Optical communications – Multiplex – Wavelength division or frequency division
Reexamination Certificate
1999-09-28
2003-12-09
Pascal, Leslie (Department: 2633)
Optical communications
Multiplex
Wavelength division or frequency division
C398S081000, C398S091000, C398S158000, C398S194000
Reexamination Certificate
active
06661974
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an optical transmitter and optical transmission system. More particularly, the invention relates to an optical transmitter and optical transmission system having a function which compensates for wavelength dispersion in order to realize larger capacity, higher speed and longer transmission distance.
Though 10-Gbps optical transmission systems are currently in practical use, there is growing demand for networks of greater capacity owing to a marked increase in utilization of networks in recent years. In particular, highly precise dispersion compensation is required at transmission speeds above 10 Gbps. This means that measuring the dispersion value of an optical transmission line accurately and then compensating for this value is essential. The present invention relates to monitoring of the dispersion value of an optical transmission line and to a technique for optimum compensation of dispersion.
Signal energy contains a variety of components (different frequency components and different mode components), and signal waveform distortion will occur at the receiving end unless signal propagation delay time is constant. The phenomenon which is the cause of this delay distortion is referred to as “dispersion” and decides the transmission capacity of an optical fiber. In order to realize a further increase in capacity, speed and transmission distance in an optical transmission system, there is need for a technique for measuring dispersion with high precision and for compensating for this dispersion.
In a 40-Gbps optical transmission system, wavelength dispersion is one factor that limits transmission distance. Dispersion tolerance declines at the square of the bit rate, At 40 Gbps, dispersion tolerance is 30 ps
m, which is much lower in comparison with a dispersion tolerance of about 800 ps
m at 10 Gbps.
FIG. 74A
illustrates the relationship between amount of dispersion compensation and power penalty based upon a transmission experiment using 40-Gbps OTDM (optical time division multiplexing), 1.3-&mgr;m zero-dispersion SMF (Single-Mode Fiber) having a length of 50 km.
FIG. 74B
shows the measurement system. The system shown in
FIG. 74B
includes an optical transmitter TX, a receiver RX, 1.3-&mgr;m zero-dispersion SMF having a length of 50 km, and −920-ps
m fixed-dispersion compensated fiber CB.
Though 1.3-&mgr;m zero-dispersion SMF has been used for optical transmission lines, dispersion at high transmission speeds imposes limitations. In recent years, therefore, dispersion-shifted optical fiber for the purpose of reducing dispersion by shifting the zero-dispersion wavelength from 1.3 to 1.55 &mgr;m has been developed and laid. The wavelength of light output from the transmitter is 1.55 &mgr;m. As a consequence, 1.55-&mgr;m light is transmitted via the 1.3-&mgr;m zero-dispersion SMF laid originally.
When a 40-Gbps baseband signal is transmitted by a 1.55-&mgr;m optical signal via a 1.3-&mgr;m zero-dispersion SMF having a length of 50 km, dispersion of 920 ps
m is produced. Accordingly, if 100% dispersion compensation is applied using the −920-ps
m fixed-dispersion compensated fiber CB, reception sensitivity degradation will be 0 dB. However, if the amount of dispersion compensation is much larger or smaller than 100% dispersion compensation (=−920 ps
m), reception sensitivity degradation rises and becomes 1 dB at amounts of dispersion compensation of −905 ps
m and −935 ps
m, as shown in FIG.
74
A. In other words, if further dispersion in excess of ±15 ps
m occurs at 100% dispersion compensation (amount of dispersion compensation=−920 ps
m), reception sensitivity degradation will exceed 1 dB.
