GRAPHENE AS SATURABLE ABSORBER FOR PHOTONICS APPLICATIONS FARAH DIANA BINTI MUHAMMAD FACULTY OF SCIENCE UNIVERSITY OF MALAYA KUALA LUMPUR 2014 GRAPHENE AS SATURABLE ABSORBER FOR PHOTONICS APPLICATIONS FARAH DIANA BINTI MUHAMMAD THESIS SUBMITTED IN FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS FACULTY OF SCIENCE UNIVERSITY OF MALAYA KUALA LUMPUR 2014 UNIVERSITI MALAYA ORIGINAL LITERARY WORK DECLARATION Name of Candidate: FARAH DIANA BINTI MUHAMMAD I/C/Passport No: 900408-03-5244 Regisration/Matric No.: SHC110042 Name of Degree: DOCTOR OF PHILOSOPHY Title of Project Paper/Research Report/Dissertation/Thesis (“this Work”): “GRAPHENE AS SATURABLE ABSORBER FOR PHOTONICS APPLICATIONS” Field of Study: EXPERIMENTAL PHYSICS I do solemnly and sincerely declare that: (1) I am the sole author/writer of this Work, (2) This Work is original, (3) Any use of any work in which copyright exists was done by way of fair dealing and for permitted purposes and any excerpt or extract from, or reference to or reproduction of any copyright work has been disclosed expressly and sufficiently and the title of the Work and its authorship have been acknowledged in this Work, (4) I do not have any actual knowledge nor do I ought reasonably to know that the making of this work constitutes an infringement of any copyright work, (5) I hereby assign all and every rights in the copyright to this Work to the University of Malaya (“UM”), who henceforth shall be owner of the copyright in this Work and that any reproduction or use in any form or by any means whatsoever is prohibited without the written consent of UM having been first had and obtained, (6) I am fully aware that if in the course of making this Work I have infringed any copyright whether intentionally or otherwise, I may be subject to legal action or any other action as may be determined by UM. (Candidate Signature) Date: Subscribed and solemnly declared before, Witness’s Signature Date: Name PROFESSOR DR HARITH AHMAD Designation Witness’s Signature Date: Name DR MOHD ZAMANI ZULKIFLI Designation ABSTRACT Graphene, a single layer of carbon atoms, is nowadays considered a great candidate to be applied as the saturable absorber (SA) with its desirable optical characteristics such as ultrafast recovery time and ultrawideband absorption due to its zero bandgap energy and linear dispersion of Dirac electrons. With its advantage over semiconductor saturable absorber mirror (SESAM) in terms of the cost, tuning range and ease of fabrication, graphene has been widely accepted to replace the usage of SESAM. In this work, several methods of graphene integration onto the fiber ferrule have been demonstrated, such as by optical deposition method, by sandwiching graphene thin film between the fiber ferrules and by adhering graphene flakes onto the fiber ferrule using index matching gel. The saturable absorption properties of the deposited graphene by each different method is also measured and analysed. Taking advantage of the unique properties of graphene, the graphene deposited in this work has been demonstrated for Q-switching operation in various setup configurations, from basic setup of a simple ring cavity of Erbium doped fiber laser (EDFL) to a more advanced configuration which enables wavelength tunability by employing different wavelength selective elements including the tunable bandpass filter (TBF), arrayed waveguide gratings (AWG) and fiber Bragg gratings (FBG). Comparison on the Q-switching output performance using different wavelength selective elements is analysed. In addition, graphene Q-switched EDFL based on distributed Bragg reflector (DBR) cavity configuration and multiwavelength graphene Q-switched Brillouin-erbium fiber laser have been demonstrated as well, with each ii approach having their own advantages. Furthermore, a Q-switched EDFL based on graphene oxide as the saturable absorber with a simple deposition method has also been introduced. Apart from Q-switching, mode-locking generation based on graphene saturable absorber is also being demonstrated and investigated using a simple ring cavity EDFL. To provide the wavelength tunability of the mode locked EDFL, a TBF is inserted into the cavity as the tuning mechanism. Further investigation on the graphene based mode locked fiber laser is carried out by using an exotic and highly doped Zirconia-erbium doped fiber (Zr-EDF) as the gain medium. It is interesting to observe that beyond a certain pump power, harmonic mode locking takes place. In addition, by