Abstract
In this paper, a T-type germanium (Ge) waveguide with an all-normal dispersion profile is designed for mid-infrared (MIR) supercontinuum (SC) and frequency comb generations. The nonlinearity coefficient of the designed waveguide is calculated as 30.48 W-1·m-1 at the initial pump wavelength of 3.0 µm. Moreover, the group-velocity dispersion is kept low and flat in the considered wavelength range. Simulation results show that with the designed waveguide, the highly coherent and octave-spanning MIR SC can be generated in the wavelength range from 1.85 to 9.98 µm (more than 2.4 octaves) when the pump pulse with wavelength of 3.0 µm, peak power of 900 W, and duration of 120 fs is launched into the 5 mm long waveguide. When the pulse train including 50 pulses at a repetition rate of 100 MHz is used as the pump source, the SC-based frequency comb is obtained.
Highlights
Mid-infrared (MIR) supercontinuum (SC) generation in optical waveguides have been extensively investigated in the field of nonlinear optics due to potential applications in chip-scale optical frequency metrology, optical coherence tomography, nondestructive testing, bio-imaging and molecular spectroscopy [1,2,3,4,5]
The generation of the SC is a result of the interaction of some nonlinear effects, which include self-phase modulation (SPM), optical wave breaking (OWB), stimulated Raman scattering (SRS), soliton fission (SF) [6,7], etc
The germanium (Ge) as one of the group IV photonics materials becomes an excellent candidate material for the MIR SC generation since its optical characteristics are similar to the Si, while its transparency window ranges from 1.5 to 14.3 μm, covering the “fingerprint” regions [14,15,16]
Summary
Mid-infrared (MIR) supercontinuum (SC) generation in optical waveguides have been extensively investigated in the field of nonlinear optics due to potential applications in chip-scale optical frequency metrology, optical coherence tomography, nondestructive testing, bio-imaging and molecular spectroscopy [1,2,3,4,5]. In 2014, Ryan et al, first reported the octave-spanning SC generation in the wavelength range from 1.5 to 3.6 μm in the Si waveguide [8]. In 2016, Yang et al, reported a low-dispersion Ge-on-Si waveguide for highly coherent SC generation from 3.7 to 9.24 μm [18]. When the pump pulse propagates in the anomalous dispersion region of the waveguide, the soliton dynamics dominate the nonlinear process of the SC generation [20,21,22]. When the pump pulse propagates in the normal dispersion region of the waveguide, the SC has good coherence since the SPM and optical wave breaking (OWB) effects play important roles in the SC generation [23,24,25]. The SC-based OFCs are obtained when a 50 pulses train at a repetition rate of 100 MHz is launched into the designed waveguide
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