Dr. Umesh

TiwariScientist

V-4 (PHOTONICS)

E.mail: umeshtiwari@csio.res.in

CSIR-CSIO,

SECTOR 30, CHANDIGARH -

160 030

Fiber Optic Sensors: Principles & Developments

OUTLINE

FIBER OPTIC SENSOR BASICS

MODULATION MECHANISMS

TYPICAL FO SENSOR SYSTEMS

FOS TECHNOLOGY AT CSIO

CURRENT TRENDS AND FUTURE SCENARIO

OPTICAL SENSOR

A device that detects events or changes in physical quantities and provides a corresponding optical output

OR

A device in which an external stimulus/measurand

e.G. Temperature, pressure, strain etc. Modulates an optical signal which when demodulated yields precise quantitative information about the measurand.

IMPORTANT CHARACTERISTICS:

NON-CONTACT / NON-INTRUSIVE AND PRECIS

E MEASUREMENTS

OPTICAL SENSORS (CONVENTIONAL)

BULK OPTICAL COMPONENTS AND LIGHT SOURCES (GAS LASERS, HALOGEN LAMP ETC.)

PROBLEMS:

PORTABILITYREMOTE MONITORING COST RUGGEDNESSEFFICIENCY

SOLUTION EMERGED THROUGH OPTICAL FIBERS & OE OMPONENTS FOR SENSING e.g. FO SENSORS

LIMITED USE

FIBER OPTIC SENSORS•

An offshoot of fiber optic communication research

•

Realization of high sensitivity of optical fibers to external perturbations (phase modulation, micro bending loss in cabling, modal noise etc) and its exploitation for development of sensors. (An Alternate School of Thought, 1975)

•

High sensitivity of fibers due to long interaction length of light with the physical variable

FO Sensors: A Boon in Disguise

FIBER OPTIC SENSORS: WHY?LARGE BANDWIDTH

EFFICIENT TRANSMISSION (LOW LOSS)

IMMUNITY TO EMI/ RFI/ EMP

SECURITY OF INFORMATION

GEOMETRIC VERSATILITY

SMALL SIZE AND LIGHTWEIGHT

FLEXIBILITY

RESISTANT TO HOSTILE ENVIRONMNT

FREEDOM FROM CROSS-TALKS

NO SPARKING AND FIRE HAZARDS

SINGLE FIBER SERVES BOTH AS SENSOR AND DATA TRANSMITTING CHANNEL

MULTIPLEXING & SPATIALLY DISTRIBUTED SENSING

HIGH PERFORMANCE

CLASSIFICATION

EXTRINSIC SENSORS

INTRINSIC SENSORS

EXTRINSIC SENSORS

Where the light leaves the transmitting fiber to be changed before it continues to the detector by means of the return or receiving fiber.

INTRINSIC SENSORS

Intrinsic sensors are different in that the light beam does not leave the optical fiber but is changed whilst still contained within it.

Optical Fiber Sensor TypesOptical Fiber Sensor Types

Point sensor: detect measurand variation only in the vicinity of the sensor

Multiplexed sensor:Multiple localized sensors are placed at intervals along the fiber length.

Distributed sensor:Sensing is distributed along the length of the fiber

Opto-

electronics

Output, M(t, Zi

)

Opto-

electronics

Output, M(t,z)

Opto-

electronics Sensing

elementOutput, M(t)

LIGHT WAVE PARAMETERS

1.

Amplitude / Intensity

2.

Phase

3.

Wavelength

4.

Polarisation

5.

Time / Frequency

1. PHASEPhysical Mechanism

Interference between signal and reference fibers (Mach- Zehnder

monomode

system) or different propagation modes

in multimode fiberDetection Circuitry

Fringe counting, or fractional phase-shift detectionMain Limitations-

Laser noise and stability

-

Measurement of small phase shifts- Elimination of unwanted spurious effects (other physical variables)

Typical Examples-

Fiber Gyroscope and Hydrophone

- Multimode Gage for Dynamic Pressure/Strain Measurement

OPTICAL MODULATION AND DETECTION TECHNIQUES

2. INTENSITYPhysical Mechanism

Modulation of transmitted light by absorption, emission or refractive index changes

Detection CircuitryAnalog (or digital for go/on-go transducers)

Main Limitations

Normalisation

for source intensity variations and, variable line and connector losses (at long distances)

Typical Examples

-

Strain/ Pressure Gage using Modulated Microbending

Loss-

Optical Encoders

OPTICAL MODULATION AND DETECTION TECHNIQUES

3. WAVELENGTHPhysical Mechanism

Spectral-dependant Variations of Absorption, Emission and Refractive Index

Detection CircuitryAmplitude Comparison at two Fixed Wavelengths, or Analogue Signal for Scanned Wavelength

Main Limitations- Suitable Scanned Wavelength Sources - Wavelength Dependant Line Loss

Typical ExamplesTemperature Measurement By:

-

Variable Fabry-Perot Cavity-

Birefringent

Crystal

-

Semiconductor Band Gap Shift

OPTICAL MODULATION AND DETECTION TECHNIQUES

OPTICAL MODULATION AND DETECTION TECHNIQUES

4. TIME RESOLVED

Physical Mechanism

Transient Absorption or Emission BehaviorsTransit Time of Closed Fiber Loop with Feedback

Detection Circuitry

-

Time-delay Pulse Analysis

Main Limitations-

Modal Time Dispersion in Fibers

Typical Examples

- Temperature Gage by Time Decay of Rare-earth Ion Fluorescence.

