Enhancement Of The Stability Of Photonic FMCW Microwave Signal Generated By An Optically Injected Semiconductor Laser Through Optical Feedback

Frequency modulated continuous wave(FMCW)is a microwave signal with the frequency changes with time.It has played an important role in modern radar system,which can be utilized in ranging,imaging,and sensing.Generally,the FMCW signal is required to be stable,adjustable,and with a large frequency sweep range.FMCW can be generated by traditional electrical and optical methods.Generally,FMCW signal generated electronically possesses limited bandwidth and poor tunability of the central frequency due to electronic bottltneck.As a result,numerous photonic techniques have been proposed,where the method of the FMCW generation based on an optically injected semiconductor laser has attracted people’s attention due to the wide frequency sweep range and large frequency tunability.However,the FMCW signal generated by this method has poor stability due to the inherent noise of laser.Therefore,it is necessary to conduct further research on this method.In this work we first theoretically investigate the FMCW generation based on a modulated light injected to a DFB-SL,and discusses the influences of injection parameters and modulation parameters on FMCW.The results show that:The injection parameters and modulation parameters will affect the frequency sweep range of FMCW.For fixed injection parameters,the sweep range increases significantly with the modulation index.When the modulation index is fixed,the injection detuning frequency increases gradually with the injection strength in order to achieve a wide sweep range.Under proper operation parameters,a photonic FMCW signal with a wide sweep range can be obtained.And the center frequency of the FMCW signal can be tuned in a wide range by changing the injection coefficient.At this time,the power spectrum of FMCW is distributed as a frequency comb and the comb spacing is equal to the modulation frequency,but the comb line contrast is low due to the influence of the inherent noise of the laser.Different stabilizing techniques have been proposed to reduce the phase noise of FMCW,where optical feedback is a promising candidate due to no needs in electronic devices.Thus we introduce an optical feedback to suppress the phase fluctuations of the generated FMCW signal and investigate the influence of feedback parameters and modulation parameters on the frequency comb contrast R.The results show that: Under a given feedback delay time,R firstly increases rapidly with the increase of the feedback ratio and then tends to a stable level.For a fixed feedback ratio,R reaches a maximum periodically when the feedback time is an integer multiple of 1/fm(fm is the modulation frequency)but the value of maximum decreases with the increase of the integer multiple number.The influence of modulation parameters on R is as follows: When other parameters are fixed,as the modulation frequency increases,the comb contrast R first increases and then becomes stable.And as the modulation depth increases,the FMCW comb contrast R will decrease.After adopting optical feedback,the frequency comb contrast can be increased to a level of more than30dB.