In modern optics, optical resonators are among the main components of any optical system. They are the basic element of lasers and nowadays, they are used in many applications such as filtering, switching, sensing, etc

In recent years Whispering Gallery Mode (WGM) optical resonators have attracted increasing attention due to their unique properties, such as high quality factor, low mode volume and applications in micrometer scales. Among different types of WGM resonators, microspheres have the simplest form, low cost fabrication process and also the highest quality factor (10¹⁰).The very high quality factor makes microspheres very attractive in different applications especially in biomedical sensing.