From photons to electrical current
A photodetector converts incident optical power into an electrical signal. In fiber communications, the detector is a critical part of the receiver; in sensing, it may measure intensity, timing, wavelength or changes in an optical field. Responsivity, bandwidth, noise, dark current and linearity are among the key device characteristics.
PIN and avalanche photodiodes
PIN photodiodes use an intrinsic region between p-type and n-type semiconductor layers to absorb light and generate carriers. Avalanche photodiodes add internal gain through impact ionization, which can improve sensitivity but also introduces excess noise and higher-voltage operating requirements. Device choice depends on reach, data rate, power budget and receiver design.
Materials and wavelength range
Silicon detectors are effective over portions of the visible and near-infrared spectrum, while indium gallium arsenide is widely used for common fiber-optic communication wavelengths. Other materials serve ultraviolet, mid-infrared and specialized sensing applications. The detector material must match the wavelength and performance requirements.
The receiver is more than a detector
A practical optical receiver includes the photodiode, transimpedance amplifier, limiting or linear amplification, clock and data recovery, and often digital signal processing. Packaging and electrical interconnects affect noise, bandwidth and thermal performance. In coherent receivers, multiple balanced detection paths recover amplitude and phase information.
What to measure
Receiver evaluation includes sensitivity, overload, bit-error performance, bandwidth, dynamic range, noise and behavior across temperature. Test conditions must specify data rate, modulation format, wavelength and error criterion. A detector datasheet alone cannot establish the performance of a complete optical receiver.
