High Sidelobe Suppression Research Articles

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.

Metal-Printing Defined Thermo-Optic Tunable Sampled Apodized Waveguide Grating Wavelength Filter Based on Low Loss Fluorinated Polymer Material

In this work, thermo-optic (TO) lateral shift apodized sampled waveguide grating for 1550 nm wavelength is designed and fabricated by the metal-printing technique based on fluorinated epoxy-terminated polycarbonates (FBPA-PC EP) and fluorinated epoxy resin (FSU-8) materials. The optical characteristics and thermal stability of the FBPA-PC EP and FSU-8 materials are analyzed. To realize periodic wide-spectrum filtering and suppress the side-lobes of grating, a lateral shift apodized sampled waveguide grating is proposed. The 3 dB bandwidth and wavelength spacing can reach 4.8 nm and 9.7 nm. The side-lobe suppression ratio of proposed device can reach 22.6 dB, which is much better than traditional Bragg grating (6.1 dB). Driving electrical powers of 42.4 mW and 87.2 mW can produce blueshifts of 1.8 nm and 3.5 nm in the measured reflection spectrum, respectively. This device realizes the aim of multiple functions, including periodic filtering, wide-spectrum filtering, and high side-lobe suppression. The device is applicable of realizing signal processing and wavelength division multiplexing (WDM )systems.

Millimetre-Wave Planar Phased Patch Array with Sidelobe Suppression for High Data-Rate Transmission

A design of 38 GHz planar phased patch array with sidelobe suppression for data-rate enhancement is proposed in the paper. The proposed array is formed of three 24-element subarrays of patches. Each patch has its own transmit/receive modules (TRM) consisting of a digitally controlled attenuator and phase shifter. In order to achieve high data-rate communications, the noise, especially due to the undesired signals received from the sidelobes, should be reduced with high sidelobe suppression of subarray. The sidelobe suppression of the proposed subarray is first improved to 17.92 dB with a diamond-shaped aperture and then better than 35 dB with a tapered radiation power distribution. The excellent sidelobe suppression of the antenna array is essential for the beam-division multiplexing applications when the signal sources are close to each other. The proposed design is validated experimentally, including the data-rate measurements showing that the 7 Gbps data transmission can be achieved with sufficient sidelobe suppression of the proposed design.

Tunable and reconfigurable microwave photonic filter based on Fourier domain optical processor and cascaded fiber rings

A tunable and reconfigurable microwave photonic filter (MPF) based on a Fourier domain optical processor (FD-OP) and cascaded fiber rings is proposed and demonstrated. A stable and multiple wavelengths source can be realized by using a broadband source and a waveshaper. The multiple wavelengths source with different wavelength spacings is achieved by programming the waveshaper. Two fiber rings of the same structure are cascaded together to achieve a high main sidelobe suppression ratio (MSSR) and a high Q factor. The FD-OP is induced to add additional phase shifts to achieve tunability. When the number of taps is 60, by introducing cascaded two-fiber rings and the FD-OP, the MSSR is 59.356 dB, the maximum Q factor is 470.895 and the 3dB bandwidth of the filter is 0.019 GHz. The simulating results show that the center frequency continuously tuned from 4.47–13.41 GHz. The tunability and reconfigurability of the MPF are well realized.

Narrow-Band Add-Drop Filter Based on Cladding-Modulated Apodized Multimode Bragg Grating

A narrow-band add-drop filter based on an anti-symmetric cladding-modulated multimode Bragg grating (CMMBG) is demonstrated on the silicon-on-insulator (SOI) platform. The CMMBG is implemented by using periodic placements of silicon cubes along a multimode waveguide. The coupling strength is varied by changing the distance between the cubes and the multimode waveguide. An unlimited free-spectral range (FSR) and weak coupling strengths for TE0–TE1 modes conversion are achieved. Sidewall-misalignment-modulation (SMM) apodization is applied to achieve a high sidelobe suppression ratio (SLSR). The experimental results show that the designed filtering device has a 3-dB bandwidth of 0.8 nm, an insertion loss of −0.65 dB and a SLSR of 20.3 dB. The devices with different grating duty cycles and multimode waveguide widths were also fabricated and analyzed. Our devices are promising for on-chip applications that require narrow bandwidths such as WDM channel filters.

