Rare-earth doped microlasers for microphotonic applications
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Rare-earth doped microlasers for microphotonic applications Lan Yang, Bumki Min and K. J. Vahala Department of Applied Physics, California Institute of Technology, Pasadena, CA 91125, U.S.A. ABSTRACT Sol gels provide a highly flexible technique for preparation of both planar and non-planar oxide thin films. They also enable the incorporation of various dopants into the films. In this work we describe the application of erbium-doped solgel films to surface functionalize optical microresonators. The resulting microlaser devices are especially interesting because their emission band falls in the important 1.5 µm window used for optical fiber communications. Both microsphere and ultra-high-Q microtoroid resonators-on-a-chip were functionalized into lasers and then characterized [1]. The erbium-doped sol-gel films were applied to the resonator surface and subsequently a CO2 laser was used to induce flow and densification of the sol-gel film on the surface. Optical quality thin films were obtained after the CO2 laser induced anneal. By varying the doping concentration and thickness of the applied sol-gel layers in microsphere resonators, we can vary the laser dynamics so that both continuous-wave and pulsation operation are possible. Single mode performance with high differential quantum efficiency was also obtained using the ultra-high-Q microtoroid resonator. These chip-based microlasers enable integration with other optical or electronic functions [2-3]. INTRODUCTION Microcavities formed by surface tension can exhibit quality factors in excess of 1×108 and are of interest in nonlinear optics, optical fiber communications, and sensing. Among these applications, Er3+-doped microlasers are especially interesting due to the erbium 4f transition 4 I13 / 2 → 4 I15 / 2 which falls in the 1.55 µm telecommunication window. The combination of ultrahigh quality factor and small mode volume can lead to ultra-low threshold microlasers. In addition, microtoroid lasers are fabricated on a silicon wafer and are thus integrable with other optical or electronic components. In this work, we study and characterize the laser performance of both microsphere and microtoroid lasers achieved by solgel surface functionalization of ultrahigh Q microresonators. To gain insight into the microlaser operation and performance, a model was developed and predictions were compared with experimental data.
EXPERIMENTAL DETAILS Two different microcavities, microspheres and microtoroids on silicon wafer [1], were used as the base resonator structure for surface functionalization. In the case of spheres, the initial silica microsphere was formed by heating the end of a tapered fiber tip with a CO2 laser as describe by Knight et al [4]. On the other hand, ultra-high Q microtoroids were fabricated upon
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silicon wafer with 2-µm layer of silica by using a combination of lithography, dry etching and selective reflow process. This process, as described in reference 1, begins by creation of a series of circular silica pads on a base silicon wafer
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