Additional Phase Modulation Research Articles

Solid-State NMR Double-Quantum Dipolar Recoupling Enhanced by Additional Phase Modulation.

Additional phase modulation (APM) is proposed to generally enhance the theoretical efficiency of homonuclear double-quantum (DQ) recoupling in solid-state NMR. APM applies an additional phase list to DQ recoupling in steps of an entire block. The sine-based phase list can enhance the theoretical efficiency by 15-30 %, from 0.52 to 0.68 (non-γ-encoded recoupling) or from 0.73 to 0.84 (γ-encoded recoupling), with doubled recoupling time. The genetic-algorithm (GA) optimized APM can adiabatically enhance the efficiency to ∼1.0 at longer times. The concept of APM has been tested on SPR-51 , BaBa, and SPR-31 , which represent γ-encoded recoupling, non-γ-encoded recoupling, and another kind beyond the former two, respectively. Simulations reveal that enhancements from APM are due to the activation of more crystallites in the powder. Experiments on 2,3-13 C labeled alanine are used to validate the APM recoupling. This new concept shall shed light on developing more efficient homonuclear recoupling methods.

Suppression of the Talbot effect in Fourier transform acousto-optic imaging.

We report on the observation and correction of an imaging artifact attributed to the Talbot effect in the context of acousto-optic imaging using structured acoustic waves. When ultrasound waves are emitted with a periodic structure, the Talbot effect produces π-phase shifts of that periodic structure at every half of the Talbot distance in propagation. This unwanted artifact is detrimental to the image reconstruction, which assumes near-field diffraction is negligible. Here, we demonstrate both theoretically and experimentally how imposing an additional phase modulation on the acoustic periodic structure induces a symmetry constraint leading to the annihilation of the Talbot effect. This will significantly improve the acousto-optic image reconstruction quality and allows for an improvement of the reachable spatial resolution of the image.

Method of spatial-temporal coding of signals in the antenna array of millimeter-wave radar

Currently, most antenna systems are built based on MIMO technology. Its main advantage compared to classical antenna arrays is the possibility of a significant reduction of the number of physical transmitting and receiving channels keeping the resolution in angular coordinates. This feature is achieved due to the fact that the transmitting or receiving array is designed to be sparse, that is, the distance between the phase centers is significantly higher than half the wavelength. The cost of simplifying the design is presence of energy losses. In addition, the presence of several transmitting signals leads to the need to separate them from each other at the receiving position. The most popular method for separating orthogonal signals in MIMO arrays is code division signal separation. To implement this method, various types of orthogonal sequences are used. The most common are the Walsh, Fourier, and pseudo-random sequences. The use of orthogonal codes leads to the arising of multiple maximums in two-dimensional spectrum of the signal reflected from the object, exactly in dimension of Doppler frequency, with regular intervals. The number of maximums corresponds to the number of orthogonal sequences. Moreover, these maximums are formed in both case when using a sequence decoder and in the absence of a decoder. This feature is explained by the fact that when the sum of signals modulated in phase by Walsh sequences is multiplied by one of these sequences, one of the signals is demodulated, and the rest receive additional phase modulation. The level of additional maximum corresponds to the level of the main maximum. As a result, the ambiguity in determining the Doppler frequency appears. The indicated problem is very actual at the present time. To solve this problem, in the present paper the method of spatial-temporal coding of signals in millimeter-wave antenna array has been considered, based on the simultaneous operation of all sparse transmitters encoded by Walsh or Fourier sequences and spatial selection of transmitting beams by receiving one. The geometry of the placement of the elements of the transmitting and receiving antenna arrays corresponds to the geometry of the elements of MIMO array. At the same time, in contrast to the classical approach of beam forming for MIMO array, when individual transmitting antennas radiate orthogonal signals, and the irradiation of the target by each of them is considered separately, in the proposed method, the target is irradiated with a beam formed for transmission due to coherent spatial addition of signals from individual sparse transmitting array elements shifted in phase in accordance with the required slope of the phase front. As a result, the directivity pattern (DP) of the transmitting and receiving antenna is formed, which is equal to the product of the DP of the transmitting and receiving arrays. In this case, the interference maxima of DP of the transmitting antenna are at the zeros of DP of the receiving antenna, and the resulting DP coincides with the DP of the virtual receiving antenna formed using the classical MIMO approach. In the course of the studies and simulations, results have been obtained that show the effectiveness of the described method in terms of Doppler frequency ambiguity elimination when encoding signals in MIMO antenna arrays of the millimeter range.

