Thermo-optical switches using coated microsphere resonators
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Thermo-optical switches using coated microsphere resonators C. Tapalian, J.-P. Laine, and P. A. Lane C. S. Draper Laboratory, 555 Technology Square, Cambridge, MA ABSTRACT We report optical switching by a silica microsphere optical resonator coated by a conjugated polymer. Microspheres were fabricated by melting the tip of an optical fiber and coated by dipping in a 1 mg/ml toluene solution of poly(2,5-dioctyloxy-1,4-phenylenevinylene) (DOOPPV). The resonator properties were characterized by evanescently coupling 1.55 µm light propagating along a stripline-pedestal anti-resonant reflecting optical waveguide into optical whispering gallery modes (WGMs). WGM linewidths less than 2 MHz were measured, corresponding to cavity Q > 108. WGM resonant frequency shifts as large as 3.2 GHz were observed when 405 nm pump light with a power density of ~100 mW/cm2 was incident on the microsphere. The time constant of the observed frequency shifts is approximately 0.165 seconds, leading us to attribute the frequency shift to thermo-optic effects. Such a system should be capable of thermo-optically switching at speeds on the order of 10 kHz.
INTRODUCTION Optical microcavity resonators show great promise for optical communication applications such as filtering, multiplexing, and switching. Most investigations have used ring and disk-type whispering gallery mode (WGM) resonators fabricated from silica or silicon-based materials by chemical vapor deposition and photolithographic methods [1]. The small size of these devices (~10 µm diameter) allows for mass-production efficiency and high device density. Sidewall roughness of such resonators, combined with bending losses for such small resonators, limits the cavity quality factor (Q) of these resonators. Such resonators typically have Q-factors of 103 to 105 in the 1550 nm wavelength region [2]. Silica microsphere resonators are an especially promising type of micro-optical cavity. Microspheres are 3-dimensional WGM resonators, typically 50-500 µm in diameter, which can be fabricated by simply melting the tip of an optical fiber. Surface tension shapes the molten silica into a near-perfect sphere before it hardens. The process creates an extremely smooth surface, which contributes directly to low optical WGM propagation losses. Furthermore, the spherical curvature perpendicular to the optical path in microspheres focuses the WGMs, reduces the mode volume, and thus increases the cavity Q. The total optical loss experienced in microsphere resonators is exceptionally low and Q-factors as high as 108 to 1010 have already been demonstrated for microspheres [3-5]. The high Q-factor translates directly into resonant optical linewidths less than 1 MHz which could form the basis for ultra-dense wavelength division multiplexing optical channel networks. In this paper we present a novel optical switch based on a silica microsphere resonator. WGMs propagate by total internal reflection around the sphere equator, remaining confined in a thin layer beneath the surface. The WGM frequencies are thus h
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