EARLY WARNING TSUNAMI NETWORK

[0001] This application claims priority to U.S. Provisional Application No. 62/855,884 filed on May 31, 2019, the entire disclosure and contents of which is hereby incorporated by reference.

Background of Invention

A. Scope of the Invention

[0002] This invention allows one to measure seismic activity from a hand-held device, in particular a mobile phone.

B. Summary of the Prior Art

[0003] This invention includes a microfabricated whispering gallery mode (WGM) based seismometer capable of operating from a mobile phone. Converting the sensor output a measurement is performed by a simple cross-correlation operation that requires low processing power.

Summary of the Invention

[0004] The present invention is directed a system and device that allows one to measure seismic activity from a hand-held device, in particular a mobile phone. A microfabricated whispering gallery mode (WGM) based seismometer operates in conjunction with an application installed on a mobile phone. By converting the sensor output from the seismometer, a measurement is performed by the application with a simple cross-correlation operation that requires low processing power.

[0005] In at least a first embodiment, a whispering gallery mode (WGM) based seismometer according to the present invention comprises: a substrate; a resonator fixedly mounted on the substrate; a waveguide fixedly mounted on the substrate and positioned adjacent to the resonator; a laser diode operatively coupled to input laser light into the waveguide; a photodiode operatively connected to receive laser light outputted from the waveguide; and a seismic detector element operatively positioned adjacent to the resonator, wherein when the seismic detector element is perturbed by an external force, whispering gallery modes (WGM) of the resonator shift such that the light transmitted through the resonator and the waveguide changes in response to the shift in the whispering gallery modes of the resonator, and the laser light outputted to the photodiode is excited in response to the changed evanescent field from the resonator.

[0006] In another embodiment, the whispering gallery mode (WGM) based seismometer incorporates a resonator that is a racetrack-shaped resonator fixedly mounted on the substrate, and a waveguide fixedly mounted on the substrate is positioned adjacent a curved portion of the racetrack-shaped resonator.

[0007] In another embodiment, the whispering gallery mode (WGM) based seismometer includes a seismic detector element that is a plate operatively positioned adjacent to the resonator and above the substrate.

[0008] In a further embodiment, the whispering gallery mode (WGM) based seismometer includes a seismic detector element that is a proof mass operatively positioned adjacent to the resonator and on the substrate.

Brief Description of the Drawings

[0009] The present invention is illustrated in the accompanying drawings, wherein:

Figure 1(A) illustrates one implementation of a first embodiment of the whispef ng gallery mode optical resonator according to the present invention;

Figure 1(B) illustrates another implementation of the first embodiment of the whispering gallery mode optical resonator according to the present invention;

Figure 2(A) shows one implementation of a second embodiment of the whispering gallery mode optical resonator according to the present invention;

Figure 2(B) illustrates another implementation of the second embodiment of the whispering gallery mode optical resonator according to the present invention;

Figure 3 illustrates a general block diagram of a WGM based seismometer according to the present invention; and

Figure 4 illustrates the general operation of the present invention.

Detailed Description of the Invention

[0010] The present invention will now be described in the detailed disclosure set forth hereinbelow in conjunction with the drawings as outlined above, wherein a compact seismometer according to the present invention is capable of operating in conjunction with an application installed on a handheld device such as a mobile phone or as part of a handheld device such as a mobile phone. A network of these seismometers will create an early warning system for tsunamis resulting from seismic activities.

[0011] The present invention exploits morphology -dependent optical resonances (MDRs), also known as whispering gallery modes (WGMs) in this case, to measure forces acting on the optical micro-resonator. The dielectric resonators can be of different geometries such as spheres, disks or rings.

Whispering Gallery Mode Resonators

[0012] The shape of whispering gallery mode optical resonators can be microspheres, silicon micro-rings and micro-disks deposited on a substrate. They can also be rings or of“racetrack” (elongated shape) shapes made of silicon deposited onto a substrate, as shown in Figures 1(A), 1(B), 2(A) and 2(B). The waveguide for coupling light into the resonator can be positioned along the length of the resonator (see Figures 1(A) and 2(A)) or the curved side of the resonator (see Figures 1(B) and 2(B)). Resonators deposited on a substrate lend themselves to many applications and are easier to package than a microsphere. In addition, a“racetrack” resonator is more sensitive to morphology changes, which is what applications such as a seismometer requires. Laser light is coupled into the resonators using a waveguide deposited onto the substrate. Typical sizes for resonators are between 500 pm and 25 mm.