-

Nuclear Radiation Diagnostics using Cerenkov

Light.

5. POLARISATIONPhysical Mechanism

Changes in the Gyratory Optical TensorDetection Circuitry

Polarization Analyzer and Amplitude Comparison

Main Limitations

- Spontaneous (Stress-induced) and Inherent Birefringence of Fibers

Typical Examples

Faraday-rotator Magnetic-field Transducer for Current Measurement of High-voltage Transmission Lines

OPTICAL MODULATION AND DETECTION TECHNIQUES

INTENSITY MODULATED SENSORS

Quasi-Distributed Sensing

•

Fiber Bragg Grating (FBG)•

Strain, Temperature, Pressure, Load

OTDR

Measurand

field M(z,t)

M(zj

,t)

z

M(t) Fiber

Sensitized regions

FO INTERFEROMETRIC SENSORS

FO SMART STRUCTURES

•

Buildings & structures that act as their own watchmen•

Aircrafts that twist themselves into optimal aerodynamic shapes•

Pipelines that find and report their own leaks

Smart structures shall be able to sense and respond to their Environment

Efforts are basically to mimic biological systems and in the coming years, this once improbable sounding scenario will become a way of life

•

FO Sensors are the key elements & Rx for infrastructure (USA)

•

Aerospace, composite materials, smart structure technology-

largest potential users of FOS technology

•

Exceptionally valuable for critical structures: bridges, dams, power plants, nuclear reactors.

Projects underway in different countries

Advanced aerospace materials (last 15 yrs)Concrete structures studies (since’84)

SENSORS FOR SMART STRUCTURES AND SKINS (BASIC SENSORS)

Extrinsic Fabry Perot Interferometer (EFPI)

Fiber Bragg Gratings (FBGs)

Long Period Gratings (LPGs)

EXTRINSIC FABRY PEROT INTERFEROMETRIC (EFPI) SENSOR

Variation of Output Intensity (in Arbitrary Units) with Change in Gap Separation `S’

(µm)

Schematic of EFPI Sensor

EXTRINSIC FABRY PEROT INTERFEROMETRIC (EFPI) SENSOR

FEATURES

•

A 2-Beam Low-finesse Interferometer•

Sensing Through Change of FP Cavity Length

I = A (1+V Cos

φ)

I : Output Intensity; V : VisibilityA : Amplitude; Φ

:Phase

Strain, ε

= mλ/2L, m: an integer (No. of fringes) L: gage length

•

Measures Strain, Displacement, Temperature and Acoustic Emission•

Zero Cross Sensitivity•

Small Size and Weight (Typically, Dia: 350 µm, Length: 4mm)•

Extremely Low Thermal Apparent Strain (0.5µm/ºc) •

Localized Axial Strain and Temp, Measurements•

Realization of Multiaxial

Embeddable/ Bondable Strain Measuring Rosettes

•

Ideal for Internal Damage Assessment of Materials/ Structures

FIBER GRATINGSFiber Gratings are sensing elements which are photo inscribed into silica fibers and are a periodic perturbations of optical fiber core refractive index created by exposure to intense uv

radiation

HIGH RESOLUTION WELL-LOCALISED SENSING REGIONABSOLUTE MEASUREMENTLINEAR OUTPUTINSENSITIVE TO OPTICAL SYSTEM INTENSITY FLUCTUATIONSCAPABILITY TO MULTIPLEX SEVERAL SENSORS ALONG ONE FIBERCOST-EFFECTIVE

FIBER GRATING SENSORS : ADVANTAGES

λB = 2neff

Λ

(Bragg Condition)

λB

:Bragg wavelength, neff.

:Effective

RI of the core ,

Λ:Grating pitch

Fiber Bragg Grating (FBG)

( ) ( ) TpeB

B Δ++Δ−=Δ αξελλ 1

Effective Photo-elastic coeffep =ε = Deformation ( )με

ξ = Thermo-optic coeff

α = Thermal expansion coeff

FBGs

-

FEATURES

LENGTH: 5 - 50 mm, PITCH (Λ) : 0.5 - 1 µm (TYPICAL)

STRONGEST INTERACTION or MODE COUPLING OCCURS AT BRAGG WAVELENGTH (λB

)

WAVELENGTH CODED INFORMATION – SELF REFERENCING FEATURE (e.g ABSOLUTE SENSORS)

BASIC SENSING IS THROUGH GENERATION OF STRAIN –

GENRIC SENSORS

SENSITIVITY TO STRAIN, TEMPERATURE AS GOOD AS OF FIBER INTERFEROMETERS

EASE OF MULTIPLEXING & DISTRIBUTED SENSING

Long Period Grating (LPG)

