@conference{
author = "Vuković, Nikola and Daničić, Aleksandar and Radovanović, Jelena and Milanović, Vitomir and Boiko, D. L.",
year = "2015",
abstract = "Quantum Cascade Lasers (QCLs) producing picosecond pulses of high peak power will find numerous novel applications for time-resolved MIR spectroscopy, free space communication, remote atmosphere sensing etc. The short upper state lifetime (~1 ps) in QCLs prohibits passive mode-locking or Q-switched operation because the gain recovers much faster than the pulse repetition rate. Gain switched pulse production has been attempted yielding 120 ps pulse width [1] and active mode-locking was achieved in QCL utilizing diagonal transition [2]. Unfortunately, diagonal transition rendered QCL operation temperature out of the practical use. Experiments reported in [3] evidence that some QCL structures exhibit features of multimode Risken-Nummedal-Graham-Haken (RNGH) instabilities [4,5] at low excess above lasing threshold (pth2<1.1). The onset of such RNGH instabilities and excitation of regular self-pulsations may provide practical means to produce picosecond pulses in the MIR spectral range. Conventional RNGH analysis [4,5] predicts that the second threshold pth2 is in the range of 9 to 16 times above lasing threshold, does not take the spatial grating of population inversion into account and barely agrees with experiments in unidirectional ring fiber lasers [6]. Here we consider Fabry-Perot (FP) cavity QCLs and conventional semiconductor quantum well (QW) LDs. Like in reference [3], we take into account the effect of the induced grating of population inversion, but we also incorporate the grating of the medium polarization and its relaxation due to carrier diffusion. This allows us to explain low second threshold in QCLs without inducing the ambiguous assumption of Ref. [3] of a built-in saturable absorber and Kerr lensing-effect in the waveguide of QCL. We further elucidate the importance of unipolar diffusion in QCLs and ambipolar diffusion in conventional LDs: without diffusion, the second threshold would be too low and independent of the lasing wavelength. Time domain simulations utilizing travelling wave rate equation model of Ref. [7] show irregular self-pulsations, in agreement with experiment in [3]. Shortening of the cavity length down to L=100μm results in the increase of the second threshold up to 2.3-2.5. Above RNGH threshold, steady state regular self-pulsations can be established, preceded by a high-amplitude burst, which might be attributed to Dicke superradiance (SR) [8]. Very high pth2 in conventional LDs is predicted by the model, attributed to ambipolar diffusion. Just like QCLs above the RNGH threshold, FP cavity LDs would exhibit SR emission burst followed by regular RNGH self-pulsations.",
publisher = "Belgrade : Vinča Institute of Nuclear Sciences",
journal = "PHOTONICA2015 : 5th International School and Conference on Photonics and COST actions: MP1204, BM1205 and MP1205 : book of abstracts; August 24-28, 2015; Belgrade",
title = "Multimode RNGH instabilities of Fabry-Perot cavity QCLs: Impact of diffusion",
pages = "115-116",
url = "https://hdl.handle.net/21.15107/rcub_vinar_10956"
}