[0013] In one implementation of a first embodiment of the optical resonator 100 as shown in Figure 1(A), the optical resonator is formed with a wafer or substrate 102. Atop the substrate 102 there are positioned a racetrack- shaped resonator 104, and a waveguide 106 positioned adjacent and parallel to the straight, long side of the racetrack- shaped resonator 104 to avoid interference that can arise over long coupling sections, while the evanescent field of the waveguide 106 couples light into the resonator 104. Alternatively, as shown in Figure 1(B), in a second implementation of the first embodiment, the waveguide 106’ is positioned along the curved side of and perpendicular to the straight, long side of the racetrack-shaped resonator 104. The gap between the waveguide 106, 106’ and the resonator 104 may be -100 nanometers. The racetrack width can range between 0.5 mm and 25 mm. A plate 108 is suspended above the resonator 104 substantially covering the resonator 104 , but not covering the waveguide 106, 106’. In either implementation, the plate 108 may be positioned 20 pm from the waveguide 106, 106’. The plate 108 may be formed from material whose index of refraction is lower than that of the resonator material. The plate 108 over the resonator 104 perturbs the evanescent field above the resonator 104. The distance between the plate 108 and the resonator 104 changes proportionally with an acceleration that is applied to the plate 108 and the resonator 104. The evanescent field perturbation by the plate 108 causes a wavelength shift in the resonance. The seismometer is microfabricated and can be five millimeter in size or smaller.

[0014] In one implementation of a second embodiment of the optical resonator 200 as shown in Figure 2(A), here was well, the optical resonator is formed as a substrate 202. Atop the substrate 202 there are positioned a racetrack-shaped resonator 204, and a waveguide 206 positioned along and parallel to the straight, long side of the racetrack- shaped resonator 204 to avoid interference that can arise over long coupling sections, while the evanescent field of the waveguide 206 couples into the resonator 204. In this first implementation, a proof mass 210 is positioned adjacent to the resonator 204 on a side opposite of the waveguide 206 and also parallel to the straight, long side of the resonator 204. The proof mass 210 may be formed from either the same material as that of the resonator substrate or material of higher density. Alternatively, as shown in Figure 2(B), in a second implementation of the second embodiment, the waveguide 206’ is positioned along a first curved side of the racetrack- shaped resonator 204 and perpendicular to the straight, long side of the resonator 204. In this second implementation, a proof mass 210’ is positioned adjacent to the resonator 204 on a side opposite of the waveguide 206’ along the second curved side of the racetrack-shaped resonator 204 and also perpendicular to the straight, long side of the resonator 204. The gap between the proof mass 210, 210’ and the resonator 204 may be 100 micrometers or more. The racetrack width can also range between 0.5 mm and 25 mm. When subjected to an acceleration, the proof mass 210 applies a force on substrate 202. The force bends the substrate 202 and stretches the resonator 204 as a result. The stretching of the resonator 204 causes a wavelength shift in the resonance of light passing through the waveguide 206.

[0015] The observed line width of the resonances, <¾ is related to the quality factor, 0=l!dl (i.e., energy transmitted/energy stored). The smaller the energy loss as the light circulates inside the sphere, the larger Q is, with Q -> ¥ as the losses vanish. In order to realize the full benefits of multiple-photon circulation, a high value of Q must be attained.

Dynamic Range [0016] The resonator free spectral range determines the dynamic range of the sensor. The free

A ²

spectral range is defined as FSR =— where L is the resonator path length. However, approaches have been recently developed for avoiding the free spectral range limitation: for instance, fast scanning of the laser bandwidth yields a series of resonances that can then be processed using cross-correlation, thus obviating the need to track one single resonance throughout the scan.

Sensitivity

[0017] The Q factor of the resonator Q = - and signal processing approach for resonance shift

DA

define the sensitivity of the sensor. Based on concurrent work, the micro-fabrication process will yield a resonator with very narrow resonances, i.e., > 10 ⁶ . Simple processing techniques such as cross correlation will allow sub-picometer spectral resolution. Other successful techniques processing techniques are peak detection and Lorentzian fit through the resonance.

Temperature Stability

[0018] The WGM element assembly (base and substrate) is sensitive to temperature. The sensitivity is related to the coefficient of thermal expansion of the WGM resonator and packaging material. It was demonstrated that temperature dependence is repeatable, such that the temperature contribution to the drift can be removed using calibration. In addition, a novel approach consisting of coating or laminating the micro-resonator with materials of negative thermo-optic coefficients (refractive index dependence on temperature) to eliminate temperature drift was theoretically demonstrated.

