Sensing with LightSensing with LightSensing with LightSensing with Light

Intensity-Based Fiber Optic SensorsIntensity Based Fiber Optic Sensors

Sensing with Fibers Fiber optic sensors are based on monitoring the change of optical signals

propagating along the sensitive area. In general, a monochromatic optic field, , can be written as: trE ,

)),(( trtietrAtrE )),((,, etrAtrE

Where: is the amplitude of the complex light field, is the angular frequ- trA ,

p p g , g qency of the monochromatic light field, and is the phase of the complex field.

,

The key parameters used to describe the light fields, may be subject

),( tr

y p g y jto change due to external perturbations. By detecting changes inthese parameters the external perturbations can be detected orsensed

1. The polarization - direction of the electric field2. The amplitude (i.e., ) or intensity (i.e., ) trA ,

2,, trAtrI

sensed.

2. The amplitude (i.e., ) or intensity (i.e., )3. The frequency (i.e., ) or wavelength of the light field (i.e., )4. The phase

, 2/f

),( tr

Fiber Optic Sensor p

Capabilities

Temperature and pressure

Capabilities

Temperature and pressure

R t ti l tiRotation, acceleration

El t i d ti fi ldElectric and magnetic fields

A i d ib iAcoustics and vibration

i h idi d i iStrain, humidity, and viscosity

Fiber Optic Sensor Advantagesp g

Li h i h / b iLightweight / nonobtrusive

Passive / low power

EMI i t tEMI resistant

Hi h iti it d b d idthHigh sensitivity and bandwidth

Environmental ruggedness

Complementary to telecom / optoelectronics

Extrinsic Fiber Optic SensorsExtrinsic Fiber Optic Sensors

Input fiber Output fiberLight modulator

p p

Environmental signal

Extrinsic fibre optic sensors consist of optical fibres that lead in to and out of a "black box" that modulates the light beam passing through it in response to an environmenbox" that modulates the light beam passing through it in response to an environmen-tal effect.

Intrinsic Fiber Optic SensorsIntrinsic Fiber Optic Sensors

O ti l fibOptical fiber

Environmental signal

Intrinsic fiber optic sensors rely on the light beam propagating through the optical fiber being modulated by the environmental effect either directly or through environmentally induced optical path length changes in the fiber itself.

Types Comparison

Extrinsic sensors are:Extrinsic sensors are: Less sensitive, more easily multiplexed, and easier to use. They also exhibit connection problems (into and out of the light y p ( g

modulator).

Intrinsic sensors are: more sensitive and more difficult to shield from unwanted external

perturbations. the all-fiber design reduces or eliminates the connection problems

experienced with extrinsic sensors, but they usually require more elaborate signal demodulation.

From a commercial standpoint, intrinsic sensors tend to be significantly more expensive than extrinsic sensors but are more versatile for installation and are more robust.

Intensity-Based Fiber Optic SensorsIntensity Based Fiber Optic Sensors

The first one developedMost commonly used nowadaysy yWorking in hazard conditions: High/Low temperturesg p Strong electromagnetic fields High/Low preasure Vibration Chemicely agresive inveroments

Easy to coupled with existing communication fiber optics networks

R l ti l h LED lti d fibRelatively cheap – LED, multimode fiberand polymer connectors could be used

Closure and Vibration Sensors B d N i l A tBased on Numerical Aperture

d

Fiber optic sensors based on numerical aperture can be used as door closure indicators and measure levels of vibration inas door closure indicators and measure levels of vibration in machinery.

