Mode Switching using Weak Aperiodic DFB Gratings within Fabry Prot Lasers

We present time domain modeling of a weak aperiodic distributed feedback (ADFB) grating incorporated into a Fabry-Pérot (FP) laser cavity. The combined FP-ADFB feedback leads to highly selective single mode emission which can be discretely tuned under varying optical sub-cavity path lengths. Tunable single-frequency semiconductor lasers are of particular interest in a wide range of applications, from optical communication to sensing systems. Multi-periodic grating structures, such as sampled gratings, have proven hugely successful for achieving tunability in lower wavelength lasers, but they are not practically scalable to terahertz (THz) lasers. For example, a 3 THz laser would require a sampled grating length of the order of centimetres, a size difficult to fabricate and electrically power. In this paper, we explore the possibility of a novel mode switching mechanism, based on a photonic structure providing multiple Bragg resonances, embedded within a pre-existing Fabry-Pérot (FP) laser cavity (Figs. 1a and 1b). In order to achieve multiple reflection peaks (Fig. 1c) we relax the structures regularity, i.e. we use an aperiodic distributed feedback (ADFB) grating. Furthermore, to achieve electronic switching, individual reflection peak values should be limited to a level comparable with the FP facet reflectivity. Here we present simulation results of a switchable FP-ADFB laser, obtained using time domain modeling (TDM). Finally, we support the FP-ADFB switching concept with experimental measurements using THz lasers. In the TDM we divide the device into many sections and then solve the time-dependent coupledwave equations by using a split-step transfer matrix method. Essentially, the coupled-wave equations are split into two sets of matrices: one containing the detuning factors and time derivatives, the other containing the coupling coefficient terms. For each section and time step, the forward and reverse rate equations for the optical fields are solved and the electron density updated. A broad Lorentzian gain shape was chosen, encompassing all three ADFB reflection bands. A complex grating coupling factor κ was chosen to model the complex nature of the surface plasmon waveguides widely used in THz lasers. The magnitude of κ was adjusted to produce the desired reflectivity peak strengths of around 0.3, commensurate with the laser facets. The facet output power of the device is recorded and Fourier-transformed to produce an output spectrum. Constructing the ADFB grating within Figure 1. (a) Illustration of the FP-ADFB architecture: An FP cavity containing an ADFB grating and two resulting aperiodic distributed Bragg reflector (ADBR) sub-cavities. (b) ADFB design. Each vertical line corresponds to a narrow, low refractive index slot. The spacing between slots is given in terms of Λ (where Λ = minimum slot separation). (c) Spectral reflectivity of the ADFB, calculated via a Fourier transform of the real-space structure, containing three peaks centred on fB = c/2neffΛ, where c is the speed of light and neff the effective refractive index. (d) Detail of a section of the ADFB grating, with the corresponding distributions of (e) the grating coupling factor (in terms of κΛ) and (f) the required phase jumps, as employed in the TDM.