Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
1999-07-26
2001-05-08
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06229599
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an apparatus for measuring characteristics of an optical fiber. More specifically, this invention relates to apparatus for measuring characteristics of an optical fiber such as beat length, correlation length, and polarization mode dispersion, at different positions along the length of the optical fiber.
BACKGROUND OF THE INVENTION
Within the field of optical fiber telecommunications, the current upper limit on the bit rate of detection arises from the birefringence distributed along the length of a single-mode optical fiber. The birefringence may be due to non-circularity of the core of the optical fiber, and to stresses within the fiber. The birefringence may also vary along the length of the fiber. However, the birefringence may also be caused by external forces acting upon the fiber, and to temperature variations, and these effects vary both along the length of the fiber and with time. The overall birefringence of a length of fiber thus varies over time with a magnitude which is random.
An ideal single-mode optical fiber guides optical power in the fundamental mode as two identical but orthogonal polarization modes, so that the modes are completely interchangeable. However, imperfections in the fiber and the effect of external parameters lead to the optical power within the two polarization modes both differing in magnitude and travelling at slightly different speeds so that a differential group delay exists between the modes. The birefringence determines the magnitude of the differential group delay, and the optical power within the two polarization, and so the width of an optical pulse travelling along the optical fiber will vary randomly by an amount determined by the random fluctuations in the birefringence. This effect is called the polarization mode dispersion, and it is of particular concern because it limits the performance of optical fiber telecommunications systems in a way that cannot be predicted accurately, and hence cannot be compensated.
Although polarization mode dispersion is usually considered to be a characteristic of the total length of an optical fiber, the effects which give rise to it may act over relatively short sections of the fiber. In particular, the polarization mode dispersion of an optical fiber cable may increase significantly after installation, arising from a change in the birefringence over one particular short length within the fiber. Accordingly, it would be useful to be able to measure characteristics related to the two polarization modes of a single-mode optical fiber, such as polarization mode dispersion, at different positions along the length of the optical fiber so that any local effect can be identified at a particular length position in the optical fiber. This would be particularly useful if the measurement were able to be made with access to just one end of the fiber in the same way that optical time domain reflectometers are used to measure the optical loss of fibers. Existing commercial apparatus can measure the overall polarization mode dispersion of an optical fiber and requires access to both ends of the fiber. The existing commercial apparatus does not enable the measurement of the polarization mode dispersion at different positions along the length of the optical fiber. The magnitude of the birefringence as it varies along the fiber may be characterized as a beat length, and the statistical correlation between two sections of fiber may be related by a correlation length. Both these parameters are useful for describing the behavior of the fiber, and the environment it is experiencing, and are inherently characteristics of length position within the fiber.
There are a number of known methods of measuring polarization mode dispersion, and associated characteristics of single-mode optical fibers, which provide a single measurement for the total length requiring access to both ends of the fiber. In addition, optical time domain reflectometry is a well-established technique for measuring the optical loss of an optical fiber at different positions along the length of the fiber and requiring access to only one end of the fiber. The present invention is based on the discovery that it is possible to apply the existing measurement techniques of polarization mode dispersion to modified versions of optical time domain reflectometry apparatus, and thus to derive useful measurements.
OBJECTS OF THE INVENTION
The present invention aims to provide apparatus to measure the beat length, the correlation length, the polarization mode dispersion, and other characteristics of single mode optical fibers related to the two polarization modes of the fiber at different positions along the length of the fiber. The present invention also aims to provide apparatus for the measurement of characteristics of an optical fiber with access to only one end of the fiber.
SUMMARY OF THE INVENTION
According to a non-limiting embodiment of the present invention, there is provided apparatus for measuring characteristics of an optical fiber, at different positions along the length of the optical fiber, which apparatus comprises:
tunable source means for providing optical pulses of light which have a variable wavelength and a narrow wavelength bandwidth;
polarization selecting coupler means which comprises an input port, a bi-directional port and an output port, and which is for conveying light between the input port, the bi-directional port and the output port, such that a state of polarization of the optical pulses of light input at the input port becomes a particular launch state of polarization of light output from the bi-directional port, and such that one or more particular receive states of polarization of light input at the bi-directional port become one or more separate channels of light output from the output port;
optical connector means for making an optical connection between the bi-directional port of the polarization selecting coupler means and one end of the optical fiber, so that the light output from the bi-directional port of the polarization selecting coupler means is launched into the optical fiber and light backscattered within the optical fiber is received as the light input at the bi-directional port of the polarization selecting coupler means;
photodetector means for converting the intensity of each of the one or more separate channels of light output from the output port of the polarization selecting coupler means into one or more separate electrical signals;
launch controller means for controlling the timing, duration and wavelength of the optical pulses provided by the tunable source means, and for specifying the particular launch state of polarization of the light output from the bi-directional port of the polarization selecting coupler means;
receive controller means for controlling the timing of measuring the electrical signals provided by the photodetector means, and for specifying the one or more particular receive states of polarization of light input at the bi-directional port of the polarization selecting coupler means; and
processor means for measuring and processing the electrical signals provided by the photodetector means into measurements of the characteristics of the optical fiber.
The tunable source means may be a semiconductor laser diode, a solid state laser, or a gas laser, with a wavelength bandwidth preferably less than 0.1 nanometers and a peak power preferably greater than one milliwatt. The tunable source means may include optical amplifiers, which may take the form of semiconductor or optical fiber amplifiers. The tunable source means may also include means for defining a specific state of polarization of the light emitted. The light emitted by the tunable source means may be any electromagnetic radiation at wavelengths appropriate for the measurements being made and the optical fiber under test. Preferably, the wavelength of the tunable source means is in the range of approximately 1100 nanometers to 1800 nanometers. The wavelength of the tunable source means may
CSELT - Centro Studi e Laboratori Telecomunicazioni S.P.A.
Dubno Herbert
Font Frank G.
Nguyen Tu T.
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