In two-dimensional photonic crystals, the photonic bandgap can modulate electromagnetic waves with corresponding wavelengths. Photonic crystal surface-emitting laser (PCSEL) realizes the vertical emission of laser beam through the inplane resonance and optical feedback, and significantly overcomes typical problems of the traditional semiconductor laser, such as large divergence angle, elliptical beam and susceptibility to higher-order modes. Therefore, by designing the lattice structure and materials of photonic crystal appropriately, the photonic bandgap in different polarization states can be regulated to produce high performance laser beams. Therefore, to obtain a deeper understanding of the impact of photonic crystal structure on the output properties, we demonstrate that by simulating the energy band structure near the Γ2 point, mode B has a wavelength closer to the emission wavelength. Subsequently, we analyze the influence of lattice structure, number of air holes and symmetry degree on the photon energy band distribution of TE mode. At last, two types of lattice structures with different symmetry degrees, i.e., rhombic lattice and bullet-like lattice, respectively, were prepared using electron beam lithography. The full widths at half maximum (FWHMs) of photoluminescence spectra of these two photonic crystals were detected to be 73 nm and 53 nm, respectively, which verified that the reducing lattice symmetry is beneficial for decreasing the FWHM. In addition, the symmetry reduction is favorable to eliminate the photon energy band degeneracy, leading to a blueshift in wavelength. Our research provides theoretical design insights for achieving high-performance PCSEL lasers.
Based on the analysis of Talbot phase-locking theory of edge emitting semiconductor lasers, a method to obtain a single in-phase mode on a tapered laser chip is proposed. A phase-locked model with 1/2 Talbot spatial filter cavity for mode selection placed between 8 emitters on each facet is set up. Based on the mode coupling rate equation theory, the parallel coupling phase-locking conditions with different fill factors is analyzed. The results show that the stable parallel coupling phase lock can be achieved for 8 emitters with the pitch of 20 um, when the fill factor is set between 0.06 and 0.12, and the phase-locking time is about 3 ns. The supermode threshold gains are also calculated under different fill factors. In the phase-locked model, when the fill factor is approximately 0.1, the threshold gain difference between the in-phase mode and out-phase mode could reach the maximum, which is around 78cm-1 . Therefore, single in-phase mode output of this novel laser with Talbot cavity becomes more robust. The simulation analysis provides a reliable theoretical support for the preparation of a coherent array laser with a single in-phase mode output.
Semiconductor laser is partially coherent beam, while the beam quality factor is based on fully coherent beam. The Wigner distribution function for partially coherent beam is used to analyze the semiconductor laser beam. The Wigner distribution function contains both spatial information and spatial frequency information in the phase space. A method for measuring the Wigner distribution function of semiconductor laser is reported. The intensity distribution of the beam caustics is measured by two focusing mirrors, and the Wigner distribution function of semiconductor laser is reconstructed. Based on the reconstructed Wigner distribution function, the light intensity of semiconductor laser is simulated. The simulated data are in good agreement with the experimental data. Through the properties of Wigner distribution function, the wavefront aberration and coherence of semiconductor laser are analyzed. The wavefront of semiconductor laser is symmetrically distributed around a point, and the wavefront on the left side of the laser diode array is larger than that on the right side. Due to the temperature difference of the laser chip, the coherence on both sides of the laser diode array is better than that in the middle of the laser diode array.
The band structure, density of states, optical properties, effective masses and loss function of AlxGa1−xAs and InyGa1−yAs were performed by the first-principles method within the local density approximation. The calculated direct band gap of the AlAs, Al0.5Ga0.5As, GaAs, In0.5Ga0.5As and InAs were 1.608 eV, 1.34eV, 1.02eV, 0.646eV and 0.316eV at G point, which were direct bandgap semiconductor materials. In addition, dielectric functions, the absorption function, refractive index, loss function and effective mass were analyzed in detail. The effective masses of AlxGa1−xAs and InyGa1−yAs were small, so they have high carrier mobility. These results make them to be promising candidates for future electronics.
GaN-based high-power laser diodes (LDs) have attracted tremendous interests in next-generation lighting applications, such as laser display, car laser light. However, high injection current usually brings inevitable drawbacks, including the well-known efficiency droop, Auger recombination and self-heating which obstruct further improvements of GaN-based optoelectrical devices. In this paper, influence of hole overflow at high injection current in an asymmetric GaN-based highpower blue LD has been comprehensively investigated and successfully suppressed by employing a new sandwiched GaN/AlGaN/GaN lower quantum barrier (GAG-LQB). Systematical simulations and measurements of structural and optical properties are carried out. As a result, the V-shaped defects induced by thick n-InGaN waveguide layer are apparently eliminated, which provides a more growth-friendly platform for deposition of the rest epitaxial layers and thus a better crystalline quality is obtained. On the other hand, the modified LD exhibits better photo-electrical properties with slope efficiency (SE) increasing from 0.98 to 1.24 and wall-plug efficiency (WPE) increasing from 18.7% to 20.5% at a high current of 1.5 A and no obvious efficiency droop is observed at a current as high as 2 A compared with the conventional one, because the middle-inserted AlGaN layer could form an extra barrier on the valence band to weaken the hole overflow and enhance the radiative recombination. Furthermore, the in-plane compressive strain induced by InGaN quantum wells (QWs) is also partially compensated by the tensile strain induced by the AlGaN layer. Therefore, the piezoelectric fieldinduced polarization is effectively alleviated and the wavelength blueshift is reduced from 7 nm to 1.6 nm.
Reliability and characterization of 850 nm 50 Gbit/s PAM-4 vertical-cavity surface-emitting lasers (VCSELs) are presented. These VCSELs have demonstrated a threshold current of 0.8 mA and a slope efficiency of 0.95 W/A. The maximum optical output power of 9 mW is achieved at a thermal rollover current of 13.5 mA. The optical power is up to 5 mW and the -3dB bandwidth is in excess of 17 GHz at 25°C and 6 mA bias. The current density and power dissipation density are low to 15 kA/cm2 and 25.5 kJ/cm2 , respectively. The standard deviations of photoluminescence peak wavelength and Fabry-Perot cavity wavelength of epitaxial wafer are 0.75 nm and 2.2 nm, respectively. After 1500 h of the reliability study no degradation or failures of the 22 VCSELs are observed at 80°C in a heating chamber at a bias of 6 mA. Considering high optical absorption of DX center, the impurity doping concentration of 3 pairs of N-DBRs that were adjacent to active region are optimized. The additional SiO2 passivation layer not only can provide moisture resistance but also provide a photon lifetime tuning. The output power increases by optimizing thickness of SiO2 layer reducing power dissipation density. Single thin oxide aperture is employed by slowing down the oxidizing rate and improving temperature during a VCSEL oxidation process to thereby reduce stress concentration of an oxidation. Single thin oxide aperture may limit the -3dB bandwidth, but the modulation characteristics can be improved by adopting advanced modulation techniques such as 4-level pulse amplitude modulation (PAM-4).
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