Optical Waveguide Sensors: Sub-Micron to Supra-Kilometer

R. A. Lieberman

ABSTRACT

The advancement of optical waveguide technology has been driven by interplay between the development of techniques for sensing and for communications. This interplay has resulted in sensors capable of measuring physical and chemical properties at length scales comparable to entire railroad tunnels, comparable to cellular organelles, and a panoply of point sensors, distributed sensors, and sensor arrays for every length scale in between. As new waveguide-related technologies emerge, exciting new opportunities are arising to extend this range even further.

Outline • Physical Sensors

• Pressure (micrometer/millimeter)

• Temperature (millimeter/centimeter)

• Ultrasound/Strain (centimeter/meter)

• Electric Field (centimeter)

• Chemical Sensors • Biomedical (micrometer/centimeter)

• Microbiology (nanometer/micrometer)

• Environmental (centimeter/meter)

• Security (meter/kilometer)

} “nanofabricated”

Fiber-tip Fabry Perot Pressure Sensor

250 μm

FULL REFLECTOR

Fiber Optic Bragg Grating Pressure Sensor

n 1 (Core)

n 2 (Cladding)

Transmitted Light

Reflected Light, λ B

Incident Light Fiber Bragg Grating

Λ , Grating Period (100’s of nm)

2a, core diameter

λ B = 2neff ΛReflected Wavelength:

Multipoint Bragg Grating Sensor Array: 10 FBGs in 5 cm

Fiber Bragg Grating Readout • Passive spectral monitoring

• “White light” plus array waveguide grating (AWG): Wavelength is directly read by CCD array

• Narrow-band source, broadband grating: Reflected intensity depends on measurand

• Active monitoring (source/receiver tracking) • Modulated DFB laser locked to grating wavelength: “Error current” directly proportional to temperature • Strain-tuned reference grating: Wavelength matches probe wavelength

Multipoint FBG Thermometry

• Bragg grating array fabricated with intra-grating spacing < 0.5 cm

• Wavelength-keyed spatial resolution:

grating reflectivity maxima successively increase by 3 nm

• Scanned DFB laser-based readout addresses each grating sequentially

• Temperature grid measured in < 1 sec

Multipoint Temperature Response: 0.1°C Resolution

K(G,T) =dλ BdT

= 2 ΛdneffdT

+ neffdΛdT

Temperature Sensitivity

Monitoring Medical Hyperthermia Treatment

FBG Medical Diathermy Monitoring

Tiss

ue P

hant

om T

empe

ratu

re (º

C)

Fiber Optic Ultrasound Detection – 1995 Cantilever Test Assembly with Surface Mounted FBGs

A

C B

D

FBG Locations Ultrasound Source

200 kHz Detected at 4 Locations

FBG A

FBG C

FBG B

FBG D

Self- Acoustic Emission - 1999 • All structures “creak” at ultrasound frequencies when

stressed.

• This self-acoustic emissions (SAE) is composed of ultrasound pulses caused by the formation of micro-cracks.

• Increase in SAE under load is a recognized precursor to catastrophic failure.

• FOAES – The fiber optic acoustic emission sensor system – is designed to detect self-acoustic emission from structures.

Aircraft Engine Mount Fatigue Tests

Typical Time Domain Signal Under Loading Conditions

FBG Array for Guided-Wave Ultrasound Detection - 2010

• Launch ultrasound with conventional transducer at any convenient point

• Conformal array of FBG transducers maps ultrasound field over large (m2) area

• Eliminates fluid bath, custom-shaped transducers, and other inconveniences of piezoelectric pickups

612-Grating Ultrasound Receiver

FBG Ultrasound Panel DAQ System

GWUS Detected with FBG-panel in F-15 Composite Skin

(Top left) at 2 in. separation; (top center) at 3 in. separation; (top right) at 4 in. separation; (bottom left) at 5 in. separation; (bottom center) at 6 in. separation; (bottom right) at 7 in. separation.

F-15 Composite Disbond Detection Ultrasound Image acquired with a grid of 612 FBGs

Color-coded GWUS amplitude map. Red shows areas where disbonds occur in F-15 vertical stabilizer.

