Novel DFB laser structure suitable for stable single longitudinal mode operation

Abstract

This paper shows that a suitable design of a single phase-shift (1PS) distributed coupling coefficient (DCC) distributed feedback (DFB) laser structure can strongly improve both the normalized mode selectivity S and the flatness of the electrical field profile of DFB lasers—usually quantified by a flatness parameter F —in order to ensure the required criteria for single longitudinal mode operation ( S ≥ 0.25 and F ≤ 0.05 ) through an extended range of biasing current injection ( I). It is shown that a symmetric longitudinal laser cavity should be used in order to accomplish the requirements imposed by the modern optical communication systems. Photon and carrier rate equations have been used to evaluate the performance of the proposed laser structure in the above-threshold regime. The variations of the selectivity, the flatness, the lasing-wavelength, the emitted power ( P) and the side-mode suppression ratio (SMSR) versus current have been assessed. For I=5 I th , where I th is the laser threshold current, substantial improvements in S (60% higher), in F (40% better), in P (45% higher) and in the SMSR (about 4 dB higher) are achieved in the proposed 1PS-DCC-DFB laser when compared to similar single phase-shifted DFB structures referred elsewhere. The improvements are even better when compared to the standard QWS-DFB laser.