Optical communications – Transmitter and receiver system – Including optical waveguide
Reexamination Certificate
2001-05-03
2004-09-07
Chan, Jason (Department: 2633)
Optical communications
Transmitter and receiver system
Including optical waveguide
C398S088000, C398S025000, C398S113000
Reexamination Certificate
active
06788901
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to beam formation for free-space optical communication systems. The improved free-space optical communication systems can be used for two-way information transfer between remote objects without any wires and/or optical fibers for connection of these objects, including the case when there are many objects taking part in the information exchange, e.g. for organization of a point-to-multipoint exchange, i.e., a two-way information exchange between a base station transceiver terminal and several subscribers.
BACKGROUND
Through fibers, optical communications provides high-speed data transmission over relatively long distances, for a wide range of applications and services. The use of fiber, however, is not always practical and/or cost effective. Radio frequency (RF) wireless solutions reduce the time, complexity and cost of installation, but those solutions are inherently limited by their use of shared RF spectrum which is narrow compared to optical spectrum. As the number of users on a given piece of spectrum, the average capacity available to any one user further declines.
Another alternative approach to data communication services involves free-space optical communications. There have been a number of proposals to supply data signals to a laser, couple the laser output to an optical system, transmit the optical signal via line of sight, and recover the information at a remote receiver. Such systems offer two-way information transfer between remote objects without use of wires and/or optical fibers. Because such systems utilize optical radiation characterized by extremely high carrier frequency and can implement non-interfering links to the individual customer premises, such systems are not subject to the limits imposed by the carrier frequency or shared capacity, as in the existing RF and microwave wireless technologies.
In many embodiments of such free-space optical communication systems, the optical radiation from a light source propagates from the transmit terminal to the subscriber from a source of light (in most cases, a laser) with a modulator, driven by a data stream, through a light guide (optical fiber) to an optical antenna (telescope or other optical collector) forming a sufficiently narrow light beam which propagates through free space to a receiving optical system and through another optical fiber to a photodetector. If all other factors are the same, the optical radiation power losses along the above path depend on the geometry (length, diameter, etc.) and type of the optical fibers used.
Free space optical communication systems implemented previously have utilized single mode fibers for transport of the beams from the laser sources to the optical emitting antenna elements. In such an implementation, the signal radiation is guided to the collector by a thin (single-mode) fiber. As known, the radiation passed through such fibers does not have any local minima. Accordingly, there are no such minima in the receive aperture plane (at least if the propagation path has sufficiently high optical quality). An example of such a system is described in Szajowski et al.,
Eight
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channel Video Broadcast Feed Service using Free
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Space Optical Wireless Technology at Sydney
2000
Olympic Games
, Optical Wireless Communications III, Proceedings of SPIE, Vol. 4214, Nov. 6-7, 2000, pp.1-10.
A drawback of a system with a single-mode fiber is that small diameter of the fiber makes it difficult to obtain high efficiency of the radiation coupling into the fiber from a radiation source, which usually is a laser diode. Commercially available devices comprising a laser diode and a single-mode fiber coupled to it (so-called “pigtailed” laser diodes) typically have a radiation coupling efficiency of 25-30%. In other words, the interface between the laser diode and the fiber is actually attenuating the light power by 3-4 times, causing a decrease in the communication range and availability.
Another drawback in using a single-mode fiber is that the light beam formed by it is not resistant to optical inhomogeinities of the free-space optical path. Experiments accomplished by the authors of this invention have demonstrated that, if there are aberrations in the optical path located close to the transmit aperture (rain drops or other small scale aberrations on protecting optical surfaces, such as windows through which the output radiation passes, etc), considerable nonuniformities appear in the transverse intensity distribution of the light on the receive aperture at the remote station. The negative effect of such spatial intensity fluctuations on the quality of communications has already been discussed above. The authors' experiments proved that the use of a multimode fiber to deliver the optical radiation to the transmitting optical antenna decreases contrast of intensity fluctuations caused by the small-scale optical inhomogeinities located close to the antenna.
Additionally, the manufacturing of the pigtailed laser diodes with single-mode fibers is difficult, and thus the cost of such devices is significantly higher than the costs of the components taken separately, that is to say a laser diode and a fiber applicable for free-space optical communication systems.
The use of a multimode optical fiber, which has a diameter significantly larger than that of a single-mode fiber strongly increases the efficiency of radiation coupling into the fiber, makes alignment substantially easier and considerably reduces the cost of a pigtailed laser diode. However, the multimode fiber creates a different set of technical problems.
In a transmitting system using a fiber with a relatively large core diameter (a multimode fiber), the optical radiation field becomes spatially non-uniform after propagating along the fiber. The radiation field has local maxima and minima in cross-sectional intensity distribution (this is so-called speckle-pattern), with large differences in magnitudes between them. Thereby an optical field with high contrast of the light intensity spatial fluctuations is formed. The fluctuations do not vanish after the radiation passes through the collector; the light beam remains spatially nonuniform along the whole propagation path, including at the receive system aperture of a remote receiving device or system, to which data stream carried by the beam is addressed.
If the receive aperture occasionally coincides with a local optical field intensity minimum, the quality of communication may degrade, which may even break communication because of insufficient received signal power entering the aperture in view of such intensity minimum.
In principle, this effect may be compensated by a manifold increase of the transmitter output power, but such compensation is not practical for technical and cost considerations.
Another way to mitigate the intensity non-uniformity effect is to increase the receive aperture size till it significantly exceeds the average speckle size in the speckle-pattern. In this case the receive aperture always captures several speckles, and the photodetector responds to the optical field intensity averaged over the cross-section of the aperture. However, for a given size of the transmit aperture, when the distance to the receiver system increases, the average speckle size also increases, thus requiring a corresponding increase of the receive aperture diameter, which is not always practical. For a given distance to the receiver, it is also possible to make the speckles size smaller by increasing the transmit aperture diameter. This solution also has the limitations related to: size, weight and cost of transmit optical telescopes (collectors).
In principle, it would be theoretically possible to suppress the fluctuations by spatial decoherentization of the optical radiation, which decoherentization leads to a reduced contrast of interference patterns, including speckle-patterns, created by the optical field. A necessary condition for the decoherentization is the radiation polychromaticity. The polychromatic radiation source
Leshev Aleksei A.
Ragulsky Valery V.
Sidorovich Vladimir G.
Vasiliev Mikhail V.
Vasiliev Vladimir P.
Chan Jason
McDermott & Will & Emery
Meklyn Enterprises Limited
Payne David
LandOfFree
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