Installing Laser Links Between Buildings

Installing Laser Links Between BuildingsIf you’re installing laser links between buildings, follow these recommendations:• The beam should not be directed near or through electrical power lines or tree branches.Consider tree growth, wind load on trees, and power lines. Power lines also sag duringwarm weather and tighten up during cold weather. Make provisions to discourage nestbuilding in the optical path by birds and insects.• Make sure the transmission path is at least 10 feet above pedestrian or vehicular traffic.This both prevents accidental viewing of the laser beam and keeps the signal from beinginterrupted. Make allowances for any unusual effects that traffic may cause, such as dustclouds.• Make sure the transmission path is not shooting through or near exhaust vents that cancause steam to be blown into the path. This has the same effect as fog on the laser beam.• Most outdoor point-to-point components are fully weatherproof. These units may also bemounted inside buildings and the signal passed through glass windows. When light particleshit a glass surface, some of the light is reflected. With a clear glass window, approximately4% of the light is reflected per glass surface. If the glass is tinted, the amount oflight reflected and absorbed increases. In the case of reflective coatings, the laser lightwill reflect off the coating, and the light will never be detected at the receiver.Another problem when shooting through glass occurs when it rains. Water droplets onthe glass in front of the transmit lens act as additional lenses and can cause the beam todiffuse. Mounting the laser near the top of the window will reduce this problem somewhat,especially if there is an awning over the window. Figure 8.4 illustrates the propercritical angle when operating through glass. As the angle of the beam with the glassincreases, more and more light is reflected until the critical angle is reached (approximately42 degrees). Above the critical angle, all of the light is absorbed into the glass,and no transmission occurs.• Avoid east-west orientations. Although LCI uses optical filters in the receiver and has asmall angle of acceptance, direct sunlight can overload the units for several minutes aday for a few days per year.• Heat from roof tops, air duct vents, air conditioners, glass-faced buildings, and so on cancause a condition known as shimmer. Shimmer will cause the light beam to bend andappear to dance around the receiver. If sufficient heat is present, the beam will deflectenough to miss the receiver altogether, usually for a few milliseconds at a time, and bursterror will occur. When mounting on rooftop locations, the preferred location is at theleading edge of the roof, with the front of the laser at least six inches over the edge. Thisminimizes the effect of roof heating, heat rising up the side of the building, and snowaccumulation in front of the unit. This also provides access to the rear of the unit for easiersetup and alignment.• The movement of laser units caused by a strong mechanical vibration could cause thesystem to go in and out of alignment intermittently. It is advisable to avoid mountinglaser equipment near vibrating machinery such as air conditioning units, compressors,motors, and so on.• The laser beam produced by laser units is not subject to the interference produced byEMI sources. However, if laser units are placed within proximity of such sources, theunit’s electronics may pick up this interference, which would then be impressed on thesignals to and from the equipment. It is advised that laser units be mounted away fromlarge microwave dishes, antennas, radio stations, or any unusual electronic equipmentthat may be radiating electromagnetic signals.• Laser units are normally designed to project a two-meter diameter beam at the receiver.This provides some latitude for beam movement. It is essential, however, that unit movementbe kept to an absolute minimum to ensure peak performance. A movement of only1mm at the transmitter can divert the beam off of the receiver if the units are installedone kilometer apart.• Ideally, a laser unit should be mounted on the corner of the building to which it will beattached and preferably to masonry construction. This will provide the most stablearrangement. When transmitting signals more than 300 meters, it is not advisable tomount LACE units anywhere except at the corner of the structure. On buildings with athin metal skin, the base for the mounts must be attached to the supporting structure or tothe metal substructure.• Do not mount laser units on structures that can sway, such as trees, fences, towers, poles,or buildings exceeding 40 stories in height. Always avoid movable camera mounts.• Do not mount laser units to wooden structures. The expansion/contraction properties ofthese materials through precipitation and temperature make them good sources for movementand should be avoided. For example, high humidity will cause the units to go out ofalignment due to the wood expanding.• Make sure that when laser is mounted there are no ledges in front of the laser that couldbe used by roosting birds. Ledges can also cause a problem in rain or snow. Waterbouncing up from the ledge onto the optics or snow buildup in front of the optics willdiminish performance.