We propose a simple scheme to realize ultra-high peak rejection notch microwave photonic filter (MPF) based on a single silicon microring resonator (MRR). Using the combination of a conventional phase modulator (PM), a tunable bandpass filter (TBF), and a silicon MRR to manipulate the phase and amplitude of optical sidebands resulting in a signal cancellation at the RF notch filter frequency, we experimentally demonstrate a notch MPF with an ultra-high peak rejection beyond 60 dB. The frequency tunability of the proposed ultra-high peak rejection MPF is also demonstrated in the experiment.
© 2015 Optical Society of America
Microwave photonic filters (MPFs) can be employed to process microwave signals in the optical domain by using photonic devices. These filters can benefit from the wide bandwidth, low loss, and natural immunity to electromagnetic interference that photonic signal processing offers [1, 2 ]. Several approaches to realizing MPFs have been proposed and demonstrated based on fiber devices [3–8 ]. However, high operating frequency with wide tuning range is difficult to be achieved using fiber devices based MPFs. Compared to these fiber devices, Silicon-on-Insulator (SOI) based waveguides can offer distinct advantages of increased stability and reliability, low cost, small footprints, and compatibility with other integrated optoelectronic devices . In the recent years, some MPFs based on SOI microring resonator (MRR), microdisk resonator (MDR), and Mach–Zehnder Interferometer (MZI) have been proposed and demonstrated showing superior characteristics [10–15 ].
Notch MPFs can be used to remove interferers in wideband radio systems, such as cognitive  or ultrawideband (UWB)  radios. Notch MPFs with high peak rejection are usually desired in these applications. However, since it is very difficult to realize high extinction ratio optical filter using integrated MRR, MDR or MZI, achieving an ultra-high peak rejection (beyond 50 dB) MPF in integrated platform is very challenging, especially if one use conventional single sideband (SSB) modulation technologies [13, 18 ] and map the optical filter spectrum to the microwave domain. Recently, D. Marpaung et al. reported a scheme to realize integrated ultra-high peak rejection MPF in silicon nitride platform by adopting novel two optical sidebands moulation with tunable phase difference and amplitude ratio and employing a low-loss tunable Si3N4 optical ring resonator as the optical filter . The proposed MPF features impressive operation performance and one key element in the MPF enabling flexible two optical sidebands moulation is a special dual-parallel Mach-Zehnder modulator (DPMZM) driven through a 90° (quadrature) radio frequency (RF) hybrid coupler . Careful DPMZM bias control is desired in this special modulation. The bias drift of the DPMZM may degrade the operation performance of the MPF, thus the long term stability of the MPF could be a problem. So far ultra-high peak rejection MPF based on a conventional modulation technique is still very challenging. In this scenario, a laudable goal would be to explore a simple approach to achieve an integrated ultra-high peak rejection MPF with increased stability using a conventional modulation scheme.
In this paper, we propose a simple yet effective approach to realizing notch MPF with an ultra-high peak rejection . We use the combination of a conventional phase modulator (PM), a tunable bandpass filter (TBF), and an SOI MRR to manipulate the phase and amplitude of optical sidebands for inducing a signal cancellation at the RF notch filter frequency. The PM is used to generate lower sideband (LSB) and upper sideband (USB) with precise π phase difference. No bias control is required in this conventional phase modulation. The TBF is used to realize an asymmetric amplitude modulation of the LSB and USB. The MRR serves as an optical notch filter. Using the proposed approach, we are able to achieve a notch MPF with an ultra-high peak rejection beyond 60 dB. Tunability of the proposed ultra-high peak rejection MPF is also demonstrated in the experiment.
