Figure 2: Chalcogenide taper coupling efficiency around 4.5 μm
From: Mid-infrared ultra-high-Q resonators based on fluoride crystalline materials
(a) Representative transmission spectrum composed of several resonance families over a wide mid-IR range when taper–resonator coupling is achieved with the MgF2 microresonator and set-up described in Fig. 1. (b) Measurement (blue curve) of a resonance linewidth at critical coupling with frequency calibration provided by a Pound–Drever–Hall (PDH) signal (green curve) and Lorentzian fit (red curve). The typical full-width at half maximum width of 
corresponds to a critically coupled quality factor of Qc∼1.0 × 107. (c) Transmission as a function of the coupling parameter 
for varying taper waist radius. The dashed line marks the critical coupling point 
. The experimental data (blue circles) are consistent with the theoretical model 
(red curve) demonstrating that the ChG taper behaves as a nearly ideal coupler in mid-IR with close to unity ideality.
