High Sidelobe Suppression Research Articles

Integrated lithium niobate Vernier micro-ring filter with wide and fast tuning capabilities

Tunable optical micro-ring filters play significant roles in optical communications, microwave photonics, and photonic neural networks. Typical micro-ring filters are based on either a thermo-optic (TO) effect with microsecond timescales or an electro-optic (EO) effect with a limited tuning range. Here, we report a continuously tunable lithium niobate on insulator (LNOI) Vernier cascaded micro-ring filter with wire-bonded packaging integrated with both TO and EO tuning electrodes, featuring a 40-nm free spectral range (FSR), 2.3 GHz EO bandwidth, and a high sidelobe suppression ratio of 21.7 dB, simultaneously. Our high-performance optical micro-ring filter could become an important element in future LNOI photonic circuits with applications in high-capacity wavelength-division multiplexing (WDM) systems, broadband microwave photonics, fast-tunable external-cavity lasers, and high-speed photonic neural networks.

Half-wavelength-pitch silicon optical phased array with a 180° field of view, high sidelobe suppression ratio, and complex-pattern beamforming

Solid-state optical beam steering devices desire a large field of view (FOV), good beam quality, and reconfigurable beamforming of complex patterns, which are not available in a single system yet. Having not been demonstrated, an active beamformer using an optical phased array (OPA) could potentially fulfill these requirements simultaneously, because it can control both the wavefront and beam pattern. Half-wavelength-pitch OPAs theoretically can achieve the three requirements concurrently, but suffer from crosstalk. Most previous efforts focus on mitigating/avoiding crosstalk. Instead, here we appreciate its existence and propose/demonstrate a programmable architecture to compensate for it. Using a tree of composite variable splitters with a full splitting-ratio range, we achieve arbitrary amplitude/phase modulation to pre-correct scrambled phase/amplitude by crosstalk. With comprehensive stray-light minimization strategies, the sidelobe suppression ratio (SLSR) is significantly improved. Our design achieves a 180∘ FOV, a peak SLSR of 24 dB, and complex-pattern beamforming simultaneously in a half-wavelength-pitch 64-waveguide array. Within the ±60∘ range, a SLSR of >20dB is achieved. Our OPA demonstrates Bayliss difference, pulse-shaped, and asymmetric three-beam patterns with high SLSRs of >20dB, ∼10dB, and >18dB, respectively. These performance metrics are important for various applications in light detection and ranging, imaging, and communication.

A Collaborative Control Method of Transmission Amplitude and Phase for Transmitarray Antenna Design

In this paper, a collaborative control method of transmission amplitude and phase for transmitarray antenna (TA) design is proposed. In this proposed method, one of the most popular hypersurface fitting models, Gaussian stochastic process (GP) model, is utilized to construct an accurate surrogate model. Following this implementation, a mapping relationship between structural parameters of TA unit cell and its transmission amplitude and phase is established. The most advantage of this method is its applicability for general TA designs because it is much difficult to control the amplitude and phase of unit cell independently through adjusting separate structural parameters. To verify the high efficiency of the proposed method, three TA antennas with different scanning angles are designed to obtain high sidelobe suppression level. Measured results show that the proposed collaborative control method of amplitude and phase is much promising for high sidelobe suppression level in TA designs.

A Sigle-Layer Dual-Band Shared-Aperture Antenna With High Gain and Sidelobe Suppression Based on High-Order Mode for Vehicular Communications

A Sigle-Layer Dual-Band Shared-Aperture Antenna With High Gain and Sidelobe Suppression Based on High-Order Mode for Vehicular Communications

High flexibility inscription of FBG via femtosecond laser

Point by point (PbP) method via femtosecond (fs) laser is considered as a highly flexible way for inscribing FBGs, which is demonstrated in different core with various central wavelengths previously. In this letter, the flexible inscription has been further expanded to regulate reflectivity, 3 dB bandwidth, and sidelobe suppression. The influence law has been discovered for the first time that there is a trade-off among the laser energy, the transverse offsets and the grating order for pursuing smaller 3 dB bandwidth and higher sidelobe suppression on high reflectivity FBGs. The results show that increasing the grating order is a more efficient way. Quasi distributed FBGs are inscribed on a continuous fiber for further confirming its flexibility.

