Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2001-08-31
2004-03-23
Nguyen, Tu T. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06710862
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to fiber optic communications where an electrical signal is converted to a light signal, which is transmitted through a fiber to a distant receiver, where it is converted back into the original electrical signal. Such communications systems have many advantages. A signal can be sent to relatively long distances without being amplified; there are no interference problems from nearby electrical fields; a relatively large number of electrical signals can be concurrently transmitted; and the fiber is relatively light and small.
One of the problems with fiber optic communications systems is that the light signal fades or loses power, in other words, becomes attenuated, as it travels along the fiber. Light is attenuated by being absorbed into the fiber, by leaking out of the fiber (due to imperfections or due to excessive bending of the fiber), by being scattered due to Rayleigh scattering, and by being reflected due to Fresnel reflection (which occurs when there is a sudden change in the density of the material through which the light is traveling, such as occurs at the ends of the fibers, and at fiber breaks).
It is important to know how much attenuation occurs in a length of fiber before the fiber is used in a communications system, and also, it is important to determine whether excessive power loss occurs once the fiber has been placed in a communications system. Such excessive power loss may be due to excessive bending of the fiber, due to fiber damage caused by excavators, hammers, and the like, and due to imperfect coupling or splicing of fiber ends.
Once the fiber is used in a communications system, it is important to assess the magnitude of any attenuation through the entire length of the fiber, and also to detect where any excessive power loss is occurring so that remedial action may be taken. Also, many job specifications require that in order for a contractor to be paid for placing the fiber optic communications system, the power loss at any splice must not exceed a certain magnitude.
A widely used method of determining light attenuation in a fiber utilizes an optical time domain reflectometer (“OTDR”). In general, an OTDR sends one or more pulses of laser light through the optic fiber. Each pulse has a predetermined width or time duration, and the interval between pulses is also predetermined. The pulse of laser light traveling through the fiber is somewhat akin to a flashlight being shined into fog (which creates a backscatter of light) or shined through a window (which causes a reflection of some light). The OTDR measures the amount of light being sent backward through the fiber as being representative of the amount of light attenuated. Although the OTDR measures only the amount of light being sent back through the fiber, and not the amount of light being transmitted through the fiber, there is a very close correlation between the two amounts.
The OTDR includes a very precise photodetector that measures the power level of light coming back through the fiber. The OTDR also includes a very precise and sensitive clock that knows when the laser pulse is fired into the optic fiber and when light is sensed by the photodetector. Since light travels in a vacuum faster than it travels in matter (the ratio between the two being called the index of refraction of the matter) and since the index refraction of the fiber is generally known, the OTDR may calculate the distance along the length of fiber where light has been attenuated and the magnitude of that attenuation.
The OTDR may be coupled with a controller to create a graph of light signal level (on the Y axis) and distance along the optic fiber (on the X axis) and to plot a series of data points based upon a sampling of the photodetector and the clock. The series of points may be connected together in what is known as a trace.
The accuracy of the signal level trace is dependent upon the accuracy of the photodetector as well as the correlation between the amount of light traveling back through the fiber as compared with the amount of light transmitted through the fiber. The accuracy of the distance of the feature in the fiber causing the signal loss from the end of the fiber into which the laser is fired is dependent upon the pulse width, the precision of the clock, and the accuracy of the index of refraction (throughout the length of the fiber) and to some degree is dependent upon the spacing of the data points that are used to form the trace.
Normally, the wavelength of the laser light in the OTDR is the same as the wavelength of light to be transmitted for communications purposes through the fiber. Also, the fiber is normally tested by sending a laser pulse down each end of the fiber in what is known as a bi-directional test.
FIG. 1
shows an exemplary trace utilizing an OTDR. The relatively gently, linearly sloped regions indicate attenuation due to backscatter, whereas the more pronounced sloped regions of the trace indicate so-called “events” which cause more severe power loss, as best shown in FIG.
2
. Note that events caused by certain phenomena (e.g., a splice) possess a characteristic, unique signature or waveform indicative of the type of phenomena. These events are of special importance because they suggest some irregularity in the fiber that might need correction or remedy. For example, a poor splice of fiber ends (usually through either melting the ends together in a fusion or through a mechanical connector) might require that the splice be remedied. Also, cracking of the fiber that might have been caused by an excavator inadvertently pressing against the fiber, might require corrective action. Consequently, it is important to locate where events happen within the fiber and to determine the magnitude of power loss at each event so that the problem may be remedied efficiently. For example, the location might be at a manhole, at a pedestal, on a particular pole, or at a construction site.
Although an event happens at a particular point or within an extremely short range of distance within a fiber (such as where a fiber end is spliced to the end of a different fiber), the trace will show that the power loss occurs over a short distance as best shown in
FIGS. 1-3
. For example, the OTDR trace of a fiber might indicate that the event of a splice starts at 10.0 kilometers and ends at 10.1 kilometers from the fiber end through which the laser is fired, where in actuality, the splice is 10.005 kilometers from such end. Thus, an event on a trace is said to have “extent”. Such extent is caused by the width of the laser pulse as well as the natural intervals caused by data point sampling of the return light. Each event is also deemed to have a “start” and an “end”, in accordance with conventional standards, for example, as indicated by the two vertical dashed lines in FIG.
3
. Also, not every relatively sudden attenuation is deemed to be an “event”. Various conventional parameters determine whether a power loss has characteristics sufficient to deem the power loss an “event”.
Most fiber optic cables include a plurality of optic fibers, with cable being currently commercially available with up to 432 such fibers in what is known as “432 count” cable. Each fiber within the cable is coded and is typically tested for light attenuation using an OTDR in the manner described above.
In addition to the imperfections of distance accuracy previously mentioned, the problem of locating an event is further compounded because a fiber optic cable may possess many strands of fiber helically wrapped around a central supporting core such that the length of the outer fibers is longer than the length of the inner fibers over the distance of the cable. When considering the different lengths of fiber wrapped in a cable, the same event may be located at different distances from the fiber ends. For example, if there is a splice of fibers at a particular manhole, the OTDR trace might show that the event happens at a location starting at 10.0 kilometers and ending at 10.1 kilomet
Soule Gregory
Wilson David Lewis
NetTest (New York) Inc.
Nguyen Tu T.
LandOfFree
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