The journey to achieve ultra-high-speed optical detection has been marked by continuous innovation in semiconductor physics and device architecture. The family of types of photodetectors​ has evolved significantly from basic PN junctions to sophisticated structures capable of operating at frequencies exceeding 100 GHz. Central to this evolution is the high speed photodetector, a critical component in modern fiber optic networks and microwave photonics systems.
The foundational technology for many high-speed applications is the PIN photodiode. By introducing an intrinsic (i) layer between the p and n regions, the depletion region width is increased, allowing for most photons to be absorbed within the high-field zone. This minimizes the slow diffusion current that plagues standard PN diodes. When fabricated using Indium Gallium Arsenide (InGaAs), these diodes become highly efficient InGaAs photodetectors​ for the 1310nm and 1550nm windows. NEON’s product line, including the FCPD and AMPD series, utilizes InGaAs PIN structures to achieve bandwidths ranging from 1 GHz to 20 GHz.
However, as bandwidth requirements pushed beyond 40 Gb/s, traditional PIN diode structures faced limitations due to the transit-time bottleneck and parasitic capacitance. To overcome this, researchers developed waveguide photodiodes. Unlike surface-illuminated devices, these guide light laterally through a thin absorption layer, allowing for a long interaction length without increasing the carrier transit time. This innovation decouples quantum efficiency from bandwidth, enabling devices with both high responsivity and ultra-wide bandwidths.
The ultimate frontier in photodetector technology is the traveling-wave photodetector. These devices integrate a slow-wave electrode structure that matches the velocity of the optical wave with the electrical wave, preventing reflections and enabling bandwidths up to 310 GHz. While NEON’s current commercial offerings focus on hybrid integrated modules like the AMPD-D and HPPD-Ku, the underlying technology is rooted in these advanced concepts.
Today, the practical implementation of these technologies involves hybrid integration. For example, NEON’s modules combine the optical chip (the photodiode) with drive circuits, impedance matching networks, and voltage stabilization. This holistic approach ensures that the raw speed of the photodiode is preserved at the module level, providing system designers with turnkey solutions for radar information processing, electronic warfare, and high-speed signal test and measurement. The progression from simple diodes to complex integrated modules illustrates the industry's relentless pursuit of faster, more reliable optical-to-electrical conversion.