Polarization-insensitive reverse-ridge AlGaAs waveguide for the mid-infrared supercontinuum generation
Introduction
Mid-infrared supercontinuum (SC) generation is of great interest for its applications in spectroscopy [1], [2], [3], [4], [5], optical coherence tomography [6], [7], [8], biomedical imaging [9], ladar [10], [11], gas sensing [12], [13], [14], etc. As well known, the interaction of dispersive and nonlinear effects, which includes self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM), stimulated Raman scattering (SRS), soliton fission (SF), contributes to the SC generation [15], [16], [17], [18].
In recent years, lots of investigations on the mid-infrared SC generations in optical waveguides have been reported. Yuan et al. numerically generated the coherent and multi-octaves SC spanning from 1.96 to 12 m in a suspended Ge-membrane ridge waveguide [19]. Saini et al. investigated the mid-infrared SC generation in the rib As2Se3 waveguides with different core shapes [20]. Sinobad et al. demonstrated the SC generation in the wavelength range from 2.8 to 5.7 m in the SiGe waveguide with an all-normal dispersion profile [21]. Lu et al. reported the SC generation at the ultraviolet to mid-infrared spectral region in the single-crystalline AlN waveguide [22]. Saini et al. generated the near-infrared to mid-infrared SC in a tellurium-oxide coated silicon-nitride waveguide through pumping in the normal dispersion region [23]. Karim et al. utilized a suspended core channel As2Se3 waveguide for the SC generation covering from 1.5 to 15 m [24].
AlGaAs is the alloy of AlAs and GaAs. The refractive index of AlGaAs can be easily adjusted by changing the alloy composition [25]. Therefore, the refractive index difference between the waveguide core and substrate and the dispersion characteristic can be flexibly changed. In addition, its large transparency window (more than 15 m [26], more than twice that of Si) and strong Kerr nonlinear index (on the order of 10−17 m2/W [27], an order of magnitude larger than that of Si [28]) make it suitable for the mid-infrared SC generation. Mei designed the AlGaAs strip waveguides to generate the broadband SC, spanning from 2 to 15.9 m [29]. Chiles et al. utilized a suspended AlGaAs waveguide to generate the second-harmonic and broadband SC covering from 2.3 to 6.5 m when pump wavelength was located at 3060 nm [30]. Kuyken et al. experimentally demonstrated an octave-spanning SC generation in the wavelength range from 1055 to 2155 nm in an AlGaAs waveguide [27]. With picojoule-level pulse energy, May et al. experimentally obtained a SC covering 544 nm at the level of −25 dB in AlGaAs-on-insulator waveguides [31].
In recent years, there are some investigations on the polarization insensitivity of the waveguide-based modulator and grating. Tabti et al. demonstrated a polarization-insensitive sampled Bragg grating by exploiting the birefringence compensation in Si3N4 waveguide [32], where the transmission spectra for both the quasi-transverse electric (TE) and quasi-transverse magnetic (TM) modes overlapped over a spectral range of 40 nm. Chen et al. proposed a polarization-insensitive graphene modulator, which was polarization-independent for the TE and TM modes [33]. Zhou et al. reported a polarization-insensitive graphene-based optical modulator integrated in a chalcogenide glass waveguide [34], where the absorption coefficient variation was almost identical in the wavelength range of 2–2.4 m for the TE and TM modes. However, there is still no report on the polarization-insensitive waveguide for the SC generation. For the SC generation, the nonlinear process is always very sensitive to the polarization state of the pump light because the dispersion and nonlinear characteristics of the quasi-TE and quasi-TM modes of the waveguides are different. Thus, the polarization state of the pump light needs to be controlled by employing the polarization controller [35], [36], [37], [38]. At this time, the complexity of the experimental system is increased, and the undesired polarization effect has an adverse influence on the energy conversion caused by the nonlinear effects. At present, the polarization-dependent problem of the pump light could be effectively solved by reasonably designing the waveguide structure.
