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Sensing & Measurement

Toward low-cost optical fiber interrogators

A novel, interrogator design based on a segment of hollow fiber partially filled with diesel may lead to new applications for optical fiber sensors.
27 July 2011, SPIE Newsroom. DOI: 10.1117/2.1201106.003717

Fiber interferometer (FI) and fiber Bragg grating (FBG) sensors are two important families of optical sensors that are attractive for many applications such as physical, chemical, and biological measurements because of their structural simplicity, excellent reliability, high sensitivity, and capability of operating under critical conditions.1, 2 However, the costs of these systems limit their wider application, especially the costs of the signal interrogators.

Traditionally, FI and FBG sensors are decoded either in the spectrum domain with an expensive optical spectrum analyzer1 or in the space domain with a scanning reference FI.2–6 The latter is usually in the form of white-light interferometry, in which a broadband light passes the sensing and reference interferometer, and the detected interference signal reaches a maximum when the optical path difference (OPD) values of both interferometers are equal.2 OPD scanning of the reference interferometer may be achieved by movable mirrors,2, 3 a piezoelectric transducer,4 a charge-coupled device (CCD),5 or a linear photodiode array.6 All these techniques suffer from structural complexity, bulky system size, low mechanical stability and, more importantly, high cost. We have developed a new device, the liquid-core fiber interrogator (LCFI), to solve these problems.7

Figure 1. Structure of a liquid-core fiber interrogator.

Our LCFI idea originated from the well-known kerosene thermometer where kerosene liquid expands or contracts in a small glass tube when the temperature changes. An optical fiber is spliced to a silica hollow fiber whose inner and outer diameters are 73 and 125 micrometers, respectively (see Figure 1). The air pressure inside the hollow fiber is pumped down to below the atmospheric pressure, and the hollow fiber is then inserted into diesel fuel so that the liquid partially fills it. The entrance end of the hollow fiber is then sealed with epoxy, with no air gap between the end and the filled liquid. The inner diameter of the hollow fiber is sufficiently small that the surface force between the liquid and the inner wall surface of the hollow fiber keeps the end facet of the liquid perpendicular to the inner wall (whether the hollow fiber is placed horizontally or vertically). When light arrives from the single mode fiber, the Fresnel effect means it is reflected at the silica-air interface and the air-diesel interface, forming a fiber interferometer. The OPD of this FI can be continuously tuned by changing the temperature of the diesel. We used a thermoelectric cooler to heat and cool the sealed diesel, and succeeded in making a tunable FI whose OPD can be adjusted across the millimeter range with nanometer resolution.

The interrogation principle is similar to Fourier transform spectroscopy.8 When a light signal passes through the tunable FI, its spectral information can be calculated from the change in the transmitted light power. The sensing information of FI sensor and FBG sensor are both spectrally encoded, so we can extract it by analyzing the spectra of the light signal from the sensors.

When we tested our LCFI as an FI sensor, we used a broadband light with a known central wavelength, as well as two reference FIs whose OPDs are very stable and known, to correct the non-linearity of the path scanning of the LCFI. We achieved an accuracy of 14 nanometers over a measurement range of 50 micrometers. We also tested our LCFI as an FBG sensor, using a broadband light to illuminate it, and a narrow-band laser with known wavelength as the reference to track the OPD values of the LCFI during scan. As the temperature of the FBG changed from room temperature to about 90°C, the decoded Bragg wavelength was linear to the temperature with a slope of 9.51pm°C, fitting well with the well-known FBG sensitivity. The measurement accuracy was about 0.37°C.

In summary, we designed a low-cost LCFI that is a segment of hollow fiber, partially filled with diesel. We are now working to develop our concept into a practical product that can be used to perform physical, chemical, and biological measurements.

Jianmin Gong, Anbo Wang
Virginia Polytechnic Institute and State University
Blacksburg, VA

Jianmin Gong received his PhD in optoelectronics from Tsinghua University (China) in 2002. Since graduation, he has worked as research staff at Virginia Tech and Northwestern University. Gong is an author of over 40 papers concerning optical sensors for physical, chemical, and biological measurement.

Anbo Wang received his PhD in applied optics from the Dalian University of Technology (China) in 1990. He is now Clayton Ayre Professor and an author/ co-author of five book chapters and 267 journal and conference papers concerning a wide range of photonic sensors for physical, chemical, and biological measurement. He is also an inventor/co-inventor of more than 60 patents or patent disclosures with more than one third licensed to industry for commercialization. Wang has chaired many international conferences and workshops in optical sensors. In the past fourteen years, he has been responsible for $18 million research funding from various federal government agencies and private companies.

Zhengying Li
Wuhan University of Technology
Wuhan, China

Zhengying Li received his PhD in optical engineering from Wuhan University of Technology (China) in 2010. Li is an author of over 10 papers concerning gas sensors and optical fiber components.

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