DESIGN AND SIMULATION OF SILICA-ON- SILICON PUMP/SIGNAL MULTIPLEXER FOR HYBRID PASSIVE-ACTIVE OPTICAL DEVICES ALVIN LAW WEN PIN

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1 DESIGN AND SIMULATION OF SILICA-ON- SILICON PUMP/SIGNAL MULTIPLEXER FOR HYBRID PASSIVE-ACTIVE OPTICAL DEVICES ALVIN LAW WEN PIN FACULTY OF SCIENCE UNIVERSITY OF MALAYA KUALA LUMPUR 2010

2 DESIGN AND SIMULATION OF SILICA-ON- SILICON PUMP/SIGNAL MULTIPLEXER FOR HYBRID PASSIVE-ACTIVE OPTICAL DEVICES ALVIN LAW WEN PIN DISSERTATION SUBMITTED IN FULFILLMENT OF THE REQUIREMENT FOR THE DRGREE OF MASTER OF SCIENCE FACULTY OF SCIENCE UNIVERSITY OF MALAYA KUALA LUMPUR 2010

3 ABSTRACT This dissertation describes the design and simulation of silica-on-silicon pump/signal multiplexer for hybrid passive-active optical devices. Uniform symmetric directional couplers and planar Bragg gratings were utilized in this work, forming the essential building blocks in a waveguide amplifier. Several designs in relation to the multiplexers are presented in this work, with the simplest application for the directional coupler as a pump/signal multiplexer. A 980/1550 nm and an 800/1310 nm pump/signal multiplexers for erbium doped and bismuth doped waveguide amplifier, respectively were modeled. These couplers were designed and simulated in Rsoft BeamPROP environment, utilizing 3D Finite Difference Beam Propagation Method (FD-BPM) to measure light propagation in the silica waveguide. Coupler parameters including edgeto-edge spacing, and the length of the central coupling region, were optimised in order to maximise the outputs. Both couplers are 11.5 mm and 8.4 mm long respectively with channel spacing of 125 m. Numerical results show that two broadly spaced wavelengths from both couplers were successfully coupled from two opposite inputs into the same output. For the purpose of broadening the amplification band for long haul optical fiber communications, a combined 800/1310 nm and 980/1550 nm pump/signal multiplexers which would be integrated into a broadband amplifier is proposed. These couplers were designed according to the silica based planar lightwave circuit fabrication tolerance and modeled using BeamPROP 3D FD-BPM. The influences of waveguide parameters such as edge-to-edge spacing, length of the central coupling region, and refractive index difference on the coupler characteristics were investigated in detail. The insertion loss for the transmission of 1310 nm and 1550 nm signal wavelengths are 0.23 db and 0.87 db, respectively after applying lateral offset. Furthermore, an optical chip i

4 consists of pump/signal multiplexer and planar Bragg grating is also presented. The chip was separate designed and modeled using BeamPROP 2D FD-BPM and GratingMOD. A 360 nm broad amplification band is obtained from this configuration in order to achieve higher data capacity and bandwidth. ii

