December 2014

www.nufern.com

Fiber Optic Sensors: Fundamentals, Principles & Applications

Dr. Mansoor Alam

Dr. Kanishka Tankala

Overview

12/4/2014 2

• Definitions and Classifications

• Fiber Optic Rotation Sensor

• Fiber Optic Current Sensor

• Fiber Optic Radiation Sensor

• Fiber Optic Biosensors

• Fiber Optic Distributed Sensors

3

Fiber Optic Sensor – Definition

• Light Injection into the Optical Fiber – Source (Laser, LED etc.)

• Transmission of Modulated Light to a Monitoring Point – Detector (PIN Diode, Avalanche Diode)

• Optical Fiber (Transmission Medium, Sensing Element) – Light modulated due to interaction with parameter of interest (Measurand)

• Signal Processing Device (OTDR, OSA, Oscilloscope, etc.)

12/4/2014

Source

Measurand

Transducer Detector

Optical Fiber Optical Fiber

Signal Processing

Fiber Optic Sensors – Measurands/Applications

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• Temperature

• Pressure, Force, Strain, Vibration

• Displacement

• Velocity

• pH, Chemical Species

• Radiation

• Acoustic Field

• Rotation, Acceleration

• Magnetic/Electric Field

Measurands

Application Areas

• Civil Engineering

• Nuclear Power Industry

• Electric Power Industry

• Navigation/Guidance

• Fly-by-Light Applications

• Oil/Gas Industry

• Biomedical

• Environmental

12/4/2014 5

Fiber Optic Sensors - Classification

Location Operating Principle

Application

Extrinsic (Point Only)

Intrinsic (Point or

Distributed)

Chemical Physical

Bio-Medical

Intensity (Bend loss,

Discontinuity, Evanescence)

Polarization I, V, B

Frequency (Grating or

Inelastic Scattering)

Phase (Interferometry)

Mach-Zehnder Michelson Fabry-Perot Sagnac

12/4/2014 6

Extrinsic Fiber Optic Sensors

Fiber is Only an Information Carrier To and From a Black Box

Light Signal Generation in Black Box Depending on the Arriving Information

Source Detector Black Box Input Fiber Output Fiber

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Intrinsic Fiber Optic Sensors

Sensing region within the fiber (Light Never Leaves the Fiber)

Measurand interaction modulates the input light signal

Source Transducer Detector

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Fiber Optic Interferometer Configurations

Laser Detector

Ref. Arm

Sensor Arm

3 dB Coupler

3 dB Coupler

Mach-Zehnder

• Ref. arm isolated from external variation

• Variation in the sensing arm induces changes in the OPD

• Phase modulation of interference signal is detected

• T, P, , Acoustic

Laser

Detector

3 dB Coupler

Sagnac

Optical Fiber Coil

• Light is split, polarized and injected in the two ends of an optical coil

• External variations of interest • Rotation rate • Magnetic field

12/4/2014 9

Rotation Rate Sensor: Fiber Optic Gyroscope

Counter rotating waves traveling through the same circular path exhibit a phase difference

∆ = (2πLD/λc) ∆ = Phase difference between the counter propagating beams (radians)

L = πND = Total length of fiber in the coil (meter)

N = Loops of fiber in the gyro coil

D = Diameter of each loop of fiber in the coil (meter)

λ = Wavelength of the propagating light (meter)

c = Speed of light in vacuum (3x108 meter/second)

= Rotation rate of the coil along its axis (radian/second)

Modulator

Source

Detector

Polarizer

2x2 Coupler

2x2 Coupler

Coil

Computer Generator

Splice

12/4/2014 10

Fiber Optic Gyroscope: Components

• Bulk fiber optic components spliced together

• All components on a single Photonic Integrated Chip (PIC)

– Fiber coil attached via V- Groove

• Fiber (PM or SM)

– PM design is more common

– Coil – Quadrupolar winding pattern

Linear Polarizer

2 x 2 Couplers

Phase Modulators

Mode Size Converters

InGaAs SLD Source

InGaAs Photodiode

12/4/2014 11

Fiber Optic Gyroscope – Optical Coil Design

∆ = (2πLD/λc) = (Scale Factor)

• Fiber Length in the coil: 100m – 6000m

• Diameter of the coil: 0.5” – 4.5”

• Operational wavelength: 850nm – 1550nm

• 850nm based gyros are more sensitive - fiber length is limited

– Components are readily available and cheaper

• Smaller footprint is a major driver

– Interest in smaller diameter fiber without penalty

Df = 170 µm, ID = 2”, H = 1”, t = 0.2” → 936.4 m

Df = 125 µm, ID = 2”, H = 1”, t = 0.2” → 1693.8 m

12/4/2014 12

Fiber Optic Gyroscope - Sensitivity

• What rotation rate can be measured?

