High-quality Frequency-modulated Continuous-wave Generation Based On A Semiconductor Laser Subject To Modulated Optical Injection

Frequency-modulated continuous-waves（FMCW）radar is a radar system which can obtain target information according to the frequency difference and phase difference of transmitted signal and echo signal through frequency modulation of continuous wave.FMCW radar has many advantages such as simple structure,small size,light weight,low cost,no range blind area,and not easy to be intercepted,and has gradually become a research hotspot in the field of radar.The measurement range of FMCW radar depends on the frequency sweep period of the transmitted FMCW signal,and its range resolution depends on the sweep band width of the transmitted FMCW signal.Therefore,in order to achieve high resolution detection of long-range targets by FMCW radar,it is required that the transmitted FMCW signal should have a large time-bandwidth product（TBWP）.Traditionally,FMCW signals are generated in the electrical domain by using electronic equipment such as a voltage-controlled oscillator（VCO）or digital synthesizer.Limited by the bandwidth of electronic equipment,the generated FMCW signal center frequency is low and the bandwidth is small.In order to improve the performance of FMCW signals,some photonic-based methods,such as space-to-time mapping,frequency-to-time mapping（FTM）and optical heterodyne beating,have been proposed successively.Although the performance of the FMCW signal can be improved by using these techniques,there is still room for improvement in terms of tunability,structural complexity,or integration.Especially,FMCW signal generation technology based on distributed feedback semiconductor lasers（DFB-SL）in a period-one（P1）dynamic state under optical injection has received additional attention from the industry due to its unique advantages.However,there are still some problems in this technology:for example,the scheme based on a Mach-Zehnder modulator（MZM）to modulate the intensity of the injected light,the resulting FMCW signal sweep band width is relatively small.Moreover,in radar detection,in addition to the accurate detection of the target’s distance,it is also necessary to detect its speed.Using dual-linear frequency-modulated continuous wave（or dual-linear chirp microwave,dual-LCM）signal as FMCW radar signal,the target range and speed can be detected simultaneously.At present,the dual-LCM signal generation methods include the generation method based on single or dual chirped baseband signal re-modulation and the generation method based on a dual optical injection DFB-SL.Although the carrier frequency can be selected flexibly by using these methods,the sweep band width,frequency tunability and linearity of the generated signal need to be improved.Focusing on the problems in the above FMCW generation scheme,a photon scheme based on a cascade-modulated optical injection DFB-SL is proposed to generate high-quality FMCW signal,and the performance of the generated FMCW signal is studied theoretically and experimentally.At the same time,a photonic scheme based on modulated light injection DFB-SL and optical heterodyne technology is proposed to generate high-quality dual-LCM signal,and its performance is experimentally studied.The specific research contents of this paper are as follows:1.A photonic scheme to generate high quality FMCW signal based on a DFB-SL subject to cascade-modulated optical injection is proposed,and the performance of FMCW signal is studied theoretically.In this paper,the P1 oscillation characteristics of the DFB-SL under continuous-wave optical injection are studied,and the effects of frequency detuning and injection strength on the oscillation frequency of P1 are analyzed.The results show that the frequency of P1 oscillation can be continuously adjustable in a large range by changing the injection strength and frequency detuning.On this basis,two MZMs cascade are used to modulate the injected light intensity of DFB-SL to generate FMCW signal,and the influence of each parameter on the performance of FMCW signal is analyzed in detail.Finally,the obtained results are compared with the theoretical results of FMCW signal generated by modulation of DFB-SL injected light intensity by a single MZM.The results show that the FMCW signal generated by the cascade-modulated optical injection has a larger bandwidth.In addition,the effectiveness of using optical feedback to improve the comb contrast of FMCW signal is analyzed.2.A photonic scheme to generate high-quality FMCW signal based on a DFB-SL subject to cascade-modulated optical injection is proposed and experimentally verified,and the influence of various parameters on its performance is discussed.In the experiment,the dynamics characteristics of P1 generated by a DFB-SL under external optical injection are studied.The variation of P1 oscillation frequency with injection strength and frequency detuning is analyzed in detail.The results show that:when the frequency detuning is fixed,the P1 oscillation frequency increases approximately linearly with the increase of injection strength.Therefore,the continuous tunability of P1 oscillation frequency f₀ can be achieved by simply adjusting injection strength.On this basis,two MZMs cascade are used to modulate the intensity of the injected light.The characteristics of FMCW signal generated by a DFB-SL subject to cascade-modulated optical injection are experimentally observed.The influences of the injection strength,frequency detuning and modulation index on the bandwidth and comb contrast of the FMCW signal are discussed in detail.The results show that the bandwidth of the generated FMCW signal can reach 25.89 GHz by adjusting the injection and modulation parameters reasonably.Furthermore,the comb contrast of FMCW signal can be significantly improved under the appropriate feedback intensity.The experimental results agree well with the theoretical prediction.3.Based on a modulated optical injection distributed feedback semiconductor laser（DFB-SL）combining with optical heterodyne,a photonic scheme for generating high-quality dual-linear chirp microwave（dual-LCM）signal is proposed and experimentally demonstrated.For such a scheme,a continuous-wave（CW）light with a frequency of f_injis split into two parts.One part is passed through a Mach-Zehnder modulator（MZM）driven by a modified sawtooth signal,and then injected to a DFB-SL for generating a single linearly chirped microwave（single-LCM）signal.The other part of the CW light with frequency of f_inj is sent to a phase modulator（PM）driven by a sinusoidal signal to generate the modulated optical sidebands,and one of higher-order sidebands is selected by a tunable optical filter and taken as the referenced light.Through heterodyning the referenced light with the single-LCM signal,a dual-LCM signal with identical（or complementary）chirp can be obtained.In this work,firstly,the characteristics of microwave frequency generated by optical heterodyne beating between the output of DFB-SL and modulated sideband are studied,and the influence of injection parameters on the microwave frequency is analyzed in detail.The result shows that,by simply adjusting the injection strength and the frequency of the sinusoidal signal loaded on the PM,the center frequency of the dual-LCM signal can be adjusted continuously in a large range.On this basis,the sawtooth signal is loaded on the MZM to generate dual-LCM signals with the identical chirp or complementary chirp.In the experiment,the linearity of the dual-LCM signal is optimized by using modified the sawtooth waveform,and the linearity of the dual-LCM signal by using unmodified signal is also given for comparison.The experimental results show that the linearity of the dual-LCM can be improved obviously by using the modified sawtooth signal.Moreover,the influences of injection parameters and modulation parameters on the bandwidth of dual-LCM signal are studied in detail.The experimental results show that,under the appropriate parameters,the bandwidth for each frequency band included in the generated dual-LCM signal with identical chirp can reach 19.36 GHz,and the time bandwidth product reach 1.936×10⁵,the bandwidth for each frequency band included in the generated dual-LCM signal with complementary chirp can reach 16.98 GHz,and the time bandwidth product reach1.698×10⁵.