UNIVERSITI PUTRA MALAYSIA DESIGN AND DEVELOPMENT OF REMOTELY PUMPED ERBIUM DOPED FIBER AMPLIFIER TRANSMISSION SYSTEM MOHD SHAHNAN BIN ZAINAL ABIDIN

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1 UNIVERSITI PUTRA MALAYSIA DESIGN AND DEVELOPMENT OF REMOTELY PUMPED ERBIUM DOPED FIBER AMPLIFIER TRANSMISSION SYSTEM MOHD SHAHNAN BIN ZAINAL ABIDIN FK

2 DESIGN AND DEVELOPMENT OF REMOTELY PUMPED ERBIUM DOPED FIBER AMPLIFIER TRANSMISSION SYSTEM By MOHD SHAHNAN BIN ZAINAL ABIDIN Thesis Submitted to the School of Graduate Studies, Universiti Putra Malaysia, in Fulfilment of the Requirements for the Degree of Master of Science December 2004

3 DEDICATION To My Mother; SitiKhadijah Bt. Kila My Father; Zainal Abidin Bin Yahaya Brothers and Sister; Mohd Faizal, Mohd Haris, Mohd Rafie, Siti Nurathirah Relatives, Friends, Teachers and Neighbours

4 Abstract of thesis presented to the Senate of Universiti Putra Malaysia in fulfilment of the requirement for the degree of Master of Science DESIGN AND DEVELOPMENT OF REMOTELY PUMPED ERBIUM DOPED FIBER AMPLIFIER TRANSMISSION SYSTEM By MOHD SHAHNAN BIN ZAINAL ABIDIN December 2004 Chairman: Faculty: Associate Professor Mohd Adzir Mahdi, Ph.D. Engineering Erbium Doped Fiber Amplifier (EDFA) has been deployed extensively in optical communication systems especially for a long haul transmission link. Basically, EDFA requires a local pump laser for its optical amplification. This design will encounter problem if the amplifier is located at the middle of transmission line where power supply for the pump laser is unavailable. Therefore, remotely pumped amplifier can overcome the problem by injecting the pump light from either side of the transmission ends; transmitter or receiver. This dissertation reveals a new technique of designing a repeaterless transmission system using a remotely pumped EDFA. By varying the length of transmission fiber before and after EDFA, its location can be optimized for a specific pump power. A bit error rate is used as the main performance parameter and its threshold value is set at better than The optimized location of EDFA will lead to the maximum transmission distance where it is found that the location of EDFA is closer to the receiver side. ii

5 In conclusion, the EDFA location on transmission line using remotely pumped technique gives major impact to the system s performance. iii

6 Abstrak tesis yang dikemukakan kepada Senat Universiti Putra Malaysia sebagai memenuhi keperluan untuk ijazah Master Sains REKABENTUK DAN PEMBANGUNAN SISTEM TRANSMISI PENGUAT GENTIAN TERDOP ERBIUM DIPAM SECARA JAUH Oleh MOHD SHAHNAN BIN ZAINAL ABIDIN Disember 2004 Pengerusi: Fakulti: Profesor Madya Mohd Adzir Mahdi, Ph.D. Kejuruteraan Penguat Gentian Terdop Erbium (EDFA) telah digunakan secara meluas di dalam sistem komunikasi optikal terutamanya untuk sistem talian transmisi panjang. Secara dasarnya, EDFA memerlukan laser pam setempat untuk penguatan optiknya. Rekabentuk ini akan menghadapi masalah jika penguat itu diletakkan di tengah talian transmisi di mana bekalan kuasa untuk laser pam tidak dapat diperolehi. Oleh itu, penguat dipam secara jauh mampu mengatasi masalah ini dengan menyuntik cahaya pam daripada mana-mana belah hujung transmisi; penghantar atau penerima. Tesis ini mendedahkan suatu teknik baru dalam merekabentuk sistem transmisi tanpa ulangan menggunakan EDFA yang dipam secara jauh. Dengan mengubah panjang gentian transmisi sebelum dan selepas EDFA, lokasinya boleh dioptimumkan bagi kuasa pam yang spesifik. Kadar kesilapan bit diguna sebagai parameter prestasi dan nilai ambang ditetapkan kepada lebih baik daripada Lokasi EDFA yang optimum akan membawa kepada jarak transmisi maksimum yang mana didapati bahawa lokasi EDFA tersebut adalah lebih dekat kepada sebelah penerima. iv