– Limited by detection of phase difference via interferometry

– Interferometers can detect ∆ 1 µradian

• For a 1550nm FOG

– = (∆)(λc/2πLD) = (74x10-6/LD) radian/s = (15.27/LD) deg/hr

– L and D are in meters

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 500 1000 1500 2000 2500

Ro

tati

on

Rat

e (

de

g/h

r)

Fiber Length (m)

Average Coil Diameter: 25 mm

Tactical Grade

Navigation Grade

0.0

0.1

0.2

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0.0 1.0 2.0 3.0 4.0 5.0

Ro

tati

on

Rat

e (

de

g/h

r)

Average Coil Diameter (in)

Fiber Length: 1600m

Navigation Grade

Strategic Grade

12/4/2014 13

Fiber Optic Gyroscope – Nufern Fibers

PM & SM Gyro Grade Fibers

PM850G-80, PM1310G-80, PM1550G-80 80/170

PM850G-SC, PM1310G-80-SC, PM1550G-80-SC 80/125

PM1550G-40 40/90

PM-ECG-1300-80 80/170

PM-ECG-850-40 40/100

S1550-80 80/130

Panda Bow Tie

r

b a

r

ab a

b

Elliptical Cladding

+1-1-1 +1

-1 +1

12/4/2014 14

Fiber Optic Gyroscope – Critical Fiber Properties

• Uniformity over fiber length in the coil (must be symmetric about midpoint)

– Counter propagating beams must see same environment at every location

– Phase accumulation over length – Only rotation contribution

• Tight tolerance on coating diameter over the length (tighter the better)

– Severe impact on Sagnac coil winding - Need fixed number of turns/layer

• Fiber NA: Governs macro-bend loss

• Coating Design and Size: Minimize PER loss

• Mode Field Diameter: As large as possible to maximize signal/noise ratio

End End Middle

Source

Detector

Polarizer

Modulator2x2 Coupler

2x2 Coupler

Coil

Splice

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Fiber Cladding and Coating Diameter Control

0.13

0.14

0.15

0.16

0.17

0 100 200 300 400 500 600 700 800

Axial Position (mm)

Co

re N

um

eri

cal

Ap

era

ture

(N

A)

-18.90

-22.72

-26.86

-20.04

-25.16

-30.0

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90

Temperature (oC)

Cro

ssta

lk a

t 980n

m (

dB

/500m

)

Fiber Length = 500 m

Spool Material = Aluminum

Spool Diameter = 40 mm

Winding Type = Dry, Helical

Winding Tension = 10 g

Clad Diameter (6 Km Section)

80.5 um

79.5 um

Coating Diameter (6 Km Section)

167 um

165 um

12/4/2014 16

Fiber Optic Current Sensor

Faraday Effect: Magnetic Field Induced Circular Birefringence

• Linearly polarized light - A superposition of two circularly polarized waves

• In PM fibers – Right and Left circularly polarized light waves travel at different speeds if a magnetic field is applied along the propagation direction

• Waves accumulate a path difference δL or equivalently a phase difference

– ∆ = VB.dL – V: Verdet constant (Radian/m-Tesla), L: Rod length (m), B: Magnetic field density (Tesla)

• For closed optical path around a current carrying conductor, Ampere’s law applies

– ∆ = V(µoNI) – µo: Magnetic Permeability (H/m), N: # of optical path loops I: Current (A)

Information from ABB: Energize, Jan/Feb 2005, p 26

EJ Casey & CH Titus: US Patent 3324393, 1967

12/4/2014 17

Practical Fiber Optic Current Sensors

• Source light passes through a polarizer → Equally split linear states of polarization

• Quarter wave-plate converts linearly polarized light into circularly polarized light

• Mirror at the end of sensing fiber returns light in opposite mode – Cancel out reciprocal effects

• Faraday loop is completed at the coupler

• Modulator allows phase-locked loop operation

Jose Miguel Lopez-Higuera: Handbook of Optical Fiber Sensing Technology,

John Wiley & Sons, 2002. P 603

Radiation Effects in Optical Fibers

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• Increased Loss - Radiation Induced Attenuation (RIA)

• Scintillation - Radiation Induced Luminescence (RIL)

– Radiation absorption excites an orbital electron to a higher energy level.

– Electron returns to its ground state by emitting the extra energy as a photon

• Thermoluminescence

– Radiation absorption creates electronic excited states that are trapped by localized defects for extended periods of time.

– Heating the material enables the trapped states to interact with phonons and decay into lower-energy states, causing the emission of photons.

• Enhanced Scattering

– Radiation absorption creates damage sites in glass that exhibit higher degree of light scattering.