High‐Performance Silicon Photonic Filter Using Subwavelength‐Structure Multimode Waveguide Gratings

AbstractOn‐chip photonic filters are one of the most important elements for spectrum manipulation in various optical systems, including wavelength‐division multiplexing and spectrum analysis. Currently, it is still very challenging to achieve scalable photonic filters with flexibly‐designed bandwidths, low excess losses, flat tops, and ultra‐wide free‐spectral ranges (FSRs). Here an unprecedented silicon photonic filter is proposed and demonstrated with high performances by using apodised antisymmetric multimode subwavelength gratings (MSWGs) structures. For the present silicon photonic filter, the MSWGs enable the index engineering in the Bragg lattice, which easily manipulates the coupling coefficient, the local effective index, and the mode profile flexibly. As a result, the strong side lobes of the grating filter are efficiently suppressed, while the bandwidth can be designed flexibly and the FSR is free. The fabricated silicon photonic filter is FSR‐free and shows a low excess loss of <1 dB and a high sidelobe suppression ratio of 20–40 dB.

Tapered high-gain Fabry–Perot cavity antenna with high sidelobe suppression for 5G industry

We propose a Fabry–Perot cavity (FPC) antenna to suppress a sidelobe level (SLL) while maintaining a reasonably high gain. Generally, conventional FPC antennas (FPCAs) produce a high SLL because waves in their FPC leak considerably through lateral openings, which is a primary reason for lowering antenna gains. We propose two design approaches to solve this problem: the reflection magnitude tapering of a partially reflective surface (PRS) and considering different incident modes for the PRS design. First, the proposed tapering can remarkably reduce an SLL by providing the PRS with more radiation opportunities. Second, the different incident modes of transverse electric (TE) and transverse magnetic (TM) can increase an antenna gain by considering a more realistic illumination environment of the PRS. We have proven that our antenna provides 19.8 dBi realized gain with high sidelobe suppression (SLS) of more than 23 dB. Consequently, the proposed FPCA can suppress sidelobes significantly while maintaining a high gain. Good agreement between simulations and experiments demonstrates the usefulness of our proposal.

Circularly Polarized High-Gain Fabry-Perot Cavity Antenna with High Sidelobe Suppression

The proposed design approach improves the circularly polarized Fabry-Perot cavity antenna (CP-FPCA) by increasing gain and sidelobe suppression (SLS) while reducing the axial ratio (AR) and cross-polarization levels. Conventional CP-FPC antennas have a high AR due to the lack of independent control over circular polarization conditions. The solution proposes a double-layered circularly polarized partially reflecting surface (CP-PRS) that independently controls the circular polarization conditions at the design frequency f0 (10 GHz) for equal magnitudes and at a ±90° phase difference between orthogonal components of the transmitted waves. The PRS and artificial magnetic conductor (AMC) unit cells are employed to satisfy Trentini’s beamforming condition, leading to increased gain and SLS and lowered AR and cross-polarization levels. Consequently, the proposed CP-FPCA provides a 15.4 dBi high gain with 25.3% aperture efficiency and more than 23.5 dB high SLS in each plane. Moreover, it achieves an AR lowered by 0.12 dB and a cross-polarization level below −42 dB. A strong correlation between the simulations and experiments proves the practicality of our proposal.

