Optical transmitter and optical transmitter control method...

Details Optical transmitter and optical transmitter control method... Optical transmitter and optical transmitter control method...

2000-06-06

2004-07-13

Pascal, Leslie (Department: 2633)

Optical communications

Transmitter

Including compensation

C398S191000, C398S193000

Reexamination Certificate

active

06763197

ABSTRACT:

BACKGROUND OF THE INVENTION
1. FIELD OF THE INVENTION
The present invention relates to an optical transmitter for generating optical signals to be transmitted through transmission media such as optical fibers having chromatic dispersion that can degrade transmission quality, and a method for controlling such an optical transmitter.
2. DESCRIPTION OF THE BACKGROUND ART
In the optical transmission system, the degradation of transmission quality caused by the waveform distortion due to chromatic dispersion of the optical fibers that are used as transmission media is of great concern. This phenomenon occurs when an optical pulse width is increased to cause interferences with respect to neighboring time-slots as a bandwidth of optical signals is affected by the group velocity dispersion of the optical fibers.
FIG. 1
shows a prior art as disclosed in Japanese Patent Application Laid Open No. 10-79705 (1998). This prior art reference proposes an optical modulation apparatus in which the group velocity dispersion is cancelled by applying a pre-chirp to optical signals at a transmitter, so as to suppress the degradation due to the group velocity dispersion. In this proposition, the waveform distortion at a receiving end is suppressed by applying a frequency chirp in an amount that nearly matches the group velocity dispersion of the optical fibers at an optical transmitter in advance.
In
FIG. 1
, the continuous-wave light generated by a light source
101
is divided into two optical clocks by a clock generator
102
. This clock generator
102
can be a Mach-Zehnder optical modulator, for example, which is to be driven by, sinusoidal wave electric signals. At this point, a value and a sign of the chirping are controlled by controlling a bias voltage to be applied to the optical modulator or the like at the pre-chirp unit
106
. Also, in the case of using an electro-absorption optical modulator as the clock generator
102
, the amount of the chirping is adjusted by controlling the bias voltage to be applied to the electro-absorption optical modulator at the pre-chirp unit
106
. Then, the chirp controlled optical clocks are encoded by first and second data modulators
103
and
104
provided as next stage optical modulators, and then multiplexed by an optical multiplexer
105
to obtain desired optical signals.
However, in this optical modulation apparatus, the frequency chirp in an amount for cancelling the group velocity dispersion is applied in advance so that a bandwidth of the optical signal spectrum is increased. The optical spectrum bandwidth is inversely proportional to the square of the tolerance with respect to the group velocity dispersion, so that the tolerance is progressively lowered when the bandwidth is increased. Consequently, this conventional scheme for applying the pre-chirp lowers the dispersion tolerance and thereby hampers a stable operation of a transmission system. In other words, the transmission quality will be degraded even by a slight difference in the group velocity dispersion.
Also, in the case of using the electro-absorption optical modulator as the clock generator
102
, the non-linear dependency of the extinction ratio with respect to the bias voltage as shown in this prior art reference can cause a problem. Namely, when the bias voltage is changed in order to control the frequency chirp, the clock pulse width is also varied so that the optical spectrum bandwidth is also changed and the dispersion tolerance is also changed. These chirp and pulse width cannot be varied independently, so that it is also difficult to set a desired amount of the chirping stably.
In addition, there is a large optical power loss at a time of extracting clock light from continuous-wave light using a gate provided by a modulator in the clock generator
102
, and the loss is further increased as two encoded clock lights extinguish each other due to the optical interference effect at a time of multiplexing at the optical multiplexer
105
. This causes a decrease in the optical power at an output end of the optical modulation apparatus, which in turn causes a lowering of S/N ratio at a time of transmission.
FIG. 2
shows three graphs indicating a relationship between the dispersion tolerance and the chromatic dispersion of optical clock having a bit rate of 80 Gbit/s, using a relative optical phase of two encoded optical clocks at a time of multiplexing at an optical multiplexer as a parameter. When the bit rate is different, the absolute value of dispersion (|D|(ps
m)) indicated on the horizontal axis is changed but a relative relationship regarding an amount of penalty with respect to the duty ratio and the relative optical phase difference remains the same. Here, “in-phase” is the case where the relative optical phase difference is 0 or integer multiple of 2&pgr;, “out-of-phase” is the case where the relative optical phase difference is an odd integer multiple of &pgr;, and “middle-phase” is the case where the relative optical phase difference is &pgr;/2 or odd integer multiple of &pgr;/2.
As shown in
FIG. 2
, the degradation of receiver sensitivity becomes noticeable when the duty ratio is excessively increased. In other words, in order to realize both a high dispersion tolerance and a low receiver sensitivity degradation, there is a need to control the duty ratio optimally. However, in the prior art, the optical clock is generated using sinusoidal waves in a frequency equal to the bit rate before the multiplexing by setting the driving point at a linear portion of the modulator, and controlling this optical clock generation at a desired value will require the change of modulation level, which in turn will cause a problem of the extinction ratio degradation. Moreover, the signal waveform will be degraded due to the interference effect at a time of the multiplexing.
Also, Japanese Patent Application Laid Open No. 10-79705 (1998) discloses an exemplary case of driving by rectangular waves, but such a driving by rectangular waves widens the optical spectrum excessively so that the dispersion tolerance will be lowered. For this reason, it is expected that the influence of the group velocity dispersion of the optical fibers with respect to the transmission distance becomes more stringent. Also, a driving by sinusoidal waves can only realize excessively broad duty ratio, so that in the case where the number of divisions is 3 or more as shown in this prior art reference, it is difficult to generate practically effective optical signals because the interference effect between adjacent optical clocks is so large that they extinguish each other.
On the other hand,
FIG. 3
shows another prior art as disclosed in Japanese Patent Application Laid Open No. 9-261207 (1997), in which outputs of optical modulators
111
and
112
are multiplexed at an optical multiplexer
113
while a low frequency signal generated by a low frequency oscillator
115
are superposed by a phase modulation to the optical signal in one of them, and a part of the optical signal after the multiplexing is monitored by an optical phase detection and control unit
116
and the phase control is carried out by an optical phase control unit
110
such that the intensity of the intensity modulated component of the low frequency signal in the monitored part becomes minimum, so as to automatically maintain the relative optical phase difference.
However, the minimum value control has a rather poor sensitivity so that it is inevitable to superpose the low frequency, signal at relatively large amplitude as already shown in this prior art reference. However, as already mentioned above, this amounts to displacing the relative optical phase difference from &pgr; intentionally so that it approaches to the case of the relative optical phase difference equal to &pgr;/2 which is associated with the severe degradation of dispersion tolerance. In other words, this controlling itself causes the degradation of dispersion tolerance.
As described, in the conventional optical transmitter that attempts to suppress the degrad

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