RF Photonic Filter Research Articles

Ultra-wideband mm-wave remote antenna unit for radio-over-fiber distributed antenna systems in silicon photonics.

This article presents what we believe to be a novel chip-scale 25-45-GHz re-configurable mm-wave remote antenna unit (RAU) for radio over fiber (RoF) distributed antenna systems. The proposed RAU architecture optimizes energy efficiency by operating directly at mm-wave frequencies, and spectral efficiency by selecting re-configurable RF photonic filters topology. Additionally, it achieves frequency agility by rejecting interferes and a small form factor by utilizing the SOI photonics process. Two photonic integrated circuits (PICs) that act as downlink and uplink units are presented while occupying a total area of 12.33 mm2. The downlink selects a single channel within the desired frequency range, while the uplink rejects up to four interferes. The building blocks of the proposed architecture are discussed and their design consideration and parameters are shown. Then, a comprehensive system analysis of the proposed RAU architecture including key performance indicators is presented. A scalable 5-channel system is demonstrated each with a 3-dB Bandwidth of 5-GHz. Moreover, this architecture can be continuously tuned and re-configured within a wide frequency range to cover all 5-channels. To the best of the authors' knowledge, this is the first wideband modular and re-configurable mm-wave RAU that covers the entire mm-wave sub-45-GHz band implemented in silicon photonics.

Programmable photonic RF filters based on an integrated Fabry–Pérot laser with a saturable absorber

Programmable photonic RF filters based on an integrated Fabry–Pérot laser with a saturable absorber

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.

Robust reconfigurable radiofrequency photonic filters based on a single silicon in-phase/quadrature modulator

Abstract Combining integrated photonics and radiofrequency (RF) signals in the optical domain can help overcome the limitations of traditional RF systems. However, it is challenging to achieve environmentally insensitive filtering in wireless communications using integration schemes. In this report, the performance of robust RF filters based on a single silicon in-phase/quadrature modulator with significantly improved temperature and optical carrier wavelength sensitivities, which were suppressed by more than three orders of magnitude compared with those of silicon resonators, was experimentally evaluated. Upconversion and the processing of signals were simultaneously realized on the modulator by setting the relative phases of the arms and the bias voltages. Moreover, the filters can be reconfigured as low-pass, high-pass, band-pass, or band-stop filters. From 25 to 75 °C, the center frequency variation was within 0.2 GHz. From 1500 to 1600 nm, the center frequency variation was within 2 GHz. The proposed scheme allows for filtering and reconfiguration without the use of optical processing modules such as resonators or delay lines, thus providing a novel approach to signal processing and a new robust filter for scenarios with dynamic environments.

High Rejection and Frequency Agile Optical Filtering of RF Signals Using a Rare Earth Ion-Doped Crystal

While photonics is broadly considered as a leading solution for analog RF processing, photonic RF filters often show a limited out-of-band rejection. In this work we demonstrate a high rejection and high steepness photonic RF bandpass filter based on spectral hole burning in a rare earth ion-doped crystal. The filter is obtained by programming a frequency-selective absorption grating in the crystal. With an experimental and theoretical study we identify the optimal parameters for this filter. We achieve a remarkable rejection of 60 dB and a rejection of 45 dB at 50 MHz from the edge of the filter, compliant with most demanding applications.

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.

Rapid Simulation of Photonic Integrated Circuits Using Verilog-A Compact Models

Silicon-based electronic-photonic integration offers several opportunities for integrated circuits (IC) designers to innovate systems architectures that leverage the advantages of optical-domain signal processing and low-loss transmission in optical fibers. However, photonic integrated circuit (PIC) design tools have evolved from the numerical Maxwell field solvers and are completely disjointed from the electronic circuit simulation tools such as SPICE and Cadence Spectre. Thus there is a gap that needs to be filled by the IC community where they can instantiate photonic building blocks to form PICs and perform co-simulation with interfacing (Bi)CMOS electronic circuits. In recent work, Verilog-A compact models have been developed for photonic device building blocks that enable transient simulations of hybrid electronic-photonic components such as lasers, modulators and detectors. However, frequency sweeps of radio-frequency (RF) photonic filters remain unwieldy due to the long simulation times of stepped frequency transient simulations. In this work, we present complex frequency chirp based methods for rapid frequency-domain simulation of PICs. The trade-offs involved in selecting the simulation parameters for a given frequency response accuracy and simulation time are studied along with the impact of windowing and frequency chirp profile. The presented method can result in over $1000\times $ improvement in simulation time of frequency sweeps of higher-order RF photonic filter topologies.

Low-RF-loss and large-rejection reconfigurable Brillouin-based RF photonic bandpass filter.

We present a high-performance radio frequency (RF) photonic bandpass filter enabled by combining on-chip Brillouin scattering with a suppressed carrier phase modulation scheme. We achieve a low RF loss of 5 dB and a large stopband rejection of more than 40 dB, which represents a significant improvement of 20 dB to the RF passband gain and 31 dB to the RF rejection ratio over traditional modulation schemes under the same optical power consumption. We further demonstrate filter reconfigurability including multiple passbands, wide frequency (1-20 GHz), and bandwidth tunability (30-350 MHz) without compromising the RF performance.

