Fabrication and investigation on microsphere laser based on er doped silica glass

VIETNAM NATIONAL UNIVERSITY OF HANOI COLLEGE OF TECHNOLOGY HOANG QUANG HUNG FABRICATION AND INVESTIGATION ON MICROSPHERE LASER BASED ON ER-DOPED SILICA GLASS Major: Nano Materials and Devices Index: M ASTER THESIS SUPERVISOR Ass Prof Dr PHAM VAN HOI Hanoi - 2005 Contents A c k n o w le d g e m e n ts In tro d u ctio n Chapter 1: M icrosphere C a v ity 1.1 Introduction 1.2 Optical modes o f a dielectric s p h e re 1.3 Intensity distribution for a microsphere W G M 1.4 Asymptotic solutions .6 1.5 Eccentricity sp littin g 1.6 Loss mechanisms in a m icrosphere 10 1.6.1 Intrinsic material loss 11 1.6.2 W hispering gallery l o s s 11 1.7 Mode volum e o f m icrospheres 13 Chapter 2: Tapered optical fiber c o u p lin g 15 2.1 Introduction 15 1.6.3 Prism s 16 1.6.4 Planar structures 17 2.2 Evanescent coupling to microspheres using tapered optical fibers 17 2.2.1 Optical properties o f tapered optical fib e rs 18 2.2.2 Mathematical description o f the waveguide-resonator coupling ju nctio n 2.2.3 Cavity-buildup factor 22 2.3 P h ase-m atch ing .22 2.4 Extension to backscattering .26 Chapter 3: M icrosphere laser based on Er-doped silica g la s s 29 3.1 Materials using for Microsphere lasers 29 3.2 Fabrication o f m icrospheres 31 3.3 Pumping and collecting techniques using half-taper coupling 33 3.4 Experiment setup 34 3.5 Experiment results 36 3.5.1 Caculations o f W GM spectra 36 3.5.2 ASE spectra and threshold laser a c tio n 40 3.5.3 Forward and backward scattering em issions 44 3.5.4 Optical power o f W G M 45 Sum ary 48 Publications Reference In tro d u ctio n Introduction Ultrahigh-quality-factors possessing whispering-gallery £ » 8, dielectric modes (WGMs) microcavity have been resonators investigated intensively for the last decade Cavity quality factors (Q factors) as high as 10J [26,19], along with small mode volumes, have been essential driving forces for fundamental research areas such as cavity quantum electrodynamics, nonlinear optics, biosensing, and microscale lasers Braginsky and Ilchenko [26] realized that the whispering-gallery modes o f dielectric silica microspheres had the potential to experimentally exhibit extremely long photon storage times, exceeding a microsecond This allowed the realization o f low threshold optical micro- cavity effects, such as lasing and nonlinear wave generation [19] Furthermore, the low optical loss o f silica microcavities allows the creation o f lasers with submicrowatt thresholds [21] Before the use o f ultra-high-Q microresonators for photonics applications could advance, the problem o f efficient excitation and extraction o f optical energy from these structures needed to be solved M ost previous studies o f silica microspheres used glass prisms and free-space laser beams to excite the cavity While this method is relatively efficient, the true advantages o f these structures could not be realized, due to both the inability to obtain complete power transfer into the cavity and to extract the optical energy in a manner convenient to further manipulation In 1997, Birks et al presented a paper which described the coupling between a fiber taper and a silica microsphere [6 ] They showed that it was possible to efficiently couple into a microsphere Vahala et al then realized that this coupling method should in principle allow near perfect coupling efficiency both into and out of a silica microresonator, while the ability to create near lossless tapers allows significant advantages in fiber compatibility However, the taper coupling technique requires the advance on experiment technique and matters For initial study on microsphere cavity, we use the half - taper coupling to pump and extract the emission from spheres Although this method is not suitable to investigate into advanced problems of microcavities, it can ■* v 'v v •’ s