Objective Flat-top filters have been widely employed as channel selectors in wavelength division multiplexing systems due to their unique flat-top response characteristics, which can reduce the crosstalk among wavelengths and improve the rapidity and accuracy of channel optical detection. A large number of integrated schemes have been proposed and demonstrated in recent years, and most of them are based on silicon-on-insulator (SOI) platforms due to their capability for integration with electronics. However, these schemes have some disadvantages, with the schemes based on photonic crystal, waveguide grating, and cascaded microring resonators having a small fabrication tolerance. Meanwhile, schemes based on multistage cascaded Mach-Zehnder interferometer (MZI) have large footprints. Schemes based on microring resonator (MRR)-assisted MZI are proposed to achieve flat-top passband and small footprints, but in most previous schemes, an external phase shift of pi or pi/2 should be applied on the MRR or the long arm of MZI, which is difficult to achieve in practical fabrication due to the variations of effective refractive index and fabrication error. Additionally, some performance indexes such as the shape factor and ripple factor are not analyzed in these schemes. Therefore, we theoretically analyze and experimentally verify a flat-top filter with a high shape factor and low-complexity fabrication processing. Our scheme is based on the SOI platform and consists of a racetrack MRR (RMRR) and an asymmetric MZI. In our scheme, no external phase shift is needed. In addition, we analyze all key indicators of the filtering performance of a flat-top filter, especially the indicators that evaluate the filter shape including the shape factor and the ripple factor. Our scheme features a high shape factor, low-complexity fabrication processing, small size, light weight, and low power consumption. It can not only be widely adopted in high-speed optical network communication but also be designed as a part of the wavelength multiplexer by multi-stage cascading. Methods Our flat-top filter consists of an asymmetric MZI coupler and an RMRR, with the MZI consisting of a pair of 2x2 multi-mode interferometers (MMIs). The input signal is divided into two light beams by the first MMI and transmits along the upper and lower arms of the MZI. The light beam in the upper arm is coupled into the RMMR to form an all-pass RMRR, and then the output light beam interferes with the light beam in the lower arm at the second MMI. A rectangular spectrum is generated ultimately. Due to the difficulty in realizing a phase shift of pi or pi/2 in practical fabrication, we ignore it and optimize the performance of our filter by adjusting other structural parameters such as the gap between RMRR and the short arm of MZI or the length of the coupling waveguide. A micro-heater is fabricated on the RMRR to investigate the effect of the phase shift introduced by RMRR on the performance of our flat-top filter. Results and Discussions The bandwidth of 3 dB of our filter is 1.94 nm. The ripple factor and the sidelobe suppression ratio are about 2.40 dB and 7.45 dB respectively. The insertion loss and FSR are about 1.82 dB and 3.94 nm respectively (Fig. 6). It is irrational to measure the bandwidths of 10 dB and 15 dB under the sidelobe suppression ratio of less than 10 dB. However, the shape factor is a crucial performance indicator of the flat-top filter, and the sidelobe suppression ratio can be significantly improved by controlling the micro-heater fabricated on the coupling area of RMRR. Therefore, we still calculate the shape factor by the same method in the simulation. The widths of the passband are 2.02 nm and 2.06 nm when the passband power declines by 10 dB and 15 dB respectively. As a result, the shape factor is 0.96 (1.94/ 2.02) and 0.94 (1.94/2.06). Additionally, we also measure the output spectra while tuning the voltage applied on the RMRR. The central wavelength of our filter gradually experiences redshift, and the filtering performance periodically varies in the trend of degradation, improvement, and degradation, which is mainly because of the periodical variation of the phase shift introduced by RMRR with the increasing temperature. It indicates that tuning the phase shift introduced by RMRR can effectively control the output spectra of the filter. Conclusions We propose and demonstrate a flat-top filter with a high shape factor and low-complexity fabrication processing, and analyze all key indicators of the filtering performance of the flat-top filter, especially the indicators that evaluate the filter shape including the shape factor and the ripple factor. A filter with 3 dB bandwidths of 1.94 nm is realized. The corresponding shape factor 1 and shape factor 2 are 0.96 and 0.94 respectively. The ripple factor, the sidelobe suppression ratio, and the insertion loss are about 2.40 dB, 7.45 dB, and 1.82 dB respectively. Our scheme does not have energy loss throughout the transmission process as the spectra of the two output ports are complementary. Furthermore, we investigate the influence of the phase shift introduced by RMRR on the performance of our flat-top filter and verify that the performance of our filter will vary periodically by adjusting the voltage applied to the RMRR. Our scheme is characterized by a high shape factor, low-complexity fabrication processing, small size, light weight, and low power consumption. Additionally, it can not only be widely utilized in high-speed optical network communication but also be designed as a part of the wavelength multiplexer by multi-stage cascading.