Ultra-narrow linewidth laser based on a semiconductor gain chip and extended Si3N4 Bragg grating.

We demonstrate ultra-narrow linewidth fixed wavelength hybrid lasers composing a semiconductor gain chip and extended silicon nitride Bragg grating. Fabricated ultra-low κ Bragg gratings provide a narrow bandwidth and high side-lobe suppression ratio. A single-wavelength 1544nm hybrid extended-distributed Bragg reflector laser with 24mW output power and a Lorentzian linewidth of 320Hz is demonstrated, providing a high-performance light source for on- and off-chip applications.

Experimental demonstration of blind equalization for OFDM/OQAM-VLC system

Orthogonal frequency-division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) in direct-detection system obtains many benefits such as free of cyclic prefix (CP) and high side-lobe suppression ratio in comparison with CP-OFDM signal. Unfortunately, for OFDM/OQAM signal, it needs an extra interference cancellation process to eliminate the imaginary intrinsic interference (IMI) when using one-tap frequency equalization known as interference approximation method (IAM). Blind equalization without a pilot overhead for OFDM/OQAM signal is proposed and experimentally demonstrated in visible light communication (VLC) system with limited modulation bandwidth. Experimental results show that blind equalization for OFDM/OQAM-VLC can effectively eliminate the IMI effect and can exhibit better symbol error rate on different subcarriers. Furthermore, compared with conventional IAM, it exhibits 3.2 and 2.7 dB overall improvement in Q2-factor using blind equalization based on the constant modulus algorithm and constant norm algorithm, respectively. In addition, the system can achieve a total data rate of 3.96 Gb / s over 5-m free space link with a bit error rate lower than hard-decision forward error correction limit of 3.8 × 10 − 3.

Silicon Multimode Waveguide Grating Filter at 2 μm

A silicon-based on-chip add-drop optical filter is realized for the wavelength-band of 2 μ m by integrating a multimode waveguide grating (MWG) and a pair of two-channel mode (de)multiplexers. The realized add-drop optical filter has low loss and high sidelobe suppression ratio in a broad band, achieved by optimizing the corrugation depth, the apodization strength, as well as the grating length. The measurement results agree very well with the theoretical simulation results. The demonstrated ∼100 μ m-long MWG has a large 0.5 dB bandwidth of 6∼26 nm and a sidelobe suppression ratio of 22∼27 dB. The thermal-tunability of the present optical filter is also demonstrated experimentally, and the measured tuning efficiency is about 0.16 nm/mW.

Narrow-Band Add-Drop Filter Based on Phase-Modulated Grating-Assisted Contra-Directional Couplers

A narrow-band silicon photonic add-drop filter is demonstrated by using phase-modulated grating-assisted contra-directional couplers (contra-DCs) on the silicon-on-insulator platform. The apodization coupling is achieved by modulating the phase of one grating in a contra-DC, while keeping the other grating uniform. As the local effective index of the gratings remains almost unchanged, the designed apodized contra-DC has a large designing flexibility and a high sidelobe suppression ratio (SLSR). To obtain a narrow bandwidth, weak coupling is designed. Single-stage and dual-stage filtering are both studied. The experimental results show that single-stage filtering device has a 3-dB bandwidth of 0.9 nm and an SLSR of 18 dB. Dual-stage filtering device has a 3-dB bandwidth of 0.5 nm and an SLSR of 23 dB. Our devices are promising for on-chip applications that require narrow bandwidths such as wavelength-division-multiplexed channel filters.