RIA Based Radiation Sensors

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• Pure silica core fibers → lowest radiation sensitivity

• Dopants increase sensitivity → B, Ge, P and Pb

• Require lower to moderate loss at the expected radiation levels

– Long Duration Sensing

• Loss → Vary linearly with accumulated dose but independent of the dose rate

• Low dose detection with high sensitivity • Long length probe at shorter wavelength

• High dose detection with high sensitivity • Small length probe at longer wavelength

12/4/2014 20

Fibers for Radiation Sensing

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5.0

6.0

7.0

0 20 40 60

Ind

uced

Att

en

uati

on

@ 1

550

nm

(d

B/k

m)

Accumulated Dose (kRad)

S1550SMF28

Radiation: GammaRadiation Source: Co60

Photon Energy: 1.117 & 1.335 MeVIrradiation Temperature: RoomMean Dose Rate: 2.02/2.13 Rad/secDose Rate SD: 0.018 Rad/secLight Power: ~ 1 µW

A = 0.323019(D)0.312243 (S1550)A = 0.478558(D)0.668710 (SMF28)

A: Ge-P B: P C; Ge

John Wallace: Laser Focus World, September 2011

12/4/2014 21

Radiation Sensing Using OTDR

• Distributed measurement over length

• Radiation sensitive fiber passes through regions of varying radiation intensity

• Measure loss of signal resulting from color center formation (defects) in silica

• Ge-P doped graded index MM fiber

– Linear Region: Up to 1000 Gy

– Sensitivity: 5 dB/km-Gy

– Expected Accumulated Dose: 100 Gy/y

– Lifetime: 1000 Gy/100Gy/y = 10 y

Stefan K. Hoffgen: 1st Workshop for Instrumentation on Charged

Particle Therapy, Fraunhofer Institute of Tech., Germany

12/4/2014 22

Luminescence Based Radiation Sensing Fibers

Requirements

• Light generated per unit fiber length

– As high as possible

– Light yield/Length increases with dF

(100-600 µm)

• % of generated light directed to the

detectors – As high as possible

– Higher NA fiber

• Benign and RIA in the UV – As low as

possible

– Decoupling of Cerenkov component

Best Choice (High OH, MM SI, Pure Silica Core)

Heraeus Product Brochure

Specialty Fiber Preforms for the Most Demanding Applications

• Fiber bandwidth for high accuracy detection – As high as possible – Fiber bandwidth decreases with increasing diameter (Issue with longer sections)

12/4/2014 23 12/4/2014 23

What is Fiber Optic Biosensor?

A device that transforms chemical information into an analytically useful signal

Optrode

Evanescent Wave

• Recognition molecule immobilized at the tip

• Optical fiber as transporting medium To and from the sensing region

• Recognition molecule immobilized on the de-clad surface of the fiber core - RI

• Evanescent light propagation changes with RI changes

Jose Miguel Lopez-Higuera: Handbook of Optical Fiber Sensing Technology, John Wiley & Sons, 2002. PP 689-690

Parameters, Immobilization & Detection

12/4/2014 24

• Measured Optical Parameters

– Fluorescence

– Absorbance

– Bioluminescence

• Immobilization of Biological Molecules – Physical Adsorption

– Encapsulation

– Covalent Attachment

– Bio-molecular Interactions

• Detection Methods

– Bio-catalytic

– Affinity Based

12/4/2014 24

12/4/2014 25 12/4/2014 25

Optical Fibers for Imaging and Spectroscopy

Arterial Image obtained by optical frequency domain interferometry (OFDI)

Courtesy: Wellman Center for photo-medicine, Harvard Medical School

OCT Image Microscope Image OCT Image of Human Eye

12/4/2014 26

Distributed Sensing - Principles

Rayleigh (Distributed Acoustic Sensing)

Brillouin (Distributed Temperature

and Strain Sensing)

Raman (Distributed Temperature Sensing)

Pulsed Laser Signal

Fiber serves as a continuous sensing element. Sensing is based on

– Elastic (Raleigh) or inelastic (Raman or Brillouin) scattering of signal

– Intensity and/or frequency shift are sensitive to temperature and strain

12/4/2014 27

Principle of Distributed Acoustic Sensing

Pulsed Laser Signal

Back Scattered Rayleigh Signal (temperature insensitive)

• Rayleigh signal intensity varies due to local changes in strain

– Temperature effects can be separated from strain effects

• Pulsed signal launched into sensing fiber

– Pulse launched only after signal from previous pulse returns

• Time of flight and repeated scans provide position and velocity

12/4/2014 28

Rayleigh Based System Considerations

Laser Pulse Attenuation

Rayleigh Signal Attenuation

• Maximum range (L) determined by strength of returning signal

• Strain resolution depends on signal to noise ratio

– Signal intensity can be enhanced with longer pulse duration (Dt)

