Showing papers on "Chirp published in 2016"
TL;DR: Mature opto-electrical/mechanical technologies have enabled laser processing speeds approaching meters-per-second, leading to a fast lab-to-fab transfer and emerging biomedical applications implementing micrometer feature precision over centimeter-scale scaffolds and photonic wire bonding in telecommunications are highlighted.
Abstract: Processing of materials by ultrashort laser pulses has evolved significantly over the last decade and is starting to reveal its scientific, technological and industrial potential. In ultrafast laser manufacturing, optical energy of tightly focused femtosecond or picosecond laser pulses can be delivered to precisely defined positions in the bulk of materials via two-/multi-photon excitation on a timescale much faster than thermal energy exchange between photoexcited electrons and lattice ions. Control of photo-ionization and thermal processes with the highest precision, inducing local photomodification in sub-100-nm-sized regions has been achieved. State-of-the-art ultrashort laser processing techniques exploit high 0.1–1 μm spatial resolution and almost unrestricted three-dimensional structuring capability. Adjustable pulse duration, spatiotemporal chirp, phase front tilt and polarization allow control of photomodification via uniquely wide parameter space. Mature opto-electrical/mechanical technologies have enabled laser processing speeds approaching meters-per-second, leading to a fast lab-to-fab transfer. The key aspects and latest achievements are reviewed with an emphasis on the fundamental relation between spatial resolution and total fabrication throughput. Emerging biomedical applications implementing micrometer feature precision over centimeter-scale scaffolds and photonic wire bonding in telecommunications are highlighted.
835 citations
TL;DR: It is demonstrated that by acting on the phase profile of the optical pulses, it is possible to introduce important conceptual and practical changes to the traditional ΦOTDR operation, thus opening a door for new possibilities which are yet to be explored.
Abstract: So far, the optical pulses used in phase-sensitive OTDR (ΦOTDR) were typically engineered so as to have a constant phase along the pulse. In this work, it is demonstrated that by acting on the phase profile of the optical pulses, it is possible to introduce important conceptual and practical changes to the traditional ΦOTDR operation, thus opening a door for new possibilities which are yet to be explored. Using a ΦOTDR with linearly chirped pulses and direct detection, the distributed measurement of temperature/strain changes from trace to trace, with 1mK/4ne resolution, is theoreticaly and experimentaly demonstrated. The measurand resolution and sensitivity can be tuned by acting on the pulse chirp profile. The technique does not require a frequency sweep, thus greatly decreasing the measurement time and complexity of the system, while maintaining the potential for metric spatial resolutions over tens of kilometers as in conventional ΦOTDR. The technique allows for measurements at kHz rates, while maintaining reliability over several hours.
250 citations
TL;DR: A scheme for nonlinear pulse compression at high average powers based on self-phase modulation in a multi-pass cell using fused silica as the nonlinear medium is demonstrated.
Abstract: We demonstrate a scheme for nonlinear pulse compression at high average powers based on self-phase modulation in a multi-pass cell using fused silica as the nonlinear medium. The scheme is suitable for compression of pulses with peak powers exceeding the threshold for critical self-focusing. At >400 W of input power, the pulses of a Yb:YAG-Innoslab laser system (10 MHz repetition rate, 850 fs pulse duration) are spectrally broadened from 1.6 to >13.5 nm bandwidth while maintaining almost diffraction-limited beam quality. The chirp is removed with a dispersive mirror compressor, and pulse durations of 170 fs at an output power of 375 W are achieved. The compression unit reaches an overall transmission of >90%.
160 citations
TL;DR: In this paper, the authors proposed an orthogonal chirp-division multiplexing (OCDM) for high-speed communication, which can efficiently exploit multipath diversity and thus outperform the OFDM, and that it is more resilient against the interference due to insufficient guard interval than single-carrier frequency domain equalization.
