Monolithic VCSEL–PIN Photodiode Integration for Bidirectional Optical Data Transmission

We present the monolithic integration, properties, and operation of 850-nm wavelength AlGaAs/GaAs-based vertical-cavity surface-emitting lasers (VCSELs) and PIN (p-doped-intrinsic-n-doped) photodetectors. The stacked layer structure requires sophisticated fabrication methods, but enables the use as, e.g., low-cost transceiver (TRx) devices. The TRx chips reported here are especially designed for bidirectional short-reach optical data links using a single butt-coupled standard multimode fiber (MMF). Photodiodes (PDs) with three different epitaxial layer structures are investigated. Devices with a 3-μm-thick intrinsic region show a responsivity of >0.6 A/W and have the lowest dark currents and highest 3-dB bandwidths of around 8 GHz. The maximum small-signal bandwidth of the VCSEL is 12.5 GHz. The parasitics of both devices are extracted by modeling the reflection spectra from S-parameter measurements. Investigations regarding the mutual influence between the closely integrated devices in full-duplex operation are carried out. The optical crosstalk is below -11 dB and the maximum electrical crosstalk between VCSEL and PIN PD of around -50 dB is nearly negligible. The butt-coupled MMF with a core diameter of 50 μm allows maximum fiber alignment tolerances in the range of 14-26 μm. Data transmission in the 10-Gb/s range is demonstrated in half-duplex and full-duplex mode.