Optical fiber sensors

Dr. Luis Mosquera Universidad Nacional de Ingeniería [email protected] [email protected]

Optical fiber sensors • INTRODUCTION  • FIBER OPTICS Fiber Types Fiber parameters Fabrication process • FIBER SENSORS types of sensors Applications • SENSORS BASED IN LPG GRATINGS Characterization Sensor design Manufacturing techniques LPG sensor applications

INTRODUCTION Energy spectrum of sunlight

History of the Fiber Optic John Tyndall demonstration in 1854

Total internal reflection is the basic idea of the optical fiber

In 1970, Corning scientists Dr. Robert Maurer , Dr. Peter Schultz, and Dr. Donald Keck developed a highly pure optical glass that effectively transmitted light signals over long distances.

The First Fiber Optics 1970: Corning gets fiber attenuation20 dB/km. 1976: NTT and Fujikura Optical Fibers obtained with attenuation of 0,47 dB/km in 1,300 nm. 1977: The first commercial installation of communication system optical fiber developed by Bell Labs is installed on the streets of Chicago. 2016: Attenuation 0,1 dB/km ( ~ 2%) The cost of fiber optic transmission of a conversation is around 1% of the cost of copper cable transmission. Therefore the optical fiber is the medium for long distance communication.

Fiber optic transmission windows

6

Loss (dB/km)

5 4

Dispersion Rayleigh & absorption ultraviolet

3

Operation windows: 825-875 nm 1270-1380 nm 1475-1525 nm

peaks caused by ions OHinfrared absorption

2 1 0 0.7 0.8 0.9

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7

λ (µm) Single-mode optical fiber (980/1550nm, 1310/1550nm, 1480/1550nm, 1550, 1625nm)

Fiber Optics: Advantages •

• • • •

Capacity: much greater bandwidth (the theoretical bandwidth of optical fiber transmission in the 1550 nm window is the terabit). Immunity to electrical interference Immunity to environmental conditions: humidity, temperature, etc. Safety: No risk of explosion or electric shock. Economy: few repeaters are needed

Disadvantages • • • • •

High initial installation cost Higher cost of interconnection low stress strength Higher cost of repair and /or maintenance specialized tools needed

FIBER OPTICS Optical fibers are circular dielectric wave guides. Have a central core surrounded by a concentric cladding with refractive index slightly smaller (≈ 1%) to the core. Optical fibers are typically made of sílice modified by dopants such as GeO2.

The geometry and composition of the fiber determines the set of electromagnetic fields, or modes, which can spread inside. Ray Paths

Index profile n1 core (8-12 µm) n2 cladding (125 µm)

Single-mode step-index Fiber n1 core (50-200 µm) n2 cladding (125-400 µm) Multimode step-index Fiber gradedindex n Multimode graded-index Fiber

Acceptance Cone & Numerical Aperture Acceptance Cone

 

n2 cladding n1 core n2 cladding

-If the angle too large  light will be lost in cladding - If the angle is small enough  the light reflects into core and propagat  

Number of Modes (NM) : In Step index: V2/2 ; Where a=radius of the core

Single-mode step-index Fiber Advantages: • Minimum dispersion: all rays take same path, same time to travel down the cable. A pulse can be reproduced at the receiver very accurately. • Less attenuation, can run over longer distance without repeaters. • Larger bandwidth and higher information rate

Disadvantages: • Difficult to couple light in and out of the tiny core • Highly directive light source (laser) is required • Interfacing modules are more expensive

Losses In Optical Fiber Cables The predominant losses in optic Fibers are: • absorption losses due to impurities in the Fiber material • material or Rayleigh scattering losses due to microscopic irregularities in the Fiber • chromatic or wavelength dispersion because of the use of a non-monochromatic source • radiation losses caused by bends and kinks in the Fiber • pulse spreading or modal dispersion due to rays taking different paths down the Fiber • coupling losses caused by misalignment & imperfect surface finishes

Scattering • • • • • •

Scattering is due to irregularity of materials. There are two main categories of dispersion, intermodal and intramodal. Scattered light passes through cladding and is lost Over 99% of the scattered radiation has the same frequency as the incident beam: This is referred to as Rayleigh scattering. A small portion of the scattered radiation has frequencies different from that of the incident beam: This is referred to as Raman scattering. Dispersion is referred to widening the pulse as the light travels through the fiber optics. The main cause of dispersion in multimode fibers is chromatic dispersion.

Microstructured Optical Fibers (MOFs) •

•

Photonic Crystal Fibers (PCFs): – Index guiding PCFs – Photonic bandgap PCFs. Other MOFs: – Suspended Core Fibers, Bragg Fibers and others.