Abstract: Chirp waveform plays a significant role in radar and communication systems for its ability of pulse compression and spread spectrum. This paper presents a principle of multiplexing a bank of orthogonal chirps, termed orthogonal chirp-division multiplexing (OCDM) for high-speed communication. As Fourier transform is the kernel of orthogonal frequency-division multiplexing (OFDM), which achieves the maximum spectral efficiency (SE) of frequency-division multiplexing, Fresnel transform underlies the proposed OCDM system, which achieves the maximum SE of chirp spread spectrum. By using discrete Fresnel transform, digital implementation of OCDM is introduced. According to the properties of Fresnel transform, the transmission of OCDM signal in linear time-invariant channel is studied. Efficient digital signal processing is proposed for channel dispersion compensation. The implementation of the OCDM system is discussed with the emphasis on its compatibility to the OFDM system; it is shown that it can be easily integrated into the existing OFDM systems. Finally, the simulations are provided to validate the feasibility of the proposed OCDM. It is shown that the OCDM system can efficiently exploit multipath diversity and thus outperforms the OFDM, and that it is more resilient against the interference due to insufficient guard interval than single-carrier frequency-domain equalization.
131 citations
TL;DR: By designing a structured gas density profile between the dual-stage gas jets to manipulate electron seeding and energy chirp reversal for compressing the energy spread, high-brightness high-energy electron beams from a cascaded laser wakefield accelerator are experimentally produced.
Abstract: By designing a structured gas density profile between the dual-stage gas jets to manipulate electron seeding and energy chirp reversal for compressing the energy spread, we have experimentally produced high-brightness high-energy electron beams from a cascaded laser wakefield accelerator with peak energies in the range of 200-600 MeV, 0.4%-1.2% rms energy spread, 10-80 pC charge, and ∼0.2 mrad rms divergence. The maximum six-dimensional brightness B_{6D,n} is estimated as ∼6.5×10^{15} A/m^{2}/0.1%, which is very close to the typical brightness of e beams from state-of-the-art linac drivers. These high-brightness high-energy e beams may lead to the realization of compact monoenergetic gamma-ray and intense coherent x-ray radiation sources.
130 citations
TL;DR: A top-injected coupled resonator based wide locking range technique is proposed in the frequency doublers to minimize the required injection power to cover the chirp bandwidth plus enough PVT variation margin, and therefore reduce the power consumption of the local-oscillating (LO) distribution network.
Abstract: A fully-integrated 77 GHz frequency doubling two-path phased-array frequency-modulated continuous-wave (FMCW) transceiver for automotive radar applications is proposed. By utilizing the frequency doubling scheme, the chirp bandwidth is improved, and the complexity of the frequency synthesizer and the insertion loss of the local-oscillating (LO) distribution network are both reduced. Top-injected coupled resonator based wide locking range technique is proposed in the frequency doublers to minimize the required injection power to cover the chirp bandwidth plus enough PVT variation margin, and therefore reduce the power consumption of the LO distribution network. Current-reused coupled resonator technique is utilized to implement the LO phase shifting in each receiving path. The digitally controlled artificial dielectric-based transmission lines are inserted in the low noise amplifiers to provide the operation frequency calibration capability. The receiving two-path signals are converted into intermediate frequency by low flicker noise current-mode passive mixers and then combined in the trans-impedance amplifier, followed by the reconfigurable analog baseband processing. Fabricated in 65 nm CMOS, the FMCW transceiver has achieved 1.93 GHz maximum chirp bandwidth, $12.9\sim 13.2$ dBm maximum transmitting power, and $47.8\sim 100.7$ dB programmable receiving conversion gain. The transceiver consumes 343 mW power and 4.64 mm2 chip area including all of the pads.
107 citations
TL;DR: The broadband chirped pulse Fourier transform spectroscopy technique enables the collection of a ≥10 GHz bandwidth spectrum in a single shot of the spectrometer, which allows broadband, high-resolution microwave spectra to be acquired several orders of magnitude faster than what was previously possible.
Abstract: Since its invention in 2006, the broadband chirped pulse Fourier transform spectrometer has transformed the field of microwave spectroscopy. The technique enables the collection of a ≥10 GHz bandwidth spectrum in a single shot of the spectrometer, which allows broadband, high-resolution microwave spectra to be acquired several orders of magnitude faster than what was previously possible. We discuss the advantages and challenges associated with the technique and look back on the first ten years of chirped pulse Fourier transform spectroscopy. In addition to enabling faster-than-ever structure determination of increasingly complex species, the technique has given rise to an assortment of entirely new classes of experiments, ranging from chiral sensing by three-wave mixing to microwave detection of multichannel reaction kinetics. However, this is only the beginning. Future generations of microwave experiments will make increasingly creative use of frequency-agile pulse sequences for the coherent manipulation and interrogation of molecular dynamics.
