Photodiode Structure with Pigtail

A pigtailed photodiode combines a semiconductor PIN or InGaAs photodiode with an optical fiber permanently attached for efficient light coupling.

Internal Structure

Most pigtailed photodiodes use a p–i–n (PIN) structure, which consists of a p-type layer, an intrinsic (undoped) layer, and an n-type layer . The intrinsic layer increases the depletion region, allowing more light to generate electron–hole pairs, which are collected as photocurrent. This structure provides high responsivity, low noise, and fast response, making it suitable for high-speed optical detection . Some devices use InGaAs for infrared detection or Si for visible to near-infrared applications .

Pigtail Fiber Coupling

A pigtail is a short optical fiber permanently attached to the photodiode, often terminated with a connector such as FC/PC or angle-polished connectors . The fiber can be single-mode or multimode, depending on the application. Single-mode fibers reduce mode interference, while multimode fibers allow higher coupling efficiency for broader beams . The fiber is actively aligned to the photodiode during assembly to maximize optical coupling efficiency and stability .

Packaging and Performance

Pigtailed photodiodes are typically housed in hermetically sealed packages or mounted on ceramic substrates to protect the diode and maintain alignment . They are designed for low dark current, low capacitance, and high-speed operation, making them ideal for optical communications, high-speed photometry, and monitoring systems . The optical alignment ensures stable responsivity over time and reduces back-reflection, which is critical in interferometric or high-bit-rate systems .

Applications

• Fiber-optic communications (LAN, MAN, WAN)
• High-speed photometry and instrumentation
• Optical monitoring and sensing
• Scientific measurements in visible and near-infrared ranges In summary, a pigtailed photodiode integrates a high-speed semiconductor photodiode with a fiber-optic pigtail, providing precise, stable, and efficient optical detection suitable for a wide range of high-performance applications .