High-speed VCSEL photonics for datacenter networks

We present a novel design of intracavity metal-aperture VCSELs toward high-speed and single-mode operations. The intracavity metal contact causes the transverse resonance which provides the modulation bandwidth enhancement. The small-signal modulation bandwidth can be double with a large mode-fi ld diameter of 10μm and single-mode operations. Introduction Vertical cavity surface emitting lasers (VCSELs) have exhibited the advantages of low cost, ease of fabrication into arrays, small footprint, waferscale testing, and low power consumption [1, 2]. Therefore, VCSELs are attracting much attention for use in data center networks. The network traffic in data centers is increasing rapidly and hence the development of high speed VCSELs is a key issue. The modulation bandwidth of VCSELs is typically less than ~ 20 GHz due to the limited intrinsic carrier-photon resonance (CPR) [3]. Therefore, many efforts have been done to push the modulation bandwidth of VCSELs further into the mm-wave band [4-14] However, there still remain difficulties in increasing the modulation bandwidth of singlemode VCSELs in comparison with multi-mode VCSELs, although single-mode VCSELs offer a longer link length of MM fibers thanks to narrower spectral widths. Also, the poor reliability has been a limiting factor for single-mode, small oxide aperture VCSELs. In this paper, we propose and demonstrate intracavity metal aperture VCSELs (MA-VCSEL) with a rectangular shaped oxide aperture. The fabrication process is exactly the same as intracavity contact VCSELs. We found that the intracavity metal contact causes the transverse resonance which provides the modulation bandwidth enhancement. We demonstrate the enhancement of the modulation bandwidth and single-mode operation thanks to the opticaltransverse coupled cavity effect. The mode-field diameter could be increased to 10 μm with stablesingle-mode operations. Device Structure Figure 1 (a) illustrates the schematic structure of the fabricated single-mode MA-VCSEL. The device is fabricated on a half-VCSEL wafer grown by MOCVD with 4 pairs of top p-type DBR. The active region includes three 850 nm quantum wells (3QWs). In order to form cavity structures, rectangular shaped mesas were formed by dryetching process and followed by wet-oxidation process. The size of an active region oxidation aperture is 9 x 10 m, as shown in Fig.1(b) which is large enough for high reliabilities. Polyamide was used for planarization and passivation. AuGe/Ni/Au was deposit d to form n-type electrodes. The p-type electrode (Au/Zn/Au) were deposited with a rectangular aperture. Finally, 8 pairs of dielectric Ta2O5/SiO2 were deposited above the surface of the mesas as a top hybrid DBR. We found that two lateral boundaries cause the transverse resonance as shown in Fig. 1(c). A key parameter is the distance d-W between two boundaries, which should be 1.5-2 μm to observe the transverse coupled cavity effect, we found.