OCW 5 Optical and Optoelectronic Sensors and...

Electronic InstrumentationOptical and Optoelectronic Sensors and Measurements

Electronic InstrumentationElectronic Instrumentation

Chapter 5

Optical and Optoelectronic Sensors

and Measurements

1Jose A. García Souto / Pablo Acedo


Module 5. Optical and Optoelectronic Sensors and Measurements

• Introduction

• Basic components and signal conditioning• Basic components and signal conditioning

• Position, Displacement and Rangefinders

• Laser interferometry

• Fiber Optic sensors

• Gyroscopes



Introduction

• In this chapter we will focus on the principles of measurement used in optical and optoelectronic sensors .

• A survey of input sensor mechanisms will be addressed but only some applications will be described in more detail.

• In this case the classification of the sensors is due to the


• In this case the classification of the sensors is due to the characteristics of the light (electromagnetic wave) that are changed by the measured physical phenomena:– Amplitude (Intensity)

– Polarization (Polarimetry)

– Optical phase (Interferometry)

– Frequency of emission / wavelength (Spectroscopy / Gratings)


Introduction

• Some advantages of optical and optoelectronic

measurements:

– Non-contact and non-invasive approaches


– Non-contact and non-invasive approaches

– EMI inmunity

– High precission measurements (e.g. interferometry)

– Withstanding harsh environments and preserving

electro-magnetic isolation (e.g. optical fiber)


Optical Conditioning

Basic Architecture for an Opto-Electronic Instrumentation Measurement System

mp1

p2

Light InputLight Output

DetectionAnalog Signal Conditioning

q Vop2

p3

Detection

• Light Input from a light source (LASER, LED), but sometimes is not necessary.

• Output from the detector can be either a current (photodiode) an impedance change (photoconductor) or a voltage (photovoltaic sensor).

• Optical conditioning and lead-in/ lead-out of the light sometimes present (e.g. optical fiber) .

• The optical conditioning frequently converts the information from an optical magnitude to another (eg. Interferometer: optical phase to intensity) .

Conditioning



Analog signal conditioning of optoelectronic sensors

• Primarily light is detected by photodiodes, APDs,

photoconductors.

• Current output with high impedance or variable high impedance

is mainly obtained.


is mainly obtained.

• Current to voltage circuits are basically used.

• Light may be externally injected into optoelectronic

measurement systems by means of Lamps, LEDs and LASERs

biased by a current source or voltage to current conversion

circuit.


Equivalent of photo-detectorsPhoto-diode and APD

Jose A. García Souto / Pablo Acedo 7

Photoconductor/Photo-resistor


Current to voltage circuit



Biasing light detectors


PHOTOVOLTAIC PHOTOCONDUCTIVE

Zero Bias Reverse Bias

No “Dark” Current Has “Dark” Current

Low Noise (Johnson) Higher Noise (Johnson + Shot)

Precision Applications High Speed Applications


Example of electronic signal conditioning

Div


Current to voltage converter

Normalization to the intensity mean

Adjustment of sensitivity


Basic Components for optical conditioning

• Emitters: LASER, LED and other light sources (lamps, SLED, etc.)

• Detectors: Photodiodes, APD, photo-conductors and others (Photo-transistors, LDR, etc.)

• Optical components: – Lenses, mirrors, beam-splitters, …

– Polarizer, optical retarder, Wollaston prism, crystals, …


– Polarizer, optical retarder, Wollaston prism, crystals, …

– Diffraction gratings

• Optical fiber:– Transmission fibers

– Intrinsic sensing

– In fiber components (e.g. in fiber Bragg gratings)

• Optical modulators– Acousto-optic modulators and other actuators (e.g. PZT onto a fiber)


• On/off intensity sensors, grating sensors and encoders:

• An optical displacement transducer can be fabricated with two overlapping gratings which serve as light-intensity modulator. They can be used as proximity sensors or position encoders.

Position, Displacement and Rangefinders




• Intensity sensor (attenuation):

Projected area

• Intensity attenuates with distance. Sometimes mirrored reflection on the target is present, but in general diffused reflection must be considered.


d

x

θ

y Detected area


d/2 x

θ

r

d/2

OF1 OF2

Example: Vibrations

Measurement Reference


Differential Synchronous detection

Carrier



• Optical Range Finders (time of flight):

• Range is obtained measuring the time difference between the sent pulse and

the received signal. Pulsed and amplitude modulation systems are used.

