EXPERIMENTAL SET UP FOR CHARACTERIZATION OF

ACOUSTO-OPTIC MODULATOR SYSTEM

NORSHAHIDA BINTI ISMAIL

A thesis submitted in fulfilment of the

requirements for the award of the degree of

Master of Science (Physics)

Faculty of Science

Universiti Teknologi Malaysia

JAN 2010

A thesis submitted in fulfilment of the

requirements for the award of the degree of

Master of Science (Physics)

iii

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iv

ACKNOWLEDGEMENT

In the name of Allah, Most Gracious, Most Merciful. Praise is to Allah, the

Cherisher and Sustainer of the worlds. For His Mercy has given me the strength and time to

complete this project.

I would like to express my sincere gratitude and appreciation to my supervisor,

Profesor Dr. Rosly Abd Rahman for his support, supervision and mentoring. Profesor Rosly

is always available to provide support and suggestions and answer questions. Without his

patience and consideration I certainly would not have finished this work.

I would like to acknowledgement the help and kindly assistance of the following

persons; Mr. Ahmad Bin Imbar, Mr. Nasir, Mr. Salehudin, Mr. Abd. Rasid Isnin, Mrs.

Ruzilah and Mr. Sakifli for assisting in carrying out experimental works and colleagues

from Optoelectronics, Laser and Advanced Optical Materials Research Group (AOMRG)

Lab for their continuing corporation, encouragement and useful comment to complete the

work

My thanks are also due to Government of Malaysia through IRPA grant vote 74534

for giving us financial support. Without this financial support, this project would not be

possible.

Thanks also to all my friends and course mates for their views, concerns and

encouragement. Last but not least, my appreciations go to my family for continuing

support, patience throughout the present work and who have favored me with

correspondence, I have much pleasure in expressing my obligation. May Allah bless those

who have involved in this project.

v

ABSTRACT

Acousto-optic effect can be used in many useful devices such as modulators, switches,

filters, frequency shifters and spectrum analyzers. In this study, the modulating effect was generated

by low cost SF6 glass with a lithium niobate transducer. Tunable Helium Neon Laser was used

as the main light source. The function generator was used to generate external input signal and to

vary the amplitude of acoustic wave. The modulated output signal was measured and analyzed

using laser beam profiler, spectrometer, Si photo detector and power meter. The investigation shows

that there was a shift of the horizontal main beam spot position when the driving frequency of the

modulator is changed. A shift of beam spot between 4.0 mm to 5.5 mm was observed for a

frequency range between 70 MHz to 90MHz. This is accordance with the expected theoretical

model of the modulator. Results also show that a modulator can produce output signals, which are

of the same type as the input signal. Increasing the amplitude of modulating signal in the range of

119 mV to 196 mV decreases the amplitude of modulated square wave signal from 2.6 V to 0.4 V.

There was a decrease in the output power of the zero order diffraction but an increase in the

first order diffraction with respect to the increase of the RF driving power.

vi

ABSTRAK

Kesan akusto-optik banyak digunakan dalam pelbagai peranti seperti pemodulasi,

pensuisan, penapisan, penganjak frekuensi, dan penganalisa spektrum. Dalam kajian ini,

kesan modulasi dijanakan oleh bahan kaca SF6 dengan pemindah aruh Lithium Niobate.

Laser Helium Neon boleh laras digunakan sebagai sumber cahaya utama. Penjana denyut

digunakan untuk menjana isyarat masukan luaran dan mengubah amplitud kuasa akustik.

Isyarat keluaran termodulasi diukur dan dianalisis menggunakan penganalisa alur laser,

pengesan spectrum, pengesan-foto dan meter kuasa. Kajian ini menunjukkan bahawa

berlaku anjakan melintang pada titik cahaya apabila pembawa frekuensi pemodulasi

diubah. Anjakan titik sinaran antara 4.0 mm hingga 5.5 mm dapat dilihat untuk jarak

frekuensi antara 70 MHz hingga 90 MHz. Ianya mematuhi jangkaan model teori

pemodulasi. Keputusan juga menunjukkan bahawa pemodulasi boleh menghasilkan isyarat

keluaran yang mana sama dengan bentuk isyarat masukan. Pertambahan amplitud isyarat

modulasi antara 119 mV hingga 196 mV akan mengurangkan amplitud isyarat termodulasi

daripada 2.6 V hingga 0.4 V. Didapati bahawa kuasa keluaran bagi pembelauan tertib sifar

menyusut tetapi ianya meningkat bagi pembelauan tertib pertama bilamana kuasa pemacu RF

bertambah.

