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Chapter 6: Optical Detectors


Optical Detectors


Now that we’ve covered optical sources, it’s time to view optical detectors (photodetectors). These are essentially devices that take optical energy and transform it into electrical energy which can be used by a telephone, computer, etc.



The discrepancies between the different types of photodetectors are small. Indeed, these devices are all variations on the same principle. Five important parameters are essential to understanding optical detectors:


  • Detector Sensitivity: this is the ratio of output current to input optical power. In other words, this measures the efficiency of the device by assessing how much energy is lost.
  • Response Time: this is a measure of how quickly the detector can respond to variations in the input light intensity.
  • Noise: the level of noise (interference) produced in the device is critical to its operation when levels of input light are low.
  • Spectral Response Range: this is the range of wavelengths (bandwidth) over which the device operates.
  • Gain: the extent to which a signal (eg an optical or electrical signal) is amplified.


Next, it is important to understand the role of these characteristics differences between the three main types of photodetectors:


  • Photodiodes avalanche

In these semiconductor devices, light is absorbed in a depletion region and generates a photocurrent. They are generally defined by their average response time and average spectral response range. They are, however, highly sensitive to light, which means that very little energy is lost. They produce some internal gain but are typically noisy.


  • Photoconductors

These are the simplest conceivable optical detectors and are also considered the slowest. The spectral response range is rather small, and these devices are usually used for long wavelengths. They exhibit below average sensitivity but produce internal gain.


  • Junction Photodetectors msm

These are characterized by their very high spectral response range and speedy response time – generally better than the previous two models. However, they are usually not as sensitive and do not produce any internal gain.


In conclusion, optical detectors transform optical signals into electrical ones. Each device is defined according to its sensitivity, response time, noise level, spectral response range, and gain. The three broad categories of devices are photodiodes, photoconductors, and junction photodetectors.


Thanks for following the LambdaGain Learning Centre. Be sure to check out the video version of this tutorial to get a more comprehensive understanding of photodetectors, as well as our next chapter which will be optical communication systems.



October 23, 2018