Fiber-Optic Acoustic Sensors (FOAS) - Northrop Grumman ...

Navigation Systems
Fiber-Optic Acoustic Sensors (FOAS)

Northrop Grumman Corporation’s Navigation
Systems Division has applied its expertise in fi beroptic
technology to develop a line of acoustic sensors it
calls Fiber-Optic Acoustic Sensors (FOAS). FOAS origi-
nated in 1981 as part of a major effort to develop both
fi ber-optic gyroscope and fi ber-optic hydrophone sensor
technologies. This effort, which has required investment
of over $140 million, has established Northrop Grumman
as the world’s leader in optical acoustic sensor technolo-
gy and exclusive manufacturer of Department of Defense
fi ber-optic acoustic sensor array systems.

FOAS Description and Capabilities
Northrop Grumman can produce fi ber-
optic acoustic sensor arrays for a variety of
applications. Northrop Grumman builds
three FOAS systems: hull arrays like the
Light Weight Wide Aperture Array
(LWWAA) currently on USS Virginia-Class
Submarines, towed arrays for submarine
and other applications, and fi xed bottom
arrays for deep ocean and shallow/littoral
water surveillance.
The arrays themselves comprise only pas-
sive fi ber-optic elements, and contain very
few parts and no electronics. One remote
transmit/receive station houses all the nec-
essary optoelectronics. The data are trans-
mitted on a few single mode fi bers, over
distances of more than 100 kilometers.
Over the past 15 years, Northrop Grum-
man and the U.S. Navy have made signifi -
cant strides in developing fi ber-optic sensor
Basic Principle of FOAS Operation
FOAS sensors use standard interferometer
principles to detect acoustic
energy. Optic fi ber is wrapped tightly
around malleable plastic spools called
mandrels. As pressure from the acoustic
event moves across the sensor mandrel, it
changes shape causing the optical fi ber to
stretch or relax.
When light is projected through the
optical fi ber wrapped around the sensor
mandrel, the acoustic energy deforms the
mandrel and changes the distance the light
has to travel. This is compared to the dis-
tance light travels through the optical fi ber
on a reference mandrel. The optical phase
difference between the two, representing
the acoustic energy, can easily be mea-
sured and passed on to the signal process-
ing equipment.
technology for passive acoustic monitoring.
This work has focused on demonstrating
the advantages of fi ber-optics over conven-
tional piezoceramic sensor arrays.
One of the primary advantages of fi ber-
optic acoustic arrays is their simplicity.
Ceramic acoustic arrays require power,
complex electronics and signal processing
to be located close to the sensor to over-
come resistance losses.
Ceramic arrays also have numerous con-
nectors and mechanical joints that reduce
their reliability. The electronic processing
elements of a fi ber-optic array are located
in a dry or controlled environment on a
submarine or on shore. Their location al-
lows them to be accessed for maintenance
purposes and technological refresh, which
is not possible with legacy ceramic arrays.