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES xi

LIST OF FIGURES xiii

LIST OF ABBREVIATIONS xvi

LIST OF SYMBOLS xvii

LIST OF APPENDICES xx

1 INTRODUCTION

1.1 Introduction 1

1.2 Background of study 2

1.3 Objective of Research 3

1.4 Problem Statement 3

1.5 Scope of Research 4

1.6 Thesis Outline 4

viii

2 THEORY

2.1 Introduction 6

2.2 Acousto-Optic Interaction 7

2.2.1 Isotropic Acousto-Optic Interaction 9

2.2.2 Anisotropic Acousto-Optic Interaction 14

2.3 Acousto-Optic Modulator 15

2.3.1 Deflection 17

2.3.2 Intensity 17

2.3.3 Frequency 18

2.3.4 Phase 18

2.4 Acousto-Optic Material selection 18

3 EXPERIMENTAL WORKS

3.1 Introduction 20

3.2 Instrumentations 20

3.2.1 Equipment used in preliminary study 20

3.2.1.1 Acousto-Optic Modulator M040-8J-FxS 21

3.2.1.2 AOM Driver 22

3.2.1.3 Newport���������� �� 24

3.2.1.4 Fiber Optic Light Source 25

3.2.1.5 Power Meter 25

3.2.1.6 NIR Diode Array Spectrometer 26

3.2.2 Equipments used in Acousto- optic Modulator system 26

3.2.2.1 Tuneable HeNe Laser 27

3.2.2.2 AO Modulator 27

3.2.2.3 AO Modulator Driver 28

3.2.2.4 Laser Beam Profiler (LBP) 29

ix

3.2.2.5 Amplified Silicon Detector 30

3.2.2.6 Fiber Optic Spectrometer 31

3.2.2.7 Polarizer and analyzer 31

3.3 Experimental works 32

3.3.1 Preliminary Experiments on the AOM 32

3.3.1.1 Investigating the effect of driving signal

on AOM output power 32

3.3.1.2 Investigating the spectral output of the AOM 33

3.3.1.3 Investigating the Light Source Sensitivity 34

3.3.1.4 Programming 35

3.3.2 Calibration of instruments 35

3.3.2.1 Calibration of the Tunable He-Ne Laser 35

3.3.2.2 Calibration of Function Generator 36

3.3.3 Experimental works on the AOM constructed 37

3.3.3.1 Set -up for calibration of function generator 38

3.3.3.2 Investigating the internal RF frequency Range 38

3.3.3.3 AOM System 39

3.3.3.4 Geometry Characteristics of AOM 40

3.3.3.5 Characteristics of Modulated Optical Signal 41

3.3.3.6 Temporal characteristics of Acousto-Optic Modulator

(AOM)(External Modulated) 42

3.3.3.7 Determining the Types of Output Signals 42

3.3.3.8 Effects of modulating signal amplitude 43

3.3.3.9 Effects of RF power 43

4 EXPERIMENTAL RESULTS AND ANALYSIS

4.1 Introduction 44

4.2 Preliminary experimental results 44

4.2.1 Observation of Driving Signal 44

4.2.2 Investigating the Characteristics of AOM 46

4.2.2.1 Spectrums 47

4.2.2.2 Graphs 48

x

4.2.3 Light Source Sensitivity 49

4.2.3.1 Spectrums 49

4.2.4 Programming 50

4.3 Experimental result and discussion of an AOM 52