105 citations
TL;DR: In this article, a complete circularly polarized 210-270-GHz frequency-modulated continuous-wave radar with a monostatic homodyne architecture is presented, which consists of a highly integrated radio-frequency transceiver module, an in-house developed linear-frequency chirp generator, and a data acquisition chain.
Abstract: A complete circularly polarized 210–270-GHz frequency-modulated continuous-wave radar with a monostatic homodyne architecture is presented. It consists of a highly integrated radio-frequency transceiver module, an in-house developed linear-frequency chirp generator, and a data acquisition chain. The radar front end featuring a fundamentally operated $\times$ 16 multiplier-chain architecture is realized as a single chip in 0.13- $\mu$ m SiGe heterojunction bipolar transistor technology with a lens-coupled circularly polarized on-chip antenna and wire-bonded on a low-cost printed circuit board. In combination with a 9-mm-diameter silicon lens, the module achieves an average in-band directivity of 26.6 dB. The measured peak radiated power from the packaged radar module is $+$ 5 dBm and the noise figure is 21 dB. For a 60-GHz frequency sweep, the radar achieves a range resolution of 2.57 mm after calibration, which is close to the theoretical bandwidth-limited resolution of 2.5 mm. With a simple scanning optical setup, this paper further demonstrates the 3-D imaging capability of the radar for detection of hidden objects with a remarkable dynamic range of around 50 dB.
95 citations
TL;DR: This work investigates the propagation of one-dimensional and two-dimensional Gaussian beams in the fractional Schrödinger equation (FSE) without a potential and introduces the Talbot effect of diffractionless beams in FSE.
Abstract: We investigate the propagation of one-dimensional and two-dimensional (1D, 2D) Gaussian beams in the fractional Schrodinger equation (FSE) without a potential, analytically and numerically. Without chirp, a 1D Gaussian beam splits into two nondiffracting Gaussian beams during propagation, while a 2D Gaussian beam undergoes conical diffraction. When a Gaussian beam carries linear chirp, the 1D beam deflects along the trajectories z = ±2(x - x0), which are independent of the chirp. In the case of 2D Gaussian beam, the propagation is also deflected, but the trajectories align along the diffraction cone z = 2√(x(2) + y(2)) and the direction is determined by the chirp. Both 1D and 2D Gaussian beams are diffractionless and display uniform propagation. The nondiffracting property discovered in this model applies to other beams as well. Based on the nondiffracting and splitting properties, we introduce the Talbot effect of diffractionless beams in FSE.
88 citations
TL;DR: In this paper, the authors considered a high-order nonlinear Schrodinger equation with competing cubic-quintic-septic nonlinearities, non-Kerr quintic nonlinearity, self-steepening, and self-frequency shift.
68 citations
TL;DR: A joint estimation approach of 3D geometry and rotation motion is presented to realize outlier removing and error reduction in ISAR imaging of maneuvering targets from sparse aperture (SA) data.
Abstract: In the current scenario of high-resolution inverse synthetic aperture radar (ISAR) imaging, the non-cooperative targets may have strong maneuverability, which tends to cause time-variant Doppler modulation and imaging plane in the echoed data. Furthermore, it is still a challenge to realize ISAR imaging of maneuvering targets from sparse aperture (SA) data. In this paper, we focus on the problem of 3D geometry and motion estimations of maneuvering targets for interferometric ISAR (InISAR) with SA. For a target of uniformly accelerated rotation, the rotational modulation in echo is formulated as chirp sensing code under a chirp-Fourier dictionary to represent the maneuverability. In particular, a joint multi-channel imaging approach is developed to incorporate the multi-channel data and treat the multi-channel ISAR image formation as a joint-sparsity constraint optimization. Then, a modified orthogonal matching pursuit (OMP) algorithm is employed to solve the optimization problem to produce high-resolution range-Doppler (RD) images and chirp parameter estimation. The 3D target geometry and the motion estimations are followed by using the acquired RD images and chirp parameters. Herein, a joint estimation approach of 3D geometry and rotation motion is presented to realize outlier removing and error reduction. In comparison with independent single-channel processing, the proposed joint multi-channel imaging approach performs better in 2D imaging, 3D imaging, and motion estimation. Finally, experiments using both simulated and measured data are performed to confirm the effectiveness of the proposed algorithm.
