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Sensing & Measurement

Detection technique uses total reflectivity of a fiber Bragg grating

Temperature-insensitive fiberoptic sensors can be made with low cost simply by measuring all the light reflected by a chirped fiber Bragg grating.
13 December 2006, SPIE Newsroom. DOI: 10.1117/2.1200610.0473

Any sensing apparatus includes two basic parts: a sensor enclosed in a suitable package and an interrogation system to detect and convert a physical parameter into a convenient signal. Optical fiber sensors are attractive both because they can be very compact and because numerous detection techniques with various levels of resolution and performance are available. Unfortunately, the packaging of the sensor and optical interrogation system is often complex and expensive. As a result, the overall cost of the optical sensing apparatus can hardly compete with other sensing technologies (e.g., electrical sensors).

Fiber Bragg gratings (FBGs) show great potential for detecting strain, pressure, and temperature variations in sensing applications.1,2 An FBG is a periodic modulation of the fiber core refractive index and is typically used as a light-reflective device. More particularly, if the period is not constant along its length, the grating is said to be chirped. The FBG reflection spectrum can then be shifted or broadened depending on how the external perturbation is applied to the grating.

The current work proposes a very simple and low-cost interrogation technique for use in FBG-based high-g accelerometers3 and dispersion compensators.4 The detection technique is based on the measurement of the total reflectivity of a uniform Bragg grating that is linearly chirped under strain.

In the high-g accelerometer application, the fiber sensor is free to stretch (or compress) under acceleration. As illustrated in Figure 1, one end of the fiber is tightly attached to a holder, while the unattached part can deform under its own weight. In this case, the applied strain is linearly distributed along the free section of the fiber sensor.5

Figure 1. When the free fiber section stretches under strain, the fiber elongation creates an uneven grating period.

If the length of the Bragg grating is short in comparison with the length of the free fiber sensor, the grating will experience only a shift of its reflection spectrum under strain. On the other hand, for similar lengths, the grating reflection spectrum will be shifted and chirped at the same time. The resulting spectral modifications are illustrated in Figure 2, where, for purposes of illustration, the spectral shift is not taken into account.

Figure 2. These reflection spectra show uniform and chirped 10mm-long Bragg gratings for the maximum reflectivities of two uniform gratings.

As seen in Figure 2, a chirped FBG spectrum has stronger sidebands and a weaker main lobe, resulting in a broadened reflection spectrum. These spectral changes are more noticeable at higher reflectivity. Figure 2 clearly shows that the total reflectivity of the spectrum (the area under the curve) depends on the chirp level and the maximum reflectivity of the grating. The chirp level is defined as the percentage variation of the period along the grating. For example, a 1% chirp means that the period at the end of the grating is 1.01 times the period at the beginning of the grating.

Two previous detection techniques based on a single shift of the FBG reflection spectrum under strain have already been proposed.5,6 In the first technique, a laser is used to interrogate the sensor. This gives good signal sensitivity but requires both temperature and frequency control and optical isolation of the laser. The second technique convolutes the spectra of a sensor grating and a reference grating, but requires that two identical gratings undergo the same temperature variation.

Our proposed detection technique, based on the measurement of the total reflectivity of a uniform FBG that is linearly chirped under strain, is illustrated in Figure 3. The sensor grating is illuminated using a broadband source, such as an LED, and all reflected light is measured using a photodiode through the 3dB coupler. The technique is insensitive to temperature because the broadband source has a much larger emitting spectrum than the grating's reflectivity spectrum. Any temperature change experienced by the fiber sensor, resulting in a shift of the FBG reflection spectrum, would create a negligible signal variation at the photodiode.

Figure 3. Broadband light from the LED is coupled to the sensor grating. When the sensor is strained, the amount of light reflected to the photodiode changes.

Figure 4 shows the ratios of the spectrum integrals calculated at 1550nm for several maximum reflectivities of a 50mm-long reference grating. As expected, the total reflectivity of a chirped grating increases with chirp level. For stronger maximum reflectivities of the grating, we also observe that the integral ratio is higher and its linear variation longer. For example, the reflected signal of a reference grating with 99.999% maximum reflectivity is increased five times when the grating chirp is changed from 0 to 0.4%. The ratio saturation observed at lower reflectivities is explained by the fact that any increase of the sidebands at the extremities of the reflection spectrum is compensated by an average reflectivity reduction of the spectrum.

Figure 4. Ratios of spectrum integrals are calculated at 1550nm for several maximum reflectivities of the reference grating. The length of the reference grating is 50mm.

Figure 5 shows three integral ratios calculated at 850 and 1550nm for two different lengths of grating. We observe that a shorter wavelength or a longer grating will give higher ratios—and thus more sensitivity—for lower values of chirp, whereas a shorter grating or a longer wavelength will give a longer linear range with chirp.

Figure 5. Ratios of spectrum integrals are calculated at 850 and 1550nm for two different lengths of grating. The maximum reflectivity of the reference grating is 99.99%. LB: grating length. λB: wavelength.

In conclusion, we have described a novel temperature-insensitive detection technique based on total reflectivity measurement of a chirped FBG. Depending on the application, sensitivity and linear range can be adjusted by proper selection of the grating length and maximum reflectivity. This technique is clearly one of the simplest and most inexpensive techniques using FBGs.

Sylvain Thériault, Anne Claire Jacob Poulin
INO (Institut National d'Optique)
Québec, Canada

Sylvain Thériault received his MSc and PhD degrees from Laval University in Québec City. He was a researcher at the Communication Research Center in Ottawa, developing fiber Bragg grating technology. He was also a professor in electrical and computer engineering at Moncton University and at the University of Québec in Hull, where his research interests focused on developing novel fiber-based components for communications and sensors. He joined Alcatel Optronics in 2001 as a group leader in fiber-based components. He is now a researcher at INO, developing novel fiber sensors.

Anne Claire Jacob Poulin joined INO as a researcher in 2000 after obtaining her PhD at Laval University in Québec City. She worked in the field of communication, fabricating passive optical components with photosensitive fibers and developing optical fiber amplifiers with specialty-doped fluoride fibers. Her current research focuses on fabrication and application of photonics devices to sensor systems in the agrifood and biomedical fields.