4.3.1 Calibration Instruments 52

4.3.1.1 Tunable He-Ne Laser 53

4.3.1.2 Determination the Polarization of

the Laser Light 56

4.3.2 Determination of Shifting of First Order Beam 56

4.3.3 Effects of input frequency on output frequency 58

4.3.4 Effects of Driving Power on Output Optical Power 59

of First Order Beam

4.3.5 Varied the RF power to determine first order power 64

5 CONCLUSIONS AND SUGGESTIONS

5.1 Conclusions 71

5.2 Suggestions 73

REFERENCES 75

Appendices A The spectrums from the spectrometer 85

Appendices B Three types of output signal at input

frequency 100 Hz to 1.8 kHz 93

Appendices C Least Square Method- Equations 96

xi

LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 Acousto � optic materials selection 19

4.1 Driving signal 45

4.2 The Characteristic of AOM 46

4.3 Light source sensitivity 49

4.4 Data Calibration for Tunable HeNe Laser 55

4.5 Data from experiments and references value 55

4.6 Determination the polarization of the laser light 56

4.7 Effect of driving frequency on horizontal shifting, d 57

4.8 Values of output frequency for square wave, triangle

wave and sine wave signals

61

4.9 Value of input amplitude for 119 mV to 870mV 62

4.10 Value of input amplitude and output amplitude 64

4.11 Ratio of output amplitude to input amplitude 65

4.12 The first order power for minimum RF power to

maximum RF power (position 1 to position 3)

67

4.13 The first order power for minimum RF power to

maximum RF power (position 4 to position 14)

68

xii

4.14 The average power for RF power position, z 70

xiii

LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 The sinusoidal variation of index of refraction 7

2.2 Raman-Nath acousto-optic diffraction geometry 11

2.3 Bragg acousto-optic diffraction geometry 12

2.4 Interaction of photon and phonon 13

2.5 Wave vector diagram for isotropic Bragg diffraction 13

2.6 Wave vector diagram for general case anisotropic

diffraction

15

2.7 Mechanisms in piezoelectric transducer for AOM 16

3.1 AOM M040-8J-F2S 21

3.2 AOM M040-8J-F2S diagramatic 22

3.3(a) AOM Driver and the diagram of the driver 23

3.3(b) Output Level section of AOM 23

3.4 Newport���������� �� 24

3.5 Kingfisher Fiber Optic Light Source KI 7822 25

3.6 Kingfisher Power Meter KI7600 25

3.7 NIR Diode Array Spectrometer 26

3.8 Tunable Laser 27

xiv

3.9 AO Modulator 28

3.10 AO Modulator Driver 29

3.11 Laser Beam Profiler ( LBP ) 30

3.12 Amplified Silicon Detector 30

3.13 Fiber Optic Spectrometer 31

3.14 Experimental setup of an acousto optic modulator

(AOM)

