Tunable radar compound coherent jamming signal generation based on microwave photonics.

A photonic-assisted scheme to generate radar compound coherent jamming signals based on a dual-parallel Mach-Zehnder modulator (DP-MZM) is proposed and experimentally demonstrated. The received linear frequency-modulated (LFM) signal is interrupted sampled and comb spectrum modulated by a DP-MZM. After photoelectric conversion, the compound coherent jamming signal, which combines the comb spectrum modulation jamming (CSMJ) and interrupted sampling repeater jamming (ISRJ), is generated. In the experiment, the generated coherent jamming signals have a 1-GHz bandwidth centered at 8 GHz and 18 GHz, respectively. The tunability of the frequency interval for CSMJ, the duty cycle, and the sampling frequency for ISRJ are verified. After pulse compression (PC), 10 false target groups are evenly distributed over a radial distance of 120 m, and the total number of false targets reaches to 50. The jamming effectiveness in radar imaging is also demonstrated. To the best of our knowledge, photonic-assisted CSMJ and ISRJ compound jamming generation is proposed for the first time. The proposed scheme is compact, wideband, and tunable, which shows a good potential application in the future electronic warfare system.

An approach for linear optical phase demodulation using a single photodetector.

In this paper, we propose and experimentally verify a phase-modulated radio-over-fiber (RoF) link capable of transmitting the radio frequency (RF) signal linearly. By executing the Kramers-Kronig (KK) algorithm at the receiver, the proposed link can accomplish linear optical phase demodulation with a single photodetector rather than a coherent receiver. In the 16-quadrature amplitude modulation (16-QAM) and 64-QAM microwave vector signal transmission experiments, measured error vector magnitudes (EVMs) are 4.14% and 4.38%, respectively, after 25-km fiber transmission, and the measured spurious-free dynamic range (SFDR) is 114.5 dB·Hz2/3, which shows a good performance in linearity.

Experimental implementation of spike-based neuromorphic XOR operation based on polarization-mode competition in a single VCSOA.

We experimentally and numerically propose an approach for implementing spike-based neuromorphic exclusive OR (XOR) operation using a single vertical-cavity semiconductor optical amplifier (VCSOA). XOR operation is realized based on the neuron-like inhibitory dynamics of the VCSOA subject to dual-polarized pulsed optical injections. The inhibitory dynamics based on the polarization-mode competition effect are analyzed, and the inhibitory response can be obtained in a suitable range of wavelength detuning. Here, all input and output bits are represented by spikes that are compatible with the photonic spiking neural network. The experimental and numerical results show that XOR operation can be realized in two polarization modes by adjusting the time offset in the inhibitory window and setting defined reference thresholds. In addition, the influences of delay time and input intensity ratio on XOR operation are studied experimentally. This scheme is energy efficient because VCSOA neuromorphic photonics computing and information processing.

Photonic approach to flexible multi-band linearly frequency modulated microwave signals generation.

In this paper, a flexible multi-band linearly frequency modulated (LFM) signal generator based on a dual-polarization binary phase-shift keying (DP-BPSK) modulator is proposed and demonstrated by experiment. Two dual-drive Mach-Zehnder modulators of the DP-BPSK modulator are driven by a local oscillator (LO) signal and an LFM signal, respectively. By appropriately changing the phase difference introduced by a polarization controller, flexible multi-band LFM signals can be obtained after photoelectric conversion. The LO signal is phase modulated to eliminate self-heterodyne, which will affect the quality of the resulting signal.

Photonic instantaneous frequency measurement using a dense wavelength-division multiplexer.

A photonic instantaneous frequency measurement receiver based on frequency to optical power mapping is proposed and experimentally demonstrated. One channel of a dense wavelength-division multiplexer (DWDM) is used as an optical filter to establish a power ratio function related to the frequency of the microwave signal. Different from most optical filters, the DWDM filter features smooth and quasilinear roll-off over a wide bandwidth. With the help of a laser of good wavelength stability and a bias controller, large measurement range and high accuracy are simultaneously achieved without multi-step operation. The instanstaneous frequency measurement receiver can measure the frequency with an accuracy of 0.2% of the signal frequency in the range of 1-40 GHz over 1.5 h in the experiment.