Bidirectional high sidelobe suppression silicon optical phased array

An optical phased array (OPA), the most promising non-mechanical beam steering technique, has great potential for solid-state light detection and ranging systems, holographic imaging, and free-space optical communications. A high quality beam with low sidelobes is crucial for long-distance free-space transmission and detection. However, most previously reported OPAs suffer from high sidelobe levels, and few efforts are devoted to reducing sidelobe levels in both azimuthal ( φ ) and polar ( θ ) directions. To solve this issue, we propose a Y-splitter-assisted cascaded coupling scheme to realize Gaussian power distribution in the azimuthal direction, which overcomes the bottleneck in the conventional cascaded coupling scheme and significantly increases the sidelobe suppression ratio (SLSR) in the φ direction from 20 to 66 dB in theory for a 120-channel OPA. Moreover, we designed an apodized grating emitter to realize Gaussian power distribution in the polar direction to increase the SLSR. Based on both designs, we experimentally demonstrated a 120-channel OPA with dual-Gaussian power distribution in both φ and θ directions. The SLSRs in φ and θ directions are measured to be 15.1 dB and 25 dB , respectively. Furthermore, we steer the beam to the maximum field of view of 25°×13.2° with a periodic 2λ pitch (3.1 μm). The maximum total power consumption is only 0.332 W with a thermo-optic efficiency of 2.7 mW/π .

Optical Phase‐Sensitive Reflectometry Based on Orthogonal Frequency‐Division Multiplexed LFM Signal in Fractional Fourier Domain

AbstractLinear frequency modulated (LFM) phase‐sensitive optical time‐domain reflectometry (φ‐OTDR) relieves the trade‐off between sensing range and spatial resolution; however, it is still limited by low sidelobe suppression ratio and signal fading. Fractional Fourier transform (FrFT) is applied to LFM φ‐OTDR for signal generation and compression. An LFM pulse is generated using the p‐order FrFT of a direct current signal and then compressed using FrFT with order 1−p to achieve a high sidelobe suppression ratio. Orthogonal frequency‐division multiplexing in the fractional Fourier domain is utilized to suppress signal fading. In experiments, fading‐free distributed phase measurement is realized using a multiplexed LFM probe pulse generated using a 0.4‐order FrFT. Dynamic sensing is demonstrated via single‐frequency or sweep‐frequency vibration, thereby validating high sensing performance. This work will not only open up a route for signal processing in LFM pulse based high‐performance φ‐OTDR, but also has significant potential for applications in other sensing systems using LFM signals.

Subwavelength structure engineered passband filter for the 2-µm wave band.

In this work, we experimentally demonstrate a passband filter for the 2-µm wave band on the silicon-on-insulator platform. The device consists of a strip waveguide and an apodized subwavelength-structured waveguide. Fabricated on a 340-nm-thick silicon membrane, the proposed passband filter shows a 3-dB bandwidth of 16-33 nm, a high sidelobe suppression ratio (SLSR) of 24 dB, and a low insertion loss (IL) of 0.4 dB.

Designing of Fiber Bragg Gratings for Long-Distance Optical Fiber Sensing Networks

Most optical sensors on the market are optical fiber Bragg grating (FBG) sensors with low reflectivity (typically 7-40%) and low side-lobe suppression (SLS) ratio (typically SLS <15 dB), which prevents these sensors from being effectively used for long-distance remote monitoring and sensor network solutions. This research is based on designing the optimal grating structure of FBG sensors and estimating their optimal apodization parameters necessary for sensor networks and long-distance monitoring solutions. Gaussian, sine, and raised sine apodizations are studied to achieve the main requirements, which are maximally high reflectivity (at least 90%) and side-lobe suppression (at least 20 dB), as well as maximally narrow bandwidth (FWHM<0.2 nm) and FBGs with uniform (without apodization). Results gathered in this research propose high-efficiency FBG grating apodizations, which can be further physically realized for optical sensor networks and long-distance (at least 40 km) monitoring solutions.

High-performance lithium-niobate-on-insulator optical filter based on multimode waveguide gratings.

A high-performance optical filter is proposed and realized with multimode waveguide grating (MWG) and two-mode multiplexers on the x-cut lithium-niobate-on-insulator (LNOI) platform for the first time, to the best of our knowledge. The present optical filter is designed appropriately to avoid material anisotropy as well as mode hybridness, and has a low excess loss of 0.05 dB and a high sidelobe suppression ratio (SLSR) of 32 dB in theory with Gaussian apodization. The fabricated filters show a box-like response with 1-dB bandwidth of 6-23 nm, excess loss of ∼0.15 dB, sidelobe suppression ratio of >26 dB. The device performance is further improved with a sidelobe suppression ratio as high as 48 dB and a low excess loss of ∼0.25 dB by cascading two identical MWGs.

