In this section, we investigate the use of the parallel MC-SLR resonator to achieve high performing wavelength interleaving / non-blocking switching functions in WDM optical communication systems. Figures 4(a-i) and (a-ii) show the wavelength de-interleaving operation achieved based on the parallel MC-SLR resonator with input from (i) Port 1 and (ii) Port 2, respectively. The corresponding power transmission spectra are shown in Figs. 4(b-i) and (b-ii). The structural parameters are LSLR = L = 346 µm, ts = 0.995, and tb = 0.707, which are designed in order to achieve a channel spacing (CS) of about 100 GHz in the C band to meet the ITU-T spectral grid standard G694.1 [38]. The input signal is separated into two spectrally interleaved signals that transmit to different output ports. Given the reciprocal optical transmission between the input and output ports, the same device can also perform a wavelength interleaving operation to combine the two sets of WDM signals input from Port 3 and Port 4.
Figure 5(a) shows the spectral response of the parallel MC-SLR resonator for various values of ts. Since the spectral responses for inputs from Port 1 and Port 2 are complementary for the same device, we only consider the spectral response from Port 1. The spectral responses at different output ports are slightly different, as shown in Fig. 5(a-i) for the output from Port 3 and Fig. 5(a-ii) for the output from Port 4. The calculated 1-dB bandwidth (BW) and normalized root-mean-square deviation (NRMSD) within the 1-dB BW range as a function of ts are depicted in Fig. 5(b). The 1-dB BW decreases with ts, and the corresponding NRMSD increases with ts. This reflects the deterioration of the filtering flatness for an increased ts. The spectral responses of the parallel MC-SLR resonator for various tb are shown in Fig. 6(a). The 1-dB BW and the corresponding NRMSD within 1-dB BW range versus tb are plotted in Fig. 6(b). Their changes with tb shows an opposite trend to their change with ts, indicating improved filtering flatness for an increased tb.
Table II compares the performance of the MZI and the parallel MC-SLR resonator in terms of 1-dB BW, NRMSD within 1-dB BW, ERs and ILs. The MZI and the parallel MC-SLR resonator are designed to have to a small CS of about 100 GHz. As compared with the MZI, the MC-SLR resonator shows an increased 1-dB BW and improved filtering flatness, at the expense of reduced ERs and increased ILs within reasonable ranges. Note that we used a moderately low waveguide propagation loss (α = 55 m− 1, i.e., 2.4 dB/cm) in our design, but well within experimental capability for SOI nanowires. For waveguides with lower propagation loss, such as is achievable silicon nitride or doped silica waveguides [46–75], for example, a more significant improvement in the 1-dB BWs and filtering flatness can be achieved.
TABLE II
PERFORMANCE COMPARISON OF THE INTERLEAVERS BASED ON MZI AND PARALLEL MC-SLR RESONATOR
Parameters
|
MZI
with input light from Port 1
|
Parallel MC-SLR resonator with input light from Port 1a
|
Output ports
|
Port 3
|
Port 4
|
Port 3
|
Port 4
|
1-dB BW (GHz)
|
59.9887
|
59.9767
|
67.8828
|
60.8639
|
NRMSD within 1-dB BW (%)
|
55.43
|
60.09
|
33.92
|
47.3
|
ER (dB)
|
45.9179
|
46.4525
|
33.7906
|
32.3885
|
IL (dB)
|
0.2065
|
0.1652
|
0.6083
|
0.3341
|
CS (GHz)
|
100.0053
|
100.0053
|
a The structural parameters are LSLR = L = 346 µm, ts = 0.995, and tb = 0.707. |
By changing LSLR and L in the parallel MC-SLR resonator, wavelength interleaving / de-interleaving with various spectral grids can be achieved, making the MC-SLR resonator versatile enough to meet the different spectral grid requirements for different WDM systems. Figures 7(a) and (b) show the power transmission spectra of the parallel MC-SLR resonator with CSs of approximately 50 GHz and 200 GHz, respectively. The corresponding device performance parameters and structural parameters are provided in Table III, together with those of the device with a CS of 100 GHz. The high 1-dB BW to CS ratios highlight the filtering flatness. The almost equal 3-dB BW to the CS ratios and the ERs for the complementary output ports also reflect very symmetric wavelength interleaving / de-interleaving for these devices.
