Longitudinal mode competition and mode clustering in (Al,In)GaN laser diodes

Abstract

Longitudinal mode competition in (Al,In)GaN laser diodes at λ = 445nm and 515 nm with mode competition frequencies from 10 MHz to 150 MHz is observed. Up to two dozen lasing modes oscillate with the lasing mode rolling from the short wavelength edge to the long wavelength edge of the gain profile. The experimental results can be described very well with a set of multi-mode rate equations including self-, symmetric and asymmetric cross gain saturation. By tuning essential parameters of the gain saturation terms, mode competition disappears and single mode operation as well as mode clustering is found. This proves that the mechanisms of gain saturation have not only a profound impact on the complex temporal-spectral behavior but also explains mode clustering in (Al,In)GaN laser diodes, both in pulsed and continuous wave (cw) operation as a natural nonlinear effect without the necessity to add noise.

AlGaInN laser diode technology for free-space and plastic optical fibre telecom applications

“…The single mode characteristics of a ~422nm GaN laser diode is measured at 24mW operation , a dominant single longitudinal mode at 421.6nm, with multiple small side modes is observed (see left hand side of figure 2). Similar single longitudinal mode characteristics has also been observed in the spectral output of other AlGaInN laser diodes and was explained by surface roughness inadvertently introduced during growth 3 and that the single mode is stabalised by longitudinal mode competition caused by optical gain saturation 4 . Similarly we observe a surface topology of the order of ~10nm in height and a periodicity of 100nm 5 , eventhough the epitaxy growth is done on very low defectivity (<5x10 4 cm -2 ) GaN substrates with a flatness of <0.1nm 6 , the surface topology features appear inadvertantly in the last epitaxy layer of growth.…”

AlGaInN laser diode technology for systems applications

“…At a higher drive current of 300mA (36mW) the dominant single longitudinal mode jumps to a spectrally wide (~1-2nm) mode comb as is more typical of a Fabry-Perot LD device (see fig.3). Similar single longitudinal mode characteristics has also been observed in the spectral output of other AlGaInN laser diodes and was explained by surface roughness inadvertently introduced during growth 4 and that the single mode is stabalised by longitudinal mode competition caused by optical gain saturation 5 . Similarly we observe a surface topology of the order of ~10nm in height and a periodicity of 100nm, eventhough the epitaxy growth is done on very low defectivity (<5x10 4 cm -2 ) GaN substrates with a flatness of <0.1nm 6 , the surface topology features appear inadvertantly in the last epitaxy layer of growth.…”

Free-space and underwater GHz data transmission using AlGaInN laser diode technology

“…The single mode characteristics of a ~422nm GaN laser diode is measured at 24mW operation , a dominant single longitudinal mode at 421.6nm, with multiple small side modes is observed (see left hand side of figure 2). Similar single longitudinal mode characteristics has also been observed in the spectral output of other AlGaInN laser diodes and was explained by surface roughness inadvertently introduced during growth 3 and that the single mode is stabalised by longitudinal mode competition caused by optical gain saturation 4 . Similarly we observe a surface topology of the order of ~10nm in height and a periodicity of 100nm 5 , eventhough the epitaxy growth is done on very low defectivity (<5x10 4 cm -2 ) GaN substrates with a flatness of <0.1nm 6 , the surface topology features appear inadvertantly in the last epitaxy layer of growth.…”