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Abstract
Narrowband microwave photonic filters based on a microring resonator are difficult to achieve because low cavity loss and low coupling loss should be satisfied simultaneously. Here, a high Q (∼2.6×106) multimode microring resonator is proposed to achieve an ultra-narrow band tunable microwave photonic filter. Combining the ultra-low loss of the silicon nitride waveguide and the ultra-low coupling coefficients of the multimode ring resonator, very narrow optical bandwidths between 72.5 MHz to 275 MHz were obtained for different order modes, which match the simulation results well. Furthermore, by introducing the two switchable modulation methods supported by the dual-drive Mach–Zehnder modulator, we achieved a narrowband passband/stopband switchable microwave photonic filter, whose 3 dB bandwidths are 180 MHz and 120 MHz, respectively. The filter frequency can be tuned from 2 ∼ 18 GHz by altering the laser wavelength, and a high out of band RF rejection ratio about 27 dB was obtained for the passband filter due to the high-quality factor. Besides, a high RF rejection ratio of about 51 dB was achieved for the stopband filter by using the RF cancellation technology.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Tunable integrated optical microring filters with low power consumption and small footprint have important applications in the field of communication [1–4]. Additionally, it can be used for an integrated microwave photonic filter (IMPF), which is an important building block of microwave photonic systems [5,6]. However, mainly due to the scattering loss and coupling loss in microring resonator, it is usually challenging to achieve a microring resonator with very narrow bandwidth, which is very important for achieving microwave photonic filter with fine frequency selectivity. So ultra-narrow bandwidth microring resonators attract many researchers’ interests. Table 1 shows some reported IMPFs based on the silicon nitride (Si3N4) [7–11], silicon-on-insulator (SOI) [12–15], silica [16], CaF2 [17], InP [18]and As2S3 [19]. In Table 1, the whispering gallery mode resonators (WGMRs) with very high Q were used to construct IMPFs in [16,17], and very narrow bandwidths were achieved. However, the discrete fiber and prism coupling were used for the Silica and CaF2 WGMRs, which cannot be integrated. In [19], an IMPF using an integrated As2S3 photonic chip that consists of a Brillouin-active element and an over coupled ring resonator was demonstrated. The bandwidth of 3 GHz was obtained based on the stimulated Brillouin scattering. However, the high power optical pump is needed, which increases the system cost and complexity. Due to the much lower propagation loss of Si3N4 waveguides comparing with the SOI and InP waveguides [18,20], it is obvious that the bandwidths of IMPFs based on Si3N4 microring resonators are typically narrower than those based on SOI and InP.
Table 1. The bandwidth comparison of reported IMPFs.
Although Si3N4 microring resonator can achieve bandwidth with several hundred MHz, it is still much broader than the bandwidth of several tens MHz realized by stimulated Brillouin scattering [21]. Typically, microring resonator’s bandwidth can be narrowed by reducing the internal cavity loss and the coupling loss between the input waveguide and microring [1]. Generally speaking, the internal cavity loss is difficult to be altered once the microring resonator is fabricated. One way to tune the coupling loss is to introduce a tunable 2×2 Mach-Zehnder coupled to the microring cavity. However, the intrinsic excess loss will reduce filter’s quality factor. Another method is to adjust the coupling gap size between the input waveguide and microring. However, very accurate coupling gap size control (typically down to tens of nanometers) is usually needed because the coupling coefficient of the microring resonator is very sensitive to the gap size. So it requires a high precision fabrication process, which is costly. On the other hand, if a multimode microring resonator is introduced, a set of coupling coefficients could exist, in which some small coupling coefficients may match the ultra-low cavity loss build from the multimode Si3N4 waveguide. Then a set of high Q resonant modes with relatively large extinction ratio could be obtained, which can be used to build microwave photonic filter (MPF) with ultra-narrow bandwidth. In this paper, a multimode microring resonator based on the double strip Si3N4 waveguide is proposed to achieve reconfigurable IMPF with ultra-narrow bandwidth. Through the experimental verification, optical filters with 3 dB bandwidths between 72.5 MHz∼275 MHz were achieved for different order modes. Similar multimode waveguide filters have been reported on the Si3N4 and SOI platforms [15,22–25]. However, only the fundamental mode was used due to its reduced propagation loss comparing with the single-mode waveguide, while the higher-order modes were not investigated in detail. Besides, the double strip Si3N4 multimode waveguide with low propagation loss and the compact bending radius was introduced, so large quality factor and free spectral range (FSR) can be obtained simultaneously, which was not achieved by the multimode microring resonators based on single strip Si3N4 [23]. Also, based on the high-order mode with hundred megahertz in Si3N4 multimode microring resonator, a reconfigurable IMPF with passband and stopband switching capability was achieved. Compared with the reported results shown in Table 1, the proposed IMPF can not only achieve ultra-narrow bandwidth but also can be switched between passband and stopband responses. The experimental results show that 3 dB bandwidths of the passband and stopband IMPF are 180 MHz and 120 MHz, respectively. The filter frequency can be tuned from 2 ∼ 18 GHz by altering the laser wavelength. In addition, the RF rejection ratio of the stopband response is enhanced to about 51 dB by using the RF cancellation technology, and the out of band RF rejection ratio of the passband response is about 27 dB. Moreover, the proposed IMPF can realize not only single-band but also multi-band filtering, which can extend the application range of the proposed IMPF.
