Microwave Waveform Research Articles

Trityl-Aryl-Nitroxide-Based Genuinely g-Engineered Biradicals, As Studied by Dynamic Nuclear Polarization, Multifrequency ESR/ENDOR, Arbitrary Wave Generator Pulse Microwave Waveform Spectroscopy, and Quantum Chemical Calculations.

Trityl and nitroxide radicals are connected by π-topologically controlled aryl linkers, generating genuinely g-engineered biradicals. They serve as a typical model for biradicals in which the exchange (J) and hyperfine interactions compete with the g-difference electronic Zeeman interactions. The magnetic properties underlying the biradical spin Hamiltonian for solution, including J's, have been determined by multifrequency CW-ESR and 1H ENDOR spectroscopy and compared with those obtained by quantum chemical calculations. The experimental J values were in good agreement with the quantum chemical calculations. The g-engineered biradicals have been tested as a prototype for AWG (Arbitrary Wave Generator)-based spin manipulation techniques, which enable GRAPE (GRAdient Pulse Engineering) microwave control of spins in molecular magnetic resonance spectroscopy for use in molecular spin quantum computers, demonstrating efficient signal enhancement of specific weakened hyperfine signals. Dynamic nuclear polarization (DNP) effects of the biradicals for 400 MHz nuclear magnetic resonance signal enhancement have been examined, giving efficiency factors of 30 for 1H and 27.8 for 13C nuclei. The marked DNP results show the feasibility of these biradicals for hyperpolarization.

Reconfigurable photonic generation of background-free microwave waveforms with multi-format

Photonic generation of background-free microwave waveforms with multi-format and parameter tunability is proposed and demonstrated. In the approach, the polarization division multiplexing Mach–Zehnder modulator (PDM-MZM) is employed to modulate the linearly polarized lightwave. After properly adjusting the modulation indices of the PDM-MZM, background-free square and triangular waveforms will be obtained after heterodyne detection; by driving the polarization modulator with binary sequence, the structure can also generate background-free digital modulation signals, including amplitude shift keying (ASK), phase shift keying (PSK) and frequency shift keying (FSK) signals via the same structure. The approach is investigated by simulations. Square and triangular waveforms with repetition rate of 20 GHz are obtained, and digital modulation signals with modulation rate of 2 Gbit/s are generated. The proposed generator shows flexible tunability of carrier frequency, bit rate and modulation format. The stability of the structure is verified by phase drift, polarization error and RF driven voltage offset.

Tunable triangular frequency modulated microwave waveform generation with improved linearity using an optically injected semiconductor laser.

A photonic approach to generating a triangular frequency modulated microwave waveform with improved linearity using an optically injected semiconductor laser is proposed and demonstrated. By controlling the optical injection strength to the semiconductor laser at the period-one oscillation state to have a triangular shape, a triangular frequency modulated microwave waveform is generated after the optical-electrical conversion. A method based on a generalized regression neural network is proposed to improve the linearity of the generated waveform. By adjusting the parameters of the low-frequency electrical triangular control signal, the tunability of the center frequency, bandwidth, and time duration of the generated waveform can be realized. In the proof-of-concept experiment, a triangular frequency modulated microwave waveform with a frequency range from 14-24 GHz and a time duration of 2 μs has been successfully generated. The improvement of the linearity of the waveform is experimentally verified. The performance of the generated triangular frequency modulated microwave waveforms for reducing the range-Doppler coupling is verified through the analyses of the ambiguity function. The tunability of the center frequency, bandwidth, and time duration is also experimentally demonstrated.

