Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2001-08-29
2003-10-14
Ngo, Hung N. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S024000, C385S037000
Reexamination Certificate
active
06633704
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to chromatic dispersion compensation, and particularly to polarization mode dispersion compensation for chirped fiber gratings.
2. Technical Background
One problem associated with upgrading the channel capacity of optical communication systems is compensating the dispersion of the optical signals. In order to increase the capacity of optical communication systems it is necessary to cancel the accumulated dispersion over the entire operational wavelength range. Thus, accurate control of dispersion control is required. Typically, optical communication systems will require dispersion control of less than 100
picoseconds per nanometer and potentially as low as 20 picoseconds per nanometer. Dispersion may be expressed as the variation in group delay, &tgr;, with respect to wavelength &lgr; or D(&lgr;)=d&tgr;/d&lgr;. Alternately, dispersion may be expressed mathematically as:
D
(&lgr;)=
D
(&lgr;
c
)+
D
′(&lgr;
c
)(&lgr;−&lgr;
c
) (1)
where,
D(&lgr;
c
) is the dispersion (second order); and
D′(&lgr;
c
) is the dispersion slope (third order) or S.
One approach to dispersion compensation in optical communication systems is to intersperse sections of dispersion compensating optical waveguide fiber between segments of transmission of optical fiber. Some factors that influence the performance and design of dispersion compensation modules include; changing traffic patterns, variations in the optical power of the optical signals, temperature fluctuations along the length of the optical fiber waveguides, installation effects on the cable, modulation format, channel spacing and irregularities with the optical waveguide fiber. Because the magnitude of dispersion in optical communication systems may change, dispersion compensation using dispersion compensating fiber will not be optimal for every channel and a tunable dispersion compensation is required.
Dispersion may be compensated for dynamically during the operation of the optical communication system in order to minimize time-dependent effects such as, for example, the variation in the dispersion characteristics of optical waveguide fibers resulting from fluctuations in temperature.
One proposed approach for dispersion compensation uses fiber Bragg gratings as tunable dispersion compensators for a single channel. In one proposed configuration, a single non-linearly chirped fiber Bragg grating is strained to vary the dispersion within the band of wavelengths of interest. Another proposed configuration uses temperature to shift the wavelength of the entire dispersion curve. This approach to dispersion compensation introduces some amount of dispersion slope across the passband. The thermally tuned configuration dynamically alters the amount of chirp along the length of the grating. The thermally tuned configurations proposed thus far require complex tuning mechanisms to control the dispersion compensation as well as requiring a second tuning mechanism to maintain a constant grating center wavelength. Thus there is a need for a less complex, tunable dispersion compensator for optical communication systems.
Polarization mode dispersion is the maximum difference of group delay as a function of polarization for an optical component. For optical components, polarization mode dispersion is a deterministic quantity and is the difference in group delay between the two principal states of polarization. In the case of linearly chirped fiber Bragg gratings group delay is a linear function of wavelength and the polarization mode dispersion is simply related to the birefringence of the fiber in the grating region by:
PMD=|CD|×&Dgr;&lgr;
B
(2)
where
PMD is the polarization mode dispersion;
CD is the chromatic dispersion; and
&Dgr;&lgr;
B
is the Bragg wavelength shift.
Equation (2) may also be written as:
PMD=|CD|×
2&Dgr;n&Lgr; (3)
where
PMD is the polarization mode dispersion;
CD is the chromatic dispersion;
&Dgr;n is the effective group index difference; and
&Lgr;is the grating period.
For nonlinearly chirped fiber Bragg gratings, which have an associated nonlinear group delay as a function of wavelength, the local slope of the chromatic dispersion replaces the quantity CD in equation (2) and equation (3).
The magnitude of polarization mode dispersion in linearly chirped fiber Bragg gratings reportedly ranges between about 0.25 picoseconds and about 8 picoseconds. Polarization mode dispersion compensation requirements for optical communication systems may be less than 1 picosecond.
FIG. 1
shows one proposed approach to polarization mode dispersion compensation for a single linearly chirped fiber Bragg grating. The approach uses a 3-port optical circulator
10
. An optical circulator is a non-reciprocating device that transports an optical signal from one port to the next port, only one direction (i.e. 1 to 2, or 2 to 3). They are used to separate forward and backward propagating signals.
An optical signal is received by the first port
12
of the optical circulator
10
. The optical circulator
10
directs the optical signal received by the first port
12
to the optical circulator's
10
second port
14
. The optical signal propagates toward a fiber Bragg grating
16
. The fiber Bragg grating
16
reflects at least a portion of the optical signal back into the second port
14
. The optical circulator
10
directs the reflected optical signal to the third port
18
of the optical circulator
10
. The optical signal exits the third port
18
of the optical circulator
10
and is directed through a &lgr;/2 waveplate
20
before entering a polarization maintaining fiber
22
. The polarization maintaining fiber
22
is connected to a transmission optical waveguide fiber
24
. The &lgr;/2 waveplate
20
in combination with the polarization maintaining fiber
22
compensate for the polarization mode dispersion in the single grating
16
.
FIG. 2
shows another proposed approach to polarization mode dispersion compensation for a single linearly chirped fiber Bragg grating. This approach also uses a three port optical circulator
10
. An optical signal is received by the first port
12
of the optical circulator
10
. The optical circulator
10
directs the optical signal received by the first port
12
to the optical circulator's
10
second port
14
. One end of a polarization maintaining fiber
22
is optically coupled to the second port
14
. The other end of the polarization maintaining fiber
22
is optically coupled to a fiber Bragg grating
16
. The optical signal propagates through the polarization maintaining fiber
22
before at least a portion of the optical signal is reflected by the fiber Bragg grating
16
. The reflected optical signal propagates back through the polarization maintaining fiber
22
and is introduced into the second port
14
of the optical circulator
10
. The optical circulator
10
directs the reflected optical signal out of the third port
18
. Typically, the third port
18
is optically coupled to an optical waveguide fiber
24
.
Pulse broadening induced by polarization mode dispersion is important in chirped Bragg gratings used as chromatic dispersion compensators in optical communication systems, and has been found to be of particular importance in high speed optical communication systems.
SUMMARY OF THE INVENTION
One aspect of the invention is a chromatic dispersion compensator. The chromatic dispersion compensator includes a first optical circulator. The first optical circulator has a first optical port; a second optical port; and a third optical port. The chromatic dispersion compensator further includes a first grating coupled to the second optical port. A polarization controller is coupled to the third optical port. The chromatic dispersion compensator further includes a second optical circulator. The second optical circulator has a fourth optical port coupled to the polarization controller; a f
Boland Daniel M.
Kohnke Glenn E.
Corning Incorporated
Ngo Hung N.
Short Svetlana Z
Smith Eric M.
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