OPTICAL DETECTORS IN FIBER OPTIC RECEIVERS.

1. OPTICAL DETECTORS IN FIBER OPTIC RECEIVERS. .

2. Introduction. • A fiber optic receiver is an electro-optic device that accepts optical signals from an optical fiber and converts them into electrical signals. • Consists typically of : -Optical detector -Low-noise amplifier -Other circuitry.

3. Block diagram of fiber optic receiver.

4. Optical Detectors. • These are transducers that convert optical signals into electrical signals. • Transducers are devices that convert input energy of one form into output energy of another. • An optical detector does so by generating an electrical current proportional to the intensity of the incident optical light.

5. Optical Detector Requirements. • Compatible in size to low-pass optical fibers for efficient coupling and packaging. • High sensitivity at the operating wavelength of the source. • Low noise contribution. • Maintain stable operation in changing environmental conditions.

6. Semiconductor Photodiodes. • Generate current when they absorb photons. The amount of current depends on ; -Wavelength of the light and responsivity of the photodiode -Size of the photodiode active area relative to the fiber core size -Alignment of the fiber and photodiode.

7. Optical detector materials. • Si,GaAs, GaAlAs – 850nm • Ge, InP, InGaAs -1300nm and 1550nm. • Materials determine the responsivity of the detector which is the ratio of the output photocurrent to the incident optical power. • It’s a function of the wavelength and efficiency of the device.

8. PIN Photodiode. • Semiconductor positive-negative structure with an intrinsic region sandwiched between the other two regions. • Normally operated by applying a reverse-bias voltage. • Dark current can also be produced which is a leakage current that flows when a reverse bias is applied without incident light.

9. PIN Photodiode.

10. Response time factors. • Thickness of the active area. -Related to the amount of time required for the electrons generated to flow out of the detector active area. • Detector RC time constant. -Depends on the capacitance of the photodiode and the resistance of the load.

11. Schematic of a Photodiode.

12. Advantage of PIN photodiodes. • The output electrical current is linearly proportional to the input optical power making it a highly linear device. • Low bias voltage(<4v). • Low noise • Low dark current • High-speed response

13. An APD internally amplifies the photocurrent by an avalanche process when a large reverse-bias voltage is applied across the active region. The gain of the APD can be changed by changing the reverse-bias voltage. AVALANCHE Photodiodes.

14. AVALANCHE Photodiode.

15. Light Emitters As Detectors. • LEDs and lasers can also be used as light detectors making them half-duplex fiber optic communication devices. • They can be used alternately as light emitters and detectors allowing transmission of information in either direction over the fiber. • In order for the LED to operate as a full-duplex, the temperatures at both ends should be carefully chosen.

16. Ping-Pong(Full-Duplex) LED.

17. Questions And Comments.