Optical communications – Multiplex – Wavelength division or frequency division
Reexamination Certificate
2001-01-24
2003-12-02
Lester, Evelyn (Department: 2873)
Optical communications
Multiplex
Wavelength division or frequency division
C359S279000, C385S014000, C385S122000, C385S140000, C250S227120
Reexamination Certificate
active
06658213
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a modulation controlling circuit for supplying a modulating signal to an external modulator that outputs a modulated optical signal, while setting a proper operating point in the external modulator.
2. Description of the Related Art
In recent years, light sources such as the DFB laser having sharp wavelength selectivity and capable of adjusting the oscillation wavelength freely have been put into practical use and techniques of suppressing a spectrum variation due to dispersion characteristics and nonlinear effects that are specific to optical fibers have been applied to optical transmission systems.
The wavelength division multiplexing, which makes it possible to adjust to increase in the demand for services of multimedia, B-ISDN, etc. flexibly in an inexpensive manner, has come to be applied positively to trunk line systems of such optical transmission systems.
Therefore, to prevent spreading of an oscillation spectrum due to a phenomenon that the refractive index varies with the modulating current or temperature when a light source as mentioned above is modulated directly, an LN (LiNbO
3
) external modulator, for example, is used in a transmitting part of a node apparatus of such a trunk line system.
FIG. 6
shows an example configuration of a transmitting part that incorporates an LN external modulator.
In
FIG. 6
, an exit aperture of a light source
60
is connected to an incidence aperture of an LN external modulator
62
via an optical fiber
61
-
1
. An exit aperture of the LN external modulator
62
is connected to the immediate succeeding transmission section of an optical transmission path via an optical fiber
61
-
2
. A data signal representing a sequence of transmission information and a clock signal that is synchronized with the transmission signal are input to first and second inputs of a modulator driving part
63
, respectively. The output of the modulator driving part
63
is connected to one modulation input of the LN external modulator
62
via a bias-T circuit
64
. The other modulation input of the LN external modulator
62
is grounded via a cascade connection of a resistor
65
and a capacitor
74
. A monitoring terminal of the LN external modulator
62
is not only grounded via a cascade connection of a resistor
66
and a battery
75
but also connected to a first input of a phase comparator
68
via a band-pass filter
67
. The output of a pilot signal generator
69
is connected to a second input of the phase comparator
68
and a third input of the modulator driving part
63
. The output of the phase comparator
68
is connected to a bias terminal of the bias T-circuit
64
via a low-pass filter
70
.
The LN external modulator
62
is composed of the following components:
Two optical waveguides
71
-
1
and
71
-
2
that are formed parallel between the incidence aperture and the exit aperture.
A photodiode
72
that is in weak optical coupling with the exit aperture, whose cathode is grounded, and whose anode is connected to the above-mentioned monitoring terminal.
A waveguide length varying part
73
for varying the difference between the optical waveguide lengths of the respective optical waveguides
71
-
1
and
71
-
2
in accordance with a current flowing between the above-mentioned first and second inputs.
In the transmitting part having the above configuration, the pilot signal generator
69
generates a pilot signal (for simplicity, it is assumed to be a sine wave having a frequency of 1 kHz) constantly. The modulator driving part
63
generates an NRZ signal by sampling, in synchronization with the clock signal, a bit string that is given as the above-mentioned data signal, and generates a modulating signal by amplitude-modulating the NRZ signal in accordance with the instantaneous value of the pilot signal.
On the other hand, in the LN external modulator
62
, a laser beam that is emitted from the light source
60
and supplied via the optical fiber
61
-
1
is branched once by the optical waveguides
71
-
1
and
71
-
2
, then recombined, and finally output to the immediate succeeding transmission section of the optical transmission path via the optical fiber
61
-
2
.
The difference between the optical waveguide lengths of the respective optical waveguides
71
-
1
and
71
-
2
is given as a periodic function corresponding to the sine of the instantaneous value of the modulating signal that is supplied from the modulator driving part
63
via the bias T-circuit
64
.
That is, the luminance of a laser beam that is output to the above-mentioned immediate succeeding transmission section via the optical fiber
61
-
2
(for simplicity, hereinafter referred to simply as “outgoing beam”) is amplitude-modulated so as to have opposite phases in periods when the NRZ signal included in the modulating signal has logical values “1” and “0” respectively, in a state that the operating point of the LN external modulator
62
(waveguide length varying part
73
) is set properly as shown in FIG.
7
A.
However, for example, in a state that the operating point of the LN external modulator
62
(waveguide length varying part
73
) is not set at a proper point as shown in
FIG. 7B
or
7
C, the luminance of an outgoing beam has phases that depend on the offset of the operating point and is given as a function of time on which the pilot signal is superimposed.
The photodiode
72
outputs a monitoring signal that represents the luminance of such an outgoing beam in the form of an instantaneous value. The band-pass filter
67
extracts the pilot signal component from the components of the monitoring signal.
The phase comparator
68
compares the phases of this pilot signal component and the pilot signal as generated by the pilot signal generator
69
and generates an error signal that indicates the difference between the two pilot signals in the forms of an instantaneous value.
The low-pass filter
70
, which has a passband that is lower than the frequency of the two pilot signals, feeds back (negatively) a component of the error signal that passes through the low-pass filter
70
to the LN external modulator
62
via the bias T-circuit
64
.
In other words, even if the operating environment such as the temperature varies, the operating point of the LN external modulator
62
(waveguide length varying part
73
) is kept proper in a stable manner and hence the transmission quality is kept high.
In many optical transmission systems of the above conventional example, the data signal is given as the product of a PN series that is used for, for example, securing confidentiality and a bit string that represents transmission information.
Therefore, a frequency spectrum of the modulating signal has a line spectrum (see
FIG. 8A
) whose envelope component is given by a function E(f) of frequency f that is represented by the following Equation (1) and whose interval &Dgr; on the frequency axis is represented by the following Equation (2) in a case where, for example, the number of stages of a shift register used for generation of the PN series is n and the period of the clock signal (i.e., the reciprocal of the bit rate of the transmission information) is T:
E
(
f
)=(sin &pgr;
fT/&pgr;fT
)
2
(1)
&Dgr;=1/{(2
n
−1)
T}
(2)
That is, the interval &Dgr; has a smaller value when the bit rate of the transmission information (i.e., the frequency of the clock signal) is smaller.
Therefore, where the bit rate is as large as 10 Gbits/sec, for example, the probability that a component (hereinafter referred to simply as “particular component”) of the line spectrum that is closest to the pilot signal component on the frequency axis exists in the passband of the band-pas filter
67
is very low (see
FIG. 8B
) and the probability that a difference component between the particular component and the pilot signal component on the frequency axis exist in the passband of the low-pass filter
70
is also low.
However, where the bit rate is as low as 15
Hayashi Akihiko
Nagase Norio
Otsuka Tomoyuki
Sano Shinichiro
Fujitsu Limited
Katten Muchin Zavis & Rosenman
Lester Evelyn
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