Demonstration of a Sub-GHz Flat-Top Comb-Based RF-Photonic Filter Enabled by Fourth-Order Dispersion Compensation

We demonstrate a sub-GHz RF-photonic filter based on phase and amplitude control of a wideband all-fiber frequency parametric comb with 25-GHz tone spacing. The narrow bandwidth filter construction based on 221 comb tones was enabled by fourth-order dispersion compensation of the dispersive element. The filter integrity was further tested by retrieval of a set of digital modulated RF channels.

Corrections to “RF-Photonic Filters via On-Chip Photonic-Phononic Emit–Receive Operations”

Presents corrections to the paper, “RF-photonic filters via on-chip photonic-phononic emit-receive operations,” (Kittlaus, E., et al), J. Lightw. Technol., vol. 36, no. 13, pp. 2803–2809, Jul. 2018.

High performance RF filters via bandwidth scaling with Kerr micro-combs

We demonstrate high-resolution photonic RF filters using an RF bandwidth scaling approach based on integrated Kerr optical micro-combs. By employing both an active nonlinear micro-ring resonator (MRR) as a high-quality micro-comb source and a passive high-Q MRR to slice the shaped comb, a large RF instantaneous bandwidth of 4.64 GHz and a high resolution of 117 MHz are achieved, together with a broad RF operation band covering 3.28 to 19.4 GHz (L to Ku bands) using thermal tuning. We achieve programmable RF transfer functions including binary-coded notch filters and RF equalizing filters with reconfigurable slopes. Our approach is an attractive solution for high performance RF spectral shaping with high performance and flexibility.

Fast Tunable Photonic Single-Bandpass RF Filter With Multiple Arbitrary Switching Flat-Top Passbands

A fast tunable single-bandpass photonic radiofrequency filter (PRF) with multiple arbitrary switching flat-top passbands is proposed and experimentally demonstrated. The PRF is comprised of a narrow, flat-top optical filter based on a fiber Bragg grating with multiple phase shifts, and a high-speed wavelength switch module by incorporating an optical carrier-suppressed single-sideband modulation unit and a direct digital synthesizer. The combination of the single flat-top passband and the high-speed digital controlled wavelength switch in the optical domain enables us to achieve a fast and arbitrarily tunable single passband in the microwave domain. In the experiments, the proposed PRF shows a single flat-top passband with a 3-dB bandwidth of 740 MHz, a rectangular shape factor around 2.27, and an out-of-band rejection ratio greater than 41 dB. In the application demonstrations, the multitone or single-tone continuous-wave RF signals with different frequency components are applied to the PRF and then the desired components or waveforms are temporally separated and successfully recovered, verifying the multiple switching passbands with arbitrarily manipulated center frequencies and numbers over the frequency range from 0 to 12 GHz at a switching speed as fast as 1.73 ns.

Influences of multiphoton absorption and free-carrier effects on frequency-comb generation in normal dispersion silicon microresonators

We investigate frequency-comb generation in normal dispersion silicon microresonators from the near-infrared to mid-infrared wavelength range in the presence of multiphoton absorption and free-carrier effects. It is found that parametric oscillation is inhibited in the telecom wavelength range resulting from strong two-photon absorption. On the contrary, beyond the wavelength of 2200 nm, where three- and four-photon absorption are less detrimental, a comb can be generated with moderate pump power, or free-carriers are swept out by a positive-intrinsic-negative structure. In the temporal domain, the generated combs correspond to flat-top pulses, and the pulse duration can be easily controlled by varying the laser detuning. The reported comb generation process shows a high conversion efficiency compared with anomalous dispersion regime, which can guide and promote comb formation in materials with normal dispersion. As the comb spectra cover the mid-infrared wavelength range, they can find applications in comb-based radiofrequency photonic filters and mid-infrared spectroscopy.

RF-Photonic Filters via On-Chip Photonic–Phononic Emit–Receive Operations

The creation of high-performance narrowband filters is of great interest for many RF-signal processing applications. To this end, numerous schemes for electronic, MEMS-based, and microwave photonic filters have been demonstrated. Filtering schemes based on microwave photonic systems offer superior flexibility and tunability to traditional RF filters. However, these optical-based filters are typically limited to GHz-widths and often have large RF insertion losses, posing challenges for integration into high-fidelity radiofrequency circuits. In this article, we demonstrate a novel type of microwave filter that combines the attractive features of microwave photonic filters with high-Q phononic signal processing using a photonic-phononic emit-receive process. Through this process, a RF signal encoded on a guided optical wave is transduced onto a GHz-frequency acoustic wave, where it may be filtered through shaping of acoustic transfer functions before being re-encoded onto a spatially separate optical probe. This emit-receive functionality, realized in an integrated silicon waveguide, produces MHz-bandwidth band-pass filtering while supporting low RF insertion losses necessary for high dynamic range in a microwave photonic link. We also demonstrate record-high internal efficiency for emit-receive operations of this type, and show that the emit-receive operation is uniquely suitable for the creation of serial filter banks with minimal loss of fidelity. This photonic-phononic emitter-receiver represents a new method for low-distortion signal-processing in an integrated all-silicon device.

