Octave-Spanning Supercontinuum Generation in AlGaAs-on-Insulator Waveguides

Introduction

Supercontinuum (SC) light sources are gaining significant interest due to their numerous applications in optical communications, spectroscopy, imaging, and metrology. While optical fiber nonlinearity was initially explored for SC generation, researchers are now focused on on-chip SC generation using various material platforms. Among these, the AlGaAs-on-insulator (AlGaAs-oI) waveguide is particularly promising due to the high third-order nonlinearity (χ(3)) of AlGaAs and strong optical confinement, which suppresses two-photon absorption at telecom wavelengths under high power pumping.

One notable advantage of AlGaAs-oI waveguides is their large second-order nonlinearity (χ(2)), enabling second-harmonic generation (SHG) and parametric down-conversion processes. These processes can further broaden the SC spectrum, particularly at shorter and longer wavelengths. This tutorial will explore the generation of an octave-spanning SC in an AlGaAs-oI waveguide by leveraging both χ(2) and χ(3) nonlinearities.

Waveguide Structure and Fabrication

The AlGaAs-oI waveguide used in this study is shown in Figure 1(a). It features a 400-nm high and 800-nm wide AlGaAs core placed on a 2-μm-thick SiO2 layer on a Si substrate. The AlGaAs core, with an Al content of 20% (bandgap around 740 nm), is covered by a 2.5-μm-thick SiOx overcladding with a refractive index of 1.51 at 1950 nm.

Figure 1: (a) Schematic cross-section of the AlGaAs-oI waveguide, (b) calculated chromatic dispersion, (c) and (d) calculated mode-field profiles at 1950 nm and 975 nm, respectively.

The calculated chromatic dispersion of the waveguide is shown in Figure 1(b), indicating the anomalous dispersion regime at the 1950-nm pump wavelength, which allows for efficient energy transfer while suppressing two-photon absorption. At the SHG wavelength (975 nm), the waveguide exhibits strong normal dispersion. Figures 1(c) and 1(d) depict the calculated mode-field profiles of the fundamental TE modes at 1950 nm and 975 nm, respectively.

To achieve low-loss and wide-bandwidth optical edge coupling, spot-size converters (SSCs) consisting of inversely tapered AlGaAs waveguides and SiOx waveguides are employed.

The fabrication process begins with the growth of an AlGaAs epitaxial wafer using metal-organic chemical vapor deposition. The wafer is then bonded to a Si wafer with a thermal SiO2 surface layer through hydrophilic direct bonding. After removing the GaAs substrate and etch-stop layer, the AlGaAs-oI substrate is obtained. The waveguide core and taper shapes are defined using electron-beam lithography and dry etching. Subsequently, a SiOx layer is deposited using electron-cyclotron-resonance plasma-enhanced chemical vapor deposition (ECR-PECVD). The SiOx core is then shaped using photolithography and dry etching, followed by SiO2 deposition via PECVD. Finally, edge facets for optical coupling are created through blade dicing.

Experimental Setup and Results

The experimental setup for SC generation is illustrated in Figure 2. A commercial fiber mode-locked laser (FML) with a center wavelength of 1950 nm and pulses shorter than 100 fs is used as the pump source. The polarized pulse is coupled to the input waveguide through single-mode and dispersion-compensation fibers (SMF and DCF). The output is then coupled to optical spectrum analyzers (OSAs) or an optical power meter (OPM) through another DCF-SMF coupling.

Figure 2: Experimental setup for supercontinuum generation.

Figure 3 shows the obtained spectra for various on-chip input pump pulse energies, along with a reference spectrum obtained without the waveguide. At 5.8 pJ input, no significant spectrum broadening is observed compared to the reference. As the input energy increases to 8.6 pJ, clear spectral broadening occurs. At 9.3 pJ input, the SC bandwidth expands further, and a new peak around 1200 nm emerges, likely originating from a dispersive wave.

When the input energy reaches 9.9 pJ, another peak appears at around 900 nm, originating from SHG. As the input energy is increased to 12.0 pJ, this SHG peak grows significantly, and the 40 dB-down bandwidth spans more than 1.2 octaves, from 881 nm to 2149 nm. The blue shift and asymmetric shape of the SHG-related peak are attributed to the strong normal dispersion of the AlGaAs-oI waveguide at the SHG wavelength range and the chirp of the SHG pulse.

Figure 3: Transmission spectra for various on-chip input pulse energies. The reference spectrum was obtained without a waveguide.

Conclusion

This article demonstrated the generation of an octave-spanning supercontinuum in an AlGaAs-oI waveguide by leveraging both χ(2) and χ(3) nonlinearities. By utilizing a 2000-nm-band pump pulse source and second-harmonic generation, a bandwidth of more than 1.2 octaves (881 nm to 2149 nm) was achieved with only 12.0 pJ of on-chip pump pulse energy. The results highlight the potential of AlGaAs-oI waveguides and their integration with SiOx waveguides as a promising technology for future SC light sources.

Reference

[1] H. Nishi, T. Segawa, H. Sugiyama, K. Wada, E. Kanno, and S. Matsuo, "Second-Harmonic-Wave Assisted Octave-Spanning Supercontinuum Generation in an AlGaAs-on-Insulator Waveguide," NTT Device Technology Labs, NTT Corporation, Atsugi, Kanagawa, Japan, and Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA, 2024.