5 ABSTRAK Disertasi ini adalah mengenai reka bentuk dan simulasi silika pada silikon pam/isyarat multiplexer untuk hibrid pasif-aktif alat-alat optik. Pengganding yang bersimetrik seragam dan jeriji Bragg adalah tumpuan dalam kerja ini dan bagaimana alat-alat itu boleh digunakan sebagai blok binaan penting dalam waveguide amplifier akan dibincangkan. Beberapa reka bentuk yang berkaitan dengan multiplexers dibentangkan. Satu aplikasi mudah untuk pengganding ini adalah seperti satu pam/isyarat multiplexer. Satu 980/1550 nm dan satu 800/1310 nm pam/isyarat multiplexers untuk erbium doped dan bismut doped waveguide amplifier, masing-masing model. Pengganding ini adalah direka dan dibuat dalam perisian bernama Rsoft BeamPROP, ia menggunakan 3D Finite Difference Beam Propagation Method (FD-BPM) untuk mengukur perambatan cahaya di waveguide silica. Parameter pengganding termasuk jarak antara dua waveguide, dan panjang yang berantau interaksi, telah disimulasikan dengan tujuan mengoptimumkan keluaran itu. Panjang kedua-dua pengganding adalah 11.5 mm dan 8.4 mm, masingmasing dengan jarak saluran yang lebih daripada 125 m. Keputusan berangka menunjukkan yang dua panjang gelombang daripada kedua-dua pengganding telah berjaya digabungkan daripada dua input bertentangan kepada output yang sama. Untuk tujuan memperluaskan amplifikasi band yang panjang untuk menyeret komunikasi serat optik, satu gabungan 800/1310 nm dan 980/1550nm pam/isyarat multiplexers yang akan diintegrasikan ke dalam satu amplifier broadband dicadangkan. pengganding ini direka mengikut silika Planar Lightwave Circuit toleransi pembuatan dan dimodel dengan menggunakan BeamPROP 3D FD-BPM. Pengaruh-pengaruh bagi parameter waveguide seperti jarak antara dua waveguide, panjang yang berantau interaksi, dan perbezaan indeks biasan pada pengganding disiasatan. Kerugian yang sisipan untuk transmisi iii

6 panjang gelombang 1310 nm dan 1550 nm masing-masing adalah 0.23 db dan 0.87 db setelah menerapkan lateral offset. Tambahan pula, satu cip optik mengandungi pam/isyarat multiplexers dan jeriji Bragg dibentangkan. Cip adalah direka bentuk dan dimodelkan menggunakan BeamPROP 2D FD-BPM dan GratingMOD. Satu jalur penguatan yang luas iaitu 360 nm diperolehi dari konfigurasi ini dengan tujuan mencapai lebih tinggi data bandwidth. iv

7 ACKNOWLEDGEMENT I would like to express my deepest gratitude to my supervisor, Professor Dr. Harith Ahmad as well as my co-supervisor, Dr. Faisal Rafiq as well as Assoc. Prof Dr. Sulaiman Wadi for their help, advice and encouragement throughout this research work. Furthermore, I would like thanks several members in Photonics Research Center including Chong Wu Yi and Nizam Tamchek from their suggestions and assistances as well as Kavintheran Thambiratnam for his help in preparing this thesis. v

8 TABLE OF CONTENTS Abstract Abstrak Acknowledgement Contents List of Figures List of Tables i ii v vi x xiv Chapter 1: Introduction Introduction Planar Lightwave Circuits Integrated Photonics Optical Amplifiers Pump/signal multiplexer Device Design and Modeling Objective of the thesis Scope of Thesis Reference 13 Chapter 2: Waveguide Analysis Methods Introduction Planar Waveguides Light Behaviour from the Point of View of Ray Optics Light Behaviour from the Point of View of Electromagnetic Theory The Finite Difference Method (FDM) Finite Difference Beam Propagation Method 33 vi

9 2.4.1 Beam Propagation Method Beam Propagation Method Based On Finite Difference Physics of Coupled Waveguide Device Coupling of Light between Waveguide Coupled Mode Theory Summary Reference 43 Chapter 3: Design and Simulation of 980/1550nm and 800/1310nm Uniform Symmetric Silica-On-Silicon Pump/Signal Multiplexers Introduction Theoretical Investigation of Directional Coupler-Type Pump/Signal Multiplexers Rsoft BeamPROP / 1550 nm Uniform Symmetric Silica-On-Silicon Pump/Signal Multiplexer Choice of Single or Multi-Mode Waveguides Structures for 980 / 1550 nm Uniform Symmetric Silica-on- Silicon Pump / Signal Multiplexer Simulation results of 980 / 1550 nm Uniform Symmetric Silicaon-Silicon Pump / Signal Multiplexer Simulation Analysis of 980 / 155 0nm Uniform Symmetric Silica-on-Silicon Pump / Signal Multiplexer Transmission Analysis Coupling Analysis Supermode Analysis Fabrication Tolerance Analysis 64 vii

