Linear frequency-modulated (LFM) waveforms have numerous applications in high-resolution radar detection, high-speed wireless communication, and high precision measurement. The generation of LFM microwave signals based on conventional electronic technologies is limited in their center frequency and bandwidth, which are usually less than a few gigahertz. Fortunately, the inherently large bandwidth offered by photonic technology is very hopeful of breaking through the electronic bottleneck. A variety of photonics-based approaches to generating the LFM waveforms have been reported, including the frequency-to-time mapping method and the external modulation method. However, these solutions suffer poor tunability or expensive RF sources. In recent years, the LFM waveform generation based on optically injected semiconductor lasers (OISLs) has attracted increasing attention. By introducing a low-speed electrical signal to control the period-one (P1) dynamics of an OISL, the LFM waveforms with a large bandwidth are generated. Nonetheless, the generated microwave signal has poor spectral purity, which restricts its many practical applications.
In this work, a high-performance microwave LFM waveform generation scheme based on an OISL with dual-loop optoelectronic feedback is proposed and demonstrated experimentally. In this scheme, the optical injection strength of an OISL is controlled first by a triangular-like voltage signal to generate LFM waveforms with a large bandwidth. Then, the quality of the generated LFM signal is comprehensively improved by introducing a delay-matched dual-loop optoelectronic feedback structure. Based on the Fourier domain mode locking principle (FDML) and the self-injection locking technique, both a short-delay optoelectronic feedback loop and a long-delay optoelectronic feedback loop are introduced to simultaneously improve the spectral purity and phase stability of the generated LFM signals. In the proof-of-concept experiment, by analyzing the spectral quality and phase deviation of the generated LFM signal, a comb contrast of 40 dB, a comb linewidth of 1 kHz, and a phase deviation ∆
φof less than π/3 are simultaneously obtained. In addition, the parameters such as bandwidth and center frequency of the generated LFM signal generated can be flexibly tuned, and an LFM signal with a large bandwidth up to 8 GHz (18–26 GHz) is generated in the experiment. The proposed scheme features a simple and compact structure, high spectral quality and flexible tuning, thus may find applications in broadband radar and high-speed communication systems.
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