Photonic Notch Filter Research Articles

Integrated microwave photonic notch filter using a heterogeneously integrated Brillouin and active-silicon photonic circuit

Microwave photonics (MWP) has unlocked a new paradigm for Radio Frequency (RF) signal processing by harnessing the inherent broadband and tunable nature of photonic components. Despite numerous efforts made to implement integrated MWP filters, a key RF processing functionality, it remains a long-standing challenge to achieve a fully integrated photonic circuit that can merge the megahertz-level spectral resolution required for RF applications with key electro-optic components. Here, we overcome this challenge by introducing a compact 5 mm × 5 mm chip-scale MWP filter with active E-O components, demonstrating 37 MHz spectral resolution. We achieved this device by heterogeneously integrating chalcogenide waveguides, which provide Brillouin gain, in a complementary metal-oxide-semiconductor (CMOS) foundry-manufactured silicon photonic chip containing integrated modulators and photodetectors. This work paves the way towards a new generation of compact, high-resolution RF photonic filters with wideband frequency tunability demanded by future applications, such as air and spaceborne RF communication payloads.

Electrically Tunable Topological Notch Filter for THz Integrated Photonics

AbstractElectromagnetic filtering is essential for emerging integrated photonic technologies and widely adaptable processing of high‐bandwidth signals. The conventional electro‐optic modulator‐based photonic approach for signal filtering at microwave frequencies cannot be implemented at terahertz (THz) frequencies due to the unavailability of the THz signal‐driven optical modulator. Here, an electrically tunable on‐chip THz photonic notch filter based on the topologically protected valley hall waveguide‐cavity platform is demonstrated. The device shows a significantly large notch suppression depth of more than 20 dB with a return loss of 13 dB in the entire tuning range of notch frequency. The feedback control circuit enables precise control of the notch frequency shift with a minimum step size of 7 MHz. This work extends the application of topological photonic crystals in developing THz‐integrated photonic devices for transformative technologies, including sixth‐generation (6G) communication and high‐resolution spectral sensing.

ANN-Assisted High-Resolution and Large Dynamic Range Temperature Sensor Based on Simultaneous Microwave Photonic and Optical Measurements

We proposed and demonstrated a temperature sensor with both high resolution and large dynamic range based on simultaneous microwave photonic and optical measurements. To simultaneously increase the sensitivity and dynamic range of optical measurement, two cascaded ring resonators (CRRs) with different temperature sensitivities and free spectral ranges (FSRs) are designed as the sensing probe and fabricated based on silicon-on-insulator (SOI) wafer. Based on the ultrahigh-Q-factor microring in the CRR, a microwave photonic notch filter (MPNF) is obtained and used to achieve a high temperature resolution. Meanwhile, the optical transmission spectrum of the CRR is used to achieve a large dynamic range. Combining the two measurements, a temperature sensor with both high resolution and large dynamic range is realized. To improve the response speed and robustness of the sensing system, the artificial neural network (ANN) is used to directly extract the temperature from the CRR spectrum, which is particularly helpful at low signal-to-noise ratio (SNR). The sensing resolution and dynamic range achieved by our scheme are <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$7.02 \times 10^{-3}$ </tex-math></inline-formula> °C and 106.94 °C, respectively.

Guided-acoustic stimulated Brillouin scattering in silicon nitride photonic circuits.

Coherent optomechanical interaction known as stimulated Brillouin scattering (SBS) can enable ultrahigh resolution signal processing and narrow-linewidth lasers. SBS has recently been studied extensively in integrated waveguides; however, many implementations rely on complicated fabrication schemes. The absence of SBS in standard and mature fabrication platforms prevents its large-scale circuit integration. Notably, SBS in the emerging silicon nitride (Si3N4) photonic integration platform is currently out of reach because of the lack of acoustic guidance. Here, we demonstrate advanced control of backward SBS in multilayer Si3N4 waveguides. By optimizing the separation between two Si3N4 layers, we unlock acoustic waveguiding in this platform, potentially leading up to 15× higher Brillouin gain coefficient than previously possible in Si3N4 waveguides. We use the enhanced SBS gain to demonstrate a high-rejection microwave photonic notch filter. This demonstration opens a path to achieving Brillouin-based photonic circuits in a standard, low-loss Si3N4 platform.

