When the tapering angle increases from ?0.2�� to 0.9��, the crosstalk decreases from approximately ?16dB to ?21.7dB.Figure 7Crosstalk as sellectchem the tapering angle a varies.Low insertion loss is also important for a practical device. Figure 8 shows the insertion loss at the central wavelength as the tapering angle a varies. Compared with the case of a conventional MMI (a = 0��), the insertion loss is reduced by approximately 1dB when the tapering angle increases to 0.9��.Figure 8Insertion loss as the tapering angle a varies.Figure 9 gives the spectral responses when the tapering angle a = ?0.2��, 0.6��, and 0.9��. This figure shows that the spectral response can be improved by adjusting the tapering angle a. When the tapering angle a reaches 0.9��, the spectral response is significantly improved compared with the other cases.

Moreover, transition is sharpened, crosstalk is reduced to approximately ?21.4dB, insertion loss is ?5.11dB, and the 3dB passband width is flattened to 1.3nm. Finally, the device is structured with WI of 2.88��m and Ltp of 10.27��m. Optical field distribution of the tapered MMI is shown in Figure 10. From the figure, distinct image points are evident. In conclusion, the structure parameters of tapered MMI are advisable.Figure 9Spectral responses when the tapering angle a = ?0.2��, 0.6��, 0.9��.Figure 10Optical field distribution of a tapered MMI (a = ?0.9��).2.3. Use of a Combination of a Tapered MMI and Tapered Input/Output Waveguides to Flatten the Spectral ResponseThis section presents a new kind of structure that can flatten the spectrum while ensuring good planarization with low insertion loss and crosstalk.

The improvements based on the structure presented in Section 2.2 include a tapered waveguide inserted before the tapered MMI, such that a preexpanded structure and a tapered waveguide are connected to the output waveguide inlet end. Figure 11 shows that wi and Ltp are the exit width and length of the input tapered waveguide, respectively, whereas wo is the entrance width of the output tapered waveguide.Figure 11Structure used to flatten spectral responses: (a) section between the input waveguide and FPR1 and (b) section between the output waveguide and FPR2. In the proposed design, the parameters of the ready-to-use AWG are the same as those listed in Table 1. The structure of the tapered MMI is similar to that designed in Section 2.

2. Considering that the interval of the AWG output waveguide is 1��m, wo = wi = 0.75��m is selected. To ensure minimal tapering and conversion Batimastat of the loss of mold spots, Ltp is optimally designed at 13��m. A comparison of the single-channel output spectra of flattened and unflattened AWGs is shown in Figure 12. The result shows that the passband is flattened to 1.31nm at 3dB. The insertion loss is decreased to ?4.36dB, and the crosstalk is approximately ?21.9dB.