An Introduction to Optical Detectors
Optical detectors play an important role in many scientific, military, and commercial applications. Solar panels, LEDs, remote controls, medical imaging, and fiber optic communication systems are just some of the many applications of optical detectors. At IZAK Scientific, our professional detectors experts can help you choose the right optical detectors no matter your application.
Types of Optical Detectors
There are several possible types of optical detectors, depending on how the detector response is triggered. Common types of optical detectors include:
- Photoconductive: Optical detectors based on photoconductivity exploit the property that a material’s conductance (and conversely, resistance) can change based on absorption of electromagnetic radiation.
- Photoemissive: Photoemissive optical detectors are based on the photoelectric effect, first observed by Heinrich Hertz in 1887. When photons strike a material, they transfer energy to that material and cause it to emit electrons (referred to as photoelectrons). These photoelectrons can be collected in a circuit such as a vacuum photodiode or photomultiplier.
- Photovoltaic: Photovoltaic optical detectors are based on the photovoltaic effect, a similar principle to the photoelectric effect. Instead of a material emitting photoelectrons, the photovoltaic effect refers to charge carriers that are excited within the material when exposed to light.
One of the most fundamental optical detectors is the photodiode, a semiconductor device that incorporates incident light as a source of current control. Similar to diodes, photodiodes consist of a p-n junction made from doped semiconductor materials such as silicon or germanium. Some photodiodes consist of a PIN junction, which adds an undoped (intrinsic) semiconductor region between the p-type and n-type regions.
Photodiodes can exploit photoconductivity, the photoelectric effect, and the photovoltaic effect, depending on the application and architecture. When operated in photoconductive mode (reverse bias), photodiodes provide a linear response with respect to the applied luminance.
There are several characteristics of photodiodes that must be considered depending on your application. At IZAK Scientific, our professional detectors experts can help guide you in choosing the proper specifications for your needs.
Optical Detector Characteristics
The response time of a photodiode is a measure of how quickly it can respond to optical input. Response time is dependent on a photodiode’s rise time (i.e., its RC time constant, the time necessary to reach 63.2% of its final steady-state output) and its charge collection time (a voltage-dependent measure of the transit time of charge carriers within the photodiode).
Two critical electrical characteristics that affect a photodiode’s response time are junction capacitance and shunt resistance. The junction capacitance depends on the width of the photodiode’s depletion layer, which varies with bias voltage. Ultimately, the junction capacitance increases with the area of the photodiode junction. This means that physically smaller photodiodes can provide faster response times than larger photodiodes. A photodiode’s shunt resistance is a measure of its resistance under zero bias.
One measure of a photodiode’s effectiveness is a property called quantum efficiency. This figure reveals the photodiode’s ability to convert incoming light energy into electrical energy. Quantum efficiency is the percentage ratio of the number of electrons in the photodiode current over the number of incident photons. It is dependent on the photon wavelength, temperature, and many other factors. High quality photodiodes can achieve quantum efficiencies of around 80%.
A similar measure to quantum efficiency is called responsivity. The responsivity of a photodiode compares the output current of the photodiode (in amps/cm2) to the incident light energy (in watts/cm2). The resulting ratio is measured in A/W. As with quantum efficiency, responsivity is wavelength dependent.
Noise in Optical Detectors
There are several common sources of noise in optical detectors.
Johnson noise, also called thermal noise, arises due to the random movement of charge carriers in a conductor. Johnson noise increases with temperature, so cooling the optical detector decreases the amount of noise in the system. Johnson noise is also dependent on the load resistance of the optical detector circuit.
Shot noise is noise that arises from random fluctuations in the arrival of photons (and/or fluctuations in the rate that charge carriers are generated and recombine, sometimes referred to as g-r noise). Shot noise can be minimized by reducing the bandwidth and DC current of the optical detector system.
1/f noise is so-called because it’s inversely proportional to the modulation frequency of the optical detector. The mechanisms underlying 1/f noise, also known as excess noise, are not fully understood. 1/f noise can be reduced by operating the optical detector at higher frequencies (commonly up to 1000Hz).
The dark current (also called leakage current) of a photodiode measures its current without any incident light. Dark current is caused by random electron/hole activity in the photodiode’s depletion region, and is considered a source of noise in optical detectors. Dark current varies with temperature and typically ranges from 1 – 10nA in silicon photodiodes up to 1000nA in germanium photodiodes.
Optical Detector Expertise
The professional detector experts at IZAK Scientific can help you better understand the needs of your optical detector system. Whether you’re looking for photodiodes, phototransistors, avalanche photodiodes, photodiode arrays, photomultiplier tubes, charge-coupled devices, or any other type of optical detectors, we’re here to help.
Contact our professional detector experts today.