USB Extender and Repeater - usb 3 over fiber
Beam wander or jitter is the amount that the centroid or peak value of the beam strength profile moves with time and can be caused by turbulence resulting in the beam becoming unfocused.
Diffraction diffractionexperiment
Think back – if we are dealing with the interference of two sources, there will be places where the waves are in phase and cause constructive interference, and other places where the waves are out of phase and interfere destructively. In an audio example, the two slits could be replaced with two loudspeakers, and the maxima and minima in the wave superposition would then correspond to locations of loudness and quiet.
Free Space Optics wireless network ranges are typically found to be between around 100m and 2km but due to the nature of the signal strength being directly affected more by atmospheric conditions over increasing distance, the shorter the range between the two unit locations the higher the performance and availability of the connection will be.
Diffraction diffractionexamples
So far we’ve only considered the case of a single slit or gap for the wave to pass through. What happens if there are two or more slits? We’ll end up with two or more diffracting waves, which we might expect to interfere with one another.
Free Space Optics provides speeds comparable to those of optical fibre connections with the flexibility and practicality of being part of a wireless network providing bandwidth speeds typically advertised as up to 10Mbps, 100Mbps, 155Mbps and 1.25Gbps, with possible speeds of up to 10Gbps becoming likely in the future with the use of WDM (Wavelength-Division Multiplexing) technology. Currently, the only other wireless technology capable of these kinds of speeds is Millimetre Wave RF Wireless Networking which, in comparison, requires licensing and can affected severely by rain in the 60GHz range. Due to the received beam being transferred onto an optical fibre to connect to the core network, trouble free integration and easy set up make Free Space Optics networking's compatibility with any system very high.
Building sway due to wind can be a problem as it disrupts the alignment between the two transceiver units causing loss of signal. Divergent beam technology can be used to allow the units to communicate in these situations but performance is still slightly affected.
To the right of the slits, the waves interfere with each other. In fact, you can generate the same patterns by placing two sources where the slits are. The sound through each slit diffracts and radiates rather like two point sources. So the patterns you are observing are very similar to those for two sources whose wave radiation interferes together. You might want to have another look at the pages on interference – all the formulations and concepts are applicable to Young’s double slit experiment. This video below nicely demonstrates this using water waves on a pond.
Diffraction diffractionformula
Beam spread, or more technically called beam divergence, is measurable angular effect of the beam's dissipation at a constant rate as it travels further through the atmosphere.
Fresneldiffraction
Unlike rain and snow, that on the whole has little effect on Free Space Optics communication, fog and water vapour droplets are a real hindrance to the operating performance. The small water droplets can at points completely stop the light beams from being received due to light absorption, refraction scattering or even complete reflection which can significantly lower data rates. Therefore in foggy areas, Free Space Optics may not be the best solution, however applications have been successfully carried out that have provided acceptable reliability with redundancy systems in place. The following five points listed all refer to the signal attenuation caused by atmospheric conditions.
Free Space Optics (FSO) is a technology that uses laser beams via a line of sight optical bandwidth connection to transfer data, video or voice communications across areas ranging typically from 100m to a few kilometres at throughput bandwidths up to 1.25Gbps at frequencies above 300GHz of wavelengths, typically, 785 to 1550nm. Using Free Space Optics wireless networks eliminates the need to secure licensing found with RF signal solutions and also the expensive costs of laying fibre optic cable; principally the concept of transferring data via light is the same as with fibre optics just through a different medium.
Definition ofdiffractionin Physics
All Free Space Optics technology is strictly controlled to make sure that standards are followed to limit any dangers. On the whole, Free Space Optics units are of low enough power not to cause long term harm when the laser is exposed to a person's eye, however precautions should be taken so that this never occurs if possible.
We’d hear these loud / quiet areas one after another as we moved in an arc in front of the loudspeakers – they’re called Young’s fringes. If the experiment is carried out using light waves, you get bright locations for constructive interference and dark locations for destructive interference. Young used this experiment to measure the wavelength of light.
Due to being located above ground unlike, for the most part, laid fibre optic cable, different challenges are presented when considering Free Space Optics performance with the biggest being atmospheric conditions. However, most drawbacks and shortcomings can be resolved through the inclusion of redundancy systems and correct wireless network planning.
Fraunhoferdiffraction
The basis of Free Space Optics communication is rather straightforward with each unit housing an optical receiver and transmitter, allowing the sending and receiving of data simultaneously, and an optical source with a focusing lens. The unit at one location transmits a beam of focused light carrying the information directly at the unit at the receiving location where the light beam is then transferred to an optical fibre from a high sensitivity receiver.
When the gap width is larger than the wavelength (bottom movie), the wave passes through the gap and does not spread out much on the other side. When the gap size is smaller than the wavelength (top movie), more diffraction occurs and the waves spread out greatly – the wavefronts are almost semicircular.
Diffractionwave examples
Free Space Optics Wireless Networks can only operate as Point-to-Point links between 2 units, however, when combined with LAN or WLAN networks they can provide very effective solutions to many scenarios such as:
Free Space Optics is a very secure method of wireless communications when compared to RF Signal Networks because the light beams cannot be detected by spectrum analysers, data transmissions can be encrypted, the laser beams are very narrow and invisible making them hard to find or detect and to receive the signal, another matching receiver would have to be aligned within the light path which is quite unlikely to happen.
When the size of the gap changes, how does this change the diffraction of the wave? When does maximum diffraction occur? (Think about your previous findings on the diffraction of sound around an obstacle).
Below is a simulation of diffraction through two slits. The experiment is named after the guy who first carried it out – Young’s double slit experiment. Have a look at what is happening to the right of the slits. Is there a pattern? What creates this? Is the amplitude larger at some places than others?
Scattering occurs when certain wavelengths experience collisions with objects and are redistributed in varying directions without energy loss (unlike absorption). Scattering is more likely to have a more frequent and larger effect over long distances where it can have a significant effect on beam strength.
Water vapour molecules in the air absorb the energy from photons (light particles) within the light beam which causes an overall loss in power density. The use of spatial diversity and correct system power helps combat this effect as absorption is more common at certain wavelength ranges of light.
Diffraction diffractionequation
Fluctuations in signal strength can be caused by variations in temperature of air pockets between the transmitter and receiver due to natural differences or objects such as buildings etc. This effect, know as refractive turbulence, causes image dancing or blurring of the signal at the receiver end which results in amplitude loss.
Huygens argued that a wavefront could be modelled as a series of wavelets. A wavelet can be described as a circular wave much like the ripple you would get from dropping a small pebble into a pond. These wavelets superimpose and interfere to form more complicated wavefronts. For example – if you dropped a number of pebbles in a straight line, all in one go at exactly the same time, a straight (in science-speak plane) wavefront would be created. The video below shows how you can use this method to work out how wavefronts are altered by a slit.
Diffraction also occurs when a wave passes through a gap (or slit) in a barrier. This is shown in the two animations below. The difference between the movies is the size of the gap.
Due to light not be able to travel through opaque mediums, objects such as birds, planes and people can momentarily cause interruptions to the service by blocking the Free Space Optics' light beam, with service resuming instantly when the light path is cleared. Multi-beam technology can be used with compatible systems to try and counter this problem.