Sidelobe Suppression Ratio Research Articles

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.

Coherence sidelobe suppression of laser diode by white-noise-current modulation and external cavity feedback.

A coherence sidelobe suppression method for a laser diode (LD) is proposed, based on the white-noise-current modulation and external cavity feedback. The theoretical model of the coherence of the white-noise-current-modulated external cavity laser (WECL) is established, which details the spectrum broadening mechanism and the coherence sidelobe suppression. At the noise current modulation, the coherence sidelobe induced by the internal cavity is lowered by the external cavity feedback, and the position of the coherence sidelobe induced by the external cavity is controlled with the cavity length, which can extend the coherence range without the sidelobes. The experimental results demonstrate that the 98.4% sidelobe suppression ratio (SLSR) of the WECL is achieved in dynamic interferometry, which is about 20% higher than that of the laser diode only with the white-noise-current modulation. This new, to the best of our knowledge, method can avoid the stray fringe cross talk caused by the multiple coherence sidelobes in the surface error measurements of parallel plates.

A short wave radar beam sharpening method based on generalised oblique projection operator with flexible parameter

AbstractBeamforming is an effective way of resolving target direction and anti‐jamming in short wave (SW) radar systems. In conventional beamforming (CBF) at a certain frequency, to get high resolution, the array aperture should be increased, and this is often not allowed in practical applications. A new narrow beam forming (NBF) method for beam sharpening based on the generalised oblique projection (GOP) filter with a flexible parameter is proposed. This method uses a GOP filter bank to form deep nulls in the undesired azimuth range on the pattern and utilises the logic product process to synthesise the GOP filters’ outputs and thus obtains a narrow beam. Compared to traditional beamforming methods, the result of NBF has the characteristics of narrower beam width and bigger side lobe suppression ratio (SLSR). Especially, a narrower beam can be obtained in the case of a small array aperture, which is valuable for practical applications. Experimental results of the range‐Doppler spectrum of short wave radar show that this narrow beam forming method can achieve super resolution of targets within a wide beam and greatly suppress clutter. Therefore, NBF can improve the azimuth resolution and achieve interference suppression in a conventional beam.

Low sidelobe silicon optical phased array with Chebyshev amplitude distribution

Abstract We propose and demonstrate a silicon photonic optical phased array (OPA) with ultra-low sidelobe level. The arbitrary ratio power splitters (ARPSs) are introduced to manipulate the amplitude distribution between different channels and suppress the sidelobe level. A 32-channel OPA has been designed and demonstrated with the amplitude distribution determined by preferred Chebyshev method. The experimental results indicate that the sidelobe suppression ratio (SLSR) can be up to 25.3 dB. The measured field of view (FOV) is 84° × 13° with divergence of 2.8° × 1.7°. Furthermore, the frequency-modulated continuous-wave (FMCW) based ranging has been also demonstrated experimentally by utilizing the OPA as the transmitter.

Twelve-channel LAN wavelength-division multiplexer on lithium niobate

Abstract A twelve-channel local-area-network (LAN) wavelength-division multiplexing (LWDM) filter is proposed and demonstrated with a uniform channel spacing of 4.5 nm (800 GHz) in the O-band of 1270–1330 nm by using x-cut lithium-niobate-on-insulator (LNOI) photonic waveguides for the first time. The present LWDM filter consists of twelve single-channel bandpass filters based on multimode waveguide gratings (MWGs) assisted with a TE0/TE1 mode (de)multiplexer. In particular, two stages of MWGs in cascade are introduced for each single-channel bandpass filter, in order to achieve high sidelobe suppression ratios, thus reducing interchannel crosstalk. For the fabricated twelve-channel LWDM filter, all the channels have very excellent box-like spectral responses with low excess losses of ∼0.6 dB, broad 1-dB bandwidths of ∼2.9–3.4 nm (which is close to 75 % of the channel spacing), and ultra-low interchannel crosstalk of <−40 dB in experiments. In addition, the present device is highly tolerant to the random variations of the etching depth (±20 nm) and the grating waveguide width (±20 nm) of the LNOI photonic waveguides, showing great potential for high-capacity WDM systems.

