Rose-Hulman Institute of Technology Rose-Hulman Scholar Graduate Theses - Physics and Optical Engineering Graduate Theses 8-10-2017 Construction and Passive Q-Switching of a Ring- Cavity Erbium-Doped Fiber Laser Using Carbon Nanotubes as a Saturable Absorber Austin Scott Rose-, [email protected] Follow this and additional works at: https://scholar.rose-hulman.edu/optics_grad_theses Part of the Optics Commons Recommended Citation Scott, Austin, "Construction and Passive Q-Switching of a Ring-Cavity Erbium-Doped Fiber Laser Using Carbon Nanotubes as a Saturable Absorber" (2017). Graduate Theses - Physics and Optical Engineering. 20. https://scholar.rose-hulman.edu/optics_grad_theses/20 This Thesis is brought to you for free and open access by the Graduate Theses at Rose-Hulman Scholar. It has been accepted for inclusion in Graduate Theses - Physics and Optical Engineering by an authorized administrator of Rose-Hulman Scholar. For more information, please contact weir1@rose- hulman.edu. Construction and Passive Q-Switching of a Ring-Cavity Erbium-Doped Fiber Laser Using Carbon Nanotubes as a Saturable Absorber A Thesis Submitted to the Faculty of Rose-Hulman Institute of Technology by Austin Murphy Scott In Partial Fulfillment of the Requirements for the Degree of Master of Science in Optical Engineering August 2017 ©2017 Austin Murphy Scott ABSTRACT Scott, Austin Murphy O. E. Rose-Hulman Institute of Technology August 2017 Construction and Passive Q-Switching of A Ring-Cavity Erbium-Doped Fiber Laser Using Carbon Nanotubes as a Saturable Absorber Thesis Advisor: Dr. Sergio C. Granieri The purpose of this thesis is to design, build, test, and achieve pulsed operation of a ring-cavity erbium-doped fiber laser using carbon nanotubes as a saturable absorber. The erbium-doped fiber is characterized first, cross-sections are calculated, and the gain value is determined. Subsequently, the ring cavity is constructed and the laser is operated in the continuous wave regime. Much time is then spent trying to characterize and utilize the carbon nanotubes successfully. Many dispersions are made using multiple solvents and dispersing media, various images are taken with both scanning electron and Raman microscopy, and attempts at purification are made. Saturable absorbers are then created both by coating the end facet of a fiber with a dispersion containing carbon nanotubes and by inserting a fabricated poly-methyl-methacrylate (PMMA) and single- walled carbon nanotube (SWCNT) polymer composite film between two fiber end facets. Once inserted into the cavity, the saturable absorbers passively Q-switch the laser in three distinct cases. A fiber end facet coating of SWCNTs dispersed into isopropanol produced pulses with duration of 17.45 ± 0.11 µs and 2.74 ± 0.14 µs, with repetition rates of 25.36 ± 0.53 kHz and 37.77 ± 0.33 kHz, respectively. A second fiber end facet coating of SWCNTs dispersed into dimethylformamide (DMF) produced pulses with duration of 12.28 ± 1.08 µs and 3.58 ± 0.12 µs, with repetition rates of 25.13 ± 0.63 kHz and 26.46 ± 0.13 kHz, respectively. The PMMA plus SWCNT polymer composite film produced pulses of 0.716 ± 0.007 µs duration and 142.8 ± 1 kHz repetition rate. Keywords: fiber laser, erbium doped fiber, single-walled carbon nanotubes, saturable absorber, Q-switching ACKNOWLEDGEMENTS I would like to say many thanks to multiple parties: Dr. Sergio Granieri for the initial project idea, lab space and laser equipment used, guidance throughout my research, and seemingly unlimited patience; Dr. Richard Liptak for providing knowledge and experimental methods which proved very helpful with my carbon nanotube research; Brian Fair for much assistance in the MiNDS facility; Roger Sladek and Ben Webster for many favors including fixing the SEM multiple times; Dr. Kathleen H. Dannelly of ISU’s Microbiology department for allowing me to use her tip sonicator, a critical piece of equipment; Cynthia Erwin and Lou Johnson for endless free access to numerous chemicals and other materials I frequently required; Dr. Daniel Morris for giving me easy access to the Raman microscope as well as teaching me how to use it, Dr. Azad Siahmakoun for his valuable suggestions which helped me greatly improve my thesis, Dr. Wonjong Joo for his part in making my journey to Seoul possible as well as helping secure an internship at KIST, an unforgettable experience; and, finally, my close friends and family members for all their emotional and financial support. i TABLE OF CONENTS Contents LIST OF FIGURES ............................................................................................................... iii LIST OF ABBREVIATIONS ................................................................................................ v LIST OF SYMBOLS ............................................................................................................ vii 1. INTRODUCTION .............................................................................................................. 