incorporating a Mach Zehnder filter into the cavity, the spectrum tunability of the mode locked Zr- EDFL is achieved and has been well demonstrated. Further development of this graphene based Zr-EDFL is carried out by demonstrating it as a pulse source for supercontinuum (SC) generation with an advantage of low cost, since only a short length of single mode fiber (SMF) is used as the nonlinear medium. Moreover, in this work, graphene has been also demonstrated as a saturable absorber for suppressing the noise and multimode oscillations in the laser cavity, and acts as the key enabler to produce the single longitudinal mode (SLM) operation in the EDFL. By heterodyning this SLM laser output and an external tunable laser source (TLS) at a photodetector, a tunable radio frequency generation can be realized. iii ABSTRAK Graphene, satu lapisan atom karbon, dianggap pada masa kini sebagai bahan kajian yang hebat untuk diaplikasikan sebagai penyerap boleh tepu (saturable absorber) yang mempunyai karateristik optik yang diinginkan seperti masa pemulihan yang sangat pantas dan penyerapan dalam lingkungan jalur lebar yang luas disebabkan jurang tenaga dalam graphene yang bernilai sifar serta serakan lelurus elektron Dirac. Dengan kelebihan yang dimiliki oleh graphene berbanding cermin penyerap boleh tepu semikonduktor (SESAM) dari segi kos, julat penalaan dan kemudahan fabrikasi, graphene telah diterima pakai secara meluas untuk menggantikan penggunaan SESAM. Dalam projek kajian ini, beberapa kaedah untuk mengintegrasikan graphene pada hujung gentian optik telah didemonstrasikan, seperti kaedah deposit secara optikal, kaedah mengapitkan filem nipis graphene antara kedua-dua hujung gentian optik, serta kaedah melekatkan kepingan graphene pada hujung gentian optik menggunakan gel bersesuaian indeks (index matching gel). Sifat-sifat penyerapan boleh tepu bagi graphene yang telah didepositkan menggunakan keadah-kaedah yang berbeza tersebut juga diukur dan dianalisis. Graphene yang didedopositkan dalam projek kajian ini juga telah didemonstrasikan untuk menghasilkan suis-Q (Q-switching) dalam pelbagai bentuk konfigurasi eksperimen, bermula daripada konfigurasi asas bagi laser fiber berdop- erbium (EDFL) dalam kaviti membulat kepada konfigurasi yang lebih berteknologi yang membolehkan penalaan panjang gelombang menggunakan pelbagai jenis alat pemilih gelombang yang terdiri daripada penapis boleh laras (TBF), parutan gelombang iv teratur (AWG) dan gentian optik berparut Bragg (FBG). Perbandingan antara prestasi keluaran suis-Q (Q-switching) menggunakan pelbagai jenis alat pemilih gelombang tersebut juga telah dicerakinkan. Sebagai tambahan, graphene Q-suis EDFL berdasarkan konfigurasi kaviti pantulan Bragg teragih (DBR) serta graphene Q-suis dalam multi gelombang berdasarkan laser Brillouin-erbium juga didemonstrasikan, dengan setiap pendekatan tersebut mempunyai kelebihahan masing-masing yang tersendiri. Selain Q-suis, penjanaan mod terkunci (mode locking) berdasarkan graphene sebagai penyerap boleh tepu juga turut didemonstrasikan dan dikaji menggunakan kaviti membulat EDFL. Untuk menghasilkan penalaan gelombang dalam EDFL bermod terkunci tersebut, satu TBF dimasukkan ke dalam kaviti tersebut sebagai alat penala gelombang. Kajian lanjut tentang laser gentian optik bermod terkunci menggunakan graphene diteruskan dengan menggunakan fiber eksotik Zirconia berdop-erbium (Zr- EDF) yang mengandungi kandungan dopan yang tinggi sebagai media gandaan. Selain itu, dengan memasukkan penapis Mach Zehnder ke dalam kavity tersebut, penalaan spektrum dapat dicapai dan didemonstrasi dengan baik. Graphene Zr-EDFL bermod terkunci ini juga dikembangkan lagi sebagai sumber denyutan (pulse) untuk penjanaan ‘supercontinuum’ (SC) yang memupunyai kelebihan dari segi kos. Dalam projek kajian ini juga, graphene juga telah didemonstrasikan sebagai penyerap boleh tepu untuk menyekat ayunan multimod dalam kaviti laser bagi membolehkan hanya satu mode membujur (SLM) untuk beroperasi dalam EDFL. Dengan menggabungkan keluaran laser SLM tersebut dengan satu sumber laser luaran (TLS), penjanaan frekuensi radio dapat direalisasikan. v ACKNOWLEDGEMENTS Praise be to The Almighty, this thesis would not have materialized without the help and guidance from my supervisor Prof. Harith Ahmad and my co supervisor, Dr. Mohd. Zamani Zulkifli. No words can describe my deepest appreciation and gratitude to Prof. Harith who has not only been patient and tolerant to my quest in seeking knowledge, but also generous in sharing