A temperature sensor link based on wavelength-multiplexed fiber Bragg grating (FBG) was designed and fabricated for distributed temperature measurement in a jet engine nozzle under field conditions. Eight FBGs with different Bragg wavelengths ranging from 1520 nm to 1560 nm were fabricated along one single-mode fiber which was packaged inside a stainless steel tube. The reflected signal from the sensor link was simultaneously collected by an optical sensing interrogator and converted into temperature information. The steel tube was embedded in a steel flange assembly attached to a jet engine. Three engine cycles were performed from 55% (idle) to 80% of the engine’s full power to test the sensor response under high temperature, vibration and strong exhaust flow conditions. Test results show good survivability of the sensor, and the temperature around the nozzle was measured up to 290 °C. The system has a temperature measurement range from 20 °C to 600 ° and the response time is less than 1 second.
The large multiplexing number of FBGs exposes a requirement for the effective and repeatable fabrication method. In
this paper we report the development of an automatic FBG fabrication system, which meets the requirement of mass
production. There are four major functional parts in the system: fiber feeding system, CO2 laser coating removal system,
FBG writing system and fiber collecting system. The fiber feeding system uses motors and gears to accurately move an
optical fiber to where the FBGs will be made. The coating removal system is based on the heat effect of a CO2 laser,
which will decompose and evaporate the selected coating of the optical fiber. The FBG writing system is based on the
UV photosensitivity of the fiber. A phase-mask is placed between the UV light and the optical fiber to produce periodic
interference pattern, which further modulates the refractive index along the fiber periodically. The fiber collecting
system is driven by a linear motor and the fiber can be wound around a spool tightly and smoothly at a moderate speed.
The whole FBG fabrication system is controlled and synchronized by a computer via some interface circuits and a
Graphical User Interface (GUI). With this system, it takes 48 seconds to fabricate one FBG, and up to 500 FBGs can be
made continuously, which is limited by the leakage of the gas inside the excimer laser. This mass production line not
only improves the fabrication efficiency but also contributes to the multiplexing capability by reducing the splicing loss.
This paper gives a review of a proposed fully-distributed fiber-optic sensing technique based on a traveling long-period
grating (LPG) in a single-mode optical fiber. The LPG is generated by pulsed acoustic waves that propagate along the
fiber. Based on this platform, first we demonstrated the fully-distributed temperature measurement in a 2.5m fiber. Then
by coating the fiber with functional coatings, we demonstrated fully-distributed biological and chemical sensing. In the
biological sensing experiment, immunoglobulin G (IgG) was immobilized onto the fiber surface, and we showed that
only specific antigen-antibody binding can introduce a measurable shift in the transmission optical spectrum of the
traveling LPG when it passes through the pretreated fiber segment. In the hydrogen sensing experiment, the fiber was
coated with a platinum (Pt) catalyst layer, which is heated by the thermal energy released from Pt-assisted combustion of
H2 and O2, and the resulted temperature change gives rise to a measurable LPG wavelength shift when the traveling LPG
passes through. Hydrogen concentration from 1% to 3.8% was detected in the experiment. This technique may also
permit measurement of other quantities by changing the functional coating on the fiber; therefore it is expected to be
capable of other fully-distributed sensing applications.
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