High-Efficiency Arbitrary Waveform Harmonic Down-Conversion Based on Optical Injection
Objective Radar plays a crucial role in both military and civilian applications,such as weather forecasting,autonomous driving,target recognition and tracking,vital signs monitoring,military surveying,and mapping.To meet the demands for high-precision and real-time detection and imaging,radar receivers must down-convert high-frequency,large-bandwidth arbitrary waveform signals to reduce the complexity of back-end processing and facilitate system miniaturization.The down-conversion system should provide high-frequency local oscillation(LO)and high gain for the modulated signal to achieve efficient down-conversion.Traditional methods of LO generation often rely on external signal sources or multiple modulators,adding to system complexity.Furthermore,broadband arbitrary waveform signals require a flat gain spectrum to preserve their original characteristics.In this paper,we propose a scheme to achieve efficient down-conversion by providing self-oscillation and high-frequency LO with flat and high gain.Methods This high-efficiency arbitrary waveform harmonic down-conversion is achieved through optical injection.First,a high-quality optical frequency comb(OFC)is generated by optoelectronic oscillation,leveraging the gain-switching semiconductor laser.The OFC's bandwidth is enhanced through the optical injection locking of one comb line.The high-frequency LO is then obtained.The arbitrary waveform signal is modulated onto the optical carrier.After being injected into the slave laser(SL),the-1st sideband is amplified and continuously locks the SL wavelength.As both the OFC comb and the sideband are coherent with the same optical carrier,the high-frequency arbitrary waveform can be down-converted by beating with the nearest comb line in a parallel structure.An optical delay line is used to adjust the optical path difference between the two branches.A wavelength selective switch filters out the unwanted comb lines of the OFC.Results and Discussions A 40 and 39 GHz microwave signal with a power level of-20 dBm is converted to 426 and 574 MHz,respectively,when the OFC frequency interval is 5.6534 GHz.The optical spectra of the combined path and electrical spectra of the down-converted signal are shown in Fig.3,with a conversion efficiency of over 13 dB.In the lower path,a linearly chirped microwave waveform(LCMW)signal with a 1 GHz bandwidth and a 20 μs period is modulated and locked to the SL wavelength,as shown in Fig.4.When the free spectral range(FSR)is 5.8 GHz,the LCMW signal is down-converted to an intermediate frequency(IF)of 1.6 and 4.2 GHz.The system's transmission bandwidth is examined,showing that it is determined by the locking range of the injection locking,which in turn is related to the injection ratio.In the experiment,while keeping the wavelength and power of the SL constant,the locking range increases with the input microwave signal power,reaching 6.3 GHz at 8 dBm input power,as shown in Fig.7.The transmission performance when the LCMW bandwidth varies under SAIL effect is shown in Fig.8.This confirms that the system can down-convert broadband LCMW signals.The effect of the LCMW signal period on output quality is also analyzed.At periods of 1,10,20,60,and 100 μs,the output signal remains clear without additional clutter,though signal quality degrades at a 0.1 μs period,as SL cavity modes are harder to lock to rapidly changing sidebands(Fig.9).Finally,Fig.10 shows that the system can effectively down-convert arbitrary waveform signals based on optical injection.Conclusions In this paper,we propose and experimentally demonstrate a scheme for improving the conversion efficiency of high-frequency arbitrary waveform signals using optical injection.High-quality and high-frequency LO signals are obtained through optically injected self-oscillating OFC,eliminating the need for an external LO source.The input arbitrary waveform signal,with wide bandwidth and high frequency,is amplified by the sideband amplification injection locking(SAIL)effect.The SAIL effect's features,including high gain and spectrum balancing,as well as the relationship between microwave signal power and locking range,are studied experimentally.Transmission tests are conducted on LCMW signals with varying periods and bandwidths,demonstrating that the system performs well with arbitrary waveform signals within the period range of 1 to 100 μs and bandwidths within the locking range.The down-conversion efficiency exceeds 13 dB.When the FSR is 5.8 GHz,an LCMW signal with a center frequency of 19 GHz is down-converted to low-frequency signals with center frequencies of 1.6 and 4.2 GHz.These results indicate that using the SAIL effect for arbitrary waveform down-conversion achieves remarkable performance.
optoelectronicsdown-conversionoptical frequency comboptical injectionradar signal
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