Ultra-compact wideband filter with sidelobe suppression based on double modulated grating-assisted microring resonator

In this paper, we propose an ultra-compact wideband filter (WBF) with high sidelobe suppression. The filter consists of a single microring resonator incorporated with a proposed new structure of double modulated subwavelength gratings. The double modulated subwavelength gratings are obtained via two different fill factors with the same period. The ultra-compact WBF achieved FSR-free, flat response with a 3 dB bandwidth of 1.0734 THz and a sidelobe suppression of 19.79 dB. In addition, a low insertion loss of <0.6 dB and a low in-band ripple of less than 0.2 dB were obtained. The filter has a compact device footprint of 380 μm2.

Ultra-Compact Band-Pass and Band-Stop Tunable Filters Based on Loop-Cascaded Nanobeam Structure

We propose an ultra-compact band-pass and band-stop filter with bandwidth tunability based on the loop-cascaded nanobeam structure in simulation, with a small coupling region of 41×1 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . It features low insertion loss, large bandwidth tunable range and high sidelobe suppression ratio. By two-stage cascading, the bandwidth of the band-pass filter can be adjusted in the range of 6 nm-24 nm. We also experimentally demonstrate the loop-cascaded nanobeam band-pass and band-stop filter with a coupling region of only 17.5×1 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

A 27 dB Sidelobe Suppression, 1.12 GHz BW−10dB UWB Pulse Generator With Process Compensation

This brief presents an on-off keying (OOK) driven impulse radio ultra-wideband (IR-UWB) pulse generator (PG) for 3-5 GHz applications, featuring high sidelobe suppression (SLS), large bandwidth ( BW <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10dB</sub> ), and robustness against process variations. To eliminate the need for a dedicated off-chip pulse-shaping filter, a transistor-sizing-based pulse shaping scheme is adopted to generate a broadband pulse with a triangular envelope, ensuring enhanced SLS with extended signal bandwidth. The proposed process compensating circuit mitigates the effects of process variations on the propagation delay of the logic gates to provide stable pulse timing characteristics. Implemented in 65-nm CMOS process, the prototype PG provides a UWB pulse with an amplitude of 634 mV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</sub> , BW <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10dB</sub> of 1.12 GHz, and 27 dB SLS, while occupying a total chip area of 964 μm × 669 μm. The proposed PG provides enhanced pulse characteristics with modest energy consumption among the reported works. Based on the proposed figure-of-merit considering the SLS and quality of pulse, the PG in this work shows a competitive FOM of 3.36×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> .

A 24 GHz Microstrip Comb Array Antenna With High Sidelobe Suppression for Radar Sensor

In this letter, a compact microstrip comb array antenna with high sidelobe suppression is presented for a 24 GHz radar sensor. Based on the printed circuit board process, several series-fed microstrip comb lines are designed on the substrate to form linear antenna arrays. Chebyshev distribution is adopted for the nonuniform excitation of each antenna element to achieve a very low sidelobe level (SLL). The excitation amplitude is adjusted by tuning the feedline width between the adjacent elements. Under such circumstances, a slimmer array with a tapering distribution is realized. The proposed linear comb array consists of eight elements and is further extended to 8 × 2 and 8 × 8 planar arrays to meet different requirements. The proposed antenna arrays are verified by the simulation and measurement, which are in good agreement. The measured SLLs are better than -28 dB. The measured gains of 8 × 1, 8 × 2, and 8 × 8 comb arrays at 24.125 GHz are 12.2, 15.0, and 19.8 dBi, respectively. The return losses are better than 10 dB and the broadside gains remain stable within the regulated industrial scientific medical band from 24 to 24.25 GHz.

Practical two-dimensional beam steering system using an integrated tunable laser and an optical phased array.

