title: Influence of the wavelength-dependence of fiber couplers on the background signal in wavelength modulation spectroscopy with RAM-nulling.
abstract: Recently a technique to optically eliminate the background residual amplitude modulation in 1f wavelength modulation spectroscopy was demonstrated, where perfect elimination throughout the scan range was not achieved due to the wavelength-dependence of couplers and that of the laser intensity modulation. This paper theoretically analyzes the technique and experimentally demonstrates that the elimination can be perfect for one of three possible experimental configurations, making this important for potential applications with some recently-developed laser sources. For the other configurations a non-zero background slope is predicted, experimentally verified, and the anomalous nature of signals is thereby explained. A common signal normalization method is devised that is independent of the signal slope, a fact that is important for industrial deployment of such systems.
authors.affiliation: Department of Electronic and Electrical Engineering, University of Strathclyde, 204 George Street, Glasgow G1 1XW, United Kingdom. arup@eee.strath.ac.uk
authors.forename: Arup Lal
authors.initials: AL
authors.lastname: Chakraborty
authors.forename: Keith
authors.initials: K
authors.lastname: Ruxton
authors.forename: Walter
authors.initials: W
authors.lastname: Johnstone
mesh_terms: D003198:Computer Simulation; D017076:Computer-Aided Design; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005336:Fiber Optic Technology / Q000295:instrumentation*; D008027:Light; D008962:Models, Theoretical; D055096:Optical Devices; D012031:Refractometry / Q000295:instrumentation*; D012542:Scattering, Radiation; D013057:Spectrum Analysis / Q000295:instrumentation*; D014159:Transducers