Study Of Gain-coupled Distributed Feedback Semiconductor Lasers Based On Surface Periodic Current Injection

The 21st century is an age of information.The rapid development of information industry cannot be separated from the comprehensive development of microelectronics technology,optoelectronics technology,communication technology,computer science and technology,automation,precision machinery and other science and technology.As an important component of microelectronic devices,distributed feedback(DFB)semiconductor laser has gradually become an indispensable light source in the fields of optical fiber communication,medical treatment,material processing and other daily life because of its characteristics of high yield,high photoelectric conversion efficiency,narrow line width,easy monolithic integration,and direct tuning.Traditional indexcoupled DFB semiconductor lasers with perfect antireflection coated facets have the intrinsic drawback of two degenerated modes symmetric with respect to the Bragg frequency.Although the single-mode and narrow linewidth laser output can be achieved with the introduction of ?/4 phase-shift grating,the uneven carrier distribution inside the laser will lead to spatial hole burning effect,which greatly affects the output characteristics of the laser.Another solution is gain-coupled DFB semiconductor laser.Its main advantage lies in the high Gain compared,single mode high yield and high stability caused by the standing wave effect,but the traditional GC-DFB semiconductor lasers requires high precision grating etching and secondary epitaxial growth fabrication steps,which makes the production cost of laser and difficulty increased.So mass production and more widely application is difficult to realize.Therefore,this paper innovatively proposes the common i-line lithography technology and plasma etching technology,using a relatively simple chip manufacturing process,to design and prepare 1045 nm purely gain-coupled DFB semiconductor laser,achieving low threshold,high efficiency,and high stability of single-mode laser output.Because the internal gain coupled effect coefficient of purely gain coupled laser is small,the internal F-P effect of the laser is significant,which will caused phenomenon of mode competition.Strong mode competition will lead to the unstable output mode of the laser and the phenomenon of multi-mode output.In order to reduce the F-P effect inside the laser,a 795 nm purely gain coupled semiconductor laser based on tilted waveguide is innovatively proposed.Through the establishment of physical model,simulation and devices,we verify the relationship between tilted waveguide gain-coupled laser diode output characteristics and corresponds tilted angle from the experiment and theoretical analysis.By changing tilted angle,tunable properties can be achieved.The results have important significance for future design of tilted waveguide structure.The specific research content and results are as follows:(1)Establish the physical model of gain-coupled semiconductor laser.Based on coupled mode theory and transfer matrix method,the internal principle of laser is analyzed and simulated by commercial software such as COMSOL Multiphysics,Rsoft and Matlab.The internal carrier distribution,laser gain curve,coupling coefficient and optical field distribution are simulated.According to the result of simulation,we optimize device structure scheme and laser fabrication parameters.(2)The design and fabrication of periodic surface electrode to realize 1045 nm purely gain coupled distributed feedback semiconductor laser.The fabricated device realized carrier periodic distribution within the active region by current periodic injection,then the larger gain contrast comparing with conventional DFB lasers can be realized.Meanwhile imaginary part of refractive index was modulated and purely gain coupled mechanism was realized.The threshold current of the device is 40 m A,and the output power of the device at 240 m A is 51.9 m W(HR and AR coatings),which is much higher than that of the single mode gain coupled DFB semiconductor laser reported in the literature(the single facet output power is 15.42 m W).The slope efficiency reaches 0.24 W/A,which is more than twice that of the Ti surface grating gain coupled DFB semiconductor laser(about 0.11 W/A).In the single mode range,the maximum SMSR is more than 35 d B,and the narrowest linewidth is 1.12 pm,which is much lower than the structure of the lateral coupled grating(about 160 pm)and the high order surface grating(linewidth is less than 40 pm)gain coupled distributed feedback semiconductor laser reported in the literature.The device adopts almost the same process flow as the Fabry-Perot laser,which greatly simplifies the fabrication method of the gain coupled DFB laser.Thanks to their simplicity and low cost,the new scheme we proposed has great potential to be widely applied in commercial DFB products.(3)Design and set up the physical model of tilted ridge waveguide.By changing tilted angle of ridge waveguide,the effective grating period is changed,also corresponding Bragg wavelength is changed.Through COMSOL Multiphysics software,we set up corresponding physical model and simulated.According to simulation results,the relationship between tilted angle and laser cavity surface reflectance was obtained.Then we calculated cavity facets loss brought by the tilted ridge waveguide,and further analyze the effect of laser performance.Unlike traditional DFB laser,output characteristics of tilted ridge waveguide purely gain coupled distributed feedback semiconductor laser are related to cavity surface reflectance: the larger tilted angle is,the smaller cavity surface reflectivity is,which means that the output peak power will decrease with the increase of titled angle,while the threshold current will increase with the increase of titled angle.(4)In order to verify the influence of titled ridge waveguide on laser output characteristics,we finish the fabrication of titled ridge waveguide purely gain coupled DFB semiconductor laser.By designing different tilted angles we realize different effective grating periods,which means different grating periods correspond to different Bragg wavelength.So that tunable characteristics of the laser are realized.The device is cleaved into a cavity length of 2 mm and contains five designed tilted angles of 0°,0.39°,1.86°,2.60 ° and 3.65° respectively.The test results at 20? show that the peak output power of laser fabricated of the five titled ridged waveguides are all more than30 m W,and the side mode rejection ratio of the output spectrum are more than 30 d B.The wavelength coverage ranges a total of 8.656 nm,from 789.392 nm to 798.048 nm,covering the absorption peak of the rubidium pump.As we expected,the results show the same conclusion with simulation results: the output peak power decreases with the increase of tilted angle while the threshold current increases with the increase of tilted angle.In addition,the test results show that the tilted ridge waveguide can reduce the drift coefficient of the laser wavelength with the current which means the employment of titled ridge waveguide will improve the stability of the laser.All of the purely gain coupled distributed feedback semiconductor lasers mentioned in this paper require only common i-line lithography technique and avoid currently nanoscale precision lithography or secondary epitaxial growth technique.They have great advantages of simple fabrication steps,high yield and consistency,low production cost,and easy to mass production and repeatability.In this paper,surface periodic electric injection gain coupled distributed feedback semiconductor lasers we designed have great performance parameters and meet the application requirements such as military and national defense,industrial production and processing,optical communications,medicine and cosmetology,and other application fields.