Study on the Optimization Design of Diffraction Grating for DFB Lasers and the Fabrication Process Using Nanoimprint Lithography
|Huazhong University of Science and Technology
|Distributed Feedback laser Nanoimprint Lithography Multiple Phase Shift Corrugation Pitch Modulated Single Longitudinal Mode Asymmetric Output
Distributed feedback (DFB) semiconductor laser is the mainstream light source of modern optical network because of its promising single longitudinal mode (SLM) performance. To overcome the drawbacks of conventional χ/4phase shifted DFB laser, in which SLM deteriorates rapidly due to spatial hole burning effect and power wastes because of the symmetric structure, this dissertation aims to design complex diffraction gratings for high performance DFB lasers and to fabricate them by nanoimprint lithography (NIL) because of its high resolution, low cost and high throuthput.First the development history of different kinds complex gratings and their merits and demerits are summarized systematically. Multiple phase shift (MPS) and multiple section corrugation pitch modulated (CPM) gratings are consider to be the best candidates for future low cost and high performance DFB laser since their great potential and suitable for NIL processes. Using transfer matrix method the MPS and CPM gratings are optimized completely for high performance DFB laser, including symmetric2PS, asymmetric2PS, asymmetric5PS,1CPM and3CPM gratings. The final optimized5PS-DFB laser presents normalized marginal gain (△athL)of1.0and field flatness factor (F) of0.01at threshold, and△αthL=0.77under even five times threshold current injection, far above the similar structure results reported in literatures. The proposed3CPM-DFB laser gives optimized results of△αthL=0.97and F=0.01, and△αthL=0.85under even five times threshold current injection. This demonstrates its wonderful SLM feature. The effects of phase error and modulated pitch error on the laser performance are also analyzed.To improve the output efficiency while maintaining good SLM performance of the DFB laser, asymmetric MPS and multiple section CPM gratings are proposed. Above threshold analysis of the optimized asymmetric3CPM-DFB laser shows that not only bigger ratio and more stable SLM performance can be obtained, but bigger power output at one facet compared with the results of literatures can be gained under the same bias current. This in essence increases the external quantum efficiency of the laser.The key processes of fabricating diffraction gratings for DFB lasers by NIL are also studied. Using these well developed techniques, dual symmetric χ/8phase shifted DFB laser is manufactured successfully. The test spectrum of the laser is well consistent with the simulated one, revealing the dual symmetric λ/8phase shifted grating is fabricated by NIL precisely. To show the versatility of NIL, eight different wavelength dual symmetric λ/8phase shifted DFB lasers on this single chip are also demonstrated through one single process procedure by slightly varying the period of adjacent gratings when the mold was fabricated by electron beam lithography.