Optical digital regenerator

Details Optical digital regenerator Optical digital regenerator

1999-02-08

2003-03-11

Pascal, Leslie (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200, C359S237000, C359S326000, C359S344000, C385S024000

Reexamination Certificate

active

06532091

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an optical digital regenerator, and more specifically, to an apparatus for regenerating an optical signal in an intact optical state.
BACKGROUND OF THE INVENTION
As an optical cross-connect node on a large capacity wavelength-division multiplexing optical network in the future, due to the increase of a bit rate per wavelength as well as the increase of the number of multiplexed wavelengths, an optical digital regenerating system for regenerating information as intact optical signals has been widely noticed instead of a system in which optical signals are electrically terminated per wavelength and regeneratively repeated.
In a WDM optical network in which an usable wavelength is assigned to each line in advance, it is necessary that an input wavelength and an output wavelength of an optical digital regenerator are the same. However, in a WDM optical network that positively adopts the wavelength conversion, reusing the wavelengths can reduce the number of wavelengths. In this case, an input wavelength and an output wavelength of an optical digital regenerator are not necessarily the same. Accordingly, conventional optical digital regenerators have employed a structure that comprises two-step wavelength converting parts so as to be widely applicable to any optical network.
A schematic block diagram of a conventional optical digital regenerator is shown in
FIG. 4. A
signal light from a trunk line system enters an input terminal
10
. The signal light (wavelength &lgr;i) having input the input terminal
10
is divided by an optical coupler
12
and enters a high-speed photodiode
14
. The photodiode
14
converts the signal light into an electric signal and applies it to a clock extracting circuit
16
. The clock extracting circuit
16
extracts a clock component of the signal light from the output of the photodiode
14
.
The input signal light (wavelength &lgr;i) of the input terminal
10
also enters a wavelength converter
18
. The wavelength converter
18
converts the signal light input from the input terminal
10
into another wavelength &lgr;j. The signal light whose wavelength have been converted into &lgr;j by the wavelength converter
18
inputs a second wavelength converter
20
. The wavelength converter
20
comprises a clock input terminal besides an optical input terminal to which the output light of the wavelength converter
18
enters. The clock extracted by the clock extracting circuit
16
inputs the clock input terminal after being amplified by an amplifier
20
and phase-shifted (adjusted) by a phase shifter
24
. The wavelength converter
20
superimposes the signal light of the wavelength &lgr;j from the wavelength converter
18
on a waveform of an RZ probe pulsed light of wavelength &lgr;k formed from the clock input through the clock input terminal. By this operation, the wavelength of the signal light is converted from the wavelength &lgr;j into the wavelength &lgr;k and at the same time the signal light is retimed and waveform-reshaped.
There are conventional structures in which retiming and waveform-reshaping is performed at a first wavelength converter using extracted clocks (for example, see B. Lavigne et al. ‘Experimental analysis of SOA-based 2R and 3R optical regeneration for future WDM networks’, OFC '98, Technical Digest, pp. 324-325, which was published at The Optical Fiber Conference held in February in 1998.).
FIG. 5
shows a schematic block diagram of the conventional embodiment.
An input signal light (NRZ optical pulse) of wavelength &lgr;
0
from a trunk line system enters an input terminal
110
. The signal light (wavelength &lgr;
0
) entered the input terminal
110
is divided by an optical coupler
112
and then inputs a clock regenerating circuit
114
. The clock regenerating circuit
114
, which comprises a high-speed photodiode, converts the optical pulse from the optical coupler
112
into an electric signal and electrically extracts a clock component contained in the signal light. An LD driving circuit
116
pulse-drives DFB lasers
118
and
120
in accordance with the clock regenerated by the clock regenerating circuit
114
. The DFB lasers
118
and
120
respectively laser-oscillate at mutually different wavelengths &lgr;
1
and &lgr;
2
and output pulse lights (probe pulse lights) locked with the regenerated clock from the clock regenerating circuit
114
. The probe pulse lights from the DFB lasers
118
and
120
are combined by a multiplexer
122
and then input one facet of a semiconductor optical amplifier (SOA)
124
.
The signal light (wavelength &lgr;
0
) having input the input terminal
110
also enters a port A of an optical circulator
126
and outputs from its port B. The input signal light from the port B enters the other facet of the SOA
124
. While the signal light of the wavelength &lgr;
0
and the probe pulse lights of the wavelengths &lgr;
1
and &lgr;
2
propagate mutually in the opposite directions in the SOA
124
, a pulse waveform or bit information of the signal light is copied to the probe pulsed light due to the cross gain modulation effect. That is, the probe pulse light being output from the SOA
124
toward the port B of the optical circulator
126
becomes an RZ pulse conveying the same bit information with the input signal light (wavelength &lgr;
0
) of the input terminal
110
.
The probe pulse light (wavelength &lgr;
1
+&lgr;
2
) having output from the SOA
124
enters the port B of the optical circulator
126
and outputs from its port C to be divided into a wavelength &lgr;
1
component and a wavelength &lgr;
2
component by a wavelength demultiplexing element
128
. The respective components of the wavelengths &lgr;
1
and &lgr;
2
demultiplexed by the wavelength demultiplexing element
128
propagate on different optical paths
130
a
and
130
b
and then multiplexed by a multiplexer
132
. The optical paths
130
a
and
130
b
are for example predetermined so that the propagation time of the wavelength &lgr;
2
component is delayed by one half bit period in comparison with that of the wavelength &lgr;
1
component. Therefore, the pulse light becomes an NRZ optical pulse after being multiplexed by the multiplexer
132
.
The output light of the multiplexer
132
inputs a first port of a first facet of a Mach-Zehnder interferometer (MZI) type wavelength converter
134
and led to one optical path in the MZI wavelength converter
134
. The CW laser light from a DFB laser
136
inputs a second facet of the MZI wavelength converter
134
. The oscillating wavelength &lgr;i of the DFB laser
136
is different from both of the oscillating wavelengths &lgr;
1
and &lgr;
2
of the DFB lasers
118
and
120
. The output light of the DFB laser
136
is divided in the MZI wavelength converter
134
. The divided lights propagate on the two optical paths and combined together again in the MZI wavelength converter
134
. The combined light then outputs from a second port of the first facet of the MZI wavelength converter
134
. On the one optical path in the MZI wavelength converter
134
, the output light of the multiplexer
132
propagates in the opposite direction. The MZI wavelength converter
134
converts the pulse signal of wavelength &lgr;
1
+&lgr;
2
into the wavelength &lgr;i. The waveforms are respectively reversed in the SOA
124
and the MZI wavelength converter
134
and nonreversing regenerated optical signal waveforms are obtained as final outputs. The purpose of the wavelength conversion in the MZI wavelength converter
134
is mainly to improve the extinction ratio.
An optical band pass filter
138
extracts the wavelength &lgr;i components alone from the output light of the second port of the first facet of the MZI wavelength converter
134
. It is for preventing the mixing of lights of the wavelengths &lgr;
0
, &lgr;
1
and &lgr;
2
due to the reflection at each part.
In the conventional structure shown in
FIG. 4
, the optical coupler
12
, the high-speed photodiode
14
, and the clock extracting circuit
16
are ind

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