We measure, for the first time, the gain compression coefficient and above-threshold linewidth enhancement factor (alpha parameter) in quantum dot (QD) distributed feedback lasers (DFB) by time-resolved-chirp (TRC) characterization. The alpha parameter is measured to be 2.6 at threshold and increases to 8 when the output power of the QD DFB is increased to 3 mW. The dependence of the above-threshold alpha parameter on the optical power is found to be stronger than the optical gain compression effect alone can predict. The inhomogeneous gain broadening, gain saturation at the ground states and carrier filling in the excited states in QDs are proposed to explain the results.
For understanding the fundamental processes in QDs and optimizing the design of QD optical devices, it is essential to obtain accurate optical gain and absorption spectra. An improved segmented-contact method is described that subtracts the unguided spontaneous emission that normally introduces error into the calculated gain and absorption. Using the technique a QD gain spectrum is measured to an accuracy of less than 0.2/cm at nominal gain values below 2/cm. This capability also enables precise measurement of waveguide internal loss, unamplified spontaneous emission spectra and Stark shift data.
The correlations between the photoluminescence (PL) wavelength, integrated intensity, peak intensity, and FWHM with laser diode performance such as the maximum gain, injection efficiency, and transparency current density are studied in this work. The primary outcome is that the variation in PL intensity within a wafer originates primarily from differences in the radiative and non-radiative recombination rates and not from dot density variation. PL generated from 980 nm wavelength pumping appears to give more consistent data in assessing the optical quality of quantum dots that emit in the 1300 nm from the ground state.
High-frequency spectra of free-running 5-mm-long triple-quantum-well graded-index separate-confinement heterostructure broad-area diode lasers emitting at ~1 μm are investigated in the range of 1-20 GHz using RF spectrum analyzer. The spectra reveal stable beat lines at ~8 and ~16 GHz, corresponding to single and double mode spacings between adjacent longitudinal modes. A current-dependent peak, varying from 0.6 to 2 GHz, is associated with the relaxation resonance. Measurements of mode beating spectra provide additional characterization of diode laser emission for coherent light applications.
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