Fiber-loop ring-down spectroscopy: A sensitive absorption technique for small liquid samples

Abstract

Cavity ring-down spectroscopy has proven to be a very sensitive gas-phase spectroscopic technique, suitable to record either very weak transitions of abundant gases or stronger transitions of trace gases. Here, an adaptation of the ring-down measurement principle to optical waveguides is presented. Fiber-loop ring-down spectroscopy ͑FLRDS͒ allows for the measurement of absorption spectra of minute quantities of liquid solutions. An optical fiber is wound into a loop using a fiber splice connector. A nanosecond laser light pulse (ϳ810 nm) is coupled into the loop and the light pulses are detected using a photomultiplier detector. It is found that once the light is coupled into the fiber it experiences very little loss and the light pulses do a large number of round trips before their intensity is below the detection threshold. The characteristic ring-down time is obtained by exponential fitting of the envelope of the wave form. This method is well suited to characterize low-loss processes in fiber optic transmission independent from power fluctuations of the light source. The strengths of the technique are demonstrated by characterization of a variety of loss processes-in particular by the measurement of the absolute loss of the optical fiber and of the fiber connector, losses due to macrobending of a section of the fiber loop, as well as losses due to lateral and longitudinal displacement in the fiber-fiber connection. Furthermore, it is shown that FLRDS is useful as an absorption spectroscopic technique for very small sample volumes and may be applied as an absorption detection method in analytical chemistry devices. A crude 47 m channel in polydimethylsiloxane polymer was fabricated between the fiber end facets and the dye 1,1Ј-diethyl-4,4Ј-dicarbocyanine iodide ͑DDCI͒ was introduced into the channel. From the concentration dependence of ring-down time the sample volume was determined as 700 pL and the detection limit as about 10 Ϫ10 mol, or 7ϫ10 Ϫ8 g of DDCI.

Cavity Ringdown Laser Absorption Spectroscopy

“…(94) Loock's group reported on a simplified fiber loop ringdown technique by introducing a micro air gap into a section of fiber in the fiber loop for detection of a small volume of dye solutions. (6,230) They demonstrated the measurement of 7 × 10 −15 mol dye solution using both a CW laser and a pulsed laser. In this study, the laser pulse was coupled into the fiber loop through a direct illumination of a bend in the fiber loop and the detection of the laser pulse leaking out of the fiber was achieved through another bend in the fiber loop.…”

Magnetic field sensing based on fiber loop ring-down spectroscopy and etched fiber interacting with magnetic fluid

“…Due to the tunable refractive index (RI) and absorption coefficient of MF, the transmission spectrum would change with the magnetic field. Research on the new FLRDS technique has been proceeding well in recent years [21,22]. In a FLRDS, optical losses of a light pulse in a fiber loop induced by changes in a quantity are measured by the light decay time constants, so FLRDS is a time-domain sensing technique used to demodulate the transmission spectrum in this letter.…”

Novel multimode fibre-cavity for ring-down experiments

“…To overcome the drawback of surface roughness due to imperfect polishing of the fibre ends, fibre bragg gratings were used as reflective elements and applied to refractive index sensing [7] and microfluid monitoring [8], [9]. Another promising approach, using fibre-loops (fibre-ring cavities) instead of reflective layers, have been the subject of many publications to date, being applied to numerous sensing purposes such as refractive index measurements [10], trace gas detection [11], pressure [12], [13], strain [14], temperature [15], and force sensing [16] as well as for fluid monitoring [17], [18].…”