Figure 1
From: Air-coupled ultrasound detection using capillary-based optical ring resonators
(a) Schematic of the experimental setup for pressure wave detection using a capillary-based ring resonator. An optical fiber taper was placed in contact with the capillary to couple the laser light around 1550 nm from a tunable laser into the WGM of the ring resonator. The transmitted light was detected by a photodetector (either DC-coupled or AC-coupled) and analyzed by an oscilloscope and a spectrum analyzer. An ultrasound transducer (800 kHz) was placed 2 cm above the resonator, aiming at the center of the resonator. (b) Photograph of the capillary-based bottle-shaped ring resonator. (c) Photograph showing the capillary-based ring resonator placed on a holder and secured by two glue dots. The distance between the two glue dots could vary, but did not affect the pressure measurement. The minimum distance tested was 3 mm. The white line indicates the position of the capillary. (d) Transmission signal shows that the Q-factor of the ring resonator (65 μm in outer radius and 10 μm in wall thickness at the most-bulged part) was 3.5 × 107. The red dot shows the spectral position where the laser was fixed. Slight WGM spectral shift resulting from a pressure wave causes the transmission intensity to change.
