A novel
MEMS pressure sensor fabricated on an optical fiber, IEEE Photonics
Tech. Letts., Vol. 13, 2001, pp. 993-995] techniques have also been
investigated. The interferometric techniques were based on
Fabry-Perot interferometer/cavity formed by etching a glass
substrate or the tip of an optical fiber and enclosing the etched
volume with a silicon diaphragm. The materials in these optical
devices were glass and silicon which will melt at the high
temperature environment in NASA planetary applications. Recently
further work in optical pressure sensor has been reported as stated
next, but all have their limitations due to the exposure of their
non-robust sensing element in the extreme NASA environment. These
are: W. Li, D. C. Abeysinghe, J. T. Boyd, "Wavelength Multiplexing
of microelectromechanical system pressure and temperature sensors
using fiber Bragg gratings and arrayed waveguide gratings," Optical
Engg., Vol. 42, 2, pp. 431-438, February 2003; W. Li, D. C.
Abeysinghe, J. T. Boyd, "Multiplexed sensor system for simultaneous
measurement of pressure and temperature," Optical Engineering.,
Vol. 43, 1, pp. 148-156, January 2004; D Guo, W. Wang, R Lin,
"Theoretical analysis and measurement of the temperature dependence
of a micromachined Fabry-Perot pressure sensor," Applied Optics,
Vol. 44, 2, pp. 249-256, Jan. 10, 2005; Y. Zhu, A. Wang, "Miniature
fiber-optic pressure sensor," IEEE Photon. Tech. Lett., Vol. 17, 2,
pp. 447-449, February 2005; J. Xu, G. Pickrell, X. Wang, W. Peng,
K. Cooper, A. Wang, "A novel temperature insensitive optical fiber
pressure sensor for harsh environments," IEEE Photon. Tech. Lett.,
Vol. 17, 4, pp. 870-872, April 2005; D. Donlagic and E. Cibula,
"All-fiber high sensitivity pressure sensor with SiO2 diaphragm,"
Optics Letters, Vol. 30, No. 16, pp. 2071-2073, Aug. 15, 2005. All
these fiber-based optical pressure sensors are non-wireless
design.
[0059] FIG. 8 shows another example probe 44 that can
simultaneously measure temperature and pressure. Temperature is
measured using the previously described probe designs (FIGS. 1-7)
where the localized central flat portion of the SiC chip 68 is
targeted and read by the laser beam 66 to estimate the chip
temperature. In the FIG. 8 design, the same principle is
implemented except that a two dimensional (2-D) large area optical
detector such as a CCD camera is used to measure the optical power.
In this embodiment, a large diameter beam 67 (e.g., 10 mm) is
created by beam expansion optics (lenses S1 127 and S2 126) in the
optical assembly such that this large beam 67 strikes nearly the
entire, or a majority of, surface area of the used large diameter
SiC chip 68. The chip 68 is laser bonded to a large diameter SiC
tube ST1 70a that is vacuum sealed at the other end using a glass
window W1 86. When external pressure P is applied to the outer
surface 69 of the SiC 68 in the probe 44, the chip 68 deforms
in-words. Recall that the SiC chip 68 acts like a Fabry-Perot
cavity. When the pressure deforms the two surfaces of the chip
cavity, like with spherical surfaces as shown in FIG. 8, the
optical interference provided by the chip 68 is not uniform over
the chip surface 71. Hence the received beam directed to the 2-D
camera 122 by the beam splitter 124 shows a given optical
interference pattern for a given pressure P and temperature T
condition. Note that the SiC front 71 and back 69 surfaces will
have unique deformations due to the external pressure effect. Hence
the optical reflections caused by these two independent surfaces
will have pressure and temperature dependent unique reflected
wavefronts that will interfere together at the CCD plane to produce
a unique pressure and temperature dependent 2-D interferogram. By
processing the central zone of the beam on the chip (and hence the
central zone of the detected interferogram), one can compute the
temperature of the chip. This is like the FIG. 1-7 temperature
probe designs. Hence, knowing the effect of temperature on the chip
without pressure effects, one can deduce how much of the global
interference pattern was produced due only to pressure P. Using
advanced image processing, such as in temperature and pressure
processor 128, methods on the measured interferogram and previously
measured temperature only and pressure only probe calibration data,
one can compute the measured temperature and pressure from the
probe design in FIG. 8. The key point to note is that temperature
is a spatially local effect on the SiC chip 68 while pressure is
mostly a spatially distributed or global effect on the SiC chip 68.
