Optics: measuring and testing – By polarized light examination
Reexamination Certificate
2000-10-05
2003-12-30
Berman, Jack (Department: 2881)
Optics: measuring and testing
By polarized light examination
C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06671045
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to optical communications, and more particularly relates to an apparatus and a method for measuring optical signal-to-noise ratio in optical communications. An optical amplifier is a device to amplify optical signals without photo-electric/electric-photo conversion, and does not depend on the transmission speed or the transmission format of optical signals. A conventional optical transmitter amplifies the signals after photo-electric conversion, and reproduces optical signals with electric-photo conversion. The optical amplifier is substituted for the conventional optical transmitter. In particular, the optical amplifier can simultaneously amplify many optical signals of different wavelengths in a wide band, and the optical amplifier plays an important role in Wavelength Division Multiplexing (WDM) optical communications. In order to increase the transmitting capacity, several channels of optical signals are multiply divided with different wavelengths in WDM
2. Description of the Related Art
The optical amplifier produces not only the amplified optical signals but also the noise of wide wavelength band. Even though optical filters are used to remove the noise included in the output beam of the optical amplifier, the noise of same wavelengths of the signals can not be eliminated. Optical signal-to-noise ratio, the power of the optical signals divided by the power of the corresponding noise, is used as a standard for a transmitting quality of optical communication network. And it is necessary to monitor the optical signal-to-noise ratio at optical lines or at optical nodes for optical communication network management.
FIG. 1
is a power spectrum of the output beam when the optical amplifier amplifies the multiplexed signals without optical filters in WDM optical communications. The spectrum is taken from a conventional spectrum analyzer. As mentioned above, the optical signal-to-noise ratio is the value of the power of the optical signals divided by the power of the corresponding noise. However, the noise can not be measured directly since the noise are detected with the optical signals as shown in FIG.
1
.
FIG. 2
is a graph to explain how to measure the optical signal-to-noise ratio of output signals shown in FIG.
1
. In order to measure the ratio, first take a power spectrum of the multiplexed signals amplified with the optical amplifier. From the obtained spectrum, find A, the peak power of the Optical Signal
1
, and measure a, b, the neighboring noise power. Calculate (a+b)/2, the average noise-power, and assume it as the noise power at the wavelength of the Optical Signal
1
. Then, the optical signal-to-noise ratio of the Optical Signal
1
is obtained using EQUATION 1.
Optical Signal-to-noise ratio
of the Optical Signal 1
=
A
-
(
a
+
b
)
/
2
(
a
+
b
)
/
2
[
EQUATION
1
]
Similarly, the ratio of the Optical Signal
2
, and Signal
3
can be obtained.
However, in some cases, it is impossible to measure the optical signal-to-noise ratio with the above method in FIG.
2
.
FIG. 3
explains these cases, and shows another power spectrum of the output beam when optical filters are used to remove the noise in WDM optical communications. According to the method in
FIG. 2
, the power (a, b) of noise nearby the peak wavelength should be known in order to measure the optical signal-to-noise ratio. In cases of
FIG. 3
, the method in
FIG. 2
can not be used since the noise is not easily distinguishable from the optical signals.
FIG. 4
is a block diagram to solve the problems, and shows a device to measure the optical signal-to-noise ratio using a polarization controller and a linear polarizer (LP). The optical signal-to-noise ratio in
FIG. 3
can be measured with the instruments in FIG.
4
. The device (
400
) for measuring the optical signal-to-noise ratio shown in
FIG. 4
is published in '98 European Conference on Optical Communication, p. 549-550, 1998' with the title of “Optical Signal-to-Noise Ratio Measurement in WDM Networks Using Polarization Extinction” by M. Rasztovits-Wiech, M. Danner, and W. R. Leeb.
In optical communications, laser diodes are generally used as a light source. The polarization state of the output beam from a laser diode is 100% linearly polarized, and the optical signals are still 100% polarized even though the polarization state is changed as the signals traveling the optical fiber. On the other hand, the noise from an optical amplifier is 100% unpolarized since the noise consists of the randomly occurred lights of all polarization states.
Therefore, the power of the interested noise can be measured when the amplified optical signals are eliminated using a polarization controller (
401
) and a LP (
402
). The polarization controller controls the polarization of the input beam, and the LP passes only the component of the light coincide with the polarization axis. The 100% polarized optical signals can be completely eliminated; the polarization controller (
401
) can control the polarization state of the optical signals even after the signals traveled the optical fiber, and the controller changes the polarization of the optical signals orthogonal to the polarization axis of the LP (
402
). However, the power of the noise passing through a LP (
402
) always reduces to the half since the noise consists of the lights of all polarization states.
Referring
FIG. 4
, the output beam (shown in
FIG. 1
) of the optical amplifier is inputted into the polarization controller (
401
). Adjust the polarization controller to maximize the power of the optical signals passing through the LP (
402
) and the variable optical band-pass filter, VOBPF (
403
) at the photo detector (
404
), and measure the maximum value. Then, readjust the polarization controller to minimize the power at the photo detector (
404
), and measure the minimum value. Repeat the process for the full spectrum range by changing the passing wavelength of the VOBPF (
403
).
FIG.
5
(
a
) is a spectrum of the output beam in
FIG. 1
when the power at the photo detector (
404
) in
FIG. 4
is maximized, and FIG.
5
(
b
) is another spectrum when the power is minimized. In FIG.
5
(
a
), the peak power is the sum of the power, D, of the optical signals and the half, d, of the noise power, while the power in FIG.
5
(
b
) is the half of the original noise. Then, the optical signal-to-noise ratio of the Optical Signal
1
is obtained using EQUATION 2.
Optical Signal-to-noise ratio
of the Optical Signal 1
=
D
-
d
2
×
d
[
EQUATION
2
]
Similarly, the ratio of the Optical Signal
2
, and Signal
3
can be obtained.
FIG.
6
(
a
) is a spectrum of the output beam in
FIG. 3
when the power at the photo detector (
404
) in
FIG. 4
is maximized, and FIG.
6
(
b
) is another spectrum when the power is minimized. In FIG.
6
(
a
), the peak power is the sum of the power, E, of the optical signals and the half, e, of the noise power, while the power in FIG.
6
(
b
) is the half of the original noise. Therefore, the optical signal-to-noise ratio of the Optical Signal
1
is obtained using EQUATION 3.
Optical Signal-to-noise ratio
of the Optical Signal 1
=
E
-
e
2
×
e
[
EQUATION
3
]
Similarly, the ratio of the Optical Signal
2
, and Signal
3
can be obtained.
However, the preceding method shown in
FIG. 4
needs to adjusts the polarization controller (
401
) to find the maximum and the minimum of the optical power at the photo detector (
404
) for each given wavelength. And the method has two major problems; (1) long operation time to measure the optical signal-to-noise ratio, and (2) a complicated active control circuit to handle the polarization controller (
401
).
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an apparatus and a method for measuring the optical signal-to-noise ratio using Stokes parameters in optical communications. The present invention
Chu Kwang Uk
Lee Chang Hee
Shin Sang Yung
Bacon & Thomas PLLC
Berman Jack
Fernandez K.
Korea Advanced Institute of Science & Technology
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