Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1999-11-26
2002-03-19
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S237000, C359S238000
Reexamination Certificate
active
06359720
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical processor.
At present, an optical processor capable of processing the highly multiplexed optical signal is required as one of indispensable elements for successfully constructing a future ultra-high speed optical communication system having a large capacity. In the circumstances like this, an optical gate device reported to the IEEE by M. Suzuki et al might be deemed useful for complying with the above requirement. Their report entitled “New Applications of a Sinusoidally Driven InGaAsP Electro-absorption Modulator to In-line Optical Gates with ASE Noise Reduction Effect, ” was published by IEEE, J. Light Wave Technol., 1992, vol. 10, No. 12, pp. 1912-1918.
In the optical gate device disclosed by the above report, there is used an electro-absorption modulator (referred to as “EA modulator” hereinafter) as an optical gate means capable of absorbing and modulating the light of a wavelength used in the optical communication system. The EA modulator has such an optical transmission characteristic that in the vicinity of zero volt, the optical transmission rate does not show any change with respect to the forward voltage applied to the EA modulator but it shows a monotonous decrease in respect of the reverse voltage applied to the same.
In the prior art optical gate device described in the above report, the voltage which is made by superimposing the sinusoidal voltage of a certain repetitive frequency on the DC biased voltage, is applied to the EA modulator in the vicinity of zero volt, thereby attaining such an optical transmission characteristic that the optical transmission rate changes along a rectangular waveform. In the optical gate device of the above report, demultiplexing of RZ (return-to-zero) optical pulse signal is carried out such that the timing of the maximum peak (“mountain top”) of the sinusoidal voltage is synchronized with the timing of the signal train taken out as the optical output.
According to the above report, as the RZ signal train formed by alternately multiplexing the RZ signal of a bit rate of Gbit/s with respect to the time axis, is given to the EA modulator which is sinusoidally driven at the repetitive or repetition frequency of 5, GHz, every other RZ signal of 5, Gbit/s is extracted from the RZ signal train passing through the EA modulator, thereby enabling another RZ signal train formed of the remaining (i.e. not extracted) RZ signals to be extracted. This way of using the optical gate device is specially referred to as “optical demultiplexing.” Also, the alternately multiplexing means the time-division multiplexing of the signal trains belonging to different series.
As explained above, the greatest feature of the optical gate device discussed in the above report is that the optical gate waveform of a rectangular shape can be formed with ease by applying the sinusoidal voltage to an EA modulator. Furthermore, the other feature of the above optical gate device is that the sinusoidal voltage is externally applied to the EA modulator, so that it is possible to make the period of the optical gate waveform variable, and thus different from the mode synchronization method. Still further, the other feature of the above optical gate device is that it is not always needed for the feeder line to have a wide-band characteristic as the modulation voltage is the sinusoidal one.
However, some problems to be solved are still left in the demuliplexing system or the optical gate system, which employs the above-mentioned optical gate device. Thus, those will be discussed in the following.
(1) First Problem
The first problem is that the optical demultiplexing system wherein the signal is extracted every other signal from the signal train alternately multiplexed with respect to the time axis, is not only less efficient in making use of the optical energy but also tends to increase the power consumption by the system and to enlarge the system scale as well.
In the optical demultiplexing system employing the above optical gate device, the alternately multiplexed signal train is branched into two parallel branches before executing the optical demultiplexing operation. Then, signals are extracted every other signal from the first signal train of the one branch, and the similar extraction is performed on the second signal train of the other branch. At this time, the phase of the second sinal train is shifted by a half period with respect to the first one. Therefore, this system comes to waste a half of the optical energy, thus being less efficient in making use of the optical energy.
In this case, still further, in order that the optical signal train can hold the same optical power for the sensitivity of the corresponding receiver before and after being processed by the optical demultiplexing, the optical power of the alternately multiplexed signal train has to be doubled by amplifying it at a certain stage either before or after the signal train being branched into two parallel branches. As a result, the power consumption and the scale of the system can not help being increased.
In association with the above first problem, when the above prior art optical gate device processes the optical signal train that is multiplexed with a multiplex factor of n (n: integer), it will encounter the following two significant problems.
One problem that exists is that it can not be expected that the operation of the EA modulator will be stable over a long period of time. In the optical demultiplexing operation of the alternately multiplexed signal train, the DC bias voltage is generally set in the vicinity of the OFF-state region. However, when the signal train is n-multiplexed, the DC bias voltage has to be deeply set in the OFF-state region. Consequently, while the high frequency voltage is deflected toward the negative side, there is applied to the EA modulator the resultant large negative bias voltage obtained as a sum of the DC bias voltage and the high frequency amplitude voltage. As the result of this, the light absorption at the end face for receiving the incident light on the EA modulator is exponentially increased, so that there is a high possibility that the element breaks down and it becomes difficult to expect the stable operation of the EA modulator over a long period of time.
The other problem exists in that the amount (n−1)
of the energy of the input optical signal is unavoidably wasted. In order to remove this problem thus far, the optical signal power has to be amplified to be n times as much as that of the incident light at a certain stage either before or after the signal train is branched into parallel branches of n. As a result, the power consumption and the scale of the system can not help being increased.
(2) Second Problem
The second problem is that in the demuliplexing system or the optical gate system employing the above prior art optical gate device, the gate width (time duration) can be changed only in a narrow range. The ratio of the gate width and inverse value of the repetitive frequency is generally called a duty ratio or cycle. In the gate system employing the above optical gate device, an obtainable duty ratio is in the range of utmost 0% through. 60%. In order to obtain the duty ratio of 60% or more, it is needed to set the DC bias voltage as a large positive voltage or to make the amplitude of the high frequency voltage zero volt and set the DC bias voltage as zero volt or more.
However, if the DC bias voltage exceeds the built-in voltage of the pin junction in the EA modulator, the abrupt current begins to flow, which causes the heat generation at the pin junction, spotaneous light emission, absorption of free carriers, etc. On the other hand, in order to make the amplitude of the high frequency voltage zero, it is need to additionally provide a new electric circuit therefor.
The present invention has been made in view of the above described and other problems in connection with the prior art optical gate device and other optical processors.
SUMMARY OF THE INVENTION
Accor
Dang Hung Xuan
Kunitz Norman N.
Oki Electric Industry Co, LTD
Venable
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