Fibre optical communication
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Hello everyone, this is Narmat Varman. Today I will talk about some of the properties or topics related to the fiber optical communication. So the first one is avalanche photodiodes. Internal multiplication of primary photocurrent occurs, therefore receiver sensitivity increases. Carrier multiplication occurs by impact ionization electrons and those generated by the photons ionize bound electrons and valence bond on colliding with

them newly generated carriers are accelerated by higher electric field causing further impact ionization resulting in avalanche effect. So avalanche effect if I just tell you in brief it's like there are holes and electrons present in it and with and by plotting all the graphs by considering the parameters such as electric field and others

So a depletion region is created and according to you can search that photo or the graph on the internet in which you will find the avalanche region and the holes and electrons. So I'll just brief you up about the graph that is the it has the heavily doped of

p+ material over which the high resistivity p material is deposits p material is diffused over this followed by the n+ material for silicon boron and phosphorus are used as dopants now let's talk about rapd reach through avalanche photodiode when reversed bias most of the potential drop across

pn+ junction as voltage is increased the depletion region widens unit it reaches through in to the pi region and is just at 5 to 10% below point of avalanche breakdown light enters through p+ region and is absorbed by the pi region and photon generated carriers collect here these electrons flow towards the pn+ junction where high electric field exists therefore carrier multiplication takes place

Average member of the electron hole pairs created by the carrier per unit distance traveled is called the ionization rate. Ionization rates are different for electrons, alpha and b and holes. Beta, so k is equal to beta by alpha is the measure of photo.

electro performance multiplication factor is equal to Im/Ip where Im is equal to the average value of the total multiplied output current and Ip is equal to primary un-multiplied photo current. Responsivity RAPD is equal to

NQM divided by HV is equal to R0 M0 where R0 is unity and responsivity. In APD an extremely high electric field region is created which is equal to 3 into 10 to the power 5 V per centimeter. The generation of the high field region causes holes and electrons to acquire sufficient energy to excite new electrons, holes

Now let's again talk about the coherent detection.

Okay, so coherent detection, we'll just brief, I'll just brief you up. So in the coherent detection, we'll just get to know about the fundamental concepts, the homodyne detection, the heterodyne detection and yes, the BER comparisons that is bit error rate.

So, first one is coherent direction the fundamental concepts. In optical fiber communication the term coherent refers to any technique which employs non-linear mixing between the two optical waves. In this technique gain is provided to the incoming optical signal by combining or mixing it with locally generated continuous wave optical field. The device used for creating the CAW signal is a narrow line width called

So, if I just talk about the

basic diagram it consists of a signal laser then ASK or PSK modulator and then optical coupler then amplifier then filter and demodulator and then the R output to send information one can modulate the amplitude frequency or phase of the optical carrier thus one of the following three modulation techniques can be implemented ASK or PSK amplitude shift keying

on or off keying, FSK, PSK. In direct detection system, the electrical signal coming into the transmitter amplitude modulates the optical power level of the light source. Thus, the optical power is proportional to the signal current level at the receiver. The incoming optical signal is converted directly into the demodulated electrical output. This directly detected current is proportional to the intensity IDD of the optical signal.

If the local oscillator field is ELO is equal to ALO cos omega LOT plus psi LOT, now the coherent electric output at the receiver will be ES plus EL0 whole square multiplication with cos omega S minus omega LO multiplication with T and then psi LT cos omega T.

Since the optical power pt is proportional to the intensity of the photo detector we can then consider

Now, let's talk about the homodyne detection. When the signal carrier and local oscillator frequency are equal, when WIF is equal to 0, it is called the homodyne detection. Homodyne detection brings the signal directly to the baseband frequency so that the further electrical demodulation is required. Homodyne receives yield the most sensitive coherent system. It is most difficult to build since the local oscillator laser to have the same frequencies. In this case, WIF is equal to 0.

Heterodyne detection. In the heterodyne detection, the intermediate frequency is non-zero and an optical phase locked loop is not needed. Heterodyne receivers are much easier to implement than homodyne receivers. 3 dB degradation and sensitivity compared to the homodyne detection either

00k FSK or PSK modulation techniques can be used. Consider that the output current of the receiver PS is less than PLO. We can ignore the first term on the right hand side. Then the receiver of the output current contains a DC term given by IDC equals to NQ by H plus PLO. A line varying IF term is given by IIFT equals to NQ by HV.

Then let's talk about the bit error rate comparison. The bit error rate depends on the SNR signal to noise ratio, probability intensity function that is PDF and the receiver output input comparator. The receiver sensitivity of for coherent detection techniques describing the terms of the average number of the photons required to achieve the 10 to the power minus 9 bit error rate comparison between the various coherent detection techniques that is direct detection.

HOMO-DYN system, PSK HOMO-DYN system, Heterodyne detection scheme. So first let's talk about the direct detection

00k system in which a sequence of 0 and 1 pulses occur with equal probability. The 00k data stream is an AND state and only half of the time the required number of photons per bit is half the number of n required per 1 pulse. If n bar and 0 electron hole pairs are created during 1 and 0 pulses then the average number of photons per bit for unity

quantum efficiency n is equal to 1 is np is equal to 1 by 2 n bar plus 1 by 2 0 then the actual number of electron hole pairs and data generated fluctuates from the average according to the poison distribution pr of n is equal to n bar multiplication e raised to power minus n bar n factorial p n is equal to probability that the data and electrons are emitted in an internal

t the receiver as zero pulse if no electron hole pairs are generated with the pulse present that is the probability that n is equal to zero electrons are emitted in a time interval t. This fundamental quantum limit is very difficult to achieve for direct detection receivers. The amplification electrons that follow the photodetector add both thermal noise and short noise so required receiver power level lies between 13 and 20 dB above the quantum limit.