FSO Products
General Information
Included below are brief descriptions on a variety of laser communication oriented topics. If you have further questions about these topics, please call or email, and we will be happy to provide further information.
Laser Safety
Our systems use Class IIIb AlGaAs lasers that emit in the near-infrared waveband. Although the emitted laser beam is invisible to the unaided eye, it can cause eye damage if viewed directly at close range for extended periods of time.
Laser emissions in the atmosphere are regulated by the FDA, and our systems comply with all federal, state and local regulations Depending on the model and configuration, our systems range from eye safe at the output aperture to eye safe at viewing distances greater than 220 feet and for exposures less than 10 seconds (ANSI Standard Z136.1).
Our systems include several safety features to limit exposure to laser radiation. These features are:-
A key control that cannot be removed if the transceivers are powered to the ON position
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An emission indicator that precedes the start of laser emission by at least 5 seconds to allow the user to avoid laser exposure
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A beam block that attaches to the front section of the transceiver
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A filter/attenuator for the alignment scope to reduce human exposure from the complementary transceiver to that of a Class I laser (completely eye safe)
Installation
Proper system installation is vital to system performance. Properly mounting the transceivers will not only result in optimum performance and increased availability, but it will also lessen the chances of operator injury and equipment damage. LSA offers two types of mounting platforms for link installation:
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For short-term installation, the systems can be mounted on heavy-duty tripods. LSA offers these tripods as an option.
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For longer-term use or permanent installation, we recommend securely attaching the systems to a mounting fixture and permanently attaching the fixture to a support such as a roof parapet, building corner, masonry wall, or other structurally rigid building section. It should not be attached to wood structures or other support structures that are dimensionally unstable or subject to flexing or vibration.
Line-of-Sight
Laser transmission of data requires a clear transmission path or line of sight. In selecting a mounting location, avoid trees, utility poles, and other objects that might obstruct the optical path.
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For building-to-building transmission, the ideal mounting location is a bolted interface to the building corner or roof parapet. The system should be pointed over the side of the building to avoid transmission across the rooftop on hot days.
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The transceivers also should not be mounted too close to a ledge or similar structure that might encourage accumulation of debris in front of the transceiver window. If obstructions such as birds and aircraft fly through the laser beam, they will impede data transmission temporarily. The systems transceivers have been designed to recover rapidly from these temporary interruptions. Most network data protocols automatically retransmit any lost data, so the user is unaware of the interruption.
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For consistent performance, the system should be mounted sufficiently high above pedestrian or vehicular traffic to avoid transmission interruption and accidental viewing. The transceivers should not be pointed at or through areas that have potential human occupation (e.g., utility poles, roofs of neighboring buildings).
Atmospheric Effects
Wireless links (both RF and laser) have real advantages, but they also have limitations due to the atmosphere that they travel through. The overall performance of a wireless communications system is described in terms of the bit-error rate (BER) that can be realized for the link. BER refers to the average rate at which bits are transmitted in error, where a BER of 10-6 means 1 bit per million is in error. Changes in the BER due to atmospheric variations are referred to as fading.
For RF links, reflections off buildings, trees, hills, vehicles, lakes, etc., tend to dominate and set the maximum achievable BER. This is further aggravated in urban areas by the dense clutter of buildings and trees that produce substantial scattering. High-data-rate systems, such as 24, 30, 32, 38 and 52 GHz microwave transmitters are susceptible to these reflections and scattering effects. For links of modest length, these effects can be reduced by using tall towers to keep the beam clear of most clutter and highly reflective surfaces, such as bodies of water. However, the need for transmission towers raises issues of increased cost, esthetic impact, and the pitfalls of local politics in gaining the necessary approvals. Another approach to reducing multipath effects is to reduce the beamwidth by going to higher frequencies, but this has the disadvantage of higher losses and lower availability in rainy weather. The BER is typically 10-6 for real-world wireless RF links without forward error correction.