Multi-Band and Frequency-Agile Chip-Based RF Photonic Filter for Ultra-Deep Interference Rejection

Unwanted high-power and frequency-agile radio-frequency (RF) signals cause saturation and nonlinear distortions in sensitive RF receivers. RF notch filters with reconfigurable responses over multiple frequency bands are highly sought after to prevent these detrimental effects. Microwave photonic (MWP) notch filters have shown increased frequency agility compared to their electronic counterparts. However, demonstrated filter schemes focus only on single notch responses, leaving them unable to simultaneously attenuate multiple interferers over a wide frequency range. In this work, we demonstrate a high-performance chip-based MWP notch filter with three independent notches widely tunable over 20 GHz. The filter exhibits low RF passband losses of 8 dB and peak notch depth greater than 40 dB with 500 MHz spectral resolution. Specifically, each notch is formed through cascaded optical processing from on-chip, low-loss Si<inline-formula><tex-math notation="LaTeX">$_3$</tex-math></inline-formula>N<inline-formula><tex-math notation="LaTeX">$_4$</tex-math></inline-formula> micro-resonators and Brillouin gain on an As<inline-formula><tex-math notation="LaTeX">$_2$</tex-math></inline-formula>S<inline-formula><tex-math notation="LaTeX">$_3$</tex-math></inline-formula> photonic chip. We use this filter to demonstrate high-performance analog RF filtering by substantially attenuating multiple interferers, and show effective image rejection during RF down-conversion in a communications receiver to enable a large reduction of error vector magnitude (EVM) from 60&#x0025; to 15&#x0025;. Finally, we provide performance analysis and design perspectives for future photonic integration of the proposed filter subsystem.

Dynamic Dispersion-Compensation for Wideband Microwave Photonic Notch Filter With High Rejection and High-Resolution

Interference-based microwave photonic notch filters (MPNFs) provide large rejection through destructive interference of radio frequency signals generated by beating a carrier with the upper and lower modulation sidebands. The two modulation sidebands are designed to have an amplitude imbalance and phase shift of π between them. In many schemes, large rejection is achieved at the desired radio frequency by balancing the amplitudes of the modulation sidebands using stimulated Brillouin scattering (SBS) based gain or loss. However, the combined dispersion of the Brillouin active medium and other components detunes the phase difference between the sidebands away from π, which causes a reduction in the notch depth. Dynamic dispersion control is, therefore, desirable for compensating dispersion-induced notch depth reduction. Here, we report dispersion-compensation exploiting bias-dependent phase of the modulation sidebands of a z-cut intensity modulator. We demonstrate a high-resolution ( ~ 50 MHz) MPNF with a large rejection of $>$65 dB over 1-25 GHz. To demonstrate the applicability of our MPNF in the presence of radio frequency interference, we suppress an interferer by 49 dB without affecting the signal. Such dispersion-compensated MPNFs are of importance in the sphere of Industry 4.0 based applications such as frequency measurement.

Electromagnetically-Induced-Absorption-Mediated Ultrahigh-Rejection Microwave Photonic Notch Filter Using Single-Sideband Modulation

