Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1999-05-19
2001-02-27
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S227140, C250S227170, C073S001850, C324S512000, C367S014000
Reexamination Certificate
active
06194706
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to determining above-ground proximity to buried fiber optic cables.
BACKGROUND OF THE INVENTION
Utility and telephony companies bury their fiber optic cables. The exact location of these cables oftentimes becomes uncertain because either the installation records are lost or the above ground landmarks change. This uncertainty presents a problem when these companies or others need to access the cables for upgrades and repairs.
In the prior art, the known methods for locating buried fiber optic cables include post-hole drilling and radio-tone transmission. Not only are these methods costly, the risk in accidentally destroying or damaging the buried cable is high because of exploratory earth drilling. In radio toning, several sparsely-spaced holes are typically dug, resulting in low accuracy and possible position misidentification. Radio toning techniques are also becoming obsolete since cables are being replaced with full dielectrics, as opposed to metal cables, and since the technique cannot reach the distances that are possible in modern fiber span transmission lengths.
It is, accordingly, one object of the invention to provide methods for determining the above-ground proximity to buried optical fiber cables. Another object of the invention is to provide systems and methods for locating buried optical fiber cable, relative to an above ground location, through generation of seismic noises on the ground. Yet another object of the invention is to provide methods and systems for determining the distance to a buried fiber optic cable to an above-ground location in near-real time. Still another object of the invention is to locate submerged fiber cables. These and other objects will be apparent in the description which follows.
SUMMARY OF THE INVENTION
U.S. Pat. Nos. 5,206,065, 5,173,139, 5,114,517, 5,106,175 and 4,697,137 provide useful background information for the invention and are thus herein incorporated by reference.
As used herein, “tap” or “optical tap” refer to known mechanisms, such as an optical splitter, which provide for obtaining a signal from a fiber optic cable. Typically, a tap of the invention is one that generates a small signal from a fiber optic cable at a ratio of approximately 1% or less as compared to the entire signal within the cable, so that significant signal traffic energy is not lost through the tap. However, other taps and ratios can be used as appropriate. An optical tap coupled to a silicon photodiode and associated electronics can be used to detect a signal within the cable.
As used herein, a “terminal” refers to a terminal end of a fiber optic cable; and generally exists for purposes of accessing signal traffic on the cable. One can utilize the terminal as a tap, if desired, to obtain polarization rotation information according to the techniques disclosed herein.
The invention of one aspect is a seismometer system that uses the buried or submerged fiber optic cable as part of the system, though the continuous wave (CW) traffic signals on the cable are not affected. An optical power tap connected to the cable (e.g., at a terminal end or connected to some location with the cable) is used to “tap” energy from the cable; and polarization selective optics (e.g., a polarization cube) isolates the polarization state from the tap for purposes of determining a polarization rotation. A seismic source then creates seismic noises or pulses on the surface (e.g., on the ground, for buried cable, or in water for submerged cable) which travel to the cable, momentarily creating micro-bending in the fiber and thus inducing a momentary rotation in the polarization state of light transmitted through the fiber cable. An optical detector and electronics coupled to the polarization optics provide for converting the polarization rotation to an electrical signal indicative of polarization rotation. As the source moves about the surface of the ground or water, the polarization rotation signal is monitored to determine a maximum rotation, thereby determining a closest proximity to the cable.
Fiber optic cables used in telecommunications are typically non-polarized; and thus the polarization state rotates freely within the fiber cable. Connectors and attached network items are generally polarization insensitive for this reason. In accord with the invention, a polarization sensitive element is connected with the cable, e.g., through an optical tap, so that the polarization rotation can be determined without affecting normal traffic signals through the cable. Note, in particular, that buried optical cables are continually bombarded by noises within the ground and yet operate well. The seismic pulse strength generated by the invention is not so large as to cause disruption or damage to the cable.
The invention thus preferably isolates the intentionally induced seismic pulses to a known frequency to improve detection of the synchronized polarization rotations. Specifically, in one aspect, by creating seismic pulses at a known frequency—e.g., one hertz to 100 hertz, or greater (up to fifty kilohertz or more may be desirable when the sonic pulses need to travel through water, in whole or in part, such as when the cable is submerged)—then synchronization electronics coupled to the tap, polarization selective element and optical detector are used to isolate the seismic pulses relative to other background noises. By way of example, if the seismic pulses are created by a jackhammer-like device that strikes the ground at 50 Hz, then the synchronization electronics seek to isolate the 50 Hz signal to maximize the detection of polarization rotation caused by micro-bending of the cable due to the 50 Hz sonic forces imparted to the cable through the ground.
Preferably, in another aspect, a telecommunications signal is transmitted synchronously with the seismic pulses. By comparing the time between (a) the receipt of the telecommunications signal and (b) the detection of the rotation in the polarization state, a distance between the location of the seismic pulse and the cable is determined. If the time is discerned to one nanosecond, the distance accuracy is approximately one foot.
In yet another aspect, a differential-measuring device such as a lock-in amplifier is used to compare the detection of the polarization rotation with the selected frequency, to reduce noise and improve signal detection. Preferably, the measuring device synchronously and differentially compares the detection with the receipt of the telecommunications signal to lock-in and triangulate on the distance between the seismic pulse and the buried cable.
By way of an operational example according to one aspect of the invention, a person moves to a location approximating the location of the buried cable and generates seismic pulses at a known frequency on the ground. These sonic pulses travel through the ground at a speed dependent upon the medium (e.g., earth or water) until they strike the buried or submerged cable, causing micro-bending in the cable. By monitoring the polarization rotation of light transmitted through the cable, the proximity of the location of the person making the seismic pulses relative to the cable is determined. The person roams the ground continuing to make seismic pulses until a maximum polarization rotation is achieved, indicating a location closest to the cable.
Those skilled in the art should appreciate that alternative detection schemes can be used that are within the scope of the invention. For example, electronic phasing can be adjusted so that a minimum signal detection is sought as opposed to a maximum—with the result being that a closest proximity of the person to the cable is determined.
In yet another operational aspect, if the person generates a telecommunications signal synchronously with the seismic pulses, then two signals are received in time and compared: the first signal representing the receipt of the telecommunications signal and the other representing the signal from indicative of the polarization rotation. The smaller the time dis
Allen Stephone B.
Duft, Graziano & Forest, P.C.
Lucent Technologies - Inc.
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