Self-calibration phase demodulation scheme for stabilization based on an auxiliary reference fiber-optic interferometer and ellipse fitting algorithm.

To solve the problem of light source jitter and asymmetric 3 × 3 coupler, a phase demodulation method with the combination of an auxiliary reference interferometer and elliptic fitting algorithm is proposed, which is verified by simulation and experiment. By introducing additional phase modulation in the auxiliary reference interferometer, the parameters of the sensing arm can be calibrated in real time, which ensures the effective operation of elliptic fitting algorithm in small signal measurement. Consequently, the experiments show that the self-calibration scheme enables a higher signal to noise and distortion ratio with an average increase of 1.65 dB and 10.47 dB compared with the traditional Arctan and cross multiplication differential, respectively. Meanwhile, the self-calibration scheme can also effectively suppress the harmonic distortion, with a total harmonic distortion of -33.64 dB in the case of small signal.

Improved Phase Noise Cancellation Technology for Auxiliary Reference Interferometer Demodulation Scheme

In an unbalanced interferometer, the phase noise level is positively related to the optical path length difference (OPD) and the wavelength instability of the light source. The effective current solution for reducing phase noise is to use an auxiliary reference interferometer to measure the phase noise signal. However, this method requires that all interferometers have the same OPD, otherwise the phase modulation depth of different interferometers with the same light source will be different and the noise reduction effect will be severely reduced. It is difficult to provide interferometers with precisely the same OPD in practical. In this paper, an improved phase-generated carrier (PGC) demodulation technique for phase noise cancellation that does not require strictly equal OPDs of interferometers is proposed. Combining an auxiliary reference interferometer scheme with an ellipse fitting algorithm (EFA) can eliminate the effect of different phase modulation depths due to different OPDs, and phase modulation depth drift on demodulation results, significantly reducing harmonic distortion. The additional phase modulation signal ensures the correct calculation of small signals by the EFA, while the OPDs of the interferometers are evaluated in real time to achieve constant phase noise cancellation capability under different OPD. Experimental results show that the proposed technique achieves a highly stable phase demodulation result with a maximum phase noise reduction of about 9dB and a minimum THD of -74.59 dB in interferometers with non-strict equal OPD.

An Advanced Approach to Improve Synchronization Phase Accuracy with Compressive Sensing for LT-1 Bistatic Spaceborne SAR

In the bistatic synthetic aperture radar (BiSAR) system, the unavoidable frequency deviation between the oscillators (USOs) will result in additional phase modulation in the demodulated radar signal, which significantly degrades the quality of the SAR image and digital elevation model (DEM) product. The innovative L-band spaceborne BiSAR system LuTan-1 (LT-1) employs a non-interrupted synchronization scheme to acquire the synchronization phase error. This advanced phase synchronization scheme avoids interrupting the normal BiSAR data acquisition and further increases the synchronization frequency. However, some non-ideal factors in the transmission link like attenuation, multipath effect, interference, etc., may cause the synchronization phase to be polluted by noise. A phase denoising approach based on compressive sensing (CS) is proposed to improve the accuracy of synchronization phase. The imaging phase with high signal-to-noise ratio (SNR) is input into the K-SVD algorithm to learn the prior information, and then the noise of the synchronization compensation phase is eliminated by maximum a posteriori (MAP) estimation. The data acquired from the ground validation system of the LT-1 synchronization module are adopted for the validation experiment. The proposed phase denoising method achieves higher phase synchronization accuracy compared with traditional ones. The processing results verify the effectiveness of the proposed method and demonstrate its potential for future on-orbit applications of the LT-1 mission.

High-stability PGC demodulation technique with an additional sinusoidal modulation based on an auxiliary reference interferometer and EFA.

