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Novel fiber optic devices with strongly enhanced graphene-evanescent field interaction for ultrafast fiber laser applications

박남훈 (일반대학원)

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Graphene, a single atomic layer of carbon atoms forming a hexagonal lattice, has been regarded as a promising material for use as a building block for future photonic, optoelectronic, and plasmonic devices owing to its outstanding electrical and optical properties. In particular, its uniform linear ...
Graphene, a single atomic layer of carbon atoms forming a hexagonal lattice, has been regarded as a promising material for use as a building block for future photonic, optoelectronic, and plasmonic devices owing to its outstanding electrical and optical properties. In particular, its uniform linear band structure, huge nonlinearity and ease of integration into diverse optical systems allow graphene to be applied to nonlinear optic devices such as nonlinear wavelength converters, optical limiters, and saturable absorbers for ultrafast fiber laser applications. However, low optical absorption (2.3%) and very thin thickness (0.34 nm) of monolayer graphene often limits its practical application. The graphene-light interaction can be greatly enhanced through the lateral interaction scheme where the evanescent field of guided light in a waveguide or an optical fiber interacts with graphene along the direction parallel to the graphene layer. Although there have been several attempts to increase graphene-light interaction based on lateral interaction scheme with evanescent field, realization of efficient fiber optic devices by evanescent-field interaction with a monolayer graphene is still challenging. In this dissertation, we investigate highly efficient linear and nonlinear all-fiber graphene devices based on strongly enhanced evanescent-field interaction with graphene, and apply these to the ultrafast fiber laser system for robust and stable operation. First, we develop an all-fiber graphene saturable absorber (SA) where high quality monolayer graphene is transferred on a side-polished fiber and a matched overcladding structure is additionally applied to it, which allows strong optical absorption of monolayer graphene up to 13.3 dB at 1550 nm wavelength. Additionally, we develop a passively mode-locked Er-doped fiber laser using our in-line monolayer graphene SA, which stably generated ultrashort pulses with pulse duration of 377 fs at a repetition rate of 37.7 MHz. The corresponding 3-dB spectral bandwidth of the laser was measured to be 8.6 nm at the central wavelength of 1607.7 nm. Moreover, the performance of the developed all-fiber graphene SA and ultrafast fiber lasers is explored in space-like environments. The monolayer graphene SA-based laser showed stable CW mode-locking operation while the inserted graphene SA was irradiated with gamma-ray of 2 kGy at a 45 Gy/hr dose rate. Lastly, an efficient electro-optic modulation is demonstrated in all-fiber graphene devices over a broad spectral range from visible to near-infrared. The ion-liquid based gating device fabricated onto a side-polished fiber with high numerical aperture significantly enhances the light-matter interaction with graphene, resulting in strong and non-resonant electro-optic modulation of up to 25.5 dB in the wavelength ranging from 532 nm to 1950 nm. In this works, the remarkable optical and electrical properties of monolayer graphene was applied to various ultrafast fiber laser applications through strongly enhanced evanescent field interaction with graphene. We believe that our study provides an opportunity for graphene to be applied in various linear and nonlinear optical devices such as broadband photodetectors, efficient SAs and electro-optic modulator in an all-fiber format.
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Chapter 1 Introduction 1
Chapter 2 Graphene 5
2.1 Electrical properties of graphene 6
...
Chapter 1 Introduction 1
Chapter 2 Graphene 5
2.1 Electrical properties of graphene 6
2.2 Linear optical properties of graphene 11
2.2.1 Optical conductivity 11
2.2.2 Optical transmittance 13
2.2.3 Optical refractive index 14
2.2.4 Tunable optical properties 15
2.3 Nonlinear optical properties of graphene 16
2.3.1 Third-order susceptibility 16
2.4 Bibliography 19
Chapter 3 Monolayer graphene saturable absorbers with strongly enhanced evanescent-field interaction for ultrafast fiber laser mode-locking 21
3.1 Introduction 22
3.2 Passively mode-locked fiber laser 23
3.2.1 Fundamentals of passive mode-locking 23
3.2.2 Saturable absorber 28
3.2.3 Saturable absorption in graphene 30
3.3 Monolayer graphene saturable absorber 31
3.3.1 Various types of graphene-based fiber optic devices 31
3.3.2 Fabrication of monolayer graphene SA 33
3.3.3 Numerical calculation of guided modes 38
3.3.4 Optical characteristics 42
3.4 Laser operation 46
3.4.1 Laser cavity 46
3.4.2 Q-switched operation 47
3.4.3 Mode-locked operation 48
3.5 Conclusion 52
3.6 Bibliography 53
Chapter 4 Graphene-based saturable absorber and mode-locked behaviors under gamma-ray radiation 56
4.1 Introduction 57
4.2 Devices and setup 59
4.2.1 Monolayer graphene-based devices 59
4.2.2 Mode-locked fiber laser with monolayer graphene SA 61
4.2.3 Gamma ray radiation environment 62
4.3 Optical properties of graphene SA in gamma-ray radiation 63
4.4 Electrical properties of graphene SA in gamma-ray radiation 67
4.5 Graphene SA for mode-locked fiber laser operation in gamma-ray radiation 69
4.6 Conclusion 73
4.7 Bibliography 74
Chapter 5 Strong electro-optic absorption spanning nearly two octaves in an all-fiber graphene device 77
5.1 Introduction 78
5.2 Graphene field effect transistor 80
5.2.1 Fundamental electrical transport of graphene FET 80
5.2.2 Ion-liquid gating 84
5.3 High NA fiber-based graphene device 87
5.3.1 numerical calculation 87
5.3.2 Fabrication of the all-fiber graphene device 89
5.4 The optical and electrical response 92
5.5 Wavelength-dependent electro-optic properties of the device 95
5.6 Dynamic characteristics of the device 99
5.7 Conclusion 101
5.8 Bibliography 102
Chapter 6 Conclusion 105