Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-07-13
2001-06-05
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06243182
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an open-air optical communication system that avoids signal degradation due to attenuation and scattering.
2. Description of the Prior Art
Open-air optical communication systems have been available for decades and cover the range from single RS-232 units operating at 1200 bps to high-speed ATM units capable of live video broadcasts. While all conventional optical communication systems operate with weather related constraints, continuing advances in bandwidth improvement, cost reductions, and the minimization of atmospheric affects have aided in bringing optical communication systems into the mainstream of communication products available to the telecommunications engineer. Exemplary developments in this regard are reported in the following recent technical papers: T. Wang, G. R. Ochs, and S. F. Clifford, A Saturation-resistant Optical Scintillometer to Measure C
n
2
, J. Opt. Soc. Am.
68, 334 (1978); S. F. Clifford, G. R. Ochs, and R. S. Lawrence, “Saturation of Optical Scintillation by Strong Turbulence”,
J. Opt. Soc. Am.
64, 148 (1974); R. S. Lawrence, G. R. Ochs, and S. F. Clifford, “Measurements of Atmospheric Turbulence Relevant to Optical Propagation”,
J. Opt. Soc. Am.
60, 826 (1970); G. R. Ochs, and Ting-I. Wang, “Finite Aperture Optical Scintillometer for Profiling Wind and C
n
2
”, Appl. Opt.,
vol. 17, No. 23, 3774-3778 (1978); R. M. Gagliardi and S. Karp,
Optical Communications,
John Wiley & Sons, Inc., New York, 1995; C. P. Primmerman, et. al., “Atmospheric-Compensation Experiments in Strong-Scintillation Conditions”,
Applied Optics
34, No. 12, p. 2081-2088, 1995; and J. H. Shapiro, “Imaging and Optical Communications Through Atmospheric Turbulence”,
Laser Beam Propagation in the Atmosphere,
J. W. Strohbehn, Ed., Springer-Verlag, New York, p.171-222, 1978.
Despite significant advances in the field of open-air optical communication, the development of such systems has been hampered by certain basic, underlying effects upon open-air optical communication systems that are unique to this type of communication. Specifically, the atmospheric optical channel may be seen as clear air, or it may contain particles from dust, fog, mist, or precipitation. When a light beam passes through the atmosphere containing fog, rain, or other particles, both attenuation and scattering occur. A collimated beam broadens due to the scattering, thus resulting in losses in signal strength. During heavy fog or snow, the light beam is totally obscured. Under such conditions no light can be transmitted to the other end of the communication system so that the open-air communication channel is interrupted. Essentially, there is no simple solution to overcome the basic limitations imposed by the laws of physics.
However, even in clean air conditions, atmospheric turbulence-induced optical scintillation may severely affect the quality of optical communications systems. Atmospheric turbulence induced optical scintillation is particularly important to understand when designing wireless communication solutions using an optical device. The shimmering eddies seen above a hot surface and the twinkling of stars are examples of turbulence induced optical scintillation. Temperature gradients within and between these eddies cause refractive index changes on the light as it passes from a transmitter to a receiver through these eddies. These changes act as additional optical lenses that orient and refocus the optical beam. Most of the light intensity fluxuation that occurs is a result of the refraction of the beam of light. That is, it results from scattering rather than attenuation.
SUMMARY OF THE INVENTION
Despite certain basic limitations, open-air optical communication systems do have some very significant advantages. Specifically they do not require any buried or overhead cable systems, which are extremely expensive to construct and which present considerable maintenance difficulties. Also, open-air optical communication systems are relatively insensitive to electrical disturbances from sources such as lightning, transmission in proximity to power lines, and fluxuations in solar radiation. Moreover, the components employed in open-air optical communication systems are typically quite reliable. Therefore, open-air optical communication systems do present considerable advantages, particularly if the problems arising from optical scintillation can be solved.
The present invention provides one of the most important and cost effective solutions to the minimization or even elimination of optical scintillation problems. This invention largely obviates the problem of optical scintillation by utilizing a “multiple optical receiver” design. A system constructed according to the invention integrates two or more receivers to overcome fading or scattering conditions that may be caused by optical scintillation or beam wandering induced by atmospheric turbulence. The multiple receiver design actually receives two or more signals through different optical paths from the same optical transmitter. All of these signals are combined to yield one composite received signal that is better than any of the individual signals.
For the open-air optical communication system of the present invention to be most effective, each optical receiving lens employed in the receiving unit should be spaced from any other receiving lens in that unit by a distance at least equal to the diameter of the lens in a plane oriented perpendicular to the collimated optical beam received. This configuration ensures that all received signals are subject to independent scintillating effects. By processing signals that are not subject to the same scintillating effect, significantly improved performance is achieved. Also, an automated gain control (AGC) circuitry is employed to further eliminate any signal fluxuations caused by atmospheric phenomena such a turbulence, fog, smoke, dust, rain, snow, etc.
For a total of n channels there are a corresponding number of independently received, incoming optical signals. That is, for n channels the system produces the following corresponding signals: S
1
, S
2
. . . S
n
.
In the automatic gain control circuitry, these signals are combined. That is, the combined signal S results from combining all of the received signals together (S=S
1
+S
2
+ . . . S
n
).
In any open-air optical communication system, there must be a minimum detection threshold for a signal in order for the system to have a satisfactory performance. This minimum threshold may be indicated by the term S
th
. For any signal S
i
(i=1, 2, . . . n)<S
th
the system will fail.
If the probability that each signal S
i
<S
th
is p, then the probability for the combined signal S<S
th
is p
n
.
This mathematical relationship provides a huge advantage for the combined signal to have a satisfactory performance. For example, if for each individual channel the failing probability p=10
−3
, a single channel system will not provide a satisfactory performance one out of a thousand times. Using a dual receiver system according to the present invention, on the other hand, the probability of failure is reduced to 10
−6
. That is, the system will fail only one time in a million. The improvement is a factor of one thousand. If more than a pair of receiving lenses and photodetectors are employed, the improvement is even more significant.
In the foregoing calculations it is assumed that all received signals are passing through independent paths. To ensure this independence of the atmospheric turbulence induced optical scintillations, the separation of the receiving lenses must be at least larger than the diameter of the lens.
In one broad aspect the present invention may be considered to be an atmospheric turbulence resistant optical communication system. This system is comprised of a transmitter and a receiver. The transmitter includes a waveform shaping modulator, an optical source, and beam forming optics for producing a co
Optical Scientific, Inc.
Pascal Leslie
Singh Dalzid
Thomas Charles H.
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