2. Silicon nitride multimode microring resonator
Figure 1(a) shows the structure of the proposed silicon nitride multimode microring resonator, in which a silicon nitride single-mode waveguide is coupled to a multimode microring resonator. Silicon nitride is a well-known waveguide platform with very low propagation loss, which is one of the critical factors to achieve integrated high Q resonator. The coupling coefficient between the input waveguide and the ring resonator is another factor that needs to be optimized because a small coupling coefficient matched with the low propagation loss must be satisfied to achieve a large optical extinction ratio, which is important for the IMPF implement. However, the coupling coefficient cannot be determined due to fabrication errors. Honestly, we cannot obtain the optimal spectral characteristics even if the structure parameters are well designed because the fabrication resolution is only 1 µm, and size deviations exist for the TriPleX technology of Lionix [26]. Under these limitations, in order to achieve high Q resonator with enough extinction ratio, we chose a multimode waveguide ring resonator because a set of coupling coefficients could exist, in which some small coupling coefficients may match the ultra-low cavity loss build from the multimode Si3N4 waveguide. Therefore, several silicon nitride multimode microring resonators with different radius and gaps were fabricated, and the obtained spectra are shown in Fig. 2. From Fig. 2, the multimode microring with R=205 µm, r=195 µm, and G=1 µm was chosen to achieve the IMPF due to its best performances.
Fig. 1. (a) The structure of the silicon nitride multimode microring resonator. (b). The cross-section of the double strip silicon nitride waveguide.
Here the double strip silicon nitride waveguide was chosen to realize the multimode microring resonator due to its low prorogation loss and relative small bending radius. The waveguide cross-section of the silicon nitride waveguide is shown in Fig. 1(b), where two trapezoid silicon nitride (Si3N4) layers with etching angle α of 82° and different thicknesses of h1 = 175 nm and h2 = 75 nm are separated by silica (SiO2) with a thickness of g=100 nm. Furthermore, top and bottom silica claddings with thicknesses of hu = hL = 8 µm are adopted to avoid radiation loss. The refractive indices of silicon nitride and silica are 1.98 and 1.45, respectively. Here the widths of the multimode and single-mode Si3N4 waveguides are chosen as w=10 µm and w=1.1 µm, respectively.
By using the finite-difference time-domain (FDTD) method, the supported modes of the bend Si3N4 multimode waveguide in the microring resonator were simulated, and the results are shown in Fig. 3. The bending direction is represented by the dotted arrow shown in the TE0 mode field distribution. The results show there are seven transverse electric modes (TE modes) in the silicon nitride multimode microring resonator, and their mode field distributions and mode effective indices are shown in Fig. 3. It should be noted that different intrinsic quality factors should be obtained according to the different scattering losses due to the different overlaps between the waveguide edges and the mode field distributions.
Fig. 3. The electric field intensity distributions of the seven supported TE modes in the bend Si3N4 multimode waveguide.
By using the Lightwave Measurement System (Agilent 81640A) and set the laser TE polarized, the optical transmission spectrum of the silicon nitride multimode microring resonator was measured, and the results are shown in Fig. 4. There are a set of resonant modes with different FSRs, which are formed by the above-simulated TE modes.
In order to resolve the corresponding TE resonant dips in Fig. 4, we used the classical transfer function method to simulate the optical transmission spectrum of the fabricated silicon nitride multimode microring resonator, whose amplitude transfer function is [1]
Fig. 5. (a)The comparison of the measured and fitted optical transmission spectra of the silicon nitride multimode microring resonator. (b) An enlarged view of one FSR.
