Fiber Optic Sensors:Providing Cost-Effective Solutions

To Industry Needs

Fiber Optic Sensors:Fiber Optic Sensors:Providing Cost-Effective Solutions

To Industry Needs

Jeffrey D. MuhsOak Ridge National Laboratory

November 2002

Properties That Can Be Sensed Using Fiber Optics

• Acceleration• Chemicals/Gases• Color• Displacement• Flow• Force• Humidity• Liquid Level• Magnetic/Electric Fields• Moisture• Motion• Position (linear, angular)• Pressure (fluid, gas, etc.)

• Proximity• Radiation• Sound• Speed• State-of-Cure• Strain• Surface Condition• Tactile Sensing• Temperature• Velocity• Vibration• Viscosity• Weight

How Does A Fiber Optic Sensor Work?

In a fiber optic sensor, one or more of the following characteristics of a propagating lightware is altered and correlated to an externally-induced physical or chemical parameter:

• Intensity• Phase• Frequency (color)

• Polarization state• Time-of-flight• Modal cross talk

Types Of Fiber Optic Sensors

• The fiber itself acts as the sensing medium, i.e., the propagating light never leaves the fiber and is altered in some way by an external phenomenon.

• The fiber merely acts as a light delivery and collection system, i.e., the propagating light leaves the fiber, is altered in some way, and is collected by the same (or another) fiber.

Extrinsic Sensors

Intrinsic Sensors

Why Are Some Optic Sensors Becoming So Popular?• Small/lightweight• Allow access into normally inaccessible areas

(often embedded) • Passive (non-electrical)

• Inexpensive• Rugged/reliable• Easy to install• Highly sensitive

• Accurate over a wide dynamic range

• Resistant to ionizing radiation• Capable of being multiplexed• Capable of distributed

measurements

In Some Cases, Fiber Optic Sensors Are Also:

What Are Some Expressed Concerns About Fiber Optic Sensors?

• New technology• Cost• Fragility• Ingress/Egress

difficulty

• Often have a nonlinear output

• Complexity• Reliability

The bottom line is FO sensors vary widely in their composition (glass plastic, rubber), sensitivity, dynamic range, size, shape, etc., each offering different value propositions

Issues Relating To Embedded Fiber Optic Sensors

Pros• Embedded rather than

attached• Sensitivity/dynamic

range• Durability

In Some Cases• Size

Pros• Embedded rather than

attached• Sensitivity/dynamic

range• Durability

In Some Cases• Size

Cons• Connectivity• Difficulty in fabrication

In Some Cases• Strength degradation• Size• Cost

Cons• Connectivity• Difficulty in fabrication

In Some Cases• Strength degradation• Size• Cost

Ex. #1: The Fabry-Perot Fiber Optic Sensor

Design of Fabry-Perot strain sensor

Fiso, Inc. sensor: see www.fiso.com

Fabry Perot strain sensors attached to a section of pipe

Metal canister instrumented with Fabry Perot strain gauges attached

Strain levels on the order of a few microstraincan be easily recorded in time periods of just a few microseconds

LE1 F2 01 (08-09) post-processed [*normalized]

-600

-400

-200

0

200

400

600

800

-0.01 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

Time (s)

Mic

rost

rain

p-p (08)*p-p (09)*

LE1-08 28.1 TPLE1-09 27.9 TP

Example fiber optic Fabry Perot module and oscilloscope arrangement

FO Patch Panel

Veloce Modules

Ex. #2: Fiber Optic Bragg Grating Temperature And Strain Sensors

Fiber Optic Bragg Grating Sensors

Pros• Multipoint in-line

measurement capability

• High sensitivity• Low-cost sensors• Small / lightweight• Linear response

Pros• Multipoint in-line

measurement capability

• High sensitivity• Low-cost sensors• Small / lightweight• Linear response

Cons• Sensitive to more than

one parameter• Require relatively

expensive processing equipment

Cons• Sensitive to more than

one parameter• Require relatively

expensive processing equipment

Ex. # 3: Fiber Optic Optical Time Domain Reflectometry (OTDR)

Optical Time-Domain Reflectometer- Based Strain Monitoring System

Mooring rope with integrated polymeric optical fiber containing reflective interfaces.

Summary Thoughts

• Fiber optics is emerging as a mainstream sensor technology capable of measuring numerous physical parameters

• Care must be taken to match a specific FO sensor technology with a particular application

• In just the past few yrs. adv. signal processing techniques have made highly sensitive COTS FO sensors a viable option in many applications

• Embedding fibers can be difficult….. but very rewarding

• New low-cost sources, detectors, and processing hardware and industry maturity are making FO sensors more cost-effective

• Sensor multiplexing is an attractive value proposition

• Don’t limit yourself to just one type of FO sensor technology