TL;DR: The fractional Fourier transform makes it possible to represent the SAR signal in a rotated joint time-frequency plane and performs optimal processing and analysis of these residual chirp signals, and the along-track defocus can be compensated for and the target's azimuthal speed estimated.
Abstract: This paper studies the effects of stationary-based processing of moving ship signatures in synthetic aperture radar (SAR) imagery and introduces a methodology to estimate and compensate for them. SAR imaging of moving targets usually results in residual chirps in the azimuthal SLC processed signal. The fractional Fourier transform (FrFT) makes it possible to represent the SAR signal in a rotated joint time-frequency plane and performs optimal processing and analysis of these residual chirp signals. The along-track defocus can thus be compensated for and the target's azimuthal speed estimated. The impact of higher order motion terms (e.g., acceleration) has been also considered. Experiments were conducted on a large number of ship signatures extracted from Radarsat-2 Multi Look Fine and Ultra Fine SAR images. An intercomparison with a standard Doppler Sublook Decomposition Method (SDM) is carried out, as well as a complete performance analysis with AIS data as ground truth.
TL;DR: This is the first demonstration, to the best of the knowledge, that 112-Gb/s PAM-4 modulation is achieved using only a 18-GHz (3-dB-bandwidth) commercial directly modulated laser.
Abstract: Single-channel 112-Gb/s PAM-4 transmission based on low-cost intensity modulation and direct detection (IM/DD) optics is experimentally demonstrated over 1-km standard single-mode fiber. By employing a digital precompensation, duobinary encode/decoding with PAM-4 signal and 7-level training-sequence-aided least mean square (TS-LMS) algorithm, we successfully achieve a receiver sensitivity of about −2 dBm with 7% overhead HD-FEC. Chirp management is applied at the transmitter side to extend the reach to 1 km for transmission at 1500 nm. This is the first demonstration, to the best of our knowledge, that 112-Gb/s PAM-4 modulation is achieved using only a 18-GHz (3-dB-bandwidth) commercial directly modulated laser. The method proposed in this paper is both bandwidth and computationally efficient, which is thought to be feasible in the low-cost short-reach optical applications.
TL;DR: It is demonstrated that two experimental channeled spectra only acquired at Master stage suffice to produce A-scans from any number of resolved depths at the Slave stage, and the utilization of complex signal processing is improved.
Abstract: A general theoretical model is developed to improve the novel Spectral Domain Interferometry method denoted as Master/Slave (MS) Interferometry. In this model, two functions, g and h are introduced to describe the modulation chirp of the channeled spectrum signal due to nonlinearities in the decoding process from wavenumber to time and due to dispersion in the interferometer. The utilization of these two functions brings two major improvements to previous implementations of the MS method. A first improvement consists in reducing the number of channeled spectra necessary to be collected at Master stage. In previous MSI implementation, the number of channeled spectra at the Master stage equated the number of depths where information was selected from at the Slave stage. The paper demonstrates that two experimental channeled spectra only acquired at Master stage suffice to produce A-scans from any number of resolved depths at the Slave stage. A second improvement is the utilization of complex signal processing. Previous MSI implementations discarded the phase. Complex processing of the electrical signal determined by the channeled spectrum allows phase processing that opens several novel avenues. A first consequence of such signal processing is reduction in the random component of the phase without affecting the axial resolution. In previous MSI implementations, phase instabilities were reduced by an average over the wavenumber that led to reduction in the axial resolution.
TL;DR: This letter demonstrates the airborne SAR experiment using an NLFM waveform and exhibits the promising power of the NLFM chirp and shows the accuracy of the proposed modified RMA.
