Characterization of 60GHz quantum well passively mode-locked Fabry-Perot laser for RoF and WPAN applications

Summary form only given. As the frequency spectrum below 10GHz becomes saturated, it will become increasingly difficult to maintain adequate bandwidth provision for the growing customer demand. For this reason, the possibility of utilizing the unlicensed 57GHz-64GHz band for services such as WPAN is an attractive prospect [1]. Furthermore, the advantages of an optical system to realize this prospect are numerous. Extremely low phase noise can be obtained from laser systems used for millimetre-wave applications, relative to the equivalent electronic components. In addition to this, the costs of such a system are much lower and the footprint smaller, not to mention the additional benefits of low loss and easy switching of carrier frequency. In this paper, a multimode quantum well Fabry-Perot laser with potential application as a carrier signal generator for WPAN is characterized. The laser is passively mode-locked, with no saturable absorber, and exhibits pulsed operation at 60GHz with only d.c. bias applied. The InP-based laser is 700μm long and exhibits multimode emission between 1555nm and 1565nm. A typical output spectrum at 70mA bias current is shown in Fig. 1. Also indicated on the graph are the measured values for optical linewidth for each mode. These values were measured using an optical self-heterodyne setup.The linewidth of the beat tone generated at a photodiode was also measured for various values of bias current. In free-running conditions, a linewidth of 1.7MHz is measured at 61.92GHz. A circulator and an optical delay line are then placed after the laser in the setup, in place of the isolator, thus creating an optical feedback loop. A filter is used to isolate two consecutive modes. The linewidth measured for the beat signal is reduced by a factor of 100, to nearly 10kHz, with lower linewidths being for longitudinal modes near to the centre of the optical spectrum. These results indicate that this laser has potential for application in RoF and- WPAN areas, due to its low linewidth, cost and the fact that it exhibits 60GHz operation using only d.c. bias.