• Spatial resolution, DL = cDt (c = speed of light in fiber)

– Longer pulses enhance range at the expense of spatial resolution

• Acquisition rate = c/2L decreases with range

12/4/2014 29

Distributed Temperature and Strain Sensing

Difficult to filter Rayleigh Signal

Brillouin Anti-Stokes Signal Brillouin Stokes Signal

Pulsed Laser Signal

• Signal light scattered due to interaction with acoustic phonons

• Intensity and frequency of stokes and anti-stokes signals depend on temperature and strain

• DTSS systems analyze signals to determine temperature and strain

12/4/2014 30

Principle of Raman DTS

Anti-stokes Signal ( Ia ) (Temperature Sensitive)

Stokes Signal ( Is )

(Temperature Insensitive)

Easy to filter Rayleigh Signal

T(z) = Tref { 1 + ∆𝛼

ln(𝐶𝑠/𝐶𝑎) z +

ln(𝐼𝑠/𝐼𝑎)

ln(𝐶𝑠/𝐶𝑎) }

• Pulsed laser signal launched into sensing fiber

• Ratio of stokes and anti-stokes intensities provides temperature

– Time of flight analysis gives position information

– Differential attenuation (Da) between stokes and anti-stokes signals needs to be considered for accurate temperature assessment

DTS Fibers for Oil and Gas

12/4/2014 31

• Fibers tolerant to harsh environments

– High temperatures and hydrogen pressures

• Fiber types:

– Graded index multimode: GR-S50/125-20P

– Single mode: S1310-P

• High temperature (200 - 350 °C):

– Pure silica fibers with polyimide coatings

• Moderate Temperatures (< 200 °C):

– Pure Silica fibers

– Ge doped fibers with hermetic carbon

– High temperature acrylate or Silicone/PFA

12/4/2014 32 12/4/2014 32

Fiber Test and Characterization Facility

• Monitor changes in spectral attenuation (600 to 1600 nm)

Periodic in-situ logging with OSA

Source power monitored with reference fiber outside H2 vessel

• Parallel testing of up to 4 fibers

Pure H2 or H2/He gas mixtures

Pressures < 150 atm.

Temperature < 300 oC

• 24x7 operation for monitoring long term effects

• Equipped with safety features and remote fault monitoring. Nufern offers hydrogen and other environmental test services

Dedicated Fiber Test Facility for Oil and Gas Industry

12/4/2014 33

Fiber Attenuation in Down Hole Environment

High temperature and hydrogen environments induce losses – Attenuation of signal and stokes and anti-stokes signals limits range

– Changes in differential attenuation effects temperature accuracy

0

5

10

15

20

25

650 750 850 950 1050 1150 1250 1350 1450 1550

Ind

uce

d A

tten

uat

ion

(d

b/k

m)

Wavelength (nm)

948 1080 1130 1170

1196

1380

Si-OH

1240

1530 1580

Interstitial H2

H2 + Si-OH

Si-OH

DTS window

Si-H

12/4/2014 34

Si-OH Related Losses in Typical Well Environments

0

10

20

30

40

50

60

70

80

800 900 1000 1100 1200 1300 1400 1500 1600

Ind

uce

d A

tten

uat

ion

(d

B/k

m)

Wavelength (nm)

Competitor recovered after 1.5 atm

NuSENSOR recovered after 1.5 atm

Pure Silica Graded Index Multimode Fibers

Significant differences in hydrogen induced-attenuation can be observed in nominally pure silica fibers due to processing differences

12/4/2014 35

Polyimide Coated Pure Silica Core Fiber

-3

-2

-1

0

1

2

0 10 20 30 40 50 60 70 80 90

Ind

uce

d A

tten

uat

ion

(d

B/k

m)

Time (Hours)

1014 NuSENSOR 1064 NuSENSOR 1114 NuSENSOR

Continued exposure to 1.5 atm H2 at 300 °C

• Polyimide coating provides thermal stability up to 300 °C

• Pure silica glass provides resistance to hydrogen induced losses

• No significant attenuation induced at typical DTS wavelengths

12/4/2014 36

Polyimide Coating Quality

After 15 minutes After 30 minutes Unexposed

40

0 °

C b

ake

After 24 hours Unexposed

IPA

(5

0°C

vap

or)

Polyimide coating show excellent resistance to temperature & chemical resistance

– Microscopic examination shows no signs of degradation

Summary

12/4/2014 37

• Optical fibers are being widely used for a variety of sensing applications

• Fiber optic sensors enable remote sensing capabilities

• Truly distributed sensing is achievable when the fiber is used as an intrinsic sensor

• Specialty fibers can be engineered to meet specific sensing applications and environments

Nufern

Brighter Fiber Solutions

Thank you