10 / 1310 nm Uniform Symmetric Silica-on-Silicon Pump / Signal Multiplexer Choice of Single or Multi-Mode Waveguides Structures for 800 / 1310 nm Uniform Symmetric Silica-on- Silicon Pump / Signal Multiplexer Simulation results of 800 / 1310 nm Uniform Symmetric Silicaon-Silicon Pump / Signal Multiplexer Simulation Analysis of 800 / 1310 nm Uniform Symmetric Silica-on-Silicon Pump / Signal Multiplexer Transmission Analysis Coupling Analysis Supermode Analysis Fabrication Tolerance Analysis Summary Reference 78 Chapter 4: Design and Simulation of Silica-on-Silicon Hybrid Pump / 81 Signal Multiplexer for Applications in Broadband Amplifier 4.1 Introduction Design of Combined 800 / 1310nm and 980 / 1550nm Uniform Symmetric Direction Couplers Structures of Silica-on-Silicon Hybrid Pump/Signal Multiplexer Simulation Results of Silica-on-Silicon Hybrid Pump/Signal Multiplexer 4.5 Simulation Analysis of Silica-on-Silicon Hybrid Pump/Signal Multiplexer Transmission Analysis Coupling Analysis 93 viii

11 4.5.3 Fabrication Tolerance Analysis Summary Reference 97 Chapter 5: Design and Simulation of Silica-on-Silicon Hybrid 98 Multiplexer for Application in Waveguide Broadband Amplifiers 5.1 Introduction Silica-on-Silicon Hybrid Multiplexer GratingMOD Bragg Grating Structures of Silica-on-Silicon Hybrid Multiplexer for Application in Waveguide Broadband Amplifiers 5.6 Simulation Results of Silica-on-Silicon Hybrid Multiplexer for Application in Waveguide Broadband Amplifiers Simulation Results of Bragg Grating Simulation Results of Uniform Symmetric-Typed Directional Coupler Discussion Reference 113 Chapter 6: Conclusion Introduction Conclusion Future works 116 List of Publications 118 ix

12 List of Figures 2.1 Three types of planar waveguide: (a) ridge waveguide; (b) rib waveguide; and (c) buried waveguide. 2.2 (a) Reflection and refraction at a plane interface, (b) propagation of light through optical waveguide by total internal reflection (TIR), (c) refractive index profile of the optical waveguide. 2.3 The basic structure of the slab waveguide where the core layer is sandwiched between upper cladding and lower cladding. 2.4 The cross-section of the waveguide is discretized with a rectangular grid of points which have identical spacing The 3D FD algorithm propagates a plane rather than a line along the z-propagation direction. 2.6 (a) the basic structure of two adjacent waveguide (b) when the lights launch into an input and propagate down a waveguide, small portions of evanescent field propagate into cladding region. 3.1 BPM mode solver showing the single/multi mode propagation within the waveguide with respect to (a) wavelength, (b) core size and (c) index difference. 3.2 A schematic showing the physical layout of the designed 980/1550nm uniform symmetric silica-on-silicon pump/signal multiplexer. 3.3 Cross-section and refractive index profiles of the 980/1550nm uniform symmetric silica-on-silicon pump/signal multiplexer. 3.4 The graph showing variation of length of central coupling region, L to edge-to-edge spacing, d for (a) 980nm and (b) 1550nm x

