Characterization of Reflective Defects in Fabry–Pérot Laser Diodes Through the Power Transmission Spectrum

Reflective defects in Fabry-Peacuterot (FP) laser diodes are characterized through the power transmission spectrum based on the Fourier transform method. From a single measurement, the defect reflection, transmission coefficients and the cosine value of the defect phase shift, are calculated through equations built on the intensities of the peaks associated with defects in the Fourier transformed transmission spectrum. Gain dispersion which is unavoidable in semiconductor laser diodes is taken into account in the calculations. Extensive numerical simulations are made which show that the calculated defect characteristics are not influenced by the defect length and position and better results are obtained as the round-trip gain of the FP laser is relatively low. The simulation also shows that the underestimation of the gain caused by insufficient resolution of the measurement system degrades the calculated parameters. A deconvolution process is accordingly introduced, which is able to greatly reduce the resolution influence. Amplified spontaneous emission from FP laser diodes with a single slot very close to the laser front facet are measured and used to characterize the slot. The slots act as reflective defects in the laser diode and are produced by etching a rectangular well into the laser waveguide. For two lasers on the same bar with nominally the same slot, good agreement in the calculated reflection coefficient and cosine value of the slot phase shift is obtained, but the slot transmission coefficients calculated are different, which is due to the round-trip gain difference observed in the two lasers under the same current injection