Role of linewidth enhancement factor on the frequency response of the synchronized quantum cascade laser

This paper presents a closed-form analysis of the synchronized quantum cascade laser (QCL) and studies analytically the effect of linewidth enhancement factor (LEF) on the frequency response of the QCL. The analysis predicts that there can be two types of response curves of the QCL-one like series resonant circuit response and the other like antiresonant circuit response. The linewidth enhancement factor introduces asymmetry in the frequency response of the synchronized QCL. The nature of asymmetry in locked QCL is seen to be different from that reported for interband semiconductor lasers. The asymmetry in series resonant-like response is inclined towards the positive detuning side while that in antiresonant circuit-like response is inclined towards the negative detuning side. The degree of asymmetry in QCL response is not only determined by the LEF but also by the upper subband electron relaxation time and the operating wavelength of the QCL. Although the value of LEF in QCL is smaller than that for interband lasers, calculation shows that the LEF has a profound effect on the response of the locked QCL.