Fiber Optics - Optical Sources And Detectors

Optical detectors- They are also known as Photodetectors. The main purpose of optical detectors is that it converts an optical signal into an electrical signal.

Properties of Photo Detectors

1. Optical detectors should have High fidelity.

2. Photodetectors should possess short response time, i.e. there should not be a longer response time.

3. Optical detectors or photodetectors should be of small size.

4. It should possess high reliability and also the cost should be cheap.

5. Photodetectors should have high sensitivity at a desired operating wavelength.

6. Optical detectors should produce the large electrical response to a received optical signal.

7. Photodetectors should produce minimum noise.

8. It should also carry stable performances.

9. Optical detectors should work at the low bias voltage, i.e. it should not be working in a high bias voltage.

10. All optical detectors used these days are semiconductor detectors. For detector without internal gain, an incident photon produces only one free electron breaking an electron-hole pair in the device, whereas for the detector with internal gain two free electrons are produced for an instant photon.

Some of the examples of Optical detectors (Photodetectors) are p-i-n photo-diode, Avalanche photodiode, etc. In simple, the photodiode without internal gain is termed as p-i-n photo-diode and the photo-diode with internal gain is termed as Avalanche photodiode.

Optical sources- What are the main purposes of optical sources? The main purpose of the optical source is to convert an electrical signal to an optical signal.

Properties of Optical Sources

1. Emit light at wavelengths where the fiber has low losses.

2. It should also emit light at wavelengths where the fiber has a low dispersion.

3. Allow direct modulation over a wide range of frequencies.

4. Temperature stable.

5. It should be stable to an environmental condition.

6. They are cheap and reliable which is comparable to the conventional technique.

7. Have compatible size & configuration in order to effectively launch light into the fiber.

8. Track the input electrical signal accurately with minimum distortion and the noise.

9. Couple sufficient power to overcome losses in fiber and connectors & provide sufficient power to drive the detectors.

10. Have a very narrow spectral bandwidth which is used to minimize operation.

Type of Optical Sources

Light emitting diodes (LED) for multimode fiber in which Radiation from Light Emitting Diodes is a case of spontaneous emission and Light amplification by stimulated emission of radiation (LASER) for single-mode fiber in which Radiation from LASER involves stimulated emission.

The electron in energy level E1 is excited to higher energy level E2 by a photon, whose energy is given by,

E = E2 - E1 = hf --------- (i)

The excited photon may return to lower energy level E1 by itself emitting a photon with a frequency given by equation (i). This process is called Spontaneous Emission.

If the electron is forced to return to the lower energy level E1 by allowing it to interact with an external photon having energy equal to the energy difference between levels (E2-E1). Then this process is known as Stimulated Emission.

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