Abstract: For synthetic aperture radar (SAR), a system impulse response with low sidelobes is very important because sidelobes may interfere with the nearby scatterers and contribute to multiplicative noise. It is well known that a nonlinear frequency-modulation (NLFM) chirp waveform can shape the signal's power spectral density and provide a radar matched filter output with lower sidelobes without loss of the signal-to-noise ratio when compared with the linear frequency-modulation chirp. These advantages make the NLFM waveform to be a promising candidate to improve the imaging quality for SAR. However, so far, to our knowledge, there is no real application of NLFM waveforms for SAR. This letter, for the first time, demonstrates the airborne SAR experiment using an NLFM waveform. In the underlying experiment, the construction of the NLFM signal is investigated and a modified range migration algorithm (RMA) is developed to adapt it for focusing the NLFM SAR data. Both simulation and experimental results exhibit the promising power of the NLFM chirp and show the accuracy of the proposed modified RMA.
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TL;DR: It is shown that the OCDM system can efficiently exploit multipath diversity and thus outperforms the OFDM, and that it is more resilient against the interference due to insufficient guard interval than single-carrier frequency-domain equalization.
Abstract: Chirp waveform plays a significant role in radar and communication systems for its ability of pulse compression and spread spectrum. This paper presents a principle of orthogonally multiplexing a bank of linear chirp waveforms within the same bandwidth. The amplitude and phase of the chirps are modulated for information communication. As Fourier trans-form is the basis for orthogonal frequency division multiplexing (OFDM), Fresnel transform underlies the proposed orthogonal chirp division multiplexing (OCDM). Digital implementa-tion of the OCDM system using discrete Fresnel transform is proposed. Based on the con-volution theorem of the Fresnel transform, the transmission of the OCDM signal is analyzed under the linear time-invariant or quasi-static channel with additive noise, which can gener-alize typical linear transmission channels. Based on the eigen-decomposition of Fresnel transform, efficient digital signal processing algorithm is proposed for compensating chan-nel dispersion by linear single- tap equalizers. The implementation details of the OCDM system is discussed with emphasis on its compatibility to the OFDM system. Finally, simula-tion are provided to validate the feasibility of the proposed OCDM under wireless channels. It is shown that the OCDM system is able to utilize the multipath diversity and outperforms the OFDM system under the multipath fading channels.
23 Feb 2016
TL;DR: In this paper, a novel Fractional Fourier Transform (FrFT) based multiplexing scheme is presented as a joint radar-communication technique, which is used to embed data into chirp sub-carriers with different time-frequency rates.
Abstract: The increasing demand of spectrum resources and the need to keep the size, weight and power consumption of modern radar as low as possible, has led to the development of solutions like joint radar-communication systems. In this paper a novel Fractional Fourier Transform (FrFT) based multiplexing scheme is presented as a joint radar-communication technique. The FrFT is used to embed data into chirp sub-carriers with different time-frequency rates. Some optimisation procedures are also proposed, with the objective of improving the bandwidth occupancy and the bit rate and/or Bit Error Ratio (BER). The generated waveform is demonstrated to be robust to distortions introduced by the channel, leading to low BER, while keeping good radar characteristics compared to a widely used Linear Frequency Modulated (LFM) pulse with same duration and bandwidth.
TL;DR: In this article, the effect of the pulse shape, bandwidth, integration angle, and signal-to-noise ratio (SNR) of the received pulse on the image resolution is comprehensively studied.
Abstract: In this paper, the design of pulse characteristics to achieve the desired image resolution for near-field synthetic aperture radar is presented. Gaussian and chirp pulses, which are the most commonly used pulses for ultra-wideband (UWB) radar applications, are considered in this paper. The effect of the pulse shape, bandwidth, integration angle, and signal-to-noise ratio (SNR) of the received pulse on the image resolution is comprehensively studied. To enhance the image resolution, preprocessing of the received pulses with envelope detection or match filtering are also studied. The range and cross-range resolutions achieved by Gaussian and chirp pulses with the same center frequency and bandwidth at various SNR values are compared. This paper shows that the Gaussian pulse with envelope detection provides better image resolution, whereas the chirp pulse with match filtering provides more resistance to noise. Closed-form equations and design guidelines are developed to design the input pulse characteristics to achieve the desired image resolution. The antennas’ effect on UWB pulses and the developed equation for cross-range resolution, are both validated using full-wave simulations and measurements.
