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Abstract
A low-loss add-drop microring resonator (MRR) with an ultra-large free spectral range (FSR) is demonstrated by introducing an ultra-sharp multimode waveguide bend and bent asymmetrical directional couplers (ADCs). The multimode microring waveguide is introduced to achieve a low bent loss, even with a small radius (e.g., R = 0.8 μm). The bent ADCs are used to suppress the resonance of higher-order modes. For the fabricated device, the transmission at the drop port has a narrow 3 dB-bandwidth of 0.8 nm and a low excess loss of 1.8 dB. A record large FSR of 93 nm is achieved to the best of our knowledge.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Silicon photonics is a promising platform for photonic integrated circuits because of the CMOS compatibility and high integration density [1]. Various silicon photonic integrated devices have been developed successfully for the applications of optical interconnects [2–4]. Due to the high refractive index contrast between the silicon core and the SiO2 (or air) cladding, ultracompact silicon photonic devices can be realized with submicron-waveguides and micro-bends [5]. In particular, a micro-ring resonator (MRR) is one of the most attractive elements for photonic integration because of its footprint compactness and functional versatility. In the past decade, various silicon photonic devices based on MRRs have been developed for wavelength division-multiplexing (WDM) add-drop filters [6], lasers [7], optical switches/modulators [8], etc.
In most applications, the size reduction of MRRs is highly desired. First, compact design allows high integration density and small circuit footprint. Second, a small footprint usually results in the reduction of power consumption. For example, the power consumed to drive thermally-tunable MRR optical filters scales down proportionally with the size of the MRR. For MRR-based optical filters used for WDM systems, more wavelength channels in a WDM system can be fit into one free spectral range (FSR), which is inversely proportional to the MRR circumference. Previously, the reported MRRs usually have radii of 5-10 μm as a compromise between the FSR and the Q-factor, and the FSR is usually about 20 nm. Great efforts have been made to reduce the MMR radii and further increase the FSR [9–12]. In 2006 [9], an add-drop optical filter was reported with a large FSR of 47 nm and a 3 dB-bandwidth of about 0.24 nm, which was realized with a SiO2-cladded MRR with a small bending radius of 2 μm. In 2008, Xu et al. demonstrated a high-performance all-pass MRR with R = 1.5 μm by using a single-mode microring waveguide with a cross section of 450 nm × 250 nm [10]. For the demonstrated MRR, the FSR is as large as 62.5 nm and the 3 dB-bandwidth is 0.17 nm. This kind of all-pass MRR might be useful for optical modulation. Later, a compact add-drop optical filter based on an MRR with R = 1.5 μm was also demonstrated with an FSR of 52 nm by using an air-cladded single-mode microring waveguide with a cross section of 300 nm × 340 nm [11]. This MRR has 3 dB-bandwidth of 1.6 nm and an excess loss of 3 dB for the transmission at the drop port. More recently, an ultracompact add-drop optical filter based on an MRR with R = 1 μm was reported with an FSR of 80.5 nm [12]. The excess loss is 1 dB and the 3 dB-bandwidth is 3.3 nm. For these ultra-compact MRRs, it is not easy to handle the coupling between the bus waveguide and the microring waveguide because of the phase mismatch and the higher-order modes excitation [10]. As a result, a narrow single-mode microring waveguide and a narrow straight waveguide were usually introduced to achieve sufficient coupling to the sharp microring waveguide [10]. This approach becomes difficult when further reducing the bending radius of MRRs to be submicron for achieving ultra-large FSR as well as low losses. It is also possible to achieve very large FSR by using the Vernier effect in cascaded multiple MRRs with slightly different FSRs [13]. However, the resonance-wavelength alignment is very critical due to the fabrication error and one usually needs to introduce the thermal-tuning technology. Another approach is to introducing some special filter inside the cavity to achieve a major resonance [14,15]. For example, in [14] an FSR-free MRR-based optical filter was realized by using bent contra-directional couplers. In this design, the design and the fabrication for bent Bragg grating is not easy, while the device footprint is as large as 1963 µm2.