A practical two-dimensional beam steering solid-state system based on the synthesis of one-dimensional wavelength tuning and a one-dimensional optical phased array is demonstrated and investigated. The system incorporates an integrated multiple-channel-interference widely tunable laser, an integrated 32-channel optical phased array, an offline phase error correction unit, and home-made control electronics. The introduction of the integrated tunable laser avoids the traditional bulky light source fed into the optical phased array, making the architecture promising to be miniaturized. In addition, a calibration method based on particle swarm optimization is proposed and proved to be effective to correct the phase errors existing in the arrayed channels and improve the emitted far-field quality. Other practical aspects, such as high-speed control and cost, are taken into the consideration of the system design as well. Under the control of home-made electronics, the laser exhibits a tuning range of 50 nm with a 44 dB side-mode suppression ratio, and the system presents the characteristics of low divergence (0.63∘×0.58∘), high side-lobe suppression ratio (>10dB), and high-speed response (<10µs time constant) in an aliasing-free sweeping range of 18∘×7∘.

Spectral-Distortionless, Flat-Top, Drop-Filter Based on Complementarily-Misaligned Multimode-Waveguide Bragg Gratings

An integrated-optics, flat-top drop-filter with low excess loss, and high sidelobe suppression ratio (SLSR), is demonstrated. The filter is based on apodized multimode waveguide Bragg grating (AMWBG) developed on a silicon-on-insulator (SOI) platform. Antisymmetric periodic refractive-index perturbations are imposed on both sides of the waveguide to couple light contra-directionally between the fundamental (TE0) and the first-order transverse electric modes (TE1). To achieve a Gaussian apodization profile, the lateral-misalignment between the two sides of the waveguide is modulated by adjusting the positions of both sides of the grating features in a complementary manner. In this way, the undesired phase errors, which exist in conventional misalignment-modulated Bragg gratings and distort the grating responses, can be eliminated. Experimental results show that our AMWBG has a high-quality flat-top response, a high SLSR of >21 dB, a 3-dB bandwidth of 8 nm, a low excess loss of 0.24 dB, and a high roll-off rate at the filter edge. We also demonstrate a dual-stage filter that has a flat-top, drop-port response with a 3-dB bandwidth > 7.5 nm, a SLSR >40 dB, and an excess loss<0.5 dB.

An Iterative Deconvolution-Time Reversal Method with Noise Reduction, a High Resolution and Sidelobe Suppression for Active Sonar in Shallow Water Environments

Matched filtering is widely used in active sonar because of its simplicity and ease of implementation. However, the resolution performance generally depends on the transmitted waveform. Moreover, its detection performance is limited by the high-level sidelobes and seriously degraded in a shallow water environment due to time spread induced by multipath propagation. This paper proposed a method named iterative deconvolution-time reversal (ID-TR), on which the energy of the cross-ambiguity function is modeled, as a convolution of the energy of the auto-ambiguity function of the transmitted signal with the generalized target reflectivity density. Similarly, the generalized target reflectivity density is a convolution of the spread function of channel with the reflectivity density of target as well. The ambiguity caused by the transmitted signal and the spread function of channel are removed by Richardson-Lucy iterative deconvolution and the time reversal processing, respectively. Moreover, this is a special case of the Richardson-Lucy algorithm that the blur function is one-dimensional and time-invariant. Therefore, the iteration deconvolution is actually implemented by the iterative temporal time reversal processing. Due to the iterative time reversal method can focus more and more energy on the strongest target with the iterative number increasing and then the peak-signal power increases, the simulated result shows that the noise reduction can achieve 250 dB in the “ideal” free field environment and 100 dB in a strong multipaths waveguide environment if a 1-ms linear frequency modulation with a 4-kHz frequency bandwidth is transmitted and the number of iteration is 10. Moreover, the range resolution is approximately a delta function. The results of the experiment in a tank show that the noise level is suppressed by more than 70 dB and the reverberation level is suppressed by 3 dB in the case of a single target and the iteration number being 8.