COMPACT INSTRUMENT FOR ACCURATE MEASUREMENT OF PRESSURE OF AN ENVIRONMENT
Apparatus and associated methods relate to measuring pressure of an external environment using a graded index (GRIN) lens and first and second Fabry-Perot interferometers, each axially aligned with one another. The GRIN lens collimates a beam diverging from a face of an optical fiber so as to direct the collimated beam to the first and second Fabry-Perot interferometers. The first Fabry-Perot interferometer is pressure isolated from the external environment but not temperature isolated. Therefore, the resonant frequency of the first Fabry-Perot interferometer is indicative of temperature. The second Fabry-Perot interferometer has a cavity that changes dimension in response to changes in pressure and temperature of the external atmosphere. The second Fabry-Perot interferometers has a second resonant frequency that is not integer multiple of the first resonant frequency. Pressure is determined based on the first and second resonant frequencies.
1 . A system for measuring pressure of an external environment, the system comprising:
an optical plate with nominal thickness L 1 (T 0 ) measured along an axis perpendicular to first and second parallel faces that are partially reflective to light normally incident thereon from within the optical plate, thereby forming a first Fabry-Perot interferometer between the first and second parallel faces;
a diaphragm having a mirrored face substantially parallel to and separated from the second face of the optical plate, the diaphragm deforming in response to changes in the pressure of the external environment thereby moving the mirrored face;
an optical cavity formed between the mirrored face of the diaphragm and the second face of the optical plate and spaced by a separating member, the separating member separating the mirrored face of the diaphragm and the second face of the optical plate by a nominal separation distance L 2 (T 0 , P 0 ) forming a second Fabry-Perot interferometer within the optical cavity between the second face of the optical plate and the mirrored face of the diaphragm; and
a graded-index (GRIN) lens that is axially aligned with and adjacent to the first Fabry-Perot interferometer, the GRIN lens configured to receive, at a first face of the GRIN lens, a diverging optical beam projected from an end face of an optical fiber and to collimate the diverging optical beam so as to transmit a collimated optical beam from a second face of the GRIN lens to the first and second Fabry-Perot interferometers.
2 . The system of claim 1 , wherein:
the optical plate is comprised of a material of index n(T) and having a non-zero coefficient of linear thermal expansion α; and
a first fundamental resonant frequency f 1 =c/(2n(T)L 1 (T 0 )(1+α(T−T 0 )) is determined by the first optical path length n(T)L 1 (T 0 )(1+α(T−T 0 )) between the first and second parallel faces.
3 . The system of claim 2 , wherein:
a second fundamental resonant frequency f 2 =c/(2L 2 (T, P)) is determined by the second optical path length L 2 (T, P) between the first and second parallel faces.
4 . The system of claim 3 , further comprising:
an optical interrogator that generates and transmits an optical beam to the optical fiber and receives a reflected portion of the optical beam reflected by the first and second Fabry-Perot interferometers, the optical interrogator further configured to determine the pressure of the external environment based on the reflected portion received.
5 . The system of claim 4 , wherein the first and second optical path lengths are different from one another.
6 . The system of claim 5 , wherein the first and second optical path lengths are not integer multiples of one another throughout the specified ranges of temperatures and pressures of the external environment.
7 . The system of claim 6 , wherein a ratio of the first and second optical path lengths is between 1.3 and 1.7 or between 2.3 and 2.7.
8 . The system of claim 6 , wherein the first and second resonant frequencies are not integer multiples of one another.
9 . The system of claim 4 , wherein the optical interrogator is further configured to scan frequencies of the optical beam generated to sweep through the first and second fundamental resonant frequencies of the first and second Fabry-Perot interferometers, respectively.
10 . The system of claim 9 , wherein the optical interrogator is further configured to determine the first and second fundamental resonant frequencies of the first and second Fabry-Perot interferometers, respectively, based on the reflected portion received.
11 . The system of claim 10 , wherein the optical interrogator determines pressure based on the first and second fundamental resonant frequencies of the first and second Fabry-Perot interferometers, respectively.
12 . The system of claim 11 , wherein the optical interrogator determines temperature based on the first fundamental resonant frequency of the first Fabry-Perot interferometer.
13 . The system of claim 12 , wherein the optical interrogator determines pressure based on the temperature determined and further based on the second fundamental resonant frequency of the second Fabry-Perot interferometer.
14 . A system for of claim 4 , wherein:
neither the first parallel face of the optical plate nor the second parallel face of the optical plate is exposed to the pressure of the external environment.
15 . The system of claim 14 , further comprising:
a hermetically sealed housing that isolates the first and second parallel faces of the optical plate from pressures of the external environment.
16 . The system of claim 4 , wherein the first face of the GRIN lens is fused to the optical fiber.
17 . The system of claim 4 , wherein the separation member circumscribes the cavity.
18 . The system of claim 4 , wherein the optical plate comprises material selected from the group consisting of:
MgAl 2 O 4 spinel ceramic,
aluminum oxynitride Al 23 N 27 O 5 ceramic,
Nd doped YAG,
LaGd doped hafnium or zirconium oxide,
polycrystalline Al 2 O 3 , or
single crystal Al 2 O 3 .
19 . A method for measuring pressure of an external environment, the method comprising:
forming a first Fabry-Perot interferometer between first and second parallel faces of an optical plate with nominal thickness L 1 (T 0 ) measured along an axis perpendicular to first and second parallel faces, which are partially reflective to light normally incident thereon from within the optical plate;
deflecting, in response to changes in the pressure of the external environment, a diaphragm having a mirrored face substantially parallel to and separated from the second face of the optical plate, thereby moving the mirrored face;
forming a second Fabry-Perot interferometer within an optical cavity formed between a mirrored face of a diaphragm and the second face of the optical plate;
separating the mirrored face of a diaphragm and the second face of the optical plate by a nominal separation distance L 2 (T 0 ) using a separating member; and
collimating a diverging optical beam projected from an end face of an optical fiber using a graded-index (GRIN) lens that is axially aligned with and adjacent to the first Fabry-Perot interferometer.
20 . The method of claim 19 , wherein:
the optical plate is comprised of a material of index n and having a non-zero coefficient of linear thermal expansion α;
a first fundamental resonant frequency f 1 =c/(2n(T)L 1 (T 0 )(1+α(T−T 0 )) is determined by the first optical path length n(T)L 1 (T 0 )(1+α(T−T 0 )) between the first and second parallel faces.