Chip-based arbitrary radio-frequency photonic filter with algorithm-driven reconfigurable resolution.

All programmable microwave photonic filters to date have been limited by the trade-off between the resolution and the operation bandwidth to construct arbitrary filter shapes. Here, we propose a new approach to synthesize a radio-frequency photonic filter with an arbitrary response and reconfigurable resolution through phase-only tailoring, which is implemented in a low-loss photonic chip. Based on an algorithm, incorporating the target transfer function of the filter optimum resolution can be intelligently chosen by tuning the coupling coefficients and the round-trip phase of cascaded ring resonators. We achieved filtering with tunable resolution from 300MHz to 25GHz along with various filtering shapes and multiple passbands.

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.

Tunable Photonic Radio-Frequency Filter With a Record High Out-of-Band Rejection

As radiofrequency filtering plays a vital role in electromagnetic devices and systems, recently photonic techniques have been intensively studied to implement radiofrequency filters to harness wide frequency coverage, large instantaneous bandwidth, low frequency-dependent loss, flexible tunability and strong immunity to electromagnetic interference. However, one crucial challenge facing the photonic radiofrequency filter (PRF) is the less impressive out-of-band rejection. Here, to the best of our knowledge, we demonstrate a tunable PRF with a record out-of-band rejection of 80 dB, which is 3 dB higher than the maximum value (~77 dB) reported so far, when incorporating highly selective polarization control and large narrow-band amplification enabled by stimulated Brillouin scattering effect. In particular, this record rejection is arduous to be achieved for a narrow passband (e.g., a few megahertz) and a high finesse in a PRF. Moreover, the proposed PRF is an active one capable of providing negligible insertion loss and even signal gain. Tunable central frequency ranging from 2.1 to 6.1 GHz is also demonstrated. The proposed PRF will provide an ultra-high noise or clutter suppression for harsh electromagnetic scenarios, particularly when room-temperature implementation and remote distribution are needed.

Lossless and high-resolution RF photonic notch filter.

A novel technique to create a lossless and tunable RF photonic bandstop filter with an ultra-high suppression is demonstrated using the combination of an overcoupled optical ring resonator and tailored stimulated Brillouin scattering gain. The filter bandwidth narrowing is counterintuitively synthesized from two broad optical resonance responses. Through a precise amplitude and phase tailoring in the optical domain, the RF filter achieves a minimum insertion loss (<0 dB), a high isolation (>50 dB), and a tunable 3 dB bandwidth (60-220 MHz) simultaneously with wide frequency tunability (1-11 GHz). This ultra-low loss RF filter paves the way toward broadband advanced spectrum management with low loss, high selectivity, and improved signal-to-noise ratio.

Enhanced processing gain via pulse polarity switching in an RF photonic phase filter.

Fast pulse polarity switching is proposed and demonstrated to enhance processing gain in a comb-based radio-frequency photonic phase filter. The polarity switching scheme overcomes previous limits on time bandwidth product and processing gain based on the number of optical frequency comb lines. In an experiment with broadband jamming noise, the peak signal-to-noise ratio of compressed RF output pulses is improved by ~30 dB compared to the input average signal-to-noise ratio.

Rapidly Tunable Dual-Comb RF Photonic Filter for Ultrabroadband RF Spread Spectrum Applications

We demonstrate a rapidly frequency-tunable radio frequency (RF) filter using microwave photonics technology for ultrawideband RF spread spectrum applications. A pair of electro-optic frequency combs is arranged as a dispersive tapped delay line in a differential detection configuration to implement a programmable finite impulse response RF filter. Our photonic scheme enables both fast frequency tuning on the order of tens of nanoseconds and wide tuning range (>7.5 GHz) with minimal variation of RF gain and passband shape. The low control voltage (ca. 1 V) and the linear relationship between control voltage and passband frequency facilitate agile frequency tuning for processing of signals with time-varying frequency content, while differential detection increases the photocurrent by a factor of two and suppresses common mode intensity noise. We exploit the rapid tunability of the implemented filter to demonstrate dynamic tracking of frequency-hopped and chirped RF signals. An experiment that performs dynamic filtering of an input chirp signal (3.92-GHz center frequency, up-chirped by >2 GHz within 100 ns) obscured by strong broadband noise achieves $\sim 11$ -dB signal-to-noise ratio (SNR) improvement. The SNR obtained is in addition to that available from standard matched filtering or pulse compression processing, suggesting strong potential for enhanced resistance against broadband noise jamming.