Simultaneous and Continuous Multiple Wavelength Absorption Spectroscopy on Nanoliter Volumes Based on Frequency-Division Multiplexing Fiber-Loop Cavity Ring-Down Spectroscopy

摘要

ABSTRACT:We demonstrate a method for measuring optical loss simultaneously atnmultiple wavelengths with cavity ring-down spectroscopy (CRD). Phase-shift CRDnspectroscopy is used to obtain the absorption of a sample from the phase lag ofnintensity modulated light that is entering and exiting an optical cavity. We performedndual-wavelength detection by using two different laser light sources and frequency-ndivision multiplexing. Each wavelength is modulated at a separate frequency, and anbroadband detector records the total signal. This signal is then demodulated by lock-nin amplifiers at the corresponding two frequencies allowing us to obtain the phase-shift and therefore the optical loss at severalnwavelengths simultaneously without the use of a dispersive element. In applying this method to fiber-loop cavity ring-downnspectroscopy, we achieve detection at low micromolar concentrations in a 100 nL liquid volume.Measurements at two wavelengthsn(405 and 810 nm) were performed simultaneously on two dyes each absorbing at mainly one of the wavelengths. The respectivenconcentrations could be quantified independently in pure samples as well as inmixtures. No crosstalk between the two channels wasnobserved, and a minimal detectable absorbance of 0.02 cm-1nwas achieved at 405 nm.