Integration of glass microspheres and planar waveguides for microsphere lasers

Microsphere resonators with sizes in the micrometer range are reported to support very high Q’s of more than 10⁹ for a fused silica microsphere. This high Q value represents many promising characteristics such as low cavity loss, long cavity life time, and narrow band width. With their remarkable characteristics, microsphere resonators can be used in various applications such as the narrow band filter, add-drop multiplexer, microlasers, and etc. In this work, the integration of microspheres with planar waveguides is the main focus. High quality neodymium-doped BK7 microspheres are fabricated and characterised to observe laser oscillation at 1.06 µm from the ⁴F_3/2 - ⁴I_11/2 transition. <br/><br/>Theoretical calculation of the microsphere mode and field and the characteristic equation which describes the relation of the sphere mode numbers <i>l</i>, <i>m, n</i> to the wavelength, are obtained. The theoretical expressions of the microsphere WGM can be described with the sphere mode numbers based on spherical Bessel and Hankel functions. Coupling and quality factors of the waveguide-coupled system are obtained by integrating the overlapped fields of the microsphere and waveguide, and the study of Q factors as a function of sphere/waveguide separation are elaborated. Theoretical models to calculate the total loss and gain of the microsphere lasers and the Q which is required for lasing action, are developed. The expressions for power threshold in the microsphere and in the input waveguide are derived, and the threshold pump power as a function of sphere/waveguide separation is obtained. <br/><br/>Experimental work to observe the WGM propagation of a passive microsphere has been carried out, and results obtained at wavelengths in the 800 nm and 1550 nm regions, and modal assignment including the evaluation of Q for each system are carried out. The experiments to observe the fluorescence and the lasing oscillation of the neodymium-doped BK7 microspheres are demonstrated. The measurement of the fluorescence lifetime of a neodymium-doped BK7 microsphere, is obtained. A laser oscillation is demonstrated with the free-space pump excitation, with the threshold pump power of 8 mW at lambda = 808nm.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2008

Author dc:creator

• Panitchob, Yuwapat

Advisor dc:contributor.advisor

• Wilkinson, James