Abstract
I. Introduction Strained layer quantum well lasers (SL-QWLs) have attracted much interest since the application of band structure engineering [1], [2] can improve lasing properties such as lower threshold currents and larger differential gains. It has been expected that the larger differential gain due to strain can enhance modulation bandwidths of QWLs [3], However, the highest bandwidths found so far are much smaller than expected. One reason for this is that gain saturation also influences the modulation bandwidths of QWLs as well as the differential gain [4], [5]. We have alreiidy shown that the gain saturation coefficient increases with compressive strain in GaAs-based SL-QWLs due to an increase of the intra-subband relaxation times [6]. Recent experimental work has also shown that the gain saturation coefficient increases with compressive strain in InGaAs/InGaAsP SL-QWLs [7]. It is, therefore, necessary to take into account the effects of strain not only on differential gains but also on gain saturation coefficients to properly evaluate the enhancement of modulation bandwidths in SL-QWLs. Strained layer quantum well lasers (SL-QWLs) have attracted much interest since the application of band structure engineering [1], [2] can improve lasing properties such as lower threshold currents and larger differential gains. It has been expected that the larger differential gain due to strain can enhance modulation bandwidths of QWLs [3], However, the highest bandwidths found so far are much smaller than expected. One reason for this is that gain saturation also influences the modulation bandwidths of QWLs as well as the differential gain [4], [5]. We have alreiidy shown that the gain saturation coefficient increases with compressive strain in GaAs-based SL-QWLs due to an increase of the intra-subband relaxation times [6]. Recent experimental work has also shown that the gain saturation coefficient increases with compressive strain in InGaAs/InGaAsP SL-QWLs [7]. It is, therefore, necessary to take into account the effects of strain not only on differential gains but also on gain saturation coefficients to properly evaluate the enhancement of modulation bandwidths in SL-QWLs. The purpose of this paper is to clarify the relationship between gain saturation coefficients and the amount of strain in InP-based SL-QWLs. The gain saturation coefficients of a tensile-strained, a lattice-matched and a compressive-strained InGaAs/InGaAsP quantum well laser are analyzed on the basis of spectral hole burning theory. The intra-subband relaxation times are calculated by taking into account the strain effects within the common framework of carrier-carrier and carrier-phonon interactions. We demonstrate here that the gain saturation coefficient in tensile-strained QWLs remains constant independent of the amount of strain, while it increases with strain in compressive-strained QWLs.
© 1993 Optical Society of America
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