Proposal and theoretical analysis of a fiber grating sensor interrogation based on dispersive Fourier transformation and a pair of chirped fiber gratings

A novel approach to interrogate a fiber grating sensor based on dispersive Fourier transformation method is proposed and simulated. In the system, two chirped fiber Bragg gratings (CFBG), with one used as a reference grating and the other one as sensing grating, are employed to form a Fabry–Perot (FP) interference structure, used as the spectral-shaping filter. The filter can be modeled as a two-tap delay-line filter. An optical pulse from a mode-locked laser source is reflected and spectrally shaped by the FP structure, and then the shaped spectrum is mapped to the temporal domain by a dispersive element. When the sensing CFBG is experiencing a strain, its wavelengths will be changed, leading to different interference spectra. Using Fourier transform, the frequency of the generated microwave waveform can be achieved, from which the wavelength shift of the sensing grating and the applied strain can be demodulated. A mathematical model to describe the microwave waveform generation is developed, by which the interrogation principle is analyzed. Numerical simulations are implemented to verify the proposed approach. The results present that the sensitivity of the system is 0.01 GHz/με. The influences of key parameters in the system on the microwave spectra are investigated and discussed, including grating length, cavity length, chirp rate and index modulation depth.

Simple and Flexible Photonic Microwave Waveform Generation With Low RMSE of Square Waveform

A simple and flexible photonics-assisted approach based on an integrated polarization-division multiplexing Mach–Zehnder modulator (PDM-MZM) and a balanced photodetector (BPD) to generating microwave waveform is proposed. The sub-MZMs of PDM-MZM are biased at peak and null points. Square and trapezoid waveforms are generated by controlling the PC and driven sinusoidal signal power. Triangle and sawtooth waveforms are also obtained with the help of a 90° microwave phase shifter (MPS). The proposed approach features simple structure, flexible frequency tunability, and easy implementation. Moreover, the problem of high root-mean-square error (RMSE) of square waveform is solved by this approach for the first time. An experiment is conducted to verify the approach. Different waveforms with the repetition rates of 3 and 5 GHz are generated. The RMSE of square waveform is reduced from 0.3328 to 0.2048 which is the lowest RMSE to the best of our knowledge.

Photonic-based microwave hybrid combiner with arbitrarily tunable phase shift and power combining ratio.

We report a broadband photonic-based microwave hybrid combiner (PMHC) using a dual-polarization dual-parallel Mach-Zehnder modulator. The broadband PMHC with arbitrarily tunable phase shift and power combining ratio can overcome several limitations of conventional microwave combiners in terms of tunability and bandwidth. By simply adjusting the biases of the modulator, the phase shift and power ratio of the two combined microwave signals are independently, continuously, and freely tunable. The proposed PMHC is theoretically analyzed and experimentally verified. In a proof-of-concept experiment, two sinusoidal microwave waveforms are successfully combined to form rectangular and triangular waveforms, respectively.

Dual-chirp Fourier domain mode-locked optoelectronic oscillator.

Optoelectronic oscillators (OEOs) have been widely investigated to generate ultra-pure single-frequency microwave signals. Here, we propose and experimentally demonstrate a dual-chirp Fourier domain mode-locked (FDML) OEO to generate dual-chip microwave waveforms. In the proposed FDML OEO, a frequency-scanning dual-passband microwave photonics filter based on phase-modulation-to-intensity-modulation conversion using an optical notch filter and two laser diodes is incorporated into the OEO cavity. Fourier domain mode-locking operation is achieved by synchronizing the scanning period of the filter to the cavity round-trip time. Tunable dual-chirp microwave waveforms with a large time-bandwidth product are generated directly from the FDML OEO cavity in the experiment, which can be used in modern radar systems to improve its range-Doppler resolution.

A Chirp-Rate-Tunable Microwave Photonic Pulse Compression System for Multi-Octave Linearly Chirped Microwave Waveform

A chirp-rate-tunable microwave photonic pulse compression system for multi-octave linearly chirped microwave waveform (LCMW) is proposed in this paper. The key components are a dual-parallel Mach-Zehnder modulator (DPMZM) and a dispersive loop (DL) structure based on a linear chirped fiber Bragg grating. By controlling the output of the DL to change the accumulated group velocity dispersion, the chirp rate of the system can be tuned. By adjusting the wavelength of the optical carrier to select the single sideband modulated optical signal (positive or negative sideband), the chirp direction (down or up chirped) can also be tuned. The performance of the multi-octave LCMW pulse compression can be improved by eliminating the second-order harmonic distortion through configuring the bias voltages of the DPMZM. Detailed theoretical analysis is conducted. A verification simulation demonstrates the pulse compression of the multi-octave LCMWs with the chirp rates of 23.877, 11.939, 3.979, and -3.979 GHz/ns. In the proof-of-concept experiment, the matched chirp rates 23.229, 12.051, -22.568, and -11.355 GHz/ns are obtained. The multi-octave LCMW pulse compressions with the compression ratios of 14.3, 29.3, 15.0, and 28.6 are verified. The simulation and experiment results are in good agreement with the theoretical analysis.