Flexible FBG Panel - 2012

100kHz Resonant PZT Source Measures at 100kHz excitation

Defect-Free Sample

Defect Present

Difference Response Shows Reflected Antisymmetric A0 Component

Subtraction of "Reflection Measurements on ISU Samples,Source: NDT PZT Resonant f: 100 kHz, Receiver: B-1025 Broadband

PZT Drive Frequency: 0.1 MHz, (2-01-11)"

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0 20 40 60 80 100 120 140 160

Time, μs

US

Res

pons

e, V

Reflected A0

Direct propagation of A0

Frequency Content of Responses with 100kHz excitation

Defect-Free

Again: See an increase in Lower-Frequency content of Defected sample response vs. Defect-free sample

Defect Present

Laser Ultrasound Technology (LUT) Inspection System

• Laser ultrasonics combines: • High-energy pulsed laser to excite ultrasound

• Adaptive interferometer that detects only ultrasound

Base Station

Measurement Head

Fiber Umbilical

I O Receiver

CW Probe Laser

Generation Pulsed Laser

Measurement Head

Sample

Principle of Operation: Photorefraction/Two-Wave Mixing

• Photorefractive “mirror” perfectly satisfies Bragg condition • Interferometer in quadrature for all distances until material’s frequency

response dies out (100kHz) • Result: nm-scale ultrasound vibrations faithfully be measured even in

presence of mm-scale low-frequency vibrations

LUT Laboratory System

Base station

Pulsed generation laser

Optical head and scanning hardware

Measurement Head Mounted Onto Weld Carriage

36” pipe

Real-Time Measurement of Steel Pipe Wall Thickness During Manufacturing

5m/s

0 2 4 6 8 10 12 14 16 18

Pipe Position (m)

3.8

2.8

3.0

3.2

3.4

3.6

Wal

l Th

ickn

ess

(mm

)

Measured by Laser-Based Ultrasonic System

Measured by Conventional Ultrasonic Gage

LUT Measurement Head

Liquid Crystal E-field Sensor

Millivolt Sensitivity of E Optrode Bias on linear portion of polarization curve

Bioelectric Field Optrodes

Liquid Crystal

Fiber Optic E Field Sensor Response to Cardiac Rhythm

Oscilloscope Lock-in Amplifier

Top trace – Input to the LC cell Bottom trace – Output of the photodetector

Outline

• Physical Sensors • Pressure (micrometer/millimeter)

• Temperature (millimeter/centimeter)

• Ultrasound/Strain (centimeter/meter)

• Electric Field (centimeter)

• Chemical Sensors • Biomedical (micrometer/centimeter)

• Microbiology (nanometer/micrometer)

• Environmental (centimeter/meter)

• Security (meter/kilometer)

} “nanofabricated”

Blood Chemistry Biosensors

• Oxygen partial pressure(PO2), carbon dioxide partial pressure (PCO2), and acidity (pH) change radically minutes before patient goes into shock

• Chemically sensitive optrodes undergo fluorescence quenching (for O2) or color change (for pH, CO2) when analyte changes

• Fluorescence lifetime and intensity, measured as for fluoroptic temperature sensor, and optical absorbance, measured by multi-wavelength methods, give continuous readings of critical parameters.

“Blood Gas” Optrode Evolution

Conjunctival Tissue Gas Sensor For Neonates Requirement to non-invasively

measure blood/tissue oxygenation in neonates:

Too low -> potential cerebral palsy

Too high -> retinopathy with risk of blindness

Pulse oximetry alone not sufficient

Standard lab tests cause anemia

Solution: Very thin, pliable sensor placed under infant’s eyelid monitoring tissue gases (pO2, pCO2, pH) due to contact with conjunctival capillary layer

Initial research funded by NIH; collaboration with Harbor-UCLA (Pediatric Ophthalmology and Pediatric Critical Care)

Tissue/Blood “Gas” Sensor

Schematic view of probe

Micro-Drilled Fiber Two orders of magnitude more signal than evanescent and fiber-tip designs

Oxygen Sensor Film Response

1.50E+05

1.70E+05

1.90E+05

2.10E+05

2.30E+05

2.50E+05

2.70E+05

2.90E+05

0 300 600 900 1200 1500 1800 2100 2400 2700 3000 3300

Time in seconds

Em

issi

on, a

.u.

2% 3% 5% 10% 20%

Tissue/Blood Gas Sensor

3000

3500

4000

4500

5000

5500

0.00 120.00 240.00 360.00 480.00 600.00 720.00 840.00

Elapsed Time

Inte

nsity

(cou

nts)

0

10

20

30

40

50

60

70

80

90

100

CO

2 %

Tes

t Pro

file

Cyc

le (s

imul

ated

)

Response to varying dissolved CO2 concentrations

Fully fabricated multi-analyte probe: pH, PCO2, PO2

Cell Chemistry Biosensor

• Multiple non-interacting optical waveguide cores can be formed in a probe that is less than 1 μm in diameter.

