Interference mitigation is crucial in modern radio frequency (RF) communications systems with dynamically changing operating frequencies, such as cognitive radios, modern military radar, and electronic warfare (EW) systems. To protect sensitive RF receivers in these systems, frequency agile RF filters that can remove interferers or jammers with large variations in frequency, power, and bandwidth are critically sought. Unfortunately, an RF bandstop or notch filter that can simultaneously provide high resolution, high peak attenuation, large frequency tuning, and bandwidth reconfigurability does not presently exist. Microwave photonic (MWP) filters are capable of tens of gigahertz tuning and have advanced in terms of performance, but most are limited in stopband rejection due to the challenge in creating a high-quality-factor optical resonance used as the optical filter. To achieve MWP filters with similar performance to state-of-the-art RF filters in terms of isolation bandwidth and rejection is still very challenging, especially in compact integrated photonic chip footprint.
Microwave photonic filters based on stimulated Brillouin scattering (SBS) have shown excellent properties in terms of high resolution and extinction. Although efficiently generated in soft glasses such as chalcogenides, there is a strong demand to harness SBS in silicon, a material platform that supports large-scale integration between photonics and electronics. For the CMOS-compatible silicon-on-insulator (SOI) platform, SBS has been elusive. The low elastic mismatch between the silicon core and the silicon dioxide substrate results in weak acoustic confinement, preventing buildup of the SBS process.
The first functional device for signal processing based on SBS from a silicon nanowire was developed that employs a novel cancellation filter technique to harness the modest SBS gain in silicon, creating a high-performance microwave photonic notch filter (Figure 1a). Only 0.98 dB of on-chip SBS gain was used to create a cancellation microwave photonic notch filter with 48 dB of suppresion, 98 MHz linewidth (Figure 1b), and 6-GHz frequency tuning (Figure 1c). This establishes the path toward monolithic integration of high-performance SBS microwave photonic filters in a CMOS-compatible platform such as SOI.
Microwave photonic cancellation notch filters have been shown capable of achieving ultra-high suppression independently from the strength of the optical resonant filter used, making them an attractive candidate for on-chip signal processing. Their operation, based on destructive interference in the electrical domain, requires precise control of the phase and amplitude of the optical modulation sidebands. To date, this was attainable only through dual-parallel Mach-Zehnder modulators that suffer from bias drifts that prevent stable filter operation. A new cancellation filter topology provides ease of control and enhanced stability using a bias-free phase modulator and a reconfigurable optical processor (implemented using a Fourier-Domain Optical Processor) as the modulation sideband’s spectral shaper. The long-term stability of the filter topology was verified through continuous real-time monitoring of the filter peak suppression over 24 hours.
Compared to previous demonstrations, the new implementation exhibited far greater stability, due to the elimination of the modulator biasing. Furthermore, the use of a Fourier-Domain Optical Processor allowed independent control over the phase and amplitude of the sidebands, greatly reducing system complexity. This enabled the realization of a simple algorithm for controlling the filter suppression, and resulted in the first demonstration of a MWP notch filter with high 40-dB suppression over a 24-hour period.
The filter can be implemented in an instantaneous frequency measurement system (IFM). IFM systems estimate unknown microwave frequencies through indirect measurement; for example, through well calibrated power measurements. These systems are useful as alternatives to direct spectrum analysis, which can be heavy, complicated, and limited in the frequency range. The frequency measurement system developed in this work simultaneously achieves the estimation of multiple frequency measurement up to 38 GHz, with errors lower than 1 MHz in a centimeter-scale chalcogenide glass waveguide.
The approach circumvents the fundamental tradeoff between measurement range and accuracy. Its channelized frequency band offers an inherent capability to resolve and process multiple simultaneous RF inputs over a wide spectrum, provided the separation between multiple microwave frequencies is larger than the channel frequency spacing. The results presented here point to new possibilities for creating a high-performance, integrated, on-chip IFM system that will help assure critical mission success at minimal costs, and enhanced security for manned and/or unmanned aircraft, surface vessels, and next-generation radar, with potential for monolithic integration in silicon chips.
The design and fabrication procedure for the photonic chip was optimized, resulting in an on-chip gain of up to 52 dB, which is an almost 1000× improvement over previous results. Crucially, these results achieved through fabrication improvements led to enhanced power handling of the devices and propagation losses of 0.5 dB/cm, allowing for long propagation lengths of 13 cm and 23 cm (fabricated in the form of spirals), and effective lengths of 6.5 cm and 8 cm, respectively.
This new class of RF photonic filters based on frequency-selective RF interference can decouple the filter peak suppression and bandwidth resolution. The technology led to several distinct advantages, including ultra-high suppression and high resolution while maintaining a wide frequency tuning. However, these filters are based on RF signal cancellation that occurs across the whole spectrum, leading to unwanted additional loss of signals in the passband, which can degrade the filter signal-to-noise ratio (SNR) performance.
Creating a Filter Prototype
A computer-controlled tunable notch filter prototype was built in this project (Figure 2). The prototype hosted various components including two integrated tunable laser assemblies (ITLAs) that acted as the SBS pump and probe lasers. The maximum output power of these lasers is 15 dBm. The modulator used in the prototype is a 40-GHz phase modulator. The SBS medium used is a 1-km standard single mode fiber spooled in a small form factor. The pump and probe signal levels can be controlled using electrical and manual variable optical attenuators (VOA). A micro-EDFA is used to boost signal level prior to photodetection. A high-power-handling detector with 50-mA output current is used to achieve low RF insertion loss. All electrical power supplies for the lasers, EDFA, photodetector, VOA, cooling system (fan), and USB communications are hosted inside the prototype enclosure.
The filter can operate with two modes; namely, with the internal phase modulator in conjunction with a Fourier-domain optical processor, or with an external dual-parallel Mach-Zehnder modulator (DPMZM). Internal reconfiguration of the optical connections should be made to switch between these two modes of operation.
The filter can be controlled and programmed through software. The communication between the prototype and the computer is done through USBs. The control software is written in LabVIEW.
Preliminary characterizations have been carried out for the prototype. The bandwidth can be tuned from 20 MHz to 46 MHz, and the typical insertion loss of the filter is -14 dB.
The long-term stability of the notch frequency and suppression was characterized, in the mode of operation, using the Fourier-domain optical processor. The frequency stability is on the order of 50 MHz, which is limited by the frequency noise and stability of the ITLAs. As a comparison, using an external cavity laser, notch stability on the order of 10 MHz can be expected. The notch suppression, on the other hand, can be maintained higher than 20 dB over a long period.
The objective of this work was to develop a novel microwave photonic (MWP) notch filter with a very narrow isolation bandwidth, an ultra-high stopband rejection, wide frequency tuning, and flexible bandwidth reconfigurability — all integrated in a compact photonic chip. The prototype demonstrated computer-control capability of reconfiguring the filter properties, such as center frequency, bandwidth, and suppression.