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An all-fiber-optical method is presented to monitor densities, viscosities, and temperatures of Newtonian liquids. The actuation is performed by photothermally heating the base of a steel cantilever with an intensity-modulated 808 nm diode laser. The cantilever vibrations are measured with an in-fiber Fabry–Perot cavity sensor attached along the length of the cantilever. When immersed in a viscous fluid, the cantilever response can be related to the fluid properties: a shift in the resonance frequency corresponds to a change in fluid density, and the width of the resonance peak gives information on the dynamic viscosity after calibration of the system. Aqueous glycerol and sucrose samples in the density range of 0.997–1.17 g cm−3 and in the viscosity range of 0.89–8.49 mPa s were used to investigate the limits of the sensor. Representative beverage samples were also analyzed as unknowns.

A wineglass has been used as an acoustic resonator to enhance the photoacoustic signal generated by laser excitation of absorbing dyes in solution. The amplitude of the acoustic signal was recorded using a fiber-optic transducer based on a Fabry-Pérot cavity attached to the rim of the wineglass. The optical and acoustic properties of the setup were characterized, and it was used to quantify the concentration of phosphomolybdenum blue and methyl red solutions. Detection limits of 1.2 ppm and 8 muM were obtained, respectively.

Multi-exponential decay waveforms are common occurrences in cavity ring-down spectroscopy and the respective ring-down times are typically obtained by fitting the ring-down waveform to the sum of exponential decay functions. In phase-shift cavity ring-down (CRD) spectroscopy the measurement of a single phase angle will not provide sufficient information and needs to be complemented by either intensity measurements or phase angle measurements at different modulation frequencies. Here, a formalism analogous to that developed for fluorescence lifetime spectroscopy is adapted to the phase-shift CRD technique and is tested for two types of waveguide CRD systems: (1) a single-mode fiber cavity in which light is confined by two identical Fiber Bragg Gratings and (2) a multimode fiber loop. By measuring the phase angle at different modulation frequencies, lifetimes for up to three different decay processes were obtained.