NA Based Displacement SensorsNA Based Displacement Sensors

NA FOS measuring small vibrations

These sensors were used to measurethermal distortions of critical componentsat low temperatures for the HubbleNA FOS measuring small vibrations

and displacements.at low temperatures for the Hubble replacement telescope imager.

http://www.philtec.com

Samba Sensors’ technologySamba Sensors technologyUlt i i t fib tiUltra-miniature fiber optic pressure sensor

http://www.sambasensors.com

Measuring hydrostatic pressureMeasuring hydrostatic pressureSpace filled with

Hydrostatic pressure

Space filled withGAS or LIQUID

Hydrostatic pressure

SSensorLow pressure 0.42mmHigh pressure 0.36mm

See TSD170 SeriesOptical fiber 0.25mm

See TSD170 Series

http://www.sambasensors.com/

Inserting transducer into a mouse heart

49http://www.sambasensors.com/

Translation Sensor Based on N i l A tNumerical Aperture

Detectors

Input lightInput light

Collection fibers

Fiber optic translation sensor based on numerical aperture uses the ratioFiber optic translation sensor based on numerical aperture uses the ratio of the output on the detectors to determine the position of the input fibre.

Rotary Position Based on Reflectancey

V i bl fl t h ftVariable reflectance shaft

Input/output fibers

Fibre optic rotary position sensor based on reflectance used to measure p y protational position of the shaft via the amount of light reflected from dark and light patches.

Critical Angle Pressure/Index of R f ti M tRefraction Measurement

Fiber coreno outsidemedium indexof refraction

MiLight input, output

Fiber claddingMirror

Fibre sensor using critical angle properties of a fibre for pressure/index of refraction measurement via measurements of the light reflected back into the fiber.

Liquid Level Sensor Based on T t l I t l R fl tiTotal Internal Reflection

Liquid

Liquid level detected by total internal reflection. The presence of a liquid causes an absence of a return light signal.

The biggest problem is that the effective shape of the sensing element changesThe biggest problem is that the effective shape of the sensing element changes when a drop or a thin film of liquid adheres to the tip of the sensor.

Liquid Level Sensor Based on Total Internal ReflectionTotal Internal Reflection

Liquid level sensor based on total internal reflection detects the presence orLiquid level sensor based on total internal reflection detects the presence or absence of liquid by the presence or absence of a return light signal.

Liquid Level Sensor Based on Total Internal ReflectionTotal Internal Reflection

Task: Using TraceProoptimise sensor shapesensor shape

0-7803-8879-8/05/ ©2005 IEEET. Eftimov, W., J. Bock

Liquid level sensor based on total internal reflection detects the presence orLiquid level sensor based on total internal reflection detects the presence or absence of liquid by the presence or absence of a return light signal.

Evanescence Based SensorsEvanescence Based SensorsActive core sensor

J.P. Singh - Institute for Clean Energy Technology (ICET) - Mississippi St. University

Evanescence Based SensorsU-bend fibre sensor:

J.P. Singh - Institute for Clean Energy Technology (ICET) - Mississippi St. University

Bending losses

Mi b di

g

Macro-bending losses Micro-bending Losses

Example bending loss1 t t 32 di t 0 5 db Property of fiber under1 turn at 32 mm diameter causes 0.5 db lossIndex profile can be adjusted to reduce loss but this degrades the fibers other

Property of fiber, under control of fabricator, now very small, usually included in the

t t l tt ti bbut this degrades the fibers other characteristicsThis loss is mode dependent

total attenuation numbers

Macro-Bending lossesgDetecting opening and closing of clamshell devices

Plastic optical fiber-based displacement sensor.

Micro Bending lossesMicro-Bending losses

Light source Microbendtransducer

Detector

Microbend fiber sensors are configured so that an environmental effect results in an increase or decrease in loss through the transducer due toresults in an increase or decrease in loss through the transducer due to light loss resulting from small bends in the fiber.

Evanescence Based SensorsEvanescence Based SensorsI i l h

LInteraction length

Light in

d

LightoutputsFiber cores outputs

Evanescence based fiber optic sensors rely on the cross coupling of light between two closely spaced fiber optic cores. Variations in this distance due to temperature pressure or strain offer environmental sensing capabilitiestemperature, pressure or strain offer environmental sensing capabilities.

Limitations of Intensity Based Fiber yOptic Sensors

Variable loss due to connectors

Microbending losses

Macrobending losses

Mechanical losses due to creep and misalignment

Fly by Light SystemFly by Light System

Blue Road Research