33

3.15 Experimental setup to investigate the sensitivity of

two types of light source; E- LED 1330nm and the

laser light source 1553nm

34

3.16 Observation on the wavelengths of Tunable He-Ne

Laser experimental setup

36

3.17 Observation on the optical power of Tunable He-Ne

Laser

36

3.18 Set up for calibration of function generator 36

3.19 Experimental setup 37

3.20 Determining the polarization of the laser light 38

3.21 Investigating the Internal RF frequency range 39

3.22 Set � up experiment for an acousto-optic modulator 40

3.23 Enlarged view of an Acousto-Optic Modulator 40

3.24 Geometry characteristics of AOM 41

3.25 Characteristic of modulated optical signal

experimental setup

41

3.26 Temporal characteristics of acousto-optic modulator

set up

47

4.7 Screenshot of the Visual Basic Programming 52

xv

4.8 Spectrum of Tunable HeNe Laser 54

4.9 Position of polarizer 56

4.10 Effect of driving frequency on first order shift angle,

z

57

4.11 Three types of output signal at input frequency 100

Hz to 1800Hz

58

4.12 Relation between input signal and output signal 61

4.13 Graph of output amplitude at various input amplitude 66

4.14 Graph of first order power from minimum RF power

to maximum RF power

71

xvi

LIST OF ABBREVIATIONS

AO Acousto-optic

AOM Acousto-optic modulator

CW Continuous wave

DC Direct current

FWHM Full wave half maximum

OSC Oscillator

RF Radio frequency

LBP Laser Beam Profiler

xvii

LIST OF SYMBOLS

c Light velocity

z Distance between zero order beam and first order

beam

� Frequency of acoustic waves

H Height of transducer

K Wave vector of photon

L AO interaction length along the direction of

propagation of light

M Figure of merit

m Diffraction order

n Refractive index of material

Q Quality factor

V Velocity of sound in material

� Planck constant

K� Wave vector of new photon

ka Wave number of acoustic wave (Wave vector of

phonon)

ki Wave number of incident light (Wave vector of

incident photon)

xviii

Kd Wave number of scattered light (Wave vector of

scattered photon)

Pa Acoustic power

� Speed of sound

� d Frequency of Scattered light (Angular frequency of

photon)

� i Frequency of incident light (Angular frequency of

photon

� o Angular frequency of new phonon

B� Bragg angle

�shift Shift angle

Io Incident optical beam density

�m Separation angle between mth diffracted order beam

and undiffracted order beam

i� Incident angle

d� Diffracted angle

0� Angle

tr Rise time

� Density of material

� Diffraction efficiency

� Wavelength of the acoustic waves

� Optical beam wavelength

xix

�a Frequency of the acoustic wave

�t Oscillation time,

�n Amplitude of the refractive index change due to the

acoustic strain

ni Refractive index of incident beam

nd Refractive index of diffracted beam

xx

LIST OF APPENDICES

APPENDIX TITLE PAGE

A The spectrums from the spectrometer 85

B Three types of output signal at input frequency 100 Hz to

1.8 kHz

93

C Least Square Method- Equations 96

CHAPTER I

INTRODUCTION

1.1 Introduction

Applications of laser light often require a means for modulating some properties of

the laser light wave, such as intensity (amplitude), phase wavelength (frequency) or

polarization (direction of propagation) (Schawlow, 1969; Hammer, 1975). A modulator is a

device that alters a detectable property of a light wave corresponding to an applied electric

signal (Hammer, 1975).

There are number of methods that can be used to modulate laser light such as

mechanical, electro-optic, acousto-optic and magneto-optic. Most mechanical methods such

as rotating mirror and mechanical shutter or chopper used for laser beam modulation are

slow, unreliable and have much inertia to allow for faster light modulation (Kaminow and

Turner, 1996; Schawlow, 1969). Thus the mechanical methods are seldom used in modern

modulation equipment. Hence, the interaction between laser wave and electric, magnetic or

acoustic fields acting through the electro-optic, magneto-optic and acousto-optic effect are

used to modulate laser-beam (Kaminow and Turner, 1996; Chen 1970). Modulation of

laser-beam by using these effects is faster and more reliable than the mechanical methods.

2

Optical modulators, using acousto-optic, magneto-optic or electro-optic effects, as

the principal components for external modulation of light wave have presently played the

important role in modern long-haul ultra-high speed optical communications and photonic

signal processing systems. Other common uses of acousto-optic media include devices for

modulating light for communication, detecting light, convolving or correlating signals,

optical matrix processing, analyzing the spectrum of signals, optical sources, laser mode

lockers, Q-switchers, delay lines, image processing, general and adaptive signal processing,

tomography transformations, optical switches, neural networks, optical computing, and

much more.

1.2 Background of Study

Brilliouin predicted light diffraction by an acoustic wave propagating in a medium

of interaction in 1922. In 1932, Debye and Sears, Lucas and Biquard carried out the first

experimentation to check the phenomena. The particular case of diffraction on the first

order, under a certain angle of incidence, (also predicted by Brillouin), has been observed

by Rytow in 1935. Raman and Nath (1937) have design a general ideal model of interaction

taking into account several orders. This model was developed by Phariseau (1956) for

diffraction including one diffraction order. Then, with development of the laser in 1960s,

acousto-optics became an engineering pursuit as devices to control photons became

necessary (Parygin, Balakshy, Voloshinov, 2001). Research and development over the last

decades has produced many types of acousto-optic devices including optical modulators

(Robert J.F., 2003).