Wideband Microstrip Comb-Line Linear Array Antenna Using Stubbed-Element Technique for High Sidelobe Suppression

This paper presents a novel wideband microstrip comb-line linear array antenna using end feeding developed in the millimeter-wave. The proposed antenna is designed for 28 GHz and has 12 elements. The reflection-canceling slit structure technique is adopted to develop a broadside beam with low reflections at the input. The proposed array is designed using stub-studded radiating elements that create more resonances to broaden the impedance bandwidth. The structure is a novel comb-line linear array antenna with stub-studded radiating elements and was developed by applying stubs to eight of the radiating elements of the array to increase the input impedance bandwidth. This helps to attain a wideband linear array antenna on a single dielectric substrate, which conventionally has a narrow bandwidth. The input impedance bandwidth is from 23.5 to 33.11 GHz, which is equivalent to a bandwidth of 33.95%. The radiation pattern bandwidth is from 27.5 to 29.25 GHz for sidelobe levels of below −20 dB equivalent to a bandwidth of 6.167%. The proposed antenna array presents features such as a simple structure with low fabrication cost and an enhanced impedance bandwidth with high sidelobe suppression. The feasibility of the proposed antenna array was evaluated by simulation and measurement in the millimeter-wave band.

Silicon photonic bandpass filter based on apodized subwavelength grating with high suppression ratio and short coupling length.

A compact silicon bandpass filter with high sidelobe suppression is proposed and experimentally demonstrated using an apodized subwavelength grating (SWG) coupler. The device is implemented by placing a SWG waveguide next to a strip waveguide, and apodization is employed with a Gaussian profile to taper the gap between the two waveguides. A high sidelobe suppression ratio of 27 dB can be obtained with a 3-dB bandwidth of 8.8 nm and an insertion loss of 2.5 dB. Owing to the large optical phase mismatch between the two waveguides and the presence of the SWG waveguide, the coupling length of the device is reduced to 100.3 μm. The experimental results validate our proposed apodized-SWG-based contradirectional coupler (contra-DC) as a promising device in suppressing out-of-band components in coarse wavelength division multiplexed (CWDM) optical communication systems.

Comparison of active-set method deconvolution and matched-filtering for derivation of an ultrasound transit time spectrum

The quality of ultrasound computed tomography imaging is primarily determined by the accuracy of ultrasound transit time measurement. A major problem in analysis is the overlap of signals making it difficult to detect the correct transit time. The current standard is to apply a matched-filtering approach to the input and output signals. This study compares the matched-filtering technique with active set deconvolution to derive a transit time spectrum from a coded excitation chirp signal and the measured output signal. The ultrasound wave travels in a direct and a reflected path to the receiver, resulting in an overlap in the recorded output signal. The matched-filtering and deconvolution techniques were applied to determine the transit times associated with the two signal paths. Both techniques were able to detect the two different transit times; while matched-filtering has a better accuracy (0.13 μs versus 0.18 μs standard deviations), deconvolution has a 3.5 times improved side-lobe to main-lobe ratio. A higher side-lobe suppression is important to further improve image fidelity. These results suggest that a future combination of both techniques would provide improved signal detection and hence improved image fidelity.

Sidelobe Suppression Enhancement of Radiofrequency Photonic Filters via Time-to-frequency Mapping

We present a multi-tap microwave photonic filter with high selectivity through applying time-to-frequency mapping and optical frequency comb shaping techniques. When arranged in the time-to-frequency mapping stage, by a Fourier transform, the deviation of the optical taps to the target profile is significantly reduced while maintaining the apodization profile, resulting in high sidelobe suppression in the filters. By applying a simple time-to-frequency mapping stage to the conventional optical frequency combs, we demonstrate a substantially enhanced (>10dB) sidelobe suppression, resulting in filter lineshapes exhibiting a significantly high (>40dB) main lobe to sidelobe suppression ratio. These results highlight the potential of the technique for implementation in various passband filters with high sidelobe suppression.

Transmission of 100-Gb/s DDO-OFDM/OQAM over 320-km SSMF with a single photodiode.

In this paper, we propose a novel approach to simultaneously receive multi-band 100-Gb/s direct-detection optical signal with only one polarization and one conventional 40-GHz photodiode. The modulation format of orthogonal frequency-division multiplexing based on offset quadrature amplitude modulation (OFDM/OQAM) is selected to provide signal spectrum with high side-lobe suppression ratio, which can effectively reduce the electrical sub-band frequency interference. The whole 100-Gb/s OFDM/OQAM signal is comprised of 6 sub-bands with 16- and 32-QAM formats loading. Only one guard band is required to accommodate the overlapped 6-band signal-to-signal beat interference (SSBI). The receiver bandwidth is mainly limited by the digital storage oscilloscope (DSO) of 33 GHz. The transmission distance over standard single mode fiber (SSMF) is up to 320 km.

