Distributed Bragg grating sensing with a direct group-delay measurement technique.

A new method of distributed in-fiber Bragg grating sensing is proposed. A method is outlined by which the strain field imposed upon a Bragg grating sensor is obtained by measurement of the sensor's reflectivity and delay characteristics. The proposal is demonstrated by interrogation of two loaded samples. Strain distributions from these experiments are compared against a theoretical estimate. The data treatment is also discussed. Strain resolution is found to be +/-24 microepsilon, with a spatial resolution defined by a minimum spatial wavelength component of the coupling distribution of 1.65 mm.

The use of personal protective measures in control of malaria in a defined community.

Malaria is one of the main health problems in the non-immune immigrant workers and army personnel of the malaria endemic areas in Myanmar. Due to changes in the vector bionomics and multiresistant strains of P. falciparum, chemoprophylaxis alone is not an effective means of control of malaria in them. So it is envisaged that the combined used of personal protective measures (deltamethrin impregnated bed-nets, scalves and hand-bands) and the chemoprophylaxis will be an effective means of control of malaria in the define group of people. The study also intended to find out the side effects of the deltamethrin and feasibility and acceptability of methods by the users. The study was conducted in Theini Township, Northern Shan State, from March to November 1993. The study population consisted of all ages of both sexes 554 and 440 persons in the test and control groups respectively. At the initial phase of the study, malaria infected persons from both the groups were treated. The experimental group received personal protective measures with impregnation of bed-nets using 25 mg ai/m2 of deltamethrin at 4 monthly intervals and the scarves and hand-bands at twice the concentration of the insecticides at monthly intervals. Chemoprophylaxis was given to both the groups at weekly intervals using age adjusted dosage of Pyrixine tablet (sulfadoxine-pyrimethamine). The parasitological, entomological, and epidemiological indices were collected at two month intervals in both the groups. The study clearly showed the impact of personal protective measures and chemoprophylaxis on malaria infection in the studied subjects. During the study period, the out patient malaria cases of the test group was 6% to 11.2% and that of the control group was 12% to 21.6% in Theini Hospital. The reinfection rate of the test group (0.9 to 4.7%) was also significantly lower than the control group (6.1 to 14.3%) from July to November. Acceptance of the treated bed-nets, scarves and hand-bands was high and good compliance was found in the follow up. The results of the study clearly showed that malaria can be controlled effectively in the defined group of persons for a malaria transmission season by using chemoprophylaxis and personal protective measures.

Distributed strain measurement based on a fiber Bragg grating and its reflection spectrum analysis.

A method of extracting the strain profile along a fiber Bragg grating from the intensity reflection spectrum is described. The procedure is based on a filter synthesis theory that relates the aperiodicity of a grating with its reflection spectrum. To illustrate the approach, we measured the strain profile near a hole in a plate and obtained a strain resolution of 80 micro. The spatial resolution depends on the strain gradient; i.e., the higher the gradient, the better the resolution. A resolution of 0.8 mm was achieved for a 5-mm grating with a gradient of 250 micro/mm.

Phase-based Bragg intragrating distributed strain sensor.

A strain-distribution sensing technique based on the measurement of the phase spectrum of the reflected light from a fiber-optic Bragg grating is described. When a grating is subject to a strain gradient, the grating will experience a chirp and therefore the resonant wavelength will vary along the grating, causing wavelength-dependent penetration depth. Because the group delay for each wavelength component is related to its penetration depth and the resonant wavelength is determined by strain, a measured phase spectrum can then indicate the local strain as a function of location within the grating. This phase-based Bragg grating sensing technique offers a powerful new means for studying some important effects over a few millimeters or centimeters in smart structures.

Bragg intragrating structural sensing.

When a fiber-optic intracore Bragg grating is subject to an appreciable strain gradient, its reflective spectrum will not only be shifted but also be distorted because of the chirp of the grating. We employed the J-matrix formalism to calculate the influence of different strain gradients on the reflective spectra of Bragg gratings and have undertaken experiments to test these calculations. The results of these experiments have confirmed that intracore Bragg gratings can be used to evaluate strain gradients and can be thought of as quasi-distributed strain sensors. This adds a new dimension to structural sensing, permitting measurements in any situation where strain gradients exist. It also provides a warning of any sensor/host debonding.