Optical Fiber. The Manufacturing  Process

The first step in manufacturing glass optical fibers is to make a solid glass rod, known as a preform. Ultra-pure chemicals primarily silicon tetrachloride (SiCl4) and germanium tetrachloride (GeCl4) are converted into glass during preform manufacturing. These chemicals are used in varying proportions to fabricate the core regions for the different types of preforms.

Areas of Application • • • • •

Telecommunications Local Area Networks Cable TV CCTV Optical Fiber Sensors

Fiber Optic Communications Time Division Multiplexing or TDM. Multiple channels are transmitted on a single carrier by increasing the modulation rate and allotting a time slot to each channel. 

An alternate method is known as wavelength division multiplexing, or WDM. By this method, capacity can be increased by using more than one optical carrier (wavelength) on a single fiber. These different wavelengths or channels, are spaced 100 GHz apart. 

Fiber to the Home •

•

Applications: – HDTV (20 MB/s ) – –telephony, internet surfing, and real-time gaming the access network (40 Mb/s) – Total dedicated bandwidth: 100 Mb/s Components (single-mode fiber optic distribution network) – optical line terminal (OLT) – central office (CO) – passive remote node (RN), – optical network terminals (ONT) at the home locations

Fiber Sensors

TYPES OF FIBER OPTIC SENSORS Fiber optic sensors can be divided by: 

Places where sensing happens  Extrinsic fiber optic sensors  Intrinsic fiber optic sensors



Characteristics of light modulated by environmental effect  Intensity-based fiber optic sensors  Spectrally-based fiber optic sensors  Interferometric fiber optic sensors

SIDE-CORE LPG CO2  m L = 14mm refractive index external 1.0 1.33 1.36 1.38 1.40 1.44 1.45

0

Transmittance(dB)

-5

-10

-15

-20

-25 500

600

700

800

Wavelength(nm)

900

1000

Intensity-based fiber optic sensors Taper SMF28 ( bending sensor) Taper SMF28 L=10mm d=30m 0 m 10 m 20 m 30 m 40 m 50 m 60 m 70 m 80 m 90 m 100 m

0

Trasmitancia(dB)

-5

-10

-15

-1

Sensitivity 181nm/m

0

-20

Wavelength shift(nm)

-2

600

-4

650

700

750

 (nm) -6

-8

-10

-12 0.29

0.30

0.31

0.32

0.33 -1

 (m )

0.34

0.35

0.36

0.37

800

850

900

Spectrally-based fiber optic sensors Wavelength shift Δλ, refractive-index, temperature, humidity, pH

5

Transmitance(dB)

0 -5

in air in water LPG L= 20mm  500 m Draka

-10 -15 -20

nm

-25 -30 1480

1520

1560

1600

 (nm)

1640

1680

LPG fiber optic sensor applied to the flexural vibration monitoring and determination of dynamic young's modulus of materials. 0.8

6c

FFT

Magnitude

0.6

0.4

0.2

0.0 0

20

40

60

80

Frequency

 

100

120

140

Young’s modulus determination by the measuring of the propagation velocities of longitudinal and transversal perturbations along a isotropic solid.  

 

E = 6.8967×1010 Pa G = 2.6432×1010 Pa ν = 0.3046

LPG fiber optic sensor applied to the measuring stress and strain in soils  

300

0

0 .2

0 .4

0 .6

0 .8 300

207 Kg

200

S tres s K P a



200

0 .2

0 .4



 0 .0 0 1

0 0 .8

0 .6

 0 .0 0 2

D e p th m

F Linear Fit of F



0

2.0 1.5 1.0

Equation

y =a +b*x

Weight

No Weighting 0.01034

Residual Sum of Squares Adj. R-Square

0.99848 Value

0.5

F F

y(mm)

0

100

75 Kg

Intercept Slope

Standard Error

-0.08335 -0.78138

0.0208 0.01363

0.0

S tr a in m

100

 0 .0 0 3  0 .0 0 4

Y=2,51MPa ν = 0,50

 0 .0 0 5  0 .0 0 6

-0.5 -1.0

 0 .0 0 7

-1.5

 0 .6

-2.0 -2.5 -2

-1

0

area(%)

1

2

3

 0 .5

 0 .4



 0 .3  0 .2 D e p th m

 0 .1

0 .0

Bragg and LPG grating fiber applied in strain - stress measurement

probeta de concreto 1.MOV

Interferometric fiber optic sensors LPG-CO2 sensor Mach-Zehnder Interferometer

Mach-Zehnder Interferometer -40

source of light Mach-Zehnder

-36

-44

-44

Trasmitance(dB)

Trasmitance(dB)

-40

-48 -52 -56 -60

-48 -52 water light air alcohol gasoline

-56 -60

-64

-64 1400

1440

1480

 (nm)

1520

1560

1540

1560

1580

 (nm)

1600

In-fiber Fabry-Perot refractometer Present an optical fiber refractometer based on a Fabry-Perot interferometer defined by two fiber Bragg gratings and an intracavity long period grating that makes the light confined in the resonator to interact with the surrounding medium. Wavelength shifts measured with a resolution of 0.1 pm have allowed to establish a refractive index detection limit of 2.1×10 -5.

