We report our research on the development of a gas detection system for environmental application. The CO2 concentration of a remote site is detected with near-infrared laser spectroscopy. Light from a thermal-tuning DFB laser, whose wavelength is close to 1.57 μm, is delivered to/from the remote gas cell via silica optical fiber. The low transmission attenuation of 0.2 dB/km promises long distance CO2 sensing in the length of the gas cell in our experiment is 20 cm only, and detection accuracy of CO2 is 1%.
To achieve pH detection of multi-points, a new pH sensor based on pH sensitive hydrogel swelling detection by a fiber
Bragg grating is proposed. The deflection of a silica membrane due to pH value change induced hydrogel swelling is
measured by the center wavelength shifts of a fiber Bragg grating epoxied on the membrane. The relation between center wavelength shifts of the fiber Bragg grating with the hydrogel swelling behavior was studied experimentally. Around 100pm shift of the fiber Bragg grating center wavelength was observed when the pH value was changed from 4 to 7 or from 7 to 10 by using pH standard solutions, which fundamentally proved the feasibility of this method for pH detection.
This paper develops a program to process the ring-down signal automatically finding peaks and calculating decay rate.
The program is tested by simulated data and analyzes the error under different SNR. Experimental system is set up to
acquire ring-down signal that is processed by data processing program.
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.
We report a low-cost interrogator for fiber-optic interferometric and Bragg grating sensors.
The interrogator is based on a compact optical path scanner which is made by splicing a
hollow fiber to a single mode fiber and by sealing a segment of air and a segment of
thermally expanded liquid inside the hollow fiber. The facets between the fiber-air
interface and the air-liquid interface reflect the light from the single mode fiber back, and
the optical path difference between the two facets can be controlled by changing the
temperature of the liquid. When the compact optical path scanner is place inside a white
light interferometer together with a sensing fiber-optic Fabry-Perot interferometer, the
optical path difference of the sensing interferometer can be decoded as the optical path
difference of the scanner when the interference signal gets maximum. The decoding
accuracy of such an interferometer interrogation system was measured to be 14 nm over a
range of 40 μm. The compact optical path scanner can also be used to form a wavelength meter, which can be applied to decode the Bragg wavelength of a fiber Bragg grating sensor. A decoding accuracy of 3.5 pm was obtained.