• Near-field optical effect is used to photopolymerize

optrodes at the ends of each sub-wavelength core. • The resulting “nanoprobe” can simultaneously

measure oxygen, pH, and metabolytes in vivo in single cells.

Nanoscale Fiber Optic Biosensor

Fiber Optic Nano-Biosensor

Photomicrograph (length approx. 100 μm)

•Optical fiber tapered to 100 nm •“Sensor bead” optically formed at tip of fiber – multiple channels •Fluorescence of sensor beads depends on local chemical environment

Multi-Core Chemical “Nanoprobe”

Central fiber end

One of six outer fiber ends

Sensor Core

Isolation Member

Three-Function Optochemical Nanoprobes

Dye-doped polymer sensing site

Three Sensor Channels in Subcellular Dimensions

Channel#1: Calcium Green; Channel #2: Texas Red; Channel #3: No Dye

Fiber Optic “Life Detector”

• Most living cells share common biochemical pathways for energy production, storage, and transport

• Spectral properties of the proteins involved in these pathways are similar

• Straightforward remote fiber spectroscopy, coupled with sophisticated spectral processing algorithms, can identify living cells, whether in biofilms, in tissue samples, or in vivo.

Fiber Optic Sensor for Tissue Viability/ Biofilm Detection

NAD/NADH Fluorescence Detected

UV Excited Planktonic Bacterial Fluorescence Captured by Fiber Optic Probe

00.0020.0040.0060.008

0.010.0120.0140.016

400 450 500 550 600

Wavelength, nm

Nor

mal

ized

Flu

ores

cenc

e In

tens

ity0th Order Dilution (1:1)

1st Order Dilution (1:10)

2nd Order Dilution (1:100)

3rd Order Dilution (1:1,000)

BioProbe Live/Dead Differentiation

Maximum Response of Biofilms on Polycarbonate Coupons Using Bioprobe

0

1000

2000

3000

4000

biofilm 1 (g=0.45) biofilm 2 (g=0.35) biofilm 3 (g=0.45)samples

Opt

ical

Sig

nal (

mV)

live samples

killed samples

Colorimetric Detection • Immobilize hydrogen-sensitive indicator in optically transparent medium • Indicator/matrix reversibly changes color in presence of H2 • Intensity of light transmitted through matrix depends upon hydrogen concentration • Intrinsically safe, electrically inert, wide temperature range, works in absence of O2

Optical Waveguide Hydrogen Sensing

Optical Sensor Formats

Integrated Optic Waveguide: Indicator imbedded in waveguides fabricated on optical chip. Multiple channels improve performance.

Optrode: Indicator immobilized in porous glass. Sensors can be packaged with electronics or remotely addressed through fibers.

Distributed Sensing Fiber: Indicator coated on entire length of sensing fiber. Wide area continuous coverage with a single cable.

Optical Fibers

Braids

Cable core

Hydrogen Safety Sensor Prototype

Hardware Package Evolution

2011 2012

Sensor Repeatability/ Reversibility Validated

Optrode response to 0.2; 1.0; 2.0%, of hydrogen at IOS

Optrode response to 0.22, 1.08 and 2.15% of hydrogen at U.S. Dep’t of Energy

H2 Partial Pressure (atm)

0.000 0.005 0.010 0.015 0.020 0.025 0.030

Signa

l/ZV

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Calibration Curve at P = 0.8, 1.0 and 1.2 atm (NREL)Calibration Curve at P = 1.0 atm (IOS Lab)

Time / min

0 100 200 300 400

Sign

al /

Cou

nts

24000

26000

28000

30000

32000

34000

36000

38000

8000 10000 12000 14000 16000 18000

Sig

nal /

Cou

nts

16000

18000

20000

22000

24000

26000

28000

30000

•Optical signals self-consistent within 2% •Optical signals at NREL within 2% of IOS signals

Hydrogen Sensor Alarm Algorithm Detail

0.2% H2

0.2% H2

0.4% H21% H2

2% H2

OFF

OFF

Optrodes Have High Sensitivity and Rapid Alarm Capability

H2 (%) Detection Time (s)*

4.0 3

2.0 3

1.0 3

0.5 3

0.1 10

0.05 10

0.02 30

0.01 120

Response Time (T90) at Various Hydrogen Levels

*RH = 45%; P = 1 atm.; T = 23 ºC; Flow Rate = 1.0 L/min.

