Modelocking Of Multisegment Semiconductor Lasers At A Repetition Frequency of 10 GHz for high-speed optical communication systems

We report on modelocking behaviour for Merent multisegment lasers grown by SAG-technology. Pulse widths of about 15ps are achieved for active loss modulation. Theoretical investigations with a 'I'LL-model show good correspondence with the experimental results. 1. IN'TRODUCTION Recently much effort has been made to integmte multisegment semiconductor lasers in order to achieve compact, mechanically stable, and high performance devices for high-speed optical communication systems.'-5 Therefore, several technologies have been developed for the fabrication of longitudinal structures consisting of materials with two or even more different bandgaps, which is necessary for an integrated activdpassive coupled cavity modelocked laser with an integrated motidator. In this paper we present experimental results of the modelocking behaviour of different multisegment laser structures grown by selective area growth (SAG) epitaxy. Finally, theoretical investigations based on the transmission line laser model (TLLM)"' which are in strong coincidence with our experimental results have been performed. 2. LASER STRUCTURE AND EXPERIMENTAL SET-UP The multisegment lasers are grown by selective area growth technology using LP-MOVPE. In a single epitaxial growth step various numbers of MQW waveguides with Werent bandgaps can be realized. Measured peak wavelengths of the photoluminescence spectra show that, between the areas forming the passive waveguide and the Bragg sections (at a wavelength of 1440nm) imd the areas of the modulation sections and the gain sections, large wavelength shifts of 80nm and 118nm are iuAieved respectively.' Typical threshold currents are 30mA to 8OmA depending on the laser structure. A more detailed description of the struictures, the growth system and procedures can be found else~here.~.' DiEerent multisegment lasers have been realized for our modelocking experiments, which will be described in the following. The gain section of the laser is driven with a constant current, and a sinusoidal voltage (modulation power approx. 25dBm) is superimposed onto a reverse lbias to the intra-cavity electroabsorption modulator section. In order to get short modelocked output pulses, the modulation fiequency of approx. lOGHz must precisely correspond to the round trip resonance frequency of the laser cavity (length of the laser is approx. 4.2"). The passive waveguides can be used to adjust the modelocking frequency and to reduce the total current. The first order gating of the DBR segment allows the selection of the operation wavelength and narrows the emission to a small spectral width. The optical output of the laser is coupled into a lensed single mode fiber. The pulse width is determined by a fast PIN-photodiode (bandwidth approx. 5OGHz) and recorded with a 5OGHz sampling oscilloscope or by an intensity autocorrelator. The use of an optical spectrum analyzer yields information on the spectral properties of the optical output. The complete experimental set-up for a four segment laser @BR passive waveguide active section modulator) is shown in Fig. 1. ' II AdwSepmant . 25 40P-ke Segment Y, + 5 20\ (3 .i3 a Powermeter Figure 2: Dependence of intensity on the reverse voltage at the loss modulator. The inset show the corresponding laser structure. or