TL;DR: In this paper, a generalized nonlinear Schrodinger equation with polynomial Kerr nonlinearity and non-Kerr terms of an arbitrarily higher order is investigated for femtosecond pulse propagation in highly nonlinear optical media.
Abstract: A generalized nonlinear Schrodinger equation with polynomial Kerr nonlinearity and non-Kerr terms of an arbitrarily higher order is investigated. This model can be applied to the femtosecond pulse propagation in highly-nonlinear optical media. We introduce a new chirping ansatz given as an expansion in powers of intensity of the light pulse and obtain both linear and nonlinear chirp contributions associated with propagating optical pulses. By taking the cubic-quintic-septic-nonic nonlinear Schrodinger (NLS) equation with seventh-order non-Kerr terms as an example for the generalized equation with Kerr and non-Kerr nonlinearity of arbitrary order, we derive families of chirped soliton solutions under certain parametric conditions. The solutions comprise bright, kink, anti-kink, and fractional-transform soliton solutions. In addition, we found the exact soliton solution for the model under consideration using a new ansatz. The parametric conditions for the existence of chirped solitons are also reported.
TL;DR: A new nonlinear frequency modulation waveform is developed that can be used as a transmitted chirp in synthetic aperture radar (SAR) imaging to improve the imaging quality compared to a linear frequency modulation (LFM) chirP signal.
Abstract: In this paper, a new nonlinear frequency modulation (NLFM) waveform is developed that can be used as a transmitted chirp in synthetic aperture radar (SAR) imaging to improve the imaging quality compared to a linear frequency modulation (LFM) chirp signal. The new NLFM is constructed based on piecewise linear functions, which are optimized using multi-objective optimization. Different signal processing algorithms are investigated in order to use NLFM as the transmitted chirp in a SAR system. In addition, a modified motion compensation (MC) algorithm using navigation data is proposed for the range-Doppler algorithm. Strip-map SAR geometry is considered to generate a SAR raw signal, in order to validate the new offered chirp signal and the proposed MC algorithm.
TL;DR: The frequency range of quantitative NMR is increased from tens to hundreds of kHz by a new pulse sequence, CHORUS, which uses chirp pulses to excite uniformly over very large bandwidths, yielding accurate integrals even for nuclei such as (19)F that have very wide spectra.
TL;DR: In this paper, the effects of nonlinearity in Thomson scattering of a high intensity laser pulse from electrons are analyzed, and analytical expressions for laser pulse shaping in frequency (chirping) are obtained which control spectrum broadening for high laser pulse intensities.
Abstract: Effects of nonlinearity in Thomson scattering of a high intensity laser pulse from electrons are analyzed. Analytic expressions for laser pulse shaping in frequency (chirping) are obtained which control spectrum broadening for high laser pulse intensities. These analytic solutions allow prediction of the spectral form and required laser parameters to avoid broadening. Results of analytical and numerical calculations agree well. The control over the scattered radiation bandwidth allows narrow bandwidth sources to be produced using high scattering intensities, which in turn greatly improves scattering yield for future x- and gamma-ray sources.
TL;DR: In this article, an orthogonal chirp division multiplexing (OCDM) technique for coherent optical communication is proposed, in which the CO-OCDM can effectively compensate the dispersion and is more resilient to fading and noise impairment than OFDM.
Abstract: In this paper, we propose an orthogonal chirp division multiplexing (OCDM) technique for coherent optical communication. OCDM is the principle of orthogonally multiplexing a group of linear chirped waveforms for high-speed data communication, achieving the maximum spectral efficiency (SE) for chirp spread spectrum, in a similar way as the orthogonal frequency division multiplexing (OFDM) does for frequency division multiplexing. In the coherent optical (CO)-OCDM, Fresnel transform formulates the synthesis of the orthogonal chirps; discrete Fresnel transform (DFnT) realizes the CO-OCDM in the digital domain. As both the Fresnel and Fourier transforms are trigonometric transforms, the CO-OCDM can be easily integrated into the existing CO-OFDM systems. Analyses and numerical results are provided to investigate the transmission of CO-OCDM signals over optical fibers. Moreover, experiments of 36-Gbit/s CO-OCDM signal are carried out to validate the feasibility and confirm the analyses. It is shown that the CO-OCDM can effectively compensate the dispersion and is more resilient to fading and noise impairment than OFDM.
