J. Eur. Opt. Society-Rapid Publ. 21, 26( 2025) 253
Figure 2. Whispering gallery modes in( a) a microsphere made of Erbium doped glass and( b) a microbubble filled with a fluorescent dye.
overlap spatially and be phase-matched. Figure 2 shows the WGM as a ring of light in a microsphere made of Erbium doped glass and a microbubble filled with a fluorescent dye.
Figure
3. Series of panels showing examples of WGMR coupling systems. Moving from left to right one has:( a) a microsphere and an LPG taper,( b)–( c) a microsphere and a standard taper( b: top view, c: front view),( d) a microbubble with an LPG taper and( e) a microbubble with a standard taper.
2.1 Coupling systems
The waveguides used to launch laser light into a WGMR are either tapered optical fibers, integrated waveguides, or prisms( and prism-like structures).
Adiabatic tapered optical fibers( also called tapers) are the most used couplers since they are easy to align and can be mode-tuned by controlling their thickness. Fiber tapers are fabricated using a heating-pulling technique to form a narrow waist [ 28 ]. To perform this task, advanced fiber splicing equipment or specific cylindrical microfurnaces are needed. The appropriate taper waist can be as small as 1 lm in diameter, with the fundamental mode extending significantly into the free space surrounding the taper. Fiber tapers are mostly used with silica WGMR, such as microspheres, integrated microtoroids, microbottles and microbubbles. The main drawback of tapers is the fragility of the tapered region and its tendency to deteriorate over time [ 29 ]. Farnesi et al. proposed a new method based on long period gratings( LPG) for improved robustness of fiber coupling to silica WGMR [ 30, 31 ]. After the LPG, the fiber was adiabatically tapered to reduce the size of the excited cladding mode and increase its evanescent tail without radiating. These tapers are up to one order of magnitude thicker than the“ standard” fiber tapers used for coupling light to WGM resonators, and therefore they are much more robust for practical applications. Figure 3 shows a microsphere and a microbubble coupled with an LPG and a regular taper. The difference in thickness of both systems can be clearly appreciated. The main drawback of this approach, despite its robustness, is its overall longitudinal size( i. e. few hundreds of millimeters), which leads to a poor integration level.
The most robust and compact system is the traditional planar waveguide, which however requires a careful alignment [ 32 ]. In 2016, Soltani et al. [ 33 ] reported on the coupling of crystalline microdisks to an integrated silicon waveguide on a silicon on insulator( SOI) platform. Silicon waveguides have a large refractive index compared to Lithium Niobate( LN) or lithium tantalate, the material of the crystalline microdisks. For achieving coupling, the waveguide had to be designed in order to provide optical
Figure 4.( a) Optical picture of a crystalline microdisk,( b) a microcrystalline disk coupled with a lithium niobate waveguide [ 32 ],( c) sketch of the prism-microdisk coupling system,( d) picture of the prism-microdisk coupling system implemented in our labs.
modes with effective refractive indexes close to the WGMR and a geometry that ensured mode overlap and the right interaction length. Zhuang et al. [ 34 ] proposed a Silicon photonic crystal waveguide as a coupler for silica toroids. In this particular case, the large group index of the photonic crystal waveguide and the small difference between the refractive indexes of the coupler and WGMR allowed critical coupling with a high efficiency. Very recently, Farnesi et al. [ 35 ] proposed Si waveguides based on subwavelength metamaterial engineering as couplers. The authors demonstrated up to 99 % coupling efficiency for microspheres and microdisks made of silica, LN, and calcium fluoride, with diameters from 300 lm to almost 4 mm.
The prism is also a very robust coupler, but achieving optimal alignment is quite challenging and proper mode beam shaping may be needed for improved efficiency. Prism coupling is typically used to efficiently couple light in and out of large WGMRs such as crystalline disks. Figure 4 shows a microcrystalline disk of several millimeters in