2. Concept and operation principle
Figure 1(a) illustrates the typical scheme of an MPF based on conventional SSB modulation using an SOI MRR resonator. The microwave signal is modulated on an optical carrier, and then processed by the SOI MRR and detected by a photodiode (PD). The optical resonance of the SOI MRR is mapped to the microwave frequency to generate a notch filter response. Since it is hard to realize high extinction ratio optical filter using integrated MRR, achieving an ultra-high peak rejection MPF is always difficult. Figure 1(b) summarizes the operation principle of the proposed ultra-high peak rejection notch MPF. An optical carrier is modulated by an RF signal with a conventional phase modulation. It is well known that the generated LSB and USB are out of phase. If the modulated signal is applied directly to a PD, due to the π phase difference between the LSB and USB, no RF signal is obtained. Here the modulated signal is sent to a TBF first, and the USB signal is attenuated after the TBF. The output field is then applied to the SOI MRR. Note that the resonant frequency of the SOI MRR is aligned with the frequency of LSB signal, thus the LSB signal will be filtered by the SOI MRR. Since the extinction ratio of the SOI MRR is not very high, the amplitude of the remaining LSB signal could be equal to the USB signal. Due to the equal amplitude and the π phase difference of the LSB and USB, the signal power at the modulated RF frequency will be cancelled after detection by the PD. This is a photonic implementation of a notch filter in microwaves which exhibits infinite rejection in principle.
The optical field at the output of the PM can be written asEq. (1) can be expanded to beEq. (4), TMRR (ω) is expressed by
We further calculate the RF response of MPF with different TBF locations to show the realization of ultra-high peak rejection MPF. In the simulations, the transmission spectrum of the TBF is fitted from the measured transmission curve as shown in Fig. 2 . The 30-dB bandwidth of the TBF is about 0.765 nm. The insertion loss of the TBF is less than 6 dB. The parameters used to calculate MRR field transmission are also extracted from the measured MRR transmission spectrum. The MPF responses with different TBF locations are shown in Fig. 3 . The optical carrier wavelength is 1581.576 nm, and the central wavelength of TBF is increased from 1581.746 nm to 1581.836 nm with a step of 0.03 nm. These wavelength values are in agreement with the experimental values. It can be clearly seen from Fig. 3(c) that an ultra-high peak notch MPF is obtained when the central wavelength of TBF is 1581.776 nm.
4. Fabrication and characterization of the silicon microring resonator
To experimentally demonstrate the proposed ultra-high peak rejection notch MPF, we fabricate the MRR on a commercial SOI wafer with a 340-nm-thick silicon slab on the top of a 2-μm silica buffer layer. The device pattern was transferred to photoresist by E-beam lithography (Vistec EBPG5000 + ES). Then the upper silicon layer was etched downward for 220 nm to form a ridge waveguide through induced coupled plasma (ICP) etching (Oxford Instruments Plasmalab System100). Figure 4(a) shows the scanning electron microscope (SEM) image of the fabricated device. The waveguide width of both bus waveguide and MRR is about 500 nm. The radius of the MRR is 10 μm. The coupling gap between the bus waveguide and MRR is about 400 nm. We use a grating coupler to couple the light in and out of the silicon chip. The coupling loss per side is less than 7 dB, so the insertion loss of the MRR is less than 14 dB. Figure 4(b) show the typical transmission spectrum of the MRR. The resonant wavelength of the MRR is 1581.737 nm, and the 3-dB bandwidth of the MRR transmission spectrum is about 0.1 nm.
5. Experimental setup
Figure 5 depicts the experimental setup. A tunable laser diode (TLD) emits a continuous wave (CW) light. An amplifier (EA) is used to amplify the RF signal from vector network analyzer (VNA). The CW light is modulated by a PM to produce an optical double sideband (DSB) signal. The TBF is used to modify the USB of the signal to obtain a modified asymmetric optical DSB signal. The output field is then applied to the SOI MRR. After the device, the optical signal is converted to electrical signal by a PD and analyzed by the VNA.
