Avalanche photodiode (APD) sensors used in near-infrared (NIR) systems underpin fibre-optic communications, LiDAR, medical imaging, scientific instrumentation, and many other applications. The terms sensitivity, responsivity, and noise frequently arise when discussing APD performance. These characteristics are strongly interrelated: the sensitivity of an APD is determined by how efficiently it converts light into current (responsivity) and by the degree of unwanted electrical fluctuations (noise) present in the device. To reliably detect weak optical signals requires high responsivity and low noise, which together yield the best sensitivity for real-world measurement.
Sensitivity in NIR APDs
Best-in-class NIR APDs, typically made with indium gallium arsenide (InGaAs) for operation around 1550 nm, routinely achieve Noise Equivalent Power (NEP) values as low as 11–17 femtowatts per square root hertz (fW/√Hz). This means that the sensors can identify extremely faint signals, which is essential for long-distance fibre transmission and high-resolution LiDAR scenarios.
Responsivity in NIR APDs
Responsivity describes how much electrical current is produced per unit of NIR light absorbed. Leading InGaAs APDs reach 0.9–1.0 amperes per watt (A/W) at 1550 nm, meaning the devices convert incoming NIR photons to electrical charge with remarkable efficiency. This attribute is fundamental for maximising detectable signal strength across a variety of applications.
Noise in NIR APDs
Noise in APDs includes random electronic disturbances (such as dark current and avalanche multiplication effects) that obscure weak signals. Modern InGaAs APDs designed for NIR operation feature an excess noise factor (ENF) as low as 1.11 at an avalanche gain (M) of 10 and is still below 3.5 at M=100. This performance delivers a good signal-to-noise ratio. In addition, dark current, a measure of baseline electrical interference, can be kept to a few tens of nanoamps, reducing background noise to minimal levels.
In summary, the interplay between sensitivity, responsivity, and noise defines the capabilities of NIR APDs. Maximising responsivity while minimising noise leads to exceptional sensitivity, allowing these devices to operate reliably in challenging low-light conditions and pushing the boundaries of precision detection.