Q4: Which laser is most popular for optic fiber communication in terms of wavelength? Is it red one? Can they also use infrared laser in optic fiber communication? Ultraviolet lasers also exist so why not use them because they can provide higher data rate than visible light lasers?
Q4: Which laser is most popular for optic fiber communication in terms of wavelength? Is it red one? Can they also use infrared laser in optic fiber communication? Ultraviolet lasers also exist so why not use them because they can provide higher data rate than visible light lasers?
Generally, in linear media, you do not expect laser beams to interfere (EDIT: on rereading my response I'm realizing we are both using the word "interfere" to mean cross coupling with each other. However, a typical use of the word "interference" in laser terminology is related to "interference patterns" which are the addition of fields which creates constructive and destructive interference. Obviously, laser beams can interfere in this way in linear media, but I assume this is not what you are asking.). Separate signals will be independent based on direction, polarization, spacial mode pattern and frequency, and will not interfere, in principle. However, the real world does not perfectly obey our idealization, and there can be cross coupling of signals. Separation by polarization and spacial modes is tricky and there is often cross coupling due to fiber imperfections. However, frequency and direction tend to be very good ways to separate different signals. The only way for these separate signals to interact is through one of the many types of non-linear processes that are well known. To keep these effects insignificant one just needs to make sure that the signal power is low enough to not induce significant nonlinearity. However, nonlinearity can happen at relativly low power because the fiber confines the light to a very narrow cross sectional area, which makes the light intensity very very high, relative to the actual power level.Q1: Suppose two laser beams being used for the purpose of communication cross each other vertically. Would there be any interference? Would the information contained in those laser beam be affected?
That is certainly one way, and probably the most significant way in practice.Q2: We know that radio communication is affected by electromagnetic interference. I think this interference comes into play at the receiving end such as antenna where both information radio signal and interference radio signal is picked up by an antenna and this combined reception of both signals produces a new noisy signal at the receiving circuit. Do I have it right?
Yes, laser diodes are the most practical lasers used in communications. They are specially designed for this use, and come in varying degrees of sophistication in design, depending on the application.Q3: What kind of laser is mostly used for optic fiber communication? I believe it's a laser diode which provides continuous laser beam and not a pulsed one, right?
It all depends on the fiber type. There are plastic fibers designed to be used with red light (or visible light). However, most glass based fibers (silica based being the most practical and common) work best in the infrared. The first fiber optic systems used 800-900 nm infrared because the sources and detectors were available at that wavelength and were cheap and reliable. Also, due to too much water content in the fiber, longer wavelengths would have had higher loss anyway. But, intrinsically, once water impurities are minimized, the longer wavelengths are better, but eventually molecular absorption (which can't be removed) takes over and the fiber becomes opaque if wavelength is too long.Q4: Which laser is most popular for optic fiber communication in terms of wavelength? Is it red one? Can they also use infrared laser in optic fiber communication? Ultraviolet lasers also exist so why not use them because they can provide higher data rate than visible light lasers?
Q2: Eq. 3.11 for attenuation, A, was derived systematically. But is this really correct that eq. 4.69 is just the definition of attenuation constant? I don't see it being derived from anything.
OK, it may be that one reference treats it better, but in fairness to the second reference, the basic formula is shown in several places (eqns. 4.59 and 4.65).Hi
I do understand the definition of decibels. The definition of decibel is dB=10log(X2/X1). Eq. 3.11 is derived systematically but on the other hand eq. 4.69 is just thrown into without any derivation. They are the same equations but in one equation we have "A" and in the other there is attenuation constant, alpha. I think I'm just confused and will get around it soon. Thanks.
Regards
PG
OK, it may be that one reference treats it better, but in fairness to the second reference, the basic formula is shown in several places (eqns. 4.59 and 4.65).
Keep in mind that it is often best to use several different sources to get the best understanding of a new or difficult topic. This is true of text books and even more true of journal and conference papers in the literature.
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