In order to improve the performance of the slotted laser, we have modelled and fabricated lasers with gratings having 2,3 and 4 periods.
The measured laser thresholds and slope efficiencies against the device-count and their respective normal distribution for 2, 3, and 4 period grating lasers are shown above. The threshold peak for the 2, 3 and 4 period lasers were around 38, 41 and 59 mA respectively and the slope efficiency peak for the 2, 3 and 4 period lasers were around 0.031, 0.028 and 0.027 mW/mA respectively. It is evident that the performance of these devices degrades as the number of periods are increased, with the 2 period grating laser performing the best. The slope efficiency measured for most devices is low because the measured fabricated devices were only cleaved and not HR/AR coated.
The slope efficiency of the device is strongly related to the transmission amplitude of the grating section. Shortening the front section by reducing the number of slots reduces the transmission losses, hence increases the slope efficiency of the device. SMSR degrades as the number of slots are reduced. Hence, there is a trade-off between higher slope efficiency and a higher SMSR Results show that for a 2 period slotted laser, as the number of slots reduces from 24 to 18, the simulated SMSR reduces by ~ 4 dB. On the other hand, as shown a simulated slope efficiency of as high as 0.2 mW/mA can be achieved which is closer to the performance of DFB lasers. Hence, the performance of these devices can be significantly improved by reducing the number of slots while keeping the SMSR within the industrial acceptable limit.
Reference
Design Optimization for Semiconductor Lasers With High-Order Surface Gratings Having Multiple Periods. Jain, Gaurav; Wallace, Michael J.; McKenna, Robert; Brazel, Kevin; Bello, Frank; Lu, Qiaoyin; Guo, Weihua; Donegan, John F. 2018 Journal of Lightwave Technology 36(22) 5121-512.