his expertise, immense knowledge and experience in this field of photonics. My personal full-hearted appreciation also goes to my family, especially my parents, Muhammad Yahya and Habsah Yusuf, as well as my siblings, especially my sisters Hamizah Muhammad and Mazhani Muhammad for supporting, encouraging and fully understanding me, in order to make sure that I could complete my Ph.D. degree till the end. Thank you for your love and for being there for me. My sincere and heartfelt gratitude goes to Mohd. Faizal Ismail for his selflessness and generous help in the lab, thank you for your kind assistance and strong encouragement to complete this thesis. Not forgetting the wonderful support from my relatives, Husin’s family and Datin Rohimah’s family. Also to those who have directly or indirectly contributed to the completion of this thesis, my gratitude goes to you. My appreciation also goes to the members of the Photonic Research Center especially Dr. Richard, Dr. Chong Wu Yi, Dr. Amirah Abdul Latif, Mohd. Hafizin Jemangin, Fauzan Ahmad, Prof. Sulaiman Wadi Harun and Encik Khairi. You all have made the lab most enjoyable to work in despite the many challenges we have to put up with. Thank you for your continuous support and friendship. vi LIST OF ISI PUBLICATIONS 1. Muhammad, F. D., Zulkifli, M. Z., Latif, A. A., Harun, S. W., & Ahmad, H. (2012). Graphene-based saturable absorber for single-longitudinal-mode operation of highly doped erbium-doped fiber laser. IEEE Photonics Journal, 4(2), 467-475. 2. Ahmad, H., Muhammad, F. D., Zulkifli, M. Z., Latif, A. A., & Harun, S. W. (2012). Tunable radio frequency generation using a graphene-based single longitudinal mode fiber laser. Journal of Lightwave Technology, 30(13), 2097-2102. 3. Ahmad, H., Muhammad, F. D., Zulkifli, M. Z., & Harun, S. W. (2012). Graphene- oxide-based saturable absorber for all-fiber Q-switching with a simple optical deposition technique. IEEE Photonics Journal, 4(6), 2205-2213. 4. Ahmad, H., Zulkifli, M. Z., Muhammad, F. D., Jemangin, M. H., Dimyati, K., Pal, B. P., & Harun, S. W. (2012). Passively Q-Switched 11-channel stable Brillouin erbium-doped fiber laser with graphene as the saturable absorber. IEEE Photonics Journal, 4(5), 2050-2056. 5. Ahmad, H., Zulkifli, M. Z., Muhammad, F. D., Samangun, J. M., Abdul-Rashid, H. A., & Harun, S. W. (2013). Temperature-insensitive bend sensor using entirely centered erbium doping in the fiber core. Sensors, 13(7), 9536-9546. 6. Thambiratnam, K., Ahmad, H., Muhammad, F. D., Zulkifli, M. Z., Zulkifli, A. Z., Paul, M., & Harun, S. W. (2014). Q-switching and mode-locking in highly-doped Zr O -Al O -Er O doped fiber lasers using graphene as a saturable absorber. IEEE 2 3 2 3 2 3 Journal of Selected Topics in Quantum Electronics, 20(1), 1100108. vii 7. Ahmad, H., Muhammad, F. D., Zulkifli, M. Z., & Harun, S. W. (2013). Graphene- based mode-locked, spectrum tunable fiber laser using Mach Zehnder filter. IEEE Photonics Journal, 5(5), 1501709. 8. Ahmad, H., Razak, N. F., Zulkifli, M. Z., Muhammad, F. D., Munajat, Y., & Harun, S. W. (2013). Ultra-narrow linewidth single longitudinal mode Brillouin fiber ring laser using highly nonlinear fiber. Laser Physics Letters, 10(10), 105105. 9. Ahmad, H., Muhammad, F. D., Pua, C. H., & Thambiratnam, K. (2014). Dual- wavelength fiber lasers for the optical generation of microwave and terahertz radiation. IEEE Journal of Selected Topics in Quantum Electronics, 20(5), 1-8. 10. Zulkifli, M. Z., Ahmad, H., Taib, J. M., Muhammad, F. D., Dimyati, K., & Harun, S. W. (2013). S-band multiwavelength Brillouin/Raman distributed Bragg reflector fiber lasers. Applied Optics, 52(16), 3753-3756. 11. Ahmad, H., Zulkifli, A. Z., Muhammad, F. D., Zulkifli, M. Z., Thambiratnam, K., & Harun, S. W. (2013). Mode-locked L-band bismuth–erbium fiber laser using carbon nanotubes. Applied Physics B, 1-6. 12. Ahmad, H., Zulkifli, M. Z., Muhammad, F. D., Zulkifli, A. Z., & Harun, S. W. (2013). Tunable graphene-based Q-switched erbium-doped fiber laser using fiber Bragg grating. Journal of Modern Optics, 60(3), 202-212. 13. Ahmad, H., Muhammad, F. D., Zulkifli, M. Z., & Harun, S. W. (2013). Q-switched pulse generation from an all-fiber distributed Bragg reflector laser using graphene as saturable absorber. Chinese Optics Letters, 11(7), 071401. 14. Ahmad, H., Zulkifli, M. Z., Muhammad, F. D., Taib, J. M., & Harun, S. W. (2013). Tunable S-band output based on Raman shift in dispersion shifted fiber. Journal of Modern Optics, 60(9), 737-740. viii