Response time < 5 sec. at 10% Lower Flammable Limit

Humidity/Temperature Effects

H2 %

0 1 2 3 4 5

Sign

al/Z

V

0.75

0.80

0.85

0.90

0.95

1.00

1.05RH 10%RH 20%RH 40%RH 75%

H2 %

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Sign

al/Z

V

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1T = 10 οCT = 20 οCT = 30 οCT = 50 οC

• Excellent response at all RH levels • Reproducible effect

• Good response at all temperatures • Reproducible effect

Fibers fabricated specifically for chemical detection “The cable IS the sensor!”

• CO2

• Water • H2S • NH3

• pH

• Cl2

• HCl • HCN • H2

• Relative Humidity • Etc..

Distributed Intrinsic Chemical Agent Sensing and Transmission (DICAST)®

DICAST® is a registered trademark of Intelligent Optical Systems

The DICAST® Paradigm

• LINEAR sensor, not a “point sensor” • Sensor cables respond to target chemical anywhere

over sensor length • Cables contain arrays of optical fibers, each

intrinsically sensitive to a particular chemical agent

• Dosimetric response • Longer exposure yields larger signals • “Integrates” concentration over time • Reversible (concentration) response also possible

Operational Features • Installation

• Continuous cable • Seamless sensing – absolutely no gaps in coverage over entire cable length

• Serpentine deployment can provide “blanket” coverage

• Concatenated segments • Sensor fiber segments interspersed with communication-grade cable

• Extends coverage range – replaces point sensor array

• Optoelectronic detection • “Alarm-style”

• Alerts user if even a single meter of cable is exposed to 10%IDLH/LCT50

• Self-referenced lock-in detection gives high sensitivity and stray light immunity

• Position-resolved • Locates chemically exposed section with 10 cm resolution

• Self-referenced optical time domain reflectometry differentiates between chemical and physical changes in fiber cable

DICAST® Sensor Principle: Chemically-Induced “Cladding Light Loss”

So S1 n1

n2 Fiber core

Fiber cladding

Chemical agent

• Interaction of chemical agent with indicator in cladding changes optical properties

• Light propagating through sensor fiber is affected by changes in exposed region

Indicator Molecules

DICAST fibers use a step-index polymer clad silica core (PCS) fiber structure Cladding material characteristics:

Polymer: • Index < 1.458 • Good adhesion to glass • Good abrasion resistance • Low moisture permeability • Suitable for high-speed fiber draw • Good host for chemical indicator

Coated fiber

Uncoated fiber

The Key: Chemically Sensitive Polymers

Indicator System: • Colorimetric response to analyte • Reversible response to analyte • No response to other substances • Narrow-band optical absorbance • Suitable for high-speed fiber draw • High stability in target environment

Sensor Fiber Performance (HCN) Fibers respond faster to higher concentrations

TEST031805-1

-5

-4

-3

-2

-1

0

1

0 60 120 180 240 300 360 420 480 540 600

Exposure Time (sec.)

Sens

or S

igna

l (dB

/m)

50ppm50ppm5ppm5ppm5ppm

Chemical Exposure Begins

Test exposure: Response of one fiber in cable to H2S--Signal vs. Reference

• Different wavelengths used to interrogate the same fiber allow self-referencing

• Signal wavelength (light blue) reacts immediately to H2S exposure

• Reference wavelength (purple) practically unaffected by exposure

• Allows alarm algorithm to compensate for non-specific effects (temperature, mechanical stress etc.)

0

0.2

0.4

0.6

0.8

1

1.2

0 200 400 600 800 1000 1200 1400 1600

Time [sec]

Val

ue

Gas OnFiber 2--visible (Signal)Fiber 2--IR (Reference)

Test exposure: Response of all fibers in cable to H2S—Signal wavelength only

• Signal wavelength shown for all fibers in cable

• Response from all fibers to exposure event provides unique “fingerprint”

• Alarm algorithm can use response trained pattern to identify individual threat

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 200 400 600 800 1000 1200 1400 1600

Time [sec]

Valu

e

Gas OnFiber 1 SignalFiber 2 SignalFiber 3 SignalFiber 4 Signal

Test exposure: Response of all fibers in cable to HCN and Cl2—Signal wavelength only

• Broad-spectrum fiber provides instantaneous warning

• Response from all fibers to exposure event provides unique “fingerprint”

