Abstract
A broadband terahertz (THz) frequency comb assisted by an optical pump in THz quantum cascade lasers (QCLs) is investigated theoretically and numerically through a Maxwell–Bloch model combined with the coupled wave theory. When an optical pump is injected into the laser cavity with dispersion, the intrinsic four-wave-mixing nonlinear process becomes not only an important elementary phase-locking mechanism during the mode proliferating process, but also the bandwidth of the frequency comb is increased and the power is amplified through the nonlinear parametric process. The relative shift between the frequency of the optical pump and the zero-dispersion frequency of THz QCLs tremendously affects the conversion efficiency of the nonlinear parametric process. The simulation results show that appropriately optical pumping could assist in generating the broadband THz frequency comb with over 1 THz and more than 80 lines, which may open many potential applications in designing and optimizing high resolution THz spectroscopy sources.
Highlights
A broadband terahertz (THz) frequency comb assisted by an optical pump in THz quantum cascade lasers (QCLs) is investigated theoretically and numerically through a Maxwell–Bloch model combined with the coupled wave theory
Compared with the optical frequency combs traditionally generated by a mode-locked laser,17 THz QCLs are capable of generating THz frequency combs through the four-wave-mixing (FWM) nonlinear effect,18 which shows constant instantaneous power instead of a short pulse in the time domain and similar frequency modulated signals, due to very fast gain recovery19 in QCLs
Note that the gain center of the THz QCLs is always set at 3.5 THz and the zero-dispersion frequency is offset by 30 modes, which is always set at mode 0 in all the following simulations
Summary
Composed of numerous highly coherent and spaced laser longitudinal modes, optical frequency combs have many unique features, such as ultrahigh frequency stability and ultralow phase noise, which make them potential candidates for large capacity radio-frequency communication, sensor imaging, and spectral metrology. In particular, since many molecules’ characteristic absorption spectral lines lie in the THz spectral range, highly accurate material detection has come into reality by detecting the absorption of comb lines through dual-comb spectroscopy technology. There are several actively and positively mode-locked methods to produce THz frequency combs in THz quantum cascade lasers (QCLs), including the microwave modulating the laser current and a graphene-coupled saturable absorber. Compared with the optical frequency combs traditionally generated by a mode-locked laser, THz QCLs are capable of generating THz frequency combs through the four-wave-mixing (FWM) nonlinear effect, which shows constant instantaneous power instead of a short pulse in the time domain and similar frequency modulated signals, due to very fast gain recovery in QCLs. Based on the high nonlinearity in QCLs and dispersion engineering, this parametric process enables amplification, and at the same time, the comb lines can be symmetrically duplicated on the other side of the pump when phase-matching condition is achieved. It could provide a wide frequency comb bandwidth in mid-infrared QCLs through phase-matching. Such a phase-matching method may be difficult to be applied to THz QCLs because of the complex dispersion profile of the laser active region. We fully utilize the time evolution simulations of the THz QCLs comb based on a Maxwell–Bloch model to represent the dynamics of the comb modes and an optically assisted pump whose frequency locates near the zero-dispersion frequency of the waveguide to investigate the possibility of broadening the bandwidth of the THz-QCLs comb
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