λi = [n01

- n(i)clad

] Λ

λi : Loss resonance wavelength coupled to the ith

cladding moden01:

: Effective index of core mode, n(i)

clad

: Effective index of the ith

cladding mode

(Phase Matching Condition)

A M Vengsarkar

& V Bhatia 1995

COUPLES LIGHT FROM THE GUIDED CORE MODE INTO CLADDING MODES IN BANDS CENTRED AT λіLength: 10 - 50 mm, Pitch: 100 – 600 μm (TYPICAL)

FUNCTION AS WAVELENGTH DEPENDENT LOSS ELEMENTS

ANY VARIATION IN STRAIN, TEMPERATURE OR EXTERNAL R.I. CAN CAUSE LARGE WAVELENGTH SHIFTS IN LOSS RESONANCES

CONCENTRATION MEASUREMENT OF ANALYTES, LIQUIDS AND BIO ORGANISMS (PROCESS CONTROL and BIOTECH INDUSTRY)

SIMULTANEOUS MEASUREMENT OF MULTIPLE PARAMETERS

LPGs: FEATURES

FBG/LPG WRITING SYSTEM LAYOUT

Integrated Vehicle Health Monitoring (IVHM) for Aerospace Vehicles

X-33 is a half scale sub-orbital experimental flight test vehicle-

a collaborative effort between NASA & Lockheed Martin

X-33 Vehicle Sensor Suite Involves:

Objectives: To provide an automated collection and paperless health decisions, maintenance and logistics systems

Greater need to reduce excessive cost associated with access to space

Focus on providing easy repair access for simplified servicing of infrastructures and expedited decision making from detected faults and anomalies

X-33 Advanced Technology Demonstrator

Distributed strain sensor (FBGs)Distributed Hydrogen Sensing (FBGs)Distributed Temperature Sensing (Raman OTDR)

302 electrical cables over 1200 meters in length and weighs over

40 kg replaced by 12 fibers, 76 meters in length weighing less than 1.70 kgMotivation: Ability to provide complete redundancy of all critical cabling

• Life cycle cost benefits

• Electromagnetic compatibility

• Improved performance and capabilities

• Other avionics demonstration with fiber optics: P3C (Navy), RC135(Air Force) and several French Aircrafts

AEROSPACE STRUCTURAL HEALTH MONITORING USING EMBEDDED FBG SENSORS

FOS Technology Developments at CSIO

FBGFBG--LPG Writing SystemLPG Writing System

FBG Sensor applications: Force, pressure, strain/stress, displacement, temperature, acceleration, vibration, acoustics, Chemical and biological sensing, Electrical and magnetic measurements

Grating Writing Modes

1.

Phase Mask (Static & Scanning)

2.

Interferometric

3.

Point-by-Point

• KrF

Excimer

Laser (248 nm) with LN module• UV beam conditioning and manipulating optics • Automated mask and fiber holder• Proximity phase mask • Optical diagnostic and feedback unit with all operation through computer

• Fiber and phase mask positioning and alignment systems• CCD camera based viewing system for monitoring and controlling mask to fiber relative position

• Fiber tension monitoring assembly • Provision for monitoring and display of the writing beam • OSA on-line monitoring of the grating inscription process• Computer control and software for the writing system

FBG/LPG Writing SystemFeatures:

FBG BASED PETROL LEAK SENSOR

1549.35

1549.4

1549.45

1549.5

1549.55

1549.6

1549.65

0 5 10 15 20Time (min)

Bra

gg w

avel

engt

h (n

m)

Dipping Drying Ph

cCurrent Science, 90(2), p 219-221, 2006

Design, development and Packaging of FBG sensors for structural Health Monitoring

0

50

100

150

200

250

300

350

-500-400-300-200-1000

Micro Strain

Load

in k

N

Strain Gage (Average)FBG (Average)

0 5 10 15 20 25 30-100

0

100

200

300

400

500

600

700

800

Com

pres

sive

Stra

in (μ

ε)Applied Load (Tonne)

FBG1 SG1 FBG2 SG2 FBG3 SG3

0 500 1000 1500 2000

0

100

200

300

400

500

600

700

Tens

ile S

train

(με)

Applied Load (Kg)

CSIO FBG ESG1 Micronoptics FBG ESG2

Current Sciences, 97, pp. 1539-42, 2009

MS Specimen

Concrete Specimen

Strain Guage

Interrogator Unit

Weldable Packaged FBG

1544.5 1545.0 1545.5 1546.0 1546.5

-70

-60

-50

-40

-30

Ref

lect

ed P

ower

(dB

m)

Wavelength (nm)

Precured FBG Sensor Postcured packaged FBG Sensor

Pre-cured and post-cured reflection spectrum of packaged FBG sensor

λB

= 1545.54 nm and a grating length of 10 mm FWHM of the FBG was 0.141 nm

Comparison of the strain response of Comparison of the strain response of packaged and unpackaged FBGpackaged and unpackaged FBG

