Fano resonances that feature an asymmetric spectral lineshape are fundamental physical phenomena that have underpinned many applications such as optical switching, data storage, sensing, and topological optics [10-12]. In this section, the spectral response of the device in Fig. 1 is tailored to realize optical analogues of Fano resonances with high slope rates (SRs) and low insertion loss (IL). The power transmission and reflection spectra with input from Port 1 is depicted in Fig. 2(a-i). The device structural parameters are LSLR = L = 100 µm, t1 = t3 = 0.82, t2 = 0.92, and t4 = 1. Clearly, the output from Port 2 shows periodical Fano-like resonances with an asymmetric resonant lineshape in each period. The high uniformity of the filter shape across multiple periods, or channels, is highly desirable for WDM systems. A zoom-in view of Fig. 2(a-i) is shown in Fig. 2(a-ii), together with another curve showing the corresponding result for another device with the same structural parameters except for a different t2 = 1. As can be seen, when t2 = 1, there is no Fano resonance, distinguishing between the device in Fig. 1 and the three cascaded SLRs in Ref. [5]. The Fano resonances in Fig. 2(a-ii) show a high extinction ratio (ER) of 30.2 dB and a high SR (defined as the ratio of the ER to the wavelength difference between the resonance peak and notch) of 747.64 dB/nm. Table II compares the performance of the Fano-like resonances generated by the coupled SLRs in our previous work [7, 8] and the device in Fig. 1. As compared with previous devices, the device reported here has a much lower insertion loss of 1.1 dB, along with a slightly improved SR. We note that a low IL of 1.1 dB is outstanding among the reported Fano-resonance devices on the SOI platform [13, 14], which renders the device here more attractive for practical applications in optical communication systems.
In Figs. 2(b)–(e), we investigate the impact of the device structural parameters including ti (i = 1–4) and length variations of the feedback loop (∆LFL, LFL = 2L + LSLR), respectively. In each figure, we changed only one structural parameter, keeping the others the same as those in Fig. 2(a-i). Figs. 2(b-i) and (b-ii) compares the power transmission spectra and corresponding IL and SR for various t1 or t3, respectively. The SR decreases with ti (i = 1, 3), while the IL first decreases with ti (i = 1, 3) and then remains almost unchanged. The spectral response and corresponding IL and SR for different t2 are shown in Figs. 2(c-i) and (c-ii), respectively. The SR decreases with t2, while the IL shows an opposite trend, reflecting that both of the two parameters can be improved by enhancing the coupling strength between SLR1 and SLR2. As shown in Fig. 2(d), both IL and SR remain almost unchanged with varied t4. In Figs. 2(e-i) and (e-ii), we compare the corresponding results for various ΔLFL. As ΔLFL increases, the filter shape remains unchanged while the resonance redshifts, indicating that the resonance wavelengths can be tuned by introducing thermo-optic micro-heaters [14] or carrier- injection electrodes [15] along the feedback loop to tune the phase shift.
TABLE II
PERFORMANCE COMPARISON OF FANO-LIKE RESONANCES
GENERATED BY DIFFERENT SLR-BASED DEVICES
Device structure
|
IL (dB)
|
ER (dB)
|
SR (dB/nm)
|
FSR (GHz)
|
Ref.
|
Two parallel
WC-SLRs a
|
6.3
|
13.9
|
389
|
692.02
|
[7]
|
Three zig-zag
WC-SLRs b
|
3.7
|
63.4
|
721.28
|
230.68
|
[8]
|
Device in Fig. 1
|
1.1
|
30.2
|
747.64
|
173
|
This work
|
a WC-SLRs: waveguide coupled SLRs.
b For comparison, the length of the SLRs (LSLRi, i = 1–3) and the connecting waveguide (Li, i = 1–4) is slightly changed from 115 µm in [8] to 100 µm.