Using a small chip diameter can help reduce pressure effects on the
chip and hence make it ideal for a temperature-only probe, while
using large chip diameters will enhance pressure effects on the SiC
single crystal chip 68 allowing both pressure and temperature
measurements. Note that since the SiC chip 68 is 2-D and the
interferogram is 2-D, one could deduce pressure and gas flow
directions/distributions on the SiC chip zone by using image
processing methods.
[0060] A remote pressure sensor is needed in many applications.
Described is Silicon Carbide (SiC) weak lensing effect based
wireless optical sensors that allows safe, repeatable, and accurate
pressure measurement suitable for harsh environments. This
completely passive front-end sensor design uses a remoted
free-space optical beam that targets a single crystal SiC chip
fitted as an optical window within a pressure capsule. With
increasing differential capsule pressure, the SiC chip forms a weak
convex mirror with a changing focal length. By monitoring the chip
reflected unique light beam fringe pattern magnification, pressure
in the capsule is determined. SiC is chosen as the front-end
all-passive sensor material due to its robust mechanical, chemical,
and optical properties when subject to extreme environments with
respect to temperature, pressure and chemically corrosive
conditions.
[0061] FIG. 9 shows an example pressure sensor 130 using high
pressure capsule (HPC). A collimated laser beam 67 passes through a
Beam Splitter (BS) 124 and after traveling a distance d.sub.1
targets the SiC chip 68 fitted as a window in an aperture 133 in a
wall 135 of the High Pressure Capsule (HPC) 132. The beam
reflections from the SiC chip 68 travel a distance of
d.sub.1+d.sub.2 and are captured by an Optical Image Detector (OID)
122. Because laser beams can be highly collimated and the pressure
effect on the SiC chip 68 is a mechanical deformation resulting in
a weak lensing effect, the distance d.sub.1 can be designed to be
rather large, e.g. several meters. Thus, only the SiC-based HPC 132
is placed in the hostile zone while the transceiver module
containing the laser source 36, alignment optics, and the OID 122
is meters away, allowing safe and remote pressure measurement. Note
that the beam reflections from the SiC chip 68 are produced as
reflections from the chip front 71 and back 69 surfaces, giving an
interferometric fringe pattern that is observed by the OID 122.
These fringes contain information about the relative phase
differences between the two SiC surfaces 69,71, and are unique for
a given SiC chip 68. In the absence of any differential pressure,
i.e., pressure inside the capsule 132 is equal to the ambient
atmospheric pressure outside the capsule 132, the SiC chip 68 acts
like a flat mirror. Thus the laser beam 66 after reflection from
the chip 68 continues to diverge in accordance with Gaussian beam
propagation and divergence. However, in the presence of
differential pressure P, the SiC chip with a circular pressure
boundary of radius "a" (in cm) bulges outwards with a maximum
central displacement of w.sub.max (in cm) given by:
w max ( P ) = Pa 4 64 D , and ( 1 A ) w max ( P ) = Pa 4 64 D ( 5 +
v 1 + v ) ( 2 A ) ##EQU00006##
for the Clamped-Edge model (Eqn. 1A) and Supported-Edge model (Eqn.
