Study Of Terahertz Master-oscillator Power-amplifier Quantum Cascade Lasers

Terahertz quantum cascade laser(THz-QCL)is a novel terahertz coherent source merging electronics and photonics band engineering.THz-QCL exhibits the advantages of high output power,compactness,frequency tunability,and the ability of monolithic integration.With very narrow linewidth,single-mode THz-QCLs have important applications in the fields of precision spectral measurement and security.To get high output power THz-QCL,studies of master-oscillator power-amplifier(MOPA)have been carried out in this thesis.The main achievements are summarized in the followings:1.The experiment investigations point out that,due to high facet reflectivity and random phase of the reflected wave,facet-emitting first-order distributed feedback terahertz quantum cascade lasers(THz-DFB-QCLs)based on metal-metal waveguides cannot realize stable single mode emission.To address this issue,the thesis have designed and fabricated novel single-mode THz-QCLs,in which the DFB grating activates the mode oscillation and the THz wave is coupled into the free space via a grating coupler.A careful design enables the grating coupler to exhibit low reflectivity(< 1%),avoiding the distribution on the mode oscillation in the cavity and resulting in stable single mode emission.2.Based on the forementioned work,the thesis realized in the world the first terahertz master-oscillator power-amplifier quantum cascade laser(THz-MOPA-QCL).The device consists of a DFB laser,a preamplifier,and a grating coupler.All components are monolithically integrated in a metal-metal waveguide.The DFB laser acts as the master-oscillator,the seed radiation generated is then highly efficiently injected into the preamplifier.After power amplification,the THz radiation is finally coupled into the free space via the grating coupler.The MOPA device demonstrates stable single-mode emission and power amplification,the peak power is 1.32 m W,and the amplification factor is about 5.3.In order to achieve high output power,this thesis have analyzed and optimized the THz-MOPA-QCL by using full-wave finite element simulations.Firstly,the thesis use buried DFB grating to reduce the contrast of the effective refractive index,which will increase the seed power from the master oscillator.Secondly,by reducing the real space overlap between the THz wave and the lateral edges of the grating coupler,the residual refle ctivity of the latter is creased down to ~10-3,which will significantly increase the factor of power amplification and simultaneously suppress the self-lasing in the power-amplifier.Finally,a tapered preamplifier is used to reduce the gain saturation.Experimentally,the peak output power of the new MOPA devices reach to 136 m W,the seed power and the factor of power amplification are respectively 10.1 m W and 13.5.The absolute power achieved is along the best values ever reported in the world.Most importantly,the peak power is about 36 times that of convenient single mode THz-QCL from the same material.It confirms that THz-MOPA-QCL is a promising architecture to realize high power single mode THz laser.4.The thesis proposed a new method to measure the net loss and gain of THz-QCL based on the THz-MOPA-QCL device architecture.In this method,the seed power from the master-oscillator is fixed,and the total output power is measured as a function of the bias applied to the power-amplifier.With the assistant of a few calculated parameters(the coupling efficiency of the grating coupler and the radiation loss of the DFB laser at threshold),this method allows one to get the net loss and gain of the device at different bias.In addition,such measurement is spectra sensitive,i.e.,it can measure the loss/gain at different frequency.The most important advantage of this method is that – since the self-lasing in the power-amplifier is completely suppressed – gain clamping is overcome and one can get the maximum available gain.Experimentally,the loss/gain curves of the fabricated devices at different temperature have been successfully characterized.5.Using 3D finite element method,the thesis have studied the propagation of THz wave in double-metal waveguide whose cross-section is deep-sub-wavelength.The simulations point out,in the quasi-static limit,the reflectivity and transmissivity are determined by the waveguide impedance at the entrance and exit ports.When the waveguide impedances are matched,the THz wave can be 90-degree bended or split with very low reflectivity and near-unity transmissivity.As a proof of concept,the thesis have proposed a terahertz integrated circuit where a quantum cascade laser is linked to two quantum cascade detectors via a T-type sub-wavelength metal-metal waveguide.In this circuit,the THz wave can be split and delivered between the laser and the detectors with very low reflection or radiation loss.At the last,the thesis discussed the approaches to further improve the performances of THz-MOPA-QCL,and the possibilities to realize terahertz amplifier and the monolithic integration of multi-functional terahertz devices.