Optical: systems and elements – Optical modulator
Reexamination Certificate
1999-02-25
2001-07-17
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
C359S264000, C359S239000
Reexamination Certificate
active
06262828
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the driving of an optical modulator and, more particularly, to the driving of an optical modulator to measure, and compensator for, dispersion in an optical transmission line.
2. Description of the Related Art
Optical transmission systems using fiber optical transmission lines are being used to transmit relatively large amounts of information. For example, optical transmission systems at 10 Gb/s are now in practical use. However, as users require larger amounts of information to be rapidly transmitted, and as more users are connected to the systems, a further increase in the capacity of optical transmission systems is required.
In an optical transmission system having a transmission speed equal to or higher than 10 Gb/s, the transmission wavelength is deteriorated by wavelength dispersion so that it becomes difficult to continue transmitting data. As a result, dispersion compensation must be performed.
For example, if a signal light having a wavelength of 1.55 &mgr;m is transmitted through an optical transmission system at a transmission speed equal to or higher than 10 Gb/s, using a 1.3 &mgr;m non-dispersion single mode fiber, then the wavelength dispersion increases by 18 ps
m/km. This increase in wavelength dispersion is relatively high, thereby requiring dispersion compensation to be performed.
In an optical transmission system having a transmission speed equal to or lower than 10 Gb/s, the range of dispersion tolerance is relatively large, and dispersion compensation can be performed by commonly applying a dispersion compensator having a predetermined dispersion value. For example, when a signal light of 1.55 &mgr;m in wavelength is transmitted through a 1.3 &mgr;m non-dispersion single mode fiber (SMF), the dispersion tolerance is about 800 ps
m. Therefore, a system can be designed in such a way that a dispersion compensator having a predetermined dispersion value, such as a dispersion compensation fiber (DCF) or a fiber grating, is commonly applied for short distance (for example, 20 km through 40 km) transmission systems using a 1.3 &mgr;m non-dispersion single mode fiber.
On the other hand, in an optical transmission system having a transmission speed equal to or higher than 10 Gb/s, the range of dispersion compensation tolerance is small, thereby requiring dispersion compensation to be performed with high precision. As a result, the change of dispersion in the transmission line must be measured and then dispersion compensation must be optimized.
FIGS.
29
(A) and
29
(B) show the results of an experiment indicating the relatively small amount of dispersion compensation tolerance at a transmission speed of 40 Gb/s.
More specifically, FIG.
29
(A) is a diagram illustrating an optical transmission system, and FIG.
29
(B) is a graph illustrating the deterioration of reception sensitivity (power penalty) versus the dispersion compensation ratio of the optical transmission system. Referring now to FIGS.
29
(A) and
29
(B), a signal is transmitted at 40 Gb/s from a transmission unit
341
through a 1.3 &mgr;m non-dispersion single mode fiber
342
for 50 km, and is then received by a receiving unit
344
. Wavelength dispersion occurs at 920 ps
m. Therefore, by adjusting the length of a dispersion compensation fiber
343
, the wavelength dispersion of the 1.3 &mgr;m non-dispersion single mode fiber
342
can be compensated for.
As indicated by FIG.
29
(B), when a power penalty equal to or less than 1 dB is an acceptable transmission condition, the dispersion compensation tolerance is only 30 ps
m. Therefore, accurate dispersion compensation is required. However, in the existing transmission line using the 1.3 &mgr;m non-dispersion single mode fiber
342
, the amount of dispersion is not correctly computed at a number of points. Furthermore, since the amount of dispersion changes with time, depending on, for example, the temperature or the stress applied to an optical fiber, the amount of the dispersion compensation should be appropriately set for each of such intermediate points by accurately measuring the amount and change of dispersion.
Recently, 1.55 &mgr;m non-dispersion shift fibers (DSF) have been adopted to perform extra-high-speed data transmission. The wavelength dispersion occurring when a signal light having the wavelength of 1.55 &mgr;m is transmitted through this fiber is equal to or smaller than ±2 ps
m/km, and the influence of this dispersion is smaller than that with the 1.3 &mgr;m non-dispersion single mode fiber.
However, when the transmission speed is equal to or higher than 40 Gb/s, the dispersion compensation tolerance is very small so that dispersion compensation is required even if the 1.55 &mgr;m non-dispersion shift fiber is used. As a result, the amount of dispersion compensation should be constantly set to an optimum value to further prevent deterioration in transmission with time, as with the 1.3 &mgr;m non-dispersion single mode fiber.
FIGS.
30
(A) and
30
(B) are diagrams illustrating the conventional measurement of dispersion in an optical transmission system.
More specifically, FIG.
30
(A) illustrates a twin-pulse method of measuring the wavelength dispersion. In the twin-pulse method, a group delay time difference is directly measured from the interval of pulses, to measure the wavelength dispersion.
Referring now to FIG.
30
(A), a pulse signal output from a pulse generator
351
is provided to laser diodes
354
and
355
through drive units
352
and
353
, respectively. Upon receipt of a pulse signal from drive unit
352
, laser diode
354
outputs an optical pulse having a wavelength &lgr;
1
. Similarly, upon receipt of a pulse signal from drive unit
353
, laser diode
355
outputs an optical pulse having a wavelength &lgr;
2
.
The optical pulse having the wavelength &lgr;
1
output from laser diode
354
and the optical pulse having the wavelength &lgr;
2
output from laser diode
355
are provided to an optical fiber
357
through a half mirror
356
, and transmitted to a detector
358
through optical fiber
357
.
When the optical pulses having the wavelengths &lgr;
1
and &lgr;
2
are transmitted through optical fiber
357
, detector
358
outputs a detection result to a sampling oscilloscope
359
. Sampling oscilloscope
359
compares the arrival time of the pulse signal received from pulse generator
351
through a delay circuit
360
with the arrival time of the optical pulses detected by detector
358
. The amount of dispersion is obtained by detecting the delay difference between the two optical pulses after transmission through optical fiber
357
.
FIG.
30
(B) illustrates a phase method of measuring the wavelength dispersion. According to the phase method, the group delay time difference is not directly measured, but the wavelength dispersion is obtained from the phase difference between the optical modulation signals generated by the group delay time difference.
Referring now to FIG.
30
(B), a synthesizer
371
modulates an optical signal output from laser diodes
372
through
374
. Laser diode
372
outputs an optical signal having a wavelength &lgr;
1
, laser diode
373
outputs an optical signal having a wavelength &lgr;
2
, and laser diode
374
outputs an optical signal having a wavelength &lgr;
3
. The optical signals output from laser diodes
372
through
374
are switched by an optical switch
375
, provided to an optical fiber
376
, and transmitted to an avalanche photodiode
377
through optical fiber
376
.
Avalanche photodiode
377
converts the optical signal transmitted through optical fiber
376
into an electric signal, and outputs the electric signal to a vector voltmeter
379
through an amplifier
378
. Vector voltmeter
379
compares the electric signal transmitted from synthesizer
371
with the electric signal transmitted from amplifier
378
, and obtains the phase difference between the optical modulation signals, thereby computing the amount of dispersion.
However, a relatively large number of
Akiyama Yuichi
Ishikawa George
Ooi Hiroki
Watanabe Shigeki
Epps Georgia
Fujitsu Limited
Lucas Michael A
Staas & Halsey , LLP
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