Paper Topic: Microwave Photonic Notch Filter
Si3N4 ring resonator-based microwave photonic . notch filter with an ultrahigh peak rejection
Microwave photonic (MWP) notch filter can be compared as the analogue of Band stop filter using electrical circuits. As name suggests, it is filter which supress or stop the passage of signal at certain range or specific frequency. MWP notch filter is explored excellently in removing the undesired signals making it efficient applicants in wireless communication, radar, radio astronomy. The ability to be tuned over many gigahertz maintaining high scale resolution (MHz), high selectivity and restriction to electromagnetic interference puts it as a better option than many electrical filters. One of the important components of filter is the filter itself which is made to pass the desired signal through. Researchers have developed many type of filters for this purpose working on different principle such as Stimulated Brillouin Scattering (SBS) based filter (example: chalcogenide waveguide), based on LiTaO3 WGM resonators and ring resonators which have been used in this paper.
Ring resonator is set of waveguides arranged in ring fashioned with atleast one waveguide is a closed loop coupled to some sort of signal input and output. It works on the principle of constructive and destructive interference along with total internal reflection. Due to observed fact that few wavelengths is observed in resonance, optical resonator works as filter. Two ring resonators can also be used in parallel or series configuration but here individual Si3N4 ring resonator is used. The problem with other filters so far observed is the tradeoff in peak rejection and bandwidth like SBS based filter 30-145 MHz of bandwidth but low rejection of 20 dB.
Silicon based resonators providing low loss, high index contrast is also one of the motivations for using in experiment to get away with this tradeoff. Here, they have incorporated Photonic Integrated Circuit (PIC) technology to lead to multi-functionalities like, modulation, signal processing and photodetection. With this, the paper demonstrates a novel method of MWP signal processing includes Si3N4 as ring resonator as optical filter with high rejection and bandwidth.
Principle and techniques
The signal processing for notch filter is done by using Novel method (reported in this paper) and compared with conventional notch filter technique. The later includes single-sideband (SSB) modulation in which optical resonance is used to remove a portion of undesired signal in the optical sidebands. And the respective optical spectrum is passed to photodetector where it is mixed with optical carrier for detection. The given equation governs the FWHM (Full Width at Half Maxima) bandwidth and peak rejections.
Novel notch filter: Unlike conventional notch filter, it involves generates two optical sidebands with tunable amplitude and phases. The electro-optic modulator (EOM) is used to encode RF signal with unequal amplitude and phase difference. The Phase difference of Δφ = ± π , is maintained between ring and optical sidebands by exploiting amplitude and phase of ring to equalise the amplitudes of the optical sidebands in desired frequency region. Then, a beat signal created by mixing of sidebands and optical carrier on photodetection giving a notch at selected frequency.
The RF insertion loss is one of the issues that should be addressed ( which is dominated by three factors).First is the insertion loss of the chip, which is relatively high (13 dB The second source of loss was the electrical-to-optical (E/O) and optical-to-electrical (O/E) conversions in the MWP link. RF loss of around 30 dB is also observed due two factors mentioned above The application of MWP Notch filter, still requires ultrahigh peak rejection with match to state of RF art filters with low loss and less pump power.
Conclusion and Future Perspectives
A efficient method is implemented to get away the trade-off between bandwidth and peak rejection using low loss Si3N4 ring resonator as MWP notch filter. The Novel scheme showed the peak rejection of greater than 55 dB in the range of 247 to 840 MHz. The Reported technique is observed to be impressive alternative than conventional SSB based filter.The filter (Si3N4 ring resonator) achieved a comparable resolution and peak rejection to state-of-the-art RF notch filters. The central frequency of laser was tuned that preserved a narrow bandwidth of 350 MHz and ultrahigh rejection of >55 dB. The technique developed here can used with fiber-Bragg gratings (FBGs) filters like SBS based and waveguide based filters.One can exploit other materials also, as resonating filters (ex-Si3N4 here) to get improved rejections in signal with low loss.