The inherent narrow bandwidth and wavelength-transparent nature of the stimulated Brillouin scattering (SBS) process make it a potential candidate for high-resolution wideband microwave photonic notch filters (MPNFs). The simplest way to realize a SBS-based MPNF is to use the narrowband Brillouin loss resonance with single-sideband modulation, where the modulation sideband is at the anti-Stokes frequency. However, using Brillouin loss with single-sideband modulation is highly inefficient because a large notch extinction requires large Brillouin loss, and thus, a large pump power. Here, we exploit an analogue of electromagnetically induced absorption (EIA) in the microwave domain to enhance MPNF rejection, at a fixed Brillouin pump power, by more than 6 orders of magnitude. We use this analogue of EIA to realize a MPNF with $&gt;75$-dB rejection up to 40 GHz, using a Brillouin loss of about 15 dB. We harness a coherent Brillouin interaction between the orthogonally polarized components of a probe, which is comprised of a laser carrier and a single-modulation sideband. Stimulated Brillouin scattering and other components used in our demonstration have recently been demonstrated on integrated platforms. The proposed scheme, therefore, has the potential for realizing efficient integrated microwave photonic notch filters with ultrahigh rejection.

Microwave photonic notch filter with integrated phase-to-intensity modulation transformation and optical carrier suppression.

We demonstrate for the first time, to the best of our knowledge, an on-chip microwave photonic (MWP) notch filter with high stopband rejection and integrated optical carrier suppression in a phase modulator-based system. The notch filter was achieved through phase modulation to intensity modulation (PM-to-IM) transformation and dual-sideband-processing using a network of three ring resonators (RRs) in a low-loss silicon nitride (Si3N4) platform. We show simultaneous PM-to-IM conversion and optical carrier processing for enhancing the filter performance using a single RR. We achieve filtering with a high stopband rejection of >55dB, an optical carrier suppression up to 3 dB, a radio frequency link gain of 3 dB, a noise figure of 31 dB, and a spurious-free dynamic range of 100dB⋅Hz2/3. These experiments point to the importance of vectorial spectral shaping of an MWP spectrum for advanced functionalities.

Microwave photonic notch filter based on polarisation multiplexing and cross gain modulation in a semiconductor optical amplifier

A coherent interference-free tunable microwave photonic notch filter with a pair of negative and positive coefficients generated by employing polarisation multiplexing and cross gain modulation in a semiconductor optical amplifier is proposed and demonstrated through simulation results. The negative and positive coefficients of the notch filter are realised through two orthogonal polarisation states of a single continuous wave laser diode. The free spectral range of microwave photonic filter is tuned by varying the differential group delay of a birefringent single-mode fibre. The proposed filter shows good tunability, easy implementation and is cost-effective.

Microwave photonic notch filter with a tunable frequency and a bandwidth based on gas absorption

We propose and demonstrate a high-performance microwave photonic notch filter (MPNF) based on gas absorption with favorable tunability and a high rejection ratio. In the demonstration experiments, acetylene gas is used to selectively absorb the single sideband produced by phase modulation, and correspondingly to suppress the generated microwave signal. The filter center frequency and bandwidth can be separately tuned by changing the carrier laser frequency and gas pressure. The proposed MPNF achieves a continuously tunable frequency from 2–20 GHz and a 10-dB bandwidth from 0.44–5.89 GHz with a high rejection ratio over 60 dB, which is expected to exceed 100 dB in theory, showing great potential for various applications such as advanced communication and radar.

Integration of Brillouin and passive circuits for enhanced radio-frequency photonic filtering

Signal processing using on-chip nonlinear or linear optical effects has shown tremendous potential for RF photonic applications. Combining nonlinear and linear elements on the same photonic chip can further enable advanced functionality and enhanced system performance in a robust and compact form. However, the integration of nonlinear and linear optical signal processing units remains challenging due to the competing and demanding waveguide requirements, specifically the combination of high optical nonlinearity in single-pass waveguides, which is desirable for broadband signal processing with low linear loss and negligible nonlinear distortions required for linear signal processing. Here, we report the first demonstration of integrating Brillouin-active waveguides and passive ring resonators on the same integrated photonic chip, enabling an integrated microwave photonic notch filter with ultradeep stopband suppressions of &amp;gt;40 dB, a low filter passband loss of &amp;lt;−10 dB, flexible center frequency tuning over 15 GHz, and reconfigurable filter shape. This demonstration paves the way for implementing high-performance integrated photonic processing systems that merge complementary linear and nonlinear properties, for advanced functionality, enhanced performance, and compactness.

