si-audio

Figure 1. Schematic of a photonics based motion detection principle. Light from a semiconductor laser such

as a vertical cavity surface emitting laser (VCSEL) illuminates a diffraction grating. A portion of the incident

light reflects directly off of the grating fingers, while the remaining light travels in between the grating fingers

and to the proof mass and back to accrue additional phase. A diffracted field results consisting of a zero and

higher orders whose angles remain fixed, but whose intensities are modulated by the relative distance

between the proof mass and grating with the sensitivity of a Michelson type interferometer.

MINIATURE OPTICAL SEISMIC SENSORS FOR MONITORING APPLICATIONS

Caesar T. Garcia, Guclu Onaran, Brad Avenson, Alex Liu, Matt Christensen, and Neal A. Hall

Silicon Audio Labs

Sponsored by the National Nuclear Security Administration

Contract No DE-FG02-08ER85106

TECHNOLOGY OVERVIEW:

Figure 2. (left) Theoretically predicted relationship between the diffracted beams labeled in Figure 1 vs.

gap distance “d” labeled in Figure 1. (right) The difference signal is then used to detect the proof mass

motion within a single interference fringe.

The Department of Energy (DOE) and the National Nuclear Security Administration (NNSA) seek revolutionary innovations with

respect to miniature seismic sensors for the monitoring of nuclear detonations. Specifically, the performance specifications are to be

consistent with those obtainable by only an elite few products available today, but with orders of magnitude reduction in size, weight,

power, and cost. This next-generation sensor technology calls upon several advanced fabrication methods and read-out

technologies being pioneered by Silicon Audio, including the combination of silicon microfabrication, advanced meso-scale

fabrication and assembly, and the use of advanced photonics-based displacement / motion detection methods. Prior development

has demonstrated 1) verified and repeatable sub 2ng/√Hz noise floor from 5 to 100Hz, 2) compact integration of 3-axis prototypes

and 3) robust deployment exercises. Ongoing developments are focusing on low frequency challenges, low power consumption,

ultra-miniature size, and low cross axis sensitivity. Successful implementation will result in a demonstration unit roughly the size of a

9-volt battery and with the ability to address the advanced needs of the monitoring community. Additional applications envisioned

include military/defense, scientific instrumentation, oil and gas exploration, inertial navigation, and civil infrastructure monitoring.

ABSTRACT:

Figure 3. Sensor block diagram including feedback control. Control circuitry conditions the dynamic

response of the output, Vout, and also provides logic instructions to ensure sensor stability.

Additionally, the control circuitry is equipped with an option for self calibration based upon the known

optical wavelength.

Microfabricated

grating region

Micro optoelectronic

components

2g tungsten proof mass

and non-magnetic low

thermal expansion springs

Figure 4. Various components used in the assembly of prototype units. Further miniaturization is

underway using Silicon Audio’s micro-optoelectronic packaging capabilities.

Figure 5. CAD image along with actual photograph of Silicon Audio’s 3-axis GeoLight prototype.

0.01 0.1 1 10 100

-10

-8

-6

-4

-2

0

2

Frequency(Hz)

No

rma

lize

d R

esp

on

se

(d

B r

ef 7

60

V/(

m/s

2))

10-1

100

101

-200

-180

-160

-140

-120

-100

-80

-60

Frequency (Hz)

PS

D (

10*l

og

10(m

2/s

4/H

z))

STS-2 EW

STS-2 NS

STS-2 Z

Si Audio X

Si Audio Y

Si Audio Z

Q330HR Digitizer (Si Audio)

Q330HR Digitizer (STS-2)

NLNM

10 20 30 40 50 60 70

-5

0

5

x 10-6

Velo

city (

m/s

)

STS-2

10 20 30 40 50 60 70

-5

0

5

x 10-6

Velo

city (

m/s

)

Si Audio

Time (seconds)

EWNSZUSGS Predicted Arrival Time

EWNSZUSGS Predicted Arrival Time

GENERAL

Topology Three Axis

Feedback Force balance with interferometric transducer

Mass centering Automatic centering

Leveling Integrated bubble level, adjusted locking leveling feet

PERFORMANCE

Noise 1ng/√Hz

Passband 100 seconds to 100 Hz

Clip level 4.2mg pk-pk

Sensitivity 760 V/(m/s^2)

Linearity (3%THD) 3.2mg pk-pk

ANALOG INTERFACE

Acceleration output ±10 volts

UVW coordinate system (Galperin orientation)

DIGITAL INTERFACE

Type Available upon request

POWER

Supply voltage 12 V

Power Consumption 20mW/channel

HANDLING

Transport No mass lock required for transport

ACKNOWLEDGEMENTS: The authors graciously thank the NNSA and the DOE SBIR program for support. We also thank the University of Texas, Institute of

Geophysics for assistance with Phase I and Phase II field test demonstrations. Finally, we would like to thank Bob Hutt and his team at

the USGS Albuquerque Seismology Lab for their assistance with prototype field testing.

A sensing structure similar to that described in Figure 1 is in development. In this embodiment, an optical detection scheme is

responsible for detecting a proof mass displacement which is proportional to ground acceleration. The sensitivity of the resulting

output signal is flat to acceleration and is therefore expressed in units of V/(m/s^2).

Figure 6. (left) Single sensors packaged as geophones for low power remote monitoring applications. (right)

Frequency response measured optically while using magnetic actuator as a self calibration signal.

Figure 7. Noise measurement conducted during visit to USGS Albuquerque Seismology Laboratory (ASL)

demonstrating Silicon Audio’s miniature 3-axis GeoLight prototype detecting down to site noise at 3 Hz

and above and capturing portions of the 0.2 Hz microseism. For comparison, the NLNM is included as are

the digitizer noise levels. Sources of 1/f noise at lower frequencies are presently being addressed.

Figure 8: P-wave arrival of Tarapaca, Chile event recorded by STS-2

and by Silicon Audio’s GeoLight prototype during testing visit to

USGS ASL, May 3-6th 2010.

Table 1: Preliminary specifications for 3-axis GeoLight

prototype.

FUTURE WORK • Improve low frequency noise (1ng /√Hz at 100 mHz) • Electrostatically actuated MEMS grating for ultra low power and miniaturization down to 1cm^3 / axis. • All MEMS embodiment for 10ng/√Hz noise floor strong motion applications.

Figure 9: a) Profilometer image of electrostatically actuated MEMS grating. b) CAD image of MEMS strong motion

embodiment with (c) top and (d) bottom photograph views of fabricated devices.

1mm

a b c d

100µm