7 Kesimpulannya, lokasi EDFA pada talian transmisi menggunakan teknik pam secara jauh memberi kesan yang besar kepada prestasi sistem. v

8 ACKNOWLEDGEMENTS Alhamdulillah, thanks to ALLAH the almighty. All blessing to Prophet Muhammad, may Allah bless upon him. My acknowledgement firstly goes to supervisor Associate Professor Dr. Mohd Adzir Mahdi, for his continuous and valuable advices since the early stages of the research. His professional thoughts, comments and opinions in have guided me on a right track. I thank also to both members of the supervisory committee, Mr. Mohd Hanif Yaacob and Mrs. Siti Barirah Ahmad Anas. A special gratitude to all my friends in Photonic and Fiber Optics System Laboratory, Engineering Faculty, University Putra Malaysia. Your constant support gives additional strength to complete the thesis. Not to forget as well staffs of Kulliyyah Engineering, International Islamic University Malaysia. Millions of appreciations are bound for Wan Farha Bt. Wan Abdul Fatah for the inspiration from the beginning of the research. Finally to my parent, Siti Khadijah Bt. Kila and Zainal Abidin Bin Yahaya, for the understanding until the end of my study. vi

9 I certify that an Examination Committee met on 30 th December 2004 to conduct the final examination of Mohd Shahnan bin Zainal Abidin on his Master of Science thesis entitled Design and Development of Remotely Pumped Erbium Doped Fiber Amplifier Transmission System in accordance with Universiti Pertanian Malaysia (Higher Degree) Act 1980 and Universiti Pertanian Malaysia (Higher Degree) Regulations The Committee recommends that the candidate be awarded the relevant degree. Members of the Examination Committee are as follows: Khairi Yusof, Ph.D. Lecturer Faculty of Engineering Universiti Putra Malaysia (Chairman) Mohamad Khazani Abdullah, Ph.D. Associate Professor Faculty of Engineering Universiti Putra Malaysia (Member) Syed Javaid Iqbal, Ph.D. Lecturer Faculty of Engineering Universiti Putra Malaysia (Member) Harith Ahmad, Ph.D. Professor Faculty of Science Universiti Malaya (Independent Examiner) GULAM RUSUL RAHMAT ALI, Ph.D. Professor/Deputy Dean School of Graduate Studies Universiti Putra Malaysia Date: vii

10 This thesis submitted to the Senate of Universiti Putra Malaysia and has been accepted as fulfilment of the requirement for the degree of Master of Science. The members of Supervisory Committee are as follows: MOHD ADZIR MAHDI, Ph.D. Associates Professor Faculty of Engineering Universiti Putra Malaysia (Chairman) MOHD HANIF BIN YAACOB Lecturer Faculty of Engineering Universiti Putra Malaysia (Member) SITI BARIRAH BT AHMAD ANAS Lecturer Faculty of Engineering Universiti Putra Malaysia (Member) AINI IDERIS, PhD Professor/Dean School of Graduate Studies Universiti Putra Malaysia Date: viii

11 DECLARATION I hereby declare that the thesis is based on my original work except for the quotation and citations which have been duly acknowledged. I also declare that it is not been previously or concurrently submitted for any other degree at UPM or other institutions. MOHD SHAHNAN BIN ZAINAL ABIDIN Date: ix

12 TABLE OF CONTENTS Page ABSTRACT ABSTRAK ACKNOWLEDGEMENTS APPROVAL DECLARATION LIST OF TABLES LIST OF FIGURES LIST OF ABBREVIATIONS ii iv vi vii ix xii xiii xvi CHAPTERS 1 INTRODUCTION Background Optical Communication System Optical Amplifiers Earth Rare Doped Fiber Amplifier Optical Amplifier Classification Problem Statement and Critical Review Research Objectives Scope of the Work Organization of the Thesis 11 2 THEORY Introduction Optical Amplification Fundamental Energy level of Erbium Ions Three Level Laser System Three Level Atomic Rate Equations System Signal with Amplified Spontaneous Emission Power Rate 22 Equations Pump Power Rate Equations Reduction to Two Level System from Three Level System Repeaterless Transmission System System Development Applications System Technologies System Configuration Summary 40 3 RESEARCH METHODOLOGY Introduction Experimental Setup Components in Remotely Pumped EDFA Isolator Wavelength Selective Coupler nm Pump Laser 45 x

