Optical: systems and elements – Optical modulator
Reexamination Certificate
2001-12-10
2002-12-24
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
C359S249000, C359S245000, C359S260000
Reexamination Certificate
active
06498671
ABSTRACT:
FIELD OF THE INVENTION
The invention is in the optical signal processing field. More particularly, the purpose of the invention is regeneration of an optical information transmission signal.
TECHNICAL BACKGROUND
FIG. 8
very diagrammatically shows a regenerator
20
in which a signal is input from a transmission line L. The function of the regenerator is to regenerate the signal while amplifying it, reshaping it, and putting it back into phase. This function is known under the name of the
3
R reconstitution standing for “reamplification, reshaping, retiming”. Regeneration may comprise solely some regeneration functions, for example reshaping or reamplification. The regenerator
20
comprises a detector
21
that converts the received optical signal into an electrical signal carrying the optical signal modulation, a clock reconstitution means
22
that reconstitutes the clock signal from the signal, a means
23
of reconstituting the shape of the signal modulation received by line L, and finally a modulator
24
receiving firstly a carrier wave from a local oscillator
100
shown in dotted lines and the reshaped electrical signal from the means
23
of reconstituting the modulated shape of the signal received by line L to give a reconstituted retransmitted signal at the output.
In this device
20
, it is noted that the carrier wave of the retransmitted signal is reconstituted locally, and therefore can differ from the initial carrier wave by modifications to characteristics such as the wave length that might be slightly different even if the two wave lengths have the same nominal value, polarization and phase.
Another example embodiment is described in the U.S. Pat. No. 5,781,326 belonging to the applicant issued on Jul. 14, 1998 (EP 813 097 A1). The purpose of the invention described in this patent is to propose a fully optical regeneration device, in other words without any optical-electrical or electrical-optical conversion.
The device for shaping a binary input signal in the shape of a first optical wave modulated between low and high power amplitudes, comprises:
a first stage to supply a modulating optical signal in the shape of a second optical wave with a determined wave length and modulated between first and second power amplitudes, as a function of the said input signal, the highest of the said first and second levels being stabilized to be only slightly dependent on fluctuations of the said low and high levels of the input signal, and
a second stage comprising an interferometric structure coupled with the said first stage to receive the said modulating signal and designed to supply an output signal resulting from constructive or destructive interference of the first and second auxiliary waves when the power of the modulating signal is equal to the said first or second levels respectively.
Each of the first and second stages of this shaping device can be used separately to produce an output signal regenerated from the input signal. The first and second stages can naturally be cascaded as described in this patent to give an improved regenerated signal, in other words capable of starting from a poor quality modulated input signal and producing an output signal in which the high levels are stabilized with a constant optical power, and in which the low levels have a practically zero power while presenting a very high signal-to-noise ratio.
For a good understanding of the benefits of the invention in the device described in this patent, each stage of the device described in this patent is shown in
FIG. 1
in this application.
The first stage is shown in part A and the second stage is shown in part B.
The first stage
61
receives an input optical signal E in the form of a continuous optical wave &lgr;e modulated in a binary manner.
The first stage
61
comprises an optical amplifier with a fiber
63
that receives the input signal E and supplies an amplified input signal AE to a clipping device
64
. The clipping device
64
comprises a semiconductor optical amplifier OA, one face of which is coupled to a laser oscillator
66
supplying an intermediate carrier wave L. The other face of amplifier OA is connected to a circulator
5
comprising a first port into which the amplified input signal AE from the fiber amplifier
63
is input, and a second port from which it is injected into the amplifier OA. A third port in the circulator
65
supplies the signal B through a rejecter filter F adjusted to the wave length &lgr;e of the input signal E. The wave length &lgr;b of the intermediate wave L is chosen to be different from the wave length of signal E.
Signal B may constitute the regenerated signal or a modulating signal for the second stage represented in part B that will now be described.
The second stage
62
of the device in
FIG. 1B
comprises an interferometric structure
67
composed of two guide branches provided with semiconductor optical amplifiers OA
1
and OA
2
respectively. A first coupler K
1
couples one end of each of these branches to a laser source
68
supplying an output carrier wave M with wave length &lgr;s. A second coupler K
2
is arranged so that the signal to be regenerated can be input into the first amplifier OA
1
. A third coupler K
3
connected to the coupler K
2
and to the second amplifier OA
2
is arranged to supply an output signal S resulting from coupling of auxiliary waves AM
1
and AM
2
supplied by amplifiers OA
1
and OA
2
respectively. The waves AM
1
and AM
2
correspond to waves M
1
and M
2
output from coupler K
1
and amplified by amplifiers OA
1
and OA
2
respectively.
Currents I
1
and I
2
are injected into amplifiers OA
1
and OA
2
respectively. According to a first option, these currents are adjusted such that the output signal S is the result of constructive interference between waves AM
1
and AM
2
when the power of the input signal is low, and is otherwise the result of destructive interference.
According to another option, the current I
2
may be adjusted to a value I
21
greater than I
20
to obtain destructive interference when the power of the input signal is low, and constructive interference when it is high.
Regardless of the specific case, as shown in
FIG. 1A
or
1
B or the case described in the patent mentioned above in which the signal B forms the input signal to the device shown in part B, the output wave length, as in the device described with reference to
FIG. 8
, is different from the input wave &lgr;e. Similarly, the polarization and phase of the output wave vary with respect to the input wave.
U.S. Pat. No. 5,781,327 delivered to TRW inc. describes an electro-optical modulation system operating in a closed loop. The carrier optical wave is generated by an optical amplifier medium
12
shown in
FIGS. 1 and 5
, and
32
in FIG.
3
. This amplifying medium is pumped by a continuous wave, which is the continuous wave at the output from the amplifier medium connected to the input of this medium through the loop.
This cycle of the loop comprises means
10
of modulating the continuous wave output from the amplifier optical medium. The loop has a loop output for the modulated optical wave. The loop cycle comprises filter or demodulation means to reduce the width of the pumping wave band. This thus improves the optical efficiency of the overall device (column
2
, row
43
). In particular, it avoids the need to reinput unwanted resonance into the modulator that would disturb modulation (end of claim
1
in this patent).
The amplifying optical medium in this patent thus behaves like a local oscillator comprising a reaction loop, in which part of the output is returned to the input. This patent is mentioned because it reveals some filter means that are used in the invention that will be described later.
In regeneration devices, variations in the phase, wave length or polarization due to local generation of the beat wave, can introduce some disadvantages particularly in a WDM (Wavelength Division Multiplex) transmission system. Similarly, devices for coherent processing of the optical signal in which the pola
Bissessur Hans
Chiaroni Domonique
Devaux Fabrice
Janz Christopher
Alcatel
Epps Georgia
Thompson Tim
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