In this paper, we reviewed and presented our latest progress of linearization methods for microwave photonic systems based on programmable photonic circuits, including the linearization in microwave photonic transmission links and programmable functional circuits.
The rapid development of radar technology requires RF transmitters with high working frequency, broad tuning bandwidth, flexible reconfigurability, and the ability of generating large time-bandwidth product signals. In this paper, we reviewed the main architecture and research status of high-frequency, broadband, multi-format radar waveform generation technology based on optical frequency combs (OFCs) in the context of microwave photonics, and we highlighted the technical route and the main problems that exist for the purpose of practical engineering applications. The main challenges and development trends of microwave photonic radar RF front-end based on optical frequency comb in future applications are also presented.
The explosive growth of information urgently requires extending the capacity of optical communication and information processing. Orbital-angular-momentum-based mode division multiplexing (MDM) is recognized as the most promising technique to improve the bandwidth of a single fiber. To make it compatible with the dominant wavelength division multiplexing (WDM), broadband equal high-efficient phase encoding is highly pursued. Here, we propose a twisted-liquid-crystal and rear-mirror-based design for ultrabroadband reflective planar optics. The backtracking of the light inside the twisted birefringent medium leads to an achromatic phase modulation. With this design, a single-twisted reflective q-plate is demonstrated to convert a white beam to a polychromatic optical vortex. Jones calculus and vector beam characterization are carried out to analyze the broadband phase compensation. A dual-twisted configuration further extends the working band to over 600 nm. It supplies an ultrabroadband and reflective solution for the WDM/MDM-compatible elements and may significantly promote advances in ultrabroadband planar optics.
A recently proposed frequency hopping receiver based on the simultaneous photonics filtering and digitizing system can provide large receiving frequency range as well as high hopping speed by rapidly tuning the optical sampling pulses. With the increase of the hopping speed, the shape of the optical sampling pulses is changed rapidly, and the receiving performance, including the off-band suppression and receiving bandwidth of the proposed frequency hopping receiver is degraded. The performance degradation is indicated in experiments, evaluated theoretically, and analyzed by numerical simulations. Simulations on the shortest frequency hopping time indicate that the receiving passbands are broadened less than 10% with more than 25 sampling periods, and less than 1% with more than 225 sampling periods. The proposed frequency hopping receiver can receive frequency hopping signals with higher hopping speed by utilizing higher sampling rates.
Rational harmonic mode-locking refers to a mode-locking state achieved at the modulation frequency that doesn’t match the fundamental frequency. In this paper, we investigated and experimentally achieved rational harmonic mode-locking in optoelectronic oscillators (OEO) for the first time through three schemes based on electric amplitude modulator (AM), electric phase modulator (PM), and Mach-Zehnder modulator (MZM), respectively. In the experiment, the fundamental frequency mode-locking as well as the 2nd-order, 3rd-order, and 4th-order rational harmonic mode-locking were obtained, all generating ultrashort microwave pulses with a repetition rate of 95 kHz and a carrier frequency of 10 GHz. Subsequently, the characteristics of the pulse signals generated by different schemes, such as pulse width, pulse amplitude, and spectral width, were systematically investigated. By comparison, we found that the AM-based mode-locked OEO generates microwave pulse signals with higher stability and narrower pulse width; the PM-based mode-locked OEO can excite more longitudinal modes in the cavity but generates signals with more spurious noise; the MZM-based mode-locked OEO has a simple structure and requires lower power of the modulation signal. We believe this paper could provide some reference for the research on the physical mechanism of the mode-locking phenomenon generated in the OEO when the modulation frequency is mismatched.
In order to solve the problem of poor spurious suppression characteristics of traditional microwave mixers when high-throughout satellite(HTS) transponder is used in broadband applications,which leads to the degradation of transponder performance. This paper presents a microwave photonics(MWP) mixer design that uses integrated photonic filters to suppress optical carriers and unused sideband. The high integration, tunability, high suppression, and low loss characteristics of the integrated photonic filters are fully utilized. By the tuning of the phase parameters of the filter’s micro-rings, the pass of the upper and lower sidebands is realized respectively, thus 30GHz to 20GHz downconversion and 30GHz to 40GHz upconversion have been realized respectively. Analysis and simulation of the MWP mixer has been done, the result shows that the conversion loss of the mixer is -20dB,noise figure is 25dB, which is close to microwave mixer. At the same time, the LO spurious suppresion is improved by 60dB, IIP3 is improved by 12dB. The design achieves a breakthrough in the technical limitations of traditional microwave mixer, and lays a theoretical and technical foundation for the future application of MWP technology in high-throughout satellite.
