Dispersion and Attenuation Phenomena - Revision history

Cmditradmin: /* Absorption losses */

2010-06-23T16:13:49Z

Absorption losses

← Older revision		Revision as of 09:13, 23 June 2010
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	=== Absorption losses ===		=== Absorption losses ===

	[[Image:Absorption_attenuation.png\|thumb\|~~300px~~\|Patterns for absorption by wavelength for SiO<sub>2</sub>]]		[[Image:Absorption_attenuation.png\|thumb\|400px\|Patterns for absorption by wavelength for SiO<sub>2</sub>]]
	Pure silicon would have virtually no absorption in the critical wavelengths. Absorption losses in the visible and near infrared regions arise mainly from the presence of impurities such as traces of transition metal ions (eg. Fe<sup>3+</sup> Cu<sup>2+</sup> or hydoxyl ions (0H<sup>-</sup>). Dramatic successes in reducing fiber losses come from better control of impurity concentrations of transition metal ions. It is very difficult to eliminate OH<sup>-</sup> bonds (from Si0<sub>2</sub>). These are responsible for overtones of vibration and absorption peaks at .95 μm, 0.24 μm and 1.39 μm. The dashed line is the background Rayleigh scattering. Above 1.6 μm there is a rapid increase in infrared absorption from lattice transitions. So there are two windows for absorption at 1.3 μm and 1.55 μm. Thus the combination of all scattering and attenuation phenomena can be optimized for these two wavelengths, which are the common telecommunications wavelengths. When working with a few miles, there is no need to regenerate the signal. In this case, it is beneficial to work at 1.3 μm since there is less dispersion loss. Theses are the types of compromises that engineers are looking at.		Pure silicon would have virtually no absorption in the critical wavelengths. Absorption losses in the visible and near infrared regions arise mainly from the presence of impurities such as traces of transition metal ions (eg. Fe<sup>3+</sup> Cu<sup>2+</sup> or hydoxyl ions (0H<sup>-</sup>). Dramatic successes in reducing fiber losses come from better control of impurity concentrations of transition metal ions. It is very difficult to eliminate OH<sup>-</sup> bonds (from Si0<sub>2</sub>). These are responsible for overtones of vibration and absorption peaks at .95 μm, 0.24 μm and 1.39 μm. The dashed line is the background Rayleigh scattering. Above 1.6 μm there is a rapid increase in infrared absorption from lattice transitions. So there are two windows for absorption at 1.3 μm and 1.55 μm. Thus the combination of all scattering and attenuation phenomena can be optimized for these two wavelengths, which are the common telecommunications wavelengths. When working with a few miles, there is no need to regenerate the signal. In this case, it is beneficial to work at 1.3 μm since there is less dispersion loss. Theses are the types of compromises that engineers are looking at.

Cmditradmin: /* Waveguide dispersion */

2010-06-23T16:13:34Z

Waveguide dispersion

← Older revision		Revision as of 09:13, 23 June 2010
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	=== Waveguide dispersion ===		=== Waveguide dispersion ===
	[[Image:Profile_dispersion.png\|thumb\|~~300px~~\|Waveguide dispersion is minimal at 1.3μm and 1.4μm wavelength for Si0<sub>2</sub>]]		[[Image:Profile_dispersion.png\|thumb\|400px\|Waveguide dispersion is minimal at 1.3μm and 1.4μm wavelength for Si0<sub>2</sub>]]
	Waveguide dispersion arises because the mode propagation velocity itself depends on wavelength regardless of any refractive index variation of the medium. Waveguide dispersion can not be neglected.		Waveguide dispersion arises because the mode propagation velocity itself depends on wavelength regardless of any refractive index variation of the medium. Waveguide dispersion can not be neglected.

Cmditradmin at 19:54, 29 December 2009

2009-12-29T19:54:27Z

← Older revision		Revision as of 12:54, 29 December 2009
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	Although there are discussions regarding this application, there is a significant investment in optical fibers that are already laid down at the bottom of the ocean. New fibers would have to be laid to take advantage of this new higher speed technology.		Although there are discussions regarding this application, there is a significant investment in optical fibers that are already laid down at the bottom of the ocean. New fibers would have to be laid to take advantage of this new higher speed technology.

	[[category:Photonics ~~Applications~~]]		[[category:Photonics applications]]
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Cmditradmin at 19:17, 29 December 2009

2009-12-29T19:17:56Z

← Older revision		Revision as of 12:17, 29 December 2009
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	Although there are discussions regarding this application, there is a significant investment in optical fibers that are already laid down at the bottom of the ocean. New fibers would have to be laid to take advantage of this new higher speed technology.		Although there are discussions regarding this application, there is a significant investment in optical fibers that are already laid down at the bottom of the ocean. New fibers would have to be laid to take advantage of this new higher speed technology.

