OPTICAL DETECTORS IN FIBER OPTIC RECEIVERS.Introduction.Block diagram of fiber optic receiver.Optical Detectors.Optical Detector Requirements.Semiconductor Photodiodes.Optical detector materials.PIN Photodiode.Slide 9Response time factors.Schematic of a Photodiode.Advantage of PIN photodiodes.AVALANCHE Photodiodes.AVALANCHE Photodiode.Light Emitters As Detectors.Ping-Pong(Full-Duplex) LED.Questions And Comments.OPTICAL DETECTORS IN FIBER OPTIC RECEIVERS.Presenter: Julius Munyantwali.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.Block diagram of fiber optic receiver.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.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.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.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.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.PIN Photodiode.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.Schematic of a Photodiode.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 responseAVALANCHE Photodiodes.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 Photodiode.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.Ping-Pong(Full-Duplex) LED.Questions And