POLYMER OPTICAL FIBER BRAGG GRATINGS By Huiyong Liu A thesis submitted in fulfilment of the requirement for the degree of Doctor of Philosophy School of Electrical Engineering and Telecommunications The University of New South Wales February 2003 U N S W 1 7 JUL 2003 LIHRARY Acknowledgment Acknowledgment First, I would like to thank my supervisors, A/Professor Gangding Peng and Professor Pak Lim Chu, for their close supervision over the years. They have given me invaluable guidance in the production of this work. I am particular graceful for A/Professor Gangding Peng in the formulation of the topic, in the development of the thesis theme, in the implementation of the detailed experimental work, and in writing up of this thesis. He has been helpful, encouraging and patient and I am indebt to him for his help. Second, I would like to give my special thanks to the colleagues and friends in Photonics and Optical Communication Group at the School of Electrical Engineering and Telecommunications for their support and encouragement. I like to thank Mr Trevor Whitbread for his valuable assistance and discussions on laboratory experiments. I also like to thank Dr Skinner for spending hours in discussing about my thesis writing and giving me helpful feedbacks. I must also give my sincere gratitude to Mr Hongbo Liu, Mr Xiaogang Li, Mr Philip Ji, Ms Margaret Lu, Ms Emily Lee, Dr Bin Wu, Mr Ruofei Peng and Dr Liguo Luo for their help discussions and support. Their friendships will be everlasting precious gifts to the author. I also want to thank Mr Yannic Aubree for his hard work in proof-reading my draft. Also thank Ms Stella Chau and Ms Vicky Hui for their helps. Last, I would like to thank my parents, Shigui Liu and Hongbin Su, my sister, Huiyun Liu, and my brother, Huiming Liu, for their support and encouragement throughout these years of my study to bring this thesis to a conclusion. Abstract Abstract This thesis presents a first systematic study on the fabrication, formation mechanism and characterization of polymer optical fiber (POF) Bragg gratings. Firstly, the investigation on the growth dynamics of PMMA-based POF Bragg gratings has been carried out. In this investigation, we found that there exist two distinctive stages in POF Bragg gratings formation. We also revealed that the growth behaviour of POF Bragg gratings bears remarkable resemblance to that of silica fiber Bragg gratings. Based on these findings, we named the two different types, i.e. Type I and Type II, of POF Bragg gratings, following the nomenclatures of silica fiber Bragg gratings. Based on the insights gained in the growth dynamics investigation, we produced POF Bragg gratings with the best result ever reported: the reflectivity of 0.999 and the line width of less than 0.5 nm, by optimizing the UV exposure fluence. We also tested the thermal and strain characteristics of Type I and Type II PMMA based POF Bragg gratings. The test showed that POF Bragg gratings have excellent thermal tunability: large tuning range, absence of thermal hysteresis and large temperature sensitivity (10 times larger than that of silica fiber Bragg gratings). The test also demonstrated that, by simply applying tensile stress, large tuning range of 32 nm in Bragg wavelength shift and large strain sensitivity (higher than that of silica fiber Bragg gratings) have been achieved. Apart from the work on PMMA-based POF, photosensitivity in the novel and low loss perfluropolymer (CYTOP) fiber material has also been examined. Significant photosensitivity has been observed and gratings have been successfully written in the Abstract CYTOP fiber material for the first time. This is an exciting start showing the possibility of writing fiber Bragg gratings in the low loss CYTOP fiber. In addition, we proposed a new scheme of the hybrid silica and polymer fiber Bragg grating sensor for simultaneous strain and temperature measurement. Detailed analysis predicted that the proposed scheme will achieve high accuracy and large perturbation tolerance. Dynamic dispersion compensation applications of POF Bragg gratings have been studied as well. Analysis and numerical simulations have been performed and the results demonstrated that the linearly chirped POF Bragg gratings fabricated by the tapered POF method can be used to achieve a dynamic dispersion compensation range from 1534 ps/nm to 66 ps/nm. Publications Publications Journal paper: 1. H.Y. Liu, G.D.Peng, P.L. Chu, "Photosensitivity in low-loss perfluoropolymer (CYTOP) fiber material", Electronics Letters, vol. 37, No. 6(2001), p. 347. 2. H.Y. Liu, G.D.Peng, P.L. Chu, "Thermal tuning of polymer optical Bragg gratings", IEEE Photonics Technology Letters, vol. 13, No. 8 (2001), p. 824. 3. H.Y. Liu, G.D.Peng, P.L. Chu, “Thermal characteristics of Bragg gratings in CYTOP fiber material,” Optics Communications, vol. 24, No. 4 (2002), p.151. 