In this paper we investigate and explore the stability and operating regime of modelocking (ML) in 4mm long Fabry-Perot type lasers, corresponding to a roundtrip frequency of 10GHz. The devices are fabricated on InAs/InP quantum dot material, operating at wavelengths around 1.5um, and are HR-coated at the absorber side. In order to find the stable ML region of operation in these devices, we have performed sweep-scans on the injection current of the gain section, and the reverse bias voltage on the absorber section. We will present the optical and electrical spectrum of devices with different absorber length. These results will be compared with the performance of earlier devices without HR coating