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
A thesis submitted in fulfilment of the requirements for the award of the degree of
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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.
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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.
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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
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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
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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
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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
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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
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LIST OF TABLES
TABLE NO. TITLE PAGE
2.1 Acoustooptic 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
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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)
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4.13 The first order power for minimum RF power to maximum RF power (position 4 to position 14)
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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
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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
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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)
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3.15 Experimental setup to investigate the sensitivity of two types of light source; E- LED 1330nm and the laser light source 1553nm
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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 Setup 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
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3.26 Temporal characteristics of acousto-optic modulator set up
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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
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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
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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
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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)
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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 betweenmth 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
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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
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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
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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).
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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).
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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
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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.
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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.