Figure 5(a) is wideband spectrum fitting results and Fig. 5(b) is the enlarged view in the red box, which is less than one FSR. From Fig. 5 and Table 2, several conclusions can be made: First, the TE0 and TE6 mode resonances are very weak and even cannot be observed in the microring resonator, this may be caused by the more significant mismatch between the amplitude transmission coefficient ti and round-trip loss factors ai; Second, for TE1 to TE5 modes, the cavity loss mainly determined by the scattering loss is getting larger when the waveguide mode order increases and this phenomenon can be explained by the electric field distributions shown in Fig. 3, where the strongest electric fields of the modes are getting closer to the waveguide edge with roughness induced by the fabrication process; Third, the resonant wavelengths of the TE2 and TE3 modes are too close to each other and beyond the wavelength resolution of the Lightwave Measurement System, which is 1 pm.
Table 2. Synthesized parameters for each mode.
In order to get better spectral resolution, the transmission spectra of the TE1∼TE5 resonant modes were measured segmentally by using the well-known optical vector network method, whose resolution can be as low as kHz level [27]. The measured transmission spectra are shown in Fig. 6. The comparison of the synthesized and measured spectra are shown in Fig. 7. Table 3 shows the synthesized and measured bandwidths of TE1∼TE5 modes as well as the central frequencies. Besides, from Table 3, it can be seen that the 3 dB bandwidth of TE2 mode is the narrowest. However, it is too close to TE3 mode to be used as a single-band microwave photonic filter, so the TE1 mode was selected to construct the tunable IMPF below.
Fig. 6. (a) The spectra of the silicon nitride multimode microring resonator measured by the optical vector network method. (b) An enlarged view of TE2 and TE3 modes.
Table 3. 3 dB bandwidths of TE1∼TE5 modes.
3. Reconfigurable microwave photonic filter implementation
From the analysis above, five visible resonant modes were obtained, and the smallest bandwidth of 72.5 MHz (corresponding to Q factor of ∼2.67×106) was achieved for the TE2 mode. This high Q factor is attributed to the very low cavity loss and coupling loss of the multimode microring resonator. It is well known that for high-performance IMPFs, integrated optical filters with bandwidth down to hundred megahertz or even sub hundred megahertz are very promising due to the high precision spectrum process capability. So by using the high Q resonant modes of the proposed Si3N4 multimode microring resonator, we constructed a reconfigurable MPF with fast passband and stopband switching capability, which is shown in Fig. 8.
As shown in Fig. 8, by using a 90° RF hybrid coupler, the RF signal output from the vector network analyzer (VNA) is split into two parts with π/2 phase difference, which is used to drive the dual-drive Mach-Zehnder modulator (DDMZM) and modulate the optical carrier emitted by the laser diode (LD). By tuning the direct current (DC) bias of the DDMZM, the optical phase difference Δφ = πVDC/Vπ,DC between the two arms in the DDMZM can be changed. Where VDC and Vπ,DC are the DC bias voltage and the DC half-wave voltage of the DDMZM, respectively. By properly adjusting the DC bias of the DDMZM, balanced and unbalanced ODSB modulation schemes were introduced to achieve the passband and stopband switchable MPFs [28].
As known, a bandstop MPF can be obtained when the optical bandstop response of the multimode microring resonator filter is introduced into optical single sideband modulation (OSSB) link. However, in this situation, the RF rejection ratio is limited by the optical extinction ratio of the optical filter. In order to improve the RF rejection ratio of the bandstop MPF, an RF cancellation method based on unbalanced ODSB modulation [29] was introduced to enhance the RF rejection ratio, and its principle diagram is shown in Fig. 9.
As shown in Fig. 9, If the TE1 resonant mode is located at a short wavelength side (high-frequency side) of the optical carrier, the +1st order optical sideband cannot be wholly filtered due to the limited optical extinction ratio. However, by properly adjusting the -1st order optical sideband of the unbalanced ODSB with DC bias of the DDMZM, the RF photocurrents generated by beating between the ±1st order optical sidebands and the optical carrier at the TE1 mode’s resonant frequency could be same amplitudes and antiphase, which satisfies the RF cancellation conditions. Therefore, a bandstop IMPF with an enhanced RF rejection ratio can be achieved.
