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.
Microcombs are revolutionizing optoelectronics by providing parallel, mutually coherent wavelength channels for time-frequency metrology and information processing. To implement this essential function in integrated photonic systems, it is desirable to drive microcombs directly with an on-chip laser in a simple and flexible way. However, two major difficulties have prevented this goal: (1) generating mode-locked comb states usually requires a significant amount of pump power and (2) the requirement to align laser and resonator frequency significantly complicates operation and limits the tunability of the comb lines. Here, we address these problems by using microresonators on an AlGaAs on-insulator platform to generate dark-pulse microcombs. This highly nonlinear platform dramatically relaxes fabrication requirements and leads to a record-low pump power of <1 mW for coherent comb generation. Dark-pulse microcombs facilitated by thermally controlled avoided mode crossings are accessed by direct distributed feedback laser pumping. Without any feedback or control circuitries, the comb shows good coherence and stability. With around 150 mW on-chip power, this approach also leads to an unprecedentedly wide tuning range of over one free spectral range (97.5 GHz). Our work provides a route to realize power-efficient, simple, and reconfigurable microcombs that can be seamlessly integrated with a wide range of photonic systems.
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