Presented at ICC-CFT, IISc

Bangalore-2011

Weldable Packaged FBGs for Structures

Mild Steel Specimen

Hysterisis Plot

Temperature response of Packaged FBG

Embeddable Packaged FBGs for Structures

0

50

100

150

200

250

300

-500-450-400-350-300-250-200-150-100-500

Micro strain

Load

in k

N

FBGSG

Concrete Specimen

Comparison between Packaged FBG Sensors with ESGs

under Compressive Loading

Field Study of Metallic Bridge in Himachal

Pradesh with NIT Hamirpur

&

HPPWD

3D design and photograph of fabricated FBG packaging fixture

Result of the FE Analysis for FBG packaging fixture

FBG Packaging

Photograph of the packaged FBG sensor for cementitious mounting

0 20 40 60 80 1000

20

40

60

80

100

120 Calibration Factor 1.3 pm/με

Shift

in W

avel

engt

h (p

m)

Measured Strain (με)

0100200300400500600700800900

25 35 45 55 65

Applied Temperature  ˚C

FBG W

avelen

gth Shift in

 pm 

Strain and temperature calibration plot

Results

BEAM TESTING IN THE LAB USING PACKAGED FBG SENSORS

FBG4 (λ4

)FBG2 (λ2

) FBG3 (λ3

)FBG1 (λ1

)

Roller end Rocker end

BeamLoad

A View of FBG sensors installed on the RC beam in Lab

Comparison of response of FBG sensor and ESG sensor on RC

beam

0

0.05

0.1

0.15

0.2

0.25

0.3

0 20 40 60 80 100 120 140 160

Applied load (KN)

Wavelen

gth shift (n

m) Δʎ1 (nm)

Δʎ2 (nm)Δʎ3 (nm)Δʎ4 (nm)

Response of FBG sensors at different locations on RC beam

Results

Photograph of the close view of the mounted FBG and other

conventional sensors

Photograph of the test site of Girder Bridge near Hapur

Test Site

FBG 8‐1558.712Strain during a vehicle movement

‐2.000

0.000

2.000

4.000

6.000

8.000

10.000

12.000

0 20 40 60 80 100 120

Samples Recorded

Strain (µ

ε)

Results

FBG Sensors Technology for Energy SectorHot Spot

Detection and Location in Transformer

Presented at ICOP -2009

FBG installed in 25 kVA

Live Transformer at Vadodara

since Sep.,2009

In Collaboration with ERDA, M/s Alstom, Vadodara

and M/s Ardison, Mohali

DIT Sponsored Project

Wdg

temp. using FBG

Top oil temp. using FBG Top oil temp. using TC

% Loading of Transformer

Ambient Temp. using TC

FBG Based Technique for Monitoring Demineralization of Bone (Bio-Mechanics Application)

0 2 4 6 8 10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0

200

400

600

800

1000

1200

1400

Stra

in G

radi

entB

1 (με/

kg)

Cumulative Ca Loss (gm)

Decalcified Bone Untreated Bone

0

20

40

60

80

100

120

140

Strain Gradient B

2 (με/kg)

Time (Days)

Comparison of strain response of normal and decalcified bone

Experimental

Setup

Results and Discussion

•Same load produced almost double strain in the demineralized

sample as compared to that in untreated sample •Calcium loss of even 0.3906 gm (treatment 1) resulted in 1.3 times/ 24% more strain for same load and a calcium loss of 1 gm resulted in 50% increase in strain. As the calcium loss was more than 2 gm the strain increase was close to 300%

Orthopaedics and Traumatology: Surgery and Research (Accepted)

Presented at ISMOT -

2009

In consultation with Orthopedic Experts from PGIMER, Chandigarh

Impact absorption capability of a mouth guard using FBG sensors

Experimental Setup

Cricket ball impact on mouthguard and Jaw model using FBG Sensor

Impact absorption capability of custom-made mouthguard investigated utilizing FBG sensors in distributed manner

The impact absorption capability was found to be more than 90% for the center impact

This study will be useful for better designing of custom-made mouthguards

Ref: Tiwari et al. Dental Traumatology (2011)

1551.0 1551.5 1552.0 1552.5-55-50-45-40-35-30-25

Reference for 30 degree Impact for 30 degree Reference for 45 degree Impact for 45 degree Reference for 60 degree Impact for 60 degree

Ref

lect

ed P

ower

(dB

m)

Wavelength (nm)

1553.6 1554.4 1555.2 1556.0-55-50-45-40-35-30-25-20

Ref

lect

ed P

ower

(dB

m)

Wavelength (nm)

Reference for 30 degree Impact for 30 degree Reference for 45 degree Impact for 45 degree Reference for 60 degree Impact for 60 degree

Fiber Bragg Grating based Hydrophone for underwater acoustic detection

Acoustic sensors are widely used in civil and military applications 

Hydrophone and SONAR devices (anti‐submarine warfare)

Ocean surveillance systems

Downhole oil and gas sensing

Oceanographic research 

Structural Health Monitoring (Crack detection etc.)

