Ask RP Photonics for advice on the origin of beam pointing fluctuations and their reduction with suitable laser designs or with feedback stabilization. With the powerful RP Resonator software one can calculate how misalignments of mirrors cause beam pointing deviations.

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For judging the angular beam stability of a laser, not only the magnitude of angular fluctuations, but also the beam radius has to be taken into account. It is instructive to compare the angular fluctuations with the diffraction-limited beam divergence, i.e., the beam divergence of a Gaussian beam with the given size. The larger the radius of such a beam is, the smaller is its divergence angle, and the more severe is the influence of pointing fluctuations with a given angular spread.

The magnitude of angular fluctuations alone is actually often not sufficient to calculate the effect of beam pointing fluctuations in an application; it can also be important how large parallel beam offsets occur, and how these are correlated with angular fluctuations.

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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.

With a good laser design, the angular beam pointing fluctuations of a laser can be a tiny fraction of the beam divergence. This corresponds to phase changes across the beam profile which are much smaller than one radian.

In multimode laser diodes, to what extent does the changing mix of modes contribute to high frequency pointing noise? Do single-mode laser diodes generally exhibit lower pointing noise than multimode laser diodes, or are other factors more important than the number of modes when considering laser diodes?

If a laser beam is sent through some optical setup, this will in general modify the magnitude and type of beam pointing fluctuations, even if the optical components are absolutely stable. The following two examples illustrate this:

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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.

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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.

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I don't have data on this, but I would expect that substantial high-frequency beam pointing fluctuations could rise from power fluctuations between the modes. That would of course not happen for single-mode diodes.

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The beam pointing stability of commercial laser products is often quantitatively specified. Unfortunately, such specifications are often not precise or even meaningless. A useful specification of angular fluctuations has to observe a number of important issues:

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.

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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.

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.

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.

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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.

A further reduction in pointing fluctuations may be achieved with an active stabilization scheme. It is possible e.g. to monitor the beam position at some point with a four-quadrant photodiode and correct it via piezo-actuated mirrors.

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.

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.

It is important to note that a certain tilt of a resonator mirror does not necessarily translate into a tilt of equal size of the output beam. Instead, it generally leads to some combination of a (larger or smaller) tilt with some shift (offset) of the beam. The type of that influence depends on the whole resonator design (as discussed in the article on alignment sensitivity). For a linear resonator, the alignment sensitivity can be very different in the two stability zones, and can even diverge near the edge of such a zone. The alignment of different resonator mirrors can also very much differ in terms of sensitivity. Such issues have important implications for the optimization of pointing stability (see below).

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The direction of the output beam of a laser is subject to some beam pointing fluctuations, which can in some cases cause significant problems – e.g., when the beam must be coupled into a single-mode fiber, or when the beam must precisely hit a target at a large distance. For such reasons, a quantitative measure for the beam pointing stability (see below) can be of importance.

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.

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.

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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.

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.

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.

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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: