We proposed a Brillouin optical fiber time domain analysis (BOTDA)-based fully-distributed temperature system as high as 1000°C and spatial resolution to 5 meters. This technique is prominent for high spatial resolution fully distributed high temperature and stress sensing over long distance.
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.
We reported a high-sensitivity CO2 gas sensing system based on wavelength scanning absorption spectroscopy. A
distributed feedback (DFB) laser was used as the light source in the system, whose wavelength was thermally tuned, by a
thermoelectric cooler (TEC), to scan around one CO2 absorption line near 1572nm. Scanning of the absorption line
spectrum is performed over a glass CO2 gas cell, 16.5 cm long with collimated optical fiber connectors. Different
concentrations of CO2 were prepared by a high-precision gas flow control meter and sealed within the gas cell. A self-designed
detection and amplification circuit was employed for absorption spectrum detection. The circuit implements
background-cancellation with a two tier amplification scheme. By cancelling the high background signal, we can
improve the CO2 sensitivity by about two orders of magnitude compared with commonly used direct detection methods
with high background signals. Reducing the high DC signal permits isolated amplification of the absorption line
spectrum. Absorption spectra of different CO2 concentrations were measured, and the results demonstrated sensing
capability of 100% to <0.1% concentrations of CO2. This sensing system is expected to be used in conjunction with a
wireless CO2 sensor network for large area CO2 monitoring. Given the very lower power consumption of the DFB laser
and the detection circuit this sensing system offers a solution for affordable long term CO2 monitoring for reliable
storage in carbon sequestration.
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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