LPG •

Fibre gratings consist of a periodic perturbation of the properties of the optical fibers, generally of the refractive index of the core. The LPG promotes coupling between the propagating core mode and co-propagating cladding modes. Phase matching between the modes is achieved at the wavelength: i   [ neff ( )  nclad ( )]

luis.mpeg

The transmission spectrum of the fiber containing a series of attenuation band corresponding to the coupling to a different cladding modes: T (   1 

k2 k2  2

Sin 2 ( k 2   2 L)

where L is the length of the LPG and κ is the coupling coefficient for the i th cladding mode.

A limitation to develop accurate sensors based on long period gratings arises from the large spectral width of the resonant bands.

Transmission spectrum over a broad wavelength range shows the varios Lpom cladding modes to which the fundamental guided mode couples

Mach-Zehnder interferometer

LPGs cascaded configuration 0  n k  k ( )    0  ( )  ( )    ( )  k 2   2 2 Tco ( )   Cos[ * Llpg ]  2  Sin[ * Llpg ] 2  2

k2 Tclad ( )  2 Sin[ * Llpg ] 2 





2nLcav   R ( )  1  4Tco ( )Tclad ( ) Sin[ ]   

2

 (  

2 co Lcav (n01  n0clm )

It is difficult to achieve resonant bands narrower than 1 nm !

BRAGG GRATING When light from a broadband source is transmited to a Bragg grating, the grating will reflect light at a single-peak wavelength that satisfies the Bragg condition:

B  2 B neff with the full width at half maximun approximately given by:

0,442B   1  0,7(kL) 2 neff L k : the depth of the refractive index modulation

sensor 2

0,7 0,6

trasmission (volts)

0,5 0,4 0,3

LPG

0,2

 LPG  374m

FBGs  B  0.5m

0,1 0,0 1490

1500

1510

1520

1530

1540

1550

Wavelength (nm)

trasmitanLPG-Bragg-13.nb

1560

1570

1580

EXPERIMENTAL SETUP

The Fabry-Perot cavity is defined by two fiber Bragg gratings located 33.5 mm apart in a conventional Boron-codoped singlemode fiber, the gratings have 7 mm in length, 9 dB reflectivity and 0.19 nm bandwidth at 3 dB . The device has a total length of 47.5 ± 1.0 mm. An over-coupled long period grating is used to transfer and recover energy between core and cladding in the resonant cavity.

The system is interrogated in wavelength and works either in transmission or in reflection.

Bragg's Tuning gratings 0,35

fiber 2 (Tensed of one of Bragg's gratings)

0,25

fiber 1 (gratings Bragg tuned) 0,20

1,2

0,15

1,0

0,10 0,05 0,00 1507,8 1508,0 1508,2 1508,4 1508,6 1508,8 1509,0 1509,2 1509,4 1509,6

Wavelength (nm)

Transmission (volts)

Transmission (volts)

0,30

0,8 0,6 0,4 0,2 0,0 1523,7

sintonization.nb

1523,8

1523,9

1524,0

Wavelength (nm)

1524,1

1524,2

Spectral response of transmission and reflection of the sensor The grating has 28 mm in length and 374 um period; the first 14 mm of the grating couple 30 dB from the core to the cladding mode at the wavelength 1521.2 nm and the resonance has an spectral width of 14 nm at 10 dB (see Fig. 2), the next 14 mm of the grating couple the energy back from cladding to core and recover the transmitted signal in the middle of the band. The final spectrum is flattened, it has a transmission band centered at 1521.2nm with a 3dB bandwidth of 20 nm. It was measured that the resonance wavelength shifted to blue when the long period grating was immersed into water, therefore the cavity reflectors were fabricated with a Bragg wavelength of 1509 nm in order to use the sensor as aqueous solutions refractometer. The Bragg gratings were inscribed scanning the fiber by a UV beam at 244nm through a phase mask.