Seismic measurements

Ultrasound and blood pressure monitoring

Marine Mammal Monitoring

Background:

55

FIBER BRAGG GRATING

SENSORS : Passive FBG

Strain,  temperature and pressure (acoustic)

variations, which changes the FBG

pitch Λ

and/or fiber refractive index n,

produce a shift in the Bragg wavelength 

λBragg

.

Bragg Wavelength shift (Δλ)

Wavelength λ

STRAIN and/or TEMPERATURE and/or PRESSURE

Δλ proportional to pressure changeThis Bragg wavelength shift property is the underlying mechanism for FBG based sensors 

56

57

For bare silica fiber, at 1543 nm,n

= 1.46, E =  72 GPa, ν

= 0.23, ρ11

=0.12,

ρ12

= 0.27,

The pressure sensitivity is ~ 3.6 pm/MPa

The pressure sensitivity of FBG can be expressed as:

∆2 1 2

22 1 2 12 11  

Where,E

is Young’s modulus of optical fiberν

is Poisson’s ratio,ρ11

and

ρ12

are elasto‐optic coefficients, n

is effective refractive index

Interrogation techniques•

Intensity based•

Wavelength based•

Phase based

58

Wavelength based Interrogation

Broadband 

Source

Coupler

Photodetector Computer 

Sensing FBG

Tunable

filter

Wavelength shift of FBG can be retrieved by scanning reflected light using an optical tunable filterPeak wavelength is recorded through out the measurement to determine the wavelength changeOptical spectrum analyzer (OSA) and tunable laser system (TLS) can be used to achieve similar measurements

59

Phase based Interrogation (or Interferometric

interrogation)

Broadban

d Source

Circulator Sensing FBG

OPD = nL

Phase 

demodulato

r

Coupler Coupler

Photodetector

Polarization 

Controller

Interferometer converts the induced wavelength shift into a phase shift which is demodulated by the phase demodulatorInterferometric detection is used in applications that require high sensitivity

Homodyne interferometric

detection

60

•

For Deep sea SONAR applications, the sensitivity of hydrophone system should be 

same 

as 

the 

background 

acoustic 

noise 

level 

of 

quite 

ocean 

represented 

by 

DSS0 

(Deep Sea State Zero) ‐‐‐

100 µPa at 1 kHz

•

Such 

strain 

sensitivity 

can 

be 

achieved 

by 

using 

resonant 

Bragg 

grating 

structure combined with interferometric

interrogation system–

Distributed 

Bragg 

reflector 

and 

Distributed 

feed 

back 

structures

are 

currently 

investigated for this purpose

In‐fiber longitudinal strain

2 1 

Where,E

is Young’s modulus of optical fiber : 72 GPaν

is Poisson’s ratio : 0.23P is Pressure

Required minimum detectable strain, εz 

= 0.75 femto‐strain  

FIBER LASER (FL) ACOUSTIC SENSORS: Active FBG

1.

Distributed Bragg Reflector (DBR) fiber laser:Two Bragg gratings with identical Bragg reflection wavelengths are placed on either side of a short section of fiber which is doped with erbium (active medium). 

This structure forms a Fabry‐Perot laser cavity, which when pumped with 980 nm, lases with emission around 1550 nm

Bragg grating at wavelength λBand ~ 99% reflectivity

Active medium (Erbium doped fiber)

980 nm pump light

Stimulated

laser emission around 1550nm

Stimulated laser emission around 1550 nm

61

2. Distributed Feedback (DFB) fiber laser:DFB is a simpler version of the DBR. 

Two identical Bragg gratings directly inscribed on an erbium doped fiber with a gap of less than one Bragg wavelength producing a phase step within the length of the grating.

Features (DBR & DFB):•

Single longitudinal mode operation

•

Very narrow line width (< 5KHz; coherence length of over 30 km)

•

Laser emission Power: 100 µW – 1 mW

Bragg grating at wavelength λBand ~ 99% reflectivity

Active medium (Erbium doped fiber)

980 nm pump light

Stimulated

laser emission around 1550 nm

Stimulated laser emission around 1550 nm

λB

/4

62

Fiber LaserWavelength shift (Δλ) 

Wavelength λ

ACOUSTIC FIELD

In the presence acoustic field both DFB and DBR act in an identical manner. The 

structure within the dynamically strained fiber undergoes dimensional changes 

that manifest themselves as a shift in the lasing wavelength.

Advantages:Intrinsically more sensitive than passive FBG sensor (because of very narrow line width)Particularly suitable for very low pressure measurementsPossibility to inscribe several laser structure on a single fiber with aim of multiplexing a few sensors which can be monitored by a single optoelectronic unit.

