Optical waveguides – Polarization without modulation
Reexamination Certificate
2002-10-23
2004-01-13
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Polarization without modulation
C385S027000, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06678431
ABSTRACT:
BACKGROUND OF THE INVENTION
This application claims the priority of Korean Patent Application No. 2002-009281, filed Feb. 21, 2002, which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a method for compensating polarization mode dispersion (PMD) occurring in optical transmission fiber in a high speed optical transmission system and an apparatus therefor.
2. Description of the Related Art
Recently, communication data has been rapidly increased due to an increase in a demand for Internet, and such a trend will be increased more in future. An optical transmission system with a wide bandwidth is required to accept a large capacity of data. In order to manufacture the optical transmission system, time division multiplexing (TDM) and wavelength division multiplexing (WDM) have been studied in a wider range. In TDM, as a bit rate increases, polarization mode dispersion (PMD) becomes influential.
In PMD, arbitrary birefringence is led to optical fiber on a transmission path due to internal causes such as an asymmetric optical fiber core structure and stress, and external causes like variations in an environment, such as variations in temperature, vibration, and pressure around a transmission path, thereby an optical signal by birefringence undergoes waveform distortion. The waveform distortion occurs due to a differential time delay of two polarization components perpendicular to each other commonly referred to as principal state of polarization (PSP) in transmission fiber. In this case, each PSP is transmitted without change of waveform. In a case where a time delay having a size same as the differential time delay of the two PSPs and having a reverse direction is forcibly applied to the two PSPs, the two PSPs are offset against each other, thereby distortion due to PMD is compensated. Several studies according to the principle are as below.
There is the article entitled “Polarization Mode Dispersion Compensation by Phase Diversity Detection”, by B. W. Hakki, and published in IEEE Photonics Technology Letters, Vol. 9, No. 1, p 121-123, and the structure of a polarization mode dispersion (PMD) compensator
1500
that is proposed by B. W. Hakki and is referred to as prior art
1
is shown in FIG.
15
. In the PMD compensator
1500
, a polarization beam splitter
1530
optically separates two polarization components passing a transmission path, and a differential time delay is obtained from a mixer
1540
, and a time delay having a size same as the differential time delay and having a reverse direction is electrically applied to the two polarization components, and the two polarization components are added by a combiner
1590
, thereby a distorted signal is compensated. However, in the PMD compensator
1500
, as a speed of a transmitted optical signal, that is, a bit rate, increases, a differential time delay should be calculated more precisely, and thus the mixer
1540
requires expensive high speed electronic devices. The physical length of the delay line
1570
is limited, resulting in restricting a compensation range, and mechanical movement is required for operation of the delay line
1570
, and thus may make a bad effect on the reliability of a system.
A technique for compensating a differential time delay due to PMD by controlling an optical delay line and a polarization transformer in a Mach-Zehnder interferometer type PMD compensator by monitoring electrical spectrum is disclosed in U.S. Pat. No. 5,930,414 entitled “Method and Apparatus for Automatic Compensation of First-Order Polarization Mode Dispersion (PMD)”, by Fishman et al., and published on Jul. 27, 1999. A compensator
1640
that is disclosed in U.S. Pat. No. 5,930,414 and is referred to as prior art
2
is shown in FIG.
16
. In the compensator
1640
, an optical signal output from a tap
1643
via a Mach-Zehnder interferometer
1642
, passes through a photodetector
1646
and an amplifier
1645
, and obtains an electrical power depending on a differential time delay. The control signal is applied to an automatic polarization transformer
1641
and an optical delay line
1642
using the electrical power as a feedback signal, the direction and size of PMD of the optical signal are controlled, and then the output of the optical signal is again monitored. As a result of this iterative process a compensated signal is eventually achieved. In the compensator
1640
, an output signal of the amplifier
1645
passes a filter to a RF power detector included in a distortion analyzer
1644
and is integrated in a given frequency range so as to obtain an unambiguous feedback signal. However, the compensator
1640
employs a method for compensating PMD by a limited range of optical delay line, and thus, there is a limitation in a PMD compensation range, and the reliability of the system may degrade due to the use of the delay line that operates mechanically. An integrator is used to obtain an unambiguous signal, and thus an additional integration process is required. The control method of adjusting all possible polarization states with the automatic polarization transformer
1641
for each given differential time delay of the delay line
1642
may require relatively much time to obtain a final compensated signal.
In addition, the configuration of the above techniques is relatively complicated.
A technique for using only one polarization component between two principal states of polarization (PSPs) as a compensated signal is disclosed in U.S. Pat. No. 6,130,766 entitled “Polarization Mode Dispersion Compensation via an Automatic Tracking of Principal State of Polarization”, by Cao et al., and published on Oct. 10, 2000. A compensator
1720
that is disclosed in U.S. Pat. No. 6,130,766 and is referred to as prior art
3
is shown in FIG.
17
. In
FIG. 17
, an optical source
1712
at a transmission terminal
1710
is frequency modulated using a dithering input signal and a driver
1711
. By controlling a polarization controller (PC) and tracking PSPs in a way that minimizes a second-order harmonic component of an interference signal between two PSPs that is detected from one output of a polarization beam splitter (PBS)
1725
, axes of the two PSPs coincide with two axes of the PBS
1725
, and therefore a undistorted compensation signal is obtained by selecting only one PSP between the two PSPs existing in the output of the PBS
1725
. However, the above compensation method requires an additional apparatus such as a driver for frequency-modulating at a transmission terminal and additional manipulation. Moreover, a relatively complicated digital signal processing method using a DSP control unit
1722
is used. A smaller signal between the two PSPs may be selected as an output during PSP tracking, and thus, the reliability of the system cannot be guaranteed.
SUMMARY OF THE INVENTION
To solve the above problems, it is a first object of the present invention to provide a method for compensating polarization mode dispersion (PMD), in which there are no limitations on a PMD compensation range, and the reliability of a system can be guaranteed by allowing the optical power of a compensated output signal to be over half of input power, apparatus, and system therefor.
It is a second object of the present invention to provide a method for tracking principal state of polarization (PSP), in which a compensated signal is eventually achieved by iterative feedback control to minimize an electrical power at a certain specified frequency such as bit-rate frequency or its harmonics for NRZ format and twice the bit-rate frequency or its harmonics for RZ format.
It is a third object of the present invention to provide a method for compensating polarization mode dispersion (PMD), in which PMD can be compensated at a high speed by a simple optical structure and a simple signal processing method without requiring high-priced high speed electronic elements, apparatus, and system therefor.
Accordingly, to achieve the first, second, and third objects, according to one aspect of the present inv
Han Ki-ho
Lee Sang-soo
Blakely & Sokoloff, Taylor & Zafman
Electronics and Telecommunications Research Institute
Sanghavi Hemang
Wong Eric
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