Technology Guides

A brief guide to APD terminology

What is an Avalanche Photodiode (APD)?

Avalanche Photodiodes are semiconductor devices designed to convert light into electrical current with high sensitivity. They operate by applying a high reverse voltage, which causes a multiplication effect (avalanche) of the charge carriers (electrons and holes) when they are generated by incident photons. This results in a significant amplification of the current, making APDs ideal for applications requiring the detection of very low levels of light, such as in fibre optic communication, medical imaging, and environmental monitoring.

What is the excess noise factor (ENF) in APDs?

ENF is a critical parameter in APDs that quantifies the additional noise introduced by the avalanche multiplication process. It impacts the overall noise performance and limits the APD’s signal-to-noise ratio (SNR).

What is a Noiseless InGaAs™ APD?

A Noiseless InGaAs™ APD is an APD with an excess noise factor low enough to achieve an APD gain of over 100 without signal-to-noise–ratio (SNR) degradation.

What is bare die in semiconductor terminology?

A bare die is a semiconductor chip in its raw form, without any packaging or housing. Bare dies are used in custom applications where specific mounting or packaging is required. They are often wire-bonded or flip-chip mounted onto substrates or PCBs. Using bare dies allows for greater flexibility in designing compact and high-performance electronic systems. This approach is commonly used in advanced applications such as high-frequency RF circuits, custom sensor arrays, and specialized photodetectors, where space constraints and performance requirements are critical.

What is dark current in photodetectors?

Dark current is the small amount of electrical current that flows through a photodetector even in the absence of light. This current is caused by thermal generation of charge carriers within the detector. Dark current contributes to the overall noise in the detector, affecting its sensitivity and performance, especially in low-light conditions. Minimizing dark current is crucial for applications that require high precision and low noise, such as astronomical observations, low-light imaging, and sensitive measurement systems.

What are fiber pigtails?

Fibre pigtails are short lengths of optical fibre with a connector on one end, used to connect photodetectors and other optical components to the fibre optic network. They simplify the integration of devices into the system by providing a ready-made connection interface. Fibre pigtails are commonly used in telecommunications, medical devices, and industrial systems. They are typically fusion-spliced to other fibres, ensuring low loss and high reliability in the connection, which is crucial for maintaining the performance of the optical network.

What does gain mean in the context of photodetectors?

Gain in the context of photodetectors refers to the amplification of the electrical signal generated by the incident light. Higher gain allows the detector to produce a stronger output signal from a given amount of light, improving the ability to detect low-intensity light. Gain can be achieved using transimpedance amplifiers (TIAs) and the avalanche multiplication process in APDs. High gain is essential for applications requiring enhanced signal strength, such as in long-distance fibre optic communication and low-light imaging.

Why is InGaAs (Indium Gallium Arsenide) used in photodetectors?

Indium Gallium Arsenide (InGaAs) is a semiconductor material known for its excellent performance in detecting infrared light, particularly in the 900 nm to 1700 nm wavelength range. InGaAs photodetectors are often preferred in applications like laser range finders, fibre optic communication, spectroscopy, and infrared imaging (including LiDAR) due to their high sensitivity, low noise, and fast response times. The material’s properties allow it to efficiently convert infrared light into electrical signals, making it ideal for use in devices that require precise detection and measurement of infrared radiation.

What is infrared (IR)?

Infrared refers to electromagnetic radiation with wavelengths longer than visible light but shorter than microwaves, typically ranging from 700 nanometers (nm) to 1 millimeter (mm). IR is used in various applications, including thermal imaging, LiDAR, ight vision, range finders, spectroscopy, and telecommunications.

What are laser range finders?

Laser range finders measure the distance to a target by emitting a laser beam and measuring the time it takes for the beam to reflect back to its source. They are used in various applications, including surveying, military targeting, sports, and autonomous vehicles. The precision and speed of laser range finders make them ideal for tasks requiring accurate distance measurements over various ranges, from a few meters to several tens of kilometers.

What is LiDAR (Light Detection and Ranging)?

LiDAR is a remote sensing technology that uses laser light to measure distances and create high-resolution, three-dimensional maps of surfaces. It works by emitting laser pulses and measuring the time it takes for the pulses to return after hitting an object. LiDAR is used in various applications, including topographic mapping, forestry, urban planning, autonomous vehicles, and archaeology. Its ability to provide precise distance measurements and detailed surface models makes it invaluable for studying and managing natural and built environments.

Concerning photodetectors, what is Noise Equivalent Power (NEP)?

Noise Equivalent Power is a critical parameter that measures the sensitivity of a photodetector. It is defined as the amount of incident light power required to produce a signal equal to the inherent noise level of the detector. The lower the NEP, the more sensitive the detector is, meaning it can detect lower levels of light. NEP is typically expressed in watts per square root hertz (W/√Hz). It helps in comparing the performance of different photodetectors, especially in applications like spectroscopy, astronomy, and low-light imaging.

What is OTDR (Optical Time-Domain Reflectometry)?

Optical Time-Domain Reflectometry is a technique used to characterize and troubleshoot optical fibres. An OTDR sends a series of light pulses through the fibre and measures the backscattered light that returns. By analysing the returned signals, the OTDR can determine the fibre’s length, identify faults, measure attenuation, and assess overall performance. OTDR is essential for maintaining and optimizing fibre optic networks, ensuring reliable communication and minimizing downtime by quickly locating issues such as breaks or splices.

What is photodetector sensitivity?

Sensitivity in photodetectors refers to their ability to convert incoming light into an electrical signal effectively. Higher sensitivity indicates that the detector can produce a significant signal even from low light levels. Sensitivity is influenced by factors such as the material of the photodetector, its quantum efficiency (the percentage of photons converted to electrons), and the overall noise characteristics. High sensitivity is crucial for applications where detecting faint light signals is essential, such as in medical diagnostics, night vision, and optical communication.