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Optical Detector Basics: Quantum Efficiency and Responsivity

By RF Wireless Expert Team

This page covers the basics of optical detectors, including discussions on quantum efficiency and responsivity. Optical detectors are crucial components in the receiver section of fiber optic communication systems.

Their primary function is to convert incoming light signals into corresponding electrical voltage pulses.

For more in-depth information, refer to a fiber optic tutorial. Common optical detectors include avalanche photodiodes (APDs) and PIN diodes.

An ideal optical detector possesses:

  • High sensitivity (ability to detect weak signals)
  • High fidelity (accurate signal reproduction)
  • Short response time (fast reaction to changes in light)
  • Stability across the operational range

Quantum Efficiency of Optical Detector

The quantum efficiency (ηD) of an optical detector describes how effectively it converts photons into electrons. It is defined as:

ηD = electron generation rate / incident photon rate

Which can also be written as:

ηD = re / rp

Where:

  • re represents the electron generation rate.
  • rp represents the photon generation rate.

Responsivity of Optical Detector

Responsivity (R) characterizes the transfer function of the detector, showing the relationship between input optical power and output photocurrent. It’s defined as:

R = Ip / Po (Units: A/W)

Where:

  • Ip is the generated photocurrent.
  • Po is the incident optical power.

We can further express the incident optical power (Po) as:

Po = Photon rate * hf

Po = rp * hf

Where ‘hf’ represents the energy of a single photon.

The electron generation rate (re) can be related to the incident optical power (Po) and quantum efficiency (ηD) as follows:

re = ηD * Po / hf

Now, we can rewrite the responsivity equation:

R = Ip / Po = e * re / Po = ηD* e / hf

Where:

  • Ip = e * re (e is the elementary charge)

It’s important to note that there’s often a trade-off between the speed of operation and the sensitivity of an optical detector. Improving one aspect can sometimes negatively impact the other.

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