Time-delay signature concealment of chaos and ultrafast decision making in mutually coupled semiconductor lasers with a phase-modulated Sagnac loop.

We propose and experimentally demonstrate the generation of dual-channels chaos with time delay signature (TDS) concealment by introducing a phase-modulated Sagnac loop in mutually coupled semiconductor lasers (MCSL). Furthermore, we demonstrate the utilization of the dual-channels chaos to solve multi-armed bandit (MAB) problem in reinforcement learning. The experimental results agree well with the numerical simulations. For the purpose of comparison, the MCSL with a conventional Sagnac loop is also considered. It is found that the TDS of dual-channels chaotic signals can be better concealed in our proposed system. Besides, the proposed system allows for a better decision making performance in MAB problem. Moreover, compared with the one-channel chaotic system, the proposed dual-channels chaotic system achieves ultrafast decision making in parallel, and thus, is highly valuable for further improving the security of communication systems and the performance of photonic intelligence.

Photonics-based multi-band linearly frequency modulated signal generation and anti-chromatic dispersion transmission.

A photonics-based anti-chromatic dispersion transmission scheme for multi-band linearly frequency modulated (LFM) signals is proposed and experimentally demonstrated. In the central station (CS), the key component is an integrated dual-polarization quadrature phase shift keying (DP-QPSK) modulator, of which the up-arm and down-arm are driven by a microwave reference signal and an intermediate-frequency (IF) LFM signal respectively. By properly adjusting the DP-QPSK modulator, optical frequency comb (OFC) and frequency shift lightwave are generated. After polarization coupling and remote transmission, the orthogonal-polarization optical signals are introduced into balanced photodetector for heterodyne detection. Thence, multi-band LFM signals are generated and transmitted to remote base stations (BS) with the largest power for the anti-chromatic dispersion ability. Experiments are conducted to verify the analysis. Multi-band LFM signals at L (1.5 GHz), C (7 GHz), X (10 GHz), Ku (15.5 GHz) and K (18.5 GHz) bands with flatness of 1.9 dB are simultaneously obtained in the CS after 50 km fiber transmission, while the normally double-sideband modulation approach experiences a significant power fading for the fiber dispersion. Tunability of the system is evaluated, and detection performances of the generated signals are also analyzed.

Experimental investigation of the time-delay signature of chaotic output and dual-channel physical random bit generation in 1550 nm mutually coupled VCSELs with common FBG filtered feedback.

Here we propose and experimentally demonstrate mutually coupled 1550 nm vertical-cavity surface-emitting lasers (VCSELs), subject to common fiber Bragg grating (FBG) feedback (FMC-VCSELs), to conceal the time-delay signature (TDS) of chaotic outputs. The autocorrelation function and delayed mutual information are used to quantitatively identify the TDS of chaotic output. For comparison, the evolution of the TDS of chaotic output in mutually coupled VCSELs (MC-VCSELs) is also presented. The effects of injection power, frequency detuning between two VCSELs, and frequency detuning between FBG and VCSELs on the TDS concealment are experimentally measured. Experimental results show that FMC-VCSELs have a better TDS suppression performance than MC-VCSELs for varying injection power. In addition, for the FMC-VCSELs system, the TDS can be suppressed to below 0.1 when the VCSELs x-polarization component is located at the edge of the main FBG lobe. Furthermore, dual-channel physical random numbers with verified randomness at a rate of 800 Gbps (2×5LSBs×80GHz) are achieved by utilizing the chaotic outputs with a low TDS from two VCSELs in the FMC-VCSELs system.

Reconfigurable photonic binary phase-coded microwave signal generator with fundamental/subharmonic carrier frequency.