Spectral response of the sensor 0,30

Transmission (volts)

0,25

glucosa 0,31% glucosa 0,60% glucosa 0,90% glucosa 1,66% glucosa 2,0%

0,20

0,15

0,10

0,05

0,00 1508,86 1508,88 1508,90 1508,92 1508,94 1508,96 1508,98 1509,00 1509,02 1509,04

Wavelength (nm)

glucosa 0,31% glucosa 0,60% glucosa 0,90%

0,20 0,19

Reflectance (volts)

0,18 0,17 0,16 0,15 0,14 0,13 0,12 0,11 0,10 1508,97

1509,00

1509,03

1509,06

1509,09

Wavelength (nm)

1509,12

1509,15

16 14

Resonance shift (pm)

12 10 8 6 4 2 0 -2 0,0

0,5

1,0

1,5

2,0

Concentration (wt%)

The wavelength displacement is linear with concentration and a linear fit gives a sensitivity to concentration of 6.79 pm/wt%.

Mechanically induced long-period interferometers in fiber gratings

Principle of Operation Pressure applied

SCG OSA 4

LPG

Trasmission (dB)

0

-4 (P1) (P2) (P3) (P4) (P5) Fiber F4 m L = 65mm

-8

-12 1200

1300

1400

1500

Wavelength(nm)

1600

1700

Properties Coupling stretch, loads, stress, strain, curvature Effect of loading P i on the spectrum of LPG

-4

-8

-12

-16

0.35kg 0.45kg 0.55kg 0.65kg 0.75kg 0.85kg 1.0 kg 1.25kg 1.5kg 2.0kg 2.25kg 2.5kg 2.75kg 3.0kg 3.25kg Fiber F4 LP G:  m L = 2,5 cm

0

Transmission(dB)

Transmission(dB)

0

-4

-8

-12

LP 11 Fiber F4 LPG:  m L = 2,5 cm

-16

0.0

0.5

1.0

1300

1400

1.5

2.0

2.5

3.0

3.5

P i(Kg)

-20 1500

1600

1700

1800

Wavelength(nm)

Increase is observed in the coupling stretch when increasing the weight applied on the grating

Applications MLPG-based Michelson Interferometer sensor Pressure applied

0

P1 -2

Couple r

OS A

Reflectance (dB)

SCG

Golden film

P2 P3 -4

-6

P1 < P2 < P3 -8 1520

-3.0

1540

1560

1580

1600

Wavelength (nm)

-4.0

4

-5.5

80% ethanol 20% gasoline

Pure gasoline

-6.0

20% ethanol 80% gasoline

Commercial Gasoline

-6.5 1556

1558

1560

1562

Wavelength (nm)

1564

0

2

1

dB

-5.0

Commercial gasoline 3



Pure ethanol

2

R efle cta n ce

-4.5

Wavelength shift | | (nm)

Reflectance (dB)

-3.5

4

6 1 .5  1 0 

0 100

80

60

40

20

0

Ethanol percentage in gasoline (%)

6

1 .5 5  1 0 

 6

W a v e le n g t h m

1 .6  1 0

 6

MLPG-based sensor MachZehnder Interferometer Pressure applied

Pressure applied 10

First LP G

5

OSA

Transmittance (dB)

SCG

0

Interferometer -5 -10 -15

Second LPG -20 -25 1400

Commercial gasoline

1450

1500

2.0

1550

1600

Wavelength (nm)



1.5

T r a n s m is s io n d B

Wavelength shift | | (nm)

2.5

1.0 0.5 0.0 -0.5 100

80

60

40

20

Ethanol percentage in gasoline (%)

0

0

5

1 0 1 5 2 0 1 .4 3  1 0 

6

1 .4 7 5  1 0 

 6

W a v e le n g th m

1 .5 2  1 0 

6

LPG in PCF

calculo de periodo de la red en PCF.nb

10

Trasmitance(dB)

5

P1 P2 P3 P 3>P 2>P 1 LP G L= 20mm 500 m P CF (Enver)

0

-5

-10

-15 1300

1400

1500

1600

 (nm)

1700

1800

LPG in Taper

0

Trasmitance(dB)

-2 -4 -6 -8 -10 -12 1100

t1 t2 t3 t4 t5 t1
1200

1300

1400

1500

 (nm)

1600

1700

1800

Surface-core LPG fiber grating

SIDE-CORE LPG CO2  m L = 14mm refractive index external 1.0 1.33 1.36 1.38 1.40 1.44 1.45

-5

895

890

885

(nm)

Transmittance(dB)

-10

l 900

-15

880

875

-20

870

865 1.0

1.1

1.2

1.3

Refractive index unit (RIU) 800

900

Wavelength(nm)

1000

1.4

1.5

Thank you very much for your attention!!