Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-06-13
2002-05-14
Negash, Kinfe-Michael (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C375S291000
Reexamination Certificate
active
06388786
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for generating a duobinary signal and an optical transmitter using the same method.
BACKGROUND OF THE INVENTION
Recently, an optical duobinary technique has attracted attention as an optical transmission manner which can overcome the waveform deterioration due to a chromatic dispersion. The duobinary technique itself has been researched for a long time and its theory system was established in the time of pulse communication with a coaxial cable. The duobinary technique is that a signal bandwidth(spectrum width) is reduced to less than ½ by mapping a binary data signal to be transmitted into a three-level signal with a redundancy in the amplitude direction. It has a merit that the waveform deterioration due to a dispersion etc. is difficult to happen since the spectrum width is narrowed. However, it had never attracted attention in high-speed optical communication since, in the receiver, a receiving circuit with a linearity is required to handle the three-level signal and a decoder for decoding the original binary data signal from the three-level signal is necessary.
A. J. Price et al., “210 km Repeaterless 10 Gb/s Transmission Experiment Through Nondispersion-Shifted Fiber Using Partial Response Scheme”, IEEE PHOTONICS TECHNOLOGY LETTERS, Vol.7, No.10, pp.1219-1221(1995) reports an optical duobinary technique where a redundancy is given to optical phase.
The optical transmitter used in this optical duobinary technique is shown in
FIG. 1. A
binary data signal is passed through a low-pass filter, which is ideally a cosine roll-off filter, with a bandwidth of about 0.25 times a clock frequency. Due to the excessive limitation of bandwidth, the interference between codes is occurred to convert the binary data signal into a three-level data signal. Similarly, a binary inverted data signal is converted into a three-level data signal. Then, these signals are input with an amplitude equal to a half-wavelength voltage V
&pgr;
to a push-pull optical intensity modulator. The push-pull optical intensity modulator is a Mach-Zehnder(MZ) interferometer with modulation terminals connected to both arms, where unnecessary chirp(phase variation) does not occur. In this technique, the bias voltage is so adjusted that a three-level signal(−1, 0, 1) corresponds to a mountain(ON), a valley(OFF) and a neighboring mountain(ON) in the voltage-extinction characteristic of the push-pull optical intensity modulator. As a result, when the amplitude and phase of light are represented by (A, &PHgr;), the data signal is mapped into three states of (1, 0), (0, indefinite) and (1, &pgr;) to generate optical duobinary signal light. This three-level signal light can be, as it is, decoded into the binary signal composed of 1 and 0 since the phase information is deleted by square-law detection when the direct detection is conducted by an optical detector. This means that direct-detection optical receivers, which are widely used, can be used as it is. It is one of the reasons why the duobinary technique has attracted attention again.
Japanese patent application laid-open No.8-139681(1996) discloses another optical duobinary system as shown in FIG.
2
. In this system, as shown in
FIG. 2
, a binary transmission data signal
50
is converted into a three-level duobinary signal by a code converter
51
. In the code converter
51
, the code conversion is first conducted by a precoder
52
composed of an exclusive-OR circuit
26
and an 1-bit delay circuit
27
, and then the duobinary signal is generated by a binary-to-three-level converter
53
composed of an 1-bit delay circuit
27
and an adder
54
. The duobinary signal is divided into two signals, where the first signal divided is input through an amplitude adjusting circuit
55
and a bias adjusting circuit
56
to the first input terminal of an optical modulator
58
and the second signal divided is input through an inverter
57
and an amplitude adjusting circuit
55
to the second input terminal of the optical modulator
58
. The optical modulator
58
is a Mach-Zehnder optical intensity modulator, where light from a light source
1
is modulated by applying the first and second signals to its two optical waveguides to generate the optical duobinary signal.
When the two electrical signals with an amplitude equal to a half-wavelength voltage(v
&pgr;
) of the optical modulator
58
are input and the bias point of signal is set at a point (a) of transmission characteristics
59
of the modulator as shown in
FIG. 3
, the middle value of the duobinary signal
60
is assigned to a minimum transmittance state and the minimum and maximum values thereof are assigned to maximum transmittance states, where the optical phase is inverted by 180 degree between the minimum and maximum values. As a result, the three levels of the electrical signal can be assigned to the optical three states, thereby narrowing the modulated light spectrum. Meanwhile, this system has a composition equivalent to the system in
FIG. 1
where the low-pass filters are replaced by the binary-to-three-level converter
53
.
However, in the conventional methods, the driving amplifier of the modulator requires a linearity since the electrical signal for driving the optical modulator is three-level. On the other hand, the driving amplifier generally needs a high-output characteristic greater than 5 Vp-p. Therefore, there is a problem that designing the circuit becomes very difficult since the linearity and the high-output characteristic are required therein.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a method for generating a duobinary signal where an electrical signal for driving an optical modulator is binary.
It is a further object of the invention to provide an optical transmitter where a binary electrical signal for driving an optical modulator is used to generate a duobinary signal.
According to the invention, a method for generating a duobinary signal, comprises the step of:
modulating individually an intensity and a phase of a carrier wave.
According to another aspect of the invention, a duobinary-manner optical transmitter, comprises:
a laser device which outputs signal light;
an optical intensity modulator which intensity-modulates the signal light according to a first data signal generated by dividing a data signal into two signals;
a precoder which inputs a second data signal generated by dividing the data signal into two signals; and
an optical phase modulator which phase-modulates the intensity-modulated signal light according to a signal which is obtained delaying 0.5 bit an output signal of the precoder.
According to another aspect of the invention, a duobinary-manner optical transmitter, comprises:
a precoder which inputs a second data signal generated by dividing a data signal into two signals;
a direct modulation phase shift keying encoder which inputs an output of the precoder after delaying 0.5 bit the output;
a laser device which outputs signal light phase-modulated by modulating an injected current according to an output of the direct modulation phase shift keying encoder; and
an optical intensity modulator which intensity-modulates the phase-modulated signal light according to a first data signal generated by dividing the data signal into two signals.
According to another aspect of the invention, a method for generating a duobinary signal, comprises the steps of:
providing two carrier waves with an equal frequency;
intensity-modulating individually the two carrier waves by first and second intensity modulators; and
coupling the two intensity-modulated carrier waves so that they have a phase difference of &pgr;.
According to another aspect of the invention, a duobinary-manner optical transmitter, comprising:
a laser device which outputs signal light;
an optical divider which divides the signal light into two light signals;
a first optical intensity modulator which inputs first signal light divided by the optical divider;
a second optical intensity modulator which inputs
Ono Takashi
Yano Yutaka
NEC Corporation
Negash Kinfe-Michael
LandOfFree
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