On the other hand, by properly adjusting the DC bias of the DDMZM, a balanced ODSB modulation (actually the equivalent phase modulation) with equal amplitudes of the ±1st order optical sidebands can also be obtained when Δφ=0 [29]. Then a bandpass MPF can be achieved by using this equivalent phase modulation, and the principle diagram is shown in Fig. 10. In Fig. 10, assume the optical carrier and the -1st order optical sideband are set at the low-frequency side of the filter resonance. The RF cancellation is satisfied out of the filter’s resonance band due to the equal amplitude and out of phase condition, while this RF cancellation condition is broken at the filter resonance. So an RF passband response can be obtained, which is equivalent to the passband MPF realized by the phase to intensity conversion using the optical stopband filter with phase modulation [30].
Here, by choosing the resonance of the TE1 mode and only adjusting the DC bias of DDMZM, a high-performance passband and stopband switchable MPF was achieved, and the measured results are shown in Fig. 11.
From Fig. 11, it can be obtained that the 3 dB bandwidths of stopband MPF and bandpass MPF are 120 MHz and 180 MHz, respectively. In addition, out of band RF rejection ratio about 27 dB was obtained for the passband filter due to the high-quality factor of the multimode ring resonator. RF rejection ratio of about 51 dB was achieved for the stopband filter by using the RF cancellation technology. Besides, by merely changing the laser wavelength, the frequency of MPF can be adjusted, as shown in Fig. 12. Due to optical double sidebands modulation adopted in the proposed MPF shown in Fig. 8, the upper-frequency boundary should be half of the frequency difference between the TE1 and TE4 resonances, which is about 40 GHz, as shown in Fig. 5(b). The lower frequency boundary is limited by the bandwidth of the multimode microring’s resonance. So a frequency tuning range of 2∼18 GHz was achieved for both the stopband and passband MPFs. The RF gain about -35 dB was achieved for the bandpass and stopband MPFs in the experiments. Also, the narrow passband/stopband MPF can be easily switched by adjusting the DC bias of the DDMZM. Besides, the overall RF performance of the proposed MPFs can be improved by introducing high-performance erbium doped fiber amplifier (EDFA), a high power semiconductor laser with low relative intensity noise (RIN), and other optimization techniques [31]. For example, with optical carrier suppressing achieved by cascade single-mode Si3N4 ring resonators, an IMPF with 260 MHz bandwidth, and excellent positive link gain about 1.8 dB was achieved in [11]. Comparing with [11], our proposed MPF has the following merits. First, only single Si3N4 multimode ring resonator is used, which is much easier to control; Second, the lower propagation loss and smaller coupling coefficient of the Si3N4 multimode ring resonator give a narrower bandwidth comparing with the Si3N4 single-mode ring resonator used in [11]; Third, by using the switchable modulation methods supported by the dual-drive Mach–Zehnder modulator, a passband/stopband switchable microwave photonic filter was obtained, which cannot be achieved by using the phase modulation method in [11].
In Fig. 12(a), the largest RF rejection ratio of the bandstop MPF is about 58 dB at 12 GHz after using the RF cancellation technique. Compared with the extinction ratio of TE1 mode shown in Fig. 6(a), the RF rejection ratio is enhanced by 41 dB using the RF cancellation technique.
Furthermore, by using the multimode resonances of the microring resonator, as shown in Fig. 13(a), (a) switchable dual-band passband and stopband MPF were also achieved, the measured results are shown in Fig. 13(b). As a dual-band stopband MPF, 3 dB bandwidths of two stopbands are 150 MHz and 240 MHz, respectively. As a dual-band passband MPF, 3 dB bandwidths of two passbands are 120 MHz and 320 MHz, respectively.
Fig. 13. (a) The selected two resonances of the silicon nitride multimode microring resonator. (b)The measured RF spectra of the dual-band bandpass and bandstop switchable MPF.
4. Conclusion
In conclusion, through theoretical analysis and experimental verification, the modes in the silicon nitride multimode microring resonator were analyzed and measured. Combing the narrowband resonances of the silicon nitride multimode microring and dual-drive Mach–Zehnder modulator, a high-performance reconfigurable microwave photonic filter with passband and stopband switching capability was achieved. The experimental results show that 3 dB bandwidths of the passband and stopband MPF are 180 MHz and 120 MHz, respectively. The frequency of the MPF can be tuned from 2∼18 GHz. Moreover, the RF rejection ratio of the stopband response is enhanced to about 51 dB by using the RF cancellation technology, and the out of band RF rejection ratio of the passband response is about 27 dB.
Funding
National Key Research and Development Program of China (2018YFB2201800); Natural Science Foundation of Jiangsu Province (BK 20161429).
Disclosures
The authors declare no conflicts of interest.
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