TL;DR: This paper fabricated two all-dielectric gratings with gradient grids in the THz regime, of which artificial birefringence is much larger than that of the equal-grid dielectric grating demonstrated by both experiments and simulations, and shows great advantages compared to the periodic gratings.
Abstract: Subwavelength dielectric gratings are widely applied in the phase and polarization manipulation of light. However, the dispersion of the normal dielectric gratings is not flat while their birefringences are not enough in the THz regime. In this paper, we have fabricated two all-dielectric gratings with gradient grids in the THz regime, of which artificial birefringence is much larger than that of the equal-grid dielectric grating demonstrated by both experiments and simulations. The transmission and dispersion characteristics are also improved since the gradient grids break the periodicity of grating lattices as a chirp feature. From 0.6–1.4 THz, a broadband birefringence reaches 0.35 with a low dispersion and good linearity of phase shift, and the maximum phase shift is 1.4π. Furthermore, these gradient gratings are applied as half-wave plates and realize a linear polarization conversion with a conversion rate over 99%, also much higher than the equal-grid gratings. These gradient gratings show great advantages compared to the periodic gratings and provide a new way in the designing of artificial birefringence material.
TL;DR: In this paper, a silicon-based on-chip electrically-tunable spectral shaper for the generation of a tunable linearly chirped microwave waveform (LCMW) based on spectral shaping and wavelength-to-time (SS-WTT) mapping is designed, fabricated, and demonstrated.
Abstract: A silicon-based on-chip electrically-tunable spectral shaper for the generation of a tunable linearly chirped microwave waveform (LCMW) based on spectral shaping and wavelength-to-time (SS-WTT) mapping is designed, fabricated, and demonstrated. The on-chip spectral shaper has a Michelson interferometer structure with two linearly chirped waveguide Bragg gratings (LC-WBGs) incorporated in its two arms. Due to the wavelength-dependent length difference between the two arms of the interferometer, the spectral response of the spectral shaper exhibits a wavelength-dependent free spectral range, which is required for the generation of an LCMW based on SS-WTT mapping. To enable electrical tuning of the spectral response, a lateral PN junction is introduced to each of the waveguides where the LC-WBGs are inscribed. Thanks to the plasma dispersion effect, the spectral response of the spectral shaper can be tuned by changing the bias voltages applied to the PN junctions, which would lead to the tuning of the generated LCMW. A theoretical analysis on the LCMW generation is performed, which is verified by an experiment in which an electrically-tunable spectral shaper is fabricated using a CMOS-compatible process with 248-nm deep ultraviolet lithography. By independently controlling the bias voltages to the PN junctions, a continuous tuning of the generated LCMW is demonstrated.
TL;DR: A novel algorithm for the analysis of the SFM signal based on the adaptive chirplet decomposition is proposed, which is accurate, efficient, and easy to be implemented.
Abstract: For synthetic aperture radar (SAR) imaging with the high-frequency band, such as the terahertz band, the platform vibration influence cannot be neglected. In general, the vibration of the platform can be viewed as a simple harmonic motion, and thus, the received signal in each range bin can be considered as a sinusoid frequency modulation (SFM) signal for the terahertz SAR imaging. This will introduce the images to be defocused in the azimuth processing. In this letter, a novel algorithm for the analysis of the SFM signal based on the adaptive chirplet decomposition is proposed. This algorithm is accurate, efficient, and easy to be implemented. Then, the high-frequency vibration of the platform can be compensated by the corresponding signal decomposition results. The simulated results demonstrate the effectiveness of the novel algorithm proposed in this letter.
25 Feb 2016
TL;DR: To obtain a 20cm-resolution image within a 15m distance using an X-band FMCW radar, an agile chirp frequency synthesizer phase-locked loop (FSPLL) with a wide chirP bandwidth greater than 750MHz and a short chir p period less than 100μs is necessary.
