We report the monolithic integration, fabrication, and electrooptical properties of AlGaAs-GaAs-based transceiver (TRx) chips for 850？nm wavelength optical links with data rates of multiple Gbit/s. Using a single butt-coupled multimode fiber (MMF), low-cost bidirectional communication in half- and even full-duplex mode is demonstrated. Two design concepts are presented, based on a vertical-cavity surface-emitting laser (VCSEL) and a monolithically integrated p-doped-intrinsic-n-doped (PIN) or metal-semiconductor-metal (MSM) photodetector. Whereas the VCSEL-PIN photodiode (PD) chips are used for high-speed bidirectional data transmission over 62.5 and 50？μm core diameter MMFs, MSM TRx chips are employed for 100 or 200？μm large-area fibers. Such a monolithic transceiver design based on a well-established material system and avoiding the use of external fiber coupling optics is well suited for inexpensive and compact optical interconnects over distances of a few hundred meters. Standard MMF networks can thus be upgraded using high-speed VCSEL-PIN transceiver chips which are capable to handle data rates of up to 10？Gbit/s. 1. Introduction Over the last decades, the well-known AlGaAs-GaAs mixed compound semiconductor material system paved the way for the tremendous development of vertical-cavity surface-emitting lasers (VCSELs) as excellent light sources. Their inherent property of light emission perpendicular to the wafer surface and therefore the ability of fabrication in densely packed two-dimensional arrays make VCSELs distinguished inexpensive candidates for many applications [1]. Further advances in VCSEL technology have enabled higher chip functionalities such as monolithic integration with photodiodes. Such a device combination can be used for monitoring the laser output power [2], for realizing an ultracompact laser self-mixing interference sensor [3], or for biomedical fluorescence sensing [4]. Favorable properties like low threshold currents and operating voltages, circular beam profiles and high modulation bandwidths are ideal for VCSEL use in optical communications [1]. In contrast to conventional interconnections with one MMF per transmission direction, one could potentially lower the link cost, weight, and size by employing a single optical fiber, as presented in [5]. Here, the VCSEL is used as a dual-purpose device, namely, an efficient laser source and a resonant-cavity-enhanced avalanche photodetector. Since it is switched between two operation modes, only half-duplex operation at 1.25？Gbit/s data rate over a 50？μm core diameter MMF with
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