Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
1999-08-30
2002-04-16
Lee, John D. (Department: 2874)
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
C385S004000, C385S008000
Reexamination Certificate
active
06374000
ABSTRACT:
BACKGROUND ART
The invention relates to a method of modulating light as defined in the introductory portions of claims 1 and 5, and a modulator circuit as defined in the introductory portions of claims 14 and 15.
Optical communication involves the need of modulating the optical signals.
This modulation may be performed by direct modulation of the light source, e.g. by direct modulation of a laser diode with data signals, but it is preferred in several connections to perform modulation on the light itself. This type of modulation is called optical modulation.
A problem involved by the direct optical modulation is that the characteristics of the modulator change over time as a function of temperature, changed properties of the material, etc. This change specifically means that the optical output as a function of the electrical driver signals is not constant, which results in undesired distortion of the optical signal.
U.S. Pat. No. 5,170,274 describes an optical transmitter of the Mach-Zehnder type, in which feedback from the modulator is obtained by means of a low-frequency pilot tone superimposed on the electrical driver signal of the modulator, thereby providing some compensation for the drift of the operating point of the modulator. However, this transmitter has the drawback that the extinction ratio is gradually impaired because of amplitude drift of the modulated signal, or more particularly a drift with respect to the amplitude of the transmission characteristic of the Mach Zehnder modulator.
SUMMARY OF THE INVENTION
When, as stated in claim
1
, the photoelectrical converter circuit is connected to and generates a feedback signal for a feedback circuit which is also connected to the pilot tone generator, said feedback circuit being adapted, in response to the pilot tone signal fed from the pilot tone generator and the feedback signal fed from the photoelectrical converter circuit, to generate driver control signals for the driver circuit to regulate the driver signal fed to the Mach-Zehnder modulator, it is possible to achieve a very well-defined regulation of a modulator circuit. Thus, the invention enables regulation, optimum in some cases, of the fed driver signal, i.e. the modulation amplitude, which is fed to the Mach-Zehnder modulator. The regulation may thus be performed automatically in connection with temperature errors or fluctuations.
This may be a particular advantage in connection with e.g. so-called bias-free Mach-Zehnder modulators, where bias corrections should be avoided, since such bias corrections will cause drift in the overall circuitry.
In case of drift or e.g. temperature fluctuations, it is thus possible according to the invention to control the amplitude of the modulation signal.
It should be noted that it is not decisive according to the invention how the modulation signals, formed by the driver signal and the pilot tone, are fed to the Mach-Zehnder modulator. The pilot tone and the driver signal may e.g. be mixed electrically before the signal is fed to the electrodes of the modulator, and the driver signal and the pilot tone may be fed to separate electrodes or sets of electrodes in the Mach-Zehnder modulator, if desired.
When, as stated in claim
2
, the pilot tone signal is fed to the logic “0” and/or logic “1” of the modulation signal, and the feedback circuit provides driver control signals in response to the pilot tone component of the feedback signal corresponding to the pilot tone signal, a particularly advantageous embodiment of the invention is obtained, as the regulation of the modulation voltage is “tuned” to either the ideal logic “0” or the ideal logic “1”. The amplitude regulation of the modulation voltage may thus be adjusted to a desired asymmetric proportion without bias regulation, which is extremely advantageous in several applications, since regulation of the bias/DC operating point per se results in electrode drift.
Regulation of the amplitude thus makes it possible to currently allow for e.g. temperature-caused changes in the transmission characteristic which particularly concern the actual amplitude of the transmission characteristic.
Thus, the regulation according to the invention may thus allow for the undesirability of applying a DC voltage to the Mach-Zehnder modulator, as this DC voltage gives rise to drift in the transmission characteristic.
When, as stated in claim
3
, the driver signal, in response to the pilot tone signal fed to the modulation signal, is regulated to the logic “0” or logic “1” of the modulation signal in accordance with either
&Dgr;
V
amp
~F
L
·k
1
or
&Dgr;
V
amp
~F
H
·k
2
where &Dgr;V
amp
is the change in the driver signal provided by the regulation, k
1
and k
2
are a suitably selected constant for the algorithm, F
L
and F
H
assume the value +1 when the pilot tone component at logic “0” and logic “1”, respectively, is in in-phase with the corresponding pilot tone generated by the pilot tone generator, and F
L
and F
H
assume the value −1 when the pilot tone component at logic “0” and logic “1”, respectively, is in antiphase with the corresponding pilot tone generated by the pilot tone generator, a very efficient regulation algorithm is achieved according to the invention, as the feedback circuit hooks on to either the logic “0” or the logic “1” and the corresponding pilot tone. It should be stressed that actually it is immaterial whether pilot tones are generated at both logic “0” and logic “1”, provided it is a pilot tone reference corresponding to the one of the above-mentioned algorithms which has now been selected.
The amplitude regulation of the modulation signal thus gives an improved extinction ratio, i.e. the ratio of logic “1” (p
1
) to logic “0” (p
0
) is increased, and the resulting signal
oise ratio in a complete application may therefore be increased. It should be noted in this connection that this improved extinction ratio is achieved, even though the amplitude regulation provides an asymmetric modulation of the transmission characteristic, as one extreme of the amplitude is adjusted to the ideal logic “1” or logic “0”, respectively, without the opposite logic level being optimized.
Usually, it is preferred to regulate to logic “0”, and thus according to the algorithm &Dgr;V
amp
~F
L
·k
1
.
As appears from the above-mentioned algorithm, the regulation is proportional to k and F, which likewise means that a special case may be that ~ is replaced by =.
When, as stated in claim
4
, the driver signal, in response to the pilot tone signal fed to the modulation signal, is regulated to the logic “0” or logic “1” of the modulation signal in accordance with either
&Dgr;V
amp
~F
L
·k
1
|A
L
|
or
&Dgr;V
amp
~F
H
·k
2
|A
H
|
where |A
L
| or |A
H
| is the numeric amplitude of the pilot tone components detected in the feedback circuit at logic “0” and logic “1”, respectively, it is additionally made possible to weight the regulation algorithm according to the actual distance from the modulation point to the desired optimum modulation point corresponding to either logic “1” or logic “0”. This may be an advantage e.g. when starting the system at modulation amplitude zero, as the algorithm tracks the desired modulation point more rapidly.
When, as stated in claim
5
, the photoelectrical converter circuit is connected to and generates a feedback signal for a feedback circuit which is also connected to the pilot tone generator, said feedback circuit being adapted, in response to the pilot tone signal fed from the pilot tone generator and the feedback signal fed from the photoelectrical converter circuit, to generate bias control signals for the bias circuit and driver control signals for the driver circuit to regulate the bias signal and driver signal fed to the Mach-Zehnder modulator, it is possible to perform a combined regulation of a modulator circuit by means of both regulation of the system bias (or DC operating point) and the modulation amplitude.
It is noted that it is not decisive according to the inv
Baker & Botts L.L.P.
DSC Communications A/S
Lee John D.
Song Sarah
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