In the reference interferometer demodulation scheme, it's difficult to guarantee in practice that both interferometers have the same optical path length difference (OPD), which makes the phase modulation depth different in different interferometers with the same laser modulation. The random shift of phase modulation depth also affects the demodulation results. An improved phase-generated carrier (PGC) technique is proposed based on an auxiliary reference interferometer and the ellipse fitting algorithm (EFA). The technique ensures the correct fitting of the EFA for small amplitude signals by introducing a sinusoidal signal as an additional phase modulation. The combination of the reference interferometer and EFA can eliminate the effect of different phase modulation depths of the two interferometers caused by different OPDs, the non-linear distortion caused by phase modulation depth shifts, and improve the accuracy of the demodulation results. The experiment results are consistent with the theoretical analysis, and the method extends the application of the EFA in the reference interferometer phase demodulation technique.

Quantitative demodulation of distributed low-frequency vibration based on phase-shifted dual-pulse phase-sensitive OTDR with direct detection.

Phase-sensitive optical time-domain reflectometry (Φ-OTDR) has been proposed for distributed vibration sensing purpose over recent years. Emerging applications, including seismic and hydroacoustic wave detection, demand accurate low-frequency vibration reconstruction capability. We propose to use the direct-detection Φ-OTDR configuration to achieve quantitative demodulation of external low-frequency vibrations by phase-shifted dual-pulse probes. Simultaneous pulsing and phase shifting modulation is realized with a single acousto-optic modulator to generate such probes, relaxing the need for an additional optical phase modulator. In the experiments, vibrations with frequency as low as 0.5 Hz are successfully reconstructed with 10 m spatial resolution and 35 dB signal-to-noise ratio. Excellent linearity and repeatability are demonstrated between the optical phase demodulation results and the applied vibration amplitudes. The proposed method is capable of quantitative demodulation of low-frequency vibrations with a cost-effective system configuration and high computation efficiency, showing potential for commercial applications of distributed seismic or hydroacoustic wave acquisition.

An Optimal Polar Format Refocusing Method for Bistatic SAR Moving Target Imaging

Bistatic synthetic aperture radar (BiSAR) has received more and more attentions because of its forward-looking imaging capability and configuration flexibility. For BiSAR moving target imaging, its non-cooperative motion leads to unknown range cell migration (RCM) and additional phase modulation. Consequently, moving target imaging in BiSAR face two main challenges: 1) The unknown RCM correction and Doppler parameter estimation are tightly coupled. 2) The Doppler parameters of the extended moving target’s different scattering points are different, i.e., the Doppler parameters are spatially variant. To cope with these problems, an optimal polar format refocusing method for bistatic SAR moving target imaging is proposed. First, the main part of tight coupling and spatial variation effects caused by the BiSAR platforms are eliminated, while the moving target is two-dimensional (2-D) defocused and shifted. Then, we analyze the characteristics of two-dimensional defocused and shifted of the moving target in BiSAR, and give the analytical expressions. On this basis, a new bistatic polar format transformation is introduced, in which the degree of freedom of defocusing result is reduced from 2-D to only one-dimension. After that, the parameter estimation and refocusing issues are transformed into a constrained optimization problem (COP), and differential evolution (DE) is applied to solve the COP and obtain the refocusing results. Finally, considering the spatial variation of the extended moving target, the compensation processing is performed to relocate each scattering point. Numerical simulations verify the effectiveness of the proposed method.

DNN With Similarity Constraint for GEO SA-BSAR Moving Target Imaging

A GEOsynchronous Spaceborne-Airborne Bistatic Synthetic Aperture Radar (GEO SA-BSAR) system has been proved to be a significant tool for moving targets monitoring. Due to the special geometry model of the GEO SA-BSAR system, there is a complex relative movement between the moving target and the bistatic radar, leading to an additional phase modulation of the echo and further, causing moving targets to be smeared in the SAR image. Recently, Deep Neural Network (DNN) shows great potential in rapid image recovery. However, most image recovery methods based on DNN concentrate on the whole image, which limits the imaging performance of sparse targets. In this letter, we propose a DNN framework with similarity constraints for GEO SA-BSAR moving target imaging. This DNN-based method optimizes the cosine similarity of azimuth signals between the ground-truth image and the predicted image in the loss function to recover the azimuth position and focusing characteristics of the sparse targets. Extensive experimental results prove that the proposed model can quickly obtain GEO SA-BSAR moving target images with small training datasets compared with some counterparts.

Scalable higher-order exceptional surface with passive resonators.