In this paper, we propose and realize a low-loss add-drop optical filter with an ultra-large FSR of 93 nm by using an MRR with bent asymmetrical directional couplers (ADCs). Here, a multimode microring waveguide is introduced to achieve low-loss propagation even with a submicron bent radius (e.g., R = 0.8 μm). Bent ADCs are introduced to minimize the coupling ratios for the higher-order modes, so that there is only one major resonance peak for the fundamental mode. For the realized optical add-drop filter, the bent radius for the MRR is as small as 0.8 μm enabling a record large FSR of 93 nm to the best of our knowledge. For the transmission at the drop port, the 3-dB bandwidth for the optical filter is 0.8 nm and the excess loss is 1.8 dB. The present ultra-compact MRR has a SiO2 upper-cladding other than air upper-cladding, and thus it not only provides a good reliability and mechanical stability, but also is very convenient for the further development of advanced photonic integration.
2. Structure and design
Figures 1(a) and 1(b) show the schematic configuration of the present MRR with bent ADCs and the cross section of a silicon-on-insulator (SOI) optical waveguide, respectively. Here an SOI wafer with 400 nm-thick device layer and 1 μm-thick buried-oxide layer is used. The operation wavelength is around 1550 nm, and the corresponding refractive indices of silicon and silica are nSi = 3.476 and nSiO2 = 1.455, respectively. Devices were designed for the transverse-electric (TE) polarization.
As it is well known, the FSR of an MRR is given by FSR = λ2/(ngL) [5], where λ is the resonance wavelength in vacuum, ng is the group index, L is the round-trip length (i.e., L = 2πR). The group index is defined by ng = neff−λ dneff/dλ, where neff is the effective index. In order to maximize the FSR, one should minimize the group index ng as well as the bent radius R. Here we calculate the group index ng and FSR for the TE0 mode as the bent radius varies from 1.4 μm to 0.7 μm, as shown in Figs. 2(a) and 2(b). Here, different silicon core widths of the microring waveguide are considered: w1 = 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 μm. As shown in Fig. 2(a), the group index ng increases as the radius R1 decreases while one has a smaller ng for a narrower waveguide. As a result, one can achieve a larger FSR by choosing a smaller core width and a smaller radius. For example, the FSR is over 120 nm in the case of R1 = 0.7 μm and w1 = 0.5 μm. In this case, however, the bending loss might be very high, as shown in Fig. 2(c). Figure 2(c) shows the calculated bent loss of the TE0 mode for the cases with different bent radii (R1 = 0.7, 0.8, 0.9 μm) as the core width w1 varies. The bent loss for a 90°-bend is given by L = 20lg[exp(nimk0Rπ/2)], where k0 is the wavenumber (k0 = 2π/λ) and nim is the imaginary part for the complex effective index neff. The scattering loss due the sidewall roughness is not included in this calculation. It can be seen that the bent loss decreases dramatically as the core width w1 increases because the wider waveguide provides stronger mode confinement. Furthermore, a wider microring waveguide can usually tolerate more fabrication errors, which helps achieve better alignment of the resonance wavelength [16]. As a result, the core width w1 should be chosen by making a trade-off between the FSR and the bent loss (Q-factor). We choose R1 = 0.8 μm with w1 = 800 nm in the following design, so that the bent loss is less than 0.01 dB per 90° bend and the FSR is larger than 90 nm.
Fig. 2 Calculated the group index ng (a) and FSR (b) of the TE0 mode for the waveguide width w1 = 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 μm as the radius R1 varies; (c) Calculated the bent loss of the TE0 mode as the core width w1 varies (R1 = 0.7, 0.8, 0.9 μm).
