Optical: systems and elements – Optical frequency converter
Reexamination Certificate
2000-12-29
2002-10-22
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
Reexamination Certificate
active
06469823
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and equipment for connecting optical communication systems using different wavelength bandwidths, for example connecting a system using 1.3 &mgr;m-band optical wavelength and a system using 1.5 &mgr;m-band optical wavelength, thereby to transmit signals therebetween corresponding to each signal type consisting of either RZ (return-to-zero) code or NRZ (non-return-to-zero) code.
BACKGROUND OF THE INVENTION
An optical wavelength converter is particularly beneficial to a wavelength division multiplexing (WDM) optical communication system, in which data are transmitted in a plurality of channels through a common optical fiber using multi-wavelength signal light.
As the amount of transmission data through a backbone optical communication system abruptly increases, the larger capacity is required in an optical communication system. As a method therefor, a wavelength division multiplexing (WDM) optical communication system has been started in use.
A WDM optical communication system is developed mainly using optical wavelengths in 1.5 &mgr;m band. On the other hand, conventional optical terminal station systems employ 1.3 &mgr;m band. Therefore, optical wavelength conversion is necessary for connecting these systems.
In
FIG. 1
, there is shown a schematic configuration diagram of a WDM optical communication system. In the existing optical terminal station system employing 1.3 &mgr;m band, optical signals having a plurality of optical wavelengths in 1.3 &mgr;m band are generated. A plurality of 1.3 &mgr;m band optical signals are respectively converted into optical signals having different wavelengths of 1.5 &mgr;m band in an optical wavelength converter
102
.
Then, optical signals having different wavelengths of 1.5 &mgr;m band are wavelength-multiplexed in a multiplexing circuit
110
of a 1.5 &mgr;m WDM transmission system
103
to transmit through an optical fiber transmission line
111
.
At the receiving side of 1.5 &mgr;m WDM transmission system
103
, wavelength-multiplexed optical signals transmitted through optical fiber transmission line
111
are received to be divided into each optical signal having respective wavelength by a demultiplexing equipment
112
.
The wavelength-divided optical signals of 1.5 &mgr;m band are converted into optical signals of 1.3 &mgr;m band by an optical wavelength converter
104
to transmit to 1.3 &mgr;m WDM transmission system
105
.
Here, the transmitted optical signals are formed of either NRZ (non-return-to-zero) code or RZ (return-to-zero) code.
In
FIG. 2
, there are shown conventional configurations of optical wavelength converters
102
and
104
illustrated in FIG.
1
. In particular,
FIGS. 2A and 2B
respectively show optical wavelength converters
102
and
104
in case the optical signal is formed of NRZ code.
FIGS. 2C and 2D
show optical wavelength converters
102
and
104
in case the optical signal is formed of RZ code.
In
FIGS. 2A and 2B
, there are shown optical wavelength converters
102
and
104
having identical configuration, provided with an optical-to-electrical converter
203
and an electrical-to-optical converter
204
coupled by a capacitor C. Electric outputs of a non-inverted signal (DATA) and an inverted signal (NDATA) having been converted into an electric signal by optical-to-electrical converter
203
are led to electrical-to-optical converter
204
.
In electrical-to-optical converter
204
, the signal is converted into a corresponding optical signal having wavelength of either 1.5 &mgr;m band or 1.3 &mgr;m band.
Also, in
FIGS. 2C and 2D
, the configurations of optical wavelength converters
102
and
104
are identical, each provided with an optical-to-electrical converter
203
and an electrical-to-optical converter
204
coupled by a capacitor C. Electric outputs of a non-inverted signal (DATA) and an inverted signal (NDATA) having been converted by optical-to-electrical converter
203
are led to electrical-to-optical converter
204
.
Moreover, in the configurations shown in
FIGS. 2C and 2D
, a bias voltage is applied to the output of the capacitor C from a bias adjustment circuit
205
.
The reason for requiring this bias adjustment circuit
205
in case of RZ code is explained later.
A configuration example of optical-to-electrical converter
203
used in the aforementioned optical wavelength converters
102
and
104
is shown in FIG.
3
.
In
FIG. 3
, optical signals transmitted through an optical fiber
201
or
202
are received by a photo diode PD to convert into an electric signal of which magnitude correspond to the magnitude of an optical signal. The electric signal is amplified by a pre-amplifier
206
, and is output to a signal having a limited amplitude adjusted by a waveform shaping circuit
207
.
In
FIG. 4
, there are shown waveforms in various parts of optical-to-electrical converter
203
shown in
FIG. 3
in the cases of RZ code and NRZ code. Waveforms in case of RZ code are shown on the left column, and waveforms in case of NRZ code are shown on the right column.
There are respectively shown outputs [a] and [b] from pre-amplifier
206
in
FIG. 4A
, inputs [a′] and [b′] to waveform shaping circuit
207
in
FIG. 4B
, and output waveform [a″] and [b″] from electrical-to-optical converter
203
in FIG.
4
C.
Now the case of RZ code is described hereafter. As shown in
FIG. 4A
, a mean value of non-inverted output DATA ([a]) output from pre-amplifier
206
locates lower than the center of amplitude [O], and a mean value of inverted output NDATA ([b]) locates higher than the center of amplitude [O]. This is because the period of L level is longer than the period of H level in consequence of the nature of RZ code.
Accordingly, a waveform of signal obtained through a capacitor C to be input to waveform shaping circuit
207
is changed with the center of amplitude shifted for a bias voltage. In an example shown in
FIG. 4B
, the waveform is shifted for approximately one-fourth of the amplitude.
Input signals [a′] and [b′] are shaped by waveform shaping circuit
207
into waveforms each having a constant amplitude. However, because the phase is shifted, the duty remains deteriorated, as shown in FIG.
4
C.
Now the case of NRZ code is described, referring to the diagrams shown on the right side of FIG.
4
. In this case, because of the nature of NRZ code, the period of H level and the period of L level is substantially identical, producing no bias voltage. Therefore, the duty is not deteriorated in the output of waveform shaping circuit
207
.
In
FIG. 5
, there is a diagram for illustrating the reason for providing a bias adjustment circuit
205
. Bias adjustment circuit
205
is provided to solve the problem of duty deterioration produced in case of RZ code as shown in
FIG. 4
, caused by the bias voltage shifted in optical-to-electrical converter
203
. Bias adjustment circuit
205
is provided in optical wavelength converter
104
as shown in
FIGS. 2C and 2D
.
In
FIG. 5A
, there are illustrated waveforms [a″] and [b″] output from optical-to-electrical converter
203
corresponding to FIG.
4
C. As understood from the figure, mean values of non-inverted output signal DATA and inverted output signal NDATA are shifted from the center of amplitude caused by RZ code.
Therefore, in
FIG. 2
, an input signal to electrical-to-optical converter
204
through the capacitor C has a deviated direct-current component resulting in a deteriorated duty, as shown in FIG.
5
B. As a measure therefor, bias adjustment circuit
205
is provided on the input side of electrical-to-optical converter
204
, as shown in
FIGS. 2C and 2D
.
Accordingly, the duty of a waveform shown in
FIG. 5B
is improved by adjusting the bias voltage, as shown in FIG.
5
C.
In the conventional transmission line, a wide range of transmission rate varying from low speed to high
Fukutomi Satoshi
Itou Sunao
Kikuchi Hideyuki
LandOfFree
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