2A), respectively. Here, D is the SiC rigidity constant and v is
its Poisson's ratio. The SiC chip 68 under differential pressure P
acts as a weak convex mirror or equivalently as a concave lens with
focal length f(P) in cm given by:
f ( P ) = w max 2 ( P ) + a 2 4 w max ( P ) .times. 10 4 cm . ( 3 A
) ##EQU00007##
[0062] FIG. 10 shows the weak lens optical ray-trace model 134 used
to design the described remote pressure sensor where the SiC chip
68 acts like a pressure dependant concave lens 136 that diverges
the input laser beam 66. Thus the beam diameter D(P) measured by
the OID provides a value for the sensed pressure P. For example, at
P=0, f=.infin. and D(P)=D.sub.0, the initial beam diameter on the
OID. Given that the illuminated SiC chip naturally produces a
specific fringe pattern via its Fabry-Perot etalon behavior, a
given chip that produces a linear fringe pattern due to its slight
wedge nature can be used under certain circumstances to design the
pressure sensor. In this case, one can essentially use the OID
measured fringe period to determine the pressure P. Moreover, using
FIG. 10, one can define a pressure dependent sensor magnification
factor M given by:
M ( P ) = D ( P ) D 0 = 1 + ( d 1 + d 2 ) f ( P ) , ( 4 A )
##EQU00008##
with d.sub.1+d.sub.2 in cm and where X(P) is the fringe period for
pressure P and X.sub.0 is the fringe period for P=0. Thus, by
measuring M using the OID, one can remotely deduce the pressure P
using the calibration data stored in the Computer Image Processor
(CIP). For sensor calibration, one uses a reference pressure gauge
to record P versus M data as P is varied over a desired calibration
range. With an increasing temperature of the SiC chip 68, one
expects an increase in chip thickness via thermal expansion and an
increase in material refractive index. Both these factors uniformly
change the optical path lengths for the interfering beams from the
SiC chip surfaces 69, 71, thus causing fringe pattern shifts with
temperature. Nevertheless, the SiC chip weak lens effect that
controls beam magnification is expected to be dominated by the
pressure-based chip deformation, making the described pressure
sensor essentially temperature independent when deploying CTE
matched packaging for the SiC chip. Note that one can tune the
laser wavelength to optimize received optical power so the receive
optical beam boundary at the OID can be clearly measured to access
receive beam magnification change due to pressure effects.
[0063] Single crystal 6H--SiC is a highly desirable front-end
sensor material as its melting temperature is around 2500.degree.
C. Moreover, apart from its resistance to chemical attack and
excellent optical properties, thick (e.g., 300 .mu.m) single
crystal 6H--SiC also has powerful mechanical properties via its
elastic, shear, and bulk modulus values and Poisson ratio.
Therefore, the SiC chip forms a robust extreme environment
front-end sensor for laser beam-based wireless access.
Nevertheless, fiber-optics can play an important role in the
described hybrid sensors by providing a wired light delivery
mechanism to the wireless port position in the sensor system. Well
protected custom Single Mode Fibers (SMFs) made of silica can
operate near temperatures reaching 1000.degree. C., thus forming an
excellent wired non-line of sight delivery mechanism to a location
near the extreme environment where temperatures are still
reasonable compared to the extreme environment temperature (e.g.,
1500.degree. C. in a combustion chamber).
[0064] FIG. 11(a,b) shows some example designs on how SMFs can be
combined with the described SiC chip-based sensing principles to
realize compact versions of the remoted hybrid sensors. In essence,
the distance between the SiC chip and the launch SMF point is
small, forming an all-in-one compact fiber remoted sensing head,
much like traditional SMF sensors. FIG. 11(a) shows a temperature
sensor embodiment 138 that is based on localized or point targeting
of the SiC chip 68 to measure temperature dependent optical path
length change (OPL). By making the light read zone 140 on the chip
68 small compared to the chip size 142, one can essentially remove
the effects of pressure on the chip localized OPL. Such targeting
is achieved using a point-to-point imaging lens 144 formation
between the SMF-free-space interface 148 and the SiC chip C 68.
Because the described chip thickness is small (e.g., 300 .mu.m)
versus the imaging lens focal length (e.g., 3 cm) deployed, the
chip front 71 and back 69 faces retro-reflect the sensing light
back to the SMF 74 for sensor signal processing. Each SMF 74 can
have an optional electronically controlled tilt control stage to
keep the SMF 74 aligned with the SiC chip 68. The entire assembly
can be mounted in an appropriate ceramic package. To enable a
pressure sensor 150, a collimating optical beam architecture shown
in FIG. 11(b) is used where a majority of chip C 68 is illuminated.
