Fast phenomena in semiconductor lasers

Abstract

Although diode lasers are almost ideal sources for ultrahigh-speed data communication systems, system performance remains critically dependent on the quality of the optical pulses that they generate. Uniquely among lasers, the output power can be modulated directly by modulating the diode current. However, this leads to relaxation oscillations and a roll-off at high frequencies that is superimposed on the frequency response of the drive circuit. This is limited by parasitics and maximum modulation frequencies range from 1 to 70 GHz, depending on the type of laser and its packaging. The higher values are obtained with short cavity lengths and tight optical confinement, the highest frequencies being achieved in vertical-cavity devices. Modulation bandwidth is usually limited by circuit parasitics, device heating and the maximum power-handling capability of the laser facets.The generation of picosecond and femtosecond pulses demands special techniques and three-gain switching, Q-switching and mode locking-are discussed in detail, with their relative advantages and disadvantages compared. Very short pulses inherently contain a significant spread of wavelengths and their generation requires the optical gain in the laser medium to extend across that wavelength range. While diode lasers satisfy this criterion better than many other types, the effect of gain non-linearities and carrier-transport effects prevent the Fourier-transform limit from being achieved in practice. As a result, external pulse-compression techniques, which exploit the detailed temporal and spectral properties of the laser pulses, such as frequency chirping, self-phase modulation and group velocity dispersion, are becoming more important, and diode lasers are increasingly challenged as primary sources by compact, efficient, diode-pumped solid-state lasers.The paper summarizes the levels of maximum pulse power and minimum duration that have been achieved using the various techniques.

Wavelength stabilisation during current pulsing of tapered laser

“…5, the different longitudinal modes of the external cavity laser can be clearly distinguished. The broadening of the spectral width compared to CW operation is believed to be caused both by the chirp during the current pulse [7] and deterioration of the beam profile from the back facet of the laser. At 10 W peak power the spectral width is as narrow as 0.04 nm (FWHM).…”

Pulse shortening of gain switched single mode semiconductor lasers using a variable delay interferometer

“…Short optical pulses with duration of tens of picoseconds can be easily generated from Gain Switched (GS) semiconductor lasers [1] and find application in a rich variety of fields [2]. Pulses from GS lasers are intrinsically characterized with a relevant amount of negative frequency chirp, due to the current induced index modulation around the threshold carrier density value.…”

Numerical Analysis of Laser Diode Longitudinally Pumped Solid‐state Laser Generation Dynamics Using Traveling Wave Model

“…But for modelling of generation dynamics the so called point laser model (PLM) is still widely used (see [9,14,16,30,35] and references therein). Due to significant mathematical difficulties traveling wave model (TWM) is rarely used for laser generation modelling [4,8,11,20,32,33]. This model is more often used for numerical analysis of fiber [11,33] and semiconductor [12,20,32] lasers.…”

“…The TWM can be successfully used for short pulse amplification modelling in the multipass amplifiers [5,10,25]. However it should be noted that very often the TWM is used in a simplified version, only during initial stage of problem investigation for setting grounds of common strongly simplified models which are used in subsequent studies [9,18,19,24,27,32].…”