• Alarm algorithm can use response trained pattern to identify individual threat

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 200 400 600 800 1000 1200 1400 1600 1800

Time [sec]

Valu

e

Gas OnFiber 1 SignalFiber 2 SignalFiber 3 SignalFiber 4 Signal

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 200 400 600 800 1000 1200

Time [sec]

Valu

e

Gas OnFiber 1 SignalFiber 2 SignalFiber 3 SignalFiber 4 Signal

HCN Cl2

DICAST® Rapid Alarm Optoelectronics

• Proprietary system architecture

• Proprietary circuit designs

• Commercial off-the-shelf components

0 10 20 30 40

trans

mis

sion

(cou

nts)

365

370

375

380

385

390

Reference Signal: 805 nmRaw Sensor Signal: 580 nm

Time (min)

0 10 20 30 40

trans

mis

sion

(cou

nts)

370

375

380

385

390

395

Compensated Signal

0.0 % CO2

0.5% CO2

1.0% CO2

3.0% CO2

6.0% CO2

Wavelengths far from the absorbance of the indicator dye are unaffected or minimally affected by the presence of the analyte, which enables the system to be self-referenced

Sensor Signal Compensation

Zone-by-Zone Deployment

• Sensor cable • Parallel sensor fiber loops

• Fed by commercial fiber

• Continuous coverage

• Can be “diced out” as short sensor cable sections joined by long conventional cable sections

• Readout • One channel per segment

• Zones monitored in real time

• Alarm triggered upon change in λ1 intensity without change in λ2

1.25 dB/m 20 meters (x4)

0 dBm

-10 dBm -35 dBm 316 nW (x4)

Silicon PIN

DICAST® System HCN Response County Regional Airport, Nov. 29, 2006

Position-Resolved DICAST®

Visible OTDR

• Principle: time-of-flight determines location sensed

• System provides user with plot of chemical concentration versus location

Position-Resolved Deployment

• Sensor cable • Single-ended sensor segments • Fed by commercial fiber • Continuous coverage • “Diced” into discontinuous segments

• Readout • Entire cable read”in real time • Optical absorbance plotted as a

function of length • Self-referenced: use of two

wavelengths minimizes false alarms • Chemical incursion localized within 10

cm

Before Exposure

4 m Exposed to Chlorine

DICAST® Applications • Environmental Monitoring

• Carbon dioxide sequestration reservoir integrity

• Groundwater contamination by halogenated hydrocarbons

• Critical Asset Protection • High-value buildings (airports, rail stations, subways, containers…)

• Transportation assets (Airplanes, trains)

• Industrial Monitoring • Pipeline leaks

• Fugitive emissions

• First Response to Chemical Events • Site characterization: Serpentine deployment over entire suspected area

• Containment verification: “Tripwire” perimeter around contaminated area

• Corrosion Monitoring • Moisture/humidity

• pH/corrosion byproducts

DICAST® Sensor Cables

• Analyte-permeable sheath • Lets ambient chemicals in to

react with fibers • Provides rugged protection

against shear stress

• Multi-fiber “core” • Grooves allow air unimpeded

access • Embedded stiffening member

adds ruggedness

DICAST® Beta Sites

DICAST® Field Deployments – 2005 - 2008

TC&C Dev3

0

0.5

1

1.5

2

2.5

3

1300 1400 1500 1600 1700 1800 1900 2000 2100

Time (Sec)

DC (V

olts

)

ExtPD2DCExtPD1DCSignal2DCSignal1DCExtPD4DCExtPD3DCSignal4DCSignal3DC

DICAST Sensor Fiber Exposure at Beta Test Site (25 PPM H2S)

Gas Flow Initiated

Controlled-Release CO2 Field Trials: 2013

CO2 Injection Well

CO2 Sensor Cable Wells

Sampling Wells

TEST PROTOCOL: Controlled release of

CO2 into ground “Grabbed sample”

measurements of CO2 transported 20m through soil

Continuous

recording of response of DICAST sensor fibers installed between release and sampling wells

Preliminary Field Trial Results

Time (hours)

0 50 100 150 200 250 300

CO

2 sat

urat

ion

(%)

0

20

40

60

80

100

120Pure CO2 is injected in the aquifer

DICAST® Sensor Fiber Response to Injected CO2 – Single-zone readout –

Submicron to Supra-Kilometer

Dye-doped polymer sensing site

Optical Waveguide Sensors are Here!