Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2001-02-26
2003-06-03
Kim, Robert H. (Department: 2887)
Optics: measuring and testing
For optical fiber or waveguide inspection
C356S484000, C356S487000, C385S123000
Reexamination Certificate
active
06573985
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a device measuring dispersion on an optical transmission line, and more particularly, to a device monitoring a dispersion value of an optical transmission line and optimally compensating for dispersion.
2. Description of the Related Art
Currently, a 10-Gbps optical transmission system starts to be put into use. With a sharp increase in network use in recent years, the demand for further increasing a network capacity has been growing. Since dispersion compensation must be made with high accuracy at a transmission speed of 10 Gbps or faster, it is essential to accurately measure the dispersion value of a transmission line.
In an optical transmission system of a transmission speed of 10 Gbps or faster, its wavelength dispersion tolerance is very small. For example, the wavelength dispersion tolerance of a 40-Gbps NRZ system is 100 ps
m or smaller. In contrast, for a ground optical transmission system, a relay section is not always uniform. A system using a 1.3 &mgr;m zero-dispersion SMF (Single Mode Fiber) of approximately 17 ps
m/km exceeds the wavelength dispersion tolerance, even if a relay section varies only by several kilometers.
However, in an optical fiber network possessed by a carrier, most of the distances and the wavelength dispersion values of relay sections are not accurately grasped. Furthermore, since a wavelength dispersion value changes with time due to the temperature of an optical fiber or the stress applied to an optical fiber, etc., the dispersion compensation amount for each relay section must be suitably set while strictly measuring the wavelength dispersion amount not only at the start of system use but also in system use. For example, if a temperature change of 100 degrees centigrade occurs on a DSF (Dispersion Shifter Fiber) transmission line of 500 kilometers, a wavelength dispersion change amount results in approximately 105 ps
m, which is nearly equivalent to the wavelength dispersion tolerance of an NRZ signal.
(wavelength dispersion change amount)=(temperature dependency of a zero-dispersion wavelength)×(temperature change amount in a transmission line)×(dispersion slope of a transmission line)×(transmission distance)=0.03 (nm/° C.)×100 (° C.)×0.07 (ps
m
2
/km)×500 (km)=105 ps
m
where the dispersion slope of a transmission line is a differentiated value (ps
m
2
/km) of a dispersion amount, which will be described later.
Accordingly, a system automatically measuring a dispersion amount is essential not only for an SMF transmission line but also for a system using a 1.55 &mgr;m zero-dispersion DSF or an NZ-DSF transmission line.
As a currently used wavelength dispersion monitoring method, the following two methods can be cited.
1. twin-pulse method
2. optical phase comparison method
FIG. 1
shows the outline of the configuration of a wavelength dispersion measuring device using the twin-pulse method.
The twin-pulse method is a method obtaining a wavelength dispersion amount (group delay) by using two optical pulse signals having different wavelengths, and by measuring the delay difference between the two pulses after being transmitted over a fiber to be measured. In this case, two LDs producing different wavelengths, and their driving units are required.
First of all, an electric signal pulse is generated from a pulse generator
10
, and at the same time, a trigger signal for starting measurement is transmitted to a group delay measuring instrument. The electric pulse transmitted from the pulse generator
10
is input to two driving units
11
-
1
and
11
-
2
, which are made to simultaneously output optical pulses to LDs
12
-
1
and
12
-
2
that respectively produce lights having wavelengths &lgr;
1
and &lgr;
2
. Optical pulses produced by the LDs
12
-
1
and
12
-
2
are multiplexed by an optical multiplexer such as a half mirror
13
, a coupler, etc., and are propagated through an optical fiber transmission line
14
. The two optical pulses that propagate through the optical fiber transmission line
14
are detected by a detector
15
, and the detection result of the optical pulses is transmitted to the group delay measuring instrument
17
. In the meantime, the trigger signal output from the pulse generator
10
is delayed in a delaying circuit
16
by an amount of time required to propagate the optical pulses through the optical fiber transmission line, and input to the group delay measuring unit
17
as a trigger signal for starting up the group delay measuring unit
17
.
The group delay measuring unit
17
detects the difference between the arrival times of the two optical pulses detected by the detector
15
, and calculates the group delay times of the optical pulses having the wavelengths &lgr;
1
and &lgr;
2
.
FIGS. 3A and 3B
show the states of optical pulses propagated with the twin-pulse method.
As shown in
FIG. 3A
, optical pulses having wavelengths &lgr;
1
and &lgr;
2
are simultaneously generated, multiplexed, and output. Since the optical pulses having the wavelengths &lgr;
1
and &lgr;
2
are simultaneously output at this time, the pulses are multiplexed into one and input to a transmission line as shown on the right side of FIG.
3
A. However, a group delay is caused by the wavelength dispersion of the transmission line. Therefore, when the optical pulses having the wavelengths &lgr;
1
and &lgr;
2
are received on a receiving side, there is a reception time lag between the optical pulses as shown in FIG.
3
B. Here, it is assumed that the group delay of the optical pulse having the wavelength &lgr;
1
is larger than that of the optical pulse having the wavelength &lgr;
2
.
Accordingly, a group delay time can be calculated based on the difference between the arrival times of the two optical pulses having the wavelengths &lgr;
1
and &lgr;
2
, and a wavelength dispersion amount can be calculated by using the difference between the wavelengths &lgr;
1
and &lgr;
2
.
FIG. 2
shows the outline of the configuration of a wavelength dispersion measuring device using the optical phase comparison method.
The optical phase comparison method is a method obtaining a wavelength dispersion amount not by directly measuring a group delay time difference, but by acquiring a phase difference between modulated optical signals, which is caused by a group delay time difference.
First of all, an electric pulse is generated by a pulse generator
10
. At the same time, a trigger signal for notifying a phase detector
18
of a measurement reference time of the propagation time of an optical pulse is transmitted by the pulse generator
10
.
The electric pulse transmitted from the pulse generator
10
is input to a driving unit
11
, and an optical pulse having a wavelength &lgr; is output from an LD
12
. This optical pulse propagates through a transmission line
14
, and is detected by a photodetector
15
. The photodetector
15
inputs the signal indicating the detection of the optical pulse to a phase detector
18
. The phase detector
18
measures the delay time of the arrival of the optical pulse with reference to the time at which the trigger signal is received from the pulse generator
101
.
Then, the wavelength of the optical pulse transmitted from the LD
12
is changed, and the above described measurement is repeated in a similar manner. As a result, the propagation time of the optical pulse, which indicates the delay time when the optical pulse transmitted with the different wavelength is detected on a receiving side from the reference time, may differ. This propagation time difference is a group delay time difference. When the group delay time difference is obtained in this way, the wavelength dispersion of a transmission line is obtained from the wavelength difference and the group delay time difference.
FIG. 3C
shows the state of optical pulses used in the optical phase comparison method.
If the input time of the trigger signal input from the pulse generator
10
to the phase detector
1
Ibukuro Sadao
Ishikawa George
Fujitsu Limited
Ho Allen C.
Kim Robert H.
Staas & Halsey , LLP
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