Evanescent wave fibre optic sensors for trace analysis of Fe³ in water

Abstract

In this communication, we discuss the details of fabricating an off-line fibre optic sensor (FOS) based on evanescent wave absorption for detecting trace amounts of Fe 3+ in water. Two types of FOS are developed; one type uses the unclad portion of a multimode silica fibre as the sensing region whereas the other employs the microbent portion of a multimode plastic fibre as the sensing region. Sensing is performed by measuring the absorption of the evanescent wave in a reagent medium surrounding the sensing region. To evaluate the relative merits of the two types of FOS in Fe 3+ sensing, a comparative study of the sensors is made, which reveals the superiority of the latter in many respects, such as smaller sensing length, use of a double detection scheme (for detecting both core and cladding modes) and higher sensitivity of cladding mode detection at an intermediate range of concentration along with the added advantage that plastic fibres are inexpensive. A detection limit of 1 ppb is observed in both types of fibre and the range of detection can be as large as 1 ppb-50 ppm. All the measurements are carried out using a LabVIEW set-up.

Light Emitting Diode Based Evanescent Wave Fiber Optic Chemical Sensor for Detection of Thiocyanate

“…There are different types of fiber optic sensors for the detection of various chemical species. The mechanism discussed in this work is modulation of optical intensity by exploiting the evanescent wave from unclad portion of the fiber to surrounding media [12,13].…”

Highly Sensitive Temperature Sensors Based on Fiber-Optic PWM and Capacitance Variation Using Thermochromic Sensing Membrane

“…A sensing membrane containing Rhodamine B was deposited on a side-polished optical fiber to form an optical waveguide; the schematic diagram is shown in Figure 1 a. When light travels through the side-polished fiber-optic device, a fraction of the radiation spreads a small distance to create an evanescent field, as defined by [ 43 ]: where is the electric field amplitude of light at the interface of core–cladding and is the distance of electric field in the cladding from the interface. is the penetration depth, and the sensitivity of the fiber-optic sensor depends on the penetration depth.…”

A High Sensitivity and Wide Dynamic Range Fiber-Optic Sensor for Low-Concentration VOC Gas Detection

“…When light propagates in an optical fiber, a fraction of the radiation extends for a short distance from the guiding region into the medium of a lower refractive index that surrounds it. This is the evanescent field and can be represented in the following form [ 42 ]: where d p is the perpendicular distance from the core-cladding interface at which the electric field amplitude has decreased to 1/ e of the initial amplitude E 0 . It is called the penetration depth and is defined by the following mathematical equation [ 43 ]: where n 1 and n 2 are the refractive indexes of the fiber cladding and the material in contact with the top surface of the overlay, respectively.…”