Wireless lasercom links also have BER limitations, but for somewhat different reasons. Bit errors in a lasercom link result from temperature variations along an atmospheric optical path due to differences in air, ground, and building thermal loading. These temperature differences result in refractive index variations of the air. This effect is easily seen as the shimmering of an object on a hot summer day when you look down a long straight road. To avoid these effects, we recommend mounting the transceivers away from hot surfaces or terrain, preferably in a location about 20 feet above ground level. For rooftop mounting, situating the transceiver at the edge of a building is optimal to avoid any rooftop induced thermal effects.
Our systems are designed to provide the user the best performance available. The unfaded BER is <10-10. We will work with our customers to deliver a solution that provides high availability and low BER when taking into account installation and local environmental conditions.
BER Impacts on Network
In most cases, a wireless link BER of 10-6 is quite sufficient. All data networks have some form of error detection, and most have a higher-layer protocol that results in retransmission of any errored data. This is to ensure the integrity of the data even for exceptionally high-quality fiber links. For typical data packet sizes of 50 to 5000 bytes network operation will be essentially unimpaired for a link BER of 10-4 or better in a data network that employs TCP/IP or similar retransmission protocols. For networks comprising many wireless links over any end-to-end connection, a BER of 10-6 ensures the network is not adversely impacted.
These conclusions are due in part to the fact that the errors occur in bursts during fades. For example, an average burst size of 1000 bits with a 10-4 BER means that, on average, there are 10,000,000 error-free bits between 1000-bit bursts. For Ethernet packets of 1500 bytes (12,000 bits), a burst size of 1000 bits and a link BER of 10-4 means that 99.9% of the packets do not have to be retransmitted. Similarly, if the average burst size is only 100 bits, a link BER of 10-6 means 99.99% of the packets do not require retransmission. These errored-packet retransmission rates have no discernible effect on network performance. ATM network performance is similar to this Ethernet example, since the 53-byte ATM cells are transported in 2430-byte SONET frames (OC-3).
Unlike data networks, real-time voice or video applications cannot retransmit errored data. However, digital voice only requires an average BER of 10-4, and a BER of 10-5 to 10-6 is necessary for end-user video reception. Hence, many of our links can readily address these applications. We have performed field tests of the our laser systems with NTSC video transmission over a link of 7.3 km and have observed excellent video and audio quality. Compressed digital video could also be transported with MPEG-2 coding for multichannel video on both of LSA's laser systems.
FCC Product Certification
The Federal Communications Commission (FCC) requires that all products sold in the United States meet certain radio-frequency emission standards. The system's transceivers, power supplies, and media converters are FCC compliant. FCC compliance assures that our laser communication systems are safe to operate in the presence of other compliant equipment, including RF equipment on microwave towers.
Uninterruptible Power Supply
Transceiver performance reliability can be increased by the use of an uninterruptible power supply that provides line conditioning, surge protection, and battery backup. LSA Photonics offers this as an option.
Lightning Arrestor
Use of a lightning arrestor is strongly recommended as with any equipment operated outdoors. Lighting arrestors are installed by electricians or building maintenance personnel. A surge protector is not a lightning arrestor.
Wireless links are used in applications that can tolerate an occasional outage since they have some availability limitations due to bad weather conditions. In general, availability is very high (95% to 100%), depending on the link range, with only occasional outages which are typically of short duration. Microwave links are affected more by rain, whereas laser links are affected more by fog. Rain tends to occur at all times of the day, but fog tends to occur mostly during the early morning hours when availability is often less crucial.
In cases where link availability is more critical, one can achieve high availability by limiting the links to shorter ranges in order to achieve better penetration in bad weather. Additionally, a redundant or backup communication path (such as a low-cost dial-up, ISDN, or T1 line) could be used to provide an alternate link during bad weather. In other cases, occasional outages are of minimal concern, or the link is not needed when the weather is exceptionally bad (e.g., video or data relay from field tests that are generally performed in good to fair weather).
The SupraConnect™ high-speed laser-communication systems are designed to communicate through moderate fog and rain. During tests at various distances within the operating ranges of both units, we routinely communicate through fog and rain that prevent us from seeing the other end of the link with the unaided eye or with a viewing scope.