The sensitivity of perturbation sensing can be effectively enhanced with higher-order exceptional points due to the nonlinear response to frequency splitting. However, experimental implementation is challenging since all the parameters need to be precisely prepared. The emergence of an exceptional surface (ES) improves the robustness of the system to the external environment, while maintaining the same sensitivity. Here, we propose, to our knowledge, the first scalable protocol for realizing a photonic high-order ES with passive resonators. By adding one or more additional passive resonators in the low-order ES photonic system, the three- or arbitrary N-order ES is constructed and proved to be easily realized in experiment. We show that the sensitivity is enhanced and the experimental demonstration is more resilient against fabrication errors. The additional phase-modulation effect is also investigated.

Phase-shifted demodulation technique with additional modulation based on a 3 × 3 coupler and EFA for the interrogation of fiber-optic interferometric sensors.

A phase-shifted demodulation technique with a 3×3 coupler and ellipse fitting algorithm (EFA) for the interrogation of interferometric sensors is proposed. To reduce the error of the EFA as to measure small phase signals, additional phase modulation is introduced. The additional modulation provides a walk of the operating point along the Lissajous ellipse large enough to permit calculation of the ellipse parameters at every moment. Experimental result shows that this technique demonstrates high accuracy and stability for measuring small phase signals. The setting of this technology expands the application of the EFA in fiber-optic phase demodulation technology.

Implementation Features of Invariant Optical Correlator Based on Amplitude LC SLM

Mathematical simulations of invariant optical-digital correlator operation are performed with the LC SLM used for the input images display and for display of the correlation filter holograms. Different phase dependences on amplitude are considered, particularly the measured dependence of the LC SLM HoloEye LC 2002. A method for optimization of correlation filters to eliminate the object recognition error due to the presence of additional phase modulation is proposed.

A CMOS Two-Element 170-GHz Fundamental-Frequency Transmitter With Direct RF-8PSK Modulation

A CMOS 170-GHz fundamental-frequency transmitter (TX) realizing the 8PSK modulation scheme directly in the RF domain is presented. The use of direct RF modulation obviates the need for high-resolution high-speed mixed-signal blocks. The proposed architecture extends the conventional quadrature modulation by performing additional phase modulation on I and Q components of the LO signal, which helps increase modulation order. The TX employs high-speed switchable phase shifters to achieve LO phase modulation and fundamental-frequency over-neutralized power amplifiers to drive an integrated two-element tapered dipole antenna array. Fabricated in a 65-nm CMOS process ( $f_{T}/f_{\text {max}} = 230$ /260 GHz), the RF-8PSK TX prototype occupies $3.2 \times 2.8$ mm2 of die area. The free-space wireless measurement of the TX over a 10-cm link range yields 15 Gb/s data rate at an error vector magnitude (EVM) of −14.8 dB. The TX achieves an EIRP of 4 dBm while consuming 560-mW power.

Broadband dispersive Ge/YbF3 mirrors for mid-infrared spectral range.

Broadband dispersive mirrors operating in the mid-infrared spectral range of 6.5-11.5μm are developed for the first time, to the best of our knowledge. The mirrors comprise Ge and YbF3 layers, which have not been used before for manufacturing of multilayer dispersive optics. The design and production processes are described; mechanical stresses of the coatings are estimated based on experimental data; and spectral and phase properties of the produced mirrors are measured. The mirrors compensate group delay dispersion of ultrashort laser pulses accumulated by propagation through 4mm ZnSe windows and additional residual phase modulation of an ultrashort laser pulse.

The influence of additional phase modulation of an amplitude liquid crystal STLM on the image recognition characteristics in the invariant optical digital correlator

We present the results of measurements of additional phase modulation characteristics of a serial amplitude liquid crystal spatial light modulator HoloEye LC 2002. It is found in which way the phase shift of the liquid crystal spatial light modulator depends on the applied signal. The mathematical simulation of the performance of an invariant diffractive optical-digital correlator based on a liquid crystal spatial light modulator with the amplitude-dependent phase shift is carried out using previously measured data. The correlation filters used in the work are an optimal tradeoff maximum average correlation height filter and a minimum noise and correlation energy optical correlation filter. A method for correlation filters optimization was proposed to compensate for the recognition error caused by the presence of the additional phase modulation. In some cases, the optimization allows one not only to compensate for the recognition error, but also to reduce it.

Removal of surface-normal spot beam from on-chip 2D beam pattern projecting lasers.

