Periodically Induced Mode Shift In Vertical Cavity Fabry Perot Etalons Grown By Molecular Beam Epitaxy

Abstract

Two dimensional multiple wavelength vertical cavity surface emitting laser (VCSEL) arrays are promising for ultrahigh capacity optical networks using wavelength division multiplexing (WDM). The emission wavelength of a VCSEL is determined by the laser cavity round trip phase condition, which can be varied across the array by varying the thickness of either the cavity or the dielectric mirror layers. In prior work, a 2D VCSEL array emitting 140 distinct wavelengths was reported [ 13 using a spatially tapered mirror layer in the VCSEL caused by the inherent beam flux gradient in a Molecluar Beam Epitaxy (MBE) system. In this work, we demonstrate an induced lateral variation in cavity thickness of a GaAs/AlAs Fabry Perot resonator. By indium bonding the substrate to patterned backing wafers we induce a lateral surface temperature gradient on the substrate, thereby altering the GaAs desorption rate across the wafer during the growth of the cavity. Above substrate temperatures of 640 C, the GaAs growth rate is a strongly decreasing function of temperature [2]. Previously, Goodhue et. al. achieved substrate surface temperature differences of 30 50 C by mounting the substrates, using indium, to molybdenum blocks machined with 1 mm deep grooves and a 10 mm period [3]. They observed a near 3 fold decrease in GaAs growth rate in the high temperature regions of the wafer. In our work we have used indium to selectively bond the GaAs substrate to GaAs wafers which have patterns ranging from 2 to 8 mm. The advantage of this technique is that we can define the patterns lithographically. We then grow passive Fabry Perot cavities consisting of AlAs/GaAs Bragg mirror stacks centered at 950 nm, 10.5 pairs on the bottom and 8 on the top, and a 300 nm thick GaAs cavity. The calculated cavity mode of this structure is 980 nm. Both mirrors are grown at a substrate temperature of 600 C and the cavity is grown at approximately 700 C. A schematic of this technique is shown in Figure 1. We expect the mirrors to be uniform since they are grown below the gallium desorption temperature. The cavity, however, will have a thickness variation across the wafer due to the induced surface temperature difference in a regime in which significant gallium desorption occurs. We characterize the material by measuring reflectivity spectra across the wafer, and mapping the Fabry Perot wavelength. The spatial resolution of the measurement is 100 pm. Figure 2 shows the measured cavity mode position perpendicular to the direction of a single 8 mm wide pattern. We see that the effect of the indium bonded central portion was a higher surface temperature, resulting in a decrease in cavity mode wavelength of 7 nm over a distance of 3 mm. In Figure 3, we plot the measured reflectivity spectra for x = 18, 19, 20 , 21 mm in Figure 2. We see that although the cavity mode shifts significantly here, the stop band of the reflectance stays nearly constant. In Figure 4 we show the cavity mode along one direction for a different wafer which was mounted to a backing with a 3 mm pattern. Again, we see a significant shift of 8 nm which follows the 3 mm period. As the pattern size decreases , however, we notice larger nonuniformities, due to the difficulty in making good thermal contact with the indium bond. We have demonstrated 8 nm cavity mode shifts across 1.5 mm in GaAs /AlAs Fabry Perot vertical cavities grown by MBE. These results are very encouraging in the pursuit of fabricating multiple wavelength VCSEL arrays. The technique of growing cavities above the gallium desorption temperature and spatially mapping the cavity mode can also be used as a tool to study substrate temperature uniformity, since the measurement is sensitive to cavity thickness variations of less than 1%.