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Free space optical communication is the most growing communication because it is easy to install and has a high speed because the signal is transmitted in the air. So that will introduce the atmospheric affect in the optical wave propagation. Atmospheric turbulence causes fluctuations in both the intensity and the phase of the received signal. So we need to study the effect and the limitation if we introduce a free space optical communication system with dual wavelength (980 nm, 1550 nm). Also studying the effect of turbulence when using two different wavelengths.
Free space optical communication is a kind of communication that use light propagation to send data between two points. Free Space Optics are capable of up to 2.5 Gbps of data, voice and video communications through the air, allowing optical connectivity without requiring fiber-optic cable or securing spectrum licenses. So we can use LED’s or Laser for transmission data. Free Space Optics (FSO) technology is relatively simple. It’s built on a laser transmitter and a receiver to provide full duplex capability.
Each FSO unit uses a high-power optical source, a lens that transmits light through the atmosphere to another lens receiving the information. The receiving lens connects to a high-sensitivity receiver via optical fiber. Because the transmission in occurred in air it is easily upgradable. FSO send a light beam from one point to another using low power lasers in the teraHertz spectrum. This beam is transmitted by laser light focused on photon detector receivers. These receivers collect the photon stream and transmit digital data. If there is a clear line between the two point FSO can operate on a distance of several kilometers as long we have a powerful transmitter.
Features of the laser communications system
Information usually in the form of digital data, data is entered to be regulated by the laser source transmitting electronics. Coding techniques can be used directly or indirectly depending on the type of laser used. Output source passes through the optical system in the channel. The visual system usually involves the transfer, beam shaping, and the telescope optics. Beam receiver comes in through the optical system and passed to the detection and signal processing electronics. There is also a terminal control electronics that must manage gimbals guidelines and other mechanisms, and machinery, to maintain and track the acquisition of the operating system designed in the mass of the process. In order to communicate, you must have received enough energy by the detector to distinguish signal from noise.
Laser power, optical transmission system losses, pointing out shortcomings of the system, transmitter and receiver antenna gains and losses, receiver, receiver and loss tracking, are all factors that force in the establishment of the receiver power. The required optical power is determined by data rate, detector sensitivity, configuration modes, noise, and detection methods. When the receiver is to detect the signals, it is in fact the decision-making regarding the nature of the signal (digital signal is sent when the distinction between the ones and zeros).
There are two types of distributions: one when the signal present (including the amount of photocurrent due to the background and the current detector in the dark), and one when there is no signal present (including sources of no signal current only). A threshold must be developed to increase the success rate and reduces the error rate. Even when there is no signal present, the fluctuation sources of no signal lead periodically to the threshold to be exceeded. This is an error stating that the signal exists when there is no signal present. Distribution of signal may also fall on the other side of the threshold, so any errors stating that the signal is going to happen even when the signal is present.
FSO systems work in the near infrared wavelength range slightly above the visible spectrum. So, the human eye cannot clearly see the transmission beam. The wavelength range is around 1 micrometer that is used in FSO transmission. The interception of FSO operating systems with narrow beam in the infrared spectral wavelength is by far the more difficult. Small diameter of the beam is usually only a few meters in diameter in the target site are one of the reasons that make it extremely difficult to intercept the communications of the FSO. Intruder must know the exact origin or target of the infrared beam and intercept only within a very narrow angle of beam propagation. Intercept packets directly from the FSO networks between remote locations is impossible mainly because the beam passes through the air usually at a higher altitude than at ground level. Due to the fact that the transmission beam is not visible, and that any attempts to block the beam can occur near the FSO point of access and the process of transition poses another obstacle.
Capture the signal from the location that does not fall directly within the path of light with photons of light scattered from aerosols, fog, rain, or molecules that may be present in the atmosphere is almost impossible because of the energy levels are very low use infrared through FSO process transmission. The main reason for the exclusion of this possibility of intrusion is the fact that light is an ally and statistically isotropic in different directions from the path of the original propagation. This specific mechanism keeps the total number of photons or the amount of radiation that can potentially be collected on the detector that is not placed directly in the beam path beyond the detection level of noise.
Atmospheric turbulence can destroy the performance of FSO systems. The changes in temperature and pressure in the atmosphere lead to changes of the refractive index along the transmission path. These changes can make the quality of received signal fade and causes fluctuations in the intensity and the phase of the received signal. These fluctuations can limit the performance of the system. The atmosphere is a viscous fluid and it has two state motions: 1) laminar (there is no mixing in the air molecules) 2) turbulent: (there is mixing that creates eddies). Atmospheric turbulence can be physically described by Kolmogorov theory. The energy of large eddies is redistributed without loss to eddies of decreasing size until finally dissipated by viscosity. The size of turbulence eddies normally ranges from a few millimeters to a few meters, denoted as the inner scale and the outer scale, respectively. So the index of refraction n is very sensitive to small scale temperature fluctuations (temperature fluctuations are combined with turbulent mixing). So, the index of refraction is the most important in optical wave propagation.
Because it behaves like a passive additive. So the spectrum of index of refraction can be described by Kolmogorove spectrum Φn (κ) = 0.033 Cn 2 κ-11/3 , 1/L0 << κ << 1/l0 Here in this model the variations in humidity and pressure are neglected. This model is the most model used in theoretical analyses but it is right only over wave number within the inertial subrange. To take into account the inner and outer scale effects, there is various models have been developed. Like Tatarskii and van Karman models. So all these models are useful for theoretical calculations but only inside the inertial range. They are not based on rigorous calculations outside the inertial range, but more on mathematical convenience and tractability. The modified atmospheric spectrum is the only model that features the high wave number rise prior to the dissipation range. Φn (κ) = 0.033 Cn 2 [1+1.802(κ/κl)-.254(κ/κl)7/6] x exp(-κ2/κ2 l)/(κ2 + κ20)11/6 , 0<= κ <∞ , κl=3.3/l0
The experiment that we need to do is to use two laser sources with different wavelength (980 , 1550) and set the receivers about 2-4 km from the transmitter and start sending the signals. We will use the same signals in both transmitters. Then we will study the performance of the system and see if that help to receive the signal in more accurate way than using one transmitter. That will help us to see the effect of optical turbulence and atmospheric effects. So we will calculate the performance of the system and measure the atmospheric turbulence. So we need to ask some questions:
What is the effect of optical turbulence?
Is losing a part of one signal will be recovered by the other signal?
Is that going to help the performance of the system?
Is the pdf that we used in the transmitter side will be the same as the pdf in the receiver side? Light wave
1. Laser beam propagation through random media by Larry C. Andrews, Ronald L. Phillips. 2. Free space optical communications class notes.
3. http://www.seminarprojects.com/Thread-freespace-optics-full-report#ixzz1KfUtl5xP 4. http://en.wikipedia.org/wiki/Free-space_optical_communication