1 2. THEORY OF RING-CAVITY ERBIUM-DOPED FIBER LASERS ............................ 3 2.1 Erbium-Doped Fibers as Gain Media ...................................................................... 3 2.1.1 Pumping as a 3-Level System..................................................................... 3 2.1.2 Pumping Schemes: Unidirectional versus Bidirectional ......................... 6 2.1.3 Gain Characteristics ................................................................................... 8 2.2 Oscillation Condition for a Fiber Laser ................................................................... 9 2.2.1 Mathematical Analysis................................................................................ 9 2.2.2 Feedback Mechanism ............................................................................... 11 3. CONSTRUCTION OF A RING-CAVITY ERBIUM-DOPED FIBER LASER ......... 13 3.1 Characterization of the EDF ................................................................................... 13 3.1.1 Signal Absorption and ASE Spectrum .................................................... 13 3.1.2 Calculating Absorption and Emission Cross-Sections ........................... 19 3.1.3 Measuring EDF Gain ................................................................................ 21 3.2 Ring Cavity Setup .................................................................................................... 24 3.3 CW Operation .......................................................................................................... 25 4. CARBON NANOTUBE INVESTIGATION AND EXPERIMENTATION................ 27 4.1 Characterization Methods and Attempts............................................................... 27 4.1.1 Scanning Electron Microscope (SEM) Imaging of SWCNTs ................ 27 4.1.2 Raman Spectroscopy of SWCNTs ........................................................... 35 4.2 Dispersions of SWCNTs into Various Organic Solvents and H 2O....................... 40 4.3 Use of SWCNTs to Make Saturable Absorbers ..................................................... 48 5. PULSING THE LASER CAVITY .................................................................................. 52 5.1 Q-Switching Theory ................................................................................................. 52 5.2 Q-Switched Laser Output Results .......................................................................... 57 ii 5.2.1 Direct-Coating of Fiber End Facets with SWCNTs in Organic Solvent Solution and Corresponding Q-Switching Results ................... 57 5.2.2 Inserting the Composite Film SA Between Two Fiber End Facets and Corresponding Q-Switching Results .................................... 65 6. CONCLUSIONS AND FUTURE WORKS.................................................................... 68 6.1 Conclusions ............................................................................................................... 68 6.2 Future Works ............................................................................................................ 72 6.2.1 Improving the Pulsed Operation to Achieve Mode-Locking ................ 72 6.2.2 Better SA Construction Techniques and SA Characterization ............. 73 LIST OF REFERENCES ..................................................................................................... 75 APPENDICES ...................................................................................................................... 79 APPENDIX A ........................................................................................................................ 79 iii LIST OF FIGURES Figure 2.1: Atomic energy band structure of Er 3+ ions ......................................................... 4 Figure 2.2: Diagram of unidirectional pumping scheme for EDFL ...................................... 7 Figure 2.3: Diagram of bidirectional pumping scheme for EDFL ........................................ 8 Figure 2.4: Diagram of ring-cavity EDFL ........................................................................... 12 Figure 3.1: Diagram of setup for measuring EDF absorption spectrum ............................. 14 Figure 3.2: Results from measuring EDF absorption spectrum .......................................... 15 Figure 3.3: Diagram of setup for measuring ASE spectrum of EDF................................... 16 Figure 3.4: ASE spectrum of EDF with unwanted oscillations present .............................