A reconfigurable photonic binary phase-coded microwave signal generator with fundamental/subharmonic carrier frequency is proposed and experimentally investigated based on a dual-electrode dual parallel Mach-Zehnder modulator (DE-DPMZM). The local oscillator (LO) signal and coding signal are intensity modulated and phase modulated, respectively, on the two arms of the DE-DPMZM. The bias voltage of the intensity modulator is set to the quadrature or maximum working points to produce fundamental or subharmonic frequency, respectively. By precisely adjusting the amplitude of the LO signal and the bias voltage of the parent modulator, the light carriers of both arms of the DE-DPMZM can be eliminated. After detection, binary phase-coded microwave signal with frequency equal to or twice the frequency of the LO signal can be generated. The proposed system has a simple structure, high reconfigurability, and good frequency tunability due to its avoidance of using optical filters. In the experiment, phase-coded microwave waveforms at 6 GHz and 11 GHz are generated by using 6 GHz and 11 GHz LO signals or 3 GHz and 5.5 GHz LO signals, respectively. Meanwhile, the performance is evaluated in terms of phase recovery accuracy and pulse compression capability. The experimental results show that the proposed generator has high peak-to-sidelobe ratio, which has practical significance for radar detection of weak targets.

Time-delay signature concealment and physical random bits generation in mutually coupled semiconductor lasers with FBG filtered injection.

We propose and demonstrate experimentally the generation of chaos with suppressed time-delay signature (TDS) and physical random bits in two mutually coupled semiconductor lasers (MCSL) by introducing an auxiliary fiber Bragg grating (FBG) filtered injection path. The measurements show that, even by simply adding a single FBG path, the TDS of chaotic signals generated by both lasers in our proposed scheme can be better concealed compared to the conventional MCSL system. Moreover, better TDS concealment can be achieved in the laser with smaller wavelength. Additionally, two random bit streams extracted from the two chaotic lasers in the proposed scheme are achieved with minimal post-processing. The total generation rate reaches 640-Gbps.

Filter-free photonic microwave upconverter with frequency quadrupling.

A novel method, to the best of our knowledge, of frequency upconversion with frequency quadrupling is presented and experimentally demonstrated. The scheme is based on an integrated polarization-division multiplexing dual-parallel Mach-Zehnder modulator (PDM-DPMZM) and an intensity modulator (IM). Polarization-multiplexed second-order sidebands are obtained by properly adjusting the bias voltage of the PDM-DPMZM. By adjusting the polarization controller, the +2-order sideband is aligned with the IM's slow axis to be modulated by the intermediate frequency (IF) signal, and the -2-order sideband is aligned with the IM's fast axis with a much lower modulation efficiency. An upconverted signal with frequency quadrupling is generated at the photodetector. The experiment result shows that the working bandwidth range is 6-40 GHz, the spur suppression ratio of the upconverted signals is 37.5 dB, and the conversion efficiency is -32 dB. 100 MSym/s quadrature phase shift keying (QPSK) or a 16QAM signal at 26 GHz is generated using a 6 GHz local oscillator signal and transmitted through 25 km SMF.

Filter-free image-reject microwave photonic downconverter based on cascaded modulators.

As essential equipment of the receiver, a novel filter-free image-reject mixer based on a Mach-Zehnder modulator (MZM) and a polarization-division multiplexing Mach-Zehnder modulator (PDM-MZM) is proposed and experimentally demonstrated in this paper. The received radio frequency signal is applied to the MZM, and quadrature local oscillator signals are sent to the PDM-MZM, which includes two submodulators in parallel. After a polarization controller and a polarization beam splitter, quadrature intermediate frequency (IF) signals are obtained by two photodetectors. Image rejection is implemented by combining quadrature IF signals via an electrical 90° hybrid coupler. The scheme uses no filter, so the working frequency can be widely tuned and effective spectrum utilization is achieved. Experimental results verify that image rejection ratios can reach 50 dB. Moreover, the variation of the conversion gain is lower than 2 dB.

All-optical and broadband microwave fundamental/sub-harmonic I/Q down-converters.

