Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2002-12-04
2004-11-16
Nguyen, Tu T. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06819412
ABSTRACT:
BACKGROUND OF THE INVENTION
Nonlinear effects, such as self-phase modulation, cross-phase modulation and four-wave mixing, are known in optical transmission systems; in particular, in transmission systems which operate on the WDM principle (wavelength division multiplexing). These cause signal distortion in the optical signal to be transmitted in the optical fiber. Nonlinear effects such as these in an optical fiber can be described by the nonlinearity coefficient.
In order to determine the nonlinearity coefficient of an optical fiber, the publication by Y. Namihira, A. Miyata, N. Tanahashi, “Nonlinearity coefficient measurements for dispersion shifted fibres using self-phase modulation method at 1.55 &mgr;m”, Electronic Letters, 1994, Vol. 30, No. 14, pages 1171-1172, for example, discloses a measurement arrangement in which the nonlinearity characteristics of an optical fiber are determined by using the self-phase modulation method. Measurement methods such as these are dependent on access to the start and end of the optical fibers to be measured, although this involves considerable measurement effort and is virtually impossible in already existing optical communications networks; that is to say, in optical fibers which have already been laid. In addition, a separate return channel is required from the fiber end to the fiber start in order to transmit the measured information.
An object to which the present invention is directed is to improve the determination of the nonlinearities in an optical fiber, and to allow the nonlinearities of an optical fiber to be measured at one end; that is to say, at the start or at the end of the optical fiber.
SUMMARY OF THE INVENTION
A major aspect of the measurement method according to the present invention is that, in a first step, at least one optical test signal is injected into the optical fiber, whose test signal power is varied, and a first onset threshold for the stimulated Brillouin scatter is determined on the basis of the change in power of the backscattered optical signal. Furthermore, in a second step, in addition to the optical test signal, at least one modulated optical pump signal is injected with a predetermined pump signal power and at a first pump wavelength into the optical fiber, and a second onset threshold for the stimulated Brillouin scatter is determined on the basis of the change in the optical test signal power. Finally, the nonlinearity coefficient of the optical fiber is determined by evaluation of at least the first and the second onset threshold, of the test and pump signal parameters, and the fiber parameters. It is particularly advantageous that the measurement method according to the present invention makes it possible to determine the nonlinearity coefficient via a measurement at only one end; that is to say, at the receiving end or transmitting end. This is an enormous advantage, particularly for the determination of the fiber nonlinearities of optical fibers which have already been laid.
In a second embodiment of the measurement method for determining the nonlinearities in an optical fiber, in a first step, at least one optical test signal is injected with a test signal power and at a test signal wavelength into the optical fiber, and the power of the backscattered optical signal is measured, and a first ratio is formed from the injected test signal power and the power of the backscattered optical signal. Furthermore, in a second step, in addition to the optical test signal which has a test signal power and is at a test wavelength, at least one modulated optical pump signal is injected with an adjustable pump signal power and at a first pump wavelength into the optical fiber, and the power of the backscattered optical signal is measured, and a second ratio is determined from the injected test signal power and the power of the backscattered optical signal. Here, the adjustable pump signal power of the modulated optical pump signal is increased or decreased until the second ratio matches the first ratio. In this case, the nonlinearity coefficient of the optical fiber is then determined by evaluation of the test and pump signal parameters as well as the fiber parameter. The variation according to the present invention of the pump signal power of the modulated optical pump signal alternatively makes it possible to determine the nonlinearity coefficient of the optical fiber by ratio formation, evaluating the observed fiber parameters and trial parameters.
A further advantage of the measurement method according to the present invention is that the test and pump signal parameters which are evaluated on the basis of the first variant of the measurement method according to the present invention are the test signal wavelength, the predetermined pump signal power, the first pump wavelength and the modulation frequency of the optical pump signal. Furthermore, the test signal power, the test signal wavelength, the pump signal power that is set, the first pump wavelength, and the modulation frequency of the optical pump signal are evaluated as the test and pump signal parameters; crucial for the second embodiment of the measurement method according to the present invention.
Theoretical principles relating to the measurement method according to the present invention for determination of the nonlinearities and the dispersion in an optical fiber will be explained in the following text.
In optical fibers, the nonlinear effect of “stimulated Brillouin scattering (SBS)” occurs as a function of the injected power of an optical test signal or signal. This narrowband SBS effect with a line width of &Dgr;&ngr;
B
≈25 MHz, which is governed by the phonon life is known (in this context, see Govind P. Algrawal “Nonlinear Fiber Optics”, Academic Press, 1995, pages 370 to 375). Furthermore, U.S. Patent Specification U.S. Pat. No. 3,705,992 disclosed the onset threshold for SBS being increased in proportion to the ratio of the spectral width &Dgr;&ngr;
s
of the optical signal which is injected into the optical fiber to the line width &Dgr;&ngr;
B
; that is to say,
I
SBS
~&Dgr;&ngr;
s
/&Dgr;&ngr;
B
where I
SBS
=intensity of the injected optical signal at the SBS onset threshold
In this case, the governing factor for reaching the SBS onset threshold is the energy which is spectrally integrated in a frequency separation of width &Dgr;&ngr;
B
. In standard monomode fibers, the SBS onset threshold occurs, for example, at slightly below 10 mW for unmodulated optical signals or test signals, and at a level which is higher by a factor of 2 to 3 dB for binary amplitude-modulated optical signals. The increase for binary amplitude-modulated optical signals is due to the fact that the optical signal power is shared between modulation sidebands and the carrier signal and, particularly at data rates in the Gbit/s range, the power of the data signal is distributed over a broad spectral band.
In the case of amplitude-modulated signals, SBS leads to signal distortion due to overmodulation (see, in particular, H. Kawakani, “Overmodulation of Intensity modulated Signals due to stimulated Brillouin scattering”, Electronic Letters, Volume 30, No. 18, pages 1507 to 1508), since the carrier of the amplitude-modulated optical signal, in which the spectral energy density for chip-free modulation is identical to the laser light source, essentially experiences severe additional attenuation due to the SBS.
The SBS onset threshold can be increased considerably by considerably reducing the spectral energy density of the optical signal, integrated over a frequency band of width &Dgr;&ngr;
B
. Thus, in the case of amplitude-modulated optical signals, the carrier signal power, measured with a resolution of &Dgr;&ngr;
B
, should be reduced to values which are considerably below the SBS threshold power. A reduction such as this can be achieved by frequency modulation or phase modulation.
The SBS effects in the optical fiber occur essentially within the first 20 km (effective length L
eff
) in a standard monomode fiber. In this case, the optical signal requi
Glingener Christoph
Gottwald Erich
Bell Boyd & Lloyd LLC
Siemens Aktiengesellschaft
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