For the subwavelength grating guided mode resonance filter (GMRF) with a spacer layer, the drift mechanism of reflective phase was analyzed, and the relation between the optical wave phase and the spectral linewidth was investigated with the planar waveguide theory and the principle of guided mode resonance. On this basis, the super narrow linewidth resonator with the wavelength of 852 nm is designed. The simulation results show that the larger slope of phase as a function of wavelength (dφ/dΛ) becomes in the resonant range, the narrower linewidth (Δλ) is shown in the reflection spectrum. Furthermore, the energy of electric field concentrated in the waveguide layer is also higher in the corresponding field distribution. Different structural parameters have different effects on reflection characteristics. As the thickness of the grating layer decreases, the slope of the phase (dφ/dλ) increases from 1.0419 to 12.2666, and the linewidth decreases by 90%. With the increase of spacer layer thickness , the phase slope (dφ/dλ) increases from 2.04 to 7.15, and then the Q (Q≈λ0/Δλ) value goes from 1058.382 to 5174.393. The variation of the period of the subwavelength grating mainly affects the location of the resonance center wavelength, but has little effect on the rate of phase swing. By optimizing these parameters, the coupled resonant cavity with a narrow linewidth 0.002nm is designed, and the Q reaches 4.26×105 .
We have designed and investigated a stable polarization microelectromechanical system (MEMS)-tunable vertical-cavity surface-emitting lasers (VCSELs) at wavelength around 980 nm with an inter-cavity subwavelength grating (ISWG). An ISWG is etched on the top of the “Half-VCSEL” consisting of the active semiconductor and the bottom mirror to select the polarization mode during the whole tuning range. The rigorous coupled wave analysis (RCWA) had be used to design and optimize the parameters of the ISWG including index, thickness, period and duty cycle to achieve a high reflectance for the desire polarization and a lower reflectance for the other polarization. Then we calculated the total reflectance of ISWG, air gap and top DBR using transmission matrix method. By designing the parameters of ISWG, we can ensure that the fundamental mode has a higher reflectance, or lower threshold current than the other modes. After calculation, we have gotten the parameters as follows. The index of the ISWG and the substrate is 3.58; thickness is 300nm; period is 500 nm; duty cycle is 0.5. The reflectance of ISWG is 0.985 for the TM polarization and 0.382 for TE polarization. The total reflectance of the whole structure is 0.995 for TM polarization and 0.902 for TE polarization in the tuning range. The difference of the reflectance between two polarization modes can select the polarization mode of lasing. This structure can achieve a tuning range of 30nm for TM single polarization.
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