It is noticed that such a microring waveguide is multimode and supports not only the fundamental mode but also the higher-order modes (e.g., the TE1 mode will appear when w1 > 600 nm). The higher-order modes will introduce some excess loss and crosstalk. Therefore, coupling region of the MRR need to be carefully designed to avoid the excitation of higher-order modes. In order to suppress higher-order modes, a potential approach is to use bent ADCs instead of straight ADCs [17], as shown in Fig. 1(a). This bent ADC is designed according to the phase-matching condition [18], neff1R1 = neff2R2, where R1 and R2 are the bent radii of the two bent waveguides, neff1 and neff2 are their effective indices of the fundamental modes. One has R2 = R1 + (w1 + w2)/2 + wgap, where w1 and w2 are the core widths for the two bent waveguides, wgap is the width of the gap between them. In our design, a relatively large gap (e.g., wgap = ~200 nm) is chosen so that the SiO2 upper-cladding can be filled into the gap easily. Regarding the core width w2 of the bent bus waveguide should be around 0.3 μm according to the phase matching condition, we choose R2 = ~1.575 μm for the bent bus waveguide. Figure 3(a) shows the calculated result for neffR with R = 0.8 and 1.575 μm, respectively. It can be seen that the bent bus waveguide is required to be as narrow as w2 = 0.22 μm to satisfy the phase-matching condition for the designed microring waveguide with R1 = 0.8 μm and w1 = 800 nm. On the other hand, one should notice that such a narrow bent bus waveguide has a high bent loss of about 4 dB/90°. Fortunately, it is not necessary to achieve very high coupling (e.g., >20%) between the bent bus waveguide and the microring waveguide, and thus some phase mismatching is allowed. Nevertheless, one should choose the width w2 carefully to make neff1R1≈neff2R2. Here we choose (R1, R2) = (0.8, 1.575) μm and (w1, w2) = (0.85, 0.325) μm as an example. Correspondingly, the bending losses of the microring waveguide and the bent bus waveguide are respectively 0.006 dB/90° and 0.009 dB/90°, which are negligibly low. Meanwhile, the gap width (wgap) is as large as 212.5 nm to avoid any difficulty in the fabrication. Here we only consider the coupling from the TE0 mode in the bus waveguide to the TE0 and TE1 modes in the microring waveguide in the calculation. This is because the TE2 mode and the vertical higher-order modes in the microring waveguide have very high loss and very weak coupling due to high phase mismatching. Figure 3(b) shows calculated power coupling ratio of the designed bent ADC when light is launched from the bent bus waveguide. From this figure, it can be seen that the coupling ratio κ0 from the TE0 mode in the bus waveguide to the TE0 mode in MRR and the coupling ratio κ1 from the TE0 mode in the bus waveguide to the TE1 mode in MRR oscillate with θ increases. The ratio κ1 can be minimized by choosing the coupling angle θ carefully. For example, κ1 = 9.46 × 10−4 with θ = 20° and the corresponding κ0 is 0.0136. As a result, one can achieve major resonances for the TE0 mode while the resonances for the TE1 mode is suppressed significantly, as shown by the calculated spectral responses in Fig. 3(c). Here a three-dimensional finite-difference time-domain (3D-FDTD) simulation was performed to calculate the spectral response. From Fig. 3(c), it can be seen that the FSR of the designed MRR is as large as 94 nm, which agrees well with the calculated result in Fig. 2(b). Figure 3(d) shows the simulated mode field distribution in the MRR when operating at the resonance wavelength 1597.3 nm. It shows that no multimode interference is observed, which indicates the designed MRR works well with the TE0 mode.
Fig. 3 (a) Calculated results of neffR; (b) Power coupling ratio as the angle θ increases. Here (R1, R2) = (0.8, 1.575) μm, (w1, w2) = (0.85, 0.325) μm; (c) Calculated spectral response of the design MRR; (d) The mode field of the MRR at the resonance wavelength (λ = 1597.3 nm). Here (R1, R2) = (0.8, 1.575) μm, (w1, w2) = (0.85, 0.325) μm, and θ = 20°.
3. Fabrication and measurement
The designed add-drop optical filters based on MRRs were fabricated with the processes of 248-nm DUV stepper and inductively-coupled plasma dry-etching. A 0.8 μm-thick silica thin film was deposited on the top as the upper-cladding by using the plasma enhanced chemical vapor deposition (PECVD) process. Figures 4(a)-4(c) show the microscope images of the fabricated four-channel add-drop filters based on MRRs with submicron radii of 0.8, 0.81, 0.82 and 0.83 μm. Here the grating couplers are used for efficient fiber-chip coupling. A broad-band amplified spontaneous emission (ASE) light source (1520~1610 nm) was used as the source and an optical spectrum analyzer (OSA) was applied to readout the output spectrum. The spectral responses of the MRRs-based optical filter was characterized by launching light from the input port and monitoring the transmissions at the through port and the drop ports. The measured spectral responses were normalized with respect to the transmission of a 300 nm-wide straight waveguide connected with grating couplers on the same chip. The resolution is 0.1 nm in the measurements.
Fig. 4 Microscope images of the fabricated silicon photonic integrated circuit (PIC) with MRRs; (a) Four-channel add-drop filters based on MRRs with submicron radii of 0.8, 0.81, 0.82 and 0.83 μm; (b) Grating couplers for chip-fiber coupling; (c) Zoom-in view of one of the MRRs.
