Ultra-wide-band saw sensor with hyperbolically frequency-modulated etched reflector
There is disclosed a surface acoustic wave sensor. An interdigital transducer (IDT) and a first reflector are formed on a surface of a piezoelectric substrate. The first reflector is displaced from the IDT in a direction of acoustic wave propagation. The first reflector includes a plurality of elongate reflective elements including a first reflective element and N additional reflective elements, where N is a positive integer. A long axis of each of the reflective elements is perpendicular to the direction of acoustic wave propagation, and a distance between adjacent reflective elements along the direction of acoustic wave propagation is a linear function of distance from the first reflective element along the direction of acoustic wave propagation.
1. A surface acoustic wave sensor, comprising:
a piezoelectric substrate;
an interdigital transducer (IDT) formed on a surface of the substrate; and
a first reflector formed on the surface of the substrate, the first reflector displaced from the IDT in a direction of acoustic wave propagation, wherein
the first reflector comprises a plurality of elongate reflective elements including a first reflective element and N additional reflective elements, where N is a positive integer,
a long axis of each of the reflective elements is perpendicular to the direction of acoustic wave propagation, and
a distance between adjacent reflective elements along the direction of acoustic wave propagation is a linear function of distance from the first reflective element along the direction of acoustic wave propagation.
2. The surface acoustic wave sensor of claim 1 , wherein the first reflector is subject to the equation:
p n =p 0 +ε·x n , and
wherein:
x n is a distance along the direction of acoustic wave propagation from the first reflective element to the n'th reflective element, where n is an integer between 1 and N,
p n =x n+1 −x n is the distance along the direction of acoustic wave propagation between adjacent reflector elements n and n+1; and
p 0 and ε are parameters that define the first reflector.
3. The surface acoustic wave sensor of claim 2 , wherein
p 0 is determined based on one of a minimum frequency F min and a maximum frequency F max of a radio frequency signal to be applied to the IDT,
p N-1 is determined based on the other of the minimum frequency and the maximum frequency, and
ε=( p N-1 /p 0 ) (1/N) −1.
4. The surface acoustic wave sensor of claim 3 , wherein
one of p 0 and p N-1 is equal to V/(2F max ), and
the other of p 0 and p N-1 is equal to V/(2F min ).
5. The surface acoustic wave sensor of claim 1 , wherein
each of the plurality of reflective elements is a groove etched into the surface of the substrate.
6. The surface acoustic wave sensor of claim 5 , wherein depths, normal to the surface of the substrate, of the plurality of reflective elements vary along the direction of acoustic wave propagation.
7. The surface acoustic wave sensor of claim 6 , wherein the depths of the plurality of reflective elements are a linear function of distance from the first reflective element along the direction of acoustic wave propagation.
8. The surface acoustic wave sensor of claim 1 , wherein
each of the plurality of reflective elements is a ridge extending from the surface of the substrate.
9. The surface acoustic wave sensor of claim 8 , wherein heights, normal to the surface of the substrate, of the plurality of reflective elements vary along the direction of acoustic wave propagation.
10. The surface acoustic wave sensor of claim 9 , wherein the heights of the plurality of reflective elements are a linear function of distance from the first reflective element along the direction of acoustic wave propagation.
11. The surface acoustic wave sensor of claim 1 , where the direction of acoustic propagation is parallel to a crystalline axis of the substrate.
12. The surface acoustic wave sensor of claim 11 , wherein the substrate is 128-degree rotated Y-cut LiNbO3 and the direction of acoustic propagation is parallel to an X axis of the substrate.
13. The surface acoustic wave sensor of claim 11 , wherein the substrate is Y-cut LiNbO3 and the direction of acoustic propagation is parallel to a Z axis of the substrate.
14. The surface acoustic wave sensor of claim 1 , wherein widths, normal to the direction of acoustic wave propagation, of the plurality of reflective elements vary along the direction of acoustic wave propagation.
15. The surface acoustic wave sensor of claim 14 , wherein the widths of the plurality of reflective elements are a linear function of distance from the first reflective element along the direction of acoustic wave propagation.
16. A passive sensor system, comprising:
a surface acoustic wave sensor comprising:
a piezoelectric substrate;
an interdigital transducer (IDT) formed on a surface of the substrate;
a first reflector formed on the surface of the substrate, the first reflector displaced from the IDT in a direction of acoustic wave propagation by a first distance; and
a second reflector formed on the surface of the substrate, the second reflector displaced from the IDT in a direction of acoustic wave propagation by a second distance different from the first distance, wherein
each of the first and second reflectors comprise respective first and second pluralities of elongate reflective elements, each plurality of reflective elements including a first reflective element and N additional reflective elements, where N is a positive integer,
a long axis of each of the reflective elements is perpendicular to the direction of acoustic wave propagation, and
for each of the first and second pluralities of reflective elements, a distance between adjacent reflective elements along the direction of acoustic wave propagation is a linear function of distance from the first reflective element along the direction of acoustic wave propagation.
17. The passive sensor system of claim 16 , wherein
the first reflector is disposed to reflect a first portion of an acoustic wave launched by the IDT as a first reflected wave,
the second reflector is disposed to reflect a second portion of the acoustic wave launched by the IDT as a second reflected wave, and
the first and second reflected waves return to the IDT separated in time by an offset time interval proportional to the difference between the first distance and the second distance.
18. The passive sensor system of claim 16 , further comprising:
a reader configured to generate a radio frequency (RF) signal to interrogate the surface acoustic wave sensor, receive a reflected RF signal from the surface acoustic wave sensor, and process the reflected RF signal to determine a duration of the offset time interval.
19. The passive sensor system of claim 18 , wherein the reader is further configured to map the determined duration to a physical parameter of the acoustic wave sensor and/or the environment of the acoustic wave sensor.
20. The passive sensor system of claim 19 , wherein the physical parameter is a temperature of the surface acoustic wave sensor.
21. The passive sensor system of claim 19 , wherein an additional sensitive layer is deposited between the first distance and the second distance, and the physical parameter is a measured changing physical property of said layer.