A new approach to the analysis of amplified spontaneous emission (ASE) spectra, based on the least-squared fitting, is proposed for characterization of broad-band multilayer antireflection (AR) coatings and semiconductor optical amplifiers (SOAs). The model formula for ASE spectra contains eight fitting parameters, representing the parabolic approximation of spontaneous emission and gain spectra, together with the linear approximation of refractive index dispersion. By zone fitting to the segmented experimental data after AR coating,a stepwise reflectivity estimate is obtained. Double-layer AR-coated 1.3 m InGaAsP MQW SOAs were fabricated and evaluated. For the best samples, minimum reflectivities on the order of 10-6 and bandwidths (for R < 1 10-4) of about 50 nm were reproducibly obtained. Reliability of the analysis procedure was checked on the basis of relevance of various physical parameters, bias current dependence, and reproducibility of the extrapolation procedure. Being noise-resistant and applicable to wider spectral range, the new method is confirmed to be a significant improvement over the standard ASE analysis procedure (i.e., the conventional Hakki-Paoli (HP) method).
Jungkeun Lee and Takeshi Kamiya, "Improved Characterization of Multilayer Antireflection Coatings for Broad-Band Semiconductor Optical Amplifiers," J. Lightwave Technol. 18, 2158- (2000)