Absorption Measurements in Liquid Core Waveguides Using Cavity

Short liquid core waveguides (LCWs) were included into a fiber-loop cavity ring-down absorption spectrometer to reduce the detection limit over, both, single pass absorption in a LCW and cavityenhanced absorption using a conventional fiber-loop cavity. LCWs of 5 and 10 cm length were interfaced with a pressure-flow system and a multimode fiber-loop cavity using concave fiber lenses with matching numerical apertures and diameters. Two red dyes, Allura Red AC and Congo Red, were detected with a 532 nm pulsed laser at a 5 nM limit of detection in a detection volume of less than 1 μL, corresponding to a minimal detectable absorbance of less than 4 × 10−4 cm−1 and a minimal detectable change in absorption cross section, σmin = Vdet × ε × CLOD, of about 14 μm2 (Allura Red AC) and 37 μm2 (Congo Red).

Loss Determination in Microsphere Resonators by Phase-Shift Cavity Ring-Down Measurements

The optical loss of whispering gallery modes of resonantly excited microresonator spheres is determined by optical lifetime measurements. The phase-shift cavity ring-down technique is used to extract ring-down times and optical loss from the difference in amplitude modulation phase between the light entering the microresonator and light scattered from the microresonator. In addition, the phase lag of the light exiting the waveguide, which was used to couple light into the resonator, was measured. The intensity and phase measurements were fully described by a model that assumed interference of the cavity modes with the light propagating in the waveguide.