Legacy
Ceramic Array
Power In
Multiplex
and Optical
Conversion
A/D
Conv.
Sensor
Signal
Cond./
Filters
∆V∆P
Preamp
Hundreds of active electronic and optical components, joints,
connectors and interfaces required for each channel
Light Weight Wide Aperture Array on
Virginia Class Submarines
The Navy’s newest Virginia-class subma-
rines employ a Light Weight Wide Aperture
Array (LWWAA) on its hull for advanced
acoustic detection. The LWWAA system is
a set of large array panels that are mounted
on either side of the submarine, which
provide sonar sensor input to the subma-
rine’s combat system. Instead of traditional
ceramic hydrophone sensors, the
LWWAA uses FOAS technologies to con-
vert a target submarine’s acoustic energy in
the water into information that can be used
to identify and track it. Northrop Grumman
designed, developed and manufactures the
key optical sensing elements of this
FOAS array.
Towed Arrays
Northrop Grumman has designed and
developed high channel count fi ber optic
towed arrays for submarine applications.
The performance of these arrays has been
Clock
Synchro
Acoustic Event ∆P
λ0
superior to equivalent ceramic arrays and
its ruggedness and reliability have been ex-
ceptional. It is expected that future towed
array systems will transition to fi ber-optic
technology to take advantage of the lower
cost, reliability and ruggedness.
Surveillance Systems
The U.S. Navy and Northrop Grumman
continue to be actively involved in the de-
velopment of all fi ber-optic acoustic sensor
surveillance arrays for anti-submarine war-
fare passive sonar. They may be deployed
in all types of terrain and marine environ-
ments to accommodate such applications
as perimeter security, harbor monitoring,
and ground surveillance. Depending on
the application, these arrays may contain
optical geophones, fi ber-optic hydro-
phones, and/or fi ber-optic microphones.
The Centurion Harbor Surveillance pro-
gram demonstrates our FOAS capabilities
in this area.
Fiber-Optic Array
λ +∆Φ
0
∆Φ/∆P
Optical Interrogation Analog Optical Output
Less than 10 passive components and splices per channel
Other advantages of fi ber-
optic arrays over convention-
al arrays include:
• Lower power requirements
• Increased reliability due to a
simplifi ed array design
• Reduced maintenance costs
• Lower cost to manufacture
• No underwater electrical compo-
nents or wires
• Light weight and small cable
diameter, which facilitate rapid de-
ployment and recovery of the array
• Immunity to electromagnetic
interference (EMI)
• Immunity to high radiation
• Diffi cult to detect since the fi ber-
optic sensor array is not operated
electrically and consists of only
non-metallic materials.

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Centurion Harbor Surveillance
Program for Homeland Security
Applications
Northrop Grumman’s FOAS technology
is a key element in the Centurion Port-
Harbor Anti-Terrorism/Force Protection
system which provides the operator with
a complete status of the harbor he
is monitoring.
This integrated sensor and display product
shows the harbor vicinity and potential
threats on a standard Navy display system
located at the test site. During a demon-
stration, divers with a battery-powered
underwater propulsion device were easily
detected attempting to penetrate the har-
bor. Surface craft traveling in the test area,
and entering the restricted Port of Huen-
eme, were also detected and tracked.
The proof-of-concept demonstration was
completed within three months of the
contract award by the U.S. Navy’s Mari-
time Surveillance Systems Program Offi ce,
utilizing passive fi ber-optic sonar arrays
and support equipment delivered by
Northrop Grumman’s Navigation Systems
Division, coupled with commercial-off-
the-shelf equipment provided by Northrop
Grumman’s Sperry Marine business unit in
Charlottesville, Virginia.
The equipment provided by Sperry Marine
included the marine radar, Automatic
Identifi cation System (AIS) shipboard iden-
tifi cation system and the digital electronic
charting system that provided the com-
mon integrated harbor picture.
The Northrop Grumman team completed
a threat analysis for the port and deter-
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mined the most effective locations to place
the underwater arrays. The array installation
was performed with the assistance of the Na-
val Facilities Engineering Support Center and
the team’s ocean engineering subcontractor,
Sound and Sea Technology, Inc. of Edmonds,
Washington and Ventura, California.
Performance benefi ts of this technology
include unsurpassed detection, identifi ca-
tion and tracking capability for the harbor
defense applications. Since no electronic
components are in the water, the fi ber-optic
arrays provide a highly reliable solution that
also offers reduced acquisition and mainte-
nance costs. The arrays employ glass fi bers,
instead of older technology piezoelectric
hydrophones, to convert sound to modulated
light for effi cient transmission to shore.

Northrop Grumman
Navigation Systems Division
21240 Burbank Boulevard
Woodland Hills, California 91367 USA
1-866-NGNAVSYS (646-2879)
www.nsd.es.northropgrumman.com
24040 FOAS Overview/01-07/2000/Crawford