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The job of the photodiode to convert the light energy received from the laser diode into electrical energy. Photons absorbed by the photodiode excite electrons within the photodiode in a process called intrinsic absorption. When stimulated with an outside bias voltage, these electrons produce a current flow through the photodiode and the external circuit providing the bias voltage.

In an electrical circuit, as shown below, light absorbed by the photodiode produces current flow through the entire external circuit. As current flows through the resistor, it produces a voltage drop across the resistor. This voltage drop is input to an amplifier for amplification.

One of the keys to the performance of the overall fiber optic receiver is the photodiode itself. The response times of the diodes govern the speed of the data that can be recovered. Although avalanche diodes provide high speed they are also more noisy and require a sufficiently high level of signal to overcome this. Testing photodiodes are critical to the reliability of the photodiode semiconductor. Various testing methods such as burn-in, accelerated life testing, are performed by Automatic Semiconductor Test Equipment designed for photodiodes.

PIN Photodiode

Greater speed means that the diode can turn on and off faster. Remember that in fiber optics the light pulses being sent by the transmitter happen at a very fast rate. The photodiode needs to be able to stop and start electron flow fast enough to keep up with the incoming light pulses.

The PIN photodiode is constructed a little differently than the PN photodiode. An intrinsic layer is used to separate the p region and the n region. This creates a large depletion region that absorbs the photons with improved efficiency when compared to the PN photodiode.

Avalanche Photodiode

The avalanche photodiode (APD) works just as its name suggests. On a snow-covered mountain, a small vibration can trigger an avalanche of snow. With the APD, a small bundle of photons can trigger an avalanche of electrons. The APD accomplishes this through a process called photomultiplication.

The APD is constructed with one more p region than the PIN photodiode. When the APD is biased very close to its breakdown voltage, it acts like an amplifier with a multiplication factor, or gain. An APD with a multiplication factor of 50 sets free, on average, 50 electrons for each photon absorbed. The free electrons produce current flow through the electrical circuit connected to the APD. The APD is typically used in receivers that operate with lower optical input power levels than those associated with PIN diodes. These receivers typically have data rates ≤1Gbps.

Photodiode Materials & Wavelength

The required materials to make a photodiode and the range of electromagnetic laser spectrum wavelength range includes the following

  • For silicon material, the electromagnetic spectrum wavelength range will be (190-1100) nm
  • For Germanium material, the electromagnetic spectrum wavelength range will be (400-1700) nm
  • For Indium gallium arsenide material, the electromagnetic spectrum wavelength range will be (800-2600) nm
  • For Lead (II) sulfide material, the electromagnetic spectrum wavelength range will be <1000-3500) nm
  • For Mercury, cadmium Telluride material, the electromagnetic spectrum wavelength range will be (400-14000) nm

Applications of Photodiodes

  • Photodiodes are used in optical communication systems for the communication system for encoding & demodulation purpose.
  • It is also used for digital and logic circuits which require fast switching and high-speed operation.
  • The photodiode is used in automotive devices.
  • The photodiode is used in medical devices.
  • Typical applications of avalanche photodiodes include receivers in optical fiber communications, range finding, imaging, high-speed laser scanners, laser microscopy, and optical time domain reflectometry (OTDR).