DFB or DBR sensor

63

64

Experimental Setup:Preliminary experimental studies performed to detect acoustic signalsWe have used an aluminium plate of dimension 175 mm X 100 mm X 0.5 mm in the cantilever configurationThe FBG is pasted on a pre-determined position in the sensor plateThe position of the FBG is determined through simulation

ASE Source

3dB 

Couple

r

Function 

GeneratorAudio 

AmplifierSpeaker

Interrogato

r Aluminium 

plateFBG

65

Experimental results:

60 80 100 1200

0.01

0.02

0.03

0.04

0.05

Frequency (Hz)

Wav

elen

gth

Shi

ft, Δλ

(nm

RM

S)

1.55 1.6 1.65 1.7 1.751549.64

1549.66

1549.68

1549.7

1549.72

1549.74

1549.76

1549.78

1549.8

Time (sec)

Wav

elen

gth

(nm

)

Response of the sensor for an acoustical 

signal of frequency 87 Hz

Fourier transform of the response 

showing a peak at 87 Hz

66

0 50 100 150 200 250 3000

0.01

0.02

0.03

0.04

0.05

0.06

Frequency (Hz)

Wav

elen

gth

Shi

ft, Δλ

(nm

, RM

S)

Frequency Response of the sensor plate for acoustical signal

Very good sensitivity is observed only in the frequency range 82 Hz to 102 Hz. At other frequencies, the wavelength shift is less than 0.01 nm.The peak at 87 Hz corresponds to the 3rd modal frequency of the sensor plate with cantilever configuration

67

Transducer design for hydrophone

FBG is intrinsically sensitive to only two parameters: Strain and TemperatureFor anything else, transduction mechanism must be provided to transform the measurand into a change in temperature or pressure Improvement in transduction mechanism allows sensitivity enhancement to corresponding perturbationElimination of sensitivity to undesired perturbations such as temperatureTailoring the frequency response

Air filled metal cylinder on which a 

fiber containing FBG is woundedFBG embedded in a polymer filled 

metal cylinderFBG mounted on flexural beam 

encapsulated in air filled housing

68

It consists of a metal cylinder with two rubber diaphragms at the two ends

The fiber containing the FBG is attached to the two diaphragms through the central holes

Pressure  variations  resulting  from  acoustic wave  acts  on  the  two diaphragms which  in  turn creates axial tension in the FBG 

Axial strain in the fiber:464

22

16

 

Where,P

is Pressure;

R

is radius of diaphragm

L

is length of metal cylinderμ

is Poisson’s ratio of diaphragm material

Ef

is Young’s modulus of fiberA is Cross‐sectional area of fiber

with 3

12 1 2  

69

EDF based edgeEDF based edge--filter interrogation filter interrogation scheme for FBG sensorsscheme for FBG sensors

•

Change in external parameters are required continuous monitoring

in the sensor network

•

Several interrogation techniques have been reported earlier based on passive optical filters, interferometer and chirped and tilted FBG

. •

Although these methods have their own advantages, but they require specific gratings and special techniques.

Motivation

70

Working principle

• The interrogation scheme demonstrated here is based on the

conversion of Bragg wavelength shift to optical intensity variation.

• Slope of the absorption spectrum of the EDF acts as the edge

detection filter.

• The edge detection filter in this scheme is a short length of un-

pumped EDF.

71

Experimental details

Schematic of experimental setup

Measured EDF Filter Characteristic at five different temperature

ASE Circulator FBG Sensors

Coupler 10 m EDF

OSA 

Detection 

System

72

FBGs

original (without filter) reflected spectrum

Output from the edge detection filter as observed on the spectrum

analyzer.

73

Comparison of measured strain with the applied strain based on EDF filter

U. Tiwari

et al, IEEE Sensors Journal, 13, 1315 (2013).

Major Contributions• EDF based FBG interrogation scheme proposed and experimentally demonstrated for the first time

• The proposed technique is established for multiple FBG interrogation

• Temperature insensitivity is also demonstrated experimentally

• Simulations have been carried out to validate the

experimental scheme

)(2/)( 21 TfBB Δ=Δ+Δ λλ

)(2/)~( 21 ελλ fBB =ΔΔ

])1()[( zepTnBiiB εααλλ −±Δ+Λ=Δ

Working Principle: Canceling equal compressive and tensile strains

Wavelength shifts for individual gratings

FBG-1

FBG-2

Fiber BraggGrating

Interrogator

Load

Isolating strain effect:

Isolating thermal effect:

Embedded Dual Fiber Bragg Grating Sensor for Embedded Dual Fiber Bragg Grating Sensor for Temperature and Strain DiscriminationTemperature and Strain Discrimination

35 40 45 50 5520

40

60

80

100

Appl

ied

L, g

m

T,o

C

20 40 60 80 100

5

10

15

20

25

App

lied Δ

T, o

C

Load, gm

Load measurement-±1gm

Temperature measurement-±10C

•

Sensor is free from cross sensitivity•

Sensitivity increased by nearly three times both for strain and temperature

•

High yield and repeatabilityMicrowave and Optical Technology Letter, 51, 1621, 2009

75

Single fiber Bragg grating sensor with two sections of different diameters for longitudinal

strain and temperature discriminationMotivation

One of the technical issues is the inability of FBG sensor to separate wavelength shift produced by strain effect and temperature effect

Accurate measurement of strain and temperature requires elimination of cross sensitivity.