Table III
Performance of the interleaver based on parallel MC-SLR resonators With different spectral grids
CS (GHz)
|
50.0025
|
100.0053
|
200.0122
|
Output ports
|
Port 3
|
Port 4
|
Port 3
|
Port 4
|
Port 3
|
Port 4
|
ER (dB)
|
31.9609
|
30.9357
|
33.7906
|
32.3885
|
34.8738
|
33.2153
|
IL (dB)
|
0.8115
|
0.4965
|
0.6083
|
0.3341
|
0.5065
|
0.2529
|
1-dB BW / CS
|
0.6773
|
0.6077
|
0.6787
|
0.6086
|
0.6780
|
0.6082
|
3-dB BW / CS
|
1.0025
|
0.9923
|
1.0005
|
0.9912
|
1.0015
|
0.9917
|
Structural parameters
|
ts
|
0.995
|
0.995
|
0.995
|
tb
|
0.707
|
0.707
|
0.707
|
LSLR (µm)
|
692
|
346
|
173
|
L (µm)
|
692
|
346
|
173
|
Given the characteristics of the parallel MC-SLR resonator as a four-port device, a 2 × 2 non-blocking switching unit was further designed based on it. We chose the Benes switching architecture since it exhibits minimum complexity among various non-blocking switching architectures [76]. Figure 8(a) shows the (i) cross and (ii) bar states of the non-blocking switching unit and the corresponding spectral responses between the different ports, shown in Fig. 8(b). The structural parameters of the parallel MC-SLR resonator were the same as those in Fig. 4(b). Two resonance channels centered at wavelengths of λ1 = 1549.4938 nm and λ2 = 1550.2945 nm were selected for the operation of the cross and bar states, respectively. When the resonance channel at λ1 is red shifted to λ2, the switching unit changes from the cross state to the bar state. Practically, the red shift can be realized by slightly increasing the chip temperature via temperature controllers or injecting a high-power pump at other resonance wavelengths [26, 32, 77, 78]. Figure 8(c) shows the shift of the center wavelengths of the resonance channels at (i) λ1 and (ii) λ2 as a function of chip temperature variation ΔT. The thermo-optic coefficient (dn / dT = 1.8 × 10− 4 / °C) of silicon used in our calculation was the same as that used elsewhere [76]. It can be seen that the resonance channel red shifts when increasing ΔT. Table IV shows the ERs, ILs and crosstalk for the 2 × 2 non-blocking switching unit based on the parallel MC-SLR resonator. As can be seen, flat-top spectral response with high ERs, low ILs and low crosstalk is achieved. When the input port is changed to Port 2, the wavelength channels for the cross and bar states remain unchanged, i.e., λ1 = λ1′ and λ2 = λ2′.
TABLE IV
PERFORMANCE OF THE NON-BLOCKING SWITCHING UNIT BASED ON MC-SLR RESONATOR
Operation state
|
Cross
|
Bar
|
Operation wavelength (nm)
|
λ1 = λ1′ = 1549.4938
|
λ2 = λ2′ = 1550.2945
|
Extinction ratio (dB)
|
PC - PB = 32.3885
|
PD - PA = 33.7906
|
Crosstalk (dB)
|
PA - PC = -34.0669
|
PB - PD = -32.1142
|
Insertion loss (dB)
|
0.3341
|
0.6083
|
PA, PB, PC, and PD denote the transmission powers at point A, B, C, and D in Fig. 9b, respectively. |
We also investigate the impact of varying ts and tb on the IL and ER, which are important parameters for the non-blocking switching unit. Figure 9 (a) plots the IL and ER of the transmission spectra from Port 1 to (i) Port 3 and (ii) Port 4 of the parallel MC-SLR resonator versus ts. The other structural parameters were the same as those in Fig. 4(b). Clearly the IL decreases with the ts while the ER shows the opposite trend, reflecting a trade-off between them. The IL and ER as functions of tb are plotted in Fig. 9(b). As shown in Fig. 9(b), the IL of the output spectrum remains almost unchanged at Port 3 and it increase at Port 4 with the tb while the ER of the output spectrum at Port 3 decreases with the tb and it remains almost unchanged at Port 4, reflecting an increase in the difference between the ERs for different output Ports with tb.