Various communication systems are known for transmitting through the atmosphere. Most commonly, microwave communication devices are used for this communication. Additionally, various optical techniques for communication are known.
Microwave technologies used for data links through the atmosphere generally suffer from low data rates. For example, an 11 to 14 or 20 to 30 gigahertz (GHz) carrier is sometimes used to transmit between a satellite and a ground station. However, modulating 1 gigabit/sec. data rates on the 18 GHz carrier is not currently practically accomplished. By increasing the carrier frequency, the data rate may also be increased. For example, a 60 GHz carrier could support modulation data rates of 1 gigabit/sec. or higher. However, the atmosphere itself attenuates the 60 GHz carrier such that transmission through the atmosphere is not practical.
Terrestrial-based, free-space optical (laser) transmission systems are being considered as a replacement for free space microwave communications systems. Laser systems generally enable data to travel between a ground-based transmitter and a ground-based receiver at high rates a data rates. In laser communication systems, an optical carrier is modulated with data to transmit information through the atmosphere. Adaptive optic subsystems may be used to focus the light on a receiver, thereby improving beam quality. The focused light can be collected into a single mode or multimode optical fiber and transmitted via the optical fiber to a de-multiplexing element.
Although laser systems can realize high data transmission rates, they can have significant drawbacks.
For example, conventional non-diffractive telescopes focus not only the optical signal but also sunlight onto other optical components. This can cause significant damage to the components, saturate the system, or introduce noise into the optical signal, causing higher bit error rates (BER).
Laser receivers can demultiplex different wavelength bands within the received power that is converging to a focus from the primary optical lens. Such demultiplexing typically requires recollimation of the received beam, reflecting the beam off a line-of-sight stabilization mirror, routing through an optical fiber, collimation after passage via the fiber to a series of bandpass filters, each of which diverts a particular wavelength to an exit lens and fiber for routing to a detector unit. This cumbersome configuration can suffer from significant optical losses, is tedious, and costly. Laser systems typically require the transmit and/or receive telescopes to be large (e.g., have an aperture of more than 20 inches) and therefore expensive. By way of example, when a telescope's aperture size is increased from 10 to 20 inches the cost also increases by a factor of 5 or 6. The need to collect the light onto the optical fiber introduces additional size and expense. The fiber can normally only receive light that is within a 30° cone, which, for a large receive aperture, requires a long optical path from the telescope to the fiber.
Laser receivers can have high BERs. BERs can be increased by the effects of atmospheric turbulence. The effects of atmospheric disturbances from thermal air currents along the optical path are typically greatest at the transmitter and receiver. For a terrestrial link, the transmitter and receiver are typically (compared to other parts of the optical path) in closest proximity to thermal sources, namely solid surfaces such as the ground and rooftops, which absorb solar flux and warm the air. The warm air rises through cooler air thereby generating turbulence and providing a non-uniform propagation medium for the optical signal. At the transmitter, such disturbances can cause different portions of the optical beam to pass through differently moving air currents, causing the propagating beam to break up, so leaving the receive aperture in an intensity minimum. Similar disturbances near the receive aperture can cause the focal spot to break up, leaving the detector in an intensity minimum. Each of these distinct processes can produce burst errors in the communicated data and must be addressed separately. As will be appreciated, a burst error refers to a period of optical extinction. The focal spot of the optical beam can be smeared. If the received optical signal power has passed through air, atmospheric turbulence will inevitably smear out the focal spot somewhat relative to the desired focal spot diameter. Imperfections in the primary optical lens can also smear out the focal spot. Since only small detectors, e.g., 60 microns in diameter, are able to respond fast enough to resolve temporarily the 400 picosecond pulses of wideband, e.g., 2.5 Gbps, data streams, the efficiency of any such detector is downgraded by its inability to collect all the power in the focal spot.