This paper presents a systematic review of long period fiber gratings (LPFGs) written by the CO2 laser irradiation technique. First, various fabrication techniques based on CO2 laser irradiations are demonstrated to write LPFGs in different types of optical fibers such as conventional glass fibers, solid-core photonic crystal fibers, and air-core photonic bandgap fibers. Second, possible mechanisms, e.g., residual stress relaxation, glass structure changes, and physical deformation, of refractive index modulations in the CO2-laser-induced LPFGs are analyzed. Third, asymmetrical mode coupling, resulting from single-side laser irradiation, is discussed to understand unique optical properties of the CO2-laser-induced LPFGs. Fourthly, several pretreament and post-treatment techniques are proposed to enhance the efficiency of grating fabrications. Fifthly, sensing applications of the CO2-laser-induced LPFGs are investigated to develop various LPFG-based temperature, strain, bend, torsion, pressure, and biochemical sensors. Finally, communication applications of the CO2-laser-induced LPFGs are investigated to develop various LPFG-based band-rejection filters, gain equalizers, polarizers, and couplers.

/pdf/review-of-long-period-fiber-gratings-written-by-co2-laser-19six9u5nt.pdf

Review of long period fiber gratings written by CO2 laser

Serveral improvements in fiber optic sensing systems are disclosed. One improvement incorporates a dielectric filter applied directly to the sensing end of an optical fiber detector which has the characteristic that it reflects back a reference light beam of one wavelength while passing a sensing signal of a different wavelength which is modulated in some predetermined fashion beyond the filter. Both light signals are reflected back through the fiber and are, accordingly, attenuated in the identical or nearly identical fashion in response to various noise sources. The ratio of the intensity of the two signals is proportional to the sensed parameter and renders the system essentially self-compensating. The dielectric filter is preferably directly coated onto the exit end of the fiber through vapor deposition techniques. Time division multiplexing is used to sequentially fire plural light sources and a single photodetector is used for measuring the intensity of the reflected back signals of the various wavelengths. Another improvement encompasses the detection unit in which the input and reflected signals are separated using an off-axis parabolic mirror which provides excellent separation efficiency. According to another principal feature of this invention, a concave surface is formed at the fiber end which increases the numerical aperture of the fiber which provides greater sensitivity to the deflection of a pressure sensitive diaphragm spaced from the confronting the fiber end.

Fiber optic sensors

We present two novel schemes for refractometry based on a long-period fiber grating– (LPG-) based Michelson interferometer These schemes are designed to overcome the measurement dependence of previously demonstrated LPG-based refractometry on the immersion depth The first utilizes an unshielded LPG and the second, a shielded one Both schemes were tested over a certain refractive-index range, and the measurement of glucose concentration in water was experimentally demonstrated In addition, the temperature sensitivity of the two schemes is discussed

/pdf/in-fiber-reflection-mode-interferometer-based-on-a-long-33lxfhvizx.pdf

In-fiber reflection mode interferometer based on a long-period grating for external refractive-index measurement

A fibre optic technique for detecting trace amounts of nitrite compounds in water is described. The off-line fibre optic sensor outlined here is based on evanescent field absorption in a test solution formed by the reaction of nitrite compounds in water with suitable chemical reagents. A short unclad portion of a plastic clad silica fibre acts as the sensing region. The experimental results clearly establish the usefulness of the present technique for detecting very low concentrations of the order of 1 ppb (parts per billion) of nitrite compounds with a large dynamic range of 1–1000 ppb. Such a high sensitivity enables the present device to be used for measuring the nitrite content in drinking water.

/pdf/a-fibre-optic-evanescent-wave-sensor-used-for-the-detection-qtow86nwpu.pdf

A fibre optic evanescent wave sensor used for the detection of trace nitrites in water

This paper presents a novel optofluidic Michelson interferometer based on droplet microfluidics used to create a droplet grating. The droplet grating is formed by a stream of plugs in the microchannel with constant refractive index variation. It has a real-time tunability in the grating period through varying the flow rates of the liquids and index variation via different combinations of liquids. The optofluidic Michelson interferometer is highly sensitive and is suitable for the measurement of biomedical and biochemical buffer solutions. The experimental results show that it has a sensitivity of 66.7 nm per refractive index unit (RIU) and a detection range of 0.086 RIU.

https://nocweba.ntu.edu.sg/laq_mems/journal%20papers/2010/A%20reconfigurable%20optofluidic%20Michelson%20interferometer%20using%20tunable%20droplet%20grating.pdf

A reconfigurable optofluidic Michelson interferometer using tunable droplet grating.

We propose and demonstrate the possibility of using a permanently microbent bare optical fiber for detecting chemical species. Two detection schemes, viz., a bright-field detection scheme (for the core modes), and a dark-field detection scheme (for the cladding modes) have been employed to produce a fiber-optic sensor. The sensor described here is sensitive enough to detect concentrations as low as nanomoles per liter of a chemical species, with a dynamic range of more than 6 orders of magnitude.

/pdf/chemical-sensing-with-microbent-optical-fiber-2byl5ebdi4.pdf

Chemical sensing with microbent optical fiber

Permanently microbent fibers are potential candidates for chemical sensing 1. They behave more or less like an unclad optical fiber in many respects and the basic mechanism involved in chemical sensing application is the evanescent wave absorption. In this manuscript we propose its usage for refractive index measurement of solutions. The sensing configuration employed here is the dark field detection configuration which essentially involves the detection of cladding modes generated in and near the microbent region.

P. Sureshkumar

Papers

Chemical sensing with microbent optical fiber

Microbent optical fiber for refractive index measurement