Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2001-10-23
2003-07-29
Pyo, Kevin (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S216000, C359S484010
Reexamination Certificate
active
06600148
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a polarization mode dispersion measuring method and a polarization mode dispersion measuring system, and relates, particularly, to a polarization mode dispersion measuring method and a polarization mode dispersion measuring system employing a technique for measuring in high precision the polarization mode dispersion of an optical device as a measured object.
BACKGROUND ART
As known, among optical devices, refractive indexes of an optical fiber, for example, vary depending on the direction of polarization.
Because of differences in refractive indexes due to this difference in polarization direction, a group delay time difference occurs in the polarization directions of light transmitted by this optical fiber.
As a result, there occurs a phenomenon that when an optical pulse or the like is incident to an input end of the optical fiber, the width of the optical pulse is expanded at an output end.
This phenomenon is called “polarization mode dispersion” (PMD), and, in a long-distance and high-speed optical transmission system, this is an important factor determining the performance of this system.
Particularly, in the case of carrying out an optical transmission of 1.55 &mgr;m by using a 1.3 &mgr;m zero-dispersion optical fiber, in mainstream use throughout the world at present, or in a wavelength-multiplexed system and a high-speed transmission (Gbit/s order) long-distance optical submarine cable system that use an optical amplifier, this polarization mode dispersion (PMD) becomes a big problem, and extremely limits the propagation distance.
Therefore, in the case of designing an optical transmission system or the like, it is necessary to measure in advance a level of the polarization mode dispersion (PMD) of optical devices including an optical fiber that are used in this system.
As a method for measuring this polarization mode dispersion (PMD), a fixed analyzer method that has a characteristic of high measuring precision while having a simple structure has been widely used among an interference method in a time region, the fixed analyzer method in a frequency region, and a polarization analysis method that have been conventionally known.
FIG. 6
shows a conventional measuring system for measuring a polarization mode dispersion by using this fixed analyzer method.
According to this measuring system, after a linearly polarized beam in a specific polarization direction has been extracted by a first polarizer
12
from a light emitted from a broad band light source
11
, this extracted linearly polarized beam is incident to one end of a measured object
1
.
Then, from a light emitted from the other end side of the measured object
1
, a linearly polarized beam in the same polarization direction as that of the light from the first polarizer
12
is extracted by a second polarizer
13
, and thereafter, this extracted linearly polarized beam is incident to an optical spectrum analyzer
14
.
In this case, a polarization direction of the linearly polarized beam incident to the measured object
1
is set to have an angle of 45 degrees with respect to the X axis (or the Y axis) by the first polarizer
12
, when the incident surface of the measured object
1
is the XY plane.
Further, the second polarizer
13
is also matched with the direction of this first polarizer
12
.
The transmission speed of an X-axis component and the transmission speed of a Y-axis component of the linearly polarized beam that has been incident to the measured object
1
do not become the same, due to a difference between the refractive index of a portion of the measured object
1
that follows the X-axis and the refractive index of a portion that follows the Y-axis.
For example, as shown in
FIG. 7A
, assume that a linearly polarized beam of a certain wavelength &lgr;a is incident to the measured object
1
, and its Y-axis component is delayed (or advanced) by 2 &pgr; from the X-axis component. Then, the polarization direction of the wavelength &lgr;a emitted from the measured object
1
becomes the same as the incident status.
Accordingly, the light of this wavelength &lgr;a is transmitted through the second polarizer
13
with a small loss.
Further, as shown in
FIG. 7B
, assume that a linearly polarized beam of a certain wavelength &lgr;b is incident to the measured object
1
, and its Y-axis component is delayed (or advanced) by &pgr; from the X-axis component. Then, the polarization direction of the wavelength &lgr;b emitted from the measured object
1
becomes orthogonal with the polarization direction extracted by the second polarizer
13
.
Accordingly, the light of this wavelength &lgr;a is attenuated large by the second polarizer
13
and cannot substantially be transmitted through.
This phenomenon occurs due to a difference in the delay time attributable to a difference between the refractive indexes of the measured object in the X-axis direction and Y-axis direction.
Further, as this delay time difference has continuity in the wavelength of a transmitted light, the intensity of the light transmitted through the second polarizer
13
changes in a constant period with respect to a change in the wavelength.
As a result, the optical spectrum analyzer
14
displays a spectrum waveform of which level changes periodically, as shown in FIG.
8
.
Then, according to this fixed analyzer method, a differential group delay time that shows a level of PMD (this is called a PMD value) is calculated by the following equation, using a first peak wavelength &lgr;1 and a last peak wavelength &lgr;2, based on the assumption that a wavelength distance between adjacent peaks (or between adjacent bottoms) of this spectrum waveform is expressed as a phase difference 2&pgr;:
&Dgr;&tgr;=
k
(
n
−1)·&lgr;1·&lgr;2/(
C·&Dgr;&lgr;)
where, k represents a mode coupling coefficient, C represents an optical speed, n represents a number of peaks, and &Dgr;&lgr;=&lgr;2−&lgr;1.
The mode coupling coefficient k is a value equal to or lower than 1 that is determined according to an inter-mode coupling status between the X-axis component and the Y-axis component of the light transmitted through the measured object
1
. When the optical path length is not very long, it is possible to set k=1.
However, according to the above-described conventional PMD measuring system, based on the principle of measurement, it is not possible to calculate a PMD value, when at least two peaks do not exist in the spectrum displayed in the optical spectrum analyzer
14
.
In other words, in the above-described equation for calculating a PMD value, it is a measurement limit of a PMD value when &Dgr;&lgr; is a maximum at the time of n=2, that is, up to when two peaks (or bottoms) exist within a bandwidth of a broad band light source.
Therefore, according to the above-described conventional PMD measuring system, the measurement limit of a PMD value is controlled by the bandwidth of the broad band light source.
Actually, a bandwidth (half-value width) that can be used in the fixed analyzer method is up to about 200 nm, and a measurement limit of a PMD in the 1500 nm band becomes &Dgr;&tgr;=50×10
−12
(second).
Therefore, in order to obtain a PMD value equal to or smaller than this by using the fixed analyzer method, there is a problem that it is necessary to use other methods having complex structures (the polarization analysis method in a frequency region, and the interference method in a time region).
DISCLOSURE OF INVENTION
An object of the present invention is to provide a polarization mode dispersion measuring method and a polarization mode dispersion measuring system capable of obtaining a PMD value up to a smaller measurement limit, in a simple structure according to the fixed analyzer method, by solving the above conventional problems.
In order to achieve the above object, according to the present invention,
(1) there is provided a polarization mode dispersion measuring method comprising the steps of:
applying a polarization mode dispersion to a linearly
Anritsu Corporation
Frishauf Holtz Goodman & Chick P.C.
Pyo Kevin
Sohn Seung C.
LandOfFree
Polarization mode dispersion measuring method and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Polarization mode dispersion measuring method and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Polarization mode dispersion measuring method and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3045811