Microwave I/Q down-converters are frequently used in image-reject super heterodyne receivers, zero intermediate frequency (zero-IF) receivers, and phase/frequency discriminators. However, due to the electronic bottleneck, conventional microwave I/Q mixers face a serious bandwidth limitation, I/Q imbalance, and even-order distortion. In this paper, photonic microwave fundamental and sub-harmonic I/Q down-converters are presented using a polarization division multiplexing dual-parallel Mach-Zehnder modulator (PDM-DPMZM). Thanks to all-optical manipulation, the proposed system features an ultra-wide operating band (7-40 GHz in the fundamental I/Q down-converter, and 10-40 GHz in the sub-harmonic I/Q down-converter) and an excellent I/Q balance (maximum 0.7 dB power imbalance and 1 degree phase imbalance). The conversion gain, noise figure (NF), even-order distortion, and spurious free dynamic range (SFDR) are also improved by LO power optimization and balanced detection. Using the proposed system, a high image rejection ratio is demonstrated for a super heterodyne receiver, and good EVMs over a wide RF power range is demonstrated for a zero-IF receiver. The proposed broadband photonic microwave fundamental and sub-harmonic I/Q down-converters may find potential applications in multi-band satellite, ultra-wideband radar and frequency-agile electronic warfare systems.

Photonic generation of microwave waveforms based on a dual-polarization quadrature phase-shift-keying modulator.

A novel microwave waveform generator based on a dual-polarization quadrature phase-shift-keying (DP-QPSK) modulator is proposed and experimentally demonstrated in this paper. The X-QPSK modulator is modulated by a local oscillator signal. For XI Mach-Zehnder modulator (MZM), odd sidebands are eliminated; for XQ-MZM, even sidebands are eliminated, while the Y-QPSK modulator is not modulated. Triangular or square waveforms can be generated by biasing the X-QPSK and adjusting the polarization controller properly. 3 GHz and 6 GHz triangular and square waveforms are successfully generated by experiment. The scheme does not need a high modulation index (m=1.25 in this paper). In addition, it features low cost and a simple structure without a filter or phase shifter.

Tunable 360° microwave photonic multichannel phase shifter with frequency quadrupling.

In this paper, a photonic scheme to generate a frequency-quadrupled microwave signal with full-range 360° tunable phase shift is proposed and experimentally demonstrated. Pure ±2nd-order sidebands with the carrier suppressed are generated by a dual-parallel Mach-Zehnder modulator. The two sidebands are separated by a fiber Bragg grating with the polarization state of one sideband rotated by 90 deg via a Faraday rotating mirror and then recombined to obtain a pair of orthogonally polarized wavelengths. The two orthogonally polarized optical sidebands are aligned into the same polarization direction by using a polarizer (Pol). Finally, a radio frequency (RF) signal with frequency quadrupling of a local oscillator signal is obtained by beating the ±2nd-order sidebands at a photodetector. The phase θ of the frequency-quadruped RF signal can be independently and arbitrarily adjusted from 0° to 360° through control of the polarization direction. Experiments are carried out to demonstrate the scheme, and frequency-quadrupled microwave signals at 12 GHz and 16 GHz are generated. A continuous phase shift from 0° to 360° of the frequency-quadrupled signal at 12 GHz is also verified. The proposed scheme not only has the merits of large operation bandwidth and simple structure but also can be extended to multi-channel applications.

Spike encoding and storage properties in mutually coupled vertical-cavity surface-emitting lasers subject to optical pulse injection.

The generation and storage properties of different spike codes in vertical-cavity surface-emitting lasers with an embedded saturable absorber (VCSELs-SA) are investigated numerically. The results show that different spike codes are generated by injecting an optical pulse into one single VCSEL-SA and can be stored in two mutually coupled VCSELs-SA. In particular, in the case of the generation of spike codes, the effects of the input strength and the temporal duration of the input optical pulse are studied; in the case of the storage of spike codes, the roles of the coupling weight and the coupling delay between the two mutually coupled VCSELs-SA are examined. Simulations reveal that spikes can be triggered if the input strength and the temporal duration exceed the threshold values, and higher values of the input strength and the temporal duration are beneficial for generating more spikes. Moreover, successful storage of a perfectly formed train of spike codes in two mutually coupled VCSELs-SA can also be realized provided that the coupling weight and the coupling delay are larger than the corresponding threshold values.