13 3.3.4 Erbium Doped Fiber Variable Optical Attenuator Test Equipment and Measurement Devices Optical Spectrum Analyzer Tunable Laser Source Characterization of Erbium Doped Fiber Amplifier Repeaterless Transmission System Using Remotely Pumped EDFA Fiber Background Loss Bit Error Rate Test Maximum Transmission Distance EDFA Location Pump Power Requirement Parameter Under Study Design Parameter Performance Parameter Summary 60 4 RESULTS AND DISCUSSION Introduction Erbium Doped Fiber Amplifier Characterization Pump Power Effect on Gain and Noise Figure Pump Power Effect on Output Power Operating Range of Pump Power Input Power Effect on Gain and Noise Figure Input Power Effect on Output Power and OSNR Wavelength Effect on System Gain and Noise Figure EDFA Transmission Performance EDFA Position on Transmission Line Optimum Parameter for Specific Transmission Total 82 Distance Effect of System Upgrade to OSNR Performance Gain Improvement over Different Wavelength Summary 92 5 CONCLUSION AND FUTURE WORKS Conclusion Future Work 95 REFERENCES 97 BIODATA OF THE AUTHOR 103 PUBLICATIONS 104 xi

14 LIST OF TABLES Table Page 3.1 Specifications of erbium doped fiber used for the experiments Required test time for BER measurement at different transmission speed 4.1 Fiber length before and after EDFA associated with maximum total transmission distance and percentage of amplifier location from transmitter side 4.2 Performance parameter summary of remotely pumped EDFA transmission system xii

15 LIST OF FIGURES Figure 1.1 Optical fiber amplifier classification; (a) Post amplifier (b) Inline amplifier (c) Preamplifier Page Scope of research work Energy level diagram Atom with respective energy level (a) with external energy and (b) releases photon Energy levels of erbium ions with possible pump bands Energy level system corresponding to three-level system Cylindrical fiber cross section Energy level of two-level system Signal evolution of distributed amplifier for different pumping schemes 2.8 Signal power evolution experienced by remotely pumped EDFA systems 2.9 Remotely pumped EDFA incorporating circulators to achieve double pass amplification 2.10 Additional pumping line utilized providing more pump power to remotely pumped EDFA Deployment of filter within pumping line Various configuration of repeaterless transmission systems Signal power evolution over transmission distance for different repeaterless systems Experimental setup of EDFA Setup of EDFA experiment for repeaterless transmission system using remotely pumped EDFA Basic operation of isolator Inputs and output associates with wavelength selective 45 coupler xiii

16 3.5 Structure of pump laser module Experimental setup associating transmitter, receiver and BERT 3.7 Output power response for a current applied to 1480 nm pump laser 3.8 Screenshot from OSA for measurement technique for gain, ASE level, noise figure and output power Fiber loss associates with respective wavelength of signal Gain and noise figure characteristics using single-pass EDFA at different signal powers Output power against pump power for various input power Gain Conversion Efficiency over consumed pump power for a range of input signal power from - 40 to - 4 dbm 4.4(a) Gain and noise figure against various pump power for -40 dbm to 0 dbm input power (b) 4.5(a) 4.5(b) ASE level over signal wavelengths at input power of -30 dbm at 40 mw pump power Output power and OSNR against input power for 30, 40 and 50 mw pump power ASE level over signal wavelengths at input power of 0 dbm at 40 mw pump power Gain and noise figure variation over wavelength. Input power is set to -20 dbm for 30, 40 and 50 mw 4.7 EDFA position configuration on remotely-pumped transmission experiment 4.8 Maximum transmission length at BER better than with respect of EDFA location on transmission line for pump powers of 30, 40 and 50 mw. Launch power from transmitter side is set to be 0 dbm 4.9 Eye diagram of the transmission for pump powers of (a) 30 mw, (b) 40 mw and (c) 50 mw Gain and noise figure performance over EDFA location 78 xiv

