Optical communications – Diagnostic testing – Determination of communication parameter
Reexamination Certificate
2000-02-14
2003-09-30
Negash, Kinfe-Michael (Department: 2633)
Optical communications
Diagnostic testing
Determination of communication parameter
C398S079000, C398S147000, C398S192000
Reexamination Certificate
active
06626588
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical transmission apparatuses and, more specifically, to an optical transmission apparatus capable of efficiently transmitting an optical signal of large power with better transmission characteristics through an optical fiber by suppressing stimulated Brillouin scattering (SBS).
2. Description of the Background Art
As known, in optical fiber transmission, when an optical signal with large optical power of 10-odd dBm is supplied to an optical fiber, part of the optical signal is reflected due to stimulated Brillouin scattering (SBS) within the optical fiber, and as a result, optical power of an output optical signal is decreased.
A conventional optical transmission apparatus capable of transmitting an optical signal of large power with better transmission characteristics through an optical fiber is disclosed in “Optical Communications System” of Japanese Patent Laying-Open No. 9-69814 (1997-69814).
FIG. 9
is a block diagram showing the configuration of the conventional optical transmission apparatus disclosed in the above publication.
In
FIG. 9
, the conventional optical transmission apparatus includes a pilot signal generation part
100
, an electro-optic conversion part
101
, an optical fiber
102
, and an opto-electric conversion part
103
.
The pilot signal generation part
100
generates a pilot signal. The electro-optic conversion part
101
converts an input signal (electric signal to be transmitted) and the pilot signal into an optical signal through direct intensity modulation. The optical fiber
102
guides the optical signal. The opto-electric conversion part
103
converts the optical signal into an electric signal.
Described below is the operation of the conventional optical transmission apparatus.
First, the electro-optic conversion part
101
is supplied with an input signal and a pilot signal, and converts these signals into an optical signal. The optical signal outputted from the electro-optic conversion part
101
goes through the optical fiber
102
to the opto-electric conversion part
103
. The opto-electric conversion part
103
re-converts the received optical signal into an electric signal.
In the above operation, when electro-optic conversion is performed through direct intensity modulation, the optical signal obtained by conversion is not only intensity-modulated but also frequency-modulated (chirping). Multiplexing the input signal with a pilot signal having a frequency lower than that of the input signal causes the spectrum of the optical signal to spread over a wide band due to frequency modulation, thereby resulting in reduction in power of a main mode (maximum optical carrier component). SBS does not occur when the power of the main mode becomes less than a predetermined threshold.
The conventional optical transmission apparatus thus suppresses SBS by multiplexing the input signal with the pilot signal.
Furthermore, in the above publication, multiplexing the input signal with the pilot signal poses two problems: deterioration of transmission characteristics, and occurrence of intermodulation distortion between the input signal and the pilot signal. To solve the former problem, according to the publication, it is preferable to set the optical modulation index of the pilot signal to an appropriate value for suppressing SBS. To solve the latter, it is preferable to set the frequency of the pilot signal less than half of a Brillouin gain band.
However, in the former problem, the appropriate value for suppressing SBS is not necessarily clear and, therefore, a good transmission characteristic can not be ensured.
Furthermore, in the latter, the intermodulation distortion is not sufficiently eliminated only by setting the frequency of the pilot signal less than half of the Brillouin gain band.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide an optical transmission apparatus capable of suppressing SBS by multiplexing an input signal with a pilot signal and further eliminating an adverse effect of intermodulation distortion between the input signal and the pilot signal, thereby allowing efficient transmission of an optical signal of large power with better transmission characteristics through an optical fiber.
The present invention has the following features to solve the problems above.
A first aspect of the present invention is directed to an optical transmission apparatus transmitting an optical signal through an optical fiber. The apparatus in accordance with the first aspect of the present invention comprises a pilot signal generation part for venerating a pilot signal. a multiplex part for multiplexing an input signal (electric signal to be transmitted) with the pilot signal, an electro-optic converter for converting an electric signal outputted from the multiplex part into an optical signal through direct intensity modulation, the optical fiber for guiding, the optical signal outputted from the electro-optic conversion part, and an opto-electric conversion part for converting the optical signal guided by the optical fiber into an electric signal. When the input signal is a frequency-multiplexed signal obtained by multiplexing a plurality of signals aligned on a frequency axis at regular intervals &Dgr;f (&Dgr;f>0), the pilot signal generation part venerates the pilot signal having a frequency {m−(1/2)}×&Dgr;f (m is an arbitrary natural number).
In the first aspect, intermodulation distortion (second order distortion) between each signal included in the input signal and the pilot signal occurs exactly at the center between two consecutive signals on the frequency axis. Therefore, of the intermodulation distortion between the input signal and the pilot signal, second order distortion, which deteriorates transmission characteristics most, can easily be cut off by a filter. It is thus possible to suppress SBS by multiplexing the input signal with the pilot signal, and also to effectively eliminate an adverse effect of the intermodulation distortion between the input signal and the pilot signal. Consequently, an optical signal of large power can be efficiently transmitted with better transmission characteristics through the optical fiber.
According to a second aspect, further to the first aspect, a frequency of each signal included in the input signal is set to {n−(1/2)+C}×&Dgr;f (n=1, 2, 3, . . . and C is 0 or an arbitrary positive integer).
In the second aspect, the intermodulation distortion (second order distortion) between the signals included in the input signal occurs exactly at the center between two consecutive signals. Therefore, such second order distortion can also be cut off when the intermodulation distortion (second order distortion) between the input signal and the pilot signal, which occurs also exactly at the center between two consecutive signals, is cut off by the filter.
A third aspect of the present invention is directed to an optical transmission apparatus transmitting an optical signal through an optical fiber. The apparatus in accordance with the third aspect of the present invention comprises a pilot signal generation part for generating a pilot signal, a level adjusting part for adjusting a level of the pilot signal, a multiplex part for multiplexing an input signal (electric signal to be transmitted) with the pilot signal level-adjusted by the level adjusting part, an electro-optic conversion part for converting an electric signal outputted from the multiplex part into an optical signal through direct intensity modulation, the optical fiber for guiding the optical signal outputted from the electro-optic conversion part, an opto-electric conversion part for converting the optical signal guided by the optical fiber into an electric signal, a reflected light monitoring part for monitoring reflected light caused by stimulated Brillouin scattering (SBS) in the optical fiber, and an optical coupler for supplying the reflected light propagating thr
Fuse Masaru
Ikushima Tsuyoshi
Sasai Hiroyuki
Matsushita Electric - Industrial Co., Ltd.
Negash Kinfe-Michael
Wenderoth , Lind & Ponack, L.L.P.
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