Accordingly, dispersion compensation tolerance when a power penalty of less than 1 dB is adopted as a condition for enabling transmission is a low 30 ps
m, meaning that precise dispersion compensation must be carried out. Further, owing to temperature and stress which acts upon the optical fiber, the amount of change in transmission-line dispersion must be measured and the amount of dispersion compensation must be optimized within this narrow tolerance in conformity with change with the passage of time. Tolerance deviation D
T
due to temperature is as follows assuming a transmission line of SMF having a length of 50 km and a temperature change of −50° C. to +100° C.:
D
T
=0.03 (nm/° C.)×150 (° C.)×0.07 (ps
m
2
/km)×50 (km)=15.8 (ps
m)
Thus there is the danger that the dispersion compensation tolerance of 30 ps
m will not be met.
FIG. 75
is a characteristic diagram of wavelength dispersion, in which the wavelength (nm) of light output from an optical transmitter is plotted along the horizontal axis and amount of wavelength dispersion is plotted along the vertical axis. When a 40-Gbps baseband signal is transmitted by a 1.55-&mgr;m optical signal via a 1.3-&mgr;m zero-dispersion SMF, dispersion of 920 ps
m is produced, as mentioned above. Accordingly, if 100% dispersion compensation is applied using the −920-ps
m fixed-dispersion compensated fiber CB, wavelength dispersion becomes zero at 1.552 &mgr;m. The zero-dispersion wavelength is 1.552 &mgr;m (=1552 nm). If the wavelength of light output by the optical transmitter deviates from the zero-dispersion wavelength, wavelength dispersion of an amount indicated by the straight line in
FIG. 75
is produced.
A method using 40-GHz component intensity in the baseband spectrum of an OTDM signal and NRZ signal has been considered as a method of wavelength dispersion compensation. This method utilizes a characteristic in which the amount of dispersion becomes zero and the eye pattern openness is maximized at a minimum point between two peaks of the 40-GHz component intensity.
FIGS. 76A and 76B
illustrate the results of simulations of 40-GHz component intensity and eye openness with respect to amount of dispersion in case of a 40-Gbps NRZ signal, in which dispersion value (ps
m) is plotted along the horizontal axis and 40-Gbps component intensity and eye openness are plotted along the vertical axis.
FIG. 76A
is for a case where &agr;>0 holds and
FIG. 76B
for a case where &agr;<0 holds, where &agr; is a chirp parameter representing direction (positive- or negative-going) and amount of fluctuation in a transmission waveform. Wavelength fluctuation (chirp) occurs when the voltage applied to an optical modulator increases or decreases owing to a rise and fall in a data pulse. Owing to the effects of chirping, (1) the rising edge of a pulse on the receiving side is delayed and the falling edge of the pulse is advanced (&agr;<0), or (2) the rising edge of a pulse on the receiving side is advanced and the falling edge of the pulse is delayed (&agr;>0). The diameter of the eye opening is reduced by being shrunk along the time axis in the case of the former and is reduced by being stretched along the time axis in the case of the latter.
In accordance with
FIGS. 76A and 76B
, when &agr;=+0.7, −0.7 holds, the 40-GHz component intensity peaks where the value of dispersion is in the vicinity of −40 ps
m and +40 ps
m, respectively, and the minimum value is obtained at the foot of the peak. Here the dispersion value is zero and the eye openness is maximum. The reason why the 40-GHz component intensity becomes zero when the dispersion value is zero (zero-dispersion wavelength=1552 &mgr;m) and the eye openness is maximum is that in the case of the NRZ signal, 40 Gbps corresponds to 20 GHz and no 40-GHz component is included. This means that the zero-dispersion wavelength can be detected by detecting the foot of the 40-GHz component intensity.
FIGS. 77A and 77B
illustrate the temperature characteristic (experimental values) of wavelength vs. 40-GHz component intensity in the case of the 40-Gbps NRZ signal. These are the results of transmission experiments at temperatures of −35 to +65° C. using a DSF having a length of
Akiyama Yuichi
Chikama Terumi
Ishikawa George
Ooi Hiroki
Fujitsu Limited
Pascal Leslie
Staas & Halsey , LLP
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