• A single Fiber Bragg Grating (FBG) sensor with two sections of different diameters is proposed and experimentally demonstrated

• A section of single FBG is etched in Hydro Fluoric acid (HF) solution to reduce diameter of the fiber to increase its strain sensitivity.

• Different shifts of the Bragg wavelengths of chemically etched and non-etched gratings caused by different strain sensitivities are used to discriminate and measure strain and temperature.

Introduction

76

(2) ][ εκλλ ε Δ+Δ=Δ kTBB T

(1) /1 2rπαεΔ

Working Principle

, εκ kT Temperature and Strain Sensitivity respectively1551.5 1552.0 1552.5 1553.0 1553.5

-65-60-55-50-45-40-35-30-25-20

Tran

smitt

ed p

ower

(dB

m)

Wavelength (nm)

FBG-2FBG-1

FBG-1

FBG-2

25 30 35 40 45 50 55 60 65

1551.00

1551.05

1551.10

1551.15

1551.20

1551.25

0

50

100

150

200

250

λ Β (n

m)

R, μm

Mic

ro S

trai

n

]1[1

εκλ ε Δ+Δ=Δ kTB T

( ]2[2

εκλ ε Δ+Δ=Δ kTB T

( )16 41

2

2

1 andAA

==ΔΔεε

0 25 50 75 100 125 150 175 200 20.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7 Etched FBG Un-etched FBG

Wav

elen

gth

shift

(nm

)

Applied strain (με)0 10 20 30 40 50 60

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Wav

elen

gth

shift

(nm

)Applied strain (με)

Etched FBG Non-etched FBG

78

(3) )1()( 12

1121 ελλ ε Δ−=Δ−Δ

AAkBB

•± 13µε (micro-strain) over 1700µε•± 10C over 60OC

Review of Scientific Instruments, 80, pp.-103106, 2009

79

Strain and Temperature Discrimination Technique by use of a FBG written in Erbium

Doped Fiber

1520 1530 1540 1550 1560 1570-45

-40

-35

-30

-25

Pow

er (d

Bm

)

Wavelength (nm)

1548.6 1548.8 1549.0 1549.2 1549.4

-56.4

-56.2

-56.0

-55.8

-55.6

-55.4

-55.2

-55.0

Tran

smitt

ed P

ower

(dB

m)

Wavelength (nm)

Spontaneous Emission Spectrum of Erbium Doped Fiber

Transmission spectrum of Fabricated FBG in EDF

Introduction

80

ΔP = AΔT

ΔλB

= Bε + CΔT

The strain ε and temperature ΔT determine by usingthe following equation*

where

ΔP is the change in transmitted power through the FBG

A is the temperature coefficient of the EDFA

B and C are the strain and the temperature sensitivities of the FBG

*Jung et al., Applied Optics,1999

Working Principle

81

Schematic of experimental setup

Experimental setup for Simultaneous measurement of strain and temperature

FBG was fixed between two translation stages

Temperature change was measured by keeping FBG in a thermal chamber with EDF, pumped using 50 mW power at 980 nm

The ASE spectrum was monitored on an OSA

Experimental Setup

82The slope was -0.0167 dBm/°C in the overall range from 50 to 110 °C.

1549.0 1549.5 1550.0 1550.5-58.0-57.6-57.2-56.8-56.4-56.0-55.6-55.2-54.8-54.4

Tran

smitt

ed P

ower

(dB

m)

Wavelength (nm)

50oC 70oC 90oC 110oC

UV

ResultsTemperature dependence of transmission spectrum of an FBG

written in EDF with no strain.

83

35 40 45 50 55 60 65 70 75 80 85 90 95 10035404550556065707580859095

100

Mea

sure

d Te

mpe

ratu

re (0 C

)Applied Temperature (0C)

0 200 400 600 800 1000

0

200

400

600

800

1000

Mea

sure

d St

rain

(με)

Applied Strain (με)

Strain and Temperature response of FBG in EDF

Results

84

Comparison between applied strain and measured stain at different applied

temperature

100 200 300 400 500 600 700 800 900 1000

100

200

300

400

500

600

700

800

900

50 oC

70 oC

90 oC Linear Fit of Data1_C

Mea

sure

d St

rain

(με)

Applied Strain (με)

Plot between applied strain and measured strain at three different

temperatures

30 40 50 60 70 80 90

200

400

600

800

Mea

sure

d St

rain

(με)

Applied Temperature (oC)

Results

Photonics -

2010, IIT Guwahati

The strain and temperature sensitivity of sensor is 0.8 pm/με and12 pm/oC

The rms deviation for measured strain and temperature is 21.2 με and 1.0 oCover ranges of 0-900 με

and 40-95 oC

Long Period Grating Based Humidity Sensor

LPFG Based Humidity Sensing

COBALT CHLORIDE/GELATINE 

BASED 

HYGROSCOPIC COATING

Sensing Probe Fabrication and Characterization

FE-SEM NSOM

RI=1.34146 nsur

<nclad

Results

1510 1520 1530 1540 1550 1560-77

-76

-75

-74

-73

-72

-71

-70

Tran

smitt

ed P

ower

(dB

m)