Silica-microsphere-cavity-based microwave photonic notch filter with ultra-narrow bandwidth and high peak rejection.

We propose and experimentally demonstrate a microwave photonic (MWP) notch filter based on a silica microsphere cavity. By using a high-Q-factor (∼1e7) cavity with a diameter of 132um, the filter bandwidth can be easily decreased to 15MHz in terms of simple fabrication and flexible coupling. Then we use the advanced modulation technique based on a dual parallel Mach-Zehnder modulator to further improve peak rejection (PR). The experimental results show that the MWP notch filter with its PR beyond 55dB and frequency tunability range over 8GHz has been achieved in combination with double-sideband modulation. To the best of our knowledge, this is a record for PR and bandwidth considered simultaneously for an MWP filter based on silica microcavities. Thus, the proposed MWP filter will be useful in the fields of microwave photonic signal processing, radar systems, etc.

Performances of Microwave Photonic Notch Filter Based on Microring Resonator With Dual-Drive Modulator

A tunable microwave photonic notch filter with ultrahigh radio frequency (RF) rejection ratio is achieved by using a dual-drive Mach-Zender modulator and a microring resonator. In addition, we dedicated to clarify the mechanisms of the microwave photonic notch filters based on a microring resonator under both overcoupled and undercoupled states and the performance differences between these two situations are investigated in detail. Furthermore, the experimental results show that the cancellation microwave photonic notch filter with ultrahigh RF rejection ratio can be achieved with both undercoupled and overcoupled microring resonators. Besides, an RF rejection ratio exceeding 50 dB was achieved for a microring resonator with an optical extinction ratio of only 4 dB, where a 46-dB enhancement from optical to RF response is obtained. Also, the bandwidth of the microwave photonic filter can tuned from 0.65 to 2.2 GHz by adjusting the coupling ratio of the ring resonator. Meanwhile, by tuning the wavelength of the optical carrier, the filtering frequency can be tuned and a tuning range of about 25 GHz was achieved. The implementation is much simpler and more economical compared with previous reports and has great potential in monolithic integration of microwave photonic filters on chip.

Microwave Photonic Devices Based on Liquid Crystal on Silicon Technology

This paper reviews the recent developments in microwave photonic devices based on liquid crystal on silicon (LCOS) technology. The operation principle, functions and important specifications of an LCOS based optical processor are described. Three microwave photonic devices, which are microwave photonic notch filters, phase shifters and couplers, reported in the past five years are focused on in this paper. In addition, a new multi-function signal processing structure based on amplitude and phase control functions in conjunction with a power splitting function in a commercial LCOS based optical processor is presented. It has the ability to realize multiple time -shifting operations and multiple frequency-independent phase shifting operations at the same time and control multiple RF signal amplitudes, in a single unit. The results for the new multi-function microwave photonic signal processor demonstrate multiple tunable true time delay and phase shifting operations with less than 3 dB amplitude variation over a very wide frequency range of 10 to 40 GHz.

Novel CMOS-Compatible Athermal and Polarization-Insensitive Ring Resonator as Photonic Notch Filter

A hybrid titanium dioxide/silicon rich nitride ring resonator with the unique feature of being simultaneously athermal and polarization-insensitive is reported for the first time to our knowledge. Although its potential application domain is extremely wide, the designed integrated microphotonic cavity, having a racetrack shape, is intended for notch filtering in a microwave photonic passband filter. A careful selection of the CMOS-compatible material system and an innovative design approach have allowed a very low dependence of the filtering shape on the input beam polarization and, simultaneously, a thermal drift of the resonance wavelength <1.5 pm/K. The numerically estimated Q-factor, free spectral range, and extinction ratio are compliant with the requirements of the selected application, being equal to 7.8 × 104, 4 nm, and 30.7 dB, respectively.

