Nonlinear Dynamics in Semiconductor Lasers - Abstract
Short optical picosecond pulses in the near-infrared range generated by diode lasers can be used for many applications like THz-scanning systems, medical technology and free space communication. A well-established method to generate picosecond pulses is passive mode-locking. Particularly short pulses can be obtained with a colliding-pulse mode-locked (CPM) laser where the saturable absorber (SA) is placed exactly in the center of the cavity surrounded by two gain sections. Further pulse shortening could be achieved by a broadening of the spectral width of the amplifying medium. To accomplish this, we investigated lasers with a double quantum well (DQW) as active layer, where the compositions x of the two InxGa1-xAsyP1-y quantum wells (QWs) differ by Dx. We determined gain spectra by fitting measured subthreshold amplified spontaneous emission (ASE) spectra of Fabry-Perot ridge-waveguide (RW) lasers. Absorption spectra were obtained from ASE spectra measured on RW devices having segmented contacts. In agreement with numerical simulations the laser with a composition difference of Dx = 0.08 (chirped DQW) has a much broader gain spectrum than the laser with unchirped QWs (Dx = 0). The CPM-RW lasers under study are 6 mm long and have a SA with a length of LSA = 200 µm. We obtained shortest full width at half maximum of the autocorrelation function of 3.2 ps and 1.9 ps for the CPM lasers with Dx = 0 and Dx = 0.08 (gain current Ig = 140 mA, absorber voltage USA = -1.0 V), respectively.