 Hello and welcome to the lecture on optical detectors part one learning outcomes of this session. By the end of the session student will be able to explain the fundamental mechanism behind the photo detection process. You may pause here the video and try to find out the answer to this question. What are the light sources used in optical communication systems? The light sources used in optical communication system are LED and laser. There are multiple types of laser, injection laser, gain-guided laser, index-guided laser, quantum well lasers. We have seen all these sources, optical sources in the previous lectures. So, we will move ahead for optical detectors. In this figure you can see the generalized optical communication system diagram, where we have three major components, optical source, optical fiber used for the transmission of light signal and optical receiver. Optical sources we have already gone through it, those are LED, laser and injection lasers. Electrical signal is fed to the optical source. This optical source converts this electrical signal into optical pulses, which is transferred or transmitted through the optical fiber to the destination. On the destination side, these optical pulses are converted into electrical signal by this optical receiver. This optical receiver has a fundamental component, which is optical detector. This detector is of area of interest for us in this session. Optical receivers convert the incident optical light, which is in the forms of pulses into the electrical signal, depending upon the circuitry involved in it, whether it will be converted into current or voltage. So, sometimes optical receiver are referred as optical to electrical converters as well. Optical detectors are used at the front end of every optical receiver to generate a photo current, which will be proportional to the intensity of the light incident on the receiver. Optical detector is a fundamental element of an optical receiver, which is followed by amplifier and signal conditioning circuitry. There are several types of photodetectors, but the mainly famous and most widely used photodetectors are semiconductor photodetectors. There are types of photodetectors, photodiode, phototransistor and photoconductors, etc. Let us see the performance and compatibility criteria of a photodetector. The photodetector should be compatible physically means it should fit with the optical fiber core size and it should be small in size. It should have high responsivity at a desired wavelength and low responsibility elsewhere means it should be wavelength selective, which means it should respond to a minute change at a desired wavelength. It should generate low noise means it should have a zero dark current ideally, but practically it is not possible, so we always refer it as a low noise. It should have high gain. The photodetector should be fast enough to get the desired high bandwidth. The high bandwidth means it should respond fast, so it should have a small response type. The photodetecting material should be insensitive to the temperature variations. Photodetecting material used in photodetectors should have long operating life, continuous and stable operations. From economic point of view, it is important for any communication system to have a low price, so it should not be expensive. It should consume low power for its operation as well as it should operate at low voltage levels. All these performance criteria are similar to the performance criteria of optical source. Let us see how the optical detection process takes place. Whenever a photon incident on a photodetecting material, it gets converted into electrical signal. Through the absorption process of photon, the conversion of optical to electrical signal takes place. The absorption of photon excites an electron from valence band to the conduction band. This excitation leaves a vacancy in the valence band, which we call it as a hole. Therefore, whenever we encounter an absorption or we speak about photo generation, we refer it as a generation of electron-hole pair. Once a photon gets absorbed in the material, an electron-hole pair has to be separated by the influence of electrical field. Let us understand this process with the help of this energy band diagram. Whenever light photon incident on a photodetecting material, there are two conditions. First, when the energy of incident photon is less than the energy band gap. If it is true, means the light photon have lesser energy than this energy band gap. Then this photon is allowed to leave as it is. It won't get absorbed by this material. In second case, when the energy of photon is greater than this energy band gap. The band gap energy between valence band and conduction band. Then this photon will get absorbed by the material, which will excite an electron from valence band to the conduction band here. This electron excitation creates a hole in valence band. So this is nothing but a generation of electron and hole pairs. When we apply an electrical field across this material, then these electrons and holes get separated in opposite direction, which constitute the photocurrent. Whenever we talk about the application of electrical field across photodetecting material to separate the electron and holes generated through this photodetection process, we apply a strong reverse bias because this reverse bias creates a strong electrical field in the junction, which increases the drift velocity of the carriers, therefore reduces the transit time for the carriers. Strong reverse bias increases the width of depletion region, thereby it reduces the junction capacitance and improve the response time. Also the width of depletion region increase leads to a larger light capturing area increasing its efficiency. So that's why always a reverse bias is applied across any photodetecting material. Let's see what are the photodetecting materials. Silicon and germanium are common semiconductor used in photodetecting materials, but these are used for short wave band. In the range of 800 nanometer to 900 nanometer wavelength, light can be detected with silicon pin diode and silicon pin diodes are widely used because these are inexpensive, reliable and easy to handle. For medium wavelength band that is from 1250 nanometers to 1350 nanometers, germanium and its different compound semiconductors are of interest in this range. Germanium has its lower band gap energy of 0.67 electron volts, so it can be used up to the wavelength band of 1600 nanometers. Of course the fabrication of compound semiconductor which are of interest for this medium wave band are expensive to fabricate. For long wavelength band the material are nothing but the ternary materials are the table 3 to 5 alloys like indium, gallium, arsenide with a band gap energy of 0.77 electron volts. After the band gap energy is small that causes a problem which is nothing but the thermal excitation problem. This thermal excitation problem can be overcome by the use of hetero junctions while fabricating the material. These are the references used. Thank you.