Directional single mode QCL emission using metal grating second order coupler at room temperature

Abstract

Low beam divergence (2.1°*4°) substrate emission from a quantum cascade laser is demonstrated with top metal grating at room temperature. An index coupled, distributed feedback laser with a SMSR of 30dB is coupled to a monolithic extraction area.Surface emission, quantum cascade laser, top metal grating, low divergence.In the last decade, strong efforts have been devoted to the development of quantum cascade lasers (QCLs) emitting in the medium infra-red region (MIR), motivated in particular by the need to improve the performances of trace-gas analysis systems. For such type of application, single-mode lasers with narrow line-width are necessary and therefore distributed feedback (DFB) QCLs have been recently developed [1][2][3][4]. In standard approach, light is emitted by a cleaved facet. To ensure that the laser is spatially single-mode, the section of the waveguide is reduced to about one wavelength. Thereby, emission from the edge of a DFB QCL is highly divergent. For this reason, lenses are required for applications such as gas sensing. The demonstration of substrate emission at room temperature with improved divergence has already been achieved in the case of a ridge waveguide configuration with a bi-periodic grating [5]. In such structure, the size of the waveguide in the direction perpendicular to the grating could not be higher than in the case of a classical QCL cavity (about 10µm in the mid-IR) in order to suppress higher order transverse modes. We have demonstrated that a combination of a classical DFB QCL, with a first order grating ( eff DFB n 2 0 λ = Λ ), and a monolithic coupler, with a second order grating ( eff c n 0 λ = Λ), allows us to design a singlemode QCL with a low beam divergence and with a Gaussian like profile in the two directions. Preliminary results obtained on InGaAs/AlInAs DFB QCLs made following this approach and emitting at 5.65µm at room temperature with a low beam divergence are reported.In order to reduce the losses introduced by the metal, and thus increase the operating temperature, the proposed configuration is based on the analysis reported in a previous paper, enabling the engineering of low loss-index coupled DFB with metal gratings [2][3]. The DFB cavity has typical dimensions (about 2mm x30µm) whereas the coupler section is larger to ensure a reduction of the divergence but should be shorter to avoid losses Fig. 1.The Fig. 2 presents a typical mono-lobe far field obtained for such structure with L E =300µm and l E =30µm. The mono-lobe far field pattern has a very low divergence in the two directions. The Full Width at Half a Maximum is only 2.1°*4°. This structure is mono-mode, Fig. 3, and exhibits a Side Mode Suppression Ratio of 30dB.