Gain and linewidth enhancement factor of distributed feedback and Fabry-Perot-type 1.55-mu m multiple-quantum-well laser diodes by the measurement of spontaneous emission spectrum from the side wall

The gain and the linewidth enhancement factor, or alpha-factor, of 1.55-mu m GaInAsP Multi-Quantum-Well (MQW) Distributed-Feedback (DFB) Laser Diodes (LD's) for high-speed optical communication were analyzed and compared to Fabry-Perot LD's, instead of measuring the ratio of the Fabry-Perot modes of the light from the facet, we measured the spontaneous emission spectrum from the side wall of a laser diode chip and transformed it into the gain spectrum. The spnotaneous emission was measured from 1200 to 1620 nm. The real part of the optical index and the linewidth enhancement actor mere calculated from the gain spectrum with the Kramers-Kronig relation. As the bias current increases from 1 mA to the threshold current I-th, the gain spectra show large differential gain with improved characteristics of gain saturation, and the linewidth enhancement factor increases from 1 to 2 at the lasing wavelength. Especially, the linewidth enhancement factor of a DFB-LD at the lasing peak is lower than that of a FP-LD.