All-optical, ultra-wideband microwave I/Q mixer and image-reject frequency down-converter.

An all-optical and ultra-wideband microwave in-phase/quadrature (I/Q) mixer, based on a dual-parallel Mach-Zehnder modulator and a wavelength division multiplexer, is proposed. Due to the simultaneous frequency down-conversion and 360-deg tunable phase shifting in the optical domain, the proposed I/Q mixer has the advantages of high conversion gain and excellent quadrature phase balance (<±1.3 deg⁡) with a wide operating frequency from 10 to 40 GHz. Assisted by an analog or digital intermediate-frequency quadrature coupler, an image-reject frequency down-converter is then implemented, with an image rejection exceeding 50 dB over the working band.

Photonic Doppler frequency shift measurement based on a dual-polarization modulator.

In this paper, a novel photonic-assisted Doppler frequency shift (DFS) measurement scheme based on an integrated dual-polarization Mach-Zehnder modulator is presented. In the proposed scheme, the DFS to be identified is transformed into a low-frequency electrical signal through an optical frequency-conversion link. The value of the DFS can be acquired by analyzing the spectrum of the low-frequency electrical signal. Meanwhile, the orientation of the DFS can be easily determined utilizing a 90° hybrid coupler. If the receiver is moving toward the transmitter, only the positive port has an output signal, while only the negative port has an output signal if the receiver is moving away from the transmitter. The scheme can simultaneously obtain the value and the orientation of the DFS. In addition, to investigate the frequency tunability of the proposed scheme, the DFS, which varies from -100 to 100 KHz at a step of 10 KHz for different microwave signals at frequencies of 10, 15, and 18 GHz, is demonstrated experimentally, and the errors are within ±5×10-6 Hz.

Photonic microwave waveform generation based on phase modulation and tunable dispersion.

Photonic generation of microwave waveforms is currently an interesting topic due to the advantages of large bandwidth and immunity to electromagnetic interference. In this paper, a photonic microwave waveform generator with tunable waveforms and repetition rates is proposed and experimentally demonstrated. A continuous-wave (CW) light is phase modulated by a local oscillator (LO) signal to generate optical sidebands. By locating the phase modulator (PM) in a Sagnac loop, we can control the intensity and phase of the carrier of the phase modulated signal. Then a compact tunable dispersion compensation module is used to introduce phase shifts to the optical sidebands. Thanks to the flexible controlling of the optical signal, the generation of microwave waveforms with tunable shapes and repetition rates can be realized. In the demonstration experiment, full-duty-cycle triangular and square waveforms with repetition rates of 5 and 10 GHz (bandwidths of 15 and 30 GHz) are successfully generated, respectively. The bandwidths are expected to be improved to above 120 GHz if larger-bandwidth measurement instruments are used. In addition to the flexible tunability, the proposed scheme also features the advantages of easy implementation and free from bias drift.

Simultaneously photonic frequency downconversion, multichannel phase shifting, and IQ demodulation for wideband microwave signals.

A single photonic system that can simultaneously perform frequency downconversion, multichannel phase shifting, and in-phase (I) and quadrature (Q) demodulation for microwave signals is proposed and experimentally demonstrated. Using an integrated polarization-division multiplexing Mach-Zehnder modulator, radio frequency (RF) signals can be frequency downconverted to multichannel intermediate frequency (IF) signals and the phase of each IF signal can be independently and arbitrarily tuned. Using two quadrature channels, the RF signal can be IQ demodulated. In the experiment, high and flat conversion gains from 8 to 40 GHz and continuously tunable phase shifts over the 360-deg range are demonstrated. In addition, vector signals with various modulation formats at 40 GHz are frequency downconverted to baseband and the IQ data are successfully extracted.