4. H. Y. Liu, G. D. Peng and P. L. Chu, “Polymer Fiber Bragg Gratings with 28dB Transmission Rejection”, IEEE Photonics Technology Letters, vol. 14, No. 7 (2002), p. 935. 5. H. Y. Liu, G. D. Peng and P. L. Chu, “Observation of Type I and Type II Bragg Grating behaviour Polymer Optical Fibre”, Optics Communications, vol. 220, No. 4-6 (2003), p. 337. 6. H. B. Liu, H. Y. Liu, and G. D. Peng, “Different Types of Polymer Fiber Bragg Gratings (FBGs) and Their Strain/Thermal Properties”, to be appeared in Optical Memory and Neural Networks special issue on "Holographic Memory and Applications”. 7. H. B. Liu, H. Y. Liu, and G. D. Peng, “Strain and Temperature Sensor using a combination of Polymer and Silica Fibre Bragg Gratings”, Optics Communications, vol. 219 (2003), p. 139. Conference paper: 1. H.Y. Liu, G.D.Peng, G.J. Destura, B.Wu and P.L. Chu, "Highly thermal tunable polymer optical fiber gratings", 9th International POF conference 2000, July, 2000, Boston, USA. Publications 2. H. Y. Liu, G.D. Peng, and P.L. Chu, "Photosensitivity and Bragg gratings in novel low-loss fluoropolymer (CYTOP) for polymer optical fiber", The 25th Australian Conference on Optical Fibre Technology (ACOFT'2000), Canberra, 16-18, June 2000. 3. H.Y. Liu, G.D.Peng, P.L. Chu, " Thermal Stability of Gratings in PMMA and CYTOP Fibres ", 10th International POF conference 2001, July, 2001, Amsterdam, Netherland. 4. H.Y. Liu, G.D.Peng, P.L. Chu, “High reflective polymer fiber gratings and the growth dynamics,” OFC 2002, March, 2002, Anaheim, USA. 5. H. Y. Liu, G. D. Peng and P. L. Chu, “Research on the Mechanism of Polymer Fiber Bragg Grating Formation”, 27th Australian Conference on Optical Fiber Technology, Sydney, Australia, July, 2002. 6. G. D. Peng, H. Y. Liu, and P. L. Chu, "Dynamics and Threshold Behaviour in Polymer Fibre Bragg Grating Creation", invited paper, presented at the the 47th SPIE conference on Photorefractive Fibre and Crystal Devices, Materials, Optical Properties and Applications VII, Seattle, WA, USA, July 2002. 7. H. Y. Liu, G. D. Peng, and P. L. Chu, “Thermal characterization of Type I and Type II Optical Fiber Bragg gratings”, Second Asia-Pacific Polymer Fiber Optics Workshop, January, 2003. 8. H. B. Liu, H. Y. Liu, R. Wang and G. D. Peng, “Strain and temperature response of polymer fiber Bragg gratings”, First international conference on Optical Communications and Networks (ICOCN 2002), Shangri-La Hotel, Singapore, November 11-14, 2002. Contents Contents Acknowledgment i Abstract ii Publications iv Contents vi Chapter 1: Introduction 1 1.1 Novelties of the thesis 1 1.2 Introduction to fiber Bragg gratings 2 1.3 Status of the research on POF Bragg gratings 10 1.4 Thesis outline 12 1.5 References 13 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings 16 2.1 Introduction 16 2.2 Basic relations of optical fiber gratings 20 2.3 POF fabrication 25 2.4 POF Bragg gratings fabrication 27 2.5 Growth dynamics of POF Bragg gratings 30 Contents 2.6 Type I POF Bragg gratings 38 2.7 Type II POF Bragg gratings 39 2.8 Conclusions 42 2.9 References 43 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings 48 3.1 Introduction 48 3.2 Set-up for thermal characterization measurement 52 3.3 Thermal tuning of Type I POF Bragg gratings 53 3.4 Thermal sensitivity of Type I POF Bragg gratings 57 3.5 Thermal tuning and thermal sensitivity of Type II POF Bragg gratings 58 3.6 Thermal decay of Type I & II POF Bragg gratings 61 3.7 Stabilization of Type I POF Bragg gratings 67 3.8 Discussion on the mechanism of Type I POF Bragg gratings decay 68 3.9 Conclusions 69 3.10 References 70 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg Gratings 73 4.1 Introduction 73 4.2 Experimental method for strain tuning characterization 76 4.3 Strain sensitivity of Type I POF Bragg gratings 76 vii Contents 4.4 Strain tunability of Type I POF Bragg gratings 78 4.5 Strain characterization of Type II POF Bragg gratings 83 4.5 Simultaneous strain and temperature measurement by using POF Bragg gratings 84 4.7 Conclusions 89 4.8 References 90 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg gratings 92 5.1 Introduction 92 5.2 Scheme for linearly chirped POF Bragg gratings fabrication 98 5.3 Dynamic dispersion compensation by linearly chirped POF Bragg gratings 103 5.4 Conclusions 115 5.5 References 116 Chapter 6: Photosensitivity Study of Low-loss Perfluoropolymer (CYTOP) Fiber Material 121 6.1 Introductions 121 6.2 CYTOP material and CYTOP optical fiber 121 6.3 Experimental methods 124 6.4 Photosensitivity of CYTOP fiber material 127 6.5 Thermal test of CYTOP gratings 130 viii Contents 6.6 Conclusions 134 6.7 References 134 Chapter 7: Conclusions and Further Work 137 7.1 Conclusions 137 7.2 Suggestions on further work 139 IX Chapter 1: Introduction Chapter 1: Introduction 1.1 Original contributions of the thesis Since photosensitivity in silica fiber was first revealed by Hill et al in 1978 [1], silica fiber Bragg grating has been developed for many important applications in optical communication systems.