Static arbitrary two-dimensional beam patterns have been demonstrated using on-chip size "integrable spatial-phase-modulating surface-emitting lasers," which use the band edge mode of a two-dimensional photonic crystal as an in-plane resonator, while the spatial phase of the lightwaves of the band edge mode are simultaneously modulated in a holographic manner by a local positional shift of holes from their lattice points. Meanwhile, the beam patterns include a spot beam in the surface-normal direction (0th-order beam), which corresponds to components of vertical diffraction of the band edge modes without spatial phase modulation. A promising method, used to remove the 0th-order beam, uses a structure that prohibits the vertical diffraction of band edge modes. For this purpose, we set the period of the virtual photonic crystal structure from the conventional Γ2 band edge to the Μ1 band edge, where vertical diffraction is prohibited. Moreover, the additional spatial phase modulation that cancels the in-plane component of the wavevectors of the lightwaves of the band edge modes at the Μ1 band edge are also imposed to output the beam patterns vertically. As a result, we successfully demonstrated two-dimensional beam patterns without a spot beam in the surface-normal direction.

Spatial multiplexing for tailored fully-structured light

Fully-structured light is an emerging approach to sculpt light in all its degrees of freedom, i.e. amplitude, phase and polarization with a high transverse resolution and complex modulation patterns. Such an attractive and versatile approach for advanced optical trapping or fabricating novel materials still poses fundamental as well as technical challenges. Though the implementation of spatial light modulators (SLMs) has opened up a promising path for this task, up to now, fully-structured light can only be achieved using interferometric methods, multiple SLMs, or split-screen techniques reducing spatial resolution. We present a sophisticated single-beam approach based on spatial multiplexing, which does not only allow joint customization of phase and polarization combined with natural and prospectively even on-demand amplitude modulation, but also ensures high spatial resolution by the use of a single standard SLM in the full-screen mode. We demonstrate the capabilities of our approach realizing double phase modulated light fields, as well as first- and higher-order vector modes with additional global phase modulation and natural amplitude shaping. These findings open new perspectives to optically trap polarization-sensitive, i.e. magnetic particles and advance laser material machining in anisotropic, birefingent matter.

ОРТОГОНАЛЬНЫЕ ФАЗОКОДИРОВАННЫЕ СИГНАЛЫ С ДОПОЛНИТЕЛЬНОЙ ОТНОСИТЕЛЬНОЙ ФАЗОВОЙ МАНИПУЛЯЦИЕЙ

The paper examines the version of a equal-energy orthogonal phase-coded signals with additional differential phase modulation of codewords. Adding differential phase modulation of codewords to the phase-coded signal made it possible to reduce the probability of bit error while maintaining the possibility of incoherent detection. A single differential quadrature phase modulation of codewords provides the lowest probability of bit error among the signals under consideration, but in the absence of a frequency offset. At small frequency offsets, the single differential binary phase modulation of codewords has the smallest bit error probability. However, two-fold differential binary phase modulation with a slight loss relative to a single differential modulation in the AWGN channel has the lowest sensitivity to the frequency offset. Additional differential phase modulation it possible to reduces the number of orthogonal signals while preserving the probability of a bit error, or reduces the probability of a bit error while maintaining the occupied bandwidth.

Stability analysis and pull-in behavior of modified dither optical phase-lock loop with loop propagation delay

This paper analyses the stability of a modified dither optical phase-lock loop (OPLL), which contains all the components of a standard dither OPLL in conjunction of an additional electro-optic phase modulator in the phase-locking branch, taking loop delay into account. Applying Nyquist criteria, expression for maximum allowable loop propagation delay of the modified dither OPLL is derived. Relative stability of the modified loop is studied in terms of the Gain margin and Phase margin of the OPLL. Also, Pull-in behavior of modified dither OPLL is analyzed in the presence of loop propagation delay. Expressions are derived analytically for computing the pull-in voltage, pull-in range and acquisition time for a second-order type-II modified dither OPLL that employs an active low pass loop filter. The influences of the dither signal, electro-optic phase modulator and loop propagation delay on the acquisition of the loop are shown. It is found that the electro-optic phase modulator completely eliminates the false locking phenomenon due to loop delay, decreases the acquisition time of the loop and allows pull-in from larger frequency offset. Simulation results are given in support of the analytical results.