We propose and demonstrate an ultra-compact photonic integrated filter via dozens of tunable basic units (TBU) which allow one to obtain reconfigurable frequency selectivity from hundreds of MHz to several GHz. The chip has been fabricated in Si3N4 TriPleXTM technology and packaged for electrical programming, optical interfacing and testing in the lab. Each subunit can be independently configured both as an active processing subsystem as well as a pure interconnecting 2x2 device by proper programming of its internal coupling units. As a proof of concept, several signal processing configurations with increasing degrees of complexity are configured including broad and narrowband flat-top filtering, coherent channelization and I-Q mixing prior to an optical detection stage. This is an advanced demonstration of a programmable RF-Photonic processor built from the interconnection of independent processing subunits encompassing functions beyond optical filtering.
Liquid crystal spatial light modulators (LC-SLMs) are usually polarization sensitive optical elements. In this paper, we propose a polarization-independent beam steering system to overcome the polarization problem of conventional liquid crystal devices by employing two polarization-dependent LC-SLMs, a polarizing beam splitter and a half-wave plate. In this system, two one-dimensional LC-SLMs are aligned orthogonally to deflect the beam in azimuthal and elevation, respectively. This system enables LC-SLMs to work in any polarization state of incident light, and can realize continuous two-dimensional laser beam pointing. Properties of polarization-independence as well as two-dimensional beam steering were mathematically and experimentally verified with a good agreement. Using the well aligned beam steering system, linearly polarized beams in different polarization angle are deflected with high accuracy and efficiency. The measured angular deviations are less than 5 micro-radians to show a high-accuracy beam steering of the system. This polarization-independent beam steering scheme is useful in the applications of nonmechanical laser communication, Lidar, and other LC-based devices.
KEYWORDS: Signal generators, Modulators, Phase shifts, Modulation, Radio optics, Signal attenuation, Interferometers, Optical engineering, Single sideband modulation, Polarization
A tunable high-order single-sideband signal generator with optical carrier suppression (SSB-OCS generator) is proposed, which is based on frequency multiplication operation using dual-parallel Mach–Zehnder modulator (DPMZM) and Mach–Zehnder interferometer (MZI). By changing all the modulators at maximum or minimum transmission point simultaneously, the amplitude of input radio frequency (RF) signals and the phase difference between input RF signals, two high-order even or odd signals with opposite amplitude of one sideband, are generated by upper DPMZM and lower DPMZM, respectively. Then, two optical signals are coupled to reserve one sideband, and the coupled SSB-OCS signal is injected into an MZI to suppress high-order intermodulation components. The simulated results show that the first-, second-, and third-order SSB-OCS signals can be achieved, and the suppression ratio is over 30 dB, which agrees well with the theoretical prediction. This scheme is featured by the capability to generate tunable high-order SSB-OCS signal with high suppression ratio, and the operation is simple and flexible. Finally, an experiment was carried out to demonstrate the feasibility of the proposed scheme, and a first- and second-order SSB-OCS signal were obtained, respectively, and the suppression ratio is over 24 dB.
A novel optoelectronic oscillator utilizing high-Q active ring resonator based on semiconductor optical amplifier and optical filter is proposed and demonstrated. The ring resonator with high Q-factor can reduce the dissipation of energy in the feedback loop to achieve oscillation with low phase noise. A high-quality microwave signal with a frequency tunable from 8 to 12 GHz is generated by employing a commercial tunable electrical bandpass filter. The single-sideband phase noise of the generated 10-GHz signal is measured to be -124 dBc/Hz at 10 kHz.
Liquid crystal optical phased array (LC-OPA) has been reported to be one of the promising methods to realize nonmechanical laser beam steering in free-space laser communication. The nonmechanical beam steering mechanism of LC-OPA in a free-space laser communication process is demonstrated. To analyze the steering performance of LC-OPA for a beam arriving from arbitrary directions, a theoretical model based on two-dimensional liquid crystal orient model and backward propagation method is proposed. In addition the phase characteristics are discussed both in normal and oblique incident cases, and a linear relationship between angle-of-arrival and detected incident angle is also derived. All of the theoretical results are verified by the following experiments with a good agreement.
Agile beam steering has been previously reported to be one of the unique properties of a liquid crystal optical phased array. We propose a stochastic scanning method using the property of agile beam steering to shorten acquisition time in building a free-space laser communication link. As a specific example, Gaussian stochastic scan enables higher acquisition probability and shorter acquisition time. In addition, there are two factors to influence the results: standard deviation of stochastic scanning angle and the width of the laser beam. Theoretical analysis is presented that the stochastic scanning method is a unique method to speed up the acquisition process in free-space laser communication.
An urban wireless optical SAC-CDMA network with MQC is proposed and analyzed. Analytical techniques for performance valuation of synchronous random access packet switching in urban wireless CDMA system are presented. Considering the nodal architecture design at higher layers, as well as the characteristics of the physical layer, such as atmospheric scintillation, phase-induced intensity noise and receiver noise, steady-state throughput characteristics using Binomial Arrival Model are obtained. Numerical results indicate that use of multi access coding can provide utilization-delay characteristics superior to that of ALOHA, and the atmospheric scintillation is an important factor affecting the throughput of urban wireless optical CDMA network when the atmospheric turbulence is comparatively strong.
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