17 4.11 OSNR and received power performance over EDFA location on transmission line 4.12 Consumed pump power associated with EDFA location, the local pump power of 40 mw is used Bit Error Rate for total transmission distance of 180 km BER for 196 km of total transmission distance for 40 mw pump power injected at input EDFA 4.15 BER for 208 km transmission distance for 40 mw pump power at EDFA input 4.16 BER for km transmission distance for 40 mw pump power at EDFA input 4.17 OSNR and received power over EDFA location at 208 km total transmission system for 40 mw pump power at EDFA input OSNR requirement for system upgrade from 2.5 to 10 Gbps Gain and noise figure difference in respect to reference point at nm 92 xv

18 LIST OF ABBREVIATIONS ASE - Amplified Spontaneous Emission BER - Bit Error Rate BERT - Bit Error Rate Tester DCF - Dispersion Compensating Fiber DUT - Device Under Test EDF - Erbium Doped Fiber EDFA - Erbium Doped Fiber Amplifier FEC - Forward Error Correction G - Gain GCE - Gain Conversion Efficiency LD - Laser Diode NA - Numerical Aperture OSA - Optical Spectrum Analyzer OSNR - Optical Signal to Noise Ratio PRBS - Pseudo-Random Bit Sequence PCE - Power Conversion Efficiency PPM - Part Per Million SOA - Semiconductor Optical Amplifier TLS - Tunable Laser Source VOA - Variable Optical Attenuator WDM - Wavelength Division Multiplexing WSC - Wavelength Selective Coupler xvi

19 CHAPTER 1 INTRODUCTION Telecommunication industries evolve rapidly since past years influenced by hungry demanding competition among manufacturers. Various telecommunication technologies have been created and innovated in order to achieve manufactures need for a reliable telecommunication quality more than just voice services. Higher bandwidths are required to satisfy a real time video on demand, live telecast and interactive applications. An optical communication is seen as one of the enabling technologies and being one of the key factors for the present and future applications. This chapter will explain briefly about optical communication systems and its development up to date. Later, optical amplifiers will be highlighted by focusing to rare-earth doped fiber amplifier. Statement of related problem and scope of the research to be conducted are discussed in details as well as its objectives to achieve. 1.1 Background Optical fiber communication systems can fulfill the bandwidth need for a practical long haul transmission distance. It is reliable in handling and transmitting data over hundreds of kilometers with an acceptable bit error rate. Worldwide researchers continuously update the technology to improve the system s performance. Typically, a link of optical fiber communication can be hundreds of kilometers and some could be extended up to several thousands of kilometers with an additional 1

20 amplifiers and repeaters [1]. Data speed of each optical fiber transmission channel could reach up to 40 Gb/s [2] and it is limited by polarization-mode and chromatic dispersions, attenuation, and nonlinearity of the fiber [3]. Some schemes included into the system to improve the transmission distance and bit rate further such as Forward Error Correction (FEC), new remote pumping schemes and utilization of dispersion compensating fibers [4]. Optical fiber communication growths impressively with over 600 millions kilometers of fiber optics have been installed worldwide [5] with USD 1.2 billion amounted only for equipment of optical transport for the year of 2000 only [6]. 1.2 Optical Communication System Basically an optical communication system consists of a link of fiber optic as a transmission medium between a transmitter and a receiver. Information is converted from electrical to optical domain, modulated and multiplexed before injecting into the fiber optic. Optical fiber carries the information by guiding the laser beam in its core utilizing a total internal reflection requirement. At the receiver end, the signal is converted back into an electrical domain by a photodetector, amplified and demodulated to produce the original signal. Between two transmission ends, an amplifier could be added to extend the link. Previously, electrical amplifiers were utilized by converting optical signal into electrical domain before it was amplified. Electrical amplifiers require external power supply to operate and this would become a big problem if a submarine transmission link is going to be deployed. The optical amplifier overcomes this 2