6. Experimental results
The measured optical spectra after the TBF and the corresponding MPF responses are shown in Fig. 6 . The optical carrier wavelength is 1581.576 nm. The optical spectra are measured by modulating a microwave signal to the optical carrier. The frequency of the microwave signal is 20 GHz, which is comparable with the central frequency of the MPF. Figures 6(a)–6(d) depict the optical spectra when the central wavelength of the TBF is 1581.746 nm, 1581.776 nm, 1581.806 nm and 1581.836 nm, respectively. The transmission spectra of the TBF in these four cases are also plotted in Figs. 6(a)-6(d) for reference. Figures 6(e)-6(h) show the corresponding MPF responses. The theoretical results shown in Fig. 3 agree well with the experiment results shown in Figs. 6(e)-(h). As shown in Figs. 6(a) and 6(e), when the USB component is slightly attenuated by the TBF, after passing through the MRR, the amplitude difference of the LSB and USB is relatively large, resulting in a shallow RF notch of about 14.5 dB. When we shift the TBF, more power of the USB component is attenuated, and the peak rejection of the MPF remarkably increases. The peak rejection of the MPF is 29.1 dB when the central wavelength of the TBF is 1581.776 nm as shown in Figs. 6(b) and 6(f). A maximum peak rejection of about 61.5 dB is observed with balanced amplitude between LSB and USB components when the central wavelength of the TBF is 1581.806 nm as shown in Figs. 6(c) and 6(g). When we further increase the central wavelength of the TBF, the USB signal will be further attenuated, thus the peak rejection decreases again. For example, when the central wavelength of the TBF is 1581.836 nm, the USB almost disappears, and the peak rejection of the MPF decreases to 40.9 dB as shown in Figs. 6(d) and 6(h).
By changing the carrier light wavelength, the operating frequency of the ultra-high peak rejection MPF can be tuned. To comprehensively describe the tunable operation of the MPF, we measure the operating frequency tunability of the MPF, as shown in Fig. 7 . When the wavelength of the carrrier light is changed from 1581.476 to 1581.626 nm, the central frequency of the MPF is tuned from 12.4 to 30.6 GHz, maintaining a ultra-high peak rejection. The obtained results shown in Fig. 7 indicate that the proposed ultra-high peak rejection MPF can operate over a large tunable frequency range.
Previous work of ultra-high peak rejection MPF  employs modulation-based SSB while the proposed approach actually exploits optically-controlled SSB. In the previous work , it relies on the generation of a specific dual sideband modulated signal with tunable sidebands amplitude and phase difference. A precise π phase difference between the LSB and USB components is achieved by carefully adjusting the modulation parameters. The key device is the DPMZM driven through a 90° RF hybrid coupler. So two MZMs, one PM and one 90° RF hybrid coupler are used in the configuration which is relatively complicated. In contrast, the proposed approach only employs a conventional PM followed by a TBF in the configuration which greatly reduces the complexity. The conventional PM generates a dual sideband modulated signal with perfect π phase difference between the LSB and USB components. The TBF adjusts the amplitude difference between the LSB and USB components. Different from DPMZM requiring careful control of three biases which may cause unstable MPF operation due to bias drift, the proposed approach is bias free and stable.
The insertion losses of the proposed MPF at system level mainly come from the modulator, TBF and MRR. With future improvement, i) the insertion loss of the TBF could be reduced by employing other kinds of low-loss bandpass filter such as fiber Bragg grating filter (insertion loss < 1 dB); ii) fully etched apodized grating coupler could be used to greatly reduce the fiber-chip-fiber coupling loss (~0.58 dB per side) ; iii) considering the possible integration of the PM, TBF and MRR on a single chip, the loss of the total system might be greatly reduced.
Remarkably, it can be seen from Eq. (4) and Eq. (5) that the final amplitudes of the three components (carrier, USB, LSB) are determined by the TBF and MRR. Since the TBF does not introduce extra phase change, the final phases of the three components (carrier, USB, LSB) are determined by the phase response of MRR. As a consequence, the amplitude and phase responses of the MRR divide the operation of MPF into two cases: i) the bandwidth (full-width at half maximum, FHWM) of the MRR is relatively smaller compared to the central frequency of MPF; ii) the bandwidth of the MRR is comparable with the central frequency of MPF.
i) When the bandwidth of the MRR is small, the typical amplitude and phase responses before and after MRR are shown in Fig. 8 .