The Influence of Dispersion on Stimulated-Brillouin-Scattering-Based Microwave Photonic Notch Filters

In this paper, the detrimental effects due to the inevitable dispersion of single-mode fibers for stimulated Brillouin scattering (SBS) based microwave photonic notch filters (MPNF) with sideband amplitude and phase control are investigated. With an inappropriate dispersion compensation, the experimental demonstration shows a notch center frequency shift of 8.3 MHz in maximum and a stopband rejection reduction by 21 dB for the MPNF. The simulation and experiment are in good agreement. Furthermore, the criteria of optimizing the filter performance are also discussed both theoretically and experimentally. These results can help design MPNF based on SBS and benefit other microwave photonic applications, which rely on the phase difference of optical waves.

Link Performance Optimization of Chip-Based Si3N4 Microwave Photonic Filters

Simultaneously achieving high link performance and advanced microwave photonic processing functionalities in compact integrated circuits is crucial but challenging for the deployment in future and existing practical applications. In this paper, we present the first comprehensive experimental study to optimize the link performance of a Si3 N4-photonic-chip-based microwave photonic notch filter. This systematic study leads to an optimized link performance of the chip-based filter, demonstrating an RF net gain, a sub-20-dB noise figure and an overall third-order spurious-free dynamic range of 115 dB.Hz2/3 over a frequency range of 0–12 GHz, in conjunction with advanced filtering functionalities with a notch rejection >50 dB. The achieved performance is based on a unique and low-loss chip-based filter scheme which offers the compatibility with existing link performance optimization techniques, while maintaining the advanced notch filter functionality. Numerical calculations are also performed to explore the future feasibility of implementing a fully integrated microwave photonic filter that approaches the same level of link performance demonstrated in this work. This study is expected to provide a feasible design route to approach fully integrated microwave photonic filters with high link performance.

All-optimized integrated RF photonic notch filter.

We report a silicon nitride chip-based radio-frequency photonic notch filter with an unprecedented performance including an RF gain of 8dB, a record-low noise figure of 15.6dB, and a spurious-free dynamic range of 116 dB·Hz2/3, with a stop band rejection of 50dB. This level of performance is achieved by using on-chip resonators' unique phase responses, and thorough optimizations of the photonic link. These record results will potentially stimulate future implementations of integrated microwave photonic subsystems for real-world applications.

Angle-of-Arrival Estimation of Broadband Microwave Signals Based on Microwave Photonic Filtering

We propose and experimentally demonstrate a photonic approach to estimate the angle-of-arrival (AOA) of broadband microwave signals. Using an integrated polarization-division multiplexing Mach–Zehnder modulator and a differential group-delay module, a microwave photonic notch filter is constructed. The relative time delay in reception of the signal at two separate antenna elements can be obtained by measuring the transmission notches over the signal spectra. The AOA is calculated from the relative time delay and the antenna spacing. The experiment results show that the measurement error is less than ±0.35 ps when the relative time delay changes from −14 to 16 ps.

Integrated reconfigurable photonic filters based on interferometric fractional Hilbert transforms.

In this paper, we present integrated reconfigurable photonic filters using fractional Hilbert transformers (FrHTs) and optical phase tuning structure within the silica-on-silicon platform. The proposed structure, including grating-based FrHTs, an X-coupler, and a pair of thermal tuning filaments, is fabricated through the direct UV grating writing technique. The thermal tuning effect is realized by the controllable microheaters located on the two arms of the X-coupler. We investigate the 200GHz maximum bandwidth photonic FrHTs based on apodized planar Bragg gratings, and analyze the reflection spectrum responses. Through device integration and thermal modulation, the device could operate as photonic notch filters with 5GHz linewidth and controllable single sideband suppression filters with measured 12dB suppression ratio. A 50GHz instantaneous frequency measuring system using this device is also schematically proposed and analyzed with potential 3dB measurement improvement. The device could be configured with these multiple functions according to need. The reconfigurable structure has great potential in ultrafast all-optical signal processing fields.