The distance between the collimating lens and the SiC chip 68 can
be designed to be as large or small as needed for the remoting
application. As relative pressure between the chip sides builds,
the chip 68 deforms and acts like a convex mirror, reducing the
optical coupling between the SMF 74 and the chip 68. Hence, a
pressure change causes wavefront optical beam spoiling. As SMFs are
very sensitive to wavefront quality, one sees a different coupling
efficiency. Thus by monitoring the SMF coupling efficiency,
pressure can be estimated assuming the chip packaging is designed
such that temperature variations cause minimal chip deformation
effects. In case temperature effects should not be ignored, both
temperature plus pressure may need to be measured to generate a
calibration table for optical coupling efficiency.
[0065] The key to pressure sensing using SiC involves global
targeting of the SiC chip with received light pattern undergoing
image (versus point zone) processing. FIG. 12 shows such a design
using a long remoting probe 44. Here, note the large beam
cross-section of the targeting beam that strikes the SiC acting as
a weak convex mirror under applied pressure. This weak lensing
changes the detected size of the optical beam image at the receiver
optics that can be an optical multi-fiber bundle 154 or a miniature
camera 122 as in FIG. 8. Using image processing, a pressure versus
image size chart can be generated for sensor calibration and
pressure measurement.
[0066] The more powerful sensor is one that can simultaneously
provide both temperature and pressure data as in FIG. 8. FIG. 13
shows an alternate implementation of the described hybrid sensor
concept that uses a remotely placed all-passive optical sensor
capsule 152 made of single crystal SiC chip 68 acting as the
capsule window and a pressure sealed capsule assembly 154 made of a
suitable high pressure high temperature material such as the
previously mentioned sintered SiC tube. The FIG. 13 design is
suited for near access sensing within the hot gas environment of a
power plant. The capsule 152 has a high pressure connector 156 that
interfaces to the high pressure hot gas flow system that is linked
to the high temperature high pressure hot gas flow 158 such as a
fossil fuel plant under test. The SiC optical window sits in a
specially designed sealed pressure seat that creates the desired
high pressure boundary conditions for the deployed SiC chip 68
within the capsule 152. One can also envision the FIG. 13 pressure
sensor module used as a pressure stick inserted into a pressure
sealed cavity to allow deep access pressure readings such as well
within a combustion chamber. Hence, depending on the application,
appropriate packaging must be deployed.
[0067] In FIG. 13, the reflected beam enters a light path
containing a pin-hole (PH1) 160 that accepts only the on-axis rays
from the SiC chip 68. After the pin-hole 160, a point detector D1
162 picks up the optical power from the selected on-axis rays.
Thus, the localized effect is captured by monitoring the on-axis
central rays from the SiC chip 68. As the temperature changes, the
SiC chip optical path length (OPL) changes dominated by the
material refractive index change. By measuring this OPL change, the
temperature of the chip 68 can be measured. On the other-hand, the
sensing straight beam passes through PBS2 164 to impinge on a two
dimensional (2-D) optical detector D2 122 such as a CCD chip. As
pressure changes inside the capsule (outside the capsule is 1 atm),
the SiC chip 68 deforms to produce a convex mirror effect. Note
that only near the on-axis condition are the faces of the chip
normal to the incident beam, hence producing true retro-reflection
to create beams that pass via the pinhole 160 to D1 162 to make the
temperature-only measurement. In effect, as pressure (P) changes,
the on-axis ray bundle remains essentially the same allowing a
pressure independent temperature measurement. It is important to
note that the SiC chip 68 and chip seating in the capsule 152 can
be designed such that the out-of-plane deflection is mainly caused
by internal pressure change with minimal contribution from
temperature (T) effects. Such a design is possible by essentially
using a chip packaging/boundary material with a similar CTE to
single crystal SiC 68 like the sintered SiC tube material where
CTE's match. Pressure (P) plus temperature data is picked up by the
optical detector 122 and then processed to determine true pressure
readings.
* * * * *