[0019] The whispering gallery modes (WGM) of the resonator are excited by a narrow-line tunable laser diode, which generates the laser light that is coupled to the waveguide. The waveguide is positioned near the resonator, within the distance of the evanescent field leaking out of the waveguide. At the output of the waveguide, a photodiode detects light level. As the laser is tuned across a narrow range, the WGM are observed as sharp dips in the transmission spectrum observed by the photodiode. When the environment surrounding the resonator or the resonator shape is perturbed, due to an external force acting on a plate suspended above the resonator or on a proof mass mounted on the substrate adjacent to the resonator, respectively, the dips in the spectrum shift allowing precise measurement of the acceleration caused by the perturbances in the environment. In an application such as for a seismometer, such a device needs to be direction- sensitive. The morphology changes that can occur in a WGM based resonator will cause the resonances to shift to either side of the resonance at rest depending on the direction of the force.

[0020] Figure 3 shows the block diagram of the WGM based seismometer 300 according to the present invention. In this embodiment, the WGM based seismometer 300 incorporates a whispering gallery mode optical resonator 304 and a waveguide 302 which receives as an input laser light 303 received from a coherent light source 301, such as a narrow-line tunable laser diode. The light source 306 is coupled to the waveguide 302 so as to input the laser light 303 thereinto. The waveguide 302 is positioned near the resonator 304, within the distance of the evanescent field leaking out of the waveguide 302. At the output of the waveguide 302, a detector 306, such as a photodiode, detects a light level of the laser light 303 that passes through the waveguide 302. As the laser is tuned across a narrow range, the WGM are observed as sharp dips in the transmission spectrum observed by the detector 306. When the environment surrounding the resonator 304 is perturbed such that an external force acts on the seismometer 300, the shape of the resonator 304 is perturbed as a result of such external force acting on the plate 108 (see Figures 1(A)-1(B)) suspended above the resonator 104 or on the proof mass 210, 210’ (see Figures 2(A)-2(B)) mounted on the substrate 202 adjacent to resonator 204, respectively, the dips in the spectrum will shift as a result of the acceleration caused by the external force. These shifts in the dips of the transmission spectrum are quantifiable allowing precise measurement of the acceleration. Because morphology changes will cause resonances to shift to either side of the resonance at rest depending on the direction of the force, such resonance shifts are also quantifiable, in addition to the degree and intensity of the acceleration, such that the seismometer 300 of the present invention is also direction-sensitive.

[0021] Further in the operation of the present invention, the detector 306 generates a detector signal 307 based on the resonances detected from the outputted 305. The detector signal 307 is digitized by an A/D converter 308 and outputted to a data buffer 309, and then to a processor 310. The processor 310 receives the digitized output 311 of the A/D converter 308 via the data buffer 309. The processor 310 then generates the appropriate spectral information based on the resonance patterns in the light received from the waveguide 302. In at least one embodiment, the processing components described above for receiving, converting and processing the detected laser light may be implemented using discrete electronic circuits, field programmable gate arrays (FPGA), or a combination thereof, all as known in the art.

[0022] For purposes of keeping the entire package compact and usable on a large scale, as shown in Figure 4, the present invention may be implemented by integrating the seismometer 400 and its processing electronics (FPGA) a mobile phone 402 and use the phone’s battery for power, or constructing the seismometer 400’ as an add-on device that connects with a mobile phone 404 using the phone’s battery for power or having its own attached battery. The mobile phone 402,404 communicates with the seismometer 400 via an app installed on the phone. The seismometer output can be a voltage range (low voltage -100 mV) proportional to the seismic activity around the individual phone. The mobile phone’s built-in gyros can keep track of the user’s physical motion and processing can extract seismic activity from the motion.

[0023] As shown in Figure 4, a plurality of similarly-equipped mobile phones 406 can form a network 408, such that simultaneous seismic signatures can be recognized from data generated by and transmitted from the network 408 of mobile phones 402, 404, 406. In at least one embodiment of the network 408, the data from the mobile phones 402, 404, 406 could transmitted and shared with each other and/or then be collected and processed by a data collection center 410, such as a server, to then generate data based on the all the data collected from the mobile phones 402, 404, 406 that is representative of the seismic activities in the area(s) in which the mobile phones 402, 404, 406 are located.

[0024] Although the present invention has been fully described in connection with the preferred embodiment thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom.