100-Gb/s coherent systems based on polarization-division multiplexed quadrature phase shift keying (PDMQPSK), with aggregate wavelength-division multiplexed (WDM) capacities approaching 10 Tb/s per fibre, are being widely deployed due to the benefits provided by coherent detection. The stringent linewidth requirements for lasers used in these systems only allow optical sources with certain phase noise characteristics to be employed. A major challenge is therefore to produce lasers with the requisite performance at low cost. Discrete mode laser diodes (DMLDs) can be designed for narrow linewidth emission and present an economic approach with a focus on high volume manufacturability of monolithic semiconductor lasers. In this paper the performance of a 112 Gb/s long-haul optical transmission system employing PDM-QPSK is investigated using a range of transmitter lasers with linewidth values ranging from 100 kHz to 5 MHz. A linewidth of 100 kHz was obtained from an external cavity laser (ECL) and linewidths ranging from 200 kHz to 5 MHz were obtained from DMLDs. The performance of the system is analysed through experimental measurements and simulations performed by Virtual Photonics Incorporated Transmission Maker (VPI™). Results are presented for back-to-back operation and after transmission through G.654 pure silica core fibre (PSCF) at distances up to 6930 km.