Due to the limitations on data transmission between memory and processing units, as well as RC latency associated with integrated circuits, traditional electronic computers face bottlenecks in power consumption, heat dissipation, and computing speed. All-optical signal processing and all-optical networks have attracted increasing research attention as alternatives to conventional electronic integrated circuits, given their advantages of high-speed parallel processing, low power consumption, high bandwidth, and low crosstalk. Efforts have been made in integrated photonic computing chips for optical computing and optical neural networks. All-optical logic gates provide basic units for all-optical computing, switching, and signal processing. Various all-optical logic gates with functions such as subtractors, differential equation solvers, storage elements, and other computational techniques have been reported. Microcavity lasers like VCSELs and DFBs are well-suited for realizing on-chip all-optical logic gates due to their small size and low power consumption. Here, we design a two-dimensional 2×4 semiconductor laser array of square microcavities with four waveguide ports, consisting of an upper row and a lower row. Integrated electrodes are designed at both rows to ensure that the same current is applied to each microcavity in the same row, simplifying experimental operations. The simple fabrication processes, flexible integration methods, easy on-chip integration, and multi-port light emission of the laser array facilitate its application in all-optical signal processing links. Larger scale on-chip integration can be achieved using high-density integration techniques. Additionally, the platform offers the possibility of wavelength multiplexing for parallel computing. This system thus sheds light on the next generation of all-optical computing systems.

Simulation and AlGaInAs/InP epitaxial wafer are employed in our study. First, we study the magnetic field pattern, frequency, longitudinal modes interval, and quality factor (Q) of the square microcavity with four waveguide ports and the 2×4 microcavity array using two-dimensional finite element method (FEM). Then, we fabricate the 2×4 semiconductor laser array of square microcavities using AlGaInAs/InP epitaxial wafer with a photoluminance wavelength of about 1517 nm. The active region with six compressively strained 6-nm-thick quantum wells and seven 9-nm-thick barrier layers is grown on the InP substrate by metal-organic chemical vapor deposition. Contact photolithography and ICP etching are used to fabricate the array of square microcavities. The microcavity is laterally confined by a BCB layer for planarization. Afterward, a Ti/Pt/Au p-electrode is deposited by e-beam evaporation followed by a lift-off process, and an Au/Ge/Ni metallization layer is deposited by magnetron sputtering as the n-electrode. Then, the output power and lasing spectra are coupled to multi-mode fiber (MMF), and the V-I curve is tested at 293 K versus continuous injection current in the upper row. The wavelengths of these eight microcavity units have been identified by the spectra at different ports. Furthermore, the mode characteristics versus continuous injection current in the upper row with a fixed injection current in the lower row are studied.

There are 16 fundamental modes found within the range of 1531.6 nm to 1531.7 nm, with Q factors ranging from 3966 to 10012. These modes exhibit different mode field distributions in different units. The mode field distribution for the highest-Q mode and the square microcavity unit are shown (Fig. 1). The threshold current is 26 mA, with a threshold current density of 1.28 kA/cm². The maximum coupled power is 8.9 μW at a continuous current of 57 mA. The emission spectra are measured from ports 1, 2, 3, and 4 at an injection current of 30 mA applied solely to the upper row. Four peaks are located at 1532.11, 1534.13, 1535.37, and 1539.39 nm, corresponding to the square microcavities in the upper row at ports 3, 1, 2, and 4, respectively (Fig. 2). When an injection current of 30 mA is applied solely to the lower row, the MMF is positioned at port 2 to ensure that the intensities of the four wavelengths are almost consistent. The four peaks are located at 1532.22, 1533.22, 1535.70, and 1537.78 nm, corresponding to the square microcavities at ports 2, 3, 4, and 1 of the lower row (Fig. 3). As the injection current of the upper row increases while the injection current of the lower row is fixed at 30 mA, the four wavelengths of the lower row disappear one by one.

In our study, a 2×4 waveguide-connected square microcavity laser array has been fabricated, and the wavelengths of these eight microcavity units have been identified by the spectra at different ports. Differences in resonance wavelengths may result from variations in the dimensions of the square cavities during the fabrication process. From these differences in wavelengths, it can be inferred that variations in the sizes of square microcavities are within the range of 9.7-71.2 nm. This laser array enables the in-plane integration of multiple light sources, and larger-scale on-chip integration of laser arrays can be expected using high-density integration techniques. The system can be used for all-optical signal processing links in applications such as complex logic calculations. Moreover, this laser array is capable of producing multiple coherent light sources simultaneously that overlap spatially due to the multiple waveguides.