In this paper we demonstrate high-power GaAs-based and InP-based superluminescent diodes (SLD) with tilted waveguides emitting in 8xx nm and 15xx nm spectral ranges respectively. The analysis of devices with different cavity lengths emphasizes the tradeoff between output power and spectral width. Power levels of about 200 mW for 820 nm SLDs and about 100 mW for 1590 nm SLDs have been demonstrated for longer cavity devices. Spectral modulation was less than 6-7% at 70-80 mW output power for both 8xx and 15xx SLDs. Simple model proposed for evaluation of spectrum modulation for both GaAs and InP devices based on semi-empirical approach is in agreement with experimental observations.
Semiconductor lasers employing intersubband transitions in quantum boxes, so-called intersubband quantum-box (IQB) lasers are found, in theory, capable to have significantly lower threshold current densities and operating voltages than quantum cascade (QC) lasers. For that to happen, an enhancement factor of about 20 in the LO-phonon-assisted electron relaxation time is necessary, which is quite possible according to recent experimental data. The increased gain for the radiative stage in an IQB laser eliminates the need for a multiradiative-stage structure (typically 25 stages in QC lasers). Due to their inherently lower input power requirements, IQB lasers operating in the mid-IR wavelength range should be capable of much higher average-output powers than QC lasers at all temperatures, and continuous-wave (CW) operation at room temperature with high wallplug efficiency.
Optical gain and spontaneous emission spectra are calculated for CdZnSe/ZnSSe single quantum well (QW) structures at room temperature with various many-body effects taken into account. It is found that Coulomb enhancement has a large effect on the gain-current relation derived from these spectra. When Coulomb enhancement is ignored, values of threshold current at various cavity lengths are overestimated by about 40 to 50% compared with the measured threshold currents. Good agreement with experiment is reached when Coulomb enhancement is included in the calculation.
It is shown that semiconductor lasers utilizing intersubband transitions in quantum boxes (IQB lasers) can have lower threshold current densities and operating voltages than quantum cascade (QC) lasers provided that a reduction factor of about 10 can be achieved in the LO phonon-assisted electron relaxation rate. The increased gain for the radiative stage in an IQB laser eliminates the need for a multi-radiative-stage structure (typically 25 in QC lasers). This allows the electron injector and Bragg mirror regions on either side of active region to be separately optimized. Due to their inherently lower input power requirements, IQB lasers operating in the mid-IR should be capable of cw operation at room temperature with high wall plug efficiency and higher average output powers than QC lasers.
Data showing the dependence of lasing wavelength on cavity length for CdZnSe single quantum well, buried ridgeguide lasers is presented. The `slope' of the data is opposite in sign to the slope calculated from conventional theory which includes carrier scattering and bandgap renormalization. The calculated slope with Coulomb enhancement included in the model has the correct sign and the correct magnitude to within 30%. Using the Coulomb enhanced model, the key spectral features reported as evidence for an excitonic gain mechanism in room temperature CdZnSe quantum well lasers are reproduced.
A liquid contact luminescence (LCL) technique is described. LCL spectral data obtained by current injection through two GaInP quantum well laser wafers are utilized to determine the internal quantum efficiency ratio for the two wafers. This measured ratio is shown to be in good agreement with the internal quantum efficiency ratio for the two wafers as determined from conventional laser slope efficiency vs. cavity length measurements.
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