In this paper, a polarization-insensitive reverse-ridge AlGaAs waveguide is proposed to generate the mid-infrared SCs. In the design, the influences of the geometrical parameters of the waveguide on the dispersion characteristic are analyzed to optimize the waveguide structure. In addition, the impacts of the pump pulse parameters and waveguide length on the SC generations are also investigated. With the designed 3.4-mm-long waveguide, for the quasi-TE and quasi-TM modes, highly coherent and octave-spanning mid-infrared SCs are generated, spanning from 2.17 to 8.53 m and 2.23 to 8.61 m, respectively, when pump pulse with wavelength of 4.2 m, peak power of 4.8 kW, and width of 90 fs is used.
Section snippets
Theoretical model
The process of the SC generation in the optical waveguides can be described by the modified generalized nonlinear Schrödinger equation (GNLSE) as following [15] where A(z, t) stands for the slowly varying envelope of the electric field, and the th order dispersion coefficient calculated from the Taylor expansion of the propagation constant is represented by (). The last term on the right side of Eq. (1) is the
Waveguide design and characteristics
Fig. 1(a) shows the three-dimensional structure of the designed reverse-ridge AlGaAs waveguide. The AlGaAs layer, where the ratio of AlAs and GaAs is 0.18 to 0.82, is embedded in the AlGaAs substrate, which enhances the mode field confinement. The width of the AlGaAs ridge core is W. The depth of the part embedded in the substrate is , and the protruding part is represented by . Fig. 1(b) shows the mode field distributions of the quasi-TE and quasi-TM modes calculated
Simulation results and discussion
The SC generation in the proposed reverse-ridge AlGaAs waveguide will be numerically investigated by solving Eq. (1) with the Runge–Kutta method. While pumping in the normal dispersion region of the waveguide, the generated SC usually has good coherence with a relatively narrow spectral range. In contrast, while pumping in the anomalous dispersion region, the SC can be easily extended to a considerable range. Furthermore, the coherence of the SC can be improved by appropriately selecting the
Conclusion
In summary, we design a polarization-insensitive reverse-ridge AlGaAs waveguide for the SC generations based on the quasi-TE and quasi-TM modes. The dispersion difference between the quasi-TE and quasi-TM modes could be reduced through exactly adjusting the geometrical parameters of the waveguide. Furthermore, by optimizing the pump pulse parameters, the generated SCs for the quasi-TE and quasi-TM modes can overlap almost completely. When the pump pulse with wavelength of 4.2 m, peak power of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (61875238).
References (44)
- et al.
Selection of detector peak wavelength for optimum infrared system performance
Infrared Phys.
(1976) - et al.
Near-and mid-infrared laseroptical sensors for gas analysis
Opt. Laser. Eng.
(2002) - et al.
Tip-enhanced infrared nanospectroscopy via molecular expansion force detection
Nat. Photonics
(2014) - et al.
Cavity enhanced absorption spectroscopy in the mid-infrared using a supercontinuum source
Appl. Phys. Lett.
(2017) - et al.
Versatile silicon-waveguide supercontinuum for coherent mid-infrared spectroscopy
Apl. Photonics
(2018) - et al.
Mid-infrared standoff spectroscopy using a supercontinuum laser with compact Fabry–Pérot filter spectrometers
Appl. Spectrosc.
(2018) - et al.
Broadband cantilever-enhanced photoacoustic spectroscopy in the mid-IR using a supercontinuum
Opt. Lett.
(2018) - et al.
Study of an ultrahigh-numerical-aperture fiber continuum generation source for optical coherence tomography
Opt. Lett.
(2002) - et al.
Dual-band infrared optical coherence tomography using a single supercontinuum source
Opt. Express
(2020) - et al.
Real-time high-resolution mid-infrared optical coherence tomography
Light-Sci. Appl.
(2019)