Dynamical Properties Of Terahertz Quantum Cascade Lasers With Optical Injection

Terahertz waves are electromagnetic waves with frequency between microwaves and infrared waves.They have the characteristics of low photon energy,strong pene-trability,and cover a large number of molecule fingerprint spectra,which have become hotspots in recent years.In recent years,with the continuous development of terahertz technology,terahertz science has gradually shown a very broad application prospect in the fields of wireless communication,imaging,radar,and defense security,and has been successfully applied to environmental monitoring,materials analysis,biomedicine,re-mote wireless communications,and other fields.Terahertz quantum cascade lasers(THz QCLs)are a new type of terahertz light source.Its radiation wavelength does not depend on the band gap and only depends on the thickness of the superlattice multiple quantum wells and has a large adjustable frequency range.However,the output power of a single THz QCL device is limited and the beam quality needs to be improved.Mu-tual injection phase-locked laser array is an important means to improve the single-mode output power and beam quality of THz QCLs.Studying the phase-locking properties of THz QCLs arrays can help to study the basic properties of side-coupled and other types of phase-locked QCLs arrays.However,compared to traditional mutual-injected semi-conductor laser arrays,research on phase-locking,noise,modulation,and self-mixing applications of lasers in arrays for THz QCLs is still limited.We mainly studies the re-lated modulation technology of terahertz wave from three aspects,the specific research contents are as follows:(1)Based on the rate equation theory,the phase-locking,noise and modulation re-sponse properties of optically injected THz QCLs are studied.The coupling coefficient?and the linewidth enhancement factor a are the main factors affecting the phase-locked range,and the phase-locked bandwidth increases as ? and a increase.In the non-locked regime,the working state of the laser exhibits nonlinear behavior such as periodic oscil-lation with different parameters,especially when the detuning frequency is sufficiently large,the laser electric field is subjected to the modulation of detuning frequency.Then,we calculated free-running,master-slave injection,and mutual injection spontaneous emission noise.In all three cases,the relative intensity noise is white noise in the low frequency range,and the noise of mutual-injected THz QCLs noise is the strongest with the same parameters.Reducing the mutual injection coupling strength and increasing the injection current are two means to reduce noise.Finally,through small-signal anal-ysis,we analyzed the modulation response characteristics of phase-locked THz QCLs The modulation bandwidth of the mutual injection array increases with the injection current and coupling strength.At low frequencies,the responses of the two lasers in the array are completely synchronized.In the high-frequency region,the phase difference of the array laser increases with the modulation frequency.These results help to deepen the understanding of the nonlinear dynamic behavior of optically injected THz QCLs and provide theoretical support for the development of THz QCL phase-locked arrays.(2)The self-mixing velocity sensing scheme of the mutual-injected THz QCLs ar-ray is theoretically studied,and the operating characteristics of the lasers with different frequency detuning and feedback strength parameters are analyzed.Within the phase-locked range,each laser in the array can be used as a self-mixing velocity sensor due to the phase-locking;when the frequency detuning is large enough,the coupling of the array laser is broken and the phase cannot be locked and only the laser receiving the feedback light can be the velocity sensor.Although THz QCLs arrays are unstable with strong self-mixing feedback,they can also be used for self-mixing velocity measure-ment;in addition,mutual injection arrays can also measure two independent moving targets simultaneously.These results provide theoretical support for THz QCLs arrays in self-mixing and multi-target rapid measurement applications.(3)The nonlinear dynamic behavior of mutual-injected THz QCLs and diode lasers is studied.When the frequency detuning of the two lasers is 0,the mutual-injected THz QCLs can maintain stable operation within the simulated coupling strength range,and its dynamic behavior is similar to the self-mixing of a single THz QCL.Although the mutual-injected diode laser could be phase-locked with small coupling strength,the electric field amplitude firstly oscillates periodically and then appears as complex nonlinear oscillations as the coupling strength increases.When the frequency detuning of the two lasers is not 0,if the coupling strength of the injected THz QCLs is not large enough,the amplitude of the electric field will show periodic oscillation.When the coupling strength is large enough,the phase locking and stable operation can be achieved.For diode laser array,the electric field undergoes a process from periodic oscillation to nonlinear oscillation with the frequency detuning increases.These results help to deepen the understanding of the operating characteristics of optically mutual-injected THz QCLs under different parameters.