KEYWORDS: Signal generators, Modulation, Laser frequency, Signal analyzers, Ka band, Single sideband modulation, Radar signal processing, Frequency modulation, Frequency combs
Optical frequency combs are commonly utilized for generating reconfigurable linearly-frequency-modulated (LFM) signals with a large operating frequency range in radar systems. However, traditional systems employing cascade delectro-optic modulators and relying on external electronic signal sources often suffer from high power consumption and bulky size. In contrast, chip-scale microcombs offer inherent advantages in terms of power efficiency and compactness. In this study, we propose and experimentally demonstrate a microcomb-based system for generating reconfigurable LFM signals. By self-injection locking a commercial distributed feedback laser chip through the backscattering of a micro resonator chip, a coherent microcomb state is achieved. Additionally, we utilize the wideband injection locking technique to enhance the signal-to-noise ratio, resulting in a significant power gain for the modulated sideband. The center frequency of the generated signals can be flexibly reconfigured by adjusting the selected comb line numbers and the frequency of the modulation signals. This enables the achievement of LFM signals with a large tunable range spanning from X- to W-bands. Furthermore, we analyze the characteristics of the produced LFM signal at Ka-band. Our measurements indicate a linearity of approximately 0.01% and a pulse compression ratio of approximately 2.1×105. These results validate the effectiveness and potential of the proposed approach.
A wideband programmable linearly frequency modulated (LFM) signal is highly desired in modern radar systems to adapt to variable environments and achieve high detection resolution. However, conventional digital microwave generation has restrictions on operation band and bandwidth. Current optical microwave generation has provided solutions to the dilemma of electronic devices, meanwhile arising new problems like insufficient time-bandwidth product and dependence on high-frequency or high-rate RF sources. Here, utilizing heterodyne-beating two phase-locked lasers, we present a new LFM signal generation method with no aid of high-frequency or high-rate electronics, featuring simple structure, large bandwidth and adjustable parameters. A frequency-swept laser (FSL) and a frequency-fixed laser (FFL) combined with a voltage-controlled oscillator are phase locked to the same oscillator to reduce phase fluctuations and employed for heterodyne-beating. An LFM waveform with an instantaneous bandwidth of 7.3 GHz cross X and Ku band is developed. The reconfigurable capability is also investigated, the bandwidth, central frequency and pulse width of the LFM signal are programmed by merely adjusting the central frequency of the FFL, voltage amplitude and the period of a low-frequency driving voltage signal driving the FSL. Measured results validate the effectiveness and prospect of the approach.
A photonic-assisted dual-band coherent radar transmitter system with a large frequency tunable range is proposed and demonstrated. This dual-band transmitter is composed of a triple-loop optoelectronic oscillator (OEO) link, a low frequency band subsystem (LFBS) and a high frequency band subsystem (HFBS). The triple-loop OEO link is developed for the generation of an ultralow phase noise microwave signal with a large tunable range, microwave photonic down-converting is used in the LFBS to change band range, microwave photonic frequency multiplying is applied in the HFBS to achieve the bandwidth extension. The band ranges of the proposed dual-band transmitter can cover from S to Ka six bands in all. Performances in the time and the frequency domains of the dual-band microwave signals are also investigated.
KEYWORDS: Radar, Receivers, Ku band, Digital signal processing, Signal processing, Signal detection, Modulators, Microwave photonics, Transmitters, Modulation
In this paper, a microwave photonic dual band radar based on a photonic-assisted de-chirp processing receiver is proposed. The dual band operation is realized independently and simultaneously with a single set of hardware. At a transmitter end, two linear frequency-modulated signals separately located in C-band and Ku-band are transmitted, echoes are collected and sent to a receiver to implement photonic-assisted de-chirp processing. At the receiver end, a main modulator with a special structure, which consists of four parallel sub-modulators, is employed. The echoes and reference signals of C-band and Ku-band are applied to two pairs of sub-modulators of the main modulator, which are biased at the peak points for C-band and biased at the null points for Ku-band. In this case, the intermediate frequency signals of C-band and Ku-band produced by de-chirp processing locate at two different frequencies. Thus operation in different bands based on a unified system is achieved. An experiment operating in C-band and Ku-band with a bandwidth of 700 MHz and 3600 MHz is conducted. The results verify the concept of the dual band radar and show the potential of photonic technology to improve the performance of modern radar system.
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