Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
2001-08-22
2004-02-17
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
C702S066000, C702S067000, C702S068000, C702S070000, C702S071000
Reexamination Certificate
active
06694273
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a receiving apparatus and a method of compensating for waveform degradation of received signals, and further to an apparatus and method for detecting waveform degradation, and an apparatus and method for measuring waveforms. More particularly, the invention relates to a technique suitable for use in compensation for waveform degradation an optical signal suffers due to an optical transmission line.
(2) Description of the Related Art
FIG. 20
is a block diagram showing one example of the existing optical transmitting system. In
FIG. 20
, an optical transmission system
100
is made up of an optical transmission apparatus
200
, an optical repeater (optical amplifier)
300
, and an optical receiving apparatus
400
, with an optical signal sent from the optical transmitting apparatus
200
being transmitted through an optical transmission line
500
to the optical receiving apparatus
400
while being repeated/amplified properly in the optical repeater
300
. Incidentally, although only one optical repeater
300
exists in the illustration of
FIG. 20
, naturally, two or more optical repeaters are employable, or no need therefor arises, depending on an optical signal transmission distance.
As the aforesaid optical transmission line
500
, there has frequently been used a single-mode optical fiber (SMF) of a wavelength (referred to as “zero dispersion wavelength”) at which chromatic dispersion becomes almost zero being in a 1.3-&mgr;m (micrometer) band (see a chromatic dispersion characteristic
600
in FIG.
22
). The “chromatic dispersion” signifies the property that the propagation speed in an optical fiber varies with optical wavelength, stemming from material dispersion (see a broken line
800
in
FIG. 22
) or structure (waveguide) dispersion (see a chain line
900
in
FIG. 22
) of the optical fiber itself (that is, with respect to the zero dispersion wavelength, the long wavelength side delays while the short wavelength side advances). In other words, the “zero dispersion wavelength” means a wavelength at which no advance nor delay occurs while light (wavelength) propagates in an optical fiber.
The reason for the frequent use of SMF as the optical transmission line
500
is because its transmission loss is at a minimum in an optical transmission band (1.55-&mgr;m band) put frequently to use for the WDM optical transmission, and long-distance transmission is feasible. However, the employment of SMF causes the waveform degradation arising from the chromatic dispersion to occur remarkably at high-speed transmission of an optical signal.
For example, in the case of long-distance transmission of a high-speed optical signal exceeding 2.5 Gb/s (gigabit/second) through the use of SMF, there occurs the phenomenon that a waveform degradation occurs in the optical signal due to the chromatic dispersion, that is, the degree of aperture of an eye pattern of the optical signal (which will be referred to hereinafter as “eye aperture”) becomes smaller. In this connection, the waveform degradation arising from the chromatic dispersion includes a case (see
FIG. 21B
) in which the eye aperture in the amplitude direction becomes smaller (waveform is rounded) as compared with the original transmission waveform (see
FIG. 21A
) and a case (see
FIG. 21C
) in which the eye aperture in the phase direction decreases (phase is compressed) with respect to the original transmission waveform.
The difference therebetween depends upon the designs (type of the optical transmission line
500
, optical transmission band, chirping setting, and others) on the optical transmission system. For example, in a common optical transmission system in which optical transmission is made in an optical transmission band of 1.55 &mgr;m through the use of SMF whose zero dispersion wavelength is in a 1.3-&mgr;m band, if the chirping setting is made such that the rise of the waveform is at the short-wavelength side (the fall is at the long-wavelength side), then the long-wavelength side intensively receives the effect of the chromatic dispersion so that the waveform tends to be rounded. If the chirping setting is made conversely, then the adverse tendency arises.
Meanwhile, such waveform degradation stemming from the chromatic dispersion becomes more noticeable as the optical signal transmission distance (repeating distance) becomes longer to cause the deterioration of the reception sensitivity characteristic of an optical receiver
402
, described later, (difficult identification/regeneration of a signal). For this reason, so far, a dispersion-shifted fiber (DSF) (see a chromatic dispersion characteristic
700
in
FIG. 22
) in which the zero dispersion wavelength is shifted to a 1.55-&mgr;m band forming an optical transmission band has been put to use for the optical transmission line
500
, thereby providing an optical transmission system
100
capable of limiting the influence of the chromatic dispersion in the optical transmission band. However, also in the case of the use of DSF, a further increase in optical signal transmission rate makes it difficult to disregard the waveform degradation arising from the chromatic dispersion as in the case of the use of SMF.
Accordingly, for whether SMF or DSF used as the optical transmission line
500
, in a case in which the optical signal transmission rate becomes high to some extent, for example, a dispersion compensation fiber (DCF)
401
designed to have a chromatic dispersion characteristic contrary to the chromatic dispersion, the optical transmission line
500
suffers, has been put at the former stage of the optical receiver
402
as shown in
FIG. 20
for compensating for the waveform degradation and enlarging the eye aperture.
However, in general, the degree of deterioration of the eye aperture acceptable to the optical receiver
402
, i.e., the range of chromatic dispersion value permissible to the optical receiver
402
, is restricted by the reception sensitivity characteristic of the optical receiver
402
, and the chromatic dispersion value increases in proportion to the transmission distance (see FIG.
22
); therefore, in the above-mentioned existing optical transmission system
100
, there is a need to place the dispersion compensation fiber
401
having a different chromatic dispersion characteristic according to optical transmission distance (repeating distance) to show the range of chromatic dispersion value acceptable to the optical receiver
402
.
Accordingly, the system sacrifices the flexibility and the needed type of dispersion compensation fiber
401
increases, which raises the cost at the system construction and the management cost after the system construction.
In addition, in the recent years, the transmission of an ultra-high-speed optical signal such as 10 Gb/s or 40 Gb/s [assuming that a 2.5-Gb/s optical signal is 16-wavelengths multiplexed (WDM: Wavelength Division Multiplex) signal, it is a 64-wavelengths multiplexed or 128-wavelengths multiplexed signal] has been realizable. In such a case, the wavelength (channel) interval becomes as extremely short as ¼ or {fraction (1/16)} and, hence, the characteristic change resulting from the external factors such as variation in temperature of the optical transmission line
500
is not ignorable.
Thus, the improvement of the reception sensitivity characteristic of the optical receiver
402
by compensating fixedly for the chromatic dispersion through the use of the dispersion compensation fiber
401
as mentioned above encounters limitation even from the viewpoint of ultra-high speed and vary-large capacity required in the recent WDM transmission technology, and the difficulty of coping with the further increase in speed and capacity in the future is easily predictable.
Moreover, for the transmission of such an ultra-high-speed optical signal such as 10 Gb/s or 40 Gb/s, even the waveform degradation arising from the dispersion (for example, polarization mode dispersion) other than the chromatic dispe
Kurooka Takashi
Miyazaki Akimitsu
Otsuka Tomoyuki
Sakamoto Hisaya
Hoff Marc S.
Staas & Halsey , LLP
Suarez Felix
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