Transmitter Pulsed Range Finder


Target

TransmitterLaser diode

Reception lens

d

APD

r tflight

φshift

Pulsed Range Finder

CW Range Finder



• Laser Doppler:TransmitterLaser diode

r

θ


]/21[]/)(2··[

cvwdt

ctdwtwdw xo

oor +=+=

Target

Reception lens

d

APD

θ

v

xv


• Triangulation

• The position of light spot on a position sensitive photo-detector is related to the

distance by triangulation. As an example, a PSD is a differential current device that

gives an output related to the position of a collimated beam on the device surface. It

provides one and two-dimensional position. Others are photo-detector arrays.



Target

Laser/LEDDiode

Receptor Lens

D

a

x

f

PSD

x = 0


Intensity

Laser Emitter

Beam-splitter

Reference path

Measurement path

MirrorLens


• Laser interferometry


0 2π 4π 6π 8π ϕ (rad)0 λ 2λ 3λ 4λ β1-β2 (µm)

( )[ ])(cos12

0 tVI

I φα+=

rmrm LtLtt −= − = )(2

)()(λπφφφ

mínmáx

mínmáx

II

IIV

+−

=

( )[ ])(cos1 tφ+( )[ ])(cos1 tφ−

DetectorBS

rm LLt

t −=)(

2)(

λπφ

Measurement of displacement Measurement of wavelength


Laser MmL

Mr

Mr

BS1

reference

measurement

L

BS2 D1

Laser interferometers

Laser

reference

BS

D

measurement

D2

BS2

Mm

D1

DM2

L M1

(b)

(a)

(c)


Laser

measurement

l∆=λπφ 4 l∆=

λπφ 2


Optical Fiber Interferometers

Laser

BS

Lens

Reference

DetectorModulator BS


nLλπφ 2=

Laser

Optical Fiber

Measurement

Sensor

Magnitude


Optical fiber intrinsic sensing

∆φFiberφoφ

∆xX

Reference Fiber

Measurement Fiber

X


βλπφ ⋅== knL

2

( )nLLnkk ∂⋅+∂⋅=∂⋅=∂ βφ

εξφ ⋅=∂kn

L

∂∂+

∂∂=

∂∂

T

n

nT

L

Lkn

TL

111 φ

x ∆xExposed sensing length

Strain

Temperature

Optical path

Gage factor (includes strain-optic)

Thermo-optic

coefficient

Thermo-elastic

coefficient


Example: Closed-loop

Intensity (Ι)

Laser

PZT

FO sensor

φ1: Measurement

φ2: Reference

+

_G

I2

I1

∫

Vo: Output

Feedback


[ ])cos(12 210

1 φφ −+= II

[ ])cos(12 210

2 φφ −−=I

I

Optical phase (φ)

φ’ 0(2)

φ’ 0(1)

∆φ’ 0

Ι1(φ) Ι2(φ)

FO sensor


PolarimetryIntensity

Ι1(φ)

Ι2(φ)

Lens

Current Cable

Detector

FO Coil


Rotation of polarization angle (2φ)

φ’ 0Ligth emitter PolarizerLens

Optical Fiber

Detector

Wollamston Prism

[ ])2cos(120 φ−= I

I


Gyroscopes• Optical gyroscope is based on the Sagnac effect and are

implemented both using fiber optics and free-space optics.

CCW

Detector

CCWCW


Ω=∆oc

AL

4 Ω=∆oo c

A42

λπφ

P

cqf o= Ω−=∆

nP

Af

qλ4

Light source

Detector

CW

Ω

Polarizer

Active medium (laser)Mirror Mirror

Ω

CCWCW


Optical fiber Bragg gratings• Wavelength based sensor.

ελλ ⋅=∂

KB

B

Strain

Light source

Detector

Optical Fiber In-Fiber GratingPower Input

Power Back-reflected Power Transmitted

n


Taa TnTlB

B ∂+=∂)(

λλ

Strain

Temperature

Detector

Λn1

n2

n3 > n2

PInput

λ

PBack-R

λλB

PTrans

λλB

δλB


• In this chapter we have focused on the principles of measurement used in

optical and optoelectronic sensors and their optical and electronic basic

signal conditioning. Examples of sensors based on intensity, polarization,

optical phase and wavelength have been presented.

Summary

optical phase and wavelength have been presented.

• A survey of input sensor mechanisms have been addressed through the

case of position, displacement and rangefinders: grating sensors and

encoders, attenuation, time of flight, laser Doppler, triangulation and laser

interferometers.

• Some applications have been described in more detail through

representative examples of fiber-optic sensing: optical fiber

interferometry, polarimetry, gyroscopes and in-fiber Bragg gratings.


OCW 5 Optical and Optoelectronic Sensors and...

Documents

Transcript of OCW 5 Optical and Optoelectronic Sensors and...