One of the earliest uses of an AOM in electro-optics system is for large screen

television images projection in theaters (Goutzoulis, Pape, 1994). Today it is not only being

used in scanning and imaging but also apply in telecommunication (Parygin, Balakshy,

Voloshinov, 2001). An effective and efficient communication system is now used in the

paperless world. The study of acousto-optic modulator design and fabrication is

increasingly important due to its high gain in modulation (Goutzoulis, Pape, 1994).

3

There are three main types of acousto-optic devices, namely, bulk acousto-optic devices,

integrated optic devices and all-fibre acousto-optic devices (Goutzoulis, Pape, 1994). Since

this technology is considered new in our country, the study will start from the most basic

level of the AOM design which is bulk acousto-optic devices. In bulk devices an optical

beam which propagates through an optical medium in the presence of an acoustic wave, can

generate a diffracted beam, producing a frequency shift in the diffracted ray. These devices

are called Bragg cell and have many advantages. The main problem in applying Bragg cells

to optical fibre is that they contribute to insertion loss interface reflection and diffraction

loss in the bulk medium.

1.3 Objective of Research

The objective of this research are:

i. Investigate the principles of an AOM

ii. Identify critical parameters in the design of AOM

iii. Construction of AOM system

iv. Evaluation of the performance of the AOM setup

1.4 Problem Statement

Acousto-optic Modulator is the most important device used to modulate signal in

optical telecommunication technology. This is an initial study in the design and

construction of an acousto-optic modulator. The success of designing and constructing

AOM will bring about new applications for use in research at UTM. Even though this type

of modulator is available in the market, but there is a need to produce or manufacture this

kind of modulator for local use. This research will be a good start for Malaysia to get

involve in AOM manufacturing.

4

1.5 Scope of Research

In this research, a equipments use in the experiments was studied. The

equipments include Tunable HeNe Laser, NEOS Technology AO Modulator ( 24080 ),

AO Modulator Driver, Laser Beam Profiler ( LBP ), PDA 55 Amplified Silicon Detector,

Fiber Optic Spectrometer, Polarizer and analyzer and Power And Energy Meter System.

A preliminaries experiment is carried out using a fibre coupled AOM using

chalcogenide glass with refractive index 2.6. This study focus on investigating the

characteristic of AOM, studying the theory and working principle of AOM and other

equipment in experimental set up, to get the relationship between driving voltage from RF

driver and output power from modulator causes by the changes in output level from radio

frequency (RF) driver, to observe several light source sensitivity.

The AOM was precisely aligned with rotating stage in order to diffract the light at

Bragg angle. The characterization of AOM was carried out in term of laser beam profile,

power and signal configuration.

1.6 Thesis Outline

This thesis composes of six chapters. The first chapter of this thesis presents an

introduction and overview of the previous research works regarding the AOM. The

objective and scope for this research is briefly address and clarify the aim of this research.

Chapter 2 presents the theoretical background related to this research. It explains the

principle of acousto-optic interaction.

5

Chapter 3 explains the equipments and how the methodology of the research is

conducted. In this chapter, the method for the characterization of the modulation output is

outline. This includes the experimental setup and procedures for Bragg angle alignment,

laser beam profiling and the measurement of output power.

The characterization of AOM output is detail out in Chapter 4. The characterization

parameters observed includes the beam profile, power and signal. In laser beam profile

characterization the RF signal is varied and details analysis that covers diffraction angle,

diffraction efficiency and optimum frequency is carried out. The optimum frequency is

important to drive the AOM for the next characterization methods. The laser beam power is

characterized by varying the RF drive power. The modulation signal is characterized based

on pulse width. This is conducted by varying the RF drive power and RF input pulses.

Finally the conclusion of the project is described in Chapter 5. This includes the

summarization of the whole project. Some works to be carried out in the future are

suggested.