21 barrier since the system uses light to amplify the information signals all optically and light can be send from a distance. 1.3 Optical Amplifiers Traditionally, an electrical amplifier converts optical signal into electrical domain first before converts back again into optical signal after amplifying it. The state of signal changes several times as there are several stages of electrical amplifiers. This would add more delay in transmission and would affect the overall system bit error rates. Every transmission medium introduces some signal losses of its signal power. The signal is attenuated along the medium as it travels from transmitter to receiver end. In an optical fiber communication, the attenuation causes the launched signal power level to decrease mainly due to signal absorption by the fiber itself. Optical amplifiers are designed in such a way that the weakened signal is boosted to a specific power level for the next transmission sequent. As its name implies, optical amplifiers operate in optical domain and maintain signal s state along fiber within the transmission distance. They eliminate a need for signal interconversion of photons to electrons. Moreover, they offer a simple setup of a single in line components arrangement which is practical for any kind of modulations and transparent to any transmission speeds [7]. Multiple optical wavelengths can be 3

22 easily amplified over a certain bandwidth that is limited by the operating range of optical amplifiers. There are two main techniques to achieve optical amplification. Semiconductor Optical Amplifier (SOA) uses a fundamental of stimulated emission to amplify an optical information signal. The information signal is amplified in a semiconductor s active region where the injection current is applied to deliver the external energy to pump electron at the conduction band. The signal stimulates the electron transition and emits photons with the same energy and wavelength as the input s [8]. SOA can be used in both nonlinear and linear modes of operation [9, 10]. But on the other hand, the SOA is unsuitable to be utilized in a repeaterless transmission system due to its operation requirement for an inline electrical source. The second type of optical amplification is the use of rare-earth doping material inside the fiber. Essentially it is a spliced active fiber connected to a pump laser within a transmission line [11]. It works on the principle of stimulated emission; the pump laser is used to provide energy and to excite ions in a special doped fiber to an upper energy level. Then, the ions are stimulated by photons of the information signal and fall down to lower level of energy; subsequently, emits photon energy exactly at the same wavelength of the signal. 4

23 1.3.1 Rare-Earth Doped Fiber Amplifier Currently research works are concentrating more on the erbium dopant, particularly in silica based fibers. This is due to the emission of Er 3+ ion lying within a set of wavelength around 1550 nm where the silica fiber exhibits the minimum attenuation of the information signal. Erbium doped fiber amplifiers (EDFA) could provide gains as high as 40 db associated with low noise, as successfully demonstrated within a pump power range of 50 to 100 mw [12-14]. The first rare earth doped material of Neodymium (Nd 3+ ) used into a single mode fiber was demonstrated in 1983 by Bell Telephone Laboratories [15, 16]. Since then, rare-earth doped fibers were fabricated in a variety of methods to suit the different designs of amplifiers. The amount of dopant inside the fiber core ranging from 100 to 2000 parts per million (ppm) up to as high as 5000 ppm [17]. Various methods used to provide a low loss optical fiber communication with an introduction of new composition to improve the performance of amplifier, as well as uniformity of doping along the longitudinal and transverse fiber axes [18-22] Optical Amplifier Classification Optical fiber amplifiers generally can be classified and categorized based on its position in transmission lines which in turn shows their respective applications. There are three basic types of amplifier; post, inline and preamplifiers. 5

24 Post-amplifier Known as a booster and power amplifier, a post amplifier is located just after a transmitter (Tx) before the launched signal goes down to fiber as depicted by Figure 1.1(a). As its location is near to the transmitter, the post amplifier handles relatively high power signals compared to other types of optical amplifiers. Mainly the post amplifier s function is to boost the signal power to the highest level hence maximizes the transmission distance. It differs than a normal amplifier as it relieves a need for a transmitter to produce a maximum optical power Inline Amplifier An inline amplifier as shown by Figure 1.1(b) is usually located in the middle of a transmission span. It functions to compensate the power losses along the fiber link and could be cascaded to extend the transmission length. As it amplifies the signal several times in the case of cascaded system, the noise level is to be kept at a minimal level. For a WDM system, its stability to produce a uniform gain among wavelengths becomes its main goal. 6

25 Tx Rx (a) Tx Rx (b) Tx Rx (c) Figure 1.1: Optical fiber amplifier classification; (a) Post amplifier (b) Inline amplifier (c) Preamplifier Preamplifier Preamplifier amplifies the incoming signal before entering a receiver (Rx) at the end of the transmission as illustrated by Figure 1.1(c). Located just before the receiver, a preamplifier increases a weak signal to an acceptable level for detection. Hence the preamplifier is designed to have a good sensitivity, high gain and low noise level. Receiver s performance is not only limited by its own noise but also contributed by the noise from preamplifier [23]. 7