Evanescent wave absorption spectroscopy using multimode fibers

Abstract

Evanescent wave absorption in an aqueous dye solution has been performed using multimode fused silica fiber which was unclad at the sensing region. Evanescent absorbance values for methylene blue in a concentration range 3×10−8 to 5×10−6 M are reported. In order to produce modes close to cutoff in the sensing region, tunneling modes were launched into the clad fiber. Spatial filtering was used to restrict the light launched to those modes which have substantial power in their evanescent field in the unclad region. The measured evanescent absorbance of the dye solution was found to vary linearly with the exposed core length and to exhibit a square root dependence on concentration. The former effect is predicted from standard theory while the latter is attributed to adsorption on the core surface which obeys a Debye–Huckel-type concentration dependence. In addition, a concentration enhancement of two orders of magnitude was observed due to this surface adsorption. While this effect limits the use of the technique for a reversible sensor it may be exploited in disposable probes.

A fibre optic evanescent wave sensor for monitoring the rate of pulsed laser deposition of metal thin films

“…FOSs can also be utilized for the online measurements of different parameters with the added advantage of the possibility of taking remote measurements (Leonard 1995). Evanescent wave (EW) spectroscopy is one of the sensing methods used in a certain class of FOS for monitoring and measurement of a variety of physical and chemical variables (Radhakrishnan et al 1993, Ruddy et al 1990. An optical fibre based EW sensor to monitor the deposition rate of thin films of an organic material (polypyrrole) produced by AC plasma polymerization has been reported by us previously (Deepa et al 1998).…”

A Novel Approach to Evaluate the Sensitivities of the Optical Fiber Evanescent Field Sensors

“…Various reasons have been assigned to this discrepancy. It is mainly caused by the deposition of the methylene blue molecules on the sensing fiber's surface and the disfigurement of the sensing fiber (Ruddy & Maccrait, 1990 ;Potyrailo & Hobbs, 1998;Zhuang et al, 2009b).The effects of the sensing fibers' disfigurements on the results are listed as follow: (1) the surface of the etched core is not perfect and scattering occurs on the interface between the analyte and the sensing fiber, which will decrease the output power of the sensor in experiment; (2）the fiber diameter is unsymmetrical and there are errors in the measurement, which will introduce mistakes to the simulation; (3) the bending loss increases the energy loss in the sensing region, and the values of / out in PP is depressed; (4) when the unclad optical fiber etched, taper will occur in the two ends of the sensing region as described in section 3.1. When light passes through the taper, energy loss happened because of the discontinuity of the modes supported by these two different parts of the fiber and the experimental results will be enhanced (Ahmad & Hench, 2005).…”

Applications of the Planar Fiber Optic Chip

“…Removing these lower order modes from the optical beam prevents collecting average absorbance measurements which are unduly weighted toward the less sensitive traveling waves inside the fiber. (Gloge, 1971;Ruddy et al, 1990) To select the modes allowed to propagate in the FOC an annular mask that only transmits a ring of light of a defined angle has been used. A mask delivering light with a low effective index, therefore working only with the highest order modes that the fiber can support, was shown to double the measured thin-film absorbance on the FOC compared to that measured for the same film without a mask.…”

“…(Anderson et al, 1993;Golden et al, 1992;Grant & Glass, 1997;Maragos & Thompson, 1999;Thompson & Maragos, 1996;Wiejata et al, 2003;Zhou et al, 1997) Previous studies, which have taken advantage of the convenience of fiber coupled instrumentation and the increased sensitivity of the total internal reflection geometry, have used a fiber optic with the cladding removed to create a sensing element around the cylindrical fiber core. The exposed core region serves as an ATR element that can be used for absorbance measurements to detect volatile organic compounds (Blair et al, 1997), probe dye solutions (Ruddy et al, 1990), monitor methane gas (Tai et al, 1987) and ammonium ion (Malins et al, 1998) concentrations, and determine solution pH using indicator doped sol-gel coatings (Gupta & Sharma, 1997;MacCraith, 1993), or an indicator doped polymer film. (Egami et al, 1996) Several investigators have worked to further increase the sensitivity of the de-clad cylindrical core fiber optic sensors by tapering the fiber optic (Guo & Albin, 2003;Gupta et al, 1994;Mackenzie & Payne, 1990;Mignani et al, 1998) or bending the sensing region (i.e.…”