Photonic generation of multi-frequency dual-chirp microwave waveform with multiplying bandwidth

Abstract A photonic approach to generate dual-chirp microwave waveform with multi-carrier frequencies and multiplying bandwidth is proposed and demonstrated by using a dual-polarization Mach-Zehnder modulator (DPol-MZM) and balanced detection. In the approach, the DPol-MZM driven by a baseband waveform modulates the injected optical signal to generate optical broad-spectra sidebands. After heterodyne in balanced photodetector (BPD), multi-frequency dual-chirp microwave waveform can be generated. The proposed generator allows the injected optical signal with arbitrary intensity modulation format. The key features of the approach are the multi-carrier frequencies generation ability and the bandwidth multiplying operation. The proposed approach is verified by simulation. Dual-chirp signal with four-carrier frequencies is generated by employing optical frequency comb (OFC) with five comb lines. Auto-correlations of the generated signals show good pulse compression ratio and high peak-to-sidelobe ratio. The maximum frequency number of the system is analyzed, and the maximum power ratio is also investigated.

Harmonically Fourier Domain Mode-Locked Optoelectronic Oscillator

We experimentally demonstrate a harmonically Fourier domain mode-locked optoelectronic oscillator (FDML OEO). Harmonic Fourier domain mode locking operation is achieved when the cavity round-trip time is equal to integer multiples of the scanning period of the filter in the OEO loop. Compared with a fundamentally FDML OEO, frequency scanning microwave signals with increased tuning speed and large bandwidth can be easily generated by the proposed harmonically FDML OEO, which can find applications in spread-spectrum communication and modern radar systems. Up to fourth order, harmonic Fourier domain mode locking operation is achieved in the experiment. The time-bandwidth product of the harmonically FDML OEO is as large as tens of thousands. The maximum chirp rate of the generated $X$ -band linearly chirped microwave waveform is 0.725 GHz/ $\mu \text{s}$ , which is four times higher than that of the fundamentally FDML OEO.

Recent Trends and Advances of Silicon-Based Integrated Microwave Photonics

Multitude applications of photonic devices and technologies for the generation and manipulation of arbitrary and random microwave waveforms, at unprecedented processing speeds, have been proposed in the literature over the past three decades. This class of photonic applications for microwave engineering is known as microwave photonics (MWP). The vast capabilities of MWP have allowed the realization of key functionalities which are either highly complex or simply not possible in the microwave domain alone. Recently, this growing field has adopted the integrated photonics technologies to develop microwave photonic systems with enhanced robustness as well as with a significant reduction of size, cost, weight, and power consumption. In particular, silicon photonics technology is of great interest for this aim as it offers outstanding possibilities for integration of highly-complex active and passive photonic devices, permitting monolithic integration of MWP with high-speed silicon electronics. In this article, we present a review of recent work on MWP functions developed on the silicon platform. We particularly focus on newly reported designs for signal modulation, arbitrary waveform generation, filtering, true-time delay, phase shifting, beam steering, and frequency measurement.