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.

An On-Chip Calibration Method for Optical Phased Array With No Blind Spot in the Steering Range

We demonstrate an on-chip phase calibration method in the silicon-based optical phased array (OPA) using multimode interferences (MMIs) and germanium silicon photodetectors (GeSi PDs), which are integrated at the end of the grating array. Two groups of MMIs with staggered symmetrical distribution are utilized to obtain the phase difference between adjacent gratings and linked to the PDs. With this method, a 1 × 32 OPA with thirty-two PDs realizes a 36.4° × 11.5° beam steering range without any blind spot, and the optimal side-lobe suppression ratio is 9.3 dB.

A tunable microwave photonic filter based on hole-assisted graded-index few mode fiber with uniform differential modal group delay

A tunable delay-line microwave photonic filter (MPF) based on hole-assisted graded index few mode fiber (HGI-FMF) is proposed. The HGI-FMF is designed with low mode crosstalk, low nonlinear effect and multiple modal groups. In particular, the designed HGI-FMF shows uniform differential modal group delay (MGD) to form finite impulse response (FIR) MPF. The uniform differential MGD is tunable with wavelength, which results in tunable MPF response. The free spectrum range (FSR) of MPF is continuously tunable from 11.73 GHz down to 10.84 GHz with 120 m designed HGI-FMF as the optical source is tuned from 1530 nm up to 1630 nm. The main sidelobe suppression ratio (MSSR) of the 4-taps filter is larger than 9.93 dB, and the 3 dB bandwidth is around 2.62 GHz.

Sparse 2-D optical phased array with large grating-lobe-free steering range based on an aperiodic grid.

Two-dimensional (2-D) optical phased arrays (OPAs) usually suffer from limited scan ranges and small aperture sizes. To overcome these bottlenecks, we utilize an aperiodic 32 × 32 grid to increase the beam scanning range and furthermore distribute 128 grating antennas sparsely among 1024 grid points so as to reduce the array element number. The genetic algorithm is used to optimize the uneven grid spacings and the sparse distribution of grating antennas. With these measures, a 128-channel 2-D OPA operating at 1550 nm realizes a grating-lobe-free steering range of 53° × 16°, a field of view of 24° × 16°, a beam divergence of 0.31° × 0.49°, and a sidelobe suppression ratio of 9 dB.

All-fiber orthogonal-polarized white-noise-modulated laser for short-coherence dynamic interferometry.

An all-fiber orthogonal-polarized white-noise-modulated laser (AOWL) for short-coherence dynamic interferometry is proposed. Short-coherence laser is achieved by current modulating of a laser diode with the band-limited white noise. A pair of orthogonal-polarized lights with adjustable delay for short-coherence dynamic interferometry are output by the all-fiber structure. In the non-common-path interferometry, the AOWL can significantly suppress the interference signal clutter with 73% side lobe suppression ratio, that improves the positioning accuracy of zero optical path difference. The wavefront aberrations of a parallel plate are measured with the AOWL in the common-path dynamic interferometers, avoiding the fringe crosstalk.

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 .

Femtosecond laser inscribed fiber Bragg gratings based on precise spatial apodization.

Plane-by-plane femtosecond laser fabricated apodized fiber Bragg gratings (FBG) are demonstrated for the first time, to the best of our knowledge. The method reported in this work provides a fully customizable and controlled inscription that can realize any desired apodized profile. By using this flexibility, we experimentally demonstrate four different apodization profiles (Gaussian, Hamming, New, Nuttall). These profiles were chosen to evaluate their performance with regard to the sidelobe suppression ratio (SLSR). Usually, a higher reflectivity of a grating fabricated with a femtosecond laser will result in a greater difficulty to achieve a controlled apodization profile due to the nature of the material modification. Therefore, the goal of this work is to fabricate high-reflectivity FBGs without sacrificing the SLSR and provide a direct comparison with apodized low-reflectivity FBGs. In our weak apodized FBGs, we also consider the background noise introduced during the femtosecond (fs)-laser inscription process which is fundamental when multiplexing FBGs within a narrow wavelength window.