Resonant wavelength (nm)

Air (Reference) 35% RH 45% RH 55% RH 65% RH 75% RH 85% RH 90% RH

The spectral signature of coated LPFG at different levels of known RH

Results

Hysteresis plot of coated LPFG w. r. t. various levels of RH

Hysteresis 

calculation wrt

increasing RH 

values ±

0.2%

Hysteresis

Results

Response at 70% RH level for 300 

minutesStability 

error   0.06%

Stability plot

LPG based Biosensor

1350 1400 1450 1500 1550 1600 1650-80-78-76-74-72-70-68-66-64-62

Tran

smitt

ed P

ower

(dB

m)

Wavelength (nm)

STRUCTURE OF A BIO-SENSOR

•

BIORECOGNITION ELEMENT

: Biomolecules

(enzymes, micro-

organisms, strand of DNA) produced by interaction of an analyte

with an interface.

•

INTERFACE

:

Surface of transducer with immobilized bioelements.

•

TRANSDUCING ELEMENTS :

Electrochemical,acoustic,piezo-

electrical, optical etc.

Biosensor

= biorecognation molecule/bioreceptor

+ Transducer

EnzymeAntibodyMembranesOrganellesCellsTissuesCofactorsDNAPeptideMicroorganism

•

Electrochemical –

Amperometric–

Potentiometric–

Conductiometric•

Piezo-electric•

Calorimetric•

Acoustic•

Optical

ReceptorsTransducers

PhysicalChemical

•Transformation•Coupling

Preliminary Investigation on Long Period Grating based bio-sensor

Reference Protein

GlucaldihideAB

SilanizationGlutaraldehydetreatment

Protein A treatmentAntibody immobilization

SEM images of LPG surface after chemical processing

Shift in wavelength for different bio-agent binding

Presented at ISMOT -

2009

1520 1540 1560 1580 1600 1620 1640 1660-77

-76

-75

-74

-73

-72

-71

-70

Tx (d

Bm

)

Wavelength (nm)

H2So4

APTES GOx 10mg/3ml Glu15 mg/10ml

1520 1540 1560 1580 1600 1620 1640 1660

-76

-75

-74

-73

-72

-71

-70

-69

Tran

smitt

ance

(dB

m)

Wavelength (nm)

H2So4

APTES GOx 10 mg/3ml Glu 20 mg/10ml

Effective Wavelength Shift = 2.52nm Effective Wavelength Shift = 2.68nm

1520 1540 1560 1580 1600 1620 1640 1660-77

-76

-75

-74

-73

-72

-71

-70

Tran

smitt

ance

(dB

m)

Wavelength (nm)

H2So4

APTES GOx 15 mg/3ml Glu 30 mg/10ml

Effective Wavelength Shift = 2.88nm

LPG Sensor based on CoLPG Sensor based on Co--valentvalent

Binding Binding Technique for Glucose DetectionTechnique for Glucose Detection

Ref: Deep

A. and

U. Tiwari

et al. Biosensors

and

Bioelectronics

(2012)

• CONFIGURING OF EXISTING OPTICAL SENSORS WITH FIBER OPTICS

• EVOLUTION OF COST-EFFECTIVE AND EFFICIENT DESIGNS

• APPLICABILITY TO NEWER AREAS

• SENSOR DESENSITIZATION AND PACKAGING

• INTEGRATION WITH MICROMACHINED ELEMENTS

• MULTIPLEXING & DISTRIBUTED SENSING

TRENDS

Remarkable possibilitiesBounds only designers’ imagination, in some cases available technologyAn interesting & promising futureThe technology behind the fabrication, packaging and installation of FBG sensors has been presented for use in the structures.

Laboratory testing has been demonstrated involving the embedment of FBG sensors in the concrete beam and their performance have been presented under variable loading conditions. Field trials using packaged FBG sensor in distributed configuration on the concrete bridge on NH24 near Hapur have also been demonstrated

CONCLUSION

Strain-temperature discrimination techniques have been presented. A new technique for FBG interrogation has been demonstrated.Bio-sensing based on LPG sensor for different applications have been experimentally demonstrated.

References1.

Fundamentals of Fiber Optics in Telecommunication and Sensors Systems, Edited by Bishu

P Pal; Wiley

Eastern Limted, New Delhi, Bangalore, Pune

2.

Optical Fibre

Sensors, Components & Subsystems, Vol. 1,2,3 & 4, Edited by Brian Culshaw

& John Dakin;

Artech

House, Boston/London

3.

Optical Fiber Sensor Technology –

Fundamentals, Edited by K.T.V. Graltan

& B.T. Meggitt; Kluwer

Academic Publishers; Boston/ London

4.

Fiber Optic Smart Structures, Edited by Eric Udd; John Wiley & Sons, Inc; New York/Tronto/Singalore

5.

Optical Fiber Sensor Technology, Edited by K.T.V.Grattan

& B.T. Megitt; Chapman and Hall;

London/Glasgow/New York/Madras