Photonic microwave waveform generation based on dual-wavelength time domain synthesis technology

<sec>Microwave waveforms, such as square waveforms, sawtooth waveforms and triangle waveforms are widely used in radar communication, electronic measurement and medical imaging and so on. Using photonic microwave technology to generate arbitrary microwave waveforms has been a research hotspot.</sec><sec>In this paper, a photonic microwave waveform generation scheme based on dual-wavelength time domain synthesis is proposed and experimentally demonstrated. Used in this scheme mainly are two lasers, two single-drive Mach-Zehnder modulators, a wavelength division multiplexer and a tunable optical delay line. The two Mach-Zehnder modulators are respectively biased at different operating points. When two beams with different wavelengths are superimposed in the time domain, different microwave waveform outputs can be generated. Therefore, by adjusting the bias voltage and modulation depth of the modulator, the phase and amplitude of the modulated optical signal can be controlled, and finally the photonic microwave waveform is generated.</sec><sec>At first, the generation mechanism of square waveform, sawtooth waveform and triangle waveform are analyzed, and the comparisons among ideal square waveform, sawtooth waveform, triangle waveform and their third-order waveforms are made through the simulation analysis. It is verified that third-order waveforms become close to the ideal waveforms. Since the proposed scheme produces higher-order components, and the waveforms of the first three orders are the same as the ideal waveforms, so the scheme has good waveform generation capability. And then square waveform, sawtooth waveform and triangle waveform with a repetition rate of 2.5 GHz are successfully generated experimentally. Thus, experimental results are well consistent with the theoretical analyses. In addition, the system also has good tunable characteristics. By changing the modulation frequency of the modulator, the frequency tuning of the output photonic microwave waveforms can be realized, and square waveform, sawtooth waveform and triangular waveform with a repetition rate of 5 GHz are also experimentally achieved. The repetition rate of the generated microwave waveform is mainly limited by the bandwidth of modulator and electrophotonic detector, so the devices with higher bandwidth can be used to generate arbitrary waveform with a higher repetition rate. Therefore, the scheme has good application prospects.</sec>

Photonic Generation and Transmission of Linearly Chirped Microwave Waveform With Increased Time-Bandwidth Product

Photonic generation and transmission of linearly chirped microwave waveform with the increased time-bandwidth product (TBWP) is proposed and demonstrated by phase-encoding and splitting parabolic waveform. In the approach, dual-polarization Mach-Zehnder modulator (DPol-MZM) along with polarization controller (PC) is employed to generate two orthogonally polarized wavelengths, and polarization modulator is driven by a parabolic signal. After splitting parabolic waveform and convolving with a binary pseudo random sequence, chirped microwave waveform with high bandwidth and large temporal duration is generated, corresponding to an improved TBWP. Dispersion-induced power fading can be effectively avoided in the approach, and chirped signal with constant initial phase can be achieved by adjusting the PC. The proposed approach is verified by simulations. Linearly chirped microwave waveform with the central frequency of 40 GHz, bandwidth of 12.5 GHz, and TBWP of 20160 is generated. RF spurious suppression ratios of the generated signal are investigated, and pulse compression performances are also analyzed. The system features filter-free, high tunability, compact structure, and large TBWP, which has potential application in the modern radar system.

Photonic approach to dual-band dual-chirp microwave waveform generation with multiplying central frequency and bandwidth

Abstract A novel photonic approach to generate frequency and bandwidth multiplying dual-chirp microwave for application in dual-band radar system is proposed using an integrated dual-polarization quadrature phase shift keying (DP-QPSK) modulator. The proposed approach is based on optical frequency multiplying and polarization selection. In the approach, the baseband signal and RF signal are injected into the DP-QPSK modulator to generate optical broad spectra and sidebands respectively. By adjusting the bias points of the DP-QPSK modulator, after polarization selection and optical to electrical conversion, frequency-quadrupling bandwidth-octupling and frequency-double bandwidth-quadrupling or frequency-tripling bandwidth-sextupling and frequency-fundamental bandwidth-doubling dual-chirp signals can be generated in the single structure respectively. The proposed approach is verified by simulations. Dual-chirp microwaves with central frequency–bandwidth of 32 GHz-8 GHz, 16 GHz-4 GHz or 24 GHz-6 GHz, 8 GHz-2 GHz are generated in the two ports respectively. Their autocorrelation results manifest good pulse compression capabilities. The central frequency, bandwidth, chirp rate and time duration can be tuned independently. The generated dual-chirp signals show good performance on the range-Doppler coupling and can significantly improve the range-Doppler resolution in dual-band radar system.