Abstract: To obtain a 20cm-resolution image within a 15m distance using an X-band FMCW radar, an agile chirp frequency synthesizer phase-locked loop (FSPLL) with a wide chirp bandwidth (BW) greater than 750MHz and a short chirp period (Tm) less than 100µs is necessary. Challenges arise as one tries to realize a triangular chirp profile in Fig. 13.1.1 with a fast chirp slope (=BW/Tm) and precise linearity. In particular, many FMCW FSPLLs exhibit increased frequency errors around the turn-around points (TAPs), degrading the effective resolution achievable. For instance, for FSPLLs modulating either the reference or feedback clock frequency [1–3], the finite loop bandwidth of the PLL limits the maximum chirp slope as well as the linearity of the chirp profile. Moreover, many of these PLLs use a multi-modulus frequency divider (FD) and delta-sigma modulation (DSM) [2], which put further constraints on the maximum PLL bandwidth to filter the quantization noise. While a two-point modulation (TPM) scheme can decouple the conflicting requirements on the PLL bandwidth [4–6], the gain or timing mismatch between the two modulation paths and limited resolution of the DSM-based FD can still limit the chirp linearity, especially when one tries to push the chirp slope faster.
TL;DR: The time-dependent phase relation explains the energy flow direction during amplification and is characteristic for this strong coupling process and is also of potential importance to understand and maybe control cross-beam-energy transfer in inertial confinement fusion.
Abstract: A detailed analysis is presented of the various stages of strong coupling Brillouin plasma amplification, emphasizing the importance of the chirp which can be of threefold origin: the intrinsic one driven by the amplification process, the one originating from the chirped-pulse-generated laser pulses, and the one associated with the plasma profile. Control of the overall chirp can optimize or quench the energy transfer. The time-dependent phase relation explains the energy flow direction during amplification and is characteristic for this strong coupling process. The study is also of potential importance to understand and maybe control cross-beam-energy transfer in inertial confinement fusion.
TL;DR: This article provides a tutorial on a novel RF steganography scheme to conceal digital communication in linear chirp radar signals to prevent an enemy from detecting the existence of such hidden information.
Abstract: The chirp signal is one of the first bio-inspired signals commonly used in RF applications where the term chirp is a reference to the chirping sound made by birds. It has since been recognized that birds communicate through such chirping sounds to attract other birds of the same species, to transmit an alarm for specific threats, and so on. However, birds of a different species, or sometime even birds in a different social group within a species, are unable to connect a specific meaning to certain calls - they will simply hear a bird chirping. Inspired by such, this article provides a tutorial on a novel RF steganography scheme to conceal digital communication in linear chirp radar signals. We first provide a review of the linear chirp signal and existing communication systems using chirp waveforms. Next we discuss how to implement the RF steganography and hide digitally modulated communication information inside a linear chirp radar signal to prevent an enemy from detecting the existence of such hidden information. A new modulation called reduced phase shift keying is employed to make the modulated chirp waveform almost identical to the unmodulated chirp signal. Furthermore, variable symbol durations are employed to eliminate cyclostationary features that might otherwise be exploited by an enemy to detect the existence of the hidden information.
TL;DR: Modulation, fiber transmission and reception of 25-Gb/s signal based on directly modulated laser and photo-detector both operating at 10 GHz are experimentally demonstrated.
Abstract: High speed data modulation based on bandwidth limited devices has been considered as a cost-effective way to upgrade 10G-EPON to the next generation 100G-EPON. In this paper, we experimentally demonstrate the modulation, fiber transmission and reception of 25-Gb/s signal based on directly modulated laser and photo-detector both operating at 10 GHz. Instead of digital signal processing, the chirp management, dispersion compensation and frequency equalization in our scheme are realized in optical domain using a single delay interferometer. Three popular formats are investigated, including NRZ-OOK, PAM-4 and duobinary. According to the experimental results, the NRZ-OOK format shows its superiority in both launch power and receiver sensitivity, which provides a cost-effective solution for the construction of 100-Gb/s TWDM-PON.