In this section, an experimental construction on the triple microrings laser configuration is presented. To make the material of these structures, 8 mg of Rhodamine-B as a gain material is solved with 16 g of SU-8. After making the material, it is time to spin coat this material on a layer of silicon dioxide as a substrate that has been washed and cleaned before. We used a spin-coat machine at a speed of 4000 rpm, which creates a thickness of 2 µm. The thickness was measured by Filmetrics (F10-RTA) device, which has high accuracy. Then we pre-baked the layer for 2 minutes at 95°C. To write the structure on the SU-8 layer, we used a direct-writing lithography method. The beam of 400 nm laser is focused on the Rhodamine-B doped SU-8 material by using a 50x lens. Laser power should be adjusted according to the width of the microrings. To create a width of 1.5 µm, the writing power of the laser was set to 0.5 mw. Then the sample was post-baked at 95°C for 8 minutes. When the sample has been exposed to the beam of laser, the desired pattern remains, and in the rest of the places, the material is removed by the solver.
This process is done by placing the sample inside the solver of SU-8 material for one minute. Finally, the sample is hard-baked for 2 hours at a maximum temperature of 150°C to increase the durability of the design on the substrate. Figure 4 (a) shows the schematic of the experimental set-up used to characterize the laser spectrum, where the 532 nm pulsed pump laser passes through a polarizer to adjust and measure the power. The spectrum scattered from the designed structure is collected by a 20x lens and passes through an optical notch filter to filter the pump light, then a 10x lens focused on the sample transmits the emission to a fiber attached to spectrometer. Figure 4 (b) illustrates a CCD camera image of a fabricated triple microrings coupled together and its pump by a 532 nm pulsed laser. The spot size of pulsed pump laser is 0.3 mm2 which assures us that the whole of the structure is simultaneously pumped uniform. The theoretical model was established using the Vernier effect. Figure 4 (c), (d) show a scanning electron-microscope (SEM) image of a coupled triple microrings arrangement. In our current analysis, the rings’ dimensions (radii, widths, and heights) are selected so as to support a high-side mode suppressed lasing in the radial direction. Compared to the double coupled microring resonators, a narrower transmission spectrum and pure single-mode with no side and high order modes (high SMSR) have been achieved in our system which means better sensitivity and high resolution for laser spectroscopy and optical fiber communications application [26].
In order to clearly examine all the advantages of the triple microrings structure presented in this paper, it is necessary to carefully determine the experimental and simulation spectra of the single, double, and triple microrings arrangements. For this purpose, in the first step, all the structures are simulated with the FDTD method, and their output spectrum is obtained. For high precision simulation, the gain is considered for all microrings. The boundary condition of perfect match layer (PML) is used to absorb the outgoing waves.
For the experimental results, the structures were fabricated with the same material on a substrate and their results were obtained with the spectrometer under the same conditions. Finally, the simulation and experimental results for the desired structures are illustrated in Fig. 5. To compare all cases, the spectra are normalized at the same pump energy (~ 58.29 µJ/mm2) and the results are indicated in Fig. 5. As it can be seen clearly, the side and high order modes exist in the double microrings output spectrums that are marked with green and orange arrows, but in the triple microrings laser, in addition to a narrower linewidth, these side and high order modes have also effectively been removed. The experimental data shows a very excellent match with the simulation data. Figure 5 (c), (e), (g), (i) show the corresponding SMSR for various configuration of asymmetric double coupled microrings and triple microring laser. As a result, we could enhance the laser intensity with high SMSR in triple coupled microring laser over 20 dB which it is much higher than that of various two-ring-coupled cavities (9, 5.65, 4.7 dB). The ratio between the threshold of the side and the main mode (Isth/Ith) is about 2 in three-ring-coupled cavity.
When the sample is pumped with a 532 nm pulsed laser, the triple microrings laser has a single-mode lasing emission at the wavelength of 612 nm with 0.6 nm Full width at half maximum (FWHM). Figure 6(a) shows the triple microrings laser spectrum in different pump powers. The stability of the laser is very suitable for pump powers much higher than the threshold pump energy. According to the FSR formula [13, 27], there is only one exciting mode in the ASE spectrum (wavelengths from 580 to 660 nm) shown in Fig. 5 (i). As can be seen in Fig. 6 (a), the triple microrings laser output spectrum has been significantly improved with high SMSR and all of high order or side modes has been eliminated by engineering the structure of triple coupled microrings, also a pure single-mode emission for different pump energies has been achieved experimentally, which is theoretically demonstrated in Fig. 2 (a) and simulated shown in Fig. 5 (j). To clearly show the SMSR for three coupled microrings arrangement Fig. 6 (b) is presented. Figure 6 (b) indicates the normalized logarithmic spectrum of triple microrings laser which represent improvement of corresponding SMSR for this configuration over 20 dB in comparison with double microrings structures. The highest SMSRs are 5.65, 4.7, and 9 for the two-ring cavities structures. Figure 6 (c) represents the normalized output emission intensity of the integrated triple microrings laser compared to the double microring resonators as a function of pumped energy. The lasing thresholds of various double microring structures are about 16, 17, 8 µJ/mm2. The solid black line in Fig. 6 (c) shows the lasing threshold of triple coupled microrings laser, which is ∼31 µJ/mm2.