Tunable Fourier Domain Mode-Locked Optoelectronic Oscillator Using Stimulated Brillouin Scattering

We propose and demonstrate a tunable Fourier domain mode-locked optoelectronic oscillator (OEO) using a frequency scanning microwave photonics filter (MPF) based on stimulated Brillouin scattering (SBS). Fourier domain mode locking operation is achieved by synchronizing the tuning period of the SBS-based MPF to the cavity round-trip time of OEO. As a result, thousands of longitudinal modes are stimulated simultaneously in the OEO cavity. Linearly chirped microwave waveforms with a chirp rate up to 0.84 GHz/ $\mu \text{s}$ and a time bandwidth product as high as 22446 are generated in the experiment. The proposed approach provides an important solution for chirped microwave waveforms generation, which is widely used in modern radar and communication systems, and has the potential to be integrated on a chip.

Integrated Microwave Photonics for Spectral Analysis, Waveform Generation, and Filtering

We review the recent work on developing integrated microwave photonic subsystems in silicon photonics for performing radio-frequency spectral analysis, generating microwave waveforms, and filtering microwave signals.

Photonic generation of binary phase-coded microwave waveforms and phase-coded pulses with ultrawide frequency tunable range

A scheme that can produce a continuous wave (CW) phase-coded signal and phase-coded pulses is proposed. The proposed scheme is based on a dual-polarization Mach–Zehnder modulator (DPol-MZM), a polarization modulator (PolM) and a balanced photodetector (BPD) without the help of an electrical phase shifter or an electrical power divider. In DPol-MZM, a carrier-suppressed double sideband (CS-DSB) signal and an optical carrier are, respectively, produced in the upper and lower arms, which are in two orthogonal polarization states. The output is then sent to a PolM. The polarization multiplexed CS-DSB and optical carrier are phase modulated by driving the RF port of the PolM with a coding signal. After beating between the modulated CS-DSB and optical carrier in a BPD, a stable CW phase-coded signal or phase-coded pulses can be obtained. The proposed scheme is experimentally verified. Phase-coded microwave waveforms at 6 and 11 GHz are generated and phase-coded pulses at 11 GHz are also produced.

Simultaneous Generation of WDM Chirped Microwave Waveforms Using Integrated Spectral Shapers in Silicon Photonics

We demonstrate a simultaneous generation of wavelength-division-multiplexed chirped microwave waveforms using an optical spectral shaping followed by a wavelength-to-time mapping. We design two types of integrated spectral shapers based on parallel distributed Fabry–Perot cavities or an arrayed waveguide grating Sagnac interferometer incorporating linearly chirped waveguide Bragg gratings. The generated waveforms have different central RF frequencies and the opposite signs of RF chirp.

Simultaneous interrogation of multiple fiber Bragg grating temperature sensors using a microwave photonic approach

In this paper, we demonstrate a microwave photonic (MWP) approach for interrogating multiple linearly chirped fiber Bragg grating (LCFBG) temperature sensors. The MWP interrogation system is based on an arrayed waveguide grating Sagnac interferometer, incorporating multiple LCFBG sensors, that is used to generate multiple chirped microwave waveforms whose characteristics are uniquely related to those of the LCFBGs. Compressing the generated microwave signals allows monitoring simultaneously and independently the shifts associated with applying temperature changes to each LCFBG in real time.

Fourier domain mode locked optoelectronic oscillator based on the deamplification of stimulated Brillouin scattering

A fast frequency scanning optoelectronic oscillator (OEO) based on Fourier domain mode locking (FDML) and the deamplification of stimulated Brillouin scattering (SBS) is proposed and experimentally demonstrated. The SBS is used to realize a fast frequency scanning microwave photonics filter (MPF). By synchronizing the scanning period of the MPF to the cavity round-trip time, the OEO can be made to operate in the FDML regime. Tunable linearly chirped microwave waveforms are generated in the experiment with a chirp rate up to 0.85 GHz/μs and a time-bandwidth product as large as 23,850. The proposed fast frequency scanning FDML OEO can find applications in modern radar and communication systems.