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.

Low-power consumption InP-based optical phase arrays with non-uniformly spaced output waveguides.

Optical Phase Arrays (OPAs) are expected to be an ideal solution to achieve beam shaping, laser radar (LIDAR), free-space optical communications, and spatially resolved optical sensors, etc. We demonstrated a low-power consumption 32-channel OPA with non-uniformly spaced waveguides based on InP substrate. The phase shifters are based on a p-i-n structure which are operated with reverse bias and have a low power consumption. Besides, in order to improve the performance especially to obtain larger steering angle and narrower beam divergence without increasing the number of channels, we have optimized the spacing between the output waveguides. The fabricated OPA achieved a steering angle of 35° with the side lobe suppression ratio more than 8.2 dB across the angle range from -20° to 20° in the far field, which is the largest phase tuning steering angle reported by InP-based OPAs as far as we know. The divergence angle is about 0.46° in the phase steering dimension and the power consumption of the OPA at each steering angle is lower than 7.5 mW.

Polarization-insensitive antisymmetric multimode waveguide Bragg grating filter based on an SiN-Si dual-layer stack.

A polarization-insensitive multimode antisymmetric waveguide Bragg grating (MASWBG) filter based on an SiN-Si dual-layer stack is demonstrated. Carefully optimized grating corrugations patterned on the sidewall of a silicon waveguide and SiN overlay are used to perturbate TE and TM modes, respectively. Furthermore, the lateral-shift apodization technique is utilized to improve the sidelobe suppression ratio (SLSR). A good overlap between the passbands measured in TE and TM polarization states is obtained. Insertion losses, SLSRs, and 3-dB bandwidths of measured passbands in TE/TM polarizations are 1/1.72 dB, 18.5/19.1 dB, and 5.1/3.5 nm, respectively.

Optical phased array with on-chip phase calibration.

Optical phased arrays (OPAs) with phase-monitoring and phase-control capabilities are necessary for robust and accurate beamforming applications. This paper demonstrates an on-chip integrated phase calibration system where compact phase interrogator structures and readout photodiodes are implemented within the OPA architecture. This enables phase-error correction for high-fidelity beam-steering with linear complexity calibration. A 32-channel OPA with 2.5-µm pitch is fabricated in an Si-SiN photonic stack. The readout is done with silicon photon-assisted tunneling detectors (PATDs) for sub-bandgap light detection with no-process change. After the model-based calibration procedure, the beam emitted by the OPA exhibits a sidelobe suppression ratio (SLSR) of -11 dB and a beam divergence of 0.97° × 0.58° at 1.55-µm input wavelength. Wavelength-dependent calibration and tuning are also performed, allowing full 2D beam steering and arbitrary pattern generation with a low complexity algorithm.

Generation of a sub-wavelength sized optical needle with arbitrary longitudinal rotation.

We show that under tight focusing conditions, arbitrarily rotating the longitudinally polarized optical needle in space is possible. Applying the Richards and Wolf vector diffraction methods, the explicit expressions underlying the simultaneous control depth of focus (DoF), intensity suppression of the sidelobes, as well as the orientation of the optical needle can be obtained, and then the strength vectors of the three-dimensional electromagnetic fields can be calculated. Calculations reveal that the sidelobe suppression ratio reaches 5.35% of the principal lobe, the optimal DoF is 5.1λ, and the maximum length is about 18.2λ. In addition, we specify the necessary conditions for rotating the sub-wavelength sized optical needle in the longitudinal field. Such an optical needle with controllable length, purity, and orientation can provide a flexible approach and additional degree of freedom for 3D precise fabrication and some other potential application areas.

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.