			[[category:Photonics Applications]]
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Cmditradmin: /* Digital modulation */

2009-11-17T23:11:24Z

Digital modulation

← Older revision		Revision as of 16:11, 17 November 2009
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	Television signals are now transmitted with digital signals. The advantage is that if the signal is degraded, information is carried accurately up to the point it is not carried at all. This means for most viewers the picture looks perfect. However other viewers may not be able to see an picture at all while they were able to get a scratchy analog signal before the digital conversion in 2009.		Television signals are now transmitted with digital signals. The advantage is that if the signal is degraded, information is carried accurately up to the point it is not carried at all. This means for most viewers the picture looks perfect. However other viewers may not be able to see an picture at all while they were able to get a scratchy analog signal before the digital conversion in 2009.

	Consider a system to carry telephone conversations from here to Japan. The first question that you have to ask is: with my input signal, how frequently do I have to sample that signal in order to have an accurate representation of the input signal? This important question is the reason why there is so much attention paid into the information theory. The sampling theorem states that you have to maintain and keep the integrity of the input information and therefore, we must sample at a rate that is twice as large as the highest frequency of your input signal. For example, the human voice can go up to about 4 kilohertz so you have to sample at a rate of 8 kilo hertz. This means that you have to sample the variation of the noise 8000 times per second (~~8 bits x~~ 8000 bps~~= 64 kbps~~). The next step is to measure the voltage corresponding to that signal. Using an 8 bit number to represent the measurement results in a bit rate that is 16 times the highest frequency. A digital scheme requires a much higher system frequency bandwidth than would be needed for a corresponding analog signal.		Consider a system to carry telephone conversations from here to Japan. The first question that you have to ask is: with my input signal, how frequently do I have to sample that signal in order to have an accurate representation of the input signal? This important question is the reason why there is so much attention paid into the information theory. The sampling theorem states that you have to maintain and keep the integrity of the input information and therefore, we must sample at a rate that is twice as large as the highest frequency of your input signal. For example, the human voice can go up to about 4 kilohertz so you have to sample at a rate of 8 kilo hertz. This means that you have to sample the variation of the noise 8000 times per second (8000 bps). The next step is to measure the voltage corresponding to that signal. Using an 8 bit number to represent the measurement results in a bit rate that is 16 times the highest frequency. A digital scheme requires a much higher system frequency bandwidth than would be needed for a corresponding analog signal.

	So 8000 times per second the result of each measurement is sent in the form of a string of 8 pulses or no pulses. This means that about 64 kilo bit of information has to be transmitted per second. This is the telecommunication bandwidth. Note: the term bandwidth in this context is completely different from the bandwidth or bandgap between the conduction band and valence band of a chemical.		So 8000 times per second the result of each measurement is sent in the form of a string of 8 pulses or no pulses. This means that about 64 kilo bit of information has to be transmitted per second(8 bits x 8000 bps= 64 kbps). This is the telecommunication bandwidth. Note: the term bandwidth in this context is completely different from the bandwidth or bandgap between the conduction band and valence band of a chemical.

			The bandwidth for optical fibers is typically on the order of 1 gigabit per second. Continuing the example from the previous slide, a phone conversation requires 64 kilobits per second. That means that you can actually have multiple signals superimposed one over the other in parallel. This means 15,000 telephone conversations that can go to the other side of the ocean. Also, if you were to fax a document with the resolution that is usually used, you can send 60,000 pages per second. The only engineering involved includes sending those 15,000 phone conversations along the same fiber without scrambling them up on the way to their destination.

			In general when one looks at all the applications of non-linear optics (or what is referred to as electro optic modulation), modulation speed is one of the most important aspects. Using pi electrons moving along a back bone, one can have a very fast response allowing modulation of the signal to occur at a rate beyond a gigabit per second.

			Although there are discussions regarding this application, there is a significant investment in optical fibers that are already laid down at the bottom of the ocean. New fibers would have to be laid to take advantage of this new higher speed technology.

Cmditradmin: /* Digital modulation */

2009-11-17T22:59:49Z

Digital modulation

← Older revision		Revision as of 15:59, 17 November 2009
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	Television signals are now transmitted with digital signals. The advantage is that if the signal is degraded, information is carried accurately up to the point it is not carried at all. This means for most viewers the picture looks perfect. However other viewers may not be able to see an picture at all while they were able to get a scratchy analog signal before the digital conversion in 2009.		Television signals are now transmitted with digital signals. The advantage is that if the signal is degraded, information is carried accurately up to the point it is not carried at all. This means for most viewers the picture looks perfect. However other viewers may not be able to see an picture at all while they were able to get a scratchy analog signal before the digital conversion in 2009.

