Wavelength Modulation Spectroscopic Chemical Sensing Using a Piezo-Electric Tunable Fiber Bragg Grating Laser
Real time gas sensing is paramount to numerous applications in industry as well as in the consumer sector. Many gas sensing applications such as fossil energy production require low-cost, multi-species sensors that are able to operate in high-temperature, high-pollution environments under the presence of strong electromagnetic fields. Optical remote gas sensing using tunable lasers is regarded as the best sensing technology for hostile environments. However, optical technology often suffers from high component and operational costs.
In this thesis, a tunable external-cavity fiber Bragg grating (FBG) diode laser was developed for spectroscopic chemical sensing. Although FBG lasers have been reported upon, previous works have focused primarily on telecommunications applications. This thesis reports the first application, to our best knowledge, of an FBG laser in chemical sensing. This fiber Bragg grating laser was comprised of an InGaAs/InP ridge-waveguide laser diode coupled to a length of SM-28 fiber bearing an FBG. The FBG is stretched via a piezo-electric actuator to allow rapid fine tuning of the output wavelength of the laser. This tunable external-cavity semiconductor laser was demonstrated with over 10 nm of tuning range. The measured spectral width of the laser was instrument-limited at less than 50 pm over the entire tuning range. The application of such low-cost tunable FBG lasers to spectroscopic chemical sensing was demonstrated in acetylene (C2H2) gas with a wavelength modulation spectroscopy technique. Both static and wavelength modulation absorption spectra of acetylene gas were observed by the tunable laser in acetylene partial pressures from 0.1 mbar to 100 mbar; the lowest detectable pressure being limited by the ultimate vacuum pressure and length of our gas cell.
In terms of other optical gas sensing devices such as Distributed FeedBack lasers, FBG lasers offer much lower manufacturing costs and better temperature stability (13pm/ºK) over DFB lasers (>100 pm/ºK). The low manufacturing cost, good temperature stability, wide tuning range, and high output power make FBG lasers excellent candidates for the application of chemical sensing in the near IR band.
Advisor:Dr. Kevin Chen; Dr. Dietrich Langer; Dr. Joel Falk
School:University of Pittsburgh
School Location:USA - Pennsylvania
Source Type:Master's Thesis
Date of Publication:01/31/2006