Figure 5(a) shows the measured spectral responses at the drop ports (O1, O2, O3, and O4) and the through ports. It can be seen that the transmissions at the drop ports have excess losses of ~1.8 dB and a 3-dB bandwidth of ~0.8 nm. The excess loss is estimated by normalizing the transmission at the drop of the MRR with respect to the transmission of a straight waveguide on the same chip. Here only one major resonance is observed per MRR in the wavelength range from 1520 nm to 1610 nm, which is limited by the wavelength range of the ASE light source available in the lab and the 3 dB-bandwidth of the grating couplers. We measured the spectral response with another light source (Agilent 81940A) to cover the resonance around 1610 nm, which can be seen in the right side of Fig. 5(a). It shows that the FSR for the MRR #4 is 93 nm, which agrees well with the previous theoretical analysis and simulation. Figure 5(b) shows the measured resonance wavelength as the radius varies. A well linearity can be observed and the resonance wavelength shift about + 31 nm when the radius increases 30 nm. The measured spectral responses are also fitted with the theoretical spectral response, as shown in Fig. 5(c), which is for MRR #2 with R = 0.81 μm. An excellent fit between the theoretical and measured spectral responses allows to extract the device parameters accurately [19]. The fitting shows that the total roundtrip loss is ~0.4 dB and the power coupling ratio is ~0.01. The relatively high loss is mainly due to the scattering loss at the rough sidewalls.
Fig. 5 (a) Measured spectral responses at the drop ports and the through port of the fabricated MRRs; (b) The resonance wavelengths for the MRRs with different radii; (c) Theoretical fitting for the spectral response of MRR #2 with R = 0.81 μm.
It is worth noting that the measured spectral responses from the drop ports have a sub-peak, about 20 nm blue-shift to the main peak, which is caused by the resonance of the higher-order mode. Figures 6(a)-6(d) show the measured spectral responses of the fabricated MRRs with different coupling angles θ. It can be seen that the peak transmission is dependent on the coupling angle θ. The peak transmission of the higher-order mode resonance become −20 dB lower than that for the TE0 mode when θ = 30°. We believe that the difference between the experimental and theoretical results is cause by the fabrication imperfections.
Fig. 6 Measured spectral responses of the fabricated MRRs with different coupling angles, i.e., (a) θ = 0°, (b) θ = 15°, (c) θ = 20° and (d) θ = 30°. Here, all the radii of MRRs are 0.8 μm.
Table 1 gives a comparison for the ultra-compact MRRs on silicon reported in the recent years. For the present MRR, the FSR is as large as 93 nm, which is the largest one reported until now. The present MRR has a moderate 3dB-bandwidth of ~0.8 nm, which is the smallest one for the MRR-based add-drop optical filters with an FSR of >50 nm shown in Table 1. The excess loss for the present MRR is also acceptably low (i.e., 1.8 dB). Furthermore, the present MRR has a SiO2 upper-cladding other than air upper-cladding, which not only provides a good reliability and mechanical stability, but also provides good compatibility and convenience to be extended and integrated with any other silicon photonic circuits.
Table 1. Comparison of reported ultra-compact MRRs on silicon.
4. Conclusion
In summary, we have realized a low-loss add-drop optical filter with an ultra-large FSR of 93 nm by using an ultra-small MRR with bent ADCs. Here, a multimode microring waveguide with a submicron bent radius R has been introduced and the achieved bent loss is as low as 0.006 dB/90° for the case of R = 0.8 μm in theory. The higher-order mode resonance has been suppressed successfully by introducing bent ADCs that was designed according the phase-matching condition for the TE0 mode. For the realized optical add-drop filter based on MRRs with R = 0.8 μm, the achieved FSR is as large as 93 nm, which is the largest one operating around 1550 nm reported until now. The transmission at the drop port has a narrow 3-dB bandwidth of 0.8 nm and a low excess loss of 1.8 dB, which is attractive for many applications, e.g., WDM systems requiring large channel count.
Funding
National Science Fund for Distinguished Young Scholars (61725503), Zhejiang Provincial Natural Science Foundation (Z18F050002); National Natural Science Foundation of China (NSFC) (61431166001, 1171101320).
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