Assuming the desired central frequency of the MPF is fr which is much larger than the bandwidth of the MRR, it can be seen that the carrier and the USB component are not affected by the MRR. For the LSB located at the resonant frequency, since the phase response of the MRR at the resonant frequency is zero, only the amplitude of the LSB is modified by the MRR. As a result, the phase relations of the three components (carrier, USB, LSB) of the microwave modulated optical signal remain unchanged after the MRR. Thus the resultant microwave modulated optical signal after the MRR can be written as
After detected by a PD, the RF signal will be a null (ultra-high peak rejection in the RF response). The simulation result is shown in Fig. 9 . The 3-dB bandwidth and central frequency of the ultra-high peak rejection notch MPF are about 60 MHz and 19.32 GHz, respectively.
ii) When the bandwidth of the MRR is comparable with the central frequency of the MPF, the typical amplitude and phase responses before and after MRR are shown in Fig. 10 .
In this case, the phase response of the MRR may affect all the three components (carrier, USB, LSB) of the microwave modulated optical signal. To achieve ultra-high peak rejection, the phase change of LSB and USB relative to the carrier should be opposite, and the resultant microwave modulated optical signal after the MRR can be written asFig. 11 . The 3-dB bandwidth and central frequency of the ultra-high peak rejection notch MPF are about 8 GHz and 19.2 GHz. The corresponding offset between the LSB and the resonant frequency of the MRR shown in Fig. 10 is about 120 MHz.
For low frequency operation, the performance of the MPF might be degraded which can be ascribed to the limition on the accuracy and edge slope of the bandpass filter. As a result, after detection by a PD, the RF response of the MPF could be asymmetric. A typical RF response working at low frequency is shown in Fig. 12 . The central frequency of the MPF is about 3.7 GHz. The operation performance of the MPF at low frequency might be further improved by employing bandpass filters with ultra-narrow bandwidth and ultra-sharp filtering edge. Bandpass filtering by stimulated Brillouin scattering (SBS) could be also considered .
In summary, we have reported a simple integrated ultra-high peak rejection notch MPF based on a single silicon MRR. We use the combination of a conventional PM, a TBF and an SOI MRR to manipulate the phase and amplitude of optical sidebands for inducing a signal cancellation at the RF notch filter frequency. In the proposed ultra-high peak MPF configuration, a conventional PM is adopted to generate LSB and USB with π phase difference. No bias control is required. The TBF is used to realize an asymmetric amplitude modulation of the LSB and USB. The MRR is employed to serve as an optical notch filter. Using the proposed simple configuration, we experimentally demonstrate a notch MPF with an ultra-high peak rejection beyond 60 dB. Moreover, tunable operation with the central frequency changing from 12.4 to 30.6 GHz of the proposed ultra-high peak rejection MPF is also demonstrated in the experiment. With future improvement, considering the possible integration of the PM, TBF, MRR and PD on a single chip, one would expect to see a fully compact ultra-high peak rejection MPF benefiting from integrated microwave photonics.
This work was supported by the National Natural Science Foundation of China (NSFC) under grant 61222502, the Program for New Century Excellent Talents in University (NCET-11-0182), the Wuhan Science and Technology Plan Project under grant 2014070404010201, the Fundamental Research Funds for the Central Universities (HUST) under grants 2012YQ008 and 2013ZZGH003, and the seed project of Wuhan National Laboratory for Optoelectronics (WNLO). The authors thank the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for the support in the manufacturing process of silicon microring resonator and the facility support of the Center for Nanoscale Characterization and Devices of WNLO. The authors would also like to thank Chao Li, Han Zhang, Chengcheng Gui and Qi Yang for their valuable technical supports and helpful discussions.
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