We investigate the high frequency modulation characteristics of mid-infrared surface-emitting ring and edge-emitting ridge quantum cascade lasers (QCLs). In particular, a detailed comparison between circular ring devices and ridge-QCLs from the same laser material, which have a linear waveguide in a "Fabry-Perot (FP) type" cavity, reveals distinct similarities and differences. Both device types are single-mode emitting, based on either 2nd-(ring-QCL) or 1st-order (ridge-QCL) distributed feedback (DFB) gratings with an emission wavelength around 7.56 mum. Their modulation characteristics are investigated in the frequency-domain using an optical frequency-to-amplitude conversion technique based on the ro-vibrational absorptions of CH 4. We observe that the amplitude of frequency tuning Delta f over intensity modulation index in as function of the modulation frequency behaves similarly for both types of devices, while the ring-QCLs typically show higher values. The frequency-to-intensity modulation (FM-IM) phase shift shows a decrease starting from similar to 72 degrees at a modulation frequency of 800 kHz to about 0 degrees at 160 MHz. In addition, we also observe a quasi single-sideband (qSSB) regime for modulation frequencies above 100 MHz, which is identified by a vanishing-1 st-order sideband for both devices. This special FM-state can be observed in DFB QCLs and is in strong contrast to the behavior of regular DFB diode lasers, which do not achieve any significant sideband suppression. By analyzing these important high frequency characteristics of ring-QCLs and comparing them to ridge DFB-QCLs, it shows the potential of intersubband devices for applications in e.g. novel spectroscopic techniques and highly-integrated and high-bitrate free-space data communication. In addition, the obtained results close an existing gap in literature for high frequency modulation characteristics of QCLs.
distributed feedback lasers; free space optics; frequency modulation; laser materials; quantum cascade semiconductor lasers; whispering gallery modes