# US 10,502,757 B2 · Apparatus and methods for photonic integrated resonant accelerometers

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- Number: US 10,502,757 B2
- Document ID: US10502757B2
- Status: Granted
- Kind: B2
- Application number: 15/895,553
- Filed: 2018-02-13
- Published: 2018-06-21
- Granted: 2019-12-10
- Earliest claimed: 2015-05-29
- Assignees: Massachusetts Institute of Technology
- Applicants: Massachusetts Institute of Technology
- Inventors: Suraj Deepak Bramhavar; Paul William Juodawlkis
- Web page: https://patentgrep.com/patent/US10502757B2

## CPC

- G01P 15/097: by vibratory elements
- G01P 15/093: by photoelectric pick-up
- H03H 3/0073: {Integration with other electronic structures}
- H03H 9/02338: {Suspension means}
- H03H 9/2431: {Ring resonators}
- H03H 2009/02346: {Anchors for ring resonators}
- G01P: MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT (measuring or recording blood flow A61B5/02, A61B8/06; monitoring speed or deceleration of electrically-propelled vehicles B60L3/00; vehicle lighting systems adapted to indicate speed B60Q1/54; determining position or course in navigation, measuring ground distance in geodesy or surveying G01C; combined measuring devices for measuring two or more variables of movement G01C23/00; measuring velocity of sound G01H; measuring velocity of light G01J7/00; measuring direction or velocity of solid objects by reception or emission of radiowaves or other waves and based on propagation effects, e.g. Doppler effect, propagation time, direction of propagation, G01S; measuring speed of nuclear radiation G01T; measuring acceleration of gravity G01V; {measuring or recording the speed of trains B61L23/00; speed indicators incorporated in motor vehicles B60K35/00; measuring frequency or phase G01R; traffic control G08G})
- H03H: IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS (waveguides, resonators, lines or other devices of the waveguide type H01P)

## Abstract

The accelerometers disclosed herein provide excellent sensitivity, long-term stability, and low SWaP-C through a combination of photonic integrated circuit technology with standard micro-electromechanical systems (MEMS) technology. Examples of these accelerometers use optical transduction to improve the scale factor of traditional MEMS resonant accelerometers by accurately measuring the resonant frequencies of very small (e.g., about 1 μm) tethers attached to a large (e.g., about 1 mm) proof mass. Some examples use ring resonators to measure the tether frequencies and some other examples use linear resonators to measure the tether frequencies. Potential commercial applications span a wide range from seismic measurement systems to automotive stability controls to inertial guidance to any other application where chip-scale accelerometers are currently deployed.

## Claims

### Claim 1

An accelerometer comprising:
a semiconductor substrate;
a proof mass, suspended from the semiconductor substrate by a first tether and a second tether, to move in a first direction in response to a force applied to the accelerometer;
a first optical waveguide, optically coupled to the first tether, to guide a first optical beam in a second direction orthogonal to the first direction such that motion of the proof mass causes a change in optical coupling between the first optical waveguide and the first tether;
a second optical waveguide, optically coupled to the second tether, to guide a second optical beam in the second direction such that the motion of the proof mass causes a change in optical coupling between the second optical waveguide and the second tether;
a first photodetector, in optical communication with the first optical waveguide, to sense a change in frequency and/or amplitude of the first optical beam caused by the change in optical coupling between the first optical waveguide and the first tether; and
a second photodetector, in optical communication with the second optical waveguide, to sense a change in frequency and/or amplitude of the second optical beam caused by the change in optical coupling between the second optical waveguide and the second tether.

### Claim 2 (depends on 1)

The accelerometer of claim 1, wherein the first optical waveguide is evanescently coupled to the first tether.

### Claim 3 (depends on 1)

The accelerometer of claim 1, further comprising:
a first reflector disposed on the first tether in optical communication with the first optical waveguide; and
a second reflector disposed in the first optical waveguide so as to form an optical cavity having a length that changes in response to the force applied to the accelerometer.

### Claim 4 (depends on 1)

The accelerometer of claim 1, further comprising:
differential detection circuitry, in electrical communication with the first photodetector and the second photodetector, to generate a difference signal based on the change in frequency and/or amplitude of the first optical beam and the change in frequency and/or amplitude of the second optical beam, the difference signal representing the force applied to the accelerometer.