Integration of optical microcavities

In this chapter, we limit our consideration to integration of open-ring micro-cavities that are characterized with the highest achievable Q-factors among the variety of the optical microcavities. These microcavities lend themselves to the planar integration. The resonant PIC were improved tremendously during last few years. On the one hand, the Q-factors of the integrated microcavities were improved beyond 107. On the other hand, planar couplers were demonstrated for bulk resonators characterized with Q-factors exceeding 109. In this chapter, we review recent developments in the field that can be divided into two categories: (i) improvement of the quality of the planar microcavities integrated on a chip (Si [51], Si3N4 [52-63], SiO2 [64], LiNbO3 [65-70]) as well as the waveguide couplers for the planar microcavities [55,71] and (ii) integration of ultra-high-Q bulk resonators with planar waveguides to make practical PIC systems involving ultra-high-Q microcavities [72-75]. Either better manufacturing procedures or new materials were utilized to improve the microcavities. Optimally engineered waveguides were designed for bulk microcavities to enable their PIC integration. This task is espe-cially intricate for the integration of microcavities made out of low refractive index materials. The chapter is organized as follows. In Section 6.2, we present the basic terms for the description of the coupling efficiency for optical microresonators and describe the major types of the bulk evanescent field couplers. In Section 6.3, we discuss recent progress in the development of high-Q (>107) planar resonators integrated in PICs. In Section 6.4, we highlight recent results on integration of bulk microcavities with Q> 109. Section 6.5 concludes the chapter.