13 3.5 Graph showing normalized output intensity as a function of the edge-to-edge spacing. (Length of central coupling region is fixed at L=6000 m). 3.6 Graph showing output intensity as a function of the length of central coupling region. (Edge-to-edge separation is fixed at 7.75 m). 3.7 Graph showing output intensity as a function of the length of central coupling region calculated by the BPM. (Edge-to-edge separation is fixed at 2.48 m). 3.8 Graph showing output intensity as a function of the length of central coupling region calculated by the BPM. (Edge-to-edge separation is fixed at 1.39 m). 3.9 BPM simulation showing the transmission of uniform symmetric silica-on-silicon pump/signal multiplexer (L=6200 m, d=7.75 m) for (a) 980nm and (b) 1550nm BPM simulation showing the fundamentals supermode of DCtyped pump/signal multiplexer excited by wavelength 1550nm BPM simulation showing the fundamentals supermode of DCtyped pump/signal multiplexer excited by wavelength 980nm The graph showing output intensity as a function of wavelength from input 1 and input Topological charts showing the resulting power outputs for (a) 800nm and (b) 1310nm wavelength with variation in length of central coupling region, L and edge-to-edge spacing, d Graph showing the optimum length of central coupling region, L and edge-to-edge spacing, d combination for our design, with =0.28%. The inset shows the normalized output intensity of both 800 and 1310nm signals as a function of length of central coupling region for a fixed edge-to-edge spacing of 4.5 m xi

14 3.15 BPM simulation showing the transmission of uniform symmetric silica-on-silicon pump/signal multiplexer (L= m, d=4.5 m) for (a) 800nm and (b) 1310nm BPM simulation showing the fundamentals supermode of pump/signal multiplexer which excited by wavelength 800nm BPM simulation showing the fundamentals supermode of pump/signal multiplexer which excited by wavelength 1310nm The graph showing output intensity as a function of wavelength from input 1 and input A 3D schematic showing the physical layout of the designed broadband amplifier. 4.2 A 2D schematic showing the physical layout of the designed broadband amplifier. 4.3 Topological charts showing the resulting power outputs for (a) 1310nm and (b) 1550nm wavelength with variation in length of central coupling region, L and edge-to-edge spacing, d. 4.4 Graph showing output intensity as a function of the length of central coupling region. (Edge-to-edge separation is fixed at (a) 3.6 m, (b) 3.7 m, (a) 3.8 m and (a) 3.9 m). 4.5 BPM simulation showing the transmission of silica-on-silicon hybrid pump/signal multiplexer for (a) 1310nm and (b) 1550nm. 4.6 BPM simulation showing the transmission of silica-on-silicon hybrid pump/signal multiplexer for (a) 1310nm and (b) 1550nm and their monitor values before and after loss compensation. 4.7 BPM showing the transmission evolution (a) before and (c) after the lateral offset optimization. (b) showing the lateral offset treatment. 4.8 The computed bending loss of fundamental mode for the s-bend with curvature radiuses R (a) 150mm and (b) 25mm xii

15 4.9 Graph showing output intensity as a function of wavelength from input Graph showing output intensity as a function of refractive index difference for 1310nm and 1550nm A schematic showing the physical layout of the silica-on-silicon hybrid multiplexer Two-dimensional sinusoidal bragg grating profile BPM mode solver showing the single/muti mode propagation within the waveguide with respect to (a) wavelength, (b) width and (c) delta. 5.4 (a) The graph showing that normalized output intensity as a function of grating length. (b) Transmission and reflection profile of a bragg grating structure at bragg wavelength 1.26 m. 5.5 The broadband spectral (reflection profile) in the range of (a) m and (b) m. 5.6 BPM simulation showing the transmission of uniform symmetric directional coupler for (a) 1350nm and (b) 1550nm. 5.7 The graph showing output intensity as a function of wavelength from 800/1350nm (dash line) and 980/1550nm (solid line) xiii

16 List of Tables 3.1 Parameters for the 980/1550nm uniform symmetric silica-onsilicon pump/signal multiplexer. 3.2 Parameters for the 800/1310nm uniform symmetric silica-onsilicon pump/signal multiplexer. 4.1 Parameters for the silica-on-silicon hybrid pump/signal multiplexer Parameters of silica-on-silicon hybrid multiplexer. 103 xiv