	Consider a system to carry telephone conversations from here to Japan. The first question that you have to ask is: with my input signal, how frequently do I have to sample that signal in order to have an accurate representation of the input signal? This important question is the reason why there is so much attention paid into the information theory. The sampling theorem states that you have to maintain and keep the integrity of the input information and therefore, we must sample at a rate that is twice as large as the highest frequency of your input signal. For example, the human voice can go up to about 4 kilohertz so you have to sample at a rate of 8 kilo hertz. This means that you have to sample the variation of the noise 8000 times per second. The next step is to measure the voltage corresponding to that signal. ~~As mentioned earlier, one uses~~ an 8 bit number to represent the measurement. So 8000 times per second the result of each measurement is sent in the form of a string of 8 pulses or no pulses. This means that about 64 kilo bit of information has to be transmitted per second. This is the telecommunication bandwidth. Note: the term bandwidth in this context is completely different from the bandwidth or bandgap between the conduction band and valence band of a chemical.		Consider a system to carry telephone conversations from here to Japan. The first question that you have to ask is: with my input signal, how frequently do I have to sample that signal in order to have an accurate representation of the input signal? This important question is the reason why there is so much attention paid into the information theory. The sampling theorem states that you have to maintain and keep the integrity of the input information and therefore, we must sample at a rate that is twice as large as the highest frequency of your input signal. For example, the human voice can go up to about 4 kilohertz so you have to sample at a rate of 8 kilo hertz. This means that you have to sample the variation of the noise 8000 times per second (8 bits x 8000 bps= 64 kbps). The next step is to measure the voltage corresponding to that signal. Using an 8 bit number to represent the measurement results in a bit rate that is 16 times the highest frequency. A digital scheme requires a much higher system frequency bandwidth than would be needed for a corresponding analog signal.

			So 8000 times per second the result of each measurement is sent in the form of a string of 8 pulses or no pulses. This means that about 64 kilo bit of information has to be transmitted per second. This is the telecommunication bandwidth. Note: the term bandwidth in this context is completely different from the bandwidth or bandgap between the conduction band and valence band of a chemical.

Cmditradmin: /* Digital modulation */

2009-11-17T22:45:04Z

Digital modulation

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 [[Image:Analogdigital.gif|thumb|550px|In binary digital modulation the information carrier can assume one two discrete states "zero" or "one"]]
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Cmditradmin: /* Digital modulation */

2009-11-17T21:41:25Z

Digital modulation

← Older revision		Revision as of 14:41, 17 November 2009
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	=== Digital modulation ===		=== Digital modulation ===

	[[Image:Analogdigital.gif\|thumb\|550px\|]]		[[Image:Analogdigital.gif\|thumb\|550px\|In binary digital modulation the information carrier can assume one two discrete states "zero" or "one"]]
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Cmditradmin: /* Digital modulation */

2009-11-17T21:29:05Z

Digital modulation

← Older revision		Revision as of 14:29, 17 November 2009
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	=== Digital modulation ===		=== Digital modulation ===

			[[Image:Analogdigital.gif\|thumb\|550px\|]]
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Cmditradmin: /* Plastic fiber */

2009-11-17T00:16:29Z

Plastic fiber

← Older revision		Revision as of 17:16, 16 November 2009
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	The progress that has been made in reducing the attenuation in plastic fibers is remarkable. Now people are discussing using plastic fibers for very long distances. The goal is to move away from those C-H bonds and find new elements that can substitute the hydrogens in these plastics so that a completely different attenuation profile can be obtained.		The progress that has been made in reducing the attenuation in plastic fibers is remarkable. Now people are discussing using plastic fibers for very long distances. The goal is to move away from those C-H bonds and find new elements that can substitute the hydrogens in these plastics so that a completely different attenuation profile can be obtained.

			See Wikipedia [http://en.wikipedia.org/wiki/Optical_fiber#Materials Fiber Optic Materials]


			== Optical communications systems ==
			An optical communication system means that there is information or a light signal that is sent somewhere by optical fiber. The goal is to modulate the signal or one of the characteristics of the signal so that the information can be obtained at the end of the line. The modulation can be made on various characteristics of the light such as variation in amplitude, irradiance or intensity (which is related to the electric field of the light), frequency, the phase of the light, and the polarization of the light. When relying on light at optical frequencies ( those on the order of visible light) modulation of the intensity (or irradiance) of signal is used.

			There are two types of modulation. Either a analog signal or digital signal (analog modulation or digital modulation) can be used. A simple example that uses modulation is sending a voice signal.

			=== Analog modulation ===

			When you pick up the phone, the voice is first transmitted into an electrical signal which is a time varying modulation of the voltage. That signal needs to be sent to the electrical fiber in the form of a modulation of the light signal. This process can be done in an analog fashion. That input information on the electrical voltage continuously varies the parameter of intensity that is modulated for the light. In this case, at any point in time, when the voltage increases, the intensity of your light signal increases; the voltage decreases, the intensity of your light decreases. There is a one to one relationship between the original signal amplitude and the magnitude of the wave parameter (irradiance).


	~~See Wikipedia [http://en.wikipedia.org/wiki/Optical_fiber#Materials Fiber Optic Materials]~~		=== Digital modulation ===


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