| With various advantages such as low power consumption,low cost and good single mode quality,VCSEL has been widely applied in the optical communication system.As technology matures,VCSEL applications diversified including laser mice,printers,medical imaging and so on.In recent years,since Apple revealed to the entire world the new iPhone X with remarkable 3D sensing technology based on VCSELs,VCSEL technology for 3D sensing has been attracting more and more interests from enterprise and research institutes.However,the new application scenarios also pose several major challenges to the traditional VCSEL technology.Firstly,with the rapid development of Internet technology,access users' demand for network bandwidth has increased by more than 70%year by year.At the same time,the transmission rate of enterprise data centers is moving towards 400 Gb/s Ethernet.The direct modulation speed of a conventional VCSEL is limited by the relaxation oscillation frequency(20 Gb/s),which cannot meet the above requirements.What's more,some possible medium-distance(10 km level)application scenarios,such as the optical transmission modules in the 6th generation mobile communication system(6G),pose a challenge on how to extend the transmission distance of VCSELs.Finally,while the output power of a traditional single-mode VCSEL is usually limited to a few mW due to its small aperture,3D sensing technology has high requirements on the accuracy,power and scan rate of the laser source.Taking lidar technology as an example,in order to achieve a sensing range of 100-200m,the light source needs to provide 100W pulse peak power.Therefore,increasing the output power of the VCSEL has become another problem to be solved urgently.In order to cope with the above-mentioned challenges,with utilizing the advantages of VCSEL such as easy to integrate,excellent single-mode performance and low manufacturing cost,this thesis focuses on the research of achieving high-speed direct modulation,external modulation,high-power optical amplification and high-speed optical scanning based on technologies including lateral integration of VCSEL,single-mode control,optical feedback and optical amplification.The main research content of this thesis is summarized as follows:1.Research on single-mode selection and lateral coupling mechanism based on slow-light transmission in VCSEL structureIn order to solve the problems of low power and unstable lateral coupling in a single VCSEL,this section studies the lateral coupling and single-mode selection mechanism based on the slow-light transmission in VCSEL structure.(1.1)Two lateral coupling mechanisms are proposed.One is based on taper-shaped-bottleneck design,in which the lateral coupling is achieved through a width difference between two cavities.The effective refractive index of the narrower cavity for the transverse mode is smaller due to coupling mode theory of waveguide.This design forms an anti-index waveguide structure which enables a lateral leakage mode from laser to the external cavity.The other one is based on wavelength detuning design.A resonant wavelength shift of the laser is achieved through inserting a phase control layer.The new structure offers a large wavelength detuning which should be capable of achiving stable unidirectional coupling.Simulation and experimental results show that this design can provide wavelength detuning over 10nm,resulting in stable(40%coupling efficiency)unidirectional optical coupling.(1.2)A mechanism of loading a Bragg grating on the surface of a long cavity VCSEL to realize the single-mode high-power slow-light lasing of the VCSEL is proposed.The proposed slow-light VCSEL has the same vertical structure as the conventional oxide-confined VCSEL except that the top surface is periodically etched to form lateral Bragg-grating mirrors.Similar to a DFB edge emitting laser,a single slow-light mode nearly at the Bragg-wavelength can be selected due to the optical feedback in the grating.Experimental results show that our proposed mechanism can effectively increase the output power of a single VCSEL(more than 80mW single-mode power under continuous wave(CW)operation).2.Research on direct modulation and external modulation based on coupled cavity VCSEL utilizing lateral optical coupling and optical feedback.In this part of the thesis,in order to solve the two major contradictions(the contradiction between the increasing demand for data traffic and the limited VCSEL modulation rate and the contradiction between the increasing demand transmission distance and the rather large VCSEL modulation chirp),we propsed a direct modulator and an external modulator based on our coupled cavity VCSEL.(2.1)In order to break the limitation of the direct modulation bandwidth brought by the relaxation frequency in conventional VCSEL,we have introduced a lateral coupling external cavity in the traditional VCSEL structure to achieve optical coupling and optical feedback.The photon photon resonance effect introduced by optical feedback helps to break the limitation of direct modulation bandwidth of VCSEL.Two models(Lang-Kobayashi model and Coupling Mode model)are used for simulations.Explanations and discussions on the simulation results are also presented.Theoretical results indicate that the 3-dB bandwidth of our new VCSEL structure can be increased by two times in comparison to that of directly modulated conventional VCSEL.At the same time the chirp reduction is clearly shown in the models due to the increased effective differential gain.Experimental measurements are also carried out on our TCC-VCSELs.The experimental results on bandwidth enhancement(23 GHz)and chirp reduction agree with our simulation.(2.2)As another method to increase the modulation bandwidth of VCSELs,we discussed the possibility of high-speed external modulation based on coupled cavity VCSELs.The slow light in the modulator enables us to reduce the modulator length as small as 30 ?m,which makes a great impact for increasing the modulation speed.Through experiments,we have achieved a small signal modulation bandwidth of 15 GHz.In the experimental measurements,high peak appears in the small signal response when the fiber is put above the laser.The high peak is assumed to be brought up by modulation noise from the laser side due to thermal lensing effect.The high peak in small signal response can be utilized to generate short pulse.We successfully get a 50 ps pulse through the gain switch method.3.Research on integrated solid-state beam scanner based on the combination of optical amplification and optical wavelength scanning in coupled cavity VCSELIn this part of the thesis,in order to solve the exsiting problems such as high cost,low scanning speed and accuracy in the existing 3D scanning technology,we propose and systematically study an integrated solid-state beam scanner.In our proposed structure,the seed light with a detuned wavelength from the lasing wavelength of the external cavity is laterally coupled into the external cavity.When the waveguide is pumped above threshold,amplification of the coupling light along the waveguide can be achieved.The amplified output is getting larger and larger with increasing the device length and the corresponding beam divergence is getting smaller and smaller.Large angle beam steering can be achieved by a scan through the input wavelength since the slow light waveguide can provide large dispersion near the cut-off wavelength.We can modulate the wavelength of the seed light to achieve large-angle light scanning.In the experiment,by applying a sinusoidal modulation current on the seed VCSEL,we were able to measure the dynamic response of the beam steering.Experimental results show that our proposed structure can achieve high-speed,highprecision,high-power optical scanning functions with a scanning frequency exceeding 10 kHz,an optical divergence angle less than 0.1°,and a single-mode power exceeding 1 watt under the condition of pulse current injection. |
PDF Full Text Request