When manufacturing SM VCSELs, developers typically make the oxide aperture in a VCSEL very small (around 2–3μm in diameter). This approach, however, may result in very low optical power, high resistance, and low manufacturing yield. To extend single-mode behavior toward more conventional aperture sizes (5–7μm), several alternative approaches have been proposed, including surface patterning, etching, overgrowth, and ion implantation in combination with photonic crystals.^{8, 9}These approaches require additional processing steps that must be precisely aligned (oxide aperture and surface pattern). The resulting complexity can reduce the yield and increase the cost of manufacturing.

Our approach uses oxide-confined leaky VCSELs, which—through the application of proper epitaxial design—enable the generation of high optical leakage losses for high-order transverse modes. Using these devices, we extend the single-mode behavior of the laser toward large oxide aperture diameters.

With our approach, we aim to create an additional cavity at a wavelength longer than the VCSEL cavity mode. Upon oxidation, the relative intensity distribution of the optical field between the coupled cavities can be strongly affected in the oxidized section. This induces a break in the orthogonality of the VCSEL mode and the second cavity mode (when at a certain tilt angle), which enables in-plane leakage to occur. High-order modes with the field intensity maxima close to the oxide periphery have thus much higher leakage losses.¹⁰

We have designed and manufactured oxide-confined leaky VCSELs and observed their leakage process through tilted narrow lobes in the far-field spectrum. The emission comes from the area outside the aperture, and thus does not suffer from diffraction-induced broadening. To model the VCSELs in 3D, we applied finite element analysis based on Maxwell’s vector equations in a rotational symmetric system.¹¹

Figure 1 shows a cross section of the simulated electric field of the fundamental and first excited optical modes of an oxide-confined aluminum gallium arsenide-based leaky VCSEL. A simulated far-field profile of the excited mode can be seen in Figure 2. The simulations show that the leakage effect results in a specific tilted emission over the VCSEL surface at ∼35–37^°. Most of the intensity of the leaking light is channeled in the direction parallel to the surface.

We manufactured and tested VCSELs according to our design. The far-field measurements of the devices at two current densities can be seen in Figure 3, which shows that at high current densities during multimode operation, narrow lobes arise at ∼35^° angles. These lobes are related to the leakage process (see Figure 2).

To summarize, we have shown that it is possible to significantly improve VCSEL spectral quality without involving any additional processing steps. Furthermore, we confirmed the occurrence of in-plane leakage through leakage lobes in the far-field profile of the device. Our findings create opportunities for engineering photonic integrated circuits, for example, by coherent coupling of two or more devices. Therefore, it may be possible to use the technique for beam steering.¹³ By operating one VCSEL in a couple under reverse bias, it is possible to realize an on-chip integrated monitor photodiode, thus drastically reducing the cost of packaging (since fewer of the elements require alignment and assembly). Our future work will focus on optimization of the leakage effect in order to manufacture high- power and high-speed single-mode VCSELs.

This project received funding from the European Union’s Horizon 2020 research and innovation program under grant 666866.