Coupler-apodized Bragg-grating add–drop filter

We demonstrate an apodization technique using tapered coupler gaps in grating-assisted asymmetric couplers, which circumvents the need for tapered waveguide perturbations and enables a higher tolerance to fabrication errors. A high sidelobe suppression ratio of 30 dB has been obtained on the submicrometer silicon-on-insulator platform using a CMOS-compatible photonics process with 193 nm lithography.

Definition of the border between the far and near field zone for printed antenna arrays with high side lobe suppression

The paper defines the border between the far and near field zone of printed antenna arrays. The conventional definition has a limited application in side lobe suppression measurements, especially in the case of high side lobe suppression. The paper particularly details the printed antenna array with 8 radiation elements. Its symmetrical tapered feed network with Chebyshev distribution enables side lobe suppression greater than -44 dB in the E-plane. In addition, the influence of tolerances in the standard photolithographic process has been investigated. The expected tolerances in the standard photolithographic process have been assumed with moderate precision in order to estimate the side lobe suppression degradation due to amplitude and phase deviations, as well as deviations to optimized values in radiating element positioning.

Simulation and analysis of Gaussian apodized fiber Bragg grating strain sensor

In this paper, the performance of various apodization profiles (uniform, hyperbolic tangent and gaussian) for un-chirped Fiber Bragg Grating is investigated. Apodization techniques are used to get optimized reflection spectra with high side lobe suppression. The simulations are done by solving coupled mode equations in MATLAB using transfer matrix method which explains the relationship between the guided modes. The result shows that Gaussian profile suppresses side lobe level much more efficiently than uniform and hyperbolic tangent profiles. Gaussian apodized Fiber Bragg Grating is used to indicate strain by producing wavelength shift. MATLAB and Opti-grating result gives an idea about the efficiency of the suggested scheme to analyze strain measurements by giving a linear response.

Highly selective and tunable microwave photonic filter using parallel Fabry–Pérot filters for universal mobile telecommunications system noise and interference suppression

The authors propose a highly selective and tunable microwave photonic filter based on parallel Fabry–Pérot filters with fibre Bragg gratings for delay. The filter is suitable for noise and interference suppression in universal mobile telecommunications system (UMTS) base stations. Simulation results demonstrate the feasibility of these filters in UMTS applications, and also show flexibility in their frequency response. Tunability of the filter has been employed for UMTS channel selection and grating length has been tuned in order to achieve appropriate delay. The filter provides a very high selective performance (in the frequency range of 1937–1976 MHz and a very high-quality factor of 494) with very high sidelobe suppression (>36 dB) in the stop band with lesser number of components. Tunability and reconfigurability of the proposed filter was obtained by changing the grating length. In this way, easy implementation of simple, compact and cost-effective filter has been demonstrated.

High Resolution Range Imaging and Sidelobe Suppression Based on FD-MIMO Radar

High Resolution Range Imaging and Sidelobe Suppression Based on FD-MIMO Radar

Compact acousto-optic filter with beam expanders constituted by photonic crystal rows of airholes

A compact acousto-optic (AO) filter with multireflector (MR) beam expanders constituted by 32 photonic crystal rows of airholes in LiNbO(3) waveguide has been proposed and numerically studied by the two-dimensional finite-difference-time-domain method. The diameter and position of every row of holes are gradually varied along MR beam expanders in order to provide high efficiency and good sidelobe suppression. Device sizes are 70 μm × 150 μm, with an FWHM bandwidth of 4.4 nm, a tuning range of 110 nm, internal loss of -3 dB, and sidelobes of -20 dB, thus providing a unique value of 0.6 μm(2) for the product FWHM bandwidth by an AO length. It is intended for use in fiber optics telecommunications and sensing.